Muyiwa Olabode

WELDABILITY OF HIGH STRENGTH ALUMINIUM ALLOYS

Acta Universitatis Lappeenrantaensis 666

Thesis for the degree of Doctor of Science (Technology) to be presented with due permission for public examination and criticism in lecture hall 1382 at Lappeenranta University of Technology, Lappeenranta, Finland on the 1st of December, 2015, at noon.

Supervisors Professor Jukka Martikainen

Laboratory of Welding Technology

LUT School of Energy Systems

Lappeenranta University of Technology

Finland

Associate Professor Paul Kah

Laboratory of Welding Technology

LUT School of Energy Systems

Lappeenranta University of Technology

Finland

Reviewers Professor Leif Karlsson

Department of Engineering Science

University West

Sweden

Professor Thomas Boellinghaus

Department of Component Safety

Federal Institute of Material Research and Testing

Germany

Opponent Professor Leif Karlsson

Department of Engineering Science

University West

Sweden

ISBN 978-952-265-865-4

ISBN 978-952-265-866-1 (PDF)

ISSN-L 1456-4491

ISSN 1456-4491

Lappeenrannan teknillinen yliopisto

Yliopistopaino 2015

Abstract

Muyiwa Olabode

Weldability of high strength aluminium alloys

Lappeenranta 2015

59 pages

Acta Universitatis Lappeenrantaensis 666

Diss. Lappeenranta University of Technology

ISBN 978-952-265-865-4, ISBN 978-952-265-866-1 (PDF), ISSN-L 1456-4491, ISSN

1456-4491

The need for reduced intrinsic weight of structures and vehicles in the transportation

industry has made aluminium research of interest. Aluminium has properties that are

favourable for structural engineering, including good strength-to-weight ratio, corrosion

resistance and machinability. It can be easily recycled saving energy used in smelting as

compared to steel. Its alloys can have ultimate tensile strength of up to 750 MPa, which

is comparable to steel. Aluminium alloys are generally weldable, however welding of

high strength alloys like the 7xxx series pose considerable challenges.

This paper presents research on the weldability of high strength aluminium alloys,

principally the 7xxx series. The weldability with various weld processes including MIG,

TIG, and FSW, is discussed in addition to consideration of joint types, weld defects and

recommendations for minimizing or preventing weld defects.

Experimental research was carried out on 7025-T6 and AW-7020 alloys. Samples were

welded, and weld cross sections utilized in weld metallurgy studies. Mechanical tests

were carried out including hardness tests and tensile tests. In addition, testing was done

for the presence of Al2O3 on exposed aluminium alloy.

It was observed that at constant weld heat input using a pulsed MIG system, the welding

speed had little or no effect on the weld hardness. However, the grain size increased as

the filler wire feed rate, welding current and welding speed increased. High heat input

resulted in lower hardness of the weld profile. Weld preheating was detrimental to AW-

7020 welds; however, artificial aging was beneficial. Acceptable welds were attained

with pulsed MIG without the removal of the Al2O3 layer prior to welding. The Al2O3

oxide layer was found to have different compositions in different aluminium alloys.

These findings contribute useful additional information to the knowledge base of

aluminium welding. The application of the findings of this study in welding will help

reduce weld cost and improve high strength aluminium structure productivity by

removing the need for pre-weld cleaning. Better understanding of aluminium weld

metallurgy equips weld engineers with information for better aluminium weld design.

Keywords: Aluminium alloys, aluminium welding processes, high strength aluminium,

anodising, Al2O3, 7025-T6, AW-7020

Acknowledgements

I would like to express my appreciation to the many people that have in one way or the

other helped me in the completion of this thesis. I gratefully acknowledge the efforts of

Dr. Paul Kah for his input in the form of research methodology, article corrections,

availability and readiness to guide. I thankfully acknowledge the efforts of Esa

Hiltunen, Antti Heikkinen and Antti Kähkönen in carrying out laboratory weld

experiments. I would also like to thank Dr. Liisa Puro and Toni Väkiparta for their

assistance in carrying out O2 composition experiments on the Al2O3 layer. I would like

to thank Peter Jones for the valuable input on the academic presentation of this thesis. I

wish also to extend my thanks to Martin Kesse for all his support.

I would like to thank the pre-examiners of this work, Professor Leif Karlsson and

Professor Thomas Boellinghaus for their valuable comments and suggestions that

helped in improving the quality of this work.

I wish to acknowledge the encouragement of friends and families. My special thanks are

extended to my families, the Olabodes, the Pöllänens and the Olamilehins for their

encouragement and support. Additionally, my appreciation goes to Bidemi Orebiyi,

Samuel Okunoye, Kevin Eyiowuawi, Edith Emenike, and others that have in one way or

the other supported my journey.

My sincere appreciation is expressed to my immediate family, especially my wife,

Olaitan Olabode, for all the encouragement, forbearance and understanding exercised

during the course of this research. I feel blessed, thank you all.

Muyiwa Olabode

October 2015

Lappeenranta, Finland

Dedication

Dedicated to almighty God, The one who was, is, and is to

come; allowing the acquisition of knowledge and giving the

wisdom to know when, where and how to apply the acquired

knowledge.

Contents

List of publications 11

Author's contribution 11

Nomenclature 13

1 Introduction 15 1.1 Research problem and research questions ............................................... 16 1.2 Scope and limitations of the study .......................................................... 17 1.3 Contribution of the work ......................................................................... 18 1.4 Social and environmental impact ............................................................ 18 1.5 Thesis outline .......................................................................................... 18

2 State of the art of Al welding 19 2.1 Alloy designation .................................................................................... 19 2.2 HSA ......................................................................................................... 19 2.3 Weldability of HSA ................................................................................. 22

2.3.1 Joint types and process limitations .............................................. 22 2.3.2 Work preparation ........................................................................ 26 2.3.3 Welding defects in HSA ............................................................. 30

2.4 Hybrid laser beam welding (HLBW) of HSA ......................................... 32 2.4.1 HLBW focusing head .................................................................. 33 2.4.2 Challenges of HLBW of Al ........................................................ 36

3 Experimental work 37 3.1 Welding metallurgy of HSA (7025-T6) .................................................. 37 3.2 Investigation of the Al2O3 layer in Al alloys ........................................... 39 3.3 Effect of Al2O3 layer on HSA (AW-7020) weld metallurgy ................... 39

4 Results 41 4.1 Findings on the welding metallurgy of HSA (7025-T6) ......................... 42 4.2 Findings on the Al2O3 layer of Al alloys ................................................. 45 4.3 Findings on the effect of Al2O3 on HSA (AW-7020) weld metallurgy .. 45

5 Discussion 49 5.1 Welding metallurgy of HSA (7025-T6) .................................................. 49 5.2 Effect of Al2O3 on HSA (AW-7020) weld metallurgy ............................ 50

6 Conclusions 53

7 Future work 55

References 56

11

List of publications

This thesis is based on the following papers. The rights have been granted by publishers

to include the papers in the dissertation.

I. Olabode, M., Kah P., and Martikainen J. (2012). Experimental review on the

welding metallurgy of HSA (7025-T6) alloy. The Paton Welding Journal, 4,

pp.88-96.

II. Olabode, M., Kah, P., and Martikainen, J. (2013). Aluminium alloys welding

processes: Challenges, joint types and process selection. Proceedings of the

Institution of Mechanical Engineers, Part B: Journal of Engineering

Manufacture, 227(8), 1129-1137.

III. Olabode, M., Kah, P., and Martikainen, J. (2015). Effect of Al2O3 film on the

mechanical properties of a welded high-strength (AW-7020) aluminium alloy.

Proceedings of the Institution of Mechanical Engineers, Part B: Journal of

Engineering Manufacture. DOI: 10.1177/0954405415600678

IV. Olabode, M., Kah, P., and Salminen, A. (2015). Overview of laser systems and

optics applicable to hybrid laser welding of aluminium alloys. Rev. Adv. Mater.

Sci, 42, (2015) 6-19.

Author's contribution

The candidate was the main author of all the publications attached to the doctoral thesis.

The candidate generated the ideas and the conclusions presented in the publications.

Revision was carried out together with the co-authors and reviewers as a joint effort.

The contribution of the author to the publications was as summarized below:

I. Made the research design and experimental design, carried out the experiment,

literature review and analysis, drew inferences and wrote the paper.

II. Made the research design, carried out the literature review and analysis, drew

inferences and wrote the paper.

III. Made the research design and experimental design, carried out the literature

review and analysis, drew inferences and wrote the paper.

IV. Made the research design, carried out the literature review and analysis, drew

inferences and wrote the paper.

13

Nomenclature

Abbreviations Explanation

7025-T6 7025 high strength aluminium alloy, thermally treated

AC Alternating current

AW-7020 7020 high strength aluminium alloy

BM Base material

CZ Composite zone

DC Direct current

DCEN Direct current electrode negative

EBW Electron beam welding

EC Experiment conditions

EDS Energy-dispersive x-ray spectroscopy

FR Feed rate

FSW Friction stir welding

HAZ Heat affected zone

HLBW Hybrid laser beam welding

HSA High strength aluminium

HV3 Hardness Vickers scale 3

I Current

IR Infrared

LBW Laser beam welding

MIG Metal inert gas welding

MZ Melt zone

PAW Plasma arc welding

PMZ Partially mixed zone

S Speed

SAW Submerged arc welding

TIG Tungsten inert gas welding

TZ Transition zone

TZ-UMZ Transition zone to unmixed zone

UHSA Ultra high strength aluminium

UMZ Unmixed zone

UTS Ultimate tensile strength

UW Ultrasonic welding

V Voltage

WI Weld interface

Nomenclature 14

YS Yield strength

Elements and chemical compounds

Al Aluminium

Al2O3 Aluminium oxide

AlMg5 5xxx series aluminium magnesium alloy filler wire

CdS Calcium sulphide

CrO3 Chromium trioxide

Cu Copper

Fe Iron

GaAs Gallium arsenide

Ge Germanium

H2SO4 Sulphuric acid

H3PO3 Phosphorous acid

He Helium

HNO3 Nitric acid

Li Lithium

Mg Magnesium

Mn Manganese

NaOH Sodium hydroxide

Ni Nickel

O2 Oxygen

Si Silicon

Ti Titanium

Zn Zinc

ZnS Zinc sulphide

ZnSe Zinc selenite

Units of measurement

MPa Megapascals

mm Millimetres

s Seconds

min Minutes

V Volts

I Amperes

15

1 Introduction

Lightweight welded metal structures are in increasing demand as a result of growing

concerns regarding efficient energy use and sustainable development (Kopp and Beeh,

2010). Al has become, after steel, the second most used material in structural

engineering (Ostermann, 2007, Schoer and für Schweisstechnik, 2002) due to its

advantageous mechanical, chemical, thermal and electrical properties. These properties

include its good strength-to-weight ratio, relatively good corrosion resistance

(Ostermann, 2007), ease of machinability, high toughness, extreme low temperature

capabilities, usability and recyclability.

Al is widely used in the transportation industry, particularly the automobile and

aerospace industries, due to its relatively low density in comparison to steel, the lower

dead-weight of Al constructions, and the resulting lower energy consumption with

minimal compromise to load carrying capacity (Davis, 1999). About 50% of Al

extrusions are used in the transportation industry (Cock, 1999). Other key industrial

sectors in which Al use is widespread include the construction industry, and power

production and power transmission (Vargel, 2004). In manufacturing industry, Al alloys

are commonly used in pressure vessels and tanks because of their relatively high

strength, good heat conductivity and beneficial properties at low temperatures.

Al alloys are categorised based on alloy composition and the manufacturing process.

The alloys are classified into two types: cast alloys and wrought alloys, with each class

comprising a series of alloys denoted as a range from 1xxx – 9xxx. This work considers

the general weldability of wrought Al alloys and more detailed weldability of high

strength Al (HSA) alloys, and uses 7025-T6 and AW-7020 alloys as case studies.

The use of Al parts is becoming increasingly common in automobile manufacture but

brings some challenges, particularly additional costs resulting from the extra care

needed when welding Al. Al and its alloys have properties that make welding

challenging, such as the presence of the Al oxide (Al2O3) layer that appears when Al

alloys are exposed to the atmosphere, high reflectivity and high heat conductivity

(Sánchez-Amaya et al., 2012a, Sánchez-Amaya et al., 2012b). A number of different

welding systems are applicable to welding of Al, for example, friction stir welding

(FSW), laser beam welding (LBW), metal inert gas (MIG), tungsten inert gas (TIG),

submerged arc welding (SAW), plasma arc welding (PAW), and hybrid laser beam

welding (HLBW). Early efforts to weld Al (HSA) alloys found poor weldability, but

more recent studies (Yeomans, 1990, Graeve and Hirsch, 2010, Dickerson and Irving,

1992) have indicated that this poor weldability was predominantly due to the presence

of copper in the alloy. Recently, it has been found that new technologies like pulsed

MIG welding, pulsed TIG welding and friction stir welding (FSW) can be more

effective than conventional fusion methods. Welding defects commonly found in Al

welds include porosity, incomplete fusion and hot cracking (ASM International

Handbook Committee, 1993, Cary and Helzer, 2005). Research (Dickerson and Irving,

1992, Volpone and Mueller, 2008) has shown that in comparison to steel, greater care is

Introduction 16

required when welding Al, especially control of weld heat input and pre-weld cleaning.

In addition, there are limitations on the weld processes that are applicable. The TIG

weld process has thus far been the most industrially accepted welding process for Al

(Olsen, 2009).

The research approach of this study consists of literature review and experimental work.

Experimental study of robotized pulsed MIG welded 7025-T6 and AW-7020 alloy is

carried out and presented to provide a better understanding of HSA weld metallurgy.

Definitions, properties, applications, weldability and welding defects of Al alloys are

presented. Particular attention is given to the issue of Al2O3 and its effects on

weldability. The Al2O3 layer and its chemical properties are studied and presented. The

formation process and the composition of the two anodic layers are described.

Properties like density, melting point and thermal conductivity are described, and the

advantages and disadvantages of Al2O3 presented. Its formation can be controlled to

gain structural advantages and improved characteristics, for example, by anodisation.

Six basic joint types, and eight common weld processes are analysed. Process

limitations, and associated welding challenges are studied, and their effects on selection

of the optimal welding process considered. A study on HLBW optics applicable to Al,

outlining the welding challenges in HLBW, is carried out and presented. New welding

technologies for Al welding are studied (because newer technologies are expected to

provide faster weld speed, cheaper welding cost and improved welding equipment

efficiency).

The study presents no single optimum process for welding Al. However, FSW, pulsed

MIG and HLBW produce better welds than TIG (Quintino et al., 2012). FSW is shown

to be the presently most favourable process, as it brings important metallurgical

advantages, for example, there is no solidification and liquation cracking, unlike with

fusion welding (Mathers, 2002). It is found in this study that the grains appear to be

reduced in size as the heat input decreases, and welding speed has no significant effect

on the hardness across the weld if heat input is kept constant. The hardness of HSA

joints is lower in the heat affected zone (HAZ) than in the parent metal.

This study is of value to practitioners and the scientific community as limited studies

are currently available about the welding metallurgy of HSA alloys. Recommendations

for selection of the optimal welding process for various Al welds can be based on

knowledge gained in this study and understanding of the limitations of each weld

process. As a contribution to scientific literature, this research provides valuable

information that is of interest when seeking to achieve effective welding of Al alloys,

specifically HSA alloys.

1.1 Research problem and research questions The motivation for this research can be found in the need for a light and strong material

that can be effectively welded. This need is particularly acute in the transportation

industry. From this starting point, a chain of questions was formed as follows:

17

1. What lighter material is commonly used in the transportation industry, especially

aerospace, which has high strength comparable to mild steel? The answers of Al and

Ti led to the next question:

2. Which of these two materials is weldable and which is cheaper? These questions

were asked because welding is the most common joining procedure in industrial

engineering and it is important that engineering solutions are economically viable,

both as regards material costs and manufacturing costs. The welding costs should

usually be as low as possible.

3. Since both materials are weldable and Al is considered to be the cheaper material,

more specific questions were asked about Al. Thus, the next question was: does Al

have other favourable properties for transportation industry uses like good corrosion

resistance and high strength-to-weight ratio? Al clearly has these qualities, which

led to the next question in the chain.

4. What groups or classes of Al alloys are most advantageous, and can the alloys be

modified for structural advantage?

Further questions then are:

5. HSA alloys are favourable from a material properties perspective, but how can they

be welded to obtain optimum weld metallurgy?

6. What are the welding challenges that need to be overcome, including the presence of

Al2O3, control of weld parameters and heat treatability?

Based on the above question chain, a niche arose that has been the subject of limited

research: welding of HSA. The relative paucity of research on HSA may be due to the

alloys being quite new, the cost of the material, and the strict weld requirements in

demanding applications such as those found in the aerospace industry.

1.2 Scope and limitations of the study The scope of this study is focused on the weldability of HSA particularly the weld

metallurgy. However the scope has the following limitations:

1. The literature review in this study is limited to an overview of Al alloys and

their classification, in addition to discussion of their general weldability and

applicable joint types.

2. The study on HLBW optics for Al alloys is limited to consideration of the

optics, the welding head and challenges in HLBW of Al.

3. The experimental study is limited to the study of HSA weld metallurgy using a

pulsed MIG robotic welding machine on 7025-T6 and AW-7020 samples. In

addition, the experiment to investigate the effect of the Al2O3 layer on Al welds

Introduction 18

is limited to AW-7020 welds while the layer composition study is limited to

7025-T6, AW-7020 and 99.9% pure Al samples.

1.3 Contribution of the work As a contribution to the body of scientific knowledge, this work provides valuable

information on HSA weldability and the effects of post-weld heat treatment, in addition

to the effect of Al2O3 on HSA welds. The effects of pre-weld heat treatment and

artificial aging are also determined. A further contribution is that the work provides

information on the necessity or otherwise of Al2O3 removal before welding, and details

under what conditions such Al2O3 removal is unnecessary. The results from the

experiments contribute empirical data to Al welding knowledge, for example, hardness

and tensile values for 7025-T6 and AW-7020. These data can be used by researchers as

background information for further research on HSA weldability, and by welding

engineers when designing welded HSA structures.

1.4 Social and environmental impact A key consideration in scientific research is the social and environmental impact of the

knowledge gained. In the case of this study, improved understanding of the weldability

of Al may generate a greater range of possible applications and thus enhance

employment prospects in the welding industry. If HSA is able to replace mild steel in

structures like motor vehicles, this in turn means lower deadweight, greater fuel

economy, reduced pollution and cost savings for consumers. The ease of recycling Al

allows high energy saving in the production process. When utilising welded materials in

new application areas such as aerospace, knowledge of the material effects of welding

techniques and welding parameters is clearly important, because of safety

considerations, and contributes to reduced risk of structural failure. The issue of the

necessity of pre-weld cleaning of joints has implications for manufacturing costs,

productivity and efficiency, and, consequently, industrial competitivity, as welding

costs are reduced. Efficient use of energy, particularly heat sources, reduces the amount

of carbon emissions allowing for a cleaner environment.

1.5 Thesis outline This thesis comprises two parts: an overall summary of the research work is followed

by reproductions of the published papers. The thesis includes both literature review and

experimental work. Chapter 2 presents state-of-the-art information on welding of Al

alloys on the basis of a review of the literature. An explanation of the terminology and

classifications is presented, and HSA is defined. The chapter also presents key aspects

of HLBW of HSA.

Chapter 3 presents the experiments that were carried out. The methodology is described,

and experimental conditions and relevant weld parameters are presented. Chapter 4

presents the findings from the experiments. Observations and inferences are discussed

in Chapter 5. The conclusions are presented in chapter 6, which also summarizes the

study as a whole, and the first part of the thesis concludes with suggestions for further

studies, given in chapter 7.

19

2 State of the art of Al welding

Trends in Al welding are discussed in this section, which covers material design,

weldability, welding processes and improvements to HSA alloys. For clearer

understanding, it is important first to present Al alloy designations and definitions of

HSA.

2.1 Alloy designation Al alloys are grouped into cast and wrought alloys and the groups are identified with a

four digit number system (e.g.7025-T6). Cast Al alloy designations are like those of

wrought Al alloys except with a decimal between the third and fourth digit (e.g. 771.0-

T71). The second part of the designation, i.e. the part following the 4-digit code

indicating the class, denotes the temper and other fabrication treatments that have been

carried out. For example, a T6 indicates that an alloy has been treated thermally and

then artificially aged. (Maurice, 1997).

The alloy group is classified by the major alloying element, as shown in Table 1. The

second digit denotes the alloy modification or the limits of impurity. ‘0’ in the second

digit denotes an original alloy. Numbers 1 - 9 signify the different alloy modifications

with slight variation in their compositions. In the 1xxx series, the second number

denotes the modifications in impurity limits: ‘0’ implies that the alloy has a natural

impurity limit, 1 - 9 imply that special control has been carried out on one or more

impurities or alloying elements. The last two numbers represent the purity of the alloy

(Campbell, 2006).

In the 1xxx series, the last two numbers signify the alloy’s level of purity. For example,

1070 or 1170 indicates that at least 99.70% Al is present in the alloy, 1050 or 1250

indicates that no less than 99.50% Al is present in the alloy, and 1100 or 1200 implies

that at least 99.00% Al is present in the alloy. For all the other series of Al alloys (2xxx

- 8xxx) the final two numbers have no special significance but are used to identify the

different alloys in the group (Campbell, 2006, Kopeliovich, 2009).

2.2 HSA Al alloys with at least 300MPa yield strength are regarded as HSA. HSA alloys are

generally in the 2xxx, 7xxx, and 8xxx series. HSA is not defined based on the series of

the alloy. For example, two alloys within the same series can have significantly

different yield strengths. For general purposes, however, an average range of the series

yield strength is used to identify HSA alloys, as illustrated in Table 1, by the series

average values.

State of the art of Al welding 20

Table 1 Wrought Al alloy classification (Kopeliovich, 2009, Matweb, 2010, Campbell,

2006)

Series Alloying elements

Percentages Tensile strength, Yield range

Series average value

1xxx 99.0% minimum 10.0 - 165 MPa 94.4 MPa

2xxx Copper 1.9% - 6.8% 68.9 - 520 MPa 303 MPa

3xxx Manganese 0.3% - 1.5% 41.4 - 285 MPa 163 MPa

4xxx Silicon 3.6% - 13.5% 70.0 - 393 MPa 275 MPa

5xxx Magnesium 0.5% - 5.5% 40.0 - 435 MPa 194 MPa

6xxx Magnesium and 0.4% - 1.5% 40.0 - 455 MPa 241 MPa

Silicon 0.2% - 1.7% 40.0 - 455 MPa 241 MPa

7xxx Zinc 1% - 8.2% 80.0 - 725 MPa 399 MPa

8xxx Others 110 - 515 MPa 365 MPa

Major characteristics of the 2xxx series include high strength (at both elevated and room

temperatures), heat treatability and high tensile strength range of 68.9-520 MPa (Gilbert

Kaufman, 2000, Matweb, 2010); some 2xxx alloys are weldable (Gilbert Kaufman,

2000). The chemical composition of 2xxx series alloys usually includes Cu and some

other elements, like Mg, Mn and Si. 2xxx alloys (e.g. 2024 alloy) are used for high

strength products such as those typically found in the aerospace industry, where they are

expected to meet high engineering standards due to stringent safety requirements. 2xxx

alloys are also used in the manufacture of truck bodies (e.g. 2014 alloy); and 2011,

2017, and 2117 alloys are extensively used for screw machine stock and fasteners.

Under naturally aged T4 condition, 2xxx series alloys have similar mechanical

properties to mild steel, with a proof strength of about 250 MPa and an ultimate tensile

strength of around 400 MPa. They also have good ductility. When thermal treatment T6

is used, the proof strength rises to 375 MPa and the ultimate stress can reach 450 MPa.

This, in turn, lowers ductility (John, 1999). Tempered alloys are generally painted or

cladded to increase their corrosion resistance. They find application in parts such as

internal railroad car structural members, tank trucks, structural beams of heavy dump

and trailer trucks, booster rockets of space shuttles and fuel tanks (Gilbert Kaufman,

2000).

The 7xxx series comprises Al-Zn alloys. Mg is also present to control the ageing

process. The alloy group possesses high strength in the high toughness versions. 7xxx

alloys have poor corrosion resistance compared to, for example, the 5xxx series and are

thus cladded in many applications. 7xxx alloys are heat treatable and can reach the 220 -

610 MPa ultimate tensile strength range. They are weldable with some welding

processes, such as pulsed MIG. Some of the highest strength alloys in the 7xxx series

have Cu in the alloy to increase the strength. However, these alloys are not

commercially weldable. The weldability reduces as the Cu content increases (Yeomans,

1990, Graeve and Hirsch, 2010, Dickerson and Irving, 1992). In commercial

applications, such alloys are commonly joined mechanically, e.g., by riveting.

2.2 HSA

21

Figure 1 Mechanical properties of aluminium alloys (Olabode et al., 2015b)

7xxx alloys are mainly used in components for which fracture resistance is a critical

design consideration. A notable example is the Foresmo bridge in northern Norway,

where Al-Mg 7xxx alloys are used in the girder system. 7xxx alloys are also found in

structures in the aerospace industry, for example, they have been used in critical aircraft

wing structures with integrally stiffened Al extrusions. Premium forged aircraft parts

are made from 7175-T736 (T74) alloys (Gilbert Kaufman, 2000).

The 8xxx series have Al and other elements such as Fe, Ni, and Li. These elements

provide the alloy with a specific property, e.g. Ni and Fe increase the yield strength in

the alloy with almost no loss of electrical conductivity. The high strength members of

the series mainly consist of alloys with Li and Cu. Li has lower density than Al and it

has a relatively high solubility. A reduction in density of about 10% compared to other

Al alloys is attainable. The 8xxx alloys have increased stiffness and are age-hardenable.

Some of the series alloys are heat treatable. They have high conductivity, high strength

(tensile strength range of 110 - 515 MPa (Matweb, 2010)) and high hardness. These

alloys are used in the aerospace industry (8090, 8091). The Al-Ni-Fe alloy 8001 is

found in nuclear power generation applications where resistance to aqueous corrosion at

elevated temperatures and pressures is required. The alloy 8017 is used as electrical

conductor (Gilbert Kaufman, 2000).

State of the art of Al welding 22

2.3 Weldability of HSA “Weldability is a measure of how easy it is to make a weld in a particular parent

material, without cracks, with adequate mechanical properties for service, and resistance

to service degradation. It varies with many factors” (TWI, 2015). Figure 1 is a

reproduction from Publication I and Publication IV that presents the fundamental

problem addressed by this research. Although the plots are not completely linear, there

are correlations that can be made. It appears that with higher yield strength of Al alloys,

the weldability, corrosion resistance, toughness, ductility, and fatigue is lower; while

modulus of elasticity and density increases. Growing industrial need for Al alloys has

resulted in considerable research on how to weld Al alloys, particularly newly-

developed alloys. In conjunction with such research, the range of welding technologies

and processes for utilization with Al alloys has increased. Based on previous studies, it

can be stated that:

1. Within the scope of current manufacturing technology, 94% of Al alloys can be

welded and over 50% have optimal weldability (Volpone and Mueller, 2008).

2. Industrially weldable thickness ranges from 0.l - 450 mm (the latter is a special

case, attained using a single pass of EBW) (Volpone and Mueller, 2008).

3. High weld rates are attainable with lower thicknesses (0.8 - 3 mm), for example,

laser butt-joint weld rates range from 5 to 3 m/min (Volpone and Mueller,

2008).

4. Weld heat input present in most fusion welding causes metallurgical problems.

In concentrated energy processes the problems are reduced due to more

controlled heat input and smaller HAZ. Few metallurgical problems are

observed in FSW (Volpone and Mueller, 2008).

5. Conventional welding processes produce welds where the metallurgical

properties of the weld zone deteriorate compared to the base metal. FSW

produces welds that have minimal or even zero deterioration (Volpone and

Mueller, 2008).

6. With the exception of FSW, fusion welding processes produce welds that have

high tendency to suffer from porosity (Volpone and Mueller, 2008).

2.3.1 Joint types and process limitations

The joint-type affects the strength, functionality and applicable welding approach.

Appropriate joint design is important from both a cost perspective and from the point of

view of producing acceptable quality welds. Six common joints and the welding

processes associated with their fabrication are considered in this research: butt joints,

cruciform joints, T-joints, edge joints, lap joints and corner joints. The joint type, joint

location, accessibility, weld processes and strength requirements determine the joint

design. A desire for high weld deposition rate of weld metal makes flat or downward

weld positions desirable and weld position is a factor that has to be considered when

choosing joint designs. In the flat position, the weld pool is usually larger, which allows

for a slower cooling and solidification rate. Trapped gases can escape from the weld

pool due to the slower cooling rate. The flat position yields good quality welds at low

weld cost. In addition, the welds are less prone to porosity and other weld defects,

2.3 Weldability of HSA 23

reducing overall welding costs. The throat thickness controls the static tensile strength

of the welds and is designed to be able to carry the workload of the welded structure. A

weld depth of 3 mm is attainable with conventional TIG and 6 mm with conventional

MIG on plate welds. Bevelling is usually carried out when welding higher thicknesses.

The bevelling can be single or double sided (Mathers, 2002).

A comparison table of the six joint types and welding process limitations as regards

their use with Al alloys was presented in Table 2, reproduced from Publication II. It was

concluded that butt and lap joints are the most applicable joint types, mainly due to the

ease of fixing the workpiece, and cruciform joints the most problematical joint type. Not

all types of welding are equally applicable to Al alloys, with each welding type having

different limitations regarding its usability for Al alloy welding. Limitations for the

different welding technologies considered in this work are given below.

Limitations in MIG welding of Al alloys include the limited weldable thickness of 25

mm when using Ar shielding gas and 75 mm with He (Mathers, 2002). The need for

proper shielding of the weld metal limits the torch distance to a range of 10 to 19 mm.

The demand for effective shielding also limits process flexibility, limiting the

applicability of MIG welding in outdoor processes as air drafts can easily disperse the

shielding gas. The heat radiation levels and arc intensity of MIG welding pose problems

for operators.

A limitation in TIG welding of Al alloys is its shallower weld penetration compared to

MIG welding. When gas shielding is used, the economically viable weld thickness limit

is 10 mm for Ar shielding and 18 mm for He shielding (DCEN). The difficulty of

obtaining adequate penetration in corners and affiliate roots also limits TIG use.

Compared to MIG, it is expensive for welding thick sections, has lower filler and base

metal tolerance and a lower metal deposition rate.

The PAW process has a plate thickness limitation of 6 mm to 60 mm for multiple pass

plasma MIG and 2.5 mm to 60 mm for plasma TIG in a single pass. The equipment is

relatively expensive in comparison with conventional TIG. The complexity of the torch

architecture demands more maintenance and accurate electrode tip setback in reference

to the orifice. Therefore it has limited operational acceptability.

FSW is claimed to be the best weld process for Al due to the minimal deterioration in

the weld metallurgy caused by the process. Welds of 1 mm- 50 mm thickness can be

achieved in a single pass. Available information on tool design, weld mechanical

properties and process parameters is rather limited. Available information on specific

alloys and thicknesses is even more limited. In comparison to LBW the productivity of

FSW is low as fixing the workpiece requires a lot of heavy duty clamps because high

downward forces are needed. The limited knowledge base also affects design guidelines

for implementation.

State of the art of Al welding 24

LBW has a limitation of the energy conversion efficiency of the electrical energy to the

laser beam, called the plug efficiency. The efficiency of lasers is generally from 10 –

30% but in fibre lasers, it can be up to 40%. LBW has low tolerance to gaps between

the workpieces. High volume production is required to justify the high initial capital

costs of LBW equipment. In critical applications, the process is sometimes

economically viable at low production volumes.

RW is limited to 0.9 mm - 3.2 mm weld plate thicknesses. The required accessibility to

both sides of the weld is a limitation, in addition to the low fatigue and tensile strength

of RW welds in comparison to other welding processes. A further limitation is the

limited number of possible joint designs and seam welds that can produce an unzipping

effect. Upset thick sectioned welds are difficult to test for weld imperfection using non-

destructive techniques.

EBW is limited by the need for the welding to be done in a chamber. There is a limit to

the size of the workpiece that can be placed in the chamber. In addition, there is a time

delay in vacuum welds. The weld cost is relatively high and the welds are prone to

defects like cracking due to the rapid solidification rate. The x-rays produced in EBW

are hazardous.

In UW, expensive high-powered transducers are needed for thick welds, which limit the

acceptability of the process. The welding configuration range is also limited. In

addition, vibration control strategies are needed to ensure quality welds across various

geometries and thicknesses, which can be challenging. Weld process limitations are

presented in a tabular form in Table 2, reproduced from Publication II.

2.3

Wel

dab

ilit

y o

f H

SA

25

Tab

le 2

Jo

int

typ

es a

nd

pro

cess

lim

itat

ion

s o

f al

um

iniu

m a

llo

ys

(Ola

bo

de

et a

l.,

20

13

)

Pro

cesses

MIG

T

IG

PA

W

FS

W

LB

W

RW

E

BW

U

W

Join

ts

Butt join

t (a

)

Lap join

t (b

)

T-j

oin

t (c

)

Edge join

t (d

)

Corn

er

join

t (e

)

Cru

ciform

(f)

Lim

itation

Lim

itatio

n

Lim

itation

Lim

itation

Lim

itation

Lim

itation

Lim

itatio

n

Lim

itation

(a)

(b)

(c)

(d)

(e)

(f)

With a

rgon,

weld

able

th

ickness is

limited to 2

5

mm

, and w

ith

heliu

m,

it is

limited to 7

5

mm

.

Lim

ited torc

h

dis

tance o

f 10

–19m

m to

ensure

pro

perly

shie

lded w

eld

m

eta

l lim

its

flexib

ility

.

Lim

ited o

utd

oor

applic

ation

because a

ir

dra

fts c

an

dis

pers

e the

shie

ldin

g g

as.

Lim

ited

opera

tor

accepta

bili

ty o

f th

e p

rocess

because o

f th

e

rela

tively

hig

h

levels

of

radia

ted h

eat

and a

rc

inte

nsity.

Lim

ited

to thin

gauges o

f up to

6m

m thic

kness.

Lim

ited

(shallo

we

r)

penetr

atio

n into

pare

nt m

eta

l com

pare

d to M

IG.

With a

rgon

shie

ldin

g g

as, th

e

econom

ical w

eld

th

ickness lim

it is

10– 1

8m

m w

ith

heliu

m (

DC

EN

).

Difficult to p

en

etr

ate

in

to c

orn

ers

an

d

into

the r

oots

of fille

t w

eld

s.

Lim

ited

by the lo

we

r depositio

n r

ate

, lo

w

tole

rance o

n f

ille

r and b

ase m

eta

l,

and c

ost fo

r th

ick

sections com

pare

d

to M

IG

Lim

ited

to 6

– 6

0m

m

ran

ge.

Pla

sm

a T

IG w

eld

th

icknesses r

an

ge c

an

be less t

ha

n 2

.5–

16m

m in a

sin

gle

p

ass.

Lim

ited

by th

e h

igh

cap

ita

l equip

ment an

d

mate

ria

l cost

com

pare

d to T

IG.

Lim

ited

to

lera

nce o

f th

e p

rocess t

o jo

int

gaps a

nd

mis

alig

nm

ent.

Lim

ited

ope

rato

r a

ccepta

bili

ty o

f th

e

pro

ce

ss d

ue to the

com

ple

x to

rch

arc

hitectu

re that

req

uires m

ore

m

ain

tena

nce a

nd

accura

te s

et-

back o

f th

e e

lectr

ode

tip

with

re

spe

ct to

th

e n

ozzle

o

rifice,

whic

h is

cha

llen

gin

g.

Weld

able

th

ickness

rang

es f

rom

1–

50m

m

(sin

gle

pass).

Too

l desig

n, pro

cess

pa

ram

ete

rs,

and

m

ech

an

ica

l p

rope

rtie

s

da

tabase

is lim

ite

d a

nd

o

nly

availa

ble

fo

r lim

ite

d

allo

ys a

nd

th

icknesses

(up

to 7

0 m

m).

Lim

ited

to low

er

pro

ductivity c

ases

com

pare

d to

LB

W.

Insu

ffic

ient d

esig

n

gu

idelin

es a

nd

lim

ited

ed

ucatio

n f

or

imp

lem

enta

tio

n.

Exit h

ole

left w

he

n t

ool is

w

ithd

raw

n.

La

rge d

ow

n f

orc

es

requ

ired

with

heavy d

uty

cla

mp

ing

necessa

ry to

hold

the

pla

tes to

geth

er

durin

g w

eld

ing.

Environm

enta

lly friendly

w

eld

ing p

rocess

be

cau

se

fum

es a

nd

spatters

are

not

gen

era

ted.

Lim

ited

convers

ion

effic

iency o

f ele

ctr

ica

l po

we

r to

focused

in

frare

d laser

be

am

als

o c

alle

d

wall

plu

g

effic

iency (

abo

ut

10%

–30

% a

nd

up to 4

0%

in

fibre

lasers

).

Lim

ited

fit u

p

tole

rance.

Pre

cis

e fit u

p

(15%

of

mate

ria

l th

ickness)

nee

de

d fo

r b

utt

and

lap join

ts.

Lim

ited

op

era

tor

accepta

bili

ty o

f th

e p

rocess d

ue

to t

he larg

e

ca

pital

investm

ent

ne

ed

ed,

the

refo

re

requ

irin

g h

igh

vo

lum

e

pro

ductio

n o

r critica

l ap

plic

ations t

o

justify

the

expe

nd

iture

.

Lim

ited

we

ld

thic

kness r

ang

e

(0.9

–3.2

mm

)

Lo

we

r te

nsile

an

d

fatigu

e s

tre

ngth

com

pare

d to

oth

er

fusio

n w

eld

ing

pro

ce

sses.

Lim

ited

join

t de

sig

ns o

r co

nfigura

tio

n.

Se

am

we

lds c

an

ge

nera

te

un

zip

pin

g e

ffect.

Lim

ited

op

era

tor

acce

pta

bili

ty o

f th

e p

rocess

be

cause

, in

th

ick-

sectio

ne

d u

pse

t w

eld

s; th

ere

is

lack o

f g

ood n

on-

de

str

uctive

we

ld

qu

alit

y testing

hig

h e

lectr

ode

we

ar

rate

and

de

teriora

tion

.

In a

dditio

n, it

req

uires a

ccess to

bo

th s

ides o

f th

e

join

t.

Hig

h c

ost of

eq

uip

ment.

Work

cham

ber

siz

e

co

nstr

ain

ts.

Tim

e d

ela

y

wh

en w

eld

ing

in a

vacu

um

.

Hig

h w

eld

pre

pa

ration

costs

.

X-r

ays

pro

duced

Durin

g w

eld

ing

can b

e a

hea

lth

risk.

Rap

id

so

lidific

ation

ra

tes c

an

ca

use c

rackin

g

in s

om

e

mate

ria

ls.

Can

we

ld u

p to

45

0m

m th

ick

pla

tes.

Expe

nsiv

e h

igh

po

we

red

tr

ansdu

cers

are

n

ee

de

d to e

nable

w

eld

ing

of th

ick

gau

ges, castings,

extr

usio

ns,

an

d

hyd

ro-f

orm

ed

com

pone

nts

.

Alte

rnative

we

ldin

g

configura

tio

ns a

re

nee

de

d to w

eld

a

wid

e v

arie

ty o

f com

pon

ent

geom

etr

ies a

nd

join

t configura

tio

ns.

Vib

ration c

ontr

ol

str

ate

gie

s a

re

needed to e

nsu

re

weld

qu

alit

y a

cro

ss

a w

ide r

ang

e o

f com

pon

ent

geom

etr

ies a

nd th

e

thic

kn

ess o

f th

e

weld

pie

ce

is

limite

d.

FU

ND

AM

ENTA

LLY

AP

PLI

CA

BLE

AP

PLI

CA

BLE

WH

EN M

AN

IPU

LATE

D

N

OT

AP

PLI

CA

BLE

NO

AP

PLI

CA

BIL

ITY

DA

TA Y

ET

State of the art of Al welding 26

2.3.2 Work preparation

Successful welding of HSA is very dependent on rigorous preparation work. The

preparation work involves selection of a suitable welding process, correct storage and

handling of the workpieces, appropriate workpiece preparation (like grinding off the

Al2O3 layer, bevelling etc.) as well as application of acceptable joint design (Yeomans,

1990, Graeve and Hirsch, 2010, Dickerson and Irving, 1992).

The Al2O3 layer found on the surface of Al workpieces occurs because Al has a strong

chemical affinity for O2 in air or moist environments. The thickness of the Al2O3 layer

is dependent on the physiochemical conditions of the environment, and the thermal

treatment or electrochemical treatment (anodisation) (Karambakhsh et al., 2010, ASM

International Handbook Committee, 1994).

Figure 2 Schematic of aluminium oxide layer and the anodised surface showing the

melting temperatures (Olabode et al., 2013)

In general welding practice, the Al2O3 layer is removed just before welding, by dry

machining or pickling. The Al2O3 layer has a melting temperature of 2050°C,

considerably above that of Al, which melts at about 660°C (as illustrated in Figure 2

from Publication II)(Mathers, 2002) The melting point difference is not a problem in

high heat density welding processes but the significant mechanical strength of the layer

can lead to Al2O3 remaining as a solid layer of fractured small particles when the

surrounding metal is molten (Riveiro et al., 2010). Removal of the layer is therefore

encouraged to avoid the possibility of the Al2O3 layer causing incomplete fusion.

In arc welding processes, the Al2O3 layer is an electrical insulator that inhibits arc

initiation. Thick layers of Al2O3 in MIG process generate erratic electrical commutation

in the contact tube of the gun thereby producing poor welds. Recommended removal

methods are brushing with a stainless steel bristled brush and other mechanical

processes like cutting or grinding using an Al2O3 grinding disk (Mathers, 2002). The

Al2O3 layer regenerates immediately if scratched. This property is responsible for the

corrosion resistance of Al (George and MacKenzie, 2003). Al2O3 is usually in hydrate

form and is hygroscopic. In some welding processes, the layer is removed by the weld

system. In UW, the vibration enhances removal of the oxide layer along with other

contaminants due to the high frequency of the vibration (Baboi and Grewell, 2010). The

layer is also removed by HLBW MIG welding systems, although cleaning by a

mechanical process is still recommended (Olsen, 2009). Cathode etching is another

2.3 Weldability of HSA 27 27

means by which the Al2O3 layer is removed. In gas shielded arcs, it is chemically

corroded thereby showing the microstructure (Novikov, 2003). It is beneficial to remove

the Al2O3 layer because (ASM International Handbook Committee, 1993, Mathers,

2002) :

1. It minimises the possibility of hydrogen porosity in the weld

2. It provides better weld stability, especially in TIG welds

3. It ensures complete weld fusion. In TIG welding, chemical etching removing the

layer is advantageous since the layer forms almost immediately after mechanical

cleaning.

The Al2O3 layer can be formed and enhanced through a process called anodisation.

Anodisation is an electrochemical process that converts the surface of metals into a

durable, decorative and corrosion resistant anodic oxide finish (Thompson, 1999,

Mukherjee, 2010). An even porous morphology of amorphous alumina (Keller et al.,

1953) is formed in acidic and alkaline electrolytes. In the process, the layer is fully

integrated into the Al substrate unlike plating or paint, so it does not chip off or peel.

The layer can be processed by painting or sealing due to the ordered and porous

structure (Thompson, 1999).

Anodisation is done to increase corrosion resistance, provide better appearance, increase

resistance to fading, improve the bonding of adhesives, provide for paint adhesion,

increase application abrasion resistance, increase lubricity, provide electrical insulation,

permit investigation of surface flaws, provide for further plating, and to provide for the

possibility of lithographic and photographic emulsion (Thompson, 1999, Mert et al.,

2011, ASM International Handbook Committee, 1994). The structure of Al2O3 is double

layered after anodization (the anodised layer and the oxide layer as in Figure 3 from

Publication III) which makes welding challenging (Thompson, 1999).

Figure 3 Schematic diagram of a cross-section of a porous anodic film on aluminium

showing the barrier, pore and other principal morphological features (Olabode et al.,

2015b)

State of the art of Al welding 28

Shielding gas is generally needed to protect the weld from atmospheric interactions.

Heated metal has an affinity to react with the atmosphere around its melting point. Al

easily reacts with O2 at room temperatures. When selecting the shielding gas, the

criteria that should be met are (Welding Journal, 2008, Boughton and Matani, 1967,

Olson et al., 1993, Mathers, 2002):

1. The shielding gas must permit plasma arc generation and promote stable arc

characteristics and mechanisms.

2. It should not degrade welding parameters like weld speed.

3. It should minimise the need for post weld cleaning.

4. It should protect the molten pool, the wire tip and welding head from oxidation.

5. It should enhance welds by providing better weld penetration and good bead

appearance.

6. It should help to prevent undercuts.

7. It should not compromise the mechanical properties of the weld.

8. It should allow for smooth detachment of molten metal from the filler wire in

addition to the provision of the required metal transfer mode.

Inert gases like He and Ar are common shielding gases used in Al welding. It is

important to understand the characteristics of the gases to enhance the shielding gas

selection process. Ar is a cheaper gas than He and it generates a stable weld arc,

producing smooth welds. On the other hand, it is more prone to producing welds with

porosity and lack of fusion due to the lower generated heat input and lower wire weld

speeds. Black sooty deposits can be left on the weld surfaces, which need to be cleaned

by brushing. He shielding gas produces hotter arcs, wider weld beads, and deeper

penetration due to its 20% greater arc voltage compared to Ar. Therefore, arc

positioning is less critical than when using He. Due to the slower cooling rate from the

hotter weld pool, hydrogen diffuses better from the weld, thereby reducing porosity. He

permits the attainment of weld speeds up to three times higher than when using Ar

shielding gas. However, because of its cost and the degree of arc instability, He is

mainly used in mechanised and automatic welding.

Ar is the recommended shielding gas for welding of HSA using a pulsed MIG process

(Boughton and Matani, 1967, Yeomans, 1990). However Ar-He mixtures can also be

used or He alone. Ar-He mixtures provide a compromise on the characteristics of both

gases and enable improved weld productivity with higher weld speeds and more

acceptable welds. A gas purity of over 99.998% at the weld torch is recommended and

the moisture level must be kept low (-50°C less than 39 ppm H2O) (Mathers, 2002).

Studies show that the shielding gas increases weld penetration by providing higher arc

energy and metal deposition rate (Blewett, 1991, Yeomans, 1990). When the section is

lower than 50 mm, He should be used (Mathers, 2002). Further details are presented in

Table 3 and the effects of the shielding gases are presented in tabular form in Table 4.

2.3 Weldability of HSA 29

29

Table 3 MIG shielding gases for Al (Welding Journal, 2008)

Metal transfer mode Shielding gas Characteristics

Spray transfer

100% Ar Good cleaning action, less spatter and has the best metal transfer In addition to having good arc stability.

35% Ar - 65% He

In comparison to Ar it generates higher heat input with an improved fusion characteristics for weld with minimal porosity

25% Ar - 75% He This mixture requires the least cleaning action, minimises porosity and produces the highest heat input.

Short circuiting Ar and Ar + He This mixture is fairly adequate for sheet metal however Ar-He is preferred on thicker weld sections.

Table 4 Effect of shielding gas on Al welding (Kang et al., 2009, Hilton and Norrish,

1988, Matz and Wilhelm, 2011, Mathers, 2002, Campana et al., 2009, Campbell et al.,

2012, Kah and Martikainen)

Shielding gas

Relative effect (100% Ar as the reference) 100%Ar Ar+He 100% He

Gas flow Minimum Higher Highest

Arc voltage (MIG) Minimum Higher Highest

Arc (MIG) Minimum stability More unstable Most unstable

Weld seam width and depth

Minimum width and standard depth

Higher width Shorter depth

Highest width Shortest depth

Weld seam appearance Minimum smoothness Smoother Smoothest

Penetration Minimum depth and roundness

Deeper and more round

Deepest and most round

Welding speed Minimum welding speed

Higher attainability Highest attainability

Lack of fusion Standard Lower Lowest

Porosity Standard Lower Lowest

Pre-heating Standard Less needed Least needed

Heat production Minimum warmth Warmer workpiece Warmest workpiece

Cost of shielding gas Minimum price More expensive Most expensive

State of the art of Al welding 30

2.3.3 Welding defects in HSA

The welding of Al requires strict control of heat input despite the fact that it has a lower

melting point compared to steel. The welding of Al is critical because of the following

considerations (Campbell, 2006, Olson et al., 1993):

1. Stable surface oxide needs to be eliminated before welding.

2. The presence of residual stresses can cause weld cracks as a result of the high

thermal expansion coefficient of Al.

3. The high heat conductivity of Al means that more heat is required to attain

welds; however, high heat input increases the possibility of distortion and

cracking.

4. High shrinkage rates on solidification increase the incidence of cracking.

5. The high solubility of hydrogen in molten Al can cause porosity.

6. The general susceptibility of HSA series to weld cracking.

Major welding defects in HSA series alloys include hot cracking, porosity, joint

softening, non-recoverable post-weld ageing, poor weld zone ductility (HAZ

degradation) and susceptibility of the joint to stress corrosion cracking. Further details

on weld defects and remedies are presented in Table 5.

Table 5 Trouble Shooting Aluminium Welds (Renshaw, 2004, Ba Ruizhang, 2004,

Olson et al., 1993, John, 1999, Joseph, 1993)

Defect Cause Remedy

Oxide inclusions

Poorly cleaned joints Ensure proper wire brushing of joints before and after each weld pass. The particles should be wiped off thesurface.

The presence of an Al2O3

layer on the filler rods of the weld metal

When possible, use fresh wire spool.Clean wires and rods to remove oxides.

The presence of sharp corners on the joint's groove

Break sharp edges during weld preparation.

Weld porosity Insufficient weld shielding Eradicate draughts.

Increase the amount of gas flow.

Reduce the electrode extension.

Presence of dye penetrants and lubricants

Use solvent to clean the surfaces of the workpiece. All lubricants should be removed from the weld area.

High welding current Reduce the current according to the recommended welding procedure.

Impure or contaminated shielding gas

Inspect the gas hoses to ensure thereare no leakages, and make sure that there is no coolant leak on the torch.

2.3 Weldability of HSA 31 31

Replace the gas cylinders if possible.

Wrong torch angle or too high travel speed

Apply the correct torch angle and travel speed based on the welding procedure.

Contaminated filler material Ensure that filler material is cleaned with solvent.

Moisture Before welding, heat and clean the surface of the workpiece.

Fusion zone porosity

Presence of hydrogen in the base material

Use pure He shielding gas, reducesodium additives and amend degassing practices.

Cold cracking Presence of too rigid joint restraints

Preheat the workpiece and slacken theclamps to reduce stress.

Hot cracking Excessive parent metal dilution

Reduce the weld current and increasethe filler wire deposition rate.

Too high interpass temperature

Reduce the weld current and introducepauses between each weld pass to increase cooling.

Undercutting Too high welding current Use the appropriate current.

Too high travel speed and inadequate filler material

Reduce the speed based on welding procedure recommendations and use recommended filler material.

Too long arc length Use arc lengths based on recommendations.

Lack of fusion

Low welding current Use appropriate welding current basedon welding procedurerecommendations.

High travel speed Reduce travel speed according to weld procedure recommendations.

Inadequate jointpreparation

Improve joint preparation.

Incorrect torch angle Use the correct torch angle based on the welding procedure.

Crater cracking

Inappropriate arc breaking Gradually reduce arc current, use ‘crater fill’ control. ‘Back weld’ at least the last25 mm of the bead.

Overlap Slow travel speed Increase travel speed according towelding procedure recommendations.

Insufficient weld current Increase the welding current.

State of the art of Al welding 32

Excessive filler material Reduce filler material.

Incorrect torch angle Use the correct torch angle based on the welding procedure.

Drop through Slow travel speed Increase travel speed.

Welding current too high Decrease the welding current.

Wide joint gap Reduce the joint gap and improve fit-up.

Too much heat build-up Reduce the interpass temperature.

2.4 Hybrid laser beam welding (HLBW) of HSA HLBW is a welding process that combines LBW with an arc welding process thereby

utilizing the advantages of both processes. It has better weld bridgability, and higher

weld speed and quality than LBW. HLBW beam delivery and focusing optics are

presented in addition to the welding head configuration and the challenges of HLBW of

Al.

The most commonly used HLBW system is laser hybrid MIG (Olsen, 2009).

Absorption of the beam by Al depends on the wavelength of the laser beam. As

presented in Figure 4 from Publication IV, due to the wave length of solid-state lasers,

Nd: YAG and fibre lasers are the most common laser power sources used in hybrid

MIG welding. Optics found in HLBW systems includes mirrors, lenses and fibre optics.

In HLBW, the laser beam needs to be focused to achieve small spot diameter. The small

spot diameter allows for higher beam density on the workpiece. The spot diameter is a

function of the lens design and focal length. Beam transfer and focusing is achievable

using diffractive optics, refractive optics or reflective optics.

Beam delivery optics before focusing utilize mirrors (for diffracting light). Mirrors can

be planar or spherical in design. The mirror is firmly fixed to an adjustable screw with

ease of accessibility for cleaning, inspection and replacement. The usability of

conventional mirror delivery is limited by the rigidity of the mechanical mounting and

the mirrors cannot move relatively to each other to avoid misalignment. Mirrors are

limited in size therefore transferring beams over a long distance with high divergence

can produce a beam diameter that is larger than the lens. Consequently there is a limit to

the distance over which beams can be transferred via mirrors

The lens is a component in beam delivery and is used for converging or diverging light.

The lens can be a simple one-element optic, generally with a focal length less than 254

mm, and can be an aspheric, plano-concave/convex or meniscus lens (Ready et al.,

2001, American Welding Society et al., 2006). Compound optics can be used, where the

lens is made of two or more separate lenses.

2.4 Hybrid laser beam welding (HLBW) of HSA 33

33

Fibre optics are another component in beam delivery and are used in Nd: YAG laser

systems to deliver beams. Fibre optics are used due to the 1.06μm wavelength

transferable over glass fibres. Fibre optics utilize the flexibility of glass fibre within the

specified bend radii for a fibre bundle. They are attractive in comparison to

conventional beam delivery due to the possibility of transporting beams over long

distances of up to 50m and around curves (Bakken, 2001).

Figure 4 Absorption of laser wavelength by metals (Olabode et al., 2015b)

Focusing optics are common in low-power welding devices. Parabolic lenses are

generally useful for focusing power above 1.5 kW in CO2 lasers. Due to the low cost

and minimal spherical aberration attributes of f-numbers above five, lenses in focusing

optics are usually plano-convex lenses. The f-number is derived by dividing the lens

focal length by the beam diameter.

Laser protection lens is a sacrificial cheaper lens placed to prevent the debris having

contact with the welding head lens. Protection lens is usually cheaper and easier to

remove and replace compared to the welding head lens. It’s usually used in laser

processes where the focal length is short or when the weld metal is volatile and

contaminated; or when weld spatter can be generated and debris can attach itself to the

welding head lens. Al is highly reflective to the laser beam wavelength, and reflected

beams can damage laser optics

2.4.1 HLBW focusing head

The performance of the beam delivery system determines the quality of laser beam

welding. The beam delivery should be as simple and as small as possible, having neither

State of the art of Al welding 34

actuators nor sensors, to allow for easy manipulation and integration with a robotic

welding system. However, available technologies for laser welding heads have

numerous advantages so consumers still tend to buy the technologies and thus the laser

heads are becoming more and more complex. Common technologies in laser focusing

heads include integrated actuators and sensors, closed loop systems, self-learning and

self-adapting systems.

Combinations of laser beam and arc can take a number of different configurations that

remarkably influence the weld performance. The principal classification criteria for

laser beam and arc combinations are presented in Figure 5 from Publication IV (based

on the heat source type), and Figure 6 from Publication IV (based on configuration).

The choice of the secondary heat source can be either arcs with consumable electrodes

or arcs with non-consumable electrodes. The former are selected when filler metal is

required to solve specific weld problems, otherwise the latter are preferred. The

arrangement plays an important role in the effectiveness and weld efficiency of the

system and the quality of the welds. The heat sources can be arranged to have a

common operation point (Figure 7 from Publication IV) or separate operation points

(Figure 8 from Publication IV).

Figure 5 Schematic of heat sources available for hybrid laser–arc combinations

(Olabode et al., 2015b).

Figure 6 Geometrical arrangements for hybrid laser–arc welding (Olabode et al.,

2015b).

Heat sources for hybrid laser–arc welding

Primary heat sources Secondary heat sources

Arcs with consumable electrodes

Arcs with non-consumable electrodes

Geometrical arrangements for hybrid laser–arc welding

Common operation point Separate operation points

Parallel technique Serial technique

2.4 Hybrid laser beam welding (HLBW) of HSA 35

35

(a) (b)

Figure 7 Schematic diagrams of hybrid laser–arc welding with a common operation

point replotted from (Olabode et al., 2015b)

Figure 8 Schematic diagram of hybrid laser–arc welding with separate operation points

replotted from (Olabode et al., 2015b)

Separated operation point arrangements are defined to be serial technique or parallel

technique or a combination of both. The serial technique is a configuration in which the

primary and secondary heat sources have an acting point distance between them, known

as the “working distance,” in a vertical or horizontal direction along the welding path.

The arc source can lead or trail the laser beam.

State of the art of Al welding 36

Other HLBW configurations with more than two heat sources have been studied and are

presented in Figure 9 from Publication IV.

Figure 9 Schematic diagrams of hybrid laser–arc processes with two secondary heat

sources (Olabode et al., 2015b).

2.4.2 Challenges of HLBW of Al

Al alloys present challenges for HLBW optics. One of the challenges limiting utilization

of such welding systems and optics is the high reflectivity of Al alloys, which limits the

choice of laser beam source to Nd: YAG and fibre lasers. Secondly, the melt zone (MZ),

and HAZ are larger in HLBW than in laser welding. The molten zone at the weld top is

wider due to the presence of arc welding process (Page et al., 2002) and the large HAZ

compromises the metallurgical properties of the weld. Thirdly, due to the wider weld

pool and higher melt temperature in HLBW, difficulties arise in covering the weld pool

with shielding gas, which can lead to contamination of the weld and porosity

(Rasmussen and Dubourg, 2005). Fourthly, volatile elements in alloys can evaporate

from the normally generated keyhole, resulting in poorer metallurgical properties of the

weld and even porosity if gas bubbles are trapped in the weld. This problem can be

mitigated by proper selection of filler material (Duley, 1999). In addition, volatile

elements present in Al alloys can generate spatter during welding, which can adhere to

the lens and damage it. A precaution is to use a protective lens. Fifthly, Al alloys have

low surface tension, and they have poor ability for root-side melt pool support. This

tends to cause difficulty in full penetration welding, specifically in thick butt welds

(Andersen and Jensen, 2001). Finally, the existence of a large number of non-

independent and interacting welding parameters compared to MIG or laser welding

processes poses control challenges, in addition to the metallurgical challenges

mentioned earlier. Therefore, HLBW of Al alloys is complicated to design and operate

(Sepold et al., 2003). Rasmussen et al. (2005) show that successful welding of Al using

HLBW demands clear understanding of the governing parameters and their effects and

interactions (Rasmussen and Dubourg, 2005) to be able to maximise the advantage of

HLBW as a robust industrial welding process (Sepold et al., 2003).

37

3 Experimental work

Three sets of experiments were carried out in this study. One focused on a study of the

microstructure of a welded HSA. Another experimental focus was a study of the

presence and composition of Al2O3 on Al alloys. The last experiment focused on

analysis of the effect of the presence of Al2O3 on the mechanical properties of welded

HSA alloys.

3.1 Welding metallurgy of HSA (7025-T6) Experimental work was carried out on Al alloy 7025-T6 using a robotised pulsed MIG

welding machine. The setup is presented in Figure 10. The robot movement was

programmed and some test sample welds were made, after which alloy 7025- T6 was

welded. Different welds trials were made and the weld parameters were varied to study

the effect of heat input and welding speed on the properties of the weld metal. A torch

angle of 10° pushing in the weld direction was used to allow for purging of the weld

area ahead of the arc. A 2 mm wire extension was used and the nozzle-to-workpiece

distance (stick-out length) was 18 mm. The shielding gas used was 99.995 % Ar and the

filler wire was 4043 Al.

Figure 10 7025-T6 weld setup

Figure 11 AW-7020 weld setup

The workpiece was a 5 mm thick plate with an area of 100 × 250 mm. The samples

were bead-on-plate welds, so there was no bevelling. The joints were cleaned

mechanically using a stainless steel bristle brush reserved for Al only. Experimental

trials were performed, from which 6 different sample sets of 7025-T6 alloy were

selected. The first three sample sets (A, B and C) had the same wire feed rate so as to

study the effect of the welding speed (10, 20 and 30 mm/s respectively). The other three

sample sets (D, E and F) had approximately the same heat input to investigate the effect

of constant heat input on the weld. The pulse current frequency was approximately 250

Hz in each weld. For sample sets A - C, the wire feed rate was constant (10 m/min) and

the heat input varied. Heat input Q for all samples was calculated as (Hirata, 2003):

Experimental work 38

𝑄 =𝑉 × 𝐼 × 60

1000 × 𝑆× 0.8

Where Q denotes heat input measured in kJ/mm; V is the voltage measured in volts; I

denotes current measured in Amperes; and S is the welding speed measured in mm/min;

0.8 is the pulsed MIG process efficiency. The heat input for samples D - F was

approximately constant and the feed rates were 10, 12 and 14 m/min, respectively. The

weld parameters are presented Table 6. The filler wire used had a tensile strength of 165

MPa, yield strength of 55 MPa and an elongation of 18%. Shielding gas supplied

through the weld torch protected the weld pool.

Table 6 Weld conditions for 7025-T6 experiment

Welding conditions for 7025-T6 welding

Weld type Bead-on-plate

Base material 7025-T6, thickness 5 mm

Filler material 4043 Al, 2 mm wire extension fro torch

Shielding gas 99.995 % Ar

Welding speed 7.5 mm/s

Nozzle distance 18 mm

Torch angle 10° to normal

Experiment specific parameter

Sample Weld speed in mm/min

Feed rate in m/ min

Heat input Q (kJ/mm)

Voltage (V) Current (A)

A 600 10 0.318 20.1 198

B 1500 10 0.127 19.4 205

C 1800 10 0.106 19.4 205

D 1200 10 0.160 19.8 202

E 1440 12 0.163 20.3 241

F 1728 14 0.158 20.5 278

The weld hardness test was carried out using HV3 scale on a hardness indenter

machine. Indentations were made on the surface of the test piece by diamond indenters.

The indenter was pyramid shaped (Figure 12) and a weight between 1 to 100 kg can be

subjected on the indenter. In this research the indenter places pressure on the workpiece

for about 10 s. The test was carried out with a 3 kg weight indentation of the diamond

tool tip on the prepared weld cross-section. 3 kg was sufficient because Al is relatively

soft and 3 kg is heavy enough to create indents. It is important that the weight is

appropriate so that the material can resist the load to some extent. The indentations were

done at about 1 mm from the weld surface in a row. The distance between each

indentation was 0.7 mm. The shape of the indentation resembled a rhombus. The depth

3.2 Investigation of the Al2O3 layer in Al alloys 39

39

of the indents depended on the material’s hardness. Note that the longer the length of

the diagonals appearing on the workpiece, the softer the material. The indenter footprint

was measured with the aid of a microscope and the averages calculated. The averaged

values were looked up from an HV3 table to determine the hardness values. The

hardness values were then plotted on a graph against the distance of each indentation

from the weld centreline.

Figure 12 Schematic of hardness testing indenter

3.2 Investigation of the Al2O3 layer in Al alloys Experiments were carried out to study the composition of the Al2O3 layer at different

distances from the alloy surfaces. 99% pure Al alloy (1xxx series), and AW-7020 and

7025-T6 samples were pre-cleaned, and then exposed to the atmosphere for one hour.

The samples were then tested for the presence of Al2O3 by placing them in an X-ray

detector (Ultra Dry EDS). Each sample was tested at a depth of 0.2µm, 1.2µm, and

3.3µm. For each depth, four measurement spots of 0.2 by 0.5 mm were selected and the

significant chemical contents analysed.

3.3 Effect of Al2O3 layer on HSA (AW-7020) weld metallurgy The purpose of this experiment was to study the effect of the presence of the Al2O3 layer

on the mechanical properties of AW-7020. The experiment was carried out as butt

welds of 2 samples each for the four weld experiment conditions (EC) 1 - 4 with the

weld parameters presented in Table 7. A robotized pulsed MIG machine was used to

weld the specimens and the weld setup is presented in Figure 11. A 5 mm AW-7020

plate was used as the workpiece. The air gap between the workpiece was 3 mm. Copper

backing was used. Pure Ar (99.5%) shielding gas supplied at a flow rate of 15 l/min was

used. A 1.2 mm diameter Elga AlMg5 filler material was supplied at 9 m/min. A nozzle

distance of 15 mm and a welding speed of 7.5 mm/s were used. The weld torch was

inclined at 15o to normal and the weld direction was such that the torch is pulling. An

average voltage of 22.7 V and an average current of 140 were used in all the

experiments.

Experimental work 40

Table 7 AW-7020 weld experiment parameters

Welding conditions for AW-7020 welding

Weld type Butt welding, I-groove, air gap 3 mm, against copper backing

Base material

AW-7020, thickness 5 mm

Filler material Elga AlMg5, Ø1.2 mm

Shielding gas Ar, flow rate 15 l/min

Wire feed rate

9 m/min

Welding speed

7.5 mm/s

Nozzle distance

15 mm

Torch angle 15° to normal

Experiment specific parameter

Experiment conditions (EC)

Current (A) Averages

Voltage (V) Averages

Al2O3 thin film

Pre-heating

Artificial ageing

1 140 22.6 Present No No

2 139 22.8 Present Yes (130

oC)

No

3 140 22.7 Present No Yes (480

0C/2 h +

quenching in water, 90

0C/8 h + 145

0C/15 h)

4 140 22.7 Absent No No

The test was carried out in a welding workshop in a controlled atmosphere and at room

temperature. The samples for the four different EC were cut, welded, and examined. In

EC 1, the weld was carried out without pre-weld cleaning of the Al2O3 in addition to the

absence of weld heat treatment. In EC 2, the weld was conducted without removal of

Al2O3. However, the workpiece was preheated at a temperature of 130oC, which is

within the recommended preheating temperature but close to the upper limit [30]. The

oxide layer in EC 3 was not removed before the welding. No preheating was carried out

but natural aging was conducted by post-weld heating at 480oC for 2 hours, followed by

quenching in water at 90oC for 8 hours and, finally, reheating and maintaining the

workpiece heat at 145oC for 15 hours. The Al2O3 layer in EC 4 was removed and no

preheating or artificial aging was carried out. In order to investigate the effect of Al2O3

on the mechanical properties, the samples were examined for ultimate yield strength,

tensile strength, elongation, and hardness values. Macrographs were taken to evaluate

the weld defects present, if any.

41

4 Results

This section presents the key findings of the research. For clarity in the result

presentation, it is important to understand fusion weld region nomenclature. Figure 13

presents the weld regions comprising of the composite zone (CZ), transition zone (TZ),

unmixed zone (UMZ), weld interface (WI) partially melted zone (PMZ), fusion

boundary, “true” heat –affected zone (T-HAZ), and base metal (BM).

The CZ is a fusion weld region where the filler metal is mixed with the base metal as

composite. The TZ is a region between the UMZ and CZ where there is compositional

gradient from the BM to the CZ. The UMZ is the region where the melted and

resolidified base metal is unmixed with the filler metal. The WI is an imaginary line that

divides the UMZ and PMZ. The PMZ is a fusion weld region just before the T-HAZ

where there is incomplete melting of the base metal. The fusion boundary is the region

that consists of UMZ, WI, and PMZ. The T- HAZ is a region where there is no melting

or liquation and the metallurgical interactions occur is the solid state. The PMZ and T-

HAZ are considered as the HAZ. The BM is the region of the base metal unaffected by

the weld heat input (Lippold, 2014).

(a)

(b)

Figure 13 Fusion weld zone nomenclature: (a) schematic diagram of fusion weld zone,

(b) micrograph of 7025-T6 fusion weld

It is sometimes challenging to distinct amongst the regions in micrographs, in this thesis

TZ and UMZ have been combined as a single region and denoted as TZ-UMZ; and

HAZ has been used to denote a combined region of PMZ and T-HAZ. The results of the

experiments are analysed and presented in sections according to the three different

experiments carried out. The findings presented are those relevant to the scope of this

study.

Results 42

4.1 Findings on the welding metallurgy of HSA (7025-T6) The results of the experiment include the microscopic, macroscopic and hardness tests

of 7025-T6 Al alloys. The results of hardness tests of samples A – C are presented in

Figure 14 from Publication I. The plots for sample A, B and C are presented on the

same graph providing for easier comparison. WI in the graph’s label represents the weld

interface. Each indentation is represented by a point on the graph's curve. The weld

regions are also displayed as CZ, TZ-UMZ, HAZ and base material (BM). Sample C

has the lowest heat input of 0.106kJ/mm, sample B has a heat input of 0.127kJ/mm and

sample A has the highest heat input of the three with 0.318kJ/mm. Sample B has about

120% heat input compared to C. In addition, sample A has 300 % more heat input than

C. The feed rate is constant (Table 6). The observations from the hardness graph of

Figure 14 are presented in Table 8.

Figure 14 Hardness testing of samples A, B and C with varying heat input. A

0.318kJ/mm, B 0.127kJ/mm and C 0.106kJ/mm (Olabode et al., 2012).

Table 8 Observations from hardness testing of samples A, B and C with varying heat

input.

Across profile

CZ TZ-UMZ HAZ WI

Highest hardness

C C C C C

Lowest hardness A A A A A

Most uniform A C C C A and B

Shortest HAZ C Longest HAZ A

Shortest WI from weld centre line

C Longest WI from weld centre line

A

4.1 Findings on the welding metallurgy of HSA (7025-T6) 43

43

Figure 15 Hardness testing of samples D, E and F with relatively constant heat input of

about 0.16kJ/mm (Olabode et al., 2012).

The hardness test results of samples D, E and F are presented as a combined graph in

Figure 15 from Publication I. The heat input is relatively constant (Table 5). The

labelling and description of the graph is the same as for samples A – C (Figure 14). The

observations from the hardness graph of Figure 15 are presented in Table 9

Table 9 Observations from hardness testing of samples D, E and F with relatively

constant heat input.

Across profile

CZ TZ-UMZ HAZ WI

Highest hardness

E E E E F

Lowest hardness F F F F E

Most uniform D D all D D

Shortest HAZ E Longest HAZ D

Shortest WI from weld centre line

E Longest WI from weld centre line

F

The macrographs of the weld samples are presented in Table 10, where the weld

penetration, weld width and WI can be seen for each sample. The macrograph shows the

interactions across the weld at room temperature, providing an understanding of the

interactions during the weld.

The micrographs of sample A - F are also presented in Table 10. The image of each

sample shows the microstructure of the CZ, TZ-UMZ, HAZ and BM at 8x

magnification. The grain transformation and transition in the WI is of paramount

significance because with even grain transition, mechanical properties like hardness will

also be uniform across the weld. The grain transformation can be seen in the

micrographs.

Res

ult

s 44

Tab

le 1

0 W

eld

app

eara

nce

, m

acro

-pic

ture

and

mic

ro-p

ictu

re o

f 7

02

5-T

6 A

l al

loy.

Fr

is f

eed

rat

e (

m/m

in),

Ws

is w

eld

ing

sp

eed

(m

m/s

), Q

is

hea

t in

put

(kJ/

mm

), V

is

vo

ltage

(V)

and

I i

s cu

rren

t (A

).

Weld

appeara

nce

Macro

-pic

ture

s

Mic

ro-p

ictu

re

Weld

Da

ta

CZ

T

Z-U

MZ

H

AZ

B

M

A

Fr:

10

Ws:1

0

Q:0

.318

V:2

0.1

I:19

8

B

Fr:

10

Ws:2

5

Q:0

.127

V:1

9.4

I:20

5

C

Fr:

10

Ws:3

0

Q:0

.106

V:1

9.4

I:20

5

D

Fr:

10

Ws:2

0

Q:0

.16

V:1

9.8

I:20

2

E

Fr:

12

Ws:2

4

Q:0

.163

V:2

0.3

0

I:24

1

F

Fr:

14

Ws:2

8.8

Q:0

.158

V:2

0.5

0

I:27

8

HA

Z

HA

Z

HA

Z

HA

Z

HA

Z

HA

Z

4.2 Findings on the Al2O3 layer of Al alloys 45

45

4.2 Findings on the Al2O3 layer of Al alloys Energy-dispersive x-ray spectroscopy (EDS) results are presented in Table 11. The

measurement acceleration voltages 3 kV, 10 kV, and 20 kV represent the calculated

depths of 0.2 µm, 1.2 µm, and 3.3 µm. In all the samples at 0.2 µm depth the presence

of O2 is highest and lowest at 3.3 µm. The values for O2 content and other elements in

AW-7020 and 7025-T6 are relatively close. This may be because they belong to the

same alloy classification series; classification is based on the chemical composition of

the alloy.

Table 11 Percentage weight composition weld samples

Oxide layer formation period 1 hour (after cleaning)

Measuring spot 0.5 x 0.2 mm

Correction method Proza (Phi-Rho-Z)

Take off angle 35.0 degrees

Measurement acceleration voltages

3kV (0.2 µm) 10kV (1.2 µm) 20kV (3.3 µm)

Test depth from surface Material O (Wt. %) Al (Wt. %)

Mg (Wt. %)

Zn (Wt. %)

0.2 µm

Al 99.90% 12.7 87.3

AW-7020 6.55 87.05 1.05 5.4

7025-T6 6.25 87.3 1.1 5.35

1.2µm

Al 99.90% 4.1 95.9

AW-7020 1.525 92 1.2 5.3

7025-T6 1.55 91.95 1.175 5.325

3.3 µm

Al 99.90% 2.75 97.3

AW-7020 1.2 93.1 1.2 4.6

7025-T6 1.075 93.35 1.15 4.4

4.3 Findings on the effect of Al2O3 on HSA (AW-7020) weld metallurgy The hardness test profile graph (Figure 16 from Publication III) presents the hardness

values of the profile across the WI. The hardness profile shows how much the hardness

in the CZ deviated from the BM and vice versa. The y-axis represents the hardness

value (HV3) while the x-axis represents the distance in mm from a common reference in

the BM to the weld centre. It is important to mention that 0 in the x-axis is located in the

BM and the scale increases towards the CZ.

Results 46

The average hardness values are denoted by the nodes on the line graph. For good

welds, the hardness from the BM to the weld centre line should have minimal

fluctuation. The WI denotes the point at which the weld fusion line appears. It can be

seen that the greatest hardness fluctuation is between the HAZ and the WI, which are

usually the areas more prone to structural failure. EC 3 has the best hardness profile of

the four ECs while EC 2 has the worst hardness profile, especially across the WI. The

observations from the hardness graph (Figure 16) are presented in Table 12.

Figure 16 Hardness profile of welded AW-7020 (Olabode et al., 2015a)

Table 12 Observations from hardness testing of AW-7020 weld samples.

Across profile

CZ TZ-UMZ HAZ WI

Highest hardness

EC 3 EC 3 EC 3 EC 3 and 4 EC 3

Lowest hardness

EC 2 EC 1 EC 2 EC 4 EC 4

Most uniform

EC 3 EC 1 EC 4 EC 2 EC 3

Shortest WI from weld centre line EC 3 Longest WI from weld centre line EC 2

Macrograph analysis of samples for each EC is presented in Table 13 from Publication

III. A 10x objective lens was used and the interaction between the weld pool and the

BM across the WI is presented. These images can show macro sized defects like

porosity and cracks, if there are any. In addition, they also show the HAZ and the

location of the WI. The macrograph samples also present the bead profile. It is

important to mention that the indentations on the macrographs are made by the

hardness-testing machine and the position of the indents from the plate surface is

approximately the same for all the experiments. For all four EC it appears that there are

4.3 Findings on the effect of Al2O3 on HSA (AW-7020) weld metallurgy 47

47

neither cracks nor porosity on the macrographs, which suggests that the welds are

acceptable.

Figure 17 from Publication III presents a comparison of the yield strength (YS) in

Re/N/mm2, ultimate tensile strength (UTS) in Rm/N/mm

2, and elongation at fraction in

A/%. The y-axis is measured in units and the x-axis represents the averages of the four

different EC and the control condition.

The tensile test measures the YS, which is the stress value at which the welded

specimen begins to deform plastically and cannot return to its original position. It is

used in this experiment to express the load bearing capacity of the weld before plastic

deformation. Welds with higher yield strength, are more desirable. Based on the YS

values, EC 1 is the best weld while EC 4 is the worst weld, which seems to be due to the

removal of the Al2O3 layer from the latter, thereby increasing the amount of weld heat

input. As seen in EC 2, preheating also seems to reduce the YS, while artificial aging in

EC 3 appears to improve the YS.

Table 13 Macrographs of welded AW-7020 (Olabode et al., 2015a)

Experiment condition 1

Experiment condition 2

Experiment condition 3

Experiment condition 4

Results 48

UTS is used to present the maximum tensile loading the weld can be subjected to before

failure. The higher the UTS, the better the weld is from the load bearing perspective. In

these experiments, EC 3 produced the highest UTS value of 273 .55 Rm/ N/ mm2. This

seems to be due to the effect of artificial aging. EC 1 has the next highest UTS value,

probably due to the low heat input to the workpiece as a result of the presence of the

Al2O3 layer. EC 2 has the next highest value, which suggests that workpiece preheating

reduces UTS values. The lowest UTS value is in EC 4, which suggests that removal of

the Al2O3 layer reduced the UTS

Figure 17 Tensile strength of welded AW-7020 (Olabode et al., 2015a)

The elongation at fracture of AW-7020 welds expresses the proportional reduction of

the cross sectional area of a tensile test piece at the plane of fracture, measured after

fracture. It is expressed as a percentage reduction of area and it shows how brittle or

ductile the weld specimen is. If the elongation is low, the weld piece will be brittle and

therefore it can easily crack or break, for example, brittle ceramics have low elongation

values and crack easily when subjected to tensile loading. On the other hand, if the

elongation at fracture value is high, the specimen is ductile and can be plastically

deformed. In many Al welds, it is desirable to have high elongation values. The best

weld is usually case specific, based on the mechanical or metallurgical properties

demanded by the application. For example, Al welds that are designed to carry torsion

loads like shafts are supposed to be rigid with minimal elongation. On the other hand,

structural Al beams are expected to have elongation so they do not break easily. EC 3

has the highest elongation values, which suggests that artificial aging increased the

malleability of the workpiece. The next highest elongation value is in EC 4, which

suggests that the absence of Al2O3 increases elongation in comparison to EC 1.

49

5 Discussion

The results show some interesting findings. 7025-T6 and AW-7020 alloys are weldable

using pulsed MIG process with little or no weld defects. Acceptable welds were

achieved with or without the removal of Al2O3 layer. This implies that with high energy

density welding processes, the removal of Al2O3 before welding is not necessary. The

Al2O3 layer composition varied in the tested alloys, suggesting that the base metal

composition influences the Al2O3 composition. The discussions are presented in

sections in the order in which the experiments and results were presented.

5.1 Welding metallurgy of HSA (7025-T6) Comparing samples A, B and C, it appears that with low heat input the grain sizes

around the WI are small. As the heat input increases the grain size increases. The

transition of cells at the WI from CZ to HAZ is smoother for higher heat input. Welding

speed is inversely proportional to the heat input so when the welding speed increases

the heat input reduces. It is also important to note that when there is high heat input

there is higher cooling rate. The higher the cooling rate, the longer the available time for

the cells to form and grow, as can be seen by comparing sample A to samples B and C.

High heat input also causes wider HAZ, as seen in sample A, where the HAZ is about

10 mm long from the weld centre line.

Comparing samples A, B and C, Sample C has the highest hardness value of 113 HV

just after the WI and the lowest value is found in sample A. With high heat input, wider

weld beads are observed and the distance of the WI from the weld centre line is further.

The weld penetration increases with higher heat input. Although the feed rate is

constant, sample C appears to have distinct grain transition at the WI. This can be a

failure point when the weld is loaded. Sample A shows that the grain sizes are bigger

when there is longer solidification time or high cooling rate. This implies that high heat

input allows for high hardness of the WI, which can be due to the solution hardening

that occurs in 7xxx series during welding. More heat causes solutionizing thereby

causing higher hardening through the solidification process. It is interesting to note that

samples D, E and F have very close hardness in the WI due to the relatively constant

heat input. In addition, the grain transition at the WI between the CZ and the HAZ is

sharp in sample C compared to A and B. This can be a failure point as the cells are not

adequately interlocked. Sample A confirms that the longer the solidification time, the

bigger the size of the dendrite.

The hardness test also shows that there is a rapid increase in the hardness value around

the WI. In samples D, E and F, it is observed that 7025-T6 shows a reduced hardness in

the WI and it increases towards the BM. At the WI it is observed that the hardness value

of sample F is higher than sample D, which is in turn higher than sample E. This implies

that the lower the welding speed at constant heat input, the lower the hardness.

Comparing sample A and F (macrographs) it can be said that large heat input causes

large weld beads, causing large distortion. Even with lower heat input it was observed

Discussion 50

that faster welding speed allows for narrower weld seams. Compared to sample A and

B, the grain growth in sample C is low, which suggests that the high heat conductivity

of Al through heat sinks has an effect on weld microstructure especially when low heat

input is used.

It can be observed from Table 10 that oxidation occurred on the surface of sample F.

However sample F appears to be the best weld with narrow seem and narrow HAZ.

The Hall-Petch effect shows that strength and toughness increases as the grain sizes

reduce. Sample C has the smallest grain size in the CZ. Thus it can be concluded that it

has the highest strength and toughness. Sample F shows that complete weld penetration

can be achieved with minimal heat input if other weld parameters are set correctly.

5.2 Effect of Al2O3 on HSA (AW-7020) weld metallurgy The effect of Al2O3 on the AW-7020 weld metallurgy is with reference to the hardness

profile of EC 1 and EC 4. These two profiles are similar (Figure 16); however, the

hardness values of EC 1 are higher than EC 4. The presence of the Al2O3 oxide layer in

the weld process (EC 1) increased the YS by 20% and the UTS by 6% but reduced the

elongation by 29% (compared to EC 4). This suggests that when the Al2O3 oxide layer

was not removed before welding, improved hardness of the AW-7020 weld was

attained. (It is important to note that there are no weld defects in the macrographs, like

porosity due to oxide inclusion in the weld pool). The question therefore arises whether

the higher strength could result from the reduced heat that gets into the weld pool due to

the heat resistivity of the Al2O3 oxide layer; in addition to the suspected absence of

chemical interaction between the molten pool and Al2O3 layer during welding (due to

the welding technology and weld parameters). This issue can be clarified by further

multiple experiments. However, it is important to mention that if there is a chemical

reaction in which the Al2O3 layer is present in the weld (causing porosity) the

mechanical properties will be lower.

The effect of pre-weld heat treatment on the AW-7020 weld appears to be that pre weld

heat treatment is detrimental to the weld, as can be seen by comparing EC 2 to the other

3 ECs in Figure 16. The hardness profile across the weld in EC 2 is more uneven with

sharp fluctuations in hardness values. For example, there is a sharp drop of the hardness

value from 81.7 HV to 51.1 HV across the WI. This is usually a failure point of the

weld piece. In EC 2, the WI is closer to the CZ (narrower HAZ), which is better when a

narrower weld seam is desired. Preheating reduced the YS by 17% and UTS by 3% but

increased the elongation by 17% (Figure 17).

The effect of artificial ageing on the AW-7020 weld appears to be that it relatively

smoothens the hardness profile, in addition to increasing the hardness values in the

HAZ, WI, and CZ. Comparing EC 1 and EC 3, the hardness value at the WI increased

from 63.6HV to 79.3HV (Figure 16). Artificial aging reduced the yield strength by 8%

but increased the UTS and elongation by 9% and 110% respectively (Figure 16). These

results therefore suggest that artificial ageing improves the mechanical properties of

5.2 Effect of Al2O3 on HSA (AW-7020) weld metallurgy 51

51

welded AW-7020 provided there are no weld defects. Based on the macrographs, the

welds in AW-7020 study appear to exhibit no defects (Table 13).

The necessity of pre-weld Al2O3 removal is examined in this study. Acceptable welds

were achieved without pre-weld removal of the Al2O3 oxide layer (Table 13, EC 1 - 3)

using a pulsed MIG welding process. This may be due to the absence of or low

chemical interaction of Al2O3 with the weld pool as the EDS result shows that the O2

content of Al2O3 in AW-7020 is about 50% lower than in pure Al. Consequently, when

using new welding technologies like pulsed MIG and friction stir welding (FSW) it may

not be necessary to remove the naturally formed Al2O3 oxide layer before welding HSA

alloys.

Good welds may have been attained due to a lower amount of O2 present on the surface

of AW-7020 compared to pure Al. A lower amount of O2 can be considered as

indicating that the Al2O3 layer is thinner, which might explain why HSA alloys have

lower corrosion resistance in comparison to pure Al.

53

6 Conclusions

This research work studied Al alloys, with particular attention being given to HSA. A

brief explanation of Al alloy series classification was presented. Possible joint

configurations and welding process limitations, in addition to the preparation work

required before welding, were also presented. Possible weld defects were discussed with

recommendations on how to prevent or at least minimize the possibility of such defects

occurring. It is not possible to propose an optimum weld process for all Al alloy

structures as the optimum weld process is case specific and can be influenced by factors

like joint design and joint accessibility.

HBLW was also studied. The optics and welding head configuration play an important

role in the quality of the welds produced. The challenges that occur when using HBLW

to weld Al were presented. Experiments were carried out to investigate the weld

metallurgy of HSA (using 7025-T6), the presence and composition of the Al2O3 layer

(using 99.9% pure Al, AW-7020 and 7025-T6), and the effect of the Al2O3 layer on

HSA weld metallurgy (using AW-7020).

The study showed that with 7025-T6 Al alloys the grain size reduces as the heat input

reduces. The transition of cells from the CZ to HAZ is smoother with higher heat input.

At constant heat input the grain size increases when feed rate, weld speed and current

increase while the hardness remains relatively constant. When heat input is high, the

HAZ is wider, nucleation is lower, and the grains around the WI are coarser.

In 7025-T6 Al, it was found that the higher the heat input, the wider the weld bead, the

further away is the WI from the weld centre line, and the deeper the weld penetration. In

addition, the dendrites became larger with longer solidification time. It was noted that a

high cooling rate allows for epitaxial cell formation. The 7025-T6 alloy, like other HSA

alloys, experiences HAZ softening but can be restored by weld post heat treatment.

Study was carried out to investigate the effect of the Al2O3 layer on the weld metallurgy

of HSA (AW-7020). The structural formation of the Al2O3 layer was briefly explained;

the O2 composition of the Al2O3 layer varies depending on the class of Al alloy. The

characteristics and properties of the Al2O3 layer were discussed, and the study presented

how the Al2O3 structure can be modified for structural advantage. Based on literature

review and experimental study, the following conclusions can be drawn:

1. Pre-weld heat treatment of the AW-7020 alloy is detrimental to the mechanical

properties of the weld.

2. Artificial aging of AW-7020 welds improves mechanical properties, including

hardness, tensile strength, and ultimate yield strength. Therefore, it is suggested that

post weld heat treatment is advantageous in high strength Al alloys.

Conclusions 54

3. Acceptable welds are attainable without pre-weld cleaning of the Al2O3 layer. It is

therefore suggested that removal of the Al2O3 layer is not necessary when using new

welding technologies like the pulsed MIG process on HSA alloys.

4. The presence of the Al2O3 layer is not detrimental to the mechanical properties of

HSA welds if there is no chemical interaction between the weld pool and the Al2O3

layer, and if there are no Al2O3 particle inclusion in the weld pool. This suggests that

new weld technology preventing Al2O3 chemical interactions during welding that

can cause weld porosity and other weld defects is advantageous.

5. The O2 composition of Al2O3 varies across the different classes of wrought Al

alloys. This suggests that the thickness of the Al2O3 layer is not the same for all Al

alloys but similar in each Al alloy class. In addition it suggests that the composition

is also dependent on the chemical composition of the parent metal.

55

7 Future work

Many questions in the area of HSA remain unanswered. Some issues requiring further

study are listed below in the form of questions (Q) followed by suggestions (S) of how

the questions could be addressed.

Q1: Do all HSA alloys have the same hardness pattern?

S1: Welding experiments should be carried out on other HSA alloys and the hardness

results correlated.

Q2: Do all HSA have the same tensile test pattern across welds?

S2: Welding experiments should be carried out on other HSA alloys and the tensile test

results correlated.

Q3: How does filler material influence the weld?

S3: Various applicable Al filler wire should be used in welding of HSA and the weld

metallurgy studied to analyse their effects.

Q4: Will changing pulsed MIG weld parameters improve HSA welds?

S4: Welding experiments in which parameters like current and voltage are varied should

be carried out on HSA to study the effect of weld parameters on weld metallurgy.

Q5: The O2 composition of Al2O3 layer appears to vary in different Al alloys. Does the

structure also vary?

S5: Structural analysis of the Al2O3 should be carried out for various Al alloy series

56

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Errata for Publications The errors in the attached published journals are presented for Publications I and III in this section.

Publications I:

1. The fusion weld regions referred to as unmixed zone (UZ) should be composite zone (CZ) as

illustrated in the Figure I below.

2. The fusion weld region referred to as partially melted zone (PMZ) should be a joint region of

the transition zone (TZ) and the unmixed zone (UZ) as illustrated in the Figure I below. The

replacement for PMZ should be TZ-UMZ.

(a)

(b)

Figure I Fusion weld zone nomenclature: (a) schematic diagram of fusion weld zone, (b)

micrograph of 7025-T6 fusion weld

3. The observed errors and corrections in Publication I are presented in Table I.

Table I Errors in Publication I

Location of error Error Correction

Page 26, column 2, paragraph 5 Unmixed zone (UZ) Composition zone (CZ)

Page 26, column 2, paragraph 5 Partially melted zone (PMZ) Transition zone to unmixed zone (TZ-UMZ)

Page 28, column 1, paragraph 2 UZ CZ

Page 28, column 2, paragraph 2 UZ CZ

Page 29, column 1, paragraph 2 UZ CZ

Page 29, column 1, paragraph 3 UZ CZ

Page 29, column 1, paragraph 4 PMZ TZ-UMZ

Page 29, column 2, paragraph 1 UZ CZ

Page 30, column 1, paragraph 4 UZ CZ

Page 30, column 2, paragraph 1 UZ CZ

Publication I, Figures 4-11 Label: UZ Label: CZ

Publication I, Figures 4-11 Label: PMZ Label: TZ-UMZ

Publications III

1. The fusion weld regions referred to as unmixed zone (UMZ) should be composite zone (CZ)

as illustrated in the Figure I.

2. The observed errors and corrections in Publication III are presented in Table II.

Table II Errors in Publication III

Location of error Error Correction

Page 7, column 1, paragraph 1 Unmixed zone (UMZ) Composition zone (CZ)

Page 7, column 1, paragraph 1 UMZ CZ

Page 8, column 1, paragraph 2 UMZ CZ

Page 8, column 1, paragraph 3 UMZ CZ

Publication III, Figure 5. Label: Weld Interface Label: TZ-UMZ

Publication I

Experimental review on the welding metallurgy of HSA (7025-T6) alloy.

Olabode, M., Kah P., and Martikainen J. (2012). The Paton Welding Journal, 4, pp.20-30.ISSN: 0957-798X (print)

© PWI, International Association «Welding», 2012

EXPERIMENTAL REVIEWOF THE WELDING METALLURGY

OF HIGH-STRENGTH ALUMINIUM ALLOY 7025-T6

M. OLABODE, P. KAH and J. MARTIKAINENLappeenranta University of Technology, Lappeenranta, Finland

In this review, various aspects such as designations, definitions, applications, properties and weldability of high-strengthaluminium alloys are presented. The effect of heat input on microstructure and hardness of the 7025-T6 alloy weldedjoints is studied. It is shown that at constant heat input the welding speed had no effect on the weld hardness. Thus,limiting heat input in welds on high-strength aluminium alloys is important to preserve their mechanical properties.

Keywo rd s : high-strength aluminium alloys, alloy 7025-T6,pulsed MIG welding, heat input, Vickers hardness, weldingmetallurgy

Light welded metal structures are in high demand,and the market keeps growing along with societalneeds. The diversification of aluminium structures alsocontinues to grow. Welding is an important processin producing these structures. The fusion welding ofhigh-strength aluminium alloys (HSA) using pulsedMIG method involves heat input and is, thus, chal-lenging but accomplishable if proper care is taken tounderstand the nature and behaviour of HSA beingwelded. A number of studies [1—3] have shown thatearlier technologies available for welding HSA presentpoor weldability due to the presence of copper in thealloy. However, new technologies like pulsed MIGwelding, pulsed TIG welding and friction stir welding(FSW) can be effectively to compared with conven-tional fusion methods. FSW proved to be presentlythe most acceptable process as it allows obtainingimportant metallurgical advantages, for example, nosolidification and liquation cracking, compared withfusion welding [4]. Based on literature review, thispaper outlines the definitions, properties, applica-tions, weldability, welding defects of HSA and studiestheir weldability with a focus on the effect of heatinput on welding metallurgy using the pulsed MIGprocess. This study adopts both a literature review ofHSA and an experimental study of 7025-T6 alloywelded by robotised pulsed MIG method. In addition,the effect of heat input and welding speed as weldingparameters on welding metallurgy of HSA are pre-sented. It was found that the grains reduce in size asheat input decreases, and welding speed had no effecton the hardness across the weld if heat input was keptconstant. The hardness of HSA joints lower in theHAZ than in the parent metal. This study is of sig-nificance as there are limited studies available aboutthe welding metallurgy of the 7025-T6 alloy.

Alloy designation. Aluminium alloys are groupedinto cast and wrought ones and are identified with a

four digit number system. Cast alloy designations aresimilar to those of wrought alloys but with a decimalbetween the third and fourth digit (123.0). The secondpart of the designation is the temper which accountsfor the fabrication process. When the second partstarts with T, e.g. T6, it means that the alloy wasthermally treated. The numbers show the type of thetreatment and other consequent mechanical treat-ment, namely T6 shows that the alloy is solution heat-treated and artificially aged [5]. In alloy designationsF denotes as fabricated and O – annealed. An addi-tional suffix indicates the specific heat treatment. Hdenotes strain-hardened (cold-worked) and it is al-ways followed by at least two digits to identify thelevel of cold-working and other heat treatments thathave been carried out to attain the required mechanicalproperties. W denotes solution heat-treated, it is fol-lowed by a time indicating the natural ageing period,e.g. W 1 h. T denotes thermally treated and is alwaysfollowed by one or more numbers to identify the spe-cific heat treatment [4].

The full designation therefore has two parts whichspecify the chemistry and the fabrication history, e.g.in 7025-T6, 7025 specifies the chemistry while T6 –the fabrication. Aluminium is classified based on thechemical composition. The classification is mainly intwo categories based on the type of production whichare wrought aluminium alloys (fabricated alloys) andcast aluminium alloys. Others can be categorised onthe basis of strain hardening possibility or heat treat-ment [6]. The wrought aluminium category is largebecause aluminium can be formed to shapes by virtu-ally any known process including extruding, drawing,forging, rolling etc. Wrought alloys need to be ductileto aid fabrication, whereas cast aluminium alloys needto be fluid in nature to aid castability [7]. Cast alu-minium alloys are identified with four digits in theirclassification. A decimal point separates the third andfourth digit. The first digit indicates the alloy groupwhich is based on the major alloying element (Ta-ble 1) [8]. The next two digits denote the aluminiumalloy itself or the purity of the alloy. In lxx.x series

© M. OLABODE, P. KAH and J. MARTIKAINEN, 2012

20 4/2012

alloys, these two digits denote the purity in percent-ages. For example, 150.0 show the minimum 99.5 %purity of the aluminium alloy. In the groups 2xx.x—9xx.x series, the two digits signify the different alloyspresent in the group. The last digit signifies how theproduct is formed. For example, 0 denotes casting,and 1 or 2 – ingot based on what chemical compo-sition the ingot has.

Further modifications from the original cast alu-minium alloy groups are identified by adding a serialletter in front of the numerical denotations. The serialletters are assigned in alphabetical order starting withA but omitting I, O, Q and X [8]. X is left out withexperimental alloys.

Wrought alloys are given four digits. The first onerepresents the alloy group which is based on the majoralloying element (Table 2). The second digit tells howthe alloy has been modified or the limits of impurity.0 in the second digit denotes an original alloy. Num-bers 1—9 signify the different alloy modifications withslight variation in their compositions. In the 1xxxseries the second number denotes the modifications inimpurity limits: 0 implies that the alloy has a naturalimpurity limit, 1—9 imply that special control has beencarried out on one or more impurities or alloying ele-ment. The last two numbers represent the purity ofthe alloy [6].

In the 1xxx series the last two numbers signify thealloy level of purity. For example, 1070 or 1170 im-plies that at least 99.7 % Al is present in the alloy,1050 or 1250 – no less than 99.5 % Al, and 1100 or1200 – at least 99.0 % Al. For all the other series ofaluminium alloys (2xxx—8xxx) the last two numbershave no special significance but are used to identifyalloys in the group [6, 8].

High-strength and ultra high-strength aluminiumalloys. Aluminium alloys with at least 300 MPa yieldstrength are regarded to be HSA, whereas ultra high-strength aluminium alloys (UHSA) are those withyield strength of 400 MPa or more. HSA and UHSAare generally included in the 2xxx, 7xxx, and 8xxxseries. There are no strict rules as to what series HSAand UHSA belong to. For example, two alloys canhave significantly different yield strengths within thesame series. To be exact, the HSA and UHSA can beclassified only specifically to certain alloys in the se-ries. For generality purpose, however, an averagerange of the series yield strength is used to identifyHSA and UHSA (see Table 2).

Properties and applications of HSA and UHSASeries. The 2xxx series includes the Al—Cu alloys.The major characteristics of the 2xxx series are heattreatability, high strength both at room and elevatedtemperatures, and high tensile strength range of 68.9—520 MPa [9, 10]. The alloys can be joined mechani-

Table 1. Cast aluminium alloy classification [6—9]

Series Alloying elements Content, % Tensile strength, MPa Series average value, MPa

1хх.х Al Min 99.0

2хх.х Cu 4.0—4.6 145—476 302

3хх.х Si 5—17 159—359 249

With added Cu and/or Mg 5—17 159—359 249

4хх.х Si 5—12 131—296 187

5хх.х Mg 4—10 138—331 232

7хх.х Zi 6.2—7.5 241 241

8хх.х Sn — 138—221 163

9хх.х Others — — —

Table 2. Wrought aluminium alloy classification [6, 8, 9]

Series Alloying elements Content, % Tensile strength, MPa Series average value, MPa

1ххх Al Min 99.0 10.0—165 94.4

2ххх Cu 1.9—6.8 68.9—520 303

3ххх Mn 0.3—1.5 41.4—285 163

4ххх Si 3.6—13.5 70.0—393 275

5ххх Mg 0.5—5.5 40.0—435 194

6ххх Mg and Si 0.4—1.5 40.0—435 241

Si 0.2—1.7 40.0—435 241

7ххх Zn 1.0—8.2 80.0—725 399

8ххх Others — 110—515 365

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cally while some are weldable [11]. The chemical com-position is usually copper and some other possibleelements, like magnesium, manganese and silicon.They comprise high strength products that are usuallytypical of the aviation industry (2024 alloy). In theindustry they are expected to meet high engineeringstandards due to high safety requirements. These re-quirements make the 2xxx series expensive. However,the alloys are also used in the manufacture of truckbodies (2014 alloy); 2011, 2017 and 2117 alloys areextensively used for fasteners and screw machinestock. Under naturally aged T4 condition, the 2xxxseries alloys have similar mechanical properties as mildsteel, with a proof strength of about 250 MPa and anultimate tensile strength of around 400 MPa. Theyalso have good ductility. When T6 conditioning isused, the proof strength gets up to 375 MPa and theultimate tensile strength can get up to 450 MPa. This,in turn, lowers ductility [11]. Moreover, they are gen-erally painted or clad to increase their corrosion re-sistance. Succinctly, the 2xxx series alloys are usedfor the construction of aircraft internal and externalstructures, internal railroad car structural members,structural beams of heavy dump and tank trucks andtrailer trucks, and the fuel tanks and booster rocketsof space shuttles [10].

The 7xxx series includes the Al—Zn alloys withmagnesium to control the ageing process. The alloygroup possesses very high strength in the high tough-ness versions. They are also heat treatable with anultimate tensile strength range of 220—610 MPa. Theycan be mechanically joined and, with selected weldingmethod like pulsed MIG process, they are weldable.Some 7xxx alloys content copper to yield the higheststrength in the series. However, these alloys are notcommercially weldable (Figure 1). The weldabilityreduces as the copper content increases [1—3]. Thus,in commercial applications they are mechanicallyjoined, e.g. by riveting.

The 7xxx alloys are mainly used when fracturecritical design concepts are important, e.g. theForesmo Bridge in northern Norway. Al—Mg alloysare used for building the girders system. Another mainapplication is in the aircraft industry [10]. They havepoor corrosion resistance compared to, for example,the 5xxx series and are thus clad in many applications.They are used for critical aircraft wing structures ofintegrally stiffened aluminium extrusions and long-length drill pipes, and premium forged aircraft partsare made from 7175-T736 (T74) alloy [10].

The 8xxx series includes alloys with aluminiumand other elements such as iron, nickel and lithium(not presented in Table 2). These elements provide aspecific property to the alloy, e.g. nickel and ironyield strength to the alloy with almost no loss toelectrical conductivity [10]. The high strength mem-bers of the series mainly consist of lithium and copper.The lithium proportion is higher than that of copper.The relatively recently developed Al—Li alloys 8090,8091 and 8093 are also included in the series. Lithiumhas lower density than aluminium and relatively highsolubility. Thus, it can be alloyed with aluminium insufficient quantities. A significant reduction in density(usually about 10 % less than other aluminium alloys)is attainable. The resulting alloys have increased stiff-ness, and they also respond to age-hardening. Someof the series alloys are heat treatable [12]. They aretherefore referred to as special alloys and have highconductivity, strength (tensile strength of 110—515 MPa [9]) and hardness. These alloys are used inthe aviation industries (8090, 8091). The Al—Ni—Fealloy 8001 is used in nuclear power generation forapplications demanding resistance to aqueous corro-sion at elevated temperatures and pressures. The alloy8017 is used for electrical conduction [10].

Weldability of high-strength aluminium alloys.The increasing industrial need for aluminium alloyshas resulted in profuse research on how to weld the

Figure 1. Mechanical properties of aluminium alloys

22 4/2012

new alloys. There are more ranges of applicable weld-ing processes available on the market. Based on studiesit can be stated that:

• within the scope of manufacturing technology,94 % of alloys can be welded and over 50 % haveoptimal weldability;

• industrially weldable thickness range is 0.1—450 mm (the latter, exceptional case, in a single passby means of electron beam welding (EBW));

• high welding speeds are attainable with reducedthicknesses (0.8—3.0 mm), for example, the laserwelding of butt joints, varying between 5 and3 m/min;

• metallurgical problems caused by welding heatinput are present with all fusion methods, but reducedin the concentrated energy processes, where heat inputis more precise and hence the HAZ is less extensive.FSW produces a low level of metallurgical distur-bance;

• with concentrated energy processes, the presenceof the Al2O3 film on the surfaces undergoing weldingdoes not compromise the quality of the weld. How-ever, pre-weld cleaning is encouraged;

• both EBW and FSW can be conducted withoutthe use of gas to protect the weld pool from oxidation;

• traditional methods give inferior mechanicalproperties with respect to those of the correspondingbase materials. The decrease varies from 20 to 35 %and is highly influenced by the metallurgical state ofthe base material. Particularly, an insignificant oreven zero reduction is only found with the FSW proc-ess, which is, at the same time, the only weldingprocess offering fatigue characteristics of butt jointsthat are entirely comparable to the base metal in theas-welded condition;

• generally all fusion welding methods, with theexception of FSW, give welds affected by widespreadporosity;

• generally, and considering similar sized weldingequipment, laser and FSW technologies involve up to10 times higher investments than traditional technolo-gies, but the level of productivity is decidedly supe-rior. Currently, large scale of the aluminium alloystructural components welded by FSW have at least10 % lower costs compared to those welded by MIGprocess [13].

Work preparation. The successful welding of HSAis very dependent on the work preparation due to theextra consideration necessary for welding aluminiumcompared to steel. It depends on using a suitable weld-

Table 3. Work preparation guide [4, 9, 14, 15]

Consideration Precautions

Stress in weld Avoid sudden changes in thickness as they act as stress raisers in the weld. It is better to taper asection in the joint if it is to be joined with a thinner sectionEnsure a good fit-up prior to welding. Aluminium is intolerant of poor fit-up and joints shouldhave the smallest gap possible to allow the penetration of the filler into the joint. In a generalfit-up, gaps of more than 1.5 mm are not acceptable. Larger gaps are easy to fill in steel butwill introduce excessive stresses in aluminium due to thermal contraction. This will compromisethe life of the weldEnsure a good alignment of the joint prior to welding. A misaligned weld will introducebending stresses, which will also shorten the life of the weldMake sure that the joint preparation is suitable for the thickness of the material and complieswith the drawing

Conditions for good qualitywelds

Make sure that the ambient conditions are suitable for welding. Aluminium is very sensitive tohydrogen contamination, so that any moisture will result in defective welds due to porosity.Welding outdoors is particularly risky as condensation can form on the joint during coldweather or the component may be left out in the rain. If welding is to be carried out duringhumid periods, moderate preheating may be usefully applied to prevent hydrogen porosity. Evenif the joint is dry, the risk of draughts destroying the gas shield must be considered. Welding ofaluminium is best carried out in a dedicated warm, dry, draught-free area indoors

Pre-weld cleaning of joint Aluminium is very intolerant of contamination in the joint. Cleaning should start with a wipeby a clean cloth soaked in a solvent such as acetone to remove oil and grease from the joint areaand 25 mm over both sides of the joint. All aluminium products have a very thin layer of oxideon the surface. This melts at about 2060 °C [4, 14] compared with 660 °C [9] for purealuminium. This oxide must be removed after degreasing and before welding by mechanicalcleaning with a stainless steel wire brush, which is reserved for aluminium use only. A grindingdisk must not be used as these are made from corundum (aluminium oxide) and will depositparticles in the surface. This is precisely the material that cleaning intends to remove. The weldshould preferably be made immediately after cleaning, but welding within 3 h of cleaning isacceptable

Suitability of weldingconsumables

Welding is normally carried out using argon or mixture of argon and helium, and the purity ofthese gases is important. A minimum purity of 99.995 % is required. Wire for MIG welding isnormally supplied clean enough and it is sufficient to always ensure that the spool is preferablyremoved from the welding machine and placed in a clean plastic bag overnight or at leastcovered to keep it clean

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ing process, storage, handling and workpiece prepa-ration as well as applying a practically acceptablejoint design [1].

The workpiece to be joined with the pulsed MIGprocess involves joint preparation which is imperativeto ensure quality welds. Based on the thickness of theworkpiece, the joints need to be bevelled and in somecases a root back-up must be applied. It is importantto clean the joint surface to remove the thin oxidelayer (Al2O3). The removal can be done by mechanicalabrasion processes like brushing with stainless steelbrushes or by chemical etching. The Al2O3 layer re-generates itself when scratched. It is responsible forthe corrosion resistance in aluminium alloys [14] andalso for the arc instability problem because it is elec-trically non-conductive. Al2O3 is hygroscopic and itis usually found hydrated. The melting temperatureis 2060 °C [4, 14] which is high when compared tothe melting temperature range of 476—657 °C of the7xxx series alloys [9]. A work preparation guide ispresented in Table 3.

Shielding gas. The primary function of shieldinggas is to protect the weld metal from the atmospherebecause heated metal (around the melting point) usu-ally exhibits a tendency to react with the atmosphereto form oxides and nitrides. For aluminium it easilyreacts with oxygen at room temperatures. In selectingthe shielding gas, the criteria that should be met areas follows [4, 16—18]:

• gas must be able to generate plasma and stablearc mechanism and characteristics;

• it should provide smooth detachment of moltenmetal from the wire and fulfil the desired mode ofmetal transfer;

• it should protect the welding head (in the arcimmediate vicinity), molten pool and wire tip fromoxidation;

• it should help to attain good penetration andgood bead profile;

• it should not be detrimental to the welding speedof the process;

• it should prevent undercutting tendencies;• it should limit post-weld cleaning;• it should not be detrimental to the weld metal

mechanical properties.The recommended shielding gas for pulsed MIG

welding of 7xxx aluminium is argon [1, 17] at flow

rate of about 20 l/min. A mixture of argon and heliumcan also be used and even helium alone. Helium in-creases weld penetration, offers higher arc energy and,thus, an increased deposition rate [1, 19]. When thesection is lower than 50 mm, helium should be used[4]. More details can be seen in Table 4.

Welding defects in HSA and UHSA. The weldingof aluminium is rather critical despite the fact that ithas a lower melting point compared to steel. The weld-ing of aluminium is critical because of the followingconsiderations [6, 18]:

• stable surface oxide needs to be eliminated beforewelding;

• presence of residual stresses causes weld crackingdue to the high thermal expansion coefficient of alu-minium;

• high heat conductivity of aluminium implies thatgreat heat is required to achieve welds, whereas highheat input increases the possibility of distortion andcracking;

• high shrinkage rates on solidification enhancecracking;

• high solubility of hydrogen in molten aluminiumcauses porosity;

• general susceptibility of the 2xxx, 7xxx and 8xxxseries to weld cracking.

Applicable major welding defects in HSA seriesinclude hot cracking, porosity, joint softening, notrecoverable on post-weld ageing, poor weld zone duc-tility (HAZ degradation) and the susceptibility of thejoint to stress corrosion cracking (Table 5).

Experimental set-up. The experiment was carriedout using a robotised pulsed MIG welding machine.The schematics of the MIG welding process are pre-sented in Figure 2.

The robot movement was programmed and sometest sample welds were made, after which alloy 7025-T6 was welded. Many different welds were made, andthe weld parameters were varied to study the effectof heat input on properties of the weld metal. Fur-thermore, the effect of the welding speed was studied.

The MIG torch used was Fronius Robacta 5000360 (max 500 A). The torch was connected to theMotorman (EA1900N) robot. The robot has 6 axesand can attain an accuracy of up to ±0.06 mm. A torchangle of 10° pushing weld direction was used to allowfor the purging of the weld area ahead of the arc. The

Table 4. Shielding gases for MIG welding of aluminium [16]

Metal transfer mode Shielding gas Characteristics

Spray 100 % Ar Best metal transfer and arc stability, least spatter, good cleaning action

Ar + 65 % He Higher heat input than in 100 % Ar, improved fusion characteristics on thickermaterial, minimised porosity

Ar + 75 % He Highest heat input, minimised porosity, least cleaning action

Short circuiting Ar or Ar + He Ar satisfactory on sheet metal, Ar + He preferred for thicker base materials

24 4/2012

Table 5. Defects in aluminium welds and their prevention [11, 15, 18, 20]

Defect Cause Remedy

Oxide inclusions Insufficient cleaning of the joint

Oxide layer on welding wire or fillerrods

Sharp edges on the joint groove

Thoroughly wire brush before welding and after each pass,then wipe cleanClean wire and rods by abrading with stainless steel wool or«Scotchbrite»Use fresh spool of wireBreak sharp edges in weld preparation

Porosity in weld Inadequate shielding

Dye penetrants, lubricants

Welding current too highContaminated shielding gas

Incorrect torch angleTravel speed too highContaminated wire or rodsMoisture

Increase gas flowEliminate draughtsReduce electrode extensionRemove any defects fullyClean surfaces with a solventKeep lubricants away from the weld areaReduce current and refer to the weld procedureCheck gas hoses for loose connections or damageCheck torch coolant to ensure no leaksReplace gas cylindersUse correct angle and refer to the weld procedureApply correct speed and refer to the weld procedureClean wire or rods with solventPreheat and clean the surface

Porosity in fusion zone Hydrogen in the base metal Improve the degassing practiceReduce sodium additionsApply 100 % He shielding

Cold cracking High joint restraint Slacken holding clampsPreheating

Hot cracking Excessive dilution by parent

Interpass temperature too high

Reduce welding currentAdd more filler wireReduce welding currentCool between passes and sequence welds

Undercutting Welding current too highTravel speed too high andinsufficient filler metalArc length too long

Reduce currentReduce speed and refer to the weld procedure, add more fillermetalReduce arc length

Lacks of fusion Welding current too lowTravel speed too highPoor joint preparationIncorrect torch angle

Increase current and refer to the weld procedureReduce travel speed, and refer to the weld procedureImprove joint preparationApply correct torch angle, and refer to the weld procedure

Crater cracking Improper breaking of arc Reduce arc current graduallyUse «Crater fill» control if available. «Back weld» over last25 mm of the bead

Overlap Slow travel speedWelding current too lowToo much filler metalIncorrect torch angle

Increase speed and refer to the weld procedureIncrease currentReduce filler metal additionChange torch angle

Drop through Slow travel speedWelding current too highJoint gap too wideToo much heat built up in part

Increase travel speedDecrease welding currentReduce gap and improve the fit-upReduce interpass temperature

4/2012 25

filler wire extension was 2 mm, and the nozzle-to-workpiece distance (stick-out length) was 18 mm. Theshielding gas used was 99.995 % argon and the fillerwire was 4043 aluminium. The workpiece was a 5 mmthick plate with an area of 100 × 250 mm. The sampleswere bead-on-plate welds, so there was no bevelling.However, the joints were cleaned mechanically byusing a stainless steel bristle brush reserved for alu-minium only.

Many experimental trials were performed, forwhich 6 different samples of 7025-T6 alloy were se-lected. The first three samples (A, B and C) had thesame feed rate so as to investigate the effect of thewelding speed (10, 20 and 30 mm/s). The other threesamples (D, E and F) had approximately the sameheat input to investigate the effect of constant heatinput on the weld. The pulse current frequency wasapproximately 250 Hz in each weld.

For samples A—C, the feed rate was constant at10 m/min, and the heat input varied. Heat input Qfor all samples was calculated as [21]

Q = VI⋅601000S

⋅ 0.8, (1)

where Q is the heat input, kJ/mm; V is the voltage,V; I is the current, A; S is the welding speed,mm/min; 0.8 is the efficiency of the pulsed MIGprocess.

For samples D—F, heat input was approximatelyconstant and the feed rates were selected as 10, 12and 14 m/min, respectively.

The base material was a 5 mm thick 7025-T6 plate,and the welding wire was ER 4043 (Table 6). Thetypical mechanical properties of the wire include the

yield stress of 55 MPa, tensile strength of 165 MPaand an elongation of 18 %. The shielding gas was99.995 % argon and it was supplied through the MIGtorch to protect the weld pool from the atmosphere,because heated metal (around the melting point) usu-ally exhibits a tendency to react with the atmosphereto form oxides and nitrides. For aluminium it easilyreacts with oxygen at room temperatures. The recom-mended shielding gas for pulsed MIG welding 7xxxseries aluminium is argon [17].

The hardness testing experiment of the welds wasdone on a Vickers hardness machine. The test methodinvolved the indentation of the test workpiece witha diamond indenter in the form of right pyramid witha square base and angle of 136° between opposite faces;subjected to a weight of 1—100 kg. The full load wasnormally applied for 10—15 s. The two diagonals ofthe indentation made on the surface of the materialafter the removal of the load were measured using amicroscope and their averages calculated [22].

This test was carried out by a 3 kg weight inden-tation of the diamond tool tip on the prepared weldcross-section. The weight can be varied for differentmaterials, but 3 kg was sufficient because aluminiumis relatively soft and 3 kg is enough to create an in-dentation. Moreover, it is important that the weightis low enough for the aluminium test piece to resistit. The indentations were done at about 1 mm fromthe weld surface in a row (Figure 3).

The distance between each indentation was0.7 mm. The shape of the indentation resembled arhombus. The depth of the indents depended on thematerial hardness. The dimension of the diagonals ofan indentation was measured and the average valuefrom the diagonals was looked up from the hardnesstable of HV3 to determine the hardness value. Thevalues were then plotted against the distance of eachindentation from the weld centreline.

Results and discussions. Effect of heat input onHSA. Micro- and macrostructure, as well as weld ap-pearance on samples A—C, are presented in Figures4—6. The picture of each sample shows the microstruc-ture using an ×8 magnification lens for analysing theunmixed zone (UZ), partially melted zone (PMZ),

Figure 3. Hardness testing on a weld sample

Figure 2. Schematics of MIG welding process: 1 – power source;2 – shielding gas; 3 – MIG torch; 4 – filler wire; 5 – aluminiumworkpiece

Table 6. Chemical composition of base metal and filler wire used, wt.%

Metal Al Be Cr Cu Fe Mg Mn Si Ti ZnOthereach

Total

7025 91.5 — 0.30 0.10 0.40 1.50 0.60 0.30 0.10 5.0 0.05 0.15

ER 4043 — 0.0001 — 0.01 0.20 0.01 0.01 4.80 0.02 0.01 — —

26 4/2012

Figure 4. Experimental results for sample A welded at vw.f = 10 m/min, vw = 10 mm/s, Q = 0.318 J/mm, U = 20.1 V and I = 198 A

Figure 6. Experimental results for sample C at vw.f = 10 m/min, vw = 30 mm/s, Q = 0.106 J/mm, U = 19.4 V and I = 205 A

Figure 5. Experimental results for sample B at vw.f = 10 m/min, vw = 25 mm/s, Q = 0.127 J/mm, U = 19.4 V and I = 205 A

4/2012 27

heat-affected zone (HAZ) and base metal (BM). Thetransition around the weld interface is of great sig-nificance. The picture shows how the grains have beentransformed, from which inferences can be made asto the mechanical properties of the weld samples.

Comparing samples A, B and C (see Figures 4—6),it can be seen that the grain sizes around the weldinterface are small when heat input is low, and viceversa. Furthermore, the transition flow of cells at theinterface as it moves from the UZ to the HAZ issmoother with higher heat input where the grain sizesare bigger. With lower heat input as in sample C (seeFigure 6) the transition is not as smooth, so the in-terface is distinct. Heat input is inversely related tothe welding speed. When the welding speed increases,heat input reduces. The higher the heat input, thehigher the cooling rate. A high cooling rate allowsepitaxial growth to occur and also for the cells togrow large, as seen by comparing sample A to samplesB and C. In sample A, the HAZ is about 17 mm fromthe weld centreline, which is the greatest distance of

the three samples (see Figure 4). Thus, it can be saidthat the higher the heat input, the wider the HAZ.

The grains of UZ in sample C compared to B andA are very fine, which shows that low heat input inA and B is insufficient to melt the pool and penetratethe weld. The high heat input and high welding speedcaused high heat energy on the weld in sample C,which makes the weld bead large with a wider root.

Sample C has fine grains compared to B and A,which shows that with high heat input and weldingspeed there is higher nucleation. In sample C, thegrain growth is low compared to A and B becausealuminium dissipates heat relatively fast through heatsinks; low heat input means that the high conductivityof aluminium strongly affects the weld microstructure(sample C cools fast).

By comparing the results from samples D, E andF presented in Figures 7—9 it can be noted that keepingthe heat input relatively constant but varying thewelding speed causes changes in the microstructure.As the welding speed and the wire feed rate increase,

Figure 7. Experimental results for sample D at vw.f = 10 m/min, vw = 20 mm/s, Q = 0.16 J/mm, U = 19.8 V and I = 202 A

Figure 8. Experimental results for sample E at vw.f = 12 m/min, vw = 24 mm/s, Q = 0.163 J/mm, U = 20.3 V and I = 241 A

28 4/2012

also the grain sizes increase. Furthermore, the in-creased welding speed gives lower nucleation andcoarser transitions of grains around the weld interface,which is similar to the effect of heat input in 7025-T6aluminium welds.

Samples D, E and F indicate that the higher thewire feed rate, the deeper the penetration. Sample Chas a constant feed rate with A and B but the graintransition at the weld interface between the UZ andthe HAZ is very sharp. This may be a possible failurepoint as the cells are not as interlocked as in sampleB. Sample A shows that the longer the solidificationtime, the bigger the size of the dendrite [23].

The grains are equiaxed with dendrites within thegrains. Fine grain sizes appear when heat input is low,and coarse grain sizes when heat input is high. Forexample, the UZ in Figure 8 has fine grains due tothe low heat input of 0.163 kJ/mm, whereas the UZin Figure 4 has coarse grains due to high heat inputof 0.318 kJ/mm. The grain size variations in the UZin Figures 4—9 are mainly due to the amount of heatinput, since high heat input means a high cooling rate.

A faster welding speed allows narrow welds evenwith lower heat input (comparing samples A—F). Sam-ple F seems to be the best weld with a narrow bead,narrow HAZ and complete penetration. On the otherhand, oxidation occurred on the surface. At a constantwelding speed, high heat input increases the weldbead size and HAZ size. The PMZ shows epitaxialgrowth, which indicates that new grains had nucleatedon the heterogeneous sites at the weld interface. Thereis a random orientation between the base metal grainsand weld grains.

As can be seen from samples A—F, since the ratioof 7025-T6 alloy temperature gradient G to the growthrate R decreases from the weld interface towards thecentre line, the solidification modes have changedfrom planar to cellular, to columnar dendrite andequiaxed dendrite across the weld interface . The ratioG/R determines the solidification modes found in the

microstructure. Sample C has the smallest grain sizein the UZ. Thus, it can be concluded that it has thehighest strength and toughness as the Hall—Petch ef-fect predicts that both strength and toughness increaseas the grain sizes reduce [24, 25]. Sample F showsthat complete weld penetration can be achieved withminimal heat input if other weld data are set correctly.

Weld defects such as porosity and oxidation werefound on the welds. Porosity could be due to gasentrapment during welding, whereas oxidation couldbe due to poor shielding gas covering (the weld poolhas contact with atmospheric air).

Hardness of HSA welded joints (7025-T6). Thehardness tests of samples A—C are presented in Fi-gure 10, where the plots for samples A, B and C arecombined on the same graph. The vertical line, la-belled WI, denotes the weld interface. The points onthe graph curve indicate the distance of each inden-tation point from the weld centreline on the horizontalaxis and the hardness value when traced on the verticalaxis. The graph also shows the weld zones, HAZ andBM. Sample C has the lowest heat input of0.106 kJ/mm resulting in a high hardness profile,sample B – relatively higher heat input of0.127 kJ/mm resulting in a lower hardness profilethan sample C, and sample A – the highest heatinput of 0.318 kJ/mm resulting in the lowest hardnessprofile.

Figure 9. Experimental results for sample F at vw.f = 14 m/min, vw = 28.8 mm/s, Q = 0.158 J/mm, U = 20.5 V and I = 278 A

Figure 10. Hardness distribution for samples welded with varyingheat input: 0.106 (C), 0.127 (B), 0.318 (A) kJ/mm

4/2012 29

Sample C also has the highest hardness at the WI,thus, implying that high heat input allows for highhardness of the WI, due to solution hardening duringwelding. High heat input causes solubility and therebyhigher hardening through the solidification process.It can also be said that the higher the heat input, thewider the weld bead and the further away from theweld centreline is the WI. The hardness test also showsthis with relatively constant heat input. The hardnesspattern of samples D, E and F are similar, but Eexhibits small variation. The hardness around 3 mmaway from the weld centreline shows a rapid increasein the value from the previous point (around 2 mmfrom the weld centreline). This is due to the closenessof the WI. From samples D, E and F it can be seenthat for 7025-T6 weld, hardness reduces in the weldzone and increases towards the base material. Thehardness graph presents half of the symmetric welds.At the WI it can be said that the hardness values ofD, E and F samples are relatively identical. This im-plies that at constant heat input, the hardness profileof 7025-T6 aluminium alloy remains the same.

The hardness tests of samples D, E and F, presentedin Figure 11, show that the hardness profiles for thethree samples are relatively similar. The WI range iswithin 0.5 mm as a result of a relatively constant heatinput. The labelling and description of the graph isthe same as for samples A—C.

CONCLUSIONS

1. The study showed that in 7025-T6 aluminium alloysthe grain size reduces as the heat input reduces. Thetransition of cells from the UZ to HAZ is smootherwith higher heat input. At constant heat input thegrain size increases when wire feed rate, welding speedand current increase simultaneously but the hardnessremains relatively constant. When heat input is high,the HAZ is wider, nucleation is lower, and the grainsaround the weld interface are coarser.

2. In 7025-T6 aluminium alloy, high heat inputresults in a low hardness profile but the hardness ofthe UZ is the same in all the selected samples. Thehigher the heat input, the wider the weld bead, thefurther away is the weld interface and the deeper theweld penetration. The longer the solidification time,the larger the dendrites and a high cooling rate allowsfor epitaxial cell formation. The 7025-T6 alloy, likeother high-strength aluminium alloys, experiencesHAZ softening but can be restored by postweld heattreatment.

Figure 11. Hardness distribution for samples welded with relativelyconstant heat input of about 0.16 kJ/mm

1. Yeomans, S.R. (1990) Successful welding of aluminium andits alloys. Australian Welding J., 35(4), 20—24.

2. Graeve, I.D., Hirsch, J. (2010) 7xxx series alloys.http://aluminium.matter.org.uk/content/html/eng/default.asp?catid=214&pageid=2144417086

3. Dickerson, P.B., Irving, B. (1992) Welding aluminium: It’snot as difficult as it sounds. Welding J., 71(4), 45—50.

4. Mathers, G. (2002) The welding of aluminium and its al-loys. Boca Raton: CRC Press; Woodhead Publ.

5. Maurice, S. (1997) Aluminum structures: Handbook ofstructural engineering. 2nd ed. CRC Press.

6. Campbell, F.C. (2006) Manufacturing technology for aero-space structural materials. Amsterdam; San Diego: Elsevier.

7. (2008) ASM Handbook. Vol. 15: Casting. Materials Park:ASM Int.

8. Kopeliovich, D. (2009) Classification of aluminum alloys.In: Substances and technology.

9. (2010) MatWeb – The Online Materials Information Re-source.

10. Kaufman, G.J. (2000) Applications for aluminum alloys andtempers. ASM Int.

11. John, D. (1999) Heat-treatable alloys. In: Aluminium de-sign and construction. New York: Taylor & Francis, 301.

12. Aluminum alloys and temper designations 101. Dayco Ind.,1—5.

13. Volpone, L.M., Mueller, S. (2008) Joints in light alloys to-day: the boundaries of possibility. Welding Int., 22(9),597—609.

14. George, E.T., MacKenzie, D.S. (2003) Handbook of alumi-num: Physical metallurgy and processes. New York: MarcelDekker.

15. Renshaw, M. (2004) The welding of aluminium castings. In:Aluminium – light strong and beautiful. A.F.o.S. Africa,11—13.

16. (2008) Choosing shielding gases for gas metal-arc welding.Welding J., 87(4), 32—34.

17. Boughton, P., Matani, T.M. (1967) Two years of pulsed arcwelding. Welding and Metal Fabr., Oct., 410—420.

18. Olson, D.L. (1993) Welding, brazing, and soldering: ASMhandbook. Metals Park: ASM Int.

19. Blewett, R.V. (1991) Welding aluminium and its alloys.Welding and Metal Fabr., Oct., 5.

20. Ba Ruizhang, G.S. (2004) Welding of aluminum-lithium al-loy with a high power continuous wave Nd:YAG laser. IIWDoc. IV-866—04.

21. Hirata, Y. (2003) Pulsed arc welding. Welding Int., 17(2),98—115.

22. Chandler, H. (1999) Hardness testing. Materials Park:ASM Int.

23. Kou, S. (2003) Welding metallurgy. Hoboken: Wiley-Intersci.24. Sato, Y.S., Urata, M., Kokawa, H. et al. (2003) Hall—

Petch relationship in friction stir welds of equal channel an-gular-pressed aluminium alloys. Materials Sci. and Eng. A,354(1/2), 298—305.

25. Vander Voort, G.F., (2004) Metallography and microstruc-tures. Materials Park: ASM Int.

30 4/2012

Publication II

Aluminium alloys welding processes: Challenges, joint types and process selection.

Olabode, M., Kah, P., and Martikainen, J. (2013). Proceedings of the Institution ofMechanical Engineers, Part B: Journal of Engineering Manufacture, 227(8), 1129-1137.DOI: 10.1177/0954405413484015

© Sage publications, 2013

Review Article

Proc IMechE Part B:J Engineering Manufacture227(8) 1129–1137� IMechE 2013Reprints and permissions:sagepub.co.uk/journalsPermissions.navDOI: 10.1177/0954405413484015pib.sagepub.com

Aluminium alloys welding processes:Challenges, joint types and processselection

Muyiwa Olabode, Paul Kah and Jukka Martikainen

AbstractAluminium and its alloys have gained increasing importance in structural engineering due to advantageous propertiessuch as light weight, ease of machining and corrosion resistance. This article presents surface-related challenges facingaluminium welding, specifically weld process limitations and joint limitations. The methodological approach is a criticalreview of published literature and results based on eight industrial welding processes for aluminium and six joint types. Itis shown that challenges such as heat input control, hot cracking, porosity and weldable thickness vary with the processused and that there is no optimal general weld process for all aluminium alloys and thicknesses. A selection table is pre-sented to assist in selection of the optimal process for specific applications. This study illustrates that knowledge of weldlimitations is valuable in selection of appropriate weld processes.

KeywordsAluminium alloys, aluminium oxide, shielding gases, anodising, aluminium welding process selection

Date received: 17 September 2012; accepted: 4 March 2013

Introduction

Aluminium and its alloys are widely used in weldingindustries due to economic advantages such as lightweight, good corrosion resistance, high toughness,extreme temperature capabilities and easy recyclabil-ity.1 Aluminium alloys are used for construction of air-planes, cars, rail coaches and marine transports.Aluminium alloys are used in manufacture of tanksand pressure vessels because of their high specificstrength, good heat conductivity and beneficial proper-ties at low temperatures.2 Aluminium is the secondmost used metal after iron and steel in the industry; forexample, aluminium is the second most used materialtaking about 15% of total body weight of average carsand about 34% in Audi A2.3 There are comprehensivereviews on the uses and applications of aluminium andits alloys.4,5 Welding is a means of joining metals bycreating coalescence due to heat. The work piece ismelted at the joint point (weld pool) that solidifies oncooling. Welding of aluminium alloys is important forfabricating structural constructions and mechanicalfabrications like aircrafts. However, welding has prob-lems and can be challenging. Welding defects commonto aluminium include porosity, hot cracking, incom-plete fusion and so on.2,6

Researches7,8 have shown that welding aluminiumdemands greater caution compared with steel, particu-larly as regards the amount of heat input and pre-weldcleaning, and that acceptable weld processes for alumi-nium joints are limited because the weldable thicknessvaries considerably with the different welding processes.It is therefore of interest to study the limitations facingaluminium welding, particularly joint- and process-specific limitations.

The aim of this article is to present a comprehensiveguide to understanding aluminium-welding challenges.In the field of aluminium welding, there are eightindustrially common welding processes and six basicjoint types that have been analysed. For comparisonpurposes, a table is designed that shows the influence ofjoint and process limitations on optimum welding pro-cess selection. The remainder of this article is dividedinto two main parts, which are surface-related weldingchallenges and joint types and process limitations.

Lappeenranta University of Technology, Lappeenranta, Finland

Corresponding author:

Muyiwa Olabode, Lappeenranta University of Technology, Skinnarilankatu

34, 53850 Lappeenranta, Finland.

Email: muyiwa.olabode@lut.fi

Evaluation of the findings shows that there is no sin-gular optimum process for welding aluminium.However, understanding of the limitations of individ-ual welding processes helps in selection of the optimalprocess for specific aluminium weld applications.

Surface-related welding considerations

A clean, smooth and protected surface is important inpre-weld aluminium structures to ensure good alumi-nium weldments except in high energy density weldingprocesses like hybrid laser beam welding (LBW) (usingpulsed metal inert gas (MIG)).9 It is therefore impor-tant to understand different surface-related phenomenaand their effect on the weldability of the work piece. Inaddition, knowledge of preventative measures ensuringthe attainment of acceptable welds, despite any adversesurface effects, is also important.

Presence of aluminium oxide surface

Oxide formation in aluminium occurs due to the strongchemical affinity of aluminium for oxygen on exposureto air. The aluminium oxide thickness increases as aresult of thermal treatment, moist storage conditionsand electrochemical treatment (anodising).10–14 It isalso important to note that Al2O3 melts at about2050 �C, while aluminium alloys melts at about 660 �C9

(as illustrated in Figure 1). Therefore, the layer isremoved by pickling or dry machining just before weld.However, the difference in melting point is not a prob-lem during the processing by means of high energy den-sity welding processes; it can also be an advantage, forexample, the presence of oxide layer during laser weld-ing increases the absorptivity of aluminium and itsalloys to laser radiation.15,16 It should be noted, that amain challenge in applying most joining technologies toaluminium is its tendency to form a thick, coherentoxide layer. This oxide layer has a melting temperaturemuch higher than that of aluminium itself; moreover, ithas a significant mechanical strength. Therefore, thisoxide layer can remain as a solid film (or fractured insmall particles) due to the flow of the molten material,16

even when the surrounding metal is molten. This canresult in severe incomplete fusion defects. Therefore,

the removal of the oxide layer just before welding isimportant.

The aluminium oxide layer is, furthermore, an elec-trical insulator, and the layer may sometimes be thickenough to prevent arc initiation. In MIG processes, athick oxide layer can produce erratic electrical com-mutation in the gun’s contact tube, resulting in poorwelds.

It is thus evident that aluminium oxide has to beremoved before welding because it compromises thequality of the weld. Generally, the oxide removal canbe done by mechanical processes like brushing with astainless steel brush, cutting with a saw or grinding withsemi-flexible aluminium oxide grinding discs.9 Somewelding processes enhance additional oxide removalprocesses, for example, in ultrasound metal weldingprocesses (UW), oxides and contaminates are removedby high-frequency motion, thus providing metal–metalcontact and allowing for the work pieces to bond prop-erly.17 In hybrid laser MIG-welding of aluminiumalloys, the MIG-welding process has a cleaning effectthat removes the aluminium oxide layer. However, it isrecommended that pre-weld cleaning of the weld sur-face should be carried out preferably by pickling or drymachining.18 In gas-shielded arc welding, aluminiumoxide removal from the weld pool can be done by cath-ode etching (which is controlled chemical surface corro-sion done to reveal the details of the microstructure).19

A direct current passes through the electrode connectedto the positive pole of the power source. There is thus aflow of electrons from the work piece to the electrodeand the ions flow in the opposite direction, bombardingthe work piece surface. The aluminium oxide film isbroken and dispersed by the ion bombardment, therebyallowing the flowing weld metal to fuse with the parentmetal. It is advantageous to remove the aluminiumoxide layer before welding because2,9

1. It significantly reduces the amount of hydrogenporosity in the weld.

2. It helps to improve the stability of the weld processespecially in tungsten inert gas welding (TIG).

3. It allows for complete fusion of the weld. Cathodecleaning is important in TIG process as the oxidestarts to form immediately after wire brushing.

Figure 1. Schematic of aluminium showing its oxide layer and the anodised surface.

1130 Proc IMechE Part B: J Engineering Manufacture 227(8)

Aluminium oxide can also be removed by chemicaletching or pickling. Table 1 presents chemical treat-ments for oxide layer removal.9

One of the causes of the oxide layer is from anodisa-tion, which is an electrochemical process by which ametal surface is converted into a decorative, durable,corrosion resistant anodic oxide finish.20,21 Anodisationutilises the unique ability of amorphous alumina to buildup an even porous morphology22 formed in alkaline andacidic electrolytes. During anodising, aluminium oxide isnot applied like paint or plating. Rather, it is integratedfully with the underlying aluminium substrate.Therefore, it cannot peel or chip off. The anodic oxidestructure is highly ordered and porous, thereby allowingfor further processing like sealing and colouring.20

The reasons for the utilisation of anodisation are toincrease corrosion resistance and ensure the metal sur-face is fade proof for up to 50 years,23 to improve dec-orative appearance, to increase abrasion resistance andpaint adhesion, to improve adhesive bonding and lubri-city, to provide unique decorative colours or electricalinsulation, to permit subsequent plating, to detect sur-face flaws, to increase emissivity and to permit applica-tion of photographic and lithographic emulsions.14,20,24

Anodising of aluminium alloys is generally advanta-geous. However, it poses challenges for aluminiumwelding because the arc cleaning effect of the AC cur-rent cannot remove the double layer (the anodised layerand oxide layer as in Figure 1). Before welding, theanodised surface needs to be removed.20

Shielding gas selection

Shielding gas protects the molten weld pool from theatmosphere, which is important because aluminium hasa tendency to react with atmospheric air to form oxideand nitrides. The shielding gases commonly used in

welding aluminium and its alloys are inert gases suchas argon and helium.

Argon is used as a shielding gas for manual andautomatic welding. Argon is cheaper than helium, andthe use of argon produces a more stable arc andsmoother welds. However, argon gives lower heat inputand lower attainable welding speed, and therefore thereis the possibility of a lack of fusion and porosity inthick sections. In addition, use of argon can result in ablack sooty deposit on weld surfaces, although this canbe wire brushed away. It has been observed that withhelium shielding gas, the arc voltage is increased by20%, resulting in a higher, hotter arc, deeper penetra-tion and wider weld beads. This implies that the criti-cality of arc positioning (aids avoidance of missed edgeand insufficient penetration defects) is lower withhelium. There is a reduction in the level of porositywhen helium shielding gas is used because the weldpool is hotter and there is slower cooling, which allowshydrogen to diffuse from the weld pool. Due to thehigher heat produced, the use of helium allows thatwelding speeds up to three times higher than withargon. The high cost of helium and the inherent arcinstability mean, however, that helium is used mainlyin mechanised and automatic welding processes.9

It is common practice to use a mixture of helium andargon as it provides a compromise on the advantages ofeach gas. Common combinations are 50% or 75% ofhelium in argon, which allow for better productivity byincreasing the welding speed and provide a wider toler-ance for acceptable welds. The purity of the shieldinggas is of importance. At the torch, not at the cylinderregulator, a minimum purity requirement of 99.998%and low moisture levels of less than 250 �C (less than39 parts per million (ppm) H2O) are expected.9

Generally, the shielding gas should be selected with thefollowing considerations.2,9,25,26

Table 1. Chemical treatments for cleaning and oxide removal.9

Solution Concentration Temp (�C) Procedure Container material Purpose

Nitric acid 50% water50% HNO3

(technicalgrade)

18–24 Immerse 15 minRinse in cold waterRinse in hot waterDry

Stainless steel Removal of thin oxidefilm for fusion welding

Sodium hydroxidefollowedby nitric acid

5% NaOH inwaterConcentratedHNO3

7018–24

Immerse for 10–60 sRinse in cold waterImmerse 30 sRinse in cold waterRinse in hot waterDry

Mild steelStainless steel

Removal of thick oxidefilm for all welding andbrazing operations

Sulphuric chromicacid

5 L H2SO4

1.4 kg CrO3

40 L water

70–80 Dip for 2–3 minRinse in cold waterRinse in hot waterDry

Antimoniallead lined steeltank

Removal of films andstains from heat treatingand oxide coatings

Phosphoric chromicacid

1.98 L of 75%H3PO3

0.65 kg of CrO3

45 L of water

95 Dip for 5–10 minRinse in cold waterRinse in hot waterDry

Stainless steel Removal of anodiccoatings

Olabode et al. 1131

1. The gas must be able to generate plasma and a sta-ble arc mechanism and characteristics.

2. It should provide smooth detachment of moltenmetal from the wire and fulfil the desired mode ofmetal transfer.

3. It should protect the welding head (in the arc’simmediate vicinity), molten pool and wire tip fromoxidation.

4. It should help to attain good penetration and goodweld bead profile.

5. It should not affect the welding speed of theprocess.

6. It should prevent undercutting tendencies.7. It should limit the need for post-weld cleaning.8. It should not be detrimental to the weld metal

mechanical properties.

The recommended shielding gas for welding alumi-nium using pulsed MIG is argon (99.998%)25,27 at aflow rate of about 20 L/min.27 A mixture of argon andhelium can also be used and even helium alone. Heliumincreases the weld penetration and offers higher arcenergy and thus increased deposition rate,27,28 and itshould be used when the section is greater than 50mm.9

More details can be seen in Table 2, which presentsMIG shielding gases for aluminium, and Table 3 pre-sents the effects of shielding gases on aluminium weld-ing. Studies have shown that welding of aluminium canbe improved (arc stability) by oxygen doping of inert

shielding gas.29 In addition, the alternating shieldinggases reduces weld porosity.30–32

Joint types and process limitations

This article considers eight industrially accepted weldingprocesses and six joint types. Joint design is importantbecause it costs money to buy weld metal. The fillet throat,weld accessibility and the functionality of the welded workpiece are taken into consideration in this design. The sixjoints considered are butt, T-joint, corner, cruciform, edgeand lap joint (see Table 4), which are derived from thethree basic welding joints (fillet, lap and butt joints). Jointdesigns are based on the strength requirements, the alloysto be joined, the thickness of the material, the joint typeand location, weld accessibility and the welding process.Before choosing the joint design, it is important to notethat welding in the flat or downward position is preferablein all arc-welding processes, as there is the easier possibilityof depositing high-quality weld metal at a high depositionrate in a flat position. Additionally, the weld pool is larger,allowing for a slower cooling and solidification rate, whichenhances the escape of trapped gases in the weld pool. Theflat position reduces weld porosity, reduces weld cost, andgives the best weld metal quality compared with otherpositions. The static tensile strength of the weld is deter-mined by the throat thickness, which must be designed toensure that it can carry the workload for which the weld isdesigned. Conventional TIG and MIG processes produce

Table 3. Effect of shielding gas on aluminium welding.9,29–34

Shielding gas Relative effect (100% argon as the reference)

100% Ar Ar + He 100% He

Gas flow Nominal Higher HighestArc voltage (MIG) Nominal Higher HighestArc (MIG) Nominal stability More unstable Most unstableWeld seam width and depth Nominal width and depth Higher width

Shorter depthHighest widthShortest depth

Weld seam appearance Nominal smoothness Smoother SmoothestPenetration Nominal depth and roundness Deeper and more round Deepest and most roundWelding speed Nominal welding speed Higher attainability Highest attainabilityLack of fusion Nominal Lower LowestPorosity Nominal Lower LowestPre-heating Nominal Less needed Least neededHeat production Nominal warmth Warmer work piece Warmest work pieceCost of shielding gas Nominal price More expensive Most expensive

MIG: metal inert gas welding.

Table 2. MIG shielding gases for aluminium.26

Metal transfer mode Shielding gas Characteristics

Spray transfer 100% Argon Best metal transfer and arc stability, least spatter, and good cleaning action.35% Argon–65% Helium Higher heat input than 100% argon; improved fusion characteristics on thicker

material; minimises porosity.25% Argon–75% Helium Highest heat input; minimises porosity; least cleaning action

Short circuiting Argon or Argon + Helium Argon satisfactory on sheet metal; argon–helium preferred for thicker basematerial.

1132 Proc IMechE Part B: J Engineering Manufacture 227(8)

Tab

le4.

Join

tty

pes

and

pro

cess

limitat

ions

ofal

um

iniu

mal

loys

.2,8

,9,1

7,1

8,3

5–52

Pro

cess

es

Join

ts

MIG

TIG

PAW

FSW

LBW

RW

EB

WU

W

Butt

join

t(a

)�

��

��

��

�La

pjo

int

(b)

��

��

��

��

T-jo

int

(c)

��

��

��

��

Edge

join

t(d

)�

��

��

��

�C

orn

erjo

int

(e)

��

��

��

��

Cru

cifo

rm(f

)�

��

��

��

�

Lim

itat

ion

Lim

itat

ion

Lim

itat

ion

Lim

itat

ion

Lim

itat

ion

Lim

itat

ion

Lim

itat

ion

Lim

itat

ion

(a)

(b)

(c)

(d)

(e)

(f)

With

argo

n,w

eldab

leth

ickn

ess

islim

ited

to25

mm

,an

dw

ith

hel

ium

,it

islim

ited

to75

mm

.

Lim

ited

torc

hdis

tance

of

10–19

mm

toen

sure

pro

per

lysh

ield

edw

eld

met

allim

its

flexib

ility

.

Lim

ited

outd

oor

applic

atio

nbec

ause

air

dra

fts

can

dis

per

seth

esh

ield

ing

gas.

Lim

ited

oper

ator

acce

pta

bili

tyofth

epro

cess

bec

ause

ofth

ere

lative

lyhig

hle

vels

of

radia

ted

hea

tan

dar

cin

tensi

ty.

Lim

ited

toth

inga

uge

sof

up

to6

mm

thic

knes

s.Li

mited

(shal

low

er)

pen

etra

tion

into

par

ent

met

alco

mpar

edto

MIG

.

With

argo

nsh

ield

ing

gas,

the

econom

ical

wel

dth

ickn

ess

limit

is10–

18

mm

with

hel

ium

(DC

EN

).

Diff

icult

topen

etra

tein

toco

rner

san

din

toth

ero

ots

offil

let

wel

ds.

Lim

ited

by

the

low

erdep

osi

tion

rate

,low

tole

rance

on

fille

ran

dbas

em

etal

,an

dco

stfo

rth

ick

sect

ions

com

par

edto

MIG

.

Pla

sma

MIG

wel

dth

ickn

esse

slim

ited

to6–

60

mm

range

.

Pla

sma

TIG

wel

dth

ickn

esse

sra

nge

can

be

less

than

2.5

–16

mm

ina

singl

epas

s.

Lim

ited

by

the

hig

hca

pital

equip

men

tan

dm

ater

ialco

stco

mpar

edto

TIG

.

Lim

ited

tole

rance

ofth

epro

cess

tojo

int

gaps

and

mis

alig

nm

ent.

Lim

ited

oper

ator

acce

pta

bili

tyofth

epro

cess

due

toth

eco

mple

xto

rch

arch

itec

ture

that

requir

esm

ore

mai

nte

nan

cean

dac

cura

tese

t-bac

kofth

eel

ectr

ode

tip

with

resp

ect

toth

enozz

leori

fice,

whic

his

chal

lengi

ng.

Wel

dab

leth

ickn

ess

range

sfr

om

1–50

mm

(sin

gle

pas

s).

Tooldes

ign,pro

cess

par

amet

ers,

and

mec

han

ical

pro

per

ties

dat

abas

eis

limited

and

only

avai

lable

for

limited

allo

ysan

dth

ickn

esse

s(u

pto

70

mm

).

Lim

ited

tolo

wer

pro

duct

ivity

case

sco

mpar

edto

LBW

.

Insu

ffic

ient

des

ign

guid

elin

esan

dlim

ited

educa

tion

for

imple

men

tation.

Exit

hole

left

when

tool

isw

ithdra

wn.

Larg

edow

nfo

rces

requir

edw

ith

hea

vyduty

clam

pin

gnec

essa

ryto

hold

the

pla

tes

toge

ther

duri

ng

wel

din

g.

Env

ironm

enta

llyfr

iendly

wel

din

gpro

cess

bec

ause

fum

esan

dsp

atte

rsar

enot

gener

ated

.

Lim

ited

conve

rsio

nef

ficie

ncy

ofel

ectr

ical

pow

erto

focu

sed

infr

ared

lase

rbea

mal

soca

lled

wal

lplu

gef

ficie

ncy

(about

10%

–30%

and

up

to40%

infib

rela

sers

).

Lim

ited

fitup

tole

rance

.Pre

cise

fitup

(15%

of

mat

eria

lth

ickn

ess)

nee

ded

for

butt

and

lap

join

ts.

Lim

ited

oper

ator

acce

pta

bili

tyofth

epro

cess

due

toth

ela

rge

capital

inve

stm

ent

nee

ded

,th

eref

ore

requir

ing

hig

hvo

lum

epro

duct

ion

or

critic

alap

plic

atio

ns

toju

stify

the

expen

diture

.

Lim

ited

wel

dth

ickn

ess

range

(0.9

–3.2

mm

)

Low

erte

nsi

lean

dfa

tigu

est

rengt

hco

mpar

edto

oth

erfu

sion

wel

din

gpro

cess

es.

Lim

ited

join

tdes

igns

or

config

ura

tion.S

eam

wel

ds

can

gener

ate

unzi

ppin

gef

fect

.

Lim

ited

oper

ator

acce

pta

bili

tyofth

epro

cess

bec

ause

,in

thic

k-se

ctio

ned

upse

tw

elds;

ther

eis

lack

of

good

non-d

estr

uct

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Olabode et al. 1133

weld metal on the surface of a plate during bead-on-platewelds to a depth of 3mm for TIG and 6mm for MIG.Therefore, to attain complete penetration for welds over3mm (MIG) and 6mm (TIG), there is the need for bevel-ling on butt joints, for example. The bevel can be single ordouble sided.9

As presented in Table 4, eight considered weldingprocesses are correlated with their applicability on sixdifferent welding joints. Butt and lap joints are applica-ble to all the selected weld processes. Cruciform jointshave the least applicability across the processes, whichis due to limited fixturing possibility during welding.Table 4 provides additional information on the viabi-lity of six joint types on the eighth selected welding pro-cesses by presenting the process-specific limitations.

An application of this review article is to use the pre-stated information to influence the selection case-specific optimum welding process. It can be challengingto determine an appropriate welding process to be usedfor aluminium. However, the challenge can be simpli-fied by considering various comparison selection factorsas presented in Table 5. The solution to the challenge iscase specific. An understanding of the selection factors

considered provides better process selection and thus abetter evaluation.

It is important to point out that the scaling is subjectto the designer’s discretion and not completely objective.The welding designer determines the importance level ofthe selected aluminium-welding project by answering aquestion like ‘how important is’ strength, elongation,chemical stability, etc., to the finished product. Thedesigner defines the importance level on a scale of 1–3(1= least, 2=moderate and 3=high). In a similarfashion, the advantageous level is determined by answer-ing a question like ‘how advantageous is’ the selectedwelding process to the selected consideration. Theimportance level is multiplied by the advantage leveland the result is called an impact factor. The impact fac-tor is summed up for each selected welding process, andthe welding process with the highest impact factor sum-mation is selected as the optimal welding process.

Case study

A welding process for high-strength aluminium for aero-space is to be selected. The available welding processes

Table 5. Weld process selection (the highest factor summation is the best of the processes considered).

Selection factors Process A (TIG) Process B (FSW) Process C (PAW) Process D (MIG)

Quality of the welded joint Imp. Ad. I. Fac. Imp. Ad. I. Fac. Imp. Ad. I. Fac. Imp. Ad. I. Fac.Strength 3 2 6 3 2 6 – – – 3 2 6Elongation 2 2 4 2 3 6 2 2 4 2 3 6Chemical stability 2 2 4 2 3 6 2 3 6 2 3 6Weld defects 2 3 6 2 1 2 2 1 2 2 1 2Penetration 1 3 3 1 3 3 1 3 3 1 3 3Distortion 1 1 1 1 2 2 1 2 2 1 2 2

Suitability for useWelding thin sheet (\1 mm) 2 2 4 2 3 6 2 3 6 2 2 4Sheet welding (.3 mm) 1 1 1 1 2 2 1 2 2 1 3 3Welding Al-Mg alloys 1 2 2 1 2 2 1 2 2 1 2 2Overhead welding 1 1 1 1 3 3 – – – – – –Variable material thickness 2 1 2 2 1 2 2 2 4 2 2 4Variable welding speed 1 1 1 1 2 2 1 3 3 1 2 2Welding of castings 2 2 4 2 3 6 2 2 4 2 2 4Joining cast to wrought alloys 1 3 3 1 3 3 1 1 1 1 2 2Repair welds on castings 2 3 6 2 3 6 2 1 2 2 2 4

Suitability for automationWith filler 1 1 1 1 3 3 1 2 2 1 3 3Without filler 2 3 6 2 1 2 2 1 2 2 1 2Butt welding \3 mm 2 1 2 2 2 4 2 2 4 2 2 4.3 mm 1 2 2 1 1 1 1 3 3 1 1 1

Suitability for joint typeButt joint 1 2 2 1 1 1 1 1 1 – – –Lap joint 1 3 3 1 3 3 1 1 1 1 1 1

Economic aspectsEquipment costs 3 2 6 3 3 9 3 2 6 3 1 3Maintenance costs 3 2 6 3 2 6 3 3 9 3 2 6Labour costs 1 3 3 1 2 2 1 3 3 1 3 3Welder’s training time 1 1 1 1 1 1 1 1 1 1 3 3

Process rating (P

) 80 89 73 76

Imp.: importance level; Ad.: advantage level; I. Fac.: impact factor; EBW: electron beam welding; FSW: friction stir welding; LBW: laser beam welding;

MIG: metal inert gas welding; PAW: plasma arc welding; RW: resistance welding; TIG, tungsten inert gas welding; UW: ultrasonic welding.

1134 Proc IMechE Part B: J Engineering Manufacture 227(8)

are as presented in Table 5. A blank table is constructedand the considered welding processes are selected andfilled into the table.

The selection factors under consideration are as pre-sented in Table 5, which are categorised under qualityof the weld joint, suitability for use, suitability of fillers,joint suitability and economics. Therefore, at this stagein the design, the processes row and the selection factorcolumn are filled in the table.

As the designer, the importance level is determinedand designed on a scale of 1–3, and using a scale of fiveis also applicable, but the calculation becomes morecomplex. Choosing a scale of 1–3 (1= low, 2=moder-ate and 3=high), a number is assigned to the consid-ered selection factor. Therefore, at this stage, theimportance level of the selection factor under consider-ation is filled into the ‘Imp.’ column (Table 5). It isimportant to note that the number is the same acrossrow (all processes) because the importance of a selec-tion factor is independent of the process.

The advantage level is determined and designed bythe designer on the same scaling used for importancelevel. If the scaling used in importance level is five, thescaling of five should be used. In this case, a scaling of1–3 is used where 1= low, 2=moderate and 3=high.At this stage, the entire advantageous level column onTable 5 is filled for all the considered selection factorsinto the ‘Ad.’ column.

The calculation for the impact factor and the processrating is carried out. The impact factor for each consid-ered selection factor is derived by multiplying theimportance level column of each process by advanta-geous level column of each process. The derived valueis filled into the ‘I. Fac.’ (impact factor) column ofTable 5. The process rating (welding process) is derivedby the summation of all the impact values column ofeach process. Therefore, the process rating row is filledin Table 5.

The optimum weld process is the process with thehighest process rating, which in this case study is pro-cess C friction stir welding (FSW).

Conclusion

This article examined the surface-related challenges,joint types and limitations of aluminium alloys with thefocus on providing a guide on how to select an optimalwelding process. Aluminium and its alloys have weldingchallenges, which include the presence of aluminiumoxide on surfaces, welding of anodised aluminium andlimited shielding gas options. The aluminium oxide sur-face is formed when aluminium is exposed to an atmo-sphere containing oxygen, and the aluminium oxide hasto be cleaned away from the surface before weldingbecause its causes weld defects like porosity.

The chemical affinity of aluminium for oxygen is uti-lised for anodising aluminium alloys and then paintingto improve corrosion resistance. However, it can be

detrimental when welding anodised aluminium as theanodised layer has to be cleaned before welding. Themelting point of aluminium alloys is generally around660 �C and the melting point of aluminium oxide is2050 �C. It is therefore recommended that the aluminiumoxide layer or the anodised layer be removed, mechani-cally or chemically, just before welding.

Aluminium alloys have high chemical affinity; there-fore only inert gases can be used as shielding gases dur-ing welding. Argon and helium gases are used inaluminium welding to protect the weld pool. The pres-ence of helium increases the arc heat input and there-fore allows for deeper penetration compared withargon gas, but on the other hand, helium is moreexpensive than argon. A mixture of helium and argonis sometimes used to improve weldability of some alu-minium alloys. A wider range of shielding gases wouldincrease the manipulation possibility for aluminiumalloy welding, but currently argon and helium are theonly gases used.

The industrial welding processes considered in thiswork include MIG, TIG, plasma arc welding (PAW),FSW, LBW, resistance welding (RW), electron beamwelding (EBW) and UW. The weldable thickness is alimitation in all the processes; the highest weldablethickness of up to 70mm is achieved with EBW. FSWproduces the best weld because the mechanical propertydeterioration is minimal, and the process is friendly asno fumes or spatters are produced during welding.

The joint configurations considered include the buttjoint, lap joint, T-joint, edge joint, corner joints andcruciform joint. The butt joint and lap joint are appli-cable to all the considered welding processes. The pos-sibility of using different joint orientations with theconsidered welding processes depends on the manipula-tion of the work piece (fixturing).

Although FSW produces the best weld for alumi-nium alloys, the optimal welding process is case spe-cific. The designed table for weld process selectionprovides information on how to select the optimalprocess based on case-specific considerations for alu-minium alloys.

Declaration of conflicting interests

The authors declare that there are no conflicts ofinterest.

Funding

This research received no specific grant from any fund-ing agency in the public, commercial or not-for-profitsectors.

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Appendix 1

Notation

EBW electron beam weldingFSW friction stir weldingLBW laser beam weldingMIG metal inert gas weldingPAW plasma arc weldingppm parts per millionRW resistance weldingTIG tungsten inert gas weldingUW ultrasonic welding

Olabode et al. 1137

Publication III

Effect of Al2O3 film on the mechanical properties of a welded high-strength (AW7020) aluminium alloy.

Olabode, M., Kah, P., and Martikainen, J. (2015). Proceedings of the Institution ofMechanical Engineers, Part B: Journal of Engineering Manufacture.DOI: 10.1177/0954405415600678

© Sage publications, 2015

Original Article

Proc IMechE Part B:J Engineering Manufacture1–10� IMechE 2015Reprints and permissions:sagepub.co.uk/journalsPermissions.navDOI: 10.1177/0954405415600678pib.sagepub.com

Effect of Al2O3 film on the mechanicalproperties of a welded high-strength(AW 7020) aluminium alloy

Muyiwa Olabode, Paul Kah, Esa Hiltunen and Jukka Martikainen

AbstractThe use in motor vehicles of lightweight metals such as aluminium and titanium provides a high strength-to-weight ratio,thereby lowering overall weight and reducing energy consumption and CO2 emissions. Aluminium alloys have thusbecome an important structural material especially high strength and ultra-high strength alloys such as AW 7020. Manystudies have shown that the presence of an aluminium oxide (Al2O3) thin film formed naturally on aluminium alloys isdetrimental to welding. This article further investigates the specific effect of the Al2O3 thin film on welding AW 7020alloy. An analytical experiment of welded AW 7020 alloy using a pulsed metal inert gas (MIG) robotic weld machine iscarried out. Four specimens were cut, butt welded, and examined. The weld parameters included pre-weld cleaning ofthe Al2O3, pre-, and post-weld heat treatment. Al2O3 was removed by wire brushing; preheating was conducted at atemperature of 130 �C; and natural ageing was conducted by post-weld heating at 480 �C for 2 h, followed by quenchingin water at 90 �C for 8 h, reheated, and sustained at 145 �C for 15 h. The result shows that the presence of Al2O3 layerappears not to be detrimental to the weld with new welding technologies, therefore suggesting that it is not necessaryto grind off the Al2O3 layer before welding. This finding implies that welding costs can be lowered and weld qualityimproved when new welding technologies are applied in the welding of high-strength aluminium alloys.

KeywordsAl2O3, AW 7020, high-strength aluminium, pulse MIG welding, mechanical properties

Date received: 23 April 2014; accepted: 13 July 2015

Introduction

The use of lightweight metals in industrial applicationshas gained importance recently as a means of achievinga greener environment with low pollution. For exam-ple, studies1,2 show that the use of lightweight materialin the construction of car bodies reduces weight, fuelconsumption, and CO2 emissions. Welding of alumi-nium is considered challenging due to the inherentproperties of aluminium alloys such as the high heatconductivity of aluminium alloys and the presence ofan aluminium oxide (Al2O3) film that appears when thealloy is exposed to the atmosphere (which is detrimen-tal to welding).3,4

Researches have shown that the presence of Al2O3 isdetrimental to the welded piece and it also presentschallenges for the welding process. The information gapof how detrimental is the Al2O3 to AW 7020 weld ifnew welding technologies that can prevent oxide inclu-sion are used exist.

This article discusses the Al2O3 layer formed on alu-minium alloys exposed to air or moisture and its

chemical properties. It describes the formation processand the composition of the two anodic layer films.Properties such as density, melting point, and thermalconductivity are presented. The advantages, disadvan-tages, and applications of Al2O3 are also presented. Itsformation can be controlled to gain structural advan-tages and improved characteristics, for example, byadonisation.

The purpose or this research is to study the effect ofAl2O3 on the mechanical properties of AW 7020. Newwelding technologies for aluminium welding are studied(because newer technologies are expected to providefaster weld speed, cheaper welding cost, and improvedwelding equipment efficiency), with focus on the effectof Al2O3 on the processes and how the welding process

Lappeenranta University of Technology, Lappeenranta, Finland

Corresponding author:

Muyiwa Olabode, Lappeenranta University of Technology, Skinnarilankatu

34, 53850 Lappeenranta, Finland.

Email: muyiwa.olabode@lut.fi

by guest on September 1, 2015pib.sagepub.comDownloaded from

is used to produce acceptable welds despite the pres-ence of Al2O3 film.

The experiments are carried out on an AW 7020alloy using a robotised pulse MIG machine. The testpieces were cut across the weld, etched, and tested forhardness and tensile strength. As a contribution to theliterature, this research provides facts on the effect ofAl2O3 on AW 7020.

Al2O3 layer of exposed aluminium

Aluminium is resistant to corrosion because alumi-nium, like all other passive metals, is covered with acontinuous and uniform natural oxide film on exposureto an environment containing oxygen (as illustrated inFigure 1). The film is formed spontaneously in oxidis-ing media according to the following reaction

2Al+3

2O2 ! Al2O3

Formation process

Al2O3 layer forms immediately, within 1ms or evenless.1 Al2O3 is a natural, non-uniform, thin, and non-coherent colourless oxide film made up of two superim-posed layers of a thickness in the range of 4–10 nm.The oxidation reaction has a free energy of reaction

21,675 kJ. This is a very high oxidation reaction energyand explains why aluminium has high affinity towardsoxygen.1

The closest layer to the aluminium alloy is called thebarrier layer and is illustrated in Figure 2. This layerhas dielectric properties2 and forms as soon as the alu-minium alloy is exposed to an oxidising media. Thislayer consists of cells and pores that are generated dueto reaction with the external environment. Attainingthe final thickness can take several weeks or evenmonths, depending on the physicochemical conditionsof the environment.3 This layer is less compacted thanthe barrier layer, and because of the presence of thepores, it reacts with the external environment duringtransformation.

Al2O3 characteristics and properties

Amorphous alumina, chemically called Al2O3, forms ata temperature range of less than 50 �C to 60 �C and hasa density of 3.40. Further principal properties are pre-sented in Table 1. It is important to note that there is adifference in the density of the aluminium alloy and theAl2O3, so the Al2O3 film is under compression. This dif-ference is responsible for the ability of the Al2O3 film toresist deformation without breaking and the excellentresistance during forming operations.4 The mechanicaland structural characteristics of Al2O3 layer are depen-dent on the oxygen partial pressure.5 It is should benoted that the composition of the oxide film dependson the chemical composition of the aluminium alloy.Therefore, the thin film oxide layer is not the same forall classes of aluminium alloys.6

Al2O3 advantages, disadvantages, and applications

The Al2O3 layer brings both advantages and disadvan-tages to the use of aluminium in structures. It is

Figure 1. Schematic of aluminium (melting temperature of660 �C) and its oxide layer (melting temperature of 2050 �C).

Figure 2. Schematic diagram of a cross section of a porous anodic film on aluminium showing the barrier, pore, and other principalmorphological features.

2 Proc IMechE Part B: J Engineering Manufacture

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advantageous as it is responsible for the corrosion resis-tance of aluminium alloys as presented in Table 2.Al2O3 allows for increased surface treatability of alumi-nium alloys using procedures such as anodising andpainting. The presence of Al2O3 can cause weld defectssuch as incomplete weld fusion and weld porosity.Incomplete fusion describes a weld that does not com-pletely merge or mix and porosity describes a weld inwhich gas bubbles are present in the weld.

Al2O3 modification for structural advantage byanodising

The thickness of the Al2O3 layer can be varied to gainstructural advantages by anodising. Anodising is a con-trolled corrosion process of aluminium alloys in alka-line and acidic electrolytes to attain a uniformcontinuous protective oxide film.13 It employs theunique ability of amorphous alumina to build up aneven porous morphology. It is important to mention

that natural self-occurring Al2O3 is very thin (0.01mm);anodising produces a higher thickness range (12–25mm).14 The advantages of anodising include faderesistance of structural aluminium alloys up to50 years,15 corrosion resistance, abrasion resistance,electrical insulation, unique decorative colours, adhe-sive bonding, decorative appearance, paint adhesion,improved lubricity, permission of subsequent plating,increased emissivity, surface flaws detection, andphotographic and lithographic emulsions applicationpossibility.16–18

The three main anodising processes are chromic ano-dising (in which the agent is chromic acid), sulphuricanodising and sometime referred to as mild adonisation(in which the active agent is sulphuric acid), and hardanodising (in which the agent is sulphuric acid, alone orin combination with additives).19

New welding technology for aluminiumalloys

Newer welding technologies are expected to providebetter and cheaper welding processes, which make itsimportant to be studied. Resistance welding (RW) canbe used in the form of seam welds or spot welds. Thefusion occurs due to the heat created by a flowing cur-rent through a resistance device for a given period oftime while the materials to be welded are pressurepressed against each other.20 The presence of Al2O3 onthe pre-weld surface influences the total resistanceacross the weld electrodes. RW is thus a surface critical

Table 1. Principal properties of Al2O3.

Property Value

Melting point 2054 6 6 �CBoiling point 3530 �CLinear expansion coefficient at 25 �C 7.1 3 1026 KThermal conductivity at 25 �C 0.46 J/cm/s/KSpecific heat at 25 �C 0.753 J/g/KDielectric constant at 25 �C 10.6Electrical resistivity at 14 �C 1019O/cm

Table 2. Advantages, disadvantages, and applications of Al2O3.

Advantages Disadvantages Application

1. It is responsible for the corrosionresistance of aluminium alloys.

2. Al2O3 allows for increasedsurface treatability of aluminiumalloys with procedures such asanodising and painting.

3. Al2O3 made through ALD is verythin and consistent. It hasexcellently controlled thicknessand composition of the film at anatomic level.7,8 ALD has excellentdielectric properties, thermal andchemical stability, and goodadhesion to various surfaces andis therefore used for siliconmicroelectronics.9

4. The presence of an oxide layerduring laser welding increases theabsorptivity of aluminium and itsalloys to laser radiation.10,11

1. The melting point of Al2O3

(approximately 2050 �C) is higherthan that of AW 7020(approximately 660 �C), whichimplies that a higher heat densitythan welding heat density isneeded to break the Al2O3

structure.2. It has a significant mechanical

strength. During welding, theoxide layer can therefore remainas a solid film (or fracture intosmall particles) due to the flow ofthe molten material,7 even whenthe surrounding metal is molten.This can result in severeincomplete fusion defects.

3. Due to its higher weightcompared to aluminium, Al2O3

can lead to weld inclusion.4. Its hygroscopic properties make it

bind to moisture, which leads tothe formation of pores in thewelds.7

1. Al2O3 is used as insulators, an ionbarrier and a protective layer inthin film industries.9

2. The Al2O3 layer is used in themanufacture of marine vehiclesrequiring high corrosionresistance.

3. Due to its high, wear resistance,Al2O3 is used in protectionagainst friction and corrosion inoptical applications.

4. The Al2O3 layer is used in micro-electromechanical systems as adielectric layer to preventelectrical short-circuit5 and ascatalyst.12

ALD: atomic layer deposition.

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process; it is therefore recommended to remove theoxide before welding.

Friction stir welding (FSW) is a mechanical solid-state welding process that softens the material to bewelded by the heat generated by friction between arotating tool and the workpiece. There is no need toclean off the oxide layer prior to welding. However,flaws due to oxide inclusion can occur if the toolshoulder selected is oversize thereby sweeping surfaceoxide into the weld. The amount of oxide inclusion canbe reduced by increasing the weld speed, resulting inlow oxide layer disruption per millimetre.21 Anothernew modification is laser-assisted FSW (in which thelaser is mainly used for preheating) which has the addi-tional advantage of using more simple and inexpensivemachines, in addition to the reduction in tool wear andhigher attainable welding speeds.22

Low-energy arc (MIG) welding methods such as coldmetal arc transfer (CMT) are a recent development of theMIG process. Low-energy arc welding uses the wire feedsystem to control the weld process. The wire is fed into theweld pool until the short circuit occurs, after which thefeed direction is reversed and the feed wire is withdrawn.The wire feed is then fed forward again and the processbegins anew.23 The process utilises high-speed digital con-trol systems to control the arc length, metal transfer, andthermal input on the workpiece. When integrated withpulsed MIG, it produces even better welds and the Al2O3

film is decomposed by the pulsed MIG process.24

Laser welding employs the use of laser beams as aheat source for welding aluminium. Newer technologi-cal modifications involve the use of an active flux thatimproves the mechanical properties and appearance ofthe aluminium weld.23,25 In addition, dual beam lasersare used, producing better weld quality (deep penetra-tion, surface smoothness, and high strength) comparedto single beam laser welds.26

Hybrid laser welding involves the use of a conven-tional arc welding process in combination with laserwelding. The process utilises the advantages of both laserand arc [Metal inert gas (MIG) or Tungsten inert gas(TIG)] welding such as high process speed, low heatinput, low thermal distortion, good gap bridging ability,and good process stability23,27 with high-precision weld-ing. It is important to mention that in TIG welding pro-cess Al2O3 film decomposition occurs by cathodeetching, however, it is still important with hybrid laserTIG welding processes to remove the Al2O3 layer justbefore welding.

Plasma arc welding (PAW) is a high-power-densityweld method for aluminium that is advantageous formaking deep welds. In addition to the general informa-tion in scientific and technical articles, aluminium weldsare stabilised using direct current (DC) power and neg-ative polarity; research has shown that stability canalso be achieved using the alternating current (AC)power source.23 Variable polarity plasma arc welding(VPPA) is a relatively new technology that usesadvanced power supply to generate rapid switches from

electrode positive (EP) to electrode negative (EN).With this technology, there is now the possibility ofadjusting the EP and EN independently to enable goodcleaning action (EP) and good penetration (EN).28

Another modification to PAW is plasma MIG welding,which uses a coaxial MIG welding torch. Plasma MIGwelding reduces spatter and fume formation andimproves the weld bead appearance (the weld bead isflatter with deeper penetration in Al–Mg weld if theplasma current is increased). With plasma MIG weld-ing, pre-weld Al2O3 layer removal is important.29

Experimental procedure

The purpose of this experiment is to study the effect ofthe presence Al2O3 of layer on the mechanical proper-ties of AW 7020. The experiment was carried out asbutt welds of two samples each for the four weld experi-ment conditions (ECs) 1–4 with the weld parameterspresented in Table 3. A robotised pulsed MIG machinewas used to weld the specimen. The weld set-up is pre-sented in Figure 3, respectively. A 4-mm AW 7020 platewas used as the workpiece. The air gap between theworkpiece was 3mm. A copper backing was used. Pureargon (99.5%) shielding gas supplied at a flow rate of15L/min was used. A 1.2-mm-diameter Elga AlMg5 fil-ler material was supplied at 9m/min. A nozzle distanceof 15mm and a welding speed of 7.5mm/s were used.The weld torch was inclined at 15� to normal and theweld direction was such that the torch is pulling. Anaverage current of 140A and an average voltage of22.7V were used in all the experiments.

The test was carried out in the welding workshop, ina well-controlled atmosphere and at room temperature.The samples for the four different EC were cut, welded,and examined. In EC 1, the weld was carried out with-out pre-weld cleaning of the Al2O3 in the absence ofpre- and post-weld heat treatment. In EC 2, the weldwas conducted without the removal of Al2O3. However,the workpiece was preheated at a temperature of 130 �Cwithin the recommended preheating temperature andclose to the upper limit.30 The oxide layer in EC 3 wasnot removed before the welding. No preheating wascarried out but natural ageing was conducted by post-weld heating at 480 �C for 2 h, followed by quenchingin water at 90 �C for 8 h, and finally, reheating andmaintaining the workpiece heat at 145 �C for 15 h. TheAl2O3 layer in EC 4 was removed and no preheating orartificial ageing was carried out. In order to investigatethe effect of Al2O3 on the mechanical properties, thesamples were examined for ultimate yield strength(YS), tensile strength, elongation, and hardness values.Macrographs were taken to evaluate the degree of welddefects present, if any.

Further experiments were carried out on the weldedsample to study the composition of Al2O3 layer at dif-ferent distances from the alloy surfaces. A sample ofover 99% pure aluminium alloy (1xxx series), AW

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7020, and 7025-T6 were placed to a thermo scientificultra dry Silicon Drift Detector (SDD) energy-dispersive X-ray spectroscopy (EDS). Three weld sam-ples were pre-cleaned and then exposed to atmospherefor 1 h. The samples were then tested for the presenceof Al2O3. Each sample was tested at a depth of 0.2, 1.2,

and 3.3mm. For each depth, four measurement spotsof 0.2 3 0.5mm were selected and each significantchemical content was analysed; the results presented inTable 4 are based on the averages.

Result

The mechanical properties of the welds were observedand the results are presented. The result consists of ten-sile test (that provides information on the YS, ultimatetensile strength (UTS), and elongation at fracture),hardness test, and macrograph examination.

Tensile tests

The tensile test bar graph in Figure 4 presents a com-parison of the YS (Re/N/mm2), UTS (Rm/N/mm2),and elongation at fraction in A/%. The y-axis is mea-sured in units and the x-axis represents the averages ofthe four different ECs and the control condition. TheYS values represent the amount of force the weldedAW 7020 can resist before plastic deformation. TheUTS value shows the amount of force needed to breakthe weld, and the elongation shows how far the weldwill stretch before breaking.

The tensile test measures the YS, which is the stressvalue at which welded specimen begins to deform plas-tically and cannot return to its original position. It isused in this experiment to express the load bearingcapacity of the weld just before plastic deformation.The higher the YS, the more desirable is the weld.Based on the YS values, EC 1 is the best weld while EC4 is the worst weld, which is due to the effect of remov-ing the Al2O3 layer thereby increasing the amount ofweld heat input. As seen in EC 2, preheating also seemsto reduce the YS while artificial ageing appears toimprove the YS in EC 3.

Table 3. AW 7020 weld experiment parameters.

Welding conditions for AW 7020 welding

Weld type Butt welding, I-groove, air gap 3 mm, against copper backingBase material AW 7020, thickness 5 mmFiller material Elga AlMg5, ; 1.2 mmShielding gas Ar, flow rate 15 L/minWire feed rate 9 m/minWelding speed 7.5 mm/sNozzle distance 15 mmTorch angle 15� to normal

Experiment-specific parameter

ECs Current (A)averages

Voltage (V)averages

Al2O3 thin film Preheating Artificial ageing

1 140 22.6 Present No No2 139 22.8 Present Yes (130 �C) No3 140 22.7 Present No Yes (480 �C/2 h + quenching in water,

90 �C/8 h + 145 �C/15 h)4 140 22.7 Absent No No

ECs: experiment conditions.

Figure 3. AW 7020 weld set-up.

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The UTS is used to present the maximum tensileloading the weld can be subjected to before failure.The higher the UTS, the better the weld is from theload bearing perspective. In these experiments, EC 3produced the highest UTS value of 273.55Rm/N/mm2. This seems to be due to the effect of artificialageing. EC 1 has the next high, which seems to bedue to the low heat input to the workpiece due to thepresence of Al2O3. EC 2 has the next high value,which suggests that workpiece preheating reduces theUTS values. The least UTS value is in EC 4 whichsuggests that the removal of Al2O3 layer reduced theUTS.

Elongation at fracture expresses the ratio as a per-centage of the final length to the original length towhich the area of the specimen stretches just before fail-ure. The elongation shows how brittle or ductile theweld specimen is. If the elongation is low, the weld

piece will be brittle, and therefore, it can easily crack orbreak, for example, brittle ceramic cracks easily whensubjected to tensile loading. On the other hand, if theelongation value is high, the specimen is ductile andcan be plastically deformed. In many aluminium welds,it is desirable to have high elongation values. The bestweld is usually case specific based on the mechanical ormetallurgical properties of the weld demanded by theapplication. For example, aluminium welds that aredesigned to carry torsion loads such as shafts are sup-posed to be rigid with minimal elongation. On the otherhand, structural aluminium beams are expected to haveelongation so they do not break suddenly. EC 3 has thehighest elongation values which suggest that artificialageing increased the malleability of the workpiece. Thenext high elongation value is in EC 4 which suggeststhat the absence of Al2O3 increases elongation in com-parison to EC 1.

Table 4. Percentage weight composition weld samples.

Oxide layer formation period 1 h (after cleaning)Measuring spot 0.5 3 0.2 mmCorrection method Proza (Phi-Rho-Z)Take off angle 35.0�Measurement acceleration voltages 3 kV (0.2 mm) 10 kV (1.2 mm) 20 kV (3.3 mm)

Test depth from surface Material O (wt%) Al (wt%) Mg (wt%) Zn (wt%)

0.2 mm Al 99.90% 12.7 87.3AW 7020 6.55 87.05 1.05 5.47025-T6 6.25 87.3 1.1 5.35

1.2 mm Al 99.90% 4.1 95.9AW 7020 1.525 92 1.2 5.37025-T6 1.55 91.95 1.175 5.325

3.3 mm Al 99.90% 2.75 97.3AW 7020 1.2 93.1 1.2 4.67025-T6 1.075 93.35 1.15 4.4

Figure 4. Tensile strength of welded AW 7020.

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Hardness test

The hardness profile graph (Figure 5) presents the hard-ness values of the profile across the weld interface (WI).The hardness profile shows how much the hardnessdeviated from the base material (BM) into the unmixedzone (UMZ) and vice versa. The y-axis represents thehardness value (HV3) while the x-axis represents thedistance in millimetre from a common reference in theBM to the weld centre. It is important to mention that0 in the x-axis is located in the BM and the scaleincreases towards the UMZ.

The hardness test is done using a diamond tip inden-ter to create indentations on the weld cross section. Theindenter has two diagonals, which are measured andused to determine the hardness value on the HV3 scale.The indenter carries a load that is enough to create anindentation on the alloy. The hardness of the materialdetermines the material resistance against the indenter.This implies that the softer the material (aluminiumalloy), the deeper the indents and the longer the diago-nals of the indents. The depression caused by an inden-ter can be seen in Figure 6.

In Figure 5, the average hardness values are denotedby the nodes on the line graph. The hardness from theBM to the weld centre line should have minimal fluc-tuation. The WI denotes the point at which the weldfusion line appears. It can be seen that the greatesthardness fluctuation is around the heat-affected zone(HAZ) and the WI, which are usually the areas moreprone to structural failure. EC 3 (Figure 5) has the besthardness profile of the four ECs while EC 2 has theworst hardness profile, especially across the WI.

Macrograph analysis

The macrograph samples of each EC are presented inFigure 6 using 103 objective lens to present the inter-action between the weld pool and the BM across theWI. These pictures are used to present weld defects such

as porosity and cracks, if there are any. In addition,they also show the HAZ and the location of the WIfrom the weld centre line. The macrograph samples alsopresent the bead profile. It is important to mention thatthe indentations in Figure 6 are made by the hardnesstesting machine and the position of the indents fromthe plate surface is approximately the same for all theexperiments. In the four ECs, it appears that there areno cracks or porosity on the macro scale which suggestthat the welds are acceptable.

EDS

The EDS result is presented in Table 4 showing the testparameters. The measurement acceleration voltages 3,10, and 20 kV represent the calculated depths of 0.2,1.2, and 3.3mm. At 0.2mm depth, the presence of oxy-gen is highest in all the samples and lowest at 3.3mm.The oxygen content in addition to the other elements inAW 7020 and 7025-T6 are relatively close. This may bedue to the same alloy series they belong to in the classi-fication. The classification is based on the chemicalcomposition of the alloy.

Discussion

The effect of Al2O3 on the AW 7020 weld is based onthe hardness profile of EC 1 and EC 4, which are simi-lar (Figure 5). However, the hardness values of EC 1are higher than EC 4. The presence of Al2O3 layer inthe weld process (EC 1) increased the YS by 20% andthe UTS by 6% but reduced the elongation by 29%(compared to EC 4). This result shows that when Al2O3

layer is not removed before welding, improved hardnessof AW 7020 weld was attained (it is important to notethat there are no weld defects such as porosity due tooxide inclusion in the weld pool). The question there-fore arises whether the higher strength could result fromthe reduced heat that gets into the weld pool due to theheat resistivity of the Al2O3 layer in addition to the

Figure 5. Hardness profile of welded AW 7020.

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suspected absence of chemical interaction of Al2O3

layer during the weld (due to the welding technologyand weld parameters). This can be clarified by furthermultiple experiments. However, it is important to men-tion that if there is a chemical reaction in which Al2O3

layer is present in the weld (causing porosity), themechanical properties will be lower.

The effect of pre-weld heat treatment on the AW7020 weld is detrimental to the weld comparing EC 2to the other three in Figure 5. The hardness profileacross the weld in EC 2 is more uneven with sharp fluc-tuations in hardness values. For example, there is asharp decrease in the hardness value from 81.7 to51.1HV across the WI. This is usually a crack failurepoint in the weld piece. In EC 2, the WI is closer to theUMZ (narrower HAZ) which is better when narrowerweld seam is desired; however, it reduced the hardnessvalues. Preheating reduced the YS by 17% and UTS by3% but increased the elongation by 17% (Figure 4).

The effect of artificial ageing on the AW 7020 weldis that artificial ageing relatively smoothens the hard-ness profile, in addition to increasing the hardness val-ues in the HAZ, WI, and UMZ. Comparing EC 1 andEC 3, the hardness value at the WI increased from 63.6to79.3HV (Figure 5). Artificial ageing reduced the YSby 8% but increased the UTS and elongation by 9%and 110%, respectively (Figure 4). It therefore suggeststhat artificial ageing improves the mechanical proper-ties of welded AW 7020 provided there are no weld

defects. Based on the macrographs, the welds in thisstudy appear to exhibit no defects (Figure 6)

The necessity of pre-weld Al2O3 removal is examinedin this study. Acceptable welds were achieved withoutpre-weld removal of the Al2O3 layer (Figure 6, EC 1-3)using a pulsed MIG welding process. This may be dueto the low chemical interaction of Al2O3 with the weldpool as the EDS result shows that the oxygen contentof Al2O3 in AW 7020 is about 50% lower than in purealuminium. It therefore suggests that with new weldingtechnologies such as pulsed MIG and FSW, it is notnecessary to remove naturally formed Al2O3 layerbefore welding high-strength aluminium (HSA) alloys.

Good welds may have been attained due to the loweramount of oxygen present on the surface of AW 7020compared to pure aluminium. This suggests why HSAalloys have lower corrosion resistance in comparison topure aluminium. It is important to mention that basedon the literature review, the effect of Al2O3 in alumi-nium welding seemed to have been active in the 1940suntil 1960. There appears to be a break in the interestfor this research, as it seems to have then picked upagain from 1990 until date, which suggest that therewas a lost in interest during the 1970s and 1980s.

Conclusion

This study was carried out to investigate the effect ofthe Al2O3 film on the mechanical properties of HSA

Figure 6. Macrographs of welded AW 7020.

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alloys using pulsed MIG-welded AW 7020 as a casestudy. The structural formation of the Al2O3 layer wasbriefly explained; the structure varies depending on theclass of aluminium alloy. The characteristics and prop-erties of the Al2O3 layer were discussed, and the studypresented how the Al2O3 structure can be modified forstructural advantage. A brief description of new weld-ing technologies used for aluminium alloys was pre-sented. In addition, weld defects in aluminium weldsassociated with Al2O3 were also presented. Based onthe literature review and experimental study, the fol-lowing conclusions can be made:

� The experiment indicates that pre-weld heat treat-ment of AW 7020 alloy is detrimental to themechanical properties of the weld because itreduces the mechanical properties of the weld.

� Artificial ageing of AW 7020 welds improves themechanical properties, including the hardness, ten-sile strength, and ultimate YS. Therefore, it is sug-gested that post-weld heat treatment isadvantageous in HSA alloys.

� Acceptable welds are attainable without pre-weldcleaning of the Al2O3 film. It is therefore suggeststhat removal of the Al2O3 is not necessary whennew welding technologies such as the pulsed MIGprocess are used on HSA alloys.

� The presence of the Al2O3 film is not detrimental tothe mechanical properties of HSA alloy welds ifthere is no chemical interaction. It suggests thatnew weld technology that prevents Al2O3 chemicalinteraction during weld that can cause weld poros-ity and other weld defects are advantageous.

� The chemical composition of Al2O3 varies acrossthe different classes of wrought aluminium alloys.This suggests that the structure is not the same inall the aluminium alloys but similar in each alumi-nium alloy classes. In addition, it suggests that thestructure is also dependant on the chemical compo-sition of the parent metal.

Declaration of conflicting interests

The author(s) declared no potential conflicts of interestwith respect to the research, authorship, and/or publi-cation of this article.

Funding

The author(s) received no financial support for theresearch, authorship, and/or publication of this article.

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Publication 4

Overview of laser systems and optics applicable to hybrid laser welding ofaluminium alloys.

Olabode, M., Kah, P., and Salminen, A. (2015). Rev. Adv. Mater. Sci, 42, 6-9.

©2015 Advanced Study Center Co.Ltd.

6 M. Olabode, P. Kah and A. Salminen

© 2015 Advanced Study Center Co. Ltd.

Rev.Adv. Mater. Sci. 42 (2015) 6-19

Corresponding author: Muyiwa Olabode, e-mail: muyiwa.olabode@lut.fi

OVERVIEW OF LASER SYSTEMS AND OPTICS

APPLICABLE TO HYBRID LASER WELDING OF

ALUMINIUM ALLOYS

Muyiwa Olabode, Paul Kah and Antti Salminen

Lappeenranta University of Technology, Lappeenranta, Finland

Received: January 27, 2015

Abstract. The need for green and sustainable energy is continually on the rise. The use of light

weight yet load bearing materials like aluminium has become important as structural materials.

Aluminium can be fabricated by welding which is challenging compared to steel due to the

presence of aluminium oxide coating and high conductivity of aluminium. The objective of this

paper is to present an overview of the optics and laser systems applicable to hybrid laser welding

of aluminium. This article is a critical review on aluminium alloys and their weld defects including

hot cracking, porosity and heat affected zone (HAZ) degradation. Furthermore, the effect of the

properties of aluminium in fusion welding, hybrid laser welding optics and the challenges alu-

minium presents to hybrid laser welding are also studied. It is observed that aluminium limited

the selection of hybrid laser welding system and optics. The configuration of the welding head is

critical to the effectiveness and efficiency of the welding system. The required weld properties

influence possible optimization of hybrid laser welding. This article can be used by welders and

welding engineers for hybrid laser welding of aluminium in addition to understanding how viable

is hybrid laser welding of aluminium.

1. INTRODUCTION

The need for lightweight metal for construction and

fabrication is on the increase due to the advantages

of sustainable energy and economy [1].Aluminium

is the second most used structural material after

steel [2,3]. The increased rate is due to advanta-

geous properties of aluminium such as its light-

weight to strength ratio, relative corrosion resistance

[2], ease of machinability. Theyare used in the trans-

portation industry, due to its relative low density in

comparison to steel, the lower dead-weight of con-

struction and low energy consumption withminimal

compromise to load carrying capacity [4]. About

50% of aluminium extrusions are used in the trans-

portation industry [5]. Other sectors include con-

struction and power transmission [6].

Aluminium and its alloys have their disadvan-

tages like high reflectivity and high conductivity that

makes welding challenging [7,8]. There are differ-

ent welding systems applicable to aluminium weld-

ing like laser beanwelding (LBW), friction stir weld-

ing (FSW), metal inert gas (MIG), tungsten inert

gas (TIG), hybrid laser beam welding (HLBW),

plasma arc welding (PAW), submerged arc welding

(SAW), and others. TIGweld process had been the

most industrially accepted welding process for alu-

minium [9]. Studies have shown that FSW, pulsed

MIGandHLBW produce better welds than TIG [10].

This paper focuses on hybrid laser welding optics

applicable to aluminium. It further presents the chal-

lenges of aluminium alloy welding in HLBW.

2. ALUMINIUMALLOYS

Aluminium and its alloys are grouped into cast alu-

minium and wrought aluminium alloys [11,12]. The

wrought alloys are usually used in fabrication be-

7Overview of laser systems and optics applicable to hybrid laser welding of aluminium alloys

cause of its high strength compared to cast alloys

[13]. This paper focuses on wrought alloys. The

wrought aluminium alloys are grouped into series

based on the chemical composition. They are de-

noted by 4 digits where the first denotes the char-

acteristic alloying element. They range from 1xxx

to 9xxx series. For example, 99% pure aluminium

belongs to 1xxx serieswhile high strengthaluminium

(HSA) alloy like 7025 belongs to the 7xxx series.

Aluminium alloys weigh about 1/3 of copper and

iron at equal volume. It is slightly heavier thanmag-

nesium and slightly lighter than titanium and it is a

relatively weak metal.Alloying of aluminium can be

done to attain high strength.Aluminium is resistant

to corrosion due to the formation of its thin oxide

layer on exposure tomoisture.Aluminium conducts

electricity, heat and reflects light and it is easy to

fabricate.

HSA alloys like the 2xxx, 7xxx, and 8xxx are

becoming of high industrial interest because their

yield strength is comparable tomild steel. However,

the higher the yield strength the more difficult it is

Fig. 1. Woodward diagram showing general relationships between some properties of aluminium alloys.

to weld (due to the chemical properties). Further

relationship between the properties of aluminium

alloys is presented in Fig. 1.

3. COMMONALUMINIUMWELDING

DEFECTS

Welding of aluminium is rather critical despite the

fact that it has lowermelting point compared to steel.

Criticality of welding aluminium is due to the:

1. Presence of a stable surface oxide formed on

exposure to oxygen

2. Presence of residual stresses that causes weld

cracks due to aluminium’s high thermal expansion

coefficient.

3. High heat conductivity of aluminium that implies

that high heat input is required for achieving sound

welds. High heat input on the other hand, increases

the possibility of distortion and cracking.

4. High shrinkage rates on solidification, that en-

hance cracking.

5. High solubility of hydrogen in molten aluminium

which causes porosity.

8 M. Olabode, P. Kah and A. Salminen

Fig. 2.Relative crack sensitivity ratings of selected aluminium (base alloy/filler alloy), redesigned from [15].

6. General susceptibilityof aluminium toweld crack-

ing [14,15] as presented in Fig. 2.

3.1. Hot cracking

The crack in aluminium welding occurs during weld

metal solidification. It mechanically involves the

splitting apart of liquid film because of the stresses

and the strain that spring up due to solidification

shrinkage and thermal contractions. The liquid film

is related to the low melting eutectics. In situations

where the difference between an alloy’s liquid film

and the lowest meeting eutectic is large, the large

solidification range makes the liquid film shrink

more. In addition, it is more demanding to feed

shrinkage over large distances. When the base of

the dendrites solidifies fully and the shrinkage is

culminated, feeding inter dendrites liquid to the

shrinkage is then critical [14].

The loss of properties due to hot cracking in an

aluminium welded joint is due to the failure in the

liquated region of the HAZ. The cracking suscepti-

bility is based on the alloying elements. When the

parent alloyadjacent to the fusion zoneexperiences

high heating rates the phenomenon of non-equilib-

rium melting arises. Micro-cracks can also arise in

the liquated regions in the presence of hydrogen

and/or sufficient strain. In additions, a change in

composition of the weld regions, toughness can be

seriously impaired following ageing. Precautions can

be taken to control liquation and liquation cracking

by controlling the grain size, the residual impuri-

ties, the degree of homogenisation, and the alloy

content.

3.2. Porosity

In high temperatures, during arc welding processes,

aluminium approaches its boiling point on the weld

pool surface. In this situation, aluminium undergoes

two order magnitude changes of hydrogen solubil-

ity. The change occurs when it cools from initial

high temperature to the onset of solidification; the

activeness of hydrogen to aluminium is due to the

temperature in themelted weld pool. Dissolution of

hydrogen in aluminium is based on the high tem-

perature equilibrium and the fast mixing of the pool

(due to the electromagnetic forces). The weld pool

therefore has high gas content relative to the sur-

face temperature [14]. This effect is vivid in alu-

minium because the arc weld region is under super

high heat and the pores canbe supersaturated such

that gas pore formation is possible without the aid

of solidification.When the weld starts to cool, there

is not enough time for the entrapped gas tomove to

the liquid’s surface, and escape from the weld pool.

The entrapped gases are the pores in aluminium

welds [14-16]. The source of porosity is usually due

to the entrapment of various gases in the weld, the

type of filler wire used, and the weld pools cooling

rate. There are numerous possibilities of gas enter-

ing the weld pool (shielding gas, air product of tur-

bulent arc action and even dissolved hydrogen).

Hydrogen or water is the source of porosity. Hydro-

gen is the typical source of porosity in aluminium

9Overview of laser systems and optics applicable to hybrid laser welding of aluminium alloys

welding; other sources include oxygen, and other

gases in the surrounding air [14].

Porosity affects the mechanical properties of

aluminium welds. The degrading effect on the ten-

sile strength and ductility depends on the size and

distribution of the pores. Elongation drops immedi-

ately as porosity level increases, tensile ductility

drops by as much as 50% from its highest level

when the porosity is about 4% of the volume. At

same porosity level, tensile strength is observed to

be very tolerant and yield strength is slightly reduced

[14]. In 7xxx series, zinc vapour is formed at the

faying surface during welding which generates gas

inclusion (porosity). aluminium hasmelting tempera-

ture of 560 °Cand high boiling temperature of 2050

°C and (compared to 420 °C and 907 °C for zinc);

thus cleaning zinc in the weld region mechanically

or using arc heat to volatize zinc ahead of the pool

helps to reduce the possibility of porosity.

3.3. Heat affected zone degradation

The HAZ is created beside the fusion zone and it

results in the degradation of the parent materials

Table 1.Welding defects and remedies, modified from [17].

Problem Causes Solutions

Porosity Turbulence of weld pool Increase welding current to stabilise transfer of

metal droplets.

Hydrogen from hydrated Keep wire covered. Store wire in a low humidity

oxide film or oil on wire, base chamber at a constant temperature. Clean basemetal

metal, drive rolls and liner. of oil and oxide immediately prior to welding.

Wet or contaminated Reject bottles above -57 °C dew point. Increase flow

shielding gas or inadequate rate. Shield from air currents.Use higher welding

flow. Fast cooling rate of current and/or a slower speed. Preheat base metal.

weld pool

Weld Cracking Improper choice of aluminium Select welding wires with lower melting and

welding wire or rod. solidification temperatures.

Critical weld pool Avoid weld pool chemistry of 0.5 to 2.0% silicon and

chemistry range 1.0 to 3.0% magnesium. Avoid Mg-Si eutectic

problems (5xxx welded with 4xxx).

Inadequate edge preparation Reduce basemetal dilution of weld through increased

or spacing bevel angle and spacing.

Incorrect weld technique Clamp to minimise stress. Narrow heat zone by

increased traverse speed. Produce convex rather than

concave bead.Minimise super-heatedmoltenmetal,

to control grain size. Proper weld size (not too small).

Preheat base metal.

HAZ degradation Excessive exposure of Control the heat input and keep it minimal by

workpiece to heat input controlling the current. Heat sinks can also be used

to hasten the heat dissipation after welding.

Optimizations that yield narrow weld seams should

be used.

caused by modification of the microstructure by

devoted temperature. The nature of the HAZ is de-

pendent on the diffusion in the region and the heat

input. Due to the thermal dependency of themetal-

lurgical transmogrification, the degradation depends

on the type of welding process and parameters.

Preheating parent metal beforeweld and using high

heat input increases the HAZ region and the degra-

dation level. The HAZ degradation may be limited

by using multi pass welding, avoiding preheating,

and by controlling the inter pass temperature [14].

A summary of welding defects and remedies

applicable to aluminium are presented in Table 1.

4. EFFECTS OF THE BASIC

PROPERTIES OF ALUMINIUM IN

FUSION WELDING

An understanding of the peculiarities associated

with aluminium fusion welding is important as the

physical and chemical properties influence theweld

[18,19]. Some of the properties considered include

the high heat conductivity of aluminium, which is

approximately three times the heat conductivity of

10 M. Olabode, P. Kah and A. Salminen

steel [2]. This implies that high energy densityweld-

ing systems like MIG, plasma and laser welding

systems are applicable. With high energy density,

there is a lower loss of strength in the HAZ and less

distortion. Another property is the extent of expan-

sion which is about twice for low alloyed steel [20].

On exposure to oxygen sources like air and water,

the surface that becomes coated with a thin layer

of naturally formed, chemically stable and thermally

stable nonconductive aluminium oxide (Al2O3)[21],

melts at about 2050 °Cwhile aluminium alloysmelts

at about 560 °C [11]. This oxide layer has amelting

temperaturemuch higher than that of aluminium it-

self; moreover, it has a significant mechanical

strength. Therefore, this oxide layer can remain as

a solid film (or fractured in small particles due to the

flow of the molten material [22]). This can result in

severe incomplete fusiondefects. It is recommended

that the layer is removed bypickling or drymachin-

ing just before weld. It is important to state that the

difference in melting point is not a problem during

the processing by means of high energy density

welding processes; for example, the presence of

oxides during laser processing increases the ab-

sorptivity of aluminium alloys to the laser radiation

[22,23]. It should be noted, that the main challenge

in applying most joining technologies to aluminium

is its tendency to form a thick, coherent oxide layer.

Another important property is solubility of hy-

drogen in aluminium. Hydrogen has high solubility

in molten aluminium as opposed to the solid alu-

minium. The solubility is reduced to one twentieth

of the solidification range in fusion welding pro-

Fig. 3.Absorption of laser wavelength bymetals, redesigned from [54].

cesses. The hydrogen gas is usually segregated

as regular spherical pores of typical diameter of 5

to 10 m [2]. They can act as crack initiation in theweld and lowering the dynamics and static strength

of the weld [24]. The sources of hydrogen in alu-

minium fusion welds include humidity and organic

contamination on the filler material and basemetal

surfaces, hydrogen content of the basematerial and

filler material, incomplete gas feeding of the weld. It

is important to suppress the level of porosity in the

weld so that the mechanical properties of the weld

do not deteriorate drastically. Finally, the high

reflectivity of aluminium to wavelengths limits the

laser beam welding science that can be used [25].

5. HYBRID LASER WELDING OF

ALUMINIUMALLOYS

Aluminium alloys can be welded by most welding

processes [2,26]. However, for fully automated sys-

tems, the common ones are MIG, TIG, LBW, and

HLBW.PlasmaMIG and other electron beamweld-

ing processes are however applicable with limita-

tions and therefore restricted to welding of special

products [26]. Newer technological developments

on the MIG process like cold arc [27] or cold metal

transfer welding (CMT) [28] are also applicable and

are growing in the industry. The most commonly

used hybrid welding system is laser hybrid MIG [9]

The usability of hybrid laser welding systems

havebeen presented byBagger andOlsen [29] then

byRasmussen and Dubourg [30]. Moreover, in the

1980s and early 1990s CO2lasers were the only

11Overview of laser systems and optics applicable to hybrid laser welding of aluminium alloys

ones with sufficient power for aluminium welding.

Therefore, CO2lasers were the most investigated

[25,31,32]. Todaymanymore investigations with the

solid-state lasers, about 80% of the laser hybrid

welding processes investigated are carried out on

solid state lasers like Nd:YAG lasers [33-35], high

power-fibre laser [36-40].As stated byUeyoma [41]

and researchedbyWanget al. [42], defocused high-

power diode laser beam can be used. There is lim-

ited research on the use of disk lasers as laser

sources in hybrid welding of aluminium alloys [9].

More than 80% of recorded research has been

carried out using MIG power source especially

pulsed MIG [43-45]. TIG power sources have also

been used but usually for basic investigations on

interactions and parameter effects [25,46-50] In

Ueyama research,ACMIG arc was applied in com-

binationwith high-power diode laser [41,51]. In some

other experiment plasma arc has been used as a

laser source [32,47,52].

The absorption of beam by aluminium depends

on the wavelength of the laser beam.As presented

in Fig. 3, due to the wave length of solid-state la-

sers Nd: YAG and fibre lasers are common laser

power sources used in hybrid MIG welding. Com-

pared to CO2lasers, Nd:YAG and fibre lasers have

approximately double the wavelengths of CO2laser

[25]. This advantageously minimises the keyhole

welding intensity needed. The delivery of fibre and

Nd:YAG lasers can be done using fibre optics which

increases the process flexibility [53] and the pos-

sibility of having a robust welding system. In addi-

tion, in Nd: YAG and fibre laser, shielding of the

laser beam by the arc plasma and laser induced

metal vapour is not expected as compared to CO2

systems.

Based on the amount of the research available,

it can be stated that Nd: YAG laser with MIG is the

most usable state of the art hybrid welding process

for aluminium alloys. In addition, the Nd: YAG la-

sers can be replacedwith solid-state lasers like the

fibre laser.

5.1. Hybrid laser welding optics

Optics found in hybrid laser welding systems appli-

cable to aluminiumwelding includesmirrors, lenses

and fibre optics. In hybrid laser welding, laser beam

needs to be focused to achieve small spot diam-

eter. The small spot diameter allows for higher beam

density on the workpiece. The spot diameter is a

function of the lens design and focal length. Beam

transfer and focusing is achievable using diffractive

optics, refractive optics or reflective optics.

5.1.1. Beam delivery optics before

focusing

Most laser welding system consists of components

likemirrors (for diffracting light). Mirrors can be pla-

nar or spherical in design. The mirror is fixed to a

firmly adjustable screwwith the ease of accessibil-

ity for cleaning, inspection, and replacement.

The usability of conventional mirror delivery is

limited by the rigidity of the mechanical mounting

and they cannot move relatively to each other to

avoid misalignment. The mirror is limited in size

therefore transferring beams over a long distance

with high divergence can produce a beam diameter

that is larger than the lens. However, the beam can

be tailored with lenses in the beam path to prevent

this phenomenon (this is called a relay system).

Mirror as reflective optics are usually found in gas

laser systems. Themirrors are generallymade from

bear metal or polishedmetal (usuallymolybdenum

or copper) to improve the reflectivity.Amaterial like

gold can also be used for coating the surface of

metalmirrors to produce high reflectivity.Metalmade

mirrors are less prone to damage compared to

lenses because they can be easily cooled by pass-

ingwater through the innerwalls of themirror thereby

resulting in higher repeatability than transmissive

lens. Usually, high-powered lasers use all reflective

water-cooled optical components ruggedly built to

survive in industrial environment and to requiremini-

mal maintenance. Themirrors can be as simple as

having one to having tenmirrors.

Retaining rings and springs are used to keep

the mirror in place thereby sustaining consistent

pressure and limitingmovement. Themirrormount-

ing plate must be flat to avoid pressure that can

force themirror towarp causing beam distortion and

difficulty in focusing the beam. Dielectric coatings

are usedonmirrors to eliminate phase shifting. This

coating can be easily damaged during cleaning so

mirrors should be cleaned using acetone and lens

tissue. Cleaning is important to prevent build up and

contamination that can result in heat absorption that

will distort and destroy the mirror. In some special

cases, the mirror’s dielectric coatings are multi-lay-

ered to rotate the polarization of the laser beam by

90°. This is common for circular polarization needed

for bidirectional welding and cutting so that beams

can create consistent kerf width in all travel direc-

tions. Thesemirrors are referred to as quarter-wave

phase retarders.

In some cases, the mirrors are coated so that it

canabsorb one component of linear polarizationand

reflect the orthogonal component. These are called

12 M. Olabode, P. Kah and A. Salminen

anti back reflection mirror and are used for beam

delivery along with phase retarders to absorb re-

flected energy that can otherwise travel back to the

laser resonator and damage it [54].

Mirror can also be adaptive designed spherical

or flat but can change surface curvature based on

the input signal. Adaptive mirror is necessary in la-

ser material processing for controlling raw beam

propagation through the guide and beam delivery

system. The principle of adaptive mirror operation

is that it compensates the axial shift of the focal

position that had been caused by the thermal load

on the optical components. Therefore, the focal po-

sition is kept constant or changed to a desired po-

sition. In aluminium laser welds of components

where ‘’flying optics” is used, the distance between

the laser source and thewelding head changes (de-

pending on the shape of work weld piece), adaptive

optics is therefore adequate [55].

Lens is another component for beam delivery

usually for converging or diverging light. The lens

can be a simple one-element optic generally with a

focal length less than 254mm. They can be as-

pheric, Plano-concave/convex or meniscus [56,57].

Lenses can be compound, where the lens is made

of two or more separate lenses that fit together to

reduce spherical aberration common in a simple

single lens. Aspheric simple one component lens

is made to reduce spherical aberration. This is

achieved by turning the lens with a diamond tool on

the lathe to a certain calculated aspheric curve. It is

important to note that glass is generally the mate-

rial used for lenses in the visible spectrum. How-

ever, glass in the infrared (IR) region does not trans-

mit. The lenses made to transmit in the IR region

are called IR lenses. IR lenses can be made from

germanium(Ge), silicon (Si), zinc selenite (ZnSe),

zinc sulphide (ZnS), and gallium arsenide (GaAs).

Other materials like diamond and calcium sulphide

(CdS) and sapphire are less common [58].

Fibre optics is another component for beam

delivery used in Nd: YAG to deliver beams due to

the 1.06 m wavelength transferable over glass fi-bres. Fibre optics utilizes the flexibility of glass fi-

bre within the specified bend radii for fibre bundle

(100-200mm). Theyare attractive in comparison to

conventional beam delivery especially due to the

possibilityof transporting beams over long distances

of up to 50 m and around curves [59]. In addition,

the optics is compact and easier to move around

particularly useful in robotic welding. A highly con-

sistent focal spot size is achievable with fibre op-

tics compared to mirror. Time sharing and energy

sharing with fibre optics is less complex and easily

achievable than with mirrors. They degrade beam

quality with larger focal spot sizes compared to

mirror delivery. The usability of fibre beam delivery

has therefore been limited tomost welding systems

where the focal size needed is larger than 100 m[56]. It is important to state that fibre optics are not

effective for transmitting ultraviolent (UV)wavelength

and can be destroyed by CO2lasers. Fibre optics

is common in diode lasers and Nd: YAG. Plastic

material are also used in place of glass for fibre

optics however, theyare used for visiblewavelength

lasers. Plastics are not effective in Nd: YAG due to

losses in transmission and lower damage thresh-

olds.

Beam delivery optics before focusing include

bendingmirrors (e.g.CO2), beam splitter (CO

2, Nd:

YAG), optical fibre (Nd: YAG, Diode lasers, Disk

and fibre laser), circular polariser and collimator.

Laser beams are delivered to theworkpiece by turn-

ing mirror system in CO2lasers and Nd: YAG la-

sers. For accurate repeatability of laser welds, it is

important that the laser optics is firm and rigid, as

misalignment and vibration are not desired. How-

ever if the laser optics is rigid then the workpiece

will need to be moved around during welding. This

becomes impracticable when welding large work

pieces. For such fixed beam systems, the floor

space for the machine must at a minimal be four

times larger than the largest work piece for which it

is designed for. On the other hand, moving optics

will save floor space but high care must be put into

controllig beam divergence, rigidity, and alignment.

Nd: YAG laser heads are small thereby allowing it

to be mounted on moving axis with limited deterio-

ration to it focus, and therefore more flexible than

CO2laser heads that are large and usually installed

to operate stationary.

Laser applications that are categorised as 1 kW

or less use transmitting optics for beam focusing in

welding. Thebeam transfer can beachieved bycon-

ventionalmirror, fibre optics, or acombinationof both.

Up-collimator or beam extenders are used to re-

duce beam divergence by increasing the beam di-

ameter. Laser beamdivergencealongwith the choice

of focus lens determine the focal spot size, research

study [9] has shown that, beam divergence can be

improved by a factor of two with half times smaller

focal size (using proper focus system) compared to

a system without collimator by doubling the beam

diameter. The usability of collimator is usually lim-

ited to low power CO2lasers and most Nd: YAG

lasers to extend beam diameter from 6 - 10 mm to

13Overview of laser systems and optics applicable to hybrid laser welding of aluminium alloys

12 - 25 mm. CO2lasers of above 500W usually do

not need collimator because of the raw beam diam-

eter and its low beam divergence.

5.1.2. Focusing optics

Focusing optics is common in low-power welding

devices. Parabolic lenses are generally useful for

focusing power above 1.5 kW of CO2lasers. Due to

the low cost and minimal spherical aberration at-

tributes of f-numbers above five, lenses are usually

Plano-convex lenses. The f-number is derived by

dividing the lens focal length with the beam diam-

eter.When the f-number is less than four, complex

optical lenses compared toPlano-convex lenses are

used. The thumb rule is that the higher the f-num-

ber the higher the problem of spherical aberration

[60].Aguide to selecting the best lens is presented

in Table 2.

Laser protection is used in laser processes

where the focal length is short or when the weld

metal is volatile and contaminated; or when weld

spatters can be generated. Debris can attach itself

to the welding head lens.Aluminium highly reflects

laser beam wavelength, and the reflected beam can

damage laser optics. The solution adopted gener-

ally to solve this is to change to a laser with differ-

ent wave length, paint or etch the surface of the

workpiece to reduce reflectivity, or to use keyhole

welding where the energy density of the spot diam-

eter is great enough to overcome reflectivity; in ad-

dition to using a cheaper protective lens.

f-number CO2

Nd:YAG

range

4+ plano-convex plano-convex

3 to 4 meniscus plano-convex

2 to 3 diffractive-convex doublet

< 2 triplet triplet

Table 2. Lens shape choices for Nd:YAG and CO2

lasers at various f-numbers, modified from [60].

The presence of foreign particle on the lens can

reduce transmission; create localized absorption on

the surface of the lens thereby destroying the lens

surface or any coating on it. Lenses can be very

expensive and in such cases where an expensive

lens life can be drastically reduced, a sacrificial

cheap optic is placed in front of it as a window or a

cover slide to protect the expensive lens. For ex-

ample, Nd: YAG and Nd: glass lasers use the pro-

tective optics due to the low cost of the cover slide.

It is less common in CO2lasers.

5.1.3. Hybrid laser focusing head

The performance of beam delivery system deter-

mines the quality of the laser beam processing. It

is desired to be simple and as small as possible

havingneither actuators nor sensors toallow for easy

manipulation and integration on to a robotic welding

system. However, the available technology for laser

welding head is attractive that consumers still tend

to buy the technology thus the laser heads are be-

coming more and more complex. Common tech-

nologies in laser focusing heads include the pres-

ence of integrated actuators and sensors, closed

loop systems, self-learning and self-adapting sys-

tems.

The combination of laser beam and arc can be

of varying configurations which remarkably influence

theweld performance. It is important tomention that

in hybrid laser welding, the primary heat source is

laser while the secondary can be any arc process.

Laser assisted arc welding is the vice versa [9].

The welding head is based on the heat source

type and relative position of the heat source to one

another [61]. The principal classification criteria are

presented in Fig. 4 (based on the heat source type)

and Fig. 5 (based on the configuration). The choice

of the secondary heat source can be either arcs

with consumable electrodes or arc with non-con-

sumable electrodes. The earlier is selected due to

the necessity of filler metal to solve specific weld

problems otherwise, the latter is preferred.

Fig. 4. Schematic presentation of heat sources available for hybrid laser–arc combinations.

14 M. Olabode, P. Kah and A. Salminen

Fig. 5. Geometrical arrangements for hybrid laser–arc welding.

Fig. 6. Schematic diagrams of hybrid laser–arc welding with a common operation point, redesigned

from [9].

Fig. 7. Schematic diagram of hybrid laser–arc welding with separate operation points, redesigned

from [9].

The arrangement plays important role for the ef-

fectiveness and efficiency of the weld system as

well as the welds. The heat source can be arranged

to have a common (Fig. 6) or separated (Fig. 7)

operation point as illustrated. In common operation

point, the arc root and laser beam spot centre are

in the same surface location of the workpiece.Many

hybrid laser-arc configurations use arc welding torch

inclined to the laser beam along the weld direction

(Fig. 6a) or across (Fig. 6b). The position of the arc

torch affects the focal point position.

(a) (b)

15Overview of laser systems and optics applicable to hybrid laser welding of aluminium alloys

Beyer et al. (1994) reported according the con-

figuration in Fig. 6a that laser power was respon-

sible for attainable weld penetration depth and the

arc parameters were responsible for the adjustment

of the weld seam width [62]. The same research

group used the setup in Fig. 6b for tailored blank of

two different plate thicknesses. The result showed

that the configuration (1) reduced the need for edge

preparation (2) increased molten material volume

and (3) generated a smooth zone transition between

the plates because the arc burns the thicker plate’s

edge therefore improving weld appearance.

A separated operation point arrangement can be

of serial technique or parallel technique or a combi-

nation of both. The serial technique is a configura-

tion inwhich the primaryand secondary heat source

has an acting point distance known as “working dis-

tance” between them in vertical or horizontal direc-

tion along path. The arc source can lead or trail the

laser beam. Leading arc source allows for preheat-

ing thereby increasing laser heat source efficiency

due to a reduction in heat loses by conduction. It

also increases the weld seam quality because of

more stabilized keyhole. Leading arc generates

deeper welds because at close working distances,

the laser beam strikes the deepest part of the weld

pool surface suppressed by the arc forces. To at-

tain deepest weld penetration, the focal point must

be set to hit at the lowest weld pool surface.

Trailing arc source with short working distance

provides stability and efficiency due to the common

phase plasma interaction between the heat sources

and also due to the thermal impact on the weld.

With greater working distances, trailing arc source

can act as heat treatment for the weld which is

favourable inHSAwelds for the improvement of joint

properties.Asummary of the principal advantages

of a leading and training arc is presented in Table 3.

Aparallel technique is aconfigurationwhere there

is a displacement between the laser focal point and

the arc acting point. The thermal load spreads in

Laser leading configuration Arc leading configuration

Useful in aluminium welds because it helps remove oxide layer Generates deeper weld penetration[9]

before arc welding [63]

Creates superior beam appearance because the laser as it gas Allows for weld preheating

does not affect the molten pool created by arc heat source[64]

Improves the homogeneity of the weldmetal[64] Requires less heat input per volume

of weld metal (J/mm3)[64].

Better stability in terms of current and voltage measurement [65]

Table 3.Configuration advantages of conventional hybrid laser welding.

different region as opposed to serial configuration

where the thermal load spread to the same region.

It is important to state that in many cases it is diffi-

cult to distinguish between a parallel and a serial

technique as they are usually applied simulta-

neously. In Seyffahrt et al. (1994) a separate opera-

tion point configuration was carried out with the pur-

pose of increasing weldablemetal sheet thickness.

Laser heat source welded the root and the top layer

was welded by the arc source Fig. 7 [66].

Other hybrid laser welding configuration with

more than twoheat sourceshavebeenexperimented

and presented in Fig. 8.Winderlich (2003) used the

configuration where a CO2laser beam and TIG arc

touch acted on the same side while the second TIG

torch acted on the opposite side of the weld. The

configuration provided attainable notch-free weld

seam useful in dynamic loading while improving fa-

tigue resistance [67].

Another configuration referred to as Hydra (hy-

brid welding with double rapid arc) is presented in

Fig. 8b. It was initially experimented byDilthey and

Keller (2001) usingCO2laser andMIGheat sources.

The configuration increased the possible deposition

rate and thus increasing attainable welding speed

and reducing thermal load, in comparison to con-

ventional hybrid laser welding configuration [68].As

illustrated, the working distance of the leading arc

can allow for preheating while the trailing arc can

provideheat treatment.Wieschemann (2001) shows

that two leading arc configuration provides optimum

gap bridgability [69].

Another configuration discovered by Stauter

(2007) is presented in Fig. 8c where the second arc

torch is a tandemhaving two consumableelectrodes.

The electrodes depositions are controlled by two

separatepowersources. This configuration increases

deposition rate and productivity and the cooling rate

is easily optimized by varying the work distance

between the conventional hybrid configuration and

the tandem torch (working distance between torch

1 and torch 2).

16 M. Olabode, P. Kah and A. Salminen

5.2. Challenges for aluminiumwelding

Aluminium alloys presents challenges for hybrid

laser welding optics. One of the challenges limiting

the welding system and the optics is; the high

reflectivity of aluminium alloy that limits the choice

of laser beam source for example to Nd: YAG and

fibre laser.

Firstly, the melts zone (MZ), and HAZ are larger

in hybrid welding, than in laser welding. Themolten

zone at the weld top is wider due to the welding arc

process [70]. This compromises the metallurgical

properties of the weld. Secondly, due to the wider

weld pool and highmelt temperature in HLBW diffi-

culty arises in covering the weld pool, which can

lead to contamination of theweld, and porosity [30].

Thirdly, alloys with volatile elements like 5xxx se-

ries can evaporate from the normally generated key-

hole thus resulting in lower metallurgical properties

of the weld and even porosity if the gas bubbles are

trapped in the weld. This can be improved byproper

selection of filler material [53]. In addition, volatile

elements present in aluminium alloys can generate

spatters during welding that can adhere to the lens

and damage the lens. Aprecaution is to use a pro-

tective lens. Fourthly, aluminium alloys have low

surface tension; they have poor ability for root-side

melt pool support. This tends to cause difficulty in

full penetration welding specifically in thick butt

welds [45]. Finally, the presence of high number of

welding parameters that is non-independent of each

other in interaction compared toMIG or laser weld-

ing process, in addition to the metallurgical chal-

lenges in aluminium fusion welds. Therefore hybrid

laser beam welding of aluminium alloys are compli-

cated to design and operate [71]. Rasmussen et al.

(2005) shows that successful welding of aluminium

Fig. 8. Schematic diagrams of hybrid laser–arc processes with two secondary heat sources, redesigned

from [9].

using a hybrid laser beam welding demands that, a

clear understanding of the governing parameters,

the effects and their interactions are understood [30]

to be able to maximise the advantage of hybrid la-

ser beamwelding as a robust industrial welding pro-

cess [71].

6. CONCLUSIONS

Aluminium alloys have become an important struc-

turalmaterial and have foundapplications from gen-

eral kitchen utensils to aerospace vehicles. They

are grouped into cast and wrought aluminium al-

loys. Thewrought aluminium alloys are sub grouped

mainly into seven. Pure aluminium is weak, light

and corrosion resistant. It conducts electricity, heat,

reflects light and easy to fabricate. When alloyed, it

can attain strength comparable to mild steel. How-

ever, some of its properties are detrimental to its

fabrication. For example, the high strength alloys

have poor weldability. They are relatively prone to

weld defects due to for example, it’s self-forming

Al2O3oxide layer, hydrogen solubility in molten alu-

minium, high shrinkage rate on solidification.

The properties of aluminium have affects on fu-

sionwelding. For example, the high heat conductiv-

ity implies that highheat energydensities areneeded

to weld the alloy that in turn increases HAZ degra-

dation. The presence ofAl2O3although can be bro-

ken with high heat energy density welding process

during welding, it has a significant mechanical

strength that it can remain solid even when the sur-

rounding metal is molten which can result into in-

complete fusion. In addition, the high solubility of

hydrogen inmolten aluminiummakesHLBW of alu-

minium prone to porosityalthoughwith proper know

how, it can be minimized.

17Overview of laser systems and optics applicable to hybrid laser welding of aluminium alloys

HLBW systems are used for welding aluminium

and the most commonly used is hybrid MIG weld-

ing. The optics are used as resonator optics, beam

delivery optics and processing optics. The optics

includeoutputwindows, foldmirror, rearmirror, beam

splitters, optical fibre, circular polarizers, collima-

tor, scanning optics, and other special optics.

The usability of HLBW system is mainly limited

in aluminium alloys due to the limited absorption of

laser wavelength byaluminium. Therefore, the com-

monly used laser power sources are fibre laser and

Nd: YAG. The focusing optics used is selected with

reference to the f - number with the aim of avoiding

spherical aberration. A rule for selecting optics is

based on the fact that the higher the f - number, the

higher the problem of spherical aberration. Beam

delivery can be done using mirror optic or fibre op-

tics; but mirrors are limited due to the need of a

rigidmechanical mounting and the difficulty of trans-

ferring beams over long distances. On the other

hand, fibre optics is limited by the bend radius and

beam quality degradation. HLBW focusing heads

are desired to be simple so that it is easy to inte-

grate.However due to thenumerous advantages the

available technologies, theyhaveonlybecomemore

complex. Some of them have mechanical moving

parts to allow for more manipulation in attaining

closed loop, self-learning and self-adapting systems.

The choice heat source and their configuration plays

important role for the effectiveness and efficiencyof

HLBW.Thechallenges faced inHLBW of aluminium

alloys are HAZ degradation, possibility of contami-

nated weld pool due to the presence of a wider weld

pool compared to LBW, the presence of a volatile

element in the alloy like zinc causing porosity and

degradation of metallurgical properties. In addition,

the presence of low surface tension that makes full

penetration welding difficult in thick butt welds. Fi-

nally, there is the presence of a high number of in-

terdependent welding parameter, in addition to the

metallurgical challenges that are present in alu-

minium fusion welds.

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