a comparative study between conventional fixed and

IN DEGREE PROJECT MECHANICAL ENGINEERING,SECOND CYCLE, 30 CREDITS

, STOCKHOLM SWEDEN 2018

A comparative study between conventional fixed and advanced adaptive control system for resistance spot welding

CAROLINE BOHLIN

KTH ROYAL INSTITUTE OF TECHNOLOGYSCHOOL OF INDUSTRIAL ENGINEERING AND MANAGEMENT

Abstract Resistance spot welding is the main welding method used in the automotive industry to weld thin

sheet metal. Today adaptive control systems have been developed for RSW, which means it can

adjust the parameters in the weld process automatically during welding. The control systems can

register the parameters and properties of the weld in real-time and from that calculate with

algorithms how to adjust to give optimal weld conditions. This project is performed at Scania

CV AB, Oskarshamn. Conducted in the part of body in white, where an adaptive control system

called HCC is used in all weld processes.

In this project, HCC was compared to the fixed control system CCR and another adaptive control

system named Master mode. First step in the comparison was to create a weld schedule for each

control system and test them on two different material combinations. The aim was to quantify

gains and benefits that adaptive resistance spot welding systems have on the welding process.

Benefits are quantified by examining the parameters and factors such as: weld time, expulsion,

robustness, electrode wear and parameters in the control system. The tests were performed by

welding as many approved spot welds as possible without tip-dressing the electrode. The

experiment followed the requirements from international standards and the Scania standard for

resistance spot welding.

The results from the experiment showed that HCC was the most robust process and the spot

welds never decreased in size, which CCR and Master mode did. It is possible to weld several

different material combinations with HCC, it increases flexibility in production and reduces the

time needed to develop new weld schedules. The same schedule can handle many combinations

with the same thickness. HCC allows the process to use several pulses and each pulse adds in

time. Therefore, the weld schedule should be well developed and optimized to avoid waste in

terms of long weld times. The results will give Scania knowledge about the processes and how to

further optimize the welding processes in production. The result can also be used as foundation

for selection of products or future investments.

Keywords: RSW, Resistance spot welding, HCC, Heat Capacity Control, adaptivity, automotive

Sammanfattning Motståndspunktsvetsning är den huvudsakliga svetsmetoden som används inom fordonsindustrin

för att svetsa tunn plåt. Idag har adaptiva styrsystem utvecklats för RSW vilket innebär att de

automatiskt kan justera parametrarna i svetsprocessen under svetsning. Styrsystemen kan

registrera parametrarna och egenskaperna hos svetsen i realtid och därmed beräkna med

algoritmer hur de bör justeras för att ge optimala svetsförhållanden. Detta projekt är resultatet av

ett examensarbete på Scania CV AB, Oskarshamn. Det utfördes i den nya karossfabriken, där ett

adaptivt styrsystem som heter HCC används i alla svetsprocesser.

I projektet jämfördes HCC med ett konstantströms styrsystem CCR samt ett annat adaptivt

styrsystem kallat Master mode. Den primära metoden var att skapa ett svetsschema för varje

styrsystem och testa dem på två olika materialkombinationer. Syftet var att kvantifiera vinster

och fördelar som adaptiva punktsvetssystem har på svetsprocessen. Testerna utfördes genom att

svetsa så många godkända punkter som möjligt utan att formera elektroden. Fördelarna

kvantifieras genom att man undersökte parametrarna och faktorerna svetstid, sprut, robusthet,

elektrodslitage och parametrar i styrsystemen. Experimentet följde kraven i enighet med

internationella standarder och Scania-standarden för punktsvetsning.

Resultaten från experimentet visade att HCC var den mest robusta processen och punkterna

minskade aldrig i storlek, vilket CCR och Master mode gjorde. Det är möjligt att svetsa flera

olika materialkombinationer med HCC, det ökar flexibiliteten i produktionen och minskar den

tid som krävs för att utveckla nya svetsscheman eftersom samma schema kan hantera många

kombinationer med samma tjocklek. HCC tillåter processen att använda flera pulser, och varje

puls adderar tid och svetsschemat bör därför vara välutvecklat och optimerat för att undvika

slöseri med avseende på långa svetstider. Resultaten kommer att ge Scania mer kunskap om

processerna och hur man kan optimera processerna ytterligare i produktionen. Resultatet kan

också användas som grund för val av produkter eller framtida investeringar.

Acknowledgements This thesis is the final examining part of the master´s program production engineering and

management at the Royal Institute of Technology (Kungliga Tekniska Högskolan), Stockholm.

This master thesis was conducted at Scania CV AB in Oskarshamn.

I want to share my gratitude to Marie Allvar and Sebastian Danielsson at Scania CV AB for

great supervising during the thesis. Sharing their time, knowledge and interest during the

progress has been very appreciated.

Many thanks also to my supervisor at KTH Joakim Hedegård from Swerea Kimab who has been

a good support during the project. Fredrik Svensson, Manager MBBEN, Scania Oskarshamn for

giving me the opportunity to complete my Master’s thesis at MBBEN. Stefan Borg, Svetsrådet,

for sharing useful information and Erik Tolf, Scania Södertälje, which contributed to yielding

results.

Caroline Bohlin

Oskarshamn, May 2018

Nomenclature

Notations

Symbol Description Unit

Q Energy [J]

U Voltage [V]

I Current [kA]

R Resistance [µΩ]

𝑑𝑛 Nugget diameter [mm]

𝑑𝑤 Weld diameter [mm]

𝑡 Thickness [mm]

T Weld time [ms]

F Force [kN]

Abbreviations

RSW Resistance spot welding

CCR Constant Current regulation

HCC Heat capacity control

NI Nugget index

BIW Body in white

UT Ultrasonic testing

AHSS Advanced high strength steel

UHSS Ultra high strength steel

DP Dual phase

HS-IF High strength interstitial free

HAZ Heat Affected Zone

CONTENTS

1 INTRODUCTION ................................................................................................................. 1

1.1 Background ........................................................................................................................... 2

1.2 Purpose .................................................................................................................................. 3

1.3 Research questions ................................................................................................................ 4

1.4 Research methodology .......................................................................................................... 4

1.5 Delimitations ......................................................................................................................... 4

1.6 Disposition ............................................................................................................................ 5

2 FRAME OF REFERENCE .................................................................................................. 7

2.1 Joining in Automotive industry ............................................................................................. 7

2.2 Resistance spot welding ........................................................................................................ 7

2.3 Equipment for resistance spot welding ............................................................................... 12

2.4 Control systems for resistance spot welding ....................................................................... 14

2.5 Standards and regulations for RSW .................................................................................... 16

2.6 Non-destructive and destructive testing .............................................................................. 18

3 MATERIAL ......................................................................................................................... 20

4 EXPERIMENTAL METHODS ......................................................................................... 24

4.1 Welding Set-up .................................................................................................................... 24

4.2 Welding experiments ........................................................................................................... 29

4.3 Analysis ............................................................................................................................... 30

5 RESULTS ............................................................................................................................. 32

5.1 Material combination 1 ....................................................................................................... 32

5.2 Material combination 2 ....................................................................................................... 40

5.3 Weld time at spot 300 .......................................................................................................... 50

6 DISCUSSION AND CONCLUSION ................................................................................. 51

6.1 Discussion ........................................................................................................................... 51

6.2 Conclusion ........................................................................................................................... 55

7 RECOMMENDATIONS AND FUTURE WORK ........................................................... 57

8 REFERENCES .................................................................................................................... 58

1

1 INTRODUCTION

This chapter introduces the thesis. It describes the background, the purpose, research

questions, the methodology and the limitations used in the presented project.

The demand for a highly efficient production makes resistance spot welding the main joining

method in the automotive industry. The process is fast, efficient and able to weld high

strength steel in a short amount of time. To be competitive in the market, Scania CV AB in

Oskarshamn strive to improve and decrease fuel consumption of the trucks. The weight of the

truck has a direct relation to fuel consumption and therefore are advanced high strength steels

(AHSS) increasingly introduced in the design of the vehicle. It enables the use of thinner steel

sheet with retained strength. The cab body is, now more than before, constructed with

different materials, from mild steel to AHSS with different thicknesses. This requires

variation of the parameters and puts higher requirements on the welding procedures. The

variation in sequence in production also makes it important to have a welding process that can

adapt flexibly between the different components.

Cycle time is one of the most critical factors to consider in mass production. A truck contains

about 3500 spot welds and by improving the process the timesaving generated from each

spot-weld can add up to a large timesaving for each truck. At Scania, it is important to know

how the process works and acts. Today Scania uses adaptive process control, to control the

spot welding processes. By using adaptive control system in the production the process can

adjust the parameters during the welding process. The control system is fairly new and the

benefits that are given are not yet investigated. It is known that the control system has a

advantageous impact on the welding process, but the benefits have not been properly

quantified.

Scania wants to know how the adaptive control system handles important spot welding

properties such as: expulsion, time, robustness and the microstructure and hardness of the

weld, and to see if the process handles different materials and combinations differently. This

will give them knowledge that will be useful in process development and for new investments

of equipment in the production.

2

1.1 Background

Scania CV AB is an international company manufacturing trucks in premium class and selling

to customers all over the world. At the plant in Oskarshamn the cab is manufactured. This

thesis will be executed in the part of Body in white (BIW). The new factory containing BIW

was built in 2015 to improve the production by investing in new technology and to meet the

increasing demand since the new generation of cabs was launched. During the spring (2018)

the new factory is taking over all production for BIW when the old BIW factory shuts down.

The production is still striving to improve and the company needs to keep updated of how

their processes work to keep production as efficient as possible [1].

The main method to join sheet metal in the automotive industry is resistance spot welding

(RSW). The control systems can either use fixed- or adaptive weld schedules, the latter is

used at Scania. The adaptive control systems can detect when the weld is complete by

knowing the leading parameter or property. One version of adaptive systems utilizes ideal

curves that have been determined for the varying parameters and the system adjusts so that the

parameters follow the curve. Following adaptive system that is used in the production is

Matuschek’s HCC ”Heat capacity control”. Another adaptive Matuschek system available is

“Master mode”. The reference welds are made by using fixed schedule CCR “Constant

current regulation”. Control systems for resistance spot welding is further described in chapter

2.4. Adaptivity keeps the welding parameters optimal for each individual weld and the defects

are therefore reduced. The gain is specific for each application and tests need to be run for

each one to know in what way and how much better it is to use adaptivity for a specific

application.

Development of computational control systems for RSW has grown a lot over the last years.

Today it is possible to customise the process by taking into account which material,

parameters, machine and electrodes that are used in the process. This enables a more cost and

time efficient process when the machine can adjust itself. The computational system can also

give important information in real-time of what happens in the weld process [2].

Science and information in this area are limited, earlier investigations and reports regarding a

comparative study like this topic has not been found. Scania is the only company using HCC

in almost all processes, they also have a special license to review the program and change

3

parameters. Since no other company uses HCC to the same extent, earlier investigations and

research in this area is lacking. However, a result of tests on boron steel using fixed weld

schedule system and adaptive weld schedule has been published. It showed that the hardness

in HAZ was different and changing unexpectedly when using adaptive control system [1].

Therefore, it would be interesting to study if it is possible to see differences in the material

properties in HAZ when evaluating the adaptive systems at Scania.

This report is the first output of finding the actual benefits adaptive spot welding has on the

production at Scania. Scania is interested in learning more about how the materials and the

combinations used in the cab are affected. The theoretical advantages are known, such as less

weld expulsion and optimal welding time, but quantifying investigations are lacking. The

cycle time for implementing the welded spots is one of the most important aspects to

consider. The large amount of spot welds in the truck means that a time gain for each spot

weld can generate a large total time gain.

1.2 Purpose

The purpose of the thesis is to increase understanding and knowledge about the true benefits

of the adaptive systems. The result will be used as a foundation or motivation for continued

work or selection of products. It can also be used to support further investments. The

objective is to identify and quantify a few of the gains and benefits that the adaptive processes

give.

Problem definition

Investigate the actual gain it allows to use adaptive systems on the welding process compared

to the traditional system, for a few selected applications. The properties and areas that will be

investigated are:

The robustness of the process: How many completed welds fulfil the requirement

stated by the standard? How does the result deviate between traditional and adaptive

system?

Weld expulsion: How do the different control systems handle weld spatter?

Weld time: Investigate the difference in weld time between the control systems.

4

Surface defects and electrode wear: Control if the surface and electrode appear

differently when using different control systems.

Other parameters that will be investigated to help answer previous areas are current,

voltage, end resistance, energy levels and nugget index.

The aim is to answer how these areas are affected by using the three different control systems

available at Scania. Another aim is to answer how some of the properties of the material and

different combinations can affect the outcome.

1.3 Research questions

I. What are the advantages/disadvantages with using the adaptive control systems, HCC

and Master mode in RSW compared to the fixed system CCR, for selected

applications?

II. Are there differences in weld properties due to the control system used?

III. Does the result comply with the Scania standard?

1.4 Research methodology

In the beginning of the project, a workplan stating the workflow and schedule were created,

where milestones and sub-goals was defined. To answer the research questions, investigations

were made in several steps. Deep and broad knowledge in fundamentals and physics about

RSW has been collected by performing a literature study regarding relevant standards, articles

and literature. Literature were found from KTH library and books from previous courses in

education. It was also important to understand the BIW production and requirement on the

process stated by Scania standard and other international standards. Beyond this, internal

education was necessary to use experimental equipment and interviews were held with people

with relevant knowledge within and outside Scania. The experimental phase included physical

weld tests and laboratory analysis of welds and electrodes, to evaluate the different outcomes

and find results.

1.5 Delimitations

Project delimitations will regard control system, type of material and material thickness. The

electrode that will be used is ISO 5821: Cap B0-16-20-40-6-45 A2/2. Also, the investigated

5

result will be limited and prioritised starting with evaluating the first prioritised combination.

At least two material combinations and at least two properties will be evaluated. Properties

that will be evaluated in the report are shown in Table 1. The different systems will be

compared with reference to the nugget size. The same nugget size will be the aim for the

samples and thereafter it will be possible to make a comparison.

Table 1 Specification of process- and material properties

Control system

The thesis will be limited to the three control systems, CCR, Master mode and HCC.

Material

The material combinations will consist of combinations existing in the cab body. Two to three

sheet combinations with different materials will be used.

The work will to the extent possible, comply with STD4429 Resistance Spot Welding -

Requirements, Scania standard.

1.6 Disposition

In this chapter the Introduction is described, it holds an overview of the subject of the thesis

and the workplan and method for accomplishing it. The second chapter gives the Frame of

reference; collection of all fields of theory which the thesis is based on regarding process,

physics and state of the art. In chapter 3, Material, the material combinations used in the

experiments are described. In Experimental methods, chapter 4, all steps of creating a weld

schedule and the experimental method is described, as well as the analysis. The Result from

the experiment is given in chapter 5. Thereafter, the result and findings from the compared

control systems are compared and discussed in chapter 6, Discussion, this chapter also

answers the thesis question stated in the beginning and conclusions from the results are

drawn. Recommendations and future work is given in chapter 7. Last chapter is references,

Process properties Weld properties

Robustness Hardness

Time HAZ

Weld expulsion

Electrode wear

6

where the literature and data from theory stated in chapter 3 is summarised. At last, all data

and information that did not fit in result are appended in appendix.

7

1 FRAME OF REFERENCE

In frame of reference, the theory regarding resistance spot welding and control systems are

presented.

2.1 Joining in Automotive industry

There are several joining methods used to join the body parts together. At Scania’s Body

shop, they use laser-brazing, MIG-brazing, stud welding and RSW to join and combine

different materials in the best way. RSW is the most commonly used method in the

production. Resistance welding is a group of processes that uses resistance, current and force

to weld pieces together. [2] Most common of these is RSW, which is used widely in steel

sheet production, due to its high efficiency. Adhesives are used in between some sheets to

distribute the load and increase fatigue life [3]. Adhesives are normally a factor to consider,

but will not be investigated in this project since the joint will not contain adhesive. This thesis

will focus on resistance spot welding and the following theory will therefore only concern

RSW.

2.2 Resistance spot welding

A resistance spot weld is accomplished by forcing the workpieces together by two electrodes

while an electrical current flows between the electrodes through the material. When the

current passes through the material, the highest resistance will occur in the middle between

the sheets, and that is where the weld will start to form. The resistance and the current creates

high energy and a localised heat. The heat makes the material melt, and a spot weld is formed.

In Figure 1, a picture shows the cross-section of a spot weld between two metal sheets. The

workpiece has higher resistance than the copper electrode and therefore only the workpiece

melts. [4]

Figure 1 Cross-section of a typical spot weld

8

RSW is used mainly to weld thin sheet of steel during a short amount of time. It is possible to

weld two-, three- and four-sheet combinations, or even more. The process is highly suited for

automation and robotics allowing highly efficient production. It therefore makes it suitable for

the automotive industry, enabling high strength steel to bond efficiently and without adding

extra material keeping the weight as low as possible. [5] It also requires little training for

operators and the process does not need filler material or shielding gas which makes it more

environmentally friendly and cost efficient. Most of the heat is restricted to the direct area and

therefore RSW gives little deformation of the workpiece [6]. Some heat is transferred away by

surrounding material due to the materials conductivity [2].

The physical fundamentals of resistance spot welding is dependent of the material property

resistance (R) across the weld, the parameters time (t) and current (I). Heat input (Q) is

defined by Joule´s law as a function of resistance, current and time. As seen in equation (1).

𝑄 = 𝑅𝐼2𝑡 (1)

Normally current and resistance is not constant during RSW, as further described in 2.2.3,

resistance changes as the material melts, this leads to a change in current during the process to

comply with the resistance. Because the resistance changes, the expression in equation (1)

must be integrated to get the correct heat input.

2.2.1 Parameters in the process

The process is depending on the parameters time, force and current

and when the weld schedule is created, these parameters must be

carefully considered and chosen. Changing one parameter will affect

another one and therefore they need to be weighed against each

other. Parameters are able to be adjusted and changed. A materials

resistivity is vital in developing the heat-effect. Therefore, current

and weld time will be adjusted to suit the specific material

properties.

Time. The total time is divided into different phases. The first one is squeeze time when the

electrodes squeeze the material to develop the right pressure and joint fit before welding. The

Time

Force

Current

9

sheets must be in the right position before the current is run, otherwise the weld will be

inadequate. The second phase is weld time when the current flows. During this time the

material melts and the nugget is created. The third phase is hold time, when the electrodes

hold pressure without flowing current. This keeps the sheet in place during the solidification.

Before and after the phases there is off time, when the electrodes have time to move to another

spot and find the right location for the weld.[8] In Figure 2 the different phases can be seen.

Electrode force. The electrode force has an impact on the resistance. When the force is

increased the sheets are pressed tighter and surface contact is improved. The surface

resistance will be lower due to this and the welding energy will be lower. That means less

heat to the weld and it gives a higher risk of loose spots. If the force is low, resistance will

become higher and the process will get more heat. This can lead to expulsion that is described

further in chapter 2.2.4. Commonly used force values in thin steel sheet spot welding

processes are 3-6 kN. [7]

Welding current. Normally, the current amplitude for resistance spot welding of thin steel

sheet is 5-10 kA [7]. If current is not set to correct values it can lead to poor strength, too

small weld nugget or discontinuities in the weld. Current is the parameter that influences the

energy the most. To keep the current in an acceptable interval, a weld lobe can be created that

generates acceptable welds with the right quality. The lobe curve shows the minimum weld

current to create a nugget of right size and the maximum weld current to avoid expulsion, e.g.

Figure 2. Spot welding cycle [21]

10

in relation to weld time. A lobe curve is created by making test welds and noting size while

systematically varying the parameters, and thereby find minimum and maximum of current

and time with an approved size and avoiding expulsion. [6]

2.2.2 Resistance

Resistance in the weld rely on the resistivity of the material. Resistivity is the materials ability

to pass current through a conductor and it varies for every steel. The resistance is given by

formula (2)

𝑅 = 𝜌𝑙/𝐴 (2)

where 𝜌 is the resistivity, 𝑙 the length of the conductor and 𝐴 the area of the conductor, which

in a resistance spot weld is the area of the electrodes tips. The higher resistance there is the

more heat is developed. The reason the workpiece melts and not the electrode is that copper

has very low resistance and the highest heat will always develop at the interface with the

highest resistance, i.e. between the sheets being welded. Therefore the nugget starts to grow

there. Total resistance between the electrodes for a two-sheet stack up is expressed by formula

(3).

𝑅𝑡𝑜𝑡 = 𝑟1 + 𝑟2 + 𝑟3 + 𝑟4 + 𝑟5 (3)

where 𝑟1 and 𝑟5 is the contact resistance between electrode and metal sheet. 𝑟2 and 𝑟4 is the

resistance in the workpiece and 𝑟3 is contact resistance between the metal sheets [6]. Figure 3

shows the principle of resistance between electrodes and metal sheets.

The resistance changes as the material melts, and is therefore called dynamic resistance. The

resistance is high in the beginning due to oxides and rough surfaces and it drops when the

material melts. Because of this, the nugget is formed rapidly after the oxides have been

Figure 3 Principle of total resistance between electrodes.

𝑟4

𝑟5

11

broken through and surfaces evened out. When the nugget starts to form, the resistance

increases and at last, the resistance drops again when the current density is spread over the

nugget. [2]

“Resistance End” is one of the parameters measured in the control system at Scania and it

indicates if the nugget has grown. It gives the value of the resistance curve when the weld

time is stopped. Therefore, the resistance end can be different for the same material and

material combinations depending on when the weld time is stopped. If the process is stopped

early, the resistance is in an early stage on the resistance curve and resistance end becomes

high. If weld time is longer, the resistance curve has had time to decrease and resistance end

becomes lower. Resistance end indicates the size of the nugget, if it is high the nugget

probably did not grow enough. If it is low the nugget has had the time to increase in size.

2.2.3 Shunting effect

Shunting effect is the phenomenon that occurs when a spot is welded close to another spot-

weld, parallel resistances occur and the current takes a path through the first weld. Figure 4

shows how the current flows through the first weld when the second weld is welded.

Normally spot-welds are welded in series so shunt effect usually occur. The result from this is

that when the subsequent spot is welded and current takes a path through the first spot the

welding current gets too low for the second spot. This needs to be considered in automotive

since there is almost always a spot close by when welding a body, and shunting effects are

considered as the normal condition. The first spot is therefore not considered when creating

weld schedules and the current is adapted to match subsequent welds. [8]

2.2.4 Weld expulsion

Weld expulsion is a problem that the automotive industry is generally striving to avoid in all

spot welding processes. Expulsion leads to loss of material, and in worst case holes in the

Figure 4 Electrical shunting between welds. Current is diverted due to the close spacing

between welds. [9]

12

sheet. It also gives marks on the metal sheet which can be unacceptable if they are visual and

can be seen on the surface of the finished body. It can therefore be a quality problem.

Expulsion happens due to too high energy and there are several reasons that it occur.

One reason is that the weld becomes too large for the electrode which causes the material to

eject out of the weld. Expulsion then happens when the weld-current and/or weld-time is too

high.

Another cause is when the electrode force is too small. When the material melts and expands

it needs to be held back by the force from the electrodes/welding gun. Otherwise, the material

will press the sheet apart allowing the material to eject and create a weld expulsion. On the

other hand, if current or weld time is too small or the electrode force is too high, it will lead to

a small nugget not being qualified according to standard. [9] Gaps between the sheets in

production, dirt on the surface or too high current in the beginning are also factors that can

lead to expulsion.

2.3 Equipment for resistance spot welding

2.3.1 Machine type

The variety of machine types is wide and the equipment is adapted to suit the application

depending on efficiency, complex shape and high production rates. From the beginning of the

RSW technology, AC was the most common electrical power source using 50 Hz, today that

technology is in many cases not efficient and precise enough. Nowadays, the power source

used in the automotive industry is normally medium frequency direct current machines

(MFDC). By using MFDC, the welding current can have a frequency between 1000-2000 Hz.

Higher frequency enables more control over the process and better quality since more

advanced algorithms can be used in the weld control as it enables fast feedback from the

process. By getting feedback for the outcome of the parameters the process can be adjusted to

get the right values. [10]

2.3.2 Weld guns

There are mainly two different types of weld guns, pneumatic and electrical servo guns.

Pneumatic guns have been widely used in the industry. Now, many industries have changed to

electrical servo guns that generally are more powerful and more precise. Weld guns can be

operated manually or automatically. The latter option is used at Scania installed on robots.

13

Weld guns are normally either linearly moving or X-shaped gun chosen to suit the

applications. [7]

2.3.3 Electrodes

Electrodes are used to lead the current through the metal sheet, cool the material after welding

and to fixate and put pressure on the sheets during welding. [9]

The electrode consist mainly of copper but can have a variety of alloying elements, such as

chrome, to increase the hardness and be able to carry the force under high pressure. Copper

has low resistivity and also a good electrical conductivity that causes the electrode to not melt

since the material will melt where the resistance is the highest. Inside the electrode, a water

coolant flows to cool the electrode and protect it from getting overheated, to avoid high

resistance build up and excessive wear. [2] The electrode comes in different shapes and sizes

to fit different applications. Figure 5 shows a picture of electrodes with different shapes.

The geometry of the electrode changes during welding due to the pressure and heat.

Deformation can lead to lower current density that causes loose spot welds. Electrode wear,

possibly leading to weld expulsion on the surface, can occur more quickly when welding

coated material since the resistance becomes higher on the surface from alloying and

oxidation. By tip-dressing the electrode regularly, it is kept in good condition. Tip-dressing

means that the outer surface is grinded away and the electrode becomes as new on the surface.

When the electrode has been tip-dressed several times the electrode becomes shorter and

needs to be replaced. [4]

In automotive the positioning of electrodes can be difficult. Many components that are welded

together require tight tolerances when they are fixed against each other. Gaps between the

sheets are hard to avoid due to the many components and it is one reason for weld expulsion

in production. In automated production, the misalignment can also be caused by poor

programming.

Figure 5 Different shapes of electrodes [18]

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2.4 Control systems for resistance spot welding

There are generally two different types of control systems, fixed and adaptive. A fixed system

uses the parameters that are set in the program during the entire process. An adaptive system

has the ability to adjust and change the parameters during the process to avoid expulsion and

an incomplete weld. Adaptive control systems are now used in most automotive industries.

Earlier, each sheet combination has required a unique optimization of parameters. Now with

adaptive control systems, each program can handle a variety of sheet combinations and the

amount of weld schedules can be decreased for the production plant.

2.4.1 CCR

Constant current regulation is a fixed control system that works in such a way that the

parameters which are set in the weld timer will be the parameters used in the process. CCR

works after a fixed schedule. CCR uses a predetermined current to create a target for current

used in the process. The limit is transformed to a target phase angle by the control system.

When the target phase angle is set, the second current is measured and compared to the target

current. The phase will adjust the current in consideration between the target current and the

measured current. Therefore, the same values of time and current will be used in the process.

[11]

2.4.2 Master mode

Master mode is an adaptive control system that can adapt some parameters during welding to

create the best weld after variation in circumstances. This enables welding of different

material combinations with the same schedule. To create a weld schedule with Master mode,

first a reference weld is created using CCR. The optimised parameters are used to weld a

reference weld and create good master curves. Master curves are the curves showing current,

time and resistance. When it has been created the system is switched to Master mode and the

current and voltage curve is saved as a function of time. Figure 6 shows an illustration of the

procedure for creating a weld schedule with Master mode.

https://docs.google.com/document/d/1AQVvlfSCd3CHPgYxDOmTmlwUDR8xPabprLmlo6FbtiI/edit#heading=h.1pxezwc

15

During welding, the parameters are adjusted to reproduce the parameters of the reference

weld. Master mode is expanded with a parameter called weld extent. Weld extent is set to a

value in percent that the process is allowed to extend the time to be able to fulfil the values of

the reference weld. When welding in master mode the process can adjust current and time.

Current will be the prioritised parameter to change and time will change if correct energy

levels are not fulfilled. Master mode can respond to changes in welding in a way that CCR

cannot. [12]

2.4.3 HCC

Heat capacity control (HCC), is an adaptive control system that compares the outcome of

actual values to the programmed parameters value of the input. It can regulate itself to match

output with input. An interval of current values is set for each pulse to let the system know

what to relate to and act within. The system uses several pulses and can adjust the amount of

pulses depending on if the weld is complete. The time from previous pulse will decide how

the next pulse will act and regulate current. Time is regulated to avoid expulsion and if it is

noted to occur the machine shuts of the current. The program has the ability to adapt weld

time and if shorter time than maximum time is used in one pulse, the current is reduced in

next pulse. If time reaches maximum in one pulse, current will be increased in the next pulse.

Master mode has been replaced by HCC at Scania and is used in all welding processes except

Figure 6 Procedure of creating weld schedule with Master mode. [14]

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16

for a few. Scania is the only company using HCC in almost all processes, and having the

permission to access the software and be able to program and change the parameters.

HCC uses the guiding quality parameter called nugget index, NI, to end the process when the

weld is considered complete. NI is calculated from the shape and amplitude of the weld

curves. The values are automatically generated from the reference curves knowing when the

nugget signifies a good size. If Nugget index ends outside the interval an error occurs and it is

considered to be a weld failure. Nugget Index can be set for each pulse and normally the value

is decreasing with every pulse. [9] Dead time is also a parameter that is set to neglect the time

in the beginning of the pulse when the curves peak.

2.5 Standards and regulations for RSW

The Scania standard STD4429 regulates the requirements on resistance spot welding in

Scania’s production. It states which requirements there are on design and construction of

resistance spot welded structures exposed to dynamic or static load. Nugget diameter 𝑑𝑛 is the

size of the diameter of a metallographic sample on an etched cross section, it is calculated by

formula 3. Approved nugget diameter that has to be fulfilled for all spot welds is:

𝑑𝑛 ≥ 3.5√𝑡 (4)

The robustness requirement has a target nugget size of dimension

𝑑𝑛 𝑡𝑎𝑟𝑔𝑒𝑡 ≥ 5√𝑡 (5)

𝑑𝑤 is the reference value of the weld diameter measured on the nugget. Due to uncertainty in

measuring, a peeled nugget, 15 percent is added to the diameter

𝑑𝑤 = 1,15 × 𝐷𝑛 𝑡𝑎𝑟𝑔𝑒𝑡 (6)

Minimum size to achieve the robustness requirement is

𝑑𝑛 𝑟𝑜𝑏𝑢𝑠𝑡 ≥ 0,9 × 5√𝑡 (7)

17

where 𝑡 is the thickness of the thinnest sheet. In production, 𝑑𝑛 𝑟𝑜𝑏𝑢𝑠𝑡 has to be fulfilled in

75% of all welds. For joints with sheets of different thicknesses, the thinner sheet is decisive

for determining the nugget size for each joining plane. [13]

Measuring the diameter of the peeled weld nugget is made in agreement with SS-EN ISO

14329:2004 “Resistance welding – Destructive tests of welds – Failure types and geometric

measurements for resistance spot, seam and projection welds”. In Figure 7 the different plug

failures are illustrated.

a and b, symmetrical plug and asymmetric plug

𝑑𝑤 = 𝑑𝑝 = (𝑑1 + 𝑑2)/2 (8)

c, partial plug

𝑑𝑤 = (𝑑1 + 𝑑2)/2 𝑎𝑛𝑑 (9)

𝑑𝑝 = (𝑑1 + 𝑑3)/2 (10)

Figure 7. The different plug failure modes that can occur. a) illustrates a

symmetrical plug, b) asymmetrical plug and c) a partial plug failure.[24]

18

SS-EN 10346 “Continuously hot-dip coated steel flat products for cold forming – Technical

delivery conditions” is the international standard which states the properties and chemical

composition for continuously hot-dip coated steel flat products for cold forming.

SS-EN ISO 10447:2015 “Resistance welding – Testing of welds – Peel and chisel testing of

resistance spot and projection welds” regulates how the peel tests of the sheets should be

done.

SS-EN ISO 14271:2017 “Resistance welding – Vickers hardness testing (low-force and

microhardness) of resistance spot, projection, and seam welds” is the standard that regulates

procedure of hardness measurement.

According to Scania standard STD 4429 the maximum hardness in the weld nugget and the

HAZ must not exceed 550 HV0.2.

2.6 Non-destructive and destructive testing

For cost reasons, it is important to keep track of the quality of the welds and make sure to find

defects in an early stage to eliminate waste and correct the problem in the beginning. To be

able to control the weld it must be tested by either non-destructive or destructive testing. For

control in production, ultrasonic testing (UT) is used at Scania. UT is a non-destructive testing

method that is able to detect defects under the surface. Ultrasonic sound waves with high

frequencies are sent into the material and waves are reflected back to the transmitter if it

detects a defect. [13] A display shows an image of the spot weld and measurements of the

diameter. If porosity or other defects exist in the weld it will show on the screen. The negative

side of UT is that the precision is important but still difficult to control while using it. The

transmitter need to be placed right in the middle of the spot weld, which is difficult to

estimate for the operator.

Destructive testing that is conducted on the cab body is peel testing. In Figure 8, a peeling tool

is shown. Complete cab tear down is performed regularly to control the nuggets. Peeling is

performed by attaching a roller tool to the metal sheet and then tearing the welds apart when

rolling it. Another destructive testing method that can be used to control the weld is chisel

testing were the joint is pressed apart by a chisel.

19

Figure 8 Peel testing using a vise and a roller. [19]

20

3 MATERIAL

In this chapter the materials and material combinations that are used in the experiments are

presented. Also weldability of the materials and RSW on coated steels are described.

Due to the importance of keeping the vehicle as light as possible, the construction of the

trucks is made out of a variety of different material that will suit the applications best. Low

weight is a request but it must also match the requirement of strength. Therefore, many

different materials are combined and welded together. The combinations can contain

materials ranging from mild steel to AHSS. In Figure 9, a graph shows different steels and

illustrates the trade-off between strength and elongation. Some parts of the body need to show

proof of strength, e.g. the A-pillar that has high strength Boron steel to support the frame and

also enables low weight. Other parts need to be ductile due to the forming it goes through

during manufacturing. On those parts mild steel or dual phase is used, if it also needs

increased strength. [4]

Further down, tables of the materials and its properties considered in this thesis are presented.

The first combination consists of two sheets of interstitial free, mild steel with the same

thickness that is included in the firewall. The second combination consists of three sheets of

different materials. One IF steel from the rear wall panel, also a DP steel from the outer sill

member and the last one is a HSLA from the upper/inner side member. The different

combinations represent two types of material combinations that exist in a Scania truck. Table

Figure 9. Relation between elongation and tensile strength for material groups. [20]

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21

2 presents the material combinations that is used in the experiment in chapter 4 and its

coating, which thickness it has and which component it is a part of.

Table 2 Material combinations

Nr Definition Designation Thickness Coating Part

1. HS-IF 260 HX260 YD 1.5 mm Z100MB Firewall

HS-IF 260 HX260 YD 1.5 mm Z100MB Firewall

2. HS-IF 220 HX220YD 0.8 mm Z100MB Rear wall panel

DP 600 HCT590X 1.2 mm Z100MB Sill member outer

HSLA 340 HX340LAD 0.9 mm Z100MB Side member, upper

inner

The chemical composition for the steels are presented in Table 3. [14]

Table 3 Chemical composition

Definition C

max.

Si

max.

Mn

max.

P

max.

S

max.

Altotal

Nb

max.

Ti

max.

HS-IF 220 0.01 0.30 0.90 0.080 0.025 ≥ 0.010 0.09 0.12

HS-IF 260 0.01 0.30 0.90 0.080 0.025 ≥ 0.010 0.09 0.12

HSLA 340 0.12 0.50 1.4 0.030 0.025 ≥ 0.015 0.10 0.15

Mechanical properties for the steels are presented in Table 4. [14]

Table 4. Mechanical properties

Definition Steel

number

Proof

strength

Rp0,2

MPa

Tensile

strength

Rm

MPa

Elongation

A80

%

min.

Plastic

strain

ratio

r90

min.

Strain

hardening

exponent

𝑛90

min.

HS-IF 260 1.0926 260-320 380 to 440 30 1.4 0.16

HSLA 340 1.0933 340 to 420 410 to 510 21 – –

HS-IF 220 1.0923 220 to 280 340 to 420 32 1.5 0.17

DP 600 1.0996 330 to 430 590 20 – 0.14

C

max.

Si

max.

Mn

max.

P

max.

S

max.

Altot

Cr +

Mo

Nb +

Ti

V

max.

B

max.

DP 600 0.15 0.75 2.50 0.040 0.015 0.015

to 1.5

1.40 0.15 0.20 0.005

22

High strength IF, interstitial free steel - HX220YD and HX260YD

IF is a conventional steel with very high ductility. The high strain hardening keeps a good

indentation in deep drawn parts and the interstitial free microstructure gives it high

drawability. HS-IF is suitable for complex parts that require high mechanical strength such as

skin parts and structural parts. [15]

High strength low alloy, HSLA, mikro-alloyed steel - HX340LAD

High strength low alloy steel is steel that is cold rolled, precipitation hardened and goes

through grain-size treatment to receive a fine grained microstructure of ferrite. The steel has

high strength and a low alloy content. Due to the high strength and the possibility to reduce

weight in applications, this sheet metal is suitable for structural components such as cross

members and chassis components. HSLA has good weldability. [16]

Dual phase steel, DP - HCT590X

Dual phase steel is an Ultra High Strength Steel (UHSS) and has a microstructure that consist

of two phases, ferrite and martensite or bainitic particles. Ferrite makes the steel ductile and

hard martensite make the steel keep a high strength. The structure is achieved by annealing

the steel and keeping the temperature under a certain time during the austenitic and ferritic

phase. It thereby ends with a microstructure of dual phases. Specific properties of dual phase

steel is high fatigue strength and energy absorption. Drawability also increases as the steel is

hardened during forming. DP steel is suitable for applications such as structural and

reinforcement components. [15]

Weldability of the material

During welding it is necessary to keep track of the parameters and control the welds in case

hard martensite is formed. The higher carbon content of the material, the higher is the risk of

forming martensite. Martensite will make the nugget brittle and decrease ductility. If the

cooling from the electrode is too fast it also leads to higher risk of creating hard martensite.

Uncoated mild steel is the simplest steel sheet to spot weld. Advanced high strength steel is

more difficult to weld due to the increased amount of alloying elements that requires a more

thorough developed weld process. Higher forces and larger electrodes are needed to keep

contact between the surfaces. Welding in boron steel requires not only higher forces, but also

more time. [4]

23

RSW on Coated steel

Steel for automotive industry is kept corrosion resistant by coating it with, most commonly,

zinc. The coating contributes to more contamination and the risk of getting defects increase. It

also leads to more electrode wear when the zinc gets stuck on the electrode. Hot-dip and

electro galvanised material is used at Scania. Hot-dip steel is rolled through a zinc-bath and

electro galvanised steel rolls through a zinc electrolyte bath connected to a current. Electro

galvanizing has a more complicated manufacturing process that enables a thinner layer of

coating. The process is more expensive and is used more in special applications that requires

higher quality. Therefore, hot dip galvanizing is used in most cases. Galvannealing can be

done after coating to decrease electrode wear. It is a subsequent heat treatment to create an

iron-zinc layer instead of just zinc. It makes it easier to weld and gives less electrode wear. [4]

24

4 EXPERIMENTAL METHODS

In this chapter, experimental methods for the three control systems CCR, Master mode and

HCC are described.

4.1 Welding Set-up

The welding Set-up that was used in the experiments was partly consisting of the hardware

and the software. Furthermore, fixtures and test coupons had to be constructed to be able to

execute the experiment.

4.1.1 Welding equipment and robot

The equipment used was a robot and a resistance spot welding gun with MFDC (1000Hz).

The inverter was a Matuschek Spatz+ M400 with a maximum short circuit control of 18 kA.

The maximum electrode force was 3.5 kN and the weld control unit was a PC based

Matuschek Spatz+ M400L. Control systems possible to use with the equipment are CCR,

Master mode and HCC. The spot welding gun was of X-type from ABB. Figure 10 below

presents an image and data of the equipment used in this experiment.

Figure 10 Table of information and data for welding equipment

Robot:

IRB 6700-300/2.70

(ID;100017, BS 010 R01)

Machine type: Spot welding gun

ID weld gun: 91401244

Max electrode force:

[kN]

3.5

Short circuit control:

[kA]

18

Throat depth: [mm] 800

Current type: MFDC (1000 Hz)

Weld control unit: PC-based Matuschek SPATZ

Water cooling: [l/min] 6

Transformer: Roman TDC-5080 91 kVA

Inverter: Matuschek Spatz+ M400L

(Type nr: M04PLM002; Serie

nr: SP-M40L 240153)

Electrodes: Cap B0-16-20-40-6-45 A2/2

25

Software

The outcome of each spot weld is visualized in “QA analysis” in Spatz studio. Useful values

and curves that can be seen in QA analysis are time, current, voltage, resistance, NI and

energy. In Figure 11 the graphic window for the parameter curves for one spot weld is shown.

It illustrates how the values change dynamically over time in the weld process. Figure 12

illustrates the graphic window where weld time changes for different spot welds.

4.1.2 Specimen manufacturing

Flat coupons were cut in sizes 600x48 mm and 150x48 mm. The smaller coupons were used

when weld schedules were created, and the larger coupons were used in the experiment when

welding larger series to be more efficient by welding many spots in a series. Measurements

are shown in Figure 13.

The fixtures were manufactured at Scania to keep the coupons in the right place during

welding with the robot. One fixture for large coupons and one for small coupons, to be placed

directly on to the weld gun, were created. Figure 14 shows pictures of the fixtures.

Figure 11 Graph in “Last event” showing parameter curves during the

welding process for one welded spot.

Figure 12 Graph in QA analysis showing average

parameters during the welding process for several

welded spots.

Figure 13 Measurements for coupons used in experiments. Large coupon to the left and small coupon to the right.

26

Thereafter, a robot program was created to follow the path for the spots over the large fixture.

The large fixture fit five large coupons side by side and each coupon fit 19 spot welds. The

spot spacing between the welds were set to 25 mm due to requirements from the Scania

standard if 1.5 mm thick materials are welded. It is important to have a sufficient overlap (14

mm in this case) between the upper and lower coupons, to enable tightening of the coupons in

a vice when doing destructive test by peeling. The large fixture is always placed in the same

position following the markings on the floor. The small fixture was placed directly on the

electrode arm enabling fast welding of small coupons.

Some unpeeled coupons were measured with UT-equipment and all peeled tests were

measured with slide calipers. The electrodes were tip-dressed right before each test started.

4.1.3 Weld Schedule

A weld schedule was created for each material combination and control system. Every trial

ended with peeling the coupon and measuring the nugget diameter. The value was

documented in a data sheet. Table 5 shows an overall layout of the different series included in

the experiment, numbered based on which material combination it is and which control

system is used.

Figure 14 Fixture for large coupons (left) and small coupons (right), with coupons placed in the small fixture.

27

Table 5 Series in welding experiment.

On each small coupon, three spot welds were welded. To avoid misleading results from

shunting effects, as mentioned in 2.2.3, the first spot weld was not included in the result. The

first spot weld to consider in the test was therefore the second spot weld on the coupon.

Starting parameters were based on the graph shown in Figure 15. The graph shows

corresponding parameters for welding time and force for a certain thickness. It is a general

graph that can be used as a guide to find parameters to start with when creating the weld

schedule. To use the graph, the thickness of the material combination was measured. The

graph was handed out to Scania from Matuschek. It is only used as a starting point and shorter

times were strived for.

CCR Master mode HCC

Weld Sch. comb 1. 1 2 3

Weld Sch. comb 2. 4 5 6

Figure 15 General graph for selecting start parameters for welding time and force.

28

When creating the weld schedules, the parameters did not follow a structured DOE due to the

limited amount of time. Instead each weld test was evaluated and from that the parameters

were altered accordingly. The parameters were optimized until they met the requirement for

10 approved spot welds without expulsion.

Target nugget size, 𝑑𝑤, that is the aim for the weld schedule is calculated from formula (6).

𝑑𝑤 for this project was 7 mm for combination 1 and 5,1 mm for combination 2. Values are

presented for both material combination 1 and 2 in Table 6. Min value were set to 90 % and

calculated from formula (7) to see when the robustness requirement fell under approved

limits, 75% must be over 𝑑𝑛 𝑟𝑜𝑏𝑢𝑠𝑡. Maximum value was set as an indication where the weld

spots started to get too large and as a limit to use when creating weld schedule. Though it was

not necessary to stop the trial if the size is greater than 110%.

Table 6 Minimum, target and maximum diameter for material combination 1 and 2

CCR

Squeeze time was set to 100 ms and hold time to 150 ms for all weld trials. From the schedule

in Figure 15 the force was set to 3 kN and the weld time to 400 ms for both combinations.

One pulse was used in the weld schedule.

Master mode

For Master mode, the same parameters as for CCR were tested to get a comparative result. In

this control system it is possible to select “weld extent”. It was set to 100, which means the

process can adapt with up to doubled time. One pulse was used in the weld schedule.

HCC

The force was set to 3 kN and intervals for current and time were chosen to let the process

adapt within. HCC uses several pulses, with a minimum of three pulses. A maximum of 12

pulses were programmed in Spatz studio.

Material thickness [mm] Min [mm] Target size [mm] Max [mm]

1.5 6.3 7.0 7.7

0.8 4.6 5.1 5.7

0.9 4.9 5.5 6.0

29

4.2 Welding experiments

For combination 1, coupons were fixed, five in the bottom and five on top in the large fixture.

For combination 2, HSLA 340 was placed in the bottom, DP600 in the middle and HS-IF220

on the top. The plates were offset by 14 mm on the long side, to accommodate peeling by

hand. 19 spots in a row were welded, the first spot was as earlier mentioned neglected due to

shunt effects. The robot welded the coupons (in total 19 spots x 5 coupons per set). Figure 16

shows one picture of how the fixture was placed in the robot cell and another picture of a set

of coupons fixed in the fixture as it is being welded.

During welding the following was observed in QA Analysis and visually:

Robustness, when the nugget diameter was smaller than minimum allowed diameter

When expulsion occur

Electrode wear

Time taken to weld one spot

Current

Voltage

Resistance end curves

Energy levels

Figure 16 To the left the fixture is placed in place for experiment and to the right the robot is welding a set of

coupons.

30

Observations were documented in a data sheet and thereafter the last coupon of the set was

peeled and nuggets measured. If the nuggets were still approved, the test continued with one

more set.

The first control system to test was CCR, series 1 and 2. Thereafter series 3 and 4 with Master

mode and at last series 5 and 6 with HCC.

4.3 Analysis

The analysis was conducted after the trials to collect data and results from the experiment.

4.3.1 Ultrasonic testing – Non-destructive testing

A number of coupons were analysed with ultrasonic testing to evaluate nugget diameter. The

UT is calibrated against the material to establish which thickness it has and thereafter the

transducer is placed above the nugget. The transducer was rotated three or four times to find

and record a relevant measurement of the nugget. Due to factors explained in section 2.6, it

requires experience and training to perform UT measurements. From experience at Scania it is

known that UT consistently shows smaller nuggets than what is seen in destructive tests, and

measured values are seen as guiding and relative rather than absolute. For material

combination 2, both sides of the nugget were inspected since the material thickness varies on

the sides. Defects were noted and documented.

4.3.2 Metallographic investigation and hardness measurements

Chosen samples of spot welds were prepared by cutting a cross-section through the middle of

the nugget. It is important to make the cut in the middle, otherwise the diameter will show a

smaller value than it is. After cutting the piece, it was mounted into a plastic cylinder,

thereafter it was grinded and polished to diamond size 3 µm and etched with Nital 2 %.

Thereafter the sample welds were examined in microscope, studying the measurements of the

nugget diameter and surface indentation. A photography was taken of the nugget. The

metallographic investigation was completed with hardness measurements using a Q-ness

hardness tester with test load HV0.3.

31

4.3.3 Peeling – destructive testing

Coupons were fastened in a vice and peeled with a roller tool according to the Scania

standard. Every fifth coupon was peeled, the mean weld diameter was measured and

calculated according to formula (8), (9) or (10) depending on geometry of the peeled plug.

32

5 RESULTS

In this chapter the results from the experiment are presented.

The aim of this work is to evaluate the welding quality by investigating the parameters,

properties and factors; robustness, current, expulsion, time, energy and resistance.

5.1 Material combination 1

Material combination 1 consist of HS-IF 260 1.5 mm. A cross-section of a spot weld is shown

in Figure 17 below.

5.1.1 Weld schedule

Series 1. CCR – Combination 1

For series 1, a weld schedule with one pulse was developed. The weld parameters and test

results are presented in Table 7. 10 samples were welded and gave approved weld size

without expulsion.

Table 7 Parameters and results for weld schedule series 1.

Weld parameters

Energy [J]

Weld size

[mm]

Current

[kA]

Force

[kN]

Squeeze

time [ms]

Weld time

[ms]

Hold time

[ms]

7.75 3 100 300 150 4745 7.3

Series 2. Master mode – Combination 1

Master modes parameters were set to the same as for CCR, series 1. Requirements were

fulfilled when welding 10 subsequent approved weld sizes. Table 8 shows the chosen

parameters and results for the weld schedule.

Figure 17 Cross-section of material combination 1.

33

Table 8 Parameters and results for series 2.

Series 3. HCC – Combination 1

Creating weld schedules with HCC required many tests with different parameters. The current

intervals were kept constant at 8.25-10.00 kA. It is higher than for CCR and Master mode

since the current is pulsed and it was not possible to select lower values of current because

then the weld nuggets became too small. 12 pulses were set with different parameters, where

the first pulse 1 is squeeze time. 10 subsequent spot welds were welded with approved size

and no expulsion. It was noticed that the weld nuggets seemed to grow when welding test

spots on long coupons. Table 9 presents the parameters and results for the weld schedule for

series 3. Some of the parameters definitions have been replaced with X, Y, Z and U due to

company secrecy. In appendix 7 definitions are shown.


Pulse

Weld parameters

Energy

[J]

Weld

size

[mm]

Force

[kN]

Paus

e

time

[ms]

Time

[ms]

𝐼𝑚𝑎𝑥

[kA]

𝐼𝑚𝑖𝑛

[kA]

Dead

Time

[ms]

X

Y

Z

U

1 3 100

25

5941

7.2

2 3 5 65 7.50

3 3 5 100 9.50 20 0.60

4 3 5 100 10.00 8.25 20 0.40

5 3 5 100 10.00 8.25 20 0.40 1.40 1.70

6 3 5 100 10.00 8.25 20 0.35 1.30 1.70

7 3 5 100 10.00 8.25 20 0.35 1.20 1.70

8 3 5 100 10.00 8.25 20 0.35 1.10 1.70

9 3 5 100 10.00 8.25 20 0.35 1.10 1.70

10 3 5 100 10.00 8.25 20 0.35 1.05 1.70

11 3 5 100 10.00 8.25 20 0.35 1.05 1.70

12 3 5 150

Weld parameters

Energy

[J]

Weld size

[mm]

Current

[kA]

Force

[kN]

Squeeze

time [ms]

Weld

time [ms]

Hold

time [ms]

Weld

extent

[%]

7.75 3 100 300 150 100 4718 7.3

34

5.1.2 Experiment


The robustness requirement was not fulfilled after 16 coupons. The test was therefore

considered stopped at the last spot weld on the previous coupon. Figure 18, presents a graph

of the measurements from the peeled coupons and the sizes are decreasing the more spot

welds that are welded. The red line illustrates the minimum size 6.3 mm. The blue vertical

line shows where the robustness requirement fell below 75 % of approved welds after spot

285.

The weld parameters set gave expulsion on the shunt spots on coupons 6, 11 and 12. Since the

parameters time and current are fixed when using CCR, current and time did not change

during the trial. In Figure 19, the graphic window illustrating the plotted values of time,

current, voltage, resistance end and energy are presented. It represents figures from 475 spot

welds. The increasing resistance had a direct impact on energy. During the experiment when

the electrodes were more worn, resistance end became higher and went from 234 µOhm to

256 µOhm. This indicates that the nuggets had not grown enough. The energy levels

decreased slightly from 4.8 kJ to 4.4 kJ.

Figure 18 Graph presenting the size decreasing the more spots welds that are welded. The process went

under robustness requirement after 285 spot welds.

285 spot

welds

35

Nugget index is measured for CCR since it is run in a program that also handles HCC. It is

only measured and not making any impact on the process. The curve for NI is shown in

Figure 20. The curve decreased during the experiment.


When welding with Master mode, it showed that the process was more robust than CCR and it

fell under the robustness requirements after 380 spot welds. Figure 21 shows when the

process fell below the limit for a robust process. Some coupons show results from only a few

spot welds because the steel sheet broke when peeling and therefore the peeling had to be

interrupted for that coupon.

Figure 19 Graphic window showing plotted values for series 1. The yellow line shows where the process went under

the robustness requirement.

Figure 20 Nugget index curve for series 1.

36

During the experiment the control system compensated with regulating time and current.

Weld time varied between 301 and 348 ms and current varied between 7.62 kA and 7.94 kA.

In Figure 22 the graphic window presents the parameters in the process for each spot in the

series. The figures in the graph represents 465 spot weld. Spots that stands out in the graphs

are shunt welds that show peaks of time and drops in current. In the beginning, the current had

a small decreasing trend and after 110 spots the trend was slightly increasing. In the figure, an

arrow shows where the falling trend turns and start to increase. However, there was no

380 spot welds

Figure 22 Graphic window of QA Analysis showing plotted values of spot welds in series 2. The arrow shoes where

the current starts to increase and the vertical line shows where the process went under the robustness requirement.

Figure 21 Robustness requirement fell under approved limit after 380 spot welds.

380 spot

welds

37

apparently increasing or decreasing trend of the parameters. From what could be seen from

the graphs the most obvious relation was that when current decreased time increased, and vice

versa. In the beginning of the experiment when the electrodes were tip-dressed, the welding

parameters kept the same value as programmed in the master curve. But very quickly as the

electrodes got worn, weld time increased and then stabilized. As the electrodes got further

worn, at the end of the experiment, time regulation started to vary more. The energy varied

with time since current was relatively stable. Master mode did not use the entire interval of

weld extent to regulate time. Resistance end went from 231 µOhm to 250 µOhm.

Nugget index is showed in Figure 23. It had a decreasing trend and as well as for CCR,

nugget index does not have an impact on the process while running Master mode.


When welding with HCC, the weld diameter never came under the robustness requirement.

The longer the experiment went, the spot welds grew larger instead of decreasing as CCR and

Master mode did. The weld size reached a diameter of up to 8 mm. It was never under

minimum diameter value and the lowest diameter measured was 6.5 mm. Figure 24 shows

plotted values for peeled nuggets. Due to breakage of the sheet metal only a small number of

spots could be measured on each coupon, because when the welds got larger size they were

Figure 23 Plotted values for nugget index for series 2.

Figure 24 Robustness requirement never fell below approved limits.

38

nearly impossible to peel. Expulsion occurred on one weld on coupon 19 and three welds on

coupon 25. After 65 coupons, 1 235 welds had been welded without showing any decreasing

trend of the weld size. Therefore, the experiment was stopped with the motivation that the

spot welds never decreased in size as well as it didn’t occur any more expulsion.

HCC regulated current and weld time more than Master mode did. Since it has the possibility

to add several pulses it went from using 4 pulses in the beginning to use 7 pulses at the end.

HCC has a large spread in current and time between the spots, because it adapted more during

the process. Weld time varied between 325 s to 627 s and current varied between 7.76 kA to

8.80 kA. Heat input has high values ranging from 5.6 kJ up to 11.4 kJ, due to the pulsed

process. Energy levels varied as the weld time varied. As seen in Figure 25, the plotted values

are more spread than the values for CCR and Master mode were.

HCC has a slightly decreasing trend in the resistance end, which is the opposite from what

CCR and Master mode had. The decreasing trend is a consequence of that weld time is longer.

The algorithm calculates from the known values when to stop.

The NI curve, shown in Figure 26, only has a slightly decreasing trend which differentiates it

from the other control systems. It is kept stable, since it is the guiding quality parameter that

HCC uses.

Figure 26 Plotted vales for nugget index for series 3.

Figure 25 Graphic window from QA Analysis showing plotted values for series 3.

39

Other observations that was noticed was that the electrode arm got very warm and the

electrode tip changed in a colour that earlier tips welded in other control systems did not.

5.1.3 Analysis - Combination 1

In table 15 results and measures of the nugget diameter from microscopic photographs and

measurements from the peeled nuggets adjacent to the analysed nugget is presented. Since the

nugget can only go through either peeling or cross-section test a comparison were made from

a nugget close by. Nugget 171 and 475 was not possible to measure due to failure when

peeling.

The results showed that the size varied depending on which method was used. The

uncertainty between a cross-section and peeled nugget is added with 15 %.This assumption

was not correct for all welds, some of the peeled nuggets were a lot larger than the cross-

section. However, the mean value of the uncertainty for the spots in the table is 12 %. Since

the sizes are not from the same nuggets the results are not fully comparative.

UT and cross-section were performed on the same nugget. The result for UT turned out to

vary a lot depending on operator. Also, the figures measured were much smaller than the real

value, which is also a known condition. No specific trend could be seen and because of the

uncertainty, the figures are not included in the result.

Table 10 Nugget and peeled weld diameter for series 1, 2 and 3.

Hardness measurement

Due to limited availability, hardness measurement was not made on combination 1. Cross-

sections of the nuggets are shown in appendix 1, 2 and 3.

Test 1 Test 2 Test 3

Spot nr 19 171 362 475 19 247 456 20 135 287 1009

Nugget

diameter

5.9 6 5.5 5.3 5.5 5.5 5.8 6.6 6.7 6.9 6.6

Peeled weld

nugget

7 - 5.8 - 7.1 7 6.1 7.3 7.2 7 7.3

40

5.2 Material combination 2

It was difficult to find parameters that resulted in welds close to target on both sides of the

nugget in the 3 sheet stack. To get weld diameters that were approved on both sides, it had to

be accepted that the nugget on the side with HSLA 340 had a diameter larger than the original

target of 90-110% of target diameter. Figure 27 shows the material combination.

5.2.1 Weld schedule


For series 4, a weld schedule with one pulse was created. The weld parameters and test results

are presented in table 11. 10 spot welds were welded with approved weld size and without

expulsion. It was accepted that HSLA 340 became larger than target diameter because it was

not possible to find parameters for both materials that were on or close to target.

Table 11 Parameters and result for weld schedule series 4.

Weld parameters

Energy

[J]

Weld size

HS-IF

220

[mm]

Weld size

HSLA 340

[mm]

Current

[kA]

Force

[kN]

Squeeze

time [ms]

Weld time

[ms]

Hold time

[ms]

6.5 3 100 350 150 4011 4.9 5.8


The same parameters as for series 4 were chosen to get a comparative result. The parameters

were tested on long coupons and they showed to give a lot of expulsion. Therefore, current

was decreased to 6.25 kA and 10 test welds were run. It showed to give no expulsion and

weld diameters within target size for both materials. Weld extent was set to 100 so the process

in the experiment would have the ability to adapt with doubled time. In table 12, weld

schedule for series 5 is presented, showing the parameters and result for heat input and weld

size.

Figure 27 Cross-section of material combination 2

41

Table 12 Parameters and result for Master mode weld schedule series


On this series, weld schedule optimization tests were run on long coupons to clearer see the

result when changing some of the parameters. Selected weld parameters and results in energy

and weld size for weld schedule series 6 are shown in table 13. Time intervals were set around

the parameters chosen for CCR and Master mode. Some of the parameters definitions have

been replaced with X, Y, Z and U due to company secrecy. In appendix 7 definitions are

shown.


Weld parameters

Energy

[J]

Weld

size

HS-IF

220

[mm]

Weld size

HSLA 340

[mm]

Current

[kA]

Force

[kN]

Squeeze

time

[ms]

Weld

time

[ms]

Hold

time [ms]

Weld

extent

6.25 3 100 350 150 100 4160 5.5 5.6

Pulse

Weld parameters

Q

[J]

Weld size

[mm]

Force

[kN]

Pause

time

[ms]

Time

[ms]

𝐼𝑚𝑎𝑥

[kA]

𝐼𝑚𝑖𝑛

[kA]

Dead

Time

[ms]

X

Y

Z

U

HS-

IF

220

[mm]

HSLA

340

[mm]

1 3 100

37

3963

5.2

6.2

2 3 5 0 5.00

3 3 5 70 7.00 20 1.20

4 3 5 70 8.00 6.00 20 0.5

5 3 5 70 8.00 6.00 20 0.35

6 3 5 70 8.00 6.00 20 0.35 1.10 1.50

7 3 5 70 8.00 6.00 20 0.35 1.05 1.50

8 3 5 70 8.00 6.00 20 0.35 1.05 1.50

9 3 5 70 8.00 6.00 20 0.35 1.05 1.50

10 3 5 70 8.00 6.00 20 0.35 1.05 1.50

11 3 5 70 8.00 6.00 20 0.35 1.05 1.50

12 3 5 150

42

5.2.2 Experiment


The robustness requirement was not fulfilled after 22 coupons and the test was therefore

considered stopped at the last spot weld on previous coupon 21. As illustrated in the graph in

Figure 28, the robustness requirement fell below 75 % of approved welds after spot 418.

The weld parameters set gave expulsion on the shunt welds on coupons 6, 11 and 12. Time

and current levels were constant (CCR). As seen in Figure 29, energy levels increased a bit in

the beginning as the voltage increased, and after about 80 welds it stabilises and then

decreases. This is also where the weld size start to get smaller. The graph represents figures

from 499 spot welds.

Since the parameters time and current are fixed when using CCR, current and time did not

change during the trial. Energy levels were relatively stable, but varied some due to changes

in resistance that affected the voltage values. Resistance end increased and went from 244

µOhm to 293 µOhm. The energy levels decreased slightly from 4.2 kJ to 3.9 kJ. This series

acted similar as series 1 did with material combination 1, CCR.

418 spot

welds

Figure 28 Graph presenting the size decreasing the more spots welds that are welded. The

process went under robustness requirement after 418 spot welds.

43

During the test a drop in the voltage- and energy curves occurred. The wrong material had by

mistake been exchanged in the middle of the sheet stack with a 1.5 mm HS-IF 260 instead of

1.2 mm DP 600. During welding it showed no specific behaviour, the exchanged material was

discovered from the voltage curve that changed because of a different resistance in the

material.

Nugget index is measured for CCR since it is run in a program that also handles HCC. The

curve for NI is shown in Figure 30. The NI decreased during the experiment.

Figure 30 Nugget index for series 4.


During the experiment the process kept the spot welds above robustness requirement until

coupon 40 and the experiment was stopped on spot 760. No expulsion occurred during the

trial. As seen in Figure 31 weld size varied a lot but had an overall decreasing trend.

Figure 29 Graphic window of QA Analysis showing plotted values for series 4. The vertical line shows where the

process went under robustness requirement.

44

Master mode varied both time and current during the experiment to compensate for the

electrode wear. Time and energy were relatively stable while current and resistance changed

opposite to each other. It had the same relation in series 2, Master mode combination 1.

Master mode prioritises to adapt current which is a function in the program. In this case it can

be seen that current is adapted and varies a lot, while time is kept stable in the beginning and

starts to adapt the longer the experiment went. The plotted values from series 5 can be seen in

Figure 32. The graph represents figures from 866 spot welds. Time was mostly stable at 351

ms and with some peaks of 370 ms. Current dipped in the beginning to 6.19 kA and then

increased and stabilised in current values round 6.7 kA. Energy is relatively stable during the

trial on levels of 4.0-4.3 kJ.

Figure 31 Graph presenting the size decreasing the more spots welds that are welded. The

process went under robustness requirement after 760 spot welds.

760 spot

welds

45

Nugget index is decreasing, it was only measured and not used in this trial. In Figure 33 the

plotted values of NI areis shown.


In the beginning of the trial, the spot welds were at its smallest and after 20 spot welds they

grew larger. As for series 3, HCC combination 1, the welds never came below the robustness

requirement. In Figure 34 the weld size is represented for the peeled coupons. No expulsion

Figure 33 Nugget index curve for series 5.

Figure 32 Graphic window from QA Analysis of plotted values for series 5. The vertical line shows where the process

went under the robustness requirement.

Figure 34 Robustness requirement never fell below approved limits.

46

occurred during the experiment. After 43 coupons the series went out of material and since

the process didn’t show any expulsion or trend of decreasing weld size, the experiment was

stopped.

Weld time increased gradually as the process added more pulses. From the beginning the

process used 3 pulses and weld time were kept round 210 ms. For each added pulse, weld

time increased 70 ms as programmed in the weld schedule and at the end of the trial weld

time was kept round 450 ms and 7 pulses. Energy increased as time increased from 3.1 kJ to

6.1 kJ. Plotted values for the parameters can be seen in Figure 35 below. The graph

represents figures from 816 spot welds welded in the series. Current varied between 6.8 kA

and 7.3 kA. Resistance end had a decreasing trend and went gradually from 280 µOhm to 245

µOhm which made impact in voltage that also kept a decreasing trend.

Nugget index is calculated from the shape and amplitude of the weld curves. HCC uses NI as

a parameter to control the other parameters and NI is more stable in this series than the other

series with CCR and Master mode. It had a slightly decreasing trend as seen in Figure 36 but

was kept relatively stable.

Figure 36 Nugget index for series 6.

Figure 11 Robustness requirement never fell under allowed limit for series 6.

Figure 35 Graphic window from QA Analysis of plotted values for series 6.

47

5.2.3 Analysis – Combination 2

In table 14, results and measures of the nugget diameter from microscopic photographs and

measurements from the peeled nuggets adjacent to the analysed nugget is presented. Since the

nugget can only go through either peeling or cross-section test a comparison was made from a

nugget close by. Material HSLA 340 does not have any available measurements because HS-

IF 220 was the only one peeled and measured. This was a decision based on the trend that the

nugget of HSLA 340 was consistently larger than HS-IF 220.

Uncertainty between a cross-section and peeled nugget is added with 15 %. As seen in table

14, the difference in measures between the methods are not as large as for combination 1. The

mean value of the uncertainty for the spots in the table is 6.7 %. Since the sizes are not from

the same nuggets, the results are not fully comparable. The results from UT varied and were

smaller than nugget diameter. Therefore, they are not included in the analysed results since

they were not correct.

Table 14 Measurements for nugget and weld diameter for series 4, 5 and 6.

Microscopic photographs of the nuggets are shown in appendix 5, 6 and 7.

Test 4 Test 5

Spot nr 19 171 361 475 19 475 931

Material 220 340 220 340 220 340 220 340 220 340 220 340 220 340

Nugget

diameter

4.7 4.9 4.9 5.3 4.6 4.8 4.9 4.7 4.6 4.9 4.8 4.9 4.3 5.1

Peeled weld

nugget

4.8 5.3 5.3 n/a 5 n/a 4.4 n/a 5.1 n/a 4.8 5.3 4 n/a

Test 6

Spot nr 20 135 287 800

Material 220 340 220 340 220 340 220 340

Nugget diameter 4.8 5 5.7 5.8 5.8 6 6 6.1

Peeled weld

nugget

4.6 6.7 n/a n/a 6.6 6.6 6.1 6.9

48

Nr of impressions: 33

Hardness measurement

Hardness measurement were completed for all control systems for material combination 2.

Series 4. CCR combination 2

For series 4, hardness measurement was made on spot 19 and 475 to compare difference in

hardness in the beginning and the end of the series. In Figure 37, the results are plotted. The

results showed that for series 4 hardness increased from 216 HV0.3 to 367 HV0.3. In

appendix 4, macroscopic photographs are appended of the spot welds. It can there be seen that

for spot 475 the weld has low penetration. Hardness indentation might have been measured on

different places of the welds for spot 19 and 475. Overall the results between spot 19 and 475

were similar.

50

150

250

350

450

550

1 4 7 10 13 16 19 22 25 28 31 34 37 40

spot 19

HS-IF 220 DP 600 HSLA 340

50

150

250

350

450

550

1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33

spot 475


Figure 37 Hardness measurement for series 4 spot 19 (left) and spot 475(right).


49

Series 5. Master mode combination 2

HS-IF 220 and DP 600 had similar hardness measurements for spot 19 and 931. The results

are plotted in Figure 38. HSLA 340 had a softer zone in the middle where it decreased from

376 HV0.3 on spot 19 to 265 HV0.3 On spot 931 for indentation 3. Microscopic photographs

are appended in appendix 5.

50

150

250

350

450

550

1 4 7 10 13 16 19 22 25 28 31 34 37 40

spot 19


50

150

250

350

450

550

1 4 7 10 13 16 19 22 25 28 31 34 37 40

spot 931


Nr of impressions: 41 Nr of impressions: 41


50

Series 6. HCC combination 2

For series 6, hardness measurements were completed for spot 20 and 800. The results showed

that the hardness were similar for spot 20 and 80. Figure 39 illustrates the plotted values from

the indentations. The only thing that can be seen changing is for spot 20 where a drop in

hardness has occurred in the weld nugget. However, it is still within the approved limits stated

by the Scania standard. Macroscopic photographs of some spots from series 6 are appended in

appendix 6.

5.3 Weld time at spot 300

The time measured for weld time at spot 300 for the series are presented in the table below.

Table 15 Weld time at weld spot 300 for respective series.

CCR Master mode HCC

Weld Sch. comb 1. 300 ms 337 ms 466 ms

Weld Sch. comb 2. 350 ms 350 ms 359 ms

cv 50

150

250

350

450

550

1 4 7 10 13 16 19 22 25 28 31 34 37 40

HCC spot 20

HSLA 340 DP 600 HS-IF 220

cv



50

150

250

350

450

550

1 4 7 10 13 16 19 22 25 28 31 34 37 40

HCC spot 800

HSLA 340 DP 600 HS-IF 220


51

6 DISCUSSION AND CONCLUSION

In this chapter a discussion of the results from the experiments in the thesis are presented. In

the conclusions, research questions that are presented in Chapter 1 are answered. Answers

are based on the information from the results and discussion.

6.1 Discussion

One of the aims with this master thesis was to investigate how the areas stated in purpose,

chapter 1.2, are affected by using the three different control systems available at Scania.

Another aim was to answer how some of the properties of the material and different

combinations could be affected by the different control systems. Those areas are discussed in

the following sections and the research questions are answered.

6.1.1 Robustness of the process

The experiment showed that HCC did not fall below the robustness requirement and it could

weld many more spot welds than CCR and Master mode. As seen in Figure 40, robustness

increased for both material combinations with master mode and further more with HCC.

Important to notice is that series 6, HCC combination 2, was stopped because of material

shortage and if the trend is right it would be able to handle the robustness for a longer time.

The graph also shows that robustness is affected by which material combination that is used.

In this case, material combination 1 required higher current and it might be therefore the

processes got lower results in robustness. The higher current made the electrode get worn

quicker. 1 4

Figure 40 Bar plot showing when material combination 1 and 2 fell under

robustness requirement for each control system

0

200

400

600

800

1000

1200

combination 1 combination 2

Amount of approved spot welds for each combination and control system.

CCR Master mode HCC

52

6.1.2 Expulsion

Expulsion occurred for all control systems but Master mode seemed to have easier to expuld

since the weld schedule for combination 1, series 2, could not be run on the same current as

series 1, therefore both weld schedules had to be reprogrammed and lowered. Also series 5,

Master mode combination 2, could not be programmed with the same parameters as CCR.

When creating weld schedule for combination 1 in CCR, current were first set to 8 kA but

when running the same parameters for Master mode the process gave expulsion on all spot

welds. Therefore the current were set to 7.75 kA for both weld schedules. CCR only spattered

on shunt spots, it could be a sign of that the parameters were a bit high programmed. For

combination 2, Master mode had to have lower current values than CCR 6.25 instead of 6.5,

to be able to handle the process without expulsion.

For HCC it was more difficult to anticipate expulsion. It occurred during series 3 on 4 spot

welds on coupon 19 and 25. Thereafter no more expulsion occurred for HCC.

6.1.3 Weld time

- CCR held constant time during the experiment

- Master mode adapted weld time a little bit but did not use its entire interval of weld

extent.

- HCC was the control system adapting weld time the most. But also being able to weld for

a short time and get approved size.

CCR held constant time during the experiment. The positive side with constant time is if the

time is critical and very important to know. E.g. in the car industry it would be important to

keep track of time but in truck manufacturing, cycle times are generally longer and therefore

more time-consuming weld processes can be accepted. Still there is a trade-off between

robustness and weld time. From the results, it is seen that HCC is more robust and can handle

welding of a large number of welds without tip-dressing. If the interval between tip-dressing

is increased from today’s 150 to 300, it would save production time. The quality of the welds

have shown to be maintained for the size and hardness at 300 spot welds with HCC.

Weld extent was used when running Master mode but the process did not use the parameter to

its full extent. It increased the weld time with 16 % for series 2 and 5 % for series 5. This

53

might be a result of the curves of the reference welds being too narrow and not optimised to

allow the curves to act within its full capacity.

HCC was the control system that adapted time most between short and long weld times. The

weld schedule could have been further optimized but it took a lot of time to create weld

schedules for HCC and due to that, the selected weld schedules were the best developed. In

the beginning of the HCC series, the processes held weld times that were low and if the weld

schedules had been further optimized the times would perhaps have been able to keep lower.

Most increase in time was HCC series 3, where it at the end at spot weld 1200 had weld times

of up to 675 ms.

6.1.4 Surface defects and electrode wear

Neither of the control systems showed surface defects that were not approved by the standard.

HCC showed larger electrode indentation that was less visual attractive, since the nugget

grew, the longer the process went. It could be a problem if a nugget with that kind of

indentation were seen on some visual surface of the cab.

The electrode got warmer for HCC than for the other control systems since it used more

energy for the spot welds. Coolant in the weld gun was not working well on the lower

electrode-tip and since the time between spot welds were round 2.5 to 4 s it did not have time

to properly cool the tip between the welds. This was a machine problem and not something

that could be caused by HCC. This problem was assumed to not have a significant impact on

the results.

6.1.5 Factors affecting the results

A likely reason for the robustness of HCC is the use of a guiding quality parameter that

controls the duration of the welding. NI is kept stable for HCC and adapted the other

parameters such as time and current. It is seen in the results for Master mode and CCR that

resistance end is increasing and for HCC it is decreasing. This can be an indicator of nugget

size changes during the welding trials. When the electrodes got worn the area of the electrode

tip increased and the current density between the electrode and the material decreased. The

current is spread over a larger area and not enough current is used to melt material and get a

54

complete weld. This is the main reason that CCR nuggets decrease in size faster than adaptive

systems.

It was more difficult to create a weld schedule for the three sheet stack, combination 2. The

sheets had different thicknesses with different target sizes and when creating weld schedule

the 340 steel became larger than the 220 steel for each parameter set up. This was a difficulty

for each control system.

It was hard to know when the process was optimised and when it was time to select final

parameters. The weld process acted differently in the beginning when the electrode-tips were

tip-dressed. The process welded with very few pulses with newly dressed electrodes and with

an increased number of pulses for the more worn they got. When creating the weld schedule

the electrodes became a bit worn for every weld parameter that was tried and due to limited

time it was not possible to tip-dress between every parameter optimisation. So, when the weld

schedule had been selected and the experiment started with new-dressed electrodes the first

welds were welded with only 3 pulses and got small weld sizes. This was a repeatedly

behaviour and might be something that is important to consider when creating weld schedule

for the production.

After HCC series 3 had been run, the control system was switched to CCR. HCC had welded

1 235 approved spot welds and a test was performed to see what would happen if the material

now were welded with CCR, with the same worn electrodes from the HCC trial. The weld

nuggets from the test were incomplete or non-existing.

6.1.6 Hardness

No specific differences could be seen in hardness between the different control systems. The

result that were found in the article [3] cannot be substantiated by this report.

55

6.2 Conclusion

The research questions have been answered and the following conclusions can be made in this

study:

I. What are the advantages/disadvantages with using the adaptive control systems, HCC

and Master mode in RSW compared to the fixed system CCR, for selected

applications?

Advantages with adaptive processes:

o More robust process

o The same weld schedule for HCC can be used for several material

combinations. It is more flexible and time is saved on not having to create weld

schedule for each material combination in the production.

o HCC has high reliability creating the right nugget size.

Disadvantages with adaptive processes:

o More difficult and time consuming to program the adaptive control systems to

find optimal parameters.

o Master mode showed to be more inclined to expuld.

o Less control of the weld time.

II. Are there differences in weld properties due to the control system used?

From what can be seen in these experiments, the material had hardness results that

normally can be expected. The hardness curves had a normal appearance and

behaviour. No further microstructural investigations were made.

When creating weld schedule, the parameters selected for HCC should be tested and

evaluated with both tip-dressed and worn electrodes. Since the process can act very

different depending on how worn the electrodes were when creating the weld

schedule.

III. Does the result comply with the Scania standard?

All processes managed to fulfil the robustness requirement within 150 spot welds

(today’s interval for tip-dressing). For all series except series 1 they were robust over

300 spot welds. Spot 300 HCC had an approved nugget size and robust process for

both material combinations.

56

The hardness of the materials was kept within hardness requirement as stated in the

Scania standard.

57

7 RECOMMENDATIONS AND FUTURE WORK

It would probably be possible to increase the interval between tip-dressing in the

production. Approximately to start with increasing the interval from 150 to 300 spots.

Further investigations can be done for different materials. To see how the time

increase caused by HCC can have an impact on different materials. Due to problems in

delivery it was not possible to investigate boron steel as the plan was from the

beginning.

Investigate how weld schedules in production handle the processes. Further

development and optimisation would probably give better weld times and less

expulsion.

Investigate the electrode and do further evaluation. It could be seen changes in

electrodes colour when using HCC that was not seen with CCR and Master mode.

Find out if it can lead to any problem or if it is unimportant.

58

8 REFERENCES

[1]: ”Scania Oskarshamn ställer om produktionen – inhyrda medarbetare berörs”, Scania, 2

february 2018, [Online] Available:

https://www.scania.com/productionunitoskarshamn/sv/home/media/nyheter/emma-en-

teknisk-lagspelare1811111112111.html [Accessed: 19 april, 2018]

[2]: Kimchi, M. and Phillips, D, H., (2018), Resistance Spot Welding: Fundamentals and

Applications for the Automotive Industry, Ohio, Morgan & Claypool publisher, ISBN

9781681731711

[3]: N. D. Raath, D. Norman, I. McGregor, R. Dashwood, and D. J. Hughes, “Effect of

Weld Schedule on the Residual Stress Distribution of Boron Steel Spot Welds,”

Metallurgical and Materials Transactions A, vol. 48A, s. 2900-2914, june 2017 [online]

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Appendix 1

Photographs of the spots from series 1 that were examined in microscope are appended in

appendix 1. The photographs show the cross-sections of the two-sheet combination. The

measurements show the diameter of the nugget and the depth of the electrode-indentation.

Series 1, spot 19 Series 1, spot 171

Test 1, spot 361 Test 1, spot 475

Appendix 2





Series 2, spot 456

Appendix 3




Series 3, spot 21 (17), spot 135 (18) Series 3, spot 287

Series 3, spot 1009

Appendix 4


appendix 4. The photographs show the cross-sections of the three-sheet combination. The




Appendix 5





Series 5, spot 931

Appendix 6




Series 6, spot 20 (21) Series 6, spot 135 (22)

Series 6, spot 287 (23) Series 6, spot 800 (24)

TRITA ITM-EX 2018:541

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