Materials

Materials and Design 25 (2004) 343–347

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&Design

Technical note

The influence of stirrer geometry on bonding andmechanical properties in friction stir welding process

Mustafa Boz a,*, Adem Kurt b

a Institute of Science and Technology, Gazi University, Ankara, Turkeyb Technical Education Faculty, Gazi University, Ankara Turkey

Received 7 July 2003; accepted 4 November 2003

Abstract

In this study, Al 1080 alloy materials were welded using friction stir welding process. The influence of stirrer design on the welding

process was investigated. For this purpose, five different stirrers, one of them square cross-sectioned and the rest were cylindrical with

0.85, 1.10, 1.40 and 2.1 mm screw pitched were used to carry out welding process. Bonding could be effected with the square, 0.85 and

1.10 mm screw pitched stirrers. Microscopic examination of the weld zone and the tension test results showed that the best bonding

was obtained with 0.85 mm screw pitched stirrer. In addition, temperature distribution with in the weld zone was also determined.

� 2003 Elsevier Ltd. All rights reserved.

1. Introduction

Friction stir welding (FSW), a solid-state welding

process invented out at TWI (Cambridge, United

Kingdom) in 1991 [1]. FSW was developed and patented

in the early 1990 [2,3]. FSW is perhaps the most re-markable and potentially useful new welding technique.

It has made it possible to weld, in a simple manner, a

number of materials that were previously extremely dif-

ficult to be reliability welded without voids, cracking or

distortion. Several industrial companies are conducting

pilot studies to use this technique in production [2,4].

Although FSW can be used to join a number of

materials, the primary research and industrial interesthas been to join aluminium alloys. Defect-free welds

with good mechanical properties have been made in a

wide variety of aluminium alloys, even those previously

thought to be �unweldable� in thickness from less than

1 mm to more than 35 mm. In addition, Friction stir

welds can be accomplished in any position [5–11]. Ex-

ploratory development work has encompassed alumin-

ium materials from 1 to 75 mm thick [12]. In this rather

* Corresponding author. Tel.: +90-312-212-39-93; fax: +90-312-212-

00-59.

E-mail addresses: mboz@gazi.edu.tr (M. Boz), ademkurt@

gazi.edu.tr (A. Kurt).

0261-3069/$ - see front matter � 2003 Elsevier Ltd. All rights reserved.

doi:10.1016/j.matdes.2003.11.005

remarkable process, a rotating steel pin having a di-

ameter commensurate with the thickness of metal plates

to be welded, is rotated at speeds >300 rpm [13,14].

The basic principle of the process is illustrated in Fig. 1

[7].

The process produces frictional heat between a ro-tating tool of harder material than the work piece being

welded, in such a manner as to thermal condition the

abutting weld region in the softer material [7,15].

However, the FSW zone is always characterized by dy-

namic recrystallization which arises through either lo-

calized or large-scale shear instabilities forming narrow

or extended adiabatic shear bands [16–18]. FSW is a

process that has been shown to produce superior as-welded mechanical properties when compared to typical

arc welding process in other aluminium alloys such as

5083, 6061 and 2219 [19].

FSW has many advantages, including the following

[4,5,8]:

• The welding procedure is relatively simple with no

consumables or filler metal.

• Joint edge preparation is not needed.• Oxide removal prior to welding is unnecessary.

• The procedure can be automated and carried out in

all positions.

• High joint strength has been achieved in aluminium

and magnesium alloys.

Fig. 1. Schematic description [7].

344 M. Boz, A. Kurt / Materials and Design 25 (2004) 343–347

• FSW can be used with alloys that cannot be fusion

welded due to crack sensitivity.

In this study, the effect of stirrer geometry on the

weldability and mechanical properties of welded alu-

minium plates using FSW process was investigated. The

stirrer was rotated at 1000 rpm and friction pressure was

held constant.

2. Experimental procedure

In this study, Al 1080 was used. The chemical com-

position of Al 1080 is given in Table 1. To carry out

FSW process, two aluminium plates, 5 mm in thickness

and 60 mm in width, were place on a flat metal plate.These two aluminium plates were then clamped with a

vice so that they would not separate during welding

process. Specially prepared stirrers, as shown in Fig. 2,

were pressed against the bonding line and the welding

process was started. The length of the stirrer was same

as the required welding depth. The welding process was

carried out by rotating the stirrer at 1000 rpm and by

moving the plates at 200 mm/min speed under a con-stant friction force. During welding, temperature mea-

surements were performed using thermocouples at

various parts of the aluminium plates, from the welding

centre to outwards.

By using the predetermined welding parameters, dif-

ferent samples were welded for mechanical tests and

metallographic examinations. Tension, bonding and

hardness measurements were made as mechanical tests.Scanning electron microscopy (SEM) and optical mi-

Table 1

Chemical composition of aluminium

Al Si Fe Cu Mn Mg Zn

99.33 0.121 0.409 0.022 0.013 0.019 0.037

croscopy examinations were carried out on the weld and

base metals, where necessary photographs were taken.

3. Results and discussion

Welding processes were carried out with five differentstirrers, four of them were screw type with 0.85, 1.10,

1.40 and 2.0 mm pitch and one was a bar with 5 mm� 5

mm square cross-section. 1.40 and 2.0 mm pitched

stirrers acted like a drill rather than a stirrer and com-

pelled the weld metals outwards. As a result, this weld

metal was accumulated towards the stirrer shoulder as

depicted schematically in Fig. 3 and therefore the

welding process could not be effected. Thus, no furtheruse of these two stirrers was made. The specimens wel-

ded using 0.85, 1.10 pitched and square cross-section

stirrers were prepared as tension test specimens and

these are seen in Fig. 4. Tension tests were performed on

these specimens and the results are given in Table 2.

As can be seen from the results, the specimen welded

with cross-section stirrer has 60 N/mm2 ultimate tensile

strength (UTS) and fracture took place within the weldmetal. This strength value is approximately 54% of that

of the base metal. During stirring, this type of stirrer

design sweeps a large amount of metal from the plasti-

cised zone and results in an inhomogeneous structure.

Intensive porosity and cracks in the stirring zone can be

seen from the micrographs in Fig. 6.

In the specimens welded using 0.85 and 1.10 mm

pitched stirrers, fractures of the both specimens tookplace in the base metals and these two specimens had

Ni Cr Pb Sn Ti Sb

0.085 0.044 0:026 < 0.050 0:021 < 0.030

Fig. 3. Weld metal accumulated to stirrer and rotating shoulder.

Fig. 2. The geometry of the stirrers used in the FSW process.

M. Boz, A. Kurt / Materials and Design 25 (2004) 343–347 345

110 N/mm2 UTS (Fig. 4). This value is in conformance

with the theoretical UTS of Al 1080. No damage was

observed in the welding zone. This result indicates the

weld metal left by the stirrer exhibits higher strength

than the base metal. The higher strength of the weld

metal can be attributed to heat generation during stir-

ring. This heat is thought to reduce hardness of Al in the

weld zone (Fig. 5) and to improve plasticity.

Fig. 4. Bending and tens

Table 2

Mechanical properties of welded specimens

Elongation (%)

Stirrers 0.85 mm screw pitch 15.36

1.10 mm screw pitch 13.84

Square 5

For three point bending, specimens of 5 mm� 25

mm� 120 mm in dimensions were prepared from the

welded specimens perpendicular to welding direction

and these specimens were subjected to 180� bending test.

The tests were performed under 3 N/mm2 load and at 2mm/min bending speed. After bending, HAZ could be

seen with naked eye. However, no micro cracks in the

weld metal and HAZ was observed. This shows that the

welding has adequate bending strength. As can be seen

from the metallographic observations, there are three

different zones in the friction stir welded specimens. The

measured hardness values show variations in these

zones, Fig. 5(a). Yutaka [1] also reported the existenceof these three zones in his study. The region labelled as c

retains the same grain structure as the base material.

Region b is characterized by recovered grains containing

a high density of sub-boundaries, which is identified as

the thermal mechanically affected zone in previous

studies [9]. Region a is characterized by recrystallization

arising from frictional heating and plastic flow during

the welding [19]. The classification of grain structure isnot consistent with the horizontal hardness profiles in

the weld. This is because, the hardness profile in the age-

hardenable aluminium depends strongly on precipitate

distributions, as stated in previous studies, rather than

on grain size [20].

The precipitate distributions and the consequent

hardness profiles are affected mainly by local thermal

hysteresis. During welding the frictional heat is generatedat the weld centre by the rotating head-pin and on the

upper surface of the weld zone by the rotating tool

shoulder [20]. Horizontal profiles of Vickers hardness in

the weld are indicated in Fig. 5(a). The location of a, b, c,

ile tests specimen.

Reduction in cross-

sectional area (%)

UTS (N/mm2)

26 110

23 111

8 60

Fig. 5. (a) Vickers hardness of sample at horizontal profiles, (b) horizontal profiles of welding direction, A: Recristallization zone; B: Recovered zone;

C: Base metal.

346 M. Boz, A. Kurt / Materials and Design 25 (2004) 343–347

are indicated by three arrows in Fig. 5(b). The average

hardness of the solution-treated base material is shown

by a line in Fig. 6(a). There is considerable softening

Fig. 6. Microstructures of the specimens welded using different

Fig. 7. SEM microstructures of (a) stir zo

throughout the weld zone, compared to the base mate-

rial. The minimum hardness is located around 12 mm

away from the weld centre. The softening can be seen

stirrers (a) 0.85 mm screw pitch stirrer, (b) square stirrer.

ne, (b) weld metal, (c) base metal.

M. Boz, A. Kurt / Materials and Design 25 (2004) 343–347 347

within about 20 mm of the weld centre. The outside re-

gion retains the base material hardness, since the hard-

ness in the base material is varied between 38 and 42 HV.

Fig. 7 gives the microstructures of the friction stir

welded Al 1080 together with the temperatures duringwelding. Temperature measurements were performed at

7 points, from the weld centre towards the base metal.

The following observations were made: 337 �C at the

weld centre, 289 �C at the transition zone, 232 �C at the

HAZ and 198 �C 35 mm away from the weld centre at

the base metal. Especially, the temperatures of the stir-

ring zone and HAZ are within the recrystallisation

temperatures of Al 1080. Thus, decreasing hardness atthe weld centre and increasing elasticity can be explained

by this temperature.

4. Conclusions

In this study Al 1080 material was bonded using FSW

process. 1.40 and 2.0 mm pitched stirrers acted like adrill rather than a stirrer and compelled the weld metal

outward in the form of chips. The weld metal was ac-

cumulated towards the stirrer shoulder as depicted

schematically in Fig. 3 and therefore the welding process

could not be effected. The best bonding was obtained

with 0.85 and 1.10 mm pitched stirrers. Both the speci-

mens welded using 0.85 and 1.10 mm pitched stirrers

exhibited the same mechanical and metallographicproperties. Bonding could be effected with square cross-

section stirrer but poor mechanical and metallographic

properties were observed. This reduction in the prop-

erties was attributed to the weld material transfer like a

large mass to the adjacent base metal.

It was seen that bonding with FWS consisted of three

regions, namely: recrystallised, recovered and HAZ

zones. The specimens welded using 0.85 and 1.10 mmpitched stirrers exhibited 110 N/mm2 UTS and fractures

took place in the base metal. This showed that weld

metal strength was higher than theoretical strength of

the base metal.

Temperature measurement during welding was car-

ried out. To increase the welding speed and to prevent

stirrer wear, the use of low heat conductive materials

(like ceramic) between the plate and the specimens canbe suggested. FSW process makes it possible that fric-

tion welding can also be applied to non-cylindrical parts.

This process is quite cost-effective in welding low melt-

ing temperatures materials like aluminium.

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