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Proceedings from Processing and Fabrication of Advanced Materials XV

October 15–19, 2006, Cincinnati, Ohio, USA

INFLUENCE OF FRICTION STIR WELDING

ON

MICROSTRUCTURAL DEVELOPMENT,

MECHANICAL RESPONSE AND FRACTURE BEHAVIOR

OF

ALUMINUM ALLOY 2024

T.S. Srivatsan, Satish Vasudevan

Division of Materials Science and Engineering

Department of Mechanical Engineering

The University of Akron

Akron, Ohio 44325-3903 E-Mail: [email protected]

R.J. Lederich

THE BOEING COMPANY

PHANTOM WORKS

Advanced Manufacturing Research & Development

P.O. Box 516 – M/C S245-1003

St. Louis, MO 63166

© 2006 ASM International. All Rights Reserved(#5201G)

www.asminternational.org

Abstract

In this paper, the salient intricacies of friction stir welding of

aluminum alloy 2024 are highlighted. Influence of welding

parameter, primarily the heat flow conduction path, on

microstructural development of a 0.125 in (3 mm) thick plate of the

aluminum alloy is presented. The conjoint influence of weld

parameter and intrinsic microstructural features on hardness,

tensile response and quasi-static fracture behavior is presented and

discussed. The tensile properties and fracture behavior of the

welded sample is compared with the unwelded counterpart. The

microscopic mechanisms governing quasi-static fracture are

rationalized in light of intrinsic microstructural features of the

welded sample, deformation characteristics of the alloy

microstructure, and nature of loading.

Key Words: Friction stir welding, aluminum alloy 2024,

Microstructure, Hardness, Tensile Properties, Fracture

© 2006 ASM International. All Rights ReservedProcessing and Fabrication of Advanced Materials XV (#05201G)


Introduction

The emergence of a novel friction welding process as a viable

solid state joining technique for the family of non-ferrous metallic

materials was termed as Friction Stir Welding (denoted henceforth

through this manuscript as FSW). The technique was invented at The

Welding Institute (TWI), United Kingdom in the early 1990s was

initially tried on aluminum alloys [1, 2]. The FSW technique did prove

itself ideal for creating good quality butt joints and lap joints [3-6] in a

number of materials to include those that are extremely difficult to weld

by conventional fusion welding processes [7]. The basic principle of the

process is shown in Figure 1 [8]. In this technique a non-consumable

rotating tool with a specifically designed pin, also known as probe, and

shoulder is inserted into the abutting edges of the plates or sheets that are

to be joined and progressively traversed along the line of the joint

(Figure 2) [9]. The pin initially makes contact as it is plunged into the

joint region. The friction arising from the initial plunging heats up a

cylindrical column of metal around the pin as well as a small region of

the metal immediately below the pin. The localized heating arising from

friction between the tool and the work piece promotes localized micro-

plastic deformation of the work piece. The localized heating aids in

softening of the material that is immediately around or adjacent to the

pin. This coupled with a combination of tool rotation and a gradual

translation through the material to be joined facilitates a movement of

material from the front of the pin to the rear of the pin. This process

results in the production of a joint in the solid state.

The depth of penetration into the material being joined is

controlled by length of the pin below the shoulder of the tool. The

shoulder, which is in contact with the material being joined, applies

additional frictional heat to the weld region and aids in preventing the

highly plastic material from being expelled during the welding operation.

The softened region of the material due to the generation of friction heat

is wider at the top surface that is in direct contact with the shoulder and

gradually tapers down to the pin diameter. The combined friction heat

from the pin and the shoulder of the tool creates a near plastic condition

around the immersed pin and at the contact surface of the shoulder with



Figure 1: A schematic of the friction stir welding technique

(Ref 8)



Figure 2: A comprehensive sketch outlining the friction stir

welding technique (Ref. 9)

the top surface of the work piece. The material gradually flows around

the tool and coalesces behind the tool as relative movement of the

rotating tool on the substrate (work piece) takes place. The noticeable

advantages of this technique are:

(a) The consolidated welds are solid phase and reveal no evidence of

fusion welding defects.

(b) No consumable filler material, shielding gas or edge preparation

is normally used to get the final weld.

(c) The distortion of the work-piece is significantly less than that

caused by the traditional fusion welding techniques.

The simultaneously rotation and translational motion of the

welding tool during the welding process creates a characteristic

asymmetry between the adjoining sides. Where the tool rotation is in the

same direction as translation of the welding tool the side is referred to as

the advancing side. On the other side where the rotation and translation

motions of the tool counteract with each other this is referred to as the

retreating side.



Overall, the technique of FSW is considered a significant

development in metal joining since its inception in the early 1990s and is

referred to as the “green technology” due to its energy efficiency,

environment friendliness and versatility [9]. When compared to the

conventional welding methods, FSW consumes considerably less energy.

Further, since no cover gas or flux is used the process can be categorized

as being environmentally friendly. Since the technique involves joining

without the use of filler material any aluminum alloy can be joined

without appreciable concern for the compatibility of composition as is

the case with conventional fusion welding processes [10-12]. The

technique can be applied to a variety of joints like butt joints, lap joints,

T-butt joints and fillet joints [13].

In this paper, we present the results of a study on

characterization of microstructural development, hardness, tensile

properties and quasi-static fracture behavior of 3 mm thick plate of

aluminum alloy 2024 (AA2024). This is an alloy that finds exhaustive

use in the aerospace industry. Two panels were welded with a different

clamping and anvil arrangement to facilitate a difference in heat

dissipation from the welded alloy plate. A rationalization of quasi-static

failure is made in light of intrinsic microstructural features of the FSW

plate, deformation characteristics of this precipitation hardened wrought

aluminum alloy and nature of loading.

Material and Sample Preparation

The material chosen was a rolled aluminum alloy 2024 plate

having a thickness of 3 mm. The nominal chemical composition of the

Al-Cu-Mg alloy is given in Table 1. The friction stir welded plates of

the alloy plate were tested in the T8 temper. Two sets of panels were

welded using a different clamping and anvil arrangement. The low

ductility panels were welded such that the heat was rapidly dissipated

through the lower backing plate. The panel with higher ductility was

clamped such that heat was drawn out towards the outside edges, rather

than downwards. The two FSW panels are referred to henceforth as

Plate A and Plate B. The weld direction was parallel to the rolling

direction of the plate.



From each welded plate, twelve specimens were precision

machined in conformance with specifications outlined in ASTM standard

E8. The surfaces of the machined specimens were fine ground with 600-

grit silicon carbide impregnated emery paper to remove scratches and

machining marks. Prior to machining of the test specimens 20 mm at the

beginning and 20 mm at the end of each welded plate (Plate A and Plate

B) were removed to exclude possible deviation from steady state start

and stop.

Experimental Procedures

Microstructural Characterization

Samples for metallography observations were cut from both the

FSW plates (Plate A and Plate B). The cut samples, 1.0 inch square in

cross-section, were mounted in Bakelite and then wet ground on

progressively finer grades of silicon carbide impregnated emery paper

using copious amounts of water both as lubricant and as coolant.

Subsequently, the ground samples were mechanically polished using

five-micron diamond suspension suspended in distilled water. Fine

polishing to a perfect mirror-like finish of the surface was achieved using

one-micron diamond solution as the lubricant. The polished aluminum

alloy sample of Plate A and Plate B were etched using Keller’s reagent (a

solution mixture of hydrofluoric acid, concentrated nitric acid and

distilled water). The etched surfaces of each sample containing the weld

region was observed in an optical microscope and photographed using a

bright field illumination technique.

Mechanical Testing

Hardness is a quantifiable mechanical property of a material. At

the microscopic level, it is a measure of resistance of the material to

indentation and cracking [14]. Macrohardness testing using a Rockwell

hardness tester is capable of providing useful information on hardness

variation through different zones of the FSW plate. In this experiment,

the Rockwell-B macrohardness of the samples was measured. Following



application of the major load the hardness value was directly read of the

scale of the hardness-testing machine. The indentation load used was

100 kgf for a dwell time of 10 seconds. Five indents were made on the

polished surface of each sample covering the regions of the base metal

(parent metal), heat affected zone (HAZ) and the weld. The results

reported are the average value for each region.

Uniaxial tensile tests were performed on fully automated, closed

loop servo hydraulic mechanical test machine [Model: INSTRON- 8500

plus] using a 100 KN load cell. The tests were conducted in the room

temperature (300 K) laboratory air environment (55% Relative

Humidity). The specimens were deformed at a constant strain rate of

0.0001 s-1. An axial 12.5-mm gage-length clip-on extensometer was

attached to the test specimen at the gage section with rubber bands. The

stress and strain measurements, parallel to the load line, and resultant

mechanical properties of stiffness, strength (yield strength and ultimate

tensile strength) and ductility (strain to failure) was provided as a

computer output by the control unit of the test machine.

Failure-Damage Analysis

Fracture surfaces of the uniaxially deformed and failed test

samples were comprehensively examined in a scanning electron

microscope (SEM) to: (a) determine the macroscopic fracture mode, and

(b) characterize the fine-scale topography of the fracture surface to

establish the microscopic mechanisms governing fracture. The

distinction between macroscopic mode and microscopic fracture

mechanisms is based entirely on the magnification level at which the

observations are made. The macroscopic mode refers to the overall

nature of failure, while the microscopic mechanisms relate to local

failure processes: (i) microscopic void formation, (ii) microscopic void

growth and coalescence, and (iii) nature and intensity of macroscopic

and microscopic cracking. Samples for SEM observation were obtained

from the deformed and failed specimens by sectioning parallel to the

fracture surface.



Results and Discussion

Macrostructure

The macrostructure of the two plates are consistent with those

reported by other authors on aluminum alloys [15]. The plates revealed a

classical nugget structure containing the presence of features that are

termed as “onion rings”. The shallow 'onion ring' like features found in

the weld zone are an evidence of the characteristic material transport

phenomenon occurring during friction stir welding. A simple extrusion

of one layer of semi-cylinder during one rotation of the tool and a cross-

sectional slice taken through such a semi-cylinder results in the onion

ring structure. The formation and presence of ‘onion ring’ pattern is also

aided by a reflection of material flow at the imaginary walls of the

groove that would exist for the situation of regular milling of the metal

[16]. The induced circular movement results in circles that gradually

decrease in radii and form a tube. An insight into the mechanism of

‘onion ring’ formation would facilitate an understanding of material flow

during FSW.

The FSW technique can be visualized as a metal working

process and comprises the conventional metal working zones of (i) pre-

heat, (ii) initial deformation (plastic), (iii) extrusion (iv) forging, and (v)

cool-down (Figure 3). The pre-heat zone occurs ahead of the pin. In

this zone, the temperature rises due to frictional heating of the rotating

tool coupled with adiabatic heating because of deformation of the

material. The heating rate in this zone and the severity of its influence is

governed by

(i) Thermal properties of the aluminum alloy,

(ii) Transverse speed of the tool.

The movement of the tool causes an initial deformation zone to form in

which the 2024 aluminum alloy is heated to above a critical temperature

and the magnitude of stress exceeds the critical flow stress of the

material resulting in conditions that are conducive for flow of the

material. The metal in this zone is forced both upwards into the zone of

the shoulder and downwards into the zone of extrusion. In the extrusion

zone, having a finite width, the metal flows around the pin from the front



to the rear. Adjacent to the extrusion zone is the forging zone where

metal from front of the rotating and translating tool is forced into the

cavity left behind by the forward moving pin under conditions of

hydrostatic pressure. Behind the moving pin and the forging zone is the

cool down zone also known as the post-heat zone, where the metal

gradually cools under passive conditions.

Figure 3: A schematic that depicts the metal flow pattern

during friction stir welding.

Initial Microstructure

The optical microstructures of the two plates are shown in

Figure 4 and 5. For this aluminum alloy (AA2024-T8) a well-defined

heat-affected zone of shallow width surrounds the weld nugget region.

The base metal revealed large recrystallized grains of non-uniform size

with a non-uniform distribution of the coarse second-phase particles

(Figure 4c and 5c). At higher magnification the weld nugget region



revealed the occurrence of full dynamic recrystallization comprising very

fine equiaxed grains (Figure 4a and 5a ).

Considering the width of the recrystallized region and its

homogeneity, the microstructural changes adjacent to the nugget, i.e.,

thermo-mechanically affected zone (TMAZ), are driven by the conjoint

influence of heat generated by the tool and the deformation induced by

the tool pin (Figure 4b and 5b). The high work hardened state of the

parent material caused by movement of the tool implies that it has a

highly unstable microstructure that readily crystallizes under the locally

high temperatures generated during welding. Exhaustive examination of

the sample in the optical microscope revealed the interface region

between the recrystallized nugget zone and the base metal to be

relatively diffuse on the retreating side of the tool, and quite sharp on the

advancing side of the tool. Beyond the TMAZ there is a heat affected

zone (HAZ). This zone experiences a thermal cycle but does not

undergo any plastic deformation.

Hardness

The observed variations in microstructure results in variations in

hardness taken through the three regions, base metal, heat-affected zone

and weld nugget. The macrohardness traverse taken across Plate A and

Plate B are summarized in Table 2. The weld nugget, TMAZ and the

HAZ are softer due to the dissolution and coarsening of strengthening

precipitates during the thermal cycle. Fine equiaxed grains in the weld

nugget have replaced the initial microstructure of the base material.

These equiaxed grains have little substructure typical of a recrystallized

microstructure and similar to that reported for 5XXX, 6XXX and 7XXX

series aluminum alloy stir welds [17-21]. .



(a)

(b)

(c)

Wel

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ure

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Tensile Properties

The room temperature tensile properties ar ummarized in

Table 3. The results reported are the mean values based on duplicate

tests. The yield strength and ultimate tensile strength of the samples, cut

traverse to the weld direction to include the weld zone at the center of the

machined test sample, are inferior to the unwelded AA2024-T8

counterpart. The degradation in yield strength of the FSW sample is

27% with respect to the u lded counterpart, while degradation in

tensile strength is 16% for sample taken from Plate B and only 8% for

sample taken from Plate A. The friction stir welded ples from both

Plate A and Plate B failed at considerably lower strain values than the

unwelded counterpart. This is rationalized to be due to localization of

strain occurring in areas softened by the friction stir welding process

resulting in a comparatively low overall strain to failure, i.e. elongation.

The degradation in strain-to-failure (tensile elongation

(i) Plate A welded specimen is 25% with respect to the unwelded

counterpart, and

(ii) Plate B specimen is 60% with respect to the unwelded

counterpart.

The degradation in ductility quantified in terms of reduction in test area

is as high as 70% for both Plate A and Plate B FSW specimens. A

comparison of the engineering stress versus engineering strain curves of

the FSW plates and the unwelded counterpart is shown ure 6.

Tensile Fracture

An examination of the tensile fracture surf es provide

useful information pertaining to the role and/or contr of intrinsic

microstructural features on strength and ductility of the FSW AA 2024-

T8. An examination of the fracture surfaces revealed a noticeable

difference in both macroscopic morphology and microscopic

mechanisms for the unwelded and welded test samples. Representative

fracture surface features of the samples are shown in Figures 7-9.

e s

the

sam

) of:

in Fig

aces do

ibution

nwe



le, tensile fracture of the unwelded alloy

) was by shear with the fracture surface inclined at 45

degrees

8a). Higher

layered t of locally brittle failure (Figure 8b). In the

features f locally ductile and brittle failure mechanisms.

weld

zone in a direction normal to the far-field stress axis. Overall

morpho her magnifications was predominantly

igure 9a) with a distinct absence of layered ledges or

groove-

On a macroscopic sca

sample (2024-T8

to the far-field stress axis. Overall morphology of the fracture

surface was rough with the surface comprising ductile and brittle failure

regions. High magnification observations of tensile fracture surface

revealed an overall transgranular failure (Figure 7a). At the higher

magnifications of the scanning electron microscope was evident fine

microscopic cracks and a population of voids of varying size distributed

through the fracture surface (Figure 7b). Cracked second-phase

particles and shallow dimples were found covering the transgranular

fracture region (Figure 7c). In the region of overload were evident shear

dimples and microscopic voids of varying size and shape, features

reminiscent of locally ductile failure (Figure 7d).

Scanning electron micrographs of the FSW Plate A of AA2024-

T8 is shown in Figure 8. Overall morphology was distinctly brittle with

failure occurring normal to the far field stress axis (Figure

magnification revealed an array of macroscopic cracks in the form of

ledges reminiscen

transgranular region was evident tear ridges and fine microscopic cracks,

reminiscent o

The tensile overload region was covered with elongated dimples and fine

microscopic voids reminiscent of locally ductile failure.

Scanning electron microscopy observations of the FSW sample

taken from Plate B revealed macroscopic failure to occur in the

logy at the hig

transgranular (F

like features. High magnification observations in the

transgranular region revealed features similar to the unwelded

counterpart, i.e. a population of fine microscopic cracks and voids of

varying size and shape (Figure 9b). The overload region was covered

with dimples of varying size and shape, reminiscent of locally ductile

failure (Figure 9c). The dimples were shallow (Figure 9d).



Mechanisms Governing Deformation and Fracture

During far-field straining in the uniaxial direction the occurrence

of localized inhomogeneous deformation is favored in the weld region.

The inhomogeneous deformation exacerbates stress concentration in this

region resulting in macroscopically brittle failure. The localization of

strain is also favored by the interactive influences of high level of critical

resolved shear stress and the shearable nature of strengthening

precipitates in the alloy microstructure in the T8 temper. The highly

localized deformation is conducive for the formation of an array of

macroscopic cracks or ledges at the microscopic level, as was observed

in the FSW sample taken from Plate A. Only few of the microscopic

voids had undergone considerable growth and their coalescence results in

process of fine

shallow dimples. The formation and presence of a sizeable population of

voids transforms the deforming AA2024-T8 sample into a composite

material at the microscopic level comprising two populations of

particles:

(1) The grains in the matrix.

(2) Voids (void being considered as a particle having zero stiffness).

Since the voids are intrinsically softer than the hardened grains in the

matrix, the local strain is exacerbated for the microscopic voids causing

as a result an increase in their volume fraction. The

microscopic cracks and a population of voids of varying size transform

the mechanical response of the FSW samples through a noticeable

degradation in elongation to failure or tensile ductility.



Conclusions

1. The plates revealed a classical nugget structure including the

presence of features that are termed as “onion rings”. The

shallow 'onion ring' like features found in the weld zone are an

evidence of the characteristic material transport phenomenon

occurring during friction stir welding.

2. Well-defined thermo-mechanically affected and heat-affected

zones of shallow widths surround the weld nugget region. The

base metal revealed large recrystallized grains of non-uniform

size with a non-uniform distribution of the coarse second-phase

particles. The weld nugget region revealed the occurrence of full

dynamic recrystallization comprising very fine equiaxed grains.

3. The yield strength and ultimate tensile strength of the samples

machined from the FSW

AA2024-T8 counterpart.

plates are inferior to the unwelded

rationalized to be

due to the localization of strain that occurs in areas softened by

the friction stir welding process.

. On a macroscopic scale, tensile fracture of the unwelded alloy

sample (2024-T8) was by shear with the fracture surface inclined

at 45 degrees to the far-field stress axis. At the higher

magnifications was evident fine microscopic cracks and a

population of voids of varying size distributed through the

fracture surface, features reminiscent of locally ductile failure.

The degradation in yield strength of

the FSW sample is 27% with respect to the unwelded

counterpart. The degradation in tensile strength is 16% for Plate

B and only 8% for Plate A friction stir welded sample with

respect to the unwelded counterpart.

4. The friction stir welded samples fail at considerably lower strain

values than the unwelded counterpart. This is

5



6. Overall mo

A revealed d

rphology of the friction stir welded sample from Plate

istinctly brittle failure occurring normal to the far

milar to the unwelded

C

field stress axis. Higher magnification revealed an array of

macroscopic cracks in the form of layered ledges reminiscent of

locally brittle failure. Observations of the FSW sample taken

from Plate B revealed macroscopic failure to occur in the weld

zone in a direction normal to the far-field stress axis. The

transgranular region revealed features si

counterpart, i.e. a population of fine microscopic cracks and

voids of varying size. The overload region was covered with

dimples of varying size and shape, reminiscent of locally ductile

failure.

Table 1: Nominal chemical composition of Aluminum

Alloy 2024 (in weight percent)

u Mg Mn Fe Si Zn Ti Aluminum

4.45 1.36 0.71 0.5 0.5 0.2 0.15 Balance



Ta

ble

2:

Ma

cro

ha

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mea

sure

men

ts o

n f

rict

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ded

pla

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02

4-T

8

Ind

enta

tion

load

on

w

: 1

00

kil

og

ram

s

We

Z

HA

Z

BM

1

ldT

MA

BM

2

BM

3

BM

4

Sam

ple

1R

B7

1

75

8

1

72

80

7

9

80

UT

S(M

Pa

)

51

0

42

74

62

44

85

10

5

03

5

03

Sam

ple

2R

B7

3

75

8

0

72

79

8

0

81

UT

S(M

Pa

)4

62

5

03

4

48

44

84

83

5

03

5

10

Sam

ple

3R

B

75

80

7

27

88

1

77

8

0

UT

S(M

Pa

)

46

25

10

4

48

48

35

10

4

83

5

04



Ta

ble

3:R

oo

m t

emp

era

ture

(27

oC

) te

nsi

le p

rop

erti

es o

f fr

icti

on

sti

r w

eld

ed

al

min

um

02

Mate

rial

Co

n

Ela

sti

cY

ield

Ult

imte

T

en

sile

(MP

a)

to (%)

Re

du in

u a

lloy 2

4

dit

ion

Mo

du

lus

(GP

a)

Str

en

gth

(MP

a)

a

Str

en

gth

Elo

ng

ati

on

Failu

re

cti

on

Are

a(%

)

Du

cti

lity

[ln

Ao/A

f]

Ten

sile

(%)

Al 2

02

4-T

8

Fri

cti

on

We

ld7

9

32

4

44

5

6.6

4

.2

4.3

ed

-A

Al 2024-T

8

Fri

cti

on

We

lde

d-B

7

2

32

5

39

9

3.4

4

.1

4.2

20

24-T

8N

o

We

ld7

14

45

47

68

.91

5.2

16

.5



Figure 6 : Engineering stress versu eer ain c es

ing th ect of fr wel n ten

p e of al num alloy 24-T8

0

100

200

300

400

500

600

0 2 4 10

Engineering Strain(%)

En

gin

ee

rin

g S

tre

ss

(MP

a)

6 8

Plate B Plate A Unwelded

s engin

iction

20

ing str

ding o

urv

silecom

res

par

ons

e eff

umi



(c) (d)

50µm 15µm

5µm 3µm

(a) (b)

Figure 7. SEM micrographs showing fracture surface featur

sample taken from unwelded aluminum alloy 2024-T8 and

es of the

deformed in tension, showing:

(a) Overall morphology.

(b) High magnification of (a) showing microscopic cracks

and a population of voids of varying sizes

(d

shape.

(c) High magnification of (b) showing cracked second

phase particle and shallow dimples

) In the region of overload shear dimples and

macroscopic voids of varying size and



igure 8. SEM micrographs showing fracture surface features

of the sample taken from friction welded aluminum

alloy 2024-T8 (Plate A) and deformed in tension,

showing:


(b) Layered ledges reminiscent of locally brittle

failure.

(c) Tear ridges and microscopic cracking.

(d) Elongated dimples and fine microscopic voids

reminiscent of locally ductile failure.

(a) (b)

(c) (d)

30µm 50µm

15µm 5µm

F



Figure 9. SEM micrographs showing fracture surface features

of the sample taken from friction welded 2024-T8

Plate B and deformed in uniaxial tension, showing:


(b) Microscopic cracks and voids of varying size.

(c) Dimples of varying size and shape.

(d) High magnification of ( c) showing shallow

nature of the dimples.

(a) (b)

(

50µm 10µm

c) (d)

5µm 3µm



References

. M. Thomas. E.D. Nicholas, J.C. Needham, M.G. Murch, P.

Templesmith, C.J. Dawes: G.B. Patent Application No

9125978.8 (December 1991).

. C. Dawes, W., Thomas: TWI Bulletin, November-December

1995, p. 124.

. W.M. Thomas: Friction Stir Butt Welding, International Patent

Application No PCT GB 92 Patent Application No 9 .8,

1991.0R

4. ,

S, Nagoya, Jaspan 1996.

5. P.L. Threadgill: Friction Stir Welds in Aluminum Alloys, TWI

Bulletin, United Kingdom, 1997.

6. W.M. Thomas, E.D. Nicholas Proceedings of the Third World

Congress on Aluminum, Lim us, 1997.

7. M.W. Mahoney: W g and Joining, 1997.

8. W.M. Thomas and E.D. Nicholas: Materials & Design

1 W.

2

3

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