TITTLE MICROSTRUCTURE AND MECHANICAL PROPERTIES OF...

i

TITTLE

MICROSTRUCTURE AND MECHANICAL PROPERTIES OF DISSIMILIAR

ALUMINUM ALLOY/STAINLESS STEEL JOINTS PREPARED BY

FRICTION STIR SPOT WELDING (FSSW)

LIM YEE KAI

A project submitted in partial

fulfillment of the requirement for the award of the

Degree of Master of Mechanical Engineering

Faculty of Mechanical and Manufacturing Engineering

University Tun Hussein Onn Malaysia

JAN 2014

v

ABSTRACT

In this paper, the effects of welding parameter (tool rotational speed and tool penetration

deep) on mechanical properties, failure mode and microstructure of dissimilar metal

welding using friction stir spot welding were investigated. The rotating tool with

different shoulder diameter of 10mm, 12mm and 14mm were used to weld aluminum

alloy A6061-T6 and stainless steel 304 sheets with thickness of 1mm. The hardness

profile and microstructure across the base metal (BM), heat affected zone (HAZ),

thermo mechanically affected zone (TMAZ) and stir zone (SZ) were obtained. The

failure mode analysis was conducted and co-related with the load displacement curve.

The hook geometry formed in joint interface was investigated. The tensile shear strength

and elongation increases with increasing of tool shoulder diameter, tool rotational speed

and tool penetration depth. The Vickers hardness profile showed a W-shaped. The

variation of Vickers hardness in each region of the weld was due to the effect of strain

hardening, dissolution of strengthening phase and grain growth under high welding

temperature. A plug type failure mode is observed at weld nugget and ductile fracture

occur at the soft region of TMAZ and HAZ, which indicated a strong metallic bonding,

was formed at the joint interface of aluminum alloy/stainless steel. The welding

parameter was found to significantly affect the hook formation. Partial metallurgical

bond (hook) was formed on the keyhole area and continues growth larger with increased

of tool rotational speed and tool penetration depth. The interface of aluminum alloy and

stainless steel weld nugget was bonded through mechanical mixing and formed partial

metallurgycal bond and kissing bond.

vi

ABSTRAK

Dalam kertas ini, kesan parameter kimpalan (kelajuan putaran dan kedalaman

penembusan) ke atas sifat mekanikal, mod kegagalan dan mikrostruktur kimpalan logam

berbeza menggunakan kimpalan friction stir spot telah disiasat. Alat berputar dengan

diameter bahu yang berbeza 10mm, 12mm dan 14mm digunakan untuk mengimpal

kepingan logam aluminium aloi AA6061-T6 dan keluli tahan karat 304 berketebalan

1mm. Profil kekerasan dan mikrostruktur base metal (BM), heat affected zone (HAZ),

thermo mechanically affected zone (TMAZ) dan stir zone (SZ) diperolehi. Analisis mod

kegagalan telah dijalankan dan ditunjuk dengan graf lengkungan anjakan beban.

Geometri hook yang terbentuk di antara muka bersama telah disiasat. Kekuatan ricih dan

pemanjangan tegangan meningkat dengan peningkatan saiz diameter bahu alat, kelajuan

putaran dan kedalaman penembusan. Vickers profil kekerasan berbentuk W. Perubahan

kekerasan Vickers di setiap zon kimpalan adalah disebabkan oleh kesan pengerasan

keterikan, pembubaran pengukuhan fasa dan pertumbuhan bijian di bawah suhu

kimpalan yang tinggi. Mod kegagalan plug diperhatikan di kumai kimpalan dan patah

secara mulur berlaku pada zon lembut TMAZ dan HAZ, yang menunjukkan ikatan

logam yang kuat telah dibentuk diantara muka bersama aluminum aloi dan keluli tahan

karat. Parameter kimpalan didapati memberi kesan yang ketara kepada pembentukan

hook. Ikatan partial metallurgycal (hook) terbentuk pada kawasan lubang kunci dan

pertumbuh besar dengan peningkatan kelajuan putaran dan kedalaman penembusan.

Kumai kimpalan aloi aluminium dan keluli tahan karat terikat melalui mekanikal mixing

dan ikatan partial metallurgycal dan ikatan kissing terbentuk.

vii

CONTENTS

TITTLE ............................................................................................................................... i

DECLARATION .............................................................................................................. ii

DEDICATION ................................................................................................................. iii

ACKNOWLEDGEMENT ................................................................................................ iv

ABSTRACT ....................................................................................................................... v

CONTENTS .................................................................................................................... vii

LIST OF FIGURES ........................................................................................................... x

LIST OF TABLES .......................................................................................................... xiv

LIST OF SYMBOLS AND ABBREVIATIONS ............................................................ xv

CHAPTER 1 ...................................................................................................................... 1

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

1.1 Research background ..................................................................................................... 1

1.2 Problem statement .......................................................................................................... 2

1.3 Research objective ......................................................................................................... 2

1.4 Scope of the research ..................................................................................................... 3

CHAPTER 2 ...................................................................................................................... 4

LITERATURE REVIEW................................................................................................... 4

2.1 FSW process principles .................................................................................................. 4

2.2 Friction Stir Spot Welding (FSSW) ............................................................................... 7

viii

2.3 Advantages of friction welding process ......................................................................... 8

2.4 Welding tools used for FSW ........................................................................................ 10

2.5 Friction stir welding pin tools ...................................................................................... 11

2.5.1 Tool geometry ...................................................................................................... 11

2.5.2 Tool shoulder material and backing material ....................................................... 12

2.6 Industrial applications of FSW .................................................................................... 15

2.6.1 Introduction .......................................................................................................... 15

2.6.2 Application of FSW in automotive industry ........................................................ 16

2.6.3 Application of FSSW in automotive industry ...................................................... 23

CHAPTER 3 .................................................................................................................... 26

METHODOLOGY ........................................................................................................... 26

3.1 Introduction .................................................................................................................. 26

3.2 Flow Chart ................................................................................................................... 28

3.3 FSSW Work Material .................................................................................................. 29

3.3.1 Work piece material ............................................................................................. 29

3.3.2 Tooling material ................................................................................................... 30

3.4 FSSW machine and equipment .................................................................................... 33

3.5 FSSW experimental process ........................................................................................ 35

3.5.1 Friction stirs spot welding procedure ................................................................... 36

3.6 Material testing and analysis ........................................................................................ 38

3.6.1 Tensile shear test .................................................................................................. 38

3.6.2 Vickers microhardness test .................................................................................. 41

3.6.3 Metallographic sample preparation ...................................................................... 43

3.6.4 Temperature ......................................................................................................... 50

3.6.5 Morphology and microstructure analysis ............................................................. 50

ix

3.6.6 Phase composition analysis .................................................................................. 52

CHAPTER 4 .................................................................................................................... 54

RESULTS & DISCUSSIONS ......................................................................................... 54

4.1 Introduction .................................................................................................................. 54

4.2 Tensile shear strength properties .................................................................................. 55

4.3 Vickers microhardness properties ................................................................................ 58

4.4 Failure modes of Al-SS weld in lap shear specimen .................................................... 63

4.5 Microstructural characterization .................................................................................. 72

CHAPTER 5 .................................................................................................................... 84

CONCLUSION AND RECOMENDATION .................................................................. 84

5.1 Introduction .................................................................................................................. 84

5.2 Conclusion ................................................................................................................... 84

5.3 Recommendation ......................................................................................................... 86

REFERENCES ................................................................................................................. 87

APPENDICES ................................................................................................................. 90

Appendix A: Properties of Aluminum alloy 6061-T6 ............................................................. 90

Appendix B: Properties of Stainless Steel 304 ........................................................................ 91

Appendix C: Gantt Chart (a) MP1, (b) MP2 ............................................................................ 92

x

LIST OF FIGURES

Figure 2.0.1: Basic principle of conventional rotary friction stirs welding. ...................... 5

Figure 2.0.2: Friction stir welded plates in aluminum 7075-T6. ....................................... 6

Figure 2.0.3: Mazda's new friction stir welder making a weld on a body assembly. ........ 6

Figure 2.0.4: (a) FSW spot welding steel and welding tool; (b) Welding spot steel ......... 7

Figure 2.0.5: Friction stir spot welding tool in PCBN (Poly Crystaline Boron Nitride) by

Mega Stir Technologies ..................................................................................................... 8

Figure 2.0.6: Schematic drawing of the FSW tool. .......................................................... 13

Figure 2.0.7: WorlTM and MX TrifluteTM tools developed by The Welding Institute

(TWI), UK (Copyright 2001, TWI Ltd) ........................................................................... 13

Figure 2.0.8: Flared-TrifluteTM tools developed by The Welding Institute (TWI), UK:

(a) neutral flutes, (b) left flutes, and (c) right hand flutes ................................................ 14

Figure 2.0.9: A-SkewTM tool developed by The Welding Institute (TWI), UK: (a) side

view, (b) front view, and (c) swept region encompassed by skew action. ....................... 14

Figure 2.0.10: Tool shoulder geometries, viewed from underneath the shoulder

(Copyright 2001, TWI Ltd). ............................................................................................. 15

Figure 2.0.11: FSW tailor welded blank produced from 6000 series aluminum in 1998

TWI, BMW, Land Rover. ................................................................................................ 17

Figure 2.0.12: Friction stir welding of the centre tunnel of the Ford GT. (Courtesy Tower

Automotive and Ford) ...................................................................................................... 18

Figure 2.0.13: The friction stir welded aluminum centre tunnel of the Ford GT houses

the fuel tank to maximize the fuel volume and reduces the number of connections to the

fuel system. (Courtesy Ford) ............................................................................................ 18

Figure 2.0.14: FSW machine with two welding heads for welding hollow aluminum

extrusions from both sides simultaneously, to produce foldable Volvo rear seats.

(Courtesy Sapa) ................................................................................................................ 18

Figure 2.0.15: FSW simultaneously with two spindles from both sides to from

suspension links with excellent fatigue properties for Lincoln stretched limousines.

(Courtesy Tower Automotive) ......................................................................................... 18

Figure 2.0.16: The rubber of the end-pieces of the suspension arms joined by FSW can

be vulcanized prior to welding due to the low heat input of the new assembly method

(Courtesy Showa Denko) ................................................................................................. 19

xi

Figure 2.0.17: Cast center part is FSW to a spin formed wheel rim to reduce wheel

weight by 20~25%. (Courtesy Hydro) ............................................................................. 19

Figure 2.0.18: Aluminum 6061-O sheet is rolled to form a cylinder and longitudinal

FSW to from wheel rim (Courtesy Simmons Wheels and UT Alloy Works) ................. 19

Figure 2.0.19: Robotic FSW of automotive parts. (Courtesy Riftec) .............................. 19

Figure 2.0.20: CNC controlled FSSW gun on an articulated arm robot. (Courtesy

Friction Stir Link) ............................................................................................................ 20

Figure 2.0.21: Prototype FSW lightweight engine cradle to reduce the weight in the front

end of the vehicle. (Courtesy Sapa) ................................................................................ 20

Figure 2.0.22: A diagram of an Accord sub-frame made using the new friction stir

welding process. These hybrid-structured front sub-frame can achieves both weight

reduction and increased rigidity. ...................................................................................... 22

Figure 2.0.23: Conceptual diagram of FSW of dissimilar metals .................................... 22

Figure 2.0.24: The pin on this friction stir welder rotates at high speed and pressure to

melt the metal. .................................................................................................................. 24

Figure 2.0.25: Friction stir spot welding of rear doors for the Mazda RX-8 (Courtesy

Mazda).............................................................................................................................. 24

Figure 2.0.26: The back side of a friction stir weld. ........................................................ 25

Figure 2.0.27: The front side of a friction stir weld. ........................................................ 25

Figure 3.0.1: Configuration of test specimen for tensile shear test. ................................. 29

Figure 3.0.2: OM shows the microstructure of the (a) aluminum alloy 6061-T4 and (b)

stainless steel AISI 304-B1 starting material. .................................................................. 30

Figure 3.0.3: Geometry of welding tool employs. ........................................................... 31

Figure 3.0.4: Conventional milling machine ................................................................... 33

Figure 3.0.5: (a) Machine setup of FSSW process; (b) Rotational welding tool with

diameter 16mm collet. ...................................................................................................... 34

Figure 3.0.6: Lap joint configuration of work material with special design base plate. .. 35

Figure 3.0.7: Schematic illustration of FSSW showing the four steps. (a) Tool rotation (b)

Plunging and heating (c) Stirring and bonding (d) Tool removal .................................... 37

Figure 3.0.8: Universal Tensile Testing machine ............................................................ 38

Figure 3.0.9: Standard tensile shear test specimen for sheet type metallic material ........ 39

Figure 3.0.10: (a) Sample firmly clamped; (b) Weld joint broken after tensile shear test

.......................................................................................................................................... 40

Figure 3.0.11: A simple model describing stress distribution at the interface and

circumference of a weld nugget during the tensile-shear test. ......................................... 40

Figure 3.0.12: (a) Vickers microhardness tester; (b) test sample on clamping stage....... 42

Figure 3.0.13: Location of two hardness traverses. The indentations were made with a

spacing of 0.5mm along each of the two parallel lines and 0.2mm above the joint

interface. ........................................................................................................................... 42

Figure 3.0.14: (a) Abrasive cutter; (b) Clamping specimen; (c) Lap joint specimen; (d)

Specimen‟s joint were cut in transverse weld zone. ........................................................ 44

xii

Figure 3.0.15: Hot mounting process ............................................................................... 45

Figure 3.0.16: Standard abrasive grinding procedure ...................................................... 46

Figure 3.0.17: Steps taken in abrasive grinding: (a) edge rounding (b) surface grinding (c)

change finer grit of sand paper (d) flushing and cleaning with tap water ........................ 47

Figure 3.0.18: Steps taken in polishing ............................................................................ 48

Figure 3.0.19: Handheld infrared thermometer (Raytex, temperature range -300C~900

0C)

.......................................................................................................................................... 50

Figure 3.0.20: Figure 3.19: Optical microscope (Olympus BX60M + JVC CCTV) ....... 51

Figure 3.0.21: JEOL JSM-6380LA Analytical Scanning Electron Microscope (SEM) .. 52

Figure 3.0.22: Sample preparation using plastic mold: (a) Test sample; (b) stick some

plasticine into the mold; (c) Press and stick the sample with plasticine in the mold (d)

press and flatten the sample with the mold ...................................................................... 52

Figure 3.0.23: X-ray Diffraction scanning machine ........................................................ 53

Figure 3.24: X-ray Diffraction scanning: (a) spinning stage (b) sample mold is clamp on

the spinning stage ............................................................................................................. 53

Figure.4.0.1: (a) Elongation, (b) Tensile shear strength, (c) Maximum welding

temperature of the weld obtained with 1.9mm tool penetration depth. ........................... 55

Figure 4.0.2: (a) Elongation, (b) Tensile shear strength, (c) Maximum welding

temperature of the weld obtained with 2000rpm tool rotational speed. ........................... 55

Figure 4.0.3: Hardness distribution profile along the cross section of the Al-SS joint

obtained with 2000 rpm tool rotational speed. ................................................................. 58


obtained with 3000 rpm tool rotational speed. ................................................................. 58


obtained with 2000rpm and 3000rpm tool rotational speed. ........................................... 59

Figure 4.0.6: Measured point of microhardness across different zone in weld area (a) Stir

zone, (b) Thermal-Mechanical Affected zone, (c) Heat affected Zone, (d) Base Metal. . 61

Figure 4.0.7: Hardness distribution profile across welding zone (retreating side)

vertically for the plate right hand side (a) Aluminum, (b) Stainless steel........................ 62

Figure 4.0.8: Appearances of FSSW sample. .................................................................. 63

Figure 4.0.9: Failure modes of FSSW after tensile shear test. ......................................... 63

Figure 4.0.10: Close up views of top and bottom weld region from typical tensile shear

test sample of Group A for increasing tool rotational speed. ........................................... 64

Figure 4.0.11: Close up views of top and bottom weld region from typical tensile shear

test sample of Group B for increasing tool penetration depth. ........................................ 65

Figure 4.0.12: OM image of the onion layers of aluminum observed at the top surface

weld region. ...................................................................................................................... 67

Figure 4.0.13: Load displacement curves for the Al-SS lap shear specimen welded with

different tool rotational speed .......................................................................................... 68

Figure 4.0.14: Load displacement curves for the Al-SS lap shear specimen welded with

different tool penetration depth ........................................................................................ 69

xiii

Figure 4.0.15: The illustration of the cross sectional fractured specimen after tensile

shear tests with different failure mode. (a) Interfacial failure mode; (b) Plug failure mode

.......................................................................................................................................... 71

Figure 4.0.16: The growth of hook geometry at different tool rotational speed using

12mm tool shoulder diameter .......................................................................................... 72

Figure 4.0.17: The growth of hook geometry at different tool rotational speed using


Figure 4.0.18: The growth of hook geometry at different tool penetration depth using


Figure 4.0.19: Comparison of the effect of different tool shoulder diameter and tool

rotational speed on the hook geometry ............................................................................ 74

Figure 4.0.20: Comparison of the effect of different tool rotational speed and tool

penetration depth on the hook geometry .......................................................................... 74

Figure 4.0.21: OM images of hook formation for FSSW using 12mm shoulder diameter,

1.9mm tool penetration depth, 5s holding time and different tool rotational speed (a)

1000rpm, (b) 2000rpm, (c) 3000rpm ............................................................................... 75

Figure 4.0.22: SEM images of hook formation for FSSW using 14mm shoulder diameter,

1.9mm tool penetration depth, 5s holding time and different tool rotational speed (a)

1000rpm, (b) 2000rpm, (c) 3000rpm ............................................................................... 76

Figure 4.0.23: SEM images of hook formation for FSSW using 14mm shoulder diameter,

2000rpm tool rotational speed, 5s holding time and different tool penetration depth (a)

1.80mm, (b) 1.90mm, (c) 1.95mm ................................................................................... 77

Figure 4.0.24: Cross section view of the bonding location in FSSW weld zone. ............ 80

Figure 4.0.25: SEM image show metallurgical on weld cross section of sample weld

using 14 mm shoulder diameter, 2000rpm and 1.90mm depth of weld penetration. ....... 80

Figure 4.0.26: Schematic illustration of material flow under the pin tool. ...................... 80

Figure 4.0.27: SEM image show interface of between Al and SS sheet for sample 12 mm

shoulder diameter, 3000rpm and 1.90mm depth of weld penetration.............................. 83

xiv

LIST OF TABLES

Table 2.1: A selection of tools designed at TWI .............................................................. 10

Table 2.2: Typical Applications for FSW. ....................................................................... 16

Table 3.1: Types of work material used in present study. ............................................... 29

Table 3.2: Nominal chemical composition of the stainless steel. .................................... 30

Table 3.3: Nominal chemical composition of 6061-T6 Al alloy. .................................... 30

Table 3.4: The chemical composition of the SKD2 tool steel. ........................................ 31

Table 3.5: Control parameter of FSSW using milling machine. ...................................... 33

Table 3.6: The abrasive size using in grinding and polishing. ......................................... 49

xv

LIST OF SYMBOLS AND ABBREVIATIONS

N - Newton

kN - Kilo Newton

mm - Millimeter

µm - Micrometer

Ø - Diameter

rpm - Rotational Per Minute

RSW - Resistance Spot Welding

FSW - Friction Stir Welding

FSSW - Friction Stir Spot Welding

Al-SS - Aluminum and Stainless Steel

BM - Base Metal

HAZ - Heat Affected Zone

TMAZ - Thermal Mechanically Affected Zone

SZ - Stir Zone

OM - Optical Micrograph

SEM - Scanning Electron Microscope

UTM - Universal Tensile Machine

HV - Vickers Hardness

ASTM - American Standard Testing Method

Tm - Melting Temperature

Teff - Effective Top Sheet Thickness

Hw - Hook Width

Hh - Hook Height

PCBN - Polycrystalline Boron Nitride

SiC - Silicon Carbide

Al2O3 - Aluminum Oxide

TWI - The Welding Institute

1

CHAPTER 1

INTRODUCTION

1.1 Research background

A new joining technique of light weight material to reduce fuel consumption by

weight savings is highly desirable in transportation industries such as aerospace and

automotive. Friction Stir Spot Welding (FSSW) is solid state welding process which

fuse material together by friction heat. The research is associated in friction based

process has considerably popular in the last few years. This in fact, can be explained by

the various advantages of these processes when compared to the conventional fusion

welding process. The advantages become more evident in situations where the

conventional welding process cannot be used due to difficulty in joining dissimilar

materials (Mazzafero & Rosendo, 2009). Friction Stir Spot Welding (FSSW) process is

suitable for joining dissimilar metals. FSSW is non material filler process and non

melting of work material which allow a low temperature or low heat input welding

process that can limit the excessive heat damage at weld zone. The joining of dissimilar

metals such as aluminum to stainless steel is used in many indutries. Hence, FSSW can

be a more efficient in terms of significant energy and cost savings. Bannets and rear

doors by aluminium are FSSW instead of resistance spot-welded by some of the

automobile company. FSSW is more efficient, less energy consumed, uses unskilled

labour etc, no consumaption of tools are prime importance.

2

1.2 Problem statement

The increasing demand for energy saving in different sector has led to the

necessity of dissimilar material joining for hybrid structure. Conventional structures

made of alloy steel have been replaced by light weigth and high strength materials such

as aluminum alloy. This new discover has greatly benefit to transportation industry.

These junctions are of great importance, because they allowed the systems, subsystems

and components manufactured in aluminum alloy and stainless steel to be structurally

united.

Conventional fusion welding of electric resistance spot welding (RSW) is

difficult to weld aluminum alloy to stainless steel. During fusion welding of Al-SS, the

high welding temperature and rapid cooling rate during the RSW process might result in

the formation of brittle intermetallic compounds at weld interface and deteriorate the

mechanical properties of the welds joint.

The difficulties in the welding of aluminum alloy with stainless steel by

conventional fusion welding process process is rather complicated and can be quite

difficult due to their different physical/chemical/mechanical properties, melting

temperature and mutual solubility. The dissimilar welding of Al-SS have been a great

challenge for engineering, because they resulted hard and brittle intermetallic phases

between aluminum and stainless steel at elevated temperatures.

1.3 Research objective

The objectives of research are:

a) To investigate the mechanical properties for lap joint aluminum alloy and

stainless steel using FSSW.

b) To analyze the effect of welding parameter on the failure mode.

c) To evaluate the hook formation at different welding parameter.

3

1.4 Scope of the research

The research work will be concentrated in the mechanical performance and the

weld zone microstructure. The FSSW is lap weld on part having 100mm x 30mm x 1mm

thick sheet aluminum alloy AA6061-T6 and austenitic stainless steel 304 using different

rotational tool shoulder diameters 10mm, 12mm and 14mm. Two sheets are welded on

an overlap area of 30 x 30 mm2. The dissimilar metals are welded using conventional

milling machine with appropriate clamping and holding fixture. The sizes of lap joint

sample are according to the studied journal while the tensile and hardness test are based

on ASTM E8M and E384 standard.

In this research, the weld strength is characterized by tensile shear test using

Universal Testing Machine (UTM). The hardness across the weld zone is measured by

vickers microhardness tester. The micrustructure is examined by optical microscope and

Scanning Electron Microscope (SEM).

4

CHAPTER 2

LITERATURE REVIEW

2.1 FSW process principles

Thomas (2006) stated the Friction Stir Welding (FSW) is invented and patented in 1991

by The Welding Institute (TWI) UK. Currently there are 120 organizations hold non-

exclusive licenses to use the FSW process and majority are from industrial companies.

These companies have filed more than 1300 patent applications related to FSW process.

Friction stir welding is a solid state hot shear joining process that conducts below

the melting point of base metal by pressing a rotating tool into joint line to generate

enough frictional heat to fuse metals together. Figure 2.1 show the basic principle and

main term definition of conventional rotary FSW process. The pin tool travel along the

length of the required weld area, stirring and forging the weld material together by

friction heat (Figure 2.2, 2.3). The rotational speed of welding tool can range from few

hundred Revolutions Per Minute (RPM) to several thousand depend on welding

parameter and type of weld material.

The interaction between the workpiece and the rotational tool generates heat due

to plastic and frictional dissipation. FSW requires less weld preparation, little post weld

dressing and produce high tensile and fatigue strength weld joint. FSW can weld plate

without any relative movement of workpiece. The rotating tool move along the joint to

5

cause coalescence and the downward pressure is required to press the rotating tool into

the workpiece.

This kind of welding process is initially implemented for low melting

temperature materials such as aluminum alloys. The application of FSW is limited for

high melting point alloys, such as stainless steel, titanium due to requirement of high

down force and long tool life. The conventional rotary FSW tool has a shoulder and

profiled probe or pin with diameter 1/3 size to the shoulder diameter. The pin length is

similar to the required weld depth.

Figure 2.0.1: Basic principle of conventional rotary friction stirs welding.

6

Figure 2.0.2: Friction stir welded plates in aluminum 7075-T6.

Source: (HBS ENGINEERING, 2013)

Figure 2.0.3: Mazda's new friction stir welder making a weld on a body assembly.

7

2.2 Friction Stir Spot Welding (FSSW)

Friction stir spot welding (FSSW) is developed by Mazda Corporation and

Kawasaki Heavy Industries in 2003 as a solid state joining technique for

aluminum alloys (Sun Y. F., 2012). FSSW is a novel variant of the "linear" FSW

process, where a rotating tool is plunged into the workpiece, hold for a certain

period of time and then retracted, hence creates a spot FSW lap-weld without

bulk melting.

FSSW is now being considered as competitive joining technique to

conventional technique such as riveting and electric resistance spot welding

(RSW). Unlike FSW, FSSW can be considered as transient process due to the

welding tool does not travel along the workpiece and it is directly press onto the

workpiece to form a spot weld in a shot cycle time (usually a few seconds)

(Badarinarayan, H, et.al, 2009).

FSSW has many advantages in energy consumption, environmental

protection and high welding quality. Similar to the FSW, the FSSW process also

consist of light materials as aluminum on an industrial scale. But more than 90%

of the global products are made of steel that makes the FSSW become an

interesting technology for the future of the industries and a substitution of

traditional fusion welding processes.

Source: Stir Zone Cold Welding (2013)

(a) (b)

Figure 2.0.4: (a) FSW spot welding steel and welding tool; (b) Welding spot steel

8

2.3 Advantages of friction welding process

a) Economical advantage

It reduces machining labor, which in turn increases capacity and reduces

perishable tooling cost. Unskilled labor can be used

No external consumables flux or filler metal or protective gases necessary

Simplification of component design

High production rate due to reduction of the welding time (less than 3

seconds)

Low metal consumption and reduced machining

Manual loading or full automation optional

Expensive material can be joined to cheaper material

the welding head can be mounted on different systems

It allows choosing of either manual loading or optional automated

loading.

It reduces maintenance cost.

It reduces cost for complex forgings or castings.

Figure 2.0.5: Friction stir spot welding tool in PCBN (Poly Crystaline Boron

Nitride) by Mega Stir Technologies

Source: HBS Engineering

9

Self-cleaning action of friction welding reduces or eliminates surface

preparation cost or time for some material combinations.

Create cast or forge like blanks, without the expensive costs of tooling

and the minimum quantity requirements.

b) Metallurgical advantage

100% metal to metal joints giving parent metal properties. The joint

strength is equal to or greater than parent material.

Dissimilar material combination

Welding of unequal cross sections can be done by friction welding

process.

As friction welding is a solid state process, possibility of porosity and

slag inclusions are eliminated.

It creates a narrow heat affected zone.

Can withstand high temperature variation.

c) Weld quality advantage

Accurate control over post weld tolerances

Consistent quality is maintained and monitored

It is highly precision and repeatable process.

d) Environmental advantage

Simple clean mechanical operation.

Does not generate fumes, gases or smoke.

e) Energy advantage

It is consistent and repetitive process. It consumes low energy and low

welding stress.

f) Safety advantage

High process-safety due to only a few process parameters

10

2.4 Welding tools used for FSW

The tool typically consists of a rotating round shoulder and a threaded cylindrical

pin that heats the workpiece, mostly by friction, and moves the softened alloy

around it to form the joint.

Table 2.1: A selection of tools designed at TWI

(Source: R, T, & H.K.D.H, 2008)

Tool Cylindrical WhorlTM

MX

trifluteTM

Flared

trifluteTM

A-skewTM

Re-stirTM

Schematics

Tool pin

shape

Cylindrical

with threads

Tapered

with

threads

Threaded,

tapered

with

three flutes

Tri-flute

with

flute ends

flared out

Inclined

cylindrical

with

threads

Tapered

with

threads

Ratio of pin

volume to

cylindrical

pin volume

1 0.4 0.3 0.3 1 0.4

Swept volume

to pin

volume

ratio

1.1 1.8 2.6 2.6 Depends

on pin

angle

1.8

Rotary

reversal

No No No No No No

Application Butt Butt Butt Lap Lap When

11

2.5 Friction stir welding pin tools

2.5.1 Tool geometry

The weld joint quality depends on tool geometry. The tool geometry play and

important rules in the rate of heat generation, traverse force, torque and thermo-

mechanical environment experienced by the tool. The tool geometry and motion of the

tool will affect the flow of plasticized material in the workpiece. The others important

factors are shoulder diameter, shoulder surface angle, pin geometry (shape and size) and

nature of tool surfaces (Rai, et.al, 2011).

A conventional FSW tool consists of a shoulder and a pin as shown in Figure 2.5.

The tool play a major role in localized heating and material flow. In the initial stage of

tool plunge, friction heat is result from the interface of pin and workpiece and material

deformation. The biggest amount of heating is resulting from the friction between

shoulder and workpiece.

The ratio size between pin diameter and shoulder diameter is important for

friction heat generation. Beside of heating, the tool also uses to „stir‟ and „move‟ the

material. The tool design determines the uniformity of microstructure, properties and

welding;

fails in lap

welding

welding

with lower

welding

torque

welding

with

further

lower

welding

torque

welding

with lower

thinning of

upper plate

welding

with lower

thinning

of

upper

plate

minimum

asymmetr

y in weld

property is

desired

12

plunging load. Generally a concave shoulder and threaded cylindrical pins are most

widely used due to it better weld quality and easy tool fabrication (Mishra & Ma, 2005).

Optimum tool design will produce desired joint quality, enable higher welding

speed and prolong tool life. In the earlier design of FSW process, the tools are in simple

geometry design. With the requirement in higher welding quality and higher weldable

thickness, complex features have been added to alter material flow, mixing and reduce

process loads. For example, complex FSW tools such as the WhorlTM

and MX Triflute

(Figure 2.6) tools had invented by TWI.

The tool shoulder‟s shape affects the material flow around the tool probe and

preventing the escape of plasticized material. The shapes of tool shoulder are available

in flat, concave or convex, smooth or grooved, with concentric or spiral grooves. The

concave shoulder has advantages over flat bottom shoulder as it directing the material

flow to the shoulder root (center close to the tool probe). The tool‟s probe (pin) diameter

is usually one third of the cylindrical tool and probe length (PL) less than workpiece‟s

thickness. (Mandal, 2009). The pin geometry are available in cylindrical or triangular,

smooth or threaded, and without pin.

2.5.2 Tool shoulder material and backing material

The tool shoulder material affect the heat generation process of FSW. The

shoulder made from Zirconia engineering ceramic able to generate 30~70% more

friction heat compare to tool steel. The heat loss through tool and backing bar also affect

the welding efficiency. Using tool materials that have low thermal conductivity with

suitable non-conductive backing bar can substantially reduce the heat loss and enable

increase for welding speed. Hence, a combination of low thermal conductivity tool

material such as SS 660 with zirconia coated tool shoulder and zirconia backing bar can

significantly improve process efficiency through increase in welding speed (Mandal,

2009).

13

Source: Mishra & Ma (2005)

Figure 2.0.7: WorlTM and MX TrifluteTM tools developed by The Welding Institute

(TWI), UK (Copyright 2001, TWI Ltd)


Figure 2.0.6: Schematic drawing of the FSW tool.

14


(a) (b) (c)

Figure 2.0.9: A-SkewTM tool developed by The Welding Institute (TWI), UK: (a)

side view, (b) front view, and (c) swept region encompassed by skew action.


(a) (b) (c)

Figure 2.0.8: Flared-TrifluteTM tools developed by The Welding Institute (TWI), UK: (a)

neutral flutes, (b) left flutes, and (c) right hand flutes

15

2.6 Industrial applications of FSW

2.6.1 Introduction

Gas metal arc welding (MIG) and resistance spot welding (RSW) are the most widely

used traditional welding processes for automotive components. Both of these processes

have well-documented issues (e.g., weld porosity, low weld strength, excessive

distortion) associated with using them on Al and Mg alloys in vehicle assembly

operations. The friction stir processes avoid melting and typically distribute heat over

wider areas than traditional welding processes. This minimizes distortion and contributes

to higher strength in FSW joints. FSW has been implemented in shipbuilding, military

and aerospace applications in joining mainly flat Al panels. Its potential benefits in truck

and automobile construction to build lightweight automotive structures.

Figure 2.0.10: Tool shoulder geometries, viewed from underneath the shoulder

(Copyright 2001, TWI Ltd).


16

Table 2.2: Typical Applications for FSW.

No Industry

category

Specific

application

Present

process

Advantages of FSW

1. Electrical Heat sinks-welded

laminations

GMAW Higher density of fins-

better conductivity

2. Electrical Cabinets,

enclosures

GMAW,

RSW

Reduced cost, Weld

through corrosion

coatings

3. Batteries Leads Solder Higher quality

4. Military Shipping

Pallets

GMAW Reduced cost

5. Extrusions Customized

extrusions

Not done

today

Can customize, reduces

need for large press

6. Boats Keel, Tanks Rivet,

GMAW

Stronger, Less Distortion

7. Golf Cars,

Snowmobiles

Chassis,

Suspension

GMAW Less distortion, Better

fatigue life

8. Tanks,

Cylinders

Fittings, Long &

Circum Seam

GMAW Higher quality - less

leaks, higher uptime

9. Aerospace Floors, wing spars Rivets Higher quality,

cheaper(no rivets &

holes)

2.6.2 Application of FSW in automotive industry

Friction stir welding technology has gained increasing interest and importance

since its invention at TWI 20 years ago. According to Thomas (2006), TWI had develop

a new concepts on FSW drive shafts and space frames and started a research on

aluminium tailored blanks for door panels (Figure 2.10) in the year 1998. These project

17

are sponsored by BMW, DaimlerChrysler, EWI, Ford, General Motors, Rover, Tower

Automotive and Volvo. As a consequence of the successful results of this project, FSW

and FSSW are being widely used in the production of aluminum automotive components.

Currently, FSW is extensively applied in automotive industry for joining and

material processing. The continuing development and recent applications of FSW

technology in the automotive industries had review by Thomas (2006). The application

of FSW is worldwide and used by many famous car manufacture and such as Ford in

Detroit (USA), Grand Rapids in Michigan (USA), Sapa in Sweden, Showa Denko in

Oyama City (Japan), Simmons Wheels in Alexandria (Australia), DanStir in

Copenhagen (Denmark), Riftec in Geesthacht (Germany) Most of the latest development

and innovation in the FSW technology are found by those companies (see Figure

2.11~2.20).

Figure 2.0.11: FSW tailor welded blank produced from 6000 series aluminum in 1998

TWI, BMW, Land Rover.

18

Figure 2.0.12: Friction stir welding of the

centre tunnel of the Ford GT. (Courtesy

Tower Automotive and Ford)

Figure 2.0.14: FSW machine with two

welding heads for welding hollow

aluminum extrusions from both sides

simultaneously, to produce foldable Volvo

rear seats. (Courtesy Sapa)

Figure 2.0.13: The friction stir welded

aluminum centre tunnel of the Ford GT houses

the fuel tank to maximize the fuel volume and

reduces the number of connections to the fuel

system. (Courtesy Ford)

Figure 2.0.15: FSW simultaneously with two spindles

from both sides to from suspension links with excellent

fatigue properties for Lincoln stretched limousines.

(Courtesy Tower Automotive)

19

Figure 2.0.18: Aluminum 6061-O sheet is

rolled to form a cylinder and longitudinal

FSW to from wheel rim (Courtesy

Simmons Wheels and UT Alloy Works)

Figure 2.0.17: Cast center part is FSW to a spin

formed wheel rim to reduce wheel weight by

20~25%. (Courtesy Hydro)

Figure 2.0.16: The rubber of the end-pieces of the suspension arms joined by FSW

can be vulcanized prior to welding due to the low heat input of the new assembly

method (Courtesy Showa Denko)

Figure 2.0.19: Robotic FSW of automotive parts. (Courtesy Riftec)

20

Figure 2.0.20: CNC controlled FSSW gun on an articulated arm robot.

(Courtesy Friction Stir Link)

Figure 2.0.21: Prototype FSW lightweight engine cradle to reduce the weight

in the front end of the vehicle. (Courtesy Sapa)

21

Honda had develops a new technology for the continuous welding of the

dissimilar metals of steel and aluminum. Honda is the first automotive industry applies

continuous FSW to weld steel and aluminum together on the sub-frame of a mass-

production vehicle body frame.

Honda focuses on Friction Stir Welding (FSW) and developed a new technology

for the continuous welding of steel and aluminum. The idea of Honda on this dissimilar

metal joint is to reduce vehicle weight in order to increase fuel economy (Figure 2.21).

The FSW generates a stable intermetallic bonding between steel and aluminum by

moving a rotating tool on the top of the aluminum which is lapped over the steel with

high pressure and high rotational speed (Figure 2.22). Hence, the welding strength

becomes equal to or beyond conventional Metal Inert Gas (MIG) welding. The

conventional welding technique most commonly used for welding of identical materials

such as steel-to-steel or aluminum-to-aluminum and impossible for dissimilar metal joint.

This FSW technology contributes to an improvement in fuel economy by

reducing body weight by 25% compared to a conventional steel sub-frame. In addition,

electricity consumption during the welding process is reduced by approximately 50%. It

also enabled a change in the structure of the sub-frame and the mounting point of

suspension, which increased the rigidity of the mounting point by 20% and also

contributed to the vehicle‟s dynamic performance (Honda Motor Co., 2012).

22

Figure 2.0.23: Conceptual diagram of FSW of dissimilar metals

Source: (Honda Motor Co., 2012)

Figure 2.0.22: A diagram of an Accord sub-frame made using the new friction stir

welding process. These hybrid-structured front sub-frame can achieves both weight

reduction and increased rigidity.

Source: (Honda Motor Co., 2012)

23

2.6.3 Application of FSSW in automotive industry

Mazda Motor Corporation is the first automotive industry that introduces friction stir

spot welding (FSSW) that does not use electric resistance to create heat. Instead, FSSW

uses a pin tool that rotates at high speeds and high pressure to create enough friction heat

to fuse metal together. This type of welding process uses a non-consumable pin tool,

requires no filler metal and no shielding gas (Figure 2.23).

Mazda in Hiroshima (Japan) uses FSSW for the rear doors and bonnet of the

Mazda RX-8 (Figure 2.25). The welding gun installed with rotating tool used to hold

both sides of weld metal. The welding tool then spins and applies high pressure to create

the frictional heat required to melt the metal. The bonnet of this sports car has an impact-

absorbing structure for pedestrian protection. Furthermore, this FSSW process also able

to avoid spatter and reduce energy consumption significantly in comparison to RSW.

This welding method is currently uses by Mazda for flanges on the aluminum

rear doors, hood of the 2004 RX-8 and the new four-door, rotary engine sports car (see

Figure 2.26 and 2.27). The major advantage of this FSSW process in welding a panel is

the significantly reduce 99% electricity consumption when compared to resistance-

welding aluminum and around 80% compared to resistance-welding steel. The

conventional resistance spot welding may require large amount of current

instantaneously pass through the aluminum to form weld nugget due to aluminum's

ability to quickly dissipate heat. Moreover, FSSW become preferences of Mazda due to

the expense of rivets, and mechanical clinching that requires large equipment (i-car,

2003).

24

Figure 2.0.24: The pin on this friction stir welder rotates at high speed and pressure to

melt the metal.

Figure 2.0.25: Friction stir spot welding of rear doors for the Mazda RX-8

(Courtesy Mazda)

87

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