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  • 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

    file:///C:/Users/user/Desktop/master%20project%20chapter/master%20full%20report/MP2%20full%20report-%20personal.docx%23_Toc377666802file:///C:/Users/user/Desktop/master%20project%20chapter/master%20full%20report/MP2%20full%20report-%20personal.docx%23_Toc377666803file:///C:/Users/user/Desktop/master%20project%20chapter/master%20full%20report/MP2%20full%20report-%20personal.docx%23_Toc377666804file:///C:/Users/user/Desktop/master%20project%20chapter/master%20full%20report/MP2%20full%20report-%20personal.docx%23_Toc377666805file:///C:/Users/user/Desktop/master%20project%20chapter/master%20full%20report/MP2%20full%20report-%20personal.docx%23_Toc377666805file:///C:/Users/user/Desktop/master%20project%20chapter/master%20full%20report/MP2%20full%20report-%20personal.docx%23_Toc377666806file:///C:/Users/user/Desktop/master%20project%20chapter/master%20full%20report/MP2%20full%20report-%20personal.docx%23_Toc377666807file:///C:/Users/user/Desktop/master%20project%20chapter/master%20full%20report/MP2%20full%20report-%20personal.docx%23_Toc377666807file:///C:/Users/user/Desktop/master%20project%20chapter/master%20full%20report/MP2%20full%20report-%20personal.docx%23_Toc377666808file:///C:/Users/user/Desktop/master%20project%20chapter/master%20full%20report/MP2%20full%20report-%20personal.docx%23_Toc377666808file:///C:/Users/user/Desktop/master%20project%20chapter/master%20full%20report/MP2%20full%20report-%20personal.docx%23_Toc377666809file:///C:/Users/user/Desktop/master%20project%20chapter/master%20full%20report/MP2%20full%20report-%20personal.docx%23_Toc377666809file:///C:/Users/user/Desktop/master%20project%20chapter/master%20full%20report/MP2%20full%20report-%20personal.docx%23_Toc377666810file:///C:/Users/user/Desktop/master%20project%20chapter/master%20full%20report/MP2%20full%20report-%20personal.docx%23_Toc377666810file:///C:/Users/user/Desktop/master%20project%20chapter/master%20full%20report/MP2%20full%20report-%20personal.docx%23_Toc377666811file:///C:/Users/user/Desktop/master%20project%20chapter/master%20full%20report/MP2%20full%20report-%20personal.docx%23_Toc377666811file:///C:/Users/user/Desktop/master%20project%20chapter/master%20full%20report/MP2%20full%20report-%20personal.docx%23_Toc377666812file:///C:/Users/user/Desktop/master%20project%20chapter/master%20full%20report/MP2%20full%20report-%20personal.docx%23_Toc377666812file:///C:/Users/user/Desktop/master%20project%20chapter/master%20full%20report/MP2%20full%20report-%20personal.docx%23_Toc377666813file:///C:/Users/user/Desktop/master%20project%20chapter/master%20full%20report/MP2%20full%20report-%20personal.docx%23_Toc377666813file:///C:/Users/user/Desktop/master%20project%20chapter/master%20full%20report/MP2%20full%20report-%20personal.docx%23_Toc377666813file:///C:/Users/user/Desktop/master%20project%20chapter/master%20full%20report/MP2%20full%20report-%20personal.docx%23_Toc377666814file:///C:/Users/user/Desktop/master%20project%20chapter/master%20full%20report/MP2%20full%20report-%20personal.docx%23_Toc377666814file:///C:/Users/user/Desktop/master%20project%20chapter/master%20full%20report/MP2%20full%20report-%20personal.docx%23_Toc377666814file:///C:/Users/user/Desktop/master%20project%20chapter/master%20full%20report/MP2%20full%20report-%20personal.docx%23_Toc377666815file:///C:/Users/user/Desktop/master%20project%20chapter/master%20full%20report/MP2%20full%20report-%20personal.docx%23_Toc377666815file:///C:/Users/user/Desktop/master%20project%20chapter/master%20full%20report/MP2%20full%20report-%20personal.docx%23_Toc377666815file:///C:/Users/user/Desktop/master%20project%20chapter/master%20full%20report/MP2%20full%20report-%20personal.docx%23_Toc377666816file:///C:/Users/user/Desktop/master%20project%20chapter/master%20full%20report/MP2%20full%20report-%20personal.docx%23_Toc377666816file:///C:/Users/user/Desktop/master%20project%20chapter/master%20full%20report/MP2%20full%20report-%20personal.docx%23_Toc377666816

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    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)

    Specimens joint were cut in transverse weld zone. ........................................................ 44

    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    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

    Figure 4.0.4: Hardness distribution profile along the cross section of the Al-SS joint

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

    Figure 4.0.5: Hardness distribution profile along the cross section of the Al-SS joint

    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

    file:///C:/Users/user/Desktop/master%20project%20chapter/master%20full%20report/MP2%20full%20report-%20personal.docx%23_Toc377666842file:///C:/Users/user/Desktop/master%20project%20chapter/master%20full%20report/MP2%20full%20report-%20personal.docx%23_Toc377666843file:///C:/Users/user/Desktop/master%20project%20chapter/master%20full%20report/MP2%20full%20report-%20personal.docx%23_Toc377666844file:///C:/Users/user/Desktop/master%20project%20chapter/master%20full%20report/MP2%20full%20report-%20personal.docx%23_Toc377666844file:///C:/Users/user/Desktop/master%20project%20chapter/master%20full%20report/MP2%20full%20report-%20personal.docx%23_Toc377666845file:///C:/Users/user/Desktop/master%20project%20chapter/master%20full%20report/MP2%20full%20report-%20personal.docx%23_Toc377666848file:///C:/Users/user/Desktop/master%20project%20chapter/master%20full%20report/MP2%20full%20report-%20personal.docx%23_Toc377666849file:///C:/Users/user/Desktop/master%20project%20chapter/master%20full%20report/MP2%20full%20report-%20personal.docx%23_Toc377666849file:///C:/Users/user/Desktop/master%20project%20chapter/master%20full%20report/MP2%20full%20report-%20personal.docx%23_Toc377666849file:///C:/Users/user/Desktop/master%20project%20chapter/master%20full%20report/MP2%20full%20report-%20personal.docx%23_Toc377666850file:///C:/Users/user/Desktop/master%20project%20chapter/master%20full%20report/MP2%20full%20report-%20personal.docx%23_Toc377666851file:///C:/Users/user/Desktop/master%20project%20chapter/master%20full%20report/MP2%20full%20report-%20personal.docx%23_Toc377666851file:///C:/Users/user/Desktop/master%20project%20chapter/master%20full%20report/MP2%20full%20report-%20personal.docx%23_Toc377666852file:///C:/Users/user/Desktop/master%20project%20chapter/master%20full%20report/MP2%20full%20report-%20personal.docx%23_Toc377666852file:///C:/Users/user/Desktop/master%20project%20chapter/master%20full%20report/MP2%20full%20report-%20personal.docx%23_Toc377666853file:///C:/Users/user/Desktop/master%20project%20chapter/master%20full%20report/MP2%20full%20report-%20personal.docx%23_Toc377666853file:///C:/Users/user/Desktop/master%20project%20chapter/master%20full%20report/MP2%20full%20report-%20personal.docx%23_Toc377666857file:///C:/Users/user/Desktop/master%20project%20chapter/master%20full%20report/MP2%20full%20report-%20personal.docx%23_Toc377666857file:///C:/Users/user/Desktop/master%20project%20chapter/master%20full%20report/MP2%20full%20report-%20personal.docx%23_Toc377666859file:///C:/Users/user/Desktop/master%20project%20chapter/master%20full%20report/MP2%20full%20report-%20personal.docx%23_Toc377666860file:///C:/Users/user/Desktop/master%20project%20chapter/master%20full%20report/MP2%20full%20report-%20personal.docx%23_Toc377666864file:///C:/Users/user/Desktop/master%20project%20chapter/master%20full%20report/MP2%20full%20report-%20personal.docx%23_Toc377666864

  • 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

    14mm tool shoulder diameter .......................................................................................... 72

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

    14mm tool shoulder diameter .......................................................................................... 73

    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

    file:///C:/Users/user/Desktop/master%20project%20chapter/master%20full%20report/MP2%20full%20report-%20personal.docx%23_Toc377666866file:///C:/Users/user/Desktop/master%20project%20chapter/master%20full%20report/MP2%20full%20report-%20personal.docx%23_Toc377666866file:///C:/Users/user/Desktop/master%20project%20chapter/master%20full%20report/MP2%20full%20report-%20personal.docx%23_Toc377666866file:///C:/Users/user/Desktop/master%20project%20chapter/master%20full%20report/MP2%20full%20report-%20personal.docx%23_Toc377666872file:///C:/Users/user/Desktop/master%20project%20chapter/master%20full%20report/MP2%20full%20report-%20personal.docx%23_Toc377666872file:///C:/Users/user/Desktop/master%20project%20chapter/master%20full%20report/MP2%20full%20report-%20personal.docx%23_Toc377666872file:///C:/Users/user/Desktop/master%20project%20chapter/master%20full%20report/MP2%20full%20report-%20personal.docx%23_Toc377666873file:///C:/Users/user/Desktop/master%20project%20chapter/master%20full%20report/MP2%20full%20report-%20personal.docx%23_Toc377666873file:///C:/Users/user/Desktop/master%20project%20chapter/master%20full%20report/MP2%20full%20report-%20personal.docx%23_Toc377666873file:///C:/Users/user/Desktop/master%20project%20chapter/master%20full%20report/MP2%20full%20report-%20personal.docx%23_Toc377666874file:///C:/Users/user/Desktop/master%20project%20chapter/master%20full%20report/MP2%20full%20report-%20personal.docx%23_Toc377666874file:///C:/Users/user/Desktop/master%20project%20chapter/master%20full%20report/MP2%20full%20report-%20personal.docx%23_Toc377666874file:///C:/Users/user/Desktop/master%20project%20chapter/master%20full%20report/MP2%20full%20report-%20personal.docx%23_Toc377666875file:///C:/Users/user/Desktop/master%20project%20chapter/master%20full%20report/MP2%20full%20report-%20personal.docx%23_Toc377666876file:///C:/Users/user/Desktop/master%20project%20chapter/master%20full%20report/MP2%20full%20report-%20personal.docx%23_Toc377666876file:///C:/Users/user/Desktop/master%20project%20chapter/master%20full%20report/MP2%20full%20report-%20personal.docx%23_Toc377666877file:///C:/Users/user/Desktop/master%20project%20chapter/master%20full%20report/MP2%20full%20report-%20personal.docx%23_Toc377666878file:///C:/Users/user/Desktop/master%20project%20chapter/master%20full%20report/MP2%20full%20report-%20personal.docx%23_Toc377666878

  • 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

    m - Micrometer

    - 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 shoulders 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 tools probe (pin) diameter

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

    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)

    Source: Mishra & Ma (2005)

    Figure 2.0.6: Schematic drawing of the FSW tool.

  • 14

    Source: Mishra & Ma (2005)

    (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.

    Source: Mishra & Ma (2005)

    (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).

    Source: Mishra & Ma (2005)

  • 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 vehicles 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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