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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
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
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
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
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)
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 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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