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FRICTION STIR WELDING OF DISSIMILAR METAL

SUJANURIAH BT SAHIDI

A project report submitted in partial fulfilment of the requirement for the

award of the Degree of Master of Manufacturing

Faculty of Mechanical and Manufacturing Engineering Universiti Tun

Hussein Onn Malaysia

JUNE 2013
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ABSTRACT

Friction Stir Welding is a solid – state thermo – mechanical joining process

(a combination of extruding and forging). Joining of steels to aluminium

alloys can be used for producing steel/aluminium bimetallic parts in a wide

range of industrial areas. The overall aim of this study is to get the optimum

parameters for the materials under considerations, to investigate the Heated

Affected Zone (HAZ), Thermo – Mechanical Affected Zone (TMAZ) and

Weld Nugget (WN) besides to study the defects occurring during welding

process by applying different parameters chosen. The welding process was

done by using conventional milling machine. Three experiments being used

are the Tensile Testing, Optical Microscopy (OM) and Electron Scanning

Microscopy (SEM) to get the strength of the joint and the metallographic

studies. The findings also found out that suitable parameters being choose

give less defect and intermetallic compounds (IMCs). Therefore, at higher

speed and lower tool plunge length, the joint strength decreased due to lack

of bonding between aluminium and steel.

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CONTENTS

TITLE i

DECLARATION ii

ACKNOWLEDGEMENT iii

ABSTRACT iv

CONTENTS v

LIST OF TABLE viii

LIST OF FIGURE ix

CHAPTER 1 INTRODUCTION

1.1 Introduction 1

1.2 Objectives 4

1.3 Scopes 4

CHAPTER 2 LITERATURE REVIEW

2.1 Introduction 5

2.2 Friction Stir Welding 6

2.3 Friction Stir Welding (FSW) Process Principles 9

2.4 Comparison of Friction Stir Welding (FSW) to

Other Welding Processes 10

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2.4.1 Improved Weldability 11

2.4.2 Reduced Distortion 11

2.4.3 Improved fatigue, corrosion and stress

corrosion cracking performance 12

2.4.4 Improved Static Strength and Ductility 12

2.5 Welding Tools 13

2.5.1 Tool Geometry 13

2.5.2 Welding Parameters 14

2.5.3 Joint Design 16

2.6 Joint Geometries 17

2.7 FSW of Dissimilar Materials 18

CHAPTER 3 RESEARCH METHODOLOGY

3.1 Introduction 21

3.2 The Flow Chart 23

3.3 Workpiece Material 24

3.4 FSW Machine and Equipment 25

3.5 Experimental Procedure 27

3.5.1 Tensile Testing 27

3.5.2 Optical Microscopy 28

3.5.3 Electron Scanning Microscope (SEM) 28

CHAPTER 4 RESULTS AND DISCUSSION

4.1 Introduction 29

4.2 Process Parameters, Temperature and Heat Loss 30

4.3 Microstructure Observations

4.3.1 Effect of Welding Speed 32

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4.3.2 The Effect of a Pin Rotation Speed on the

Tensile Strength of a Joint 39

4.4 Tunnel Formation 40

4.5 Effects of Tool Tilt Angle 43

4.6 Analysis and Observation of Cross – Section

Microstructure of a Joint 44

CHAPTER 5 CONCLUSION 46

CHAPTER 6 RECCOMENDATION 48

REFERENCES 49
 viii

LIST OF TABLES

2.1 Key Benefits of Friction Stir Welding 7

2.2 Main FSW Process Variables 15
3.1 Types of Work Material Used In Present Study 24
3.2 Nominal Chemical Composition of the
Stainless Steel 24
3.3 Nominal Chemical Composition of the 6061
Aluminium Alloy 24
3.4 Welding Parameters and Tool Properties 25
3.5 Summary of the Welding Parameters
and Tool Plunge Depth 26
4.1 Tensile Strength of Weld Using Different
Tool Shoulder Diameter and Different Tool
Rotational Speed for Butt Joint 31
4.2 Tensile Strength of Weld Using Different
Tool Shoulder Diameter and Different
Tool Penetration Depth 31
4.3 Welding Temperature by Using Different
Tool rotational Speed with Different Welding Speed
and Different Plunge Depth 34
4.4 Vickers Microhardness Data for Sample
Weld at 2000 rpm and 3000 rpm 37
 ix

LIST OF FIGURES

2.1 Schematic Diagram of Friction Stir Welding 7

2.2 Schematic Drawing of the FSW Tool 14
2.3 Joint Configuration for Friction Stir Welding 17
2.4 A Schematic Illustration of FSW Butt – Joint,
The Two Sheets Are Transparently Presented
To Show the Probe 19
3.1 Micrographs of the Microstructure of the (a)
6061 Aluminium Alloy and (b) AISI 301
Stainless Steel 24
3.2 Conventional Milling Machine 25
3.3 QC – 3A Universal Testing Machine 27
3.4 Optical Microscopy (OM) 28
4.1 Tensile Strength of the Weld Obtained
With Different Tool Rotational Speed
Using Different Shoulder Diameter 32
4.2 Tensile Strength of the Weld Obtained
With Different Tool Penetration Depth
Using Different Shoulder Diameter 33
4.3 Displacement of the Weld Obtained With
Different Tool Rotational Speed Using
Different Shoulder Diameter 33
4.4 Welding Temperature by Using Different Tool
Rotational Speed with Different Plunge Depth 35
4.5 Welding Temperature by Using Different
Welding Speed with Different Plunge Depth 35
4.6 Hardness Profile for the Studied Welds 38
 x

4.7 OM Images of Weld S1 Showing Different

Grain Size of WN, TMAZ and HAZ 39
4.8 OM Images of Weld S1 Showing Al/Fe
Interface and Heavily Deformed Fe Fragment
In Al 41
 1

CHAPTER 1

INTRODUCTION

1.1 Introduction

Joining of steels to aluminium alloys can be used for producing steel/aluminium

bimetallic parts in a wide range of industrial areas. (Movahedi, et al.,2013).
According to Elrefaey, et al., (2005), the friction stir butt and lap welding of steels to
various aluminium alloys have been studied. However, it is difficult to obtain a
sound steel to aluminum joint by using the conventional fusion welding processes
due to the large difference between the melting points of steel and aluminum alloys
and also the formation of thick brittle Al/Fe intermetallic compounds at the joint
interface. Based on Taban, et al., (2010), joining of aluminium to steel is generally
difficult due to differences between their physical and chemical properties. Both
alloys have incomparable melting point, thermal conductivity, coefficient of linear
expansion and heat capacity. Compared to the fusion processes, low-heat generation
during solid state welding makes it a highly potential approach for aluminum to steel
joining.

Friction Stir Welding is a solid-state thermo-mechanical joining process (a

combination of extruding and forging), invented by The Welding Institute (TWI) in
 2

1991, that has become a viable manufacturing technology of metallic sheet and plate
materials for applications in various industries, including plate materials for
applications in various industries, including aerospace, automobile, defense and
shipbuilding.

According to Thomas WM, et al. (1991) Friction Stir Welding (FSW) is a

relatively new technique developed by The Welding Institute (TWI) for the joining
of Aluminium alloys.

Friction Stir Welding (FSW) process is relatively a new joining process that
is presently attracting considerable interest. FSW is emerging as an appropriate
alternative technology with high efficiency due to high-processing speeds. Since the
joint can be obtained below the melting temperature, this method is suitable for
joining a number of materials those are extremely difficult to be welded by
conventional fusion techniques. (Gene M., 2002). The process is solidstate in nature
and relies on the localized forging of the weld zone to produce the joint.

FSW produces welds by using a rotating, non-consumable welding tool to

locally soften a workpiece, through heat produced by friction and plastic work,
thereby allowing the tool to “stir” the joint surfaces. (Lohwasser and Chen, 2010). In
this welding process, a rotating welding tool is driven into the material at the
interface of, for example, two adjoining plates, and then translated along the interface.
FSW offers ease of handling, precise external process control and high levels of
repeatability, thus creating very homogenous welds. No special preparation of the
sample is required and little waste or pollution is created during the welding process.
Furthermore, its applicability to aluminium alloys, in particular dissimilar alloys or
those considered “unweldable” by conventional welding techniques, such as tungsten
inert gas (TIG) welding, make it as an attractive method for the transportation sector.
The friction stir process involves the translation of a rotating cylindrical tool along
the interface between two plates. Friction heats the material which is then essentially
extruded around the tool before being forged by the large down pressure. The weld is
formed by the deformation of the material at temperatures below the melting
temperature. The simultaneous rotational and translational motion of the welding tool
during the welding process creates a characteristics asymmetry between the
 3

adjoining sides. On one side, where the tool rotation is with the direction of the
translation of the welding tool one peaks of the advancing side (AS), whereas on the
other hand, the two motions, rotation and translation counteract and one speaks of the
retreating side (RS) (M. Steuwer A, M. Withers PJ, 2003).

According to Bhadeshia and Debroy (2009), the level of activity in research

on the friction stir welding of steels is dwarfed when compared with that on
aluminium alloys. The relative weakness of aluminium makes it ideally suited for the
process which requires, at high strain rates, the permanent flow and mixing material
without melting. It is apparent that the torment that an FSW tool would have to go
through in the case of steel would be much greater than that for aluminium unless
temperatures are achieved in excess of some 800 ºC; the steel must be sufficiently
plasticized to permit the material flow to enable a sound weld to be fabricated.

In recent years, numerical modeling of FSW has provided significant insight

about the heat generation patterns, materials flow fields, temperature profiles,
residual stress and distortion, and certain aspects of tool design. The development of
new welding tool materials and geometries has made it oossible to join materials
such as steel and titanium in the laboratory environment and in a limited number of
production applications. In FSW, of steel it has been shown that the lower welding
temperature can lead to very low distortion and unique joint properties. FSW of steel
is an area of active research, so it is reasonable to expect other production
applications to emerge over time. A very attractive application is FSW of steel plate
for shipbuilding applications, based primarily on the reduction of welding distortion,
but the development of low-cost welding equipment and more robust welding tool
materials is required before this application can be exploited.

Buffa and Fratini (2009), have applied the method of applying the role of tool
geometry to steels, with validating consisting of a comparison of the far field thermal
profiles against published experimental data on the austenitic stainless steel.
 4

1.2 Objectives

For this research, the objectives that are tried to achieve by the researcher are:

1. To get optimum parameters for the materials under considerations i.e.

alloy steel and Austenitic Stainless Steel

2. To investigate the Heated Affected Zone (HAZ) and Thermo-Mechanical

Affected Zone (TMAZ)

3. Defects occurring during the welding process

1.3 Scope of Study

The focus of the research work will be concentrated in the mechanical performance
and the stir zone microstructure by FSW lap and butt welded part having 100mm ×
100mm × 3mm thick sheet Aluminium (A6061) and 100mm × 100mm × 3mm thick
sheet Austenitic Stainless Steel using different pin diameters. All the testing of
welded part will be tested by ASTM standard. Different pin diameters tool will used
to conduct experiments.

In this research, Universal Testing Machine (UTM), Optical Microscope (OM)

to get the microstructure properties and Scanning Electron Microscope (SEM) will
also be used to measure HAZ and TMAZ zone.
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CHAPTER 2

LITERATURE REVIEW

2.1 Introduction

Friction stir welding (FSW) is a relatively new solid-state joining process. This
joining technique is energy efficient, environment friendly and versatile. In particular,
it can be used to join high-strength aerospace aluminum alloys and other metallic
alloys that are hard to weld by conventional fusion welding.

FSW is considered to be the most significant development in metal joining in

a decade. Recently, friction stir processing (FSP) was developed for microstructural
modification of metallic materials.

Due to high corrosion resistance and exceptional mechanical properties and

the reference phase diagram of Al-Fe systems, Baker (1993) states that the low
 6

solubility of iron in aluminium promotes the formation of brittle intermetallic

compounds (IMCs) such as Fe2Al5, FeAl3 and FeAl, in the weld zone. Therefore, it
seems that obtaining strong joint between aluminium and steel sounds impossible or
very difficult by using common fusion welding techniques. Different techniques such
as diffusion welding and friction welding have been used to join aluminium to steel.
Based on the research done, it is proven that at low melting speeds due to the
formation of thick IMCs (which was characterized as Al6Fe and Al5Fe2) in the weld
zone the tensile strength of joints was very poor. Even at low welding speeds the
tunnel defect was formed. At higher welding speed and lower tool plunge depth, the
joint strength decreased due to lack of bonding between aluminium and steel.

According to Mishra and Ma (2005), particular emphasis has been given to (a)
mechanisms responsible for the formation of welds and microstructural refinement
and (b) effects of FSW/FSP parameters on resultant microstructure and final
mechanical properties have been studied. The technology diffusion has significantly
outpaced the fundamental understanding of microstructural evolution and
microstructure-property relationships between metals and alloys. Moreover, the use
of lightweight metals (for example, Al alloy) as the structural components to replace
the heavier steel alloy in automotive have been thought to be a promising approach.
(Sun,et al., 2013)

2.2 Friction Stir Welding

Friction Stir Welding (FSW) is considered to be the most significant development in

metal joining in a decade and is a “green” technology due to its energy efficiency,
environment friendliness and versatility (Mishra and Ma, 2005). As compared to the
conventional welding methods, FSW consumes considerably less energy. No cover
gas or flux is used, thereby making the process environmentally friendly.

The joining, does not involve any use of filler metal and therefore any
aluminum alloy can be joined without concern for the compatibility of composition,
 7

which is an issue in fusion welding. When desirable, dissimilar aluminum alloys and
composites can be joined with equal ease.

Figure 2.1: Schematic diagram of friction stir welding

In contrast to the traditional friction welding, which is usually performed on

small axisymmetric parts that can be rotated and pushed against each other to form a
joint, FSW can be applied to various types of joints like butt joints, lap joints, T butt
joints and fillet joints. The key benefits of FSW are summarized in Table 2.1.

Table 2.1: Key benefits of friction stir welding (Mishra and Ma, 2005)

Metallurgical benefits Environmental benefits Energy benefits

Improved materials use (e.g.,
joining different thickness)
Solid phase process No shielding gas required allows reduction in weight
Only 2.5% of the energy
Low distortion of workpiece No surface cleaning required needed for a laser weld

Good dimensional stability and

repeatability Eliminate grinding wastes
Eliminate solvents required for
Decrease fuel consumption in
No loss of alloying elements degreasing
light weight aircraft,
Excellent metallurgical automotive and ship
properties in the joint area applications
Consumable materials saving
Fine microstructure such as rugs, wire or any other
Absence of cracking gases
Replace multiple parts joined
by fasteners
 8

Before the invention of FSW, there had been some important technological
developments of non – fusion welding processes, which have found some limited
industrial uses. A significant process of these is friction welding developed at the
time just before laser was invented. During friction welding, the pieces to be welded
are compressed together and are made to more relative to each other. Thus frictional
heat is generated to soften the material in the joining region. The final step is made
by applying increased pressure to the softened material to yield a metallurgical joint
without melting the joining material. However, the relative movement during the
stage of heat generation and material softening can practically only be rotational or
linear. Although friction welding operation is simple, the welding geometry is quite
restricted and thus its use is also limited.

For solid state welding, the thermomechanical principle of friction welding

had actually laid an important base for the later invention of FSW. The Welding
Institute (TWI) in the UK had for years engaged in various R&D and industrial
activities of friction welding and surfacing. Wayne Thomas and his colleagues in
TWI had long worked on and developed friction extrusion, friction hydropillar
processing and third-body friction joining processes.

To date it is with aluminium alloys that FSW is most successfully applied.

The reason for the predominant use of FSW on aluminium alloys is a combination of
process simplicity in principle and the wide use of aluminium alloys in many major
industries. It is especially the case where some aluminium alloys are difficult to
fusion weld as, for example, is clearly evident in FSW application made by Boeing
for making the Delta 2 rocket tanks. FSW allowed them to dramatically reduce their
defect rate to nearly zero. Maximum temperature during FSW can reach just below
the solidus of the workpiece alloy. For most aluminium alloys, it is significantly less
than 660 ºC. Thus, H13 tool steel or high-speed tool steel, which is quite inexpensive,
is a satisfactory tool material. Thus, FSW of aluminium alloys is relatively
straightforward, although FS engineering, particularly for components and structures
of high geometry complexity, can be quite challenging.

According to Ghosh et al.(2011), friction stir welded advanced high strength

steel (AHSS) joints are scanty. However, FSW and friction spot stir welding (FSSW)
 9

allow the possibility of joining advanced high strength steels and reduce problems
associated with resistance spot welding (RSW). In principle, FSW could be applied
for welding of all solid metallic materials. During FSW of steels, the local operating
temperature generated by both friction and deformation needs to be at 1100 ºC –
1200 ºC so that the workpiece material is sufficiently plasticized for stirring and
welding. Such high operating temperatures and the necessary forces acting on the
tool during FSW create an extraordinary demand on the mechanical properties of the
tool material.

2.3 Friction Stir Welding (FSW) Process Principles

Friction stir welding (FSW) produces welds by using a rotating, non-consumable

welding tool to locally soften a workpiece, through heat produced by friction and
plastic work, thereby allowing the tool to “stir” the joint surfaces. The dependence on
friction and plastic work for the heat source precludes significant melting in the
workpiece, avoiding many of the difficulties arising from a change in state, such as
changes in gas solubility and volumetric changes, which often plague fusion welding
processes. Further, the reduced welding temperature makes possible dramatically
lower distortion and residual stresses, enabling improved fatigue performance, new
construction techniques, and making possible the welding of very thin and very thick
materials.

FSW has also been shown to eliminate or dramatically reduce the formation
of hazardous fumes and reduces energy consumption during welding, reducing the
environmental impact of the joining process. FSW can be used in any orientation
without regard to the influence of gravitational effects on the process. These
distinctions from conventional arc welding processes make FSW a valuable new
manufacturing process with undeniable, economic, and environmental benefits.

According to Najafabadi et al. (2010), FSW is an innovative solid state

bonding technique. In early years, it was introduced for light alloys. Recently, high
 10

performance tools materials are employed for FSW of high melting temperature
materials such as titanium, nickel and steels.

2.4 Comparison of friction stir welding (FSW) to other welding processes

Comparison of FSW to other welding processes is typically done within the context
of justifying the use of the process over other, more conventional techniques.
Successful application of FSW depends upon a clear misunderstanding of the
characteristics of the process, so favourable technical and economic justification can
be developed.

The unique, favourable characteristics of FSW compared to traditional arc

welding methods provide several sources for technical justification for use of the
process.

The main points for technical justification of FSW compared to arc welding
processes are:

 Improved weldability

 Reduced distortion

 Reduced residual stress, improved fatigue, corrosion, and stress corrosion

cracking performance

 Improved cosmetic performance

 Elimination of under matched filler metal

 Improved static strength and ductility

 Mechanized process

 High robustness, few process variables

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2.4.1 Improved Weldability

According to Mishra and Ma (2005), a solid-state welding process patented by The

Welding Institute (TWI), in 1991, is a potential candidate for the joining of dissimilar
materials due to the lower processing temperature over conventional fusion welding.
(Sato et al.,2004). This is especially the case in certain aluminium alloys.

Some aluminium alloys or material forms, such as castings, are difficult or

impossible to weld by traditional arc welding processes due to problems with the
formation of brittle phases and cracking. For these alloys, weldability alone may be
sufficient to form a justification for the use of FSW over conventional arc welding or
other joining techniques, such as mechanical fasteners. Further, FSW makes possible
the joining of some dissimilar alloys, which can be of significant benefit in certain
applications.

Besides, defect-free welds have now been made by FSW in the joining of
different Al alloys (for example Al 2024/Al 7075) (Cavaliere et al., 2008), Al/steel
( Lee et al., 2006), and Al/Mg (Kwon et al., 2008).

2.4.2 Reduced Distortion

The reduced peak temperature reached in FSW compared to arc welding processes
also generally leads to reduced longitudinal and transverse distortion, although FSW
weldments are certainly not free of residual stress. The balance if residual stress in
FSW can result in essentially flat weldments in materials of virtually any practically
weldable thickness, although this is affected by welding tool design, joint design,
welding parameters and fixture design.
 12

2.4.3 Improved fatigue, corrosion, and stress corrosion cracking performance

The reduced maximum temperature and residual stress can also lead to improved
performance under cyclic loading conditions. Typically, joints produced by FSW
have fatigue strength, but below base metal strength. FSW joints that are machined
after welding have been shown to approach base metal fatigue strength. Based from
the studied by D.M. Rodrigues et al. (2009), the base material is characterized by a
recrystallized microstructure with equiaxed grains, with relatively uniform grain size.

According to P. Cavaliere et al. (2009), the studied friction stir welded joints
offer the best fatigue performances only when optimal microstructure configurations
are reached. With a revolutionary pitch in the range of 0.07-0.1, the process is in the
optimal temperature and strain rates conditions to produce good microstructure
quality without defects for butt joints and therefore sound welds are achieved. Based
on the studied longitudinal residual stresses, the residual stresses values differences
depend on the asymmetry of the FSW process, where higher deformation across the
weld line are achieved.

2.4.4 Improved static strength and ductility

Even in cases where adequate filler metals are available, the higher temperature
reached and limited material deposition rates in arc welding can degrade the HAZ
sufficiently to reduce the joint strength compared to FSW. It is often the case in thin
section aluminium alloys that the joint strength in arc welding and FSW are
comparable. However, in thick materials, up to 75mm thick, the fact that FSW can be
accomplished in a single pass can result in significantly improved joint strength and
ductility. In some applications, this may be sufficient to justify the use of FSW over
arc welding and mechanical fastening.
 13

2.5 Welding Tools

Many of the advanced made in friction stir welding have been enabled by the
development of new welding tools. The welding tool design, including both its
geometry and the material from which it is made, is critical to the successful use of
the process.

2.5.1 Tool Geometry

Welding tool geometry development led to the first sound welds made in aluminium
alloys and this has led to higher weld production speeds, higher workpiece thickness,
improved joint properties, new materials and new welding equipment.

According to Mishra and Ma (2005), tool geometry is the most influential

aspect of process development. The tool geometry plays a critical role in material
flow and in turn governs the traverse rate at which FSW can be conducted. An FSW
tool consists of a shoulder and a pin as shown schematically in Fig. 2.2 below. The
tool has two primary functions: (a) localized heating, and (b) material flow. In the
initial stage of tool plunge, the heating results primarily from the friction between pin
and workpiece. Some additional heating are the results from deformation of the
material. The tool is plunged till the shoulder touches the workpiece. The friction
between the shoulder and workpiece results in the biggest component of heating.
From the heating aspect, the relative size of pin and shoulder is important, and the
other design features are not critical. The shoulder also provides confinement for the
heated volume of material. The second function of the tool is to ‘stir’ and ‘move’ the
material. The uniformity of microstructure and properties as well as process loads is
governed by the tool design. Generally, a concave shoulder and threaded cylindrical
pins are used.
 14

Figure 2.2: Schematic drawing of the FSW tool

With increasing experience and some improvement in understanding of

material flow, the tool geometry has evolved significantly. Complex features have
been added to alter material flow, mixing and reduce process loads. Thomas et al.
(2001) suggested that the major factor determining the superiority of the whorl pins
over the conventional cylindrical pins is the ratio of the swept volume during rotation
to the volume of the pin itself, i.e., a ratio of the “dynamic volume to the static
volume” that is important in providing an adequate flow path. Typically, this ratio for
pins with similar root diameters and pin length is 1:1:1 for conventional cylindrical
pin.

For lap welding, conventional cylindrical threaded pin resulted in excessive

thinning of the top sheet, leading to significantly reduced bend properties.
Furthermore, for lap welds, the width of the weld interface and the angle at which the
notch meets the edge of the weld is also important for applications where fatigue is
of main concern.

2.5.2 Welding Parameters

With the general principles of the effect of process variables on the friction stir
welding process have much in common with other welding processes, the details are
 15

completely different, as one might expect. The main process variables in friction stir
welding are listed in Table 2.2.

Table 2.2: Main FSW process variables

Tool design Variables Machine Variables Other Variables

Shoulder and pin materials Welding speed Anvil material
Shoulder diameter Spindle speed Anvil size
Pin diameter Plunge force or depth Workpiece size
Pin length
Thread pitch Tool tilt angle Workpiece properties
Feature geometry

These variables all act to determine the outcome of the welding process. The
welding process affects these joint properties primarily through heat generation and
dissipation, so primary attention should be given to the effect of the welding process
variables on heat generation and related outcomes.

For FSW, two parameters are very important: tool rotation rate ( ω, rpm) in
clockwise or counterclockwise direction and tool traverse speed ( v, mm/min) along
the line of joint. The rotation of tool results in stirring and mixing of material around
the rotating pin and the translation of tool moves the stirred material from the front to
the back of the pin and finishes welding process. Higher tool rotation rates generate
higher temperature because of higher friction heating and result in or intense stirring
and mixing of material. However, it should be noted that frictional coupling of tool
surface with workpiece is going to govern the heating. So, a monotonic increase in
heating with increasing tool rotation rate is not expected as the coefficient of friction
at interface will change with increasing tool rotation rate.

In addition to the tool rotation rate and traverse speed, another important
process parameter is the angle of spindle or tool tilt with respect to the workpiece
surface. A suitable tilt of the spindle towards trailing direction ensures that the
shoulder of the tool holds the stirred material by threaded pin and move material
efficiently from the front to the back of the pin. Further, the insertion depth of pin
into the workpieces (also called target depth) is important for producing sound welds
with smooth tool shoulders. The insertion depth of pin is associated with the pin
 16

height. When the insertion depth is too shallow, the shoulder of tool does not contact
the original workpiece surface. Thus, rotating shoulder cannot move the stirred
material efficiently from the front to the back of the pin, resulting in generation of
welds with inner channel or surface groove. When the insertion depth is too deep, the
shoulder of tool plunges into the workpiece creating excessive flash. It should be
noted that the recent development of ‘scrolled’ tool shoulder allows FSW with 0 º
tool tilt. Such tools are particularly preferred for curved joints.

Preheating or cooling can also be important for some specific FSW processes.
For materials with high melting point such as steel and titanium or high conductivity
such as copper, the heat produced by friction and stirring may be not sufficient to
soften and plasticize the material around the rotating tool. Thus, it is difficult to
produce continuous defect-free weld. In these cases, preheating or additional external
heating source can help the material flow and increase the process window. On the
other hand, materials with lower melting point such as aluminium and magnesium,
cooling can be used to reduce extensive growth of recrystallized grains and
dissolution of strengthening precipitates in and around the stirred zone.

2.5.3 Joint design

The most convenient joint configurations for FSW are butt and lap joints. A simple
square butt joint is shown in Figure 2.3a. Two plates or sheets with same thickness
are placed on a backing plate and clamped firmly to prevent the abutting joint faces
from being forced apart. During the initial plunge of the tool, the forces are fairly
large and extra care is required to ensure that plates in butt configuration do not
separate.

A rotating tool is plunged into the joint line and traversed along this line
when the shoulder of the tool is in intimate contact with the surface of the plates,
producing a weld along abutting line. On the other hand, for a simple lap joint, two
lapped plates or sheets are clamped on a backing plate. A rotating tool is vertically
plunged through the upper plate and into the lower plate and traversed along desired
 17

direction, joining the two plates (Fig 2.3d). Many other configurations can be
produced by combination of butt and lap joints. Apart from butt and lap joint
configurations, other types of joint designs, such as fillet joints (Fig. 2.3g), are also
possible as needed for some engineering applications.

It is important to note that no special preparation is needed for FSW of butt

and lap joints. Two clean metal plates can be easily joined together in the form of
butt or lap joints without any major concern about the surface conditions of the plates.

Figure 2.3: Joint configurations for friction stir welding: (a) square butt, (b)
edge butt, (c) T butt joint, (d) lap joint, (e) multiple lap joint, (f) T lap joint, and
(g) fillet joint

2.6 Joint Geometries

A variety of joint geometries are possible with FSW; however, there are certain
limitations and requirements that are unique to the process.

In each of joint designs and fixture arrangements, it is necessary to:

 Provide sufficient area for the welding tool shoulder path

 Provide sufficient containment of softened weld metal

 Provide sufficient force to prevent motion of the workpieces

 18

 Provide adequate heat sink to dissipate the heat of welding

The area required for the welding tool shoulder is a function of material
thickness and alloy. For aluminium alloys, the area required for the shoulder is about
three to five times the material thickness. Steel and titanium typically would require
less shoulder area, since these materials have lower thermal conductivity and
therefore require a smaller shoulder diameter.

2.7 FSW of dissimilar materials

According to Coelho et al. (2012), the use of light-weight materials for industrial
applications is a driving force for the development of joining techniques. Friction Stir
Welding (FSW) inspired joints of dissimilar materials because it does not involve
bulk melting of the basic components. In the research by Coelho et al., two different
grades of high strength steel (HSS), with different microstructures and strengths,
were joined to AA6181-T4 alloy by FSW and the study has proved that the influence
of the distinct HSS base material on the joint efficiency.

Dissimilar materials welding are an indispensible technique for many

industrial sectors, offering the possibility to optimize the welded component
performances with the different material properties for the local loads within a given
parts. The melting phase absence allows joining dissimilar materials with the
achievement of sound welds (Scialpi et al., 2008). Dissimilar fusion welding between
Al alloy and steels is a challenge in welding control because of the large differences
in melting temperature and physical and mechanical properties of the alloys involved.
The process often results in complex weld poor shapes, inhomogeneous solidification
microstructures and segregations in addition, the extremely low solubility of Fe in Al
leads to the formation of brittle and excessive Al – rich FexAly intermetallic phases
which are detrimental for the mechanical properties of the joint.
 19

Figure 2.4: A schematic illustration of FSW butt-joint, the two sheets are
transparently represented to show the probe

FSW is based on extreme plastic deformation in the solid-state where no

associated bulk melting is involved. At early stages of the process development,
FSW appears especially attractive for joining Al alloys and other light-weight
materials like Mg alloys. This is connected with two main reasons:

1) The process prevents melting and solidification, minimizing residual stresses,

cracking, porosity and loss volatile solutes;

2) The plastic deformation (stirring) of such light-weight materials (e.g. Al and

Mg alloys) can be realized using relatively simple welding tools (e.g. made of
tool steel)

The FSW of steels involved high temperatures; Ohashi et al.(2009), found the
base dual phase steel to suffer contamination with Si, N and O when friction stir spot
welding using a silicone nitride tool. The contamination with oxygen could be
mitigated using an argon shroud, and that from the tool (Si,N) by coating the tool
 20

with TiC and TiN. According to Lee et al. (2009), a steel tool is used to make good
joints between aluminium alloy sheet of 1mm thickness and underlying steel sheet.
The tool does not have to be an exotic material because its penetration during friction
stir spot welds did not exceed half the thickness of the aluminium. The underlying
steel was never touched by the tool. Nevertheless, a mixed layer just 2 µm in
thickness, formed at the aluminium/steel interface, with some intermetallic
compound formation, resulting in a metallurgical bond between the dissimilar
materials. Furthermore, shear tests demonstrated that with this configuration, it is
possible to achieve properties similar to those when the steel is friction stir spot
welded to itself.
 21

CHAPTER 3

METHODOLOGY

3.1 Methodology

Proper planning should be taken by every individual in creating successful report

writing. Before carrying on the report writing, studies must be performed related to
the problem prevailing surrounding issues and create an idea to solve the problem.
Besides that, irregular planning will create problem in producing the thesis report
writing. Methodology method can be used as guidelines for every step in completing
the thesis report writing. The report writing produced based on two main concepts.
They are PRIME and 9P concepts.

The PRIME concepts require Problem, Research, Invention, Modification and

Evaluation. Meanwhile, for 9P concept, they are Problems, Idea Development, Idea
Selection, Material Selection, Prototype Development, Manufacture, Testing,
Modification and Recording.
 22

The study of methodology is a method to identify how the project from the
early stages up to the final presentation. In this chapter, aspects of the report writing
will be described greater depth and detail so that it will be easy to understand.

Identification of the problem is central in the production of report writing. Based on

the identified problems, it is necessary to study methods to solve problems.

In the process of preparing the thesis report, researcher has carried out some
of the rules and procedures for obtaining a good yield and quality. First of all, when
selected the suitable title, the researcher have observed and examined the problems
and materials that can used in this project. Once the problems have been identified,
the researcher managed to get the problem statement, objectives, scope and
categories of projects that will be produced later.

Then after carried out few literature reviews in order to get the basic view for
the project, the selection of materials used for the project was identified and the
testing equipment used in this research was identified. The selection of materials is
not only seen in terms of cost, but also from the quality and durability of material
when used on the project to be produced. With the provision of adequate materials
and proper, the installation process on a project to produce to be going well soon.
 23

3.2 The Flow Chart

Start

Project Proposal

Problem Statement / Objectives /

Scope

Background Study

Project
Understanding

Literature Review

Methodology

Welding Process

(Butt Joint and Lap Joint)

Testing Process
(UTM, SEM, Bend Test
Equipment, Surface
Roughness) NO

0 Tester)

YES

Result / Discussion

End
 24

3.3 Workpiece Material

Chosen materials for FSW technique are a commercial 6061 aluminium alloy
and austenitic stainless steel 304 was used as starting material for friction stir
welding technique. The chemical composition of work materials are listed in Table
3.1, 3.2 and 3.3

Table 3.1: Types of work material used in present study

No Item Specifications
1 Sheet metal A6061 100 mm (length) × 100 mm (width) × 3 mm (thick)
2 Sheet metal AISI 304 100 mm (length) × 100 mm (width) × 3 mm (thick)

Table 3.2: Nominal Chemical Composition of the Stainless Steel

Element C Cr Ni Mn Si P S Fe
wt (%) <0.08 17.5 - 20 8-11 <2 <1 0.045 0.03 Balance

Table 3.3: Nominal Chemical Composition of 6061 Aluminium Alloy

Element Si Fe Cu Mn Mg Cr Zn Ti Al
wt (%) 0.59 0.38 0.26 0.03 0.96 0.25 0.02 0.04 Balance

Figure 3.1: Micrographs of the microstructure of the (a) 6061 aluminium alloy
and (b) AISI 304 stainless steel
 46

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 47

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