1. 1
Chapter 1: Introduction
Friction stir spot welding (FSSW) is variation of the “linear” friction stir welding
(FSW) process. By using this process spot weld and lap-weld can be created without bulk
melting.
As being derivative of FSW, FSSW is widely used in automotive and aerospace
sectors. Another advantage is that unlike FSW, FSSW can be considered as short-lived
process since it has short cycle time. The unique feature of FSSW is that solid state
bonding process takes place between two surfaces in contact. The main parameters that
has to be studied in FSSW process are:[1]
a. Tool geometry
b. Tool rotational velocity
c. Downward force applied to the tool
d. Dwell time
e. Plunge depth
Friction-Stir Spot Welding (FSSW) is proved to be a better alternative to
Resistance Spot Welding (RSW). This is because FSW makes a linear weld, FSSW
makes a spot weld. Hence by eliminating the tool translation FSSW is similar to a typical
resistance spot weld.[1]
Over the past several years, industry and university contributions to FSSW
technology are continued to grow. The studies have increased the understanding of this
process on a fundamental level. This has led to its adoption by industry, mostly for
automotive applications. FSSW techniques have been gaining a lot importance in recent
year because of its investigations involved by the industry and universities. FSSW of
aluminum has been executed in automotive manufacturing production. This demonstrates
that it is a higher quality and cost saving process as compared to resistance spot welding.
By considering the applications and importance of friction stir spot welding,
joining of stainless steel plates with optimization of parameters was chosen. This gives
sound metallurgical welds.

2. 2
Chapter 2: Literature survey
2.1 Friction Stir Spot Welding:
Friction Stir Spot Welding (FSSW) is a general term. A single spot weld creates a
discrete, localized joint of limited size. Spot welds are typically proposed to behave in same
manner with other spot welds at the joints of a structure.
FSSW is a solid-state welding process. It involves specially designed rotating cylindrical
tool with varying end geometry and a probe pin. This pin is first plunged into the upper sheet.
When the rotating tool contacts the upper sheet, a downward force is applied on the upper sheet.
But the backing tool beneath the lower sheet supports this downward force. The downward force
and the rotational speed are maintained for an appropriate time. This generates frictional heat.
Due to this frictional heat, sheets get heated. Then, material adjacent to the tool softens and
deforms plastically. A solid-state bond is made between the surfaces of the upper and lower
sheet. Finally, the tool is drawn out of the sheets and projected pin leaves a characteristic exit
hole in the middle of the joint. Heat and plastic flow coming from tool rotation determine
remarkable microstructural modifications. This results in local modification of mechanical
characteristics of material around the joint. Main parameters are:
 Tool geometry
 Tool rotational speed
 Downward force applied to the tool
 Time
Fig.1: Basic Friction Stir Spot Welding[1]

3. 3
FSSW equipment requires significantly less surrounding setup. This means that water,
compressed air, complex electrical transforming equipment not required for FSSW process.[1]
FSSW is also a variation of friction stir seam welding where two overlapping sheets are
joined without traversing the tool. During the plunge stage, material in contact with the tool is
heated and plasticized. Once the joining operation has been completed, the tool is withdrawn.
This leaves a keyhole depression created by the rotating pin. The welding operation is completed
in 1-5secs. Within this short time it involves very rapid heating and cooling rates. A bond is
created between the overlapping sheets in the stir zone region. This region has been plasticized
and consolidated by the tool.
When the pin plunges into the contacting sheets, the material below the pin is
compressed, and extruded upward around the pin periphery. Material flows during friction stir
spot welding have been investigated by fused tracer particles. Bonding between the contacting
sheets occurs prior to penetration into the lower sheet because a layer of dynamically
recrystallized material forms beneath the tip of rotating pin. Once the rotating pin begins to
penetrate into the lower sheet, a bond is established across the diameter of the tool pin. This
causes start of intermixing between the upper and lower sheets.
Fig.2: Schematic representation of FSSW process [2]

4. 4
2.1.1 Tool Geometry:
FSW and FSSW tools have similar characteristics, such as body, shoulder, and probe.
These may have a range of different features and shapes. Features on the probe, such as flats
flutes and threads, can promote the flow of material around the probe. A shoulder with a flat
face and scrolls will tend to capture the material displaced by the probe and redirect it inward
toward the probe.
Probes of different cross-sectional shapes are shown in Fig.3. These shapes serve to
change the ratio of the physical volume of the probe to the swept volume of the probe.
In plunge FSSW, the plunge stage creates a hooking defect at the lap joint interface due
to displacement of probe‟s volume of material. In addition to this, features on the probes such as
threads provide a predicting effect. This effect causes material to recirculate toward the shoulder,
further increase the lifting and hooking, and create a large weld nugget. [3]
The shoulder of pin tool has three main functions:
1. To capture material displaced by the probe
2. To apply Z-force or forging force, and
3. To create frictional heat
A concave shoulder has a small pocket volume which captures the displaced material and
keeps it pressed against the probe. A large shoulder diameter is favorable to create adequate
frictional heat and avoid a large heat-affected zone. A large diameter creates a wider HAZ,
compared to small shoulder diameter. Since low Z-force is the primary goal of this research,
the pin tool shoulder diameter needs to be reduced for low process forces. This provides
Fig.3: Different probe shape with same effective swept area: (a)rectangular (b)triangular
(c)square (d)pentagon (e)hexagon (f)octagon (g)circular

5. 5
sufficient forging force to ensure consolidation of the weld nugget. A large shoulder diameter
requires more Z-force compared to small shoulder diameter. This creates similar forging
pressure for sufficient consolidation of weld nugget.
Fig 4 and Fig. 5 show the results of the heating rate with RPM and temperature with
RPM respectively during the tool penetration stage. It shows an almost linear variation of
both the parameters with RPM.
Fig.4: Variation of tool RPM and heating rate during tool
penetration stage[4]
Fig.5: Variation of tool RPM and temperature during tool
penetration stage[4]

6. 6
2.1.2 Microstructure of FSSW:
Fig.6 shows the partition of welded joint in three parts which is visible in the schematic
representation of axial section of it. Sheets thicknesses t1 and t2 of base material (BM) are 1.5
mm, while t0 is due to tool penetration and is 0.02 mm. BM zone represents material that shows
no modification in properties. The innermost zone, known as Stir Zone is located all around the
blind hole generated by tool pin. This zone presents a complete re-crystallization. As you move
along the radius direction towards external radius, the material has been modified mechanically
by the tool and thermally by the generated heat when the friction between metal and tool occurs.
This zone is termed as Thermo-Mechanically Affected Zone (TMAZ). Also in TMAZ, the
external diameter defines the nugget zone. Beyond this zone, the material is subjected
exclusively to the effects of heat dissipated during the welding process. This zone is also an axial
symmetric one and is called heat affected zone (HAZ). The outer material beyond this zone is
considered as not modified by the welding process. Hence is called as Base Metal (BM). [5]
Fig.6: Junction zones with different interfaces [5]

7. 7
The microstructure analysis of the welds is summarized as follows:[4]
Fig.7(a) shows the microstructure of the spot weld and Fig.7(b) shows the close up of regions I,
II, III and IV as marked in Fig.7(a). Fig.7 (a) shows indentation profile which shows probe pin
and flat tool shoulder. From Fig.it is clearly seen that except near the center hole, the bottom
surface is kept almost flat. In the sir zone the upper and lower sheets are bonded. Two notch tips
can be seen near points „C‟ and „D‟. The notch tips extend into the weld and appear to be formed
from the unwelded interface between the two sheets. The weld joint has no defect in the stir
zone. Fig.7 (b) shows a close up view of region I, II and III shows relative coarse grains in the
base metal, finer grains in TMAZ and very fine equiaxed grains in the stir zone. In the stir zone
due to stirring and recrystallization equiaxed grains are formed. In Fig.7 (b), a close up view of
the region IV shows that the curved interface become vague and disappear close to the stir zone.
As the tool continues to plunge in the upper and lower sheets, the material near the tool shoulder
and near the probe pin is stirred. The shoulder indentation squeezes out a portion of the upper
sheet material which decreases the thickness of the upper sheet material under the shoulder
indentation. However, due to the strain of the neighboring material, the sheet is bent along the
outer circumference of the shoulder indentation. The bending of the sheets creates a gap between
the upper and the lower sheets. The bend is marked by „A‟ and „B‟ and the gap is marked by „C‟
and „D‟ in Fig.7 (a). The squeezed out material from the shoulder indentation forms a ring along
the outer circumference of the shoulder indentation on the top surface of the upper sheet. The
squeezed out material can be seen in Fig.7 (a).[4]

8. 8
In Fig.8 the temperature distribution at the surfaces of both workpieces which is measured
perpendicular from the center of the work piece to the edge is shown. At the tip of the tool, the
temperatures of both plates are the same, which is about 4850
C. The peak temperature of the top
surface of the upper plate is almost constant at the interface of tool‟s shoulder and work piece,
which is within the radius between 1.5 mm and 5 mm away from the center of the work piece.
Then, the temperature starts to decease to about 1500
C at the edge of the work piece.
Fig.7: (a) Micrograph of the cross section of the FSSW weld (b) close up views of regions I, II, III and IV[4]

9. 9
Fig.8: Temperature contours at various times [6]

10. 10
At the top surface of the bottom plate, lower peak temperature is observed. This is due to
conduction heat transfer from the upper plate. The temperature of the top surface of the upper
plate and the top surface of the bottom plate become uniform as we move from 6 mm away from
the centre of the workpiece towards to the edge.
2.1.3 Process Parameters:
The process parameters of FSSW are similar to FSW. It includes spindle speed (rpm), travel
speed, plunge speed, tilt angle (degree), dwell time (sec), and forge load or normal load. The
process parameters of plunge FSSW include plunge speed, dwell time and spindle speed. Since
FSSW is thermo-mechanical process, these three process parameters are chosen as the main
interest of investigation in this study. The term “cold” weld is associated with a weld which is
made with relatively high travel speed and low spindle speed. On the other hand a “hot” weld is
described as weld with relatively low travel speed and high spindle speed. These relative terms
of hot and cold welds do not correlate with peak temperature. One would assume a “hot” weld
should reach a higher peak temperature compared to cold weld. But the higher conductivity of
Fig.9: Plot of temperatures versus distance away from the center of the tool.[6]

11. 11
aluminum tends to disperse the heat of hot weld. This is due to the slow travel speed, hence
lower peak temperature. FSSW need a consistent normal load to produce a good FSSW joint. [3]
2.1.4 Tool Rotation Speed:
The tool rotation speed has profound effect on several parameters like dwell time, heating
rate, temperature, strain rate which finally affect the weld strength and quality.
2.1.5 Interdependency of parameters:
There is interdependency of the welding parameters with respect to variations in Tool
geometry, Plunge depth and Tool rotational speed. Cross tension load increase when tool
penetration increases from -0.2 mm to 0.2 mm and then decreases due to thinning of plate as
shown in fig.12. Effect of tool rotation for cylindrical tool the tensile shear load increases with
rotational speed as depicted in fig.13. For other tools the tensile shear load decreases with
rotational speed (above 1000 Rpm). [8]
Fig.11: Effect of dwell time[7]
Fig.10: Effect of plunge depth[7]

12. 12
Fig.12: Plunging depth Vs tensile shear load using different tool geometry [9]
Fig.13: Plunging depth versus tensile shear load using different
tool geometries[9]

13. 13
1.1 Comparison with Resistance Spot Welding
FSSW can be an efficient alternate process to electrical resistance spot welding.
The FSSW joints which were obtained with higher probe insertion depth/pin heights
resulted in higher tensile shear strength. When the welding time increased from 5s to 10s, the
tensile shear strength of the FSSW joints reduced. Fig.14 and Fig.15 shows the graphical results
of the tensile shear strength of the RSW and FSSW welded joints respectively. The
microhardness of the base plates was measured to be 32 HV50 before welding. Fig.16 and Fig.17
shows the microhardness of the RSW and FSSW. [10]
Fig.14: Tensile shear strength of RSW[10]
[][]joints[11]
Fig.15: Tensile shear strength of FSSW Joints [10]

14. 14
Fig.16: Microhardness distribution of RSW welded joint (a) Upper
Sheet (b) Lower Sheet[10]

15. 15
Fig.17: Microhardness distribution of FSSW welded joint (a)
Upper Sheet (b) Lower Sheet[10]

16. 16
The hardness increase in the FSSW process is higher than in the RSW process. The
tensile shear strengths of the FSSW welded joints are higher than those of the RSW welded
joints. FSSW can be a more efficient alternate process than the electrical RSW process. As
compared to RSW process FSSW has higher plastic deformation in welding zone. Major factor
that affects the tensile shear strength in FSSW process are Probe insertion depth/Pin height. The
welding time and tool rotation is the second and third respectively in the FSSW process.
2.3 Applications:.
Riveting and resistance spot welding are the major competitors to FSW and FSSW. It is
doubtful that FSW will completely replace riveting and resistance spot welding. Rather, efficient
structures will probably use a combination of these technologies. In commercial purpose
applications field Mazda Motor Corporation has been a pioneer. They were the first automobile
manufacturer to apply FSSW to manufacture aluminium body assemblies. They used FSSW for
rear doors and hood of their RX-8 models, pictured in Fig.20. [11]
Fig.18: Centre tunnel on the Ford GT

17. 17
Welding of two different metals, such as aluminium and steel has been a difficult task till
now. However this joining is possible by optimizing the rotating tool face and joining
characteristics. Also by using galvanized steel sheets on one side, joining of aluminium and steel
becomes easier. Galvanic corrosion occurs when two different types of metals come in contact
with each other. Hence use of galvanized sheets prevents the galvanic corrosion. This helps
greatly in weight reduction of the final assembly.
Fig.19 : Mazda MX-8‟s rear door panel

18. 18
2.4 Friction stir spot welding of Stainless steel:
Stainless steel is engineering materials capable of meeting a broad range of design
criteria. They exhibit excellent corrosion resistance, strength at elevated temperature, toughness
at cryogenic temperature, fabrication characteristics and they are selected for a broad range of
consumer, commercial and industrial application. They are used for demanding requirements of
chemical processing to the delicate handling of food and pharmaceuticals. They are preferred
over many other materials because of their performance in even the most aggressive
environments, and they are fabricated by methods common to most manufacturers.[12]
In the fabrication stainless steel products, components or equipment, manufacturers
employ welding as the principle joining methods. Stainless steel is weldable materials and a
welded joint can provide optimum corrosion resistance, strength and fabrication economy.
However, designers should recognize that any metal including stainless steel may undergo
certain changes during welding. Hence it is necessary to exercise a reasonable degree of care
during welding to minimize or prevent any deleterious effect that may occur, and to preserve the
same degree of corrosion resistance and strength in the weld zone that is an inherent part of base
metal.[12]
Structural steel connections are traditionally comprised of a combination of bolted and/or
welded elements. The mild steels used in structural applications are considered to be reasonably
easy to weld, whether in the shop or in the field. However, for steel to remain competitive among
rapid advances in other materials, including composites, advances in other joining technologies
can be examined to establish when special processes may offer an advantage over traditional arc
welding. One of such development was for such advances in steel fabrication and joining
technology is friction stir welding (FSW), a solid state welding process originally
developed by the Welding Institute for welding aluminum.
As the technology becomes more commercially viable, knowledge of its application to
Traditional structural steels have the potential for application in steel construction; for example,
FSW of steel could broaden the potential grades and combinations of steels used in structural
applications, because FSW is particularly suitable to join highly dissimilar materials. By
expanding options for fabrication of steel members with the potential for increased fatigue

19. 19
strength and a reduction in residual stresses, FSW could provide new avenues for steel
construction.[13]
Cracking and porosity occur during fusion welding. This leads to distortion and defects
in the weld. The required energy input is also noticeably low, as melting phenomena are also
fully avoided in FSSW and FSW. As a result, the heat affected zone (HAZ) and residual stresses
are comparatively small.
Even though FSSW provides various technical advantages over conventional welding
processes, FSSW of ferrous alloys including stainless steels (SS) and other advanced high
strength steels is still known to be very difficult for various reasons including high temperatures
and severe tool wear conditions encountered during the process. Due to these difficulties,
resistance spot welding is still the most commonly used spot joining method for steel metal
sheets in automotive applications. However, once a cost-effective, adequate wear resistant tool
material becomes commercially available, the desirable qualities of FSSW joints and those of
ferrous alloys may result in rapid implementation of FSSW of ferrous alloys in various
engineering applications, especially in the automotive industry While the development of
adequate tool materials for FSSW (or FSW) of ferrous alloys is still in progress, a clear
understanding of the characteristics of FSSW joints of ferrous alloys is also important.[14]
2.4.1 Properties of stainless steel[12]
Table 1: Comparison Of welding Characteristics of austenitic steel with carbon steel
Sr.
No.
Carbon
steel Type 304 Remarks
1 Melting point 2800 2550-2650 It requires less heat to
produce fusion which means
faster welding for the same
heat or less heat input for
the same speed
2 Electrical
resistance(annealed) At
20 0
C
At 885 0
C
This is of importance in
electric fusion methods. The
higher electrical resistance
of type 304 results in the
generation of more heat for
12.5 72
125 126

20. 20
the same current of the same
heat with lower current, as
compared with carbon steel.
This together with its low
rate of heat conductivity
accounts for the
effectiveness of resistance
welding methods on type
304.
3
Rate of heat
conductivity(Compared
with percent) At 100
0
C
At 648 0
C
Type 304 conducts heat
much more slowly than
carbon steel thus promoting
sharper heat gradients. This
accelerates warping,
especially in combination
with higher expansion rates.
Slower diffusion of heat
through the base metal
means that weld zones
remain hot longer, one result
of which may be longer dual
in carbide precipitation
range unless excess heat is
artificially removed by chill
bars etc.
28%
100% 66%
4
Coefficient of
expansion per 0
F Over
range indicated
0.0000065
(68- 1162
0
F)
0.0000098(68-
945 0
F)
Type 304 expands and
contracts at faster rate than
carbon steel, which means
that increased expansion and
contraction, must be
allowed for an order to
control warping and the
development of thermal
stresses upon cooling.

21. 21
2.4.2 Influence of alloying elements on weld structure:
The metallurgy of all stainless steel weld metals is controlled by both composition and
solidification rate. But composition is major factor. The structures of welds in austenitic stainless
steel are either full austenite or ferrite in matrix of austenite. The soundness of the weld or
freedom from crack defects is related to the presence of ferrite in austenitic matrix. Therefore, it
is required to know in the case of each element its effect in forming austenite or ferrite. In
addition, some elements have an effect on austenite stability which is important in maintaining
toughness of weld.
Chromium and molybdenum perform dual roles, acting first as strong ferrite formers and
second as strong as austenite stabilizer. Nickel, carbon and nitrogen also perform dual roles,
acting first as strong austenite formers and second as strong austenite stabilizers. Silicon and
columbium perform only as ferrite formers. Manganese performs a dual role, acting first as a
weak austenite former and second as a strong austenite stabilizer.[12]
2.4.3 Friction stir spot welding of Ferritic Stainless steel:
Because of high temperatures and severe tool wear conditions involved during process
this process is known to be very difficult. Due to these difficulties, resistance spot welding is still
the most commonly used spot joining method for steel metal sheets in automotive applications. If
cost-effective and adequate wear resistant tool material is available, the desirable qualities of
FSSW joints will obtain. This results in rapid application of FSSW of ferrous alloys in various
engineering applications, especially in the automotive industry.
The process parameters are rotation speed, depth, and dwell time. A tool of
polycrystalline cubic boron nitride with convex scrolled shoulder was used instead of concave
shoulder tool. One of the advantages of a convex scrolled shoulder tool is a 0° lead angle as
shown in Fig. 20(a) and (b), respectively. Hence, a complicated joining process composed of
spot joining, i.e. FSSW, and linear joining, i.e., FSW, can be conducted without changing the
lead angle during the process. In contrast, a concave shoulder tool generally requires a lead angle
of around 2° for FSW (Fig. 20(c)), while FSSW using a concave shoulder tool is naturally

22. 22
conducted with a 0° lead angle (Fig.20(d)). Another benefit of a convex shoulder tool is that one
may vary the depth of welding with some amount of flexibility, which is more difficult with a
concave shoulder tool. This would allow for welding materials of different thicknesses using the
same tool. [13]
Optical micrographs of base metal and at different regions of FSW joint are displayed in fig. 21.
The base metal (fig. 21a) exhibits a microstructure of coarser ferrite grains. Very fine equiaxed
ferrite grains with grain boundary martensite is observed at the top of the stir zone (fig. 21b),
whereas semi elliptical banded structure of ferrite and martensite is observed in the bottom
region of the stir zone (fig. 21c). In the TMAZ, a distorted structure with grains re-oriented
perpendicular to the transverse direction is observed (fig. 21d and 21e). The high temperature
Fig.20: Schematics of the change of the lead angle during FSW/FSSW
processes: (a) FSW and (b) FSSW with a convex shoulder tool; (c) FSW and (d)
FSSW with a concave shoulder tool.[13]

23. 23
heat-affected zone (HTHAZ) was observed to be similar to the base metal, whereas the
microstructure of the low temperature heat-affected zone (LTHAZ) consisted of finer equiaxed
grains (fig.21f).[14]
The defect free joint of the given ferrous alloy suggests that a convex shoulder tool may
be used in FSSW as well as in FSW. Microstructural analysis shows that the microstructure,
which is quite similar to that in the stir zone of FSW joints of the same SS, has been induced in
the stir zone of the FSSW joint even though the stirring mechanism of FSSW could be quite
different from that of FSW. The microstructural analysis also suggests that both continuous
dynamic recrystallization and recovery occurred in the stir zone during the FSSW.
As schematically shown in Fig.22, the rotational speed is kept constant and the
downward force is controlled by the riveting machine control unit. Initially, the downward force
increases almost linearly for a period of time. Then the downward force is kept nearly constant
for a period of time and finally decreases almost linearly to zero.(ti represents the time that the
tool contacts to the top surface tf represents the time that tool extracts from the top)
Fig.21: Optical micrographs of friction stir welded 409M ferritic stainless steel joint.[14]

24. 24
2.5 Material flow:
The material softened by frictional heat is pushed down to near the bottom surface, which
then moves upwards outside of the material that flows downward, as indicated by the arrow.
Such region is defined as the mixed zone in the present study.
It was found that the morphology of the weld zone depended on processing parameters
such as tool rotational speed and tool holding time. [15]
Fig.22 : A schematic plot of the processing
parameters as a function of time.[15]
Fig.23: Schematic illustration of the cross-section of
friction stir spot weld.[15]

25. 25
2.6 Tensile shear strength
It can be seen that the tensile shear strength increases with increasing tool holding time at a
given tool rotational speed. [15]
The failure of the friction stir spot weld in the lap-shear specimen may be initiated in the
upper sheet near the middle part of the nugget. It is possible that the curved crack growth
occurring in the TMAZ is due to asymmetrical weld nugget geometry and inhomogeneous
material properties in the TMAZ.
Fig.24 : Tensile shear force as a function of tool
holding time.[15]

26. 26
2.7 Dynamic recrystallization:
During hot forming process alloy is liable to undergo work hardening, dynamic recovery
and dynamic recrystallization, three metallurgical phenomenon for controlling microstructure
and mechanical properties. At microstructural level dynamic recrystallization begins when strain
hardening plus recovery can no longer store immobile dislocations. Hot working behavior of
alloys is generally reflected on flow curves which are a direct consequence of microstructural
changes. During deformation dislocations formed and work hardening takes place. Then
rearrangement of dislocations and their self-annihilation causes their absorption by grain
boundaries known as dynamic recovery. Further nucleation and growth of new grains is called as
dynamic recrystallization. The latter is one of most important softening mechanism at high
temperature. This is characteristics of low stacking fault energy materials e.g., austenitic stainless
steel and copper.
The most significant changes in structure sensitive properties occur during the primary
recrystallization stage. In this stage the deformed lattice is completely replaced by a new
unstrained one by means of nucleation and growth process. Then practically stress free grains
grow from nuclei formed in the deformed matrix. The orientation of the new grains differs
considerably from that of crystals they consume i.e. it takes place by the advance of large-angle
boundaries separating the new crystals from the strained matrix.
During dynamic recovery, the original grains get increasingly strained but the sub –
boundaries remain more or less equiaxed. This implies that substructure is dynamic and re-adapts
continuously to the increasing strain. In low SFE metals like austenitic stainless steel, the process
of recovery is slower and allows sufficient stored energy build up. At a critical strain and
variation in driving force, dynamically recrystallized grains appear at the original grain
boundaries resulting in necklace structure. With further deformation more and more potential
nuclei are activated and new recrystallized grains appear. The cause lies in the fact that higher
strain rate and lower temperature provide shorter time for energy accumulation and lower
mobilities at boundaries which result in nucleation and growth of dynamically recrystallized
grains and dislocation annihilation.

27. 27
.
2.8 Cause and Effect diagram (Ishikawa diagram):

28. 28
2.9 Objective:
Welding of Stainless Steel through FSSW is still under development. The process is an energy
saving joining method.
The characteristic of FSSW resulted in elimination of cracking and porosity. Hence
distortion and defect are avoided. FSSW of Stainless Steel Joint provides an optimum corrosion
resistance, strength and fabrication economy so a simple functional process is to be developed.
The objective of this experiment was to study welding of austenitic stainless steel sheets
with lap configuration. Parameters such as tool rotation speed, dwell time and plunge depth was
varied with various combination derived from Tagauchi method of design of experiments.
Optimized samples with the lap weld were further characterized with their micro structure,
hardness and material flow. The other objective was to study the tool wear and its duration of
work. The number of samples and its deformation was studied with reference to its macrograph
and hardness values.

29. 29
Flow chart:
Problem Statement
Parameter Analysis
Material Selection
Tool Selection FixtureSample Dimension
Design and Manufacturing Cutting Design and Manufacturing
Friction Stir Spot Welding of samples using
Vertical Milling Machine
Parameter Variation
Dwell Time Tool RPMPlunge Depth
Characterization of Weld Joint
Hardness
tranverse
Tensile shear
test
Microstructure
analysis
Result and discussion
Conclusion
3. Experimental Plan

30. 30
Activity Bar chart :
Activity bar chart
Activity/ Period Jul Aug Sept Oct Nov Dec Jan Feb Mar April May
Problem Definition
Literature survey
Objective
Fixture Design and manufacturing
Tool Design and manufacturing
Raw Material
Chemical Analysis
Experimental Planning
Machine Training
Experiment
Characterization
Result & Discussion
Conclusion

31. 31
3.1 Fixture design:
In order to perform FSSW using the available Vertical Milling Machine, a proper fixture
was designed to hold the lap joint in place. Referring to the various fixtures designed and used by
earlier researchers, a suitable functional design was finalized.
The other main criterion for the fixture is the durability. The fixture had to withstand
tremendous forces in the z axis during the welding process. Thus a suitably stiff design which
does not deform, fail or fracture during the entire process was taken into account. The fixture
will also be subject to these conditions repeatedly, hence demanding a good shock absorbent
material
The dimensions of the fixture were fixed based on the space and geometrical constraints
in the VMC machining bay. The constraints here were the horizontal slots for securing the
fixture to the base plate of the VMC and also to make sure that any projections above the fixture
were well clear of all the moving machine parts. Along with these the fixture had to be within the
working area of the bay.
These were the geometrical constraints and along with them the fixture had to do the
important function of holding the lap joint samples firmly in place during the entire welding
process. This fixture can at a time hold 4 samples with nut and bolts with backing plate. Hence it
is much easier to operate. It does not play any role in weld joints throughout the cycle. It had to
be designed to configurations of lap joint samples tensile shear test.
3.2 Fixture Manufacturing:
Taking into considerations the above aspects, a fixture design was finalized. A mild
Steel plate of dimensions 250 mm × 200 mm × 10 mm was procured and then machined as per
requirement. The steps involved in the machining of the fixture from the mild steel plate were
1. Milling 2. Drilling
3. Slot cutting 4. Tapping

32. 32
Face Milling –
The surface was machined to get rid of the surface roughness and to give a flat surface for
sample mounting by milling.
Drilling–
Through holes were drilled in the base plate using the drilling machine. The fig shows the
position and dimensions of the holes drilled in the Base Plate.
Slot Cutting –
The Slots to secure the fixture to the base plate were marked and cut.
Tapping–
The last step involved the tapping of the drilled holes which was done using a hand tap.
Side Plate Base Plate

33. 33
Fig. 27 shows the schematic of the manufactured fixture. Also the positions and the dimensions
of all the drills and slots are clearly depicted in the figure. Fig. 28 is the image of the actual
fixture. It can be seen that the sheets are clamped on the top of the base plate using two steel
plates. These plates are also drilled and the holes are matched with the base plate and bolts are
used to clamp the sheets between base plate and the steel sheets.
Figure28:Actual fixture used in FSSW process using VMC
Figure27: Schematic diagram of base plate and side plate

34. 34
3.3Tool Design
Tool material:
Requirements of the tool for FSSW of austenitic stainless steel are high hardness, good
thermal conductivity, and good wear resistant and high melting point. Hence tungsten carbide
was selected as the tool material for experimentation of this process.
Tool Geometry :
Factors affecting the quality of weld in FSSW are tool shoulder diameter, Pin
length and diameter, Concavity of shoulder, tapering pin, flutes on shoulder and pin and
shape of pin. The diameter of the Shoulder was set to 12 mm and the pin diameter was 5mm. The
length of the pin was 1.7 mm from the surface of the shoulder. Shoulder of the tool was provided
with concavity of 100
for better flow of metal. Chamfers and fillets were provided wherever
necessary. Schematic diagram shows the details of tool geometry.
Fig. 29: Circular pin profile tool
1.7

35. 35
3.4 Machine specifications:
To set up the Friction Stir Spot Welding process a suitable CNC controlled milling
machine was required. The machine used during the project course was Premier make, model
number PVM 40. A few specifications of the machine are given in Table 2.
Table 2 Technical specifications of PVM-40
Particulars Values Unit
Work Area
450 ×
850 mm2
Distance from table top to spindle face 150 / 170 mm
Traverse
X axis 510 mm
Y axis 430 mm
Z axis 550 mm
Spindle Speed
60 -
6000 rpm
Feed Rate
1 -
10000 mm/min
Max load on Table 400 Kg
Total connected load 20 kVA
A programme was written to get the carry out FSSW. The programme is as below-
G91 G28 X0 Y0 Z0;
G90 G54 G21;
G54 G00 X0 Y-25 Z50;
M03 S3200;
G98 G82 X0 Y0 Z-2.5 R1 P12000 F1;
G00 G80 Z100;
M05;
G28 G91 X0 Y0 Z0;
M30;
%
The G and M codes and their functionality is mentioned in Appendix I.
The fixture was fixed in place on the machining bay with the samples clamped firmly on it. The
references for the machine were set and the programme was run.

36. 36
3.5 Sample Dimension:
With the literature review sample dimensions were finalized. They were cut out from
respective metal sheets using sheer cutting for precision and cuts without deformation of the sample
plates. A schematic of the sample dimensions for tensile shear test and cross tension test is shown in
Figure30.
Fig.30: Tensile Shear sample

37. 37
3.6 Experimental Planning:
After the entire set up is made ready the actual welding will be done with the help of the set up.
Initially by trial and error method, the various conditions which the set up allowed to vary will be
checked and a broad understanding of the impact of various process parameters will be noted. This
helps in the subsequent steps and also in deciding the exact process variables for the final
experiments.
Of all the process variables chosen for analysis were:
1. Tool RPM
2. Dwell Time
3. Plunge Depth
By simple combinations the number of conditions needed to analyse the interdependency of the
above parameters using experimental samples and characterizing them would be an arduous task.
Thus to help in ease of experimentation, Design Of Experiments approach was resorted to. Using
Taguchi analysis the number of experiments to be done reduced drastically.
Of the above chosen parameters, four individual conditions were assigned for the experiments based
on the experience with earlier trial and the varied conditions used in the different publications.
The list of 16 different experiments designed using Taguchi methods of Design of Experiments are
mentioned in Table 3. The selected sixteen experiments were executed and the joints for each
condition were analyzed and characterized using various methods discussed in the following
sections.

38. 38
Characterization:
Characterization of the spot welds was done by various tests like tensile test, microhardness
traverse and microstructural analysis.
Table 3 Experiment parameter variation
No Tool rotation
(rpm),
Plunge depth (mm) Dwell time (seconds)
1 2800 2.3 4
2 2800 2.4 8
3 2800 2.5 12
4 2800 2.6 16
5 3000 2.3 8
6 3000 2.4 4
7 3000 2.5 16
8 3000 2.6 12
9 3200 2.3 12
10 3200 2.4 16
11 3200 2.5 4
12 3200 2.6 8
13 3400 2.3 16
14 3400 2.4 12
15 3400 2.5 8
16 3400 2.6 4
Sample dimension:
Previous studies on friction stir spot welding showed that the standard samples for the tensile shear
test were 100 mm×30×1.5 mm. Hence the samples which were previously cut were welded and then
tested.
Sample for tensile shear test is shown in Fig.30.The actual sample prepared is shown in Fig.31

39. 39
Testing of samples:
16 conditions by variation of parameters were obtained by design of experiments (Taguchi L16).
Two samples of each condition were tested for tensile shear test.
Samples were tested on Universal testing machine. The maximum load offered by UTM was 10
tons. Figure32 shows the sample and the force direction during tensile shear test.
Samples were tested and Peak Load was noted down. From the value of peak load the strength of
the weld joint was calculated. As the diameter of the spot was 8mm the strength of the joint was
calculated by the formula
Strength= peak load/πr2
Lap Shear test:
Strength testing is an important aspect of a weldability study in Friction Stir Spot
welding. Among all tests, tensile-shear testing is the most common laboratory test used in the
determination of weld strength because of its simplicity. In this test the sample is welded and
then it is pulled in tension until failure occurs.
Fig.31: Actual sample for lap shear test

40. 40
Microstructure Preparation -
The welded lap joints had to be cut along the cross section in order to analyses the
microstructural changes induced by the welding process and to characterize the hardness traverse
profile of the weld region.
During the cutting operation the important consideration was to not alter the structure of
the welded joints by the cutting operation. Thus a cutting process with minimal HAZ was to be
selected. The conventional abrasive cutter was thus not a viable option due to the extensive HAZ
created by it. The best suited method of sample cutting was Electric Discharge Machining
(EDM) which has a very thin HAZ, generally in microns.
The cut samples were then cold mounted ground and polished. Polishing was done on
Emery papers 220.320,400,600,800,1000,1200 followed by lapping on velvet cloth using
suspension of Alumina particles. Then samples are etched with different reagent like Marbles
reagent, but it doesn‟t reveal grain boundaries. Also electrolytic etching is carried out which is
insufficient with required results. Finally etching reagent of composition 45 ml HCl, 15 ml
HNO3 and 20 ml methanol is used. Then image analyzer was used to view the microstructure of
the polished samples. Images of different zones viz. fusion zone, Heat affected zone and base
metal were captured. Panoramas of the entire samples were made by capturing multiple images
of successive zones of the sample and by joining them in Adobe Photoshop. Panoramas were
Fig.32: Schematic of Tensile shear sample testing.

41. 41
helpful to view all the zones of welded specimen and the gradual changes in the structure from
weld center to base metal. Also the sizes of various zones were measured in the panoramas.
The micro-hardness of the composites was measured using the Vickers hardness tester (FM
700 Future Tech) with a Vickers diamond pyramidal indenter.
The micro hardness tester used during the analysis. Load was fixed to 100g and dwell
time was 10 sec. Horizontal hardness traverse was taken from the welds center to the base metal
and vertical hardness traverse was taken at the weld center in vertical direction. Fig.33 shows the
hardness traverse profile.
Fig.33: Schematic of Hardness traverse profile

42. 42
Chapter 4: Results and Discussion
Optimization was done by L16 orthogonal array as listed in Table 3. The effect of each
individual parameter was identified. Lap separation load was taken as output characteristic.
Table 6 shows the response of mean with respect to tool rotation speed, plunge depth and dwell
time.
4.1 Lap Shear Test:
The lap separation load for various parameters are given in the following table.
Table 4: Lap separation results of various parameter
Sr.
No
Tool rotation
(rpm)
Plunge depth
(mm)
Dwell time
(seconds)
Load (kg) Load (N) S/N ratio
1 2800 2.3 4 465.01 4557.098 85.21
2 2800 2.4 8 318.17 3118.066 81.92
3 2800 2.5 12 452.77 4437.146 84.98
4 2800 2.6 16 440.54 4317.292 84.75
5 3000 2.3 8 299.81 2938.138 81.40
6 3000 2.4 4 122.37 1199.226 73.62
7 3000 2.5 16 171.32 1678.936 76.54
8 3000 2.6 12 152.96 1499.008 75.56
9 3200 2.3 12 397.71 3897.558 83.86
10 3200 2.4 16 312.05 3058.09 81.75
11 3200 2.5 4 318.17 3118.066 81.92
12 3200 2.6 8 244.74 2398.452 79.64
13 3400 2.3 16 189.68 1858.864 77.43
14 3400 2.4 12 330.46 3238.508 82.25
15 3400 2.5 8 91.78 899.444 71.12
16 3400 2.6 4 391.59 3837.582 83.72

43. 43
Where signal to noise ratio is calculated by:
S/N ratio (η) = -10 log 10 ((1/n) Σ (1/ (y ij)
2
))
Where n is no of replication, yij is the observed response value. i = 1, 2, 3….n; j=1, 2, 3….k
The objective of this experiment was to maximize lap shear separation load. So it was beneficial
to select a larger value of S/N ratio. S/N ratio was calculated for individual process parameters
and listed in Table 4. Table 5 and 6 shows the response table for S/N ratio and response table for
mean respectively.
Table 5: Response table for S/N ratio
Level RPM Plunge Depth
Dwell
time
1 84.21 81.97 81.11
2 76.78 79.88 78.52
3 81.79 78.64 81.66
4 78.63 80.91 80.11
Table 6: Response table for mean
Level RPM Plunge Depth Dwell time
1 4107.4 3312.91 3177.99
2 1828.83 2653.47 2338.52
3 3118.04 2533.37 3268.05
4 2458.59 3013.08 2728.29

44. 44
Fig.34 : Lap shear load as function of tool rotation in rpm
1000
1500
2000
2500
3000
3500
4000
4500
2500 3000 3500
Lapseparationload(N)
Tool rotation(rpm)
Fig.36 : Lap separation load as function of plunge depth in mm
0
1500
3000
4500
2.2 2.3 2.4 2.5 2.6 2.7
Lapseparationload(N)
Plunge depth(mm)
Fig.35: Lap shear load as function of dwell time in seconds
0
1500
3000
4500
0 5 10 15 20
Lapseparationload(N)
Dwell time(sec)

45. 45
4.1.1 Effect of Plunge depth:
It can be seen that as the plunge depth increases the tensile strength increases initially due to
increased diffusion bonding. But it decreases further with increasing Plunge depth due to
excessive thinning of bottom plate. Hence optimum plunge depth lies somewhere in between the
maximum and minimum.
4.1.2 Effect of tool rotation:
An increase in tool rotation speed the heat output also increases. This high amount of heat
causes coarsening of grain size resulting into depreciation of lap separation load.
Fig.37: Effect of plunge depth on lap separation load
0
500
1000
1500
2000
2500
3000
3500
4000
4500
5000
0.6 0.7 0.8 0.9
Lapseparationload(N)
Plunge depth(mm)
2800 rpm
3000 rpm
3200 rpm
3400 rpm
Fig.38: Effect of tool rotation on lap separation load (N)
0
500
1000
1500
2000
2500
3000
3500
4000
4500
5000
2800 3000 3200 3400
lapseparationload(N)
tool rotation(rpm)
Plunge depth 0.6mm
Plunge depth 0.7mm
Plunge depth 0.8mm
Plunge depth 0.9mm

46. 46
4.1.3 Effect of Dwell time:
Graph of Tensile Strength V/s Dwell time, shows that there is a lot variation in lap separation
load due to different dwell times. A general observation is that the dwell time must not be more
or less.
4.2 Microstructure examination:
The microstructures of the welded regions were seen, analysed to study and understand the effect
of the various process parameters on the quality of the weld and the effect made on the structure
of the metal.
The microstructures show a distinct stir zone where the grain size has undergone significant
refinement due to dynamic recrystallization phenomenon and subsequent grain growth with the
increase in temperature. This region is the one immediately in contact with the rotating tool and
is the region of maximum temperature. This is also the region which promotes the diffusion of
the metal atoms among the two sheets the most. The larger the length of this region, greater is the
ultimate diffusion and in turn better joining between the sheets.
The next region is the heat affected zone (HAZ).
Fig.39: Effect of dwell time on lap separation load
0
500
1000
1500
2000
2500
3000
3500
4000
4500
5000
4 8 12 16
Lapseparationload(N)
Dwell time(sec)
Plunge depth 0.6mm
Plunge depth 0.7mm
Plunge depth 0.8mm
Plunge depth 0.9mm

47. 47
The zone immediately next to the fusion zone which due to the surrounding heat has undergone
grain coarsening. This region is recognised by the coarser grain structure in the micrograph.
The thermo mechanically affected zone adjacent to the HAZ reveals finer grain structure due to
mechanical work between tool and metal and the heat generated by friction.
The base metal shows twin grains of austenitic stainless steel having fine structure. Only grain
boundaries are revealed by the reagent used which proves to be the main interest during process.
The ASTM grain size number of various zones are calculated by linear intercept method which are as
shown below:
Table 7: ASTM graib size number
Zones ASTM grain size number
Base metal 5.5 to 6
TMAZ 6.5 to 7
HAZ 3 to 3.5
Stir zone 5
(c)HAZ (d)TMAZ
(a) (b)
Fig.40: Microstructure of various zones: (a) Base metal (b) Stir zone (c) HAZ (d)TMAZ
(a)Base
metal
(b)Stir
zone

48. 48
Panaroma of some conditions are as below:
Condition 12
S1200rpm 2.6mm,4 s
Condition11:
3200rpm,2.5mm,4s
Condition 7:
3000rpm,2.5mm,8s

49. 49
4.3 Hardness traverse:
A hardness traverse was taken along the cross section of the weld nugget as depicted in the
figure. For hardness testing a load of 100 grams was used and a dwell time of 10 seconds.
It was observed that there was increase in the hardness of work pieces near the weld nugget.
This is due to martensite formation below the pin. The hardness traverse graphs show that peak
hardness is noted in the fusion zone where fine grain structure was observed due to dynamic
recrystallization. The hardness decreases as we move away from the weld center and finally base
metal hardness is observed further. The value of base metal hardness was around 175Hv and the
peak hardness was in the range of 200 -230 Hv.
In vertical hardness traverse the hardness below the pin is highest due to martensite formation
exactly below it. The hardness decreases as we move from upper surface to lower surface. But
all hardness values are higher than the base metal hardness.
The following are the graphs for some of the conditions which were considered during the
experiments.
Condition 2
0
50
100
150
200
250
0 0.5 1 1.5 2 2.5
HardnessHv
Distance from top(mm)

50. 50
Condition 4
0
50
100
150
200
250
-15 -10 -5 0 5 10 15
HardnessHv
Distance from center(mm)
0
50
100
150
200
250
0 0.5 1 1.5 2
HardnessHv
Distance from top(mm)

51. 51
Fig. 41 Hardness traverse
0
50
100
150
200
250
-20 -15 -10 -5 0 5 10 15 20
HardnessHv
Distance from center(mm)

52. 52
4.4 Tool wear:
In accordance with the sample material, tool material i.e. tungsten carbide rods of 12 mm
diameter were manufactured.
During the welding process it was seen that after every weld wear in the tool was observed. This
is caused due to martensite formation below the pin. Gradually with increase in parameters the
tool pin decreased in its length and bulging of shoulder took place. This was due to large heat
produced and also the Z-force acting over tip of the pin. There was contamination observed
which formed a thin layer over the tool surface. This increase in temperature due to friction heat
made the tool red hot, hence coolant was used at regular interval. This sudden quenching gives
some surface cracks which resulted into fracture of some tools during process. The tool sustain
for only 4 samples during process. The wearing of various tools which were used during the
process are as shown below:
The hardness of tool near pin at 500 gram load for 10 sec dwell time are 75.4 HRc, 76.2 HRc and
75.9 HRc. The observed hardness of tool was more than base metal but due to frictional heat it
causes wearing of the tool.
IVIII
I
As
received
I II
Fig.42 : Mode of tool wear at various condition; as received (I)2800rpm,PD2.3mm (II)
2800rpm,PD2.4mm (III) 2800rpm,PD2.5mm (IV)2800rpm,PD2.6mm
As
received
As
received I
Fig.43: Fractured tool after 1st
sample with parameter 3400rpm,
PD2.5mm
Bulging of shoulder

53. 53
Chapter 5: Conclusion
The austenitic stainless steel sheet, 1.5mm thick were successfully welded using Friction stir spot
welding. The optimized results obtained are 3000 rpm, PD 2.4 8sec. The lap separation load for
this joint is obtained of a value 4.557 kN. The microstructure observed shows change in grain
size from base metal to the stir zone. The stir zone showed a fine structure as compared to base
metal due to dynamic recrystallsation in the stir zone. The HAZ region showed coarser grain due
to high friction heat developed. The hardness traverse shows high hardness in the stir zone.
Further decrease in hardness is observed in the HAZ region with a sudden increase in the TMAZ.
The tool showed a deformation with increase in parameter values viz. high tool rotation speed
and high plunge depth. This deformation was characterized with an decrease in pin length and
the bulging out of tool at the shoulder region. The tool of tungsten carbide sustained for 4
sample. This was due to the martensite formation exactly below the pin causing this wear.