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International Journal of Mechanical & Mechatronics Engineering IJMME-IJENS Vol:19 No:01 14
190201-7676-IJMME-IJENS © February 2019 IJENS I J E N S
Effect of Tool-Pin Geometry on Microstructure and
Temperature Distribution in Friction Stir Spot Welds
of Similar AA2024-T3 Aluminum Alloys
Haider Kamal Ibrahim 1, Abdul Wahab Hassan Khuder
2, Muhammed Abdul Sattar Muhammed
3
1 Technical Affairs Directorate, Ministry of Science and Technology, Baghdad, Iraq,
2 Middle Technical University, Engineering Technical College-Baghdad, Baghdad, Iraq,
3 College of Engineering, Nahrain University, Baghdad, Iraq
Corresponding Author: Abdul Wahab Hassan Khuder, Power Mechanical Engineering Department, akhuder@toc.edu.iq
Abstract-- Friction stir spot welding (FSSW) is a solid state
joining process derived from the linear friction stir welding as an
alternative method for the single-point joining processes. Three
tool rotational speeds of (535, 980, 1325rpm) and two types of
tool-pin geometry (straight cylindrical and triangular) were used
to fabricate the FSSW joints of similar AA2024-T3 aluminum
alloy sheets with 2mm thickness. This study was aimed to
investigate the effects of pin geometry and tool rotational speeds
on the lap shear strength and microstructural characteristics of
friction stir spot welds. In addition to that, temperature
distribution during FSSW, using both tool-pin shapes at the best
rotational speed, was evaluated by two thermocouples K-type
which were positioned at two different locations within welded
area. Three dimensional non-linear numerical models using
ANSYS program were built to simulate the temperature
distribution. Tensile-shear test results showed that the weld
strength was improved in all FSSWed joints by changing the tool
pin profile from cylindrical to triangular. Scanning electron
microscopy of the failed specimens observed that the pin
geometry influence fracture mode under tensile shear loading.
Microscopic examinations revealed that grain size in the stir zone
increased with increasing rotational speed in the welds made by
the cylindrical pin. While, conversely, was occurring in the other
welds fabricated by the triangular pin. Temperature distribution
obtained from the experimental and numerical model results was
directly affected by the pin geometry during the welding process.
Index Term-- Friction stir spot welding, AA2024-T3 aluminum
alloy, Pin geometry, Temperature distribution, ANSYS, Fracture
mode.
I INTRODUCTION
The increasing demands for energy saving and weight
reduction in automotive and aerospace industry has led to
replacing certain ferrous parts with lightweight alloys and
advanced high strength steels (HSS). This interest has created
the need to develop a reliable joining technology which can
produce aluminum-steel dissimilar joints of high quality
instead of conventional fusion welding processes [1,2]. The
highly alloyed 2XXX series (aluminum-copper alloys) are
used extensively in aerospace industries. These aluminum
alloys are generally classified as non-weldable alloys due to
the deterioration of their mechanical properties causes of the
dissolution of strengthening precipitates during fusion welding
[3,4]. Therefore, solid state welding processes, such as friction
stir welding (FSW), were developed in order to overcome a
variety of problems and weld defects in fusion welding of
aluminum alloys [1,5]. During FSW, the maximum
temperature of the welded plates is typically ranges from 70%
to 80% of the melting temperature of the workpiece material
[6,7]. Based on friction stir welding (FSW), Mazda
Corporation of Japan proposed a friction stir spot welding
(FSSW) method in an attempt to transfer some of the
advantages of FSW to spot welding [8,9]. In FSSW process, a
non-consumable rotating tool is plunged into the overlapped
workpieces that to be joined, hold for a certain duration time
and finally retracted from the workpiece with no lateral
movement or transitional. Figure 1 shows the conventional
friction stir spot welding phases. The frictional heat generated
at the tool-workpiece interface softens the surrounding
materials. The rotary action and the downward force of the
tool causes the material flow and mixing of the plasticized
materials of upper and lower sheets result in the formation of a
solid-state weld region [10]. Recently, the FSSW has shown
that it is an alternative method for spot welding of aluminum
alloys, magnesium alloys, copper alloys and polymers due to
its advantages such as low heat input, low operating costs and
short cycle time (usually a few seconds) [11,12]. Tool
rotational speed and tool-pin geometry are among the most
important factors which influence mechanical and
microstructural characteristics of FSSW joint. Some
researchers have found that increase in the tool rotational
speed strengthened the joint [13,14]. On the other hand, Zhang
et al. [15] observed that weld strength decreased with
increasing tool rotational speed. Urso et al. [16] found that
maximum tensile-shear load when the FSSW tool is rotated at
the lowest rotational speed. Patel et al. [17] found that the
width of bonding area decreased with increasing tool
rotational speed. Paidar et al. [18] investigated the effect of
tool-pin geometry on the weld characteristics of FSSW
specimens. Their microscopic examination revealed that
triangular pin resulted in a welded zone with finer grain
structure than that formed by the cylindrical pin. Whereas,
Badarinarayan et al. [19] indicated that tensile shear strength
of welds made by the triangular pin is twice as much as that of
welds fabricated by the cylindrical pin. Despite various
studies, the effect of tool-pin geometry on temperature
distribution during FSSW has not completely been clarified.
Hence, the objectives of this investigation are to study the
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effect of different parameters (tool rotational speed and tool-
pin geometry) on the microstructure and tensile-shear strength
of the friction stir spot welds of AA2024-T3 aluminum alloy
sheets. As well as, to predict the numerical transient
temperature distribution in the workpieces during the welding
process by a finite element based thermal model using
ANSYS.
Fig. 1. Schematic illustration of the FSSW process [20]
II EXPERMENTAL WORK
AA2024-T3 aluminum alloy, with 2mm sheet
thickness, was chosen as the base material in this
work. The chemical composition and mechanical
properties are listed in Tables (1&2), respectively.
The temperature-dependent properties of AA2024-T3
aluminum alloy at various temperatures are given in
Table 3 [21].
Table I
Chemical composition of AA2024 aluminum alloy sheets used in this work
Si
(%)
Fe
(%)
Cu
(%)
Mn
(%)
Mg
(%)
Cr
(%)
Ni
(%)
Zn
(%)
Ti
(%)
V
(%)
Ga
(%) Al
0.103 0.197 4.84 0.645 1.5 0.027 0.006 0.181 0.042 0.017 0.016 Bal.
Table II
Mechanical properties of AA2024-T3 aluminum alloy used in this work
Tensile Strength
(MPa)
Yield Strength
(MPa)
Elongation
(%)
486 360 20
Table III
Temperature dependent material properties of AA2024-T3 [21,22]
Temperature
( )
Thermal
Conductivity
(w.m-1
.K-1
)
Specific
Heat
(J. Kg-1
.K-1
)
Elastic
Modulus
(GPa)
Poison's
Ratio
Yield
Strength
(MPa)
Density
(Kg.m-3
)
Thermal
Expansion/
10-6
20 164 881 72.4 0.33 473 2780 14
100 182 927 66.5 0.33 416.5 2780 23.018
200 194 1047 63.5 0.33 293.5 2780 24.509
300 202 1130 60.4 0.33 239.5 2780 25.119
400 210 1210 56.1 0.33 150 2780 25.594
500 220 1300 50 0.33 100 2780 26.637
The FSSW tools, fabricated from tool steel AISI D3 with
hardness of 60HRC. The chemical composition of the tool
material is listed in table 4. Two different tools were used.
They have straight cylindrical and triangular tool pin profiles;
each one of them consists of a flat shoulder with a diameter of
18mm. Figure 2 shows the FSSW tools used in this work with
details.
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Table IV
Chemical composition of tool steel AISI D3 used in this work
C
(%)
Mn
(%)
P
(%)
S
(%)
Si
(%)
Cr
(%)
V
(%)
Mo
(%)
2.11 0.35 0.06 0.02 0.21 12 0.11 0.17
Fig. 2. Shapes and dimensions of the FSSW tools used in this work (All dimensions in mm)
To achieve a FSSW joint, the rotating tool plunged for
0.4mm with manual feed rate of ( 0.47 – 0.55)mm/min for
a specified time of ( 2 – 3) sec as a dwell time. Three tool
rotational speeds (535, 980, 1325) RPM was used. After
welding, macro and microstructure examinations were
conducted on the friction stir spot welded specimens. Keller’s
microetchant consisting of (190 ml distilled water, 5 ml HNO3,
(3 ml HCl, and 2 ml HF) was used in this work. The tensile-
shear test specimens were prepared according to American
Welding Society (AWS) [23], as shown in Figure 3. They
were tested at room temperature by a computerized universal
testing machine at the constant crosshead speed of (1mm/min).
For each welding condition, three specimens were tested in
order to obtain the average of three values to evaluate the weld
strength.
Fig. 3. Dimensions of the overlap specimen that to be friction stir spot welded (All dimensions in mm)
Thermal cycles and temperature distribution in the weld area were measured during FSSW process using the cylindrical - and
triangular - tool-pin profiles at the best tool rotational speed. Two thermocouples K-type (-100 to 1100 ) were positioned at
two locations within the area to be welded, as schematically shown in Figure 4. One of the thermocouples (T1) was located at the
center of the nugget zone (NZ), and the other thermocouple (T2) was inserted at a distance of 7mm from the center of the NZ.
Thermocouples were connected to digital readers used to display the measured temperature.
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Fig. 4. A schematic cross-section shows thermocouples sites in the welding zone
Two cases were considered with different tool-pin profiles under the best tool rotational speed for measuring the axial load during
the FSSW process. A component dynamometer based on load cell was fixed on a special rig designed for this work, see Figure 5
(a, b). The analog force data which transmitted from the load cell are displayed using Weight Indicator.
Fig. 5. Axial load measuring system used in this work, (a) Overview, (b) load cell
III FINITE ELEMENT MODEL
A multi-purpose finite element program ANSYS®15. 0 was
used for numerical simulation of the temperature distribution
during FSSW process. A three dimensional transient,
nonlinear heat transfer model was developed to determine the
temperature fields.
A. Thermal model
Thermal model is used for the purpose of calculating the
transient temperature fields that developed in the weld area
during the FSSW. The transient temperature field is a function
with the time and spatial coordinates (x, y, z) are estimated by
the three dimensional nonlinear heat transfer equation:
( ) (
) ( ) ( )
……… equation (1)
Where T is the absolute temperature, t is the time, k is a
coefficient of thermal conductivity along x, y, z directions,
Qint is the internal heat source rates in the three axis, C is a
specific heat of the workpiece, and 𝜌 is temperature dependent
density of the workpiece [21,24].
B. Assumption
Several simplifying assumptions have been made in
developing the FEM thermal model:
Workpiece material is isotropic and
homogenous.
No melting occurs during FSSW.
Thermal boundary conditions are symmetrical
across the spot centerline.
Heat transfer from the workpiece to the
sample fixture is negligible.
Initial temperature is assumed to be 25oC .
It is assumed that 100% of the dissipated
energy caused by friction is transformed into
heat.
The tool and workpiece fixture are assumed
rigid and no deformation occurs in these parts.
C. Boundary conditions
Boundary conditions for FSSW thermal model were applied as
surface loads through the ANSYS program. Assumptions were
(a) (b)
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made for various boundary conditions based on data collected
from various published papers [25,26,27]. Convection and
radiative heat losses to the ambient take place across all free
surfaces of the workpiece, while conduction losses occur from
the upper and bottom surfaces of the workpiece to the
workpiece fixture. To account the convection and radiation
effects on all free surfaces of the welded sheets, the heat loss
(qs) is calculated by equation (2).
𝑞𝑠= 𝛽 (T −𝑇o) + η ξ ε (𝑇4−𝑇o
4) …..... equation (2)
Where T is the absolute temperature of the workpiece
surfaces, 𝑇o is the absolute temperature of the ambient, 𝛽 is
the convection heat transfer coefficient, ε is the emissivity of
the workpiece surfaces, η is the radiation factor, and ξ is the
Stefan-Boltzmann constant (5.67×10-12 W/m2.k
4). In the
current model, the typical values of 𝛽, ε, and η were taken to
be 30 w/m2 °C, 0.5, and 1 for aluminum, respectively, with an
ambient temperature (𝑇o) of 300°k.
In order to account for the conductive heat loss through the
bottom and upper surfaces of weld sheets, a high overall heat
transfer coefficient was assumed based on previous study [28].
The heat loss was modeled approximately using heat flux loss
by convection qb , which given by equation (3).
𝑞𝑏=𝛽𝑏 (𝑇−𝑇o) ……… equation (3)
Where 𝛽𝑏 is a fictitious convection coefficient. In this work,
the optimized value of, between the workpiece and the fixture,
was taken to be 418 w/m2 k. Figure 6 shows a schematic
diagram of the boundary conditions applied in the FEM
model. The edges of upper and lower workpieces were
restrained in the x direction; the tool can only have translation
and rotation in the y direction, while the backing anvil was
fixed in all degrees of freedom to avoid rigid body motion.
Fig. 6. Schematic diagram of boundary conditions [29]
Heat generation during FSSW arises from three heat sources: friction work at the tool shoulder and the upper sheet
interface, friction work on the upper and the lower sheet interface, and plastic deformation of the material around the
tool pin [29]. The heat generated due to the friction work at the interface of the tool and the upper sheet contributes
the most heat to the welding process, which is about (96.84%) of the total heat generated from FSSW, while the rest
of the heat energies come from the friction force at the interface between the overlapped sheets (0.02%), and the
plastic deformation in the material (3.14%) [26]. Thus, the heat generation is assumed occur predominantly under the
tool shoulder as a main heat source, and the other sources were almost negligible.
Frictional heat from the rotating tool is:
( ) ( ) ……
equation (4) Where is the slip factor that compensates for tool/material
relative velocity. Typical values for slip factor were found in
the ranges of 0.6 – 0.85.
The frictional heat of the tool-shoulder will be:
Qshoulder = ∫
( )
……
equation (5) In similar concept, heat generated by lateral surface of the
tool-pin is:
Qpin ( ) Lp …… equation (6)
The temperature gradient in two points, which are located at
similar positions of thermocouples T1 and T2, was calculated
to compare these data with that measured experimentally to
validate the results. Heat transfer coefficient is 300w/m2
k,
convection heat transfer coefficient is 30w/m2k [28], ambient
temperature and initial temperature To= 300 k, slip factor =
0.8, and friction coefficient = 0.4 [30]. The heat generation
(Q) was calculated from equations (5) & (6) based upon the
coefficient of friction ( ), pressure (P) in addition to other
welding parameters such as the tool rotational speed and tool
pin geometries. Then, the heat calculated was used as input
load to ANSYS model.
In order to evaluate the temperature distribution during the
welding process, the frictional heat was simulated by change
the heat source location (FSSW tool) through the overlapped
sheets under a constant tool plunging rate. Depending on the
overall welding time and the tool plunging depth, the heat
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source location (tool plunging depth) was divided into four
steps during four welding times.
IV. RESULTS AND DISCUSSIONS
A. Tensile-shear stress
A comparison between tensile-shear test results of the FSSW
specimens in terms the effects of tool pin profile and tool
rotational speeds on the weld strength is shown in Figure 7.
Fig. 7. Effects of tool pin profile and tool rotational speed on the weld strength of FSSW specimens
From the general comparison shown in the above figure, it is
clear that friction stir spot welds made by the cylindrical and
triangular pin profiles showed the maximum tensile shear
failure load (TSFL) at the lowest rotational speed of (535rpm).
These results are compatible with several studies [15,16,31]
which were indicated that the weld strength decreases with the
increase of tool rotational speed. Also, it can be seen that weld
strength was improved in all FSSWed joints by changing the
tool pin geometry from cylindrical to triangular, especially at a
rotational speed of 980rpm, the lowest value (2807N) was
increased to (5722N). The variation in weld strength may be
due to the difference in the plastic flow and frictional heat
generated in the weld zones which attributed to the difference
in tool pin profiles and tool rotational speeds [18]. Two
different failure modes were observed in failed specimens
under tensile shear loading: shear, and tensile – shear mixed
failure modes. Figure 8 shows close-up top views of the lower
sheets and close-up top and bottom views of the upper sheets
of FSSW joints under tensile-shear tests.
Fig. 8. Close-up views of fracture modes under tensile-shear tests
Shear failure is observed in FSSW specimens welded with the
cylindrical pin at all tool rotational speeds. The fracture is
propagated in horizontal direction along the boundary between
the welded sheets due to tensile loading. The final fracture is
occurring only through the stir zone because the tool-pin
indentation can be recognized without any damage. Tensile-
shear mixed failure mode occurred in FSSW specimens
fabricated by the triangular pin profile at all rotational speeds.
In this failure mode, fracture partly occurred in the stir zone
on one side of the spot weld and partly in the upper sheet in
Failure modes Close-up top view
(Lower sheet)
Close-up bottom view
(Upper sheet)
Close-up top view
(Upper sheet)
Shear fracture
Tensile-shear mixed
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the opposite side. In this work, the FSSW specimens which
are failing in a tensile-shear mixed mode showed higher weld
strength than those failed with the other failure mode. This is
due to the increasing in tensile-shear strength of welds that
obtained with an altering tool pin profile in the FSSW process.
Figure 9 shows scanning electron micrographs of fracture
surfaces of the lower sheet of a welded specimen failed in
shear fracture mode. The fracture surfaces are characterized by
the presence of small dimples elongated in the direction of
loading, as shown in Figures 9 (b-d, f). The dimple shape
indicates that state of applied load in these regions is primarily
tensile-shear loading. The fracture surface shown in region (C)
exhibited more elongated dimples in various sizes with the
same direction, see Figure 9 (d). This indicates that the
fracture has occurred in ductile shear failure mode at the final
stage of failure [15,19,32].
Fig. 9. SEM fractographs of a FSSW specimen failed in shear mode, (a) Overview of fractured nugget in the lower sheet, (b) magnified view of the region A
marked in (a), and (d) magnified view of region D marked in (view - C -).
Figure 10 shows SEM micrographs of the fractured nugget in the lower sheet of a FSSW specimen failed in tensile-shear mixed
mode. The presence of small shallow dimples distributed among the tear ridges, as shown in Figure 10 (b, and c), indicates that
the mixed fracture in this region contains shear ductile fracture and quasi-cleavage fracture [15]. As revealed from Figure 10 (d),
there are smaller, elongated dimples as well as the existence of cleavage facets. This indicates the occurrence of mixed-type
fracture mode in region B at the final stage of failure. From figures 9 and 10, it can be observed that the FSSW tool with a
triangular pin resulted in a spot weld with harder and stronger stir zone than that produced by the cylindrical pin during the
welding process.
b
View - C -
D
d
a
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Fig. 10. SEM fractographs of a FSSW specimen failed in tensile-shear mixed failure mode, (a) Overview of fractured nugget in the lower sheet, (b) magnified
view of region C marked in (view -A-), (c) magnified view of regions E and F marked in (b), (d) magnified view of region G marked in (view -B-)
B. Macro and Microstructure
Figure 11 shows the optical microstructure of the base
material of AA2024 aluminum alloy in T3 condition. The base
material consists of elongated grains and a large number of
second-phase particles (Al2Cu) distributed in the matrix. The
average grain size of the base material was approximately
( 35 m).
Fig. 11. Microstructure of AA2024-T3 base material (longitudinal section)
Due to high pressure and severe plastic deformation during stirring, the upper and lower sheets are compressed
together to form an effective FSSW joint. Figure 12 shows the macroscopic appearance and microstructures of a
cross-section of the best welded specimen fabricated by the triangular tool-pin profile at the lowest tool rotational
speed (535rpm). The cross-section of the welded joint can be divided into three distinct regions, which are, in
sequence, the stir zone (SZ), thermo mechanical affected zone (TMAZ), and the heat affected zone (HAZ), as shown
in Figure 12 (a1), (a2) and (a3), respectively. The stir zone
consists of refined grains due to dynamic recrystallization
which occurred in the periphery of the tool-pin, caused by
high frictional heating and intense plastic deformation during
FSSW process, see Figure 12 (a1). The TMAZ is characterized
by highly deformed and elongated grains. This region is
affected by severe plastic deformation of materials occurred in
the vicinity of the rotating pin, but the temperature in TMAZ
is not sufficient enough to cause recrystallization. Thus, the
grain structure in this region is coarser than SZ and finer than
50 m
Second phase precipitates
View - A -
View - B -
b
C
c
E
F
d
G
a
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HAZ, see Figure 12 (a2). The heat affected zone (HAZ)
consists of undeformed coarse grain structure, because it has
only exposed to the frictional heat. The grain structure in HAZ
was not mechanically affected by plastic deformation, as
shown in Figures 12 (a3).
Fig. 12. (a) A macroscopic appearance of the cross-section of FSSWed specimen made by the triangular pin at 535 rpm, (b) close-up top view of the spot, (c) close-up views of regions a1 – a3 marked in (a), respectively.
In friction stir spot welded joint, bonding condition between
two welded sheets in overlap configuration can be categorized
into three main regions; named completely bonded region,
partially bonded region, and unbounded region, as shown in
Figure 13 (a). The extension of these three regions is called in
most previous studies as Hook [32].
Fig. 13. Typical cross-sectional views show the bonding regions in FSSW specimens fabricated in this work, (a) a cross-sectional macrograph, (b, c, and d) close-up views of completely bonded region, partially bonded region, and unbounded region, respectively
In completely bonded region, the interface of the welded sheet
surfaces cannot be identified and the refined grains from both
sheets are completely stirred, see Figure 13 (b). This is due to
severe plastic deformation and stirring action which occurred
in this region. Next to completely bonded region is a partially
bonded region, where non uniform mixing is observed due to
insufficient stirring and frictional heating, see Figure 13 (c). In
the last region (unbounded), there is no stirring occurred on
the interface of the two sheets because it is away from the
rotating pin [32], as shown in Figure 13 (d). The plastic flow
of upper and lower sheet materials during FSSW leads to the
formation of the above regions [33].
Figures 14 (a) & (b) show the effect of tool-pin geometry on
the microstructure of the SZ in different FSSWed specimens at
a constant rotational speed of 535rpm. It can be observed that
triangular pin resulted in a stir zone with finer grain structure
than the cylindrical pin. This indicates that a triangular pin
causes more severe plastic deformation in welding material
than the cylindrical pin because the triangular pin caused
flowing of the plasticized material back and forth in the radial
direction over a wide region during FSSW process. In
(c)
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contrast, the cylindrical pin caused flowing of material only around its own axis [19].
Fig. 14. Microstructure of the SZ in different FSSW specimens welded by different tool pin profiles at constant rotational speed of 535 RPM, (a)
cylindrical pin, (b) triangular pin
An optical macroscopic examination was carried out
on FSSW specimens welded by the same tool-pin
profile (cylindrical shape) using different rotational
speeds, as shown in Figure 15. From the macroscopic
appearance, it is revealed that the completely bonded
region decreased with the increase of tool rotational
speed from 535rpm to 980rpm. Width of bonded
region (W) is found to be about (3mm) at a rotational
speed of 535rpm, while it is (0.9mm) at 980rpm. The
increasing in tool rotational speed resulted in an
increase in the frictional heat to undesirable limit,
which in turn caused an excessive plastic deformation
which led to the decrease in completely bonded
region [32].
Fig. 15. Macroscopic appearance of the cross-section of FSSW specimens welded by the cylindrical pin at different tool rotational speeds, (a) 535
rpm, (b) 980 rpm
In the same context, the grain size in the SZ increased with
increasing rotational speed (from 535rpm to 1325rpm) in the
welded specimens made by the cylindrical pin. While in the
welds made by the triangular pin, the grain size in SZ
decreased with increasing tool rotational speed, see Figure 16.
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Fig. 16. Microstructure of the SZ in FSSWed specimens made by different tool-pin profiles and rotational speeds
This is attributed to the geometric characteristics of the triangular pin, where the frictional area at the interface between the pin and the plasticized material is small in the triangular pin due to the presence of three sharp edges. Hence, frictional heat generated in the SZ is less than that generated by the cylindrical pin
during FSSW [34].
C. Temperature Distribution
During FSSW process, temperature rises when the rotating pin
penetrates the upper sheet through plunging of FSSW tool into
the workpiece. After that, temperature increases at a rapid rate
when the shoulder makes contact with the upper surface of the
workpiece. At the final stage, the tool is retracted and the
welded joint is cooled to the room temperature. In this work,
the temperature distribution measurements of the FSSW
process were conducted using the cylindrical and triangular
tool-pin profiles at the best rotational speed (535 RPM).
Figure 17 (a, and b) shows the experimental temperature
profiles that measured for the complete welding cycle using
the cylindrical and triangular tool-pin profiles, respectively.
Fig. 17. Experimental temperature profiles for FSSW process using different tool pin shapes, (a) cylindrical pin, and (b) triangular pin
As evident in the above figures, the temperature profile of
thermocouple (T1) is higher than that of thermocouple (T2) in
both cases measured. For the cylindrical pin-welds, the
maximum temperature measured by T1 is 319 (0.71Tm),
while the maximum temperature of T2 is 225 (0.6Tm). Also,
for the triangular pin-welds, the peak temperature measured
from T1 is (234 ), while the peak temperature from T2 is
(194 ). This difference in thermocouple readings between T1
and T2 may be attributed to variation of heat generated in the
FSSW regions during the welding process. The NZ is
subjected to severe plastic deformation of welding materials
caused by rotation of the tool pin, in addition to the frictional
heat generated between the deformed materials and the pin
surfaces, which causes an increase in temperature of this
(a) (b)
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region as compared with the other welding zones. While the
region located at a distance of 7mm (T2) is only exposed to
frictional heating resulting from the rotation of the tool
shoulder [17], so the temperature profile of (T2) is lower than
that of thermocouple at the center of the spot (T1). It is clear
from the above figures that the temperature profile of the
cylindrical pin is higher than that of the triangular pin. This is
because the cylindrical pin has a larger surface projected area
at the pin tip than the tip surface of the triangular pin, see
Figure 18, which causes more frictional heat at the periphery
of the cylindrical pin as a result of the large frictional area
between the lateral surfaces of the pin and the welding
material [34].
Fig. 18. A sketch for comparison between the frontal area of the cylindrical and triangular tool pin shapes [35]
ANSYS 15.0 was used for simulation of thermal distribution
during FSSW process. The temperature gradient in two points,
which are located at similar positions of thermocouples T1
and T2, was calculated. Figures (19 & 20) show the cross-
sectional views of calculating the temperature distribution in
the workpiece at different tool plunging depths and welding
times during FSSW using the cylindrical and triangular tool-
pin profiles, respectively. The highest temperature was
observed at the center of weld nugget. This is because the
rotation of the tool pin and shoulder contribute to the highest
heat flux in this region. From these figures, it can be seen that
the workpiece welded by the cylindrical pin had higher
temperature distribution than that observed in the other one
welded by the triangular pin.
Fig. 19. Contour of a FSSW using the cylindrical pin at different tool plunging depths and welding times, (a) 0.85mm at 22.5sec, (b) 1.7mm at 45sec, (c) 2.55mm at 67.5sec, and (d) 3.4mm at 90sec
(a) (b)
(c) (d)
Max. Temperature 142.4 Max. Temperature 156.5
Max. Temperature 172.3 Max. Temperature 315.9
International Journal of Mechanical & Mechatronics Engineering IJMME-IJENS Vol:19 No:01 26
190201-7676-IJMME-IJENS © February 2019 IJENS I J E N S
Fig. 20. Contour of a FSSW using the triangular pin at different tool plunging depths and welding times, (a) 0.85mm at 22.5Sec, (b) 1.7mm at 45Sec, (c)
2.55mm at 67.5Sec, and (d) 3.4mm at 90Sec.
Figures (21&22) show the comparison between temperature
distribution profiles, which were calculated by ANSYS model
and those experimentally measured. At the center of weld
nugget, maximum temperature measured experimentally using
the cylindrical pin was (319 ), whereas at ANSYS model, it
was found to be (315 ). On the other hand, the maximum
temperatures obtained at center of the spot in the welds using
the triangular pin was (234 ) and (242 ) in the experimental
and ANSYS model results, respectively.
Fig. 21. Comparison of the experimental and ANSYS model results obtained at different
tested regions during FSSW using the cylindrical pin, (a) T1 location, (b) T2 location
Max. Temperature 76.3 Max. Temperature 88.4
(a) (b)
Max. Temperature 98 Max. Temperature 242.5
(c) (d)
(a) (b)
International Journal of Mechanical & Mechatronics Engineering IJMME-IJENS Vol:19 No:01 27
190201-7676-IJMME-IJENS © February 2019 IJENS I J E N S
Fig. 22. Comparison of the experimental and ANSYS model results obtained at different
tested regions during FSSW using the triangular pin, (a) T1 location, (b) T2 location
From the above comparison results, it can be observed that the
discrepancy between the experimental test and ANSYS model
results in maximum temperatures was (1.25% - 3.3%) at the
center of weld nugget (T1) during FSSW using different tool-
pin profiles. Also, it can be seen that the trend of the
temperature profiles between the experimental and ANSYS
model results was almost identical. This indicates that the
variable inputs and the assumptions adopted in ANSYS model
were reasonable.
V. CONCLUSION
1. The 535rpm tool rotational speed and the triangular
tool-pin profile are the best welding parameters
which gave the higher failure loads in all FSSWed
specimens.
2. Two distinct failure modes observed in fractured
specimens of FSSW under tensile shear loading:
shear and tensile-shear mixed failure modes. The
tool-pin geometry has the strongest effect in
determining failure modes in FSSW specimens
than the effect of rotational speed.
3. The triangular pin resulted in finer microstructure
in the SZ as compared with the cylindrical pin.
4. Temperature distribution during FSSW is directly
affected by the tool-pin geometry at a constant tool
rotational speed of 535rpm. The maximum
temperature measured at the center of the spot
during the welding process using the cylindrical
and triangular tool-pin profiles are 319 (0.71Tm)
and 234 (0.6Tm), respectively.
5. A good agreement obtained between the maximum
temperatures measured experimentally and those
obtained numerically from the simulation results at
the center of the spot with a discrepancy percentage
of (1.25 – 3.3)% during FSSW using different tool
pin profiles.
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