Effect of Tool-Pin Geometry on Microstructure and ...ijens.org/Vol_19_I_01/190201-7676-IJMME-IJENS.pdf · Abstract-- Friction stir spot welding (FSSW) is a solid state joining process

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, [email protected]

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



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.



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.



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)



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



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



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



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



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)



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.



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)



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




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


(a) (b)

Max. Temperature 98 Max. Temperature 242.5

(c) (d)

(a) (b)



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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Effect of Tool-Pin Geometry on Microstructure and ...ijens.org/Vol_19_I_01/190201-7676-IJMME-IJENS.pdf · Abstract-- Friction stir spot welding (FSSW) is a solid state joining process

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