Applied Engineering Letters Vol.8, No.

2, 60-69 (2023) e-ISSN: 2466-4847

EFFECT OF FRICTION STIR-WELDING TOOL PIN GEOMETRY ON THE

CHARACTERISTICS OF AL-CU JOINTS
UDC:621.791.051
Original scientific paper https://doi.org/10.18485/aeletters.2023.8.2.3

Hammad T. Elmetwally1 , Mostafa A. Abdelhafiz1 , M. N. El-Sheikh1 , Mahmoud E.

Abdullah1*
1
Mechanical Department, Faculty of Technology and Education, Beni-Suef University, Beni-Suef 62511,
Egypt

Abstract: ARTICLE HISTORY

Friction stir welding (FSW) is considered to be a solid-state welding Received: 8 March 2023
technique that is suitable well for joining copper and aluminium sheets. The Revised: 3 May 2023
current experimental study focused on the influence of pin geometry on the Accepted: 6 June 2023
Published: 30 June 2023
micro-structural and mechanical characteristics of such joints. An aluminium
sheet was welded to a copper sheet at a constant rotational speed of 1280
rpm and a traverse speed of 16 mm/min. The welding tool was made from
KEYWORDS
W302 steel with four different pin profiles: straight cylindrical, tapered,
FSW, dissimilar joint, pin
triangular, and squared. When the squared pin was utilized, the optimum geometry, joint strength,
joint was produced as the specimen prepared from this joint had a defect- microstructure, peak
free structure and a tensile strength of 107.2 MPa (80% of the aluminium temperature, ductile-brittle
strength). On the other hand, the pin with a triangular profile was utilized to fracture
determine the minimum characteristics, and the specimens' structures
revealed dislocations, separations, and cracking in copper particles inside
the aluminium matrix. The microhardness trend is consistent across all
specimens. Moreover, specimens welded using squared and cylindrical pin
tools have the maximum hardness values obtained at the stir zone of the
copper side. The inspection of fractured surfaces showed well mixing
between aluminium and copper as well as ductile fracture when a squared
pin tool was used while it showed a combination of ductile fracture and
brittle fracture for the specimen welded with a triangular pin tool. Based on
this study, the use of the squared pin tool gives the most favourable results
compared with other pin profiles.

1. INTRODUCTION gave FSW an advantage over fusion techniques for

welding many metals, such as welding aluminium,
The Friction Stir Welding (FSW) method is copper, and titanium alloys [7-9]. Welding different
widely recognized as a crucial technique for joining metals, such as aluminium and copper, are highly
metals and alloys that possess ultra-oxidation important for reducing weight by replacing copper
layers during the welding process through fusion with aluminium, saving welding costs, and
techniques. For this reason, FSW is used for considering the safety factor of welded joints
welding aluminium, copper, steel, and dissimilar [10,11]. Many industrial applications, such as
joints; it can also be used underwater [1-4]. FSW electrical applications in power plants, tend to
procedure includes heating the ends of two parts reduce production costs when minimized copper
of the joint to a temperature below the minimum usage as it was replaced by aluminium. Several
melting point of the base metals [5,6]. This feature previous works studied the effects of FSW process

*CONTACT: M.E. Abdullah, e-mail: bsu_mahmoud@techedu.bsu.edu.eg © 2023 Published by the Serbian Academic Center
 H.T. Elmetwally et al. / Applied Engineering Letters Vol.8, No.2, 60-69 (2023)

parameters to obtain the best welding conditions 0, 0.3, and 0.6 mm from the center tool axis. It was
of the FSW process. Esmaeili et al. [12] studied the concluded that the tool offset has significant
effect of welding parameters, such as tool welding parameters which enhance the hardness
rotational speed and tool offset, on the and welding strength besides grain refinement in
microstructure and mechanical properties of pure the stir zone at an optimum tool offset of 0.3 mm.
aluminium and brass. They obtained the maximum García-Navarro et al. [20] evaluate the effect of
efficiency at a rotational speed of 450 rpm with a tool rotational speed and tool travelling speed on
1.6 mm tool offset that produces proper material both welding temperature and electrical properties
flow due to the presence of an intermetallic layer of welded joints of aluminium and copper; the
at the interface in addition to crack deflection by welding tool was a threaded cylindrical pin with a
the occurrence of a lamellar composite structure in flat shoulder. The workpiece fixation for all welded
the stir zone. Mehta & Badheka [13] reviewed the specimens, the copper put on the advancing side,
effect of welding parameters on material flow, and the aluminium put on the retreating side. The
microstructure, and welding defects. It can be seen results carried that the electrical resistivity
that the use of copper and aluminium joints increased with decreasing the tool travelling speed
welded by FSW is still limited due to the low and the higher value of temperature recorded
mechanical properties and formation of IMCs in occurred at a welding pitch of 65 rev/ mm.
large amounts. Imperfections, such as fragmental Ghiasvand et al. [21] evaluate the impact of tool
defects, voids, pores, and cracks, are commonly offset, pin offset, and material position of different
found in dissimilar Cu–Al FSW systems which are aluminium series on welding temperature. They
formed due to improper process parameters. The carried out the pin offset as the main effective
effect of single pass and dual pass on the welding parameter recorded high welding
microstructure of different aluminium series has temperature. Bokov et al. [22] studied the impact
been studied [9]. It was concluded that a of pin shape on the thermal cycle for welding
significant growth in the grain size during the aluminium and steel sheets. The steel is put on the
second pass reduces the hardness at the heat- advantage side and the aluminium is put on the
affected zone and; consequently, reduces the joint retreating side. They found that 15:26% of the heat
strength by 4.8%. Dhanesh Babu et al. [14] studied generated during the friction welding process was
the effect of pin geometry on the heat generation, generated by the tool pin. Elmetwally et al. [23]
mechanical properties, and microstructure of suggest an optimization between tool rotational
AZ80A Mg alloy. It was found that the cylindrical speed and tool travelling speed to improve the
pin gave a high welding temperature mechanical and microstructural properties for
approximately 83%. In contrast, the triangular pin welding aluminium to copper using the FSW
profile gave a lower value of 79%. Although the process. The results have shown that the
high temperature is an effective factor in friction maximum strength of the welded joint was carried
stir welding, the optimum grain size appeared at a at lower travelling speeds and higher rotational
temperature of 81:82 % using a Taper cylindrical speeds. Chupradit et al. [24] studied the effect of
pin profile. Celik & Cakir [15] studied the influence pin geometry on heat generation and mechanical
of welding conditions, i.e. rotation speeds, tool working during the FSW process. It was found that
traverse speeds, and tool position on the welding the mechanical working increased with increasing
properties of aluminium and copper. They carried the pin tilt angle. In addition, rising in the welding
out the optimum welding strength at a welding temperature was observed as increasing the
pitch 66.5 rev/mm, and 1 mm tool offset. Msomi & contact area between the tool and the workpiece.
Mabuwa [16] evaluate the position of material as a Recent works developed a mathematical model to
welding parameter, which is put on the advancing estimate the effect of pin geometry on the heat
side once and other on the retreating side on generated during the FSW process [25,26].
fatigue strength during FSW/FSP. They found that FSW process is still under development. Many
the location of strong material put in the articles deal with FSW process parameters, such as
advancing side refinement welding stir zone, in rotational speed, traverse speed, tool offset, tool-
contrast, weak material put in the advancing side tilt angle, positions of aluminium and copper
has an undesirable effect on welding efficiency. sheets, and shoulder shape. Limited articles
The effect of tool offset on the stir zone properties studied the effect of pin geometry on the
of copper plates has been investigated [17-19]. The mechanical and microstructural properties of the
tool pin was taper shape; the tool offset was taken Al-Cu joints. Therefore, the present work aimed to
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investigate the impact of pin shape (as a category

of tool geometry) on the mechanical and
microstructural properties of FSW process used to
weld copper with aluminium. The present study
includes the effect of pin shape on the heating
cycle and thus on the mechanical and
microstructural properties.

2. THE EXPERIMENTAL

2.1 Materials and tools

Pure copper and aluminium (as received) with 3

mm in thickness plates were used as base metals
for the joints. The chemical composition of the
base metals is laboratory checked and given in
Table 1 and in Table 2. The mechanical properties
of the base metals are determined using tensile
and hardness tests and given in Table 3. Four FSW Fig. 1. Pin Configurations and Design; I) Taper, II)
tools made from W302 steel are machined to their Square, III) cylindrical, and IV) Triangle
final dimensions and heat treated to achieve the
required hardness value (50-52 HRC) to retain its 2.2 Procedures and tests
surface shape and prevent pin corrosion. The FSW
tool is designed with a flat shoulder shape and Rectangular strips with dimensions of 75x120
different pin geometries (shapes). The selected pin mm are cut from (as received) aluminium and
geometries (as shown in Fig. 1) were cylindrical, copper sheets. The aluminium and copper strips
taper, straight triangle, and straight square. For all are fixed together on a fixture device. The fixture
shapes, the pin length was 2.4 mm i.e., 80% of device and the welding tool were held at a milling
base metal thickness. machine milko-r35 as shown in Fig. 2a. Based on
literature work, the tool tilt angle was selected as 2
Table 1. Chemical composition of aluminium (as degrees for all welded specimens [13,18,27]. A
received) travelling speed of 16 mm/min and rotational
Element Si Fe Co Zn Pb speed of 1280 rpm is selected for welding all
Wt.% 0.02 0.54 0.013 0.02 0.031 specimens according to the best results and
Element Sn V Ti Other Al recommendations from research [23]. The
Wt.% 0.04 0.015 0.024 0.031 99.3 temperature was measured and checked using a
thermocouple type K and an IR camera (BOSHTM,
Table 2. Chemical composition of copper (as received) thermos detector GIS 1000 °C). Each of the
Element Fe Sn Pb S Zn measuring instruments has been calibrated (±1 °C).
Wt.% 0.11 0.099 0.016 0.01 1.73 The thermocouple bulb (sensor) was fixed with the
Element Al C Cr Other Cu help of Epoxy adhesive on the copper side at the
Wt.% 0.04 0.01 0.015 0.07 97.9 middle of the welding pass at 0.5 mm depth. The
thermocouple was connected to the Avometer to
Table 3. Mechanical properties of the base materials obtain the temperature value directly, which was
recorded every 5 seconds. The welding
Ultimate
Yield temperature was measured and checked by using
Tensile Hardness
Property strengt E% a calibrated thermocouple. During the welding
strength (HV)
h (MPa) process, the copper strip was put on the advancing
(MPa)
Al side (AS) and the aluminium strip was put on the
85 134 6 48
(as received) retreating side (RS). After the welding process was
Cu
90 241 54 129 done, the tensile, hardness and microstructure
(as received) samples were cut from the welded joint as shown
in Fig. 2b.

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The tensile samples were designed according to 3. RESULTS AND DISCUSSION

ASTM-E8M standard. The tensile test was done on
a universal testing machine QUASAR 100-VAV201 3.1 Morphology and Microstructural analysis
with a capacity of 100 KN. The microhardness was
measured at a direction perpendicular to the The surface morphology of the optimum and
welding direction and carried out on a Vickers worst Al-Cu joints can be seen in Fig 3. The
testing machine, VMH tester model number 1600- optimum surface was obtained when the joint was
4981, with a dwell time of 15 s and 500 gf. The welded using a squared pin tool (Fig. 3a). The
tensile test results are conducted by using welded surface of such joint displays no grooves,
microstructural analysis using an optical holes, or surface imperfections, with a few flashes
microscope and scanning electron microscope. The on the copper side. Other pin profiles, such as
microstructural specimens were ground, polished, triangular pins, display longitudinal fractures and
and chemically etched before the microstructural flashing flaws (Fig. 3b).
analysis. The sanding process was performed on
the cross-section area perpendicular to the
welding direction then polished with 5μm and 2μm
diamond paste and etched by using an etching
solution that consists of 190 ml H2O, 5 ml HNO3, 3
ml HCl, and 2 ml HF acid for the aluminium side
and 25 ml H2O, 10 ml H2O2, and 25 ml NH4OH for
the copper side. The etched surfaces were
examined using an optical microscope (type:
Olympus PMG 3). The fracture behaviour of the
tensile fractured surfaces was evaluated via
scanning electron microscope (SEM), “type: Carl Fig. 3. The appearance of the joint surface welded
Zeiss-SIGMA 1500VP”. using: (a) squared pin, (b) triangular pin

Fig. 4 depicts an EDX study at point on the

interface line between Cu and Al sides of a
specimen welded using a squared pin tool,
demonstrating excellent mixing between Al and
Cu.

Fig. 2a. FSW process setup; (a) Recorder temperature,

(b) IR camera, and (c) Thermocouple

Fig. 4. EDX analysis of SZ of the specimen welded by

squared pin (at the interface between Al and Cu)

Fig. 5 displays optical images of square pin

configurations at various zones, whereas Fig. 6
provides optical images of the stir zone for the
triangular pin. The heat-affected zone (HAZ)
structure of Al side is shown in Fig. 5a, while Fig. 5b
presents the structure of the interface between
Fig. 2b. Schematic view of FSW specimen design; (#1) the stirring zone (SZ) and the thermal-mechanical
Hardness and Microstructure sample, (#2) Tensile test affected zone (TMAZ) in the Al side. In the squared
samples pin case, there is a good mixing of aluminium and
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copper in the stir zone; little Cu bits are noted in consistent with prior research [20,21]. The stirring
the stir zone on the aluminium side and get greater motion in the stir zone ensures proper mixing of
towards the end of the stir zone near the copper fine aluminium particles and coarse copper
side (Fig. 5c and 5d). On the copper side, copper pieces/particles (Fig. 5c). On the copper side, small
particles are small in the stir zone (SZ), elongated copper particles are seen in the stir zone. These
and compressed in the thermal-mechanical particles grow to become coarser and harder in the
affected zone (TMAZ), and eventually bigger in the TMAZ and HAZ, as illustrated in Fig. 5e. The
heat-affected zone (as shown in Fig. 5e and 5f). microstructure produced in the specimen welded
by the triangular pin tool is completely different;
surface defects grow inside the stirring zone, as
shown in Fig. 6a and 6b, and these defects include
cavities and cross and longitudinal cracks, which
are expected due to the impulse action of the
triangular profile. Fig. 7 shows the SEM images for
the microstructure of different zones. The fine
particle structure of the heat-affected zone for the
squared pin is given in Fig. 7a while a mixed
structure between the Al particles and Cu pieces at
the TMAZ is shown in Fig. 7b. The SEM images of
the flaws formed in the stir zone when the
triangular pin used are shown in Fig. 7c, 7d and 7e.
These flaws include large voids and cavities, as
indicated in Fig. 7c, separation and fractures in
certain copper particles (Fig. 7d), and
agglomerations of fractured copper layers inside
the aluminium matrix (Fig. 7e).

Fig. 5. Microstructural images of squared pin profile: (a)

Al-matrix, (b) Al-side SZ&HAZ & TMAZ, (c) RS of SZ, (d)
nugget zone, (e) Cu-side SZ, TMAZ and HAZ, (f) Cu
matrix

Fig. 6. Microstructural images of triangular pin profile:

(a) SZ of Cu side, (b) TMAZ & HAZ of Cu side

Fig. 7a and 7b show SEM images of the

aluminium side's HAZ and TMAZ. The
microstructure of the HAZ and the TMAZ shows
the presence of fine particles which indicates
Fig. 7. SEM images for the microstructure of different
dynamic recrystallization. The stirring motion
zones of Al-Cu joint welded by FSW for: (a) squared pin
raises the temperature to the recrystallization tool at HAZ (b) squared pin at TMAZ, (c), (d) and (e)
point of aluminium, resulting in the formation of a defects at stir zone of the triangular pin; cavities and
new and tiny particle. Recrystallization occurred dislocation (c), fracture in Cu particles (d), and fracture
entirely in the HAZ and only partially in the TMAZ and separation of Cu layer (e)
of the aluminium side. These findings are

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3.2 Thermal cycle 3.3 Mechanical properties

The differences in transient temperature for 3.3.1 Joint strength

each pin profile were studied and presented in Fig.
8. In general, the temperature-time shapes were The tensile test is employed to determine joint
similar for all pin profiles. The thermal cycle strength for each pin profile, and the results are
includes the heating region starting as the tool shown in Fig. 9.
engaged in the workpiece until the transient
temperature reached its peak value followed by
the cooling region which continued until the tool
leaves the workpiece. The maximum heating rate
was 1.573 °C/s for the square profile while the
minimum heating rate was 1.151 °C/s for the
triangle profile. On the other hand, the maximum
cooling rate was found as -1.205 °C/s for the
square profile while the minimum cooling rate was
-0.832 °C/s for the triangle profile also. The peak
temperatures were 316 °C, 288 °C, 255 °C, and 231
°C for square, tapered, cylindrical, and triangle Fig. 9. Joint strength in MPa versus pin shape
profiles respectively. The contact area between the
tool profile and the workpiece (as the friction The joint strength varies significantly depending
influence) plays a significant factor in evaluating on the tool pin profile. The largest strength was
the amount of transferred heating energy. Square achieved in the square pin case, which was 80%
and cylindrical profiles have the maximum aluminium strength, followed by the tapered pin,
contacted area and then the tapered profile while which was 60% aluminium strength. Since the
the triangle profile has the minimum contacted minimum strength value is 22.5% of aluminium
area (see Table 4). strength, the pin shape has a significant impact on
joint efficiency. Due to the friction effect, the
material in front of the tool is heated, agitated,
and pushed behind the tool, while the forward
motion extrudes another layer around the tool
[21]. The pin with flat faces (square and triangular)
is associated with eccentricity which causes
incompressible material to pass around the pin
profile [28] and make multiple pulses for each tool
rotation which swept the material around the tool
[29]. The stirred material produces dynamic swept
which largely differ according to tool profile
[28,30]. The maximum dynamic swept was found
for triangular pin and then for the squared pin. The
Fig. 8. Thermal cycle for different pin profiles ratio between the dynamic swept material volume
to the static material volume is 2.3, 1.56, 1.09, and
Table 4. Contacted area and peak temperature for
1 for triangular, squared, tapered, and cylindrical
different pin profiles
pin profiles respectively [28-30]. The higher
Total pulsating effect and dynamic swept of squared pin
Peak
contact produce higher localized plastic deformation which
Tool Symbol Temperature
Area maximizes the heat generated during the welding
(°C)
(mm2) process and therefore improves the joint strength
Taper I 274.58 255 and efficiency. Although the higher values of
Square II 288.41 316 dynamic swept of the specimen welded by
Cylindrical III 292.17 282 triangular pin profile, number of defects (as shown
Triangle IV 285.65 238 in Fig. 5) produced through stirring action caused a
weak joint and lower strength value.

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Khodaverdizadeh et al. [31] showed that the in the weld zone, with associated hardness
recrystallized grain size in the SZ is the main reason distribution and strained area. Fig. 11 depicts the
for improving the mechanical properties of the fracture surface of joints welded with triangular
specimen welded by squared pin over specimen and squared cylindrical pin shapes. By micro void
welded by other pin profile; however, the grain coalescence, both the samples had ductile fracture
size in the SZ is finer in the case of squared profile. morphology. Nonetheless, the triangular pin
On contrary, samples welded using other pin profile joint (Fig. 11a and 11b) displays some
profiles have weaker properties because of their cleavage type fracture mode and bigger voids, as
coarse grains. Improvement in mechanical well as the presence of intermetallic components
properties, when squared pin profile, was used (IMCs) indicating poorer ductility of the joint. In
over other pin profiles are in agreement with other the case of the joint welded employing square pin
references [7,28-30]. profile (Fig. 11c and 11d); however, fully dimple-
like fracture mechanism and lack of huge voids are
3.3.2 Joint hardness seen, indicating better ductility of the joint. Shear
and deep dimples in Fig. 11c and 11d indicate the
The hardness of the joint is measured existence of a ductile fracture.
perpendicular to the welding line closest to the top
surface, and the results for various tool profiles are
shown in Fig. 10. The hardness decreased slightly
in the stir zone (SZ) around the welding line and
reached a minimum value at 2 mm from the datum
toward the copper side before suddenly increasing
to a maximum value at 3-4 mm from the welding
line around the circumference or pin edge closer to
the copper side's thermal-mechanical affected
zone (TMAZ). The hardness value decreases as a
result of stirring action, which softens the particles
and raises strain amount in these places, followed Fig. 10. Micro-hardness of welded joints at different pin
by strain hardening in the area close the pin profiles
surface. The peak hardness corresponded to the
peak temperature, i.e. the maximum hardness
obtained for the squared and tapered pin
configuration and the minimum hardness obtained
for the triangular and cylindrical pin configuration,
indicating the effect of welding temperature in
hardening the grains in the stir zone. The lowest
hardness number for all specimens is less than
aluminium hardness while the maximum hardness
number is greater than copper hardness. Thermal
exposure induces a significant softening effect,
reducing the hardness of the SZ. The substantial
grain refinement generated by welding with a
square pin profile; on the other hand, enhances
the hardness of the SZ [19]. The hardness test
results can show the failure location and form of
the tensile test, which occurs in the stir zone but Fig. 11. SEM images for the fractured surfaces of tensile
near to the aluminium side and seems to be a specimen welded by: triangular pin profile (a and b) and
ductile fracture, as detailed in the following item. squared pin profile (c and d)

3.3.3 Joint Fracture Phases of the worst specimen in mechanical

and microstructural properties are examined via
The fracture was generated on the joints’ XRD. The resulted XRD pattern is given in Fig. 12.
retreating side (aluminium) due to variations in the The XRD pattern demonstrates the existence of
temperature distribution and flow of the material intermetallic components (IMCs) in the form of
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Al2Cu at the cracked surface. These IMCs are 4. When a squared pin profile is utilized, ductile
almost harder than the neighbours’ particles and fracture occurs, but a mixed brittle-ductile
layers [32]. As IMCs founded, joint defects were fracture occurs owing to the presence of flaws
initiated and grew in these areas. or intermetallic components when a triangular
pin profile is employed. The fracture happened
towards the end of the aluminium side's SZ.

ACKNOWLEDGMENT

The authors extend their thanks and appreciation

to the technical staff, Faculty of Technology and
Education, Beni-Suef University, Egypt.

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