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Research Article
Influence of Friction Stir Welding Process on the Mechanical
Characteristics of the Hybrid Joints AA2198-T8 to AA2024-T3
Ahmed Samir Anwar Alemdar , Shawnim R Jalal,
and Mohammedtaher M Saeed Mulapeer
Mechanical and Mechatronics Department, College of Engineering, Salahaddin University, Erbil, Iraq
Correspondence should be addressed to Ahmed Samir Anwar Alemdar; ahmed.anwar@su.edu.krd
Received 8 August 2022; Revised 6 September 2022; Accepted 26 September 2022; Published 8 October 2022
Academic Editor: Sonar Tushar
Copyright ©2022 Ahmed Samir Anwar Alemdar et al. is is an open access article distributed under the Creative Commons
Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is
properly cited.
e study presents the hybrid joining of the third generation AA2198-T8 aluminum lithium alloy to AA2024-T3 aluminum
copper alloy, which has been highly demanded recently in the aerospace industry. is investigation aims to reduce the cost of
production in the industrial sector. As a result, an affordable alternative is to use hybrid designs using AA2198-T8 alloy in crucial
parts and AA2024-T3 alloy in the rest of the structure. A joining method is required to create hybrid structures composed of last-
generation and standard aluminum alloys. e joining process was successfully friction stir-welded using five different welding
travel speeds—36, 76, 102, 146, and 216 mm/min—with an invariable spindle speed of 960 rev/min. Two reversed steps, double-
sided friction stir welding (DS-FSW) and single-sided friction stir welding (SS-FSW) techniques with two appropriate tool
designs, were employed to investigate the dissimilar material mechanical properties and their morphological changes. e
experimental outcomes show that DS-FSW of the reversed steps has a higher joining strength than SS-FSW for all the welding
parameters studied. e variation in travel speeds provided the highest strength at 102 mm/min travel welding speeds for DS-
FSW. erefore, it is found that, from the three tensile samples, tensile strength, yield strength, and elongation of the joint were
407.1 MPa, 271.2 MPa, and 9.5%, respectively. e joint efficiency reached 87% compared with the base material tensile strength of
AA2024-T3. Furthermore, fractures of the tensile samples were found in the vicinity of the thermomechanically affected zone
(TMAZ) of the AA2198-T8 side. e microhardness and morphology results correspondingly have precise predictions for the
fracture zone of the joints in this research examination.
1. Introduction
Aluminum alloys are the most desirable in several appli-
cations, especially in aerospace industries, because of their
high strength-to-weight ratio and excellent corrosion re-
sistance [1]. e recently developed third generation of
aluminum alloys, such as AA2198 aluminum lithium alloys,
become a choice for lightweight structural applications for
the fuselage of aerospace industries [2]. AA2198 has lower
density and improved mechanical characteristics than their
conventional counterparts, AA2024 aluminum-copper al-
loys. Nevertheless, gaining these benefits costs material
expenses. Meanwhile, a reasonable solution is using a hybrid
joining of AA2198-T8 to AA2024-T3 alloys individually for
critical regions through the remaining structure retaining
AA2024-T3 alloys [3]. However, welding aluminum alloys
via the conventional fusion welding process are inappro-
priate and not recommended as various weld defects such as
porosity formation, cracking during solidification, signifi-
cant loss of strength in the weld region, and high distortion.
Friction stir welding (FSW) is a green hybrid solid-state
joining welding process [3]. FSW offers considerable ad-
vantages for joining aluminum alloys in similar and dis-
similar combinations over conventional welding [4]. FSW
utilizes a rotating pin to mix the materials of the two sides of
the joint locally below the melting point. us, welding flaws
such as hot cracking are avoided from forming. Selecting the
best FSW tool and process parameters is crucial for pro-
ducing dependable joints for aerospace applications, par-
ticularly for dissimilar alloys with variable mechanical and
Hindawi
Advances in Materials Science and Engineering
Volume 2022, Article ID 7055446, 11 pages
https://doi.org/10.1155/2022/7055446
thermal characteristics [5]. Friction stir welding tools can be
made out of various materials. Each material has favorable
and bad qualities depending on the material being welded.
Steels, carbide particles, reinforced composites, tungsten
carbide with cobalt, titanium carbide, tungsten carbide,
nickel alloys, cobalt-base alloys, refractory metals, ceramic
materials, and polycrystalline cubic boron nitride (PCBN)
are among the materials in this category. Steel tools such as
AISI H13 are used for soft welding materials like aluminum.
Tungsten carbide with cobalt-based material and polycrys-
talline cubic boron nitride (PCBN) is employed for hard
materials such as titanium and its alloys [6]. However, these
materials have superior melting properties. ey are more
challenging to fabricate into the complex shapes that the tool
may have. Difficulties in fabrication lead to increased tool
costs; therefore, it is often desirable sometimes to sacrifice
some durability for a reasonably priced tool [3, 7]. Hardened
tool steel AISI (H13, D2) based tools are perfectly acceptable
for welding workpiece thicknesses less than 12 mm in FSW
of aluminum. However, in some case studies, the AISI H13-
based steel tool is generally recommended for welding
aluminum alloys within 0.5–50 mm but with more chal-
lenging or highly abrasive materials [8]. Many researchers in
the tool geometry investigation have proved that FSW tool
profile designs have crucial effects on enhancing welding
sound joints [9]. Moreover, the discrepancy of the tool
geometry proves that the parameters change, besides the
type of material and the thickness used in FSW are further
concerns in this regard. erefore, the FSW tool geometry
design proved in the most experimental research to be a
threaded taper pin with concaved shoulders [10].
FSW is currently used as an alternative solution for the
traditional welding process and riveting for the fuselages in
the aerospace industry. Some weld defects such as excessive
flashing, excavating (tunneling), and kissing bonds can also
occur in friction stir welding if optimum weld parameters are
not used [11]. e core of the work is to produce reliable
hybrid joints via the friction stir welding process to select
appropriate weld parameters, which are essential to improve
the joining strength. us, the selection of the type of friction
stir welding process, whether single-sided friction stir welded
welding (SS-FSW) or double-sided friction stir welding (DS-
SFW), should be done based on the experimental test results.
us, mechanical behavior and microstructural changes
should be inspected to determine the best technique for a set
of fixed optimum weld parameters. In recent years, DS-FSW
(double side friction stir welding) has drawn the attention of
many industrial sectors and researchers to improve the
mechanical properties of weldment materials. DS-FSW has
been used as an additional parameter for friction stir welding
techniques to improve joint efficiency and mechanical
characteristics of the problematic welded materials. DS-FSW
has become strongly recommended for material thickness
exceeding 50 mm for a successful joining process [8]. us,
the mechanical properties of reversed passes have been
considered an additional parameter for the DS-FSW process
[12]. Moreover, the effects of the tool size as geometrical
design and the whole of the applicable mechanical parameters
can be considered. Meanwhile, microstructural
characterization and mechanical test results should be con-
ducted to advise the applicable weld parameters [13].
is study aims to achieve the utmost efficient hybrid
joints via FSW and reduce the cost of production in the
industrial sector. As a result, an affordable alternative is to
use hybrid designs using AA2198-T8 alloy in crucial parts
and AA2024-T3 alloy in the rest of the aerospace structure.
e examination of reversed DS-FSW and SS-FSW case
techniques has been run into several crucial tests in com-
pliance with particular international standards. us, the
specimens were conducted into tensile, morphology,
microhardness, and scanning electron microscope (SEM)
examinations. e mechanical test as a central pillar for the
work has been examined to specify the best defectless joint
parameter.
2. Materials and Experimental Work
In this research examination, two base materials—AA2198-
T8 aluminum lithium alloys and AA2024-T3 aluminum-
copper alloys (300 mm ×75 mm ×6 mm)—were used to join
via the FSW process. e chemical composition of the base
material for AA2198-T8 and AA2024-T3 are analyzed by
Spectro Maxx and shown in Tables 1 and 2, respectively. In
this case study, the milling machine model (NK-IWA-
SHITA) was used to perform FSW for both cases of welding
techniques. e dissimilar plates are fixed on the specially
made rigid base plate via a clamp to prevent slipping and
shifting during FSW processes. e FSW tool selected for the
hybrid joining process was made of hot-worked AISI H13
tool steel, which has been proven successful for the FSW
process for aluminum alloys, and after heat treatment, the
hardness reached 66 HRC [9]. Both FSW design cases are
tapered threaded pins with concaved shoulders, per rec-
ommendations for experimental work case studies [3, 9, 14].
Simultaneously, the tool geometry for each method of FSW
for hybrid welding joints has been designed in different pin
and shoulder dimensions regarding material thickness and
friction stir welding processes [2, 9]. e geometrical designs
of the tool for reversed DS-FSW and SS-FSW are illustrated
in Figure 1. us, the design of both techniques is upgraded
with the fins section to improve tool life performance and
reduce the heat rise of the tool. As a result, the research
intends to find the best FSW process technique between
reversed (advanced side AS and retreating side RS) DS-FSW
and the traditional SS-FSW. In the meantime, many trials
and experimental studies recommendations have been
reviewed to address this aim. e rotation of the spindle
speed of 960 rev/min, the tilt angle of 2
o
, the plunge-in depth
of 0.3 mm, and the dwell time DT of 20sec of welding
parameters were kept constant through examining five
different travel speeds or welding speeds of 36, 76, 102, 146,
and 216 mm/min [15–17]. Before each joining process for
reversed DS-FSW and SS-FSW, the base material intertouch
surface has been machined 0.3 mm and swapped by ethanol
to remove any oxide or dirt particles to perform a clean
welding process.
Examination of reversed DS-FSW and SS-FSW case
techniques has been run into several crucial tests in
2Advances in Materials Science and Engineering
compliance with particular international standards. e
mechanical test as a central pillar for the work has been
examined to specify the best defectless joint parameter.
Tensile machine model is HUALONG 600 kN. e tensile
specimens have been prepared according to the American
welding society standard specification AWS D17.3/17.3 M
2016. Meanwhile, the joint efficiency has been achieved
through the ultimate tensile test compared with the base
material of AA2024-T3. e Vickers hardness HV05 profile
was measured via a Digital Vickers Hardness tester machine
in compliance with ASTM E92. At the same time, it reveals
the typical microstructure according to ASTM E3 standard.
e specimen was sliced and ran into a sequence of grinding
and polishing via grit papers (600, 800, 1000, and 3000 grits)
and diamond suspension solutions (0.5 and 0.25 microns) to
remove minor scratches on the surface during the polishing
process. us, for etching purposes, the double etchant was
employed, and two solutions were prepared: solution A, the
composition of 25 ml of HNO
3
and 75 ml of H
2
O; solution B,
the composition of 0.5 g of NaF, 1 ml HNO
3
, 2 ml of HCL,
and 97 ml of H
2
O. e prepared sample was immersed for
60 sec in solution A at 70°C and quenched significantly by
cold water and then immersed in solution B for 30 sec and
washed in a stream of warm water. However, Metkon
Microscopic model M902 is used to capture the micro-
structure photos of the hybrid joint. Furthermore, the
fracture surfaces of the utmost hybrid joint after the tensile
test were examined through the scanning electron micro-
scope (SEM) to reveal the propagation of defects and failure
points.
3. Results and Discussion
3.1. Mechanical Characteristics. Tensile samples were sliced
perpendicular to the welding line representing long gauge
length. e ultimate tensile strength (UTS) yields tensile
strength (YTS), and elongation percent (EL) has been
considered for each welding speed in both reversed DS-FSW
and SS-FSW. e joint efficiency for the reversed DS-FSW
and SS-FSW has been compared with the parent metal. e
tensile properties result in various welding speeds for re-
versed DS-FSW and SS-FSW, as shown in Figure 2. e
hybrid joint tensile characteristics were significantly de-
creased compared to the base material. Furthermore, the
tensile strength increased and decreased by increasing the
welding speed from 36 mm/min to 216 mm/min in both
cases of the welding process (reversed DS-FSW and SS-
FSW). Since at lower welding speeds, the temperature rises
18.0
30.0
2.0
2.0
6.0 5.0
5.5 4.0 Concave shoulder
readed Pin
(DS-FSW) Tool readed Pin
Concave Shoulder
16.0
30.0
2.0
2.0
5.0
5.0
3
3Concave shoulder
readed Pin
(DS-FSW) Tool readed Pin
Concave Shoulder
Fins
Figure 1: Dimensions and the geometrical design of double side friction stir welding (DS-FSW) and single side friction stir welding (SS-
FSW) tools.
Table 1: Chemical composition of BM AA2198-T8 (wt%).
Element Cu Li Fe Mg Mn Si Ti Ag Zn
Concentration 3.4 1.02 0.08 0.75 0.3 0.05 0.07 0.5 0.3
Table 2: Chemical composition of BM AA2024-T3 (wt%).
Element Cu Li Fe Mg Mn Si Ti Ag Zn
Concentration 3.83 — 0.45 1.84 0.75 0.5 0.12 0.5 0.23
Advances in Materials Science and Engineering 3
cause a change in grain structure with excessive flash ma-
terial, whereas higher welding speeds lead to propagation of
defects like incomplete penetration in SS-FSW or mis-
alignment of the double pass in reversed DS-FSW. e
utmost joining efficiency in mechanical characteristics was
carried out in reversed DS-FSW at 102 mm/min, where the
maximum ultimate tensile (UTS), yield strength (YTS), and
elongation (EL) are 407.1MPa, 271.2MPa, and 9.5%, re-
spectively. e minimum values are obtained at 36 mm/min,
257.4 MPa, 160.1 MPa, and 3.7%, respectively. On the other
hand, for the SS-FSW joints, the maximum UTS, YTS, and
EL are found at 76 mm/min, 293.3 MPa, 190.2 MPa, and
4.8%, respectively. e minimum values are found at
216 mm/min, 210 MPa, 75.9 MPa, and 2.2%, respectively, as
shown in Figure 2.
3.2. Macrostructure Examination. Excessive flash material
in friction stir welding has crucial side effects on the joining
process. e flash material of the friction stir-welded
material in both welding processes reversed DS-FSW and
SS-FSW have been examined via macroexamination. As
illustrated in Figure 3, the macrostructure examination was
revealed via double etching solution according to the
ASTM E3 standard. e sliced section surface has been
prepared through the sequence of polishing grits and di-
amond suspension solutions to remove the minor scratches
and glitter the surface. Macroexamination presented that
the increase in the transverse speed of welding processes
leads to misalignment of the tool axis, especially in the DS-
FSW process, and the “S” shape curve appears in the weld
center for SS-FSW. e root cracks were observed in the SS-
FSW at 76 and 102 mm/min transverse welding speed.
rough macrostructure examination at 36, 76, and
102 mm/min of transverse welding speeds, no misalign-
ment was observed, and moderate flash material was no-
ticed at 102 mm/min for DS-FSW. On the other hand, the
heat-affected area in SS-FSW welding was more significant
than the reversed DS-FSW. However, a larger heat-affected
area increases the risk of losing mechanical properties and
efficiency reduction in FSW of the hybrid joint AA2024-T3
to AA2198-T8.
466
257.4
376.4
407.1 389.1
274.3
265.2
293.3
248.3
215.5
210
AA2024 BM 36 76 102 146 216
Welding Speeds (mm/min)
DS-FSW
SS-FSW
AA2024 BM
0
100
200
300
400
500
600
Ultimate Strength UTS (MPa)
(a)
DS-FSW
SS-FSW
AA2024 BM
330
160.1
203.9
271.2 246.3
162.2
150.8
190.2
118.2
111.5
75.9
AA2024 BM 36 76 102 146 216
Welding Speeds (mm/min)
0
100
200
300
400
Yield Strength YTS (MPa)
(b)
DS-FSW
SS-FSW
AA2024 BM
21
3.7
6.7
9.5
5.3 3.8
4.6 4.8
2.8 22.2
AA2024 BM 36 76 102 146 216
Welding Speeds (mm/min)
0
5
10
15
20
25
Elongation (%)
(c)
Figure 2: Tensile properties of the DS-FSW and SS-FSW hybrid joints in various welding speeds. (a) UTS, (b) YTS, and (c) EL.
4Advances in Materials Science and Engineering
3.3. Microstructure Examination. Double etching solutions
revealed the microstructure of the reversed DS-FSW hybrid
joint. Figure 4 shows the microstructure of the utmost
hybrid joining process for reversed DS-FSW at 102 mm/min
assembled after 372 captures. e grain size and distribution
in SZ are found in refined equiaxed grains due to recrys-
tallization behavior in the reversed DS-FSW process, as
shown in Figure 4. is area corresponds closely to the route
of the pin during welding. Compared to the base metal, the
grain size of the substance in this zone is very small. is
zone’s microstructure contains elements of both metals in
the welding process. is zone is characterized by numerous
concentric rings, often known as “onion ring structure.” On
both sides of the stir zone, the TMAZ is located. During the
welding process, plastic deformation of the base metals
creates it. Here, the strain and temperature are lower than in
the SZ, and as a result, the impact of welding on the mi-
crostructure is less distinct. In all welding procedures, the
HAZ is created owing to the heat cycle of the weld. is area
endures a heat cycle, but no plastic deformation occurs.
Temperatures are lower than in the TMAZ but still signif-
icantly impact the zone’s microstructure and mechanical
behavior.
Meanwhile, parameters change significantly affect grain
size and microstructure changes in three zones, SZ, TMAZ,
and HAZ. e grain size measurement was done in com-
pliance with international standard ASTM E112. ough it
is noticed that the size of these equiaxed grains variants, the
average grain size of the reversed DS-FSW for the SZ of
unaltered spindle speed of 960 rev/min of the selected
variable welding speeds 36, 76, 102, 146, and 216mm/min
are 4.2, 2.4, 2.1, 2.9, and 3.6µm, respectively. e regular size
in SZ was reduced when the welding speed increased from 36
to 102 mm/min and increased when the welding speed in-
creased from 102 to 216mm/min, which encircling 4.21µm
to 4.2 µm, and 2.1 µm to 3.6 µm, respectively. Consequently,
the grain is significantly coarser at the lowest welding travel
speed, as shown in Figure 5. e average grain size in SZ of
the SS-FSW for the same selected welding speeds was 4.9,
2.4, 3.4, 4.1, and 6.2µm, respectively. However, the overall
average grain size in SZ for reversed DS-FSW was observed
that it is severe and smaller than SS-FSW. Moreover, minor
and major defects like tunnel defects and kissing bonds have
been noticed in SS-FSW at welding speeds of 146 mm/min
and 216mm/min, as shown in Figure 6.
3.4. Microhardness. e microhardness profile of the re-
versed DS-FSW and SS-FSW of the hybrid joints is fabri-
cated at different welding speeds, showing the
microstructure and grain size changes. Conferring to ASTM
E92, the microhardness values were measured via HV05.
Correspondingly, the microhardness distribution for the
joints as a “W” shape specified a gradual reduction in TMAZ
and HAZ regions [18, 19]. On the other hand, the HAZ for
both cases at various speeds indicated a minimal micro-
hardness value. In addition, the base material microhardness
measured values were higher than the three weld zone
values. us, this demonstrated that the three zones of the
DS-FSW; ω = 960 rpm ; v = 102 mm/min
AS
AS RS
RS
DS-FSW; ω = 960 rpm ; v = 76 mm/min
AS
AS RS
RS
SS-FSW; ω = 960 rpm ; v = 76 mm/min
ASRS
SS-FSW; ω = 960 rpm ; v = 36 mm/min
AS
RS
AA2198AA2024
DS-FSW; ω = 960 rpm ; v = 36 mm/min
AS
AS
RS
RS
AA2198AA2024
SS-FSW; ω = 960 rpm ; v = 146 mm/min
AS
RS
“S” Line
SS-FSW; ω = 960 rpm ; v = 216 mm/min
AS
RS
3 mm
“S” Line
DS-FSW; ω = 960 rpm ; v = 216 mm/min
AS
AS RS
RS
Misalignment
SS-FSW; ω = 960 rpm ; v = 102 mm/min
AS
RS
Root crack
DS-FSW; ω = 960 rpm ; v = 146 mm/min
AS
AS RS
RS
Misalignment
Figure 3: Macrostructure of reversed DS-FSW and SS-FSW joints AA2198-T8 to AA2024-T3 at various welding speeds.
Advances in Materials Science and Engineering 5
Onion rings
Figure 4: Microstructure examination of the DS-FSW at 102 mm/min welding speed.
4.2
2.4 2.1
2.9
3.6
4.9
2.4
3.4
4.1
6.2
36 76 102 146 216
Welding Speeds (mm/min)
0.0
2.0
4.0
6.0
8.0
Grain Size (µm)
DS-FSW
SS-FSW
Figure 5: Average grain sizes of the stir zone for the FSW process.
6Advances in Materials Science and Engineering
weld are softened: nugget zone (NZ), the thermomechani-
cally affected zone (TMAZ), and the heat-affected zone
(HAZ), as shown in Figures 7 and 8.
Figure 9 presents the mean microhardness values for
reversed DS-FSW and SS-FSW hybrid joints for various
welding speeds in the nugget zones. e maximum average
microhardness value measured for reversed DS-FSW in the
NZ is 127.4 HV at 102 mm/min welding speed. Meanwhile,
the minimum microhardness value of the NZ is 113.6 HV at
36 mm/min welding speed. On the other hand, the micro-
hardness mean values for the NZ of the SS-FSW hybrid
joints are at the same welding speeds. us, the maximum
microhardness value of SS-FSW was found to be 108.1 HV at
76 mm/min and the minimum 91 HV at 36 mm/min.
erefore, comparing reversed DS-FSW with the SS-FSW
microhardness values in NZ, the measured values in re-
versed DS-FSW were higher than the SS-FSW at various
welding speeds. Generally, microhardness values distribu-
tion in the three zones is correlated to the microstructure
variations [20]. e main reason for the microhardness
values of the NZ lower than the parent metals is the original
particles that strengthen aluminum alloys which are dis-
solved in the FSW process; however, it precipitates in the NZ
after the FSW process. Due to the dynamic recrystallization
caused by the rotation tool in the stir zone forming refined
equiaxed grains in NZ. e microhardness values in NZ are
higher than the TMAZ and HAZ, as shown in Figures 7 and
8. Exclusively, when the welding speeds used 102 mm/min in
reversed DS-FSW, the average microhardness value is higher
due to fine equiaxed grain sizes. While due to the coarse
grain size results of 36 mm/min, the lowest microhardness
value was observed [21]. e grain size and microhardness
values are closely related to the mechanical properties of
precipitation strengthening of aluminum alloys. e
strength and elongation of the base material scored a higher
value due to the presence of a large amount of the pre-
cipitation phase. e superior tensile property and micro-
hardness values in the NZ compared to the HAZ and TMAZ
are due to the contribution of equiaxed fine grain size.
On the other hand, the grain size and reprecipitated
phase distribution of various FSW joints generated by
different welding speeds vary significantly. e favorable
input of more refined equiaxed grains and more repreci-
pitated particles into the NZ in reversed DS-FSW at a
welding speed of 102 mm/min may enhance the mechanical
characteristics of FSW joints. e joints manufactured at
36 mm/min exhibit the reverse impact of coarse grain and
reprecipitated particles in the NZ. Figures 7 and 8 dem-
onstrate the prediction of the fractured locations of all
welding joints for reversed DS-FSW and SS-FSW achieved
during tensile testing at various welding speeds. e welding
speeds of 36 mm/min, 146 mm/min, and 216 mm/min are
used during the FSW process fractured in the NZ. It is
evident to predict the fracture location via microhardness
profile in Figure 8 that the hybrid joint generated at 102 mm/
min and 76 mm/min breaks in the TMAZ with noticeable
necking phenomena.
3.5. Fractured Locations. Figure 10 presents the fractured
locations of the reversed DS-FSW and SS-FSW hybrid joints
achieved during tensile testing for optimum and minimum
strengths. As illustrated in Figure 10(a), it is evident that the
joints generated in reversed DS-FSW at 36 mm/min break in
the NZ with noticeable minimum tensile strength. However,
the maximum tensile strength was achieved at 102 mm/min
welding speed for reversed DS-FSW. Meanwhile, the frac-
ture of the tensile test was in the TMAZ with noticeable
necking phenomena, as presented in Figure 10(b). On the
other hand, in SS-FSW, at welding speeds of 216 mm/min, as
shown in Figure 10(c), the fracture locations were in NZ with
minimum tensile strength. However, except for 76 mm/min,
the fracture was in TMAZ, as illustrated in Figure 10(d), with
higher tensile strength, nevertheless, still irresistible with the
utmost tensile strength of the hybrid joint made by reversed
DS-FSW.
3.6. Fractured Surface. Figure 11 illustrates surface mor-
phology SEM of the fractured surfaces of specific tension
tests resulting from specific welding speeds, giving the
optimum and minimum tensile properties.
Kissing Bond
(a)
Tunnel Defects
(b)
Figure 6: Microstructure of the defects for SS-FSW: (a) 146 mm/min and (b) 216 mm/min.
Advances in Materials Science and Engineering 7
36 mm/min
76 mm/min
102 mm/min
146 mm/min
216 mm/min
70
90
110
130
150
170
Microhardness HV05
AA2024 AA2198
HAZ HAZ
TMAZ
TMAZ
NZ
BM
BM
–20 –15 –10 –5 0 5 10 15 20 25–25
DISTANCE FROM WELDING CENTER (mm)
Figure 8: Microhardness values of single-sided friction stir welding (SS-FSW).
113.6 119.6 127.4 121.3 117.6
91.2
108.1 101.6 97.6 96.6
36 76 102 146 216
Welding Speeds (mm/min)
0
50
100
150
Microhardness HV05
DS-FSW
SS-FSW
Figure 9: Microhardness value of stir zone for various welding speeds.
70
90
110
130
150
170
Microhardness HV05
36 mm/min
76 mm/min
102 mm/min
146 mm/min
216 mm/min
AA2024 AA2198
HAZ HAZ
TMAZ
TMAZ
NZ
BM BM
–20 –15 –10 –5 0 5 10 15 20 25–25
Distance from welding center (mm)
Figure 7: Microhardness values of double-sided friction stir welding (DS-FSW).
8Advances in Materials Science and Engineering
10 mm
(a)
10 mm
(b)
10 mm
(c)
10 mm
(d)
Figure 10: Fracture locations of minimum and maximum tensile strength. DS-FSW at (a) 36 mm/min and (b) 102 mm/min. SS-FSW at
(c) 216 mm/min and (d) 76 mm/min.
Dimples
(a)
Deep Dimples
(b)
Figure 11: Continued.
Advances in Materials Science and Engineering 9
In Figure 11(a), the fractured surface of the hybrid joint
reversed DS-FSW acquired at 36 mm/min has some river
patterns and cleavage platforms with some dimples. It is a
typical mixed fracture mode representing ductile and brittle
fractures. Figure 11(b), the fractured surface of the hybrid
joint of reversed DS-FSW, at 102 mm/min, has depicted
many deep dimples and ripping ridges on the fractured
surface, showing that this fracture mechanism is a complete
ductile fracture. Figure 11(c), fractured surface of the joint
SS-FSW at 36 mm/min, presents some river patterns and
tearing edges with few deep dimples, indicating that it is a
typical mixed fracture mode. Figure 11(d) shows that the
fracture surface of 76 mm/min welding speed for SS-FSW
joint presented fine dimples, and minimal tearing ridges on
the flat facets suggest a mixed fracture mode in the NZ.
4. Conclusion
Hybrid joints of AA2024 to AA2198 have been successfully
produced via reversed DS-FSW and SS-FSW using various
welding speeds at a fixed rotational rate. e microstructure
evolution and mechanical characteristics of the friction stir
welded joints have been examined; thus, regarding experi-
mental outcomes, the following have been concluded:
(1) e welding speed increased from 36 mm/min to
216mm/min, and the area of the heat-affected zone
(HAZ) initially increased and then decreased due to
different welding temperatures. At 102 mm/min, the
size of the HAZ appeared to be the smallest for the
joint of DS-FSW, and the “S” curves in the nugget
zone (NZ) started to disappear due to the plastic
flow. Generally, the DS-FSW typical HAZ size is
narrower than the SS-FSW at different welding
speeds.
(2) e main reason for the microhardness values of the
NZ lower than the parent metals is the original
particles that strengthen aluminum alloys are dis-
solved in the FSW process; however, it precipitates in
the NZ after the FSW process. Due to the dynamic
recrystallization caused by the rotation tool in the stir
zone forming refined equiaxed grains in NZ. At
102 mm/min in DS-FSW, the average microhardness
value is recorded as the highest value, 127.4 HV, due
to fine equiaxed grain sizes. While due to the coarse
grain size results of SS-FSW at 36 mm/min, the
lowest microhardness value was observed.
(3) e utmost mechanical properties and microstruc-
ture evolution have been noticed at various welding
speeds in reversed DS-FSW compared to the con-
ventional SS-FSW. However, the joint efficiency of
reversed double-sided friction stir welding DS-FSW
recorded a higher value than the single-sided friction
stir welding SS-FSW, reaching up to 87.3% com-
pared to the base material of AA2024-T3.
(4) e joints formed at 102 mm/min exhibit excellent
tensile characteristics, such as ultimate tensile (UTS),
yield strength (YTS), and elongation (EL) for the DS-
FSW joints, recorded as 407.1 MPa, 271.2 MPa, and
9.5%, respectively. On the other hand, UTS, YTS,
and EL for the SS-FSW joints were found maximum
at 76 mm/min, 293.3 MPa, 190.2 MPa, and 4.8%,
respectively. Only the joints cracked at 76 and
102 mm/min in the TMAZ, accompanied by severe
Tearing ridges
(c)
Fine Dimples
(d)
Figure 11: SEM morphology pictures of the fracture surface of tensile test joints. DS-FSW at (a) 36 mm/min and (b) 102mm/min. SS-FSW
at (c) 36 mm/min and (d) 76 mm/min.
10 Advances in Materials Science and Engineering
necking phenomena. us, reversed DS-FSW has
superior mechanical characteristics over traditional
SS-FSW.
Data Availability
e data supporting the findings of this study are included
within the article.
Conflicts of Interest
e authors declare that they have no conflicts of interest.
Acknowledgments
is study was mainly supported by the Department of
Mechanical and Mechatronics Engineering, College of En-
gineering, Salahaddin University-Erbil, Ministry of Higher
Education and Scientific Research, Kurdistan Regional
Government (KRG), Iraq.
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