S. APAY et al.: EFFECT OF DIFFERENT APPLICATION PRESSURES ON ROTARY-FRICTION-WELDED AA2024-T6 JOINTS
519–523
EFFECT OF DIFFERENT APPLICATION PRESSURES ON
ROTARY-FRICTION-WELDED AA2024-T6 JOINTS
VPLIV UPORABE RAZLI^NIH TLAKOV NA ZVARE AA2024-T6,
IZDELANE S POSTOPKOM VRTILNO-TRENJSKEGA VARJENA
Serkan Apay
1
, Fatih Özen
2*
, Volkan Onar
3
1
Faculty of Engineering, Department of Mechanical Engineering, Düzce University, 81620 Düzce, Turkey
2
Beºiri Organized Industrial Zone Vocational College, Batman University, 72000 Batman, Turkey
3
Faculty of Technology, Pamukkale University, Department of Mechanical and Manufacturing Engineering, 20260 Denizli, Turkey
Prejem rokopisa – received: 2023-03-23; sprejem za objavo – accepted for publication: 2023-06-29
doi:10.17222/mit.2023.834
An AA2024-T6 aluminium alloy was welded with a rotary-friction-welding technique using different forging pressures under
constant friction pressure. It was found out that the increasing forging pressure has an adverse effect on the tensile strength of
the welded joint. The maximum tensile strength was 366.22 MPa for a forging pressure of 80 MPa. However, the failure ener-
gies and elongations were decreased as the forging pressure increased. The minimum elongation was 15.45 %, while the mini-
mum failure energy was 4.35 J with a forging pressure of 120 MPa. This situation is attributed to the loss of ductility up to a de-
gree in high forging pressures and temperatures induced in the HAZ. In microstructural examinations the existence of the S
phase has dominant role in determining the local hardness. The S phase is affected by the welding heat in the heat-affected zone,
the thermomechanically affected zone and the dynamically recrystallized zone. The hardness is increased up to the middle of the
TMAZ. In this zone the heat input caused an aging effect and increased the dispersed S phase in the intergranular zones. The ag-
ing mainly governed by the heat input increased the hardness up to beginning of the recrystallization zone.
Keywords: AA2024, rotary friction welding, forging pressure
Avtorji v ~lanku opisujejo medsebojno rotacijsko-trenjsko varjenje palic premera 12 mm iz Al zlitine AA2024-T6 s pomo~jo
tehnike razli~nih kova{kih tlakov in konstantnega tlaka trenja. Avtorji so ugotovili, da nara{~ajo~i kova{ki tlak {kodljivo vpliva
na natezno trdnost zvarnih spojev. Maksimalno natezno trdnost zvara so dosegli pri kova{kem tlaku 80 MPa in sicer
366,22 MPa. Vendar pa se energija poru{itve in rastezek zmanj{ujeta z nara{~anjem kova{kega tlaka. Najman{i vrednosti
raztezka15,45-% in najmanj{a energija poru{itve 4,35 J sta bili dose`eni pri tlaku 120MPa. Te razmere avtorji pripisujejo
zmanj{anju duktilnosti pri visokem kova{kem tlaku in induciranim temperaturam v toplotno vplivani coni (HAZ; angl.: heat af-
fected zone). Mikrostrukturne preiskave so pokazale, da ima prevladujo~o vlogo na lokalno trdoto prisotnost sulfidne faze.
Nanjo vpliva toplota, nastala med trenjskim varjenjem v toplotno vplivani coni, termodinamsko vplivani coni (TMAZ) in coni
dinami~ne rekristalizacije. Trdota nara{~a do sredine TMAZ. V tej coni vnos toplote povzro~a u~inek staranja in pove~ano
porazdelitev sulfidne faze na meje kristalnih zrn. Staranje je v glavnem kontrolirano z vnosom toplote in trdota v zvaru nara{~a
do za~etka rekristalizacijske cone.
Klju~ne besede: Al zlitinaAA2024, rotacijsko-trenjsko varjenje, kova{ki tlak
1 INTRODUCTION
The aluminium alloy AA2024 is a member of the
heat-treatable aluminium series.
1
Al-Cu precipitation is
the main strengthening mechanism in the Aluminium
2xxx series. AA2024 has good fracture toughness, high
corrosion resistance and specific strength.
2
This alloy is
generally employed in lightweight applications to save
weight such as in airplanes, spacecraft, and high-speed
trains.
3,4
Since the 2xxx series has a high thermal conductivity,
a high thermal expansion coefficient and a low melting
point, they are hard to weld with traditional welding
methods.
5
Therefore, solid-state welding techniques to
weld aluminium alloys is of high importance.
6
Among
the solid-state welding techniques, rotation friction weld-
ing (RFC) is a promising method, especially for the parts
with cylindrical cross-section Department of Machinery
and Metal Technologies, s.
7–9
In RFC, two cylindrical
cross-section materials are forced to rotate to generate
heat with the pressure applied by axial forces.
10–13
This
technique can be regarded as a relatively quick technique
that generally lasts for tens of seconds.
14,15
Since the principles of friction welding are the same,
there are some variants of this welding method such as
friction stir welding (FSW), linear friction welding
(LFW), and rotary friction welding (RFW). There are
also various studies regarding friction welding in the lit-
erature.
17–19
Prashanth and others studied RFW of
Al–12Si parts manufactured with the selective laser
melting method.
16
The shape and size of the Si phase in-
creased in the weld zone. This variation has resulted in
significant changes to the mechanical properties of the
weld joint. Rafi and others investigated the mechanical
and microstructural properties of RFW applied to a
AA7075-T6 joint.
17
They found that the spindle speed,
friction pressure, and burn-off length affect the joint
strength with 89% joint efficiency. Mogami and others
Materiali in tehnologije / Materials and technology 57 (2023) 5, 519–523 519
UDK 669.715:539.378.2:52-335.5 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 57(5)519(2023)
*Corresponding author's e-mail:
fatih.ozen@batman.edu.tr (Fatih Özen)

investigated high-frequency LFW of 5052 and 6063 alu-
minium alloys.
18
Material flow during welding was af-
fected by the thermal conductivity at elevated tempera-
tures. They also noted that the grains are refined and the
hardness was increased at the interface of the Al 6063 al-
loy. The joints with low heat inputs yielded higher joint
strengths.
In this work AA2024-T6 aluminium rod materials
were welded with a rotary-friction-welding technique.
The effect of forging pressure under constant parameters
were investigated. Tensile tests were applied to rotary-
friction-welded specimens. Failure energies, elongations
and tensile results were evaluated in terms of different
forging pressures. The microstructure was characterized
by optical microscope, SEM, SEM/EDS devices. Phase
formations was evaluated and supported by the literature.
2 MATERIALS AND METHOD
In this study, AA2024-T6 with an 18-mm diameter
was employed. The specimens were machined to 12-mm.
The tensile tests were performed on a SHIMADZU
AGS-50kN Universal testing device according to the
ATSM E8 standard. The crosshead speed was adjusted to
5 mm·min
–1
. The tensile strength and maximum strength
are measured at room temperature as 354 MPa and
522 MPa, respectively. The elongation at break was
23 %. There are no specific applications on the surface
of the specimens. The chemical composition of the
AA2024-T6 is presented in the Table 1.
Table 1: Chemical composition of the AA2024-T6
Element (w/%)
Cu Mg Si Fe Cr Mn Zn Ti Al
3.7 1.22 0.5 0.51 0.15 0.32 0.03 0.05 Balance
The specimens were welded with rotary-friction-
welding machine. Table 2 shows the welding parameters
used in RFW tests for all experiments, the spindle speed
and friction pressure were 1200 min
–1
, 60 MPa, respec-
tively. The total friction time was adjusted to 8 s,
whereas the duration of the forging time was 4 s.
Table 2: Welding parameters employed in RFW experiments
Experiment
No.
Forging
pressure
(MPa)
Friction
pressure
(MPa)
Forging
time (s)
Friction
time (s)
18 0
60 4 8 2 100
3 120
The specimens were cut from a plane that is parallel
to the axis of rotation for metallographic explorations.
Conventional metallographic procedures were applied to
the joined specimens. The microstructure specimens
were etched with Keller solution with an application
time of 8 s. A Nikon Eclipse L150A light optical micro-
scope was employed for micro and macrostructural
investigations. Also, FEI Quanta 200 FEG scanning elec-
tron microscope (SEM) with energy dispersive spectros-
copy (EDS) device was employed for metallurgical ex-
ploration. A Wilson hardness tester was used for
microhardness measurements. Hardness measurements
were taken with 0.25-mm intervals with a 10-gF load.
3 RESULTS AND DISCUSSION
The effect of the friction pressure on tensile-elonga-
tion behaviour of the rotary-friction-welded AA2024-T6
aluminium joints is shown in Figure 1. According to the
figure, the tensile strength of the base metal exhibited the
best tensile performance among all tensile tests with
472.26 MPa maximum tensile bearing capacity. None of
the rotary-friction-welded specimens has achieved the
tensile strength of the base metal. Changing the forging
pressure during welding has an effect on the tensile
strength. The tensile strengths increased as the forging
pressure is decreased. The maximum tensile strength is
obtained at a forging pressure of 80 MPa. The tensile
strengths for forging pressures of (120, 100 and 80) MPa
were obtained as 345.37, 359.10, and 366.22, respec-
tively.
The elongations represent the degree of ductility for
the material being tested. The elongation has similar
characteristics compared to tensile behaviour of the
joints. Figure 2 shows the effect of forging pressures on
the elongation at break and failure energies. The elonga-
tions were decreased as the forging pressure increases.
On the other hand, the base metal has an elongation of
19.21 % that is similar to the elongation at a forging
pressure of 80 MPa, achieving 19.28 % elongation. The
lowest elongation was obtained in forging pressure of
120 MPa as 15.37 %. There was an approximately 20 %
loss in elongation compared to the application pressure
of 80 MPa. Namely, there is an important ductility loss
in the experiment that has forging pressure of 120 MPa.
The failure energy is another important aspect for
evaluating toughness and the energy-absorption capacity
S. APAY et al.: EFFECT OF DIFFERENT APPLICATION PRESSURES ON ROTARY-FRICTION-WELDED AA2024-T6 JOINTS
520 Materiali in tehnologije / Materials and technology 57 (2023) 5, 519–523
Figure 1: Effect of different forging pressures on tensile strengths

of the joint. Therefore, measuring the failure energy is of
great importance. The failure energy was measured with
an integration of the area under the tensile-elongation
curve. According to the failure-energy results, the maxi-
mum failure energy was obtained in the base metal. The
failure energy has an inverse relation to the forging pres-
sure. Since the elongation and tensile strength were de-
creased with an increase in the forging pressure, the fail-
ure energies were decreased as the forging pressure
increases.
A rotary welded and trimmed microstructure speci-
men with 80 MPa forging pressure is presented in Fig-
ure 3a and 3b. On the other hand, light optical micro-
scope microstructure images from the cross-section of
this specimen are presented in Figures 3c to 3e. The
microstructure of the rotary-friction-welded AA2024 can
be divided into four section for both jointing side; i) base
metal (BM), ii) heat-affected zone (HAZ), iii) thermo-
mechanically affected zone (TMAZ) and iv) dynamically
recrystallized zone (DRZ).
Given the grain size and distribution, BM has a
coarse grain size in the microstructure between 10 μm
and 240 μm. These grains are not equiaxed and elon-
gated through the rotation axis. Due to the tempering ef-
fect, the grains in the HAZ are coarsened as well. With a
combination of forging and friction force that is parallel
to the rotation axis, the grains are enforced to extend out-
side. Namely, the elongation axis was deflected 90° due
to these effects. The friction heat and dynamic stirring of
both friction surfaces causes both recrystallization and
mechanical deflection of the grains simultaneously.
Therefore, elongated grains perpendicular to the rotation
axis and finer grains compared to HAZ are obtained. As
for DRZ, this region represents the fusion and solidified
zone. Since each side of the specimen is cold, the DRZ
underwent quick solidification. Thereby, a finer grain
size in the weld joint is obtained.
Line hardness measurement from the cross-section of
the rotary-friction-welded joint is illustrated in Figure 4.
T h eB Mh a s1 1 7±5H Vhardness. In the HAZ, the hard-
ness increases up to 130 HV due to the tempering effect.
Then, the hardness reaches the maximum point in the
TMAZ due to the combined effect of strain hardening
and tempering. However, as we move into the DRZ, the
hardness decreases. Partial recrystallization affects the
S. APAY et al.: EFFECT OF DIFFERENT APPLICATION PRESSURES ON ROTARY-FRICTION-WELDED AA2024-T6 JOINTS
Materiali in tehnologije / Materials and technology 57 (2023) 5, 519–523 521
Figure 2: Effect of forging pressure on elongation at break and failure
energy
Figure 3: Macro image of a) untrimmed and b) trimmed rotary-friction specimen, c) schematic of the LOM image, microstructure image from d)
cross-section and e) HAZ and BM interface

strain hardening. As the recrystallization effect increases
due to friction heating, the hardness decreases at the
same time. This recrystallization ratio reaches a maxi-
mum in the DRZ. For this reason, the finer grains were
obtained in the DRZ. Although the DRZ has finer grains,
the hardness of this region is the lowest. The recryst-
allized grains had the lowest hardness since these grains
are free of strain hardening and tempering effects.
Energy dispersive x-ray spectroscopy (EDS) is a use-
ful technique to characterize the phases in the micro-
structure. Figure 5 and Table 3 show line and point EDS
results from the TMAZ. The line EDS measurements
were taken from intergranular formations. As seen from
the figure, there are some Mg, Cu and Si responses.
While the first EDS measurement point exhibited a Si re-
sponse, 0.04 % of the Si from the second EDS measure-
ment was obtained. The content of the first EDS point
can be attributed to the presence of Al
2
C u+A l
3
Mg
2
+
Mg
2
Si or Al(CuMgSi) phases. On the other hand, the
second EDS result reflects a clear indication of Al
2
CuMg
(S phase) phase. The S-phase existed the intergranular
regions in TMAZ and HAZ. As well as the coarse S
phase, the solid-solution clusters that converted into the
GPB zones increase the hardness of the HAZ. Due to se-
vere plastic flow and heating, the S phases and GBP
zones are coarsened in the TMAZ. The natural aging is
the main driving force for the coarsening effect. How-
ever, the hardness near the DRZ and in the DRZ de-
creased since the S-phases and GBP zones are dissolved
in the microstructure due to the recrystallization effect.
Similar results were also obtained by various research-
ers. It was reported that the S phases are generally at
intergranular locations in the TMAZ.
19
Due to the
recrystallization effect stemmed from friction heating,
the S phase is dissolved in the grains.
Geng noted that the Al
2
CuMg (S) and Al
2
Cu ( )
phases are present in the HAZ and TMAZ.
20
These
phases are dissolved in the Al matrix of the DMZ due to
the heating effect.
4 CONCLUSIONS
In this work, AA2024 aluminium rods were success-
fully welded with the rotary-friction-welding technique.
According to the results, following conclusions can be
drawn:
• Friction increased the severity of the heat input in the
HAZ and TMAZ, causing wide, brittle regions. These
wide HAZ and TMAZ resulted in less tensile
strength. Therefore, the forging pressure has an ad-
verse effect on the tensile strength of the RFW speci-
mens.
• Increasing the friction pressure elevated the hardness,
especially in the TMAZ region. Brittleness in the
DRZ, HAZ and TMAZ caused a local loss of ductil-
ity. For this reason, the elongation at break and the
failure energy tend to decrease at high friction pres-
sures.
• The Al(CuMgSi) and Al
2
CuMg phases are present in
the microstructure. Al
2
CuMg plays an important role
in the strengthening mechanism. These phases pre-
cipitated at intergranular locations.
• The amount of Al
2
CuMg and other natural aging
phases tend to increase near to the DRZ due to the
heating effect. Also, the maximum hardness is gener-
ally obtained in the TMAZ. However, the hardness is
suddenly decreased due to the recrystallization effect
induced by welding heat. During recrystallization,
the secondary phases are dissolved in the solid solu-
tion.
• Since the newly recrystallized grains have a mini-
mum amount of precipitation phases, the lowest hard-
ness is obtained at the DRZ. The main reason for fail-
S. APAY et al.: EFFECT OF DIFFERENT APPLICATION PRESSURES ON ROTARY-FRICTION-WELDED AA2024-T6 JOINTS
522 Materiali in tehnologije / Materials and technology 57 (2023) 5, 519–523
Figure 4: Line hardness measurement from cross-section of the ro-
tary-friction-welded AA2024
Figure 5: Line EDS and point EDs measurements from TMAZ
Table 3: Point EDS measurement results
Measure-
ment No.
Elements (Conc.-%)
Mg Cu Si Al
1 1.68 2.86 0.44 95.02
2 1.87 3.86 0.04 94.22

ure from the DRZ in tensile specimens can be
attributed to this softening effect.
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S. APAY et al.: EFFECT OF DIFFERENT APPLICATION PRESSURES ON ROTARY-FRICTION-WELDED AA2024-T6 JOINTS
Materiali in tehnologije / Materials and technology 57 (2023) 5, 519–523 523