M. MIL^I] et al.: THE INFLUENCE OF PROCESS PARAMETERS ON THE MECHANICAL PROPERTIES...
771–776
THE INFLUENCE OF PROCESS PARAMETERS ON THE
MECHANICAL PROPERTIES OF FRICTION-STIR-WELDED
JOINTS OF 2024 T351 ALUMINUM ALLOYS
VPLIV PROCESNIH PARAMETROV, IZDELANIH S TORNO
VRTILNIM VARJENJEM, NA MEHANSKE LASTNOSTI SPOJEV
ALUMINIJEVE ZLITINE 2024/T351
Miodrag Mil~i}
1
*, Toma` Vuherer
2
, Igor Radisavljevi}
3
, Dragan Mil~i}
1
,
Janez Kramberger
2
1
University of Ni{, Faculty of Mechanical Engineering, Aleksandra Medvedeva 14, 18000 Ni{, Republic of Serbia
2
University of Maribor, Faculty of Mechanical Engineering, Smetanova ulica 17, 2000 Maribor, Slovenia
3
Military Technical Institute, Ratka Resanovi}a 1, 11000 Beograd, Republic of Serbia
Prejem rokopisa – received: 2019-03-18; sprejem za objavo – accepted for publication: 2019-05-17
doi:10.17222/mit.2019.062
The aim of this paper is to analyze the influence of the dominant parameters of Friction Stir Welding (FSW) on the mechanical
properties of welded joints. Experimental investigations were carried out on 6-mm-thick plates made of the aluminum alloy AA
2024 T351. The rotation speed of the FSW tool was a constant 750 min
-1
, and the welding speed was (73, 116 and 150)
mm/min. The obtained welds were free of imperfections and with an acceptable flat front surface. The testing of the mechanical
properties of the FSW welded joint concerned the Vickers hardness test, tensile testing, bending tests, and Sharpy pendulum
impact tests. The profile of the hardness distribution across the welded joint is made along three horizontal directions: near the
face, in the middle and near the root of the weld. For the Sharpy impact testing, an instrumented pendulum Amsler RPK 300
was used. The FSW process generates three different microstructural zones: the nugget zone (NZ), the thermomechanically
affected zone (TMAZ), and the heat-affected zone (HAZ), which were examined by optical microscopy and scanning electron
microscopy (SEM). All the results of the experimental research indicated that the welding parameters n = 750 min
–1
and v = 116
mm/min give welded joints with the best mechanical properties.
Keywords: friction stir welding, aluminum alloy 2024 T351, mechanical properties, welded joints
V ~lanku avtorji opisujejo in analizirajo vpliv prevladujo~ih procesnih parametrov na mehanske lastnosti spojev, ki so izdelani s
torno vrtilnim varjenjem (FSW). Eksperimentalne raziskave so izvajali na 6 mm debelih plo{~ah iz Al zlitine tipa AA 2024, ki
so bile toplotno obdelane s postopkom T351. Varjenje so izvajali pri konstantni hitrosti vrtenja FSW orodja, 750 obratov na
minuto in hitrostih varjenja (73, 116 in 150) mm/min. Izdelani zvari so bili brez napak in s sprejemljivo kakovostjo povr{ine.
Mehanske lastnosti zvarov (FSW spojev) so ovrednotili z nateznim in upogibnim preizkusom, preizkusom udarne `ilavosti po
Charpyju in dolo~itvijo trdote po Vickersu. Profil porazdelitve trdote v zvaru so dolo~ili v treh vodoravnih smereh: blizu ~ela
zvara, v sredini in v korenu zvara. Za dolo~itev udarne `ilavosti so uporabili in{trumentirano kladivo Amsler RPK 300. FSW
proces je ustvaril tri razli~ne mikrostrukturne cone, ki so jih avtorji preiskovali z opti~no in vrsti~no elektronsko mikroskopijo; z
grobo zrnato cono (NZ), termomehansko vplivano cono (TMAZ) in toplotno vplivano cono (HAZ). Na osnovi eksperimentalnih
rezultatov avtorji prispevka ugotavljajo, da so najbolj{e mehanske lastnosti dosegljive pri naslednjih parametrih varjenja: n =
750 min
–1
in v = 116 mm/min.
Klju~ne besede: torno vrtilno varjenje, Al zlitina 2024/T351, mehanske lastnosti, varjeni spoji
1 INTRODUCTION
Wayne Thomas, and his team of researchers, at The
Welding Institute (TWI) in the UK patented the Friction
Stir Welding (FSW) process in 1991. As a solid-state
joining technique it was initially applied to aluminum
alloys.
1
This process has a particularly common appli-
cation in welding aluminum alloys used in the auto-
motive and aerospace industries. This welding process
allows joining of the materials that are hardly weldable
with a conventional welding processes. The FSW
process can provide better mechanical properties of the
compound than conventional methods. In order to
achieve a quality joint, a lower amount of energy is
required than the melting process of the material. It is a
relatively clean welding technology that does not pollute
the environment. The great importance of applying this
welding process is the appearance of very small
deformations when welding thin sheets of high length,
which is not obtained by any other method.
FSW is most commonly used for welding aluminum
alloys, which are difficult to weld with conventional
welding processes. The principle of obtaining in-
separable compounds by FSW is shown in Figure 1.
FSW is performed by a special tool on the parts to be
joined. The FSW process consists of four phases:
1) plunging phase, 2) dwelling phase, 3) welding phase,
and 4) exit or retract phase.
2
Materiali in tehnologije / Materials and technology 53 (2019) 6, 771–776 771
UDK 620.1:67.017:621.791.1:669.715 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 53(6)771(2019)
*Corresponding author's e-mail:
miodrag.milcic@masfak.ni.ac.rs (Miodrag Mil~i})

The tool rotates around its axis and is further axially
moving relative to the base metal parts to be joined. The
function of the tool is:
• pre-heating of the base metal in the welding zone,
• deforming and mixing of materials,
• making friction stir weld.
Pre-heating of the base metal in the welding zone
involves friction on the contact of the tool and the base
metal, which is why the heat is generated in the contact,
which facilitates the deformation of the base material
and its mixing into the monolithic joint. In the initial
phase (plunging phase) of the penetration of the tool,
heating is primarily a result of the friction between the
tool pin and the workpiece. The tool penetrates the
contact of the shoulder of the tool and workpiece. After
the completion of the plunging phase, the tool continues
to rotate in order to stabilize the material and prepare for
the phase of welding (dwelling phase). By rotation of the
tool pin, the material of the workpieces is cut, deformed
and mixed with the previously deformed material. The
tool pin causes the movement of parts of the base metal
around the tool and towards the axis of the tool. By
translating the tool, a portion of the deformed material
moves and deposits it in the zone behind it, thus forming
a weld joint (welding phase).
The welding cycle ends with the fourth phase, with
the tool exiting the material (by vertical movement)
(Figure 1).
Welding parameters, tool geometry, and joint con-
struction have a significant impact on the structure of
materials flow and temperature distribution, influencing
the development of the microstructure of the material.
For the FSW process, two parameters are very im-
portant:
4–8
• the tool’s rotational speed (min
–1
) in the clockwise
direction or in the opposite direction,
• the speed of the tool along the line of the joint.
A peculiar feature of friction stir welds is the com-
plex microstructure of the welded joint in which they
form different welding zones that are drastically different
from the basic metals that are welded. The microstruc-
ture of the welded joint obtained by the FSW method
significantly depends on the structural tool design, the
rotating speed, the welding speed, the pressure of the
tool on the plates along the vertical axis, the angle under
which the tool acts on the material and the characteristics
of the material being welded. In the microstructure of the
welded joint, several zones can be identified: the heat-
affected zone (HAZ), the thermo-mechanical affected
zone (TMAZ), the nugget zone (NZ) and the base ma-
terial.
9–11
It is very important to choose the right combination
of welding parameters in order to obtain a welded joint
of the corresponding quality without defects.
12
The main objective of the research is the analysis of
the influence of the most important parameters of FSW
welding, tool rotating speed and welding speed, on the
structural and mechanical properties of FSW joints from
aluminum alloy AA 2024 T351.
13
2 EXPERIMENTAL PART
Experimental investigations were focused on deter-
mining the impact of the FSW on the metallurgical and
mechanical properties of the welded joints. The basic
material is an AlCu4Mg1 AA 2024-T351 alloy, which is
M. MIL^I] et al.: THE INFLUENCE OF PROCESS PARAMETERS ON THE MECHANICAL PROPERTIES...
772 Materiali in tehnologije / Materials and technology 53 (2019) 6, 771–776
Figure 1: Illustrated scheme of friction stir welding: 1 – base metal, 2
– direction of tool rotation, 3 – weld tool,4–downwardmov ement of
tool, 5 – tool shoulder, 6 – pin, 7 – advancing side of weld, 8 – axial
force, 9 – direction of welding, 10 – upward movement of tool, 11 –
exit hole, 12 – retreating side of weld, 13 – weld face and 14 – base
plate
Figure 2: a) Conventional milling machine for FSW and b) FSW tool

difficult to weld with conventional welding processes.
The chemical and mechanical properties of the basic ma-
terial are given in Table 1.
Table 1: Chemical composition of AA 2024-T351
Chemical
composition
Cu Mg Mn Fe Si Zn Ti
w/% 4.70 1.56 0.65 0.17 0.046 0.11 0.032
The dimensions of the welding samples were
500 mm length, 65 mm width and 6 mm depth. Below
the welding samples was placed an austenitic plate in
order to maintain the heat in the welded joint area. A
milling machine was used for welding. The length of the
weld is about 400 mm.
Figure 2 shows the conventional milling machine
that was used for welding, and a tool for welding a butt
FSW joint. The most important parameters of the FSW
welding are the welding speed and the tool’s rotational
speed.
Experimental research was performed with a constant
number of tool rotations, and a variation of the welding
speed (Table 2).
Table 2: Friction-stir-welding parameters
Sample
Rotation
rate
n
min
–1
Welding
speed
v
mm/min
Ratio
n/v
rev/mm
Ratio
v/n
mm/rev
Ratio
n
2
/v
rev
2
/m·min
A–I
750
73 10.27 0.0974 7705.5
B – II 116 6.47 0.155 4849.14
C – III 150 5 0.2 3750
After completion of the welding process, welded
joints were tested with non-destructive methods. For this
purpose, visual and radiographic control of samples was
performed. On the welded samples there were no
detected welding imperfections.
From the FSW samples we made specimens for the
tensile testing, specimens with a V-notch at the different
position of the FSW joint for the Charpy pendulum
impact testing and specimens for the fracture mechanics
parameter testing. The dimensions of the specimens for
the tensile testing and for the Charpy pendulum impact
testing are shown in Figure 3.
For the metallographic examination of the FSW
welded joint, preparation of the specimens by grinding,
polishing and chemical treatment using Tucker’s reagent
(45 mL HCl, 15 mL HNO
3
, 5 mL HF and 25 mL H
2
O)
was carried out. The metallographic observation was
performed by optical microscopy using the optical
microscope Leica M205A. Tensile testing was performed
at room temperature on standard ASTM E8M specimens
made perpendicular to the welded joint. Bending
specimens and the bending test procedure on the welded
joints are defined by the EN 910 standards. Bending
specimens were loaded on the faces and roots of the
welded joint.
The Vickers hardness measurement was carried out in
a cross-section of the welded joint, perpendicular to the
direction of welding, using a digitally controlled hard-
ness tester (Model HVS-1000 digital display microhard-
ness tester). The aim of this testing of the welded joints
is to determine the hardness of the welded seams, the
HAZ, the TMAZ, the nugget zone and the basic ma-
terials for their mutual comparison. The hardness
diagrams were obtained by measuring the hardness along
three horizontals (close to the face of the weld, in the
middle of the weld and near the welding root). The hard-
ness was measured at every 0.5 mm.
Impact toughness testing using Sharpy’s pendulum
was performed at room temperature using an instru-
mented pendulum Amsler RPK 300. The impact tough-
M. MIL^I] et al.: THE INFLUENCE OF PROCESS PARAMETERS ON THE MECHANICAL PROPERTIES...
Materiali in tehnologije / Materials and technology 53 (2019) 6, 771–776 773
Figure 4: Positions on FSW joint in which V-notch was made on the
impact test specimens
Figure 3: Dimensions of the tensile specimens: a) and b) the Sharpy
impact-test specimens

ness according to the Sharpy’s method is determined by
testing a (6 × 10 × 55) mm specimens with a V-notch
having a depth of 2 mm and a radius of 0.25 mm.
The Sharpy specimens were made with a V-notch in
the middle of the welded joint and with V-notch +4 mm
and –4 mm from the middle of the FSW welded joint
(Figure 4).
3 RESULTS
Table 3 shows the macrostructures of cross-sections
of the FSW joints with different welding parameters.
Typical FSW zones: NZ, TMAZ and HAZ are observed
on all the FSW joints. The temperature of the FSW does
not exceed 80 % of the melting temperature of the base
metal. Depending on the welding parameters, different
forms of grumble are observed. The grain size in the
grumene zone is the smallest, while the grain is larger in
the TMAZ and HAZ zones. The hottest grain is in the
heat-affected zone (HAZ) near the thermo-mechanical
zone (TMAZ). At this location, plastic deformations
during welding are not pronounced, and thermal
influence causes the formation of a smaller number of
larger particles, which again causes a decrease in the
hardness and the tensile strength.
Table 3: Macrostructure of cross-sections of FSW welded joints with
different welding parameters
The tensile testing of FSW welded joints was per-
formed for each of the welding parameters. The results
of the tests are given in Table 4.
Table 4: Tensile testing results
Sample
No.
Yield
strength
(YS) MPa
Ultimate ten-
sile strength
(UTS) MPa
Elongation
%
Joint
efficiency
%
A-I 303.7 398.33 2.3 0.83
B-II 336.6 469.09 7.2 0.97
C-III 339.7 373.34 0.65 0.77
By comparing the yield strength (YS), ultimate
tensile strength (UTS) and % elongation of the FSW
joints, the welded samples B-II with the welding para-
meters 750/116 rpm/(mm/min) have the highest values in
relation to the welded samples A-I and C-III with the
parameters 750/73 and 750/150. This indicates that n/v =
750/116 rpm/(mm/min) are the optimal welding
parameters of the AA2024 alloy with the selected tool.
M. MIL^I] et al.: THE INFLUENCE OF PROCESS PARAMETERS ON THE MECHANICAL PROPERTIES...
774 Materiali in tehnologije / Materials and technology 53 (2019) 6, 771–776
Figure 5: Hardness distribution across the welded joint

The welded joint efficiency with these parameters is
97 %, which is extraordinary.
The results in Table 4 indicate that the greatest
elongation occurred in the B-II welding parameter.
The testing of welded joints was also performed on
bending, around the face, and around the root. The
welded FSW joint has poor bending characteristics.
Comparing the obtained bending test results, the largest
bend angle to the first cracking phenomenon is for
welding parameters 750/116 and amounts to 42°.
Figure 5 shows the hardness distribution in the
welded zones for all three welding parameters.
The hardness values in all the zones of the friction
stir welded joint are less compared to the values of the
hardness of the base metal. The heat generated during
the FSW process causes softening of the welded joint
due to the dissolution and the coarsening of the precipi-
tates.
The distribution of the microhardness depends on the
amount of heat generated under the shoulder of the tool
and around the tool pin and plastic deformations. For the
used friction stir welding parameters C-III, B-II and A-I,
it can be concluded that the largest amount of heat is
generated in the mixing zone for the welding parameters
A-I.
By comparing the distribution of hardness, it can be
seen that the FSW welded joint at a high welding speed
(150 mm/min) showed higher hardness values than at
lower welding speeds (116 and 73 mm/min).
Comparing the hardness distribution for the welding
parameter A-I (Figure 5a), B-II (Figure 5b) and C-III
(Figure 5c), it is noted that the hardness of the sample
A-I in the stir region is uniform across the entire height.
The FSW sample C-III shows the hardness difference in
the height of the weld: the hardness of the root in the stir
zone is between 110 HV and 115HV, while the hardness
near the face of the weld is between 130 HV and 135
HV.
The Charpy impact test results for the FSW joints are
shown in Table 5.
The impact energy values were measured at three
points of the notch: in the weld center, the notch on the
advancing side in the 4 mm position from the weld
center (+4), and the notch on the side retreating at the 4
mm position from the weld center (-4). For all the
welded samples, the maximum impact-energy values are
measured for the score on the advancing side.
The highest values of the impact energy were
measured on samples welded with welding parameters
n = 750 min
–1
and welding speed of 116 m /min for the
notch on the side of theB+4progression side.
On all the welded samples, the smallest impact
energy is on the samples with a notch on the retreating
side.
Table 5 shows the F-t and E-t diagrams for speci-
mens with a notch in the weld center for welded samples
A-I, B-II and C-III.
4 CONCLUSIONS
This paper analyzes the effect of the process para-
meters on the mechanical properties of butt joints
obtained by friction stir welding (FSW). On the basis of
the examinations performed, given the results of the
experiment and their comparison, the following con-
clusions can be drawn:
The largest frictional heat generated is for the weld-
ing parameters is A-I, and the minimum amount of heat
for the welding parameters is C-III. The largest grain
size was measured in the welding parameter A-I.
The profile of the distribution and allocation of the
microhardness depends on the level of the temperature
and the plastic deformation, which is the highest under
the tool shoulder and around the pin.
A joint efficiency as high as 97 % of the base metal
could be achieved at B-II. The highest elongation of the
welded joint is achieved with the welding parameters
B-II and is 7.2 %.
The properties of FSW joints on bending are poor.
The largest bend angle to the first cracking phenomenon
is for welding parameters B-II and amounts to 42°.
The asymmetry of the welded joint and the changes
in the metallurgical transformations occurring around the
pin and under the shoulder of the tool during its com-
bined moving, influence the value of the impact strength
in various areas of the welded joint.
The relation between the number of revolutions of
tools n velocity of welding v directly influences the value
M. MIL^I] et al.: THE INFLUENCE OF PROCESS PARAMETERS ON THE MECHANICAL PROPERTIES...
Materiali in tehnologije / Materials and technology 53 (2019) 6, 771–776 775
Table 5: Results of impact toughness and F-t and E-t diagrams for
experimental samples in weld center
KV / J/cm
2
E/J E
i/J E
p/J
OM 19.5 7.8 3 4.8
negx-3A-S 12.62 5,05 1.64 3.41
A+4 17.1 6.8 3 3.8
A-4 13.6 5.5 2.1 3.4
B-S 12.62 6.7 1.7 5
B+4 21.3 8.5 3.2 5.3
B-4 11.1 4.5 1.4 3.1
C-S 14.2 5.7 2.4 3.3
C+4 20.7 8.3 3.1 5.2
C-4 12.8 5.1 1.9 3.2
A-S
B-S
C-S

of the fracture toughness and energy that is required for
the initiation and propagation of the crack;
The highest values of the impact energy were
measured on samples welded with welding parameters
n = 750 min
–1
and welding speed of 116 mm/min for the
notch on the side of the B +4 progression side.
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776 Materiali in tehnologije / Materials and technology 53 (2019) 6, 771–776