Friction Stir Welding of Non-Heat Treatable Al Alloys: Challenges and Improvements Opportunities

Abstract

Friction stir welding (FSW) is an effective solid-state joining process that has the potential to overcome common problems correlated with conventional fusion welding processes. FSW is used for the joining of metallic materials, in particular Al alloys (non-heat-treatable and heat-treatable). The heat produced by the friction between the rotating tool and the workpiece material generates a softened region near the FSW tool. Although the heat input plays a crucial role in producing a defect-free weld metal, it is a serious concern in the FSW of work-hardened non-heat-treatable Al alloys. In this group of alloys, the mechanical properties, including hardness, tensile properties, and fatigue life, are adversely affected by the softening effect because of grain growth and reduced dislocation density. Considering this challenge, work-hardened Al alloys have been limited in their industrial use, which includes aerospace, shipbuilding, automotive, and railway industries. The current comprehensive review presents the various approaches of available studies for improving the quality of FSW joints and expanding their use. First, the optimization of welding parameters, including the tool rotational and traverse speeds, tool design, plunge depth, and the tilt angle is discussed. Second, the incorporation of reinforcement particles and then underwater FSW are stated as other effective strategies to strengthen the joint. Finally, some supplementary techniques containing surface modification, bobbin tool FSW, copper backing, and double-sided FSW in relation to strain-hardened Al alloys are considered.

Full text

Citation: Abnar, B.; Gashtiazar, S.;
Javidani, M. Friction Stir Welding of
Non-Heat Treatable Al Alloys:
Challenges and Improvements
Opportunities. Crystals 2023,13, 576.
https://doi.org/10.3390/
cryst13040576
Academic Editor: Bolv Xiao
Received: 3 March 2023
Revised: 23 March 2023
Accepted: 23 March 2023
Published: 28 March 2023
Copyright: © 2023 by the authors.
Licensee MDPI, Basel, Switzerland.
This article is an open access article
distributed under the terms and
conditions of the Creative Commons
Attribution (CC BY) license (https://
creativecommons.org/licenses/by/
4.0/).
crystals
Review
Friction Stir Welding of Non-Heat Treatable Al Alloys:
Challenges and Improvements Opportunities
Behrouz Abnar 1, Samaneh Gashtiazar 2and Mousa Javidani 1, *
1Department of Applied Science, University of Quebec at Chicoutimi, Saguenay, QC G7H 2B1, Canada
2School of Metallurgy and Materials Engineering, College of Engineering, University of Tehran,
Tehran 1417614411, Iran
*Correspondence: mousa_javidani@uqac.ca
Abstract:
Friction stir welding (FSW) is an effective solid-state joining process that has the potential
to overcome common problems correlated with conventional fusion welding processes. FSW is used
for the joining of metallic materials, in particular Al alloys (non-heat-treatable and heat-treatable).
The heat produced by the friction between the rotating tool and the workpiece material generates
a softened region near the FSW tool. Although the heat input plays a crucial role in producing a
defect-free weld metal, it is a serious concern in the FSW of work-hardened non-heat-treatable Al
alloys. In this group of alloys, the mechanical properties, including hardness, tensile properties,
and fatigue life, are adversely affected by the softening effect because of grain growth and reduced
dislocation density. Considering this challenge, work-hardened Al alloys have been limited in their
industrial use, which includes aerospace, shipbuilding, automotive, and railway industries. The
current comprehensive review presents the various approaches of available studies for improving
the quality of FSW joints and expanding their use. First, the optimization of welding parameters,
including the tool rotational and traverse speeds, tool design, plunge depth, and the tilt angle
is discussed. Second, the incorporation of reinforcement particles and then underwater FSW are
stated as other effective strategies to strengthen the joint. Finally, some supplementary techniques
containing surface modification, bobbin tool FSW, copper backing, and double-sided FSW in relation
to strain-hardened Al alloys are considered.
Keywords: friction stir welding; aluminum alloys; non-heat-treatable; mechanical properties
1. Introduction
Friction stir welding (FSW) is a widely used solid-state joining process for metals
and alloys developed at The Welding Institute (TWI) in 1991. The FSW process, for is
used to integrate two metal pieces; the general principles are schematically shown in
Figure 1. During the FSW process, a rotating tool is inserted into the interface between
two workpieces, which is then traversed along the welding line. The rotating tool is
usually composed of a pin and a shoulder. The applied plastic deformation and heat-
induced by friction between the rotating tool and workpiece lead to the formation of a
solid-state weld [
1
–
5
]. Although FSW is applicable for various metals [
6
–
10
], it is mostly
used in the joining of Al alloys [
1
,
11
–
14
]. FSW of Al alloys has many advantages compared
with conventional fusion welding processes (e.g., MIG and TIG). These include finer
microstructure, better dimensional stability, lower processing defects, reduced residual
stresses, and superior mechanical properties [3,15,16].
Non-heat-treatable Al alloys, which are also called strain-hardened or work-hardened
alloys, include a group of alloys that are usually strengthened via cold work and/or solid
solution hardening. The various combinations of additions used for these alloys are shown
in Table 1[
3
,
17
,
18
]. Temper designations for the alloys strengthened by strain hardening
consist of an H followed by two or more digits (e.g., AA3103-H13). The first digit following
Crystals 2023,13, 576. https://doi.org/10.3390/cryst13040576 https://www.mdpi.com/journal/crystals

Crystals 2023,13, 576 2 of 40
the H indicates whether the strain-hardened alloy has been thermally treated, and the digit
following the H1, H2, and H3 (e.g., H1xx, H2xx, or H3xx conditions) indicates the degree
of the applied strain hardening. Furthermore, the letter “O” is used to present the annealed
conditions by which the lowest strength is achieved [19,20].
Crystals 2023, 13, x FOR PEER REVIEW 2 of 41
hardening consist of an H followed by two or more digits (e.g., AA3103-H13). The first
digit following the H indicates whether the strain-hardened alloy has been thermally
treated, and the digit following the H1, H2, and H3 (e.g., H1xx, H2xx, or H3xx conditions)
indicates the degree of the applied strain hardening. Furthermore, the letter “O” is used
to present the annealed conditions by which the lowest strength is achieved [19,20].
Figure 1. Schematic of the FSW Process [21]. Reprinted from: Quantitative wear analysis of H13
steel tool during friction stir welding of Cu-0.8%Cr-0.1%Zr alloy, Wear 378 (2017): 82–89. Sahlot,
Pankaj, Kaushal Jha, G. K. Dey, and Amit Arora, Copyright 2017, with permission from Elsevier.
Table 1. Non-heat-treatable Al alloys [17,18].
Non-Heat-Treatable
Aluminum Series
Common Alloys
(Number) Alloy System Tensile Strength Range
(MPa)
1xxx 1050, 1060, 1100 Pure Al 70–175
3xxx 3003, 3004, 3105 Al-Mn 140–280
5xxx 5005, 5052, 5056, 5083, 5086,
5454, 5456, 5657, 5754 Al-Mg 140–380
The plastic flow applied by the rotating pin leads to stirring and mixing of material
around the weld zone, while friction between the tool and the workpiece provides the
main contribution to heat generation [3,22,23]. The generated heat activates softening
mechanisms by which the mechanical properties of the weld area are deteriorated com-
pared to the base metal [1,24–28]. Overall, four distinct macrostructural zones can be iden-
tified in the FSWed joints, including: heat-affected zone (HAZ), thermomechanical af-
fected zone (TMAZ), stir zone (SZ), and base metal (BM) [29]. Figure 2 shows a common
cross-section of FSWed joints [29].
Figure 1.
Schematic of the FSW Process [
21
]. Reprinted from: Quantitative wear analysis of H13 steel
tool during friction stir welding of Cu-0.8%Cr-0.1%Zr alloy, Wear 378 (2017): 82–89. Sahlot, Pankaj,
Kaushal Jha, G. K. Dey, and Amit Arora, Copyright 2017, with permission from Elsevier.
Table 1. Non-heat-treatable Al alloys [17,18].
Non-Heat-Treatable
Aluminum Series
Common Alloys
(Number)
Alloy
System
Tensile Strength Range
(MPa)
1xxx 1050, 1060, 1100 Pure Al 70–175
3xxx 3003, 3004, 3105 Al-Mn 140–280
5xxx
5005, 5052, 5056, 5083, 5086,
5454, 5456, 5657, 5754 Al-Mg 140–380
The plastic flow applied by the rotating pin leads to stirring and mixing of material
around the weld zone, while friction between the tool and the workpiece provides the
main contribution to heat generation [
3
,
22
,
23
]. The generated heat activates softening
mechanisms by which the mechanical properties of the weld area are deteriorated compared
to the base metal [
1
,
24
–
28
]. Overall, four distinct macrostructural zones can be identified in
the FSWed joints, including: heat-affected zone (HAZ), thermomechanical affected zone
(TMAZ), stir zone (SZ), and base metal (BM) [
29
]. Figure 2shows a common cross-section
of FSWed joints [29].
Crystals 2023, 13, x FOR PEER REVIEW 2 of 41
hardening consist of an H followed by two or more digits (e.g., AA3103-H13). The first
digit following the H indicates whether the strain-hardened alloy has been thermally
treated, and the digit following the H1, H2, and H3 (e.g., H1xx, H2xx, or H3xx conditions)
indicates the degree of the applied strain hardening. Furthermore, the letter “O” is used
to present the annealed conditions by which the lowest strength is achieved [19,20].
Figure 1. Schematic of the FSW Process [21]. Reprinted from: Quantitative wear analysis of H13
steel tool during friction stir welding of Cu-0.8%Cr-0.1%Zr alloy, Wear 378 (2017): 82–89. Sahlot,
Pankaj, Kaushal Jha, G. K. Dey, and Amit Arora, Copyright 2017, with permission from Elsevier.
Table 1. Non-heat-treatable Al alloys [17,18].
Non-Heat-Treatable
Aluminum Series
Common Alloys
(Number) Alloy System Tensile Strength Range
(MPa)
1xxx 1050, 1060, 1100 Pure Al 70–175
3xxx 3003, 3004, 3105 Al-Mn 140–280
5xxx 5005, 5052, 5056, 5083, 5086,
5454, 5456, 5657, 5754 Al-Mg 140–380
The plastic flow applied by the rotating pin leads to stirring and mixing of material
around the weld zone, while friction between the tool and the workpiece provides the
main contribution to heat generation [3,22,23]. The generated heat activates softening
mechanisms by which the mechanical properties of the weld area are deteriorated com-
pared to the base metal [1,24–28]. Overall, four distinct macrostructural zones can be iden-
tified in the FSWed joints, including: heat-affected zone (HAZ), thermomechanical af-
fected zone (TMAZ), stir zone (SZ), and base metal (BM) [29]. Figure 2 shows a common
cross-section of FSWed joints [29].
Figure 2.
A common cross-section of FSWed joints that shows SZ, TMAZ, HAZ, and BM. Dotted
and dashed lines indicate the outer boundaries of TMAZ and HAZ [
29
]. Reprinted under the
CC BY 4.0 License.

Crystals 2023,13, 576 3 of 40
In heat-treatable (e.g., age-hardened) Al alloys, the softening mechanism is associated
with the dissolution of strengthening precipitates and grain growth during the welding
thermal cycle. Loss of the mechanical properties in this group of Al alloys can be somewhat
mitigated by applying subsequent natural or artificial aging treatment [
17
,
26
,
30
–
33
]. For
instance, Sato et al. [
34
,
35
] reported higher density of strengthening precipitates and
superior mechanical strength of AA6063 after applying post-weld aging relative to the
as-received base metal. Furthermore, Kalemba et al. [
36
] investigated the FSW of AA7136-
T76 under three and six years naturally aged conditions, and stated that natural aging
remarkably improved the mechanical properties of FSWed joints of heat treatable alloys.
In non-heat-treatable, work-hardened Al alloys, the softened region leads to consider-
able deterioration in the tensile properties, hardness, and fatigue resistance of the FSWed
joints. Recovery and recrystallization are the principle softening mechanisms [
31
,
37
,
38
]. It
should be noted that the FSW process often does not lead to a reducing effect on mechanical
properties of the annealed and H111 temper Al alloys. “H111 Temper” Al alloys receive
cold-work hardening after annealing but not enough for the alloys to meet the mechanical
properties of full work-hardened products such as H11 or H12 temper [
19
,
39
,
40
]. Unlike
the heat-treatable alloys, applying the post-weld heat treatment does not work and can
significantly deteriorate the mechanical properties of the FSW joints of the strain-hardened
Al alloys [17,38,41,42]. Therefore, a comprehensive literature review is required to outline
the promising strategies and processing routes that can improve the weld performance, and
to identify the optimum process parameters that can provide high quality FSWed joints.
Several efforts have been made to identify and introduce novel processing routes
which can improve the mechanical properties of FSWed joints of non-heat-treatable Al
alloys. The first practical approach is the optimization of FSW parameters, including
rotational and traverse speeds, welding tool geometry, tool tilt angle, and plunge depth.
Minimizing heat generation by applying higher traverse speeds and/or lower rotational
speeds seems to be a successful solution for alleviating the softening problems. However,
there are concerns about insufficient heat generation which can lead to different types of
defects in the weld area where the mechanical properties of the joints can be remarkably
reduced [
1
,
22
,
43
,
44
]. Unfavorable residual stress, distortion, and high surface flash are also
negative effects of the excessive heat input that mostly affect the fatigue performance of
FSWed joints [
2
,
26
,
45
,
46
]. Therefore, an appropriate combination of traverse and rotational
speeds is required to achieve a defect-free joint with minimum heat input.
Another important FSW parameter is tool geometry, which plays a key role in localized
heating and material flow and can remarkably influence the tensile properties, hardness,
and fatigue life. Designing appropriate tool geometry is necessary to control the heat input
of the FSW joint. The common tool pin design consists of a cylindrical and tapered/conical
pin that can be with or without thread [
12
,
47
–
49
]. By designing an appropriate tool pin
design, it is possible to fabricate FSWed joints with symmetrical mechanical properties.
Furthermore, the tool tilt angle is associated with the effective transfer of material
from the front to the back of the pin. Its effect on the formation or disappearance of defects,
peak temperature, the material flow, and changing the shapes of the nugget zone has
also been shown. The tool tilt angle depends on the tool pin design and the degree of its
mixing during FSW. In addition, surface defects that are mainly responsible for fatigue
crack initiation are affected by tool tilt angle [
44
,
50
,
51
]. The plunge depth is another factor
that affects the surface quality of the joints. Plunge depths that are either too shallow or too
deep can negatively affect the weld quality (e.g., by insufficient plastic deformation, lack of
penetration, local thinning, and excessive flash) [2,27,52].
The incorporation of reinforcement nano- or microparticles in the microstructure of
non-heat-treatable Al alloys can improve the mechanical strength of the stir zone, provided
that an appropriate ratio of rotational and traverse speeds is chosen. In addition to the
rotational and traverse speeds, the number of FSW passes and direction of the multi-passes
are important variables that affect the FSWed joint quality in the presence of reinforcement
particles [
53
–
55
]. The reinforcement particles can be ceramic-based particles, that are

Crystals 2023,13, 576 4 of 40
intrinsically hard, or intermetallic compounds formed in the microstructure by in situ
reaction between metal powder and aluminum matrix. The homogeneous distribution of
the reinforcement particles in the aluminum matrix is critical to have high strength FSWed
joints [56,57].
The use of artificial cooling (e.g., water) during the course of the FSW process can
minimize heat input and the associated softening effect [
2
]. Underwater friction stir welding
(UFSW) improves the mechanical properties of joints by preventing grain coarsening in
different weld zones, especially in the HAZ. Using water cooling may require revising the
welding parameters of the conventional FSW processes (e.g., rotational speed) to obtain
high-quality defect-free joints [
58
,
59
]. Water cooling media can greatly improve the fatigue
performance of FSWed joints by controlling residual stresses and distortions [25,60].
Several other measures have been suggested to improve the quality of FSWed joints of
work-hardened Al alloys. Surface defects, which act as the site of fatigue crack initiation, can
be removed and replaced by favorable compressive residual stress and applying suitable
surface treatments [
61
,
62
]. The Bobbin tool FSW was developed to solve the insufficient
tool penetration in conventional tool FSW [
26
,
63
]. Furthermore, using Cu backing as a
cooling agent was introduced to FSW of Al alloys to improve hardness, tensile properties,
and fatigue life [
64
,
65
]. Double-sided FSW is an efficient method for joining Al thicker
plates, which maximizes symmetry and minimizes the root flaws [10,66].
The FSW process can be used in an individual or combined manner to improve
the joints’ efficiency and performance. These techniques are thoroughly reviewed in the
following sections.
2. Application
FSW is extensively used in many industries (e.g., shipbuilding, marine, aerospace,
railway vehicles, and automotive sectors) in joining non-heat-treatable Al-based prod-
ucts [
13
,
67
–
71
]. Although the FSW process is mostly used in butt welds, other joint designs
such as spot welds and T-joint welds are also being performed. In most cases, FSW is ap-
plied for large-scale products that are welded by setting the workpiece on a worktable. Even
though the applications are in a one-dimensional form, the facility is under development
to conduct FSW in a three-dimensional form [
2
,
72
,
73
]. The following are some common
examples of FSW, which imply the strong need for FSW use in various industrial sectors.
2.1. Marine
One of the main applications of FSW is in shipbuilding, where it is often used for
joining the boat’s hulls and its stiffeners, decking, bulkheads, and superstructure made
from corrosion resistant AA5XXX aluminum alloys, such as AA5086, AA5454, AA5456,
AA5059, and AA5383 AA5083 products [
74
–
76
]. In addition, the FSW process is used to
join honeycomb panels which have been developed with a high noise-absorbing coefficient
for the walls of the ship cabin. In shipbuilding construction, prefabricated panels fabricated
by FSW lead to reduced problems for retaining highly skilled welders, thus reducing labor
costs. [
67
,
68
,
77
,
78
]. Furthermore, a portable prototype FSW machine has been recently
used in manufacturing the bow section of a new type of ocean viewer vessel made from
the AA5083-H321 alloy [67,78].
2.2. Aerospace
The various advantages of the FSW process, such as the weight of the structure, the
strength of the joint (particularly fatigue performance), and finished cost, led to interest from
the aerospace industry [
2
,
78
–
80
]. Various joining processes, especially FSW, are performed
to join the main structural areas in a transport aircraft, namely fuselage and pressure cabins,
wings, and empennage (horizontal and vertical stabilizers). The engineering properties
required for these structures are strength, stiffness, fatigue crack growth, fracture toughness,
and corrosion [
81
]. FSW is used in the manufacturing of aircraft (A3xxx Airbus series) for

Crystals 2023,13, 576 5 of 40
the production of longitudinal joints in the fuselage, links, and central container of the
wing [68].
2.3. Railway
The use of FSW is also increasing in the railway vehicle industry and is an ideal
process for butt welding of lengthy longitudinal extruded section profiles for high-speed
trains [
82
,
83
]. In train and tram structures, FSW is now used for roof panels, car-body, and
railway wagons, which are made from longitudinal hollow Al extrusions [78,84,85].
2.4. Automotive
The FSW of different Al alloys has been extensively used in the automotive industry
for the high-volume production of vehicle components for years due to the high integrity of
the technique [
51
,
72
]. Among the innovations used in joining aluminum alloy body panels,
prototype frames, and structural components in automobiles are FSW and friction stir spot
welding (FSSW) [86].
3. Optimization of Welding Parameters
FSW includes complex metal movement and severe plastic deformation through the
mechanical (spinning) motion of a rotating tool. The tool rotational speed, traverse speed,
tool design, plunge depth, and tilt angle are the major welding parameters that have
considerable impact on the material flow pattern and temperature distribution, thereby
affecting the microstructures and mechanical properties of joints [
1
,
27
,
87
]. Evidently, the
proper selection and adjustment of the process parameters is critical and can remarkably
affect the final quality of the FSWed joints.
Based on what was previously discussed for non-heat-treatable, work-hardened Al
alloys (i.e., H-temper), the main issue with defect-free FSWed joints is the severe softening
effect and deterioration of mechanical properties in the welded joints, especially in the stir
zone (SZ). For example, Yazdipour et al. [
88
] reported that the hardness around the weld
line of AA5083-H321 alloys can be reduced to 50% relative to the base metal. In addition,
for a defect-free FSW joint of H-temper Al alloys, the mechanical properties of welds are
close to the properties of the annealed temper (i.e., O temper) alloys where the yield and
tensile strength can be reduced to 75% and 50%, respectively [
31
,
88
,
89
]. Furthermore, the
fatigue strength of the FSWed joints can be equal to or less than the base metal, depending
on the applied welding conditions [
2
,
43
,
51
,
90
]. For example, Aydin et al. [
91
] reported
lower fatigue resistance for the FSWed joints relative to the base material, whereas Ue-
matsu et al. [
92
] demonstrated that the fatigue strengths of the welds can be nearly the
same as those of the parent materials.
The softening problem, which is caused by dynamic recrystallization, grain growth,
and low-temperature annealing, results from the heat input rising from the frictional motion
of the rotating welding tool. Therefore, controlling the amount of heat input is a key factor
in reducing the softening effect in FSWed joints [
2
,
26
,
88
,
93
]. On the other hand, producing
a solid and defect-free joint requires adequate heat input. When the amount of heat input
is too low, welding defects occur in the joint, and the mechanical properties, such as tensile
strength and fatigue resistance, are adversely affected [
31
,
94
]. Therefore, in joining the
strain-hardened Al alloys using FSW, the welding parameters must be selected so that the
heat input is optimized to achieve a defect-free high strength joint [
31
,
38
,
94
,
95
]. Effects of
the main process parameters on mechanical properties of non-heat-treatable Al alloys and
possible improvement strategies are discussed in the following sections.
3.1. Effect of Traverse and Rotational Speeds
Tool rotation speed and tool traverse speed are the two principal parameters generating
heat during FSW. Depending on the amount of heat input, three types of problems in FSWed
joints of H-temper Al alloys can be encountered: (1) softening (high heat input), (2) process

Crystals 2023,13, 576 6 of 40
defects (low heat input), and (3) distortion and residual stress [
43
,
44
,
96
]. The effects of
these shortcomings on joint quality will be discussed in the following clauses.
3.1.1. Softening (High Heat Input)
The ratio of “traverse to rotational speed”, which is called revolutionary pitch (RP),
controls the heat input to the joint. A higher revolutionary pitch leads to lower heat input in
the joint, whereas a lower revolutionary pitch produces higher heat input [
31
,
94
,
97
,
98
]. As
shown in Figure 3, Khorrami et al. [
42
] indicated that increasing the rotational speed (higher
heat input) led to lower tensile strength and larger elongation in defect-free FSWed joints
of both work-hardened (as-welded) and annealed 1XXX pure Al alloy [
42
]. Additionally,
an increase in ultimate tensile strength has been reported when the revolutionary pitch
increases from 0.07 to 0.47 mm/rpm in the FSW of AA1050-H24 pieces [
99
]. Therefore,
decreasing the rotational speed and increasing the traverse speed leads to an increase
in the revolutionary pitch. Subsequently, the amount of heat input reduces under this
condition. When the heat input is alleviated, the annealing of the work-hardened Al base is
reduced. This is desirable for the strength of the defect-free welds because the softening
effect caused by annealing is reduced. The mechanical properties, including tensile strength
and hardness, can therefore be improved in the defect-free FSWed joints by rotational and
traverse speeds [1,41,97,98].
Crystals 2023, 13, x FOR PEER REVIEW 6 of 41
Effects of the main process parameters on mechanical properties of non-heat-treatable Al
alloys and possible improvement strategies are discussed in the following sections.
3.1. Effect of Traverse and Rotational Speeds
Tool rotation speed and tool traverse speed are the two principal parameters gener-
ating heat during FSW. Depending on the amount of heat input, three types of problems
in FSWed joints of H-temper Al alloys can be encountered: (1) softening (high heat input),
(2) process defects (low heat input), and (3) distortion and residual stress [43,44,96]. The
effects of these shortcomings on joint quality will be discussed in the following clauses.
3.1.1. Softening (High Heat Input)
The ratio of “traverse to rotational speed”, which is called revolutionary pitch (RP),
controls the heat input to the joint. A higher revolutionary pitch leads to lower heat input
in the joint, whereas a lower revolutionary pitch produces higher heat input [31,94,97,98].
As shown in Figure 3, Khorrami et al. [42] indicated that increasing the rotational speed
(higher heat input) led to lower tensile strength and larger elongation in defect-free
FSWed joints of both work-hardened (as-welded) and annealed 1XXX pure Al alloy [42].
Additionally, an increase in ultimate tensile strength has been reported when the revolu-
tionary pitch increases from 0.07 to 0.47 mm/rpm in the FSW of AA1050-H24 pieces [99].
Therefore, decreasing the rotational speed and increasing the traverse speed leads to an
increase in the revolutionary pitch. Subsequently, the amount of heat input reduces under
this condition. When the heat input is alleviated, the annealing of the work-hardened Al
base is reduced. This is desirable for the strength of the defect-free welds because the sof-
tening effect caused by annealing is reduced. The mechanical properties, including tensile
strength and hardness, can therefore be improved in the defect-free FSWed joints by rota-
tional and traverse speeds [1,41,97,98].
Figure 3. Variations in ultimate tensile strength and elongation of FSWed joints of 1XXX pure Al
alloy in as-welded (work hardened) and annealed conditions versus rotation speed [42]. Reprinted
from: Thermal stability during annealing of friction stir welded aluminum sheet produced by con-
strained groove pressing, Materials & Design 45 (2013): 222–227, Khorrami, M. Sarkari, M.
Kazeminezhad, and A. H. Kokabi, Copyright 2012, with permission from Elsevier.
The mechanical behavior of different weld zones (SZ, HAZ, and TMAZ) varies with
the heat input induced by traverse and rotational speeds. Several studies on FSW of non-
heat-treatable wrought Al alloys have shown that the various traverse speeds at a constant
rotational speed do not have a considerable impact on the average hardness value of the
stir zone [25,38,94,100]. However, the variation of rotational and traverse speeds not only
results in a change in the mechanical behavior of the HAZ, but also influences the softened
Figure 3.
Variations in ultimate tensile strength and elongation of FSWed joints of 1XXX pure Al alloy
in as-welded (work hardened) and annealed conditions versus rotation speed [
42
]. Reprinted from:
Thermal stability during annealing of friction stir welded aluminum sheet produced by constrained
groove pressing, Materials & Design 45 (2013): 222–227, Khorrami, M. Sarkari, M. Kazeminezhad,
and A. H. Kokabi, Copyright 2012, with permission from Elsevier.
The mechanical behavior of different weld zones (SZ, HAZ, and TMAZ) varies with
the heat input induced by traverse and rotational speeds. Several studies on FSW of
non-heat-treatable wrought Al alloys have shown that the various traverse speeds at a
constant rotational speed do not have a considerable impact on the average hardness value
of the stir zone [
25
,
38
,
94
,
100
]. However, the variation of rotational and traverse speeds
not only results in a change in the mechanical behavior of the HAZ, but also influences
the softened HAZ width. It has been widely reported that the hardness values of HAZ
decrease with decreasing traverse speed and increasing rotational speed [
3
,
31
,
38
,
75
,
96
,
101
].
This combination of parameters (lower revolutionary pitch or higher heat input) results in a
wider softened HAZ due to a higher holding time at the peak temperature since the cooling
rate is noticeably reduced. Furthermore, the grains also have enough time to grow, leading
to a decrease in the average hardness in this region. It is well-known that grain growth leads
to a decrease in the hardness value in Al alloys [
38
,
41
,
88
,
102
,
103
]. As observed in Figure 4,
the average grain size values of HAZ in the FSW joint of AA3003-H18 are remarkably

Crystals 2023,13, 576 7 of 40
affected by traverse and rotational speeds. The largest grain size (41.7
µ
m) belongs to
Figure 4g with speeds of 1200 r/min and 40 mm/min (i.e., the lowest revolutionary pitch
or highest heat input). In contrast, Figure 4c shows the smallest grain size (20.4
µ
m) at the
highest revolutionary pitch [
38
]. Mohammadi Sefat et al. [
103
] studied the size of HAZ for
FSW of AA5052-H18 and showed that increasing the traverse speed (linear speed) from
63 mm/min
to 315 mm/min and decreasing the rotational speed from 2500 rpm to 800 rpm
narrowed the size of HAZ (softening area) (Figure 5) [
103
]. Thus, a controlled combination
of the traverse and rotational speeds provides the possibility of reaching a defect-free joint
with minimum heat input. Table 2outlines a list of process parameters which are optimized
to achieve high tensile properties in the FSWed joints of non-heat-treated Al alloys.
Crystals 2023, 13, x FOR PEER REVIEW 7 of 41
HAZ width. It has been widely reported that the hardness values of HAZ decrease with
decreasing traverse speed and increasing rotational speed [3,31,38,75,96,101]. This combi-
nation of parameters (lower revolutionary pitch or higher heat input) results in a wider
softened HAZ due to a higher holding time at the peak temperature since the cooling rate
is noticeably reduced. Furthermore, the grains also have enough time to grow, leading to
a decrease in the average hardness in this region. It is well-known that grain growth leads
to a decrease in the hardness value in Al alloys [38,41,88,102,103]. As observed in Figure
4, the average grain size values of HAZ in the FSW joint of AA3003-H18 are remarkably
affected by traverse and rotational speeds. The largest grain size (41.7 µm) belongs to Fig-
ure 4g with speeds of 1200 r/min and 40 mm/min (i.e., the lowest revolutionary pitch or
highest heat input). In contrast, Figure 4c shows the smallest grain size (20.4 µm) at the
highest revolutionary pitch [38]. Mohammadi Sefat et al. [103] studied the size of HAZ for
FSW of AA5052-H18 and showed that increasing the traverse speed (linear speed) from
63 mm/min to 315 mm/min and decreasing the rotational speed from 2500 rpm to 800 rpm
narrowed the size of HAZ (softening area) (Figure 5) [103]. Thus, a controlled combination
of the traverse and rotational speeds provides the possibility of reaching a defect-free joint
with minimum heat input. Table 2 outlines a list of process parameters which are opti-
mized to achieve high tensile properties in the FSWed joints of non-heat-treated Al alloys.
Figure 4. Optical microscope images and average grain size values of HAZ in FSW joint of AA3003-
H18 at various rotation and travel speeds: (a) 800 r/min, 40 mm/min; (b) 800 r/min, 70 mm/min; (c)
800 r/min, 100 mm/min; (d) 1000 r/min, 40 mm/min; (e) 1000 r/min, 70 mm/min; (f) 1000 r/min, 100
mm/min; (g) 1200 r/min, 40 mm/min; (h) 1200 r/min, 70 mm/min; (i) 1200 r/min, 100 mm/min [38].
Reprinted from: Effects of heat input in friction stir welding on microstructure and mechanical prop-
erties of AA3003-H18 plates, Transactions of Nonferrous Metals Society of China 25, no. 7 (2015):
2147–2155, Abnar, B., M. Kazeminezhad, and A. H. Kokabi, Copyright 2015, with permission from
Elsevier.
Figure 4.
Optical microscope images and average grain size values of HAZ in FSW joint of AA3003-
H18 at various rotation and travel speeds:
(a) 800 r/min,
40 mm/min;
(b) 800 r/min
, 70 mm/min;
(c) 800 r/min
, 100 mm/min; (
d
) 1000 r/min, 40 mm/min; (
e
) 1000 r/min,
70 mm/min
;
(f) 1000 r/min
,
100 mm/min; (
g
) 1200 r/min, 40 mm/min; (
h
) 1200 r/min, 70 mm/min;
(i) 1200 r/min
,
100 mm/min [38]
. Reprinted from: Effects of heat input in friction stir welding on microstruc-
ture and mechanical properties of AA3003-H18 plates, Transactions of Nonferrous Metals Society of
China 25, no. 7 (2015): 2147–2155, Abnar, B., M. Kazeminezhad, and A. H. Kokabi, Copyright 2015,
with permission from Elsevier.
Crystals 2023, 13, x FOR PEER REVIEW 8 of 41
Figure 5. The effect of rotational and linear speed (traverse speed) on the size of HAZ in the FSW of
AA5052-H18 [103]. Reprinted from: Friction Stir Welding of 5052-H18 aluminum alloy: Modeling
and process parameter optimization." Journal of Materials Engineering and Performance 30 (2021):
1838–1850, Mohammadi Sefat, Mohammad Javad, Hadi Ghazanfari, and Carl Blais. Copyright 2021,
with permission from Springer Nature.
Table 2. A summary of selected optimization parameters reported achieving high tensile properties
in FSW joints of non-heat-treated Al alloys.
Al Alloy Plate Thickness Welding Tool Ge-
ometry
Optimum Parame-
ters
for Tensile
Strength
Results and Joint
Efficiency % Ref.
5083-O 5 mm
Shoulder diameter:
20 mm, Pin diame-
ter: 5 mm, Pin
height: 4.5 mm, cy-
lindrical with
thread pin, tilt an-
gle: 2°
Rotational speed:
800 rpm
Traverse speed: 124
mm/min
UTS: 340 MPa
Joint efficiency:
92%
[95]
5083 6 mm
Shoulder diameter:
24 mm, Pin diame-
ter: 8 mm
Taper with thread
pin, tilt angle: 1.5°
Rotational speed:
710 rpm
Traverse speed: 40
mm/min
UTS: 185 Mpa
Joint efficiency:
66%
[101]
5052 4 mm
Shoulder diameter:
17.8 mm, Pin diam-
eter: 4 mm, Pin
height: 3.8 mm, Ta-
per pin
Rotational speeds:
1300 rpm, Traverse
speed: 50 mm/min
UTS: 221 MPa
Joint efficiency:
85%
[104]
5052-H18 2 mm
Shoulder diameter:
12.8 mm, Pin diam-
eter: 1.8 mm, cylin-
drical pin
Rotational speeds:
1233 rpm, Traverse
speed: 107 mm/min
UTS: 229 MPa
Joint efficiency:
Not reported
[103]
5052 1 mm
Shoulder diameter:
7.5 mm, Pin diame-
ter: 2.5 mm, square
shape pin
Rotational speeds:
3250 rpm, Traverse
speed: 200 mm/min
UTS: 184 MPa
Joint efficiency:
80%
[105]
Figure 5.
The effect of rotational and linear speed (traverse speed) on the size of HAZ in the FSW of
AA5052-H18 [
103
]. Reprinted from: Friction Stir Welding of 5052-H18 aluminum alloy: Modeling
and process parameter optimization. Journal of Materials Engineering and Performance 30 (2021):
1838–1850, Mohammadi Sefat, Mohammad Javad, Hadi Ghazanfari, and Carl Blais. Copyright 2021,
with permission from Springer Nature.

Crystals 2023,13, 576 8 of 40
Table 2.
A summary of selected optimization parameters reported achieving high tensile properties
in FSW joints of non-heat-treated Al alloys.
Al Alloy Plate
Thickness Welding Tool Geometry Optimum Parameters
for Tensile Strength
Results and Joint
Efficiency % Ref.
5083-O 5 mm
Shoulder diameter: 20 mm,
Pin diameter: 5 mm, Pin
height: 4.5 mm, cylindrical
with thread pin, tilt angle: 2◦
Rotational speed: 800 rpm
Traverse speed: 124 mm/min
UTS: 340 MPa
Joint efficiency: 92% [95]
5083 6 mm
Shoulder diameter: 24 mm,
Pin diameter: 8 mm
Taper with thread pin, tilt
angle: 1.5◦
Rotational speed: 710 rpm
Traverse speed: 40 mm/min
UTS: 185 Mpa
Joint efficiency: 66% [101]
5052 4 mm
Shoulder diameter: 17.8 mm,
Pin diameter: 4 mm, Pin
height: 3.8 mm, Taper pin
Rotational speeds: 1300 rpm,
Traverse speed: 50 mm/min
UTS: 221 MPa
Joint efficiency: 85% [104]
5052-H18 2 mm
Shoulder diameter: 12.8 mm,
Pin diameter: 1.8 mm,
cylindrical pin
Rotational speeds: 1233 rpm,
Traverse speed: 107 mm/min
UTS: 229 MPa
Joint efficiency:
Not reported
[103]
5052 1 mm
Shoulder diameter: 7.5 mm,
Pin diameter: 2.5 mm, square
shape pin
Rotational speeds: 3250 rpm,
Traverse speed: 200 mm/min
UTS: 184 MPa
Joint efficiency: 80% [105]
5086-H34 6 mm
Shoulder diameter: 16 mm,
Pin: square shape, Pin height:
5.7 mm, square shape pin
Rotational speeds: 1250 rpm,
Traverse speed: 80 mm/min
UTS: 310 MPa
Joint efficiency: 85% [106]
1100-H14 4 mm
Shoulder diameter: 18 mm,
Pin diameter: 6 mm, Pin
height: 3 and 1.5 mm,
cylindrical pin
Rotational speed: 900 rpm
Traverse speed: 40 mm/min
UTS: 94 MPa
Joint efficiency:
Not reported
[107]
5083-H111 6 mm
Shoulder diameter: 20 mm,
Pin diameter: 7 mm, Pin
height: 5.8 mm, triangular
shape pin, tilt angle: 2◦
Rotational speed: 800 rpm
Traverse speed: 125 mm/min
UTS: 299 MPa
Joint efficiency: 92% [100]
1050-H24 5 mm
Shoulder diameter: 15 mm,
Pin diameter: 6 mm, Pin
height: 4.7 mm, cylindrical
pin, tilt angle: 3◦
Rotational speed: 1500 rpm
Traverse speed: 400 mm/min
UTS: 84 MPa
Joint efficiency: 80% [31]
Failure behavior of defect-free FSWed joints under cyclic loading is also influenced by
the degree of softening induced with different rotational and traverse speeds [
46
,
108
,
109
].
It is generally believed that fatigue crack is more likely to be initiated by the applied plastic
deformation localized in the softened area of the FSWed joints [
110
,
111
]. Kulekci et al. [
43
]
reported that increasing the tool rotational speed for a fixed tool pin diameter reduces the
fatigue strength of FSWed lap defect-free joints of AA5754 (Figure 6). This reduction in
fatigue resistance, as illustrated in Figure 6, is attributed to the heat input and corresponding
softening induced in the welding region. Similar to hardness and tensile strength, extreme
heat input during the FSW process negatively affects fatigue strength. Therefore, tool
rotational and traverse speeds need to be optimized to improve fatigue performance. The
rotational speed of 1000 rpm and traverse speed of 100 mm/min have been reported as
optimum parameters for AA5754 FSWed joints [43].

Crystals 2023,13, 576 9 of 40
Crystals 2023, 13, x FOR PEER REVIEW 10 of 41
Figure 6. Fatigue performance of FSW lap joints was obtained with (a) 3 mm (b) 4 mm, and (c) 5
mm pin diameter and different tool rotations [43]. Reprinted from: Effects of tool rotation and pin
diameter on fatigue properties of friction stir welded lap joints, The International Journal of Ad-
vanced Manufacturing Technology 36 (2008): 877–882, Kulekci, Mustafa Kemal, Aydin Şik, and
Erdinç Kaluç. Copyright 2006, with permission from Springer Nature.
Furthermore, the heterogeneity of mechanical properties has been reported for dif-
ferent thickness layers (top, middle, and bottom) of FSWed joints [94,112–114]. For in-
stance, Liu et al. [94] indicated that the hardness, ultimate strength, and yield strength of
the middle layer were slightly smaller than the upper and lower layers of defect-free joints
of AA1050-H24 sheet with a thickness of 5 mm. The lower heat output from the middle
layer relative to the upper/lower layers, and the subsequent softening, was responsible for
the weaker mechanical strength [94,112,113]. For thinner plates, the mechanical strength
of the top, middle, and bottom is closer and more uniform ]114[. However, for thicker
plates, the heterogeneity of mechanical properties across the plate thickness is expected
as the cooling rate and heat output vary from surface layers to central zones. In general,
the increase in revolutionary pitch (low heat input) can help to minimize this heterogene-
ity in defect-free FSWed joints. Process cooling, such as water spray or underwater FSW,
is another method for reducing the heterogeneity across the plate thickness, which is de-
scribed in Section 5.
Figure 6.
Fatigue performance of FSW lap joints was obtained with (
a
) 3 mm (
b
) 4 mm,
and (c) 5 mm
pin diameter and different tool rotations [
43
]. Reprinted from: Effects of tool rotation and pin
diameter on fatigue properties of friction stir welded lap joints, The International Journal of Advanced
Manufacturing Technology 36 (2008): 877–882, Kulekci, Mustafa Kemal, Aydin ¸Sik, and Erdinç Kaluç.
Copyright 2006, with permission from Springer Nature.
Furthermore, the heterogeneity of mechanical properties has been reported for differ-
ent thickness layers (top, middle, and bottom) of FSWed joints [
94
,
112
–
114
]. For instance,
Liu et al. [
94
] indicated that the hardness, ultimate strength, and yield strength of the
middle layer were slightly smaller than the upper and lower layers of defect-free joints of
AA1050-H24 sheet with a thickness of 5 mm. The lower heat output from the middle layer
relative to the upper/lower layers, and the subsequent softening, was responsible for the
weaker mechanical strength [
94
,
112
,
113
]. For thinner plates, the mechanical strength of the
top, middle, and bottom is closer and more uniform [
114
]. However, for thicker plates, the
heterogeneity of mechanical properties across the plate thickness is expected as the cooling
rate and heat output vary from surface layers to central zones. In general, the increase in

Crystals 2023,13, 576 10 of 40
revolutionary pitch (low heat input) can help to minimize this heterogeneity in defect-free
FSWed joints. Process cooling, such as water spray or underwater FSW, is another method
for reducing the heterogeneity across the plate thickness, which is described in Section 5.
3.1.2. Defects (Low Heat Input)
High traverse speed and low rotational speed may result in an inappropriate joint due
to low heat input and the formation of defects in the weld zone [
44
,
91
,
115
]. The values
of these high and low speeds are not specific and depend on several parameters, such as
the thickness of the workpiece, the design of the welding tool, the backing material, the
title angle, and the plunge depth. For defect-free joints, the tensile properties are most
likely correlated with the hardness values distributed across the weld. On the other hand,
for defective joints, the tensile properties (e.g., ultimate tensile strength, yield strength,
elongation, and fracture location) of the joints are primarily affected by defects such as
cracks, voids, tunneling, surface imperfection, hooking, flashes, and kissing bonds or zigzag
lines (Al
2
O
3
oxide layer) [
44
,
109
,
116
,
117
]. The presence of a defect in the stir zone not only
causes a significant reduction in elongation, but also results in a deterioration of the tensile
and yield strength of the FSWed joints. Moreover, in a defective weld, fracture locations are
often located in the stir zone where cracks are initiated [31,95].
It should be noted that both insufficient and excessive heat input can lead to process
defects in the joints [
118
]. According to the Han et al. study on the FSW of AA5083-O [
95
],
when the rotational speed of the probe was too high, the peak temperature was stabilized.
Therefore, more pressure on the probe shoulder became ineffective and led to a defect in the
welded joint through chip formation. Based on their report, the optimum FSW conditions
were a welding speed of 124 mm/min and a rotational speed of 800 r/min for AA5083-
O [
95
]. In addition, Jamalian et al. [
106
] indicated that both low (500 and 800 rpm) and high
(1600 rpm) rotational speeds at various traverse speeds (41.5, 80, and 125 mm/min) led
to tunneling and wormhole defects in the FSWed joints of AA5086-H34. However, FSW
at rotational speeds of 1000 and 1250 rpm showed a defect-free weld. Figure 7shows the
macrostructure of the defective joints obtained with different rotational speeds in their
work. Figure 7a–f shows tunneling and wormhole defects at low rotational speeds, which
were caused by insufficient heat generation and poor plasticity flow. At rotational speeds
that were too high, these defects were generated as a result of considerable turbulence in
the plasticized metal, as shown in Figure 7g–i [
106
]. Moreover, Javad Rasti [
119
] studied
the welding parameters’ effect on the tunnel void area during a FSW of AA1060 at different
rotational (500 and 1000 rpm) and traverse (250, 500, 650 mm/min) speeds. According to
his findings, the area of tunnel cavity defect was zero for a rotational speed of
1000 rpm
and traverse speed of 250 and 500 mm/min [
119
]. According to various works on FSW
of non-heat-treatable Al alloys, it is concluded that the optimum rotational speeds are
approximately
1000 rpm
to achieve a defect-free joint, but the optimum traverse speed
varies for different alloys. For example, in order to obtain a defect-free joint, 5xxx Al alloys
generally need higher heat input caused by lower traverse speed compared to the 1xxx
Al alloys [
120
]. This can be attributed to the mechanical properties and different flowing
behavior of base metals. In high-strength Al alloys, such as AA5083 (275–385 MPa), it takes
more time to reach the desired heat input and temperature for stirring materials. Therefore,
the transverse speed should be slow. However, AA1060 requires lower heat input because
of the lower tensile strength (55–130 MPa).
One of the main factors that influence the fatigue behavior of joints is a weld defect.
Although the high traverse speeds and low rotational speeds (low heat input) are helpful
to control residual stresses and distortion, there is concern about the formation of weld
defects under this process condition, which can reduce fatigue life [
44
,
91
]. Process defects
accelerate the fatigue crack initiation and propagation, which are the main contributors to
the sudden decline of fatigue life in the weld joints [2,46].

Crystals 2023,13, 576 11 of 40
Crystals 2023, 13, x FOR PEER REVIEW 12 of 41
Figure 7. Cross-section of the defective welds with rotational and traverse speeds of (a) 500 rpm,
41.5 mm/min; (b) 500 rpm, 80 mm/min; (c) 500 rpm, 125 mm/min; (d) 800 rpm, 41.5 mm/min; (e) 800
rpm, 80 mm/min; (f) 800 rpm, 125 mm/min; (g) 1600 rpm, 41.5 mm/ min; (h) 1600 rpm, 80 mm/min;
(i) 1600 rpm, 125 mm/min [106]. Reprinted from: Study on the effects of friction stir welding process
parameters on the microstructure and mechanical properties of 5086-H34 aluminum welded joints.
The International Journal of Advanced Manufacturing Technology 83 (2016): 611–621, Moham-
madzadeh Jamalian, H., M. Farahani, M. K. Besharati Givi, and M. Aghaei Vafaei, Copyright 2015,
with permission from Springer Nature.
One of the main factors that influence the fatigue behavior of joints is a weld defect.
Although the high traverse speeds and low rotational speeds (low heat input) are helpful
to control residual stresses and distortion, there is concern about the formation of weld
defects under this process condition, which can reduce fatigue life [44,91]. Process defects
accelerate the fatigue crack initiation and propagation, which are the main contributors to
the sudden decline of fatigue life in the weld joints [2,46].
A continuous S-shaped line in the stir zone is called S-line, zigzag line, or kissing
bond, which is mainly formed by the accumulation of the fine oxide particles resulting
from the initial oxide layer on the butt surface [110,121]. Sato et al. [122] investigated the
characteristics of the kissing bond defect on FSW of AA1050-H24. They reported that
welding parameters have a substantial effect on the formation of the kissing bond. For
example, the higher the welding heat input, the lower the kissing bonds (as shown in
Figure 8). At the lower heat input, the distribution of the oxide layers is weakened, and
Figure 7.
Cross-section of the defective welds with rotational and traverse speeds of (
a
) 500 rpm,
41.5 mm/min; (
b
) 500 rpm, 80 mm/min; (
c
) 500 rpm, 125 mm/min; (
d
) 800 rpm, 41.5 mm/min;
(
e
) 800 rpm, 80 mm/min; (
f
) 800 rpm, 125 mm/min; (
g
) 1600 rpm, 41.5 mm/ min; (
h
) 1600 rpm,
80 mm/min; (
i
) 1600 rpm, 125 mm/min [
106
]. Reprinted from: Study on the effects of friction stir
welding process parameters on the microstructure and mechanical properties of 5086-H34 aluminum
welded joints. The International Journal of Advanced Manufacturing Technology 83 (2016): 611–621,
Mohammadzadeh Jamalian, H., M. Farahani, M. K. Besharati Givi, and M. Aghaei Vafaei, Copyright
2015, with permission from Springer Nature.
A continuous S-shaped line in the stir zone is called S-line, zigzag line, or kissing
bond, which is mainly formed by the accumulation of the fine oxide particles resulting
from the initial oxide layer on the butt surface [
110
,
121
]. Sato et al. [
122
] investigated the
characteristics of the kissing bond defect on FSW of AA1050-H24. They reported that
welding parameters have a substantial effect on the formation of the kissing bond. For
example, the higher the welding heat input, the lower the kissing bonds (as shown in
Figure 8). At the lower heat input, the distribution of the oxide layers is weakened, and
they appear with localized accumulation in the microstructure. The oxide layers at the
higher heat input can be broken, homogeneously distributed in the microstructure, and do
not adversely affect the mechanical properties of the joints [
51
,
109
,
122
]. Zhou et al. [
109
]
studied the effect of the kissing bond on the fatigue behavior of FSW on AA5083-H321.
They concluded that the kissing bond (bonded welds) accelerated the crack initiations

Crystals 2023,13, 576 12 of 40
under fatigue loading, and subsequently, the fatigue life of the joints containing kissing
bonds was 21–24 times shorter relative to the sound welds [109,123].
Crystals 2023, 13, x FOR PEER REVIEW 13 of 41
they appear with localized accumulation in the microstructure. The oxide layers at the
higher heat input can be broken, homogeneously distributed in the microstructure, and
do not adversely affect the mechanical properties of the joints [51,109,122]. Zhou et al.
[109] studied the effect of the kissing bond on the fatigue behavior of FSW on AA5083-
H321. They concluded that the kissing bond (bonded welds) accelerated the crack initia-
tions under fatigue loading, and subsequently, the fatigue life of the joints containing kiss-
ing bonds was 21–24 times shorter relative to the sound welds [109,123].
Figure 8. (a) Cross-section of the as-polished weld and (b) Optical micrographs of the welds pro-
duced by two welding parameters before and after etching [122]. Reprinted from: Characteristics of
the kissing-bond in friction stir welded Al alloy 1050. Materials Science and Engineering: A 405, no.
1–2 (2005): 333–338, Sato, Yutaka S., Hideaki Takauchi, Seung Hwan C. Park, and Hiroyuki Kokawa,
Copyright 2005, with permission from Elsevier.
Apart from the defects observed in butt welds, friction stir lap welding (FSLW) also
has special features at the edges of the bonded region, which is called a “hooking defect”
[2]. In lap joints, one sheet is laid on top of another, and a slight overlap region is created.
The FSW probe plunges completely through the upper sheet and slightly into the lower
sample, traversing through the overlap region [124–126]. A schematic of the FSLW process
is shown in Figure 9a. Shirazi et al. [127] observed such defects in their study on the FSLW
of AA5456. Figure 9b shows an example of the hooking defect on both the advancing and
the retreating sides of the joint conducted on 5 mm thick AA5456-H321 and 2.5 mm thick
AA5456-O. According to their findings, the height and shape of the hooking were affected
by rotational and traverse speeds [127]. Increasing the rotational speed enhanced the ver-
tical material flow, leading to an increase in the hooking height. At a constant rotational
speed, the height and direction of the hooking changed with the increase of the traverse
speed. At low rotational speeds, the increase in the traverse speed resulted in decreasing
the hooking height on both the advancing and retreating sides due to less vertical material
flow [127]. Moreover, Salari et al. [128] suggested that a larger hooking defect had a more
Figure 8.
(
a
) Cross-section of the as-polished weld and (
b
) Optical micrographs of the welds produced
by two welding parameters before and after etching [
122
]. Reprinted from: Characteristics of
the kissing-bond in friction stir welded Al alloy 1050. Materials Science and Engineering: A 405,
no. 1–2 (2005): 333–338,
Sato, Yutaka S., Hideaki Takauchi, Seung Hwan C. Park, and Hiroyuki
Kokawa, Copyright 2005, with permission from Elsevier.
Apart from the defects observed in butt welds, friction stir lap welding (FSLW) also
has special features at the edges of the bonded region, which is called a “hooking defect” [
2
].
In lap joints, one sheet is laid on top of another, and a slight overlap region is created. The
FSW probe plunges completely through the upper sheet and slightly into the lower sample,
traversing through the overlap region [
124
–
126
]. A schematic of the FSLW process is shown
in Figure 9a. Shirazi et al. [
127
] observed such defects in their study on the FSLW of AA5456.
Figure 9b shows an example of the hooking defect on both the advancing and the retreating
sides of the joint conducted on 5 mm thick AA5456-H321 and 2.5 mm thick AA5456-O.
According to their findings, the height and shape of the hooking were affected by rotational
and traverse speeds [
127
]. Increasing the rotational speed enhanced the vertical material
flow, leading to an increase in the hooking height. At a constant rotational speed, the
height and direction of the hooking changed with the increase of the traverse speed. At
low rotational speeds, the increase in the traverse speed resulted in decreasing the hooking
height on both the advancing and retreating sides due to less vertical material flow [
127
].
Moreover, Salari et al. [
128
] suggested that a larger hooking defect had a more negative
impact on the mechanical properties of FSLW AA5456. They also reported that an increase
in rotational speed was a reason for increasing the hooking defect height [128].

Crystals 2023,13, 576 13 of 40
Crystals 2023, 13, x FOR PEER REVIEW 14 of 41
negative impact on the mechanical properties of FSLW AA5456. They also reported that