Characteristics of Intermetallic Compounds in Dissimilar Friction Stir Welding: A Review

Abstract

In the present paper, mechanical and metallurgical characteristics of different dissimilar weldments fabricated by friction stir welding were investigated. Existence of lamellar composite structure within the stir zone in addition to observation of interfacial intermetallic compounds (IMCs) was the main characteristics that were investigated throughout this research. Results indicated that the optimum IMCs layers, resulting in enhanced mechanical properties, met three criteria, thinness, uniformity, and continuity.

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Metallography, Microstructure, and Analysis (2019) 8:445–461
https://doi.org/10.1007/s13632-019-00557-w
REVIEW
Characteristics ofIntermetallic Compounds inDissimilar Friction Stir
Welding: AReview
A.Esmaeili1,2 · C.Sbarufatti2· A.M.S.Hamouda1
Received: 6 October 2018 / Revised: 25 April 2019 / Accepted: 3 July 2019 / Published online: 15 July 2019
© ASM International 2019
Abstract
In the present paper, mechanical and metallurgical characteristics of different dissimilar weldments fabricated by friction
stir welding were investigated. Existence of lamellar composite structure within the stir zone in addition to observation of
interfacial intermetallic compounds (IMCs) was the main characteristics that were investigated throughout this research.
Results indicated that the optimum IMCs layers, resulting in enhanced mechanical properties, met three criteria, thinness,
uniformity, and continuity.
Keywords Friction stir welding· Intermetallic compounds· Tensile strength· Composite· Hardness distribution·
Dissimilar materials
Introduction
Joining dissimilar metallic materials is a requirement for
a variety of industrial sectors, including automotive and
marine applications [1]. Excessive heat input [2], forma-
tion of welding defects specifically solidification defects,
and macrosegregation [3] along with different chemical and
physical properties of the joint materials have made the con-
ventional fusion welding methods inappropriate for dissimi-
lar material welding. Formation of thick IMCs is one of the
main challenges in conventional fusion welding arisen from
elevated temperature during fusional welding [4]. Some
solid-state welding methods such as explosive welding [5]
and ultrasonic welding [6] were used for joining dissimilar
materials. However, some restrictions such as initial prepa-
ration, geometrical constraint, time-consuming processes,
expenses, and specific equipment raised challenges for
the efficiency of these methods. Since its invention by the
The Welding Institute (TWI) [7], the FSW has been taken
into consideration as one of the most efficient methods for
joining dissimilar metals. In the past decade, many attempts
have been carried out to investigate weldability of dissimi-
lar materials by means of FSW, including aluminum with
copper, Al/Cu [8–43], aluminum with steel, Al/St [44–83],
aluminum with titanium, Al/Ti [84–91], titanium with steel,
Ti/St [92–98], aluminum with magnesium, Al/Mg [99–124],
magnesium with steel, Mg/St [125–128], magnesium with
titanium, Mg/Ti [129], steel with nickel, St/Ni [130, 131],
steel with copper, St/Cu [132–134], and steel with brass, St/
brass [135]. Most of the above-mentioned studies achieved
proper results in terms of mechanical properties, i.e., tensile
strength along with proper hardness distribution and metal-
lurgical characteristics. The occurrence of IMCs along with
the creation of a composite structure throughout the stir zone
was inevitable microstructures observed in dissimilar fric-
tion stir welding (DFSW).
Although many attempts have been conducted on DFSW,
based on the available scientific literature, it is quite difficult
to identify common guidelines delivering optimized joint
characteristics. To date, few reviews were done on FSW of
similar base metals [3, 136–143]. Mishra etal. and Nandan
etal. [136, 137] reviewed FSW applications, mainly focus-
ing on the feasibility of FSW for similar materials, e.g., alu-
minum-to-aluminum alloys, leaving dissimilar friction stir
welding out of their investigation. Cam etal. [138] reviewed
capability of FSW in joining dissimilar alloys with the same
base material, including steel with steel, copper with copper,
and aluminum with aluminum alloys. Some review papers
* A. Esmaeili
esmaeili.64@gmail.com; Ali.Esmaeili@qu.edu.qa;
Ali.esmaeili@polimi.it
1 Department ofMechanical andIndustrial Engineering,
College ofEngineering, Qatar University, Doha, Qatar
2 Department ofMechanical Engineering, Politecnico di
Milano, Milan, Italy

446 Metallography, Microstructure, and Analysis (2019) 8:445–461
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were published on dissimilar friction stir welding for spe-
cific applications, including Al/Cu joints [144–146] and Al/
St joints [147, 148]. However, no review paper is available
in the literature comparing the characteristics of different
dissimilar joints fabricated by FSW, specifically deriving
conclusions for the optimization of the method efficiency.
To this aim, this paper reviews DFSW applications to iden-
tify common guidelines for optimizing the metallurgical and
mechanical characteristics of the joint, i.e., tensile strength,
hardness distribution, and formation of IMCs and composite
structures.
Macrostructure andMicrostructure
oftheDissimilar Weldments
Macroscopic investigation of the dissimilar butt friction
stir welding reveals several welding zones, i.e., base met-
als (BS), heat-affected zone (HAZ), thermo-mechanical
affected zone (TMAZ), and nugget zone (NZ) or stir zone
(SZ) (Fig.1). These regions are unique and can be seen in
any dissimilar joints regardless of the type of base materials
used as shown in Fig.1a–e for Al/brass, Ti/St, Al/St, Al/Ti,
and Al/Mg, respectively.
Microstructures of the welding shown in Fig.2 demon-
strate a significant grain refinement in the stir zone as well
as grain elongations in the TMAZ. Severe plastic deforma-
tion arisen from tool action leads to shortening of the grains
within the NZ [27]. Furthermore, the HAZ presents larger
grain size, which is associated with moderate cooling rate.
Occurrence of composite-like structure formed in various
patterns such as onion rings is typical microstructure observed
during DFSW as shown in Fig.3. This lamellar pattern acts
like a reinforcement filler, i.e., a crack barrier, thus resulting
in improved mechanical properties of the stir zone especially
concerning microhardness. In addition, the formation of an
interfacial metallurgical bond is another typical characteris-
tic observed in DFSW. However, this interfacial bond should
be kept to its ideal conditions in order to limit its detrimen-
tal effect on mechanical properties. It is worth mentioning
that conventional fusion welding methods are not taken into
consideration for dissimilar welding due to the formation of
Fig. 1 (a) Al1050-brass. Reprinted from [32], with permission from
Elsevier. (b) CP-Ti-to-304 stainless steel. Reprinted from [98], with
permission from Elsevier. (c) Al 6013-T4-to-X5CrNi18-10 stain-
less steel. Reprinted from [77], with permission from Elsevier. (d)
TiAl6V4-to-aluminum alloy AA2024-T3. Reprinted from [91], with
permission from Elsevier. (e) AZ31B to Al6061. Reprinted from
[110], with permission from Elsevier

447Metallography, Microstructure, and Analysis (2019) 8:445–461
1 3
substantially thick interfacial IMCs arisen from elevated heat
throughout welding [149]. Therefore, it is of importance to
find out the typical properties of IMCs formed in DFSW in
order to draw a conclusion respecting the ideal characteristics
of IMCs which result in proper mechanical properties.
Mechanical andMetallurgical
Characterization
Dissimilar welding fabricated by FSW will be discussed
in detail throughout this section, particularly focusing
on IMCs, composite-like structure, tensile strength, and
hardness distribution. Al/Cu joint is one of the typical
dissimilar joints made by FSW [8–43, 150, 151]. Table1
summarizes the outcomes in various dissimilar weldments
made by FSW. The typical characteristics appeared in
majority of the Al/Cu joints are as follows: First, the stir
zone is composed of dispersed fine particles of harder mate-
rial, i.e., copper particles, within the matrix of aluminum as
softer materials mainly in the form of onion ring as shown
in Fig.3a and b. Second, inhomogeneous hardness distribu-
tion is shown in Fig.4a–e, arisen from the onion ring acting
like crack propagation barriers and intensifying microhard-
ness of the stir zone due to its lamellar pattern. This also
results in inhomogeneous hardness distribution in the NZ
Fig. 2 Microstructure of DFSW: (a–f) Al1350 to pure cop-
per. Reprinted from [28], with permission from Elsevier. (g–l) Al
6013-T4-to-X5CrNi18-10 stainless steel. Reprinted from [77], with
permission from Elsevier. (m–p) Ti–6Al–4V to AISI 304. Reprinted
from [94], with permission from Elsevier. (q–t) AA5754 to AZ31.
Reprinted from [103], with permission from Elsevier

448 Metallography, Microstructure, and Analysis (2019) 8:445–461
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as shown in Fig.4a–e. From Fig.4a–e, it can be concluded
that the peak hardness is observed at the top region rather
than at the bottom and middle regions in butt joint design.
This can be attributed to two reasons: (1) the severe plastic
deformation arisen from the tool shoulder in the vicinity of
the top surface, making the grain size smaller than in those
regions far from the top, and (2) the effect of onion rings
in microhardness enhancement as shown in Fig.4b and c.
Third, as shown in Table1, Al/Cu joints demonstrate ten-
sile strength smaller than tensile strength of the softer base
metal, though some studies showed that tensile strength of
the Al/Cu joint is higher than tensile strength of the softer
base material [22].
Fourth, the formation of IMCs is another common char-
acteristic observed in Al/Cu joints as shown in Fig.5a–c.
Al4Cu9, Al2Cu3, and Al2Cu are typical IMCs formed in Al/
Cu joints. Likewise, the dispersed particles within the stir
zone partially or completely transform into IMCs depending
on their size. In fact, the particles with less than 2µm nomi-
nal dimension transform into IMCs, whereas a tiny layer of
IMCs forms around the particle when the particles dimen-
sion is higher than 2µm, as shown in Fig.5b. As a result,
the interface and dispersed particles within the stir zone
are two possible locations for IMCs formation (Fig.5a–c).
From Fig.5a and c, it can be also noticed that the IMCs
formed at the interface of AL/Cu joint are thin, uniform,
and continuous.
Al/St joint is another typical weldment created by FSW.
Many studies have been carried out on the weldability of
Al/St joints in various configurations [46–55, 57, 59–66,
68–77, 80, 81, 83, 152–162]. Similar to Al/Cu joints, the
formation of IMCs and composite-like structure are obvious
from Fig.5d–f. In addition, two possible locations can be
highlighted for the formation of IMCs, including the inter-
face (Fig.5d and f) and around the steel particles (Fig.5e).
FeAl, FeAl3, Fe2Al5, and FeAl6 are the most common IMCs
formed in Al/steel joints demonstrating thinness, continu-
ity, and uniformity as shown in Fig.5d and f. Moreover,
stir zone of Al/steel joints presents inhomogeneous hardness
distribution, as for Al/Cu joints (Fig.4g and h). Regardless
of the joint configuration, i.e., lap, spot, or butt, the tensile
strength of the Al/St joints was typically lower than tensile
strength of the aluminum as softer base material as shown
in Table1. Finally, the Al/St joints showed a brittle fracture
at the interface, demonstrating low ductility.
Joining of aluminum to titanium is another typical
weldment fabricated via FSW as it has many applications
in industry, specifically for aerospace applications. Similar
distinctive microstructural characteristics are observed in
Al/Ti joints including formation of IMCs, composite struc-
ture, and inhomogeneous hardness distribution along with
proper tensile strength. Figure5j demonstrates the forma-
tion of extremely thin, continuous, and uniform IMCs in
Al/Ti joints. According to Table1, the IMCs are Al3Ti and
Fig. 3 Formation of onion ring within the stir zone, i.e., compos-
ite structure: (a) aluminum to bronze. Reprinted from [23], with per-
mission from Elsevier. (b) Aluminum to brass. Reprinted from [27],
with permission from Elsevier. (c) Aluminum to titanium. Reprinted
from [88], with permission from Elsevier. (d) Aluminum to copper.
Reprinted from [37], with permission from Elsevier. (e) Aluminum
to steel. Reprinted from [74], with permission from Elsevier. (f) Alu-
minum to titanium. Reprinted from [85], with permission from Elsevier

449Metallography, Microstructure, and Analysis (2019) 8:445–461
1 3
Table 1 Dissimilar joints fabricated by FSW: Th: thickness (µm), TS: tensile strength ratio
Row Joint IMCs TSFracture location Refs.
Type Th (µm)
Al/Cu joint
1 AA6061-T6/ pure copper … < 5 80% of Al Interface [9]
2 Al1060/commercial pure copper … < 1.5 80% of Al HAZ [31]
3 5052 aluminum/ pure copper CuAl2, CuAl, Cu9Al4, Cu3Al < 10 60% of Al Interface—IMCs [38]
4 1060 aluminum/ pure copper Al2Cu, Al4Cu9< 2 67% of Al Interface [40]
5 AA6063/ Cu-DHP Al4Cu9, AlCu4< 3.6 86.5% of Al SZ/TMAZ [41]
6 AA6061-T651/ electrolytic copper Al4Cu9, AlCu3< 2 55% of Cu TMAZ [42]
7 Al6061-t6/pure copper-T6 Al2Cu, Al4Cu9< 3.2 59% of Cu … [43]
8 AA 1050/ pure copper Al2Cu, Al4Cu9< 1 71% of Al HAZ [16]
9 Al3003 and pure Cu pipes Al2Cu, < 0.7 89% of Al SZ [18]
10 1050 AA/pure copper Al4Cu9, Al2Cu3, Al2Cu, AlCu < 3 95% of Al NZ /TMAZ [19]
11 AA6063/HCP copper Al4Cu9, Al2Cu, AlCu4, AlCu … 78.6% of Al SZ/TMAZ [21]
12 Al1060/annealed pure copper Al2Cu, Al4Cu9< 1 113% of Al Interface [22]
13 AA5052 /C22000 bronze Al2Cu < 2 64% of Al Aluminum side [23]
14 electrolytic touch pitch copper/AA6061-
T651
CuAl, CuAl2, Cu3Al, Cu9Al4< 3 43% of Cu TMAZ [24]
15 AA6351/pure copper Al4Cu9, AlCu, Al2Cu, Al2Cu3< 1 77% of Al NM [25]
16 AA5083 /pure copper Al2Cu < 1 78% of Cu SZ [26]
17 Al1050H16 /brass Al2Cu, Al4Cu9, CuZn < 2 71% of Al TMAZ/HAZ [27, 32]
18 Pure copper/Al1350 NO NO 79% of Al SZ [28]
19 Al 5A02/pure copper Al4Cu9, Al2Cu3, Al2Cu < 1 76.5% of Al SZ [29]
20 Al1060 /pure copper Al4Cu9, AlCu, Al2Cu … 2709N BM [36]
21 Al1060/pure copper Al2Cu, Al4Cu9< 1 84% of Al HAZ [37]
Al/St joint
22 AA5083/A316L ~Al5Fe2 or Al–Fe < 0.5 0.93% of Al Aluminum side [44, 58]
23 Al 2024/St37 FeAl3< 0.8 85% of St SZ [45]
24 Al6016-T4/ DC04 … … 85% of Al TMAZ [56]
25 CP Al/SS304 FeAl3< 4.8 78% of Al TMAZ of Al [78]
26 Al1100/St37 FeAl, Fe3Al < 14 50% of Al SZ of Al [79]
27 AA6061-T6/AISI304 FeAl, Fe2Al5, AlFe2Cr < 12 64% of Al SZ [82]
28 A3003-H112/SS304 ~Fe3Al < 0.15 54% of Al Aluminum side [47]
29 Al1100/1Cr18Ni9Ti FeAl3< 1 110% of Al Aluminum side [48]
30 AA1100/ A441 AISI steel Fe3Al < 4 90% of Al Aluminum side [49]
31 A6061/ SUS 304 Al8Fe2Si < 0.08 97% of Al SZ [52]
32 Al-5083/St-12 Al5Fe2< 2.6 42% of st Steel side [53]
33 Al-5083 H321/St12 … < 2.3 74% of St Aluminum side [54, 63]
34 AA6061/HIF steel Al13Fe4< 6.4 71.4% of st … [57]
35 Al 5083-H321/316L FeAl3< 3 79% of Al … [60]
36 Al 3003/mild steel Al5Fe2· Al6Fe < 0.8 73% of Al Interface [61]
37 DP600/AA6181-T4 Al Fe2Al5< 0.05 80% of Al BM-HAZ-TMAZ [62]
38 HC260LA/AA6181-T4 Al Fe2Al5< 0.05 77% of Al BM-HAZ-TMAZ [62]
39 Al 6061-T6/TRIP 780/800 steel FeAl, Fe3Al < 1 85% of Al HAZ [64]
40 IRSM42-93 /AA5052 H32 FeAl, Fe2Al5, FeAl3< 2.8 91% of Al Interface—SZ [66]
41 AA5052/HSLA steel FeAl2, FeAl3< 1.6
Most: 0.7
91% of Al Aluminum side [70]
42 Mild steel/A7075-T6 … < 0.1 75% of St Interface [73]
43 Al 5186/mild steel Al5Fe2· Al6Fe < 0.5 90% of Al Interface [74]
44 A5083/SS400 FeAl, FeAl3< 3 86% of Al Interface—SZ [75]
45 Al 6016/IF-steel FeAl3, Fe2Al5, FeAl2< 8 4500N Aluminum side [76]

450 Metallography, Microstructure, and Analysis (2019) 8:445–461
1 3
Al2Ti. Similar properties, i.e., formation of IMCs, lamellar
composite structure, and inhomogeneous hardness distri-
bution, can be seen for other joints, including aluminum-
to-magnesium, steel-to-titanium, and magnesium-to-steel
(Table1). Al3Mg2 and Al12Mg17 are the most frequent
IMCs formed in joining of aluminum-to-magnesium, while
FeTi and Fe2Ti are the most common ones in titanium-to-
steel weldments.
Figure6 depicts correlation between IMCs thickness and
the corresponding tensile strength ratio of the joint. It should
be mentioned that the data presented in Figs.6 and 7 are
extracted from Table1. Tensile strength ratio (TS) is defined
as the ratio of the tensile strength of the weldment (TW) over
the tensile strength of softer base metal (Tb) (Eq1).
(1)
T
s=
T
w
T
b
Table 1 (continued)
Row Joint IMCs TSFracture location Refs.
Type Th (µm)
46 Al 6013-T4/X5CrNi18-10 … … Fatigue :70% of Al … [77]
Al/Ti joint
47 Al2024-T4+All7475-T6/Ti6Al4V ~ TiAl3< 5 119% of Al2024
92% of Al7475
… [84]
48 Ti6Al4V/A6061 dissimilar TiAl3,< 0.5 62% of Al HAZ [85]
49 AA2024-T3/pure Ti TiAl3< 4 71% of Ti Interface [86]
50 Ti–6Al–4V/Al–6Mg TiAl, Ti3Al < 2 92% of Al SZ [88]
51 Al1060/Ti–6Al–4V TiAl3< 2 100% of Al Al side [89]
52 Al–Si alloy /pure titanium TiAl3< 5 62% of Al–Si Interface [90]
53 AA2024-T3/ TiAl6V4 TiAl2< 1 73% of Al Interface [91]
Ti/St joint
54 CP-Ti/SPCC FeTi or FeTi + Fe2Ti < 1 ~ 69% of Ti Titanium side [92]
55 CP-Ti/304 b-Ti (+ x-Ti), Ti2Ni,
FeTi + Fe2Ti, and r-FeCr
< 1 ~ 69% of Ti titanium side [95]
56 CP-Ti/304 TiFe < 1.5 73% of Ti Interface [96]
57 Pure titanium/ structural steel Fe2Ti, FeTi mixed β titanium < 0.2 82% of Ti Titanium side [97]
58 CP-Ti/304 TiFe < 3 73% of Ti Interface [98]
Al/ Mg joint
59 Al6061(T6)/AZ31B Al12Mg17 < 2 68% of Mg … [99]
60 Al-A6061/Mg-Z31B Al12Mg17, Al3 Mg2< 4.5 54% of Mg Interface [100]
61 Al2024-T3/AZ31B-O Al12Mg17, Al3 Mg2< 1.2 44.5% of Mg Interface [118]
62 Al6013/pure magnesium Al3 Mg2< 2 30% of Mg Interface [120]
63 Al6013/pure magnesium Al3 Mg2< 4 50% of Mg Interface [120]
64 AA6022-T4/AM60B, Al12Mg17, Al3 Mg2< 2 3300 SZ [124]
65 AA1100/ AZ31 Al12Mg17, Al3 Mg2< 7 70% of Al … [101]
66 Al6061/AZ31B Al12Mg17, Al3 Mg2< 2 ~ 52% of Mg BS [102]
67 Al6061-T6/AZ31-O …. … 76% of Mg Interface [104]
68 AA6061-T6/Mg Al12Mg17 < 3.5 ~ 67% of Mg Interface [109]
69 Al 6061-T6/ AZ31B Al12Mg17, Al3 Mg2< 8 55% of Mg Interface [110]
70 Al 6061/ AZ31B Al12Mg17, Al3 Mg2< 8 98% of Al–Al Al/Mg layer [112]
71 Al 6061-T6/AZ31B-O Al12Mg17, Al3 Mg2< 3 70% of Mg … [113]
72 A5052P-O/AZ31B-O … ~ < 2 70% of Mg … [114]
73 AC4C/AZ31 Al12Mg17, Al3 Mg2, Mg2Si < 20 27% of Mg Interface [117]
Mg/Ti joint
74 Titanium/ ZK60 … < 1 69% of Mg SZ [129]
St/Cu joint
75 Copper (ETP)/SS304L … … 76% of Cu … [132]
76 Cu/ SS304L … … 79% of Cu HAZ [134]

451Metallography, Microstructure, and Analysis (2019) 8:445–461
1 3
It can be seen that increasing the thickness of IMCs
decreases the final strength, as the trend line in Fig.6 is
descending in response to increasing IMCs thickness. It can
be concluded that the developed interfacial IMCs should be
relatively thin in order to provide proper tensile strength. It
is worthwhile mentioning that IMCs thickness is depend-
ent on the diffusion rate during the welding which itself
associates with welding parameters, particularly rotation
speed and traverse speed [32]. Taking into account different
base metals, Fig.7a and b represents IMCs thickness and
tensile strength for various weldments, respectively. From
Fig.7, it can be pointed out that the appropriate tensile
strength is achieved when the IMCs thickness is less than
3µm. Thickening of the IMCs can be attributed to excessive
heat generation resulting from improper selection of weld-
ing parameters. As a result, optimum welding parameters
account for the formation of appropriate IMCs, thus result-
ing in enhancement of tensile strength of the bond.
Moreover, according to Fig.7a and b, A/Cu, Al/St, and
Al/Ti joints have the highest tensile strength compared
with other dissimilar joints such that IMCs thickness is
between 2 and 3µm. On the other hand, tensile strength
Fig. 4 Hardness distribution: (a–c) Al–brass. Reprinted from [32],
with permission from Elsevier. (d) Al–Cu. Reprinted from [29], with
permission from Elsevier. (e) Al–Cu. Reprinted from [33], with per-
mission from Elsevier. (f) Al–Mg. Reprinted from [104], with per-
mission from Elsevier. (g) Al–St. Reprinted from [77], with permis-
sion from Elsevier. (h) Al–St. Reprinted from [60], with permission
from Elsevier. (i) Al–Ti. Reprinted from [91], with permission from
Elsevier

452 Metallography, Microstructure, and Analysis (2019) 8:445–461
1 3
of the Ti/St joints drops by 11 percent in comparison with
Al/Ti joints, which can be attributed to the formation of
thick IMCs approximately 5µm in Ti/St joint. Therefore,
regardless of the base metals, IMCs in the range of 2–3µm
demonstrate better mechanical properties in terms of ten-
sile strength.
Fig. 5 Formation of IMCs: (a, b) Al–brass. Reprinted from [27], with
permission from Elsevier. (c) Al–Cu. Reprinted from [33], with per-
mission from Elsevier. (d) Al–St. Reprinted from [64], with permis-
sion from Elsevier. (e) Al–St. Reprinted from [74], with permission
from Elsevier. (f) Al–St. Reprinted from [63], with permission from
Elsevier. (g) Al–Mg. Reprinted from [124], with permission from
Elsevier. (h) Al–Mg. Reprinted from [113], with permission from
Elsevier. (i) Al–Mg. Reprinted from [123], with permission from
Elsevier. (j) Al–Ti. Reprinted from [85], with permission from Else-
vier. (k) Ti–St. Reprinted from [96], with permission from Elsevier.
(l) Ti–St. Reprinted from [95], with permission from Elsevier

453Metallography, Microstructure, and Analysis (2019) 8:445–461
1 3
Challenges andFuture Work
As mentioned in the previous sections, FSW can be an effec-
tive way to join dissimilar metals, as the tensile strengths
listed in Table1 were promising. In contrast, majority of
the dissimilar joints in this review showed a brittle fracture
at interface along with a low elongation, as shown in Fig.8
that can be attributed to IMCs formation. In order to assure
safe usage of dissimilar joints for industrial applications, ten-
sile strength and ductility of the weldments should be kept
in appropriate ranges. For example, appropriate toughness
and ductility should be met for some industrial applications
where adequate resistivity against shock–impact loading is
mandatory. In fact, most of the joints listed in Table1 are not
sufficiently strong for such applications. As a result, future
researches should emphasize on simultaneous enhancement
of tensile strength along with ductility and toughness.
In this direction, Zhang etal. [40] slightly improved elon-
gation and tensile strength using new configuration design,
i.e., tooth-shaped joint configuration (TJC), at the interface
as shown in Fig.9. They compared the effect of TJC with
routine butting joint configuration (BJC). From Fig.8c, it
can be pointed out that TJC joint could slightly improve the
ductility of Al/Cu joints, i.e., changing fracture mode from
brittle to ductile and from BJC to TJC joints, respectively.
SEM images of the microstructure and fracture pattern
of BJC and TJC are shown in Fig.10. As it can be seen
from Fig.10b and d, both BJC and TJC present ideal mul-
tilayer IMCs in terms of reduced thickness, uniformity, and
continuity as discussed in the previous section. The whole
thickness of IMCs was 2.1µm and 2.6µm for BJC and TJC,
respectively. Although the total thickness of latter is higher
compared with the former, TJC generated more IMCs sub-
layers, each one characterized by a smaller thickness as
shown in Fig.10b and d. As a result, the formation of thinner
multi-sublayers of IMCs for TJC in comparison with BJC
leads to enhanced ductility of the joint. This enhancement in
tensile strength can be also appreciated from Fig.10e–f and
Fig. 6 IMCs thickness and
tensile strength of various weld-
ments (a) IMCs thickness vs.
weldment, (b) ratio of tensile
strength vs. weldments, and
(c) ratio of tensile strength vs.
IMCs thickness
Fig. 7 (a) Average IMCs thickness—various weldments; (b) average tensile strength—various weldments

454 Metallography, Microstructure, and Analysis (2019) 8:445–461
1 3
g–h demonstrating morphology of the fracture surface of
the BJC and TJC, respectively. Even surfaces and cleavage
patterns (Fig.10e and f) show brittle fracture in the former,
while dimple fracture in Fig.10g and h illustrates ductile
behavior in the latter.
Another approach to improve ductility of the welding
is heat treatment. Joshi etal. [132] successfully enhanced
elongation of the Al/steel joint using gas tungsten arc weld-
ing torch in order to provide additional heating (Fig.11a
and b) whereas tensile strength decreased. In other words,
tensile strength was sacrificed to obtain a more pronounced
elongation, as attributed to the formation of welding defects
resulting from heating-assisted FSW. In another study by
Pourahmad etal. [120], they dramatically improved tensile
strength of the welding in Al/Mg joints by post-weld heat
treatment (Fig.11c). However, increment rate of elonga-
tion in comparison with tensile strength was not noticeable,
due to stress relief caused by heat treatment. In addition,
they also slightly increased elongation. Increasing heating
time resulted in deteriorating mechanical properties due to
thickening of IMCs (ß-phase).
Ultrasonic-assisted FSW was used to improve tensile
strength and ductility of Al/Mg joint [111, 119]. As shown
in Fig.7b, aluminum-to-magnesium joints showed the
lowest tensile strength among dissimilar joints. Simul-
taneous enhancement of tensile strength and elongation
was carried out by ultrasonic-assisted FSW, as shown in
Fig.12. According to their results, ultrasonic-assisted
FSW could successfully break continuous IMCs formed at
the interface into smaller pieces, resulting in a remarkable
enhancement of the tensile strength and elongation of the
weldments. It can be pointed out that tensile strength and
elongation of the ultrasonic-assisted FSW were increased
by 82 and 200 percent, respectively, using ultrasonic-
assisted FSW in comparison with non-assisted FSW.
Although the aforementioned studies could successfully
enhance and improve tensile strength and elongation of the
dissimilar welding, further investigation is still required
in order to extend results to other dissimilar welding such
Fig. 8 Low elongation in various weldments: (a) Al1050/brass.
Reprinted from [27], with permission from Elsevier. (b) Al/bronze.
Reprinted from [23], with permission from Elsevier. (c) Al1060/
pure copper. Reprinted from [40], with permission from Springer. (d)
AA5754 /AZ31. Reprinted from [103], with permission from Else-
vier. (e) AA6181-T4/HC260LA. Reprinted from [62], with permis-
sion from Elsevier. (f) AA6181T4/DP600. Reprinted from [62], with
permission from Elsevier
Fig. 9 Tooth-shaped joint configuration (TJC) in joining aluminum to
copper. Reprinted from [40], with permission from Springer

455Metallography, Microstructure, and Analysis (2019) 8:445–461
1 3
as Al/St and Al/Cu joints. Hence, as gap of the knowl-
edge, further attempts should be carried out on improve-
ment of elongation as well as ductility without sacrificing
other mechanical properties such as tensile strength and
hardness. In other words, DFSW can be safely used for
industrial application once it meets the requirements of
industrial application, including proper tensile strength,
elongation, and hardness.
Conclusions
FSW showed excellent efficiency for joining dissimilar
metals in comparison with other welding methods. Micro-
structure of the weldments showed that two main charac-
teristics mainly appeared in dissimilar friction stir welding:
(1) formation of IMCs through a metallurgical bond and
(2) development of a composite structure within the nugget
zone using a mixed flow of softened materials. Formation
of IMCs was a typical phenomenon in dissimilar friction
stir welding. However, there were some conditions to be
met in order to achieve maximum strength. Thinness, uni-
formity, and continuity were taken into account as the most
important characteristics for achieving proper IMCs. As a
result, IMCs were not always detrimental when they met the
above-mentioned criteria. Regardless of base metals used in
DFSW, the average thickness and tensile strength ratio taken
from all studies reviewed in this paper were 2.84µm and
73 percent of the softer base metal, respectively. Therefore,
the existence of IMC is a critical factor to obtain a sound
joint. Occurrence of a composite structure in the nugget zone
via the distribution of fine particles of harder metals within
the matrix of softer material was another phenomenon in
dissimilar friction stir welding. Inhomogeneous hardness
Fig. 10 (a, b) Microstructure in BJC, (c, d) microstructure in TJC, (e, f) brittle fracture in BJC, (d) ductile fracture in TJC. Reprinted from [40],
with permission from Springer
Fig. 11 Effect of heating-assisted FSW of Al/steel joint: (a, b) ten-
sile strength and elongation, respectively. Reprinted from [132], with
permission from Springer. (c) Effect of post-weld heat treatment on
tensile strength and elongation of Al/Mg joint. Reprinted from [120],
with permission from Elsevier

456 Metallography, Microstructure, and Analysis (2019) 8:445–461
1 3
distribution within the stir zone was ascribed to composite
structures in which the grain size was relatively fine in com-
parison with other welding zones. Likewise, the majority of
the dissimilar joints fabricated by FSW showed lower tensile
strength with respect to the base metals, where interface
and HAZ were the most likely regions for failure initiation.
In the end, brittle fracture or low elongation was not prop-
erly improved in DSFW. Therefore, further attempts require
enhancing ductility of dissimilar weldment along with other
properties, including tensile strength and hardness.
Acknowledgments This publication was made possible by GSRA
Grant No. GSRA2-1-0609-14024 from the Qatar National Research
fund (a member of Qatar foundation). The findings achieved herein
are solely the responsibilities of the authors.
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