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Effect of welding speed on microstructural and mechanical properties of laser lap weld joints in dissimilar Al and Cu sheets

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Abstract

Conventional fusion welding of aluminium and copper dissimilar materials is difficult because of poor weldability arising from the formation of brittle intermetallic compounds on the weld zone as well as different chemical, mechanical and thermal properties of welded joints. Joining of Al and Cu plates or sheets offers a metallurgical challenge due to unavoidable formation of brittle intermetallic compounds. Therefore, it is necessary to effectively suppress the formation and growth of Al–Cu intermetallic compounds. For welding of dissimilar Al and Cu sheets, no systematic work has been conducted to reduce these defects. Thus, this paper focuses on the effect of welding speed on the quality of a lap weld joint in the Al and Cu sheets with a single mode fibre laser. It was found that consequently sound strong weld joints could be produced by suppressing the formation of intermetallic compounds in the interface zone at extremely high speeds.
Effect of welding speed on microstructural and
mechanical properties of laser lap weld joints
in dissimilar Al and Cu sheets
S. J. Lee*, H. Nakamura, Y. Kawahito and S. Katayama
Conventional fusion welding of aluminium and copper dissimilar materials is difficult because of
poor weldability arising from the formation of brittle intermetallic compounds on the weld zone as
well as different chemical, mechanical and thermal properties of welded joints. Joining of Al and
Cu plates or sheets offers a metallurgical challenge due to unavoidable formation of brittle
intermetallic compounds. Therefore, it is necessary to effectively suppress the formation and
growth of Al–Cu intermetallic compounds. For welding of dissimilar Al and Cu sheets, no
systematic work has been conducted to reduce these defects. Thus, this paper focuses on the
effect of welding speed on the quality of a lap weld joint in the Al and Cu sheets with a single
mode fibre laser. It was found that consequently sound strong weld joints could be produced by
suppressing the formation of intermetallic compounds in the interface zone at extremely high
speeds.
Keywords: Aluminium, Copper, Dissimilar welding, Lap welding, Single mode fibre laser, Intermetallic compounds, Weldability, Extremely high welding
speed
Introduction
Many emerging applications in power generation,
chemistry, petrochemical and nuclear plants, aerospace,
transportation and electronics industries require the
joining of dissimilar materials by various joining
methods.
1–6
Aluminium (Al) and copper (Cu) joints
are indispensable in the fabrication of electronic
components and solar collectors due to their unique
performances such as high electric conductivity, heat
conductivity, corrosion resistance and mechanical prop-
erties. Especially, development of dissimilar Al and Cu
welding for rechargeable batteries has been received
great attention. Thus, the joining of Al and Cu is one of
the key fabrication techniques for the manufacture of
industrial components. However, it is difficult to
produce a sound joint by joining these two metals
because of the formation of brittle intermetallic com-
pounds near their interface.
1–3,7–11
Especially, a general fusion joining of Al and Cu has a
metallurgical drawback due to unavoidable formation of
brittle intermetallic compounds. Many researchers have
focused on the production of a stable Al and Cu joint to
overcome the difficulties in joining these two dissimilar
metals. In spite of successes achieved in defect free
dissimilar welding of mentioned systems, previous
investigations showed that friction stir welding of Al
to Cu did not result in the production of sound welds. It
is claimed that there is usually a large void formation,
cracks and other distinct defects throughout the weld.
The direct friction stir welding of Al to Cu was
performed to produce a weld by mixing mechanically,
but the intermetallic compounds of CuAl
2
, CuAl and
Cu
9
Al
4
were formed, and consequently, it was found
that friction stir welding was also difficult due to their
brittle nature.
1–3,7–11
Fusion welding of Al and Cu is
difficult due to unavoidable formation of brittle inter-
metallic compounds. Therefore, it is essential to
suppress the formation and growth of Cu–Al inter-
metallic compounds effectively. No systematic works
have been conducted to reduce these defects of dissimilar
Al and Cu welding under the full penetration lap
welding conditions so far. Furthermore, possibility and
properties of full penetration lap joint were not
sufficiently studied until now. This paper focuses on
the effect of the welding speed on the quality of a lap
joint made with a single mode fibre laser of high energy
density. And the weld and interface shapes and the
amount of upward or downward transport of elements
were observed.
Laser welding has a lot of advantages such as the
achievement of high power density by easy focusability
to a small spot, a precision welding, a high weld quality,
small strains of a weld, a non-contact process and a valid
process for automation, robotisation and systematisa-
tion in comparison with arc welding and resistance
welding.
12,13
As a heat source of materials processing,
laser has been widely applied in the fields of welding,
cutting and cladding. Especially, laser welding provides
some advantages such as high energy density, rapid
Joining and Welding Research Institute (JWRI), Osaka University, 11-1
Mihogaoka, Ibaraki, Osaka 567-0047, Japan
*Corresponding author, sujin@jwri.osaka-u.ac.jp
ß2014 Institute of Materials, Minerals and Mining
Published by Maney on behalf of the Institute
Received 11 July 2013; accepted 31 August 2013
DOI 10.1179/1362171813Y.0000000168 Science and Technology of Welding and Joining 2014 VOL 19 NO 2111
heating and cooling and easy accessibility to the heating
zone. The welding with a high energy/power density laser
can produce a deep penetration weld at high speeds.
14
In
this study, a single mode fibre laser was used as a heat
source. The single mode laser can be widely applied in
various industrial productions such as welding, cutting
and heat treatment. It has high efficiency, reliability and
quality of a laser beam because it can be easily
concentrated to extremely small spot than the others. It
also has a compact size and a long life, and therefore, its
use is expected in various fields. The energy/power density
of a single mode fibre laser is exceedingly high. Thus, it
has magnificent merits in comparison with the others.
In this study, therefore, laser lap welding of Al and Cu
dissimilar sheets were performed using single mode fibre
laser at extreme high welding speed with the objective of
elucidating the effect of welding speed on the microstruc-
tural characteristics of the welds fusion zones and the
mechanical properties of welded joints of Al and Cu sheets.
The mechanical properties of Al and Cu dissimilar weld
joints were evaluated by the tensile shear test. Moreover,
microstructural characteristics and formation phases of the
weld fusion zones in Al and Cu were investigated.
Materials and experimental procedures
The materials used in the experiments are pure copper
and commercially available aluminium A1050. The sizes
of the metal sheets are 0?3 mm thick, 30 mm wide and
60 mm long. Their chemical compositions are over
99?96% pure Cu and about 99?57% Al respectively. Both
metals are described as Cu and Al. The melting points of
pure Cu and Al are about 1358 K (1085uC) and 933 K
(660uC) respectively. Both metals have high light
reflection. The effect of the upper sheet of Cu or Al
was investigated as an important viewpoint of laser
absorption and melting in dissimilar laser lap welding.
Figure 1 shows a schematic representation of the
experimental apparatuses and laser welding situation. The
Cu (upper)–Al (lower) or Al (upper)–Cu (lower) sheets
were fixed to a jig on the high speed stage. A single mode
fibre laser with the maximum power of 2 kW, the
wavelength of 1070 nm and the beam parameter product
(BPP) of 1?05 mm mrad was utilised. The laser beam was
delivered by the optical fibre and focused on the specimen
surface by a lens of 150 mm in focal length. The spot size
of a laser beam was about 20 mm at the focal point. The
laser beam was directly irradiated on the lapped sheets.
The conditions used were the laser power of 1 kW, the
travelling speed of 5 to 50 m min
21
and the defocused
distance of 0 mm. Ar shielding gas of 35 L min
21
was
used to suppress the oxidation of an upper molten surface
during laser welding.
This study was performed with the objective of
proposing an efficient Al and Cu dissimilar full
penetration lap welding by elucidating governing factors
and mechanisms of laser welding. The interface zone of
dissimilar Al and Cu metals was analysed by means of
scanning electron microscopy (SEM), energy dispersive
X-ray spectroscopy (EDX) and X-ray microdiffraction
method (XRD). The strengths of the dissimilar weld
joints were evaluated by the tensile shear test at the
moving speed of 0?1mms
21
.
Results and discussion
Effect of travelling speed on dissimilar welding
results of Cu and Al lap sheets with single mode
fibre laser
The welding of dissimilar Cu and Al metals has been
known to be too difficult to produce sound welded joints
because of the formation of brittles intermetallic
compounds. Full penetration lap welding of Cu and Al
sheets using single mode fibre laser was performed at
extremely high welding speed with the objective of
knowing a possibility of the suppression of intermetallic
compounds formation. Figure 2 shows the stereo
microscope photos of the top and bottom surface
appearances and SEM images of cross-sections of laser
lap welds made in Cu and Al sheets at the laser power
of 1 kW and the welding speed of 10 m min
21
(167 mm s
21
), 20 m min
21
(333 mm s
21
), 30 m min
21
(500 mm s
21
) and 50 m min
21
(833 mm s
21
). Joining of
Al and Cu sheets and vice versa was feasible at the high
speed of 50 m min
21
or slower, and full penetration
welds were produced under all the welding conditions
except the upper sheet of Cu at more than 40 m min
21
.
At 50 m min
21
welding speed, a partial penetration lap
weld was formed for the upper sheet of Cu, while a full
penetration lap weld was partially produced for that of
Al. These are attributed to extremely fast speed of
50 m min
21
and a higher reflection and higher thermal
conductivity of Cu. The top and bottom weld beads
were narrower with an increase in the welding speed,
1 Schematic experimental set-up for lap welding of dissimilar Al and Cu sheets with single mode fibre laser
Lee et al. Effect of welding speed on weldability of laser lap welding of Al–Cu
Science and Technology of Welding and Joining 2014 VOL 19 NO 2112
and accordingly, the areas of the weld fusion zones were
smaller. The weld beads of Al sheets were wider than
those of Cu ones under the same conditions. This may
be due to the lower melting point of Al.
Microstructural characteristics and formation
phases in interface zone
According to the results in Fig. 2, the microstructural
movement of weld fusion zone near the interface these
two combinations is quite different. Since the density of
Cu is 8?96 g cm
23
which is higher more than three times
of Al (2?70 g cm
23
), the liquid Al would not sink into
Cu side in Al (upper)–Cu (lower) combination, though
there are the effect of molten pool movement and
gravity action. In Al (upper)–Cu (lower) mode, the Al
was floating on Cu. However, liquid Cu would easily
sink into Al side in Cu (upper)–Al (lower) combination
due to difference of material properties of Cu and Al.
Thus the fluid flow in the welding pool would be quite
different, and as a result, the formation of intermetallic
compound may differ widely.
The intermetallic phases formed in the laser weld
fusion zones were first detected using micro XRD
method. Figure 3aand bshows micro-XRD results
near the interface fusion zones of laser lap weld joints in
Al (upper)–Cu (lower) sheets at 10 and 50 m min
21
, and
Fig. 3cand dshows XRD results of laser lap weld joints
in Cu (upper)–Al (lower) sheets at 10 and 50 m min
21
respectively. Figure 3ashows peaks from intermetallic
compounds such as CuAl
2
,Cu
9
Al
4
and CuAl in addition
to the ones from Al and Cu base metals. On the other
hand, Fig. 3bindicates high base metal peaks from Al
and Cu and a low peak from CuAl
2
intermetallic
compound. Figure 3cand dshows peaks from inter-
metallic compounds such as CuAl
2
,Cu
9
Al
4
and CuAl in
addition to the ones from Al and Cu base metals. But
Fig. 3chas stronger peaks of intermetallic compounds.
These results suggest that the amount of intermetallic
compounds should be reduced at the higher welding
speed of 50 m min
21
. Figure 3bindicates stronger Cu
base metal and CuAl
2
intermetallic compound peaks
than Fig. 3abecause Cu sink into Al side in Cu (upper)–
Al (lower) combination. On the other hand, stronger Al
base metal peaks is indicated in Fig. 3aAl (upper)–Cu
(lower) combination. Figure 3dindicates peaks from
several intermetallic compounds even though it is weld
2 Photographs of top and bottom surface appearances and SEM cross-sectional photos of dissimilar welds made in Al–
Cu or Cu–Al lap sheets with single mode fibre laser at different welding speeds of 10–50 m min
21
Lee et al. Effect of welding speed on weldability of laser lap welding of Al–Cu
Science and Technology of Welding and Joining 2014 VOL 19 NO 2113
at 50 m min
21
high welding speed because of different
weld fusion phenomena of partial penetration compare
with Fig. 3b.
In order to confirm the difference in microstructural
characteristics of laser welds depending upon the welding
speed, the laser weld fusion zones of Al and Cu lap sheets
were observed by SEM and analysed by EDX in detail.
Figures 4 and 5 show the SEM cross-sectional photos
indicating the EDX analysis spots in the laser lap weld
fusion zones of dissimilar Al (upper)–Cu (lower) metal
sheets produced at the laser power of 1 kW and the
welding speeds of 10 and 50 m min
21
representatively. It
is clearly seen that no cracks were formed.
The EDX analytical results of the areas named a to j
and a9to f9in Figs. 4 and 5 respectively, are given in
Tables 1 and 2. A phase diagram of Al–Cu binary
system for the prediction of the formation phases in the
weld fusion zones is also indicated in Fig. 6, showing
chemical compositions of EDX analytical results of
special points in Tables 1 and 2. The areas a to h in
Fig. 4B and the areas a9to f9in Fig. 5B’ are located near
the weld interface of sheets. In the case of 10 m min
21
,
from the results of EDX analyses in Table 1, CuAl
2
compounds were formed all over the laser weld fusion
zones on the upper side of Al sheet, while the
compounds were hardly formed in the fusion zone on
the lower side of Cu sheet. In particular, rapid
compositional variation from about 10 to 95 at-% was
observed in the weld fusion zone near the interface, and
accordingly, various intermetallic compounds might be
formed. In the case of 50 m min
21
, according to the
results of Fig. 5B and Table 2, Al and Cu solid solutions
were formed on the respective sheet sides, and the
formation zones of intermetallic compounds were only
confined near the interface. The width of intermetallic
compounds was about 5 mm narrow. The formation
zones of intermetallic compounds of Al and Cu was
dramatically decreased comparison with previous inves-
tigations results.
15–18
It is concluded from the SEM observations and EDX
analyses that single mode fibre laser welding could
suppress the formation zones of intermetallic com-
pounds at extremely high travelling speeds. It is
consequently expected that sound strong welds can be
produced at such high speeds.
18
Tensile shear test results of laser lap weld
joints made in dissimilar Al and Cu sheets at
various welding speeds
Similar or dissimilar welding of Cu–Cu, Al–Al, Cu–Al
or Al–Cu lap sheets was performed with a single mode
fibre laser beam at the welding speed of 5–50 m min
21
.
The mechanical properties of all the weld joints were
evaluated by the tensile shear test. Figure 7 indicates the
3 Micro-XRD analytical results of laser weld fusion zone produced in dissimilar Al (upper)–Cu (lower) lap sheets at weld-
ing speeds of a10 m min
21
and b50 m min
21
and Cu (upper)–Al (lower) lap sheets at welding speeds of c10 m min
21
and d50 m min
21
Lee et al. Effect of welding speed on weldability of laser lap welding of Al–Cu
Science and Technology of Welding and Joining 2014 VOL 19 NO 2114
loads in N and the strengths in MPa (N mm
22
)
respectively, of the tensile shear test results of the similar
or dissimilar laser lap weld joints as a function of
welding speed. In Fig. 7b, the cross-sections of fractured
specimens after the test are also shown. The tensile shear
loads of laser welded joints of Cu–Cu and Al–Al similar
sheets decreased from 900 and 580 to 290 and 260 N
with an increase in the welding speed from 5 to
50 m min
21
respectively, except the load of Cu–Cu
sheets at 40 m min
21
. In the case of laser welding of Cu,
a partial penetration lap weld was formed at more than
40 m min
21
because of the higher reflection and higher
thermal conductivity of Cu, and the width of a partial
penetration weld at the joint interface at 40 and
50 m min
21
was slightly wider and extremely narrower
respectively, than a full penetration weld bead at
30 m min
21
. The reduction in the tensile shear load is
generally attributed to the decrease in the weld bead
width at the joint interface. On the other hand, the
tensile shear loads of dissimilar Cu–Al and Al–Cu
sheets were almost constant at about 480–540 N at the
welding speeds of 10–50 m min
21
although the loads of
dissimilar welds were low at about 350 and 380 N at the
speed of 5 m min
21
. Thus, the tensile shear strengths of
dissimilar weld joints of Cu–Al and Al–Cu lap sheets
increased from about 25 and 40 MPa to more than
90 MPa through 160 and 205 MPa with an increase in the
welding speed respectively. It is noted that the tensile
shear loads of dissimilar weld joints were equal to or
higher than those of similar weld joints of Cu–Cu and Al–
Al sheets at the high welding speed of 20 m min
21
or 30–
50 m min
21
. This is attributed to the formation of a small
amount of harder intermetallic compound in the softer
weld fusion zone. The strength of the dissimilar weld joint
was higher at higher welding speed. The strength
properties of weld fusion zones were improved by welding
at high welding speed. The fracture occurred across the
weld fusion zone near the interface.
4 SEM cross-sectional photos of dissimilar weld joint made in Al (upper)–Cu (lower) lap sheets at 10 m min
21
welding
speed, showing analyses locations as marks: a–j in B and C
5 SEM cross-sectional photos of dissimilar weld joint made in Al (upper)–Cu (lower) lap sheets at 50 m min
21
welding
speed, showing analyses locations as marks: a9–f9in B9
Lee et al. Effect of welding speed on weldability of laser lap welding of Al–Cu
Science and Technology of Welding and Joining 2014 VOL 19 NO 2115
Figure 8 shows SEM cross-sectional photos of tensile
shear test specimens produced in Al–Cu and Cu–Al lap
sheets under 5, 10, 40 and 50 m min
21
welding speed
conditions to confirm difference of fracture behaviours
by a change specimen set-up. In the most cases of Al–
Cu and Cu–Al welds the fracture of the specimens
occurred in the weld fusion zone near the interface.
When Al was located on the upper side of Cu during
laser welding, better weldability could be obtained than
that of the opposite Cu–Al combination except in the
case of 40 m min
21
. The cross-sectional photos of
fractured specimens of the tensile shear test show that
the specimen made at 40 m min
21
welding speed was
fractured in weld fusion zone near the interface, and
Cu–Al had wider fracture surface because of similar
case of anchor effect due to proper mixture zone.
Workpiece of Cu–Al dissimilar welds at 10 m min
21
was fractured at Al base metal nearby weld zones; the
weld joints was strongly jointed with enough welds size
without fracture from intermetallic compounds zone. The
tensile shear test specimens at 5 m min
21
were fractured
from overall of weld fusion zone in SEM cross-sectional
photos; especially, Cu–Al was easily fractured and had
larger weld fusion zone. The different mixture phenomena
on weld parts were observed because Cu element was
fully distributed to Al sheet, but Al element was not
fully distributed to Cu sheet due to their material
properties. It is confirmed that the set-up of sheets and
kind of metals were important factors of dissimilar
welding phenomenon. And it demonstrates the diffi-
culty in welding of dissimilar materials due to the
formation of brittle intermetallic compounds.
The specimens of Al (upper)–Cu (lower) joints made at
the welding speed of 10 and 50 m min
21
after the tensile
shear test are shown as photos (b) and (f) in Fig. 7b, and
are also representatively exhibited to understand the
effect of welding speed on microstructural behaviour of
weld fusion zone at higher magnification in Fig. 9. Phases
of nine points on the cross-sections of the fractured
specimens were analysed using EDX method. In the
specimen made at 10 m min
21
, the EDX analysis results
show that ‘1’ and ‘2’ near the fractured location were
regarded as CuAl and CuAl
2
intermetallic compounds,
because they contained about 50 at-%Al and 50 at-%Cu,
and about 70 at-%Al and 30 at-%Cu respectively. Also,
Table 1 EDX analytical results of marked points: a to j in
Fig. 4B and C, in laser weld fusion zone of
dissimilar Al (upper)–Cu (lower) lap sheets
produced at 10 m min
21
welding speed
Atomic percentage
PhaseAl/at-% Cu/at-%
a10?489?6 Cu (solid solution)
b41?758?4Cu
11
Al
9
zCu
33
Al
17
c52?247?8 CuAlzCuAl
2
d70?329?7 CuAl
2
zAl
e79?820?2 CuAl
2
zAl
f85?414?6AlzCuAl
2
g91?78?3AlzCuAl
2
h94?35?7AlzCuAl
2
i83?116?9 CuAl
2
zAl
j93?26?8AlzCuAl
2
Table 2 EDX analytical results (compositional ratios of Al
and Cu) of marked points: a9to f9in Fig. 5B, in
laser weld fusion zone of dissimilar Al (upper)–
Cu (lower) lap sheets produced at 50 m min
21
welding speed
Atomic percentage
PhaseAl/at-% Cu/at-%
a92?997?1 Cu (solid solution)
b945?954?1Cu
11
Al
9
zCu
33
Al
17
c958?641?4 CuAl
2
zAl
d981?019?0 CuAl
2
zAl
e982?317?7AlzCuAl
2
f998?21?8Al
6 Phase diagram of Al–Cu binary system, showing locations of chemical compositions of a to h and a9to f9in Tables 1 and 2
Lee et al. Effect of welding speed on weldability of laser lap welding of Al–Cu
Science and Technology of Welding and Joining 2014 VOL 19 NO 2116
‘3’, ‘4’ and ‘5’ were judged to be a mixture of CuAl
2
and
CuAl (containing 60 at-%Al and 40 at-%Cu), CuAl
and CuAl
2
compounds respectively. Similarly, ‘6’, ‘7’, ‘8’
and ‘9’ of the specimen at 50 m min
21
may be CuAl,
AlzCuAl
2
(containing 90 at-%Al and 10 at-%Cu), CuAl
and AlzCuAl
2
respectively. It is clearly understood that
the fracture during tensile shear testing occurred in the
intermetallic compounds near the fusion zone interface. In
the case of higher welding speed, the area of intermetallic
compounds decreased, while that of Al increased.
Consequently, the reason for the improvement in
mechanical properties at higher welding speeds was
attributed to the decrease in the intermetallic com-
pounds and the increase in Al solid solution near the
weld interface.
Conclusions
Lap welding of dissimilar Al and Cu sheets using single
mode fibre laser of very high energy density was carried
out at various welding speeds with the objectives of
confirming a possibility of welding of such dissimilar
metals and elucidating the microstructural characteris-
tics of weld fusion zones. According to EDX and micro-
XRD analytical results, intermetallic compounds were
easily formed in the wider areas in the weld fusion zones
produced at 1 kW and 10 m min
21
, while they were
extremely reduced at the higher welding speed of
50 m min
21
. The width of intermetallic compounds
was dramatically decreased to about 5 mm narrow.
According to the results of the tensile shear test, the
loads and strengths of dissimilar Al (upper)–Cu (lower)
and Cu (upper)–Al (lower) weld joints were almost equal
in most cases except for the welding speed of
50 m min
21
, the loads of dissimilar weld joints were
higher than Al–Al similar ones and almost equivalent to
Cu–Cu similar one at the speed of more than 10 and
20 m min
21
respectively, and the tensile shear strength
of a weld metal increased with increasing welding speed.
It was therefore confirmed that sound, strong laser
welded joints could be produced at the extremely high
welding speed by suppressing the formation of inter-
metallic compounds.
Acknowledgements
This work was conducted as a part of A-STEP ‘High-
quality and high-efficiency processing technology of
titanium alloy using a high brightness laser process
control method’.
7 Results of tensile test for similar and dissimilar weld joints made with single mode fibre laser, showing effects of
welding speed on atensile shear load and btensile shear strength
8 SEM cross-sectional photos of tensile shear test specimens of dissimilar weld joints produced in Al–Cu and Cu–Al lap
sheets at 1 kW under 5–50 m min
21
welding speed conditions
Lee et al. Effect of welding speed on weldability of laser lap welding of Al–Cu
Science and Technology of Welding and Joining 2014 VOL 19 NO 2117
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afracture side of Al side at 10 m min
21
;bfracture side of Cu side at 10 m min
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;cfracture side of Al side at
50 m min
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;dfracture side of Cu side at 50 m min
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9 SEM cross-sectional photos of tensile shear test specimens of dissimilar weld joints produced in Al–Cu lap sheets at
1 kW and 10 or 50 m min
21
, showing EDX analytical points
Lee et al. Effect of welding speed on weldability of laser lap welding of Al–Cu
Science and Technology of Welding and Joining 2014 VOL 19 NO 2118
... This increased the IMCs formation due to the uneven melting within the fusion zone and led to IMCs such as Al 2 Cu and Al 3 Cu 4 in the weld as shown in the SEM micrographs in Fig. 13b (i-iii). Similar studies have suggested that higher welding speeds can decrease the IMCs due to rapid cooling and by controlling excessive melting of Al and Cu Ma et al., 2023;Lee et al., 2014). However, in this study, the irregular molten flow at the rear part of the weld resulted in uneven cooling which, while freezing the formation of IMCs, led to pores and cracks compromising the strength of the joint. ...
... At lower S the higher heat input increases the melting and evaporation of bottom Al busbar. The molten Al tries to escape away from the top of the fusion zone as observed in Fig. 12b, increasing the intermixing and hence the formation of IMCs during welding Lee et al., 2014). Table 5). ...
... On the other hand, researchers have studied the impact of welding parameters and advanced laser welding techniques on the improvement of joint performance. Yan and Shi [17] reported that both reduced laser power and greater welding speed can reduce the mixing of Al and Cu. Lee et al. [18] proposed that sound and strong laser welded joints can be produced at a extremely high welding speed by suppressing the formation of Al-Cu IMCs. Shin et al. [19] reported a reduced mechanical strength and increasing electrical resistance in the laser welded joint with promoted formation of Al-Cu IMCs. ...
... It was suggested by Zhou et al. [5] that the brittle IMCs can introduce the local stress concentration during the tensile lap shear test, accelerating the failure of weld. Furthermore, it was determined by Yan and Shi [17] and Lee et al. [18] that a thicker layer of IMC can further degrade the joint strength. As evidenced in Fig. 11 (g)-(i), the failure sites in weld with BSS = 0 and 1 are associated with the presence of dense distributed and thicker layers of Cu 9 Al 4 /Cu 3 Al, which leads to a greater extent of the stress concentration as compared to the weld with BSS = 0.6, where the morphology of IMCs is characterized with thin and dispersed serial layers. ...
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Al-Cu dissimilar laser welding has gained substantial attention in the electric vehicle battery manufacturing, while challenged by the formation of brittle Al-Cu intermetallic compounds (IMCs). This study investigated the impact of beam shaping technology on the characteristics of Al-Cu dissimilar laser welds by using a coaxial core and ring dual beam laser system. The investigation assessed multi-level weld characteristics including weld geometry, IMC formation, mechanical and electrical properties. Results showed that the dual laser beam can effectively avoid imperfections observed with single beam, such as full penetration related to core-only beam and surface discontinuity defects associated with ring-only beam, respectively. For the core-only weld, extensive upward migration of Cu promoted the formation of brittle CuAl 2 and Cu 9 Al 4 /Cu 3 Al 2 , leading to a considerable reduction in joint strength and a sharp increase in the electrical resistance. For the ring-only weld, the insufficient bonding area between Al and Cu sheet, leads to a significant degradation of joint strength, although a minimal electrical resistance was identified. An optimized power splitting was determined at 40% for the core beam and 60% for the ring beam, which allows the minimal formation of Al-Cu IMCs and consequently, the improved mechanical strength and reduced electrical resistance.
... Thus, during cyclic loading, cracks are easily deflected into the formed IMC interface, leading to the early failure of the bonds due to their brittle nature. Closer to a battery assembly scenario in [36], the authors showed that IMC formation can be suppressed by increasing the LW speed of Al-Cu sheets, resulting in mechanically stronger welds. In addition, in LW, spatial beam oscillation, weld shape, and the number of welds also seem to affect the maximum mechanical breaking load [24]. ...
... This conclusion was also reached in [28], where the authors found that a thin intermetallic layer ensures low values of electrical resistance and high mechanical strength by studying the LW of Al on Cu sheets. Increased welding speed has been identified as the factor that suppresses the formation of IMCs, which cause high electrical resistivity, reduced mechanical strength, and high brittleness [36]. A more recent and unusual study is presented in [69]. ...
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Electric vehicles’ batteries, referred to as Battery Packs (BPs), are composed of interconnected battery cells and modules. The utilisation of different materials, configurations, and welding processes forms a plethora of different applications. This level of diversity along with the low maturity of welding designs and the lack of standardisation result in great variations in the mechanical and electrical quality of the joints. Moreover, the high-volume production requirements, meaning the high number of joints per module/BP, increase the absolute number of defects. The first part of this study focuses on associating the challenges of welding application in battery assembly with the key performance indicators of the joints. The second part reviews the existing methods for quality assurance which concerns the joining of battery cells and busbars. Additionally, the second part of this paper identifies the general trends and the research gaps for the most widely adopted welding methods in this domain, while it renders the future directions.
... However, porosity formation in laser welded aluminium alloy affects its tensile strength and fatigue resistance. In addition, the high reflectivity of aluminium alloys reduces the laser energy absorption, which potentially affects the weld quality [6][7][8]. Vänskä et al. [9] showed the effect of beam oscillation (BO) in laser welding of 5-mm-thick stainless-steel tubes. They used an oscillated beam to remove the small gaps and misalignment in the welding, though tiny pores were found afterwards. ...
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The present study evaluates the effect of beam oscillation on the mechanical and electrochemical properties of electron beam welded commercially pure aluminium. The circular beam oscillation diameters of 1 mm and 2 mm have been used while keeping all the other welding parameters constant. The churning effect of beam oscillation led to the formation of a broader fusion zone compared to static beam joint. The fusion zone of a static beam weld consists of equiaxed and columnar structures, while the fusion zone of an oscillated beam weld mainly consists of equiaxed structures. The joints produced using beam oscillation have less porosity (0.01%) than static beam joint (0.02%). Also, the pores were more evenly distributed in the oscillated beam joints. The application of circular beam oscillation of diameters 1 mm and 2 mm increased the microhardness (56 VHN and 58 VHN) as compared to the static beam joint (52 VHN) and base metal (45 VHN). The tensile strength of aluminium (102 MPa) decreased slightly after electron beam welding (99 to 91 MPa). Beam oscillation reduced the tensile strength further (91 MPa and 93 MPa) as compared to the static beam joint (99 MPa), whereas the percentage elongation increased (9 to 17%) due to beam oscillation. Beam oscillation has reduced the corrosion rate from 0.02 mm/year (base metal) to 0.001 mm/year (oscillated beam weld). The mechanism of variation in mechanical and electrochemical properties of electron beam welded aluminium with the application of beam oscillation has been established.
... Understanding the induced changes in the activity coefficient in the liquid metal phase in the absence of conventional explanations points to the possibility that the laser irradiation may be significantly altering the electronic states, and perhaps the entropy, directly. In Abere et al [35], we have corroborating evidence from the solidified non-equilibrium structure not observed at the Cu/Al interface in IR laser welds [36] [37] that complements our findings for the oxide. Our explanation for the observed behavior introduces several important questions that will need to be answered before we may substantiate this theory. ...
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