Microstructure and mechanical properties of cold metal transfer welded aluminium/dual phase steel

Abstract

In this work, an attempt was made to join A6061-T6 aluminium alloy to Dual Phase 800 steel using AlSiMn filler by cold metal transfer (CMT) processes. The effect of process parameters on the microstructures and tensile strength of the joints are analysed. Microstructural analysis revealed the formation of the Fe–Al intermetallic (IM) layer. The IM layer thickness was found to be very small. Electron microscopy analysis revealed the presence of long plate shape (Al5FeSi) and fragmented particles in the IM layer. At the interface, ⊖-FeAl3 and η-Fe2Al5 phases were formed. Pulsed process resulted in better joint strength when compared to conventional CMT method. X-ray diffraction study revealed the formation of the Fe3Al and η-Fe2Al5 phases in the weld. The welding currents and arc length correction factor had significant effects on the joint strength. Tensile failure occurred at the heat affected zone.

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Microstructure and mechanical properties of cold
metal transfer welded aluminium/dual phase steel
S. Madhavan, M. Kamaraj & L. Vijayaraghavan
To cite this article: S. Madhavan, M. Kamaraj & L. Vijayaraghavan (2016) Microstructure and
mechanical properties of cold metal transfer welded aluminium/dual phase steel, Science and
Technology of Welding and Joining, 21:3, 194-200, DOI: 10.1179/1362171815Y.0000000082
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Microstructure and mechanical properties
of cold metal transfer welded aluminium/dual
phase steel
S. Madhavan*
1,2
, M. Kamaraj
2
and L. Vijayaraghavan
1
In this work, an attempt was made to join A6061-T6 aluminium alloy to Dual Phase 800 steel using
AlSiMn filler by cold metal transfer (CMT) processes. The effect of process parameters on the
microstructures and tensile strength of the joints are analysed. Microstructural analysis revealed
the formation of the Fe–Al intermetallic (IM) layer. The IM layer thickness was found to be very
small. Electron microscopy analysis revealed the presence of long plate shape (Al
5
FeSi) and
fragmented particles in the IM layer. At the interface, fi-FeAl
3
and g-Fe
2
Al
5
phases were formed.
Pulsed process resulted in better joint strength when compared to conventional CMT method.
X-ray diffraction study revealed the formation of the Fe
3
Al and g-Fe
2
Al
5
phases in the weld. The
welding currents and arc length correction factor had significant effects on the joint strength.
Tensile failure occurred at the heat affected zone.
Keywords: Aluminium–steel joining, Microstructure, Intermetallic layer, Tensile strength, Failure
Introduction
Metal joining is a solution for multimaterial product
design.Dissimilar metal joining provides significant
advantage to the whole structure with exceptional
mechanical strength. In spite of incorporating compo-
sites in the automotive structure for achieving light-
weight requirements, steel continues to be the primary
component.
1
As a result, tailored welded blanks com-
bining steel and aluminium alloys are still very advan-
tageous for enhancing functional performance and
lightweight engineering. Fusion joining of aluminium to
steel is impeded due to huge difference in their melting
point, coefficient of thermal expansion, chemical com-
position, zero solubility of Fe in Al and formation of
brittle intermetallic (IM) layer.
2
Because of these diffi-
culties, aluminium and steel are joined by
electromagnetic pulse crimping,
3
friction welding,
4
dif-
fusion bonding,
5
laser keyhole,
6
vacuum brazing
7
and
resistance spot welding.
8
Reports suggest that alu-
minium rich IMs are very brittle compared to iron rich
IMs, and if the IM layer thickness is between 5 and
10 mm, good mechanical strength can be obtained.
9
So
fusion welding method with reduced heat input and
short circuiting metal transfer may be the answer to this
issue. A recent development is cold metal transfer
(CMT) welding, which is a modified metal inert
gas welding and is preferable to join aluminium and
steel owing to spatter free ignition, short circuit transfer
and reduced heat input.
In previous studies, galva annealed, galvanised and
cold rolled steel sheets (0.7 mm) were joined to Al
6
k21-
T4 (1.4 mm) alloy using Al4043 filler.
10
Galva annealed
steel was not able to make a sound joint even by utilising
all the welding parameters used within the range. Even
cold rolled steel showed the same result as galva
annealed steel. It was possible to produce a sound joint
in hot dip galvanised sheet. The tensile shear strength
was found to be very low when welded with Al4043
filler. Engineering gap between the sheets had significant
effect on the weld strength and prevented weld metal
porosity. Cold metal transfer waveform produces an
increase in the short circuit frequency and decrease in
the drop weight deposited during each short circuit. This
waveform is best suited to join aluminium to steel
because the Fe–Al IM layer thickness is reduced.
11
A prior study led to the statement that *10 mm is the
scientific thickness limit for the IM layer. However, these
investigations were based only on interfaces with a Cu
interlayer between aluminium and steel and should not
be taken for granted for every system.
12
By varying the
welding parameters, Al/steel joints were tested in which
the IM layer thickness was varied. The IM phase present
at the interface was claimed to be Al
13
Fe
4
based on
energy dispersive X-ray (EDS) measurements, but no
relationship between the joint strength and the IM layer
thickness was established.
13
Laser beam joining of steel
to aluminium sheets by heat conduction produces IM
layer v10 mm but makes use of flux and steel sheet
thickness up to 1.5 mm to ensure heat transfer, high
requirements on flatness and restrictions in size of the
sheets to be joined, and high investment costs makes it
techno-economically unviable.
14
A prior literature also
1
Manufacturing Engineering Section, Department of Mechanical
Engineering, Indian Institute of Technology Madras, Chennai-600036,
Tamil Nadu, India
2
Department of Metallurgical and Materials Engineering, Indian Institute
of Technology Madras, Chennai-600036, Tamil Nadu, India
*Corresponding author, email:::::: k
Ñ2016 Inst itute of Materi als, Minerals a nd Mining
Received 18 June 2015; accepted 30 July 2015
DOI 10.1179/1362171815Y.0000000082
194
Published by Taylor & Francis on behalf of the Institute
kamara j@iitm.ac.in
j
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suggests that welding speed has an influence on the IM
layer thickness.
23
Intially, the thickness reduces and
gradually increases with increase in speed. So an opti-
mum range of speed has to be maintained to get a
minimum compact interface.
Keeping in view the above facts, the present investi-
gation is aimed at studying the effect of different process
parameters (welding current, welding speed, arc length
correction factor, wire feedrate starting and ending
current) in making dissimilar aluminium/steel lap joints
with CMT and pulsed CMT (P-CMT) processes on the
tensile strength of the joints, microstructure of weld
nugget, IM layer formed and phase formation at the
interface. Especially, the IM layer compounds were cri-
tically analysed by scanning electron microscopy (SEM),
X-ray diffraction (XRD) and high resolution trans-
mission electron microscopy, and the fracture behaviour
of the joints are also discussed.
Materials and experimental techniques
Materials
Aluminium alloy A6061-T6 and galvanised dual phase
(DP) steel DOGOL DP800 sheets with thickness of 2.0
and1.6 mm were joined using AlSi3Mn wire with a
diameter of 1.2 mm were chosen as the filler metal. The
chemical composition of the base materials and filler
metal is given in Table 1.
The experiments were carried out based on L
9
orthogonal array and were divided into two groups. For
the sake of clarity, process parameters that yielded low,
medium and high values of tensile strength are alone
reported. In the first group, aluminium and DP steel
sheets are joined by the CMT process. In the second
group, the sheets were joined by the P-CMT process.
Experimental
The dimensions of the plates were 40 mm|200 mm. The
oxide film at the welding location was removed before
welding using wire brush and cleaning with acetone.
Joints were produced by CMT and P-CMT welding
processes in lap configuration, with aluminium sheet
placed over the DP steel. Welding parameters of both the
processes are shown in Tables 2 and 3. Shielding gas used
in this experiment was commercially pure argon (purity of
99.9%), and the gas flowrate was 18 L min
21
.Bothpro-
cesses were carried out with a starting I
S
and ending
current I
E
of 150 and 50% of the operating current
respectively. The heat input is calculated by considering
the welding currents, voltage and welding speed.
Metallographic specimens of weld cross-section were
cut, and the specimens were polished according to
standard metallographic procedures. A solution (5 mL
HF +3 mL HNO
3
+92 mL ethanol) was used to etch the
metallographic specimens to reveal the general micro-
structure. The etchant used in the current study was not
a standard one, but optimised after extensive trials to
suitably reveal the dissimilar weld microstructure.
The metallographic specimens were observed under
optical microscopy and scanning electron microscopy
(SEM). A slice was also cut from the weld nugget and
IM layer, and then reduced by ion beam milling to ob-
serve in a transmission electron microscopy (TEM) with
EDS. The microhardness distribution was measured
with Wilson Wolpert microhardness tester with a load of
0.5 kg at regular interval of 15 s. Lap shear tensile test
samples were also cut out from the resulted Fe–Al joints.
The specimens were tested by INSTRON 3367 30 kN
testing machine with a constant loading rate of 0.5 mm
min
21
.
Results and discussion
Radiographic qualification
Full exposure of the weld bead by
192
Ir c-ray using
ASTM NO: 10 penetrameter shows the sensitivity cal-
culated as per 1 T, which is clearly visible. The radi-
ography shows very fine discontinuous gas porosities to
the tune of 50 mm with intermittent distance of 100 mm.
No spatter or weld discontinuity is observed. The
radiographic quality as per ASTM reference radiograph
for gas porosity is level I. Figure 1 shows the radio-
graphic image of the aluminium–steel weld produced by
the CMT process.
Macrostructure and microstructure
Figure 2aand bshows appearances of the joints between
aluminium and DP800 steel sheets made by CMT and
P-CMT processes. The spreading of molten aluminium
on steel surface was limited in the case of CMT process
with some porosity. The P-CMT process produces
aesthetic and sound welds. It may be due to high welding
speed and reduced heat input of the process. According
to the microstructural characteristics, the welds can be
divided into the nugget (fusion zone) and interfacial
reaction layer. Microstructural comparison for both the
processes is made for the samples with the highest value
of tensile strength.
Figure 3 shows the optical microstructure for the
Al–Steel welds produced by the CMT and P-CMT
processes. It is evident from Fig. 3athat there is clean
interface between the Al base metal and the nugget with
minimal diffusion. Weld nugget zone microstructure of
the aluminium–steel weld produced by the CMT process
has an obvious solidification character, while the DP800
steel still remains unaffected by low heat input during
welding. The weld nugget shows dendritic coring of
Al–Si towards the cooler part of the process. The den-
drites are finer between the primary arms. Interfacial
reaction layer is formed and can be seen in Fig. 3c. The
nugget at the interfaces shows a sharp fine needle-like
features. Probably, the constituents of the weld,
particularly Al, Si and Mn, could have diffused into the
zone. The microstructural changes are seen both at
th nugget side and at the steel side. This indicates the
mutual fusion of the constituents.
Table 1 Chemical composition of A6061-T6, DP800 steel and AlSi3Mn wire/wt-%
Alloys Al Fe Mg Si C Cr Ti Nb Ni Mn Cu Zn
A6061-T6 97.4 0.45 0.8 0.6 ... 0.20 0.10 ... ... ... 0.25 0.2
DP800 steel ... 98.13 ... .20 0.14 ... 0.011 0.019 0.028 1.47 ... ...
AlSi3Mn filler 95.6 0.22 0.15 3.2 ... ... 0.05 ... ... 0.72 ... ...
195
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In the case of the P-CMT process (Fig. 3d), the micro-
structure at the interface shows least formation of the IM
layer can be the observed. The interface between the Al
matrix and the nugget shows fusion without any distinct
layer. This has been due to insufficient time for the growth
of IM layer during welding. The nugget shows spheroidal
partially recrystallised zone instead of dendritic pattern of
grains (Fig. 3e), which might be due to the heat input of the
process. The low heat input of pulsed nature of CMT
process causes insufficient time for enough melting; to form
dendrites, low volume of weld pool and also fast freezing of
pool result in spherical dendritic structure. This sort of
solidified structure generally results in higher tensile
strength. Further, the P-CMT process offers less scope for
mutual diffusion of the constituents.
A uniform weld nugget can be seen from Fig. 4a, with
Si enriching the grain boundary.
15
Secondary phase
particles are also present. These particles have the same
crystal structure as a-AlFeSi particles but contain Mn.
Figure 4bshows IM b9-Mg
2
Si particles present along
with coarse Si particles and elliptical fi-FeAl
3
. The peak
strength is usually associated with b9particles. Trape-
zoidal, faceted
16
and single crystal fi-Fe
2
Al
5
is also
present distinctly in the weld metal. Selected area elec-
tron diffraction of these particles results in an imperfect
ring pattern because the volume fraction and size of the
particles are small and are not sufficient to give a com-
plete ring pattern. The TEM-EDS analyses of these fine
particles show that they contain Al, Si and Fe as shown
in Fig. 4c. Presence of the Fe
3
Al and g-Fe
2
Al
5
phases is
evident from the XRD analysis as shown in Fig. 4d.
Characterisation of IM compound layer
Figure 5ashows the SEM image of the IM compound
layer of the joints. The interface between the IM layer and
steel is relatively flat because the IM layer formation
originates from the steel side to the weld nugget, whereas
the interface between aluminium and the IM layer is
wavy. In order to clarify the complete structure of the IM
layer, the joint interface was also observed with TEM.
Thickness of the IM layer varied from 1.49 to 3 mm
depending on the process parameters used in the exper-
iments (Fig. 5b). From the analysis of selected area elec-
tron diffraction patterns, it is found that the interfacial
layer is composed of the AlFeSi phase. The presence of Si
reduces the IM layer thickness by dissolving into them
and prevents cracking of the weldment. The absence of
Mg in the filler wire also prevented hot cracking.
17
The
EDS traces also suggest that some amount of interdiffu-
sion occurred during the welding process. Figure 5c
shows the typical image of long (plate-like) Al
5
FeSi phase
with fragmented fi-FeAl
3
(monoclinic) and adjacent
g-Fe
2
Al
5
(orthorhombic) (trapezoidal) particles taken in
the seam region close to the joint interface (Fig. 5d). It
also appears as FeAl
3
hasformedonFe
2
Al
5
layer where
Fe
2
Al
5
is identified by tooth-like morphology in SEM.
This morphology is due to anisotropic diffusion, which
results in epitaxial growth of Fe
2
Al
5
into the steel sub-
strate. It can be understood that Fe
2
Al
5
has formed first
during the chemical reaction because of its wavy interface
over FeAl
3
.Furthermore,Fe
2
Al
5
possesses the lowest free
energy of formation compared to FeAl
3
, and to conclude,
according to the Pretorius model of effective heat devel-
opment, Fe
2
Al
5
is formed.
17
There is good agreement for
the above mentioned phenomena, as several FeAl
3
grains
are linked to a single Fe
2
Al
5
crystal. So, it can be
established that fi-FeAl
3
and g-Fe
2
Al
5
are present in the
IM layer.
Table 2 Welding parameters of CMT process
Current/A Speed/cm min
21
Arc length
correction
Wire
feedrate/m min
21
70 36.85 0.0 4
75 36.85 15.0 4.2
80 34.30 10.0 4.7
Table 3 Welding parameters of P-CMT process
Current/A Speed/cm min
21
Arc length
correction
Wire
feedrate/m min
21
55 34.30 15.0 2.7
60 31.75 15.0 3.0
65 31.75 20.0 3.2
1 Radiograph of lap welded aluminium–steel joint produced
by CMT process
2aimage (SEM) of weld produced by CMT process; bP-CMT process
Madhavan Microstructure and mechanical properties of col d metal transfer welded aluminium/dual phase steel
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Mechanical properties
The Vickers micorhardness measured at different
locations on the sample is shown in Fig. 6. The
Aluminium/weld interface shows slight reduction
hardness due to the coarsening of precipitates. There
was increase in hardness at weld nugget due to existence
of alloying elements in solution condition. Further, the
interface near the steel side shows higher hardness due to
a,dAl–heat affected zone (HAZ); b,eweld nugget; c,fnugget/steel interface
3 Optical microstructure of CMT and P-CMT welded Al–steel sheets
4aweld nugget with Si enrichment along grain boundaries; bIM b9-Mg
2
Si particles are present along with coarse Si particles
and elliptical fi-FeAl
3
;cEDS pattern showing presence of AlSiFe and Mn; dXRD analysis revealing presence of Fe
3
Al and g-
Fe
2
Al
5
phase
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existence of FeAl
3
and also Mg
2
Si. This is due to dif-
fusion of alloying elements and formation of IM com-
pounds. The comparison between the strengthening
agents, namely Mg
2
Si, in parent metal is higher than at
the interface; hence, the interface hardness is lower.
Similarly, the hardness at the nugget interface is also
lower. Whereas in the case of the P-CMT process, the
nugget shows spheroidal non-dendritic grains. This is
due to reduced welding current and rapid cooling.
The tensile shear strength values of aluminium–steel
joints obtained with different welding currents and
processes are listed in Fig. 7. Since the geometry of the
tensile samples is not identical due to different process
conditions, hence the tensile strength of joints are
presented in N mm
21
(peak load divided by breadth).
21
The joint made with the P-CMT process has the highest
tensile strength of 320 N mm
21
. In the P-CMT process,
welding current is lower than CMT, so a small amount
of wrought aluminium base metal fuses and the dissol-
ution of strengthening phase in the nugget zone is also
reduced. The nugget softening primarily determines the
joint tensile strength, which is obtained by longer arc
lengths. Whereas in the CMT process when the welding
current is raised, grains in the nugget zone near the HAZ
5aimage (SEM) of intermetallic reaction layer; bTEM image of IM layer; cTEM image showing interfacial IM compound
Al
5
FeSi phase and elliptical particles; dTEM image of trapezoidal, faceted single crystal of g-Fe
2
Al
5
6 Distribution of hardness in cross-sections for A6061-T6-
DP800 steel welds produced by CMT and P-CMT process
7 Tensile shear strength for CMT and P-CMT welded joints
for various welding current
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gets coarsened and joint strength gets reduced as
observed from Fig. 7. So, the welding current had sig-
nificant effect on the tensile strength of joints; hence, the
welding current has to be optimised not only to inhibit
the formation of brittle IM compound but also to
improve the microstructure and improve the joint
strength. In both cases, fracture occurred at the
Al/nugget interface. In both the processes, starting cur-
rent I
s
rapidly heated the base metal, and the ending
current I
E
prevented local overheating. However, with
the increase in the wire feeder speed, the heat input
increases leading to greater softening of the HAZ of Al
and results in reduced tensile strength of the joints.
The tendency of tensile strength to increase with
decreasing IM layer thickness can be clearly observed.
If the IM compund layer thickness is *8mm, then
microcracks are generated inside the layer, but they do
not induce fracture.
19
In this work, the IM layer is only
1.5 mm, at any point of time the tensile sample does not
fracture at the IM layer and the mechanical property of
joint is acceptable. Moreover, good joint strength would
be obtained by creating the IM layer with discontinuous
thickness, and the similar result is obtained by the P-
CMT process in the present study.
20
This result illus-
trated that thickness and morphology of the IM layer
play an important role in determining the joint strength.
Failure modes
Failure modes of welds produced by the P-CMT and
CMT processes for various welding conditions were
determined by visual examination. Tensile loading
resulted in failure at Al HAZ
22
and is shown in Fig. 8a.
Since the thickness of the IM layer was very minimal in
both the processes (CMT and P-CMT), none of the
samples failed at the interface. Softening of HAZ is
attributed to the dissolution of strengthening precipi-
tates and grain coarsening. This is evident from the
microstructural transitions shown in Fig. 3 for both the
processes. This failure mode has ductile fracture
characteristics with small size dimples and voids. Frac-
ture surface shows the pits with small spherical grains
mainly made of Mg
2
Si (denoted by arrows) strengthen-
ing precipitates.
Conclusions
Dissimilar aluminium/steel lap joints were successfully
produced by the CMT and P-CMT processes. The effect
of welding parameters (welding current, arc length cor-
rection factor, wire feedrate, starting and ending
current) on the microstructure of weld nugget, the IM
layer formed at the nugget seam/steel interface, the
tensile strength of the joints and fracture mode are
analysed. From this investigation, the following
important conclusions have been derived.
1. Pulsed-CMT process resulted in better joint
strength and hardness with reduced welding current
when compared with conventional process.
2. Owing to high heat input in the CMT process, the
grains in nugget zone near HAZ coarsens and the
Mg
2
Si phase dissolves, which results in Al softening
and as a result the joint strength gets reduced.
The welding current had a significant effect on the
mechanical strength of the joints followed by the wire
feedrate on increasing the heat input. In both pro-
cesses, starting current rapidly heated the base metal,
and the ending current prevented local overheating.
3. The presence of the Fe Al and -Fe Al phases in
3 2 5
the weld nugget was evident from the XRD and
electron microscopy analysis. The IM b9-Mg
2
Si
particles were also present with coarse Si particles.
Particles having similar crystal structure as a-
AlFeSi are present but contain Mn.
4. Thickness of the IM layer varied from 1.49 to 3 mm
for the P-CMT and CMT processes respectively.
This is appreciably less than similar IM layers
obtained by prior literatures and joining processes
dealing with the same system.
5. At the interface, fi-FeAl
3
and g-Fe
2
Al
5
phases were
formed. g-Fe
2
Al
5
is characterised by trapezoidal,
faceted and single crystal surrounded by aluminium.
Whereas adjacent fi-FeAl
3
is found to be elliptical.
Electron diffraction patterns revealed that the inter-
facial layer is also composed of long plate-like Al
5
FeSi
phase. It is also concluded that g-Fe
2
Al
5
forms first
followed by fi-FeAl
3
.This illustrates that the thick-
ness and morphology of the IM layer play an im-
portant role in determining the joint strength.
6. The CMT and P-CMT welds failed at the Al HAZ.
This failure mode has ductile fracture character-
istics with dimples and voids.
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