s11663-022-02574-7

ORIGINAL RESEARCH ARTICLE
Analysis of Metal Transfer Characteristics
in Low-Heat Input Gas Metal Arc Welding
of Aluminum Using Aluminum–Silicon Alloy Fillers
DIVYA NALAJALA, RAMA KISHORE MOOKARA,
and MURUGAIYAN AMIRTHALINGAM
Welding of thin aluminum sheets with high heat input conventional welding processes often
results in heat-affected zone softening, distortion, and burn-through problems. In this study,
metal transfer characteristics are optimized in a gas metal arc-welding process to achieve low
heat input and high productivity to weld aluminum sheets. The main objective of this study is to
achieve maximum droplet transfer rate at low heat input in AA 1050 alloy using short circuiting
(CMT) and short circuiting with pulse (CMT-P) gas metal arc-welding processes. Metal transfer
characteristics are studied while depositing the AlSi5 and AlSi12 fillers on to an AA 1050
aluminum plate. Experiments are conducted to investigate the influence of mean current (90 to
110 A) and pulse frequency (5, 6, and 7) on heat input, droplet transfer rate, and its diameter.
Kinetics of the metal transfer during welding is correlated with the corresponding current and
voltage waveform and bead geometry. Results showed that for a given heat input, the droplet
transfer rate is high in CMT-P at all mean current amplitudes compared to short-circuiting
process for both the fillers. Analysis of the bead geometry indicated that for a given mean
current, bead width of the AlSi12 filler is always higher than the AlSi5 filler in both the modes of
metal transfer due to the near complete eutectic solidification in the former case. It is also found
that the filler wire composition does not affect the droplet transfer rate for a given mean current
in CMT and CMT-P processes used in this study.
https://doi.org/10.1007/s11663-022-02574-7
Ó The Minerals, Metals & Materials Society and ASM International 2022
I.
INTRODUCTION
THIN gage (1 to 3 mm) aluminum alloys are widely
used in the automotive industry due to their good
strength-to-weight ratio and corrosion resistance. Welding of the thin aluminum sheets is in general difficult
than steel due to its high thermal conductivity (222 W
m1 K1 ), high coefficient of thermal expansion
(24 106 K1 ), and refractory oxide formation.[1]
Moreover, welds of aluminum are also susceptible for
porosity formation due to the major difference in
solubility of the atomic hydrogen in the solid and liquid
states of aluminum (1 and 22 atomic parts per million,
appm in solid and liquid aluminum, respectively, at 933
DIVYA NALAJALA, RAMA KISHORE MOOKARA, and
MURUGAIYAN AMIRTHALINGAM are with the Department of
Metallurgical and Materials Engineering, Indian Institute of
Technology (IIT) Madras, Chennai, Tamil Nadu 600036, India.
Contact e-mail: [email protected]
Manuscript submitted September 19, 2021; accepted May 24, 2022.
Article published online June 16, 2022.
2914—VOLUME 53B, OCTOBER 2022
K and 1 atmosphere pressure).[2] Aluminum plates with
thickness more than 3 mm are generally welded with
pulsed gas metal arc welding with good joint efficiency.
High heat input associated with the pulsed gas metal arc
welding leads to burn through and distortion during
welding of the thin aluminum sheets. Gas metal arc
welding with short-circuiting metal transfer with relatively low heat input is employed to weld the thin sheets.
However, this process often results in lack of fusion in
the weld zone.[3] It is, therefore, essential to optimize the
metal transfer characteristics as a function of current–voltage waveform to join the thin aluminum sheets
with low heat input.
Cold metal transfer (CMT) welding process with low
heat input is reported to be an ideal process to weld thin
aluminum sheets.[4] Integration of wire feed motion into
process control and the modified current and voltage
waveform makes this process unique from other welding
processes. This process can be used to weld thin sheets
of heat-sensitive materials with low heat input. CMT-P
process is developed by introducing current pulses in
CMT to weld thicker sections with increased deposition
rate.
METALLURGICAL AND MATERIALS TRANSACTIONS B
Feng et al., used the CMT process to weld pure
aluminum sheet and reported that the weld bead
appearance was found to be good due to reduction in
spatter. Distortion of the sheet was also minimized
because of the low heat input.[4] Pickin et al., examined
the suitability of the CMT process for welding of the
aluminum alloys and found that the deposition rate has
increased in CMT compared to pulsed metal inert gas
(MIG) welding for a given heat input.[5] Zhang et al.,
used the CMT process to join aluminum to zinc-coated
steel by brazing using the Al–Si filler to evaluate the
suitability of this process for dissimilar joining. Results
indicated that the CMT process was found suitable to
join steel to aluminum due to the reduction in intermetallic formation resulting from low heat input.[6] Cao
et al., also developed the CMT-welding procedure to
join aluminum to galvanized steel and found that
thickness and composition of the intermetallic compound layer vary with the heat input.[7] Pang et al.,
evaluated the suitability of short circuiting with pulse
transfer (CMT-P) for welding thick aluminum plates. It
was found that the weld penetration depth decreased
when the number of short circuits increased for a given
pulse frequency.[8] It was reported that at lower current
(below 110 A), the transfer mode in CMT process was
based on short circuiting. A combination of spray and
short-circuit transfer was observed when the current was
increased above 100 A for the ER 4043 filler with 1.2
mm diameter.[1] Alaattin Ozan Irizalp et al., reported
that porosity decreased with the heat input during
overlap welding of 2-mm-thick AA1050 aluminum using
the CMT-welding process. This was attributed to the
rapid cooling rates at low heat input which suppressed
the nucleation and growth of pores during solidification.[9] Yang et al., used the CMT process for dissimilar
overlap welding of 2-mm-thick AA6061-T6 and
1.2-mm-thick zinc-coated low carbon steel using
ER4043. Porosity was observed in the aluminum alloy
due to the entrapment of hydrogen, Zn vapor, and other
shielding gases during the welding process.[10] Cong
Baoqiang et al., analyzed the influence of arc mode
[Conventional CMT, CMT-pulse (CMT-P), CMT
advanced (CMT-ADV), and CMT-pulse advanced
(CMT-PADV)] in CMT process on the weld bead
geometry and porosity distribution in AA2219 aluminum alloy. Heat input was very less in CMT-ADV
and CMT-PADV processes compared to conventional
CMT and CMT-P processes. Porosity was decreased in
CMT-P and CMT-ADV whereas completely eliminated
using CMT-PADV process due to reduction in depth of
penetration which was beneficial for the escape of
generated gas bubbles during solidification. Hence, weld
bead geometry plays an important role in controlling the
porosity in aluminum alloy welds which was controlled
by the heat input.[11] It was reported that the filler
composition, arc current and length, and weld pool
surface area influence the porosity distribution in
aluminum alloy welds.[12,13] There were attempts to
develop low-heat-input welding processes to weld thin
aluminum sheets and repair of the aluminum components with minimal damage to base metal microstructures.[14–17] However, a detailed analysis on the influence
METALLURGICAL AND MATERIALS TRANSACTIONS B
of current–voltage amplitude on heat input, droplet
transfer rate, and its diameter while welding of aluminum alloys using the CMT process was still not made.
It is essential to optimize metal transfer behavior as a
function of the heat input to increase the deposition
rate, achieve good bead characteristics, minimize distortion, and spatter formation. The objective of this
work is, therefore, to achieve the maximum droplet
transfer rate at low heat input during bead-on-plate
deposition of AA 1050 aluminum using AlSi5 and
AlSi12 fillers by varying the mean current and pulse
frequency to increase the productivity. Bead-on-plate
deposition is performed using CMT and CMT-P processes. Effect of mean current and pulse frequency on
droplet transfer rate and its diameter is investigated and
correlated with the heat input. Bead geometry is also
analyzed and compared between the two fillers as a
function of mean current and pulse frequency to
investigate the wetting characteristics.
II.
EXPERIMENTAL PROCEDURE
Bead-on-plate deposition was carried out on a
6-mm-thick AA 1050 aluminum plate using AlSi5 and
AlSi12 filler wires of 1.2 mm diameter. Pure argon was
used as a shielding gas. Fronius CMT Transpulse
Synergic 4000 power source was used for welding. The
power source was integrated with computer numerical
control (CNC) table which can move in three directions
(X, Y, and Z) to deposit material layer by layer based on
the G codes. In this work, welding was done using the
CMT and CMT-P processes in the X-direction. Base
plates were cleaned with acetone before the experiment.
A high-speed camera at a minimum shutter speed of 1/
10,000 seconds was used to record metal transfer in 6000
frames per second. A 700 W xenon light was used to
illuminate weld zone. Blue and orange band pass filters
were used to mask the arc and to visualize only the
droplet transfer. The high-speed camera and xenon light
source were placed opposite to each other in line with
the welding torch and work piece. During welding,
shadow images of the arc, electrode, molten droplets
transfer, and weld pool were recorded at a frame rate of
6000 per second and resolution of 896 * 752 using a
FASTCAM high-speed imaging system. The current
and voltage waveform during welding were recorded
using 75-MHz two-channel InfiniiVisionä digital storage oscilloscope (DSO) with separate current and
voltage clamps connected to the channels. The voltage
was measured by connecting the probe terminals at the
contact tip and the base material, respectively. The mean
current and voltage at various stages in CMT and
CMT-P processes were measured from the obtained
waveform and reported in this study. The measured
current–voltage waveform was correlated with the
high-speed camera images to analyze the metal transfer
behavior. The chemical compositions of the base and
filler materials (AlSi5 and AlSi12) are given in Tables I
and II, respectively. Process parameters used during the
bead-on-plate deposition are given in Tables III and IV.
VOLUME 53B, OCTOBER 2022—2915
Table I. Chemical Composition of AA 1050 Base Material (Wt Pct)
Base Material
AA 1050
Ni
Fe
Cu
Zn
Mg
Mn
Cr
Si
Al
0.03
0.637
0.081
0.004
0.005
0.04
0.001
0.073
99.2
Table II.
Fillerwire
AlSi5
AlSi12
Chemical Composition of Fillers Used (Wt Pct)
Fe
Mn
Cu
Si
Zn
Mg
Ti
Al
<0:6
<0:6
<0:15
<0:15
0:3
<0:3
5
12
<0:1
<0:2
<0:2
<0:1
<0:15
<0:15
balance
balance
Table III. Process Parameters Used During the
Bead-on-Plate Deposition in CMT and CMT-P Processes
IP ¼
Description
Parameters
Welding Process
Electrode
Electrode Diameter (mm)
Shielding Gas
Shielding Gas Flow Rate (L min
Mean Current (A)
Number of Current Pulses
Arc Length (mm)
CTWD (mm)
Speed (mm s1 )
1
)
CMT
AlSi5
1.2
Ar
25
90 to 110
—
10
15
5
CMT-P
AlSi12
1.2
Ar
25
90 to 110
5, 6, 7
10
15
5
Table IV. Wire Feed Rate for the Mean Current Used
During Bead on Deposition in CMT and CMT-P Processes
Wire Feed Rate (m min1 )
Mean Current (A)
CMT
CMT-P
5.1
5.3
5.3
5.4
5.4
4.1
4.4
4.6
4.7
4.9
90
95
100
105
110
Note: wire feed rate was identical for both AlSi5 and AlSi12 fillers.
The droplet transfer rate and its diameter were
measured using a FASTCAMä viewer software. Droplet transfer rate was measured by counting manually
the number of drops transferred to the molten pool per
second. The transfer rate was compared over 3 seconds
for all mean current amplitudes and averaged. The
droplet diameter in CMT process was measured during
the background phase where it attains a spherical shape
just before the onset of short-circuiting event. In CMT-P
process, the freely detached droplet during pulsing event
was considered for the diameter measurement. The
recorded current and voltage waveform was used to
calculate the instantaneous power (IP) in Watts:[18]
2916—VOLUME 53B, OCTOBER 2022
n
1X
ðIi Ui Þ;
n i¼0
½1
where Ii the instantaneous current (A) and Ui is the
instantaneous voltage (V) and heat input (J mm1 )
HI ¼
IP g
:
S
½2
g process efficiency (0.8), S welding speed (5 mm s1 ).
Calculations were carried out over seven cycles, and
average value was reported as the heat input.
The quasi-binary phase diagram for Al–Si system was
calculated using the TCAL5 database of ThermoCalcä. Al–Si system is well known to exhibit eutectic
reaction at a defined temperature and composition. The
deviation from eutectic composition can greatly influence the fluidity of the alloy which in turn can result in
change in the droplet diameter due to the prevailing
surface tension forces. A correlation was attempted
using the phase diagrams to analyze the droplet transfer
characteristics. Transverse sections were cut from the
deposits using electrodischarge machine (EDM) and
polished for metallography analysis. Bead width and
height were measured on these transverse sections using
ImageJ software.
III.
RESULTS
A. Analysis of the Phase Diagram
The quasi-binary phase diagram of Al–Si system is
shown in Figure 1. The eutectic solidification temperature for Al–Si system is 577 C at a eutectic composition
of 12.6 wt pct Si. Two fillers chosen in this study are the
aluminum alloys with 5 wt pct Si (AlSi5) and 12 wt pct
Si (AlSi12). AlSi5 filler is, therefore, expected to solidify
as a mixture of pro-eutectic and eutectic microstructure
with a solidification temperature range of 60 C, and
AlSi12 exhibits near eutectic solidification. The changes
in solidification temperature range can significantly
influence the weld bead geometry and its effect is
discussed in the subsequent sections.
METALLURGICAL AND MATERIALS TRANSACTIONS B
Fig. 1—Phase diagram of Al–Si system generated using the Thermo-Calc software.
B. Metal Transfer Characteristics in CMT and CMT-P
Processes
Metal transfer characteristics are analyzed using the
high-speed camera images while depositing AlSi5 and
AlSi12 fillers by CMT and CMT-P processes with mean
currents of 90, 95, 100, 105, and 110 A and correlated
with the current and voltage waveform. All mean
currents used in this study contain one short-circuit
event along with the number of pulses as 5 for 90 to 100
A, 6 for 105 A, and 7 for 110 A.
Figure 2 shows the metal transfer in CMT process at a
mean current of 90 A for AlSi5 filler. It can be seen that
the droplet transferred to the work piece in three stages:
(i) arcing phase, (ii) background phase, and (iii)
short-circuit phase in a single cycle (Figure 2).
During the arcing phase (1 to 4 ms), high current and
arc voltage of about 130 A and 17 V (Figure 2(a)) are
maintained to generate sufficient arc energy to stabilize
the arc. The arc heated the electrode to form a droplet at
the electrode tip as shown in Figure 2(b). Simultaneously, work piece is also heated which leads to the
formation of a weld pool. In the background phase (9 to
13 ms, Figure 2(c)), current is reduced to about 70 A to
avoid the droplet detachment. As a result of decrease in
the current, size of the arc is also reduced. The
METALLURGICAL AND MATERIALS TRANSACTIONS B
background current is maintained in such a way that
droplet attained a spherical shape. During this stage, the
filler wire started to move toward the work piece and
came into contact with the work piece during short-circuiting phase as shown in Figure 2(d). Arc is extinguished, and the current and voltage are further reduced
which can be seen from in Figure 2(a). Metal transferred
to the work piece during retract motion of the filler wire
(at 16 ms) at low current of about 40 A. Arc re-ignites
and this cycle is repeated. Throughout this transfer
mode, stable and spatter-free metal transfer is observed.
Experimental analysis of the metal transfer behavior is
in line with the commercially used current–voltage
waveform characteristics.[4,19] It is observed from the
high-speed images that mode of metal transfer is only by
short circuiting below 100 A mean current. However,
when the mean current is increased to above 100 A, an
occasional free flight pulsing transfer is also observed in
CMT mode as shown in Figures 3(a) and (b).
Metal transfer behavior of AlSi5 filler during CMT-P
process at 90 A mean current is shown in Figure 4(a) for
AlSi5 filler. The current and voltage waveform shown in
Figure 4(b) represents five pulses and one short circuit
per cycle. In this mode, metal transfer is made by short
circuiting with free flight pulsing transfer.
VOLUME 53B, OCTOBER 2022—2917
Fig. 2—(a) Current–voltage waveform and corresponding metal transfer behavior in a cycle showing three stages during CMT process with
AlSi5 filler at 90 A mean current. (b), (c), and (d) represent arcing phase, background phase, and short-circuiting phases, respectively.
Fig. 3—Observed free flight pulsing transfer during arcing phase in
AlSi5 filler in CMT mode along with short-circuiting transfer at (a)
100 A and (b) 110 A mean currents. The red line separates the metal
transfer images showing the droplet detachment during arcing phase
and short-circuiting phases (Color figure online).
2918—VOLUME 53B, OCTOBER 2022
First row of Figure 4(a) shows ignition of arc,
formation, and detachment of droplet in a single pulse.
In second row, remaining drops formation is shown
only by the arc ignition and droplet detachment. Third
row indicates the onset of short circuiting. The process
of metal transfer behavior is described in two stages.
During pulsing (stage 1), peak current (Ip ) is about 280
A and corresponding voltage is about 23 V and the
droplet is detached at this stage as shown in the first row
of Figure 4(a). Subsequently, the background current
(Ib ) and voltage are reduced to about 75 A and 19 V,
respectively, to ensure that the arc is ignited as shown in
Figure 4(b). Upon raising the current to peak value
again, another droplet detachment is observed at 4 ms.
This process is repeated till five drops are detached. At
the end of the fifth pulse, current is maintained at 75 A
for a short duration and further reduced to 50 A. The
voltage and current decreased further indicating the
onset of short circuiting (stage 2) at 22 ms as explained
earlier in CMT process. In this cycle, the metal transfer
is achieved with a combination of five pulses and one
short circuit. Arc is re-ignited and this process is
repeated for several cycles.
AlSi12 filler also exhibited identical metal transfer
characteristics as shown in Figures 5 and 6 in both CMT
and CMT-P processes for all the mean currents used
here. However, the drop transfer is observed at a mean
METALLURGICAL AND MATERIALS TRANSACTIONS B
Fig. 4—(a) Metal transfer behavior in a cycle contains 5 pulses and 1 short circuit and (b) corresponding current and voltage waveform during
CMT-P process at a mean current of 90 A with AlSi5 filler. Stage 1 and 2 represents pulse and short circuiting phases respectively. In (b), stage
1 and 2 corresponds to first row of stage 1 and 2 in (a) respectively.
current amplitude of 105 A as shown in Figure 7(a) in
CMT process as compared to AlSi5 filler in which drop
transfer is observed at 100 A (Figure 3(a)).
C. Effect of the Mean Current on Droplet Transfer Rate
and Heat Input in AlSi5 and AlSi12 Fillers
Variation of the droplet transfer rate and heat input
as a function of the mean current is analyzed during
bead-on-plate deposition of aluminum alloy using AlSi5
and AlSi12 fillers (Figures 8 and 9). For both the fillers,
increase in mean current resulted in increased droplet
transfer rate and heat input.
In CMT mode, the droplet transfer rate is nearly
constant till 100 A for AlSi5 filler. At 100 A, occasional
drop transfer is observed for AlSi5 filler as shown in
Figure 3(a) and slight enhancement in droplet transfer
rate is observed above 100 A. In CMT-P process,
continuous increase in the droplet transfer rate is
observed with mean current. The current pulsing frequency is identical for 90 to 100 A. But at 100 A, the
droplet transfer rate is (162 s1 ) higher than that of 90
and 95 A which had transfer rates of 128 and 145 s1 ,
respectively. Similarly, in AlSi12 filler, drop transfer is
observed at 105 A which enhanced the droplet transfer
rate (Figure 7(a)). In CMT-P, the droplet transfer rate is
increased with number of pulses. Unlike in AlSi5 filler,
the droplet transfer rate is identical from 95 to 100 A for
AlSi12 filler.
METALLURGICAL AND MATERIALS TRANSACTIONS B
Analysis of droplet transfer rate as a function of mean
current indicated that there was no significant difference
in the droplet transfer rate between both the fillers in
both CMT and CMT-P processes for the range of mean
current used in this study (Figure 8).
Figure 9 shows the variation in the heat input for both
the fillers as a function of mean current in both CMT
and CMT-P processes. Heat input increased with the
mean current for both the fillers in CMT and CMT-P
processes. AlSi5 filler in CMT mode, exhibited a spike in
the heat input from 100 A whereas this increment is
observed from 105 A for AlSi12 filler. There is no
significant difference in heat input between both the
fillers in CMT process. In CMT-P mode, AlSi12 filler
exhibited slightly higher heat input compared to AlSi5
filler at all mean current values (270 and 350 J mm1 in
AlSi5 and AlSi12 fillers, respectively, at 90 A mean
current). This is attributed to the difference in the arc
current and voltage which can be seen from Figures 4
and 6.
D. Effect of Mean Current on Droplet Diameter
High-speed image analysis indicated that, drop diameter is large (2 mm at 90 A) in CMT process compared
to CMT-P (1.1 mm at 90 A) at all mean current
amplitudes for both the fillers (Figures 10 and 11).
Droplet diameter is decreased with increase in mean
current in both CMT and CMT-P processes for AlSi5
VOLUME 53B, OCTOBER 2022—2919
Fig. 5—(a) Current–voltage waveform and corresponding metal transfer behavior in a cycle showing three stages during CMT process with
AlSi12 filler at 90 A mean current. (b), (c), and (d) represents arcing phase, background phase, and short-circuiting phases, respectively.
Fig. 6—(a) Metal transfer behavior in a cycle contains 5 pulses and 1 short circuit and (b) corresponding current and voltage waveform during
CMT-P process at a mean current of 90 A with AlSi12 filler. Stage 1 and 2 represent pulse and short-circuiting phases, respectively. In (b), stage
1 and 2 correspond to first row of stage 1 and 2 in (a), respectively.
2920—VOLUME 53B, OCTOBER 2022
METALLURGICAL AND MATERIALS TRANSACTIONS B
Fig. 7—(a) Observed free flight pulsing transfer during arcing phase
in AlSi12 filler followed by short circuiting at 105 A and (b) 110 A
mean currents. The red line separates the metal transfer images
showing the droplet detachment during arcing phase and
short-circuiting phases (Color figure online).
Fig. 9—The heat input as a function of mean current in CMT and
CMT-P processes for AlSi5 and AlSi12 fillers. (i), (ii), and (iii)
correspond to 5 pulses, (iv) and (v) correspond to 6 and 7 pulses,
respectively, in the current–voltage waveform of CMT-P process.
Fig. 10—Droplet diameter as a function of mean current in CMT
process with AlSi5 and AlSi12 filler wires. The error bars and
symbol sizes indicate uncertainty in the measurements.
Fig. 8—The droplet transfer rate as a function of mean current in
CMT and CMT-P processes for AlSi5 and AlSi12 fillers. (i), (ii), and
(iii) correspond to 5 pulses, (iv) and (v) correspond to 6 and 7
pulses, respectively, in the current–voltage waveform of CMT-P
process. The error bars and symbol sizes indicate uncertainty in the
measurements.
and AlSi12 fillers. For a given current (for example at 90
A), drop diameter is small (1.7 mm) for AlSi12 when
compared with AlSi5 filler wire (2 mm).
E. Analysis of Bead Geometry
Bead dimensions are measured as a function of mean
current and pulse frequency during the bead-on-plate
deposition by CMT and CMT-P processes for both the
fillers. Macrographs of the beads made using AlSi5 and
AlSi12 fillers with the corresponding mean current
amplitudes are shown in Figures 12 and 13. Measured
bead width and height of both the fillers are compared
and shown in Figures 14 and 15, respectively, for CMT
METALLURGICAL AND MATERIALS TRANSACTIONS B
and CMT-P processes. It can be seen that as the mean
current increases, increase in the bead width is observed
in both CMT and CMT-P processes for both the fillers
(Figure 14). Bead width is found to be high in CMT-P
process as compared with CMT process at all mean
current amplitudes used here. In CMT process, bead
width is slightly higher for AlSi12 filler as compared to
AlSi5 filler at all mean current amplitudes whereas,
almost similar bead width is observed in CMT-P process
for both the fillers. Bead height is reduced slightly with
increase in the mean current for both the fillers in both
CMT and CMT-P processes (Figure 15). For a given
mean current, bead height is always higher for AlSi5
filler when compared to AlSi12 filler.
Porosity is observed in the deposits made by both the
fillers during CMT and CMT-P processes. It is observed
that, pore size is large in the beads made with AlSi12
filler compared to AlSi5 (Figures 12 and 13). As the
analysis is focused toward optimizing metal transfer
VOLUME 53B, OCTOBER 2022—2921
Fig. 11—Droplet diameter as a function of mean current in CMT-P
process with AlSi5 and AlSi12 filler wires. The error bars and
symbol sizes indicate uncertainty in the measurements.
Fig. 14—Bead width as a function of mean current in CMT and
CMT-P processes for AlSi5 and AlSi12 fillers.
Fig. 12—Macrographs of the CMT beads deposited using AlSi5 and AlSi12 fillers.
Fig. 13—Macrographs of CMT-P beads deposited using AlSi5 and AlSi12 fillers.
2922—VOLUME 53B, OCTOBER 2022
METALLURGICAL AND MATERIALS TRANSACTIONS B
Fig. 15—Bead height as a function of mean current in CMT and
CMT-P processes for AlSi5 and AlSi12 fillers.
kinetics, reasons for this difference in porosity are not
investigated. But, this variation in porosity distribution
can be due to the difference in filler composition and arc
mode as reported in the literature.[11–13]
IV.
DISCUSSION
Metal transfer characteristics observed from the
high-speed camera images during bead-on-plate deposition of AlSi5 and AlSi12 fillers onto aluminum plate are
consistent with the reported process characteristics.[5,19]
For AlSi5 filler, change in mode of metal transfer is
observed at 100 A in CMT process (Figure 3(a)). It is
generally perceived that metal transfer occurs only by
short circuiting in CMT process. However, when the
mean current is increased, metal transfer changes from
complete short circuiting to short circuiting with intermittent free flight transfer due to the influence of
Lorentz force in aiding the droplet detachment.[1]
However, it should be noted that this transition from
a complete dip to mixed-mode transfer is a characteristic
of the filler metal and depends on many parameters such
as electrode diameter, chemical composition, shielding
gas, and electrode tip to work-piece distance.[20] For
AlSi5 filler of 1.2 mm diameter, drop spray transfer is
observed at 100 A mean current (Figure 3(a)). Significant reduction in the droplet diameter (from 1.85 to 1.4
mm) beyond 100 A (Figure 10) shows the influence of
Lorentz force on droplet detachment. It is identified as
drop transfer by the formation of conical neck at the
electrode tip (at 12 ms in Figure 3(a)) and corresponding
droplet diameter (1.4 mm) which is slightly larger than
electrode diameter (1.2 mm). However, continuous
droplet detachment is not observed for this filler. Above
105 A mean current, there is a continuous droplet
detachment by the drop transfer, 2 to 3 drops followed
by short circuiting. This resulted in significant change in
the heat input beyond 100 A as shown in Figure 9.
METALLURGICAL AND MATERIALS TRANSACTIONS B
In general, the pulse parameters should be chosen in
such a way that one droplet is detached per pulse. In this
experiment, the number of pulses is found to be same
from 90 to 100 A but the background current and
duration are varying. One droplet detachment per pulse
is observed from the high-speed camera images at 100 A
mean current resulting in stable and maximum transfer
rate (162 s1 ) compared to 90 and 95 A. There is a
continuous increase in the background current and
decrease in the background duration to support increase
in the mean current.[20]
For all mean current amplitudes, the droplet transfer
rate is maximum (for example, 175 s1 at 110 A mean
current) in CMT-P process compared to CMT (89 s1 at
110 A mean current). Even though the metal transfer is a
mixture of short circuiting with intermittent free flight
pulsing transfer in CMT process beyond 100 A, significant difference in transfer rate between CMT and CMT-P
processes is observed. As explained in Section 3.2, droplet
is transferred to the work piece in 18 ms (Figure 2) during
CMT, whereas, in CMT-P (Figure 4), the droplet detachment is achieved in 0.36 ms due to which the droplet
transfer rate is maximum in CMT-P process.
Analysis of the droplet diameter indicated that for a
given mean current, the droplet diameter is always small
in CMT-P process compared to CMT for both the fillers.
In CMT process, the droplet diameter is small during
arcing phase due to the high current whereas reduction in
current during the background phase resulted in growth
of the droplet. As the current magnitude is less, the
droplet attained large size and detached under the
influence of gravitational force. Analysis of the droplet
diameter in the current range of 90 to 110 A in CMT
process for AlSi5 filler indicates, droplet diameter is large
till 100 A (1.8 mm). As the current increases beyond 100
A, the increase in Lorentz force aids the droplet detachment resulting in reduction in the droplet diameter as
shown in Figure 10. In CMT-P process, droplet diameter
is small due to the influence of Lorentz forces at higher
pulse peak currents.[21]
From this study, it is found that heat input is high in
CMT-P process compared to CMT. In CMT-P process,
droplet is transferring to the weld pool at high current
and voltage compared to CMT process for the same
mean current as explained in Section 3.2. This leads to
increase in heat input in CMT-P process.
Analysis of the variation of metal transfer behavior
with mean current (Figure 7(a)) shows a change in mode
of metal transfer at 105 A mean current for AlSi12 filler.
The composition of the fillers used in study differs only
in silicon concentration while other elemental concentration is similar. Therefore, it can be concluded that
difference in current at which intermittent free flight
pulsing observed is due to the difference in chemical
composition. Above 105 A, there is a continuous
detachment of 2 to 3 drops followed by short circuit
which enhanced the transfer rate. In CMT-P, one
droplet detachment per pulse is observed at all mean
current amplitudes due to which the droplet transfer
rate is more when compared to CMT-P deposition of
AlSi5 filler (Figure 8).
VOLUME 53B, OCTOBER 2022—2923
AlSi12 filler also shows a similar behavior in the metal
transfer as AlSi5, as the current increases, there is a
reduction in droplet diameter (Figures 11 and 10). For a
given mean current, the droplet diameter is small in
AlSi12 filler as compared to AlSi5 in CMT process
(Figure 10). As discussed in Section 3.1, AlSi12 is eutectic
alloy which has greater fluidity compared to the hypo-eutectic AlSi5 alloy which has a solidification range of
around 60 C. Viscosity of AlSi5 alloy is also found to be
slightly higher (1.16 mPa s) than AlSi12 (1.1 mPa s) at 973
K.[22] It can be inferred that the increase in Si content in
AlSi12 filler results in decrease in droplet diameter.
Increase in bead width is observed with mean current
in CMT and CMT-P processes for the both fillers used
in this investigation (Figure 14). Melting rate of the
electrode increases with current amplitude which
resulted in increased bead width. In CMT-P process,
deposition rates are high when compared with CMT
process due to the introduction of pulses in the current
waveform. Hence, bead width is high in beads made
using CMT-P process as compared with CMT for both
the fillers. For given mean current, bead width is slightly
higher for AlSi12 filler than AlSi5 filler due to the near
eutectic solidification in the former case.
It is observed that for a given mean current, high
droplet transfer rate (Figure 8) and high heat input
(Figure 9) are observed in CMT-P mode as compared to
the CMT mode for both fillers. For a given mean
current, difference in the droplet transfer rate is significant for both fillers in CMT and CMT-P modes. AlSi5
filler does not exhibit significant differences in the heat
input for a given mean current as compared to AlSi12
filler in both modes (Figure 9). In AlSi5 filler, for the
range of mean currents used in this study, CMT-P mode
provides higher droplet transfer rate at low heat input as
compared to CMT mode. In AlSi12 filler, for a given
mean current, high droplet transfer rate and heat input
are observed in both modes as compared to AlSi5 filler.
This correlates with the observed changes in the droplet
diameter, corresponding heat input characteristics, and
bead geometry in the deposits made using AlSi12 filler.
V.
CONCLUSIONS
Bead-on-plate welding is performed on AA 1050
commercial aluminum plate using AlSi5 and AlSi12
fillers by CMT and CMT-P processes with argon as a
shielding gas. The droplet transfer rate, its diameter, and
bead geometry are analyzed as a function of mean
current and pulse frequency. The following conclusions
are drawn based on the results obtained from this study.
(1) For a given mean current, the droplet transfer rate
and associated heat input are always high in CMT-P
process as compared to the CMT process.
(2) For the AlSi5 filler, the droplet transfer rate is always high in CMT-P mode at low heat inputs as
compared to CMT mode for the current ranges used
in this study.
(3) On the contrary, in the AlSi12 filler, both droplet
transfer rate and heat input are high in CMT-P
2924—VOLUME 53B, OCTOBER 2022
(4)
(5)
(6)
(7)
mode as compared to CMT process for the range of
mean current used in this study.
In CMT process, above 100 A mean current, metal
transfer is found to be short circuiting with an
intermittent free flight pulsing transfer for AlSi5
filler. This transition is observed at 105 A mean
current for AlSi12 filler.
Filler wire composition does not show significant
influence on the droplet transfer rate. However, heat
input characteristics is observed to change with the
filler composition for a given mean current used in this
study.
Bead width in CMT process is found to be lower
than CMT-P process for a given mean current,
whereas bead height is higher in CMT as compared
to CMT-P process for both fillers.
Small droplet diameter and higher bead width are
observed in AlSi12 filler compared to AlSi5 at all
mean current amplitudes.
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