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Determining the critical transition current for metal transfer in gas metal arc welding (GMAW)

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It is of great significance to determine the critical transition current from globular transfer to spray transfer in gas metal arc welding (GMAW) because metal transfer modes affect the weld quality and welding productivity. In this study, a simple model is developed to calculate the critical transition current based on the analysis of various forces exerted on a pendent droplet at the electrode tip. It is found that the force exerted by the incoming molten metal from the wire on the drop, i.e., the term ṁ cv c, plays an important role in determining the critical transition current. For mild steel wires and argon shielding gas, the critical transition current is predicted with different levels of wire diameter and extensions. The calculated results match the experimental ones.
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RESEARCH ARTICLE
Determining the critical transition current for metal transfer in
gas metal arc welding (GMAW)
Chuan-song WU (*), De-gang ZOU, Jin-qiang GAO
Institute of Materials Joining, Shandong University, Jinan 250061, China
EHigher Education Press and Springer-Verlag 2008
Abstract It is of great significance to determine the crit-
ical transition current from globular transfer to spray
transfer in gas metal arc welding (GMAW) because metal
transfer modes affect the weld quality and welding pro-
ductivity. In this study, a simple model is developed to
calculate the critical transition current based on the ana-
lysis of various forces exerted on a pendent droplet at the
electrode tip. It is found that the force exerted by the
incoming molten metal from the wire on the drop, i.e.,
the term _
mcvc, plays an important role in determining
the critical transition current. For mild steel wires and
argon shielding gas, the critical transition current is pre-
dicted with different levels of wire diameter and exten-
sions. The calculated results match the experimental ones.
Keywords critical transition current, metal transfer, gas
metal arc welding (GMAW)
1 Introduction
Gas metal arc welding (GMAW) has been widely used for
automatic and robotic welding applications in the materials
processing industry due to its higher productivity, lower
cost and easier operation. During the GMAW process,
consumable electrode (wire) is continuously melted, and
the molten metal from the wire is transferred to the work-
piece, thereby providing filler besides simple heating to melt
the substrates. At a low welding current in argon-richer
shielding gas, molten metal from a small diameter wire is
transferred in the form of drops having a diameter larger
than the wire. This is called globular transfer. When the
welding current reaches a critical level, the tip of the wire
becomes pointed and many very small droplets detach from
the wire tip at a rate of hundreds per second. This mode of
transfer is called spray transfer. The current at which this
transition from globular to spray transfer occurs is referred
to as the critical transition current [1]. To ensure weld qual-
ity and efficiency in welding thick-section steels, the globu-
lar transfer mode is usually used with mixed shielding gas
composed of argon and carbon dioxide. Thus, determining
the critical transition current for globular transfer turning
into spray transfer is of great significance.
There are many factors affecting the mode of metal
transfer. Several theories, such as static force balance
[2,3], pinch instability [4,5], as well as ‘‘mass-spring’’ mod-
eling [6], have been proposed to explain the droplet trans-
fer processes. However, all the available models can not be
directly used to calculate the critical transition current. It
was found that the transition from globular to spray
transfer strongly resembles the transition from drop to
jet type emission, observed in experiments with liquids
issuing from a capillary tube. These experiments show
that if the velocity of the liquid is beyond a critical value,
no single drops are detached, but a jet emerges. This jet is
generally unstable and disintegrates into small drops [7].
In this study, taking the physics of droplet transfer in
GMAW as analogous to water dripping or flowing from
an orifice, the critical transition current in GMAW from
globular to spray transfer is analyzed and determined.
2 The transition from globular to spray
transfer
As the arc current is increased, the diameter of the droplet
pendent on the wire tip decreases and approaches a value
less than the wire diameter. The arc plasma jumps to the
upper part of the droplet (Fig. 1). The drop is then formed
at the pointed tip of the wire, and liquid metal flows down-
wards along the conical surface into the drop.
During the growth and detachment of the droplet, the
forces acting on the droplet contain gravitational force,
electromagnetic force, plasma drag force, surface tension
and so on. The transition from globular to spray occurs
when the total downward force is equal to the sustaining
force due to the surface tension, i.e., when
Received July 3, 2008; accepted July 15, 2008
E-mail: wucs@sdu.edu.cn
Front. Mater. Sci. China 2008, 2(4): 397–401
DOI 10.1007/s11706-008-0059-8
_
mcvczFemzFdzFg~psdnð1Þ
where _
mcis the mass flow rate for v5v
c
,v
c
is the critical
liquid velocity of the neck of the droplet, F
em
is the electro-
magnetic force, F
d
is the plasma drag force, F
g
is the gravity
of the droplet, psd
n
is the sustaining force due to the surface
tension, and d
n
is the diameter of the neck of the droplet.
Equation (1) describes a critical instance when the sus-
taining forces equal the downward forces. At this
moment, the velocity of the liquid achieves the critical
value v
c
, and the value of the current calculated is the
critical transition current.
The electromagnetic force on the droplet results from
diverging or converging current flows within the elec-
trode. When the current path diverges in the drop, the
Lorentz force, acting normal to the current path, creates
a detaching force (Fig. 2). By assuming that the current
density is uniform across the drop, the equation for the
axial electromagnetic force F
em
is as follows [8]
Fem~m0I2
4p
1
4{ln Rsin h
rz1
1{cos h
{2
1{cos hðÞ
2ln 2
1zcos h#ð2Þ
where Iis the welding current, m
0
is the permeability of
free space, Ris the droplet radius, ris the radius of the
electrode, and his the angle of the anode area.
Equation (2) can be used to calculate the value of F
em
for
the instance that all the welding current flows into the drop-
let, but at the critical situation from globular to spray trans-
fer, because of the neck of the droplet transfer, a part of the
current, I
1
, will pass through the drop, and the remainder,
I
2
, through the surface above it, as shown in Fig. 3. To
define x5I
1
/I
c
, Eq. (2) turns into the following form:
Fem~ex2m0I2
c
4pð3Þ
e~1
4{ln Rsin h
rz1
1{cos h
{2
(1{cos h)2ln 2
1zcos h
ð4Þ
where eis a geometrical factor. Waszink et al. [7] gives a
survey about part of the parameters in the situation when
globular transforms to spray transfer, as shown in Table 1.
Thus, through combining Table 1 with Eq. (3), the electro-
magnetic force can be calculated.
The plasma drag force on the droplet is similar to the
force acting on a spherical particle immersed in a fluid
stream. The drag force F
d
may be calculated by
Fd~CdAp
rfv2
f
2
 ð5Þ
where C
d
is the dimensionless coefficient, A
p
is the pro-
jected area on the plane perpendicular to the direction of
Fig. 1 Sketch of the forces acting on the droplet
Fig. 2 Sketch of electromagnetic force
Table 1 The critical liquid velocity v
c
and force terms [7]
d
w
/mm v
c
/m?s
21
ex
2
_
mcvc/10
23
NF
em
/10
23
NF
s
/10
23
N
0.8 1.0 0.23 0.8–1.6 0.3–0.5 1.3–1.8
1.1 1.1 0.11 0.8–1.3 0.2–0.4 1.2–1.6
1.6 1.0 0.05 0.9–1.4 0.2–0.4 1.3–1.6
398 Chuan-song WU, et al.
fluid flow, r
f
is the density of the plasma, and v
f
is the
velocity of the plasma. To calculate the value of the
plasma drag force, r
f
is defined as the density of argon,
and v
f
5100 m/s
2
,C
d
50.16 [8].
The surface tension force, F
s
, which acts to retain the
droplet on the electrode, is given by
Fs~psdnð6Þ
where d
n
is the diameter of the neck of the droplet and s
is the coefficient of the surface tension for the liquid
metal.
Mass in the presence of a gravitational field causes a
gravitational force, F
g
. The equation for this force is
Fg~4
3pR3rgð7Þ
where ris the density of the liquid droplet, and gis the
acceleration of gravity.
For the force exerted by the incoming liquid on a pen-
dent drop _
mcvc, the value of v
c
is taken from Table 1, and
_
mcis determined from the following expression
_
mc~dn
d

2
:5:5|10{13 I2L
Sw
z2:4|10{6I{5:0|10{5

ð8Þ
where dis the wire diameter, Lis the wire extension,
S
w
5pd
2
/4, and Iis the total current flowing through the
wire.
After all the forces in Eq. (1) are expressed, the follow-
ing equation is obtained:
_
mcvczex2m0I2
c
4p
zCdAp
rfv2
f
2

z4
3pR3rg
~2ps
_
mc
prlvc

1=2
ð9Þ
where only I
c
is unknown. The critical transition current I
c
can be calculated with different values of electrode dia-
meter and electrode extension. The analysis and calcula-
tion are based on the following assumptions:
1) The physical parameters of the liquid metal are con-
stant;
2) Melting rate is constant throughout the time of drop-
let growth;
3) The system is symmetrical about the electrode;
4) When globular transfer transforms to spray transfer,
the radius of the droplet equals the radius of the electrode.
3 Results and discussion
Tables 2–4 show that the calculated values of both critical
transition current and various force terms under different
levels of wire diameter and wire extension. It can be seen
that the term _
mcvc, the electromagnetic force F
em
and the
surface tension force F
s
are the main forces which affect
the critical transition current most greatly. If the plasma
drag force F
d
and the gravitational force F
g
are ignored,
Eq. (9) can be written as:
b
_
mc
I2
c
za~
_
m1=2
c
I2
cð10Þ
where aand bare constants, a~ecx2m0
4p
:ffiffiffiffiffiffiffiffiffi
prvc
p
2ps,b~
vcffiffiffiffiffiffiffiffiffi
prvc
p
2ps.
Fig. 3 Sketch of welding current distributing
Table 2 The critical transition current and force terms (electrode
diameter 1.6 mm)
L/mm I
c
/A _
mcvc/
10
24
N
F
em
/
10
24
N
F
d
/
10
24
N
F
g
/
10
24
N
F
s
/
10
24
N
10 281 8.41 3.95 0.90 1.47 14.7
15 277 9.30 3.84 0.90 1.47 15.5
20 272 10.1 3.70 0.90 1.47 16.1
25 267 10.8 3.56 0.90 1.47 16.7
Table 3 The critical transition current and force terms (electrode
diameter 1.1 mm)
L/mm I
c
/A _
mcvc/
10
24
N
F
em
/
10
24
N
F
d
/
10
24
N
F
g
/
10
24
N
F
s
/
10
24
N
10 198 7.17 4.32 0.42 0.48 12.4
15 193 8.12 4.11 0.42 0.48 13.2
20 189 9.00 3.93 0.42 0.48 13.8
25 185 9.75 3.74 0.42 0.48 14.4
Determining the critical transition current for metal transfer in gas metal arc welding (GMAW) 399
Consequently, the model predicts a linear relationship
between _
m1=2
c.I2
cand _
mcI2
c, that is in accord with the
data measured by Lesenwich [9], as Fig. 4 illustrates.
The current at which transition from globular to spray
transfer begins (the critical transition current) depends on
a number of factors, including: (i) the composition of the
electrode wire, (ii) the electrode wire diameter, (iii) elec-
trode extension, and (iv) the composition of the shielding
gas [1]. For an argon shielding gas and mild steel electrode
wire, Eq. (10) is employed to predict the critical transition
current. Figure 5 shows that the critical transition current
from globular transfer to spray transfer decreases linearly
as the electrode extension becomes longer, and Fig. 6
demonstrates that the critical transition current increases
with an increase in the electrode diameter. These results
agree well with those observed experimentally.
4 Conclusions
A simple model is developed to determine the critical current
at which transition from globular to spray transfer begins. It
considers various forces exerted on the pendent droplet at
the electrode tip, especially the force acted on the drop by the
incoming molten metal through the neck of the droplet from
the wire, i.e., the term _
mcvc. Judging from the order of the
magnitude, the term _
mcvc, the electromagnetic force and the
surface tension force are the main forces which affect the
critical transition current most greatly. The model can be
used to predict the critical transition current at different
Table 4 The critical transition current and force terms (electrode
diameter 0.8 mm)
L/mm I
c
/A _
mcvc/10
24
NF
em
/
10
24
N
F
d
/
10
24
N
F
g
/
10
24
N
F
s
/
10
24
N
10 163 6.30 6.08 0.17 0.18 12.8
15 162 7.70 6.03 0.17 0.18 14.1
20 160 8.95 5.89 0.17 0.18 15.2
25 157 10.0 5.70 0.17 0.18 16.1
Fig. 4 The relationship between _
m1=2
c.I2
cand _
mcI2
cmea-
sured by Lesenwich [9]
Fig. 5 The influence of electrode diameter on the critical
transition current for mild steel
Fig. 6 The effect of wire extension on the critical transition
current for mild steel
400 Chuan-song WU, et al.
welding conditions, such as varying electrode diameters and
extensions. The calculated results are in agreement with
experimentally measured data in literature.
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353
Determining the critical transition current for metal transfer in gas metal arc welding (GMAW) 401
... Unfortunately, this transfer mode occurs only in a small range of welding current [13], and there is a transition region between this melting transfer mode and the globular transfer mode. The variation of transfer frequency and droplet size due to the current variation in this transition mode is high, and consequently the metal transfer is irregular and the weld quality is very low [10,14,15]. Therefore, when it is required to work in the projected transfer mode, it would be necessary to control the process to prevent from entering the transition region. ...
... The critical transition current I ac is the current level at which a transition from globular transfer mode to spray transfer mode occurs. It is therefore necessary to determine its value to specify the suitable transfer mode [14]. Based on the conclusions derived in [33], it is affirmed that F em equals F mf at this current level. ...
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New estimators are designed based on the modified force balance model to estimate the detaching droplet size, detached droplet size, and mean value of droplet detachment frequency in a gas metal arc welding process. The proper droplet size for the process to be in the projected spray transfer mode is determined based on the modified force balance model and the designed estimators. Finally, the droplet size and the melting rate are controlled using two proportional-integral (PI) controllers to achieve high weld quality by retaining the transfer mode and generating appropriate signals as inputs of the weld geometry control loop.
... Spray transferring occurs at high current density. A spray consists of many small droplets that are detached from the electrode by the action of a powerful electrodynamic force caused by high welding current (about 200 A when welding with a 1.1 mm wire) [7]. The droplet transfer process has great stability and no spatter. ...
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Principles of Welding
  • Rw Jr
Heat generation and heat flow in the filler metal inGMAwelding
  • J H Waszink
  • G J P M Van Den Heuvel
  • J. H. Waszink
Waszink J H, Van Den Heuvel G J P M. Heat generation and heat flow in the filler metal in GMA welding. Welding Journal, 1982, 61(8): 269-282
An analysis of the gas-shielded consumable metal arc welding system
  • J Amson
  • J. C. Amson
Amson J C. An analysis of the gas-shielded consumable metal arc welding system. Welding Journal, 1962, 41(4): 232-249
Experiment investigation of the force acting on a drop of weld metal
  • J H Waszink
  • L H J Graat
  • J. H. Waszink
Waszink J H, Graat L H J. Experiment investigation of the force acting on a drop of weld metal. Welding Journal, 1983, 62(4): 108-116
Control of melting rate and metal transfer in gas shielded metal arc welding
  • A Lesenwich
  • A. Lesenwich
Lesenwich A. Control of melting rate and metal transfer in gas shielded metal arc welding. Welding Journal, 1958, 37(4): 343-353