pulsed current gma welding ss
TRANSCRIPT
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j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / j m a t p r o t e c
Arc characteristics and behaviour of metal transfer in pulsed
current GMA welding of stainless steel
P.K. Ghosh a,, Lutz Dorn b, Shrirang Kulkarni a, F. Hofmann b
a Department of Metallurgical & Materials Engineering, Indian Institute of Technology Roorkee, Roorkee 247 667, Indiab F ugetechnik und Bechichtungtechnik, Sekr. PTZ 6, Pascal strasse 8-9, TU Berlin, 10587 Berlin, Germany
a r t i c l e i n f o
Article history:
Received 20 January 2008
Received in revised form
19 March 2008
Accepted 23 March 2008
Keywords:
Austenitic stainless steel
P-GMAW
Welding parameters
Arc characteristics
Metal transfer
Arc pressure
Stability in shielding
a b s t r a c t
The variation in arc characteristics, stability in shielding of arc environment and behaviour
of metal transfer with a change in pulse parameters have been studied by high speed video-
photography during pulsed current gas metal arc (P-GMA) weld deposition using austenitic
stainless steel filler wire. A comparative study of similar nature has also been carried out
duringgas metal arc (GMA) weld deposition in globular andspray transfer modes. Theeffect
of pulse parameters has been studied by considering their hypotheticallyproposed summa-
rizedinfluence defined by a dimensionless factor = [(Ib/Ip)ftb], mean current andarc voltage
and correlation between welding parameters and arc characteristics have been established.
The arc characteristics studied by its root diameter, projected diameter, length and stiffness
measured in terms of arc pressure and the behaviour of metal transfer noted by the droplet
diameter and velocity of droplet at the time of detachment have been found to vary sig-
nificantly with the variation in . At a given the experimentally measured values of the
behaviour of metal transfer are found well in agreement to their corresponding theoretical
values estimated through mathematical expressions reported earlier. The increase of and
the ratio of (Ib/Ip) have been found to adversely affect the stability of shielding jacket and
arc profile especially at high arc voltage.
2008 Published by Elsevier B.V.
1. Introduction
The advent of pulsed current gas metal arc welding (P-GMAW)
in critical applications of different ferrous (Dorling, 1992)
and non-ferrous alloys (Ghosh and Dorn, 1994) to improve
the weld characteristics over those observed in case of con-ventional continuous current gas metal arc welding (GMAW)
(Lyttle, 1983) is fairly well established by several workers.
But it is often pointed out that the quality of P-GMA weld
very much depends upon arc characteristics (Ghosh et al.,
1999) and behaviour of metal transfer (Randhawa et al., 2000)
affecting the energy distribution in welding, as discussed in
case of preparation of weld (Ghosh et al., 2000a) and bead on
Corresponding author. Tel.: +91 1332 285699; fax: +91 1332 285243.E-mail address: [email protected] (P.K. Ghosh).
plate weld deposition (Ghosh et al., 2000b), dictated by the
pulse parameters. Generally two kinds of metal transfer, such
as one droplet detachment per pulse and multiple droplets
detachment per pulse are considered in P-GMAW process
by keeping the pulse current at just above (Subramanium et
al., 1999) and far higher than (Wu et al., 2005) the transitioncurrent respectively. The transition current is defined by the
current shifting the behaviour of metal transfer (Wang et al.,
2004) from the gravitational to spray mode. The process of
one drop transfer per pulse with relatively low rate of metal
transfer is popularly used in joining of thin section, whereas
the multiple droplet transfer per pulse with proper control
of arc characteristics and the behaviour of metal transfer
0924-0136/$ see front matter 2008 Published by Elsevier B.V.
doi:10.1016/j.jmatprotec.2008.03.049
mailto:[email protected]://dx.doi.org/10.1016/j.jmatprotec.2008.03.049http://dx.doi.org/10.1016/j.jmatprotec.2008.03.049mailto:[email protected] -
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resulting in high deposition rate finds wide spread application
of P-GMAW process in weld fabrication of different materials
of varied section size. But, the control of arc characteristics
and the behaviour of metal transfer by appropriate selection
of pulse parameters in P-GMAW process (Ghosh et al., 2004)
are quite critical due to simultaneous influence of relatively
large number of parameters on each other during welding
(Ghosh et al., 2006). It involves the pulse parameters as meancurrent (Im), pulse current (Ip), base current (Ib), pulse time
(tp),base time(tb), pulse frequency (f) as well as arc voltage (V).
However, the difficulties in controlling the pulse parameters
with respect to appropriate operation of P-GMAW process
have been well addressed by considering a summarised
influence of pulse parameters defined by a hypothetical factor
= [(Ib/Ip)ftb] derived from the energy balance concept (Ghosh
et al., 2007a) where, tb =[(1/f) tp].
The difficulties in proper selection of pulse parameters
adversely affecting the weld quality get further compounded
when theP-GMAW processis applied to heat-sensitivemateri-
alsof lowthermal conductivity and high coefficient of thermal
expansion such as austenitic stainless steel (ASS). Due to itscomparatively lower thermal conductivity and higher coef-
ficient of thermal expansion than the structural steel, the
HAZ and weld of arc welded ASS becomes comparatively
more prone to sensitization and development of consider-
able stresses respectively. Thus, during welding of ASS the
pulse parameters should be more critically selected for effi-
cient energy distribution in the arc leading to comparatively
low heat built-up in weld pool. In this regard the knowledgeof
correlation of with the arc characteristics and behaviour of
metal transfer may be very much useful for more precise andwide spread application of P-GMAW process in welding of ASS
of improved weld quality. But, hardly any understanding has
been reported so far in this area.
In view of the above an effort has been made to study the
effect of at various pulse parameters and arc voltage on
the characteristics of arc and behaviour of metal transfer in
P-GMAW process with the help of high speed video-grapy of
arc environment during bead on plate weld deposition using
1.2 mm diameter SG-1.4316-2CrNi199 ASS filler wire in argon
shielding. A similar study has also been carried out during
GMAW process operated in globular and spray transfer modes
of metal transfer to further analyse and compare the influence
of weld parameters on arc characteristics. The study providesa basic understanding to analyse the primary mechanisms
of P-GMAW process dictated by the summarised influence
Table 1 The pulse parameters giving stable arc at different
Wire feed rate (mm/s) Im (A) Ip (A) Ib (A) (Ib/Ip) tp (ms) tb (ms) f(Hz) Arc voltage (V)
158 0.07 255 370 55 0.15 2.7 2.3 200 25
158 0.11 258 396 95 0.24 2.7 2.3 200 24
142 0.18 244 374 108 0.29 2.7 4.4 140 24
125 0.19 246 400 124 0.31 2.7 4.0 150 24
158 0.20 248 320 106 0.33 2.7 4.0 150 24
158 0.24 256 300 102 0.34 2.3 5.7 125 25
92 0.27 248 355 157 0.44 3.4 5.5 113 25
142 0.09 212 410 59 0.14 2.8 4.3 140 23
125 0.17 214 274 84 0.31 3.7 5.0 110 22
100 0.17 215 414 108 0.26 2.7 5.3 125 22
125 0.21 220 320 109 0.34 2.9 4.6 133 25
100 0.23 210 360 122 0.34 2.7 5.3 125 24
125 0.07 200 390 45 0.16 2.7 4.0 150 23
92 0.08 206 430 58 0.14 3.7 4.8 118 23
125 0.10 205 376 57 0.15 2.8 4.9 130 24
100 0.15 205 400 88 0.22 2.7 5.3 125 23
125 0.15 204 331 80 0.24 2.8 4.9 129 24
92 0.18 207 380 93 0.25 2.7 6.8 105 24
92 0.25 200 310 109 0.35 2.6 6.6 109 25
142 0.06 211 319 32 0.10 2.6 4.6 138 19
125 0.06 199 333 32 0.10 2.7 5.0 130 19
125 0.12 208 313 58 0.19 2.8 4.9 130 20
125 0.21 209 290 104 0.36 2.7 4.0 150 19
125 0.23 201 315 123 0.39 2.7 4.0 150 21
125 0.26 207 280 124 0.44 2.7 4.0 150 20
92 0.05 181 362 40 0.11 6.4 5.5 84 18
92 0.11 182 354 57 0.16 2.9 6.2 110 19
108 0.16 189 305 76 0.25 2.9 5.2 124 20
92 0.20 179 303 85 0.28 2.7 6.8 105 21
92 0.26 188 310 110 0.36 2.6 6.8 107 21
100 0.08 160 340 40 0.12 2.7 5.3 125 20
100 0.12 161 350 64 0.18 2.7 5.3 125 19
100 0.19 165 311 90 0.29 2.7 5.3 125 19
100 0.23 173 304 106 0.35 2.7 5.3 125 20
100 0.27 175 295 121 0.41 2.7 5.3 125 20
http://dx.doi.org/10.1016/j.jmatprotec.2008.03.049http://dx.doi.org/10.1016/j.jmatprotec.2008.03.049http://dx.doi.org/10.1016/j.jmatprotec.2008.03.049http://dx.doi.org/10.1016/j.jmatprotec.2008.03.049http://dx.doi.org/10.1016/j.jmatprotec.2008.03.049http://dx.doi.org/10.1016/j.jmatprotec.2008.03.049http://dx.doi.org/10.1016/j.jmatprotec.2008.03.049 -
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of pulse parameters which may affect the weld characteris-
tics.
2. Experimental
2.1. Welding
Studies on arc characteristics and behaviour of metal trans-
fer were carried out by bead on plate weld deposition on
10 mm thick stainless steel plate by P-GMAW and conven-
tional GMAW processes using direct current electrode positive
(DCEP) CLOOS Quinto GLC 403 welding power source. The
weld deposition was made by employing 1.2mm diame-
ter SG-1.4316-2CrNi199 stainless steel filler wire at electrode
extension of 12mm under argon shielding at a flow rate of
18 l/min. The welding was performed with stable arc at wide
variation of pulse parameters as typically shown in Table 1
and the process characteristics have been studied as a func-
tion of welding parameters. The deposition of weld bead wasmade by operating the power source at certain pulse param-
eters of varying lying in the range of 0.0530.27 at different
arc voltages. The pulse characteristics, such as Ip, Ib, tp and f
were measured with the help of a transient recorder (max-
imum resolution of 1 MHz) fitted with the electrical circuit
of the welding set up. The arc voltage (V) and the Im were
estimated as mean values of the voltage and current plots
respectively of the pulse behaviour captured by the transient
recorder as typically shown in Fig. 1(a) and (b) respectively. In
order to compare the observations of P-GMAW the arc charac-
teristics of conventional GMAW at different parameters have
also been studied by keeping welding current of some of its
parameters similar to certain mean currents of P-GMAW, asshown in Table 2. During welding the arc environment was
video-graphed with the help of a high speed camera operated
at a speed 104 frames per second. The camera was placed on
a rigid fixture in front of the arc along the line of welding. The
observations on the photographs are analysed with respect
to the P-GMA welding parameters by classifying the Im into
three different ranges of 2506 A, 2144A and 2043A at
the arc voltage of the order of 24 1 V and also into three dif-
ferentranges of 2064A,1844and1677 A at the relatively
lower arc voltage of 20 1V. However in GMA welding, the
Fig. 1 Typical behaviour of pulse observed during P-GMA
welding.
Table 2 The GMA welding parameters studied inglobular and spray transfer modes
Wire feedrate (mm/s)
Weldingcurrent (A)
Arc voltage(V)
Remarks
67 150 23 Stable arc
83 157 23 Stable arc
92 166 23 Stable arc
92 170 24 Stable arc
100 190 24 Stable arc
117 195 24 Stable arc
158 205 25 Stable arc
photographs have been analysed with the variation in weld-ing current in the range of 150205 A at a given arc voltage of
241 V.
2.2. Measurements of arc characteristics and metal
transfer
The nature of variation in arc characteristics and the
behaviour of metal transfer with the change in pulse param-
eters in P-GMAW and similarly with the change in welding
current in conventional GMAW have been studied on the high
Fig. 2 (a) Schematic diagram showing different dimensions of arc and (b) typical nature of electrode tip.
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Fig. 3 Schematic diagram showing measurement of arc
blow in terms of arc deflection from electrode axis.
speed video-graphs of the welding operation. The arc charac-
teristics defined by its root diameter (DR), projected diameter
(DP) and length (L) was suitably measured during pulse on
period by appropriatecomputerizedscaling technique applied
on number of photographs of each welding parameter as
schematically shown in Fig. 2. In order to maintain a unifor-
mity in comparablemeasurement the DR and L were measured
with the help of a computerized projection on the arc profile
through the point of curvature of downward flow of arc cav-
ern as depicted in Fig. 2. Whereas, during pulse off period the
observed deviation of arc from the electrode axis (appeared to
be arc blow) was measured by a similar computerized scaling
technique as schematically shown in Fig. 3. The behaviour ofmetal transfer was also studied by measurement of diameter
(D) of the droplets using the similar computerized technique.
The droplets occasionally revealed in the background of the
glair of arc during their transfer to the weld pool at different
welding parameters. In most of the cases they became visible
for a while immediately after a pulse approaching transition
of current from the peak to the base level with diminishing
brightness of the arc. However, just as a matter of chance the
droplets were also visible in few cases on certain consecu-
tive frames of video-graphs during their initiation of travel
towards weld pool just after detachment from the electrode
tip. During their entire path of travel the droplets could not be
visible primarily due to arc glair and excessive glow of molten
metal resulting from high heat intensity. Thus, only in some
cases the droplet detachment velocity (Vi) could be estimated
by measuring the shifting of position of a droplet as it travels
further with respect to electrode tip towards the weld pool on
consecutive frames at the given speed of video-graphy.
3. Results and discussion
The arc characteristics and behaviour of metal transfer affect-
ing the quality of weld is largely dictated by the influence
of respective welding parameters of the P-GMAW and GMAW
processes on arc profile, arc pressure, stability in shielding of
arc environment as well as nature of droplets transferred dur-
ing welding. The nature of arc defined by its root diameter,
projected diameter and length largely denotes the degree of
constriction and stiffness of arc affecting the weld character-
istics. However in contrast to GMAW process wherein a steady
arc exists during welding, the arc characteristics of P-GMAW
process are generally considered in two primary phases of
strong and weak arc of the pulse on and pulse off periodsrespectively of the process as typically shown in Fig. 4(a) and
(b) respectively.
3.1. Arc characteristics of P-GMAW process
3.1.1. Arc profile of pulse on period
Although the arc profile observed in video-graphs may not be
a true profile of the arc causing over estimation in measure-
ment due to covering by glare of plasmatic part of shielding
gas around it, but its nature of response to welding parame-
ters is a matter of great interest to understand its influenceon
weld quality. During pulse on period (tp) the typical arc char-
acteristics at the , Im and arc voltage of 0.27, 248 A and 25 Vrespectively has been shown in Fig. 5(a). Similarly at a com-
paratively lower , Im and arc voltage of 0.05, 181A and 18 V
respectively the arc characteristics of the pulse on period has
been shown in the photograph presented in Fig. 5(b).The pho-
tographs (Fig. 5(a) and (b)) reveal that the variation in , arc
voltage andmean current significantly influences thearc char-
acteristics of pulse on period. Such a variation in the nature
of arc characteristics at a close range of Ip of the order of
355362 A may have primarily occurred due to difference in
energy distribution in entire pulse system balanced by the
application of lower arc voltage and Ib of 20 V and 40A respec-
tively at longer tp of 6.4 ms (Table 1). It is also observed that at
a given of the order of 0.050.06, a stable, but relatively weak
Fig. 4 Typical appearance of arc during (a) pulse on time at = 0.06, Im =211A and V= 19 V and (b) pulse off time at =0.21,
Im =220A and V=25V.
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Fig. 5 Typical change in arc characteristics at different welding conditions (a) = 0.27, Im =248A, V= 25V and (b) = 0.05,
Im =181A, V=18V.
arc exists at a relatively lower Im as shown in Fig. 5(b) than
that observed in Fig. 4(a).
At a given arc voltage of the order of 24 V the influence of
on the arc length (L), arc root diameter (DR) and projected
arc diameter (DP) has been shown in Fig. 6(ac) respectively
when the Im is varied in a range of the order of 250204A assaid earlier. Similarly at a given relatively low arc voltage of
the order of 20 V the influence of on the L, DR and DP has
been shown in Fig. 7(ac) respectively when the Im is varied in
a range of theorderof 206167 A. The Figs.6 and 7 show that at
both the comparatively high and low arcvoltages the L, DR and
Dp enhances significantly with the increase of irrespective of
variation in Im at the relatively high and lowlevels in the range
of the order of 250167A. Whereas at a given and arc voltage
the increase ofIm has been found to reduce the DR and Dp but,
to enhance the L appreciably. However, at a given close range
of Im of 204206A (Figs. 6 and 7) the rate of increase of L, DRand Dp with respect to varies significantly with the change
in arc voltage from 24 to 20 V. It is marked that the arc lengthbecomes comparatively more sensitive to and Im at the lower
arc voltage of 20 V, while an opposite behaviour is observed in
case of the DR and Dp. Thus, it can be inferred that the arc
characteristics with respect to its stiffness and spreading over
theweld canbe significantlycontrolled byvarying, Im andarc
voltage by following the empirical correlation as given below.
For arc voltage of 241 V
L(24V) = 19.95 0.0086Im 0.0631Im + 7.56 (i)
DR(24 V) = 7.28 + 0.025Im + 0.054Im 2.49 (ii)
DP(24V) = 22.85+ 0.032Im 0.014Im + 4.02 (iii)
For arc voltage of 201 V
L(20 V) = 3.38 0.017Im + 0.017Im + 7.97 (iv)
DR(20 V) = 14.7+ 0.027Im 0.037Im 0.655 (v)
DP(20V) = 26.23+ 0.069Im 0.075Im 1.17 (vi)
The expressions (i) and (iv) of the arc voltages of 24 and
20V respectively resolves that during welding under the given
conditions of relatively low and high (Table 1) of 0.05 and
0.25 respectively the arc extinguishes (L =0) at the Im of 728
and 515 A respectively with the arc voltage of 24 V and at the
Im of 504 and 691 A respectively with the arc voltage of 20 V.
It may primarily happen due to high wire feed rate when it
burns off or touches the job without allowing enough time to
transfer droplet from the electrode. Such a logical agreement
of expressions (i) and (iv) with the physical implications to agreat extent justifies their use for control of arc characteristics
in specific application of P-GMAW. From the solutions of the
Eqs. (i) and (iv) it appears that at the higher arc voltage of 24 V
the increase of from 0.05 to 0.25 extinguishes arc at compara-
tively lower Im (corresponds to lower wirefeed speed), whereas
the situation becomes opposite in case of the lower arc volt-
age of 20V. Thus, it may be inferred that keeping a lower
and higher arc voltage of about 0.05 and 24 V respectively is
more beneficial for P-GMAW of thicker section with stable arc
at higher wire feed rate (higher Im) but, at lower arc voltage of
the order of 20 V maintaining a higher of the order of 0.25
is more useful in this regard. This may have primarily hap-
pened because at higher arc voltage the increase of arc lengthbecomes significantly more sensitive to at lower mean cur-
rent (Fig. 6(a)), whereas at lower arc voltage the increase of arc
length becomes relatively more sensitive to at higher mean
current (Fig. 7(a)).
3.1.2. Arc profile of pulse off period
At the Ib, tb and arc voltage of 109 A, 4.62ms and 25 V (Fig. 3(b))
respectively a short deflected arc exists in pulse off time with
no appreciable geometry of extension in between the electrode
and base material. However, depending upon the magnitude
of Ib, tb and arc voltage an arc of recognisable geometry may
exist in pulse off period. In that case knowledge about its pro-
file at different pulse parameters may be further useful tounderstand the thermal efficiency of P-GMAW process.
During pulse off time (tb) the characteristics of weak arc
observed at different pulse parameters and arc voltages have
been qualitatively studied. At a given Im of 204206 A an arc
of appreciable geometry has been found to exist in pulse off
period having Ib and lying in the range of 32109A and
0.060.25 respectively as shown in Figs. 8 and 9 for the arc
voltages of the order of 24 and 20 V respectively. The typical
nature of arc characteristics depicted in Figs. 8 and 9 reveal
that the arc stability considered by its nondeflected intense
white appearance, is comparatively better at relatively lower
of the order of 0.06 in comparison to that observed at higher
of 0.25.
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Fig. 6 At differentIm the effect of on (a) arc length (b) arc
root diameter and (c) arc projected diameter during pulse
on time at a given arc voltage of the order of 24 V.
Fig. 7 At differentIm the effect of on (a) arc length (b) arc
root diameter and (c) arc projected diameter during pulse
on time at a given arc voltage of the order of 20 V.
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Fig. 8 At a given Im and arc voltage of the order of 204 A and 24 V respectively the arc characteristics of pulse off time
under different pulse parameters of (a) = 0.07, Ib = 45A and (b) =0.25, Ib =109A.
Unlike to that observed in case of P-GMAW of aluminium
alloy (Ghosh et al., 2007a), the weak arc at low Ib has been
found to be deflected from its vertical axis appearing as arc
blow as a function of and Ib. The effect of and Ib on the
arc deflection at pulse off period has been shown in Fig. 10(a)
and (b) respectively. It is observed that irrespective of the
Ib varying in the range of about 40160 A an increase of
predominantly enhances the arc deflection almost linearly.
However, at a given close range of an increase ofIb reduces
the arc deflection significantly. Even a small arc in pulse
off time also shows such behaviour as revealed in Fig. 4(b).
It is further observed that at a moderate Im of the order
of 204206A the increase in ratio of (Ib/Ip) in the range of
0.3510.443 enhances the arc blow significantly at both the
high and low arc voltages of the order of 24 and 20 V as shown
in Figs. 8(b) and 9(b) respectively. But, at the similar orders of
Im no such behaviour of arc has been observed at low (Ib/Ip) in
the range of 0.10.115 when the arc voltage varied to the same
levels of the order of 24and 20V asshown in Figs. 8(a) and 9(a)
respectively. As the increase of enhances (Table 1) the (Ib/Ip)
the arc blow also happened primarily to be there at higher
of 0.25 and 0.17 as shown in Figs. 8(b) and 9(b) respectively. At
this stage it appears to be interesting to point out that such
biasness in directionality of arcing also sometime observed
in pulse on period at high (Ib/Ip), especially at high arc voltage
of 241V, as shown in Fig. 5(a). The occurrence of such
behaviour of arc may create irregularities in heat distribution
and related thermal characteristics of weld joint.
Fig. 9 At a given Im and arc voltage of the order of 206 A and 20 V respectively the arc characteristics of pulse off time
under different pulse parameters of (a) = 0.06, Ib = 32A and (b) =0.17, Ib =84A.
3.1.3. Arc profile of conventional GMAW
The variation in arc profile with a change in welding parame-
ter is also marked in conventional GMAW process. The typical
changes in arc characteristics at a given welding current range
of 150205A with constant arc voltage of 24 1V have been
compared in Fig. 11(a)(d). The photographs reveal that arc
blow primarily occurs at a comparatively lower welding cur-
rent of the order of 150170 A wherein globular metal transfer
exists. In this range of welding current the arc deflection from
the electrode axis comes down from about 22 to 15 with
the increase of current upto 170 A. However, at the high weld-
ing current beyond about 190 A, establishing spray mode of
metal transfer, the arc deflection practically becomes neg-
ligible (Fig. 12). Such a variation in arc characteristics may
have primarily occurred during globular transfer mode possi-
bly because the arc does not cover the relatively larger dropletformed at the electrode tip due to comparatively lower elec-
tromagnetic pinch force (Rhee and Asibu, 1991).
3.2. Arc stiffness
The stiffness of arc as a function of welding parameters plays
an important role to avoid its deflection from central axis
which adversely affects the energy concentration in weld. The
arc stiffness is generally considered as a direct function of arc
pressure. The arc pressure (Pa) can be estimated with the help
of an equation derived from total pressure distribution at the
perturbed boundary of solidliquid interface (Lancaster, 1987)
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Fig. 10 Influence of (a) and (b) base current on arc
deflection in P-GMAW process.
by assuming arc as a hollow conducting fluid cylinder of inner
and outer radius equal to equilibrium radius of the moltenmetal (R) and arc rootradius (Ra) respectively (Rhee and Asibu,
1991). The Pa is expressed as follows
Pa =0J
2a
4[R2a R
2 2oR cos(t) cos(kz)] (vii)
where Ja = Ip/R2a is the arc current density during pulse on
period, is angular frequency, k is a wave number and o is
the amplitude of the perturbation parameter. Eq. (vii) can be
resolved (Rhee and Asibu, 1991) as follows with the help of the
expression of pressure (P1) due to surface tension attributed
to the cylindrical radius (R1) at theperturbed boundary as pro-
posed earlier.
P1 =
R1=
R
1
0
Rcos(t) cos(kz)
(viii)
o cos(t) cos(kz) = RR2
R1(ix)
3.2.1. Arc stiffness in P-GMAW
Considering Eqs. (vii) and (viii) the expressions for estimationof arc pressure Pa in P-GMAW is finally derived as follows.
Pa =0I
2P
42R4a
R2a 3R
2+
2R3
R1
(x)
where, the R and R1 are assumed as the size of droplet
radius (D/2) and effective radius (r) of tapering of electrode
respectively. The performance of P-GMAW process is primar-
ily characterised by its pulse current where the back ground
current maintains the continuity of the process. Thus, the
arc pressure Pa as a measure of arc stiffness of P-GMAW pro-
cess may be primarily considered as a function of IP and the
geometry of effective part of the arc along its vertical axis cor-roborating the metal transfer as shown in the expression (x).
However, in case of existence of an arc of noticeable geometry
at low current (Ib) ofpulseofftime the Pa can also be estimated
by the expression (x) for comparatively higher Ra, R and R1 at
negligible electrode tapering. In case of welding with or with-
out metal transfer in pulse off time the R may be assumed as
equal to R1 and accordingly the Pa at Ib becomes appreciably
lower than that of Ip. The intensity of this fluctuation on Paunder the pulsed current depends upon the ratio of (Ib/Ip) at
different Im and . However, the effect of such fluctuation of
arc pressure on the arc environment is comparatively more
significant in case of a softer long arc at high arc voltage than
a stiffer arc of low arc voltage.At the arc voltages of the order of 24 and 20 V the influ-
ence of variation in at different Im on the arc pressure (Pa)
estimated at IP has been shown in Fig. 13(a) and (b) respec-
tively. The figure shows that at both the comparatively high
and low arc voltages the arc pressure or stiffness decreases
significantly with the increase of at any mean current lying
in the range of 167250 A. At a given a significant enhance-
ment in arc stiffness occurs with theincrease ofIm at both the
arc voltage of 24 and 20 V. However, at a given Im of the order
of 204206 A themaintaining of higher arc voltage of the order
of 24 V gives rise to larger arc stiffness than that observed in
case of working at lower arc voltage about 20 V when the
is kept constant. In view of these, the arc stiffness in termsof arc pressure as a function of and Im at the high and low
arc voltages of the order of 24 and 20 V have been empirically
correlated as follows.
For the arc voltage of 24 1 V
ln(Pa) = 3.87 ln() 5.88 ln(Im) 0.95 ln(Im) ln()+ 28.64
(xi)
For the arc voltage of 20 1 V
ln(Pa) = 4.01 ln() 1.09 ln(Im)+ 0.65 ln(Im) ln()+ 3.59
(xii)
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Fig. 11 Typical arc characteristics observed in GMAW at a current of (a) 150 A (b) 170 A (c) 190 A and (d) 205 A and constant
arc voltage of 241 V.
3.2.2. Arc stiffness in conventional GMAW
The arc pressure (Pa) of conventional GMAW process under
globular and spray modes of metal transfer has been esti-
mated by substituting Ip of Eq. (x) by the welding current
(I). The effect of welding current on arc pressure has been
shown in Fig. 14. The figure depicts that at a given arc volt-
age of 24 V the arc pressure enhances considerably upto about
0.30.35Kpa with the increase of welding current upto thetransition current of spray mode of metal transfer of the order
of 170190A followed by a insignificant change in it with a
Fig. 12 Arc deflection observed with the change in
welding current at a given arc voltage of 24 V in GMAW
process.
further increase of welding current. Whereas at a given arc
voltage of 24 V a similar range of arc pressure can be obtained
even at a mean current and of about 204 A and 0.2 respec-
tively, when the arc pressure can be considerably increased
further by increasing the mean current and lowering down the
to about 0.05 as shown in Fig. 13(a). Thus, it may be realised
that the use of P-GMAW process at appropriate Im and may
be beneficial to achieve higher penetration in weld pool bymaintaining a spray mode of metal transfer at peak current
than that can be achieved by employing conventional GMAW
process.
3.3. Behaviour of metal transfer
The diameter of droplet (D) as observed in the video-graph
is typically revealed in the photograph shown in Fig. 15.
The detachment of droplet from the electrode followed by
its transferring movement captured in consecutive frames of
video-graphs, facilitating the measurement of its detachment
velocity (Vi), is typically shown in Fig. 16. The reliability of
the measured D and Vi has been verified by comparing themwith their theoretical values estimated at the same welding
parameters with the help of the expressions reported earlier
(Randhawa et al., 2000) as stated below.
Vi = (2/dr)1/2[1+ 0.187 1.2260.142]
1/2(xiii)
D = 4r/(1 + 3/16) (xiv)
= 0 I2p/(
2r) (xv)
The expressions have been evaluated with the help of
the experimentally measured values of effective radius (r) of
tapering of electrode (Fig. 2(a) and (b)) as 0.770.16 mm and
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Fig. 13 At differentIm the effect of on arc pressure (Pa)
during pulse on time at a given arc voltage of the order of
(a) 24V and (b) 20 V.
arc length (L) as stated above where, is coefficient of sur-
face tension (1.2 N m1), d is density of molten filler metal
(7507kgm3) (Wang et al., 2004) and 0
is the permeability of
free space (4107 N A2) (Lancaster, 1987).
Fig. 14 At a given arc voltage of the order of 24 V the effectof welding current on arc pressure (Pa) during globular and
spray modes of metal transfer.
Fig. 15 Typical appearance of a droplet as revealed in the
video-graph at =0.18, Im = 244 A and arc voltage of 24 V.
The comparison given in Table 3 depicts that in spite of the
inherited heterogeneity of welding process and considerable
difficulties in proper revealing of droplet size due to intense
glare, in most of the cases the measured D and Vi are well
in agreement to their theoretically estimated values with an
average difference of 20.7% and 7.25% respectively. During the
use of pulsed current in GMAW theD and Vi
primarily depends
upon Ip. Thus, the influence ofIp on measured D and Vi under
Fig. 16 Video-photographs showing typical tapering of electrode followed by transfer of droplet at a pulse parameter of
=0.08, Im = 206 A and arc voltage of 23 V.
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Table 3 Comparison between estimated and measured diameter (D) and detachment velocity (Vi) of droplet at different
Wire feedrate (mm/s)
Im (A) IP (A) Vi Est. (m/s Vi Obs. (m/s) Difference (%) D Est. (mm) D Obs. (mm) Difference (%)
158 0.20 248 320 2.21 2.57 16.3 0.20 0.25 25
158 0.24 256 300 1.22 1.29 5.7 0.53 0.50 5.7
92 0.27 248 355 1.98 1.72 13.1 0.25 0.32 28
125 0.21 220 320 2.09 2.14 2.4 0.23 0.28 21.7142 0.06 211 319 1.90 1.93 1.6 0.27 0.33 22.2
125 0.17 214 274 1.61 1.54 4.4 0.23 0.28 21.7
Average difference (%) 7.25 Average difference (%) 20.7
different lying in the range of 0.060.26 and arc voltages of
24 and 20 V has been shown in Fig. 17(a) and (b) respectively.
The figures depict that the D and Vi of metal transfer in P-
GMAW process predominantly depends upon Ip irrespective of
and arc voltage. However, in agreement to the observations
Fig. 17 Under different and arc voltages the effect of IPon (a) droplet diameter (D) and (b) detachment velocity (Vi)
of droplet.
(Table 4) of earlier works on steel ((Khim and Eagar, 1993) and
aluminium (Subramanium et al., 1998)) it mostly appears that
at a given arc voltage the increase of shows a tendency to
enhance D but to reduce Vi, whereas at a given the increase
of arc voltage generally shows an affinity to enhance both the
D and Vi relevantly with respect to their corresponding Ip.
3.4. Stability in arc shielding
The stability of shielding of arc environment in gas metal arc
welding process may be primarily considered through uninter-
rupted arc profile. The interruption of arc profile and shielding
in P-GMAW process at an appropriate gas flow rate largely
depends upon the degree of fluctuation in arc pressure at dif-
ferent arcstiffness and arc length. The arclengthand stiffness
variesas a function of, Im andarc voltage,whereas thedegree
of fluctuation of arc pressure significantly depends upon the
ratio of (Ib/Ip). During pulse on period at a given Im and arc
voltage of the order of 250 5 and 241 V respectively, a con-
siderable disturbances in shielding environment and on nat-
ural bell shape of the arc has been observed with the increase
of from 0.18 to 0.27 as shown in Figs. 5 and 18 respectively,
which is in agreement to an earlier observation ( Ghosh et al.,
2007a) on aluminium alloy. Although such disturbance in arc
profile with the increase of from 0.06 to 0.26 is also marked
at a relatively low Im and arc voltage of 1936A and 201 V
respectively, but its intensity has been found to be practically
in significant as shown in Fig. 19(a) and (b),whichis alsoin the
line of earlier observation on aluminium alloy as stated above.
In this context here it may also be noted ( Table 1) that
the increase of enhances the ratio of (Ib/Ip) but reduces
the arc stiffness as function of its pressure (Fig. 13), when
both of them favours the adverse influence of fluctuation
Fig. 18 At a given Im and arc voltage of 243 A and 24 V
respectively the typical turbulence in arc shielding at
=0.18.
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Table 4 Estimated at different pulse parameters for droplet diameter reported in earlier works
Pulse parameters Calculated Observed droplet diameter (mm) Reference
Ip (A) Ib (A) f(Hz) tb (ms)
500 180 20 47.5 0.34 2.16 Khim and Eagar
(1993)400 180 10 95 0.43 2.40
300 180 5 190 0.57 2.96
400 70 400 1.9 0.13 0.90 Subramanium et al.
(1998)250 100 400 1.5 0.24 0.91
0 150 225 3.3 0.45 1.2
Fig. 19 At a relatively low Im and arc voltage of 1936 A and 201 V respectively the typical nature of arc profile with the
variation in (a) = 0.06 and (b) =0.26.
of arc pressure on stability in arc environment under the
pulsed current. As it is reported earlier in case of aluminium
alloy (Ghosh et al., 2007a) here also an increase in ratio of
(Ib/Ip) to about 0.289 and beyond has been found to signif-
icantly enhance the fluctuation in arc pressure at low arc
stiffness, especially at the high arc voltage of the order of
241 V, which considerably enhances the turbulence in
shielding environment and disturbs the arc profile as shown
in Figs. 5(a) and 18. A low , (Ib/Ip), Im and arc voltage ofthe order of 0.050.06, 0.10.11, 181199 A and 1820V is
generally found suitable for maintaining a practically stable
arc environment (Figs. 5(b) and 19(a)) but, a low arc voltage
may allow the increase of and (Ib/Ip) upto certain extent
with practically insignificant disturbance in arc environment
as it is shown in (Fig. 19(b)) with 0.26 and 0.355 respectively.
Thus, it can be inferred that at any level of Im the variation in
, (Ib/Ip) and arc voltage significantly influences the stability
of arc environment when their low values are beneficial to
maintain a stable arc environment in P-GMAW process.
As it occurs in case of welding of aluminium alloy (Ghosh et
al., 2007a), the fluctuation in arcing due to large variation in Ip
and Ib causing high (Ib/Ip) ratio creates instability in shieldinggas jacket resulting into formation of vortex at the boundary
of arc profile penetrating the arc environment. A less stiff arc
at longer arc length of higher arc voltage may become more
prone to such occurrence. This behaviour is typically marked
by arrows in Figs. 5(a) and 18. In both the Figs. 5(a) and 18
it also appears that in addition to fluctuation of arc pressure
some mechanism occurring at the area of contact of the arc
with the base plate is further evolving turbulence in shielding
gas disturbing the arc environment. However, during conven-
tional GMAW at high arc voltage of 241 V such disturbance
of arc environment has not been found to occur (Fig. 11(d)) at
high welding current beyond 200 A but upto a certain extent
at a relatively low welding current (Fig. 11(ac)) than this. This
mechanism may be studied further considering the charac-
teristic of plasma flow in arc environment of stainless steel
welding under the pulsed current of GMAW. An improper
shielding with formation of such vortex penetrating the arc
may cause air aspiration in arc environment and introduce
porosity and inclusion in the weld (Ghosh and Hussain, 2002).
In consideration of the observations of this work a control
of arc characteristics in P-GMAW by varying the pulse param-
eters can be effectively explored to use this process in variousapplications of welding of austenitic stainless steel ranging
from joining of thin sheet to thick sections. This is corrobo-
rating some earlier reported works where it is proposed that a
comparatively lower , Im and arc voltage can be satisfactorily
used in joining of thin sheets (Ghosh et al., 2007b), whereas an
intermediate range of Im and arc voltage with lower which
provides comparatively lower energy input should be used for
joining of thick sections considering the facts that lower
gives higher arc stiffness and droplet velocity (Ghosh et al.,
2006).
4. Conclusions
The study provides a physical realisation with basic under-
standing of the effects of at various pulse parameters
including the arc voltage on the characteristics of arc and
behaviour of metal transfer in P-GMAW process. It also crit-
ically compares the arc environment of the P-GMAW and
conventional GMAW processes during bead on plate weld
deposition of stainless steel.The various aspects of the studies
may be primarily concluded as follows.
1. Hypothetical factor defined as a summarised influence of
pulse parameters mayacts as a key to controlthe behaviour
of arc and metal transfer in P-GMAW process.
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2. Theincrease in from 0.05 to 0.27 enhances thearc length,
root and projected diameters of arc and droplet diameter,
whereas it reduces the arc pressure and velocity of droplet.
3. Especially at high arc voltage of 241V the increase of
from 0.05 to 0.27 enhances the arc blow irrespective of
mean current of P-GMAW. However, in case of conventional
GMAW an appreciable arc blow appears during low welding
current of globular metal transfer.
Acknowledgement
The authors thankfully acknowledge the financial support
provided by the Alexander von Humboldt Foundation, Bonn
to Prof. Dr. P.K. Ghosh for his stay in TU Berlin to carry out this
work.
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