Pulsed Current GMA Welding SS

8/2/2019 Pulsed Current GMA Welding SS

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j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 9 ( 2 0 0 9 ) 12621274

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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j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 9 ( 2 0 0 9 ) 12621274 1263

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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1264 j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 9 ( 2 0 0 9 ) 12621274

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







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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