---Effect of Pulse Current in GMAW

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Introduction

The distortion and shrinkage stress giv-ing rise to thermal straining of a weld jointlargely results from differential contrac-tion of the weld and neighboring basemetal arising out of the cooling cycle ofthe welding process. The thermal strainproduced along the direction of weldingresults from the longitudinal shrinkage,

whereas the strain produced in the com-ponent normal to the direction of weldingis caused by the transverse shrinkage

(Refs. 13). A combined stress resultingfrom the thermal strains reacting to de-velop internal forces causes weld distor-tion, commonly observed as bending(Refs. 1, 3). Welding distortion has a neg-ative effect on the dimensional accuracy ofassembly as well as external appearanceand mechanical properties of the weldjoint, which can add additional cost to rec-tify in commercial practices. Dependingupon length of the weld, the linear bend-ing stress may be due to longitudinalshrinkage (Refs. 1, 2). However, the trans-verse bending always remains significant

in reference to plastic upsetting in thezone adjacent to the weld. As a conse-quence of the in-homogeneous permanentdeformation of the weld and its adjacentarea, some residual stresses develop in theweld joint (Refs. 46). The presence ofresidual stresses in the weld joint may ad-versely affect the fatigue, stress corrosioncracking, and fracture mechanics proper-ties of the weld joint depending upon char-acteristics of the material (Refs. 58).Thus, a critical look into the developmentof shrinkage indulging distortion in weld-ing depending upon the process, proce-

dure, and parameter has always been felt

imperative to produce welds of superiorquality. A significant amount of experi-ments and numerical analyses have been

carried out for measurement and estima-tion of shrinkage and deformation of the weld joint, which explore basic under-standing of this area (Refs. 911). Buthardly any systematic work has been re-ported on estimation and measurement ofwelding deformation in thick plate withdifferent sizes of weld grooves and weld-ing parameters using the gas metal arcwelding (GMAW) process. This is espe-cially true in the case of pulsed current gasmetal arc welding (GMAW-P), where thesituation becomes more complex due toinvolvement of a relatively large number

of simultaneously interactive pulse pa-rameters, such as mean current (Im), pulsecurrent (Ip), base current (Ib), pulse time(tp), base time (tb), and pulse frequency (f)at different arc voltages (V). However, asolution to the critical control of pulse pa-rameters for the desired operation of theGMAW-P process has been well ad-dressed by considering a summarized in-fluence of pulse parameters defined by adimensionless hypothetical factor =(Ib/Ip) ftb derived on the basis of energybalance concept (Refs. 5, 8, 1214) where,tb is expressed as [(1/f) tp]. Thus, the con-

trol of thermal and metal transfer behav-ior as well as efficiency of the process,which may largely depend on interactivepulsed parameters, can be accomplishedin consideration of the factor .

In view of the above, an effort has beenmade in this work to estimate transverseshrinkage stress and bending stress bymeasurement of transverse shrinkage anddistortion under different weldingprocesses, procedures, and parametersduring welding of 16-mm-thick controlled-rolled high-strength low-alloy (HSLA)steel plates. At a given heat input and weldgroove size the effect of welding processes,

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KEYWORDS

Welding ProcessesWeld Groove SizeWeld DistortionShrinkage StressesBending StressesPulsed Current

GMAW

P. K. GHOSH ([email protected] ), K. DE-VAKUMARAN ([email protected]), and A.K. PRAMANICK ([email protected])are with Department of Metallurgical & MaterialsEngineering, Indian Institute of Technology Roor-

kee, Roorkee, India.

ABSTRACT

Multipass butt joining of 16-mm-thick microalloyed high-strength low-alloy (HSLA) steel plates was carriedout by gas metal arc welding (GMAW) with and without pulsed current ondifferent sizes of conventional andnarrow grooves. Effect of welding pa-rameters, thermal behavior, andgroove size on shrinkage stress, distor-tion, and bending stress were suitablymeasured or estimated in appropriateoccasions. It was observed that the useof pulsed current with controlled pa-

rameters can improve the characteris-tics of the weld joint with respect to itsdistortion and stresses, especially inthe case of suitable narrow groove welds. Influence of the concernedfunctions on the weld characteristicsstudied are appropriately correlatedand discussed.

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Effect of Pulse Current on Shrinkage Stressand Distortion in Multipass GMA

Welds of Different Groove SizesThe pulsed gas metal arc process was found to be beneficial in reducing shrinkage

stress and distortion in welds on 16-mm-thick HSLA steel

BY P. K. GHOSH, K. DEVAKUMARAN, AND A. K. PRAMANICK


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procedures, and parameters on variationof estimated shrinkage stresses in the welddeposit causing bending of the weld jointwere corroborated by the estimation ofbending stresses developed in the weldjoint through the measurement of platebending during welding. This provides anopportunity to physically confirm the ef-fect of various welding processes and pro-cedures on the stress generation in the weld joint. These observations may bebeneficial for using the GMAW-P processto produce desired weld quality, and alsomay form a basis for improvement in itsautomation.

Experimentation and Analysis

Welding

Controlled-rolled 16-mm-thick micro-al-

loyed HSLA steel plates of specificationSAILMA-350 HI/SA533 grade having thechemical composition given in Table 1 wereused in this work. Typical microstructure ofthe base metal is shown in Fig. 1. The platesin size of 110 110 mm were butt jointwelded by multipass deposition techniqueusing a single V-groove with an includedangle of 60 deg, which conformed to AWSspecification (Ref. 15), and also by using asuitably designed narrow groove with 11-and 8-mm groove openings, designated byNG-11 and NG-8, respectively, as schemat-ically shown in Fig. 2AC. The plates were

welded by continuous current gas metal arcwelding (GMAW) and pulsed current gasmetal arc welding (GMAW-P) processes atdirect current electrode positive (DCEP).The welding was performed by using 1.2-mm-diameter mild steel welding wire of

specification AWS/SFA 5.18ER-70S-6under argon (99.97%) gas shielding at aflow rate of 1618 L/min. During weldingthe distance between the nozzle to work-piece was maintained within 1718 mm. Arelatively long electrode extension was usedto facilitate welding gun manipulation in thenarrow groove for weld deposition with sat-isfactory root and groove wall fusion. Weld-ing of the plates was carried out usingsemiautomatic welding with a mechanizedwelding gun travel. The details of the weld-ing parameters used in this work are pre-sented in Table 2. Prior to welding, theplates were preheated to about 125130C,and after each pass, the deposit was allowedto cool down to room temperature followedby reheating to maintain an interpass tem-perature similar to that of preheating forsubsequent weld passes. The welding wascarried out by rigid clamping of one side ofthe weld joint, where the other side of it wasleft free to respond to any distortion result-ing from weld deposition, as schematicallyshown in Fig. 3. During multipass GMAwelding, the deflection of the free end of the

weld joint per weld pass was measured usinga dial gauge having least count of 0.01 mmplaced at a given distance of 100 mm fromweld centerline (LC) Fig. 3. The trans-verse shrinkage was measured at the centerof the weld groove at a given strain length(LS) (Ref. 2) of 100 mm Fig. 3. Aftereach pass of welding at any heat input, thetransverse shrinkage and deflection of theplate from its initial position as well as theamount of weld deposition were measured,followed by an estimation of transverseshrinkage stress and bending stress usingstandard mathematical expressions below.

The amount of weld deposition was esti-mated by measuring the weight gain of theplate after each weld pass. To maintain reli-ability in the study for weld characteristics,experiments were carried out on three to sixwelds for each welding parameter.

Estimation of Heat Input ()

The heat input () of GMAW andGMAW-P processes were estimated withconsideration of the heat generated by thewelding arc as a function of welding ormean current (I or Im), arc voltage (V),welding speed (S), and its efficiency (a)

as follows (Refs. 15, 16):

(1)The mean current (Im) of the GMAW-Pprocess may be expressed (Refs. 2, 17) as

(2)The a of the GMAW process using mildsteel filler metal under argon gas shieldinghas been considered as 70% (Ref. 18)whereas, according to the earlier statment,

the a in the case of the GMAW-P processoperating at different pulsed parametershas been worked out as a function of andIm as follows (Ref. 19).

a = 94.52 0.118Im 107.61

II t I t

t tm

b b p p

b p

=+( )+( )

= ( ) a mV I or I

S

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Fig. 1 Typical microstructure of base metal.

Fig. 2 Schematic diagram of single (A) V-groove,(B) narrow groove of 11-mm opening (NG-11),and (C) narrow groove of 8-mm opening (NG-8).

A B

C

Table 1 Chemical Composition (wt-%) of Base Metal

C Si Mn Cr Ni Cu Nb+Ti+V Al P S0.178 0.37 1.57 0.003 0.037 0.25 max 0.08 0.012 0.002

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+ 0.348Im (3)

Estimation of Total Heat Transferred to

Weld Pool (QT)

The total heat transfer to the weld pool(QT) of the GMAW and GMAW-Pprocesses was estimated (Ref. 12) withconsideration of the arc heat transfer tothe weld pool (QAW), heat of the filler

metal transferred to the weld pool (Qf),and welding speed (S) as follows.

(4)The QAW of the GMAW-P process is esti-mated as

QAW = (VIeff Ieff)a (5)

Where and Ieffare the effective melt-ing potential at anode and effective cur-

rent (root mean square value of the pulsedcurrent wave form), respectively. TheQAW of the GMAW process was estimatedby substituting Ieff of Equation 5 withwelding current (I). The Ieff is estimated(Refs. 2, 20) by the following expression:

(6)Where, the pulse duty cycle is

kp = tp/tpul (7)

The Qf for the GMAW and GMAW-Pprocesses was estimated (Refs. 12, 13, 17)

as follows:For GMAW: Qf= Qde mt (8)

For GMAW-P: Qf= Qde mt f (9)

Where mt is mass of filler metal trans-ferred per pulse (kg), Qde is heat contentper unit mass of the welding wire (Jkg1)at the time of deposition, and f is pulse fre-quency (Hz). The modeling detail for esti-mating Qf of the GMAW-P process hasbeen reported elsewhere (Refs. 12, 13).

The calorimetrically measured values ofQde per unit mass of a deposited dropletin the GMAW process using 1.2-mm-diameter mild steel filler metal and cover-ing the array of globular to spray metaltransfer have been reported (Refs. 21, 22)by earlier workers.

Studies on Geometrical Characteristics of

Weld Joint

A transverse section of the weld col-lected from the central part of the weldjoint, assuring an area of stable welding,was polished by standard metallographicprocedure and etched in 2% nital solutionto reveal the weld geometry. Typicalmacrographs of the weld joints of V-

groove, NG-11, and NG-8 produced byusing the GMAW and GMAW-Pprocesses are, respectively, shown in Fig.4AC and Fig. 5AC. The geometricalcharacteristics of weld joint such as totalarea of weld deposit (AWD) including thejoint root opening (G), area of top (ATR)and root (ARR) reinforcement, area ofbase metal fusion (ABF), and dilution(%DL) of base plate, as schematicallyshown in Fig. 6, was measured by graphicalmethod. The estimation of AWD, ABF, and

DLwas carried out as follows:

AWD =AWA +ATR +ABF+ARR (10)A

BF=A

WDA

WAA

TRA

RR(11)

(12)

Estimation of Transverse Shrinkage Stress

The transverse shrinkage stress (tr)developed in the weld joints during theirpreparation under the varied thermal be-haviors of different welding processes,procedures, and parameters was estimated with the help of measured transverseshrinkage (tr) as follows (Refs. 2, 23):

(13)Where E is the Youngs modulus of basematerial (210 109 Nm2), R is the shapefactor considered as 0.1 (Ref. 2), and LS isthe straining length of 100 mm (Fig. 3).

Estimation of Bending Stress

The bending stress (b) developedthrough physically observed distortion ofweld joints during their preparation under

QQ Q

SrT

AW f =

+( )

I k I + 1-k Ieff p p2

p b2= ( )

%DA

AL

BF

WD

= 100

tr tr

S

R EL

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Fig. 3 Schematic diagram of welding fixture.

Table 2 Estimated Thermal Behavior of Welding at Different Parameters

Welding Process Measured Welding Parameters Estimated Thermal Behavior

V S I(a)/Im Pulse Parameters QAW Qde Qf QT(cm/min) (A) Ip Ib f tb tp (kJ/cm) (J/s) (J/kg) (J/s) (kJ/cm)

(A) (A) (Hz) (s) (s)

GMAW 28 2404(a) 8.28 0.5 2930 1834000 2156 10.89241 0.15 420 104 50 0.012 0.008 7.45 0.4 4294 1755157 2059 13.61

23030.23 440 140 100 0.007 0.003 7.22 0.5 4145 1702060 1996 13.16

GMAW-P 24 0.15 372 88 100 0.006 0.004 7.1 0.6 3789 1677370 1731 13.671824

0.23 420 128 50 0.015 0.005 6.77 0.5 3673 1625076 1677 13.38


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varied thermal behavior of different weld-ing processes and procedures was also es-timated (Ref. 23) as follows:

(14)Where M is the bending moment, Im is themoment of inertia, and t is plate thickness.The M and Im are estimated (Ref. 23) asfollows:

M = FLC (15)

(16)

Where LC is the distance of the measuringpoint (dial gauge tip, Fig. 3) from the cen-tral axis of the weld joint, and bw is theplate width. The force (F) generated dueto distortion of the plate was estimated(Ref. 23) with the help of measured de-flection () by considering it as a case sat-isfying cantilever beam theory in view ofone end of the plate is fixed and other endis free to deflect as explained earlier (Fig.

3).

(17)The F has been appropriately estimatedunder different welding processes, proce-dures, and parameters.

Results and Discussion

GMAW

Geometrical Characteristics of Weld Joint

At a given heat input () of 8.28 0.5kJ/cm and a similar amount of weld depo-sition of 2.5 gm/cm per pass, the effect ofwelding procedure on geometrical charac-teristics of the weld with respect to its totalarea of weld deposit (AWD), total area ofbase metal fusion (ABF), and dilution ofbase metal (DL) is shown in Fig. 7AC.Figure 7A and B shows that the V-grooveweld joint gives comparatively higher AWDand ABF than those of narrow groove weldjoints. It may be due to the comparativelyhigher amount of weld deposition re-quired in filling a V-groove weld joint than

that of filling a narrow groove weld joint.

However, it is further observed that ABFof the comparatively narrower weld joint(NG-8) is relatively higher than that ofNG-11, but lower than V-groove weldjoints. It is possibly attributed to a com-paratively larger amount of base metal fu-sion on both sides of the groove wall in theroot region of the narrower groove widthof NG-8 than that of the V-groove andNG-11 welds at a given and similar

order of metal deposition. This may havecaused more dilution of weld metal withbase metal in the NG-8 weld compared tothe NG-11 and V-groove weld joints asshown in Fig. 7C. The variation in extentof base metal fusion may also significantlyaffect the temperature of the weld pooland consequently its solidification behav-ior, influencing transverse shrinkage anddeflection of weld joints.

Transverse Shrinkage Stress

At a given welding current (I) and heat

input () of 230 4A and 8.28 0.5kJ/cm respectively, resulting in a similarrate of weld deposition (2.5 gm/cm), theeffect of the number of weld passes on thecumulative transverse shrinkage (tr)measured during multipass GMA weldingin different size grooves is shown in Fig. 8.The variation of results shown in the fig-ure maintains a standard deviation (SD)lying in the range of 0.090.2. The figureshows that at a given heat input the in-crease in the number of weld passes en-hances the cumulative transverseshrinkage of the weld joint due to increase

in the amount of weld metal deposition.

FEI

L

m

C

=3

3

Ib tm w=

3

12

b m

R M t

I=

2

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Fig. 4 Typical macrograph of GMA weld joints having different sizes of weld grooves produced at a given of 8.28 0.5 kJ/cm. A V-groove; B NG-11; C NG-8.

Fig. 5 Typical macrograph of pulsed GMA weld joints having different sizes of weld grooves produced at a given Im = 230 3 A, = 0.23, and = 7.22 0.5 kJ/cm. A V- groove; B NG-11; C NG-8.

A

A

B

B

C

C

Fig. 6 Schematic view of different locations of weldjoint considered in graphical measurement of area ofweld deposit (AWD ) and area of base metal fusion(ABF).

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The figure further reveals that at a given, the influence of weld pass on transverseshrinkage appreciably reduces as one pro-ceeds from the root pass to subsequent fill-ing passes. However, at a given weld pass,the cumulative shrinkage of the weld jointwas found to be more in the NG-8 weld,

but relatively less in the NG-11 weld withrespect to that of the V-groove weld. Onthe basis of these observations, the changein estimated (Equation 13) transverseshrinkage stress (tr) with the variation inweld groove size of GMA weld joint wasstudied and is shown in Fig. 9. The figureshows that the V-groove weld joint has acomparatively higher transverse shrinkagestress than that of the narrow groove weld joints, with the NG-11 weld having thelowest among them.

Deflection and Bending Stress

In line with the earlier observation(Fig. 8) on cumulative shrinkage (tr), thevariation in cumulative deflection () ofthe weld joint in the same range of given and I with the increase in number of weldpasses in multipass GMA weld depositionin different weld grooves was found to fol-low a similar trend as shown in Fig. 10. Itwas further observed that the graduallyincreases with the progress in number ofweld passes from the root pass and at agiven number of weld passes, it is relativelymore in the NG-8 weld, but less in the

NG-11 weld than that of the V-grooveweld. On the basis of measured deflectionof the weld joints as discussed above, it isa measure of bending of the plate from itsneutral axis. At a given order of, the ef-fect of welding procedure on variation ofshrinkage stresses in the weld depositcausing distortion of the weld joint can becorroborated by estimation of bendingstress developed in the weld joint throughmeasurement of degree of bending of theplate. This may provide a physical confir-mation of the effect of welding process onthe stress generation in the weld joint. Ata given , the effect of welding procedure

on estimated bendingstress is shown in Fig.11. It was observedthat the estimatedbending stress of nar-row groove welds is always considerably

lower than that of theV-groove weld. How-ever, the estimatedbending stress of theNG-11 was found to bemarginally lower thanthat of the NG-8 weld,which is in agreement with the observedtrend of estimatedtransverse shrinkagestress (Fig. 9) with re-spect to variation ingroove size. In this

context, it is furthernoted that the magni-tude of the estimatedtransverse shrinkage lies in close approxi-mation to the bending stress estimated onthe basis of distortion generated in weldjoints (Figs. 9 and 11) of different groovesizes.

GMAW-P

Geometrical Characteristics of Weld Joint

At a given Im and close range of of

230 3A and 7.227.45 kJ/cm, respec-tively, and a similar amount of weld depo-sition of 2.5 gm/cm per pass, the effect of welding procedure using different weldgrooves at varied of 0.15 and 0.23 onweld deposit, base metal fusion, and dilu-tion of weld are shown in Fig. 12AC. Inagreement with those observed in GMAW,it was found that V-groove weld jointsshowed comparatively higher AWD andABF but lower dilution than that of thenarrow groove NG-8 weld joint. However,the NG-11 weld joint shows comparativelyhigher AWD but lower ABF and DL thanthat of the NG-8 weld joint. It is also in-

terestingly observed that at a given andIm, the measures of aforesaid geometricalcharacteristics of all the V-groove, NG-11,and NG-8 weld joints reduce with the in-crease of. This may have primarily hap-pened due to a decrease in total heattransfer to the weld pool (QT) (Table 2)with the increase of (Ref. 24). The low-ering of QTwith the increase of may re-duce the weld groove shrinkage and ABF,thus, decreases the AWD and DL. At a

given welding or mean current (I or Im),arc voltage, and welding speed, the isrelatively lower (Table 2) with GMAW-Pthan the GMAW process due to compara-tively low process efficiency (a) of theformer one causing rather low heat con-tent per unit mass of the filler metal (Qde)at the time of deposition. This behaviorconsiderably reduces the AWD, ABF, andDL of a pulsed GMA weld compared tothe GMA weld at a higher of 0.23, but atthe same Im, arc voltage, and weldingspeed, a decrease of to 0.15 makes theabove-mentioned geometrical characteris-tics of the pulsed GMA weld comparable

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A B

C

Fig. 7 At a given , Im , and arc voltage, the effect of different weldgrooves. A Measured area of weld deposit; B measured area of basemetal fusion; C dilution (%) of GMA weld deposit.


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to those of the GMA weld.Similar observations on the variations in

AWD, ABF, and DL of the narrow grooveNG-11 and NG-8 weld joints with a changein pulse parameters were noted as shown inFig. 13AC during welding at a relatively

lower range of and Im of 6.777.1 kJ/cmand 182 4A, respectively, with a weld dep-osition of 2.5 gm/cm per pass at different of 0.15 and 0.23. However, a comparison ofFigs. 12 and 13 shows that at a given theAWD, ABF, and DL of NG-11 and NG-8weld joints reduces significantly with a de-crease of Im from 230 to 182 A. This mayhave primarily happened due to a compar-atively low heat content per unit mass ofweld deposition (Qde) at lower Im (Table 2),which can consequently make a differencein transverse shrinkage and deflection of theweld joint.

Transverse Shrinkage Stress

At a given Im and close range of of230 3 A and 7.227.45 kJ/cm, respec-tively, at different of 0.15 and 0.23, theeffect of number of weld passes on cumu-

lative transverse shrinkage (tr) measuredduring multipass pulsed GMA welding indifferent sized weld grooves is shown inFig. 14. In order to maintain reliability incomparitive observations on relativelysmall variations in tr, a statistical analysiswas made. In agreement to the earlier ob-servations on cumulative transverseshrinkage of the GMA weld joints at dif-ferent weld groove sizes (Fig. 8), here alsoit was observed that the transverse shrink-age of the pulsed GMA weld joints in-creased with the increase of weld passes.However, at a given weld pass and similar

order of I and Im of about 230 A, the use

of GMAW-P in place of GMAW appre-ciably lowers the transverse shrinkage. Inspite of a similar rate of weld deposition(2.5 gm/cm per pass), this may be primarilyattributed to the phenomena of heatbuildup (Refs. 5, 8, 25) with GMAW-P,

which with the influence of interruption inmetal deposition results in a comparativelymilder thermal behavior of weld joints. Ata given weld pass, the cumulative transverse shrinkage of the weld joint wasfound to be more in the NG-8 weld andrelatively less in the NG-11 weld with re-spect to that of the V-groove weld. But, itwas further observed that at a given Im and, the cumulative transverse shrinkage inall the V-groove, NG-11, and NG-8 weldjoints reduced with the increase of Fig. 14. This may have primarily happeneddue to the decrease in total heat transfer

to the weld pool (QT) (Table 2) with the

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Fig. 8 The effect of number of weld passes on measured cumulativetransverse shrinkage during GMA weld deposition in different sizes of weldgrooves at a given , Im, and arc voltage.

Fig. 9 The effect of different weld grooves on estimated transverseshrinkage stress in GMA weld joints at a given , Im, and arc voltage.

Fig. 10 The effect of number of weld passes on measured cumulativedeflection during GMA weld deposition in different sizes of weld grooveat a given , Im, and arc voltage.

Fig. 11 The effect of different weld grooves on estimated bending stressof GMA weld joints at a given , I, and arc voltage.


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increase of as explained earlier. On thebasis of these observations, the change inestimated (Equation 13) transverseshrinkage stress (tr) with the variation in weld groove size of pulsed GMA weldjoints at different has been studied asshown in Fig. 15. The figure shows that theV-groove weld joint is having compara-tively higher transverse shrinkage stressthan that of the narrow groove weld joints,where the NG-11 weld depicts the lowestshrinkage among them. However, Fig. 15

further shows that the transverse shrink-age stress developed in the weld joint pre-pared by the GMAW-P process withdifferent weld groove sizes is compara-tively lower than that observed in GMAweld joints Fig. 9. This may be primarilyattributed to the lower thermal intensity ofthe pulsed GMA weld due to a reductionin heat buildup in the weld deposit as a re-sult of the interruption in weld depositionunder pulsed current. It is interesting tonote that at a given Im and , the trans-verse shrinkage stress of all the V-groove,NG-11, and NG-8 weld joints reduced

with the increase of Fig. 15.

At the comparatively lower range of Imand of 182 4 A and 6.777.1 kJ/cm,respectively, the variation in cumulativetransverse shrinkage with the increase ofweld passes in different sizes of narrowgroove welds observed at relatively lowerand higher of 0.15 and 0.23 is shown inFig. 16. In order to study the effect of Im atdifferent on transverse shrinkage ofpulsed GMA welds, the observations inFigs. 14 and 16 were compared. It is ob-served that at a given , the decrease of

Im from 230 to 182 A at a close range ofsignificantly reduces the transverse shrink-age of both the NG-11 and NG-8 welds,possibly due to a comparatively low heatcontent per unit mass of the filler wire(Qde) at the time of deposition (Table 2)as explained earlier. The figures furtherdepict that at a given Im and , the in-crease of relatively reduces the transverse shrinkage, especially in narrowerweld NG-8. However, at a given weld pass,the cumulative shrinkage of the weld joint was found comparatively more with anNG-8 weld than with an NG-11 weld. On

the basis of these observations, the varia-

tion in estimated (Equation 13) transverseshrinkage stress ( tr) with the change ingroove size of pulsed GMA weld joints atrelatively lower range of Im and at dif-ferent was studied and is shown in Fig.17. The figure shows that at a given of0.15, the NG-8 weld has a comparativelyhigher transverse shrinkage stress thanthat of the NG-11 weld, whereas at thehigher of 0.23, an opposite trend is ob-served. However, Fig. 17 further depictsthat the transverse shrinkage stress devel-

oped in the weld prepared by GMAW-Pusing different weld groove sizes is com-paratively lower than that observed (Fig.15) in the pulsed GMA weld produced atrelatively higher Im and of 230 3 Aand 7.227.45 kJ/cm, respectively. In viewof the results depicted in Figs. 9, 15, and17, the significant role different weldgroove sizes plays on transverse shrinkagestress developed during welding is clearlyunderstood, along with some further criti-cal observations at varying , Im, and . Itis noted that the NG-8 weld gives the low-est transverse shrinkage stress among all

the welds of different groove sizes studied,

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Fig. 12 The effects of different weld grooves at a given , Im, and arc voltage of GMAW-P. A Onmeasured area of weld deposit; B area of base metalfusion; C dilution of weld deposit.

A

C

B C

A B

Fig. 13 The effect of different weld grooves at a given relatively lower range ofand Imof GMAW-P. A On measured area of weld deposit; B area ofbase metal fusion; C dilution of weld deposit.


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when a relatively higher of 0.23 is usedat relatively lower range of Im and of

182 4 A and 6.777.1 kJ/cm, respec-tively. However, to have a clearer under-standing of all the phenomena regardingthe influence of welding process, proce-dure, and parameters on transverseshrinkage stress developed during weld-ing, it should be studied further in detailusing primarily more variation of at dif-ferent groove sizes and plate thicknesses.

Deflection and Bending Stress

In line with earlier observation (Fig.14) on cumulative shrinkage (tr), the vari-ation in cumulative deflection () of the

weld joint with an increase in number ofweld passes using GMAW-P in different

sized weld grooves in the same range ofgiven , Im, and was found to follow asimilar trend as shown in Fig. 18 for dif-ferent of 0.15 and 0.23, respectively. Thefigure shows that the deflection developedin the weld joint prepared by GMAW-P,especially at higher of 0.23, is signifi-cantly lower than that observed (Fig. 10)in the GMA weld joint prepared with sim-ilar current voltage and heat input. Thismay be primarily understood by the com-paratively lower estimated shrinkage stress(Fig. 15) of the pulsed GMA weld deposit, which generates comparatively lowerbending force in the weld joint as a result

of relatively smaller heat buildup than thatnoted (Fig. 11) in the GMA weld joint.

However, at a given weld pass, the cumu-lative deflection of the weld joint wasfound to be more with the NG-8 weld, butrelatively less with the NG-11 weld withrespect to the V-groove weld. It was fur-ther observed that at a given and Im, thecumulative deflection of the weld jointwith all the V-groove, the NG-11 and NG-8 welds reduced with the increase of Fig. 18. At a given range of Im and of230 3 A and 7.227.45 kJ/cm, respec-tively, and a different of 0.15 and 0.23,the effect of welding procedure on bend-ing stress developed in the weld joint esti-mated (Equation 14) on the basis of

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Fig. 14 The effect of number of weld passes at a given , Im,, and arc volt-age of GMAW-P on measured cumulative transverse shrinkage of weld depositunder different sizes of weld groove at different f.

Fig. 16 At a relatively lower range ofand Imof GMAW-P, the effect of num-ber of weld passes on measured cumulative transverse shrinkage of weld deposi-tion in different sizes of weld groove at different .

Fig. 15 At a given , Im, and arc voltage of GMAW-P, the effect of dif-ferent weld grooves on estimated transverse shrinkage stress of weld jointsat different .

Fig. 17 At a relatively lower range ofand Imof GMAW-P, the effectof different weld grooves on estimated transverse shrinkage stress weld jointsat different .


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measured bending during multipasspulsed GMA welding in different weldgroove sizes is shown in Fig. 19. It was ob-served that the estimated bending stress ismarginally lower in the NG-11 weld thanthe NG-8 weld, but in both cases it is con-siderably lower than the V-groove weld,which follow a similar trend of estimatedtransverse shrinkage stress Fig. 15. Inthis context, it was further corroboratedthat the estimated transverse shrinkage iswell in agreement to the bending stress es-timated on the basis of distortion gener-

ated in weld joints Figs. 15 and 19.At the comparatively lower range of Imand of 182 4 A and 6.777.1 kJ/cm,respectively, the increase in cumulative de-flection with the increase of weld passes indifferent sizes of narrow grooves observedat relatively lower and higher of 0.15 and0.23 is shown in Fig. 20. The figure showsa similar trend of variation in transverseshrinkage as explained earlier (Fig. 16) inthe case of variation in cumulative shrink-age under the same conditions of welding.But, it is observed that at any , the cumu-lative deflection is marginally lower inNG-11 than NG-8 weld. However, it wasnoted that at a given , the decrease of Imfrom 230 to 182 A reduces the deflectionsignificantly, and the effect is compara-tively more with NG-11 weld as revealedin Figs. 18 and 20, respectively. On thebasis of these observations, the variationin estimated (Equation 14) bending stress(tr) with the change in weld groove sizeof pulsed GMA weld joints at the rela-tively lower range of Im and at different was studied as shown in Fig. 21. In linewith earlier observations on the change intransverse shrinkage stress (Fig. 17) withan increase of under the same condi-

tions of welding, a similar trend was ob-

served concerning bending stress of theweld joints especially at a lower of 0.15.

In view of the above discussions regard-ing transverse shrinkage stress and bendingstress generated under different weldingprocesses and weld groove sizes, it is under-stood that the NG-8 weld develops compar-atively higher transverse shrinkage stressand bending stress than the NG-11 weld,especially at higher thermal influence ofmore or lower (Refs. 12, 13, 25). Thismay have happened due to a comparativelylarger proportion of groove filling during

the root pass deposition in the compara-tively narrower groove of NG-8 than NG-11weld Fig. 23. Accordingly, the force (FR)generated in the weld contributing to de-velopment of bending moment (MR) in thegroove wall causing its distortion resultingfrom root pass deposition in different sizesof weld grooves using different weldingprocesses have been estimated by usingEquations 15 and 17, respectively. The vari-ation in FR and MR with respect to thewelding process, groove size, and weldingparameters is shown in Fig. 22 A and B, re-spectively. The figure shows that throughthe root pass the NG-8 weld generates rel-atively higher FR and MR than that ob-served with the V-groove and NG-11 weldjoints, where the NG-11 weld is the lowestamong them. Mechanism of the force (FR)and bending moment (MR) generated dur-ing root pass weld for V-grooves as well asNG-11 and NG-8 grooves is schematicallyshown in Fig. 23AC. Figure 23 clearlyshows that a decrease in weld groove sizesignificantly enhances the proportionate fill-ing of the weld groove by root pass, ad-versely affecting the shrinkage stress andbending stress of the weld. However, thisphenomenon has to be studied further in

order to exploit the use of narrow groove

welding of thick plates with appropriateamount of weld deposition per pass.

Conclusions

The investigation revealed that the useof GMAW-P in multipass butt joining ofthick (16-mm) steel plate is beneficial toreduce the shrinkage stress of the weld de-posit as well as distortion and bendingstress of the weld joint compared to usingconventional GMAW at a given welding

heat input. During GMAW-P at a givenheat input, a control of pulse parametersgives rise to a relatively higher and fur-ther improves the weld characteristics inthis regard. The use of a narrow weldgroove instead of a conventional V-groovealso improves the situation in this respect.The variation of weld characteristics inquestion with a change of pulse parame-ters primarily happens due to decrease intotal heat transfer (QT) to the weld poolas a function of the transfer of arc heat(QAW) and heat of the filler metal (Qf) toit with a change in at a given weldingspeed (S), where the increase of de-creases the QT. The change in said weldcharacteristics with groove size is largelydictated by the amount of weld deposit.However, the use of a too narrow weldgroove, where the root pass can fill thegroove more than half of its depth, may re-strict the beneficial effect of narrowgroove to minimize the bending stress ofweld joint.

Acknowledgments

The author thankfully acknowledges

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Fig. 18 At a given , Im, and arc voltage of GMAW-P, the effect of num-ber of weld passes on measured cumulative deflection of weld depositionunder different sizes of weld groove at different .

Fig. 19 At a given , Im, and arc voltage of GMAW-P, the effect of differ-ent weld grooves on estimated bending stress of weld joints at different .


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the Council of Scientific and IndustrialResearch (CSIR), India, and for financial

support to K. Devakumaran, research as-sociate during analysis of the work.

References

1. Masubuchi, K. 1980. Analysis of WeldedStructures. New York, Pergamon Press, p. 235.

2. Radaj, D. 1992. Heat Effects of Welding.Berlin, Springer-Verlag, p. 182.

3. Parmer, R. S. 1997. Welding Engineeringand Technology. New Delhi, Khanna Publishers,p. 214.

4. Teng, T. -L., and Chang, P. -H. 2004. Ef-fect of residual stresses on fatigue crack initia-tion life for butt welded joints. J Mater Process

Technol (145): 325335.

5. Ghosh, P. K., and Ghosh, A. K. 2004.Control of residual stresses affecting fatigue life

of pulsed current gas metal arc weld of high-strength aluminum alloy.Met. Mater. Trans. A(35A): 24392446.

6. Deng, D., and Murakawa, H. 2008. Pre-diction of welding distortion and residual stressin a thin plate butt welded joint. Comp Mater Sc(43): 353365.

7. Deng, D., Murakawa, H., and Liang, W.2008. Numerical and experimental investiga-tions on welding residual stress in multi passbutt welded austenitic stainless steel pipe.Comp Mater Sc (42): 234244.

8. Kulkarni, S. G., Ghosh, P. K., and Ray, S.2008. Improvement of weld characteristics by

variation in welding processes and parametersin joining of thick wall 304LN stainless steel

pipe.ISIJ Int. (48): 15601569.

9. Sattari Far, I., and Javadi, Y. 2008. In-fluence of welding sequence on welding dis-

tortions in pipes. Int J Pressure Vessel andPiping (85): 265274.

10. Tang, T. L., and Lin, C. C. 1998. Effectof welding conditions on residual stresses dueto butt weld. Int J of Pressure Vessel and Piping(75): 857864.

11. Panin, V. N. 2007. Experimental calcula-tion estimation of residual welding distortion inshells of turbine penstocks at hydraulic powerstations. The Paton Welding J, pp. 711.

12. Goyal, V. K., Ghosh, P. K., and Saini, J.S. 2008. Analytical studies on thermal behaviorand geometry of weld pool in pulsed current gasmetal arc welding. J Mater Process Technol(209): 13181336.

13. Ghosh, P. K., Goyal, V. K., Dhiman, H.

K., and Kumar, M. 2006. Thermal and metal

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Fig. 20 At a relatively lower range ofand Imof GMAW-P effect of numberof weld passes on measured cumulative deflection of weld deposition in differ-ent sizes of weld groove at different .

Fig. 21 At a relatively lower range ofand Imof GMAW-P effect of dif-ferent weld grooves on estimated bending stress of weld joint at different .

Fig. 22 Effect of root pass deposition using different welding processes and weld groove sizes. A Force (FR) generated in weld; B bending moment (MR)developed in groove wall.

A B


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transfer behavior in pulsed current GMA welddeposition of Al-Mg alloy. Sci. Technol. Weld.Joining 11(2): 232242.

14. Ghosh, P. K., Randhawa, H. S., andGupta, S. R. 2000. Characteristics of a pulsed-current vertical up gas metal arc weld in steel.Met. Mater. Trans. A 31A(12): 22472259.

15. Jenney, C. L., and OBrien, A., eds. 2001.AWS Welding Handbook. 9th ed. Vol. 1, p. 110,American Welding Society, Miami, Fla.

16. Messler, Jr., R. W. 1999. Principles ofWelding. New York, John Willey & Sons, p. 181.

17. Lancaster, J. F. 1984. The Physics ofWelding. New York, International Institute ofWelding, p. 109.

18. Christensen, N., de L. Davis, V., andGjermundsen, K. 1965. Distribution of tem-perature in arc welding.British Welding J12(2):5475.

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20. Joseph, A., Harwig, D., Farson, D., andRichardson, R. 2003. Measurement and calcu-lation of arc power and heat transfer efficiencyin pulsed gas metal arc welding. Sci. Technol.Weld. Joining 8(6): 400406.

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als. 467, India, S. Chand & Company.

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25. Ghosh, P. K., Ravi Reddy, M., and De-vakumaran, K. 2006. Distortion and transverseshrinkage stress in butt welds of steel platesunder different welding procedure and param-eters of GMAW and SMAW processes. IndianWelding J. 38(4): 1523.

Nomenclature

Symbol Description

AWD Total area of weld deposit (mm2)

AWA Initial area of weld groove(mm2)

ATR Area of top reinforcement (mm2)

ABF Area of base metal fusion (mm2)

ARR Area of root reinforcement (mm2)

bw Plate width (mm)DL Dilution (%)E Youngs modulus (MPa)F Force generated due to

distortion (N)FR Force generated at the root pass

weld due to distortion (N)f Pulse frequency (Hz)G Root opening (mm)I Welding current (A)Ieff Effective current (A)Ip Peak current (A)Ib Base current (A)Im Mean current (A)

Im

Moment of inertia (mm4

)kp Pulse duty cycleLS Straining length (mm)LC Distance of the measuring

point (mm)M Bending moment (N-mm)MR Bending moment developed in

groove wall (kN-mm)mt Mass of filler metal transferred

per pulse (kg)QT Total heat transferred to weld

pool (kJ/cm)QAW Arc heat transferred to the weld

pool (Js1)Q

deHeat content per unit mass ofthe filler metal at the time ofdeposition (J kg1)

Qf Heat of the filler metaltransferred to the weldpool (Js1)

R Shape factorS Welding speed (m/min)SD Standard deviationt Thickness of the base plate (mm)tp Pulse on time (s)tb Pulse off time (s)V Arc voltage (V) Summarized influence of pulse

parameters factor

a Arc efficiency (%)

Effective melting potential atanode (mild steel = 5.8 V)

Heat input (kJ/cm)tr Transverse shrinkage stress (MPa)b Bending stress (MPa)tr Measured transverse

shrinkage (mm) Measured deflection (mm)

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Fig. 23 Schematic diagram showing the proportionate filling of groove with the root pass contributing to force generated and bending moment developed inweld with different groove sizes. A V-groove; B NG-11; C NG-8.

A CB

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