Effect of Pulsed Current Welding on Mechanical Properties

ORIGINAL ARTICLE

Effect of pulsed current welding on mechanical propertiesof high strength aluminum alloy

V. Balasubramanian & V. Ravisankar &

G. Madhusudhan Reddy

Received: 9 August 2006 /Accepted: 18 October 2006 /Published online: 4 January 2007# Springer-Verlag London Limited 2006

Abstract High strength aluminum alloys (Al-Zn-Mg-Cualloys) have gathered wide acceptance in the fabrication oflightweight structures requiring high strength-to-weight ratio,such as transportable bridge girders, military vehicles, roadtankers and railway transport systems. The preferred weldingprocesses of high strength aluminum alloy are frequently thegas tungsten arc welding (GTAW) process and the gas metalarc welding (GMAW) process due to their comparatively easyapplicability and better economy. Weld fusion zones typicallyexhibit coarse columnar grains because of the prevailingthermal conditions during weld metal solidification. Thisoften results in inferior weld mechanical properties and poorresistance to hot cracking. In this investigation, an attempt hasbeen made to refine the fusion zone grains by applying apulsed current welding technique. Rolled plates of 6 mmthickness were used as the base material for preparing singlepass welded joints. A single ‘V’ butt joint configuration wasprepared for joining the plates. The filler metal used forjoining the plates was AA 5356 (Al-5Mg (wt%)) gradealuminum alloy. Four different welding techniques were usedto fabricate the joints: (1) continuous current GTAW(CCGTAW), (2) pulsed current GTAW (PCGTAW), (3)continuous current GMAW (CCGMAW) and (4) pulsedcurrent GMAW (PCGMAW). Argon (99.99% pure) was usedas the shielding gas. Tensile properties of the welded joints

were evaluated by conducting tensile tests using a 100 kNelectro-mechanical controlled universal testing machine.Current pulsing leads to relatively finer and more equi-axedgrain structure in GTA and GMA welds. In contrast,conventional continuous current welding resulted in predom-inantly columnar grain structures. Grain refinement isaccompanied by an increase in tensile strength and tensileductility.

Keywords High strength aluminum alloy . Gas metal arcwelding . Gas tungsten arc welding . Pulsed current welding .

Grain refinement . Tensile behaviour

1 Introduction

The weldability characteristics of aluminum alloys varywidely from alloy system to alloy system. Hot cracking orsolidification cracking tendency is the most importantfactor governing the weldability of aluminum alloys.Almost all the heat treatable alloys are unfortunately proneto hot cracking. The susceptibility to solidification crackingis greatly influenced by the composition of the weld metaland hence the proper choice of filler material is animportant aspect in controlling solidification cracking [1].The use of non-heat treatable fillers which can resist hotcracking is more meaningful in welding 7xxx series alloys.In these alloys, as long as the weld metal contains 3% Mgor more, hot cracking is not a serious problem. Post weldsolution treatment is often not necessary for 7xxx seriesalloys and thus a direct aging treatment can be employed.Another way of controlling solidification cracking is torefine the fusion zone grain structure. Coarse columnargrains are often more susceptible to solidification crackingthan fine equi-axed grains. This may be because fine equi-

Int J Adv Manuf Technol (2008) 36:254–262DOI 10.1007/s00170-006-0848-0

V. Balasubramanian (*) :V. RavisankarDepartment of Manufacturing Engineering, Annamalai University,Annamalai Nagar,608 002 Tamil Nadu, Indiae-mail: [email protected]

G. Madhusudhan ReddyMetal Joining Section,Defence Metallurgical Research Laboratory,Kanchanbag (P.O.),Hyderabad 560 058, India

axed grains can deform to accommodate contraction strainsmore easily [2].

Weld fusion zones typically exhibit coarse columnargrains because of the prevailing thermal conditions duringweld metal solidification. This often results in inferior weldmechanical properties and poor resistance to hot cracking.While it is thus highly desirable to control solidificationstructure in welds, such control is often very difficultbecause of the higher temperatures and higher thermalgradients in welds in relation to castings and the epitaxialnature of the growth process. Nevertheless, several methodsfor refining weld fusion zones have been tried with somesuccess in the past: inoculation with heterogeneous nucle-ants, microcooler additions, surface nucleation induced bygas impingement and introduction of physical disturbancethrough techniques such as torch vibration [3].

The use of inoculants for refining the weld fusion zoneswas not as successful as in castings because of theextremely high temperatures involved in welding and alsodue to the undesirable effects of the inoculating elements onweld mechanical properties at the levels required forproducing grain refinement. Other techniques like surfacenucleation and microcooler additions were also turneddown because of the complicated welding set-ups andprocedures associated with their use. In the process, tworelatively new techniques, current pulsing and magnetic arcoscillation, have gained popularity because of their strikingpromise and the relative ease with which these techniquescan be applied to actual industrial situations with onlyminor modifications to the existing welding equipment [4].

Pulsed current welding, developed in the 1950s, is avariation of constant current welding which involvescycling of the welding current from a high level to a lowlevel at a selected regular frequency. The high level of thepeak current is generally selected to give adequatepenetration and bead contour, while the low level of thebackground current is set at a level sufficient to maintain astable arc. This permits arc energy to be used efficiently tofuse a spot of controlled dimensions in a short timeproducing the weld as a series of overlapping nuggets andlimits the wastage of heat by conduction into the adjacentparent material as in normal constant current welding. Incontrast to constant current welding, the fact that heatenergy required to melt the base material is supplied onlyduring peak current pulses for brief intervals of time allowsthe heat to dissipate into the base material leading to anarrower heat affected zone (HAZ). The technique hassecured a niche for itself in specific applications such as inwelding of root passes of tubes, and in welding thin sheets,where precise control over penetration and heat input arerequired to avoid burn through [5] .

Current pulsing has been used by a few investigators [6,7] to obtain grain refinement in weld fusion zones and

improvement in weld mechanical properties. Significantrefinement of the solidification structure has been reportedin aluminum alloys and titanium alloys. Most of thereported literature is focused on pulsed current welding ofmedium strength aluminum alloys and the publishedinformation on pulsed current welding of high strengthaluminum alloys could be counted with fingers. Hence, thepresent investigation has been carried out to understand theeffect of the pulsed current welding technique on tensileproperties of high strength aluminum alloy (AA 7075).

2 Experimental work

High strength aluminum alloys (Al-Zn-Mg-Cu alloys) havegathered wide acceptance in the fabrication of lightweightstructures requiring a high strength-to-weight ratio, such astransportable bridge girders, military vehicles, road tankersand railway transport systems. The rolled plates of AA7075 aluminum alloy were cut into the required sizes(300×150 mm) by power hacksaw cutting and grinding.The single ‘V’ butt joint configuration, as shown in Fig. 1,was prepared to fabricate GTA and GMA welded joints.The initial joint configuration was obtained by securing theplates in position using tack welding. The direction ofwelding was normal to the rolling direction. All necessarycare was taken to avoid joint distortion and the joints weremade after clamping the plates with suitable clamps. Asingle pass welding procedure was applied to fabricate thejoints. AA 5356 (Al-5%Mg) grade filler rod and wire wereused for GTA and GMA welding processes, respectively.High purity (99.99%) argon gas was used as the shieldinggas. The chemical composition and mechanical propertiesof the base metal and weld metals are presented in Table 1.The welding conditions and process parameters presentedin Table 2 were used to fabricate the joints by continuouscurrent and pulsed current welding processes.

The welded joints were sliced using a power hacksawand then machined to the required dimensions as shown inFig. 2 for preparing tensile specimens. American Society

Fig. 1 Dimensions of single ‘V’ butt joint configuration (alldimensions in mm)

Int J Adv Manuf Technol (2008) 36:254–262 255

for Testing of Materials (ASTM) guidelines were followedfor preparing the test specimens. Two different tensilespecimens were prepared to evaluate the transverse tensileproperties. The smooth (unnotched) tensile specimens wereprepared to evaluate yield strength, tensile strength andelongation. Notched specimens were prepared to evaluatenotch tensile strength and notch strength ratio of the joints.Tensile tests were carried out in a 100 kN, electro-mechanical controlled universal testing machine. Thespecimen is loaded at the rate of 1.5 kN/min as per ASTMspecifications, so that the tensile specimen undergoesdeformation. The specimen finally fails after necking andthe load versus displacement has been recorded. The 0.2%offset yield strength is derived from the diagram. Thepercentage of elongation was also evaluated and the valuesare presented in Table 3. A Vicker’s microhardness testingmachine (Model 3212; Zwick, UK) was employed formeasuring the hardness of the weld metal with 0.5 kg loadand the values were recorded and presented in Table 3.

Microstructural examinations were carried out using alight optical microscope (VERSAMET-3; Union Optical,Japan) incorporated with image analyzing software(Clemex-Vision, Canada). The specimens for metallograph-ic examination were sectioned to the required sizes from thejoint comprising weld metal, HAZ and base metal regionsand polished using different grades of emery papers. Finalpolishing was done using the diamond compound (1 μmparticle size) in the disc polishing machine. Specimenswere etched with Kellers reagent to reveal the microstruc-ture. The fractured surface of the tensile tested specimenswas analyzed using a scanning electron microscope (Model5610LV; JEOL, Japan) at higher magnification to study thefracture morphology to establish the nature of the fracture.

Table 1 Chemical composition (wt %) of base metal and weld metal

Type of material

Base metal(AA 7075)

Weld metal(GTAW)

Weld metal(GMAW)

Zn 5.61 0.12 0.13Mg 4.52 5.02 5.04Mn 0.03 0.12 0.14Fe 0.29 0.40 0.42Si 0.08 0.25 0.25Cu 1.61 0.11 0.10Cr – 0.10 0.12Ti 0.02 0.17 0.20Al Bal Bal BalMechanical properties of base metal and weld metalYield strength (MPa) 417 308 303Ultimate tensilestrength (MPa)

520 376 380

Elongation (%) 14.2 7.2 6.8Reduction in crosssectional area (%)

9.1 4.7 4.5

Notch tensile strength(MPa)

610 – –

Notch strength ratio 1.18 – –Hardness (VHN) 140 121 114

Table 2 Welding conditions and process parameters

Process CCGMAW PCGMAW CCGTAW PCGTAW

Weldingmachine

Lincoln Lincoln Lincoln Lincoln

Tungstenelectrodediameter (mm)

– – 3 3

Filler rod/wirediameter (mm)

1.6 1.6 3.0 3.0

Voltage (volts) 30 30 24 24Current (amps) 200 – 150 –Welding speed(mm/min)

150 150 120 120

Heat input(kJ/mm)

4 3 3 2

Peak current(amps)

– 200 – 150

Base current(amps)

– 100 – 75

Pulse frequency(Hz)

– 6 – 6

Pulse on time(%)

– 50 – 50

Shielding gas Argon Argon Argon ArgonGas flow rate 16 l/min 16 l/min 16 l/min 16 l/min

(a) Unnotched (Smooth) Specimen

(b) Notched Specimen Fig. 2 Dimensions of tensile specimen (all dimensions in mm). aUnnotched (smooth) specimen. b Notched specimen

256 Int J Adv Manuf Technol (2008) 36:254–262

Samples were preserved by coating with commercial oxideproof oil. Before examining under SEM, the specimenswere cleaned ultrasonically with carbon tetra chloride andacetone.

3 Results

3.1 Tensile properties

The transverse tensile properties such as yield strength, tensilestrength, percentage of elongation, percentage of reduction incross-sectional area, notch tensile strength, notch strengthratio and joint efficiency of continuous and pulsed currentwelded AA 7075 aluminum alloy joints were evaluated. Ineach condition, three specimens were tested and the averageof three results is presented in Table 3. The yield strength andtensile strength of unwelded parent metal are 417 MPa and520 MPa, respectively. But the yield strength and tensilestrength of CCGMAW joints are 180 MPa and 235 MPa,respectively. This indicates that there is a 55% reduction instrength values due to CCGMA welding. However, thePCGMAW process is found to be beneficial to enhance thestrength of welded joints and the yield strength and tensilestrength of PCGMAW joints are 195 MPa and 254 MPa,respectively. This shows that there is an 8% increase instrength values due to pulsed current welding. Similarly, theyield strength and tensile strength of CCGTAW joints are205 MPa and 272 MPa, respectively, which are 48% lowercompared to the parent metal. However, the PCGTAWprocess is found to be beneficial to improve the strength ofwelded joints and the yield strength and tensile strength ofPCGTAW joints are 220 MPa and 295 MPa, respectively.This shows that there is a 7% increase in strength values dueto pulsed current welding. Of the four types of welded joints,the joints fabricated by PCGTAW exhibited very highstrength values and the enhancement in strength value isapproximately 25% compared to CCGMAW joints and 15%compared to PCGMAW joints.

The elongation and reduction in cross-sectional area ofunwelded parent metal are 14.2% and 9.1%, respectively.But the elongation and reduction in cross-sectional area of

CCGMAW joints are 6.5% and 4.2%, respectively. Thissuggests that there is a 55% reduction in ductility due toCCGMA welding. However, the PCGMAW process isfound to be useful to enhance the ductility of welded jointsand the elongation and reduction in cross-sectional area ofPCGMAW joints are 8.5% and 6.75%, respectively. Thisshows that there is a 30% increase in ductility due to pulsedcurrent welding. Similarly, the elongation and reduction incross-sectional area of CCGTAW joints are 10.4% and7.3%, respectively, which are 20% lower compared to theparent metal. However, the PCGTAW process is found tobe beneficial to improve the ductility of welded joints andthe elongation and reduction in cross-sectional area ofPCGTAW joints are 12.3% and 8.55%, respectively. Thispoints out that there is an 18% increase in ductility due topulsed current welding. Of the four types of welded joints,the joints fabricated by PCGTAW exhibited very highductility values.

Notch tensile strength (NTS) of unwelded parent metalis 610 MPa. But the notch tensile strength of a CCGMAWjoint is 240 MPa. This reveals that the reduction in NTS isapproximately 60% due to CCGMAwelding. However, thePCGMAW process is found to be useful to enhance thenotch tensile strength of welded joints and the NTS ofPCGMAW joints is 279 MPa. This shows that there is a16% increase in NTS due to pulsed current welding.Similarly, the NTS of CCGTAW and PCGTAW joints are304 MPa and 342 MPa, respectively. This points out thatthere is a 12% increase in NTS due to pulsed currentwelding. Of the four types of welded joints, the jointsfabricated by PCGTAW exhibited very high NTS valuesand the enhancement is 40% compared to CCGMAW and20% compared to PCGMAW.

Another notch tensile parameter, NSR, is found to begreater than unity (>1) for all the joints. This suggests thatthe AA7075 alloy is insensitive to notches and they fall intothe ‘notch ductile materials’ category. The NSR is 1.18 forunwelded parent metal but it is 1.02 for CCGMAW joints.NSR is improved by the pulsed current welding techniqueirrespective of the welding processes. Of the four types ofwelded joints, the joints fabricated by PCGTAW exhibited arelatively higher NSR (1.16) and the improvement in NSR

Table 3 Transverse tensile properties of welded joints

Joint type Yield strength(MPa)

Ultimate tensilestrength (MPa)

Elongation(%)

Reduction inc.s.a (%)

Notch tensilestrength (MPa)

Notch strengthratio (NSR)

Jointefficiency(%)

CCGMAW 180 235 6.5 4.25 240 1.02 45.2PCGMAW 195 254 8.5 6.75 279 1.10 48.8CCGTAW 205 272 10.4 7.30 304 1.12 52.3PCGTAW 220 295 12.3 8.55 342 1.16 56.7


is 12% compared to CCGMAW and 5% compared toPCGMAW.

Joint efficiency is a ratio between tensile strength ofwelded joint and tensile strength of unwelded parent metal.The joint efficiency of CCGMAW joints of AA7075aluminum alloy is merely 45%. The pulsed current weldingtechnique is found to improve the joint efficiency irrespec-tive of the welding processes. Of the four types of weldedjoints, the joints fabricated by PCGTAW exhibited arelatively higher joint efficiency (57%) and the improve-ment in joint efficiency is 25% compared to the CCGMAWjoints, 16% compared to PCGMAW joints and 8%compared to CCGTAW joints.

3.2 Hardness

The hardness across the weld cross-section has beenmeasured using Vicker’s micro-hardness testing machineand the values are presented in Table 4. The hardness of thebase metal (unwelded parent metal) in its initial T6 (solutionheat treated and naturally aged) condition is approximately140 VHN. But the hardness of the CCGMAW joints in theweld metal region is 70 VHN. This suggests that thehardness is reduced by 70 VHN in the weld center due towelding heat and the usage of lower hardness filler metal(Al-5%Mg). However, the pulsed current welding tech-nique has enabled regaining of the hardness level to someextent in the weld metal region. Of the four joints,PCGTAW joints show hardness of 100 VHN which is30 VHN higher than the CCGMAW joints. Similarly, theCCGTAW joints recorded hardness of 85 VHN, which is15 VHN higher than the CCGMAW joints. PCGMAWjoints exhibited hardness of 80 VHN, which is 10 VHNgreater than the CCGMAW joints. Similar trends have beenobserved in PMZ, HAZ and BM regions also. The hardnessis relatively higher in the PMZ and HAZ regions comparedto the WM region and this may be due to the formation ofvery fine recrystallised grains in that region.

3.3 Microstructure

Microstructures of all the joints were examined at differentlocations and it was observed that all the joints invariably

contained three distinctive regions: (i) a weld metal (WM)region, (ii) a partially melted zone (PMZ), and (iii) a heataffected zone (HAZ). An appreciable difference in grainsize (average grain diameter) was observed in the weldmetal region but the grain size was almost similar in thePMZ and HAZ regions of all the joints. Hence, themicrographs taken at the weld metal region (weld centre)alone are displayed in Fig. 3 for comparison purposes.From the micrographs it can be understood that the currentpulsing is very effective in the fusion zone region. Further,an attempt has been made to measure the average graindiameter of the weld metal region (fusion zone) of all thejoints applying Heyn’s line intercept method and the mea-sured average grain diameter is presented in the respectivemicrographs.

The measured average grain diameter of CCGMAWjoints is 80 μm but the average grain diameter ofPCGMAW joints is 60 μm; this indicates that reduction ingrain diameter is 25% due to pulsed current welding of theGMAW process. Similarly, the measured average graindiameter of CCGTAW joints is 40 μm but the average graindiameter of PCGTAW joints is 20 μm; this also points outthat the reduction in grain diameter is 50% due to pulsedcurrent welding of the GTAW process. Of the fourtechniques, the PCGTAW technique produces very finegrains in the weld metal region compared to all othertechniques and 75% reduction in grain diameter has beenobserved compared to CCGMAW joints; 66% reduction ingrain diameter has been observed compared to PCGMAWjoints.

Generally, in Al-Zn-Mg-Cu alloys two different strength-ening precipitates will form and they are: (i) MgZn2 and (ii)CuAl2. The black particles seen in the weld metal regionare strengthening precipitates but there is an appreciabledifference in size of the precipitates. Fine precipitatesgenerally belong to CuAl2 and coarse precipitates belongto MgZn2 because the maximum available copper for theprecipitation reaction is only 1.6% in this alloy but theavailable magnesium and zinc for the precipitation isplenty. However, both the precipitates are uniformlydistributed throughout the matrix of the weld metal inpulsed current welds but in continuous current welds theprecipitates are more densely available in the grainboundaries than in the grain interior.

3.4 Fracture surface

The fractured surfaces of tensile test specimens of weldedjoints of AA7075 aluminum alloy have been analyzedusing SEM to reveal the fracture surface morphology.Figures 4 and 5 display the fractographs of unnotched andnotched tensile specimens, respectively. The displayedfractographs invariably consist of dimples which are an

Table 4 Micro-hardness values (VHN)

Joint type Location

WM PMZ HAZ BM

CCGMAW 70 80 95 136PCGMAW 80 90 104 138CCGTAW 85 100 116 140PCGTAW 100 110 122 142


indication that most of the tensile specimens failed in aductile manner under the action of tensile loading. In thetension test, voids form prior to necking but after a neck isformed the void formation becomes much more prominent.An appreciable difference exists in the size of the dimpleswith respect to the welding processes. Fine dimples areseen in PCGTAW joints (Figs. 4d and 5d); coarse dimplesare seen in CCGTAW joints (Figs. 4c and 5c); coarse andelongated dimples are seen in PCGMAW joints (Figs. 4band 5b). However, an intergranular fracture feature hasbeen observed in CCGMAW joints (Fig. 4a). This may bedue to the combined influence of a coarse grained weldmetal region and a higher amount of precipitate formationat the grain boundaries. Fine dimples are a characteristicfeature of ductile materials and the PCGTAW joints haveshown higher ductility compared to all other joints. The

dimple size exhibits a directly proportional relationshipwith strength and ductility, i.e., if the dimple size is finer,then the strength and ductility of the respective joint ishigher and vice versa.

4 Discussion

During tensile tests all the specimens were found to fracturewithin the weld region. Thus it may be assumed that theultimate tensile strength presented in Table 3 is primarilythe ultimate tensile strength of the weld. The use of pulsedcurrent welding improves the strength of the weld over thatobserved for the case of continuous current welding. Therefinement of microstructure due to the pulsed currentwelding results in a uniform distribution of the fine

Fig. 3 Optical micrographs ofweld metal region. a CCGMAW(average grain diameter=80 μm).b PCGMAW (average graindiameter=60 μm). c CCGTAW(average grain diameter=40 μm).d PCGTAW (average graindiameter=20 μm)


precipitates more effectively governed by its zinc pick-upenhancing the amount of precipitates in the matrix. Similarobservations have been made by other investigators [8, 9].

In general, hardness in the fusion zone is the lowest dueto the ‘as cast’ nature of the microstructure, which ischaracterized by coarse dendritic grains, interdendriticsegregate phases, and the lack of strengthening phases.Hardness is slightly higher in pulsed current welds ascompared to continuous current welds and this could be dueto the refined microstructure and low segregation ofstrengthening phases. The moderately higher hardness ofpulsed current welds close to the fusion boundary ispossibly due to a large fraction of alloying elements insolid solution at the end of the weld thermal cycle, therebygiving conditions for extensive age hardening. This can beexplained as follows: at the fusion boundary, precipitatedissolution occurs as the particles are exposed to temper-atures higher than 400°C during heating and cooling as aresult of welding. The dissolution process enriches the solidsolution of the aluminum matrix with Mg, Zn and Cu. Thisresulted in increasing hardness.

However, from the results it is evident that the pulsedcurrent parameters have greater influence on mechanical

and metallurgical properties. The enhancement in mechan-ical properties such as tensile properties and weld metalhardness are mainly due to the refinement in fusion zonegrain size. Hence, the basic reason for the improvement inmechanical properties is the refinement produced in fusionzone grain size by the pulsed current welding technique.The mechanism involved in grain refinement by pulsingcurrent is explained in detail below.

The amplitude of thermal oscillations has been found toincrease with increasing ratio of the peak to base currentsand to decrease with rising pulse frequency. The conse-quence of the thermal fluctuations leads to periodicinterruption in the process of solidification. As the pulsecurrent decays, the solid liquid interface advances towardsthe arc and increasingly becomes vulnerable to anydisturbances in the arc form. Heat transfer experienced bythe weldment during welding can alter the microstructureand thus the property of the weldment. Therefore the heattransfer and fluid flow in the weld pool can significantlyinfluence factors such as weld pool geometry, temperaturegradient local cooling rates and solidification structure [10].

In general, the formation of equi-axed grain structure inCCGMAW weld is known to be difficult because of the

Fig. 4 Fractographs of unnotchedtensile specimen. a CCGMAW.b PCGMAW. c CCGTAW.d PCGTAW


remelting of heterogeneous nuclei or growth centers aheadof the solid-liquid interface. This is due to the hightemperatures in the liquid, thus making survival nucleidifficult. The evolution of microstructure in the weld fusionzone is also influenced in many ways by current pulsing,principally, the cyclic variations of energy input into theweld pool cause thermal fluctuations, one consequence ofwhich is the periodic interruption in the solidificationprocess. As the pulse peak current decays the solid-liquidinterface advances towards the arc and increasinglybecomes vulnerable to any disturbances in the arc form.As current increases again in the subsequent pulse, growthis arrested and remelting of the growing dendrites can alsooccur. Current pulsing also results in periodic variations inthe arc forces and hence the additional fluid flows thatlower temperatures in front of the solidifying interface [11].Furthermore, the temperature fluctuations inherent inpulsed welding lead to a continual change in the weld poolsize and shape favouring the growth of new grains. Itshould also be noted that effective heat input for unitvolume of the weld pool would be considerably less in

pulse current welds for which reason the average weld pooltemperatures are expected to be low [12].

It is important to note that while dendrite fragmentationhas frequently been cited as a possible mechanism,evidence for the same has not been hitherto established/demonstrated. It has been sometimes suggested that themechanism of dendrite break-up may not be effective inwelding because of the small size of the fusion welds andthe fine interdendrite spacing in the weld microstructure.Thus grain refinement observed in the PCGTAW welds istherefore believed to be due to other effects of pulsing onthe weld pool shape, fluid flow and temperatures. Thecontinual change in the weld pool shape is particularlyimportant. As the direction of maximum thermal gradient atthe solid-liquid interface changes continuously, newergrains successively become favourably oriented. Thus,while each grain grows only a small distance, more grainsgrow resulting in a fine-grained structure [13].

The superior mechanical properties of PCGTAW joints aremainly due to the following reasons: (i) Finer grains in theweld metal region on the order of 20 μm; (ii) More grain

Fig. 5 Fractographs of notchedtensile specimen. a CCMIG.b PCMIG. c CCTIG. d PCTIG


boundary area due to fine grains (enhances resistance todeformation); and (iii) Uniform distribution of precipitates allover the matrix (enhances resistance to indentation).

5 Conclusions

In this paper, the effect of a pulsed current welding techniqueon tensile properties of welded AA7075 aluminum alloy hasbeen analyzed in detail. From this investigation, the followingimportant conclusions have been derived:

(i) Of the four types of welded joints, the joints fabricatedby PCGTAW exhibited very high strength values andthe enhancement in strength value is approximately25% compared to CCGMAW joints, 15% compared toPCGMAW joints and 8% compared to CCGTAWjoints.

(ii) PCGTAW joints exhibited relatively higher jointefficiency (57%) and the improvement in jointefficiency is 25% compared to CCGMAW joints,16% compared to PCGMAW joints and 8% com-pared to CCGTAW joints.

(iii) Hardness is lower in the weld metal (WM) regioncompared to the PMZ, HAZ and BM regionsirrespective of welding technique. Very low hardnessis recorded in the CCGMAW joints (70 VHN) andthe maximum hardness is recorded in the PCGTAWjoints (100 VHN).

(iv) The PCGTAW technique produced very fine grains(20 μm) in the weld metal region compared to allother techniques and the reduction in grain diameterwas 75% compared to CCGMAW joints, 66%compared to PCGMAW joints and 50% comparedto CCGTAW joints.

Acknowledgement The authors are grateful to the Department ofManufacturing Engineering, Annamalai University, Annamalainagar,Tamil Nadu, India for extending the facilities of the Metal JoiningLaboratory and Materials Testing Laboratory to carry out this investiga-tion. The authors also wish to express their sincere thanks to theDirectorate of Extramural Research & Intellectual Property Rights,Defence Research & Development Organisation (DRDO), New Delhifor the financial support to carry out this investigation through sponsored

project No.ERIP/ER/0203351/M/01/633. The first two authors are verygrateful to the third author, Dr. G. Madhusudhan Reddy, Scientist-F,Defence Metallurgical Research Laboratory (DMRL), Hyderabad for hisvaluable suggestions, guidance and discussion.

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Effect of Pulsed Current Welding on Mechanical Properties