Coating behaviour and nugget formation ... - Dulal Saha, PhD. · force (2.25 kN) and weld current (3.0 kA) were reduced to half of that of the original values (4.5 kN and 6.0 kA)

Coating behaviour and nugget formation duringresistance welding of hot forming steels

D. C. Saha1,2, C. W. Ji3,4 and Y. D. Park*1

In this work, the effects of the coating composition and the welding parameters on the heat

generation and nugget formation during resistance spot welding of the hot press forming steels

have been evaluated. Two types of coated steels were used, termed as Al–Si and Zn coating.

Al–Si coated steel showed rapid nugget growth toward the electrode direction, which is the

probable reason of the higher heat conduction from the electrode–sheet surface toward the bulk

material. In Zn coated steel, heat was generated and localised at the faying interface and

uniformly propagated to the steel substrate. High speed camera images showed that the

presence of oxide at faying interface provides inhomogeneous current flow and violent heat

generation in the Zn coated steel.

Keywords: Resistance spot welding, Hot press forming steels, Coating layer, High speed camera, Nugget formation

IntroductionIn recent years, automotive manufacturing companiesare embraced to use high strength steels for weight re-duction, high impact resistance and less CO2 emissions.Hot pressed boron steels have been developed to servethese purposes, which provide unique properties, such asno spring back effect, ultrahigh strength (ultimate tensilestrength>1.5 GPa), etc. To ensure the corrosion pro-tection issue, Al–Si based precoated hot press forming(HPF) steel was developed. Hot press forming steel isusually austenised in a furnace maintaining temperaturebetween 900 to 930uC and hold for a period of 5–10 min;as a result, Fe diffusion occurs, and thereby, the coatingtransformed into several layers of intermetallic com-pounds; among these, the Fe–Al–Si layer existingbetween the coating and steel substrate has a thicknessof about 5–7 mm.1–3 It has been reported that there are atotal five layers; the first layer was identified to beFe2Al5, which was followed by FeAl2 and Fe2SiAl2.Details of the coating microstructure can be foundelsewhere.4 Additionally, zinc based coating has beendeveloped in Europe, which offers excellent scaling aswell as cathodic corrosion protection.5

In the last decades, researchers have dedicated theirinterest to develop microstructure, phase transformationdiagrams and mechanical properties relationship of theHPF steels; limited information was obtained regardingheat treatment, coating process, formability and phasetransformation.1,4,6,8,9 Apart from the microstructural

analysis, proper joinability and weldability of the com-ponents must be ensured in order to commercialise thesteel sheets. The joining of the press hardened steel bymeans of mechanical clinching is difficult due to thehigher base metal strength; therefore, a short laser pulseis applied to heat up the sheets locally before or at thesame time while clinching.10 Resistance spot welding(RSW) is one of the fundamental joining process forautomotive components, it being said that every auto-motive contains about 5000–8000 spot welds. The vitalissue associated with the RSW of HPF steels is thecomposition and properties of the existing surfacecoating and oxides, which degrades electrode life andenhances surface and interface expulsion.4,9,11 There-fore, it is a matter of concern to remove the meltedcoating from the faying interface before nugget growth.Therefore, prepulse condition is sometimes consideredto be an effective way to get the desired weld proper-ties.11 Hwang et al.12 proposed to use a pulse currentwaveform to reduce the expulsion to weld Al–Si coatedHPF. Jong et al.13 evaluated the microstructure andmechanical properties of boron containing ultrahighstrength steel. Resistance spot weldability of dissimilarcombination of 22MnB5 and GADP780 was investi-gated and reported by Choi et al.14 Although surfacecoating of HPF steels solved the surface oxidation anddecarburisation problem, the existence of the highsurface coating introduces a high contact resistance,therefore degrading the spot weldability.4,7,15 In order toget the desired weld quality, a sandblasting or a dry icecleaning process had been used to remove the coatingbefore weld.5,7,11 Therefore, it has been granted that thecoating properties are the main spot welding issue,which needs to be understood properly to obtain a highreliable weld properties.

In the present work, the spot welding characteristicsof Al–Si and Zn coated HPF steels have been investi-gated by observing the nugget formation and heatgeneration. The nugget formation was monitored using

1Department of Advanced Materials Engineering, Dong-Eui University,995 Eomgwangno, Busanjin-gu, Busan 614-714, Korea2Department of Mechanical and Mechatronics Engineering, University ofWaterloo, 200 University Avenue West, Waterloo, Ont N2L 3G1, Canada3Department of Materials Science and Engineering, Pusan NationalUniversity, Busan, Korea4Department of Materials Science & Engineering, The Ohio StateUniversity, Columbus, OH 43221, USA

*Corresponding author, email [email protected]

� 2015 Institute of Materials, Minerals and MiningPublished by Maney on behalf of the InstituteReceived 16 April 2015; accepted 27 May 2015DOI 10.1179/1362171815Y.0000000054 Science and Technology of Welding and Joining 2015 VOL 20 NO 8 708

a high speed video camera, as well as by cross-sectioningthe nugget. The coating layer melting sequences, elec-trode degradation, elements distribution on the electrodetips and the effect of coating on expulsion behaviourhave been evaluated.

Materials and methods

MaterialsTwo different coated HPF steels (thickness, 1.0 and1.50 mm) were used in this investigation, which aretermed as Al–Si coated HPF and Phs-ultraform Z140(in this study, from here and onwards, Al–Si coated HPFand Phs-ultraform Z140 will be referred as Al–SiHPF and Zn HPF respectively). The chemical compo-sitions and coating properties of both of the steels areshown in Table 1. It can be noted here that all materialsused in this investigation were in hot stamped condition.The Al–Si HPF was austenised in a furnace to 900uC for5 min and quenched through a metallic flat dies with acontact pressure of*40 MPa for 20 s; the temperature ofthe dies were about 40–50uC, as measured with apyrometer. Zn HPF was austenised by the followingsimilar procedure as of the Al–Si HPF. In non-hardenedcondition, coating thicknesses were about 9–13 mm,which were transformed to 15–30 mm as hardenedconditions. Cross-sectional views and coating layercomposition of both Al–Si HPF and Zn HPF areshown in Fig. 1.

MethodsThe ISO 18728-21216 was employed to evaluate theweldability, and detailed welding schedules are tabulatedin Table 2. The welds were performed on a pedestal typemedium frequency dc inverter spot welder with Cu–Crdome radius type electrode (ISO 5821:2009)17 under aconstant water cooling rate of 6 L min21. Before weld,the Zn HPF steel undergoes a dry ice cleaning process toremove the oxide layer from the surface of the sheets,which lowered the contact resistance.4 Dynamic resist-ance data between the copper electrodes were obtainedfrom the machine readout software (Rexroth, BoschGroup, BOS6000 version: 1.35.2). For metallographicevaluation, samples were mounted and polished by thefollowing standard metallographic procedure; thepolished samples were etched with 4% nital (4%HNO3on 96% ethanol) to reveal the microstructure. Stereo-and optical microscopes were used to perform micro-structure observation. Furthermore, an electron probemicroanalyser was used to get elements distributioninformation on the electrode tips as well as the fayingsurface of the welded coupons.

A high speed camera was used to monitor the nuggetgrowth; the monitoring system includes a processor,camera with different lenses, power supply, lightingsources, etc. Half cross-sectioned Cu–Cr alloy made8.0 mm electrodes (dome radius type) were used tocapture the nugget formation sequences. The electrodeforce (2.25 kN) and weld current (3.0 kA) were reduced

to half of that of the original values (4.5 kN and 6.0 kA)as suggested elsewhere.18,19 A 1.50 mm thickness ofAl–Si HPF and Zn HPF steels was used to observe thenugget growth in a high speed camera; the front faces ofthe sheets were polished with 600 grade silicon carbidepapers before the experiment. The sheets were placedbetween the two half sectioned electrodes, and the elec-trodes were brought into contact of the sheets with apredefined electrode force; afterward, a weld current wassupplied to make the weld. To magnify and visualisethe liquid nugget properly, a high speed camera wasequipped with series of lenses: a combination of Nikon200 mm, 1:1.4 D 50 mm, ultraviolet lens and an endfilter. The images were captured at a rate of 2000 fps andshutter time of 1/6400.

Results and discussion

Effect of weld currentOwing to rich chemistry, high stiffness, high strength andcomplex microstructure, advanced high strength steels(including HPF steels) usually provide a narrow weldprocess window in contrast to mild and low carbonsteels.20,21 Figure 2 indicates the proportional relation-ship between the weld current and nugget diameter,i.e. nugget diameter increased with weld current; similarresults were obtained elsewhere.9,21,22 An appropriateweld current was determined according to 4t 1/2 condition(where t is the sheet thickness).16 The graph also illustratesthe effect of coating and the sheet thickness on the nuggetsize; a larger nugget was observed in the Al–Si HPFcompared to the Zn HPF steel, regardless of the sheetthicknesses. Contrariwise, the sheet thicknesses greatlyinfluences the expulsion (i.e. ejection of themetal from thefaying interface, star symbols in Fig. 2) condition of theAl–Si HPF steel. It was noticeable that no weldable cur-rent range (current range between the nugget diameterequal to 4t 1/2 to expulsion) was obtained in thinner gaugeAl–Si coated steel. Before expulsion, Zn HPF provided alarger nugget diameter (1.0 mm sheets) compared to theAl–Si HPF, which is attributed to the effect of surfacecondition, as Zn HPF steel surfaces were dry ice cleaned.It can be noted here that thinner gauge sheets showedearlier expulsion compared to thicker gauge sheets.Electrode tip diameter is one of the influencing factors,which is inversely proportional to the heat input. As asmaller electrode tip (6.0 mm) was used to weld 1.0 mmgauge sheets, therefore, the contact diameter at the elec-trode/sheet (E/S) interface would be smaller compared tothe larger electrode as used for thicker gauge sheets. As aresult, the current density (ratio of the applied current tothe contact area) will be higher; therefore, a high currentdensity offered earlier expulsion, as expected. Through-out the study, a larger nugget diameterwas obtained in theAl–Si HPF compared to the ZnHPF steel, and the nuggetsize differences were more pronounced when thickergauge and higher weld current (above 6.0 kA) were used.The bulk resistance of the sheets is related to its thickness,as thicker gauge sheet has a higher bulk resistance

Table 1 Nominal chemical composition and coating properties of used hot pressed steels

Materials C Mn Si P Ceq33 Fe Al Si Zn

Al–Si HPF 0.23 1.24 0.26 0.015 0.45 Coating ... 90 10 ...Zn HPF 0.3 2.0 0.5 0.02 0.75 40–65 0.7 ... Balance

Saha et al. Coating behaviour and nugget formation during RSW

Science and Technology of Welding and Joining 2015 VOL 20 NO 8 709

compared to the thinner one. High bulk resistivity of thethicker gauge sheet allowed more heat to be generated;therefore, the nugget size would be larger at the samewelding condition as depicted in Fig. 2. While heat wasgenerated at the sheet/sheet interface, the thicker gaugesheet can sustain at a high heat; on the other hand, thinner

sheets have less ability to withstand generated heat as theunmelted area around the molten nugget reduced; there-fore, early expulsion could be expected.

Weld nugget formation and growthThe presence of coating layer on the sheet surfaces canhave adverse effects on the nugget formation. Generally,forweldingof coated steels, a higherweld current, a longerweld time and a higher weld force are recommended byseveral researchers.23,24 Unlike hot dipped galvanised or ahot dipped galvannealed, the coating layer on the HPFsteel offers an additional resistance at the faying surface(F/S, the contact interface between the sheets), aswell as atthe E/S interface. The additional resistance influencesthe nugget formation and heat generation as can be seenin Fig. 3.

Two important factors should be considered toexplain the heat generation and nugget formationphenomenon; these are surface coating and sheetthickness. It can be observed in Fig. 3 that heat wasgenerated at F/S interface as well as in the E/S interfacesin both of the steel types. In contrast to the Zn HPF,more heat was being accumulated at the E/S interfacecompared to the F/S interface in Al–Si HPF, as canbe compared by the contrast of the zones affected by theheat (Fig. 3a and c, at 1 cycle). It is evident that2 Nugget diameters as function of weld currents

Table 2 Resistance spot welding conditions used in experiments (ISO 18278-2)16

Schedule (cycle, 60 Hz) Electrode

Materialsthickness/mm

Electrodeforce/kN Squeeze Pulse Weld Cooling Hold Materials

Tipdiameter/mm

Waterflowrate/L min21

1.0 3.5 70 1 15 ... 15 Cu–Cr 6 61.5 4.5 70 3 11 2 20 Cu–Cr 8 6

1 Optical micrographs and change of Fe, Al, Si and Zn concentrations in thickness direction of samples a Al–Si HPF and

b Zn HPF; all of materials considered in present study were in hot stamped condition



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the current passing path (current passing width) wasnarrower in the case of the Zn HPF; therefore, it isexpected that the nugget would form earlier due to thehigher current density. The earlier nugget formation wasobserved in the Zn HPF at weld cycles of 2 (Fig.3b) and4 (Fig. 3d). Although the earlier nugget formation wasidentified in the Zn HPF, the rapid nugget growth wasobserved in the Al–Si HPF (at 3 cycles, Fig. 3a). Figure 2also depicts same phenomenon with respect to sheetthickness; a slower nugget growth was observed inZn HPF. Initially, a large amount of heat was con-centrated at the E/S interface in Al–Si HPF, which actedas a thermal barrier to prevent heat from being dis-sipated through the water cooled electrode and allowedlarger nugget to be formed.

Dynamic resistance (the change in resistance duringwelding, measured between the electrodes)25 shown inFig. 4 was measured between two electrodes duringwelding of two different steels. The graph indicates higherinitial contact resistance26 and dynamic resistance in theAl–Si HPF, for a specific weld time. The higher contactresistance at the beginning of the weld is believed to be theeffect of existing surface oxides in theAl–Si coated sample.It has been reported that the Fe–Al intermetallic phasesexisting on the Al–Si coating is electrically conductive;therefore, the coating is considered to be compatible to the

RSW process.4 It should be noted here that higherdynamic resistance during welding allows to generatemore heat, which eventually enlarges the nugget diameteras corroborated with Figs. 2, 3 and 5. Although the otherfactors such as coating thickness, surface roughness and

4 Comparison of dynamic resistance of Al–Si coated HPF

and Zn coated HPF at welding current of 6.0 kA

a Al–Si HPF (1.0 mm); b Zn HPF (1.0 mm); c Al–Si HPF (1.5 mm); d Zn HPF (1.5 mm)3 Nugget cross-sections are showing heat generation and nugget formation with respect to weld time of 1–4 cycles at 6.0 kA

weld current




resistance of the base metal influences the contact resist-ance during RSW, it was found that these factors haveinsignificant effect in the hot stamped material, as the re-sistance of the sheets is about half of the contact resistancebetween the electrode and sheet. It should be reportedthat the surface oxides play an important role forincreasing the contact resistance; the higher the oxidelayer, the larger the contact resistance.4 Therefore, some-time, it is recommended to optimise the heating cycle ofthe hot stamping process to minimise the formation of thesurface oxides; a slowheating cycle produces thinner oxidelayer compared to the fast heating cycle. However, thesurface oxides have an adverse effect on spot weldability;therefore, the hot stamped sheets are always followed by

either dry ice cleaning or shot blasting process to reducethe contact resistance within acceptable range.

A high speed camera was assisted to visualise the gen-erated heat and the nugget formation mechanism; thecaptured images are shown in Fig. 5. The coating meltingand nugget formation were compared with respect to thewelding cycle. At the relatively low welding time (1 cycle),at the start of the weld current, fume generation wasobserved at F/S, which was due to the presence ofimpurity and coating layer on the sheets’ surface.There were no significant differences; both samples showsimilar behaviour. At 3 cycles, it was clearly visible thatthe bulk materials were affected by the weld current andheat propagated from the faying interface to the outward



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direction. Although bulk materials were heated up, thecoating melting in the Al–Si HPF was not detected sig-nificantly; conversely, the Zn coating was melted at F/Sand simultaneously heat spread outwardly. This impliesthat the Zn HPF provided additional resistance at F/Sand helps to accumulatemore heat compared to theAl–SiHPF. At 5 cycles, Al–Si coating melted and liquidformed; afterwards, the molten coating was squeezed outat the F/S as can be visualised in Fig. 5a. Conversely, theearlier nugget formation was found in the Zn HPF(Fig. 5b), at the same weld time, at 5 cycles. This obser-vation (Fig. 5) well agreed with Fig. 3d, where the earlynugget formation was also detected in the Zn HPF at 4cycles. With the further progress of the weld cycle, thecontrolled nugget formation and uniform coatingmeltingwere examined in the Al–Si HPF. In contrast to the Al–SiHPF, inhomogeneous current flow and violent heatgeneration were detected in the Zn HPF. Non-uniformityof the coating layer and the violent heat generationmainly occurred due to the presence of the oxide onthe sheets’ surface.4,27 At the end of the weld, at 15 cycles(end of the applied weld current), a bigger nugget can befound in the Al–Si HPF, which could be the fact of thestable and uniform heat generation/propagation.This phenomenon well agreed with the experimental datapresented in Fig. 2, where Al–Si HPF provides a largernugget diameter. On the other hand, the nugget for-mation phenomenon is completely different in the ZnHPF, where unstable heat generation can be detected,which was the major reason for getting earlier expulsionas shown in Fig. 2. In addition, a lack of molten liquid inthe fusion zone (FZ) was observed in this case (Fig. 5b, at15 cycles),27 which suggested that the expulsion mayoccur in the full weld schedule. Dirty sheet faces and thecoating adhesion onto the electrodes were visualised inFig. 5 (15 cycles); conversely, the clean electrode tip canbe identified in the Al–Si HPF.

Effects of coating on heat generationAs previously described, in addition to the F/S interface,the presence of surface coating influences the heat gen-eration at the E/S interface. The combination of thesegenerated heat plays an important role on the finalnugget size and shape. As the heat generation patternwas different for both materials, therefore, it is quiteimportant to take into account the thermal distributionacross the weldments. Based on the experimental ob-servation, the predicted temperature distributions acrossthe sheets during welding are presented in Fig. 6. Points1 and 7 represent the temperature of the electrodes insidethe cooling water circulating system, points 2 and 6 arethe E/S interface temperature, points 3 and 5 are thebulk materials temperature and point 4 is the F/Sinterface temperature. In Al–Si HPF, it was observedthat higher heat was being accumulated at the E/Sinterface due to the presence of oxides; therefore, thetemperature distributions between points 2 and 3, and 5and 6 were higher compared to the counterpart. This is

6 Schematic representation of temperature distribution

during resistance spot welding of Al–Si HPF and Zn HPF

a Al–Si HPF; b Zn HPF5 Nugget propagation photographs monitored using high speed camera with respect to weld time of 1–15 cycles




reasonable as the Al–Si intermetallic phases that existbetween the steel substrate and coating layer are elec-trically less conductive;26 therefore, Al–Si coated HPFare generally considered to be compatible with RSWprocess.4 Another study9 reported that Al–Si HPF steelprovided good spot weldability due to the high meltingpoint of the Fe–Al alloy phase on the surface. Theexistence of higher heat at the E/S interface allows heatto be soaked into the bulk material and eventuallyfacilitates nugget growth toward the electrode direction.According to the temperature distribution presented inFig. 6, it can be asserted that the higher heat build-up inthe E/S (in Al–Si HPF) will enlarge the electrode tips.At the same time, the higher temperature will allowmore coating to be penetrated onto the electrode tip;these issues have been discussed in the section on ‘Effectsof electrode contact area on heat generation’.

Figure 7 shows the coating melting sequences at theF/S interface with respect to the weld current. The weldswere performed with varying weld currents from 6.0 to

10.0 kAwhile keeping other parameters constant (weldingtime, t51 cycle). The melting temperature of the Al–Sicoating layer is *600uC;9 therefore, during welding, thecoating layer melted quickly and squeezed out from theinterface as can be seen in Fig. 7a. The molten coatingpushed out intensely to the periphery of the interface,which implies that melting occurred uniformly. Therefore,a high volume of themoltenmetal can be accumulated andejected, which made a trace on the sheet surface as can bevisualised in Fig. 7a (at 8.0 and 10.0 kA). A close obser-vation showed that in the Al–Si coated HPF, melting in-itiated at various spots (Fig.7a; at 8.0 kA) rather than atthe centre of the F/S; as the weld current increased, thenumber of spot increased; finally, all spots combined eachother. Conversely, in the Zn HPF, heat was concentratedat the centre of the F/S, andmolten coatingwas pushed outslowly towards the periphery of the localised heatedarea (Fig. 7b).

To investigate the element composition distributions atF/S, the samples were welded with a higher weld current

a Al–Si HPF; b Zn HPF7 Optical stereomicroscope images of faying interfaces showing coating melting sequences at 1 cycle with increasing weld

current from 6.0 to 10.0 kA




(10.0 kA) and a lower weld time (1 cycle), so that com-plete nugget formation can be avoided. Afterward,element distributions at faying interface were observedusing electron probe microanalysis as presented in Fig. 8.The results illustrate a less aluminium concentration circleat the centre of the F/S of the Al–Si coated sample(Fig. 8a). Conversely, iron was exposed at the centre ofthe F/S, which is a good indication of the removal of theAl–Si layer from the weld centre; in addition, lean oxygen(removal of oxides) concentration annular zone wasformed, which offered a stable nugget formation.Another noticeable feature was that silicon was uniformlydistributed throughout the periphery of the annular zone.

Contrariwise, even though nugget was formed at thecentre of the F/S, still a non-uniform zinc distribution canbe found in Zn coated sample (Fig. 8b). Rich concen-tration of oxygen was observed at the periphery of thewelded zone, which must be removed before nugget for-mation; otherwise, it may create a barrier to heat gener-ation and nugget formation. Presence of oxides at theinterface suggests inhomogeneous current flow;28 there-fore, an unstable heat generation and propagation can beexpected as shown in Fig. 5b.

The coating removal/melting process were furtheranalysed by observing the interface microstructureof the cross-sectioned samples as presented in Fig. 9.

a Al–Si HPF; b Zn HPF8 Electron probe microanalysis maps of elements distribution at faying surface (3.5 kN, 10.0 kA, 1 cycle)




Distinguishable coating removal features were detected;coatings were removed from the centre to the F/S andformed uniform intermetallic coating layers (Fe–Al–Si)throughout the F/S interface. In Al–Si HPF, a uniformcontinuous melted intermetallic layer of *8+3 mm(zones ‘b’ and ‘c’ in Fig. 9a) was observed throughoutthe F/S interface, and the molten coating was pushedfrom the interface towards the two ends (corona bond

area) as indicated by zones ‘a’ and ‘d’ (Fig. 9a). At theinitial stage of welding process, the Al–Si coating layermelted and removed easily due to its low melting point9

and accumulated into the edge of the electrode contactarea. The Fe–Al–Si intermetallic phase is electricallyconductive;4 therefore, a uniform coating of this inhi-bition layer eventually helped to maintain uniformelectrical current path.

a Al–Si HPF; b Zn HPF9 Optical microscope examination of interface coating layer during RSW at 6.0 kA, 2 cycles

10 Electrode tips diameter and cross-sectional views for a and c Al–Si HPF (1.0 mm) b and d Zn HPF (1.0 mm) after 20 welds at

3.0 kA weld current




Conversely, in theZnHPF, theF/S coating layerwasnotremoved uniformly, as can be seen in Fig. 9b. Althoughnugget was formed in this case, a non-uniformly meltedcoating can be examined at F/S, as can be seen from zonesB and C in Fig. 9b. Most importantly, the Zn–Fe inter-metallic layer was not squeezed out as a liquid form, as itwas in Al–Si HPF. A high electrical resistivity of thecoating layer did not permit sufficient current (as currentalways passes through the least resistance path) tomelt thecoating uniformly throughout the F/S rather than tolocalise heat at the weld centre. Another feature was thatthe coating layer was not much affected by the electric

current at the heat affected zone (HAZ) area (zones A andD in Fig. 9b).

Effects of electrode contact area on heatgenerationElectrode indentation onto the surface of the sheets isanother important feature that indicates the degree ofdeformation in the E/S interface under a predefined elec-trode force.Ashigherheatwasbeingconcentratedat theE/Sinterface in theAl–SiHPF(Fig. 3aand c), therefore, ahigherdegree of plastic deformation can be expected. The averagepenetration/indentation of the electrodes into the sheets was

a Al–Si HPF; b Zn HPF11 Electron probe microanalysis maps of elements distribution at electrode tips




measured tobe*220+12 mmand170+15 mmin theAl–SiHPF andZnHPF (at weld current of 6.0 kA, 1.5 mm sheet)respectively. As a result, more surfaces were brought intocontact to the electrode tip, which eventually enlarged theelectrode tip surface area as shown in Fig. 10 (contact di-ameter, 4.15 mm compared to 3.95 mm).

Figure 10 illustrates the electrode contact diameterand the coating penetration into the electrode faces.The experiments were carried out on 1.0 mm thicknesssteel sheets with a weld current and time of 3.0 kA and1 cycle respectively. After 20 welds, the electrode con-tact diameters were measured on a stereomicroscope,and the contact diameters were found to be 4.15 and3.95 mm in the Al–Si HPF and Zn HPF respectively.It was obvious that a larger electrode contact facewould reduce the current density,29 hence allowing awide current passing width. Therefore, it will requiremore time to form nugget initially as detected in Fig. 3.After breakdown of the surface coating, a wider currenttransient area helps to expand the nugget in all direc-tions as examined metallographically by cross-section-ing the welded part (Fig. 3a for 3 and 4 cycles), and itwas further confirmed by examining the nugget growthand heat generation using a high speed video camera,as presented in Fig. 5.

Comparing the effective contact diameter between F/Sand E/S among the two different samples, it can befound that the E/S contact areas (Fig. 10) were largercompared to the F/S interface (Fig. 7) in the Al–Si HPF.In addition, this statement can be verified by visualisingthe heat affected area in the E/S interface (Fig. 3,1 cycle). In Fig. 1b, a lot of surface cracks were visible inthe Zn HPF, and the cracks were extended from thecoating surface to the coating/steel substrate interface.These surface cracks might help to concentrate heat in alocalised place (weld centre) by quick breakdown.Figure 2 illustrates that, to obtain the same weld size, theZn HPF requires a higher weld current in identicalwelding condition. Lesser heat at E/S cannot providesufficient thermal barrier to help nugget growth in theelectrode direction.

Figure 11 indicates the element map of the electrodetip surface for both steels. As it can be seen in Fig. 11a,aluminium, oxygen and silicon were adhered into theelectrode tip uniformly. However, aluminium andoxygen showed higher concentration at the centre of theelectrode tip. Figure 11b illustrates similar element mapin Zn HPF, and it represents a different phenomenon ascompared to the Al–Si HPF. In this case, Zn stronglypicked up from the sheets onto the centre of the




electrode tip, and oxygen, iron and aluminium accu-mulated at the annular zone around the periphery of thetip. It can be visualised that, although zinc was pickedup at the electrode face, it had not formed brass (Zn–Cu)alloy composition, as copper was totally absent wherezinc was in rich concentration.

Microstructural observation of weldMicrostructure has changed fromas received condition tothe FZ depending on the thermal gradient experiencedduring welding. Five distinct zones can be found in thepost-welded weldment, which were categorised as BM,subcritical HAZ (SCHAZ), intercritical HAZ (ICHAZ),upper critical HAZ (UCHAZ) and FZ.30–33 Microstruc-tures obtained in this investigation are presented inFig. 12; zones inAl–SiHPFare denoted a–e and that inZnHPF,A–E. The distinguishable features ofmicrostructurewere not observed between the used two types of steels, asthe microstructure of the as received materials weresimilar and identicalwelding conditionswere applied.TheBM consists of typical lath martensite for both of thematerials; however, the prior c grains were larger in Al–SiHPF, as canbe compared fromzones ‘a’ and ‘A’ inFig. 12.

The transition zone was formed between BM and HAZ,which referred to the SCHAZ, and it is clearly visible as ablack shade in the half section of the weld nugget. In theSCHAZ,microstructure experienced temperature close totheAc1 temperature line, and theBMmartensite structureundergone tempering process or softening (zones b andB).34,36 During tempering, a virgin martensite micro-structure was destroyed, and it was transformed intoferrite and cementite.34 The tempered martensite usuallyprovides lower mechanical properties compared to theBM; it was reported that the reduction of hardness in thiszone has detrimental effect on joint performance.13,35 Thezone next to the SCHAZ is called ICHAZ (zones ‘c’ and‘C’), where temperature reached between the temperatureof Ac1 and Ac3 line, and austenite phase is formed alongthe prior austenite grain boundary; austenite phase istransformed into new formed martensite and/or bainite21

upon cooling to room temperature. The zone beside theICHAZ is the UCHAZ (zones ‘d’ and ‘D’), where tem-perature is well above theAc3 temperature line; therefore,complete austenisation occurred. Full austenised micro-structurewas retransformed intomartensite structure dueto the high cooling rate involved in the RSW process.

12 Microstructural change from base metal to weld metal for a Al–Si HPF, base metal, b SCHAZ, c ICHAZ, d UCHAZ and

e FZ and A Zn HPF, base metal, B SCHAZ, C ICHAZ, D UCHAZ and E FZ




However, microstructures are coarser than that of the asreceived materials (zone ‘a’ and ‘A’), and the finalmicrostructure in the FZ was 100% martensite; direc-tional solidification and typical lath type martensite wereobserved in this zone (zones e and E).30–33

ConclusionsIn this investigation, the effect of different coating typeson nugget formation and heat generation during RSWof hot pressed forming steels is reported. On the basis ofexperimental results and discussion, the major con-clusions can be drawn here.

1. Coating types of the sheets play an important role inheat generation and nugget formation in RSW of theHPF steels. Al–Si coating helps to grow nugget moreuniformly compared to the Zn coating; therefore,nugget diameter was higher in the former case.

2. Although early nugget formed in Zn HPF, thenugget growth rate was rapid in the Al–Si HPF, atthe initial stage of welding. In the Al–Si HPF,both faying interface and the bulk resistance areresponsible for nugget formation and growthtoward the electrode direction.

3. Higher heat was concentrated at E/S interface,which provides larger electrode contact diameter aswell as larger nugget sizes for Al–Si HPF. UnlikeZn HPF, for Al–Si HPF, heat generated severalpoints in F/S, and finally, these points merged andformed a larger nugget. Existence of oxide at F/Swas the main hindrance to obtain homogeneouscurrent flow and results in violent heat generationin Zn HPF, in contrast to Al–Si HPF, where con-trolled heat generation was observed.

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