quasistatic spot weld strength of advanced high ...€¦ · gws-5a (refs. 6, 7). welds were made...

WELDING RESEARCH

WELDING JOURNAL / MARCH 2017, VOL. 96104-s

Introduction The development of advanced high-strength steels (AHSS) has been a keyenabler for meeting future environ-mental regulations and safety require-ments in the automotive industry. De-velopment of AHSS has been accompa-nied by increased ultimate tensilestrength along with good ductility.Light weighting the automotive bodystructure can be achieved by reducingsheet thickness. However, any materi-

al intended for automotive use shouldbe readily weldable and exhibit goodweld strength. Resistance spot welding(RSW) is a preferred welding processfor body-in-white automotive joining(Ref. 1). The spot weld strength of devel-oped steels should be examined inboth static and dynamic modes if in-tended for application in the automo-tive industry. Quasi-static spot weldstrength is one of the key propertiesfor qualifying new steels, and it is usu-

ally evaluated in tension-shear andcross-tension loading modes. Quasi-static tension-shear and cross-tensionstrengths are two important aspects ofspot welded joints, which are expectedto increase with increasing base metaltensile strength. Tension shearstrength (TSS) and cross tensionstrength (CTS) depend on the proper-ties of the weld-affected zone, whichincludes the spot weld nugget andHAZ. The AHSS nugget microstruc-ture typically consists of martensitebecause of the rapid cooling rate char-acteristic of the spot welding process.The microstructure changes graduallyfrom martensite to the base metal mi-crostructure with increasing distancefrom the nugget, depending on thepeak temperature experienced duringwelding. These microstructuralchanges result in significant changesin crack propagation resistance, frac-ture modes, and maximum load capac-ity. Based on this change in fracturemode, different analytical formulashave been provided to correlate TSSwith sheet thickness (Ref. 2). Also,CTS has been related to base metalproperties and fracture toughness ofthe nugget (Refs. 3–5). In this paper, TSS and CTS are eval-uated for different fracture modes,and analytical techniques are used tonormalize TSS and CTS from dataavailable for different thicknesses andweld diameters. Then, materialstrength in the weld nugget and HAZsare evaluated by hardness measure-ments, and correlated with base metalUTS. Finally, any HAZ softening that

QuasiStatic Spot Weld Strength ofAdvanced HighStrength Sheet Steels

This study highlights the spot weld strength in tensionshear and crosstensionloading modes, and HAZ strength as a function of base metal strength, in sheet steels

BY H. GHASSEMI-ARMAKI, S. BHAT, S. KELLEY, AND S. SADAGOPAN

ABSTRACT Quasistatic spot weld strength is expected to increase with base metal tensilestrength. This paper highlights the spot weld strength (evaluated in tensionshear andcrosstension loading modes) as a function of the base metal ultimate tensile strength(UTS). The studied steels included those with ferritic, dualphase, and/or martensitic microstructures with tensile strengths ranging from ~ 300 to 1700 MPa. Tensionshearstrength (TSS) increases with weld diameter. However, there was an inflection point inthe trendline where the failure mode changed from interfacial fracture to plug failure.The weld diameter where inflection occured in the trendline increased with an increasein sheet thickness and changed slightly with the base metal characteristics. Overall, theeffect of sheet thickness seems more pronounced than that associated with the chemistry/microstructure in the plug failure mode. The TSS and crosstension strength (CTS)data were normalized to offset differences in sheet thickness and weld diameter. Examination of the data shows that TSS increases linearly with base metal tensile strength upto about 800 MPa, and then deviates from linearity. In contrast, CTS appears independent of the base metal UTS. For further analysis, the influence of the heataffected zone(HAZ) softening (the difference between the base metal hardness and minimum hardness in the subcritical HAZ) on the spot weld strength was studied. The correlation ofHAZ softening and spot weld mechanical behavior shows that the TSS increases, evenwith increasing HAZ softening, although there is a discontinuity in the trendline with theonset of HAZ softening. However, CTS appears to be independent of HAZ softening.

KEYWORDS • Spot Weld Strength • Tension Shear • Cross Tension • Base Metal Strength • HeatAffected Softening

H. GHASSEMIARMAKI ([email protected]), S. BHAT, S. KELLEY, and S. SADAGOPAN are with ArcelorMittal Global R&D, E. Chicago, Ind.

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occurs in AHSS is correlated with basemetal UTS and its quantitative rela-tion with TSS and CTS. Note that allTSS and CTS testing during the studyhave been performed on homogenouswelding stackups under quasi-staticconditions.

Materials Studied andExperimental Procedures

The steels in this study ranged fromthose with a low base metal tensilestrength and a ferritic microstructureto materials possessing UTS as high as1700 MPa. Steels with UTS between ~ 600 and 1200 MPa have a primarilydual-phase microstructure consistingof martensite in a ferrite matrix. Asthe UTS of materials with ferrite andmartensite microstructure increases,the volume fraction of martensite in-creases. Steels with UTS above ~ 1200MPa typically have a fully martensiticmicrostructure. Materials with thesame UTS can have somewhat differ-ent microstructures depending on thestrengthening mechanisms employedto achieve the desired strength. In this study, material thicknessranged from 1 to 2.5 mm, and all sheetsteels were cold rolled. Products wereseparated by surface condition intothe following two categories: 1) bare(without coating) and electrogalva-nized coating and 2) galvanized, gal-vannealed, and AlSi coated (for presshardening steels). The identifyingcodes used were as follows: a) DP =

dual-phase (mainly martensite andferrite), b) M = fully martensitic, c) IF= interstitial free, and d) HSLA = high-strength low alloy. All sheets weretested in the as-received condition,and oil cleaning was not applied forany studied sheet steel. Spot welding practices were basedon commercially available standards,mainly AWS D8.9 and General Motor’sGWS-5A (Refs. 6, 7). Welds were madeusing a pedestal type AC spot weldingmachine. Welding parameters werechosen from a standard based on thecategory of thickness and materialstrength (Refs. 6, 7). Cross-weld mi-crohardness, tension-shear, and cross-tension tests were performed based onAWS D8.9 (Ref. 6) using an Instrontensile machine equipped with hy-draulic grips at a cross-head speed of0.4 in./min. The ultimate load wasrecorded for tension-shear and cross-tension tests. Weld diameters for ten-sion-shear and cross-tension datawere measured from samples after me-chanical testing. The measured weldsize after mechanical tests depends onfracture mode. The measured weld di-ameter for fully interfacial failure isclose to the fusion diameter andnugget size, while its weld button isfor plug failure. However, all measuredweld sizes have been called weld diam-eter, regardless of failure mode. Failuremodes can be fully interfacial fracture(failure mode 7), plug failure (failuremode 1), or a combination of two fail-ure modes (failure modes 2, 3, 4, 5,and 6) (Ref. 6). However, to simplify

the interpretation of TSS and CTS re-sults in this study, the failure modeswere divided into plug failure modes(mode 1) and interfacial failure modes(modes 2–7). Also, all welded test samples werewithin the safe current range and nowelded sample above expulsion currentwas considered in this study. While themajority of tests carried out used thestandard spot weld protocols (Refs. 6,7), special temper cycles were developedto improve CTS values of AHSS. Theseresults are described in the section ti-tled improving CTS in AHSS by post-weld heat treatment (PWHT).

Results and Discussions

TensionShear Strength

Evaluating TSS as a Function ofWeld Diameter

Figure 1 shows tension shearstrength as a function of weld diameterfor high-strength interstitial free sheetsteel having UTS ~ 300 MPa and 1.2mm thickness. While the TSS increaseslinearly with increasing weld diameter,the trend line deviates at higher welddiameters. The failure mode changesfrom interfacial at lower weld diame-ters (indicated by solid symbols) to plugfailure (PF) at higher weld diameters(indicated by open symbols). So, thechange in trend of TSS appears to be as-sociated with a change in failure modefrom interfacial to plug failure.

Fig. 1 — Tensionshear strength as a function of weld diameter forhighstrength IF steel.

Fig. 2 — Tensionshear strength as a function of weld diameterfor two grades of DP980 GA (low C and mid C) having thicknessesof 1.2 and 1.6 mm.

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Figure 2 shows TSS as a function ofweld diameter for two grades of gal-vannealed (GA) DP980 with differentchemistries. One is a low-carbon grade(LC, ~ 0.09 wt-% C), and the other is amedium-carbon grade (MidC, ~ 0.16wt-% C). Data was given for two differ-ent thicknesses for each grade (1.2 and1.6 mm). Solid symbols represent in-terfacial failure mode, while open sym-bols indicate plug failure. As seen inFig. 2, TSS increases linearly as a func-tion of weld diameter for each chem-istry/thickness combination. However,the slope of the trend line suddenlydecreases at a point where the failuremode changes from interfacial fractureto plug failure. All four steels show al-most similar TSS for a given weld di-ameter in the interfacial failure modezone; however, the weld diameterwhere the deviation occurs appears todepend both on the sheet thicknessand material grade (mainly carboncontent). In plug failure mode, lowerthickness sheets show lower TSS,while higher thickness sheets followthe same trend in interfacial fracturemodes until deviation starts at a high-er weld diameter. On the other hand,for a given sheet thickness, TSS ishigher in the plug failure mode regionfor steel with lower carbon content.Therefore, the chemistry, microstruc-ture, and sheet thickness affect TSS athigher weld diameters where the TSStrend transitions from the interfacialfailure to plug failure mode. Overall, the effect of sheet thick-ness seems more pronounced thanthat associated with the chemistry/mi-crostructure in the plug failure mode.

Interfacial failurehappens as longas the weldnugget strength ishigher than theHAZ or base ma-terial strength.However, weldnugget strengthincreases linearlywith an increasein weld diameter,and plug failuretakes place whenthe weld nuggetstrength goesabove the HAZ orbase materialstrength. The behaviorof TSS as a func-tion of weld diam-eter was also evaluated for othergrades having different combinationsof microstructure and chemistry re-sulting in a range of base metal ulti-mate tensile strengths. Figure 3A–Cshow TSS for the following: 1) HSLA,2) steels having nominal 590 MPaUTS, and 3) fully martensitic grades(M1500 and M1700) with differingthicknesses. Figure 3 shows the sameTSS vs. weld diameter trend (and de-pendence on failure mode) previouslyobserved in Figs. 1 and 2 for othersteel grades. Therefore, these resultssuggest that the TSS behavior is inde-pendent of the steel grade, UTS, mi-crostructure, chemistry, thickness,and surface condition (uncoated andcoated steels). As seen in Fig. 3B, three steels with

minimum nominal UTS of 590 MPashow the same TSS at a given weld di-ameter in the interfacial failure mode.The weld diameter where inflectionoccurs increases with an increasingsheet thickness from 1.4 to 1.6 mm. Figure 3C shows the TSS for M1500and M1700 martensitic steels, andTSS is higher for M1700 steel in theinterfacial failure region. However, theweld diameter at which inflection hap-pens, as well as the slope of the regres-sion line in the plug failure mode, isapproximately similar for both M1500and M1700 steels with 1.0 mm thick-ness. The inflection point and slope ofregression analyses in the plug failuremode change mostly with sheet thick-ness, as it was apparent for M1500from 1 to 1.2 and 1.5 mm.

Fig. 3 — Tensionshear strength as a function of weld diameter forthe following: A — HSLA 420, B — steels having nominally minimum590 MPa UTS, and C — martensitic grades (M1500 and M1700).

A B

C


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TSS as a Function of Base Metal UTS

Figure 4A and B show TSS as afunction of weld diameter for 1.2-mm-thick steels showing interfacialand plug failure, respectively. In Fig.4, two groups of steels have been pre-sented. Data for group 1 steels hadthe nominal UTS below 440 MPa withno martensite phase, and data forgroup 2 steels had the nominal UTSabove 980 MPa and containedmartensite phase. As seen in Fig. 4A,TSS for interfacial failure mode in-creases as weld diameter increases,and interfacial failure mode for group1 steels stops at a lower weld diame-ter as compared to group 2 steels

where interfa-cial failure modewas still ob-served at a high-er weld diame-ter. The TSS forthe plug failuremode (Fig. 4B)showed thatthis mode hadshifted to ahigher weld di-ameter forgroup 2 steelswith a higherUTS, and TSSincreased fromgroup 1 to 2steels for a giv-en weld diame-ter. However,the TSS for

group 2 steels didn’t seem to increaselinearly as the UTS increased. To eval-uate the effect of UTS on TSS for allmaterials having different thickness,the analysis of data can be accom-plished by normalizing with respectto sheet thickness and weld diameter.

In Figs. 1–3, TSS had been ana-lyzed as a function of weld diameterby linear regression and indicated bysolid and dashed lines regarding tointerfacial and plug failure modes, re-spectively. As found, there was linearrelation between TSS and weld diam-eter d in both the failure mode re-gions. On the other hand, results inFigs. 1–3 indicate that there was noeffect of sheet thickness on TSS for

the interfacial failure mode. But, TSSincreased as sheet thickness in-creased, and it had a linear relationwith sheet thickness t as described inRef. 2. Therefore, TSS in interfacialand plug failure modes can be pre-sented analytically in the followingequations:

TSSIF = IFd (1)

TSSPF = PFd (2)

where IF and PF coefficients corre-spond to the interfacial failure andplug failure mode. These coefficientsare determined by experimental data;however, they depend on the proper-ties and characteristics of the basemetal. Plug failure mode is a desirablefailure to increase the spot weld’s ener-gy absorbed during crash, and it is ex-pected to increase with an increase inthe base metal strength. TSS data for arange of material thicknesses can beanalyzed by Equation 2 and normal-ized with respect to sheet thicknessand the weld diameter, which is identi-fied by PF. The analyzed weld diame-ter has been defined as a function ofsheet thickness, 6√t and 7√t, and bothof these weld diameters were associat-ed with plug failure in our experimen-tal data. Figure 5 shows PF as a function ofbase metal UTS for 6√t and 7√t welddiameters. Here, UTS was experimen-tally determined rather than nominalminimum values. As seen in Fig. 5, PF

increases with increasing base metal

Fig. 4 — Tensionshear strength as a function of weld diameter for 1.2mmthick ferritic steels having an UTS less than 440 MPa and steelscontaining martensite and UTS above 440 MPa for the following: A — Interfacial and B — plug failure modes.

Fig. 5 — Tensionshear strength divided into td (t for the thicknessof sheets and d as the weld diameter) as a function of base metalUTS studied steels for 6√t and 7√t weld diameter.

A B


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UTS in the range from 300 to ~ 800MPa, and then there was unknown be-havior from 800 to 1200 MPa UTS,but it increased again linearly from1200 MPa up to maximum studiedsteel, which was M1700CR. The re-gression analysis in Fig. 5 presents PF

as the following:

PF 2.7 (UTS — ) (3)

This analytical equation presentsPF in Equation 2 and depends on basematerial UTS. However, the significanteffect of UTS on TSS decreases forsteels showing HAZ softening, as indi-

cated in Fig. 5. As shown in Fig. 5, TSSincreases linearly with base metal ten-sile strength for the strength ranges of0 to 800 MPa and beyond 1200 MPa.However, in the range of 800 to 1200MPa, the TSS behavior was uncertain.Data for dual-phase (DP980) and fullymartensitic steels (M900) show thatthis uncertainty was independent ofthe microstructure. However, furtherinvestigation is necessary for therange of 800 to 1200 MPa to correlatePF with the base metal UTS.

CrossTension Strength

Figure 6 shows the CTS of twoDP980 GA low-carbon materials withthicknesses of 1.2 and 1.6 mm. Not

only does the CTS increase linearlywith increasing weld diameter, butalso with increasing thickness. A simi-lar trend was observed for other steelgrades, and CTS can therefore be mod-eled by the following equation (Ref. 8):

CTS = dt (4)

where d is the weld diameter, t is thesheet thickness, and is a coefficientcharacteristic of each steel but inde-pendent of the sheet thickness andweld diameter. The trend lines in Fig.6 are modeled based on a value for specifically determined for the studiedDP980 GA steel. Alternatively, using Equation 4, thevalue of can be determined for dif-

where

� = 0 for UTS < 800 MPa

� = 600 for UTS > 1200 MPa

Unknown for 800 < UTS < 1200 MPa

�

��

��

Fig. 6 — Crosstension strength as a function of weld diameterfor DP980 GA steels with gauge thicknesses of 1.2 and 1.6 mm.

Fig. 7 — Base metal UTS dependency of for double stackup, homogenous welding, and different steel thicknesses at 5√t, 6√t, and7√t weld diameter.

Fig. 8 — A — Improvement in CTS through the application of temper pulse and B — microhardness changes in the spot welding profile associated with the application of temper pulse.

BA


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ferent stackups. Figure 7 shows as afunction of the base metal UTS forthree different weld diameters, relatedto sheet thickness, given here as 5√t,6√t, and 7√t. This approach results ina wide range of values for given eachweld diameter/base metal UTS combi-nation. The multitude of data pointsfor each UTS value corresponds to ma-terials having different microstruc-tures, chemistries, or steel processingparameters. While Fig. 7 indicates thatCTS is relatively independent of thebase metal UTS, other factors havebeen reported that can affect CTS(Refs. 2, 3, 9, and 10). Krajcarz et al.have investigated the fracture tough-ness of sheet steels after welding andfound that CTS decreases with a de-creasing nugget fracture toughness(Ref. 11). The current authors have ex-plored in-situ PWHT, which tempersthe microstructure of the nugget andincreases CTS. The PWHT is not typi-cally a part of a welding schedule spec-ified by commercially available weldingstandards (Refs. 6, 7).

Improving CTS in AHSS by PWHT

The properties of most ultrahighstrength sheet steels (UHSS) areachieved through adding significantamounts of alloying elements, espe-cially carbon. This enriched chemistryincreases the material’s hardenabilityand promotes the formation of hardand brittle microstructures (marten-

site) during spot welding. As a result,CTS was reduced, despite the en-hanced strength of the unwelded prod-uct (Ref. 11). In the simplest spot weldingprocess, a pulse of welding current wasapplied to the stackup to be joined.The sheets were fused together by anugget of melted and resolidified ma-terial produced at the faying interface.If the weldment was kept clamped be-tween the water-cooled electrodes sub-sequent to the welding pulse, the weldcooling is accelerated. Given sufficienttime for the weld/HAZ temperature tofall below the martensite finish tem-perature, an appropriate current pulsecan then be applied to reheat the weldand HAZ, and temper the martensite. Figure 8A illustrates the CTS in-crease obtained through the applica-tion of temper pulsing to 1.2 mmM1500 EG. The data shown was forwelds made for three conditions with-in the welding current range: a) IMWS

— current to produce the minimumweld size, b) IAWS — current from themiddle of the current range, and c) IEXP

— the minimum current to produceexpulsion. Four temper current condi-tions are included: a) no temper (aswelded), b) 80% WC — 80% of thewelding current, c) 87.5% WC —87.5% of the welding current, and d)95% WC — 95% of the welding current. In Fig. 8A, any CTS improvement issignified by the vertical separation be-

tween the as-welded data points (solidblue) and the symbols correspondingto the varying temper pulse condi-tions. Note that the 87.5% WC data(solid black) falls most consistently atthe high end of the CTS range for IAWS

and IEXP , signifying that the 87.5% WCcondition provides the greatest in-crease in CTS. Figure 8B depicts the changes inthe weld hardness profile resultingfrom the application of the temperpulses referred to in Fig. 8A. Note thatthe 80% WC and 87.5% WC pulsesproduce a hardness reduction with thegreatest decrease corresponding to the87.5% WC condition. In contrast, thehardness peak evident in the 95% WCpulse hardness profile indicates hard-ness recovery in the weld center. Thisrecovery was associated with sufficientheating to reaustenitize the weld areaduring tempering, allowing the forma-tion of hard, untempered martensiteduring post-temper cooling. There-fore, the improvement of CTS as seenin Fig. 8A was contributed to temper-ing of the martensitic microstructurein the nugget and results in improvingthe nugget fracture toughness.

HAZ Softening and Spot WeldStrength

Effect of Base Metal UTS on theHardness of Welding Zones andHAZ Softening

Figure 9A shows typical cross weldmicrohardness profiles for DP590CRand M1700CR steels. As seen forDP590CR, while the hardness increas-es as one moves from the base metal,through the HAZ and into the weldnugget, there is no HAZ softening. Incontrast, the M1700CR microhardnessdrops rapidly from a high base metalvalue to a minimum in the HAZ, andthen increases quickly to a maximumvalue in the nugget. The sharp hard-ness drop in the HAZ, quantified bysubtracting the minimum HAZ hard-ness from the average base metalhardness, was due to martensite tem-pering (Ref. 12). Because theM1700CR microstructure is fullymartensitic, the tempering effect inresponse to the welding-induced ther-mal cycle is maximized. In contrast,the volume fraction of martensite inthe DP590CR is very low, providing lit-

Fig. 9 — A — Microhardness profiles of DP590 CR and M1700 CR steels; B — crosssection view for maximum temperature distribution of M1700 CR by using SORPAS simulation; C — corresponding crosssection metallography; and D — thermal history for a finite element node close to the Ac1 line marked in B.

A

B

C

D


tle martensite to be softened and min-imizing the HAZ softening effect.Also, the difference of HAZ softeningbetween these two steels can be corre-lated to the kinetics of martensitetempering, which was higher inM1700CR because of higher carboncontent (Refs. 5, 11, and 12). To better understand the correla-tion between the hardness drop in theM1700 CR and the time/temperaturedistribution associated with the weld-ing process, SORPAS software wasused to simulate the correspondingwelding condition. Figure 9B showsthe maximum temperature distribu-tion from a cross-section view. Bound-ary lines signifying the Ac1 as well asthe melting temperature were drawn,and the center of the Fig. was the weldnugget. Figure 9C shows an etchedcross-section metallography of thesame relative weld location as that inFig. 9B. A comparison of Fig. 9B and Cshows a good agreement between theprediction of the nugget size and HAZzone by SORPAS and the actual weld.As indicated by the arrows, the HAZhardness drops in Fig. 9A correspondto material exposed to a maximumtemperature near the Ac1. The thermalhistory of an existing finite element

node close to the line marked as Ac1 inFig. 9B (but a little bit far from the lineand closer to the base material) wasplotted in Fig. 9D. As shown, the maxi-mum temperature reached ~ 680°C,which is close to Ac1 for M1700 CR (~ 730°C). Base metal hardness, minimumHAZ hardness, and weld nugget hard-ness were plotted as a function of thebase metal UTS in Fig. 10 for the stud-ied steels. The hardness of these threezones can be formulated by linear re-gression analysis as a function of basemetal UTS:

Hv = A(BMUTS) + B (5)

where BMUTS is the ultimate tensilestrength of the base metal, and A andB are coefficients (Table 1). The comparison of A coefficientsregarding the slope of regressionanalysis shows that the slope of thenugget hardness curve was lower thanthat for the base metal hardness (0.28vs. 0.15). Nugget hardness has beentied to the Yurioka carbon equivalent(CEY) (Ref. 4). Figure 10 shows thatthe trend lines for the nugget hard-ness and the minimum value at HAZwere approximately parallel, exhibiting

a constant hardness difference inde-pendent of the base metal tensilestrength. In contrast, the nugget hard-ness and base metal hardness trendlines converge as the UTS increases.Higher strength steels (UTS above ~ 1300 MP) in this study have fullymartensitic microstructures, so solidi-fied nugget and base metal will consistof martensite, resulting in almost sim-ilar hardness. Minimum hardness inthe HAZ (HvMHAZH) increases with basemetal UTS, but at a slower rate due toHAZ softening during welding (Ref.13). HAZ softening is attributed tomartensite tempering that is associat-ed with thermal history experiencedduring welding (Refs. 5, 12, and 13). The difference in hardness betweenthe base metal and minimum hardnessat the HAZ, which was expressed asmaximum HAZ softening (HAZMAX),has been plotted as a function of basemetal UTS in Fig. 11. As shown, HAZMAX increases with an increasingbase metal UTS, regardless of the sur-face condition of the product (coatedand uncoated). HAZMAX can be ex-pressed in terms of base metal UTS(BMUTS) with the following equation:

Hv = C(BMUTS) – D (6)

where BMUTS was the ultimate tensilestrength of the base metal, and C and Dwere coefficients that have been report-ed for bare/EG and GA/GI/AlSi surfaceconditions, respectively (Table 2). The increase in HAZ softening athigher strength steels can be attrib-uted to a richer chemistry (mainly car-bon content) of the steels. The effect

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Fig. 10 — Hardness of the base metal, nugget, and minimumHAZ as a function of base metal UTS.

Fig. 11 — Maximum HAZ softening as a function of base metal UTSfor bare/EG and other coatings (GA/GI/AlSi).

Table 1 — A and B Coefficients and R2 of Equation 5 for Different Zones

Hardness of Different Zones A B R2

Hv Base Metal (base metal) 0.28 31 0.98

Hv Nugget (nugget) 0.15 265 0.98

Hv MHAZH (minimum hardness in HAZ) 0.12 131 0.89


of alloying elements on the kinetics ofHAZ softening was explained in Refs.12 and 13. Equations 5 and 6 forbare/EG surface conditions can becombined to produce:

HAZMax = 0.6 HvBaseMetal – 155 (7)

Equation 7 shows that maximumHAZ softening in the Vickers hardnessincreases by 60% of the base metalhardness. This relationship holds formaterials with ferritic, dual-phase,and/or fully martensitic microstruc-tures.

Effect of HAZ Softening on TSSand CTS

Concerns have been voiced in theautomotive industry regarding the po-tential effect of HAZ softening on spotweld strength as measured via TSS andCTS. To address this issue, the influ-ence of maximum HAZ softening(HAZMax) was studied. Normalized TSSand CTS have been plotted as a func-tion of maximum HAZ softening(HAZMax) in Fig. 12A and B, respective-ly. In normalized form, TSS and CTSwere expressed in terms of PF (forTSS) and (for CTS) coefficients asshown in Equations 2 and 4. Data inboth plots were presented for weld di-ameters of 6√t and 7√t with all studied

steels showing plug failure. The trendline in Fig. 12A shows that TSS in-creases regardless of the appurtenanceof HAZ softening, although there is atrendline inflection point where HAZsoftening appears. In contrast, Fig. 12B shows thatCTS becomes relatively independent ofHAZ softening as HAZMAX increases.As seen, maximum HAZ softening forM1700 CR is much higher than DP590CR, but CTS was similar for bothsteels. This observation suggests thatCTS depends on other factors, e.g.crack propagation resistance from thenotch formed by the intersection ofthe fusion line and the faying inter-faces, which may be attributed to thefracture toughness of the weld nugget(Refs. 3, 11, and 14).

Conclusions

Tension-shear strength (TSS),cross-tension strength (CTS), andheat-affected zone (HAZ) hardnesseswere investigated for a wide range ofsheet steels having ultimate tensilestrengths in the range of ~ 300 to1700 MPa. The results of this studycan be summarized as follows: 1) TSS increases with weld diame-ter, but there was an inflection pointin the trendline where the failure

mode changed from interfacial frac-ture to plug failure. The weld diameterat which inflection occurs in thetrendline increases with an increase insheet thickness and changes with basemetal characteristics. But, overall, theeffect of sheet thickness seems morepronounced than that associated withthe chemistry/microstructure in theplug failure mode. However, despitethe presence of the inflection point,TSS continues to increase until the ex-pulsion current is reached. 2) TSS increases linearly with basemetal tensile strength up to about 800MPa as well as beyond 1200 MPa, whilethe trend line is not well defined for TSSin the UTS range of 800–1200 MPa.However, CTS appears to be relativelyindependent of the base metal UTS. 3) The as-welded CTS of steels ex-hibiting martensitic weld nuggets canbe improved through the application ofan appropriate PWHT, which tempersthe martensite, increasing the nuggettoughness and improving the CTS. 4) The nugget hardness, minimumHAZ hardness, and base metal hardnesshave been correlated with base metalUTS. The results show that the mini-mum hardness in the HAZ, which referto the location of HAZ exposed to theAc1 temperature, doesn’t increase as thebase metal or nugget hardness increase.This deviation results in a difference ofbase metal strength and HAZ strength,called HAZ softening. HAZ softening in-creases with the increasing base metalUTS for steels having dual-phase andmartensitic steels. Maximum HAZ soft-ening (HAZMax) increases with almost60% increase in base metal hardness. 5) The shape of the TSS trendline is

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Fig. 12 — Normalized TSS (PF) and CTS () as a function of maximum HAZ softening (HAZMax) for weld diameters 6√t and 7√t.

Table 2 — C and D Coefficients and R2 of Equation 6 for Different Surface Conditions

Material C D R2

Bare/EG 0.17 136 0.95 GA/GI/AlSi 0.165 107 0.92

A B


affected by the presence of HAZ soft-ening. However, TSS continues to in-crease despite escalating HAZ soften-ing, whereas HAZ softening does notseem to affect CTS.

The writers acknowledge Arcelor-Mittal Global R&D management forencouragement and support to com-plete this paper.

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