Materials and Design 85 (2015) 461–470

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Materials and Design

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Optimised design of electrodemorphology for novel dissimilar resistancespot welding of aluminium alloy and galvanised high strength steel

Weihua Zhang a, Daqian Sun b,⁎, Lijun Han c, Yongqiang Li d

a Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, Chinab Key Laboratory of Automobile Materials, Ministry of Education, School of Materials Science and Engineering, Jilin University, Changchun 130022, Chinac Department of Product Management, FAW-Volkswagen Automobile Co. Ltd., Changchun 130011, Chinad Department of Technology, FAW Car Co. Ltd., Changchun 130012, China

⁎ Corresponding author.E-mail addresses: psdq@sina.com, sundq@jlu.edu.cn (

http://dx.doi.org/10.1016/j.matdes.2015.07.0250264-1275/© 2015 Elsevier Ltd. All rights reserved.

a b s t r a c t

a r t i c l e i n f o

Article history:Received 16 December 2014Received in revised form 2 July 2015Accepted 6 July 2015Available online 11 July 2015

Keywords:Dissimilar material weldingMicrostructureMechanical propertyOptimised designElectrode morphology

A novel resistance spot welding method of dissimilar materials of 6008-T66 aluminium alloy and H220YDgalvanised high strength steelwas presented, and themorphology ofwelding electrodeswas designed optimally.Macrostructure, microstructure and mechanical property of the welded joints obtained with optimised elec-trodes were studied. Numerical simulation of current density distribution and temperature field during weldingwas also performed. The optimised electrodes were a planar circular tip electrodewith tip diameter of 10mmonthe steel side and a spherical tip electrode with spherical diameter of 70 mm on the aluminium alloy side. Thewelded joint obtained with optimised electrodes could be regarded as a special welded-brazed joint, and an in-termetallic compound layer composed of Fe2Al5 and Fe4Al13 with maximum thickness of about 4.0 μm wasformed at the aluminium/steel interface in the welded joint. Tensile shear load up to 5.4 kN was achieved forthewelded joint obtainedwith optimised electrodes. Current density distribution duringweldingwith optimisedelectrodes was more homogeneous than that with F type electrodes. Furthermore, interfacial temperature in thewelded joint duringweldingwith optimised electrodes (about 915 °C)was lower than thatwith F type electrodes(about 985 °C).

© 2015 Elsevier Ltd. All rights reserved.

1. Introduction

Imposed by the urgent situation of energy scarcity and environmentalpollution, automobile lightening which could reduce fuel consumptionand exhaust emissions has attracted increased interests. The use of light-weight materials such as aluminium alloy in automobile manufacturingprovides an effective means to achieve automobile lightening [1,2]. At-tributing to its inherent problems in terms of cost and security perfor-mance, the design and manufacturing of an all-aluminium vehicle bodystructure which could obtain a weight reduction of nearly 50% comparedwith traditional steel vehicle body structurehave not yet been extensivelyadopted in automotive industry [3]. Nevertheless, the employment of hy-brid vehicle body structure composed of both aluminium alloy and highstrength steel parts turns out to be a compromise between cost and secu-rity performance [4]. The hybrid vehicle body structure signifies weldingand joining of dissimilar materials of aluminium alloy and high strengthsteel. It is difficult to obtain a reliable welded joint of aluminium alloyand steel by traditional fusion welding due to their discrepancies inthermophysical properties, which readily favouring the formation ofwelding defects such as cracks, porosities and cavities [5–7]. Furthermore,the formation of brittle Al-rich intermetallic compounds involving FeAl3

D. Sun).

[5,6], Fe2Al5 [5–7] and FeAl2 [5,7] facilitated by the nearly zero solid solu-bility of iron in aluminium would degrade mechanical properties of thewelded joint. Mechanical joining techniques such as clinching, screwing,self-piercing riveting (SPR) and adhesive bonding are used to join dissim-ilarmaterials of aluminium alloy and steel in automotive industry to date.SPR could be regarded as an effective means to join aluminium alloy andsteel parts [8,9], whereas the inherent problems in regard to joint appear-ance, potential weight increase, access constraint and ease of rupturehave restricted its application [10].

Several solid state welding methods such as explosive welding [11,12], ultrasonic welding [13], magnetic pulse welding [14], frictionwelding [15] and friction stir welding [16–18] are adopted to weld alu-minium alloy and steel. The brittle intermetallic compounds are desiredto be diminished by using these solid state welding methods. Interme-tallic compounds composed of AlFe, Al2Fe, Al3Fe and Al6Fe were formedin the explosive welded joint of aluminium alloy and steel, meanwhilethickness of the intermetallic compound layer could be reduced withthe employment of stainless steel or aluminium interlayer due to thedecrease in collision energy [12]. Prangnell et al. suggested that the in-termetallic compounds (Fe2Al5 and FeAl3) formed in the ultrasonicwelded joint of A6111 aluminium alloy and DC04 steel would reducemechanical properties of the welded joint if their thickness exceeded1 μm [13]. Lee et al. found that an intermediate layer composed of fineAl grains, fine fragments of SPCC and dispersive intermetallic particles

Table 1Chemical compositions of 6008-T66 aluminium alloy and H220YD high strength steel.

Elements, wt.% C Si Cu Fe Mg Mn Zn Ti V S P Nb Al

6008-T66 … 0.56 0.15 0.19 0.45 0.07 0.007 0.02 0.08 … … … BalanceH220YD 0.007 0.09 … Balance … 0.51 … 0.01 … 0.007 0.05 0.02 …

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(Fe2Al5 and FeAl3) was formed in the magnetic pressure seam weldedjoint of SPCC steel and A6111 aluminium alloy [14]. Intermetallic com-pounds composed of Fe2Al5 and FeAl with overall thickness of 350 nmwere formed in the friction welded joint of 6061-T6 aluminium alloyand AISI 1018 steel [15]. Watanabe et al. revealed that interfacial inter-metallic compounds involving FeAl and FeAl3 were formed in the upperfriction stir welded joint of 5083 aluminium alloy and mild steel, andthat tensile strength of the welded joint reached 86% that of the basemetal of aluminium alloy. Besides, cracks inclined to propagate throughthe intermetallic compounds [16,17]. The adaptability of these solidstateweldingmethods is restrained to a certain extent in automotive in-dustry due to the limitation on configuration and capacity of weldingequipments. Besides, arc welding–brazing [19,20] and laser brazing[21–23] are also employed to weld aluminium alloy and steel. Theresults indicated that composition, morphology and thickness of the in-termetallic compounds could be regulatedwith the introduction of fillermetals, whereas cracks also tended to initiate at the intermetalliccompounds and then propagate through them.

Few studies corresponding resistance spot welding of aluminiumalloy and steel have been reported to date, although it is a majorweldingmethod in automobile manufacturing [24,25]. Some additionalmaterials such as aluminium interlayer [26], cover plate [27] and coldrolled aluminium clad steel strip [28,29] were used during resistancespot welding of aluminium alloy and steel. Hwang et al. illustratedthat the introduction of aluminium interlayer could reduce thicknessof the intermetallic compound layer in the welded joint of A5086 alu-minium alloy and SS400 steel [26]. Qiu et al. suggested that an increasein nugget diameter of thewelded jointwas achieved due to the employ-ment of cover plate located between electrode and aluminium alloy[27]. Sun et al. revealed that an intermetallic compound layer withthickness of 8.5 μm was formed in the welded joint of AA5182 alumin-ium alloy and SAE1008 steel with the employment of aluminium cladsteel strip, and that a hybrid fracturemodewas obtained for theweldedjoint [28]. A tensile shear load up to 3.5 kNwas obtained for the weldedjoint of Al–Mg alloy and steel with the employment of aluminium cladsteel interlayer [29]. Nevertheless, the introduction of additional mate-rials brings in weight increase, which deviates from the requirementby automobile lightening.

Fig. 1. Schematic diagram of resistance spot welding arrangement (not to scale,dimensions in mm).

In the study, a novel resistance spotweldingmethod of dissimilarma-terials of aluminium alloy and galvanised high strength steel is presented.The morphology of welding electrodes is designed optimally, and the ef-fect of electrode morphology on weldability of the dissimilar materials isstudied. Thewelding current density distribution and temperature field ofthe welded joints during welding with optimised electrodes and tradi-tional F type electrodes are both revealed.Macrostructure,microstructureand tensile shear load of the welded joints obtained with optimised elec-trodes are studied as well.

2. Experimental procedure

The dissimilar materials used were 6008-T66 aluminium alloysheets with thickness of 1.5 mm and H220YD galvanised high strengthsteel sheetswith thickness of 1.0mm. Chemical compositions of the dis-similar materials are shown in Table 1. The steel sheets were galvanisedbymeans of hot dipping, and the thickness of the zinc layer coatingwas7 μm on both surfaces. Both aluminium alloy and steel specimens weremachined into a size of 100mm× 25mm. Prior to welding, the alumin-ium alloy specimens were grinded by using abrasive paper (grade 800)and degreased in acetone in sequence. Meanwhile the steel specimenswere just degreased in acetone. Lap joint configuration was employedduring resistance spot welding. The cap-type electrodes made ofCuCrZr alloy were used, and the morphology of the electrodes wasdesigned optimally to solve the inherent problems [24,30] in resis-tance spot welding of aluminium alloy and galvanised high strengthsteel and improve weldability of the dissimilar materials. A sphericaltip electrode on aluminium alloy side and a planar circular tip elec-trode on galvanised high strength steel side were employed duringwelding. Specifically, spherical diameter (D1) of the spherical tipelectrode on aluminium alloy side used was 30 mm, 50 mm,70 mm and 100 mm, respectively. In the meantime, tip diameter(D2) of the planar circular tip electrode on steel side used was8 mm, 10 mm, 12 mm and 14 mm, respectively. Fig. 1 shows theschematic diagram of the resistance spot welding arrangement.Welding experiments were carried out by using a median frequencydirect current (MFDC) resistance spot welding machine quippedwith Rexroth PSI 6300 controller and Nimak LHN4 welding gun.MFDC resistance spot welding was superior to industrial frequencyalternative current resistance spot welding in dissimilarmaterial weldingof aluminium alloy and galvanised high strength steel as the formerweldingmethod could supplymore stable current and higher power effi-ciency than the latter one. The welding parameters used were shown asfollows: welding current (19 kA–25 kA), welding time (250 ms–350 ms), electrode force (3 kN–3.5 kN), squeezing time (400 ms) and

Table 2Optimised resistance spot welding parameters used under different tip diameters of pla-nar circular tip electrode on steel side with spherical diameter of spherical tip electrodeon aluminium alloy side being fixed as 70 mm.

Tipdiameter(mm)

Weldingcurrent(kA)

Weldingtime(ms)

Electrodeforce(kN)

Squeezingtime(ms)

Keepingtime(ms)

8 21 250 3 400 40010 22 300 3.5 400 40012 24 350 3.5 400 40014 25 350 3.5 400 400

Fig. 2. Appearances of welded joints of 6008-T66 aluminium alloy and H220YD galvanised high strength steel obtained with different electrode tip diameters on steel side. (a) Steel sidewith 8 mm; (b) aluminium side with 8 mm; (c) steel side with 10 mm; (d) aluminium side with 10 mm; (e) steel side with 12 mm; (f) aluminium side with 12 mm; (g) steel side with14 mm; (h) aluminium side with 14 mm.

Fig. 3. Cross-sectional macrostructure of welded joints obtained with different electrode tip diameters on steel side. (a) 8 mm; (b) 10 mm; (c) 12 mm; (d) 14 mm.

Fig. 4. Effects of electrode tip diameter on steel side on nugget diameter and indentationratio of welded joints.

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keeping time (400 ms). A commercial finite element (FE) analysis soft-ware was adopted to study current density distribution and temperaturefield during resistance spot welding with optimised electrodes and tradi-tional F type electrodes (DIN 5821-F16 × 20; the experimental resultsconcerning F type electrodes were suggested in [24], and the optimisedwelding parameters are shown as follows: welding current (9 kA),welding time (250 ms), electrode force (2.5 kN), squeezing time(200 ms) and keeping time (200 ms)). After welding, metallographicsampleswere obtained by cross-sectioning thewelded joint through cen-tre of the weld nugget perpendicular to the weld surface plane, followingby grinding and polishing in sequence. The metallographic samples wereetched in 4 vol.%HNO3ethanol solution for 5–10 s andKeller's reagent for3–8 s successively to attackmacrostructure and interfacialmicrostructureof the welded joints.

Macrostructure of the welded joints was investigated by means ofstereomicroscope (ZEISS SteREO Discovery). Microstructure and chem-ical compositions of the welded joints were studied by using scanningelectronmicroscopy (ZEISS EVO18/Hitachi S-3400N) equippedwith en-ergy dispersive spectroscopy (EDAX/Link), respectively. Phase structureof the interfacial zone in the welded joints was analysed by utilisingmicro X-ray diffractometer (D8 Discover with GADDS). According tothe standard of ASTM: E8/E8M-13a, mechanical properties of thewelded joints were examined at a crosshead speed of 1.0 mm min−1

by using MTS 810 material test system. Furthermore, tensile shearload of the welded joints was evaluated by the average value of threesamples per welding condition.

3. Results and discussion

3.1. Optimised design of electrode morphology on steel side

Table 2 shows resistance spotwelding parameters used under differ-ent tip diameters of the planar circular tip electrode on steel side withspherical diameter of the spherical tip electrode being fixed as 70 mm

Fig. 5. Effect of electrode tip diameter on steel side on tensile shear load of welded joints.

Table 3Optimised resistance spotwelding parameters used under different spherical diameters ofspherical tip electrode on aluminium alloy side with tip diameter of planar circular tipelectrode on steel side being fixed as 10 mm.

Sphericaldiameter(mm)

Weldingcurrent(kA)

Weldingtime(ms)

Electrodeforce(kN)

Squeezingtime(ms)

Keepingtime(ms)

30 19 300 3 400 40050 21 300 3.5 400 40070 22 300 3.5 400 400100 24 350 3.5 400 400

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on aluminium alloy side. The welding parameters had been optimisedthrough previous trials according to the optimisation criteria, i.e., lessexpulsion, lower indentation depth, larger weld nugget diameter andhigher tensile shear load for the welded joints being achieved simulta-neously. Fig. 2 shows appearances of the welded joints of 6008-T66 al-uminium alloy and H220YD galvanised high strength steel obtainedwith different electrode tip diameters on steel side. As shown inFig. 2(a), a deep indentation and severe alloying between electrodeand galvanised steel on the surface of the welded joint were observedwhen 8 mm-diameter electrode on steel side was employed, in themeantime, severemelting on the aluminium alloy surface of theweldedjoint occurred duringwelding (Fig. 2(b)), whichmade it hard to detachthe electrode from the welded joint and degraded the electrode life.With increasing tip diameter of the electrode on steel side, the appear-ance quality of the welded joint was improved, i.e., indentation depth

Fig. 6. Appearances of welded joints of 6008-T66 aluminium alloy and H220YD galvanised higside. (a) Steel sidewith 30mm; (b) aluminium sidewith 30mm; (c) steel sidewith 50mm; (d)

became shallower, alloying phenomenon was restrained and theattachment between electrode and welded joint was inhibited as well,as illustrated in Fig. 2(c)–(h). The improvement concerning the appear-ance quality of the welded joint was due to the decreased currentdensity on the contact surface between welded joint and electrodewith the increase in electrode tip diameter, which reduced heat gener-ated on the contact surface. Fig. 3 shows cross-sectional macrostructureof the welded joints obtained with different electrode tip diameters onsteel side. As is shown, the welded joint could be regarded as a specialwelded-brazed joint to some extent due to the existence of a clearstraight interface between aluminium nugget and steel, the welded-brazed joint being formed throughwetting and spreading of themoltenaluminium nugget on solid steel during welding. A deep indentation ofthe welded joint was observed. Meanwhile inner expulsion occurredduring welding, which favoured the formation of interfacial shrinkagecavity in the welded joint as indicated in Fig. 3(a). The shrinkage cavityexhibited an irregular shape, and it was formed near aluminium/steelinterface as a result of the lack of liquid metal during the solidificationprocess due to the severe inner expulsion. As electrode tip diameterincreased, shrinkage cavity-free welded joint was obtained, meanwhileindentation depth of the welded joint decreased correspondingly(Fig. 3(b)–(d)). Fig. 4 shows effects of electrode tip diameter on steelside on nugget diameter and indentation ratio (i.e., the value of indenta-tion depth divided by thickness of the welded joint) of the weldedjoints. As is illustrated, with increasing electrode tip diameter, the nug-get diameter increased first, and then it decreased gradually, the maxi-mumvalue reaching 10.1mmwith electrode tip diameter being 10mm.Moreover, the indentation ratio decreased greatly first then kept quasi-stable as electrode tip diameter increased. Fig. 5 shows effect of elec-trode tip diameter on steel side on tensile shear load of the weldedjoints. As is shown, the tensile shear load of the welded joint obtainedwith electrode tip diameter of 8 mm reached 4.8 kN. With increasingelectrode tip diameter, the tensile shear load of the welded joint in-creased dramatically first and then decreased gradually, the highestvalue up to 5.4 kN being achieved with electrode tip diameter of10 mm. In addition, variation tendency of the tensile shear load of thewelded joints fittedwell with that of the nugget diameter of theweldedjoints.

3.2. Optimised design of electrode morphology on aluminium alloy side

Table 3 shows resistance spot welding parameters used under dif-ferent spherical diameters of the spherical tip electrode on alumini-um alloy side with tip diameter of the planar circular tip electrodebeing fixed as 10 mm on steel side. The welding parameters had

h strength steel obtained with different electrode spherical diameters on aluminium alloyaluminium sidewith 50mm; (e) steel sidewith 100mm; (f) aluminiumsidewith 100mm.

Fig. 7. Cross-sectional macrostructure of welded joints obtained with different electrode spherical diameters on aluminium alloy side. (a) 30 mm; (b) 50 mm; (c) 100 mm.

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also been optimised according to the optimisation criteria suggestedin Section 3.1. Fig. 6 shows appearances of the welded joints of 6008-T66 aluminium alloy and H220YD galvanised high strength steel ob-tained with different electrode spherical diameters on aluminiumalloy side. As shown in Fig. 6(a)–(d), obvious alloying between elec-trode and galvanised steel as well as serious aluminium alloymeltingon the surface of the welded joints occurred with relatively lowerelectrode spherical diameters (30 mm and 50 mm). With increasingspherical diameter of the electrode on aluminium alloy side, the ap-pearance quality of the welded joint was improved greatly (Figs.2(c)and (d), 6(e) and (f)). Fig. 7 shows cross-sectional macrostructure ofthe welded joints obtained with different electrode spherical diame-ters on aluminium alloy side. The results indicated that deep inden-tation and shrinkage cavity induced by expulsion were both formedwith electrode spherical diameter of 30mm and 50mm (Fig.7(a) and(b)), and that shrinkage cavity-freewelded joint with lower indenta-tion depth was obtained as electrode spherical diameter increased(Figs. 3(b) and 7(c)). Fig. 8 shows effects of electrode spherical diam-eter on aluminium alloy side on nugget diameter and indentationratio of the welded joints. As is shown, the nugget diameter experi-enced an increased tendency with increasing electrode sphericaldiameter from 30 mm to 70 mm, whereas the nugget diameterdecreased with further increasing electrode spherical diameter. Be-sides, the indentation ratio decreased gradually as electrode spheri-cal diameter increased. Fig. 9 shows effect of electrode sphericaldiameter on aluminium alloy side on tensile shear load of the weldedjoints. The tensile shear load of the welded joint increased greatlyfirst with electrode spherical diameter increasing from 30 mm to

Fig. 8. Effects of electrode spherical diameter on aluminium alloy side on nugget diameterand indentation ratio of welded joints.

70 mm, then it decreased with further increasing electrode sphericaldiameter. Moreover, variation tendency of the tensile shear load wasin good accordance with that of the nugget diameter.

It should be noted that the optimised electrodes were a planar circu-lar tip electrode with tip diameter being 10 mm on steel side and aspherical tip electrodewith spherical diameter being 70mmon alumin-ium alloy side. Furthermore, the weldability with optimised electrodesin terms of welded joint appearance, nugget diameter and interfacialdefects involving shrinkage cavity was promoted compared to thatwith traditional F type electrodes [24,30] due to the improved weldingcurrent density distribution and interfacial temperature field of thewelded joint duringweldingwith optimised electrodes (to be discussedin Section 3.4).

3.3. Interfacial microstructure of welded joint obtained with optimisedelectrodes

Fig. 10 shows microstructure of aluminium/steel interface in thewelded joint of 6008-T66 aluminium alloy and H220YD galvanisedhigh strength steel obtained with optimised electrodes, analysis posi-tions being plotted from A to C in Fig. 3(b). An intermetallic compoundlayer was formed at the aluminium/steel interface, its characteristics interms of morphology and thickness varying greatly with regions of theinterface in the welded joint. As shown in Fig. 10(a), an intermetalliccompound layer with dual-layered structure was formed at the inter-face in the central region of the welded joint, which was composed ofa tongue-shaped structure layerwith thickness of 3.6 μmapproximatelybeside steel and a successive acicular structure layer with thickness ofabout 0.4 μm beside aluminium nugget. With increasing distance fromthe central region of the welded joint, thickness of the intermetallic

Fig. 9. Effect of electrode spherical diameter on aluminium alloy side on tensile shear loadof welded joints.

Fig. 10. Microstructure of aluminium/steel interface in welded joint obtained withoptimised electrodes, analysis positions being plotted from A to C in Fig. 3(b). (a) RegionA; (b) region B; (c) region C.

Fig. 11. Distribution of alloying elements across interface with line scanning path markedusing a white solid line in Fig. 10(a).

Table 4EDS compositions of interfacial intermetallic compound layer in welded joint with analy-sis positions taken from A and B in Fig. 10(a) and C in Fig. 10(c).

Elements Al Fe Zn Mg Si

wt.% at.% wt.% at.% wt.% at.% wt.% at.% wt.% at.%

A 53.23 69.88 45.99 29.17 0.06 0.03 0.05 0.08 0.67 0.84B 62.88 77.48 36.12 21.50 0.31 0.16 0.18 0.25 0.51 0.61C 63.53 78.01 35.81 21.24 0.05 0.03 0.01 0.01 0.6 0.71

Fig. 12.Micro X-ray diffraction profile of interfacial region in welded joint taken from A inFig. 3(b).

466 W. Zhang et al. / Materials and Design 85 (2015) 461–470

compound layer decreased (Fig. 10b and c). A tongue-free intermetalliccompound layer with thickness of approximately 1.0 μmwas formed atthe interface in the peripheral region of the welded joint as shown inFig. 10(c). Fig. 11 shows distribution of alloying elements across the in-terfacewith line scanning pathmarked using awhite solid line as exhib-ited in Fig. 10(a). The result indicated that growth of the intermetalliccompound layer was regulated by reactive mutual diffusion betweenliquid aluminium nugget and solid steel during welding. Consequently,the characteristics of the intermetallic compound layer variedwith loca-tions at the interface in the welded joint due to the discrepancy oftemperature history in different locations. Table 4 shows EDS composi-tions of the interfacial intermetallic compound layer in thewelded jointwith analysis positions taken from A and B in Fig. 10(a) and C inFig. 10(c). The results indicated that the tongue-shaped structure and

acicular structure might be Fe2Al5 and Fe4Al13, respectively, and thatan individual intermetallic compound of Fe4Al13 was formed probablyat the interface in the peripheral region of the welded joint. Fig. 12shows micro X-ray diffraction profile of the interfacial region in thewelded joint taken from A in Fig. 3(b). The result identified the existenceof intermetallic compounds of Fe2Al5 and Fe4Al13 at the interface in thewelded joint. Furthermore, the results demonstrated that the thicknessof the interfacial intermetallic compound layer in the welded joint ob-tained with optimised electrodes was far lower than that obtainedwith F type electrodes which exceeded 10 μm [24].

3.4. Numerical simulation of welding process with optimised electrodes andtraditional electrodes

Fig. 13 shows numerical simulated distribution of current densityin the welded joint and electrodes during resistance spot welding of

Fig. 13. Numerical simulated distribution of current density during welding. (a) With Ftype electrodes; (b) with optimised electrodes.

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6008-T66 aluminium alloy and H220YD galvanised high strengthsteel with optimised electrodes and traditional F type electrodes atthe welding time of 50ms. As illustrated in Fig. 13(a), current densityin the welded joint, electrodes and the contact surfaces betweenelectrodes and specimens was distributed inhomogeneously duringwelding with F type electrodes, meanwhile the maximum currentdensity up to 2.2 × 103 A mm−2 approximately was obtained in theperiphery of the contact surface between electrode and steel,which facilitated the formations of deep indentation, severe meltingof aluminium alloy surface and alloying between galvanised steel

Fig. 14. Thermal cycle curves at aluminium/steel interface in central region of welded jointduring welding with optimised electrodes and F type electrodes.

and electrode. Nevertheless, current density distribution was morehomogeneous during welding with optimised electrodes, and themaximum current density (about 1.0 × 103 A mm−2) was less thanhalf that with F type electrodes, which decreased heat generationand improved the weldability greatly (Fig. 13b). Moreover, thewider current density distribution facilitated the formation of a larg-er nugget diameter for the welded joint obtained with optimisedelectrodes compared with that obtained with F type electrodes.Fig. 14 shows thermal cycle curves at the aluminium/steel interfacein the central region of the welded joint during welding withoptimised electrodes and F type electrodes. The results exhibitedthat the interfacial temperature experienced a rapid increase imme-diately welding current was fed, then it decreased slowly after a peakvalue (about 985 °C for F type electrodes and 915 °C for optimisedelectrodes) due to the heat extraction by means of water coolingthrough electrodes. Immediately the welding current was turnedoff, the interfacial temperature dropped sharply. It should be notedthat a lower interfacial temperature favoured the formation of athinner intermetallic compound layer for the welded joint obtainedwith optimised electrodes compared with that obtained with Ftype electrodes. Fig. 15 shows numerical simulated temperaturefield of the welded joint with optimised electrodes and F type elec-trodes and corresponding experimental results. The experimentalresults of resistance spot welding with F type electrodes weregiven in [24]. As shown in Fig. 15(a), the distribution of temperaturewas inhomogeneous in the welded joint, the maximum temperatureoccurring at the inner of the high strength steel. The aluminium alloynear aluminium/steel interface melted as the temperature exceededits melting point, and an aluminium nugget contour was formed.Meanwhile, the serious distortion of the welded joint was also indi-cated in the simulated result under the applied electrode pressure.As shown in Fig. 15(b), a more homogeneous temperature fieldwas achieved with optimised electrodes than that with F type elec-trodes. The interfacial temperature with optimised electrodes waslower than that with F type electrodes, and also the dimension ofthe aluminium nugget contour with optimised electrodes was largerthan that with F type electrodes. Besides, the degree of distortion ofthe welded joint with optimised electrodes was far lower than thatwith F type electrodes. In addition, the weld nuggets numerical sim-ulated were both in good accordance with those obtained in the ex-periments with the same welding parameters used.

3.5. Mechanical behaviour of welded joint with optimised electrodes

Fig. 16 shows cross-sectional macrostructure of the partiallyfailed welded joint of 6008-T66 aluminium alloy and H220YDgalvanised high strength steel obtained with optimised electrodesduring tensile shear testing. As can be seen, cracking initiated atbase metal of aluminium alloy beside the aluminium nugget attrib-uting to the located stress concentration, and then it propagatedthrough the aluminium alloy base metal. With tensile shear forcefurther acting, the aluminium alloy substrate was separated fromthe aluminium nugget which was welded to the steel counterpartfirmly, thereby the welded joint fracturing in nugget pullout failuremode (Fig. 17). As illustrated in Fig. 17, region A was the fractureorigin site, and the cracks ended in regions B and C. Fig. 18 showsSEM morphology of the fractured surface of the welded joint withanalysis positions marked as B and C in Fig. 17. As shown inFig. 18(a), enormous elongated dimples induced by shear stresswere observed on region B, presenting ductile fracture features. Re-gion C included a certain amount of cleavage planes (Fig. 18(b)),which indicated that the intermetallic compounds also acted ascrack propagated sites partially due to the relatively higher stressconcentration at the interface in the periphery region of the weldedjoint.

Fig. 15. Numerical simulated temperature field of welded joints and experimental results. (a) With F type electrodes; (b) with optimised electrodes.

468 W. Zhang et al. / Materials and Design 85 (2015) 461–470

The results demonstrated that promotedmechanical property of thewelded joint of 6008-T66 aluminium alloy and H220YD galvanised highstrength steel could be achieved with the employment of optimisedelectrodes compared with that obtained with F type electrodes, whichexhibited interfacial fracture mode, i.e., the crack originated from thebrittle interfacial intermetallic compound layer (Fe2Al5), and then it in-clined to propagate through the intermetallic compound layer, inducingfracture of the welded joint eventually [24].

4. Conclusions

In the present study, a novel resistance spot welding method ofdissimilar materials of 6008-T66 aluminium alloy and H220YDgalvanised high strength steel was presented. The morphology ofelectrodes was designed optimally. Current density distributionand temperature field during welding were revealed. Macrostruc-ture, microstructure and tensile shear load of the welded jointsobtained with optimised electrodes were studied. The weldingmethod was proposed based on the precise control of welding ener-gy distribution by means of the employment of optimally designedelectrodes. The method could solve the inherent difficulty in resis-tance spot welding of dissimilar materials of aluminium alloy andsteel as the thickness of intermetallic compound layer could be reg-ulated accurately, and cavities as well as other defects could be

Fig. 16. Cross-sectional macrostructure of partially failed welded joint obtained withoptimised electrodes.

inhibited. Furthermore, the optimised electrodes could be obtainedeasily by using CNC machining. Hence the method possesses poten-tial application prospect.

Fig. 17.Appearance of fracturedwelded joint obtainedwith optimised electrodes. (a) Steelside; (b) aluminium side.

Fig. 18. SEM morphology of fractured surface of welded joint with analysis positionsmarked as B and C in Fig. 17. (a) Region B; (b) region C.

469W. Zhang et al. / Materials and Design 85 (2015) 461–470

Several results could be summarised as follows:

(1) Morphology of electrodes used in resistance spot welding of6008-T66 aluminium alloy and H220YD galvanised highstrength steel was optimised, which were a planar circulartip electrode with tip diameter of 10 mm on steel side and aspherical tip electrode with spherical diameter of 70 mm onaluminium alloy side.

(2) The welded joint obtained with optimised electrodes could beregarded as a special welded-brazed joint with a straight in-terface between aluminium nugget and steel, which wasformed by means of wetting and spreading of molten alumin-ium nugget on solid steel.

(3) An intermetallic compound layer composed of Fe2Al5 andFe4Al13 with overall thickness of about 4.0 μm was formed atthe aluminium/steel interface in the central region of thewelded joint obtained with optimised electrodes, meanwhilethe interfacial intermetallic compound layer in the peripheralregion of the welded joint with thickness of merely 1.0 μmwas identified to be Fe4Al13. The growth of the intermetalliccompound layer was regulated by reactive mutual diffusionbetween liquid aluminium nugget and solid steel.

(4) The current density distribution with optimised electrodeswas more homogeneous than that with F type electrodes,and the maximum current density with optimised electrodes(about 1.0 × 103 A mm−2) was less than half that with Ftype electrodes. The interfacial temperature in the weldedjoint during welding with optimised electrodes (about915 °C) was lower than that with F type electrodes (about985 °C). The improved welding current density distribution

and interfacial temperature field of the welded joint withthe employment of optimised electrodes facilitated the pro-motion of the weldability.

(5) Tensile shear load up to 5.4 kN was achieved for the weldedjoint obtained with optimised electrodes, and the weldedjoint exhibited nugget pullout failure mode during tensileshear testing, which was superior to the interfacial failuremode for the welded joint obtained with F type electrodes.

Acknowledgements

The authors would like to thank the National Natural ScienceFoundation of China (No. 51275204) and the Young Talent Projectof Shenyang National Laboratory for Materials Science (No.Y40663) for the financial support. They would also like to acknowl-edge Prof. Zheshi Yuan from Jilin University for critical comments.They also thank Mr. Qinglei Chen and Mr. Qun You for their assis-tance to conduct resistance spot welding experiments.

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