materials Article Effect of Welding Current on Weld Formation, Microstructure, and Mechanical Properties in Resistance Spot Welding of CR590T/340Y Galvanized Dual Phase Steel Xinge Zhang 1,2, Fubin Yao 3, Zhenan Ren 2,* and Haiyan Yu 4 1 School of Mechanical and Aerospace Engineering, Jilin University, Changchun 130025, China; [email protected] 2 College of Materials Science and Engineering, Jilin University, Changchun 130025, China 3 Doosan Infracore China Co., Ltd., Yantai 264006, China; [email protected] 4 Sanyou Automobile Parts Manufacturing Co., Ltd., Changchun 130022, China; [email protected] * Correspondence: [email protected]; Tel.: +86-431-8509-4376 Received: 20 September 2018; Accepted: 13 November 2018; Published: 17 November 2018 Abstract: During resistance spot welding, the welding current is the most important process parameter, which determines the welding heat input and then has a great influence on the welding quality. In present study, the CR590T/340YDP galvanized dual phase steel widely used as automobile material was carried out using resistance spot welding. The effect of welding current on the weld formation, microstructure, and mechanical properties was studied in detail. It was found that the quality of weld appearance decreased with the increase of welding current, and there was a Zn island on the weld surface. The microstructure of the whole resistance spot welded joint was inhomogeneity. The nugget zone consisted of coarse lath martensite and a little of ferrite with the columnar crystal morphology, and the microstructure of weld nugget became coarser when the welding current was higher. There was an optimum welding current value and the tensile strength reached the maximum. This investigation will provide the process guidance for automobile body production. Keywords: Resistance spot welding; galvanized dual phase steel; microstructure; mechanical properties; weld formation 1. Introduction In the automotive field, reducing the body weight is one of the most fundamental ways to save energy and reduce pollution. Therefore, more and more lightweight and high-strength materials have been developed for body manufacturing [1–3]. Galvanized dual phase steel has the advantages of low yield ratio, good formability, high tensile strength, good matching of strength and plasticity, corrosion resistance, and so on, which has been used in automobile body manufacturing [4–6]. The wide application potential of galvanized dual phase steel in the automotive field mainly depends on the welding methods and its welding quality. In previous literature, several welding methods, such as laser welding [7–10], gas metal arc welding (GMAW) [11–13], and friction stir welding (FSW) [14–16], have been employed to weld the galvanized dual phase steel, and these welding methods have their own advantages and disadvantages, as well as being applicable to different base metal materials and joint shapes. Resistance spot welding has the advantages of high efficiency and low cost, which is an important welding method for manufacturing the automobile sheet structures [17,18]. Many scholars have studied the microstructure, softening zone characteristics, mechanical behavior, and welding spatter defect of resistance spot Materials 2018, 11, 2310; doi:10.3390/ma11112310 www.mdpi.com/journal/materials Materials 2018, 11, x FOR PEER REVIEW 2 of 13 Materials 2018, 11, 2310 2 of 13 studied the microstructure, softening zone characteristics, mechanical behavior, and welding spatter defect of resistance spot welding of dual phase steels [19–23]. The CR590T/340YDP galvanized dual weldingphase steel of dual sheet phase used steels as an [19 automobile–23]. The CR590T/340YDP manufacturing material galvanized not dual only phase has steelgood sheet corrosion used asresistance, an automobile but also manufacturing high strength, which material can not effect onlyively has reduce good the corrosion weight of resistance, the car body, but alsoand some high strength,research on which resistance can effectively spot welding reduce have the been weight carried of the out car [24–26]. body, andWang some et al. research [27] investigated on resistance the spoteffect welding of base material have been chemical carried compositions out [24–26]. Wang on the et properties al. [27] investigated of the resistance the effect spot ofwelding base material joint of chemicalDP590 steel, compositions and the results on the indicated properties the of thetensile resistance strength spot and welding toughness joint ofof DP590welded steel, joints and were the resultsaffected indicated by the chemical the tensile compositions strength and of toughnessthe base material, of welded especially joints were the carbon affected content. by the chemicalNamely, compositionsfor the same grade of the DP590 base material, steel, the especially weld formation, the carbon microstructure, content. Namely, and for mechanical the same gradeproperties DP590 of steel,resistance the weld spot formation,welding will microstructure, be different if andthe chemical mechanical compositions properties ofof resistancethe base material spot welding are not will the besame. different During if thethe chemicalresistance compositions spot welding of process, the base it materialis well known are not that the the same. welding During heat the generation resistance 2 spotcan be welding expressed process, as Q it= isI2Rt well (I knownis the welding that the current; welding R heat is the generation resistance; can and be t expressed is the welding as Q =time).I Rt (InI is general, the welding the welding current; timeR andis the welding resistance; current and aret 10is− the1 s level welding and kA time). level, In respectively. general, the Therefore, welding −1 timethe welding and welding current current is considered are 10 ass thelevel key and factor kA level,to determine respectively. the welding Therefore, heat the input welding and influence current isof consideredthe welding as quality the key [28]. factor Wang to determineet al. [29] theestablished welding a heatfinite input element and model influence for ofresistance the welding spot qualitywelding [28 of]. DP590Wang steel, et al. and [29] theestablished nugget formation a finite element process model was investigated. for resistance The spot simulative welding ofresult DP590 for steel,nugget and size the was nugget obviously formation bigger process than that was of investigated.the experiment The under simulative a large resultwelding for current, nugget although size was obviouslythey were biggerwell fitted than under that of a thesuitable experiment welding under curren a larget. Therefore, welding the current, experimental although investigation they were well on fittedthe effect under ofa the suitable welding welding current current. on properties Therefore, in the resistance experimental spot investigationwelding of DP590 on the steel effect has of thean weldingimportant current significance. on properties in resistance spot welding of DP590 steel has an important significance. ToTo exploreexplore the the weldability, weldability, and provideand provide the process the process with guidance with guidance for automobile for automobile body production, body theproduction, CR590/340Y the CR590/340Y galvanized dualgalvanized phase steeldual sheetsphase employedsteel sheets for employed the automobile for the front automobile longitudinal front beamlongitudinal part were beam carried part were out by carried resistance out by spot resistance welding, spot and welding, the influence and the of influence the welding of the current welding on weldcurrent formation, on weld formation, microstructure, microstructure, microhardness, microhardness, and tensile and strength tensile was strength studied was in detail.studied in detail. 2. Materials and Methods 2. Materials and Methods In this study, the base metal is galvanized dual phase steel (Trademark: CR590T/340Y) sheet, In this study, the base metal is galvanized dual phase steel (Trademark: CR590T/340Y) sheet, whichwhich isis alwaysalways usedused toto manufacturemanufacture thethe automobile’sautomobile's frontfront longitudinallongitudinal beambeam part.part. The thickness of thethe basebase metalmetal isis 22 mm,mm, andand itsits .hemical.hemical compositionscompositions areare listedlisted inin TableTable1 1.. The The XRD XRD pattern pattern result result indicatesindicates thatthat thethe basebase metalmetal consistedconsisted ofof ferriteferrite (86.2(86.2 vol.%) and martensite (13.8(13.8 vol.%), as shown in FigureFigure1 1.. TheThe galvanizedgalvanized dualdual phasephase steelsteel sheetssheets were were conducted conducted with with double-side double-side hot hot galvanizing galvanizing and the weight of the Zn layer is 80 g/m3. The yield strength and tensile strength of the base metal are and the weight of the Zn layer is 80 g/m3. The yield strength and tensile strength of the base metal 356are 356 MPa MPa and and 605 MPa,605 MPa, respectively. respectively. Table 1. Chemical compositions of base metal (wt.%). Table 1. Chemical compositions of base metal (wt.%). Component C Si Mn S P Al Cr Nb Mo Fe Component C Si Mn S P Al Cr Nb Mo Fe wt% 0.0870.087 0.0080.008 1.6941.694 0.0070.007 0.0010.001 0.0300.030 0.1600.160 0.0120.012 0.0110.011 BalanceBalance FigureFigure 1.1. XRD pattern of base metal. TheThe resistanceresistance spotspot weldingwelding robotrobot (Shougang(Shougang MOTOMANMOTOMAN RobotRobot Co.,Co., Ltd.,Ltd., Beijing,Beijing, China)China) waswas employedemployed toto weldweld