SUPPLEMENT TO THE WELDING JOURNAL, MARCH 2016 Sponsored by the American Welding Society and the Welding Research Council Resistance Spot Weldability of Deformed Introduction Transformation-induced plasticity steel (TRIP) is an advanced highstrength steel (AHSS). It has been applied widely in the automotive industry to reduce weight and improve vehicle safety (Refs. 1 –3). Its structure usually consists of bainite, martensite, and retained austenite embedded in a continuous soft ferrite matrix (Refs. 4–8). Decomposition of the austenite phase into martensite during plastic deformation improves ductility (Refs. 3, 8–11). This phenomenon involves formation of strain-induced martensite by deformation of metastable austenite and leads to an increase of strength, ductility, and toughness of the steel (Refs. 12, 13). Among the welding methods used in the automotive industry, resistance spot welding is the most widely used joining technology for manufacturing auto body structures. In practical applications, steels are frequently subjected to deformation prior to welding. As mentioned earlier, during plastic deformation and straining, the retained austenite phase is also transformed into martensite in TRIP steel. High elongation values of the TRIP steels are explained by the interactions of the deformation and phase transformation mechanisms during straining of the material (Ref. 4). The effectiveness of the deformation can be reduced significantly during welding in the heat-affected zone (HAZ), where the peak temperatures are too low to cause melting but high enough to cause a change in the microstructure and properties of the materials (Ref. 14). The mechanical properties of the cold-deformed TRIP WELDING RESEARCH steels can be also affected by the phase transformations associated with the thermal cycle during welding. An extensive literature survey found no study about spot weldability of colddeformed TRIP steel. In order to simulate automotive manufacturing conditions, the spot weldability of deformed TRIP800 steel was investigated in detail. For this purpose, the tensile shear strength, weld button geometry, and electrode indentation were determined for a quality assessment of the weldment and obtained weld lobe diagram. The results were compared with the weldability of as-received welded TRIP800 samples that were published elsewhere (Ref. 15). The hardness profile and structure of the weldment were also evaluated for optimization of the welding parameters. Experimental Method Materials and Welding Processes The chemical composition of TRIP800 steel sheet in 1.5 mm thickness is given in Table 1. The deformation in tension by 10% was carried out by using a Shimadzu-type tensile test machine with a cross-head speed of 5 mm/min. Test samples were spot welded in a pneumatic, phase-shift controlled AC spot welding machine with 60 kVA capacity in 50 Hz electrical circuit. Spherical tip electrodes (CuCrZr) having 5.5 mm diameter were used. The welding schedule applied in the experiment is MARCH 2016 / WELDING JOURNAL 77-s TRIP800 Steel Various parameters were investigated for obtaining optimum tensile shear strength, weld button geometry, and electrode indentation BY H. E. EMRE AND R. KAÇAR ABSTRACT In order to simulate automotive manufacturing conditions, the spot weldability of colddeformed, in tension by 10%, TRIP800 steels was investigated in this study. Tensile shear strength and failure mode associated with button geometry and electrode indentations were also evaluated, and weld lobe was drawn for resistance spotwelded deformed TRIP800 steel. The microstructure of the weldment was evaluated and the hardness profile of the weldment was also determined. It was found that the properties of the weldment are directly related to parameters used for the process and deformation of base metal prior to welding. It was found that the button diameter and button size ratio of deformed TRIP800 steel spot welds should be at least 4√t and 0.14–0.30, respectively, for pullout failure mode, acceptable shear strength, and surface quality. KEYWORDS • Resistance Spot Weld • Cold Deformation • Welding Parameters • Weld Lobe Diagram H. E. EMRE (hayriyeertek@karabuk.edu.tr) and R. KAÇAR are with the Department of Manufacturing Engineering, Karabük University, Karabük, Turkey.
Welding Journal | March 2016
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