Resistance spot welding of dissimilar DP600 and DC54D steels

The microstructure in the fusion zone (FZ) was determined to be full lath martensite because of the high cooling rate. The heat-affected zone (HAZ) of DP600 was divided into coarse-grained, fine-grained, and inter-critical zones, and the volume fraction of martensite in HAZ was higher than that in the DP600 base metal (BM). The HAZ of DC54D was characterized by the presence of intensive ferrite. As the welding current, welding time, and electrode force increased, the tensile-shear strength of the welded spot experienced two stages of a notable increase and a subsequent decrease. The maximum value of 6.59 kn was obtained at a welding current of 9 kA, welding time of 14 cycles, and electrode force of 2.6 kn. Interfacial fracture occurred because of the small nugget diameter. Plug failure took place along the plastic deformed DC54D dominated by abnormally grown grains discovered from the interface of HAZ/BM through electron back-scattered diffraction.

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