Resistance spot welding of high-strength DP steel and nano/ultrafine-grained IF steel sheets

In the current study, microstructural characteristics, mechanical behavior, and failure analysis of resistance spot welded joints for high-strength dual-phase (DP) steel and nano/ultrafine-grained interstitial free (IF) steel was investigated. The DP steel side indicated three different heat-affected zones including intercritical, fine-grained, and coarse-grained. The volume fraction of martensite in the coarse-grained heat-affected zone was higher than in other regions due to more growth of γ grains. The IF steel side exhibited three different subregions in the heat-affected zone, namely, ultrafine-grained, fine-grained, and coarse-grained heat-affected zone. In the coarse-grained heat-affected zone, the iron oxide was effective on the microstructure by suppressing the grain growth. The uniformity of microhardness in the fusion zone of 10 kA welding current was more than other samples due to faster homogenization of melt. All welded samples indicate hardening at fine-grained heat-affected zone (on both sides) and ultrafine-grained heat-affected zone (on the interstitial free steel side). With increasing the current of welding from 6 kA to 10 kA, the peak load increased from 7.38 kN to 11.23 kN and the energy absorption improved from 6.01 kJ to 11.72 kJ due to increasing the diameter of the fusion zone. All samples failed through partial thickness-partial pullout mode. Cleavage facets and shear dimples were observed in all samples, which indicated a mixture of brittle and ductile fractures.