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This study focuses on improving the mechanical performance of third-generation Q&P steel resistance spot welds using a double-pulse welding cycle. Single and double-pulse welding schedules were implemented to assess the mechanical performance of the welds. Single-pulse welds exhibited poor cross-tension strength (CTS) values, failed around the fusion zone, and were accompanied by poor energy absorption capability. However, the double-pulse schedule showed improved CTS values by 33%, with an associated 110% increase in absorbed energy. The failure path observed from interrupted cross-tension tests showed that, in welds made using both pulsing schedules, failure proceeded along the fusion boundary and CGHAZ. In the single-pulse welds failed in brittle fashion, whereas the welds made with a double-pulse schedule exhibited a mixed (ductile and brittle) fracture morphology. The high-density microhardness mapping confirmed the presence of a localized softened zone (halo ring) adjacent to the fusion boundary in single-pulse welds. Strong elemental partitioning of Mn, Si, and C in the vicinity of the fusion boundary during long welding time was the primary cause for the halo formation. However, the halo ring was eliminated by performing a double-pulse weld schedule with 30 ms cooling time in between pulses; resulting in improved mechanical properties.

This study consisted of a comparative evaluation of the mechanical properties and failure behavior of QP980 steel with those of DP980 steel spot welds. Moreover, this paper contributes to the existing knowledge of the resistance spot welding behavior of QP steels by analyzing their microstructure, hardness profile, tensile shear strength, cross tension strength, and failure behavior. It was found that the cross tension and tensile shear test samples all failed in a partial or full button pull-out mode, with an acceptable ductility ratio and cross tension strength as per AWS. Fractography of the testing samples showed that the crack propagated along the fusion boundary, an occurrence previously reported in the literature for QP980, but with no comprehensive explanation regarding the mechanism behind the behavior. This work discusses the similarities between this behavior and welds exhibiting a possible softened region at the fusion boundary, a phenomenon known as the halo ring. The role of this softened region on the failure behavior of the welds is described.