Determination of resistance spot weld failure path in ultra-high-strength press-hardened steel by control of fusion boundary transient softening

The use of coated press-hardened steels (PHS) in body-in-white (BiW) components provides significant potential for weight reduction, but leads to decrease in joinability, thus creating challenge to produce welds meeting commercial quality requirements. Welding with longer welding times can improve joinability, but causes softening at the fusion boundary of spot welds due to local decarburization, referred to as a “halo” at the fusion boundary. In the present work, RSW with and without a softened region at the fusion boundary were fabricated. The geometry and hardness of the halo were obtained from high-resolution hardness mappings and transferred to an FE model, which was used to predict mechanical performance and fracture mode of spot welds in lap-shear and cross-tension tests. The developed FE model was verified experimentally and allowed for an accurate prediction of failure modes while demonstrating that the presence of the halo ring affects the location of failure as well as the load–displacement response of spot welds.