A Critical Nugget Size Prediction Model for Al–Si-Coated Press Hardened Steel Resistance Spot Welds

For the automotive industry, it is important to predict the fracture mode of resistance spot-welded joints. Traditional 4t^0.5 (where t is the sheet thickness) criterion for the transition from interfacial fracture (IF) to button pullout fracture (BPF) mode is not applicable for press hardened steel, press-hardened steel (PHS), welds. This study aims to investigate the effect of Al–Si coating and critical heat affected zone (CHAZ) location on the lap-shear fracture mode transition mechanism of Al–Si-coated PHS welds. It was found that in-situ tempering pulse after the welding stage could change the relative location of the CHAZ and weld nugget, which in turn, had a significant effect upon the fracture mode transition. Thus, a new analytical model was built to predict the critical nugget size for Al–Si-coated PHS, wherein the Al–Si coating and the CHAZ location are considered as critical factors for predicting the fracture mode transition of PHS spot welds and are incorporated into this model. A reasonable correlation of the model to experimental data was achieved.