Influence of heat-treated Al–Si coating on the weldability and microstructural inhomogeneity for hot stamped steel resistance nut projection welds

Resistance nut projection weldability of Al–Si coated hot stamped steel (HSS) was investigated under the viewpoint of weldable current range and joint strength (pull-out load). The microstructural inhomogeneities in the welds were also studied in order to elucidate the factors affecting the joint strength of the welds. The weldability of the given Al–Si coated HSS was compared with the weldability of an identical HSS without the Al–Si coating (Al–Si coating was polished out) and Zn coated dual phase steel. The weldable current range of Al–Si coated HSS was found to be narrower than that of the other materials. Furthermore, the average pull-out load within the weldable current range of the Al–Si coated HSS was the lowest among the three materials. The reason for poor weld mechanical property of the Al–Si coated hot-stamped steel was attributed to the microstructural inhomogeneities such as unmixed Al–Si coating layer at the edge of the nugget and the second phase Fe₃(Al, Si) intermetallic compound. The formation of Fe₃(Al, Si) phase was attributed to the solidification segregation of Al and Si during the weld solidification and was confirmed with the numerical analysis of solidification segregation.