Improving weldability of Al-Si coated press hardened steel using stepped current pulse schedule

Abstract

Al-Si coated press hardened steel (PHS) were introduced into vehicle body structures for a balance of lightweighting and crashworthiness. However, the weldability of PHS is relatively poor, due to its high carbon equivalent and high strength and is characterized by severe heat affected zone, HAZ softening, and interfacial fracture. The interfacial fracture mode is defined in part by the presence of retained Al-Si coating as a sharp notch at the faying interface following the welding process, which limits the mechanical performance of the joint. To improve the weldability of PHS, a stepped current pulse schedule was proposed for the resistance spot welding (RSW) of 1.5 mm Al-Si coated PHS as a means to effectively promote nugget growth, inhibit expulsion and eliminate the residual Al-Si layer. In this manner the mechanical performance of joints were significantly improved and the desirable button pullout fracture mode was realized.