Failure behavior of resistance spot welded advanced high strength steel: The role of surface condition and initial microstructure

The continuous galvanizing process (CGP) under high dew point temperature has been recognized as a viable technique for improving the coating quality of advanced high-strength steels (AHSS). However, the effect of this surface modification technique on resistance spot welding (RSW) process is not clear. The aim of this study is to investigate the role of different surface conditions and initial microstructures obtained through CGP on mechanical properties and failure behavior of resistance spot welded AHSS. It has been shown that the formation of internal oxides during the annealing and subsequent coating process reduced the heat input during RSW, resulting in smaller nugget size and consequently, lower tensile-shear peak load values. Finite element modeling (FEM) and experimental results indicated that in the case of the as-received sample, higher fusion zone/base-metal (FZ/BM) hardness ratio leads to a failure location closer to heat-affected zone (HAZ). However, the lower FZ/BM hardness ratio in the case of annealed-bare and annealed-coated specimens leads to failure location more towards FZ. It was shown that nugget size will not control the failure location during lap-shear loading but does affect nugget load-carrying capability. However, failure behavior is controlled by microstructural characteristics of different weld regions.