New insights into the fracture behavior of advanced high strength steel resistance spot welds

While the automotive industry is striving for the reduction of car body weight to increase the fuel efficiency and to reduce CO₂ emission without compromising the safety and crashworthiness of vehicles, a new generation of advanced high strength steels (AHSS) have emerged as excellent candidates to meet these requirements. However, their integration into the car body structure is associated with welding-related problems. This research utilizes a novel approach to establish a fundamental correlation between welding parameters, microstructure and mechanical performance of AHSS resistance spot welds. In-situ micro-cantilever bending experiments are executed and analyzed in a quantitative manner to evaluate the effect of texture and post-welding heat treatment on the local fracture toughness of spot welds. A striking finding is that, through a switch from single to double pulse weld scheme, the texture of martensite formed in the fusion zone becomes responsible for a significantly higher fracture toughness of the area in front of the pre-crack. In addition, it is found that paint baking heat treatment also results in a much enhanced fracture toughness through tempering of the martensitic microstructure. A quantitative correlation is made between the micro-scale fracture toughness and macro-scale mechanical performance of advanced high strength steel welds.