Factors affecting weld quality in resistance spot welding of advanced high strength steels

This study investigates the significant factors on weld quality in the resistance spot welding of quench and partitioning advanced high-strength steel. Attention is laid emphasis on nugget formation and growth, weldability range (lobe curve), nugget size and shape (macrostructural features), coating effects, mechanical properties, and fracture behavior of resistance spot welds. As zinc-coated steel sheets are widely applied in body-in-white in automotive industry, coating's effect on weld quality is particularly significant. The research establishes correlations of nugget macrostructure, coating effects, and the following mechanical properties and fracture characteristics of the welds. Key conclusions recognize that Zn coating significantly alters the nugget formation mechanism by reducing the electrical contact resistance. Compared with uncoated samples, Zn-coated steels require a larger welding current for nugget formation because the major heat generation transforms from contact resistance to bulk resistance within the steel sheet. This reduction in contact resistance heating leads to larger nuggets for the same weld conditions. The uncoated samples show greater hardness of fusion zone and tensile-shear strength than coated samples, recording peak load-bearing capacities of 28 kN and 24 kN for medium heat input conditions, respectively. The critical welding current to transition into pullout failure mode was 9.5 kA for uncoated samples and 9.0 kA for coated samples. These findings highlight the complex interplay among coating presence, process conditions, and nugget shape attributes, all of which are crucial for predicting the mechanical performance and failure mode of resistance spot welds in Zn-coated QP980 steel.