Influence of Pulse Number on Nugget Formation and Joint Performance in Resistance Spot-Welded 2 GPa Press-Hardened Steel

The resistance spot welding (RSW) of 2 GPa press-hardened steels, which have been widely used in industrial applications in recent years, poses significant challenges due to brittleness caused by the fully martensitic microstructure, subcritical heat-affected zone softening, and interfacial cracking influenced by the coating. The effect of the number of pulses during welding on nugget formation and fracture behavior remains a critical issue. In this study, microstructural characterizations using optical microscopy and SEM, microhardness testing, tensile-shear tests, and fractography analyses were conducted across different pulse conditions (insufficient, moderate, optimal, and excessive heat input). The obtained results demonstrate that the pulse number is a decisive parameter in nugget formation and joint behavior due to its governing effect on heat input and thermal history. While increasing heat input promotes nugget development, excessive pulsing induces localized degradation, severe indentation, and centerline flaws. Under optimal pulsing conditions (Pulse 7), uniform fusion, refined martensitic transformation, an FZ hardness of approximately 580–600 HV, and a maximum shear strength of approximately 790 MPa were achieved with complete button pull-out failure. In contrast, under low pulse conditions, the narrow fusion zone resulted in hardness approaching 700 HV and brittle interfacial fracture along the faying interface driven by the fully hardened martensitic matrix and micro-shrinkage voids. These findings establish clear processing guidelines for pulse modulation to enhance weld integrity and joint reliability in 2 GPa press-hardened steels.