Resistance Spot Welding of Quenching and Partitioning (Q&P) Third-Generation Advanced High-Strength Steel: Process–Microstructure–Performance

This paper investigates process–microstructure–performance relationships in Q&P980 third-generation advanced high-strength steel (AHSS) resistance spot welds. The hardening and softening phenomena during welding are discussed in terms of weldment microstructure. The fusion zone (FZ) microstructure was mainly lath martensite with an average hardness of 500 HV due to the high cooling rate resulting from resistance spot welding. No significant softening was observed in the sub-critical heat-affected zone which was related mainly to the presence of the low volume fraction of fresh martensite in the initial microstructure of the base metal. The factors controlling the tensile–shear peak load, failure energy, and failure mode transition of Q&P980 resistance spot welds are discussed. The assessment of the tensile–shear peak load of the welds failed in the IF mode verified that interfacial failure strength is governed by FZ hardness rather than FZ fracture toughness. In the case of pullout failure, two competing failure mechanisms were identified: (i) ductile cracking from the notch tip and (ii) through-thickness localized necking, with the former was found to be the dominant failure mechanism for the Q&P980 steel resistance spot welds. The high susceptibility of the Q&P980 resistance spot welds to interfacial mode was due to its low hardening ratio, lack of significant softening in the HAZ, and its propensity to shrinkage void formation.