Resistance Spot Welding and In-Process Heat Treatment of Hot Stamped Boron Steel

Abstract

Escalating environmental concerns have prompted efforts to reduce vehicle weight and carbon emissions, resulting in increased application of advanced high strength steels (AHSS). 22MnB5 hot stamping grade AHSS, namely USIBOR 1500P, provide high strength to weight ratio allowing sheet thickness reduction to decrease weight, while maintaining high safety characteristics. Resistance spot welding (RSW) has been the predominant welding process for automotive assemblies. Welding hot-stamping AHSS has introduced new challenges for achieving acceptable welds. The added alloying elements and high hardenability characteristics resulting in low weldability and weld toughness complicates this initiative. The current study examines the effects of in-process weld tempering with secondary current pulse has on the weld toughness during RSW of USIBOR steels. RSW and weld tempering were tested on USIBOR at two different surface conditions; as-delivered and hot-stamped. Joint performance properties, micro-hardness map profiles, and failure modes of welds for both tempered and non-tempered conditions are detailed. Furthermore, a relationship between resulting joint performance and microstructural evolution is produced. The objective of this work is to optimize in-process tempering parameters, analyze metallurgical evolution of the weldments, and compare the effects on mechanical performance for both tempered and non-tempered welds.