Challenges and advances in resistance spot welding of ultra-high strength hot-stamped steels: Coating effect, softening phenomena, and mechanical performance

Abstract

Hot-stamping (HS) has emerged as an effective process for producing steel components with ultra-high strength (UHSS) and complex geometries while minimizing or eliminating springback. HS steels present significant advantages for weight reduction and improved crashworthiness, making them increasingly favored for critical crash-relevant components in body-in-white (BIW) structures, such as A-pillars, B-pillars, and door reinforcements. To integrate these components into BIW structures, they must be welded, particularly using resistance spot welding (RSW), which is the predominant sheet-assembly process in the automotive industry. However, the RSW of these steels is very challenging, primarily due to their high alloying content and ultra-high strength. Furthermore, metallic coatings, especially Al–Si coatings, are applied to protect the steel from oxidation, decarburization, and corrosion. However, these coatings complicate the RSW of HS steels even further. This paper reviews current practices in the RSW of HS steels, highlighting the associated challenges and potential solutions. It examines the influence of coating presence, type, weight, and heat treatment conditions on weldability. Furthermore, it analyzes the microstructural transformations and hardness characteristics in various regions of the joints to enhance the understanding of mechanical properties and failure behavior. Key factors affecting the failure behavior, such as subcritical heat-affected zone softening, fusion boundary softening (halo ring), and nugget edge grain morphology, are discussed. Finally, the paper explores various techniques developed to improve the weldability, including the application of pre-pulse, double pulse (''dual nugget''), three-pulse stepped-up current, magnetically assisted three-pulse stepped-up current, individually optimized three-pulse, in-situ post-weld tempering, in-situ nugget edge grain refinement, and paint baking.