Subcritical heat affected zone softening in hot-stamped boron steel during resistance spot welding

Abstract

Highly non-uniform temperature gradients experienced by workpieces during resistance spot welding (RSW) can lead to the formation of a subcritical heat affected zone (SCHAZ), a region prone to “premature” failure for ultra-high strength steels. Accurate prediction of spot weld properties such as surface electrode indentation and local hardness is essential for computer-aided engineering (CAE) based design of light-weight and impact-resistant structures. In this study, a 3D fully coupled electro-thermo-mechanical model incorporating an improved electrical contact resistance formula from the literature is developed for resistance spot welding of aluminium‑silicon coated hot-stamped boron steel. The temperature profiles, contact pressure distribution, nugget formation, and electrode indentation during RSW are numerically investigated. Tempering kinetics of base metal martensite is experimentally measured by isothermal tempering tests and used to extract kinetics parameters for a Johnson-Mehl-Avrami-Kolmogorov (JMAK) equation. The non-isothermal JMAK equation coupled with the process model is shown to accurately predict local SCHAZ softening.