Enhancing crashworthiness in multistep resistance spot welding of Al–Si-coated 22MnB5 steel and 201 stainless steel

With the recent reinforcement of crashworthiness standards for automotive bodies, combining high-strength Al–Si-coated 22MnB5 hot stamping steel (HS) and high-toughness austenitic stainless steel (ASS) has gained attention for producing components with excellent strength and elongation, despite the trade-off between these properties. However, intrinsic properties of each material, including high contact resistance and stiffness of HS and high alloying element contents (e.g., Ni and Mn) in ASS, can deteriorate welding quality when they are joined, thereby affecting crashworthiness of weld joints. Therefore, a new approach must be developed based on clear understanding of metallurgical–mechanical characteristics of dissimilar weld joints. For the first time, this study analyzed metallurgical–mechanical characteristics in resistance spot welding (RSW) of HS and ASS, and a method for achieving pullout failure was developed. The characteristics required to achieve pullout failure include an increased nugget diameter, enhanced strength of fusion zone (FZ), and a softened stress concentration region. To this end, multistep RSW was introduced by adding pre- and/or post-heating steps to one-step RSW consisting of only welding. First, the increase in nugget diameter was attributed to increased total heat input via preheating. Next, martensite and -ferrite consistently formed in the FZ under different process parameters, showing similar strength. Finally, the softening of the stress concentration region was attributed to the tempering of the martensite in heat-affected zone (HAZ) during post-heating. Consequently, among metallurgical–mechanical characteristics of weld joint between HS and ASS, increasing nugget diameter and softening HAZ were shown to effectively induce pullout failure.