Influence of surface layers on resistance spot joinability of partially hardened steel 22MnB5 with aluminum-silicon and zinc coatings

Abstract

One of the methods to achieve local change in material properties when producing high strength parts with tailored mechanical properties is variating temperature histories in the furnace prior to hot stamping. This causes changes in surface layer development of coated steels and thus differences in contact resistances,which determine heat development during the resistancewelding process. To get a deeper understanding of the influences, surface layers have on the welding process, parts with temperature histories, typical for industrially manufactured partially hot-stamped parts with aluminum-silicon and zinc coatings were investigated. Static contact resistance was determined; measurements of dynamic resistances in welding process were conducted. Welding current ranges were determined for different heat-treated conditions. No correlation between static contact resistance and welding current range was found; however for Zn coating, the welding current range was moved towards lower welding currents and shrinked for the measured total static contact resistance of about 4000 μΩ. For aluminum-silicon coating, no dependence between welding current range and static contact resistance was evident. For Al-Si-coated steel, a novel criterion, based on the fraction of the resistive phases in the surface layer, determined by metallographic investigations was developed, allowing to determine material joinability.