12th International Conference on Zinc & Zinc Alloy Coated Steel Sheet - GALVATECH 2021

In zinc coated 3rd Generation advanced high strength steels (3G-AHSS), liquid metal embrittlement (LME) occurs due to liquid zinc or zinc alloy penetration along the grain, grain boundaries and pre-existing cracks of the steel sub-surface area in the presence of critical amount of external tensile stress during resistance spot welding. Due to penetration of liquid metal, cohesion between grains of steel decreases and causes embrittlement. Most of the zinc coated 3G-AHSS are used in the automotive industry where multiple layers of coated steel sheets are joined together using resistance spot welding (RSW) technique. High temperature exposure, presence of tensile stress during welding and liquid metal are the three key parameters responsible for LME during spot welding of zinc coated AHSS. The LME crack initiation and propagation depend on the states and condition of the above three factors. Present study deals with the determination of onset of LME crack formation and the propagation during interrupted spot welding of Zn coated 3G-AHSS. All the spot welding has been carried out at constant electrode force along with constant welding current, only the welding cycle (time) has been varied. In addition, to evaluate the effect of external stress along with the stress required for RSW on LME crack formation and propagation, interrupted RSW has been carried out in the presence of external tensile stress. Results show that LME crack formation and propagation are very much dependent on the welding cycle which eventually controls the total heat input and stress condition. It has been also observed that with Zn coated 3G-AHSS the welding cycle (time) as well as stress affects the structural change at the steel/coating interfacial area.