Effect of Multiple Pulse Resistance Spot Welding Schedules on Liquid Metal Embrittlement Severity

Zinc-coated advanced high strength steels (AHSS) used in automotive applications are susceptible to liquid metal embrittlement (LME) during resistance spot welding (RSW). This study examines the impact of multiple pulse welding schedules on LME severity in welds of TRIP1100. Two different types of pulsing methodologies have been proposed to reduce LME severity: applying a pre-pulse before the welding current to remove the zinc coating and pulsing during the welding current to manage heat generation. However, the mechanisms by which these methods affect LME severity have not been fully explored. This work showed that a welding schedule consisting of two equal length pulses resulted in the least severe LME because it reduced the amount of free zinc available for LME without creating too much tensile stress. The majority of pre-pulse welding schedules caused an increase in LME cracking due to the additional heat introduced into the weld. However, a 4 kA (low current) pre-pulse applied for 3 cy (low time) reduced LME cracking by almost 30%. The pre-pulse allowed zinc to diffuse into the coating and stabilize the zinc, without introducing too much additional heat into the weld. These results indicate that multiple pulse welding schedules may be successfully used to reduce LME cracking, although the mechanisms by which they impact LME are more complicated than previously thought.