Ultrasonic resistance welding of TRIP-780 steel

A hybrid ultrasonic resistance spot welding process is developed, which integrates high-power ultrasonic vibrations into conventional resistance spot welding process. Weldability of galvanized transformation induced plasticity (TRIP-780) steel is studied based on this process. Ultrasonic resistance welds showed nugget pullout failure mode and more ductility compared with conventional resistance spot welds. Optical and scanning electron microscopic analysis revealed the refined microstructure in the fusion zone and heat affected zone of ultrasonic resistance welds. Coalesced bainite is present the fusion zone of conventional resistance spot welds while possible tempered martensite and feathery bainite is observed in ultrasonic resistance welds, which accounts for their higher ductile behavior.