Understanding process–microstructure–property relationship of multilayered Cu foil welds by conductive heat ultrasonically assisted resistance spot welding

A novel conductive heat ultrasonically assisted resistance spot welding (CHURW) process is developed and applied to a battery cell internal assembly consisting of multilayer pure Cu foils and a Ni-plated Cu tab. In CHURW, graphite interlayers are placed between weld stacks and electrodes. Furthermore, ultrasonic vibration (UA) is applied perpendicularly to the weld interfaces simultaneously as the electrical current passes through. High quality welds are achieved for the 96 layered foils–tab stacks (96 t), with enlarged nugget size, no nugget detachment along the fusion boundary, and suppressed porosity. Effects of the number of foil layers and processing parameters on the weld geometry, microstructure, mechanical and electrical properties are comprehensively evaluated. In 96 t, increasing UA amplitude refines the grains near fusion boundary, whereas increasing welding time refines the grain structure throughout the nugget. Lap shear tensile tests show double peak loads in 96 t. The first fracture initiates at the nugget surface where the columnar dendrites are located, and the second occurs at the tab-side heat affected zone (HAZ). Higher UA amplitude mitigates the HAZ softening, whereas longer welding time leads to more HAZ softening while increasing the nugget hardness. Increasing UA amplitude first reduces and then increases the weld electrical resistance.