Ultrasonic Spot Welding of Galvanized Steel Sheet

Abstract

Ultrasonic spot welding of galvanized steel sheets has been conducted to investigate formation process of the weld microstructure, and effect of zinc coat on joint property. The joint strength showed a rapidly increase at the weld time of 0.8s, and it has the maximum value over 1.0 s. In the weld time range from 0.1s to 0.8s, fracture occurred in zinc coat/ steel interface. In contrast, the fracture partially occurred in the base steel in the weld time over 1.0s, and concaves in which the steel substrate of one specimen partially remains on the other specimen were observed. The formation process of weld microstructure of ultrasonic welding was proposed to be a two-steps process. First, zinc coat in the interface was mechanically removed by the vibration and clamping force, and some zinc particles dispersed in the weld interface. Second, the zinc particles melted due to the friction, and partially bonded regions were simultaneously formed around the zinc particles. The partially bonded regions were stirred with the steel substrate as the weld time increased. The joint strength increased by development of the stirred zone. At the weld time over 1.2s, the joint strength decreased due to growth of crack around the stirred zone.