Influence of Welding Speed on Microstructures and Properties of Ultra-high Strength Steel Sheets in Laser Welding

Abstract

Ultra-high strength steels (UHSS) are welded using an Nd:YAG laser welding system. The influence of welding speed on joint appearance, microstructure, hardness and tensile properties are investigated. The results indicate that the size of the fusion zone decreases with the increasing of the welding speed and partial penetration occurs if the welding speed exceeds 100 mm/s. The microstructure of the fusion zone and its approaching heat affected zone (HAZ) is lath martensite leading to the high strength and hardness. There exists a tempered zone (softening area) in the HAZ approaching to the base metal, where the microstructure is tempered maretensite and ferrite. When changing the welding speed from 70 to 130 mm/s, the width of the tempered zone decreases from 100.4 to 48.2 μm and the degree of softening slightly decreases with the minimum hardness ranging from 310 to 336 HV. The highest tensile strength and the maximum elongation at fracture are both obtained at the welding speed of 90 mm/s, the specimen fails in a ductile fracture mode.