Failure Mode and Fatigue Behavior of Dissimilar Laser Welds in Lap-Shear Specimens of Low Carbon Steel and HSLA Steel Sheets

In this paper, failure modes of dissimilar laser welds in lap-shear specimens of low carbon steel and high strength low alloy (HSLA) steel sheets are investigated based on experimental observations. Micro-hardness tests across the weld zones of dissimilar laser welds were conducted. The hardness values of the fusion zones and heat affected zones are significantly higher than those of the base metals. The fatigue lives and the corresponding failure modes of laser welds as functions of the load ranges are then examined. Optical micrographs of the laser welds before and after failure under quasi-static and cyclic loading conditions are then examined. The failure modes and fatigue behaviors of the laser welds under different loading conditions are different. Under quasi-static loading conditions, a necking failure occurred in the upper low carbon steel sheet far away from the laser weld. Under low-cycle loading conditions, the dominant kinked fatigue crack was initiated from the right crack tip and grew into the upper low carbon steel sheet. Under high-cycle loading conditions, the dominant kinked fatigue crack was initiated from the left crack tip and grew into the lower HSLA steel sheet. The experimental results indicate that the failure modes of the laser welds under cyclic loading conditions strongly depended on the load range amplitude.