Fatigue Behavior and Fracture Mechanisms of Dissimilar Laser‐Welded Lap Joints of High‐Strength and Mild Steels

ABSTRACT This study was aimed at clarifying the fatigue fracture mechanism in dissimilar laser‐welded lap joints of 1‐GPa‐grade steel (H) and 280‐MPa‐grade steel (M). Various mechanical properties were evaluated, and plane‐bending fatigue tests were conducted on four types of joints: (1) M/M, (2) H/H, (3) H/M, and (4) M/H. In the fatigue tests, H/H joints showed significantly higher fatigue strengths, whereas H/M dissimilar joints displayed similar fatigue strengths. On the other hand, though the fatigue strength of H/M is higher than that of M/H in the finite life range, the fatigue limits of M/H and H/M are almost agreed. This difference arises because, after the fatigue tests, all joint types except H/M generated welding fractures, whereas H/M joints fractured in the base material of mild steel parts. Therefore, the results indicate that placing a high‐strength material on the laser‐irradiated side ensures high reliability.