Effect of Welding Methods on Fatigue Properties of Welded Joints of High Strength Steel Sheets for Automobiles
Hiroshi TSUDA, Yoshihiko SUNAYAMA, Masashi DAIMARUYA and Hidetoshi KOBAYASHI
Abstract:In high strength steel sheets, it seems to be possible to improve the fatigue strength of joints welded by using laser welding due to the lower heat input of welding and the flatter weld beads compared with conventional welding methods such as arc welding or resistance spot welding. Therefore, in this paper, fatigue tests were conducted for the specimens with the butt joints of 780 Mpa grade steel sheets welded by using laser welding, plasma welding and MAG welding and for the specimens flattened the weld beads to investigate the effect of welding methods on the fatigue properties of welded joints of high strength steel sheets for automobiles. And the macrostructure, the microstructure and the hardness distribution around the weld metal were observed to study the mechanism for the difference of the fatigue strength by the welding methods. It was found that the fatigue strength in arc-welded joints and plasma-welded joints is lower than that in laser-welded joints mainly due to the occurrence of softening in the heat affected zone. On the other hand, it was found that the stress concentration at the toe of the weld bead significantly deteriorates the fatigue strength even in the laser-welded joints. Key Words:High strength steel, Welded joint, Fatigue strength, Tensile strength, HAZ softening, Stress concentration, Laser welding, Plasma welding, MAG welding