Laser welding of ultra-high strength steel with different oscillating modes

To obtain high-quality welded joints with low heat input and high welding efficiency, a laser oscillating mode was proposed for welding D406A ultra-high strength steel (UHSS). The effect of four oscillating modes on morphology, microstructure and mechanical properties of the 3.5 mm-thick UHSS welded joints were discussed, including transverse, longitudinal, circular and without oscillation. The morphologies of the welded joints with different oscillating modes were significantly different. Among them, circular oscillation obtained the deepest weld penetration, and an asymmetric phenomenon of cross-section was observed. Furthermore, both transverse and circular oscillation obtained equiaxed grains, while circular oscillation had the finest columnar grains and the largest equiaxed grain area. Subsequently, mechanical properties of the welded joints with circular oscillation were improved obviously, which has the highest tensile strength of 809.44 MPa and the largest ductility of 21.8%. In comparison to without oscillation, the tensile strength and ductility were increased by 9.58% and 11.4% respectively. This study provides guidance for the selection of optimal oscillating modes for laser welding of UHSS.