Formation, microstructure and mechanical properties of double-sided fiber laser welded ultra-high strength steel

The ultra-high strength steel with a thickness of 20 mm was welded by double-sided high-power fiber laser welding. The welding parameters were optimized according to the behavior of molten metal and formation of the weld. Necking, spatters and uneven formation occurred when the welding parameters were improper. Microstructure of the weld at most region was made up of martensite, while austenite could be found at the final solidified position where the solidification rate was relatively slow. The thermal effects of the second welding process resulted in the austenization of original martensite and then transformed to ferrite and carbide within the interlaminar weld. Hardness of the weld was much higher than that of base metal owing to the formation of martensite. Hardness decreased at the austenite zone and the interlaminar zone made up of massive ferrite. Solid solution of C element within the martensite improved the tensile strength but weakened the toughness of the weld, leading to the formation of cleavage planes at fracture surfaces of the top and the mid welds. Large angle between the grain growth direction and force loading direction was benefit to improve the tensile properties.