Fiber laser welding of HSLA steel by autogenous laser welding and autogenous laser welding with cold wire methods

6 mm thickness high strength low alloy (HSLA) steel butt joints were fabricated by fiber laser welding. The influence of welding parameters on morphologies, weld process behavior, microstructure and mechanical properties were investigated. Defects (undercut, spatters and humping) were observed with the increase of heat input (HI) during autogenous laser welding (ALW), and weld morphologies changed from partial penetration to root humping first, then fine result, and finally over penetration (cutting) via high-speed images (HSI). Three typical transition modes were observed during autogenous laser welding with cold wire (LWACW), including spreading-transition, liquid-bridge transition and explosion transition. Sound process stability was achieved in liquid-bridge transition with the optimized parameters of 6 kW laser power, 0.8 m/min welding speed and 3.5 m/min feeding wire speed. The microstructure in the fusion zone was composed of acicular ferrite with some equiaxed grains, while that of autogenous laser welding was mainly made up of the acicular ferrite. The microhardness of base metal was relatively lower than that of fusion zone in both types, and both joints fractured at the side of base metal during tensile test.