Microstructure and Microhardness of Fiber Laser Welded Dual-Phase Steels with High-Strength Low-Alloy Steels

The fiber laser welding of dual-phase DP980 steels with high-strength low-alloy LA340 steels was studied with a concentration on microstructure, microhardness, and tensile properties of butt joints. The microstructure in the fusion zone of joints was predominantly built of lower bainite and martensite. Mainly martensite was found in the coarse-grained region of HAZ near DP980 steel, but equiaxed ferrite and tempered martensite were observed in fine-grained region of this HAZ. The microstructure consisted of acicular ferrite, upper bainite and equiaxed ferrite in the high tempered coarse-grained region of HAZ near LA340 steel. The fine-grained equiaxed ferritic microstructure was found in the low tempered region of HAZ near LA340 steel. The microhardness increased in the fusion zone and in both heat-affected zones compared to the base metals, 335 HV_0.1 for DP980 steel and 186 HV_0.1 for LA340 steel. The microhardness reached values from 380 to 441 HV_0.1 in the fusion zone, from 314 to 464 HV_0.1 in HAZ near DP980 steel, and from 181 to 380 HV_0.1 in HAZ near LAD340 steel. The tensile strength of the fiber laser welded joints was 460 MPa and it reached the strength of the LA340 steel base material with failure occurring in this steel.