Microstructure and mechanical properties of laser welded joint of dissimilar advanced high strength steels QP980-DP980

Fiber laser welding was used to join QP980 and DP980 advanced high strength steels. The microstructure,hardness distribution and tensile properties of the dissimilar welded joints were analyzed. The fracture morphology was observed and analyzed. The results showed that the fusion zone was composed of full martensite and showed the highest hardness（ 535 HV）. The heat affected zone（ HAZ） on both sides could be divided into super-critical zone,inter-critical zone,and sub-critical zone. The super-critical zone in the HAZ on both sides was all martensite due to the fast cooling rate,and the hardness was improved. The inter-critical zone consisted of martensite and ferrite. The sub-critical zone on the QP980 side consisted of tempered martensite,ferrite and retained austenite. The sub-critical zone on the DP980 side consisted of tempered martensite and ferrite. The hardness of the sub-critical zone was decreased due to the appearance of tempered martensite. The tensile fracture of the joint occurred in the HAZ of DP980 and exhibited ductile fracture mode. The tensile strength could reach 98. 9% of DP980 base metal,and the elongation of joints was 70. 9% than that of DP980 base metal.