Microstructure and Mechanical Behaviors of Fiber-Laser-Welded QP980-QP1180 Steels

Abstract

Advanced high-strength steels are considered the first choice when manufacturing lighter vehicles. Quench-partitioning (QP) steels are good candidates that fulfill manufacturing and performance requirements with their outstanding strength and formability. Laser welding offers a productive solution to the challenges of liquid metal embrittlement due to a low heat input and higher welding efficiency. This study investigated the microstructural evolution and mechanical performance of dissimilar laser-welded joints between QP980 and QP1180 steels. The microstructure of the joint mainly consisted of martensite phase in the fusion zone (FZ) and super-critical heat-affected zone (HAZ). In the mid and sub-critical HAZ, the microstructure consisted of tempered martensite along with ferrite and retained austenite on both sides. Due to these microstructural evolutions, FZ and HAZ are strengthened, and thus, laser welds can be achieved without the formation of a visible soft zone. Fracture of the joints occurred in softer base metal (BM) with ductile characteristics without any considerable strength loss. However, the ductility of the joints was lower than that of BMs because of deformation localization due to microstructure, yield strength, and thickness variations in the tensile and Erichsen test specimens. These results show that laser welding can be considered an effective alternative for joining QP steels.