Investigation of laser-flux cored arc hybrid welding of Q690D high strength steel: weld formation, microstructure evolution and mechanical performance

Q690D high-strength low alloy (HSLA) steel, renowned for its exceptional mechanical properties, is extensively employed in fabricating marine engineering equipment. Traditional laser-arc hybrid welding (LAHW) of medium-thick plates with solid wires exhibits limited alloying capabilities and wettability, causing undercut and humping defects. However, the unique structure and molten slag protection effect of flux-cored wires (FCWs) in the laser-flux cored arc hybrid welding (L-FCAHW) process demonstrate significant enhancement of the weld formation. In this work, L-FCAHW technique was developed to weld 10 mm thick Q690D HSLA steel. The interaction mechanisms between laser-induced keyhole dynamics, molten pool dynamics and flow behavior, as well as the metallurgical performance were investigated in detail. The results indicate that the slag covering effect of FCWs markedly improves the wettability of the molten pool and suppresses the undercut defects. Additionally, the formation of humping defects is influenced by the equilibrium between the vapor recoil force and the Marangoni effect. The underlying mechanism involves a dynamic balance among the vapor recoil force, surface tension, gravity, and arc pressure. The Fe–Ti–O slag system enhances the wettability of the molten metal, significantly refining the grain size. Microstructure in the arc zone (AZ) was predominated by martensite and quasi-polygonal ferrite, while the laser zone (LZ) was lath martensite. The tensile properties of the welded specimens obtained by L-FCAHW process were comparable to those of the base material. The impact toughness of the weld specimens is able to meet industry standards and ensures adequate impact toughness even at low temperature.