Study on the microstructure and properties of the swing laser-MIG composite welding joint of TC4B titanium alloy

The microstructure of the welded joints was studied for the 20 mm thick TC4B titanium alloy plate welded by the swing laser-MIG hybrid welding. The effects of swing on the tensile strength and impact properties of the welded joints were compared and the fatigue properties of the welded joints were analyzed. The results show that the thermal influence of welding is divided into the coarse-grained and fine-grained regions. The high-temperature area of the coarse-grained region has a long residence time, the α phase can be completely transformed into the high-temperature β phase, the β phase transform into the α' phase at the beginning of cooling, forming a martensitic structure; the heating temperature of the fine-grained region is not high enough to completely transform the α phase into the high-temperature β phase, and the transition from the β phase to the α' phase at the beginning of cooling is incomplete, resulting the formation of α and α' phases. The tensile strength of the swing welded joint is 985 MPa, and the impact energy is 42.6 J. The ultimate fatigue strength of the swing welded joint is 464 MPa, and the fatigue cracks originate from internal pores, and the fatigue crack propagation zone is mainly ductile fatigue cracks, and a large number of dimples appear in the instantaneous fracture zone.