Microstructural evolution and impact properties of vacuum laser welded near-alpha Ti-6Al-3Nb-2Zr-1Mo titanium alloy: Effect of base metal microstructure

The advancement of high-power laser welding has significantly improved the penetration capability for welding titanium and its alloys. In this study, four 25 mm-thick near-alpha Ti-6Al-3Nb-2Zr-1Mo titanium alloy plates with two distinct base metal microstructures (equiaxed and bimodal) were welded using vacuum laser beam welding. The microstructure evolution and impact properties of the laser welded joints were investigated. The results show that the impact toughness of the weld zone, heat affected zone, and base metal in joints with bimodal base metal is higher than that in joints with equiaxed base metal. The lowest impact toughness for the joints with bimodal base metal is in the weld zone, which is 41.63 J, owing to the formation of abundant acicular α′ martensite. For the joints with equiaxed base metal, the impact toughness of the weld zone is comparable with that of the base metal, which is attributed to a more tortuous crack propagation path. The microstructure of the base metal significantly influences the microstructure evolution of the joints. In joints with bimodal base metal, the grain size of the primary β in the weld zone is larger due to fewer nucleation sites in the heat affected zone near the fusion line. This results in the formation of thicker acicular α’ martensite, which enhances twin generation and dislocation movement, thus increasing the absorbed impact energy and impact toughness.