Microstructure and properties of underwater laser welding of TC4 titanium alloy

Underwater local dry laser welding of TC4 alloy was carried out through a double-side gas-shielding nozzle, and the heat input and defocus distance were optimized by analysing the weld quality. With increasing heat input, the fusion zone (FZ) and heat-affected zone (HAZ) expanded, while the amount of vapour increased, resulting in an increase in the degree of oxidization of the underwater joint; the mechanical properties improved initially and subsequently deteriorated, influenced by bead oxidization and welding defects. With increasing defocus distance, the width of the FZ increased gradually, whilst the width of the HAZ remained largely unchanged. Excellent tensile strength and impact toughness were obtained when the laser beam was focused on the upper surface. In the FZ of the underwater laser welding (ULW) joint, the microstructure comprised full martensite α′ with an acicular morphology, whereas a small amount of grain boundary α-phase appeared in the FZ of the in-air joint, and the thickness of the α′ lamellae decreased compared to that of the in-air joint due to the higher cooling rate of the ULW joint. The impact toughness and tensile strength of the underwater joint reached ˜90% of the values for the in-air joint, and a mixed fracture mode was operative in the fracture surfaces.