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In this study, a Ti-6Al-4V/Ni-Ti functionally graded material was successfully prepared by multi-wire arc additive manufacturing. Its microstructural, electrochemical, and mechanical properties were studied. The results show that the microstructure is transformed from an α-Ti + β-Ti basket structure to an NiTi columnar crystal. With an increase in Ni content, Icorr decreases from 0.203 ± 0.017 μA/cm2 to 0.145 ± 0.003 μA/cm2, and Ecorr increases from −409 ± 5 mV to −259 ± 3 mV. The microhardness increases from 335 ± 7 HV0.2 to 615 ± 17 HV0.2 in the gradient region and then decreases to 250 ± 8 HV0.2. The ultimate compressive strength of the gradient region is 1253 ± 62 MPa. [ABSTRACT FROM AUTHOR]

In order to explore the feasibility of underwater wet laser welding of the TC4 titanium alloy, research on the underwater laser self-fusion welding process was carried out. The weld structure and mechanical properties in both the air environment and the underwater environment were compared and analyzed. The results show that increasing the laser power and reducing the welding speed are beneficial to obtain a larger water depth threshold. Off-focus amount has little effect on water depth threshold; when the laser power is 3000 W and the welding speed is 5 mm/s, and the water depth exceeds 7 mm, a continuous weld cannot be formed. Compared with welding in the air, underwater welding has narrower weld width, smaller heat affected zone and finer crystal grains. The weld structure is mainly composed of α′ martensite and secondary acicular α′ phase, it is distributed in a net basket shape and the grain size at the top of the weld is finer. The hardness of the weld center is above 600 HV0.1, and the residual stress of the underwater welding weld is approximately symmetrically distributed. There is a large tensile stress along the welding direction at the weld, reaching 458 MPa. The larger residual tensile stress leads to the decrease of weld tensile strength, the tensile strength and elongation of the middle sample are only 52% and 77% of the base metal. Furthermore, the fracture mode is typical brittle fracture. [ABSTRACT FROM AUTHOR]