Microstructure evolution and tribological properties of (TiB+TiC)/Ti–6Al–4V composites fabricated via in situ laser-directed energy deposition of wire and powders in an underwater environment

Titanium-matrix composite-reinforced restored layers are used to repair cracks and dents on titanium alloy shells to impart the deposition layer with the same properties as the base metal. However, there are few studies on their underwater applications. Underwater laser-directed energy deposition is a rapidly-developing technique that shows great application potential for underwater restoration projects. This paper reports the effect of the Ti/(B+C) ratio on the microstructure and mechanical properties of a layer restored underwater using CWPLDED. The results showed that the ceramic-reinforced phase transitioned from a quasi-continuous mesh distribution to a multi-point distribution as the Ti/(B+C) ratio changed. The matrix grain was refined because the reinforcing phase promoted the non-spontaneous nucleation of β-Ti and α-Ti and limited grain growth during both solidification and solid-state phase transformation. The composite layer restored underwater had a maximum ultimate tensile strength of 1231.18 ± 6.37 MPa and a maximum yield strength of 1158.11 ± 8.71 MPa. When Ti: B: C = 3: 4: 4, the improved wear resistance of the restored layer was attributed to the higher microhardness caused by an increased precipitated phase content and Ti matrix grain refinement. When Ti: B: C = 3: 4: 1, the restored layer exhibited better wear resistance due to the excellent spatial configuration of the reinforced phase. The intrinsic relationship between the (B+C) content, reinforced phase space, and dry sliding wear performance was revealed. This provides a design window for preparing (TiB+TiC)/Ti64 composite restored layers with excellent comprehensive performance by CWPLDED and offers new insights for repairing titanium alloys underwater.