Effect of the laser-wire relative position on the wire-based laser-directed energy deposition of in-situ synthesis Ti/TiB/TiC composite coatings

Laser-directed energy deposition (LDED) with a Ti-B4C cored wire was utilized to synthesize the TiB- and TiC-reinforced titanium matrix composite coatings for enhancing the tribological performance of Ti alloys. The wire transfer behaviors, microstructural characteristics and wear performance of the coatings were investigated at two different laser-wire relative positions. The wire deposition better adapted to varying processing parameters when the wire tip was placed in the anterior portion of the laser-irradiated region, which resulted in the continuous bridging transfer mode. This enabled favorable process stability, track formability, and microstructure homogenization. The representative LDEDed coatings were dominated by TiB/TiC composite structures characterized by the embedding of TiC dendrites into TiB hollow prisms. The formation of TiB/TiC composite structures was related with the heterogeneous nucleation on the coherent (010)TiB//(101)TiC interface, of which the two-dimensional lattice mismatch was only 5.500%. A well-defined three-phase orientation relationship between the α-Ti matrix and reinforcements of TiB and TiC was identified as [112¯1]α-Ti//[01¯0]TiB//[101]TiC. This relationship improved interface stability and restrained the interfacial debonding under an imposed load. The typical LDEDed coating produced by the continuous bridging transfer mode possessed the desired combination of high hardness and low wear rate, which was primary attributable to the direct load-bearing effect of the tightly combined TiB/TiC intergrowth structures and the indirect load-sharing effect of the TiBw/TiCp eutectic phases.