Laser-directed energy deposition of in-situ titanium-matrix coatings with a Ti-B4C cored wire

To improve the wear resistance of TC4 alloy, laser-directed energy deposition combined with in-situ synthesis method was developed to fabricate titanium carbides and titanium (di)borides reinforced titanium matrix composite coatings, utilizing a Ti-B4C cored wire as feedstocks. The processing characteristics of typical metal transfer modes during wire deposition process were investigated. A process map was established to reveal the correspondence between the operating parameters and the wire transfer behaviors including melt spreading transfer, slag column transfer and wire inserting transfer. The deposited coatings comprised of the Ti matrix and the in-situ synthesized reinforcing phases of TiB2, TiB and TiC, with the majority of the reinforcing phases combining into a sort of intergrowth structures. In this study, the formation mechanism of the intergrowth structures as well as the microstructure evolution of the coatings produced with different transfer modes were systematically analyzed. The deposition with slag column transfer provided an appropriate mass flux matching with the laser energy, forming a coating with dense microstructures dominated by the TiB2/TiB/TiC intergrowth structures, while the other two modes resulted in the limited volume fraction of reinforcing phases or un-melted defects in the coatings. The TiB2/TiB/TiC and TiB/TiC intergrowth structures provided a large load-bearing capacity, leading to the enhancement of the wear resistance. The coating fabricated under the slag column transfer mode displayed the best wear resistance which was 11.71 times higher than that of TC4 substrate, and the wear mechanism was slight adhesive wear.