In-situ pre-alloying of Ti6Al4V composite coating fabricated by laser melting deposition: Synergistic improvement of microstructure and property

For laser melting deposited (LMD) Ti-based coatings, rapid solidification of titanium melt promotes the growth of columnar grains aligned along the path of maximum heat dissipation, resulting in the dominant 〈001〉 texture of β phase and the deterioration of surface strength and fracture toughness. Pre-alloying of Ti powders enables in-situ grown nano-reinforcements which can significantly influence the solidification, microstructure and properties of LMD-composites. Here, comparative studies of mixed Ti6Al4V + TiB₂ powder and Ti6Al4V-TiBw composite powder on the columnar-to-equiaxed transition and strengthening efficiency for Ti6Al4V-TiBw composite coatings fabricated by LMD were performed. By a combination of microstructural characterizations, at optimal printing condition, the microstructure of different coatings fabricated using Ti6Al4V, mixed Ti6Al4V + TiB₂ powder and Ti6Al4V-TiBw composite powder displayed a clear transition from coarse columnar grain, semi-columnar, to equiaxed grains with a small portion of dendrites, respectively. With strongly pinning effect of finer TiBw, the ultimate tensile strength of Ti6Al4V-TiBw composite powder coatings increases by ∼100 MPa relative to mixed Ti6Al4V + TiB₂ powder coatings, accompanied with a 2% loss of plasticity. According to fractography analysis, microcracks in the mixed Ti6Al4V + TiB₂ powder coating initiate and propagate through TiBw clusters and alloy matrix, while microcracks in the Ti6Al4V-TiBw composite powder coating initiate at the dendritic structure. More importantly, the underlying mechanism of TiBw introduction strategy on microstructural evolution of composite coating has been discussed based on the phase diagram of Ti-B and thermodynamics in terms of heterogenous-phase-related convection and undercooling in rapid solidification process.