Development of core–shell-structured Ti-(N) powders for additive manufacturing and comparison of tensile properties of the additively manufactured and spark-plasma-sintered Ti-N alloys

Abstract In this study, Ti-(N) powders with a core–shell structure were prepared via a nitriding process of titanium (Ti) powders for metal additive manufacturing (AM). Nitriding of spherical Ti powders (D50 = 130 µm and D50 = 63 µm) was carried out at different temperatures from 873 K to 1373 K for 10 min. The nitriding process enabled the manufacture of a core–shell-structured Ti-(N) powder, where an N-enriched shell surrounds a lower N core. The thickness of the N-rich Ti shell increases exponentially with increasing nitriding temperature. Depending on nitriding temperature, a shell layer consisting of Ti2N and TiN compounds can form. Based on X-ray diffraction (XRD) results, it was identified that the N-rich shell is composed of (i) only Ti(N) solid solution when nitrided below 1023 K; and (ii) Ti(N) solid solution, Ti2N and TiN compounds if nitrided at or above 1023 K. The core–shell-structured Ti-(N) powders were subsequently used to manufacture bulk samples by spark plasma sintering (SPS) and laser metal deposition (LMD) processes for comparison. The microstructure was characterized and discussed. LMD-fabricated Ti-N alloy samples exhibited high tensile strength (965 MPa) with good tensile ductility (7.6%) due to the homogeneous distribution of N and fine grain size. In contrast, SPS-fabricated Ti-N alloy samples using the same nitrided Ti-N powder showed much lower tensile strength (708 MPa) with essentially no tensile ductility (0.3%) due to the inhomogeneous distribution of N. LMD can allow the fabrication of strong and ductile Ti-N alloys while it is not possible by SPS.