Microstructure and Deformation Behavior of Metastable Ti-Mo Alloys Fabricated by Powder Feeding Strategies in Laser Additive Manufacturing

Additive manufacturing (AM) of binary alloys with dissimilar powder characteristics poses significant challenges in achieving compositional homogeneity and microstructural uniformity. These challenges are particularly critical in the fabrication of metastable β-Ti alloys, where compositional deviations at macro- and microstructural scales directly influence phase stability and deformation mechanisms. This study systematically evaluates the impact of powder feeding methodologies- mechanical powder mixing (PM) and double-hopper (DH) delivery-on the microstructure, mechanical properties, and deformation behaviour of Ti-15Mo (wt.%) alloys fabricated via directed energy deposition (DED) at a laser power of 800 W. Microstructural analysis revealed that the PM approach, utilizing mechanically blended Ti and Mo powders, resulted in heterogeneous phase distribution along the build direction, with localized α-phase precipitation due to Mo segregation with higher hardness (470 ± 30 HV) variation along the print direction. Conversely, the DH method, employing controlled, separate powder feed rates, facilitated uniform elemental dispersion, yielding a predominantly β-phase microstructure with minimal compositional fluctuations which resulted in limited hardness variation of 325 ± 25 HV. Mechanical characterization under compressive loading demonstrated that PM specimens exhibited higher yield strength (850 MPa) compared to DH specimens (450 MPa), attributed to α-phase-induced precipitation strengthening. However, alloys processed via the double hopper (DH) method displayed a 30% enhancement in work-hardening capacity, attributed to the retention of metastability achieved through precise compositional control. Deformation analysis confirmed twinning-induced plasticity (TWIP) as the dominant mechanism in both conditions, with {332} <113> as the predominant twin system but the twin distribution was more uniform in DH samples. The DH approach is particularly suited for systems where parent powders differ significantly in density, as it effectively mitigates elemental segregation. This enables precise microstructural control and predictable mechanical behavior, supporting the use of multi-hopper systems in DED to ensure compositional fidelity and reproducibility in high-performance alloy fabrication.