Microstructure and metastable phase formation of Mo-added Ti6Al4V alloys under laser micro-alloying

Understanding the influence of alloying elements on the solidification stage of Ti-based alloys during additive manufacturing (AM) is crucial for subsequent microstructural control and composition optimization. In this work, a laser micro-alloying (LMA) technique is employed to retain the initial solidification microstructure of the alloys. The laser micro-alloying parameters are first optimized, after which Ti6Al4V-xMo (x = 0–8 wt%) samples are fabricated using a selected processing parameter (2400 W, 0.2 s) to investigate the influence of Mo content on the microstructure of Ti6Al4V. The results indicate that as the Mo content increases, the alloy's microstructure evolves from a uniform distribution to a distinctive cellular morphology, which is attributed to the enrichment of Mo in the cell interior. The transition from the β phase to the α/α′ phases is inhibited, ultimately resulting in a microstructure predominantly composed of the β phase. Importantly, the increase in Mo content induces a higher proportion of the α″ phase, which is due to the larger lattice distortion that makes atomic migration more difficult and favors the transformation of the bcc-β phase into the orthorhombic-α″ phase. Furthermore, the microhardness of the alloy exhibits a trend of initial increase followed by a decrease, with heterogeneous phases contributing to uneven hardness distribution. This work provides theoretical insights into the microstructure formation and regulation of Mo-added Ti6Al4V alloys in laser additive manufacturing.