The Laser based powder-bed fusion (LPBF) in additive manufacturing has attracted various industries in delivering near-net-shaped products with complex geometries. Titanium alloys, especially Ti-6Al-4V alloy which is a widely biomedical material for load bearing implants. Unfortunately, the high energy laser used in the process is associated with high heating and cooling rates. The prevailing conditions give rise to formation of non-equilibrium phases such as martensite (α′; hcp) which exhibits higher strength, but offer a poor elongation. The post annealing is generally followed that leads to transformation of α′ to α- and β- phases resulting in an improved elongation with compensation in strength levels. The post annealing was done below β-transus temperature of the alloy (995ᵒC). The understanding of underlying phase transformation is very crucial in depicting the bulk mechanical properties. Therefore, the current study aimed to investigate at atomic level with aid of advanced level characterization techniques such as Atom Probe Tomography (APT) and Transmission Kikuchi Diffraction (TKD). The as-printed condition revealed an enrichment of V and Si (β-stabilizers) and depletion of Ti and Al (α-stabilizers) at the interfaces and grain boundaries. While the annealed condition shows a strong enrichment of V and Si (β-stabilizers) at the α/β interfaces. In addition, the STEM-EDX analysis revealed the segregation of precipitates near the α/β interface. It was observed that the phase transformation is accompanied by a significant segregation of solutes as a consequence of element partitioning effect in Ti-6Al-4V alloys. Therefore, this work would provide more insights on the decomposition of α′-phase to an equilibrium α- and β- phases by annealing. Furthermore, a general diffusional phase transformation path during annealing (via decomposition of α′) is clearly revealed through a systematic characterization.

Keywords: LPBF, Ti-6Al-4V, annealing, phase transformation, segregation.