Possible globularization mechanism in LPBF additively manufactured Ti-6Al-4 V alloys

Abstract

Owing to the great technical significance such as better balance of strength and toughness, the conversion of lamellar to globular microstructure finds its great importance. In order to obtain globular microstructure, it is necessary to shed light on the globularization mechanisms. This paper attempts a systematic study on globularization of Laser Powder Bed Fusion (LPBF) processed Ti-6Al-4 V alloy including microstructure evolution and globularization mechanism. Annealin g was performed in the α + β field, in attempting to transform the acicular α’-martensite into globular microstructure. The required driving force for static globularization during annealing is achieved with the presence of defects such as dislocations and twins which is inherently present within α’-martensite. The static globularization fraction increases with increasing annealing temperature. During annealing, the static globularization involved the evolution of sub-boundaries, phase transformation, and coarsening of α-phase. Furthermore, the possible mechanism was explored through various characterization techniques such as SEM, EBSD and TEM analysis. Globularization mechanisms such as boundary splitting, direct cylinderization, edge spheroidization, termination migration, and Ostwald ripening have been observed. Most of the mechanism activates at higher annealing temperature close to β-transus temperature. The globularization has not taken place homogeneously in the microstructure causing globularization heterogeneity. The inability of achieving a full globularization in LPBF additively manufactured Ti-6Al-4 V alloy is due to its characteristic α-lamellae microstructure such as aspect ratio and morphology.