Effect of Scanning Strategy on Variant Selection in Additively Manufactured Ti-6Al-4V  

P.L. Stephenson, N. Haghdadi, R. DeMott, X. Liao, S.P. Ringer, S. Primig
Additive Manufacturing 36 (2020) 101581.

Additive manufacturing (AM) of the Ti-6Al-4V alloy is increasingly popular for making complex shaped parts, especially for biomedical and aerospace applications. The widespread commercial adoption of AM parts is driving increasing interest in a more in-depth understanding of the links between AM processing and the resultant microstructures so as to consistently fabricate products with desired properties.

Ti-6Al-4V undergoes a complex microstructural evolution during the AM process including a transformation from the body-centred cubic β phase to the hexagonally close-packed α phase (or martensitic α’) after solidification. This phase transformation is usually governed by the Burgers orientation relationship. Crystal symmetry allows for twelve possible α orientation variants to transform from one prior β orientation. It has been widely observed that the selection of these twelve crystallographic variants is not always random. This phenomenon is known as variant selection and strongly influences the microstructural and mechanical properties.

Considering the unique thermal and stress gyrations during AM, it is critical to determine the nature of any variant selection and the underlying mechanisms during this processing, and establishing these links is the focus of this work. This is the subject of this study. Our detailed focus is on exploring variant selection in electron beam powder bed fusion printed Ti-6Al-4V through the dominant mechanisms of interfacial energy minimization at β grain boundaries and strain minimization during phase transformation. Our study shows different variant selection mechanisms are operational throughout the AM builds. Moreover, it is possible to control variant selectin through altering AM operating parameters for example by employing different electron-beam scanning strategies.