Effect of scanning strategy on variant selection in additively manufactured Ti-6Al-4V

Abstract

Abstract Additive manufacturing of the Ti-6Al-4 V alloy is increasingly popular for making complex shaped parts. This alloy undergoes a transformation from the body-centred cubic β phase to the hexagonally close-packed α phase following solidification. There are currently gaps in the understanding of relationships between the processing conditions and the final material microstructure. In particular, the role of α variant selection mechanisms within additively manufactured parts is not well enough understood to assure product quality when varying processing parameters such as the scanning strategy. In this study, Ti-6Al-4 V samples were fabricated via the electron beam powder fusion (E-PBF) process under three different scanning strategies (linear scan, random and Dehoff point fills). Electron back-scatter diffraction revealed that the scanning strategy employed directly affects which variant selection mechanism dominates during the β→α transformation. Faster cooling rates in the linear scan produce microstructures which are influenced heavily by self-accommodation, while the microstructure of the slower cooling random fill strategy is dominated more by prior β grain boundary effects. This, in turn, dictates the microstructural evolution of the material, leading to the prevalence of different microstructural features such as macrozones or intragranular 3-variant clustering. These insights will enable optimisation of processing strategies in additive manufacturing to produce tailored product microstructures.