Mechanical anisotropy and its evolution with powder reuse in Electron Beam Melting AM of Ti6Al4V

Powder reuse is recognized as a key to industrialization of metal additive manufacturing (AM), which necessitates that changes in the structural behavior of metal with reuse are clearly understood. In this investigation, the mechanical properties of Ti6Al4V resulting from Electron Beam Melting (EBM) were evaluated as a function of powder reuse over 30 build cycles. The metal was characterized in the horizontal and vertical build orientations under uniaxial tension to failure in the as-built and machined conditions. Results showed that there was an increase in strength and decrease in ductility of the metal with powder reuse, resulting from rising oxygen content of the powder. The elongation at failure and tensile toughness of the metal exhibited anisotropy, which increased substantially with powder reuse. For the horizontal orientation, the elongation at failure decreased by over 60% in the 30 builds, which was one and a half times greater reduction than that for the vertical orientation. The anisotropy in ductility with reuse is suspected to result from changes in microstructure with oxygen content, specifically changes in the grain boundary alpha phase. Additionally, the powder recovery system (PRS) was identified as a contributor to powder oxidation.