Grain Scale Investigation of the Mechanical Anisotropic Behavior of Electron Beam Powder Bed Additively Manufactured Ti6Al4V Parts

Abstract

Numerous factors, including variable grain structures and different inherent defects, impact the mechanical behavior of Ti6Al4V parts fabricated using metal Additive Manufacturing (AM) processes. This study focuses on an in-depth analysis of how different microstructural features, such as crystallographic texture, grain size, grain boundary misorientation angles, and inherent defects, as byproducts of the electron beam powder bed fusion (EB-PBF) AM process, impact its anisotropic mechanical behavior. Standard tensile testing, conducted on samples produced at different orientations relative to the build table, showed significant anisotropy in elastic-plastic constitutive characteristics. Furthermore, X-ray computed tomography (CT) and electron back-scattered diffraction (EBSD) anal-yses were conducted on as-built samples to assess the effects of inherent defects and microstructural anomalies on such behavior. The samples arranged vertically and parallel to build direction had an average porosity of 0.05%, while the horizontally built samples, which were perpendicular to the build direction, had an average porosity of 0.17%. Moreover, the vertical samples showed larger grain sizes, with an average of 6.6 µm, wider α lath sizes, a lower average misorientation angle, and subsequently lower strength values than the other two horizontal samples. Among the three strong preferred grain orientations of the α phases, <1 1 2 1> and <1 1 2 0> were dominant in the horizontally built samples, whereas the <0 0 0 1> orientation was dominant in vertically built samples. Finally, larger grain sizes and higher beta-phase volume ratios were observed in the areas located at distances further away from the build plate. This was possibly due to the change in thermal gradients, cooling rates, and some thermal annealing phenomena resultant from the elevated build chamber temperature.