Anisotropy of Plasticity in Ti-6al-4v Alloy Processed by Electron Beam Direct Energy Deposition

Abstract

Additive manufactured titanium alloys are generally characterized by columnar grain structures along the vertical direction (VD), thereby causing a relative lower plasticity in the horizontal direction (HD) than that in the VD direction. However, herein we report that such columnar grain structures in the Ti-6Al-4V alloy produced by electron beam direct energy deposition (EB DED) can induce a superior uniform elongation in the HD direction than that in the other loading directions (22.5°,45°, 67.5° and VD directions). Besides the orientation of α laths with a favorable soft and hard match when the loading direction is along the HD, such anisotropic plasticity also can be attributed to the columnar grain structures which can also effectively prevent dislocation slip and suppress the necking of the HD sample, thereby triggering a greater work-harding rate and strain hardening exponent in the stage of uniform deformation, and increasing the uniform elongation in the HD. The greater total elongation in the VD is mainly reflected in its higher non-uniform elongation, rather than uniform elongation, which is mainly caused by the α laths possessing easily activated basal slip system inducing strain softening. As for the 22.5° and 45° samples, most of the α laths under such loading directions present hard orientation, thereby leading to both lower non-uniform elongation and total elongation. The above information offers a deeper understanding about anisotropic plasticity in additive manufactured titanium alloys, thereby providing a theoretical basis for optimizing the consistency of mechanical properties.

Keywords: Electron beam direct energy deposition, Anisotropy, uniform elongation, columnar morphology, Ti-6Al-4V alloy