Additive friction stir deposition of a Ti-6Al-4 V alloy: Microstructure and texture

Abstract

The microstructure and crystallographic texture of a Ti-6Al-4 V alloy produced through additive friction stir deposition were thoroughly investigated in the current study. The deposition was largely performed above the β transus temperature (T_β), leading to β parent grain refinement, which subsequently transformed to fine colonies of α + β lamellar structure following the Burgers orientation relationship upon cooling. The α overall texture exhibited multiple components, closely inherited from the parent β phase subjected to severe shear deformation during deposition above T_β. The deformation of β led to the variant selection during subsequent phase transformation upon cooling, promoting V₁, V₇ and V₄ variants. The variant selection persisted throughout the process, though each deposition layer experienced multiple transformation cycles through subsequent depositions, due to the texture memory effect. This enhanced the population of 60°/$\left(11\overline{2}0\right)$ and 63.3°/$\left(\overline{10}55\overline{3}\right)$ boundaries at the expense of other α intervariant boundaries. The distribution of α boundary plane was strongly anisotropic, mostly terminated on the prismatic plane orientations (i.e., $\left(\mathrm{hki}0\right)$, ranging from $\left(4\overline{1}\overline{3}0\right)$ and $\left(5\overline{2}\overline{3}0\right)$). This was due to the constraints associated with the phase transformation crystallography rather than low energy arrangement. The high temperature parent β grain boundaries largely terminated at {111} plane orientation, rather than low energy {110} close packed plane, due to the promotion of 〈111〉 fibre in β overall texture. This restricted the nucleation of grain boundary α having Burgers OR with either side of parent β boundaries. This ultimately led to the simultaneous enhancement in the ductility (∼ 18 % total elongation) and strength (∼ 1040 UTS) of material along both hatch and building directions.