An in situ study on the role of heterogeneous microstructure on anisotropic tensile deformation behaviour in an additive manufactured Ti6Al4V alloy

Additive manufacturing (AM) processes are known to produce anisotropic microstructures and thereby, anisotropic mechanical properties. However, fundamental understanding on the anisotropic mechanical behaviour of AM-built Ti6Al4V is limited, particularly for high-deposition rate wire feed directed energy deposition AM processes. The present study provides insights into the role of anisotropic microstructure and associated texture on the tensile deformation and damage accumulation in wire feed directed energy deposition Ti6Al4V. Materials were deposited using oscillation-pass and parallel-pass build strategies. In situ neutron diffraction studies were performed on samples with tensile loading applied parallel and perpendicular to the built layers. Dissimilar thermal histories experienced in the parallel-pass strategy resulted in thinner columnar β grains and finer transformed microstructure, the latter leading to higher yield strength compared to the oscillation strategy. The presence of strong columnar β fibre textures in both build strategies led to anisotropic deformation. When loaded perpendicular to the columnar grains, elastic strain accumulation is more crystallographically homogeneous and includes strain accumulation between basal, prismatic, and pyramidal planes in both build strategies. Conversely, when loaded parallel to the columnar β fibre texture, the majority of the pyramidal orientations preferentially aligned along the loading axis and were subjected to significant elastic strains. Similar anisotropy was inferred under plastic deformation where tensile strain appeared to be accommodated primarily by prismatic slip but was not detected when loaded parallel to the columnar grains.