β Grain refinement by yttrium addition in Ti-6Al-4V Wire-Arc Additive Manufacturing

Wire-Arc Additive Manufacturing (WAAM) of large near-net-shape titanium parts has the potential to reduce costs in aerospace applications. However, with titanium alloys, such as Ti-6Al-4V, conventional WAAM processing conditions generally result in epitaxial solidification from the melt pool fusion boundary, which over many layers can generate coarse cm-scale, <001> //ND fibre textured, columnar beta grain structures within the deposited metal. The mechanical anisotropy caused by this coarse primary grain structure cannot be eliminated by subsequent solid-state phase transformations. In order to attempt to refine the size of the solidified beta-grains and reduce their strong texture, the growth restriction efficiency of low addition levels of the strongly partitioning element (k = 0.1) yttrium (Y) has been investigated. Less than 0.8 wt.% Y was sufficient to reduce the widths of the solidified columnar beta grains from 1 to 2 mm to 100-300 mu m. Y was also found to induce a columnar-to-equiaxed transition (CET) in the latter stages of melt pool solidification, which benefits from a lower liquid thermal gradient and higher solidification velocity. Inter-dendritic segregation of Y was also found to be significant and oxygen scavenging led to the formation of Y2O3 particles in the inter-dendritic liquid, with a previously unreported irregular eutectic morphology. High resolution EBSD analysis showed these particles exhibited specific orientation relationships with the solidified beta grains, which were confirmed experimentally. (C) 2021 Elsevier B.V. All rights reserved.

Automated summary

Adding low levels of yttrium (Y) can reduce the width of solidified columnar beta grains in large titanium parts and induce a columnar-to-equiaxed transition (CET) in the latter stages of melt pool solidification, leading to reduced texture and increased solidification velocity.