β grain refinement during solidification of Ti-6Al-4V in Wire-Arc Additive Manufacturing (WAAM)

Constructing titanium aerospace parts by near-net-shape processing has the potential to greatly reduce cost and lead time, one method for this is Wire-Arc Additive Manufacturing (WAAM). Conventional WAAM processing with the most common Ti alloy, Ti-6Al-4V, results in solidification by epitaxial growth from previously deposited layers and a structure dominated by columnar β grains which are heavily fibre textured and cm's in scale. In order to prevent these large grains from forming, while maintaining deposition parameters, the solidification conditions were modified by the additions of particles to the melt; either using inoculant, TiN particles, or the solutal growth restrictor, Y, also added as elemental powder that dissolved in the melt. The powder particles were added by adhering them to the deposited tracks to avoid the costs of manufacturing new wires. With TiN inoculants the morphology of β grains was completely modified to equiaxed grains averaging 300 μm in diameter. Y additions narrowed the columnar grains from 1-2mm to 100-300 μm. Y also induced a change to equiaxed grains, late in solidification, in the region which was remelted by subsequent deposition. However, Yttria particles were found to have formed interdendritically with an interconnected skeletal morphology. High-resolution EBSD analysis showed both TiN and yttria particles exhibit specific orientation relationships with the solidified β grains, which were confirmed experimentally.