In-situ grain structure control in directed energy deposition of Ti6Al4V

Abstract

In metal additive manufacturing (AM), long columnar grains along the building direction due to highly directional thermal gradients often lead to a strong texture and severe anisotropy in mechanical properties of the deposits, which significantly impair part qualification and targeted applications. Here, a novel depositing strategy by periodically alternating processing parameters is designed to partially preserve the equiaxed grains resulted from the columnar-to-equiaxed transition at the top of each deposited layer and, at the same time, to interrupt the epitaxial growth of columnar grains in AM titanium alloy. With the help of the competitive growth of the new grains and the potential coarsening effect during subsequent thermal-cycles, a microstructure of full equiaxed prior-β grains in Ti6Al4V fabricated by laser directed energy deposition (DED) is finally obtained without either using any auxiliary equipment or adjusting alloy chemistry like previous researches. This further contributes to a superior mechanical property and a remarkable reduction on both the crystallographic textures and the property anisotropy.