Microstructure, microsegregation and mechanical properties of directed energy deposited Ti-32Mo titanium alloy

Abstract: The intrinsic directional positive thermal gradient of additive manufacturing usually leads to columnar grains crossing several layers and corresponding anisotropy in mechanical properties. However, β titanium alloys usually contain high concentration of β stabilizing elements, which may lead to different microstructure from other titanium alloys by affecting the solidification and solid-state phase transformation process. This research investigated a β Ti-32Mo titanium alloy fabricated by directed energy deposition to analyze its microstructure, microsegragation, mechanical properties and grain structure formation mechanisms in detail. It is found that the as-built alloy, which contains high Mo concentration, consists of near columnar grains in the top layer, however for the rest of the alloy, the grains transform into a near equiaxed structure. Both the near columnar grains and near equiaxed grains show typical dendrite segregation, besides, the microsegregation in deposited layers attenuates gradually under subsequent thermal cycles. There is no anisotropy in mechanical properties in the as built alloy, which is determined by the untextured near equiaxed structure. The near columnar grains can be explained by the constitutional supercooling promoted by high Mo concentration, which encouraged nucleation inside the molten pool and suppressed epitaxial growth from former deposited layer. On the other hand, the near equiaxed grains were transformed from the near columnar grains during the layer by layer manufacturing process, which is first found in titanium alloys fabricated by additive manufacturing. Furthermore, the equiaxed grains formation mechanism was analyzed based on experimental results and a hypothetical mechanism was proposed. Graphical abstract: [Figure not available: see fulltext.]