Effect of manganese on grain morphology and microstructure of Ti-6Al-4V alloy by laser wire deposition

The addition of growth restricting elements in additive manufacturing of Ti and Ti-based alloys can result in the formation of equiaxed structures. However, an excessive incorporation of these elements frequently impairs the mechanical properties. Obtaining the minimum necessary amount of addition is crucial to achieve the transition from columnar to equiaxed microstructure. This ensures that the desired equiaxed structure is obtained while minimizing any potential negative effects on the material's properties. Here manganese was taken as the potential grain refiner in Ti alloy. Then, evolutions of grain morphologies and microstructures as well as the mechanical properties various of Ti-6Al-4V alloy in laser wire deposition additive manufacturing with the growth restriction factors of 26, 32, 39 were explored respectively. And the average hardness of the deposits were 495 HV, 512 HV, and 530 HV, respectively, showing an increase of approximately 15%, 19% and 23%. When the growth restricting factor value reached 39, the columnar prior β grains underwent a complete transformation into equiaxed grains. Mn elements promoted the decomposition of β→α+Mn₂Ti during the multi-pass deposition and the microhardness of the deposits were significantly increased. Additionally, Mn elements were expelled outwards during the β→α transition. Nevertheless, the diffusion rate of Mn elements was low and thereby there was a low growth rate of α phases, thus refining α phases.