Fabrication and study of innovative Ni-added Ti-6Al-4V through directed energy deposition

In general, Ti-6Al-4V alloy components fabricated by wire arc additive manufacturing (WAAM) have columnar grains grown across layers, which reduces the mechanical properties of the obtained components, and severely limits its application. In this study, the novel Ti-6Al-4V-7.5Ni components were fabricated for the first time using Ti-6Al-4V and pure Ni welding wires as feedstock and in-situ alloying by dual-wire arc additive manufacturing (D-WAAM) through controlled wire feeding speed of wire. After fabrication, the effects of designed Ni addition on microstructure and mechanical properties of Ti-6Al-4V were studied. The results show that appropriate Ni promotes the transformation from columnar to equiaxed grains. The prior beta grains and alpha laths were significantly refined. Nano-scale Ti2Ni phases with a scale range of 20-120 nm were formed between alpha-Ti and beta-Ti. The solid solution strengthening caused by the Ni, the precipitation strengthening caused by Ti2Ni and grain refinement jointly promote the improvement of the strength of Ti-6Al-4V-7.5Ni components. The hardness of Ti-6Al-4V-7.5Ni was 457 +/- 6 HV0.2, which was 38.9% higher than that of Ti-6Al-4V. Besides, the average ultimate tensile strength of Ti-6Al-4V-7.5Ni was 1012 +/- 38 MPa, which was 26.5% higher than that of Ti-6Al-4V.