Grain refinement and columnar-to-equiaxed transition of Ti6Al4V during additive manufacturing via different laser oscillations

Conventional additive manufacturing produces coarse columnar grains, which affect the mechanical properties of additively manufactured titanium alloys. This study developed a novel integrated additive manufacturing technology termed oscillation laser melting deposition, including linear, circular, 8-shape, and infinite, was developed to modify the microstructure and improve the mechanical properties of Ti6Al4V. The results showed that significant grain refinement and columnar-to-equiaxed transition (CET) can be induced by laser oscillation. The prior β grain size of the sample with infinite laser oscillation decreased by 54.24% in the single-track zone and by 42.55% in the overlap remelting zone. The ultimate tensile strength of the sample with infinite laser oscillation increased by 16.95% and 32.37% in the parallel and vertical directions, and the elongation also increased by 83.60% and 13.77%, respectively. The anisotropy of (10-10) and (11-22) was also significantly eliminated. The temperature variation and thermal field evolution were also investigated, and the complex oscillation changed the fluid flow velocity orientation, reduced the temperature gradient, and promoted the nucleation of equiaxed grains. In addition, the strengthening mechanisms of the different laser oscillations were revealed. Therefore, the oscillation laser melting deposition technology can become a new approach for overcoming the key bottlenecks of additive manufacturing.