An investigation on the effect of different multi-step heat treatments on the microstructure, texture and mechanical properties of the DED-produced Ti-6Al-4V alloy

This work deals with the effect of different heat treatments on directed energy deposition (DED)-produced Ti-6Al-4V samples. Annealing treatments at 1050 degrees C followed by different cooling rates were conducted to allow a complete recrystallization of the microstructure and remove the columnar prior-beta grains, thus increasing the overall isotropy of the material. An agine treatment at 540 degrees C was also performed for further microstructural stabilization. The microstructures, textures and mechanical properties were then assessed. Due to the heat treatments, greatly differing microstructures were achieved in an equiaxed grain morphology. However, a "grain memory" effect was detected which resulted in the grains size increasing along the height of the samples. This effect was correlated to the intrinsic prior-beta grain width variation along Z on the as-printed specimens, typical of the DED technology. Electron backscatter diffraction analyses proved that the intensity of the preferential directions increased after the heat treatments, likely due to the crystallographic variant selection mechanisms taking place when the samples cool down from the annealing temperature. This effect is also influenced by the significant difference in terms of prior-beta grains sizes between the heat-treated and the as-printed specimens. To sum up, a complete homogenization of the material via heat treatment above the beta-transus temperature proved to be challenging. In fact, the data suggest that the intrinsic texture-related anisotropy granted by the manufacturing process is very difficult to be eliminated.