Formation mechanism of massive phase in the heat affected zone of Ti-6Al-4V fabricated by forging-additive hybrid manufacturing

The heat affected zone (HAZ) of Ti-6Al-4V forging-additive hybrid manufacturing parts is always featured with large-sized massive phase (α_m) embedded in the previous equiaxed α region. However, the formation mechanism of such a unique microstructure remains unclear. To reveal the formation mechanism of massive phase in HAZ, we investigated both the microstructures of HAZs of single-pass and multi-pass samples, and also carried out a finite element (FE) simulation to obtain the temperature-field evolution of the whole process. Our results show that, in the single-pass sample, patchy α_m forms in previous equiaxed α region of HAZ, while needle-like martensite α' forms in previous lamellar α regions close to the equiaxed α region. Electron probe microanalysis (EPMA) results further show that, the high temperature β phase transformed from previous equiaxed α can inherit the initial composition due to the large size of previous equiaxed α and the insufficient diffusion during rapid heating, while the β phase transformed from previous lamellar α possesses the bulk average composition due to the small distance between neighbor lamella. It is this difference in composition that resulting in the formation of α_m and α' in the previous equiaxed α region and the previous lamellar α region, respectively, during subsequent cooling. In the following thermal cycles of multi-pass deposition, some α_m grains remain during the next heat cycle and finally evolve into a few large-sized α_m within the previous equiaxed α region, as observed in the multi-pass sample. These findings are helpful for understanding and controlling the microstructural evolution of HAZ in hybrid manufactured Ti alloys.