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 (αm) 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 αm 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 αm 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 αm grains remain during the next heat cycle and finally evolve into a few large-sized αm 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.