Grain Morphology Evolution of TiAl Alloy During Electron Beam Powder Bed Fusion

Electron beam powder bed fusion (PBF-EB) of TiAl alloys exhibits different grain morphologies: equiaxed grains and columnar grains. However, the rule for controlling grain morphologies has not been found. In addition, the grain morphology is closely related to thermal gradient and growth velocity during PBF-EB. Here, thermal gradient and growth velocity are controlled by preheating temperature, beam current, and scanning speed. Based on the numerical simulation, the relationship between solidification parameters (thermal gradient and growth velocity) and process parameters (preheating temperature, beam current, and scanning speed) has been revealed. Meanwhile, the grain morphology of the topmost region of the PBF-EB-built Ti-48Al-2Cr-2Nb alloy can be controlled. In addition, the columnar-to-equiaxed transition is found in the PBF-EB-built Ti-48Al-2Cr-2Nb alloy. This is attributed to the PBF-EB technology belonging to the layer-by-layer stacking process, and the stacked layer reduces the thermal gradient and increases growth velocity in the remelting region. These findings will contribute to understanding the grain morphology evolution of TiAl alloys during PBF-EB, providing the principle for controlling the grain morphology of PBF-EB-built TiAl alloys.