Thermokinetics driven microstructural evolution during laser-based additive manufacturing of γ-TiAl alloy

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This work investigates effects of thermokinetics on evolution of microstructure in additively manufactured Ti4822 alloy fabricated with identical processing parameters under intrinsically different thermokinetic conditions associated with heights of 4 mm and 10 mm with a same base cross-sectional area. Despite similar printing conditions, distinctly different microstructures were observed due the different thermokinetics experienced by each one of them. While 4 mm component possessed a fine scale lamellar γ+α 2 microstructure, the 10 mm component generated coarsened γ grains with spheroidized α 2 pockets. A component-scale thermal model was employed to explain the thermokinetics driven phase transformations. The difference in microstructure is attributed to thermal histories experienced during fabrication by components of different volumes. Specifically, rapid cooling from above the α-transus temperature promoted fine γ-lath formation in the 4 mm component, whereas slower cooling through the γ+α phase field in the 10 mm component resulted in coarsened γ grains. The nanoindentation based hardness and elastic modulus correlates well with the microstructural changes observed in the fabricated components. These findings offer valuable insights into tailoring microstructures by exploiting the novel thermokinetics intrinsic to additive manufacturing processes.