Evolution of microstructure and mechanical property of Ti-47Al-2Cr-2Nb intermetallic alloy by laser direct energy deposition: From a single-track, thin-wall to bulk

Abstract

The complex thermal history during the laser direct energy deposition (L-DED) can result in the inhomogeneous microstructure of the Ti-47Al-2Cr-2Nb (TiAl) alloy. This work investigates the grain structure and metastable microstructure evolution of L-DED TiAl samples with different deposit shapes of single-track, thin-wall, and bulk. Numerical simulations were performed for all samples to reveal the influence of thermal history on microstructural evolution. Among the three samples, the single-track sample has unique dendritic and cellular structures due to the highest cooling rate. Meanwhile, incompletely precipitated lamellar γ was found at the boundary and inside the α2 grains. And residual B2 phase is also present in the single-track sample. As for the thin-wall and bulk samples, they exhibit fully lamellar structure, and contain only γ and α2 phases at room temperature. During the L-DED process, the temperature gradient and cooling rate inside both samples decreased with the elevation of the deposition height. However, due to the different thermal histories, the temperature gradient and cooling rate do not change in the same way. This leads to a large difference in grain structure and metastable microstructure between the thin-wall and bulk samples. Also, these changes affect the mechanical properties of TiAl alloys. In summary, this study can provide potential guidance for a tailored microstructure in L-DED TiAl alloy.