Microstructure evolution and tensile property of TiAl alloy using continuous direct energy deposition technique

Attributing to the fast solidification and thermal cycling during additive manufacturing process, massive phase transformation could take place thus leading to the formation of unpredictable microstructures with large variation in mechanical properties. In this paper, a continuous direct energy deposition technique (CDED) is adopted to prepare TiAl alloy seceding from the thermal cycling, and the evolution of micro-structure, microhardness and tensile properties of alloy are investigated. Results indicate that the micro-structure of alloy composes of columnar grains with very fine (alpha 2 +gamma) lamellae, and no heat-affect band is observed. A heat transfer model is established to assist in explaining the solid-phase transformation and microstructure formation mechanism of TiAl alloy. Furthermore, a special duplex gamma microstructure (DP gamma) forms which composes of massive gamma phase surrounded by feathery-like gamma phase. By dissecting crystallographic orientation from EBSD results, the formation mechanism of DP gamma is clarified as the result of sequential solid-state phase transition in (alpha + gamma) phase region, in which the feathery-like microstructure forms firstly followed by the massive gamma phases nucleating at the inter-phase boundaries of feathery-like gamma. Tensile property of as-deposited alloy reveals of 535 MPa with 1% elongation, and the highest hardness of around 320HV is detected at the top region of alloy with the finest interlamellar spacing. (C) 2022 Elsevier B.V. All rights reserved.