Microstructure stability of beta-solidifying TiAl alloys produced by selective laser melting

Abstract

γ-TiAl alloys are well-suited for aerospace applications due to low density, high temperature strength and Young’s modulus as well as high corrosion and oxidation resistance [1]. Since current products are restricted by conventional processing routes, new opportunities arise from the prospect of processing γ-TiAl by selective laser melting (SLM). The development of suitable SLM processing conditions for γ-TiAl alloys would allow the realisation of more complex geometries, such as parts with varying cooling channels. However, SLM of γ-TiAl is demanding: cracking due to residual stresses from high cooling rates and Al evaporation during manufacturing. Increasing the substrate plate temperature followed by slow cooling after the process [2, 3] reduces temperature gradients and, thus, thermal stresses. In this work we employed in situ high-temperature heating (between 400-800 °C) to manufacture beta-solidifying TiAl alloys by SLM and compared the effects of intrinsic heat-treatment during SLM with extrinsic post-annealing. Evolution of the microstructure during heat-treatment of as-built samples studied by in situ synchrotron radiation diffraction shows various phase transformations below the eutectoid temperature as a consequence of microstructural stabilization. The intrinsic heat-treatment during SLM leads to graded microstructures characterized by an augmentation of  phase with increasing building time [4]. [1] H. Clemens, et al., Design of Novel ß-solidifying TiAl Alloys with Adjustable ß/B2-phase Fraction and Excellent Hot-Workability, Advanced Engineering Materials, 10, 707-713 (2008). [2] J. Gussone, et al., Microstructure of γ-titanium aluminide processed by selective laser melting at elevated temperatures, Intermetallics, 66, 133-140 (2015). [3] W. Li, et al., Effect of substrate preheating on the texture, phase and nanohardness of a Ti–45Al–2Cr–5Nb alloy processed by selective laser melting”, Scripta Materialia, 118, 13-18 (2016). [4] Gussone, J. et al., Microstructure stability of γ-TiAl produced by selective laser melting. Scripta Materialia, 130, 110-113 (2017).