Microstructure stability of gamma-TiAl produced by selective laser melting

Kurzfassung

γ-TiAl alloys are well-suited for aerospace applications due to their 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 (e.g. casting) new opportunities arise from the prospect of processing by selective laser melting (SLM). The development of suitable SLM processing conditions for γ-TiAl alloys would allow the realisation of more complex geometries. However, SLM of γ-TiAl is very demanding: fast crack formation due to residual stresses arising from high cooling rates and Al evaporation occur during manufacturing. In order to reduce the residual stresses, we employed in situ high-temperature heating to produce Ti-44.8Al-6Nb-1.0Mo-0.1B by SLM. Increasing the substrate plate temperature followed by slow cooling after the process [2, 3] reduces temperature gradients and, thus, also thermal stresses. In this work we compare the effects of intrinsic heat-treatment during SLM of Ti-44.8Al-6Nb-1.0Mo-0.1B with extrinsic post-annealing. Evolution of the microstructure during heat-treatment of as-built samples studied by in situ synchrotron radiation diffraction shows stabilization of 2 as well as transformations to  and  below the eutectoid temperature. These observations contrast the marginal microstructural changes found for the microstructure thermodynamically stabilized by hot isostatic pressing. The intrinsic heat-treatment during SLM leads to graded microstructures characterized by an increase of globular  and at the 2/colony boundaries. [1] H. Clemens, W. Wallgram, S. Kremmer, V. Güther, A. Otto, A. Bartels, “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, Y.-C. Hagedorn, H. Gherekhloo, G. Kasperovich, T. Merzouk, J. Hausmann, “Microstructure of γ-titanium aluminide processed by selective laser melting at elevated temperatures”, Intermetallics, 66, 133-140 (2015). [3] W. Li, J. Liu, Y. Zhou, S. Wen, Q. Wei, C. Yan, Y. Shi, “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).