Effect of HIP treatment on mechanical properties of TiAl4822 alloy additively manufactured by electron beam melting

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One of the lightweight heat-resistant alloys, Ti-48Al-2Cr-2Nb (TiAl4822), has already been adopted to turbine blades for jet engines, but is manufactured by casting which requires expensive post process. Aerospace industry is intensively studying the additive manufacturing (AM) process for the engine parts. In this research, tensile and creep properties of additively manufactured TiAl4822 by electron beam melting (EBM) were studied. The mechanical tests were performed at 750 ℃ which is similar to the actual condition in low pressure turbine module. The effect of hot isostatic pressing (HIP) treatment on these properties was investigated because such information is very limited, even though HIP treatment is required in many aerospace parts. Near net shape specimens were prepared for both tensile and creep tests. The effect of the HIP treatment on the microstructure in the specimens was observed by scanning electron microscope (SEM). Electron back scatter diffraction (EBSD) was used to analyze the effect of the HIP treatment on microstructure such as area fraction of each phase and grain size in the specimen. The results showed that the benefit of sintering during the HIP treatment decreased the voids with a size of up to 40 μm and the defects with a size of millimeter order due to lack of fusion in the specimens. The HIP treatment improved both tensile and creep properties at 750 ℃. It was found that the HIP treatment improved creep life by 15% and elongation by 60%, and delayed the starting point of tertiary creep. This is mainly because the HIP treatment eliminated the defects caused by lack of fusion and made the material more ductile than as-built specimen. On the other hand, the microscopic analysis by EBSD revealed that the HIP treatment increased the average diameter of the grain and widened the distribution of grain size, with the increase of the area fraction of Ti3Al(α2). The increase of grain diameter could decrease the strength according to the Hall-Petch Law. In this research, however, the elimination of the defects such as the voids and the defects due to lack of fusion improved the tensile and creep properties. The results suggested that the post heat treatment including cooling process after HIP treatment to control the microstructure is important.