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In the aviation industry, materials are being developed to improve the performance of jet engines. TiAl is used in turbine blades because of its high strength at high temperatures and its light weight. However, due to the difficulty of machining TiAl, additive manufacturing has been expected in recent years. Electron-beam-melted TiAl4822 has a unique layered microstructure composed of duplex-like fine-grain layers and a band of coarser γ-grains. This unique microstructure is known to bring about the anisotropy of mechanical properties. This study investigated the effects of building direction on creep properties. The creep test was conducted in accordance with the ASTM standard. The creep life of an additive manufacturing specimen was longer than that of a cast specimen due to the higher percentage of lamellar structure in the former. A specimen with building angle θ = 0° (vertical direction) had the longest creep life, which is attributed to the deformation of layers. When the building angle is θ = 45° or 90°, slip occurs between the layers, resulting in the formation of a large void along the vertical grain boundary of the γ band, which may cause high creep elongation.

TiAl (titanium aluminide) has been applied to turbine blades in aerospace industry, because it is a promising lightweight material with high strengths at high temperatures. In recent years, the production method of TiAl by additive manufacturing has been studied. In this research, the effect of dimensions of TiAl specimens by additive manufacturing had on the tensile and creep properties was studied. It was confirmed that the near-net specimen showed higher tensile strengths and lower creep resistance and life than those of the specimen cut from the block with larger cross section and volume than near net specimen. The results showed that the difference of the mechanical properties was because of the different microstructure caused by the powder melting process including hatching to melt inside the cross section and contouring perimeter. The difference in microstructure is considered to be due to the energy density. The energy density of contours is nearly 4 to 10 times higher than that of hatching. Therefore, it is considered that rapid heating and rapid cooling occur in the contour process, resulting in a fine structure.

The application of Ti-48Al-2Cr-2Nb (TiAl4822) in such industries as aerospace and automotive has been studied and commercialized in some mass production parts. Additive manufacturing (AM) technology is one of the research areas to expand the application using super alloys. Electron beam melting (EBM), with a vacuum chamber and a heating system, is currently a major technique to build TiAl4822. The present study shows the mechanical properties of additively manufactured TiAl4822 by EBM. There are many reports describing the specific mechanical property or microstructure, but very few reports included the holistic investigation including different mechanical properties such as tensile, fatigue, and creep with observation of microstructure in TiAl4822. In addition, the previous researches reported by different groups applied relatively small specimens with different dimensions and conditions. This makes it difficult to compare the results. In this research, the ASTM specimens for tensile and fatigue tests were prepared adopting same conditions in EBM process to understand the important mechanical properties at 23℃ and 750℃. The effect of test direction on tensile and fatigue properties were evaluated because it has been known that AM gives anisotropic properties. Microstructure was analyzed by scanning electron microscope (SEM) and electron back scatter diffraction (EBSD) to understand the effect of the differences in microstructure on the mechanical properties. The results of tensile test at 750℃ showed that the elongation tested in 45° to the build direction was the largest due to the ductile γ phase is parallel to 45°. Regardless of the stress amplitude, fatigue life in 0° showed the shortest because the relatively low strength between layers could affect the tensile strength in 0°. The fracture surface and cross section of specimen after fatigue test indicated that the delamination between layers can propagate in the material. Fatigue limit in 45° was the greatest, reflecting tensile property in 45°. However, the observation of the cross section after high cycle fatigue test revealed that the delamination between the layers can also occur in 45° because shear stress along the layers is maximum in 45°. In was also found that the anisotropy in EBM specimens in fatigue property depends on the number of cycles, while all the EBM specimens with relatively small grain size showed the longer fatigue life than that of casting specimens. Understanding these anisotropic mechanical properties and microstructure is important when the AM parts are designed.

Titanium aluminide (TiAl) alloy is attracting attention in the automobile and aviation industries as a promising lightweight heat-resistant alloy material. Parts used in high-temperature environments must have ductility and toughness at room temperature in addition to high strength at high temperatures, however, some alloys, such as TiAl alloys, are known to be difficult materials for casting and machining. Recently, manufacturing processes using additive manufacturing (AM) have been studied to solve these problems. In the present research, the effect of energy density during the AM process on surface roughness and on the strength of the built parts was studied by changing build parameters. As the energy density increased, the porosity and surface roughness decreased. Additionally, higher energy density tended to increase the fraction of lamellar structure. It was found, however, that an increase in lamellar structure does not necessarily lead to any improvement in tensile or creep strength at 750°C. We found that a constant energy density of 15 J/mm³ showed better tensile and creep properties than those of the standard parameter. Our results suggest that it is important to optimize the parameters according to the required properties of the parts. [ABSTRACT FROM AUTHOR]