Effect of build direction on microstructure and mechanical properties of TiAl4822 alloy additively manufactured by electron beam melting

Abstract

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.