Microstructure Control of TiAl Alloys via Metal Additive Manufacturing

Metal additive manufacturing (AM) techniques, such as electron beam-powder bed fusion (EB-PBF), have attracted much attention as a novel fabrication technology for difficult-to-machine materials in the aerospace and medical implant industries because it is possible to build 3D objects with complex shapes directly from 3D-CAD data. Another important feature of AM techniques is unique microstructures induced by unusual solidification conditions and thermal history during the process. The alloys with a unique microstructure prepared by the AM techniques exhibit better mechanical and/or functional properties than conventional cast or forged materials. In this study, we developed new microstructure control techniques for TiAl alloys using the EB-PBF process by focusing on the repeated thermal effects from the melt pool and the ultra-rapid cooling which are important features of the process.<br/>We found that the thermal effects from the melt pool have a significant influence on the microstructural evolution of practical Ti-48Al-2Cr-2Nb (4822) alloys. Owing to the repeated thermal effects, 4822 alloys with a unique layered microstructure consisting of equiaxed γ grain regions (γ bands) and duplex regions perpendicular to the building direction can be obtain. Moreover, in β phase-containing TiAl alloys fabricated by the EB-PBF process, a unique microstructure is formed due to a peculiar phase transformation induced by ultra-rapid cooling rate that reaches up to 10⁶ K/s. The α₂/γ nano lamellar grains with lamellar spacing of a dozen nm are formed via massive α transformation caused by ultra-rapid cooling. In addition, we also found that the strength-ductility balances and fatigue properties of these TiAl alloys are improved significantly by each unique microstructure.