Electron Beam Additive Manufacturing of Ti <sub>6</sub>Al <sub>4</sub>V: Evolution of Powder Morphology and Part Microstructure with Powder Reuse

Abstract

Additive Manufacturing (AM) processes for metals are advancing at a rapid pace. Among many attractive qualities, AM relaxes design constraints and can significantly reduce material waste in comparison to subtractive manufacturing processes. However, there are some fundamental issues that must be addressed for metal AM to become prevalent in aerospace. In powder bed fusion AM, powder reuse from previous build cycles is desired to improve process economics. However, there is limited understanding of the contributions from powder reuse to particle and part quality. The present study investigates this topic in electron beam melting (EBM) powder fusion AM of a titanium alloy (Ti6Al4V) over 30 cycles of build and powder reuse. Results show that nearly all aspects of the process are influenced by powder reuse. Specifically, the particle size distribution tightens, largely due to fewer with small diameter. Particle damage increases with reuse, which includes surface deformation (reduction in sphericity), partial melting and/or particle fusion and fracture. In regard to the built metal, the microstructure exhibits increasingly finer basket weave and greater surface area to volume ratio of a with reuse. Yet, there are no apparent trends in a-lath thickness or volume. In the analysis of composition, while substantial changes in the Al and V content are apparent, or in Fe, H and N, the O concentration of the powder increased significantly with reuse. In fact, it exceeded the concentration limit (0.2%) in just 11 build cycles. Overall, powder reuse should be considered carefully in the development of titanium parts for performance critical applications by EBM AM.