Powder Reuse in Electron Beam Melting Additive Manufacturing of Ti6Al4V: Particle Microstructure, Oxygen Content and Mechanical Properties

Thesis (Master's)--University of Washington, 2020Metal Additive Manufacturing (AM) is being adopted in the aerospace and orthopedic industries. While the cost of powder feedstock has raised interest in reusing unmelted powder in subsequent builds, powder reuse could degrade part properties. In this study the properties of Ti6Al4V metal powder are evaluated over 30 build cycles in Electron Beam Melting (EBM) AM. The morphological, microstructural, mechanical, and chemical changes are evaluated in cross-sectioned powder particles and compared to control samples. Results show that in response to the elevated build chamber temperature, the powder undergoes a sub-beta-transus aging heat treatment with reuse. Nanoindentation shows that the particles undergo an increase in near-surface hardness (up to 2 GPa) with respect to the core. Tint etching revealed an oxidized surface layers consistent with alpha case formation. The particle hardening appears to result from oxygen diffusion during powder recovery and not work hardening via mechanical aspects of powder recovery