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

Abstract

As metal Additive Manufacturing (AM) becomes more widely adopted in the aerospace and orthopedic industries, there is increasing demand to improve part quality and reduce overall cost. The high cost of powder feedstock has raised interest in recovering unmelted powder in the build chamber and its reuse in subsequent builds. While degradation in powder properties with recovery and reuse can cause degradation in part properties, this topic has received rather limited attention. 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 isolated control samples to understand the mechanisms of degradation. Results show that in response to the elevated build chamber temperature, the powder undergoes a sub-beta-transus aging heat treatment with powder reuse. Based on nanoindentation hardness measurements, the particles undergo an increase in near-surface hardness (up to 2 GPa) with respect to the core. Moreover, 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 related to the mechanical aspects of that process. These results demonstrate the importance of managing/mitigating oxidation of metal powder feedstock to improve its reusability and increasing its overall lifetime.