Rheological Behavior of Inconel 718 Powder for Electron-Beam Melting

Understanding the impact of powder reuse in powder-bed-fusion electron beams (PBF-EB) is key to maintain the processability and yield. Powder oxidation, due to exposure to high temperatures for a prolonged period of time, can lead to a decrease in electrical conductivity of the powder and, hence, electrostatic forces that originate during interaction with the electron beam. The effect of oxidation on physical properties as powder rheological properties, apparent/tap density and charging are studied in this work. The analysis using Scanning Electron Microscopy (SEM) shows thermodynamically stable Al-rich oxide particulates (sized 100-200 nm) covering the surface of the reused powder particles, with an increase of 20% in bulk oxygen in comparison to the virgin powder and, measured by X-ray Photoelectron Spectroscopy (XPS), average oxide thickness of circa 13 nm in the reused powder. On the one hand, reusing the powder positively impacted the flowability studied using the Revolution Powder Analyzer (RPA), in which the avalanche angle was decreased from 37 deg to 30 deg, for virgin and reused powder, respectively. The volume fraction of loose powder was similar for both virgin and reused powder, 57% and 56%, respectively, while the packed volume fraction was measured lower in the reused (57%) than the virgin powder (60%). On the other hand, the charging behavior, studied using the ION Charge Module of the powder, worsened; this almost doubled in the reuse powder (-9.18 V/g) compared to the virgin powder (-5.84 V/g). The observation of ejected particles from the build volume is attributed to the charging behavior and lower packing volume fraction in the reused powder.