Contamination-Controlled Recycle of Inconel 718 Swarf through Y₂O₃-HfO₂ Dispersion Engineering and Spark Plasma Sintering

Abstract

Machining swarf from Inconel 718 retains valuable nickel, chromium, and niobium, yet conventional remelting increases energy demand and risks oxidation and compositional loss. Direct solid-state recycle offers a route that combines critical-metal recovery with oxide-dispersion strengthening. Existing routes have not integrated contamination-controlled swarf, deliberate Y₂O₃-HfO₂ dispersion engineering, pressure-assisted consolidation, and material-genealogy-based oxygen accounting. This investigation aims to establish relationships among swarf contamination, oxide chemistry, spark-plasma-sintering response, and high-temperature performance in recycled ODS-IN718. Swarf was cleaned, fragmented, mechanically alloyed with controlled oxide additions, consolidated by spark plasma sintering, heat treated, and assessed using chemical analysis, X-ray diffraction, electron microscopy, and mechanical characterization. Stoichiometric calculations separated intentional oxide-derived oxygen from processing-derived excess oxygen. Y₂O₃ additions produced intentional oxygen concentrations of 638, 1063, and 1488 ppm, while combined Y₂O₃-HfO₂ formulations yielded 1215 and 1443 ppm. These calculations established that total oxygen alone could not represent contamination in oxide-containing material. The genealogy framework also distinguished recycled, virgin-powder, and wrought conditions without assigning unsupported experimental performance values. This recycle strategy supports circular superalloy manufacturing, resource efficiency, and chemically defensible assessment of ODS feedstocks. Future research should validate the processing window through independent industrial swarf streams, long-duration creep, cyclic oxidation, fatigue, and nanoscale dispersoid-stability measurements.