Direct observation of surface chemistry evolution for Ti-6Al-4V powders in calciothermic reduction recycling process

Titanium alloy powders are susceptible to oxygen pickup during handling, storage, and usage in additive manufacturing (AM), which degrades the 3D-printed parts’ mechanical properties and severely limits their reuse. Industrial AM titanium alloy feedstocks are subjected to stringent oxygen requirements, leading to large volumes of high-oxygen powder waste, which increases carbon footprint and cost. Thermochemical reduction with strong oxygen-affinity metals offers a viable route for titanium powder recovery, yet the oxygen removal mechanism from powder surfaces remains to be confirmed by direct experimental observation. Here, calciothermic reduction of AM-grade spherical Ti-6Al-4V powder is performed, followed by acid pickling and drying. Multiscale characterization is employed to correlate changes in bulk oxygen content, surface morphology, oxide-layer structure, and near-surface chemistry at the individual-particle level. Atom probe tomography reveals a graded oxygen profile extending from a TiO₂ outer surface through progressively oxygen-depleted subsurface regions into the α-Ti64 matrix. The oxidized and rejuvenated powders exhibit oxide regions of approximately 10 and <5 nm, respectively, while the matrix oxygen concentration decreased from about 3.2 to 0.5 at.%. A Ca–O-rich reaction-product layer and a narrow Ca–Ti–O interfacial region were directly observed after reduction, indicating that oxygen transfer occurs through a surface-mediated interfacial reaction.