This project will develop a new, cost-effective, and environmentally friendly plasma etch process to recondition Nb-based alloy C103 powder procured from Allegheny Technologies Incorporated (ATI), identify end-of-life criteria for recycled refractory metal laser powder bed fusion (LPBF) powder, and demonstrate adequate powder reconditioning utilizing a final LPBF build. To accomplish these tasks, the project team will collaborate with ATI (Monroe, NC) to perform 10 to 15 builds, and the project team (at Purdue University, West Lafayette, IN) will characterize the powder in terms of as-built component size, morphology, microstructures, surface roughness, compositions, and mechanical properties. For reconditioning, we will identify the adequate gas, pressure, and powder handling conditions to recondition the powder and remove deleterious products such as oxides and oxygen-rich layers on the powder surface. The combination of successful recycling and an environmentally friendly reconditioning process will enable cost and energy savings and significant reductions in workplace hazards by eliminating contact with hydrofluoric acid and other caustic wet etch solutions.

The aim of this project is to develop a powder reconditioning process that safely and cost-effectively removes oxygen-rich layers in refractory additive manufacturing (AM) powder. This will be accomplished by three, hypothesis-driven tasks. Task #1: Identify baseline and end-of-life properties of recycled refractory metal powder. In this task, the project team will characterize the rate and mechanisms of oxygen uptake in both C103 powder and as-built components. The project team hypothesizes (hypothesis #1) that oxygen uptake in the reused powder will be in the form of thin amorphous surface oxides resulting from high temperature oxidation, and this increased oxygen is incorporated into the bulk material by the LPBF process. Powder morphology, size, and flow properties will be measured, and as-built mechanical properties, microstructures, and compositions will be measured. The project team further hypothesizes (hypothesis #2) that degradation in mechanical properties will be due to the formation of hafnium(IV) oxide in the form of thin films or precipitates at grain boundaries which promote grain boundary decohesion. All properties will be compared to baseline wrought sheet and virgin powder properties. In Task #2: Develop and deploy refractory alloy powder reconditioning methods, the project team will develop a plasma etching procedure to remove surface oxides on reused powder, and they hypothesize (hypothesis #3) that surface oxides can be removed by plasma etching via (a) ion bombardment and ablation for (relatively) non-reactive gasses (e.g. Ar and H₂) and (b) chemical reaction with halogen-based gasses (e.g. Cl and F) to form volatile metal-halogen species. Etching parameters including drum rotation rate, power, gas type and flow rate, and pressure will be optimized, and an initial small-batch experiment will be performed. Finally, in Task #3: Demonstrate feasibility of powder reconditioning, the project team will recondition the large batch of powder, perform a final LPBF build, validate the reconditioned powder and as-built component properties utilizing the same methods outlined in Task 1. The project team will also perform reconditioning on other select refractory AM powder provided by collaborators.

The benefits of this project include the identification of degradation mechanisms in high-value C103 refractory AM powder during LPBF processing and development of a cost-effective, safe, and environmentally friendly plasma etch process that removes surface oxidation/oxides from a wide variety of powder material. This process can be used to recycle and recondition refractory AM powder, and also pre-treat waste streams for other recycling or reclaiming processes. The project team expects this process to be highly flexible and amenable to many materials.