Influence of supercritical debinding and processing parameters on final properties of injection-moulded Inconel 718

Abstract

In the metal injection-moulding process, the thermoplastic polymer binder plays an essential role as it provides fluidity to the high-loaded feedstock and strength to maintain the moulded shape. The purpose of this study is to develop an environmentally friendly feedstock loaded with a super-alloy Inconel 718 powder. Different binder formulations based on polyethylene glycol (PEG), for its water solubility, and bio-sourced polymers were investigated. Poly(lactic acid)/(polyhydroxybutyrate-valerate) was investigated as a bio-sourced polymer because its miscibility with the PEG. The results are compared to a standard formulation using polypropylene and PEG developed by our research group. A micro powder of Inconel 718 (nickel-based super-alloy) was chosen to elaborate the feedstock. The chemical and rheological behaviour of the feedstock during the mixing, injection, and debinding processes were investigated, with tight control of each process. The comparative impacts of the two different debinding processes and optimum sintering parameters were investigated: one by water and one by CO2 in supercritical state. The supercritical debinding caused no damage to the components for all types of feedstock and decreased the time to remove the PEG from 48 h to 4 h. Finally, the density, microstructure, and hardness of the different samples after final heat treatment were compared. The microstructure was clearly optimized, as the γ″ phases were promoted inside the grains. The results show that the well-adapted binder and debinding process produces an Inconel 718 component with high mechanical properties (Vickers hardness of 341 ± 19 HV). Moreover, this approach can be used with other formulations, powders, and binder systems.