Metal Injection Moulding of Inconel 718 using a Water Soluble Binder System

Inconel 718 components are typically cast or machined from solid in various industries. The properties of Inconel 718 make the processing of small complex parts very difficult.1–4 There is an emerging interest in metal injection moulding (MIM) of Inconel 718.5–11 MIM is a manufacturing process best used for the batch production of small complex components. The process involves mixing fine metal powders with a series of binder components to produce a pelletised feedstock. This feedstock can then be processed using an injection moulding machine to produce moulded parts. The parts then need to be solvent debinded and then sintered close to the melting temperature to fully densify the metal powder. Polyethylene glycol (PEG) is a common component of water soluble feedstock for metal injection moulding. The processability of Inconel 718 MIM feedstocks using PEG with varying molecular weight was investigated in this work. Inconel 718 powder was formulated with different molecular weight PEG, poly (methyl methacrylate) (PMMA) and stearic acid (SA) to produce a homogenous pelletized feedstock. The powder loading for each feedstock as well as the concentration of PMMA and SA were unchanged for the entirety of this work, the only variable being the molecular weight of PEG used. The rheological behaviour of each of these feedstocks was analysed to determine injection moulding parameters and stability. Investigations into the mouldability of tensile bar components, differential scanning calorimetry (DSC) characteristics as well as debinding behaviour were all carried out. Parts were sintered and heat treated with density and dimensional measurements taken throughout, in order to identify the effect of PEG molecular weight on the different processing steps. Metallography and mechanical testing was carried out to compare the properties of each feedstock with industrial standards. One of the key findings from this work was that the molecular weight of PEG used had an effect on the debinding time of the moulded parts. Parts moulded from a higher molecular weight PEG require a longer period of time in a water bath in order to achieve 100% weight loss. It was found that a debinding temperature of 40°C produces sintered parts with a higher density, compares with debinding at 50°C or 60°C. There was no trend in sintered density across the different molecular weights of PEG, when the same debinding temperature was used. This was the same for the mechanical testing results, which showed no clear trend across the range of parts. The mechanical properties of the injection moulded tensile bars exceed the properties of AMS 5383 (for cast specimens) and are inferior to AMS 5662 (wrought specimens). The mechanical properties are also currently inferior to that of MIM standard AMS 5917, however HIP’ing of the components could improve the mechanical strength and this could result in the standards being met.