Electrolytic processing of superalloy aerospace castings using choline chloride-based ionic liquids

A new methodology is described for the removal of surface oxide scale from single crystal aerospace castings of nickel based superalloys using anodic electrolytic etching (electropolishing) with a novel choline chloride based ionic liquid, also known as deep eutectic solvent. This method enables the removal of scale from as cast components facilitating further critical quality checks and assessments, for example grain structure inspection, before the heat treatment. In addition, removal of scale prior to heat treatment also helps to mitigate other defect formation in single crystal castings such as surface incipient melting. Consequently, this methodology may offer significant potential process advantages over current manufacturing practice in that defective components can be identified earlier and, critically, before the expensive and time consuming heat treatment process. Here details are presented of some of the challenges, methodologies as well as characterisation of model test pieces and turbine blade components using the scale-up facilities at the ionic liquids demonstrator facility at the University of Leicester. It is shown that oxide scale as well as residual casting mould shell can be effectively removed under mild conditions using liquids that are of low toxicity, environmentally sustainable, relatively low cost and without the use of strong inorganic acids or chemical etchants.