Removal of casting defects from CMSX-4®and CMSX-10®alloys by electropolishing in a novel electrolyte; Deep Eutectic Solvent

Neil Dsouza,Robert Harris,Karl S. Ryder,Matthew Appleton,Amy Cook,Andrew Ballantyne

doi:10.1051/matecconf/20141413007

Abstract

Here we describe the use of environmentally benign Deep Eutectic Solvents (DESs) to electropolish oxide casting scale from Ni based superalloy turbine blades. These new electrolytes are a viable alternative to hazardous and aggressive aqueous acids and etchants currently used to process these components. In addition we show that the DES electrolytes selectively expose the underlying surface structure of these superalloys. We demonstrate the ability of these electrolytes to preferentially etch either γ or γ′ phases depending on the choice of electrolyte formulation and the applied potential during the electrolytic etch on CMSX-4 ® and CMSX-10 ® alloys.

Highlights

Single crystal superalloy casting methodologies are common place for the manufacture of turbine components of aerospace engines where the components are subjected to extreme thermal and mechanical stresses
The removal of casting scale is an important part of the processing of the aerofoil castings, the effects on the surface morphology of the underlying metal postelectropolishing, have not previously been discussed
We show that electrochemical polishing of CMSX-4 and CMSX-10 using a range of Deep Eutectic Solvents (DESs) electrolytes results in selectivity of dissolution for the γ and γ phases, we show that selectivity is a function of the formulation of the electrolyte as well as the applied potential and that the two alloys respond differently to other electropolishing candidates

Summary

Introduction

Single crystal superalloy casting methodologies are common place for the manufacture of turbine components of aerospace engines where the components are subjected to extreme thermal and mechanical stresses Such components, for example turbine blades, can be expected to operate at temperatures 200–300 ◦C above the melting point of the alloy and so demands in performance are great. In addition to obscuring grain structure inspection, residual casting scale is associated with instances of surface incipient melting during heat treatment. This can result in the formation of melt pools and new RX grains in the surface of the casting. The electrolytes used for this process are highly hazardous and corrosive, for example mixtures of nitric/phosphoric and sulphuric acid, which can be difficult and unpleasant to handle, are environmentally damaging and can cause unwanted pitting and uneven etch rates of the surface of the casting during the electrolytic etch [13]

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Results

Conclusion