Optimization of the Anodic Dissolution of the Superalloy Inconel 718
Authors: Msakni Malouche Mariem , Stein Nicolas , Lecomte Janvier , Boulanger Clotilde , Rancic Mickael 
Published: February 2020

Due to its particular mechanical and thermal properties, nickel-based superalloys such as Inconel 718 are widely used for aerospace industry. They are known to be the among the most difficult-to-cut- materials[1], so that conventional machining methods such as milling are inefficient and hard to proceed[2].

Because of its ability to generate high quality surfaces and to reproduce complex shapes. Pulse or Precise ElectroChemical Machining (PECM), an unconventional machining process (no cutting tools), can be useful to manufacture those alloys. This technic is based on the anodic oxidation of metals through a strong applied pulse current. The electrical signal is synchronized to the oscillation of the cathode corresponding to a rectilinear translational movement. Throughout the process, the electrolyte is continuously renewed that allows the removing of the solid residues of the dissolved metal and guarantees a good dissolution efficiency. The PECM is characterized by a very low inter-electrode distance (gap), from 0.01 to 0.2 mm, which ensures the dimensional accuracy.

First step was to understand the physical-chemical phenomena related to the anodic dissolution of the Inconel 718 in the usual electrolyte NaNO3. Thus, polarization curves associated with current constant dissolution have been studied in 2,35 mol/L (20 % wt) NaNO3. Energy dispersive X-ray spectroscopy analysis of cross-sections were used to highlight the solid anodic products formed during anodization. They are mainly constituted of Ni and Nb oxides as well as γ '' (Ni3Nb) inclusions. From ICP analyses of the electrolyte, the dissolved elements in solution are mainly those of the austenitic matrix.

Consequently, in order to optimize the dissolution process, the individual anodic behavior of both main components forming the superalloy, namely the matrix and the γ'' phase were studied in a mixture of NaNO3 and EDTA as a chelating agent. Linear Sweep Voltammetry (LSV) was performed to study the global behavior of each phase in the chelating medium. Chronopotentiometry was carried out to determine the dissolution yield. Finally, electrochemical impedance spectroscopy (EIS) measurements were performed to evaluate the electrochemical interface following the anodization process. It appears that, in presence of EDTA, the hydroxides formation in the electrolyte is limited, whereas the charge transfer resistance and the anodizing layer resistance decrease.

As a final step, the electrolyte containing the chelating agent was subsequently tested for Inconel 718 in PECM configuration using a home – made experimental set up in order to determine the influence of the gap, the electrolyte flow and the pulse time on anodic dissolution. In addition, metallographic sections and dissolution products analysis were also made to identify the material integrity.

[1] D. Dudzinski, A. Devillez, A. Moufki, D. Larrouquere, V. Zerrouki, et J. Vigneau, « A review of developments towards dry and high speed machining of Inconel 718 alloy », Int. J. Mach. Tools Manuf, vol. 44, no 4, p. 439-456, mars 2004.

[2] M. Burger, L. Koll, E. A. Werner, et A. Platz, « Electrochemical machining characteristics and resulting surface quality of the nickel-base single-crystalline material LEK94 », J. Manuf. Process, vol. 14, no 1, p. 62-70, janv. 2012.