Electropolishing Behavior of Additive Layer Manufacturing 316L Stainless Steel in Deep Eutectic Solvents
Authors: Rotty Chloé , Doche Marie-Laure , Mandroyan Audrey , Hihn Jean-Yves 
Published: February 2020

The development and improvement of new manufacturing processes during the four last decades, lead up to consider new post-treatment techniques in order to have surface finishing similar to the one obtained by mechanical technics such as milling. Actually, conventional mechanical machining is no longer suitable in terms of cost and time for the geometric complexity of the additively manufactured structures, which may present inner channels. The aim of this study is then to develop an electropolishing process as a method to improve the surface finish dedicated to Additive Layer Manufacturing (ALM) parts.
Electropolishing is usually carried out in concentrated acid media according to metal type, such as phosphoric acid, sulfuric acid and their mixtures [1]. These electrolytes are difficult to handle because of their relative toxicity and hazardousness. In order to improve working conditions and safe disposal of waste solutions, the use of alternative electrolytes is welcome. In this respect, ionic liquids and especially Deep Eutectic Solvents [2] based on Choline Chloride are herein tested.
Electropolishing of ALM and “cast” 316L stainless steel was studied in a mixture of Choline Chloride and Ethylene Glycol. The mechanism was investigated by Linear Sweep Voltammetry and Electrochemical Impedance Spectroscopy. The limiting current density on the electropolishing plateau follows the Levich law showing that this reaction is under diffusional control in DES. The electropolishing window is wider for cast materials in DES than for ALM. The best conditions were obtained for potential at the end of the electropolishing range for both materials. Roughness, morphology and structure of samples were evaluated by respectively Stylus Profiler, Secondary Electron Microscopy, Atomic Force Microscopy and X-Ray Diffraction. Electropolished surface was bright and smooth for cast and ALM materials despite differences in their microstructures, with a composition being equal. In order to complete the study, corrosion measurements were performed before and after electropolishing (for the best conditions) for each manufacturing process. Benefits of the process were confirmed on corrosion resistance.
The main operating parameters determined from laboratory experiments, allowed to design and build a pilot for electropolishing of larger and complex geometry parts. This new reactor provide optimisation of hydrodynamic conditions to improve surface finishing in particular in hollow area. Forced convection generated by ultrasound and jet flow can remove bubbles produced by reactions which are trapped into the viscous layer. Thus, electropolishing process is heightened on these materials.
[1] P.J. Núñez, E. García-Plaza, M. Hernando, R. Trujillo, Characterization of Surface Finish of Electropolished Stainless Steel AISI 316L with Varying Electrolyte Concentrations, Procedia Eng. 63 (2013) 771–778.
[2] E.L. Smith, A.P. Abbott, K.S. Ryder, Deep Eutectic Solvents (DESs) and Their Applications, Chem. Rev. 114 (2014) 11060–11082.

Figure 1