Development of an Electropolishing Process for Additively Manufactured 316L Parts Under Direct and Pulsed Potential Control

Abstract

The initial purpose of additive manufacturing was mainly rapid prototyping, but all industrial sectors have adopted this technique that has increased in profitability and allows innovative shapes in almost all alloys. Advances in technology have led to AM components whose mechanical and structural properties are equivalent to machined parts. However, a major impediment to the extension of the process is due to the bad surface finish of the as built parts, characterized by a high roughness (Ra up to 40 µm) and the presence of un-melted particles and weld beads. These defects are incompatible with the planned applications (like inner channels for fluid flow), and also induce a degradation of the material functional properties such as an increased sensitivity to fatigue and corrosion. Electropolishing, an electrochemical method for planarization of metals based on their anodic dissolution in an appropriate electrolyte, is a solution to consider. During this work a specific electropolishing process dedicated to 316 L AM parts was developed with the originality of being under potentiostatic control [1,2]. First, a preliminary study done at laboratory scale in a three electrodes electrochemical cell, allowed us to determine the process parameters including the working potential range. Then, the same type of arrangement was subsequently up-scaled in a 3 L reactor in order to electrochemically polish bigger parts ( ≈40 cm2). The potential control is made possible thanks to the introduction of a reference electrode in the pilot and by the use of a high current/high voltage potentiostat able to monitor both direct and pulse potentials. When transposing the process at pilot scale, new difficulties are emerging, due to the large initial roughness of the parts. If a satisfactory roughness decrease is easily achievable in less than one hour, it goes with an inhomogeneous loss of dimension. This geometric deformation of the parts needs to be considered as least as much as roughness parameters. In this context, pulsed potential are emerging as an attractive way to performed electropolishing, by controlling the dissolution while getting interesting final surface properties and limited deformation [3]. Samples present a very bright finishing and low roughness values (Fig.1) In this study a special attention was paid to the quantification of the deformation on especially designed grooved test samples by 3D optical microscopy and laser scanning [4]. Acknowledgments : the authors would like to thank the IRT-M2P program AFTER ALM for its financial support. [1] Rotty C., Mandroyan A., Doche M.-L., Hihn J. Y., Surf. Coat. Technol., 307 Part A, 125–135 (2016) [2] Rotty C, Doche M.-L, Mandroyan A.,Hihn J-Y., Montavon G., Comparison of electropolishing behaviours of TSC, ALM and cast 316L stainless steel in H3PO4/H2SO4, Surfaces and Interfaces, 6, 170-176 (2017) [3] Taylor EJ., Sequential electromachining and electropolishing of metals and the like using modulated electric fields, US Patent 6,558,231 (2003) [4] C.M. Sulyma, P.C Goonetilleke, D.Roy, Journal of materials processing technology, 1189-1198 (2009) Figure 1