Modelling Electrochemical Polishing of Additive Manufactured Parts. Limits and Improvements of Secondary Distribution Model in Circulating Flow

Abstract

Additive manufacturing allows to design and to realize parts with complex geometries, which were difficult, even impossible, to obtain by conventional means. Nevertheless, the final surface finish obtained at the end of the 3D printing process do not fit with technological specifications especially due to high roughness levels. Thus, an additional finishing step must be implemented in order to get the expected surface finish. Electrochemical polishing (ECP) is showing promising results for smoothing SLM parts with high complex shapes. ECP is an electrolytic process based on the anodic dissolution of the workpiece under constant current or potential [1]. As it is done for electroplating systems, it is nonetheless necessary to uniformize the distribution of current lines in the treatment cell, in order to make the dissolution as homogeneous as possible on the whole surface. Indeed, the concentration of the current lines on the edges causes a greater dissolution which is detrimental to the geometric integrity of the parts. The ability to provide a trustworthy simulation of the process by the help of an accurate modelling is a major issue to ensure its industrial deployment through the development of specific tools and the selection of relevant operating parameters. To this end, the use in a first approach of a secondary current distribution model gives interesting results in term of dissolution rates. However, this approach faces limitations and it is difficult to predict roughness evolution as a result of the specificity of electropolishing processes i.e. wide potential ranges and the presence of a viscous layer. Moreover, parts after treatment present heterogeneities depending to their position in the electrolyte flow circulation, which can be explained by the presence of turbulences in the vicinity of the impact zone. In order to understand and contribute to the global model improvement, mechanisms involved at a microscopic scale have been investigated with a special attention paid to the influence of hydrodynamic on mass and heat transfer exchanges. Microscopic behaviors are described, and their sensitivity to global parameters (temperature, liquid flow velocity..) are studied to help to take into account the impact of fluid dynamics and / or heat transfers on the viscous layer, which governs the polishing.[1] C. Rotty, A. Mandroyan, M-L. Doche, J-Y. Hihn, Surface & Coatings Technology vol.307 p125–135 (2016).[2] V. Urlea, V. Brailovski, Int. J. Adv. Manuf. Technol. vol.92 p4487–4499 (2017).