Use of Pulsed Potentials for Electropolishing TA6V Parts Elaborated By Additive Manufacturing in Deep Eutectic Solvent
Authors: Roy Florian , Doche Marie-Laure , Orecchioni Quentin , Hihn Jean-Yves , Tardelli Joffrey 
Published: December 2024

It is now possible to achieve complex geometries by additive manufacturing of various metals and alloys, without compromising their core mechanical and structural properties. Nevertheless, their degraded surface state remains a critical issue which constitutes a key obstacle for its broad deployment. SLM parts are characterized by their high roughness (Ra up to 40 µm), a strong texture (directly related to the manufacturing process and parameters) and to the presence of potentially detachable unmelted particles. For almost all applications, relevant finishing post-treatments must be carried-out on the surface, to improve its behavior against fatigue, ageing, corrosion and abrasion. Due to oxygen contamination of the surface, titanium and titanium alloys have the ability to form an oxide layer at its surface, called alpha case. In addition to a high hardness which makes the part difficult to machine, this layer alters the mechanical properties (such as loss of ductility, degradation of static and fatigue strength) and corrosion resistance. The degradation of the properties caused by the formation of this layer forces manufacturers to carry out additional operations such as chemical attack, machining, sandblasting, ...etc. Removal of alpha case by chemical polishing is often done with fluorine ions, which creates soluble complex with titanium leading to mass loss, with the drawbacks linked to this kind of processes.

Electropolishing then becomes an interesting alternative, especially if it is carried out in an electrolyte presenting a low health hazard such as Deep Eutectic Solvent (DES). This kind of solvent, used for the first time by Abbott et al [1], are more tolerant to water and less expensive than Room Temperature Ionic Liquids. Pure metallic titanium plates were also successfully electropolished in a DES mixture made of choline chloride-propylene glycol, improving the brightness of the sample and a reduction of the roughness from a few hundreds of nanometers to about 40 nm [2]. The problem is however very different considering additively manufactured parts which show very high roughness (going up to several dozen µm), which leads to the risk of deformation of the geometry of the parts due to very long processing times. In addition, the very high natural viscosity of the electrolyte, combined with the formation of a viscous layer on its surface, creates fluid movements. Indeed, when it reaches too much thickness, the viscous layer detaches itself to flow, which has the effect of creating inhomogeneities on the removal of matter and the reduction of roughness. To overcome this problem, the use of pulsed potentials cycles, at two frequencies, was evaluated. The first one based on short pulses, already experienced on stainless steel, aims to limit the geometric deformation during long processing time [3]. The second one, based on longer pulses allow to limit the growth of the viscous layer and prevent its flow. These sequences were designed through the study of transient curves, and were applied to test pieces on RDE electrodes, leading to bright and homogeneous titanium alloy samples. This process was implemented into a 65L tank, were different parts were successfully finished.

[1] A.P. Abbott; G. Capper; K.J. McKenzie; A. Glidle; K.S. Ryder; “Electropolishing of stainless steels in a choline chloride based ionic liquid: an electrochemical study with surface characterisation using SEM and atomic force microscopy” Phys. Chem. Chem. Phys., 8 (2006), pp. 4214-4221

[2] A. Kityk; V. Protsenko; F. Danilov; V. Pavlik; M. Hnatko; J. Šoltýs; “Enhancement of the surface characteristics of Ti-based biomedical alloy by electropolishing in environmentally friendly deep eutectic solvent (Ethaline); Colloids and Surfaces A: Physicochemical and Engineering Aspects, 613 - (2021), pp126125

[3] M. L. Doche; J-Y Hihn; E. Drynski, F. Roy; A. Boucher; Jason Rolet; J. Tardelli; “Electropolishing of 316L stainless steel parts elaborated by selective laser melting: from laboratory to pilot scale,” Procedia CIRP, vol. 108, no. C, pp. 722–727, 2022