Anodic dissolution mechanism of TA15 titanium alloy during counter-rotating electrochemical machining

Counter-rotating electrochemical machining (CRECM) is a novel shaping method with advantages in processing revolving parts, especially engine casings. However, few researchers have studied the anodic behaviour of the counter-rotating state. This paper aims to analyse the anode dissolution behaviour of TA15 and obtain desired surface qualities in CRECM. The anodic characteristics were measured by polarization and cyclic voltammetry curves, and the passive-trans passive behaviour of TA15 was revealed. The electrode surface structures at different stages were analysed using electrochemical impedance spectroscopy (EIS), and a quantitative dissolution model was established to illustrate the electrochemical dissolution and structural evolution of the revolving surfaces. A series of CRECM experiments were conducted, and three stages (pitting corrosion, pitting expansion, and smoothing) were detected according to the current signals. In the first stage, an oxide film with small pores was formed initially on the metal surface when exposed to air. This air-formed oxide film was broken down locally during the electrochemical reaction process, resulting in the occurrence of pitting. With the increase of electricity, the oxide layer became thinner, and the pitting areas expanded and began to connect with each other. In this stage, the surface quality was poor owing to the uneven material dissolution. When the amount of electricity approximately reached a constant, the oxide layer was completely removed, and a very thin salt film was generated at the metal-electrolyte interface. In this stage, the material was dissolved uniformly, and the surface was smooth without pitting corrosion. Based on the obtained results, anode workpieces with grid-like convex structures were fabricated using CRECM. Both the revolving surface and the sidewalls of the convex structures can be controlled from pitting to smoothness.