Improvement on the machining accuracy of titanium alloy casing during counter-rotating electrochemical machining by using an insulation coating

Abstract

Counter-rotating electrochemical machining (CRECM) is an innovative ECM method that can be used to manufacture revolving parts, especially thin-walled casings with complex convex structures. As titanium alloys are easy to passivate and difficult to machine, active NaCl solution is often used to obtain a good surface quality from ECM. However, the top surfaces of convex structures will be subjected to severe over cutting using NaCl solution, resulting in poor machining accuracy. Therefore, an insulation coating is employed to protect the non-processed area by shielding stray currents in this paper. A simulation model is established to investigate the evolution of convex structures with an insulation coating. According to the simulated convex structures, the top of the convex structure suffers from serious stray corrosion without the protection of insulation coating, and the sidewall inclination angle is 25.42°. However, the insulation coating enables convex structures without over cutting, achieving a sidewall inclination angle of only approximately 1.1°. Under the protection of insulation coating, the width and height decrease linearly with time, and the radius of the root fillet increases towards an approximately constant value. The experimental results indicate that stray currents can be shielded completely with an insulation coating, and no electrochemical dissolution occurs on the top of the convex structure. The deviation between simulation and experimental results does not exceed 10 %, demonstrating reliability of the proposed model. Cylindrical and conical parts with a grid-like convex structure are successfully produced with a surface roughness value of Ra1.8 μm. This fully verifies the effectiveness of an insulation coating and demonstrates the excellent processing capability of CRECM.