“Confined Etching-Electrolysis” Composite Electrochemical Machining On TC18

Titanium alloy has been widely used in aviation, automobile, chemical industry and other fields for its good comprehensive performance. However, it also has the characteristic of high viscosity, which makes it difficult to obtain good machining quality by traditional machining method, especially the machining for complex three-dimensional structure. Electrochemical machining is a best way to solve the above problems. Electrochemical Machining(ECM) has high processing efficiency, but it has the disadvantages of poor machining stability and poor machining precision, while high precision machining of three-dimensional microstructures can be achieved by the Confined Etchant Layer Technique(CELT). In order to achieve high efficiency, good precision and surface quality of machining, the present paper put forward a perfect integration of two machining methods, forming a new type of composite(hybrid) processing method, named “Confined etching – electrolysis” composite electrochemical machining technology.In this paper the present situation and development trend of several typical composite electrochemical machining technologies and electrochemical micromachining methods were evaluated. The principles and characteristics of ECM and CELT were analyzed and discussed emphasisly. At the same time, their advantages and disadvantages of this two machining methods were summarized. The necessity and feasibility of compositing the two kinds of machining technologies was confirmed. Through a series of experiments, such as weight loss, Tafel polarization curve and cyclic voltammetry, the appropriate electrolyte system for confined etching of TC18 was determined.The composite mode designed in this paper was conducted by alternating between the ECM and CELT, which was achieved through a double pole double throw switch(DPDT) to control polarity conversion. Etching agent was generated at the anode by design. In the ECM duration, anodic dissolution reaction of the workpiece occurred, while the mold worked as cathode. In the CELT duration, the confined etching reaction occurred, the workpiece was in the open circuit state(additionally setting an external cathode) and the mold worked as anode.Based on a set of single factor tests and orthogonal tests for the composite electrolyte system of machining TC18,a composite electrolyte system was ultimately determined which was suitable for both ECM and CELT. The composite electrolyte system was: 25 g / L NaBr + 15 g / L NaNO3 + 20 g / L NaNO2 + 7g / L NaF + 10 g / L NaOH + 10 g / L sodium citrate + 0.2 g / L cetyl trimethyl ammonium bromide. Through the investigation and analysis of the processing parameters, the pulse power supply was selected as processing power. Under a set of processing conditions such as the voltage 6 V, a frequency 320 Hz, the duty ratio 1:16, the pumping speed of electrolyte(Control electrolyte flow) 120 r / min, a hemispherical surface was obtained on TC18 sample with a processing accuracy of 68 μm and a surface roughness of 0.72 μm.The research shows that “Confined etching – electrolysis” composite electrochemical machining technology has good advantage in the complex 3D structure machining. But its present machining accuracy is still much lower than that of pure Confined Etchant Layer Technique in micromachining field. It is believed that this processing technology will be successfully applied in industry after further investigation.