| Electrochemical machining(ECM) can be used to achieve a desired workpiece surface by anodic dissolution principle with a shaped cathode. This machining method might be applied to the micromachining, precision finishing, and to the nano-fabrication in principle because of removing with one atom by one atom from the surface of the workpiece. So it can be considered that ECM can be developed as a kind of micro-manufacturing technology.However, the ECM process involves electrolytic solutions such as salt, acid and alkali that may contaminate and erode the surface, which is a signification drawback for precision machining and micromachining. In addition, treatment of waste electrolyte is a problem too. In order to overcome these problems, the researchers proposed a new idea that the general used electrolyte will be replaced by ultrapure water. But at the ordinary condition, the density of OH~- and H~+ ions in ultrapure water is about 10-7mol/L, through which the allowed passing current density is only the order of 10-5A/cm~2, which is too low to produce the ECM removal rate suitable. However, the basic means is to use some kind of catalyst to promote the action of dissociating water into H~+ and OH~-. Then under the same electric potential gradient, the electrolytic current density will reach more than 1A/cm~2, and the ECM removal rate will reach in the order of 1-10μm/min when current density enters into the range of 1-10A/cm~2. So the practical micro-machining rate can be realized, and it is showed well applying foreground in micromachining and precision machining fields.In this paper, according to the theory of ECM in ultrapure water (PW-ECM) and water dissociation mechanism, it is found that PW-ECM can be realized for some necessary conditions. Water dissociation model is set up and analyzed, the process of water dissociation is explained by the hypothesis named the forming of cavities and double electric layer, and this is the theoretical basis that PW-ECM can keep on researching.It is analyzed that characteristic of ion exchange material and process of water dissociation for utilizing ion exchange resin, ion exchange membrane and bipolar membrane. Considering the characteristics of PW-ECM, cation exchange membrane is chosen as the catalyst of accelerating water dissociation. The basis is found for development of PW-ECM set-up.PW-ECM set-up is developed independently for engineering application. Its key components are analyzed in detail and designed, and it is explained how to use the different set-up. Micro-position feeder of piezoelectric ceramic and the ordinary NC system are chosen as the micro-feed drive system, their characteristics satisfy machining requirement.Finally, the basic principle and technological experiments of PW-ECM were studied. It was proved that PW-ECM will be probably applied to 2D/3D shape machining by finding theηω—i curve, machining a triangular hole, a quadrate hole and a set of letters"PW-ECM". In order to solve the problems in the technological experiments, DC pulse and ultrasonic assistance were applied to PW-ECM. As a result, machining accuracy was better than by DC, and the process stability was ensured, because electrolyte in interelectrode gap was periodically renewed during pulse interval and electrolytic sludge of the machining zone were carried away in time. In addition, passivation films were removed by actions of the impacting and cavitating.PW-ECM not only is a new idea but also has come true in accordance with foregoing theoretical analysis and experimental investigation. So it might become a new way for green microfabrication. |
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