Electrochemical machining (ECM) of Cr-containing materials such as stainless steel produces hexavalent chromium, which is often considered a problem in ECM. In this study, using the relationship of the standard electrode potential, we investigated whether the formation of hexavalent chromium itself could be suppressed during ECM and developed the method of preventing the formation of hexavalent chromium using divalent iron ions in the electrolyte. If the electrolyte contains much easily oxidized ion, the oxidation of trivalent chromium is prevented by the oxidation of the ion. To supply divalent iron ions to the electrolyte, an iron-ion-supplying device composed of iron chips was fabricated. In our first report, it was shown that the formation of hexavalent chromium can be prevented by this method, but it was also found that it is difficult to maintain the prevention effect. In this report, a method to continuously prevent the formation of hexavalent chromium is described. It was found that the required amount of Fe²⁺ ions could be supplied to the electrolyte by temporarily acidifying the electrolyte. And it was shown to be possible to carry out ECM with a neutral electrolyte while preventing a decrease in Fe²⁺ ion concentration by neutralizing the acidic electrolyte before supplying it to the machining tank.

Recently, additive manufacturing (AM) with selective laser melting (SLM) method using metal powder is attracting attention as a promising technique for producing various complex shapes efficiently. In this paper, we describe the application of the AM method to the fabrication of tool electrodes for Electrical Discharge Machining (EDM). The EDM properties of a copper electrode fabricated by the AM method (AM-Cu electrode) were investigated, and the applicability as a tool electrode for EDM was examined. Then, the AM-Cu electrode was applied to deep slot machining with a high aspect ratio, and its effectiveness was verified. As a result, we confirmed that the EDM properties of the AM-Cu tool electrode were similar to those of the conventional Cu tool electrode. Moreover, it was shown that using the AM-Cu tool electrode with microholes inside reduced the EDM processing time and significantly improved the machining accuracy compared with the case of using the conventional solid Cu tool electrode.

In scanning electrochemical machining, in which the shape of the product is formed by scanning a tool with a simple shape, every machining point comes in touch with the current density in the order of low-current-density region, high-current-density region, and low-current-density region. In the machining of titanium alloys, which are prone to passivation, a strong passivation film is formed before the tool moves over the machining point. Therefore, the material cannot be eluted even when it comes in touch with the high-current-density region because of the thick film. In this study, to realize the practical use of scanning electrochemical machining, the thickness and residual stress of the passivation film generated on the surface of titanium alloy were measured. In addition, the effect of the passivation film on machining was investigated and discussed.