It is well known that electrochemical machining (ECM) of Cr-containing materials such as stainless steel produces hexavalent chromium, which is often taken up as a problem in electrolytic processing. In this study, using the relationship among the standard electrode potential, we investigated whether the formation of hexavalent chromium itself could be suppressed during ECM and developed the method of using divalent iron ions in the electrolyte. If the electrolyte contains many easily oxidized ions, it can be considered that the oxidation of trivalent chromium is prevented by the oxidation of these ions. To supply divalent iron ions to the electrolyte, an iron-ion supply device composed of iron chips was fabricated. It was clarified that the electrolyte must be kept non-alkaline to contain a large amount of iron ions and that with a sufficient amount of divalent iron ions, the formation of hexavalent chromium can be suppressed.
The influence of the concentration of sodium nitrate aqueous solution electrolyte on the machining characteristics of wire electrochemical finishing using bipolar pulses was investigated. The material removal rate and surface roughness obtained remained almost the same as the concentration was varied from 0.2 wt% to 20 wt%. However, smaller surface roughness was obtained with a smaller depth of cut under a lower electrolyte concentration. Hence, the deterioration of the geometrical accuracy of the workpiece owing to the electrochemical finishing operation can be better suppressed using a lower electrolyte concentration. The measured glossiness of the finished surface obtained with a lower electrolyte concentration was higher. To understand the reason for the particular characteristics obtained from a lower electrolyte concentration, the temperature and current density distributions were analyzed. The results showed that the current density is more localized in a gap with lower electrolyte concentrations owing to Joule heating. Consequently, considering environmental issues, the use of the 0.2 wt% electrolyte is the most appropriate under the conditions adopted in the present work.
To achieve high-speed machining of sintered carbide, a new process of milling using an electrochemical reaction is proposed. First, the phenomena on the sintered carbide surface occurring in the electrochemical reaction were investigated. The reactions that could occur on the sintered carbide surface are the elution of Co, the oxidation of Co, and the oxidation of WC. It was found that the elution of Co occurs first, followed by the oxidation of WC. Then, the cutting resistance and the machining phenomena were evaluated and compared in the cases with and without the electrochemical reaction. It was shown that the cutting resistance becomes significantly lower when the electrochemical reaction occurs. The analysis results showed that the reason for the reduction in cutting resistance was the elution of Co and that most of the electric charge was used for the elution of Co, not the oxidation of Co and WC, when machining was carried out under certain conditions.
Electrolyte jet machining is carried out by jetting the electrolyte from a nozzle toward the workpiece while applying voltage between the nozzle and workpiece. This machining method can selectively process metallic materials regardless of their hardness. Therefore, difficult-to-cut materials, such as a cemented tungsten carbide (WC) rod, which is used as a fine tool, can be machined effectively if a suitable electrolyte and power supply are used. In this research, a method for removing the material on a rotating cylindrical workpiece by using the side surface of an electrolyte jet was proposed. The purpose of this study is to determine the optimal machining parameters, such as the rotating speed of the workpiece and the machining time for the shaping of a WC alloy rod by electrolyte jet machining. First, machining experiments were carried out without a nozzle scan to clarify the machining characteristics by changing the rotation speed of the workpiece. Then, shaping characteristics were investigated by changing the scanning speed of the nozzle.