Electrical discharge machining (EDM) proceeds by the accumulation of series of single discharges. By combining a band-pass filter and a laser beam, we have constructed a system for observing the discharge gap phenomena in more detail by using the high-speed camera equipped with high-magnification optical lens and we have used this system to observe material behavior during a single discharge. In the present study, single discharge experiments were conducted on steel, zinc alloy, tungsten, copper, and titanium, and the behavior of each of these materials on positive polarity side was examined. With some of these materials, the workpiece was found to be intermittently removed during a single discharge. The mechanism of carbon adhesion to copper is discussed by comparing the contact area between the arc plasma and the workpiece and the size of the discharge crater.

Electrochemical milling has a drawback that the machining depth increases as the scanning speed decreases at the point where acceleration and deceleration occur. In this study, a method for controlling the pulse on-time of the pulse voltage according to the scanning speed of the tool electrode was developed to solve this problem. In this method, the duty ratio of the applied pulse voltage is varied according to the scanning speed. The simulation results of groove machining showed that by controlling the pulse voltage on-time according to the speed, the machined shape was almost the same as that in the ideal case where there was the infinite acceleration/deceleration. Experimental results of groove machining showed that controlling the pulse voltage on-time according to the speed solved the problem of increased machining depth at the point where acceleration/deceleration occurred and improved the machining accuracy.

A coupled analysis of electric fields and a circuit equation analysis of the pulse-power supply circuit incorporating stray inductance was undertaken to determine the reason for not achieving machining with a short pulse duration with the increase in the machining area in pulse electrochemical machining. The analysis results show that as the machining area increased, the current flowing through the pulse-power supply circuit increased, and the influence of the inductance increased. As the machining area increased, the current was inhibited from flowing owing to the influence of inductance, and the maximum value of the Faradaic current density decreased. The total charge density of the Faradaic current on the workpiece surface decreased, along with the decrease in the total charge of the Faradaic current per pulse, which was expected to reduce the workpiece removal. Therefore, it was confirmed that the inductance of the pulse-power supply circuit significantly affects why a large area cannot be machined with a short pulse duration.

In additive manufacturing (AM) process, three dimensional objects can be produced. AMed Al-Si alloy has been widely applied for automobiles and aircrafts because of its low density and high strength. However, AMed metal products have large surface roughness. Surface irregularities such as spatter and small cavity are also generated on the surface. The rough surface and surface irregularities would lead to reduction in surface strength. Therefore, surface smoothing and surface repairing methods are required. On the other hand, large-area EB with 60 mm in diameter can be used for melting metal surface uniformly and instantly. Surface smoothing and surface repairing of AMed metal products can be done by the EB irradiation. In this study, improvement in mechanical properties of AMed Al-Si alloy surface by large-area EB irradiation was investigated. Experimental results show that tensile strength and surface hardness of AMed Al-Si alloy can be increased by the EB irradiation.

Diamond composites which are made of diamond grains and metals has high thermal conductivity and low coefficient of linear thermal expansion. Therefore, a possibility of diamond composite as electrode for EDM was highly expected. In this study, tool electrode performances of copper-diamond (Cu-Dia) composite and silver-diamond (Ag-Dia) composite in EDM die-sinking were experimentally investigated. The results show that the metal around the diamond grains in the electrode is preferentially removed during EDM under large discharge current condition, which causes the diamond grains to expose on the electrode surface. Consequently, the undulation of electrode surface increases and the discharging state becomes unstable. In order to exert maximum effect of high thermal conductivity of diamond grains and prevent its sign ificant exposure, EDM performance was investigated under the conditions with small discharge current and small discharge area. As a result, the electrode wear ratio of Cu-Dia became lower than that of pure copper. Therefore, Cu-Dia composite has a possibility as an electrode for EDM with small discharge area.

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.