Observation of craters formed by single pulse discharge by stacking cross sectional shapes: comparison of craters in liquid and air

This paper describes the results of comparing craters formed by single pulse discharge in liquid and in air at comparatively short pulse durations. Investigations were conducted by stacking cross-sectional shapes that the authors developed to clarify the volumes of resolidified metal and molten metal at the crater. In liquid, most of the resolidified layer remained at the bottom of the crater or in its vicinity. In air, the resolidified layer existed only at the downstream side of the high-speed gas flow. The removal ratio was found to be minus in liquid as most of the molten material remained on the crater. In air, more than 70 per cent of the molten material was removed at pulse durations above 2.5 mu s. Thermal analysis results indicate that about 26 per cent of the discharge energy was used for melting the workpiece regardless of the pulse duration in liquid. In air, the discharge energy was found to decrease with longer pulse durations (i.e. from 20 to 10 per cent for 2.5 to 2.8 mu s) due to the decrease in the heat flux in the workpiece as a result of the arc column being moved at high speed by the high-speed gas flow in the discharge gap.