Numerical study on the splitting of a vapour bubble in the process of EDM

Abstract

Electrical discharge machining (EDM) is a powerful technique for machining of hard and brittle materials. In this process, because of electrical discharge, a vapour bubble is generated in the dielectric liquid between the tool and the workpiece. The growth and collapse phases of the vapour bubble have significant effect on the hydrodynamic behaviour of the dielectric liquid domain between the tool and the workpiece and cause molten material to escape from the crater. Previous numerical studies on the dynamics of an electrical discharge-generated vapour bubble have simulated the growth and collapse of the bubble until it has taken the shape of an hour-glass. This is necking phenomenon which is followed by splitting of the bubble into two parts. In this paper dynamics of an electrical discharge-generated vapour bubble between the tool and the workpiece after its splitting are investigated by using the boundary integral equation method. Development of a liquid jet on the boundary of the each of the upper and lower parts of the bubble and the impingement of the liquid jets on the nearby rigid surfaces are sought. This paper consists of two parts. In part one, the vapour bubble is initially located between the tool and the workpiece. Consequently the dynamic behaviour of the two parts of the bubble in the absence of the buoyancy forces are symmetric with respect to a horizontal axis through the mid-point between the tool and the workpiece. In part two, the elrecrical discharge-generated vapour bubble is initially located in the vicinity of the workpiece. Therefore during the necking phenomenon the upper part of the bubble is smaller than its lower part. Consequently the dynamic behaviour of the two parts of the bubble after its splitting are significantly different.