José Duarte Marafona

How Hydrogen Dielectric Strength Forces the Work Voltage in the Electric Discharge Machining

An electro-thermal model based on the Joule heating effect is proposed to simulate a single discharge in an electric discharge machining process. Normally, the dielectric strength of the hydrocarbons oil is approximately 20 MV/m, but it varies with both the thickness of the film and its decomposition. After the breakdown, the hydrocarbon oil has an average dielectric strength value of 2 MV/m. This value is close to the dielectric strength of the hydrogen, which is the main gas that results from the hydrocarbon oil decomposition, at temperatures between 6000 K and 9000 K. Therefore, the electric discharge occurs in a hydrogen atmosphere that imposes both the discharge gap and the work voltage. A 200 V voltage is associated to a 100 μm discharge gap, leading to a 20 V work voltage. Therefore, the 3 V work voltage control corresponds to approximately 15 μm. In other words, the increase of the discharge gap originates other discharge during the discharge pulse. The work voltage control, together with the multiple discharge method, is taken into account. The 100 μm discharge gap corresponds to the higher value of the transitory discharge gap that over evaluates the material removal and the tool wear rates. The results of the numerical simulations are validated with experimental data.

Electrical discharge machining plasma developed in a solid or gaseous medium

The article aims to show that the electrical discharge machining plasma can be developed in solid or gaseous medium, through the numerical and experimental evaluation of process performance. The plasma channel developed in gaseous medium is based on an electrical discharge developed in a gas bubble and the plasma channel developed in solid medium is based on underwater explosions. The main electrical difference between both mediums is on its electrical resistivity. However, if the radius of plasma channel increases, its electrical resistivity should decrease because its electrical resistance and applied current intensity are constant, or in other words, the applied electrical power is constant during discharge duration. Thus, the plasma channel is developed in gaseous and solid mediums, with same electrical resistivity and Joule factor, because the radius of plasma channel is considered constant during discharge duration. The comparison of numerical results of electrical discharge machining performance obtained through an electrical discharge machining plasma developed in gaseous and solid mediums shows high agreement with the experimental results. Therefore, the electrical discharge machining plasma developed in solid and gaseous mediums is reliable when hydrocarbon oil is used as a dielectric fluid due to the high degree of agreement of numerical and experimental results of electrical discharge machining performance.

EDM performance is affected by the white layer

The aim of this article is to show that the thickness and thermal conductivity of white layer affect the material removal rate and electrode wear rate in the process of electrical discharge machining. To study this effect a semi-empirical model was used and the assumption was made that the material of both the workpiece and electrode is removed by evaporation. The results of the model show an increase in the material removal rate with the increase of the white layer thickness. There is a value of thermal conductivity of the white layer that minimizes the material removal rate. Moreover, there is always an increase in the material removal rate when comparing with workpiece without white layer. There is a value of thermal conductivity of the white layer that minimizes the electrode wear rate. The white layer thickness does not have a significant effect on the electrode wear rate. This article demonstrates that the process of electrical discharge machining is not only influenced by the thermal properties of the material of the workpiece but also by the metallurgic modifications that occur during the machining.