Advances In Simulation of Electro - Discharge Machining

 Advances In Simulation of Electro-   Discharge Machining

In this blog, you will find information about the possibilities of acquiring near to accurate results in the simulation of EDM. The blog focuses on different simulation techniques incorporated during modelling.

BACKGROUND

Electro Discharge Machining process is one the precision machining processes which gives accuracy of machining in micrometers. EDM processes are extensively complicated, and require a trained professional for operational purposes. In a research - based scenario, individuals incorporate their own ways of modelling the setup to obtain desired results. This blog discusses various methods in various parts of simulating EDM, which one may use for modelling his/her own setup.

EDM is an extremely complex heat transfer. The heat induces melting and a partial evaporation of the cathode and anode surface, which is transmitted from a high temperature plasma channel. With the enthalpy method, one can solve the problem of latent heat. Workpiece material is analyzed based on energy conservation and Fourier heat conduction law, heating due to a single spark is usually assumed.

Vapor pressure and temperature

In the electrical discharge machining, the workpiece is evaporated and removed by the heat of plasma. In many studies, research is carried out without considering evaporation due to latent heat. However, that is inaccurate. The evaporating pressure can be predicted if the evaporating temperature is predicted considering the latent heat. Provided that the plasma pressure distribution is equal to the Gaussian distribution alike the heat source, it can be said that :

Simulation with ANSYS

A thermal based analysis is performed when the EDM conditions and the material properties are given. The heat flux of the Gaussian distribution is applied to the workpiece. The heat flux intensity varies with discharge gap current and the diameter of plasma is specified. In this type of analysis, temperature distribution which is calculated in thermal analysis is applied as load. Simultaneously, the plasma pressure in the Gaussian distribution is applied.

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Simulation of the crater geometry (ANSYS)

& K Factor

The simulation of crater geometry refers to the machining surface starting at the circumference of the plasma channel. A heat source is considered to be affecting the workpiece. The change of the quantity of heat Q(t) is calculated by the voltage drop at workpiece(cathode), and tool(anode). Factor k is considered, whether the heat is constant or spatially unequally scattered within the channel diameter. The influences can also be examined by the difference in the drop voltages with k-factor. For testing, k = 1 is usually set. Another important factor is the dependence of the plasma channel geometry on the electrical parameters of discharge. In many  simulations, no real connection between the radius change and the physical processes is established. Newer modelling techniques are characterized by taking real current and voltage curves and practically calculated original radii, and radius change differentials drf/dt. Results of the different research methods (COMSOL and ANSYS) are combined in order to extend the realistic physical conditions for the discharge process and surface heating.

THERMAL MODELLING

Thermal erosion is one of the most accepted theories by various researchers for prediction of material removal from electrodes during the EDM process. During the EDM process, due to a high electric potential difference between electrodes, electrons from the anode surface start to move towards the cathode and create ions on the plasma channel. The plasma channel creates enough heat to start thermal heating of the workpiece surface. The highly charged ionized particles on the plasma channel increase the work surface temperature up to the melting point, sometimes even up to a boiling point.

Heat Flux Distribution

In most of the research and literature, it is assumed that there are two types of heat sources: point heat source and uniform disc heat source, but both the models don't give the accurate results. Thus, recent research has shown that the Gaussian distribution of heat flux predicts results more closely to experimental results. Sparks produced by the plasma channel go to the workpiece surface and highten the temperature of the workpiece. It has been found that Gaussian distributed heat energy is more accurate than the disc heat source.

Spark Radius

The radius of spark during pulse-on time does not remain constant instead it tends to increase with time. Ikai and Hashiguchi showed that the discharge radius changes with the current and discharge duration. There are various approaches to find the heat input radius or plasma channel radius. Phase Change in the EDM process, during discharge time material removes due to melting and evaporation of material. Hence, considering the effect of latent heat will increase the probability to get more accurate results.. Determination of Crater Shape MRR calculation depends on the assumption of crater shape. Crater shape is assumed to be a spherical dome shape.  

One specific characteristic which distinguishes different models is the way the heat source is defined : A point heat source, for example. Solvers that describe the melt front can be easily formulated. Nevertheless, the shapes of simulated craters geometries are generally different to the measured ones. In many models, a disc heat source is used for modeling the heat flow to the workpiece. Solutions describing the melt pool are derived, but generally susbtantial differences between simulated and measured craters are seen. Gaussian distributed heat source on the workpiece needed to calculate the temperature distribution, provided an analytical solution for the partial differential equation that describes heat conduction. Simulation results are better deduced for intensive discharges, wheater higher currents with longer pulses were applied. For lesser energetic discharges, high discrepancies in simulation and experimental results were found. It is stated that the plasma efficiency is lower for less energetic pulses, i.e. the efficiency of molten material removal is lower in this case, the reason for the aforementioned discrepancies.

Another characteristic that differentiates models is by taking material properties into consideration. Generally, constant thermo and physical properties for the workpiece are considered, i.e. its dependence on temperature is not taken into account. Most of the erosion based models do not consider the latent heats of fusion and vaporization. 

 Concept of erosion model

 The WEDM process is assessed in this method. For modeling the transmission of the heat generated in a single discharge to the workpiece/anode, a heat conduction phenomenon is taken into account. The equation describes the partial differential for heat conduction in Cartesian coordinates.

where T is the temperature, x, y and z represent the Cartesian coordinate system, ݍሶ is the rate of thermal energy generation per unit volume (W/m3 ), k is the thermal conductivity, D is the thermal diffusivity, t represents the time, U the mass density and cp is the specific heat.

Thermophysical properties of workpiece materials 

These properties play a decisive role in EDM, which affect its machinability. In this method, the melting and boiling temperatures are of immense importance. Materials having comparatively less melting temperatures erode faster.

References:

Review on modelling and optimization of electrical discharge machining process using modern Techniques Manish Gangil, M. K. Pradhan, Rajesh Purohit

Experimental and finite element analysis of EDM process and investigation of material removal rate by response surface methodology Mehrdad Hosseini Kalajahi & Samrand Rash Ahmadi & Samad Nadimi Bavil Oliaei