Experimental and numerical investigations of material removal process in electrochemical discharge machining of glass in discharge regime

Highlights

• The fraction of discharge energy transferred to the workpiece was found to be 30.5% in ECDM.

• The material removal expands inward from the projected contour of the electrode edge.

• The effective discharge rate was found to be 10.1% in ECDM.

Abstract

Electrochemical discharge machining (ECDM) can be applied as a non-traditional processing technology for machining non-conductive materials such as glass and ceramics, based on the phenomena of evoked electrochemical discharges around the tool electrode. The material removal mechanism of ECDM is noticeably complex and difficult to experimentally characterize. In this paper, finite element models were proposed to predict the material removal in the ECDM discharge regime. First, the single-pulse discharge on a tapered electrode was modeled. It was found that about 30.5% of the discharge energy is transferred to the workpiece. The continuous discharge on a cylindrical electrode was thereafter modeled according to this phenomenon, in which the removal of a layer of the workpiece material starts from the projected contour of the edge of the electrode end and extends inward during the ECDM processing. The effective discharge ratio for material removal was calculated to be 10.1%. The drilling depths of holes at different applied voltages were predicted by the proposed finite element method. It was found that the predicted values were consistent with the experimental results.

Introduction

Electrochemical Discharge Machining (ECDM) for non-conductive materials, known as well in the literature under the names Spark Assisted Chemical Engraving (SACE) and Electro Chemical Spark Machining (ECSM), is a non-traditional machining technology for those hard and brittle materials, such as glass, quartz and ceramics [1]. Electrochemical Discharge Machining (ECDM) for non-conductive materials is different from the hybrid processes of electrochemical machining (ECM) and electrical discharge machining (EDM) for conductive materials, which also termed ECDM [2]. During the ECDM of non-conductive materials process, the gas film formed around the tool electrode is broken down under the applied voltage, and consequently electrochemical discharges occur [3,4]. Material removal takes place mainly due to thermal melting, in combination with chemical etching. The material removal rate in the ECDM is affected by many factors, including the electrolyte, the gas film formed on electrode, electrochemical discharge, and workpiece [5,6]. The combination of thermal process and chemical reaction makes the material removal mechanism in ECDM complex and difficult to characterize through experiments, therefore, a set of models have already been proposed to study the mechanism behind this process.

Basak et al. proposed an analytical model and predicted the material removal rate under different conditions based on the model [7]. They assumed that the electrochemical discharge only occurred at the edge of the electrode end; however, this did not conform to actual discharge activities. Jain et al. developed a finite element-based model and simulated the heat transfer process in the workpiece under the intense action of a single spark [8]. In their model, a prismatic heat source was used, and the material with a temperature above the melting point or softening point on the workpiece was assumed to have been removed. The temperature distribution was also calculated, and the simulation results were found to a certain extent different from the experimental results. One of the reasons is that the shape of the heat source differed from the actual one. Jiang et al. proposed a stochastic model for spark energy estimation and found that the average spark energy was approximately 3.8 mJ [9]. Jalali et al. developed a thermal analysis model and studied the material removal mechanism in the gravity-feed micro-hole machining [10]. By comparing their study with experimental results, it was determined that the equivalent critical temperature for material removal of glass was approximately 600 °C considering both thermal melting and chemical etching in the material removal process. Panda et al. proposed a three-dimensional finite element transient model and simulated the temperature field and material removal rate in travelling wire electrochemical discharge machining (TW-ECDM) [11]. Their study showed that the material removal rate has a complex relationship with machining parameters. They only considered the thermal removal process of the material and the critical temperature for material removal of glass during processing was set as the softening temperature point (820 °C). Wei et al. proposed a finite element-based model using a Gaussian distribution heat source [12]. They combined the single-spark finite element simulation and the machining depth analytical model to study the machining characteristics under specific parameters, and found the fraction of the thermal power transferred to the workpiece is 29.1%. The single-spark finite element simulation results, however, were not verified by experiments. Behroozfar et al. investigated the characteristics of the plasma channel and material removal in electrochemical discharge machining (ECDM) of glass [13]. They found that the average diameter of the plasma channel was about 260 μm under an applied voltage ranging from 30 V to 45 V. Hajian et al. proposed a thermo-physical model and found that the equivalent temperature was about 720 °C in electrochemical discharge milling (ECD milling) [14].

In the ECDM models proposed in literature, the energy input did not use the real-time discharge current and voltage for a single-spark or single-pulse discharge. Behroozfar et al. used the average discharge current and applied voltage for calculating the input energy [13]. While Jiang et al. used the mean energy of the discharge as an input parameter [9]. However, the actual discharge activity is uncertain due to the instability of the gas film, which results in fluctuations in discharge currents. The average discharge current or the mean discharge energy, as a statistical result, is accurate for continuous discharge, but will produce larger errors for a single-spark or single-pulse discharge. To understand the ECDM process more in detail, it is necessary to use the real-time discharge current and voltage as the input parameters of single-pulse discharge model to obtain more accurate simulation results, and thus to obtain more accurate value of the energy transfer ratio (the fraction of discharge energy transferred to the workpiece).

The distribution of discharge locations on the tool electrode is also an important factor affecting the accuracy of the model. In the previous models of ECDM drilling with cylindrical tool electrode, the discharges were simplified to be distributed either at the edge of the electrode or at the bottom surface of the electrode. However, the actual material removal process of ECDM drilling is a dynamic process, which means that the discharge locations also change during the material removal process, especially for gravity-feed micro-drilling. Therefore, the dynamics of discharge location should be considered to develop a refined FEM model for ECDM drilling. In addition, the electrochemical discharge takes place between the tool electrode and the electrolyte, the discharges close enough to the workpiece were effectively used for material removal. Thus, the effective discharge ratio (EDR, the number of discharges used for material removal as a percentage of the total number of discharges) should also be taken into account in a FEM model of ECDM drilling.

In this paper, a two-step approach has been applied to study the material removal process in ECDM drilling through finite element modelling combined with experimental validation, i.e. 1) single-pulse discharge FEM modeling and validation, 2) continuous discharge FEM modeling and validation. The single-pulse duration is set to 10 ms to have enough time for gas film formation. In the continuous discharge step, the DC power is maintained at the given voltage during the machining process. The energy transfer ratio was obtained through the first step and used in the subsequent continuous discharge FEM modeling and simulation. Usually a cylindrical electrode is used in machining micro-hole by electrochemical discharge, and the spark position on the electrode is randomly distributed along its bottom and sidewall, making it difficult to quantitatively analyze the discharge. Therefore, in this research a tapered tool electrode was fabricated in order to concentrate the discharges at the electrode tip in the phase of modeling single-pulse discharge and experiments. In the modeling of the continuous discharge, normal cylindrical electrodes were used. The overall procedure of the finite element modeling of the ECDM material removal process is illustrated in the flowchart shown in Fig. 1. First of all, the single-pulse discharge on the tapered electrode is modeled. The simulation results, obtained with input parameters of actual discharge current and voltage, were compared with the experimental results to obtain the energy transfer ratio. In the second step, the continuous discharge on the cylindrical electrode is modeled while considering the dynamic process of material removal. An effective discharge ratio can be obtained in this simulation. Finally, simulation of hole-drilling by ECDM was carried out and further validated through experiments.