Electrochemical Discharge Phenomenon Research Articles

Experimental Investigation of Discharge Phenomena in Electrochemical Discharge Machining Process

Electrochemical discharge machining (ECDM) is a promising non-traditional processing technology used to machine non-conductive materials, such as glass and ceramic, based on the evoked electrochemical discharge phenomena around the tool electrode. The discharge in ECDM is a key factor that affects the removal of material. Moreover, the discharge current is an important indicator reflecting the discharge state. However, the discharge characteristics remain an open topic for debate and require further investigation. There is still confusion regarding the distinction of the discharge current from the electrochemical reaction current in ECDM. In this study, high-speed imaging technology was applied to the investigation of the discharge characteristics. By comparing the captured discharge images with the corresponding discharge current, the discharge can be classified into three types. The observations of the discharge effect on the gas film indicate that a force was exerted on the gas film during the discharge process and the shape of the gas film was changed by the force. In addition, the energies released by different types of discharge were calculated according to the voltage and current waveforms. The discharge frequency was found to increase with the increase in applied voltage and the frequency of the second type of discharge was approximately equal to that of the third type when the applied voltage was higher than 40 V.

Electrochemical Manufacturing to Support Industry 4.0 – Spark Assisted Chemical Engraving (SACE)

Manufacturing industry is facing new challenges as there is a growing demand for mass-personalized products at low cost. A new kind of processes need to be developed. This new revolution was recognized recently by industry and in Germany the key word industry 4.0 [1] was introduced to characterize this “fourth industrial revolution” for the entire manufacturing value chain. A major hurdle to overcome for successful production of mass personalized products are setup and tooling costs. According to recent studies (made by the Universities of Michigan and Cincinnati for the World Economic Forum) hybrid technologies, including electrochemical technologies, are promising to address these manufacturing challenges [2]. Among the established electrochemical technologies can be cited electrochemical (discharge) machining, electro-deposition/-forming and electropolishing e.g. as post-process for metal additive manufactured (AM) parts. None is yet used for mass personalisation, where the custom products are no longer an assembly of individual parts (i.e. modular design), but they are fully personalized, i.e. the shapes of the parts change too. Suitable manufacturing processes for personalized batch-size-1 production must be highly flexible and have little overhead, particularly for the tooling. Hybrid technologies are very promising as these processes require little to no specialized tooling and can handle virtually any shape, including inner surfaces. At the same time, glass, existing for millions of years in its natural form, has fascinated and attracted much interest from both the academic and industrial world. The application of glass science to the improvement of industrial tools occurred only in the past century, with a few exceptions. Glass has been employed in many forms to fabricate glazing and containers for centuries while it is now entering new applications that are appearing in micro and even nanotechnology like fibers, displays and Micro-Electro-Mechanical-System (MEMS) devices [3]. Many qualities make glass attractive since it is transparent, chemically inert, environmentally friendly and its mechanical strength and thermal properties. In fact, no other materials being mass-produced have shown such qualities over so many centuries. Nowadays glass offers recycling opportunities and allows for tailoring new and dedicated applications. Moreover, glass is radio frequency (RF) transparent, making it an excellent material for sensor and energy transmission devices. Another advantage of using glass in microfluidic MEMS devices [4] is its relatively high heat resistance, which makes these devices suitable for high temperature microfluidic systems [5] and sterilization by autoclaving. However, glass is a hard to machine material, due to its hardness and brittleness. Machining high-aspect ratio structures is still challenging due to long machining times, high machining costs and poor surface quality [6]. Hybrid methods like Spark Assisted Chemical Engraving (SACE) [7] perform well to machine high aspect ratio and smooth surface structures on glass. These assets of SACE technology combined with its relative high machining speeds compared to chemical methods and low-cost compared to femto-laser technologies make SACE perfectly suitable for rapid prototyping of micro-scale glass devices. In this thermochemical process, a voltage is applied between tool- and counter-electrode dipped in an alkaline solution (typical NaOH or KOH). At high voltages (around 30 V), the bubbles evolving around the tool electrode coalesce into a gas film and discharges occur from the tool to the electrolyte through it. Glass machining becomes possible due to thermally promoted etching (breaking of the Si-O-Si bond) [7]. In the present communication, it is shown how electrochemical processes can be used to design new high precision manufacturing processes for industry 4.0. In particular hybrid machining by SACE technology is discussed for hard-to-machine materials like glass. Some other examples are highlighted as well in the field of post-processing technologies for metal AM parts and fabrication of high-precision complex metal structures based on 3D printed high resolution polymer models. [1] Deloitte, “Industry 4.0. Challenges and solutions for the digital transformation and use of exponential technologies”, Deloitte, pp. 1–30, 2015. [2] J. Ni, J. Lee “Emerging and Disruptive Technologies for the Future of Manufacturing” Case study no.7 World Economic Forum Global Agenda Council on the Future of Manufacturing [3] E. Le Bourhis, “Glass, Mechanics and Technology.”, Wiley-VCH, 2014. [4] G. M. Whitesides, “The origins and the future of microfluidics.”, Nature, vol. 442, no. 7101, pp. 368–373, 2006. [5] D. Sinton, “Energy: the microfluidic frontier.”, Lab Chip, vol. 14, no. 17, 2014. [6] L. Hof, J. D. Abou Ziki, “Micro-hole drilling on glass substrates – a review”, Micromachines, vol. 8, no.53, 2017. [7] R. Wüthrich and J. D. Abou Ziki, “Micromachining Using Electrochemical Discharge Phenomenon.”, Elsevier, 2015. Figure 1

Application of electrolyte injection to the electro-chemical discharge machining (ECDM) on the optical glass

B270 is a special type of optical glasses which has found sophisticated applications in the lens and prism manufacturing while keeping the cost under the control. Machining process through electro-chemical discharge is an advanced machining technology to produce deep holes, micro-grooves, and micro-channels on the non-conductive materials. The process is a non-traditional machining method, including melting at high temperatures because of electrical discharges on the electrode tip and chemical etching through the electrolyte. A major problem in drilling operations, using electro-chemical discharge phenomenon, is electrolyte access to the machining area in a deep section of the hole. As an electro-chemical discharge phenomena, discharge creation and chemical etching are main parts of electro-chemical discharge phenomenon. This subject plays an important role especially in deep holes because of limited electrolyte circulation. Therefore, this research addresses to improve the performance of electro-chemical discharge process in mesoscale drilling operations. For this purpose, hollow electrodes with high-pressure electrolyte injection system are employed to provide the electrolyte to the machining area in deep holes. The experiments at various levels of pressure and electrode diameter were done. The results showed that by injection of electrolyte through the hollow cathode to the drilling area, material removal rate (MRR) and drilling speed increased. Also, mentioned effects rose by increasing the electrode diameter and injection pressure. Improvement the hole depth versus the effective pressure for different electrode diameters was about 50–70%. More injection pressure resulted in larger hole diameter and creation of conical shape hole.

Enhancement of electrochemical discharge machining accuracy and surface integrity using side-insulated tool electrode with diamond coating

Electrochemical discharge machining (ECDM) is an emerging non-traditional processing technology used to machine electrical non-conductive material like glass and ceramics, by using the electrochemical discharge phenomena around the tool electrode. In the general ECDM drilling process, the gas film forms and electrical discharge appears at both the tool end and the tool sidewall that were exposed to the electrolyte. The undesirable sidewall discharge enlarges the hole entrance diameter and destroys the hole surface integrity. In order to prevent the sidewall discharge, a side-insulated tool electrode with a 4 μm-thick diamond coating layer was used in ECDM micro-hole drilling. The gas film formation and electrical discharge mostly happened on the tool end due to the insulation layer on the tool sidewall. Experiments showed that, compared with the traditional tool electrode, the side-insulated tool electrode achieves a smaller hole diameter and better surface integrity without an obvious heat affected zone at the hole entrance. Furthermore, the hole diameter nearly remains the same as the machining depth increases from 50 μm to 500 μm. The side-insulated electrode has an advantage in enhancing shape accuracy by reducing the taper angle of the micro hole. When the machining depth is 600 μm, the side-insulated electrode achieves a much smaller hole taper angle () than the traditional tool electrode does ().

Sparc assisted electrochemical machining: a novel possibility for microdrilling into electrical conductive materials using the electrochemical discharge phenomenon

This paper details the fundamental principles of the machining method spark assisted electrochemical machining (SAEM). SAEM is a further development of the electrochemical arc machining (ECAM) which makes use of the electrochemical discharge phenomenon to machine electrically conductive materials. ECAM is not able to drill micro-holes, but now, using SAEM a postponed electrochemical finishing of the SAEM drilled micro-hole can be realized. The mass flow calibration of a throttle plate can be done immediately after SAEM microdrilling by electrochemical machining (ECM) with a still axis-symmetrically located tool electrode with the same machining equipment, working media, and power source. To enable ECAM to drill micro-holes, its material removal mechanism is improved by combination with the contact arc as an additional mechanism. This mechanism is presented and explained herein. The machining efficiency of the newly developed SAEM is optimized by adjusting the gap control in such a way that the contact arc is generated with an adequate frequency. This reduces the machining time by almost 50 %. Some examples of SAEM drilled micro-holes, for which different electrolytes are used, are presented and the surface and heat affected zone of those micro-holes are examined.

Modeling and experimental investigation of gas film in micro-electrochemical discharge machining process

Electrochemical discharge machining (ECDM) is a promising machining technology that effectively machines non-conducting and brittle materials, featuring good material removal rate, flexibility, and accuracy of machining. ECDM makes use of the electrochemical discharge phenomenon to trigger the discharging by the gas film surrounding the tool electrode. As the fundamental of electrochemical discharging, gas film is essential to the machining quality and efficiency. However, modeling of gas film in electrochemical reaction is not well established. This paper presents analytical modeling of the gas film, involving bubble growth and departure on electrode, gas film evolution, and electrolysis characteristics. Experiments were carried out to compare models to the actual discharging phenomenon. High speed camera imaging demonstrated the formation of a gas film on the tool electrode. The range of thickness of gas film found in experiments indicated good consistency with the range of film thickness estimated from analytical models. Experiments on critical voltages and currents further revealed the characteristics of the gas film in electrochemical reaction.

Study of Micro Drilling on Pyrex Glass Using Spark Assisted Chemical Engraving

Spark assisted chemical engraving (SACE) technology has the potential for micro machining glass with advantages of simplicity, high aspect ratio and less micro cracks. Typical micro SACE process utilizes cylindrical tool and suffers from poor electrolyte circulation along with machining depth. This work employed Φ200µm tungsten carbide drill to experimentally fabricate micro holes on Pyrex Glass by applying SACE process. On the rotation of the drill, the micro grooves of the drill are capable of enhancing electrolyte circulation within machining zone and thus improve the processing performance. Several effects of process factors on drilling results were evaluated correspondently. The results revealed that machining time, drill rotation direction and rate, and the contact force affected the processing performance markedly. A deep hole of 245 ¿m in entrance diameter and 8.8 in aspect ratio was obtained by optimal process parameters. Tool wear has also been discussed herein. Keywords: Electrochemical discharge phenomenon, micro drilling, micro machining, non-conductive material, pyrex glass, spark assisted chemical engraving.

Parametric analysis and a soft computing approach on material removal rate in electrochemical discharge machining

Electrochemical discharge machining (ECDM) is a very recent technique in the field of non-conventional machining to machine electrically non-conducting materials efficiently and effectively using the electrochemical discharge phenomenon. In the present paper, an experimental setup of ECDM has been developed and experiments are performed to optimise the process parameters for higher material removal rate (MRR). As there are many process parameters affecting to the process, the Taguchi methodology of robust design of experiments is used for optimization of these process parameters. From the obtained experimental results it was noticed that the material removal mechanism in ECDM is non-linear – the volume of material removed decreases with increasing machining depth. Hence, mathematical modelling is difficult. A soft computing approach called adaptive neuro fuzzy inference system (ANFIS) is adapted to model the non-linear material removal rate.

A mechanistic model of the gas film dynamics during the electrochemical discharge phenomenon

A model for the prediction of the current–voltage characteristics of a two electrodes cell incorporating the dynamics of the gas film formed during the electrochemical discharge phenomenon is developed. In its mean-field version, the model presents good qualitative agreement but overestimates the hysteresis effect and predicts too large current densities for the cell operation once the gas film is formed. An improved stochastic model, which assumes gas film departures from the electrode surface according to a Poisson process, addresses these issues and gives significantly better predictions. Two relations are presented which allow estimating the mean gas film detachment time and its variance from the experimental study of the hysteresis in the forward and reverse scan of a two electrode cell operated at high current densities.

Building micro and nanosystems with electrochemical discharges

Since the discovery of the electrochemical discharge phenomenon by Fizeau and Foucault, several contributions have expanded the wide range of applications associated with this high current density electrochemical process. The complexity of the phenomenon, from the macroscopic to the microscopic scales, led since then to experimental and theoretical studies from different research fields. This contribution reviews the chemical and electrochemical perspectives where a mechanistic model based on results from radiation chemistry of aqueous solutions is proposed. In addition applications to micro-machining and fabrication of nanoparticles are discussed.

Gas film formation time and gas film life time during electrochemical discharge phenomenon

The formation of a gas film around an electrode is at the origin of the electrochemical discharges in aqueous solutions and in molten salts. The study of the electrochemical discharge phenomenon from the current signal point of view is conducted to identify the gas film formations and the following discharges. This is performed using the wavelet analysis as a denoising tool with the discrete Meyer wavelet as base function. The proposed algorithm allows to measure experimentally the gas film life time and its necessary building time prior to each series of discharges. The accuracy of the algorithm is evaluated using generated signals and its application to real data is demonstrated. From the experimental data, it is concluded that the gas film is more stable at high terminal voltages whereas its formation time decreases with it. Electrochemical and thermal interpretations for the measured statistics are proposed.

Electrochemical discharges—Discovery and early applications

Electrochemical discharge phenomenon, as well known as plasma electrolysis, electrode effects or contact glow discharge electrolysis, was first described in literature more than 150 years ago. Today this effect is studied only in quite specialised applications. This was not always the case. During the 19th century and early 20th century, electrochemical discharges were intensively studied and a broad spectrum of applications, ranging from X-ray imaging to wireless telegraphy, was developed. This communication retraces this early history of electrochemical discharges by highlighting the interesting electrochemical effects and applications linked to them. The paper ends by shortly mentioning today's applications of electrochemical discharges which are in the field of micro-machining, surface engineering, nanotechnology and waste water treatment.

The effect of thermal conductivity of the tool electrode in spark-assisted chemical engraving gravity-feed micro-drilling

Spark-assisted chemical engraving (SACE) is a non-traditional micro-machining technology based on electrochemical discharge phenomena. In SACE gravity-feed micro-drilling, various parameters including the thermal properties of the tool electrode play a significant role in the process. Based on a series of experiments using tool electrodes with different thermal properties, the effect in SACE gravity-feed micro-drilling is discussed. It is demonstrated that machining with higher thermal conductivity tool electrodes results in faster machining during the discharge regime and slower machining during the hydrodynamic regime of SACE gravity-feed micro-drilling.

Physical principles and miniaturization of spark assisted chemical engraving (SACE)

Spark assisted chemical engraving (SACE) is an unconventional micromachining technology based on electrochemical discharge phenomena for glass and various ceramics. The limits of SACE with respect to small dimensions in the particular case of glass are explored. It is found, using a specially developed set-up based on an AFM, that even using extremely sharp tool-electrodes does not allow us to produce a smaller pattern than typically 25 µm. It is concluded that the gas film thickness, in which the electrochemical discharges take place, is the main limiting factor. Further experimental investigations on its formation are investigated. By adding surfactants to the electrolyte, in order to increase the wettability of the tool-electrode and therefore to reduce the gas film thickness, it is observed experimentally that the critical voltage reduces significantly. This observation may lead to a novel method of characterizing the gas film thickness in SACE.

Machining of non-conducting materials using electrochemical discharge phenomenon—an overview

Machining with electrochemical discharges is an unconventional technology able to machine several electrically non-conductive materials like glass or some ceramics. After almost 40 years of its first mention in literature, this technology remains an academic application and was never applied in industrial context. The knowledge about machining of non-conducting materials using electrochemical discharge phenomenon is reviewed up to this date with some particular attention to the electrochemical point of view. Some main limiting factors are highlighted and possible solutions are discussed.

Spark assisted chemical engraving in the light of electrochemistry

A novel technology for glass microstructuration called spark assisted chemical engraving (SACE) is presented. This technology is based on electrochemical discharge phenomena. Beyond a critical voltage, electrical discharges occur through the gas film around the electrode and the glass machining is possible. The SACE process is similar to the electrode effects. SACE has been analysed using an electrochemical approach ( U– I plots), the experimental results clarify the onset of the discharge activity. Additionally, two theoretical models of the SACE phenomenon were introduced. Firstly a model, based on percolation theory, is briefly described and predicts the critical parameters (voltage and current). Secondly a method to estimate the spark’s characteristics (amplitude and duration) is presented and with the help of these data the machining depth was estimated by a numerical simulation. Finally, some glass microstructuring examples are presented to show the potential of this technology.

Mechanism of material removal in electrochemical discharge machining: a theoretical model and experimental verification

Electrochemical discharge machining is a very recent technique in the field of non-conventional machining to machine electrically non-conducting materials using the electrochemical discharge (ECD) phenomenon. If a beyond-critical voltage is applied to an electrochemical cell, discharge initiates between one tool of the electrodes and the surrounding electrolyte, which is termed here ‘electrochemical discharge’. In the present work a simplified model is developed to predict the characteristics of the material removal rate for varying input parameters with the objective of finding the possibility of enhancing the capability of the process. The ECD phenomenon has been analysed as a switching process between the tool (one of the electrodes) and the electrolyte and it has been found that an extra control parameter can be obtained by introducing an additional inductance in the circuit. The theoretical and experimental results indicate that a substantial increase in the material removal rate can be achieved due to the additional inductance introduced.

Microwelding using electrochemical discharge

Use of electrochemical discharge (ECD) phenomenon for generating intense localized heat in a controlled manner is not unknown in the field of unconventional machining, especially when the work material is electrically non-conducting. In this paper the authors investigate the feasibility and the characteristics of microwelding using BCD. One of the major applications of microwelding is the fabrication of thermocouples using very thin wires, and this investigation is dedicated to microwelding thermocouple wires to produce satisfactory joints. It has been shown that such welds can be very conveniently achieved using ECD in a self-controlled manner. The major process parameters have been identified and their influence on the weld characteristics has been investigated. An attempt has been made to develop a theoretical model. The results are in good qualitative agreement with the experimental observations.

Mechanism of spark generation during electrochemical discharge machining: a theoretical model and experimental verification

The use of the electrochemical discharge phenomenon to machine materials is a very recent technique in the field of non-conventional machining. When the machined material is electrically conducting, the process is usually termed ‘electrochemical arc machining’ (ECAM), whereas for non-conducting work materials it is termed ‘electrochemical discharge machining’ (ECDM). Although in both cases electrical discharge takes place through the electrolyte and plays a critical role, the mechanism of spark generation has not been investigated adequately. In the present work a theoretical model of this discharge phenomenon is developed. With the help of the model, the critical voltage and current required to initiate discharge between the electrode and the electrolyte are estimated. The theoretically-predicted values compare quite well with experimental observations.

Effect of the circuit parameters on the electrolytes in the electrochemical discharge phenomenon

The electro-chemical discharge (ECD) phenomenon is a process producing electrical discharges in electrolytes by the application of voltage to an electrolytic cell. This process has the advantage of machining non-conducting material kept in the vicinity of the discharge. The study in this paper explores the influence of external circuit parameters on the discharge process. The experimental results of average voltage and current with NaOH and KOH electrolytes of 2, 5, 10 M concentrations with different circuit configurations are reported. The instantaneous voltage—current behaviour during discharge at 60 V of NaOH, KOH and HCl electrolytes are reported also for concentrations of 2 and 5 M. The best circuit configuration for the machining of non-conducting material is discussed.