Surface modification by electrical discharge machining: A review

The last decade has seen an increasing interest in the novel applications of electrical discharge machining (EDM) process, with particular emphasis on the potential of this process for surface modification. Besides erosion of work material during machining, the intrinsic nature of the process results in removal of some tool material also. Formation of the plasma channel consisting of material vapours from the eroding work material and tool electrode; and pyrolysis of the dielectric affect the surface composition after machining and consequently, its properties. Deliberate material transfer may be carried out under specific machining conditions by using either composite electrodes or by dispersing metallic powders in the dielectric or both. This paper presents a review on the phenomenon of surface modification by electric discharge machining and future trends of its applications.     

Machining Performance and Surface Integrity of AISI D2 Die Steel Machined Using Electrical Discharge Surface Grinding Process

The aim of this study is to establish optimum machining conditions for EDSG of AISI D2 die steel through an experimental investigation using Taguchi Methodology. To achieve combined grinding and electrical discharge machining, metal matrix composite electrodes (Cu-SiC_p) were processed through powder metallurgy route. A rotary spindle attachment was developed to perform the EDSG experimental runs on EDM machine. Relationships were developed between various input parameters such as peak current, speed, pulse-on time, pulse-off time, abrasive particle size, and abrasive particle concentration, and output characteristics such as material removal rate and surface roughness. The optimized parameters were further validated by conducting confirmation experiments.     

Surface integrity and material removal mechanisms associated with the EDM of Al2O3 ceramic composite

Electric discharge machining (EDM) has been proven as an alternate process for machining complex and intricate shapes from the conductive ceramic composites. The performance and reliability of electrical discharge machined ceramic composite components are influenced by strength degradation due to EDM-induced damage. The success of electric discharge machined components in real applications relies on the understanding of material removal mechanisms and the relationship between the EDM parameters and formation of surface and subsurface damages. This paper presents a detailed investigation of machining characteristics, surface integrity and material removal mechanisms of advanced ceramic composite Al₂O₃–SiC_w–TiC with EDM. The surface and subsurface damages have also been assessed and characterized using scanning electron microscopy (SEM). The results provide valuable insight into the dependence of damage and the mechanisms of material removal on EDM conditions.     

Feasibility study of rotary electrical discharge machining with ball burnishing for Al2O3/6061Al composite

This study investigates the feasibility and optimization of a rotary EDM with ball burnishing for inspecting the machinability of Al₂O₃/6061Al composite using the Taguchi method. Three ZrO₂ balls attached as additional components behind the electrode tool offer immediate burnishing following EDM. Three observed values (machining rate, surface roughness and improvement of surface roughness) are adopted to verify the optimization of the machining technique. In addition, six independent parameters are chosen as variables for evaluating the Taguchi method; these variables are categorized into two groups: (1) electrical parameters, i.e. peak current, pulse duration and non-load voltage; and (2) non-electrical parameters, i.e. flushing pressure of dielectric, rotational speed of electrode and residual height of hump. Experimental results indicated a feasible technique for applying rotary EDM with ball burnishing in machining the Al₂O₃/6061 composite. Optimization of this technique is also discussed.     

Examination of wire electrical discharge machining of Al2O3p/6061Al composites

Alumina particle reinforced 6061 aluminum matrix composites (Al₂O₃p/6061Al) have excellent physical and chemical properties than those of a traditional metal; however, their poor machinability lead to worse surface quality and serious cutting tool wear. In this study, wire electrical discharge machining (WEDM) is adopted in machining Al₂O₃p/6061Al composite. In the experiments, machining parameters of pulse-on time were changed to explore their effects on machining performance, including the cutting speed, the width of slit and surface roughness. Moreover, the wire electrode is easily broken during the machining Al₂O₃p/6061Al composite, so this work comprehensively investigates into the locations of the broken wire and the reason of wire breaking.The experimental results indicate that the cutting speed (material removal rate), the surface roughness and the width of the slit of cutting test material significantly depend on volume fraction of reinforcement (Al₂O₃ particles). Furthermore, bands on the machined surface for cutting 20 vol.% Al₂O₃p/6061Al composite are easily formed, basically due to some embedded reinforcing Al₂O₃ particles on the surface of 6061 aluminum matrix, interrupt the machining process. Test results reveal that in machining Al₂O₃p/6061Al composites a very low wire tension, a high flushing rate and a high wire speed are required to prevent wire breakage; an appropriate servo voltage, a short pulse-on time, and a short pulse-off time, which are normally associated with a high cutting speed, have little effect on the surface roughness.     

Study of electrical discharge grinding using metal matrix composite electrodes

An investigation was made into the combined technologies of electrical discharge machining and grinding (EDMG). A metal matrix (Cu/SiC_p) electrode with a rotating device was made and employed to study the EDMG technology. It was found that 3–7 times the normal electrical discharge machining (EDM) material removal rate (MRR) could be achieved in EDMG under suitable conditions of electrode rotating speed, SiC_p particle size and current. This novel achievement is attributed to the fact that, under appropriate conditions, the hump-shaped melted material created by the EDM mechanism is vulnerable to attack by the grinding mechanism during the EDMG operation, greatly increasing the removal rate. Conversely, under inappropriate conditions, in which hump-shaped material solidifies prior to the non-conductive ceramic particle grinding, the above function becomes negligible and results in much lower MRR.     

A study on the fine-finish die-sinking micro-EDM of tungsten carbide using different electrode materials

In recent years, tungsten carbide (WC) and its composites (WC–Co) are widely used in the die and mold industries due to their unique combination of hardness, strength and wear resistance. Micro-EDM is one of the most effective methods for machining these extremely difficult-to-cut materials. However, numerous applications of WC often involve intense mechanical demands at the surface. Therefore, fine-finish micro-EDM of WC is becoming an imminent and important issue. In this study, investigations have been conducted with view of obtaining fine surface finish in the micro-EDM of WC using tungsten (W), copper tungsten (CuW) and silver tungsten (AgW) electrodes. It was found that the surface characteristics are dependent mostly on the discharge energy during machining. The fine-finish micro-EDM requires minimization of the pulse energy supplied into the gap. In addition, the surface finish was found to be influenced greatly by the electrical and thermal properties of the electrode material. The performance of the electrodes for the finishing micro-EDM was evaluated based on the achieved surface roughness and surface characteristics with respect to material removal rate (MRR) and electrode wear ratio (EWR). It was found that AgW electrode produces smoother and defect-free nanosurface with the lowest R_a and R_max among the three electrodes. Besides, a minimum amount of material migrates from the AgW electrode to the WC workpiece during the finishing micro-EDM. On the other hand, CuW electrodes achieved the highest MRR followed by AgW. In the case of electrode wear, the W electrode has the lowest wear followed by CuW and AgW. Finally, considering all the performance parameters, AgW appears to be the best choice for finish die-sinking micro-EDM of WC.     

Some Studies into Electrical Discharge Machining of Nimonic75 Super Alloy Using Rotary Copper Disk Electrode

The present study reports the rotary disk electrical discharge machining of Nimonic75 super alloy, extensively used in aerospace industries. The experiments have been performed using Taguchi’s orthogonal array L18 (2¹ × 3⁵) with copper disk electrode. The control factors considered were, viz., peak current, pulse on time, pulse off-time, gap voltage, and rotational speed of disk electrode with three levels each, and aspect ratio (AR) of the disk electrode having two levels, as noise factor. The novel approach of this article is to study the effect of the AR of the disk electrode on the performance measures, viz., material removal rate, disk electrode wear rate, and surface roughness. The results based on Taguchi’s analysis show that among the considered process parameters, the AR and peak current significantly affect the machining characteristics. Furthermore, the rotating disk electrode easily flushes off the debris, resulting in better machining and reducing the chances of re-solidified layer formation.     

Probability of precision micro-machining of insulating Si3N4 ceramics by EDM

Electrical discharge machining (EDM) is used as a precision machining method for the electrically conductive hard materials with a soft electrode material. But recently we succeeded to machine on insulating material by EDM. The technology is named as an assisting electrode method. The EDMed surface is covered with the electrical conductive layer during discharge. The layer holds the electrical conductivity during discharge. For micro-EDM, the wear of tool electrode becomes lager ratio than the normal machining. So the micro-machining is extremely difficult to get the precision sample.

In this paper to obtain a fine and precise ceramics sample, some trials were carried out considering the EDM conditions, tool electrodes material and assisting electrode materials. Insulating Si₃N₄ ceramics were used for workpiece. The machining properties were estimated by the removal rate and tool wear ratio. To confirm the change of micro-machining process, the discharge waveforms were observed. The micro-machining of the Ø0.05 mm hole could be machined with the commercial sinking electrical discharge machine.     

Developments in the non-traditional machining of particle reinforced metal matrix composites

The non-traditional machining of particulate reinforced metal matrix composites is relatively new. However, researchers seem to pay more attention in this field recently as the traditional machining of particulate reinforced metal matrix composites is very complex. This research investigates different non-traditional machining, such as electro-discharge, laser beam, abrasive water jet, electro-chemical and electro-chemical discharge machining of this composite materials. The machining mechanism, material removal rate/machining speed and surface finish have been analysed for every machining process. This analysis clearly shows that vaporisation, melting, chemical dissolution and mechanical erosion are the main material removal mechanisms during non-traditional machining. The thermal degradation and the presence of reinforcement particles mainly damage the machined surface. The understanding of electro-discharge, laser beam and abrasive water jet machining is more developed than that of electro-chemical and electro-chemical discharge machining for particulate reinforced MMC.     

Effect of machining fluid on the process performance of electric discharge milling of insulating Al2O3 ceramic

Wire electric discharge machining (WEDM) and electrical discharge machining (EDM) promise to be effective and economical techniques for the production of tools and parts from conducting ceramic blanks. However, the manufacturing of insulating ceramic blanks with these processes is a long and costly process. This paper presents a new process of machining insulating ceramics using electrical discharge (ED) milling. ED milling uses a thin copper sheet fed to the tool electrode along the surface of the workpiece as the assisting electrode and uses a water-based emulsion as the machining fluid. This process is able to effectively machine a large surface area on insulating ceramics. Machining fluid is a primary factor that affects the material removal rate and surface quality of the ED milling. The effects of emulsion concentration, NaNO₃ concentration, polyvinyl alcohol concentration and flow velocity of the machining fluid on the process performance have been investigated.     

A Taguchi and experimental investigation into the optimal processing conditions for the abrasive jet polishing of SKD61 mold steel

This study introduces an abrasive jet polishing (AJP) technique in which the pneumatic air stream carries not only abrasive particles, but also an additive of either pure water or pure water with a specified quantity of machining oil. Taguchi design experiments are performed to identify the optimal AJP parameters when applied to the polishing of electrical discharge machined SKD61 mold steel specimens. A series of experimental trials are then conducted using the optimal AJP parameters to investigate the respective effects of the additive type and the abrasive particle material and diameter in achieving a mirror-like finish of the polished surface. The Taguchi trials indicate that when polishing is performed using pure water as an additive, the optimal processing parameters are as follows: an abrasive material to additive ratio of 1:2, an impact angle of 30°, a gas pressure of 4 kg/cm², a nozzle-to-workpiece height of 10 mm, a platform rotational velocity of 200 rpm, and a platform travel speed of 150 mm/s. Applying these processing parameters, it is found that the optimal polishing effect is attained using #8000SiC abrasive particles and a 1:1 mixture of water-solvent machining oil and pure water. The experimental results show that under these conditions, the average roughness of the electrical discharge machined SKD61 surface is reduced from an original value of R_a=1.03 μm (R_max: 7.74 μm) to a final value of R_a=0.13 μm (R_max: 0.90 μm), corresponding to a surface roughness improvement of approximately 87%.     

Effect of micro-powder suspension and ultrasonic vibration of dielectric fluid in micro-EDM processes—Taguchi approach

Solutions are needed for increasing the material removal rate without degrading surface quality in micro-electrical discharge machining (μ-EDM). This paper presents a new method that consists of suspending micro-MoS₂ powder in dielectric fluid and using ultrasonic vibration during μ-EDM processes. The Taguchi method is adopted to ascertain the optimal process parameters to increase the material removal rate of dielectric fluid containing micro-powder in μ-EDM using a L₁₈ orthogonal array. Pareto analysis of variance is employed to analyze the four machining process parameters: ultrasonic vibration of the dielectric fluid, concentration of micro-powder, tool electrode materials, and workpiece materials. The results show that the introduction of MoS₂ micro-powder in dielectric fluid and using ultrasonic vibration significantly increase the material removal rate and improves surface quality by providing a flat surface free of black carbon spots.     

短电弧加工不同直径电极间隙流场研究

在短电弧加工过程中,利用流体软件Fluent对不同的电极直径加工间隙流场进行仿真,通过极间流场中的压力场与速度场间接得出加工屑排除的变化规律,并通过实验进行验证。结果发现:电极直径的增大可以促进加工屑从加工间隙排出,减少因间隙颗粒的堆积而产生的短路现象,从而避免“二次放电”;当加工深度不变时,电极直径的大小与工件材料的去除率呈正相关,证明短电弧加工过程中存在“面积效应”;电极直径的相对变化对工件表面质量没有明显的改善,即对表面粗糙度的影响不大。     

Material Removal Rate,Kerf, and Surface Roughness of Tungsten Carbide Machined with Wire Electrical Discharge Machining

In this article, the effects of varying seven different machining parameters in addition to varying the material thickness on the machining responses such as material removal rate, kerf, and surface roughness of tungsten carbide samples machined by wire electrical discharge machining (WEDM) were investigated. The design of experiments was based on a Taguchi orthogonal design with 8 control factors with three levels each, requiring a set of 27 experiments that were repeated three times. ANOVA was carried out after obtaining the responses to determine the significant factors. The work piece thickness was expected to have a major effect on the material removal rate but showed to be significant in the case of surface roughness only. Finally, optimization of the machining responses was carried out and models for the material removal rate, kerf, and surface roughness were created. The models were validated through confirmation experiments that showed significant improvements in machining performance for all investigated machining outcomes.     

金属球阀合金球面电火花机械复合磨削加工新方法

耐高温（≥500℃）、高硬度（≥62 HRC）、耐磨损是大型金属球阀内芯合金球体的特性,球体表面必须进行硬化处理。通常,在球阀球体表面喷WC或镍基合金等高硬度涂层材料,其表面硬度≥62 HRC,但使用传统机械磨削方法加工效率极低。在分析电火花机械复合加工原理的基础上,设计了适用于高硬度回转球面精密磨削加工的方法,并设计制作了一种专用复合工具电极和专用夹具,通过使用青铜结合剂金刚石砂轮的复合工具电极来实现合金球体表面的电火花机械复合磨削加工。     

Material removal mechanisms of cemented carbides machined by ultrasonic vibration assisted EDM in gas medium

A new electrical discharge machining (EDM) technology named tool electrode ultrasonic vibration assisted electrical discharge machining in gas medium (UEDM in gas) is proposed and its principle is introduced. Relevant experimental equipment was designed by which a series of machining experiments of cemented carbide material were carried out. The mechanisms of cemented carbide material removal are discussed in detail through observing and analyzing the microstructures of machined surface. Five material removal mechanisms of cemented carbides machined by UEDM in gas were proposed, which are melting and evaporation, oxidation and decomposition, spalling, the force of high-pressure gas and the affection of ultrasonic vibration.     

Evolutionary programming method for modeling the EDM parameters for roughness

The method of electrical discharge machining (EDM), one of the processing methods based on non-traditional manufacturing procedures, is gaining increased popularity, since it does not require cutting tools and allows machining involving hard, brittle, thin and complex geometry.

By using different EDM parameters (current, pulse on-time, pulse off-time, arc voltage), the R_a (μm) roughness value as a result of application of a number of copper electrode-hardened powder metals (cold work tool steel) to a work piece has been investigated, in this study. At the same time, roughness values obtained from the experiments that have been modeled by using the genetic expression programming (GEP) method and a mathematical relationship has been suggested between the GEP model and surface roughness and parameters affecting it. Moreover, EDM has been used by applying copper, copper–tungsten (W–Cu) and graphite electrodes to the same material with experimental parameters designed in accordance with the Taguchi method. Results obtained from this study have been compared among each other and similar studies in the literature.     

Optimization of machining parameters in magnetic force assisted EDM based on Taguchi method

A versatile process of electrical discharge machining (EDM) using magnetic force assisted standard EDM machine has been developed. The effects of magnetic force on EDM machining characteristics were explored. Moreover, this work adopted an L18 orthogonal array based on Taguchi method to conduct a series of experiments, and statistically evaluated the experimental data by analysis of variance (ANOVA). The main machining parameters such as machining polarity (P), peak current (I_p), pulse duration (τ_p), high-voltage auxiliary current (I_H), no-load voltage (V) and servo reference voltage (S_v) were chosen to determine the EDM machining characteristics such as material removal rate (MRR) and surface roughness (SR). The benefits of magnetic force assisted EDM were confirmed from the analysis of discharge waveforms and from the micrograph observation of surface integrity. The experimental results show that the magnetic force assisted EDM has a higher MRR, a lower relative electrode wear ratio (REWR), and a smaller SR as compared with standard EDM. In addition, the significant machining parameters, and the optimal combination levels of machining parameters associated with MRR as well as SR were also drawn. Moreover, the contribution for expelling machining debris using the magnetic force assisted EDM would be proven to attain a high efficiency and high quality of surface integrity to meet the demand of modern industrial applications.     

工艺参数对电熔爆大长径比异形深孔加工影响规律研究

采用电熔爆加工技术进行大长径比异形深孔加工时,加工表面质量和电极损耗是影响加工效果的主要因素。采取自主开发的异形深孔加工电极,通过单因素试验法研究大长径比异形深孔加工过程中峰值电流、电源电压、电极材料、冷却液压力等工艺参数对加工表面质量和电极损耗的影响;通过正交试验的方法确定各因素影响加工表面质量的主次顺序及最佳工艺参数组合。结果表明：在电极材料为铜钨合金时,峰值电流120 A、电源电压21 V、冷却液压力6 MPa为最佳工艺参数组合。研究结果为电熔爆异形深孔实际加工中的参数选择提供了参考。