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.     

Pulse electrochemical discharge machining of glass-fiber epoxy reinforced composite

The effect of pulse current and tool immersion depth on gas film formation and its consequences on machining quality in the pulse electrochemical discharge machining (PECDM) of glass-fiber epoxy reinforced composite are studied. The frequency and duty cycle of the pulse current were controlled for discharging at no more than single spark per cycle. As compared to ECDM with DC current, the PECDM results in smaller hole diameter and smaller heat affected zone (HAZ). Also, lower tool immersion depth results in thinner gas film and smaller HAZ in the workpiece.     

Voltage pulse frequency and duty ratio effects in an electrochemical discharge microdrilling process of Pyrex glass

Electrochemical discharge machining (ECDM) is used to microdrill glass wafers. One of the drawbacks of ECDM is the heat-affected zone (HAZ) left on the microdrilled hole surface. To reduce the HAZ, a series of rectangular voltage pulses, were applied in this study instead of the rectified or full-wave DC voltages. The effect of the frequency and duty ratio of the voltage pulse on the ECDM of Pyrex glass was experimentally investigated. The experimental results show that the thermal damage of the microdrilled hole decreases as the frequency increases and as the duty ratio decreases. It was also found that the clearance increases as the tool diameter decreases. This experimental investigation provides a new method that exhibits several advantages over conventional ECDM.     

On the analysis of the electrochemical spark machining process

The electrochemical spark machining (ECSM) process has been proved as a potential process for machining of low machinability high-strength electrically non-conducting materials, but the mechanism of material removal during the process, by and large, is not yet understood. In the present work, the electrochemical discharge is modelled as a phenomenon similar to that which occurs in arc discharge valves. This phenomenon is used to explain various experimental results, on the basis of circuit and arc discharge valve characteristics. The spark energy and the approximate order of hydrogen gas bubble diameter are computed by the proposed valve theory. Material removal rate is evaluated by modelling the problem as a 3-D unsteady state heat conduction problem. The problem is solved by the finite element method to compute the temperature distribution which is post-processed for estimating material removal per spark, overcut obtained in the machined cavity, and attainable maximum penetration depth. The conclusion drawn is that the application of valve theory to the ECSM process seems to be realistic. Estimated material removal rate, overcut and maximum penetration depth show a good agreement with experimental findings.     

Trepanning of Al2O3 by electro-chemical discharge machining (ECDM) process using abrasive electrode with pulsed DC supply

Electro-chemical discharge machining (ECDM) of electrically non-conductive high-strength–high-temperature-resistant ceramics such as aluminium oxide (Al₂O₃) by trepanning method (i.e. orbital motion of tool) has shown the possibility of drilling large size holes by comparatively smaller electrodes efficiently and economically. However, at greater machined depth, the conventional electrode configurations and machining parameters show that machining performance gradually deteriorates with increase in tool depth and finally cause micro cracks on the machined surface due to thermal shocks at high voltage. To reduce this problem and to enhance the machining performance during trepanning operation of Al₂O₃, a spring fed cylindrical abrasive electrode of 1.5 mm diameter has been used under the effect of the three most influential parameters, namely, pulsed DC supply voltage, duty factor and electrolyte conductivity, each at five different levels to assess the volume of material removed, machined depth and diameteral overcut. The results obtained from this study revealed that pulsed DC has reduced the tendency of cracking at high supply voltage compared to smooth DC and the machining ability of the abrasive electrode was better than copper electrode as it would enhance the cutting ability due to the presence of abrasive grains during machining. In addition to this, trepanning provides the scope for drilling bigger holes.     

Effect of surface roughness of tool electrode materials in ECDM performance

Electrochemical discharge machining (ECDM) is an emerging non-traditional machining process that involves high-temperature melting assisted by accelerated chemical etching. In this study, the tool electrode (200 μm in diameter) is fabricated by wire electrical discharge grinding (WEDG). After the tool electrode is machined, the surface roughness of tool electrode materials (stainless steel, tungsten carbide, and tungsten) is different because of the physical properties. However, the surface roughness affects the wettability on tool electrode, and also changed the coalesce status of gas film in ECDM. Hence, this study explores the wettability and machining characteristics of different tool electrode materials and their impact on gas film formation. Their machining performance and extent of wear under gravity-feed micro-hole drilling are also examined. Experimental results show that the optimal voltage of different tool electrode can shed light on the machining performance. Moreover, wettability of tool electrode is determined by surface roughness of tool material, which in turn affects the coalesce status of gas film, machining stability and micro-hole diameter achieved. In addition, differences in tool material also results in variations in machining speed. Significant tool wear is observed after repeated gravity-feed machining of 50 micro-holes.     

绝缘涂层构件电化学放电穿孔技术

研究了绝缘涂层构件的电化学放电加工原理与5种放电过程及其电压电流波形特点。分析了绝缘涂层构件穿孔技术中的绝缘涂层材料加工、绝缘涂层与基体金属过渡加工及基体金属加工3种不同状态及其特点,采用不同的进给速度和加工电压,以提高加工效率和质量。对钛合金基体的氧化锆陶瓷涂层材料进行了电化学放电穿孔实验,验证了该方法是可行的。     

Analysis and performance of slotted tools in electrical discharge drilling

Accumulation of machining debris due to inadequate gap flushing severely limits the material removal rate and impairs the quality of the machined surface in electrical discharge machining. This is particularly pronounced in electrical discharge drilling of holes with a high aspect-ratio, wherein conventional flushing techniques essentially cease to be effective. To this end, this paper proposes the application of novel tool electrodes comprising geometric features specifically designed to promote tool rotation-induced debris egress. The corresponding flow fields are modelled numerically to optimize said designs. Relative to conventional rotating cylindrical tools, removal rate enhancements on the order of 300% are demonstrated.     

电火花块反拷加工控制系统设计与实验

对电火花块反拷加工的运动控制系统进行了设计,提出了可行的数据采集策略与运动控制策略。基于LabView开发的运动控制系统主要包括初始化模块、粗对刀模块、精确对刀模块、加工控制模块与坐标实时显示模块。应用设计的控制系统对碳化钨工具电极进行了反拷加工实验,正交实验结果得出:材料去除率随着脉宽的增大先增加后减小,脉宽有一个最佳值,脉宽小了能量不足,脉宽大了容易产生电弧放电,影响了材料去除。材料去除率随着脉间相对于脉宽倍数、主轴转速与进给率的增大而增大,足够大的脉间以及主轴转速,有利于电火花加工区域的消电离与加工产物的排除,在材料能够有效去除的前提下,进给速度越快材料去除率越高。     

Reliability of electrode wear compensation based on material removal per discharge in micro EDM milling

This paper investigates the reliability of workpiece material removal per discharge (MRD) estimation for application in electrode wear compensation based on workpiece material removal. An experimental investigation involving discharge counting and automatic on the machine measurement of removed material volume was carried out in a range of process parameters settings from fine finishing to roughing. MRD showed a decreasing trend with the progress of the machining operation, reaching stabilization after a number of machined layers. Using the information on MRD and discharge counting, a material removal simulation tool was developed and validated.     

雾中电火花加工的极间能量分配系数研究

对雾中电火花加工中铜-钢电极对分别采用正-负和负-正接法时,工件端所分配的放电能量系数进行了研究。首先通过实验测量了放电加工中工件的温升、单蚀坑尺寸及放电平均电压、电流等,然后建立了热场与流场相耦合的物理模型,结合有限元分析软件COMSOL3.5,得到了内喷雾电火花加工在不同脉宽下工件的能量分配系数范围。从理论分析的角度证明了前期实验中得到的在喷雾电火花加工中采用铜-钢电极对时,为获得较高的材料去除率,适宜采用钢工件接正极的结论。     

Enhancement of ECDM efficiency and accuracy by spherical tool electrode

Electrochemical discharge machining (ECDM) is an emerging non-traditional processing technique that involves high-temperature melting and accelerated chemical etching under the high electrical energy discharged. However, there are still several obstacles to overcome. First, both machining time and hole entrance diameter were found to increase with increasing machining depth. In particular, the increase becomes drastic when machining depth exceeds 250 μm. In addition, achieving both high efficiency and accuracy in drilling a through hole in hard and brittle materials by ECDM poses even greater difficulty. To solve the above problems, this study proposed using a tool electrode with a spherical end whose diameter (150 μm) is larger than that of its cylindrical body (100 μm). Experimental results show that the curve surface of the spherical tool electrode reduces the contact area between the electrode and the workpiece, thus facilitating the flow of electrolyte to the electrode end, and enables rapid formation of gas film, resulting in efficient micro-hole drilling. Moreover, the curve surface does not cause excessive concentration of current density; and hence, bubbles grow at a more uniform speed; thus, increasing the discharge frequency. Comparison between machining depth of 500 μm achieved by conventional cylindrical tool electrode and the proposed spherical tool electrode shows that machining time was reduced by 83% while hole diameter was also decreased by 65%.     

混粉电解电火花复合加工工艺研究

在电解电火花加工技术的基础上,提出了在电解液中加入导电粉末加工非导电材料的新方法。通过对高硼硅玻璃非导电材料进行混粉电解电火花加工实验,对加工过程中各影响参数(电压、电解液浓度、粉末浓度)进行分析,结果表明:采用混粉电解电火花加工技术可高效地加工脆硬非导电材料的表面,并能获得较好的表面质量。     

Modeling study of the hydrodynamic arc breaking mechanism in BEAM

Blasting erosion arc machining (BEAM) utilizes thermal effect of the arc rather than spark to remove workpiece material. This paper investigates the expansion and distortion rules of the discharge channel based on the analysis of high speed photography and the micrographs of craters to study mechanism of BEAM. Based on these investigations, a thermal finite element model is developed. Then the temperature distributions in workpieces, both with and without flushing, are obtained. The comparisons of the obtained temperature distributions explain the formation of the trailing crater and the effect of hydrodynamic arc breaking mechanism on the efficiency of BEAM.     

Finite element prediction of material removal rate due to electro-chemical spark machining

Electro-chemical spark machining (ECSM) is an innovative hybrid machining process, which combines the features of the electro-chemical machining (ECM) and electrodischarge machining (EDM). Unlike ECM and EDM, ECSM is capable of machining electrically non-conducting materials. This paper attempts to develop a thermal model for the calculation of material removal rate (MRR) during ECSM. First, temperature distribution within zone of influence of single spark is obtained with the application of finite element method (FEM). The nodal temperatures are further post processed for estimating MRR. The developed FEM based thermal model is found to be in the range of accuracy with the experimental results. Further the parametric studies are carried out for different parameters like electrolyte concentration, duty factor and energy partition. The increase in MRR is found to increase with increase in electrolyte concentration due to ECSM of soda lime glass workpiece material. Also, the change in the value of MRR for soda lime glass with concentration is found to be more than that of alumina. MRR is found to increase with increase in duty factor and energy partition for both soda lime glass and alumina workpiece material.     

Improving machining performance of wire electrochemical discharge machining by adding SiC abrasive to electrolyte

The use of wire electrochemical discharge machining (WECDM) to slice hard brittle materials has recently been studied because its effectiveness is independent of the mechanical characteristics of the machined materials. Therefore, materials with high hardness, brittleness, strength and electrical insulation, which are difficult-to-cut, can be machined. In ECDM, the electrochemical reaction produces hydrogen bubbles, which accumulate around the cathode. A thin gas layer forms on the surface of the electrode and isolates the electrode from the electrolyte. When a voltage that exceeds the critical voltage is applied, continuous discharge occurs. The material near the electrode is removed by the discharge erosion and chemical etching. The use of WECDM to cut electrically insulating materials has only recently been investigated. However, the breakdown of the gas in the bubbles and the vibration of the wire in WECDM strongly affect the shape accuracy. This work aims to improve the over cut quality by adding SiC abrasive to the electrolyte. A mechanism that combines discharge, chemical etching and abrasive cutting is studied. The effects on expansion, roughness and material removal rate (MRR) are discussed. The experimental results reveal that adding abrasive reduces the slit expansion because it increases the critical voltage. The particles disrupt the bubble accumulation to form an isolating layer around the wire, increasing the critical voltage and reducing the discharge energy. The surface roughness is improved because the abrasive helps to refine the micro-cracks and melted zone that is formed by discharge heat erosion. Meanwhile, smaller grit produces lower roughness. The quality of the slit can be controlled; its expansion and roughness of the slit are 0.024 mm and 0.84 um Ra, respectively.     

Study of gas film quality in electrochemical discharge machining

Electrochemical Discharge Machining (ECDM) has been demonstrated to be an alternative spark-based micromachining method for fabricating microholes and microchannels in non-conductive brittle materials. However, the mechanism for attaining accurate control of the contour shape and dimensions remains to be explored. In ECDM process, the gas film on the electrode surface is used as the dielectric medium required for discharge generation. Quality of gas film is the dominant factor that determines the machining qualities such as geometric accuracy, surface roughness and repeatability. Nevertheless, it is difficult to assess the gas film quality of ECDM. In this study, current signals and machined contours were taken as indexes of gas film quality. Experimental results showed that a stable and dense gas film could be obtained when the applied voltage exceeded the critical voltage and reached a specific level, which is called the “transition voltage” in this study. At the transition voltage, a stable electrochemical discharge activity could be generated, thus producing the smallest deviation of contour dimensions. Moreover, when the drilling process reached a certain critical depth, bubbles inside the hole could not easily escape. In order to reduce the interface energy between bubbles, a thicker gas film is formed at the hole entrance, resulting in unstable discharge performance that undermined machining results. In summary, information provided by current signals can shed light on the changes in gas film structure, which serve as useful reference for varying process parameters to achieve better efficiency and accuracy.     

气中微细电火花沉积加工的单脉冲放电过程热力学仿真(英文)

建立气中微细电火花沉积加工过程电极材料的热物理模型。利用有限元分析软件ANSYS对单脉冲条件下的工具电极和工件的瞬态温度场进行数值模拟,分析热源形式、初始边界条件和放电能量分配对工具电极和工件材料蚀除形式的影响,并预测适合微细电火花沉积加工的工艺参数。采用仿真预测得到的工艺参数,在高速钢工具表面稳定沉积出直径约200μm、高度约1.2mm的微圆柱结构。对沉积材料微观组织结构的测试分析表明,沉积材料与基体结合紧密。工艺实验和测试分析证明了所建立的微细电火花沉积加工过程的单脉冲放电热物理模型和有限元求解过程的正确性。     

Experimental investigation of the process behaviour in Mechano-Electrochemical Milling

The increasing demand for high-performance materials, in for example aerospace and biomedical industries, calls for more efficient and capable technologies. This paper describes a new technology, namely Mechano-Electrochemical Milling (MECM), which combines electrochemical machining (ECM) with a mechanical cutting process. The process behaviour has been investigated experimentally based on the machining of two Titanium alloys, Titanium grade 2 and Titanium grade 5. The material removal mechanism was investigated through analysis of the machined surface and removed material. Besides the slightly higher material removal rate in MECM compared to ECM, the MECM process results in more stable process conditions.     

超声振动辅助气中放电加工实验分析

超声振动辅助气中放电加工技术避免了常用的煤油等工作液作为介质带来的环境污染问题，具有工作环境清洁、适用范围广、加工效率高、工具电极简单等优点。实验研究了电压、脉冲宽度、峰值电流、超声振幅及气体介质压力等参数对加工效率、工件表面粗糙度及电极损耗的影响．并对试验结果进行了分析。