Workpiece surface modification using electrical discharge machining

Electrical discharge machining (EDM) is a widely used process in the mould / die and aerospace industries. Following a brief summary of the process, the paper reviews published work on the deliberate surface alloying of various workpiece materials using EDM. Details are given of operations involving powder metallurgy (PM) tool electrodes and the use of powders suspended in the dielectric fluid, typically aluminium, nickel, titanium, etc. Following this, experimental results are presented on the surface alloying of AISI H13 hot work tool steel during a die sink operation using partially sintered WC / Co electrodes operating in a hydrocarbon oil dielectric. An L8 fractional factorial Taguchi experiment was used to identify the effect of key operating factors on output measures (electrode wear, workpiece surface hardness, etc.). With respect to microhardness, the percentage contribution ratios (PCR) for peak current, electrode polarity and pulse on time were ˜24, 20 and 19%, respectively. Typically, changes in surface metallurgy were measured up to a depth of ˜30 μm (with a higher than normal voltage of ˜270 V) and an increase in the surface hardness of the recast layer from ˜620 HK_0.025 up to ˜1350 HK_0.025.     

混粉电火花放电蚀坑及温度场仿真研究

混粉电火花加工是一种新型加工工艺,通过在工作液中添加微细粉末,显著改善加工表面粗糙度。电火花加工表面粗糙度的形成与放电蚀坑大小有直接关系,而放电蚀坑大小与单次脉冲放电温度场有密切联系。为了进一步提高混粉加工表面质量,利用ANSYS软件对放电点温度场进行了模拟与分析,得到了工件表层温度场的分布规律,揭示了材料去除机制,阐明了加工表面粗糙度与温度场的关系,对预测和改善混粉加工表面质量及日后生产实际应用提供了一定的理论依据。     

Electrical discharge machining using simple and powder-mixed dielectric: The effect of the electrode area in the surface roughness and topography

Electrical discharge machining (EDM) is one of the most widely disseminated manufacturing technologies, in particular as regards the generation of accurate and complex geometrical shapes on hard metallic components. Nevertheless current EDM technologies have major limitations when dealing with fine surface finish over large process area. Indeed this is one reason that explains the need of final manual polishing of mould cavities performed by EDM. Recently EDM with powder-mixed dielectric (PMD-EDM) has been a focus of an intense research work in order to overcome these technological performance barriers. This paper presents a research work within the objective to acquire deep knowledge on EDM technology with powder mixed dielectric and to compare its performance to the conventional EDM when dealing with the generation of high-quality surfaces. In particular the analysis of the effect of the electrode area in the surface quality measured by the surface roughness and craters morphology was carried out for both technologies. The results achieved evidenced a linear relationship between the electrode area and the surface quality measures as well as a significant performance improvement when the powder mixed dielectric is used.     

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.     

A study of EDM and ECM/ECM-lapping complex machining technology

EDM (electrodischarge machining) and ECM (electrochemical machining)/ECM-lapping complex machining is investigated in this paper. First, EDM shaping and ECM finishing technology are investigated. These processes are carried out in sequence on the same machine tool with the same electrode (copper) and the same machining liquid (water). Two types of EDM and ECM complex machining are investigated. One is with a formed electrode, and the other is with simple-shape electrode scanning. The complex machining with electrode scanning is applied to produce small and various-shaped components without making a formed electrode. The EDM surface of 1 μm Ra is improved to 0.2 μm Ra by applying ECM. Second, in order to get a smoother surface, a new EDM and ECM-lapping complex machining technology is developed. The surface roughness of a machined hole is improved to 0.07 μm Ra by applying 2 min of ECM lapping. The surface finishing of a hole shape is demonstrated with the complex machining technology.     

工作介质粘度对钛合金电火花加工的影响研究

通过分析工作介质的粘度对电火花加工时电蚀产物的抛出量、抛出力及工作介质传热性能的影响,研究了钛合金电火花加工的加工效率、电板相对损耗率和表面粗糙度Ra值随介质粘度的变化规律。结果表明：粘度增大能在一定程度上提高加工效率。但粘度过大反而使加工效率下降;电极相对损耗率和表面粗糙度Ra值均随粘度增大而上升;粘度越大的介质传热性能越差,使工件表面不会因快速冷却而产生大量微裂纹。     

Study on the characteristics of electrical discharge machining using dielectric with surfactant

This research mainly explores the influence of surfactant on the characteristics of electrical discharge machining (EDM) process on mold steel (SKD61). In this study, particle agglomeration is reduced after surfactant molecules cover the surface of debris and carbon dregs in kerosene solution. Debris is evenly dispersed in dielectric to improve the effects of carbon accumulation and dreg discharge, and reduce the unstable concentrated discharge. The EDM parameters, such as peak current, pulse duration, open voltage and gap voltage are studied in this paper. The experimental results show that after the addition of Span 20 (30 g/L) to dielectric, the conductivity of dielectric is increased. The machining efficiency is thus increased due to a shorter relay time of electrical discharge. When proper working parameters are chosen, the material removal rate is improved by as high as 40–80%. Although the improvement of surface roughness is not obvious, the surface roughness is not deteriorated since the material removal rate is great.     

Influence of silicon powder-mixed dielectric on conventional electrical discharge machining

Electrical discharge machining (EDM) is a technological process with a large industrial implementation. Its use is particularly intense when very complex shapes on hard materials with a high geometrical and dimensional accuracy are required. However, the technological capability of the process has limited its application when the specification of the part surface quality imposes polished and mirror-like characteristics. The addition of powder particles in suspension in the dielectric modifies some process variables and creates the conditions to achieve a high surface quality in large areas. This paper presents a new research work aiming to study the performance improvement of conventional EDM when used with a powder-mixed dielectric. A silicon powder was used and the improvement is assessed through quality surface indicators and process time measurements, over a set of different processing areas. The results show the positive influence of the silicon powder in the reduction of the operating time, required to achieve a specific surface quality, and in the decrease of the surface roughness, allowing the generation of mirror-like surfaces.     

A comparative study of surface integrity of Ti–6Al–4V alloy machined by EDM and AECG

The electrical discharge machining (EDM) process produces the recast layer with or without cracks on the surface that requires a remedial post-treatment in the manufacture of critical or highly stressed surfaces. One of the frequently used post-treatment processes is also the abrasive electrochemical grinding (AECG) and it has been widely used in the precision machining of difficult-to-cut materials due to an enhanced surface integrity and productivity. The aim of this study is to investigate improvability of surface integrity in terms of machining voltage, electrolyte flow rate and table feed rate parameters of AECG in EDMed Ti6Al4V alloy. Scanning electron microscopy (SEM), X-ray diffraction (XRD), energy dispersive spectrograph (EDS) and surface roughness measurement were performed to study the surface characteristics of the machined samples. Experimental results indicate that the AECG process effectively improves the surface roughness and eliminates the EDM damages completely by setting suitable grinding parameters.     

硅电极电火花表面改性的研究

以工业纯硅为电极,通过电火花放电对试样进行表面改性处理.电火花设备型号为EDM7125,工作液为普通煤油,电极材料含硅99.6%,工件材料为45钢和球铁.采用扫描电镜及金相显微镜对试样表面形态进行了研究,对试样表面进行了能谱分析,对试样进行了耐蚀及表层结合强度等试验.结果表明:试样加工后表面形成一层含硅超过16%的合金层.电火花加工球铁石墨基本未脱落,但石墨周围易产生或扩展显微裂纹.高硅层结合强度非常高,进行永久变形时,无剥离和裂纹产生.而且,经电火花加工过的试样耐蚀性得到极大提高.另外,电火花加工,当参数合理时,可达到粉末混入加工的目的,试样可获得好的表面质量.     

Effect of electrode material on electrical discharge machining of alumina

Technologies for machining advanced insulating ceramics are demanded in many industrial fields. Recently, several insulating ceramics, such as Si₃N₄, SiC and ZrO₂, have been successfully machined by electrical discharge machining (EDM). As unstable discharges occur during the machining of Al₂O₃ ceramics, inferior machining properties have been obtained. The formation mechanism of the electrical conductive layer on the EDMed surface is much different as compared to other ceramics. In addition to this, the electrically conductive layers are not formed sufficiently to adhere to the EDMed workpiece surface and keep a stable and continuous discharge generation on the ceramics. Graphite is widely used as electrode material in EDM. It is expected that carbon from graphite electrode implant and generate a conductive layer. Copper, graphite (Poco EDM-3) and copper-infiltrated-graphite (Poco EDM-C3) electrodes were used to compare the effects of generation of a conductive layer on alumina corresponding to EDM properties. The electrical discharge machining of 95% pure alumina shows that the EDM-C3 performs very well, giving significantly higher material removal rate (MRR) and lower electrode wear ratio than the EDM-3 and copper electrodes. The value of MRR was found to increase by 60% for EDM-3 with positive electrode polarity. As for EDM-C3, MRR was increased by 80% under the same condition. When the results were investigated with energy dispersive spectroscopy (EDS), no element of copper was observed on the conductive layer with both EDM-3 and EDM-C3. However, surface resistivity of a conductive layer created with EDM-C3 is less than with EDM-3. Surface roughness was improved to 25 μm with positive polarity of EDM-C3.     

混粉电火花镜面加工表面性能研究

利用自行研制的混粉电火花镜面加工装置,以淬火状态的8407模具钢为例,对多种添加粉末下的混粉电火花镜面加工表面性能进行了研究。研究结果表明,混粉电火花镜面加工不但能够显著降低加工表面粗糙度值,而且能够提高加工表面的耐磨性、耐蚀性和表层显微硬度。     

The use of the orthogonal array with grey relational analysis to optimize the electrical discharge machining process with multiple performance characteristics

In this paper a new approach for the optimization of the electrical discharge machining (EDM) process with multiple performance characteristics based on the orthogonal array with the grey relational analysis has been studied. A grey relational grade obtained from the grey relational analysis is used to solve the EDM process with the multiple performance characteristics. Optimal machining parameters can then be determined by the grey relational grade as the performance index. In this study, the machining parameters, namely workpiece polarity, pulse on time, duty factor, open discharge voltage, discharge current, and dielectric fluid are optimized with considerations of multiple performance characteristics including material removal rate, surface roughness, and electrode wear ratio. Experimental results have shown that machining performance in the EDM process can be improved effectively through this approach.     

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.     

微细液态添加剂光整电火花加工技术初探

提出了一种在普通电火花工作液中加入微细液态添加剂进行电火花光整加工的新方法.初步的试验结果表明,此法能显著改善工件的表面粗糙度,达到镜面效果.从液相添加剂的作用机理对微细液态添加剂电火花加工改善表面粗糙度的原因进行了解释.     

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。     

EDM performance of Cr/Cu-based composite electrodes

Electrode materials for electrical discharge machining (EDM) are usually graphite, copper and copper alloys because these materials have high melting temperature, and excellent electrical and thermal conductivity. The electrodes made by using powder metallurgy technology from special powders have been used to modify EDM surfaces in recent years, to improve wear and corrosion resistance. However, electrodes are normally fabricated at high temperatures and pressures, such that fabrication is expensive. This paper proposes a new method of blending the copper powders contained resin with chromium powders to form tool electrodes. Such electrodes are made at low pressure (20 MPa) and temperature (200 °C) in a hot mounting machine. The results showed that using such electrodes facilitated the formation of a modified surface layer on the work piece after EDM, with remarkable corrosion resistant properties. The optimal mixing ratio, appropriate pressure, and proper machining parameters (such as polarity, peak current, and pulse duration) were used to investigate the effect of the material removal rate (MRR), electrode wear rate (EWR), surface roughness, and thickness of the recast layer on the usability of these electrodes. According to the experimental results, a mixing ratio of Cu–0wt%Cr and a sinter pressure of 20 MPa obtained an excellent MRR. Moreover, this work also reveals that the composite electrodes obtained a higher MRR than Cu metal electrodes; the recast layer was thinner and fewer cracks were present on the machined surface. Furthermore, the Cr elements in the composite electrode migrated to the work piece, resulting in good corrosion resistance of the machined surface after EDM.     

Surface modification of Al–Zn–Mg alloy by combined electrical discharge machining with ball burnish machining

The study investigated the feasibility of modifying the surface of Al–Zn–Mg alloy by a combined process of electric discharge machining (EDM) with ball burnish machining (BBM). A novel process that integrates EDM and BBM is also developed to conduct experiments on an electric discharge machine. Machining parameters of the combined process, including machining polarity, peak current, power supply voltage, and the protruding of ZrO₂, are chosen to determine their effects on material removal rate, surface roughness and the improvement ratio of surface roughness. In addition, the extent to which the combined process affects surface modification is also evaluated by microhardness and corrosion resistance tests. Experimental results indicate that the combined process of EDM with BBM can effectively improve the surface roughness to obtain a fine-finishing and flat surface. The micropores and cracks caused from EDM are eliminated during the process as well. Furthermore, such a process can reinforce and increase the corrosion resistance of the machined surface after machining.     

混粉电火花加工表面显微裂纹的研究

对混粉电火花加工表面的显微裂纹进行了研究.选用铝粉工作液和硅粉工作液,在不同的参数条件下对两种工件材料进行了加工实验,得到了相应的裂纹分布图.结果表明,脉冲宽度对加工后表面显微裂纹的影响十分明显,峰值电流、粉末种类、工件材料等因素对显微裂纹的影响则并不明显.     

A new method of optimising material removal rate using EDM with copper–tungsten electrodes

Electrical discharge machining (EDM) is widely used in the production of dies. This paper describes an investigation into the optimisation of the process which uses the effect of carbon which has migrated from the dielectric to tungsten–copper electrodes. This work has led to the development of a two-stage EDM machining process where different EDM settings are used for the two stages of the process giving a significantly improved material removal rate for a given tool wear ratio.