喷雾电火花铣削加工的能量分配与材料蚀除模型

针对喷雾电火花铣削加工(Electrical discharge milling, ED-milling)建立电蚀坑形成过程的热-流耦合模型,并改进电火花加工中放电能量在阴极及阳极分配系数的判断方法来为所建立的蚀除模型提供边界条件。此外,通过试验得到不同放电参数下的电蚀坑半径,并对蚀坑半径随脉宽和电流变化的规律采用最小二乘法进行拟合作为等离子体扩张方程。基于材料蚀除的热-流耦合二维模型,应用仿真-试验比对的方法得到雾中电火花铣削加工时放电能量在正极的分配系数近似为0.29,负极分配系数约为0.025。根据相关加工参数及所建立的模型,对喷雾电火花加工的电蚀坑尺寸进行计算并与试验测量结果进行对比,两者误差约在8%,证明该模型是可信的。通过对比试验和分析结果,可知喷雾电火花铣削加工中放电通道中的能量分配在阳极的比例远大于阴极,从而揭示了在雾中电火花加工中工件接正极时材料去除率更高的原因。通过所建立的阴、阳极的蚀除模型,用于对喷雾电火花加工的材料去除率、表面粗糙度等进行推导和预测,从而优化工艺参数并减少加工成本。此外,所建立的模型可进一步扩展应用到液中、气中等多种电火花加工(Electrical discharge machining, EDM)中,并为EDM加工机理的研究提供了一种可行的方法。     

基于圆台形热传导模型的慢走丝线切割仿真与试验

基于放电通道中等离子体的形成机理,根据慢走丝线切割在短脉冲放电加工时放电通道中电子流与离子流散射速度的差异,提出了圆台形热传导模型。采用基于圆台形热传导模型的有限单元法对航空材料Inconel 718的典型工况进行了仿真计算,系统地分析了放电能量对放电通道温度以及放电蚀坑深度的影响规律,并采用声发射检测技术在线监测慢走丝线切割的加工表面粗糙度。通过仿真结果与试验测得工件表面粗糙度Rt值的对比,再结合试验测得的声发射信号波形图特征及声发射信号均方根值发现：仿真计算得到的放电蚀坑深度与表面粗糙度Rt值吻合较好;声发射信号的强度随着放电能量的增加而增强,声发射信号强度随着放电温度变化速率的变小而减弱。最后回归分析得到材料表面粗糙度与声发射信号均方根值的数学预测模型,预测结果与测得的表面粗糙度误差仅为4.4%。     

Grinding-aided electrochemical discharge machining of particulate reinforced metal matrix composites

A grinding-aided electrochemical discharge machining (G-ECDM) process has been developed to improve the performance of the conventional ECDM process in machining particulate reinforced metal matrix composites (MMCs). The G-ECDM process functions under a combined action of electrochemical dissolution, spark erosion, and direct mechanical grinding. The tool electrode has a coating containing a hard reinforcement phase of diamond particles. The MMC employed in this study was Al₂O₃ particulate reinforced aluminum 6061 alloy. The material removal mechanism of this hybrid process has been analyzed. The results showed that the grinding action can effectively remove re-cast material deposited on the machining surface. The surface roughness (R _a) measured for the G-ECDM specimen was ten times smaller than that of the specimen machined without grinding aid (i.e., ECDM alone). Moreover, the material removal rate (MRR) of G-ECDM was about three times higher than that of ECDM under the experimental conditions of this study. The voltage waveform and crater distribution were also analyzed, and the experimental results showed that the G-ECDM process operates in a stable condition. The relative importance of the various processing parameters on MRR was established using orthogonal analysis. The results showed that MRR is influenced by the machining parameters in the order of duty cycle?>?current?>?electrolyte concentration. This study showed that the G-ECDM process is superior to the ECDM process for machining particulate reinforced MMCs, where a higher machining efficiency and a better surface quality can be obtained.     

Multi-objective optimization of material removal rate and surface roughness in wire electrical discharge turning

Wire electrical discharge turning (WEDT) is an emerging area, and it can be used to generate cylindrical forms on difficult to machine materials by adding a rotary axes to WEDM. The selection of optimum cutting parameters in WEDT is an important step to achieve high productivity while making sure that there is no wire breakage. In the present work, the WEDT process is modelled using an artificial neural network with feed-forward back-propagation algorithm and using adaptive neuro-fuzzy inference system. The experiments were designed based on Taguchi design of experiments to train the neural network and to test its performance. The process is optimized considering the two output process parameters, material removal rate, and surface roughness, which are important for increasing the productivity and quality of the products. Since the output parameters are conflicting in nature, a multi-objective optimization method based on non-dominated sorting genetic algorithm-II is used to optimize the process. A pareto-optimal front leading to the set of optimal solutions for material removal rate and surface roughness is obtained using the proposed algorithms. The results are verified with experiments, and it is found to improve the performance of WEDT process. Using this set of solutions, required input parameters can be selected to achieve higher material removal rate and good surface finish.     

Development of an intelligent process model for EDM

This paper reports the development of an intelligent model for the electric discharge machining (EDM) process using finite-element method (FEM) and artificial neural network (ANN). A two-dimensional axisymmetric thermal (FEM) model of single-spark EDM process has been developed based on more realistic assumptions such as Gaussian distribution of heat flux, time- and energy-dependent spark radius, etc. to predict the shape of crater cavity, material removal rate, and tool wear rate. The model is validated using the reported analytical and experimental results. A neural-network-based process model is proposed to establish relation between input process conditions (discharge power, spark on time, and duty factor) and the process responses (crater geometry, material removal rate, and tool wear rate) for various work—tool work materials. The ANN model was trained, tested, and tuned using the data generated from the numerical (FEM) simulations. The ANN model was found to accurately predict EDM process responses for chosen process conditions. It can be used for the selection of optimum process conditions for EDM process.     

Characterization and geometric modeling of single and overlapping craters in micro-EDM

In the micro-electro discharge machining (micro-EDM) process the machined surface consists of tiny overlapping craters. The shape of the crater has significant influence on the characteristics of the machined surface. However, existing models on crater shapes generally do not represent characteristic features of the craters, such as the bulging rim around the crater and the asymmetrical bowl-shape. In this study, experiments were conducted to examine three-dimensional (3D) geometrical features of craters in micro-EDM. Based on the experimental data, a geometric model of craters is proposed to better represent and understand the 3D shapes of single craters. A uniform model and a more realistic nonuniform model of crater geometry are developed, taking into account the bulging rim and the asymmetrical shape in craters. Furthermore, a method of constructing overlapping craters using nonuniform models of single craters is proposed and presented. This method is capable of representing major geometrical features of actual crater clusters.     

An experimental study on micro wire-EDM of polycrystalline diamond using a novel pulse generator

This paper presents a new pulse generator for cutting of polycrystalline diamond (PCD) by micro wire electrical discharge machining (micro wire-EDM). The pulse generator using anti-electrolysis circuitry and digital signal processor-based pulse control circuit was developed to suppress damages on the machined surface of PCD while achieving stable machining. A novel pulse control method was proposed to provide high-frequency pulse control signals with a period of off duty cycle for reionization of the dielectric in the spark gap so as to reduce the consecutive occurrence of short circuits. A series of experiments were carried out to investigate the effect of open voltage on machining performance in terms of material removal rate, slit width, thickness of the damaged layer on machined surface, and surface finish. An increase of open voltage increases peak current, thus producing greater discharge energy and, thereby, contributing to improvements in material removal rate, but leading to larger slit width and thickness of the damaged layer and worse surface finish. Experimental results not only demonstrate that the developed pulse generator could achieve satisfactory machining results but also have verified the applicability of this new technique in micro wire-EDM.     

Study on erosion mechanism of graphite electrode in two-electrode spark gap switch

In a high-powered single pulse system, the graphite electrode is better than other common metal electrodes for high energy transfer and pulse discharge. In this paper, the erosion mechanism of graphite electrode is investigated with the thermodynamics theory and the experimental results. Based on a simplified mathematical model, the graphite electrode erosion process of high-powered spark gap switch is also analyzed. The analysis results show that the relationship of the graphite electrode erosion and the charge transfer is linear, which is accordant with the experimental results.     

Experimental verification of a model of erosion due to the impact of rigid single angular particles on fully plastic targets

A previously described rigid-plastic model of the erosion of ductile targets by the impact of single angular particles was experimentally verified over a wide range of particle angularities, incident angles of attack, and incident orientation angles. The model assumes that the particle is perfectly rigid and thus is non-deforming, while the target material response is fully plastic, so that elastic rebound effects are neglected.Measurements of particle rebound kinematics, crater volume, and crater shape revealed generally good agreement with those predicted by the rigid-plastic model, and erosion mechanisms resulting from particles tumbling either forwards or backwards, were identified. For highly angular particles, target material removal sometimes occurred due to tunnelling of the particles below the target surface, leading to early break-off of a machined chip, behaviour that could not be predicted by the rigid-plastic model. Besides providing insights into fundamental erosion mechanisms, the results of the present study can be used to predict particle rebound kinematics, crucial for simulations of erosive streams which take into account interference between incident and rebounding particles.     

Appearance of craters on track surface of rolling element bearings by spark erosion

A theoretical investigation has been carried out to determine the time span for the appearance of craters by spark erosion on the track surfaces of rolling-element bearings operating under the influence of shaft voltages. An analytical model has been developed using the continuum theory of Griffith to determine energy per unit area required for the development of craters on the track surfaces. Also, an expression is deduced for the net energy transmitted per unit area on the track surfaces by the leakage of stored charge energy resulting in the appearance of craters under the effect of shaft voltage and bearing capacitance depending on operational and dimensional parameters of a bearing in a given span of operation. By the formulation of capacitive stored energy in a bearing using high resitivity lubricant, along with the developed model and expressions using the ratio of cycles to accumulate and discharge of the accumulated charge energy at the various levels of bearing to shaft voltage, the time span/cycles for the appearance of craters of specified dimensions by spark erosion has been determined. Matching time span/cycles has been ascertained by the investigation of a case history of the diagnosis of bearings that failed by crater formation. The analysis can also be used for determination of shaft voltage if the time span/cycles for the formation of known size craters and bearing capacitance are established.     

Effect of the powder concentration and dielectric flow in the surface morphology in electrical discharge machining with powder-mixed dielectric (PMD-EDM)

The addition of powder particles to the electrical discharge machining (EDM) dielectric fluid modifies some process variables and creates the conditions to achieve a higher surface quality in large machined areas. This paper presents a new research work that aims to study the improvement in the polishing performance of conventional EDM when used with a powder-mixed-dielectric (PMD-EDM). The analysis was carried out varying the silicon powder concentration and the flushing flow rate over a set of different processing areas and the effects in the final surface were evaluated. The evaluation was done by surface morphologic analysis and measured through some quality surface indicators. The results show the positive influence of the silicon powder in the reduction of crater dimensions, white-layer thickness and surface roughness. Moreover, it was demonstrated that an accurate control of the powder concentration and flushing flow is a requirement for achieving an improvement in the process polishing capability.     

Surface integrity study of WEDM with various wire electrodes: experiments and analysis

Abstract

Wire electrical discharge machining (WEDM) is always significant for its high-precision machining. However, due to the generation of high discharge energy during machining, machined surfaces are often got distorted. These might be upgraded by choosing the correct tool with proper machining condition. The effects of the electrode materials and process parameters on different responses of WEDM like average surface roughness, recast layer thickness, and surface morphology are systematically examined here to enhance the knowledge of WEDM and its correlation with electrode property. The experiments have been carried out on one of the expensive steel namely Maraging steel 300 due to its applicability in tooling and aerospace industries. Plain brass wire, zinc-coated brass wire (ZCB), and silver-coated brass (SCB) wires are used as a tool electrode for analysis. Comparative experimental studies prove that among BW, ZCB, and SCB, the overall performance of SCB is commendable owing to the high-quality surface considering control parameters in low discharge energy level. However, the second-best performance is shown by ZCB.     

Observation and modeling research of high-velocity flushing effect on the performance of BEAM

Blasting erosion arc machining (BEAM) is a novel high efficiency electrical erosion process, which is characterized by a powerful multi-hole inner flushing. In order to study the influence of the high-velocity flushing on the performance of BEAM, a single discharge experiment is carried out on a specially designed observation apparatus, and the phenomena that occurred during the discharge is observed using a high-speed video camera. During the discharge process, a spray cloud of the removed debris near the downstream side of the discharging point is captured by the camera. Tail shaped crater and erosion-corrosion features are also found on the workpiece and electrode surfaces, respectively. These observed phenomena imply that the high-velocity flushing can greatly improve the melting metal removal from the molten pool thereby improving the material removal rate (MRR). For purpose of exploring the mechanism behind, the flow field and pressure distribution of the high-velocity flushing are analyzed by using computational fluid dynamic (CFD) method. The analyzed results indicate that a low-pressure suction effect exists on the downstream side of the electrodes. With the help of the low-pressure suction effect, the high-velocity flushing can continuously take the molten metal out of the molten pool during the discharge, resulting in the reduction of the overheating of the molten metal and improve the efficiency of the discharging energy. Thereby, the work in this paper helps to explain why the high-velocity flushing can markedly promote the material removal rate.     

基于C8051单片机的电火花线切割自适应电源研究

对电火花线切割脉冲电源的原理进行了探讨与分析,介绍了一种基于C8051单片机的新型电火花线切割自适应电源的设计方法.对放电状态进行在线检测,该系统能够通过对放电脉冲的自适应调整,使极间放电状态总是处于理想状态,能够较好地适应加工条件的剧烈变化,大大提高了线切割加工过程的稳定性和加工品质.     

Study on the nano-powder-mixed sinking and milling micro-EDM of WC-Co

Present study investigates the feasibility of improving surface characteristics in the micro-electric discharge machining (EDM) of cemented tungsten carbide (WC?CCo), a widely used die and mould material, using graphite nano-powder-mixed dielectric. In this context, a comparative analysis has been carried out on the performance of powder-mixed sinking and milling micro-EDM with view of obtaining smooth and defect-free surfaces. The surface characteristics of the machined carbide were studied in terms of surface topography, crater characteristics, average surface roughness (R _a) and peak-to-valley roughness (R _max). The effect of graphite powder concentration on the spark gap, material removal rate (MRR) and electrode wear ratio (EWR) were also discussed for both die-sinking and milling micro-EDM of WC?CCo. It has been observed that the presence of semi-conductive graphite nano-powders in the dielectric can significantly improve the surface finish, enhance the MRR and reduce the EWR. Both the surface topography and crater distribution were improved due to the increased spark gap and uniform discharging in powder-mixed micro-EDM. The added nano-powder can lower the breakdown strength and facilitate the ignition process thus improving the MRR. However, for a fixed powder material and particle size, all the performance parameters were found to vary significantly with powder concentration. Among the two processes, powder-mixed milling micro-EDM was found to provide smoother and defect-free surface compared to sinking micro-EDM. The lowest value of R _a (38?nm) and R _max (0.17???m) was achieved in powder-mixed milling micro-EDM at optimum concentration of 0.2?g/L and electrical setting of 60?V and stray capacitance.     

Constraining the shear strain in viscoelastic materials and utlization of the “incompressible” properties for damping treatment in hybrid joint interface module to improve their effect for vibration control in machining

Features of chip formation can inform the mechanism of a machining process. In this paper, a series of orthogonal cutting experiments were carried out on unidirectional carbon fiber reinforced polymer (UD-CFRP) under cutting speed of 0.5 m/min. The specially designed orthogonal cutting tools and high-speed camera were used in this paper. Two main factors are found to influence the chip morphology, namely the depth of cut (DOC) and the fiber orientation (angle ??), and the latter of which plays a more dominant role. Based on the investigation of chip formation, a new approach is proposed for predicting fracture toughness of the newly machined surface and the total energy consumption during CFRP orthogonal cutting is introduced as a function of the surface energy of machined surface, the energy consumed to overcome friction, and the energy for chip fracture. The results show that the proportion of energy spent on tool-chip friction is the greatest, and the proportions of energy spent on creating new surface decrease with the increasing of fiber angle.     

An Experimental Study and Statistical Analysis of the Effect of Laser Pulse Energy on the Geometric Quality During Laser Precision Machining

In laser precision machining, process parameters have critical effects on the geometric quality of the machined parts. Due to the nature of the interrelated process parameters involved, an operator has to make a host of complex decisions, based on trial-and-error methods, to set the process control parameters related to the laser, workpiece material, and motion system. The objective of this work is to investigate experimentally the effect of laser pulse energy on the geometric quality of the machined parts in terms of accuracy, precision, and surface quality. Experimental study of formation of machined craters on thin copper foil with variation in laser pulse energy, the geometry and the surface topology of craters is presented. The geometrical parameters were measured and statistically analyzed with respect to incident pulse energy. Statistical analysis including pattern recognition was employed to analyze the experimental data systematically and to serve proper selection of the process parameters to achieve the desired geometric quality of the machined parts. Plausible trends in the crater geometry with respect to the laser pulse energy are discussed. The technique has been verified experimentally on simple geometrical features such as circles and grooves, and the geometric quality is evaluated.     

MEASUREMENT OF HYDROGEN CONTENT IN ELECTRICAL DISCHARGE MACHINED COMPONENTS

During electric discharge machining (EDM) process, localized high temperature electric discharges occur in the presence of hydrocarbon dielectric such as kerosene. Hydrogen that can get incorporated into the workpiece during the process can dramatically affect the mechanical properties of the machined workpiece. In the present work the hydrogen content in workpieces machined by EDM and electric discharge abrasive drilling (EDAD) have been measured by a relatively new diagnostic technique called Elastic Recoil Detection (ERD), which utilizes He⁺ ions of mega electron volt energy obtained from a particle accelerator. Workpieces made up of high-speed steel and cemented carbide were machined at different pulse-on-times, and their hydrogen content was measured. From the experiments reported in this paper, it is concluded that hydrogen diffusion takes place in the workpiece surface.     

线切割机床加工铝件电极丝进电装置研究

针对普通线切割机床加工铝件常出现的问题,对普通线切割机床加工铝件的切割过程特性进行了研究.对普通线切割机床加工铝件时产生的Al2O3损坏机床电极丝进电装置导电块的避免方案进行了分析,提出了取消普通线切割机床电极丝进电装置导电块,重新设计普通线切割机床电极丝进电装置的方案.为提高切割效率,进电位置选在了两前导轮轴处;为避免在导轮轴处产生火花放电,腐蚀导轮,影响导轮寿命,对导轮结构进行改进,选择了加长导轮轴;为不影响导轮的运转,选择用石墨与轴接触,达到既能导电,同时还能自润滑以提高寿命.研究结果表明,改进后的普通线切割机床,在切割铝件时,可避免导电块的损坏问题,降低断丝、短路的机率,提高被切割铝件的表面质量.     

An Experimental Study and Statistical Analysis of the Effect of Laser Pulse Energy on the Geometric Quality During Laser Precision Machining

In laser precision machining, process parameters have critical effects on the geometric quality of the machined parts. Due to the nature of the interrelated process parameters involved, an operator has to make a host of complex decisions, based on trial‐and‐error methods, to set the process control parameters related to the laser, workpiece material, and motion system. The objective of this work is to investigate experimentally the effect of laser pulse energy on the geometric quality of the machined parts in terms of accuracy, precision, and surface quality. Experimental study of formation of machined craters on thin copper foil with variation in laser pulse energy, the geometry and the surface topology of craters is presented. The geometrical parameters were measured and statistically analyzed with respect to incident pulse energy. Statistical analysis including pattern recognition was employed to analyze the experimental data systematically and to serve proper selection of the process parameters to achieve the desired geometric quality of the machined parts. Plausible trends in the crater geometry with respect to the laser pulse energy are discussed. The technique has been verified experimentally on simple geometrical features such as circles and grooves, and the geometric quality is evaluated.