State of the art in wire electrical discharge machining (WEDM)

Wire electrical discharge machining (WEDM) is a specialised thermal machining process capable of accurately machining parts with varying hardness or complex shapes, which have sharp edges that are very difficult to be machined by the main stream machining processes. This practical technology of the WEDM process is based on the conventional EDM sparking phenomenon utilising the widely accepted non-contact technique of material removal. Since the introduction of the process, WEDM has evolved from a simple means of making tools and dies to the best alternative of producing micro-scale parts with the highest degree of dimensional accuracy and surface finish quality.Over the years, the WEDM process has remained as a competitive and economical machining option fulfilling the demanding machining requirements imposed by the short product development cycles and the growing cost pressures. However, the risk of wire breakage and bending has undermined the full potential of the process drastically reducing the efficiency and accuracy of the WEDM operation. A significant amount of research has explored the different methodologies of achieving the ultimate WEDM goals of optimising the numerous process parameters analytically with the total elimination of the wire breakages thereby also improving the overall machining reliability.This paper reviews the vast array of research work carried out from the spin-off from the EDM process to the development of the WEDM. It reports on the WEDM research involving the optimisation of the process parameters surveying the influence of the various factors affecting the machining performance and productivity. The paper also highlights the adaptive monitoring and control of the process investigating the feasibility of the different control strategies of obtaining the optimal machining conditions. A wide range of WEDM industrial applications are reported together with the development of the hybrid machining processes. The final part of the paper discusses these developments and outlines the possible trends for future WEDM research.     

HS-WEDM加工工艺指标的主要影响因素及其控制

高速走丝电火化线切割机床是目前我国生产和使用的主要线切割机种。本文就笔者在教学实习、生产实践中总结的影响HS—WEDM加工工艺指标的一些主要因素及其相关控制进行了分析、说明。     

An experimental study of discharge mechanism in electrochemical discharge machining

The basic mechanism of the electrochemical discharge machining process is not yet completely understood and is still a subject of investigations. In the present work an attempt has been made to identify the underlying mechanism through experimental observations of time-varying current in the circuit. Based on these observations the basic mechanism of temperature rise and material removal is proposed.     

电火花线切割NC代码编译与运动轨迹仿真研究

针对目前电火花线切割数控系统NC代码编译复杂并且几乎不具备扩展性和通用性的问题,提出一种简单、清晰的线切割NC代码编译方案。基于.NET开发平台,采用C#开发语言,通过正则表达式进行词法分析,使用自定义函数进行语法分析。采用该方案能对简单平面图形、锥度切割和上下异形切割进行编译。通过.NET绘图功能显示图形,利用逐点比较插补法,对直线和圆弧进行插补,实现运动轨迹仿真。     

The surface characteristics of P/M high-speed steel (ASP 23) multi-cut with wire electrical discharge machine (WEDM)

High-speed steel (ASP 23), which was made by powder-metallurgy (P/M) process and widely used as the material for IC-packaging mold was subjected to multi-cutting with wire electrical discharge machine (WEDM) by using a brass wire electrode. Before WEDM, the steel was quenched at 1180 °C and then three-times tempered at 560 °C to acquire suitable mechanical properties. The cut surface was examined with scanning electron microscope (SEM) and analyzed with anodic polarization measurement in 1 M CH₃COOH. The results show that surface alloying between the steel and wire electrode materials was observed in the recast layer; meanwhile, martensitic transformation can be found. Surface alloying and phase transformation of the cut surface can be characterized with their anodic polarization curves measured in 1 M CH₃COOH. An obvious passive current can be detected in the martensitic structure by anodic polarization curve in higher overpotential, while such passive current did not occur in the tempered structure. A wide potential range corresponding to the potential range of passive current and higher breakdown potential of the anodic polarization curve was found on the cut surface after WEDM. This effect was caused by surface alloying in the recast layer and martensitic transformation in the workpiece adjacent to the cut surface.     

A new method for monitoring micro-electric discharge machining processes

Micro-electric discharge machining (μ-EDM) is a very complex phenomenon in terms of its material removal characteristics since it is affected by many complications such as adhesion, short-circuiting and cavitations. This paper presents a new method for monitoring μ-EDM processes by counting discharge pulses and it presents a fundamental study of a prognosis approach for calculating the total energy of discharge pulses. For different machining types (shape-up and flat-head) and machining conditions (mandrel rotation and tool electrode vibration), the results obtained using this new monitoring method with the prognosis approach show good agreement between the discharge pulses number and the total energy of discharge pulses to the material removal and tool electrode wear characteristic in μ-EDM processes. On applying tool electrode vibration, the machining time becomes shorter, because it removes adhesion. The effect of tool electrode vibration in order to remove adhesion can be monitored with good results. In order to achieve high accuracy, the tool wear compensation factor has been successfully calculated, since the amount of tool electrode wear is different in each machining type and condition. Consequently, a deeper understanding of the μ-EDM process has been achieved.     

An investigation of ultrasonic-assisted electrical discharge machining in gas

This study focuses on using ultrasonic to improve the efficiency in electrical discharge machining (EDM) in gas medium. The new method is referred to as ultrasonic-assisted electrical discharge machining (UEDM). In the process of UEDM in gas, the tool electrode is a thin-walled pipe, the high-pressure gas medium is applied from inside, and the ultrasonic actuation is applied onto the workpiece. In our experiment, the workpiece material is AISI 1045 steel and the electrode material is copper. The experiment results indicate that (a) the Material Removal Rate (MRR) is increased with respect to the increase of the open voltage, the pulse duration, the amplitude of ultrasonic actuation, the discharge current, and the decrease of the wall thickness of electrode pipe; and (b) the surface roughness is increased with respect to the increase of the open voltage, the pulse duration, and the discharge current. Based on experimental results, a theoretical model to estimate the MRR and the surface roughness is developed.     

A macroscopic mechanical model of wire electrode deflection considering temperature increment in MS-WEDM process

In medium-speed wire electrical discharge machining (MS-WEDM), wire in the area near the guide wheel and between the two guide wheels obviously form the wire bending deformation due to wire tension, electrostatic force, electrodynamics force, hydrodynamic force, temperature increment, etc. Besides, the wire deflection would have a direct influence on the machining accuracy, productivity and stability. In this paper, first of all, main causes of wire electrode deformation are proposed to better understand its fundamental mechanism. Second, two macroscopic mechanical models of wire deflection are developed in the area near the guide wheel and between the two guide wheels considering temperature increment and wire vibration in machining 20 mm-thickness workpiece process, respectively. Moreover, the numerical solution of deflection in the area near the guide wheel and the theoretical solution of deflection between the two guide wheels has been worked out. Then, the analysis of the variation trend of wire deflection and the influences of wire deflection on the machining process have been conducted. Eventually, from the confirmation experiment and comparison with other researchers’ models, it has been proved that the macroscopic mechanical models of wire deflection in MS-WEDM process are reasonable and reliable. In addition, according to macroscopic mechanical models, some of the practical approaches of reducing wire deflection have been proposed to improve machining accuracy, and these high-precision models can be applied into NC system to set a compensation for wire deflection in the future.     

PtIr高温合金电火花线切割工艺参数优化研究

为了提高PtIr高温合金电火花线切割加工的表面质量及减少切割边缘毛刺,研究电流、脉宽、脉间距对PtIr合金电火花线切割加工效果的影响。结果表明：加工过程的参数对材料的表面粗糙度及边缘毛刺有重要影响。对电火花线切割加工工艺参数进行了优化,电流为5 A、脉宽5μs、脉距5μs时、加工速度150步/S、钼丝直径0.1 mm时加工效果最佳,表面粗糙度Ra值为0.14μm,切割精度公差可控制在±0.006 mm内。     

Optimization of the WEDM process of particle-reinforced material with multiple performance characteristics using grey relational analysis

This paper presents an effective approach for the optimization of the wire electric discharge machining (WEDM) process of Al₂O₃ particle-reinforced material (6061 alloy) with multiple performance characteristics based on the grey relational analysis. The machining information for the difficult-cutting particle-reinforced material is inadequate and complicated. The response table and response graph for each level of the machining parameters are obtained form the grey relational grade, and select the optimal levels of machining parameters. In this study, the machining parameters namely the cutting radius of working piece, the on time of discharging, the off time of discharging, the arc on time of discharging, the arc off time of discharging, the servo voltage, the wire feed and water flow are optimized with considerations of multiple performance characteristics, such as the surface removal rate and the maximum surface roughness. It is clearly shown that the above performance characteristics in the WEDM process are great improved together through this approach.     

Experimental determination of convective heat transfer coefficient in WEDM

An experimental determination method of the convective heat transfer coefficient in wire electro-discharge machining (WEDM) is introduced. A special device is developed to measure the average temperature increment of the wire after a period of short circuit discharges, and the thermal load imposed on the wire is also tracked and recorded in advance. Then, based on the thermal model of the wire, the convective coefficient can be calculated accurately. Some tuning experiments are carried out inside and outside a previously cut profile to examine the influence of the kerf on the convective coefficient. As soon as the wire cuts into the workpiece, the convective coefficient will decrease more than 30%. With this method, the effect of the coolant flushing pressure on the convective coefficient is also estimated. If the pressure is raised from 0.1 to 0.8 Mpa, the convective coefficient will increase more than 20%, and thus ameliorate the cooling condition of the WEDM process.     

聚晶金刚石电火花超声机械复合加工技术研究

聚晶金刚石具有优良的力、热、化学、声、光、电等性能,在现代工业、国防等领域中得到日益广泛的应用。但是,聚晶金刚石硬度高,耐磨性好,其成型加工非常困难,超声电火花机械复合加工技术是一种有效的加工方法。本研究在分析电火花超声机械复合加工原理的基础上,采用青铜结合剂金刚石砂轮实现了聚晶金刚石的超声电火花机械复合加工。实验分析了超声电火花机械复合加工过程工艺参数与加工效果之间的关系,结果表明,脉宽、脉间、峰值电流、超声振幅、开路电压等参数对复合加工过程工艺结果的影响程度较明显。     

Cr12MoV钢冲模模板线切割开裂失效分析及工艺改进

采用化学成分分析、硬度测试、扫描电镜和光学显微镜等方法,对Cr12MoV钢冲模模板在线切割加工时发生的开裂失效原因进行分析。结果表明,模板原材料共晶碳化物偏析严重,大块碳化物尺寸超标是导致其在线切割时开裂的主要原因。通过改进锻造及热处理工艺,解决了模板线切割开裂失效的问题。     

An analytical model to predict specific shear energy in high-speed turning of Inconel 718

Machining of Inconel 718 at higher cutting speeds is expected to provide some relief from the machining difficulties. Therefore, to understand the material behavior at higher cutting speeds, this paper presents an analytical model that predicts specific shearing energy of the work material in shear zone. It considers formation of shear bands that occur at higher cutting speeds during machining, along with the elaborate evaluation of the effect of strain, strain rate, and temperature dependence of the shear flow stress using Johnson–Cook equation. The model also considers the ‘size-effect’ in machining in terms of occurrence of ‘ploughing forces’ during machining. The theoretical results show that the shear band spacing in chip formation increases linearly with an increase in the feedrate and is of the order of 0.2–0.9 mm depending upon the processing conditions. The model shows excellent agreement with the experimental values with an error between 0.5% and 7% for various parametric conditions.     

线切割加工质量的分析及改善措施

线切割加工在模具制造业中常用于加工精密、微细的模具零件。通过介绍线切割的加工原理,研究了线切割加工过程容易产生的加工质量缺陷,分析了加工面变形与开裂、变质层及表面粗糙度产生的原因,提出了进一步提高工件表面质量的改善措施和方法。实践中可以通过合理选材、优化电参数、选择正确的加工路线等方面综合分析和控制线切割加工,在保证生产率的前提下,改善和提高线切割加工质量。     

电火花铣削加工工艺与算法研究

介绍了电火花铣削加工工艺及其有关计算方法。在大量实验的基础上,研究了微细电火花分层铣削加工,采用电极底面放电方式、电极轨迹规划、电极的损耗及补偿策略和合理选择分层厚度及电极微进给量等关键技术,提高了加工精度和效率。     

Specific energy and temperature determination in abrasive flow machining process

A model has been proposed for the determination of specific energy and tangential forces in abrasive flow machining (AFM) process. It accounts for the process parameters of AFM e.g., grain size, applied pressure, hardness of workpiece material, number of cycles and number of active grains. Heat transfer in AFM has also been analysed considering heat flow to the workpiece and medium, and theoretical results compared with experimentally observed values of workpiece temperatures. The dependence of the workpiece temperature on the AFM parameters has been discussed. The model also predicts the fraction of heat entering into the workpiece and medium.     

微细电火花加工实现条件的研究

介绍了微细电火花加工的原理和特点,从加工表面质量、脉冲电源、微细工具电极的制造和安装、放电面积效应的影响、伺服控制系统等方面对微细电火花加工的实现条件进行了研究,并给出了微细轴的电火花加工等具体加工实例.研究结果对微细电火花加工技术的具体应用具有重要的参考价值和指导意义.