Investigation of the spark cycle on material removal rate in wire electrical discharge machining of advanced materials

The development of new, advanced engineering materials and the need for precise and flexible prototypes and low-volume production have made the wire electrical discharge machining (EDM) an important manufacturing process to meet such demands. This research investigates the effect of spark on-time duration and spark on-time ratio, two important EDM process parameters, on the material removal rate (MRR) and surface integrity of four types of advanced material: porous metal foams, metal bond diamond grinding wheels, sintered Nd-Fe-B magnets, and carbon–carbon bipolar plates. An experimental procedure was developed. During the wire EDM, five types of constraints on the MRR due to short circuit, wire breakage, machine slide speed limit, and spark on-time upper and lower limits are identified. An envelope of feasible EDM process parameters is generated for each work-material. Applications of such a process envelope to select process parameters for maximum MRR and for machining of micro features are discussed. Results of Scanning Electron Microscopy (SEM) analysis of surface integrity are presented.     

Evaluations of spark distribution and wire vibration in wire EDM by high-speed observation

In fine wire EDM using thin wire electrode, uniform distribution of spark location is necessary to achieve stable machining performance. However, it is difficult to precisely evaluate the distribution of spark location by the conventional branched electric current method when the workpiece is thin. A new evaluation method by using a high-speed video camera is proposed. The locations of spark are measured by analyzing the recorded images. Then the effects of machining parameters, such as servo voltage, pulse interval time, wire running speed and others on the distribution of spark location are investigated. The possibility of evaluating the wire vibration is also discussed.     

Sub-nanosecond monitoring of micro-hole electrical discharge machining pulses and modeling of discharge ringing

Monitoring the gap voltage and current in micro-hole electrical discharge machining (EDM) using high-speed data acquisition with 0.5 ns sampling period is conducted. The spark and arc pulses at three stages, namely electrode dressing, drilling, and penetration, of the micro-hole EDM are recorded. The EDM process parameters are setup to use negative polarity to blunt the electrode tip and positive polarity for micro-hole drilling and penetration. A new phenomenon of pre-discharging current is discovered. In the first 20–30 ns of spark and arc pulses, the current starts to rise while the voltage remains the same. Effects of EDM process parameters, including the open voltage, electrode diameter, and polarity, on the rate of spark and arc pulses and electrode feed rate are investigated. A model based on the RLC circuit is developed to study the ringing effect at the end of a discharge. The intrinsic parasitic capacitance and resistance of a RLC circuit are calculated from the decaying voltage signal and compared under two sets of experiments with varying wire electrode diameter and gap voltage to validate the ringing model. The calculation and experimental results validate the proposed RLC model for ringing phenomenon. The model shows the electrode diameter has negligible effect on ringing and high open voltage increases the parasitic resistance and damping in ringing. The monitoring technique and ringing model developed in this research can assist in the selection and optimization of micro-hole EDM process parameters.     

Near dry electrical discharge machining

This study investigates the near dry electrical discharge machining (EDM) process. Near dry EDM uses liquid–gas mixture as the two phase dielectric fluid and has the benefit to tailor the concentration of liquid and properties of dielectric medium to meet desired performance targets. A dispenser for minimum quantity lubrication (MQL) is utilized to supply a minute amount of liquid droplets at a controlled rate to the gap between the workpiece and electrode. Wire EDM cutting and EDM drilling are investigated under the wet, dry, and near dry conditions. The mixture of water and air is the dielectric fluid used for near dry EDM in this study. Near dry EDM shows advantages over the dry EDM in higher material removal rate (MRR), sharper cutting edge, and less debris deposition. Compared to wet EDM, near dry EDM has higher material removal rate at low discharge energy and generates a smaller gap distance. However, near dry EDM places a higher thermal load on the electrode, which leads to wire breakage in wire EDM and increases electrode wear in EDM drilling. A mathematical model, assuming that the gap distance consists of the discharge distance and material removal depth, was developed to quantitatively correlate the water–air mixture's dielectric strength and viscosity to the gap distance.     

Ultrasonic and electric discharge machining to deep and small hole on titanium alloy

Being a difficult-to-cut material, titanium alloy suffers poor machinability for most cutting process, let alone the drilling of small and deep holes using traditional machining methods. Although electric discharge machining (EDM) is suitable to handle titanium alloys, it is not ideal for small and deep holes due to titanium alloys’ low heating conductivity and high tenacity. This paper introduces ultrasonic vibration into micro-EDM and analyzes the effect of ultrasonic vibration on the EDM process. A four-axis EDM machine tool which combines ultrasonic and micro-EDM has been developed. A wire electric discharge grinding (WEDG) unit which can fabricate a micro-electrode on-line, as well as a measuring unit, is set up on this equipment. With a cylindrical tool electrode, made of hard carbide, which has high stiffness, a single-side notch was made along the electrode. Ultrasonic vibration is then introduced into the micro-EDM. Experiments have been carried out and results have shown that holes with a diameter of less than Ø0.2 mm and a depth/diameter ratio of more than 15 can be drilled steadily using this equipment and technology.     

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.     

A fundamental study on Ti–6Al–4V's thermal and electrical properties and their relation to EDM productivity

There are strong needs for productive/quality machining strategies of notoriously “difficult-to-machine” aerospace materials. The current means of machining these materials is dominated by mechanical cutting methods, which are costly due to high tooling costs, poor surface quality and limitations in the workpiece features and operations that can be machined. The newest EDM technology may be able to circumvent problems encountered in mechanical machining methods. In this paper, the EDM technology has been used to machine titanium alloy Ti–6Al–4V to investigate the effect of Ti–6Al–4V's thermal and electrical properties on the EDM productivity. In the study, temperature measurements have been made for Ti–6Al–4V workpieces with various duty factors to clarify the essential causes of difficulty in machining titanium alloys and observe the optimal duty factor in terms of productivity and quality.     

Fabrication of micro-electrodes by multi-EDM grinding process

In this study, a multi-EDM grinding process is adapted to fabricate micro-electrodes. Equipments such as a wire EDM machine and a traditional CNC-EDM machine are used for machining micro-electrodes. Rod electrodes of copper with diameter 3.0 mm were cut to be 0.15 mm on wire-EDM machine at first step. EDM grinding process was used to grind micro-electrodes to fine diameter bellow 20 μm on a CNC-EDM machine at second step. For EDM grinding, rotating mechanisms are mounted on both the WEDM machine and the CNC-EDM machine. A CCD camera is provided for viewing and for on-line dimensional controlling, when micro-electrodes were cutting. Fine electrodes could be processed to a smaller size using proposed two-steps EDM grinding process. Higher L/D ratio could be also achieved by this method. The processed fine electrodes can be used for drilling micro-holes, micro-deep holes, micro-milling, micro-punching, and manufacturing of micro-nozzles.     

Performance and Surface Integrity of Ti6Al4V After Sinking EDM with Special Graphite Electrodes

Titanium and its alloys have high chemical reactivity with most of the cutting tools. This makes it difficult to work with these alloys using conventional machining processes. Electrical discharge machining (EDM) emerges as an alternative technique to machining these materials. In this work, it is investigated the performance of three special grades of graphite as electrodes when ED-Machining Ti6Al4V samples under three different regimes. The main influences of electrical parameters are discussed for the samples material removal rate, volumetric relative wear and surface roughness. The samples surfaces were evaluated using SEM images, microhardness measurements, and x-ray diffraction. It was found that the best results for samples material removal rate, surface roughness, and volumetric relative wear were obtained for the graphite electrode with 10-μm particle size and negative polarity. For all samples machined by EDM and characterized by x-ray (XRD), it was identified the presence of titanium carbides. For the finish EDM regimes, the recast layer presents an increased amount of titanium carbides compared to semi-finish and rough regimes.     

面向三维形貌特征的钛合金电火花成形加工工艺参数优化

由于钛合金的超高硬度,使得切削加工十分困难,且成本巨大,即使耗费很大代价加工出来的产品,其加工质量往往不尽人意。因而,在研究国内外电火花加工技术最新成果的基础上,设计钛合金电火花加工试验,将加工速度、三维表面粗糙度作为评价加工性能的指标,分析了占空比、峰值电流、电压对钛合金加工的影响规律之后,深入研究了钛合金电火花成形加工的工艺特性;以MATLAB R2007b软件为平台,利用BP神经网络建立钛合金电火花加工工艺参数优化模型,并通过加工实验,对其预测结果进行评价。     

Broaching of Inconel 718 with cemented carbide

Broaching is the standard process for machining complex-shaped slots in turbine discs. More flexible processes such as milling, wire EDM machining and water-jet cutting are under investigation and show promising results. In order to further use existing resources and process knowledge, the broaching process has to be improved towards higher material removal rates. Taking into account that the state-of-the-art broaching process is working with high-speed-steel tools, the higher thermal resistant cemented carbide cutting materials offer the potential to significantly increase cutting speeds, which lead to increased process productivity. The following article presents a broad study on broaching with cemented carbide tools. Different cutting edge geometries are discussed on the basis of process forces, chip formation and tool wear mechanisms. Furthermore, a detailed comparison to the standard process is drawn.     

High-speed milling of titanium alloys using binderless CBN tools

The performance of conventional tools is poor when used to machine titanium alloys. In this paper, a new tool material, which is binderless cubic boron nitride (BCBN), is used for high-speed milling of a widely used titanium alloy Ti–6Al–4V. The performance and the wear mechanism of the BCBN tool have been investigated when slot milling the titanium alloy in terms of cutting forces, tool life and wear mechanism. This type of tool manifests longer tool life at high cutting speeds. Observations based on the SEM and EDX suggest that adhesion of workpiece and attrition are the main wear mechanisms of the BCBN tool when used in high-speed milling of Ti–6Al–4V.     

电火花往复走丝线切割在高速加工中的电极丝寿命初步研究

分析了电火花往复走丝线切割在高速加工中电极丝寿命的主要影响因素,通过在一定电源参数且平稳切割的前提下,典型电极丝对不同厚度工件切割作对比,同时初步定义电极丝寿命,计算和分析实验数据来探究厚度是影响电极丝寿命的主要因素之一。     

Wire electrical discharge machining of metal bond diamond wheels for ceramic grinding

The application of cylindrical wire Electrical Discharge Machining (EDM) for profile truing of metal bond diamond wheels is presented. Instead of using the mechanical force to break the diamond and matrix in the grinding wheel, the wire EDM process uses the thermal energy or electrical sparks between the wire and rotating grinding wheel to remove the metal bond and form the wheel. The design and manufacture of a corrosion-resistant, precise spindle with the high-electrical current capability for wire EDM truing of grinding wheel is first introduced. Three truing configurations were designed to study effects of wire EDM process parameters and to investigate the level of form accuracy and corner radii achievable by the wire EDM truing of diamond wheels. Results show that the wire EDM process can efficiently generate the μm-scale precision form on the diamond wheels. The wheel, after truing, was used to grind the silicon nitride workpiece. Grinding forces and wheel wear rate were measured. In the beginning of the grinding, high wheel wear rate was identified. The subsequent wheel wear rate was considerably lower and stabilized.     

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.     

A study on the machining of high-aspect ratio micro-structures using micro-EDM

Micro-electro-discharge machining (micro-EDM or μ-EDM) has been gaining popularity as a new alternative method to fabricate micro-structures. The main advantages of the micro-EDM method are its low set-up cost, high accuracy and large design freedom. Compared to etching or deposition techniques, micro-EDM has the advantage of being able to fabricate complex three-dimensional shapes with high-aspect ratio. However, there are many operating parameters that affect the micro-EDM process. The fabrication of micro-electrodes on the machine is also an important process to remove the clamping error to maintain high accuracy in the machined micro-structures.

In this paper, the machining of micro-structures is divided into two basic processes. One is the on-machine fabrication of the micro-electrodes with high-aspect ratio, and the other is the EDM of the workpiece in micrometer range. An optical sensor has been developed to measure and control the dimension of the thin electrode during the tool fabrication process. Different methods have been investigated to fabricate a thin electrode into the desired dimension without deflection. The performance of the micro-EDM process is evaluated in terms of the material removal rate (MRR), tool wear ratio (TWR), and the stability of the machining. Influences of the various operating parameters of the micro-EDM process, such as the operating voltage, gap control algorithm, and resistance and capacitance values in the R–C spark control circuit, are discussed.     

Micro Wire Electrochemical Cutting by Using In Situ Fabricated Wire Electrode

This study develops a micro wire electrochemical cutting process. Because of no electrode wear in this process a very thin metal wire can be used as the wire electrode. The tungsten wire electrode with the diameter of 5 μm is electrochemically in situ fabricated for the production of micro structures with the slit width of less than 20 μm. Effects of process parameters on the wire diameter in the wire etching and on the gap variation in the electrochemical cutting have been studied theoretically and experimentally. Many complex micro metal parts with structures of several ten micrometers have been obtained.     

Al6061/TA2异种金属冷金属过渡焊接性分析

以AlSi₅ 焊丝对6061铝合金和TA2纯钛进行CMT熔钎焊,采用SEM,EDS分析焊接接头的微观组织特征,并通过拉伸试验对接头进行力学性能评定.结果表明,焊接过程稳定,焊缝成形美观.所得到的焊接接头具有熔焊和钎焊两部分,其中局部熔化的铝母材与熔融的焊丝混合后形成焊缝,焊缝金属与微熔的钛母材形成三个钎焊界面.钎焊界面主要成分为TiAl₃金属间化合物,其厚度较薄.此外,界面附近还有一些随机分布的棒状的TiAl₃金属间化合物.焊接过程中,随着焊丝偏移量的增加,焊缝力学性能提高.参数优化后的接头抗拉强度较高,且断裂在铝热影响区.     

Composite Wires with High Tensile Core for Wire EDM

In recent years wire EDM cutting speeds and final surface roughness have been continuously improving. Much higher cutting speeds during roughing and surface roughness values lower than 0.1 μm Ra during finishing can be obtained. These enhancements are mostly attributed to the use of more advanced generator technology, but also to the type of wire. This paper discusses the use of new composite wires comprising a high tensile core and several coatings. Several wire compositions are introduced and experimentally tested. The use of a very high tensile core, the application of a layer that prevents the process heat from weakening the wire core and the presence of a superficial top coating with different possible functions are discussed. Results obtained while cutting with prototype wires show that a significant rise in accuracy especially in corner cutting is attained, while the cutting rate is at a comparable level as commercial reference wires.     

Electrical Discharge Surface Alloying of Ti and Fe Workpiece Materials Using Refractory Powder Compact Electrodes and Cu Wire

The paper reviews the use of metal powders dispersed in the dielectric fluid and refractory PM electrodes, to initiate workpiece surface modification during EDM. Experimental work details the effects of EDM parameters (up to 270 V) on the hardness/composition of the white layer following die sink machining of AISI H13 tool steel and roll texturing of 2% Cr steel using partially sintered PM electrodes. Similar data are presented following EDM scanning and wire cutting of standard TI alloy TI-6AI-4V and a y TIAI. With AISI H13, recast layers were 5-20 μm thick and up to ∼ 1350 HK_0.025. When machining TI-6AI-4V with WC/Co electrodes, recast microhardness was 600-2900 HK_0.025. Wire cutting y TIAI generated porous alloyed layers up to 115 μm thick with extensive cracks and no increase in bulk hardness.