An analysis of the debris in ultrasonic-assisted electrical discharge machining

This paper endeavours to examine the influence of ultrasonic irradiation of the spark gap on electrical discharge machining debris. An analysis using scanning electron microscopy shows that most of the particles are globular in shape with slight ellipticity. Except for a few isolated hollow particles, the rest are solid. There is evidence of the collisions of particles in the form of splats, attached satellites, cracks and dents, etc. Compositional analysis showed pickup of tool electrode material by the debris. The size distribution was near normal, indicating the stochastic nature of particle formation. The pulse characteristics significantly influence the particle size. The influence of ultrasonics was highly significant with markedly high spheroidicity, size, copper pickup and incidence of collisions.     

Effect of processing parameters on structural features of tungsten carbide after electrical discharge machining (EDM)

Abstract

Tungsten carbide (WC) is an extremely hard material which is used extensively in the manufacturing of tools and dies. In the presence of cobalt as a binder its machining becomes a difficult task because of interfacial bonding. In the EDM process, where electrical energy is used for the machining of the substance, the heat generated due to the plasma is responsible for removal of the substance at the interface. The heat generated is conducted differentially because of the composite structure of the tungsten carbide cermet. In order to improve the technological performance it is essential to understand the morphological features of tungsten carbide after machining. The studies have been conducted using different machining parameters. The objective of this study is to analyse the impact of machining parameters on the morphology of tungsten carbide suitable to withstand impact load on press forging for small components during operation. Experiments have been performed with the specially designed fixtures with proper flushing arrangements, to avoid arcing during the process. WC of P20 grade which is one of the most suitable grade substances to withstand load after EDM, has been used as work piece material for the entire study. Copper, graphite and copper tungsten electrodes have been used for the present study. The morphological features were studied with the help of the scanning electron microscope (SEM). It was observed that structural features varied with variation in electrode under similar experimental conditions. Phenomenon of such structures is discussed at length. The formations of cracks on WC have also been studied in detail. The detail of this study is presented in the paper.     

Effect of ultrasonic vibration of tool on electrical discharge machining of cemented tungsten carbide (WC-Co)

This paper deals with the effect of copper tool vibration with ultrasonic (US) frequency on the electrical discharge machining (EDM) characteristics of cemented tungsten carbide (WC-Co). It was found that ultrasonic vibration of the tool (USVT) was more effective in attaining a high material removal rate (MRR) when working under low discharge currents and low pulse times (finishing regimes). In general, the surface roughness and the tool wear ratio (TWR) were increased when ultrasonic vibration was employed. It was observed that application of ultrasonic vibration significantly reduced arcing and open circuit pulses, and the stability of the process had a remarkable improvement. This study showed that, there were optimum conditions for ultrasonic assisted machining of cemented tungsten carbide, although the conditions may vary by giving other input parameters for those which had been set constant in the present work.     

Material removal rate and electrode wear ratio study on the powder mixed electrical discharge machining of cobalt-bonded tungsten carbide

In this article, a material removal rate (MRR) and electrode wear ratio (EWR) study on the powder mixed electrical discharge machining (PMEDM) of cobalt-bonded tungsten carbide (WC-Co) has been carried out. This type of cemented tungsten carbide was widely used as moulding material of metal forming, forging, squeeze casting, and high pressure die casting. In the PMEDM process, the aluminum powder particle suspended in the dielectric fluid disperses and makes the discharging energy dispersion uniform; it displays multiple discharging effects within a single input pulse. This study was made only for the finishing stages and has been carried out taking into account the four processing parameters: discharge current, pulse on time, grain size, and concentration of aluminum powder particle for the machinability evaluation of MRR and EWR. The response surface methodology (RSM) has been used to plan and analyze the experiments. The experimental plan adopts the face-centered central composite design (CCD). This study highlights the development of mathematical models for investigating the influence of processing parameters on performance characteristics.     

Analysis of energy distribution during electric discharge machining of tungsten carbide

Abstract

During electrical discharge machining (EDM) process, electrical energy is used for the machining of the components. Energy distribution in electrical discharge machining process is the distribution of input energy supplied during machining to various components. In order to improve the technological performance during EDM process, it is essential to understand the distribution of input energy in the entire system. An experimental study on the effect of EDM energy distribution parameter for tungsten carbide is presented. The copper tungsten electrode has been used for the study. Experiments have been performed in specially designed dielectric insulated tank. To minimise the energy wastage, workpiece as well as the electrode was covered with Teflon. Current and pulse duration have been selected as variable parameters. The objective of this study is to analyse the amount of electrical energy used for machining effectively. The detail of this study has been presented in this paper.     

Influence of electrical discharge machining on sliding friction and wear of WC-Ni cemented carbide

Tribological characteristics of fine-grained WC-8 wt%Ni with different surface finishing conditions corresponding to sequential wire electrical discharge machining (wire-EDM) or grinding were investigated by performing dry reciprocating sliding experiments on TE77 pin-on-plate equipment with WC-6 wt%Co as mating material. Surface finishes and wear surfaces were characterized using surface topography scanning, SEM analysis and EDX spectrometry. The decreased wear resistance due to wire-EDM could be linked to flexural strength properties as well as XRD measurements revealing tensile residual stresses on wire-EDM surfaces, in contrast with the compressive stress state on ground surfaces. Substantial improvement of wear resistance was accomplished with finer-executed EDM steps.     

Accretion of titanium carbide by electrical discharge machining with powder suspended in working fluid

A surface modification method by electrical discharge machining (EDM) with a green compact electrode has been studied to make thick TiC or WC layer. Titanium alloy powder or tungsten powder is supplied from the green compact electrode and adheres on a workpiece by the heat caused by discharge. To avoid the production process of the green compact electrode, a surface modification method by EDM with powder suspended in working fluid is proposed in this paper. After considering flow of working fluid in EDM process, the use of a thin electrode and a rotating disk electrode are expected to keep powder concentration high in the gap between a workpiece and an electrode and to accrete powder material on the workpiece. The accretion machining is tried under various electrical conditions. Titanium powder is suspended in working oil like kerosene. TiC layer grows a thickness of 150 μm with a hardness of 1600 Hv on carbon steel with an electrode of 1 mm in diameter. When a disk placed near a plate rotates in viscous fluid, the disk drags the fluid into the gap between the disk and the plate. Therefore, the powder concentration in the gap between a workpiece and a rotational disk electrode can be kept high. A wider area of the accretion can be obtained by using the rotational electrode with a gear shape.     

Non-linear mechanism in electrical discharge machining process

Electrical discharge machining (EDM) is an advanced non-traditional manufacturing technology that has many advantages over other machining methods. Many papers have discussed the machining mechanism and modeling of the EDM process. However, previous mechanism models have mainly been linear, which contradicts their precondition that EDM is a stochastic process. In this paper, a non-linear mechanism model is proposed for the EDM process. A threshold condition that leads to chaos is calculated using the Melnikov theory. The theoretical results indicate that the EDM system can generate varied chaos in the evolution of electrical discharge. To verify this conclusion, validation experiments are implemented. Several sets of complete EDM processes’ real-time series are analyzed by multiple chaotic numerical criteria, including power spectrum analysis, principle component analysis (PCA), correlation dimension analysis, and Lyapunov exponent analysis. The experimental results provide further qualitative and quantitative evidence that a complete EDM process has dynamical chaotic characteristics.     

Influence of electrical conditions on performance of electrical discharge machining with powder suspended in working oil for titanium carbide deposition process

This paper describes the influence of the discharge current and the pulse duration on the titanium carbide (TiC) deposition process by electrical discharge machining (EDM) with titanium (Ti) powder suspended in working oil. Although the influence of the electrical conditions for removal EDM has been investigated, the criteria for deposition have not been discussed. In the experiments, a 1-mm copper rod was used for an electrode to prevent the flushing of working oil from the gap between the electrode and a workpiece. Ti powder reacted with the cracked carbon from the working oil, then depositing a TiC layer on a workpiece surface. A major criterion of the deposition or removal was the discharge energy over a pulse duration of 10 μs. A thickness of the TiC layer became the maximum at a certain discharge current and pulse duration. Larger discharge energy and power promoted the removal by heat and pressure caused by the discharge. The removal was classified further into two patterns; cracks were observed on the Ti-rich surface in removal pattern 1 and a workpiece was simply removed in removal pattern 2. The maximum hardness of the deposition was 2000 Hv. The workpiece about 10 μm beneath its surface was also hardened because of the dispersion of TiC. The machining conditions for the hardest deposition did not coincide with those for the highest one. Therefore, the discharge current and pulse duration should be optimized for the deposition.     

Super-high-thickness high-speed wire electrical discharge machining

High-speed wire electrical discharge machining (WEDM-HS) of materials of super-high thickness (more than 1000 mm) is a challenging problem. First, sufficient energy is required to maintain the inter-electrode normal discharge. Next, there must be adequate inter-electrode dielectric fluid. Third, in order to generate a smooth cut surface, it is necessary to suppress the vibration of the wire electrode to reduce vibration lines on the cutting surface. To better understand these challenges, the energy and the flow of the medium between two electrodes were analyzed, allowing the establishment of a relevant model. The results indicated that for super-high-thickness machining, the pulse energy must be adequate to compensate for the energy consumed in the molybdenum wire and inter-electrode working liquid. In addition, the running speed of the wire electrode should be improved to ensure that there is a sufficiently high flow rate of the dielectric fluid. The servo control mode of the existing machine tools and dielectric fluid were improved and then a process experiment was performed. The experimental results show that the process can be carried out efficiently and stably and the workpiece surface can be cut smoothly using the improved working liquid and servo control mode.     

Critical wall thickness in electrical discharge machining

Electrical discharge machining (EDM) is capable of fabricating thin wall structures due to the absence of physical contact between the tool and workpiece. However, the presence of impulsive force does exist during the sparking process, which could give rise to limitations in fabricating small features. The L9 Taguchi method together with the analysis of variance was used to determine the significance of various machining parameters on the specific impulsive force. Through the statistical analysis, it was observed that discharge energy was the most significant factor that influenced the specific impulsive force. Hence, it is important to understand the effects of specific impulsive force in EDM and to investigate its influence that limits the fabrication of thin wall structures. The limit in which the machined part is able to maintain its geometrical integrity is described as the critical wall thickness. The effects of specific impulsive force on thin wall structure were studied through determining the critical wall thickness for various discharge energies. The experimental work showed that both electrothermal erosion and brink erosion, which is attributed by the specific impulsive force was responsible for the loss of geometrical integrity of the wall structure. As the wall thickness falls below a critical value in a given discharge energy setting, the brink erosion was observed. The limit that established the critical wall thickness was elucidated by conflating the thermal erosion and brink erosion.     

微细电火花加工装置及加工工艺的研究

为适应现代加工小型化的需求,在开发微型电火花加工装置的基础上,通过加工微孔、微轴及微三维结构,对加工工艺进行系统研究;分析RC脉冲电源中参数R、C对加工效率、电极损耗比和表面质量的影响;验证电火花伺服扫描加工微三维结构的可行性.     

Investigating surface properties of OHNS die steel after electrical discharge machining with manganese powder mixed in the dielectric

Although electrical discharge machining is essentially a material removal process, efforts have been made in the recent past to use it as a surface treatment method. An additive powder in the dielectric medium affects the sparking action and helps in improving the surface properties. It may melt at the high temperature of the plasma channel and alloy with the machined surface under appropriate machining conditions. Breakdown of the hydrocarbon dielectric contributes carbon to the plasma channel. In this paper, changes in surface properties of oil-hardening non-shrinkable die steel after machining with manganese powder suspended in kerosene dielectric medium have been investigated. Results show improvement in microhardness by 73%, and no microcracks on the machined surface. X-ray diffraction analysis of the machined surfaces reveals the transfer of manganese and carbon from the plasma channel in the form of manganese carbide. Quantitative analysis of chemical composition by optical emission spectrometer confirms significant increase in the percentages of manganese and carbon.     

Role of stabilizers on agglomeration of debris during micro-electrical discharge machining

Micro-electrical discharge machining (µ-EDM) as an advanced mechanical micro-machining method has been widely used in the generation of micro-features by removing debris through melting and evaporation phenomena, and during this time some amount of debris is dispersed in the dielectric medium. These debris are treated as unwanted materials and later will be disposed of from the system. But, after critical evaluation of the debris through transmission electron microscope (TEM), it is found that the debris are agglomerated and in the size range of 40–600?nm (mean size: 242?nm). This makes curious to search for any alternatives to overcome the agglomeration to result into ultrafine particles in the range of nanometres. In the present article, an attempt has been made to incorporate ultrasonication during machining process and also resorted to use stabilizers. As per chemistry aspect of metallic aluminium particles, Poly Ethylene Glycol (PEG), Bael Gum (BG) and ACacia Gum (ACG) stabilizers are used along with deionized (DI) water and studied their effect individually on the interaction between the nucleated nanoparticles. The characterization results reveal that using PEG, BG and ACG, the debris of aluminium generated are in the size range of 45–500?nm (mean size: 196?nm), 25–70?nm (mean size: 45?nm) and 3–30?nm (mean size: 10?nm), respectively. ACG stabilizer was found to be strongly reduce the agglomeration resulting in excellent stable dispersion of the colloidal aluminium nanoparticles. The nature of colloidal samples was studied by ultraviolet–visible (UV–vis) spectroscopy after 2 weeks of their generation. Furthermore, thermal conductivity and viscosity properties are ascertained, and the heat transfer effect has also been studied experimentally using a developed thermal management system. The obtained results are presented in the article.     

Analytical modeling of ductile-regime machining of tungsten carbide by endmilling

The aim of the present research was to compare the efficacy of two powder based 3D printing technologies for rapid casting of light alloys. The technologies of ZCast process and investment casting were employed to cast aluminium A356 alloy and zinc ZA-12 alloy. The split pattern shells were printed in ZCast501 powder and used directly as mould with outside sand support in case of ZCast process. Two commercially available powders starch-based ZP14 and plaster-based ZP100 were infiltrated with two infiltrants acrylate and wax resulting in four different material systems for making investment casting patterns. A standard method was premeditated by identifying and designing the benchmark component. The proposed concept was presented in physical form by fabricating prototypes to appraise the impact of technology used on dimensional accuracy. The dimensional accuracy was acceded by assigning tolerance grades as per UNI EN 20286-1 ISO. In addition, the working suitability of castings was analysed by comparing important mechanical properties, and further, the results were supported by radiography and microstructure analysis. The feasibility of decreasing shell wall thickness for ZCast process was also checked so as to make the process more economical and fast.     

Advantages of carbon nanotubes in electrical discharge machining

Carbon nanotubes (CNTs) have a small specific gravity and a straight-pin shape, which allow them to continuously float and to uniformly disperse throughout the entire dielectric-filled cavity with little agglomeration during electrical discharge machining (EDM). In the past, powder mixtures of silicon, aluminum, and chrome have been used in the EDM process. However, there are concerns about flushing the controlled gap between the electrode and the workpiece because of their heavy specific gravity and their associated non-uniform dispersion in the dielectric. In this study, the effect of adding CNT powders to the dielectric on the surface integrity and the machining efficiency of the workpiece were investigated. CNTs can avoid the agglomeration problem. The CNTs were fabricated by chemical vapor deposition and added to the dielectric at a concentration of 0.4?g/l. The average surface roughness of 0.09?μm was achieved within 1.2?h, and the material defects of the recast layer and the micro-cracks were considerably reduced. The adopted processing parameters were a negative electrode polarity, a discharge current of 1?A, a pulse duration of 2?μs, an open-circuit voltage of 280?V, and gap voltage of 70?V. This technology improved the surface finish by 70% and the machining time by 66%. The achievement is attributed to the nanoscale characteristics of the CNTs in the dielectrics. The surface force became large and was able to balance the gravity body force of the CNTs. Consequently, the electric arcs were well dispersed and more uniform across the electrode gap, thus significantly enhancing the performance of the electrical discharge. It is expected that carbon nanotubes will be used in many EDM applications.     

Development of parallel spark electrical discharge machining

This paper describes the development of parallel spark EDM method. In the discharge circuit, the electrode is divided into multiple electrodes, each of which is electrically insulated and connected to the pulse generator through a diode. A capacitor is inserted parallel to each discharge gap between each electrode and workpiece (here workpiece is common for each electrode). Compared with conventional EDM in which only a singular discharge can be generated for each pulse, multiple discharges can dispersively be generated for each pulse in parallel spark EDM. Results of experiments on parallel spark EDM and conventional EDM show that not only is the machining process more stable, but the machining speed and surface roughness can also be improved with parallel spark EDM.