Repetitive Spark Research Articles

Thermal simulation of microelectrode machining process during block electrical discharge grinding

Electrical discharge machining is widely used in aerospace industries, semiconductor applications, micro-electromechanical systems, and other fields of the machining of micro-holes, micro-shafts, and micro-structures due to non-contact stress during the electrical discharge machining process. In the electrical discharge machining process, the machining precision of electrical discharge machining depends on the dimensional precision of the tool electrode, therefore, the machining process of microelectrode has attracted growing concern. In this study, a thermal model of the microelectrode machining process was proposed to simulate the formation process of microelectrode based on the temperature distribution during block electrical discharge grinding of the microelectrode process. According to the microelectrode machining process, the thermal model considers the main influential factors such as random distribution of discharge points, time-dependent discharge channel radius, Gauss heat source, phase transformation latent heat, etc. Through the thermal simulation analysis, the temperature distribution and the dynamic spark-erosion process of the microelectrode based on random repetitive spark discharges were obtained during block electrical discharge grinding of the microelectrode. Besides, it is found that the maximum temperature value and discharge crater volume of the electrode surface fluctuate and gradually decrease with the increase of discharge times. In addition, the effect of pulse width on the machining process of microelectrode was investigated by numerical simulation and block electrical discharge grinding of microelectrode experiment. The results indicated that the crater volume and the maximum temperature value on the surface of the microelectrode increased with the increase of pulse width. The formation process of microelectrode was considered a significant indicator to determine the machining precision and machining efficiency of the electrical discharge machining process.

Multi objective optimization of EDM process parameters for HCHCr steel

Electric discharge machining (EDM) or spark machining involves controlled erosion of electrically conducting materials by an interrupted, repetitive electric spark creating discharge between the cathode- tool and anode- work-piece, separated by a dielectric fluid medium. The objective of this work is to study the influence of machining characteristics during EDM of high carbon and high chromium steel (HCHCr) with copper electrode. EDM of HCHCr steel is conducted according to Taguchi orthogonal array (OA) design of experiment to evaluate the material removal rate (MRR), tool wear rate (TWR) and surface finish (Ra).The experiments are conducted on die-sinking EDM with varying current, voltage, and pulse-on-time to measure the above responses. Finally, the multi objective optimization technique namely principal component analysis (PCA) is used for optimizing the process parameters. The experimental results indicate the optimum parameters as: current 9 Amps, pulse-on time 15 µs and pulse-off time 10 µs.

Formation mechanism of Fe-based amorphous powders produced by spark erosion

Spark erosion is a convenient, flexible, and low-cost method to quickly produce fine powders of metals, alloys, and semiconductors in size ranging from nearly a hundred micrometers to submicrometer by repetitive spark discharges. Due to the complexity of the powder-forming journey, normally accompanied by high temperature, high pressure, decomposition, diffusion and rapid quenching caused by discharge plasma, the mechanisms of powder formation and possible contaminant infiltration are still controversial, posing a significant challenge to control particle size and chemical composition of the powder produced by this method. In this study, Fe-based amorphous powders in different particle-size distributions with high sphericity were fabricated by spark erosion under different discharge-energy conditions. The maximum particle size of the resultant powders can be correlated with discharge parameters, crater depth, and crater radius, respectively. A multi-ring-breakup model is proposed to reveal the particle-size distribution of the powder formed from the electrode melt under a single-pulse discharge. Furthermore, a dielectric-element infiltration model is provided to quantitatively evaluate the infiltration mass ratio of the contaminant elements, stemming from the decomposed products of dielectric liquid, in the resultant powder with different particle sizes. The models verified through the experimental data are significant for the development of high-performance fine Fe-based amorphous powder with controlled particle size and chemical composition.

Hydrogen-assisted spark generation of silver nanoparticles: The effect of hydrogen content on the signal intensity in surface-enhanced Raman spectroscopy

We employed the so-called hydrogen-assisted spark generation method to produce silver nanoparticles by means of a repetitive spark discharge in a flowing gas atmosphere. Different compositions of H2–Ar mixtures were used as carrier gas to systematically investigate the effect of the hydrogen content on the signal enhancement obtainable with silver nanoparticles in Raman spectroscopic measurements. To this end, the particles were collected on filters and used as SERS substrates without further preparation. It was shown that a ca. 50-fold signal-increase can be achieved by adding at least 30% H2 to the Ar gas. The potential causes of the enhancement were also investigated, and they were associated partly with more pronounced aggregation on the filter and partly with the increased resilience of the silver particles against surface oxidation. The latter effect was supported by control experiments with non-oxidizable gold nanoparticles, which did not exhibit considerable enhancement-variation regardless of the hydrogen content.

Investigations of Surface Qualities of Wire EDM Machined Tool Steels

Abstract In the EDM process an electrical discharge occurs between an anode (graphite or copper) and a cathode (material to be machined) in a dielectric medium. Developments in EDM technologies have opened a new path towards high-precision, more productive machining of high alloyed, high hardness and high strength steels [1]. Wire EDM offers an efficient and cost effective alternative to traditional machining in tool and component manufacturing [2]. EDM machining removes electrically conductive material with rapid and repetitive spark emissions using a dielectric fluid. As a result, the surface of the raw material is subject to erosion. The quality of the machined surface depends to a large extent on the parameters of the machining technology [3]. In the testing experiments, wire EDM machining was performed on two tool steel grades using different machining parameters. After each machining step, hardness and roughness tests were performed on the surface of the workpiece. Scanning electron microscopy was performed on the machined surface. The tests confirmed that the surface quality of the tools is significantly influenced by the parameters set during the EDM. The change of parameters was achieved by performing multiple cutting threads. By using the multiple cutting thread, tools with favourable surface roughness have been produced for tool steels with a high hardness of quenched and tempered heat treatment state.

Recent advances in compact repetitive high-power Marx generators

AbstractThis paper introduces recent activities on Marx-based compact repetitive pulsed power generators at the Institute of Applied Electronics (IAE), China Academy of Engineering Physics (CAEP), over the period 2010–2018. A characteristic feature of the generators described is the use of a simplified bipolar charged Marx circuit, in which the normal isolation resistors or inductors to ground are removed to make the circuit simpler. Several pulse-forming modules developed to generate a 100 ns square wave output are introduced, including thin-film dielectric lines of different structures, a pulse-forming line based on a Printed Circuit Board, and non-uniform pulse-forming networks. A compact repetitive three-electrode spark gap switch with low-jitter, high-voltage, and high-current was developed and is used in the generators. A positive and negative series resonant constant current power supply with high precision and high power is introduced. As an important part of the repetitive pulse power generator, a lower jitter pulse trigger source is introduced. Several typical high-power repetitive pulsed power generators developed at IAE are introduced including a 30 GW low-impedance Marx generator, a compact square-wave pulse generator based on Kapton-film dielectric Blumlein line, a 20 GW high pulse-energy repetitive PFN-Marx generator, and a coaxial Marx generator based on ceramic capacitors. The research of key technologies and their development status are discussed, which can provide a reference for the future development and application of miniaturization of compact and repetitive Marx generators.

Note: Erosion of W-Ni-Fe and W-Cu alloy electrodes in repetitive spark gaps.

A pair of W-Ni-Fe and W-Cu electrodes were tested under 100 kA level pulsed currents for 10 000 shots, respectively. Surface roughness and morphology characteristics of the two pairs of electrodes were obtained and compared. Experimental results indicated cracks divided the W-Cu electrode surface to polygons while the W-Ni-Fe electrode surface remained as a whole with pits and protrusions. Accordingly, the surface roughness of W-Ni-Fe electrodes increased to ∼3 μm while that of W-Cu electrodes reached ∼7 μm at the end of the test. The results reveal that the W-Ni-Fe alloy has a better erosion resistance and potential to be further applied in spark gaps.

High-efficiency synthesis of nanoparticles in a repetitive multigap spark discharge generator

We describe a method of obtaining aerosol nanoparticles in a repetitive spark discharge generator with 12 interelectrode gaps between tin electrodes, which operates at a pulse repetition frequency of 2.5 kHz. During synthesis of tin oxide nanoparticles in air, the mass productivity of the gas discharge generator reaches up to 9 g/h for primary particles with characteristic sizes within 5–10 nm and agglomerate size on the order of 50 nm.

Interface Engineering in Hybrid Organic-Inorganic Composites for Flexible and High-Performance Thermoelectrics

Thermoelectric devices are typically rigid and their performance and reliability could be compromised by thermal stress. There is also a growing need to develop flexible thermoelectric devices for integration with wearable electronics. By using inherently soft polymers, hybrid organic/inorganic composites are considered a promising candidate for flexible thermoelectrics. In addition, the composites also provide an opportunity for enhanced thermoelectric performance by taking the advantages of low thermal conductivity and high Seebeck coefficient from the organic and inorganic components, respectively. Experimentally, however, enhanced thermoelectric performance in hybrid composites has not been observed, primarily due to inefficient carrier transport across the organic/inorganic interfaces. In particular, if the composites are energetically heterogeneous, their thermoelectric performance will be dictated by the more conductive component and there is no contribution from the less conductive component, preventing higher power factor in the composites . Here we demonstrate a strategy for interface engineering to improve the charge carrier transport from a conducting polymer, i.e., poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS), which has an electrical conductivity greater than 1000 S/cm [1-3], to nanostructures of a prototypical inorganic thermoelectric material, i.e., Bi0.5Sb1.5Te3 (BST), which is the state-of-the-art thermoelectric materials at room temperature (with thermoelectric figure of merit ZT~1) [4]. PEDOT:PSS consists of hydrophobic PEDOT in core and hydrophilic PSS in shell, where PSS is an insulator but it is required to dope EDOT as an oxidizing agent during the polymerization process of EDOT. Therefore, conduction in PEDOT:PSS is mediated by phonon-assisted hopping that requires an activation energy due to the delocalized carriers within the backbones of PEDOT. The key to enhance this hopping process in a BST-PEDOT composite is to reduce the hopping distance for transport of PEDOT-to-PEDOT and PEDOT-to-BST. This could be achieved in a uniform dispersion of BST nanostructures in PEDOT, in which the discrepancy in the activation energy is minimized. To demonstrate this concept, we fabricated BST particles by using a spark erosion method, which involves repetitive spark discharges to produce fine powders [5], and then mixed the particles in PEDOT:PSS (CLEVIOS PH1000) using a non-ionic surfactant and controlling its micelle concentration (concentration ranging from 0.5 wt % to 10 wt%). We then characterized thermoelectric properties and morphologies of the hybrid films fabricated under various conditions. We demonstrated homogeneous dispersion of BST in the PEDOT matrix and consequently efficient interfacial charge transport by optimizing the surfactant. Our results show that hybrid BST/polymer composites are promising for flexible and high-performance thermoelectrics. The strategy developed in this work may also be applicable to other heterogeneous energy conversion architectures consisting of organic and inorganic materials.

Self-pulsing discharges in pre-heated air at atmospheric pressure

The paper presents investigations of self-pulsing discharges in atmospheric pressure air pre-heated to 300–1000 K. Despite using a direct-current power supply, two self-pulsing discharge regimes, a repetitive transient spark (TS) and a repetitive streamer (RS) were generated. The pulse repetition frequency, on the order of a few kHz, can be controlled by adjusting the generator voltage. The TS is a discharge initiated by a streamer, followed by a short (tens of ns) spark current pulse (∼ 1 A), associated with the total discharging of the internal capacity of the electric circuit. The TS is suitable for the study of ‘memory’ effects (pre-heating, pre-ionization) on the mechanisms of streamer-to-spark transition and electrical breakdown in atmospheric pressure air. The TS regime was stable below ∼600 K. Above ∼600 K, a stable repetitive streamer (RS) regime was observed. In this regime, the breakdown and spark did not occur. After the initial streamer, the internal capacity of the electrical circuit discharged partially. With further pre-heating of the gas, the stable TS appeared again at ∼1000 K.

Parametric optimization of wire cut electrical discharge machining

Wire Electrical Discharge Machining is a manufacturing process whereby a desired shape is obtained using electrical discharges (or) by repetitive spark cycle. Precision and intricate machining are the strengths. Machining parameters tables provided by the machine tool manufacturers often do not meet the operator requirements. Selection of optimum machining and machining parameters combinations is needed for obtaining higher cutting efficiency and accuracy. In this present study, machining is done using Wire-Cut EDM and optimization of surface roughness is done using Taguchis design of experiments. Experimentation was planned as per Taguchis L16 orthogonal array. Each experiment has been performed under different cutting conditions of gap voltage, pulse ON time, and pulse OFF time and Wire feed. Dielectric fluid pressure, wire speed, wire tension, resistance and cutting length are taken as fixed parameters. Inconel 800 was selected as a work material to conduct the experiments. From experimental results, the surface roughness was determined for each machining performance criteria. Signal to noise ratio was applied to measure the performance characteristics deviating from the actual value. Finally, experimental confirmation was carried out to identify the effectiveness of this proposed method. Keywords: Optimization; Taguchis L-16 Orthogonal Array; Surface Roughness; S/N Ratio.

State of art in wire electrical discharge machining process and performance

Wire electrical discharge machining (WEDM) is one of the most widely used electro-thermal-based non-traditional machining process, which can be applied for almost the whole range of electrically conductive materials irrespective of their hardness. The rapid, repetitive and discrete spark discharge between the wire tool and job immersed in a dielectric medium leads to build up rapid heat and remove the unwanted material from the parent material by means of melting, spalling and chemical reaction. In recent years, researchers have explored a number of ways to improve the WEDM process performance by analysing the different factors that affect the quality characteristics. The experimental and theoretical studies show that process performance can be improved considerably by proper selection of tooling, material and operating parameters. This paper reviews the research work carried out so far in the area of machining different materials by WEDM. The review presents a report about the influence of WEDM process parameters and studies the improvement of the process performance.

Surface Integrity of Tool Steels Multi-cut by Wire Electrical Discharge Machining

Electrical discharge machining removes electrically conductive material by means of rapid and repetitive spark discharges resulting from local explosion of a dielectric liquid. The electrical discharge machining has been widely used for cutting fine blanking tools which are made of especially hard tool steels. Whereas, its thermal nature causes great concerns regarding surface integrity, which matters a lot to tool life. In the present study, the evolution of surface integrity of the S390 and SKD11 with multi-cutting is comprehensively compared. The result shows that the surface roughness, white layer thickness and surface residual stress decrease with the increase of cutting pass. Compared to the finish trim cut surface of S390 sample, more pinholes and slightly larger craters were produced and the micro cracks could be found on the finish trim cut surface of SKD11 sample.

HYBRID MODELING OF PROCESS PARAMETERS PRODUCED BY ELECTRIC DISCHARGE MACHINING USING DIFFERENTIAL EVALUATION AND PARTICLE SWARM OPTIMIZATION

One of the important advanced manufacturing technologies in machining hard-to-cut complicated contour is electrical discharge machining (EDM) and it is highly appreciated because it is economical and effective for moulding materials of heat resistance and high strength into complex one. The EDM process deals with erosion of electrically conductive materials by the way of initiating rapid and repetitive spark discharge between the work piece and electrode tool, with the separation of gap of 0.05 to 0.5 mm which is called spark gap. A die electric fluid soakes this spark gap. In EDM process machining parameters are considered to be important to achieve optimal machining performance in high heat resistance and hard material. The hand book provides all the details about the opted machining parameters but it does not guarantee that the obtained parameters are close to optimal performance. The EDM process should have the features of robustness, accuracy, process capability to confront with the challenges of the changing world. The EDM process should be modeled and optimized to achieve perfection as mentioned above. The researcher uses RSM to investigate the effect of five controllable input variables (viz) peak current, discharge voltage, pulse on time, pulse off time, and oil pressure. MRR and SR, two output variables are the responses. Experiments were administered on AISI 1020 steel with copper electrode. The response is modeled using RSM on experimental data. The remarkable coefficients are received by performing analysis of variance (ANOVA) at the level of 5%. This study shows the MRR and SR have been significantly influenced by peak current, pulse on time, pulse off time, discharge voltage and oil pressure. The soft computing tools differential evolution and PSO are framed, developed, experimented as predictive models.

Effect of Boron Carbide (B<sub>4</sub>C) Additive during Micro-Electrical Discharge Machining of Ti-6Al-4V Alloy

Micro-electrical discharge machining (μEDM) deals with micro dimensional material removal from the work sample due to the repetitive spark discharge between the micro-tool electrode and the workpiece. In the present paper, an attempt has been made to machine through micro-holes on Ti-6Al-4V superalloy plates to investigate the effect of mixing of Boron Carbide (B 4 C) powder additive in kerosene dielectric. A detailed comparative study has been made showing the effect on micro-ED machining criteria i.e. material removal rate (MRR), tool wear rate (TWR), overcut (OC) and taper by varying two most influencing micro-EDM process parameters i.e. peak current and pulse-on-time keeping other parameters such as flushing pressure, duty ratio, gap voltage, etc. as constant. It has been found that B 4 C powder can enhance material removal rate while it can reduce tool wear rate. Analyses have also been done showing some optical and SEM micrographs of the micro-tool electrode tips as well as machined micro-hole walls to study the micro-hole surface features during experimentation.

A Gliding Discharge in Open Air Sustained by High-Voltage Resonant AC Power Supply

Gliding discharges can generate nonthermal plasma at atmospheric pressure. Therefore, they are widely used in plasma-assisted ignition and combustion. In this paper, the gliding discharge sustained by a resonant ac power supply was obtained with a pin-to-pin electrode geometry, and the characteristics of such discharge were investigated through its voltage-current and discharge images. Inceptive, nonstable, and stable stages involved in this gliding discharge were studied. Experimental results showed that the discharge was first ignited by a spark with a current peak of tens of amperes and soon turned into a stable stage, where the discharge respectively behaved in repetitive spark mode with a current peak of several amperes in a small gap and glowlike mode with a current peak of several microamperes in a relatively large gap spacing. Under a certain situation, a nonstable discharge was observed, which changed from a spark discharge to a glowlike discharge after some time. In addition, both the discharge stability and the breakdown voltage were influenced by the air flow. The discharge became unstable when the air flow exceeded 3 L/min.

English

Electrical discharge machining is considered as one of the main non-conventional machining processes used for manufacturing geometrically complex or hard material parts that are extremely difficult to machine by conventional machining processes. The process involves a controlled erosion of electrically conductive materials by the initiation of rapid and repetitive spark discharges between the tool and work piece in the presence of dielectric medium. The most important process parameters in EDM are material removal rate, electrode wear and surface roughness. The present work is focused to obtain a comprehensive study of surface roughness parameters (Ra, Rz, Rzmax & Rsm) of work materials at different currents in sink EDM process. The experiments were carried out on Al & Die steel components at different currents using cu as a common tool material. The surface roughness parameters under study included average roughness (Ra), average maximum height of the profile (Rz), maximum roughness depth (Rzmax) and the mean spacing of the asperities (Rsm). Machining time of each component and micro hardness of the work pieces before and after machining ware also obtained. In this work the Surface integrity was characterized by roughness parameters and micro hardness values. The results of this investigation revealed that the surface roughness parameters, machining time and micro hardness of work pieces were influenced by current and could be utilized to achieve desired quality of surface finishing thereby increasing the EDM efficiency.

DC Discharges in Atmospheric Air and Their Transitions

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> We present images and basic characteristics and describe transition mechanisms between three dc discharges in atmospheric air point-to-plane or point-to-water gap. With increasing applied voltage, a streamer corona transits to a transient spark: a repetitive spark with very short (<formula formulatype="inline"><tex>$\sim$</tex></formula> 100-ns) current pulses (<formula formulatype="inline"><tex>$\sim$</tex></formula>1 A) of very limited energy. With an appropriate ballast resistor, this transient regime evolves into a pulseless glow discharge. These three discharges generate nonequilibrium plasmas of high chemical activity interesting for environmental and biomedical applications. </para>

Temporal evolution of micro-plasma in a low-current, repetitive vacuum spark soft X-ray source

Temporal evolution of micro-plasma and soft X-ray emission in a short vacuum gap, operated with a repetitive low-current (/spl sim/100 A) discharge was investigated. The device consists of two resonant circuits having different resonant frequencies for energy storage. Micro-plasma of diameter less than 1 mm is formed during spark phase with a very high-frequency (VHF) voltage, and electrodes are heated by successive high-frequency (HF) current. Thermo-field electrons are accelerated by an electric field in a cathode sheath that is formed after arc extinction. Soft X-rays were emitted mainly in the period between arc extinction and subsequent breakdown.

Elemental analysis of metals under water using laser-induced breakdown spectroscopy

Laser-induced breakdown spectroscopy (LIBS) has been evaluated for the detection of metals located under water. Repetitive laser sparks were formed on the metals by focused pulses from a Q-switched Nd:YAG laser (1064 nm). Using repetitive single sparks (RSS) at 10 Hz, only weak elemental emission signals were generated from even strongly emitting species at high concentrations (e.g. Al(I) from commercially pure aluminum). The spectrum in these cases was dominated by spectrally broad emissions (continuum radiation) from the plume ( T < 3000 K). The use of a repetitive spark pair (RSP), however, produced a plasma ( T ∼ 8,880 K) on the second pulse useful to monitor the elemental composition of the metals. In the RSP, a pair of pulses was focused on the metals at 10 Hz with the separation between pulses adjusted from 30 to 180 μs. The pulse pair was generated from a single laser. Only a weak dependence of the emission intensities on the separation between the pulses of the spark pair was observed although signal enhancements over two orders of magnitude were obtained compared to the RSS. Using the RSP, detection limits of 367, 520, 1200, and 1190 ppm were obtained for Cr, Cu, Mn, and Si in steel. Using time-resolved imaging of the laser plumes/plasmas formed on the metal samples under water, some characteristics of the excitation process were evaluated. It was determined that the first pulse of the RSP forms a large bubble or vapor cavity (maximum diameter ∼ 8 mm) on the metal surface which is then interrogated by the second pulse. The second pulse then forms a plasma resulting in excitation similar to that produced by a single laser spark formed on metal in air.