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As a thermal erosion process, the performance of electrical arc machining (EAM)—including material removal rate (MRR) and tool wear ratio (TWR)—is greatly influenced by arc plasma in the gap between the workpiece and the electrode. Therefore, it is necessary to study plasma channel expansion behaviour (PCEB) to disclose the EAM mechanism. By far, negative polarity EAM is usually adopted to achieve a higher MRR; most current research concentrates on the PCEB of the anode side for it directly relates to the MRR. However, in electrical machining, the plasma ignites from the cathode side, and positive polarity EAM is also recommended to improve the surface quality. Thus, the PCEB of the cathode is also very important. In this paper, a set of single discharge observation experiments was implemented to investigate the PCEB at cathode side by utilizing a four-frame high-speed camera. Based on time solved imaging of single discharge, the relationship between the PCEB and primary electrical parameters, including discharge time and peak current, was studied. Besides, the PCEBs of three conventional electrode materials were compared to find the mutual relationship between the material properties and expansion behaviour. The PCEB formulations of these materials under the specific parameters were further deduced and applied to simulate the single discharge crater. Finally, to elaborate the validity of the formulations, the simulation results above were compared with the results obtained by using uniform plasma diameter.

The machining efficiency and productivity of the micro-electrical discharge machining (μ-EDM) process can be enhanced by the fundamental insights on the plasma-material interaction, which can be appraised via experimental and numerical analysis of discharge crater formation. The high-power density with low pulse-duration results in the transformation of the solid workpiece into liquid and even vaporisation phase to generate a crater. The crater dimension largely depends on the extent of vaporization induced recoil pressure as well as the pressure exerted by the expanding plasma channel. Efforts have been made by the researchers to analyse the plasma characteristics viz. its temperature, pressure and the role of plasma pressure on the material removal. However, there is a scarcity of the realistic data pertaining to precise plasma pressure which consequences in less accurate estimation of the numerically simulated crater. Moreover, the extent of evaporation is delayed due to activation of plasma pressure. Therefore, it is imperative to conduct a sensitivity analysis of variation of plasma pressure to improve the predictive nature of three-phase multi-physics numerical modelling of the μ-EDM. In the present paper, an endeavour has been made to consider the multi-phase transformation corresponding to the high-power density applied to the workpiece. Further, the effect of parameters such as Marangoni convection, Young Laplace surface tension pressure, vapour induced recoil pressure and a time-varying plasma pressure on the melt-pool hydrodynamics are considered. A 2D phase-field numerical model for the machining of Ti-6Al-4V has been formulated considering the aforementioned parameters. The numerical model is implemented in the commercial finite element method (FEM) software COMSOL Multiphysics. The simulation results reveal that the material removal in the initial phase of pulse-duration is dominated by vaporisation along with the melt expulsion due to recoil pressure which governs the crater size. The vaporisation of the Ti-6Al-4V is delayed to 4250 K due to active plasma pressure in the melt-pool. In the remaining pulse period, the surface tension induced backflow of molten metal along with Marangoni convection result in non-uniform perturbation at the bottom of the crater. Finally, the simulated crater is validated with the experimentally determined one-off crater using a dedicated single spark EDM circuit.

This study proposes a new explanation of how tool wear is reduced during powder-mixed micro electrical discharge machining. Owing to its lower discharge energy than a transistor discharge circuit, an RC discharge circuit outperforms in micro EDM. Given that the energy distribution of a cathode is lower than that of an anode during discharge, a tool is negatively charged to reduce tool wear and increase the material removal rate. However, due to the stray inductance of an RC discharge circuit, the tool polarity and direction of the current fluctuate before the discharge plasma channel is extinguished, which increases the tool wear. In this study, it was found that graphite powder in a dielectric fluid disperses the charged energy into several small discharge plasma channels. These small channels were extinguished quickly before the tool polarity and direction of the current fluctuate; which decreases the amount of tool wear. The machining time and tool wear length in powder-mixed micro ED-drilling were evaluated with various particle diameters and concentrations of powder-mixed dielectric fluid. With the optimal tool feeding conditions, the machining time and tool wear length of graphite-powder-mixed kerosene were reduced by 30.9% and 28.3%, respectively, as compared to those in non-powder-mixed EDM.

Electrical arc machining (EAM) is a developing thermal process which benefits from high energy density arc column for machining process. In EDM though, many researches have been conducted to make a better understanding of material removal mechanism, plasma temperature distribution, plasma density etc. using simplified tool and workpiece as pin and plate electrode in single discharge. However, in EAM due to high energy density and long discharge duration, plasma arc column behavior is different from plasma discharge in EDM. It can be further deducted that besides the electrical parameters, it is highly possible that the electrode shape and some other conditions have significant influence on the discharging process and formation of the crater. Therefore, in this paper effects of different electrode shapes and size on plasma channel growth and crater formation was investigated based on time resolved imaging of single arc discharge and crater foot prints. Using an ultra-fast four-channel ICCD camera, changes in plasma channel in presence of different electrode tips and diameters was studied. Different currents were also applied to investigate light emission of plasma column during the discharge. Afterwards, shape of craters in different conditions were compared. Observation of images in definite time steps from the beginning of breakdown to the end of discharge showed that plasma channel diameter changes over the time and along the gap distance. Comparing crater geometry, tool erosion and plasma arc column in different conditions give an insight to efficient design of discharge equipment for precise observation of arc discharge in EAM.

Electrical discharge machining is a promising non-traditional machining process, which is quite well investigated in relation to the formation of the plasma channel and the formation of single erosional craters. However, the influence of geometrical and energetic heterogeneities of electrodes on the formation of the workpiece shape and surface finish is insufficiently investigated. In this work the analysis of the influence of geometric and energetic heterogeneities of the surfaces of the electrodes on the discharge and its location, the shape and size of the erosion craters is performed. Theoretical studies based on multidischarge numerical simulation of electrical discharge machining, which includes a cyclical execution of the following steps: 1) modeling of thermal processes for a single discharge; 2) modeling of the workpiece material removal and crater formation during the pause; 3) determining of the location of the subsequent discharge, are carried out. For the simulation of thermal processes the equation of transient heat conduction in a medium with variable physical-chemical properties was used. Thus the workpiece material removal was replaced by an anisotropic material with low heat capacity, which allowed to eliminate the necessity of changing the computational domain when this material was removed. The experimental investigation on the distribution of discharges over the workpiece surface at the initial stage of EDM was performed. The emergence of the phenomenon of “nesting” craters in the zones of maximum energy disposition (bulges, zones of differential surface and temperature) was shown. The process of how these zones are distributed over the entire workpiece surface was shown.

A novel machining process of high-speed EDM milling with moving electric arcs was developed, which can remove material continuously without discharge intervals of pulse power supply and achieve extremely low electrode tool wear for the direct-current power supply employed. An EDM equipment was devised to explore machining characteristics and removal mechanisms of moving electric arc in high-speed EDM milling process. Firstly, the motion characteristics and resistance properties of moving electric arc were investigated. The theoretical analysis shows that the arcs maintain a relatively independent movement between the electrode and workpiece. Then, the plasma channel of electric arc was captured by high-speed video camera to explore the machining mechanisms of the moving electrical arc. Finally, the machining experiments were carried out based on titanium alloy, which verified the above mechanisms and a much higher material removal rate could be easily achieved by moving electric arc. Moreover, the tool electrode wear in high-speed EDM milling is much lower than conventional EDM due to the protective layer formation.

Blasting erosion arc machining (BEAM) is a promising method for machining difficult-to-cut materials. In BEAM, high pressure inner flushing is implemented to increase machining efficiency in terms of material removal. However, effects of flushing on plasma arc evolution and material removal mechanism in BEAM are still unknown. In this research, air was blown to a single arc discharge as an alternative to water based dielectric in BEAM process. Using an ultra-fast four channel ICCD camera, influence of cross directional air jet on plasma channel development and crater formation was studied. Observation showed that plasma arc channel is obviously disturbed in presence of high speed air flushing. In other word, arc plasma column moves along the air flushing direction. Consequently, crater geometry and electrode wear will highly be influenced by air flushing in the gap. High speed air flushing increases molten metal flushed away from molten pool compare to the stationary air medium condition. This indicates that there is a decrease in recast layer on machined surface and as a result, volume of removed material will increase. Moreover, using high speed air flushing in single arc discharge decreases tool electrode wear volume significantly. Finally, based on the experimental results and theoretical analysis, high speed air flushing proves aerodynamic arc disturbing phenomenon during single arc discharge which is a potential evident to hydrodynamic arc breaking mechanism in BEAM.

An RC-type generator circuit is suitable for micro-electrical discharge machining (micro EDM) owing to its low single discharge energy. In micro EDM, negative tool polarity has the advantage of reducing the tool wear rate and the relative ratio. However, when using an RC-type generator circuit in micro EDM, the direction of the current fluctuates at the plasma channel between the tool and the workpiece due to stray inductance. Because the current flowing from the tool to the workpiece can intensify the tool wear rate, it is preferable to prevent the fluctuation of the current direction. In this study, we propose the RC-type generator circuit using an n-channel MOSFET to prevent the fluctuation of the current direction and to increase duty ratio to enhance the material removal rate. Using a micro ED drilling process, the material removal rate, relative ratio, and surface roughness were measured with respect to the feeding control conditions and electrical conditions. As a result, by using the RC-type generator circuit with a MOSFET, the relative ratio is reduced by up to 40%, with the same material removal rate and surface roughness, compared with the results without a MOSFET.

In recent years, researches on Electrical Discharge Machining (EDM) plasma mainly focus on the reasons and the characteristics of plasma oscillation and discharge wave oscillation. The mechanism of EDM plasma channel oscillation and discharge wave oscillation, however, has not been explained in detail. This paper chiefly analyzes the geometry change of EDM gap, the diameter change of EDM plasma channel and the electromagnetic field change of EDM plasma. These three factors contribute to the EDM plasma oscillation after affected by one another. On the base of analyzing, a new method of machining by active-oscillation-pulse in EDM has been put forward. The active-oscillation-pulse, which is generated by adding high frequency oscillation component to conventional EDM pulse, can put EDM plasma channel into forced oscillation state. Then the EDM plasma channel will move more frequently and harmful arc discharge will decrease accordingly. Experiment shows that the machining efficiency can be improved remarkably by machining of active-oscillation-pulse.

The polarity effect in EDM drilling was investigated using a water based dielectric. EDM drilling machines are hardly designed to change the polarity, although it is known from the EDM sinking process that the polarity is linked to the discharge time. It was found out that there is a current during the delay time and the open circuits when processing with negative tool electrode polarity. A separate experimental setup showed a water electrolysis. The electrolysis effect occurs simultaneously to the electrical discharge process when the tool electrode has a negative polarity. It could be shown that the tool electrode is fully covered with gas bubbles when the electrical voltage is applied. This phenomenon leads to a modified energy distribution in the narrow gap. The plasma channel growth rate at the tool electrode is much higher, leading to reduced thermal stress. Therefore a lower wear and a higher feed rate could be observed when using a negative tool electrode.