Study On The Process And Mechanism Of Powder Mixed Near Dry Electrical Discharge Machining

In the traditional electrical discharge machining (EDM) process, the discharge is induced in the liquid medium. However, in the dry EDM process, the liquid medium is replaced by the gas medium. Dry EDM has many advantages. For example, the tool wear rate (TWR) of it is low, its acting force on the machined surface of the workpiece is low, its material removal rate (MRR) with oxygen gas as its dielectric medium is high, and environmental pollution is reduced. So the dry EDM is paid close attention to. Nonetheless, dry EDM has disadvantages, such as small discharge gap and low specific heat of the gas. Its residual heat transfer is difficult, the cooling of the molten materials produced during its discharging is slow, and it is hard for the corrosion particles to be expelled out of the discharging areas. Therefore, its short circuit rate is high, its utilization efficiency of pulse energy is low, and the machining stability of it is poor. Only by taking appropriate measures to overcome shortcomings of the dry EDM process while taking advantages of it, can we promote the practical application of this process. In the powder mixed near dry electrical discharge machining (PMND-EDM) process, trace amounts of powder particles and liquid phase are added into the gas continuous phase. Its MRR is higher while the TWR is lower. So it is thought to be a potential green processing technology with high machining efficiency. In this paper, the mechanism of PMND-EDM process is researched, its basic processing parameters are determined, its technological characteristics are studied, and its processing parameters are optimized.Start from the function of the solid phase and the liquid phase, research is done on the mechanism of the PMND-EDM process. Based on the Wagner model, the dielectric coefficient of the solid-liquid mixed phase is calculated. The dielectric properties of the solid phase, the liquid phase and the solid-liquid mixed phase are analyzed. And it is verified that the dielectric coefficient of the continuous gas phase is lower than its inner dispersed phases. Research is done on the medium breakdown mechanism of the three phase dielectric medium based on the influence of variation of electric field strength on the discharge characteristic of dielectric medium in non-monolithic medium. Analysis is done on the electric potential distribution in the three phase dielectric medium based on the Laplace’s equation of differential form of the Gauss’s law in the spherical coordinates. Results show that the distortion of the uniform electric field can be induced due to the addition of dispersed phases. The expression for electric field strength is established under a certain assumed conditions. The calculation formula for the discharge gap is obtained. And the change of the discharge gap, which is caused by the three phase dielectric medium, is analyzed. The theoretical analysis results indicate that the discharge gap can be enlarged by the three phase flow, the breakdown of the medium can be improved, and the expelling of the debris formed during discharging can be facilitated. The enlargement of discharge gap for the three phase flow is verified by the comparison test. The discharge voltage is recorded during the comparison test and the comparative analysis is done. It is confirmed that the medium breakdown frequency is increased. The theoretical calculation reveals that serious distortion of the electrical field strength is easier to be found at locations with dispersed phased, and the series electrical discharge among the tool electrode, the dispersed phases and the workpiece electrode can be induced easily in these places. Based on the above, the theory model of series electrical discharge is establised. Research is done on the phenomena of material attachments on the surface of tool electrode. The surface micro-morphology of the used tool electrode is observed by the scanning electron microscope (SEM). The composition of the thin-film attachment region, the lamellate attachment region, the region with spherical attachments and the porosity attachment region are analyzed by the energy dispersive spectrometer (EDS), respectively. It is verified that there are attachments, which are elementary compositions of the workpiece, on the surface of the tool electrode. And the mechanism of tool electrode wear for the PMND-EDM process is proposed. The research results show that the tool electrode wear in the PMND-EDM process is caused by the material removal from the tool electrode and the material attachments on the surface of tool electrode.The technical parameters of the PMND-EDM process are divided into two categories:basic processing parameters and machining parameter. And relevant researches are done.Starting from the general powders in EDM process, comprehensively consider the addition of powder particles on the EDM machining, the dielectric medium stability, and the synthesized mechanical properties of the machined workpiece surface, and do analysis on the influence of powder ingredient and powder size on the dispersion stability. From the perspective of giving full play to the effect of powder properties on the improvement of electrical discharging, silicon powders with grain size of2.5μm are selected as the solid phase of the PMND-EDM process. Starting from the general tool electrode materials in EDM process, copper is selected as the tool electrode material for the PMND-EDM process. Comparative test is done on the influence of polarity of electrodes on the machining. And it is found that the higher MRR and the lower TWR can be obtained when the tool electrode is connected to the cathode of the pulse electrical source, the machining polarity for which is normal. Therefore, normal polarity is more appropriate for the PMND-EDM process. Theoretical analysis is done on the polarity effect of the PMND-EDM process based on the conventional theory of polarity effect. The difference in the kinetic energy for the positive ion swarm and the electron swarm, when they impact the negative electrode and the positive electrode respectively, is got by calculating. The theory of polarity effect for the PMND-EDM process is established. And the experimental results coincide with the theory analysis results based on the theory of polarity effect for the PMND-EDM process.Based on the single factor experimental design, research is done on the technological characteristics of the PMND-EDM process. The influences of peak current, pulse on time, pulse off time, flow rate of solid-liquid mixture, concentration of powder particles and rotational speed of tool electrode on the MRR and the surface roughness (SR) are achieved, and the reasons for these influences are revealed. Research results reveal that the effect of peak current on the EDM machining mainly based on its effect on the obtained energy density of the machined workpiece surface. With the increase of peak current, the crater becomes larger and deeper, both the MRR and the SR for the machined workpiece increase. The effect of pulse on time on the EDM machining mainly based on its effect on the supply time of the pulse discharge energy. With the increase of pulse on time, the supply time of pulse discharge energy increases and the plasma channel expands in its radius direction; as a result, the crater becomes larger and deeper, both the MRR and the SR of the machined workpiece increase. The influences of pulse off time on EDM machining are mainly based on two main aspects. One is its influence on the deionization effect of the dielectric medium in the discharge gap. The other is its effect on the duty circle. The deionization effect indirectly influences the discharge course of single pulse and the discharge energy of single pulse, and then influences the MRR and the SR of the machined workpiece. With the increase of pulse off time, both the MRR and the SR decrease. Flow rate of solid-liquid mixture determines the volume of solid phase and liquid phase that are delivered into the discharge gap. The volume of the liquid phase and the solid phase influence the breakdown of the dielectric medium and the deionization effect of the dielectric medium. With the increase of the flow rate, the MRR of the workpiece increases, however, the SR of the machined workpiece decreases. The concentration of powder particles influences the breakdown of the dielectric medium and the deionization effect of the dielectric medium. With the increase of the concentration, the MRR of the machined workpiece increases first and then decreases, however, the SR decreases first and then increased. The existence of rotation for the tool electrode is helpful to improve the machining process of the PMND-EDM technology. However, the excessive increase of the rotational speed for the tool electrode will deteriorate the machining stability, which results in the decrease of the MRR and the SR for the machined workpiece along with the increase of the tool rotational speed. A low tool rotational speed should be selected from the perspective of stabilizing the machining stability and then improving the machining.Based on the theory of orthogonal experimental design, systematical research is done on the MRR, the SR and the TWR. The MRR obtained in the orthogonal design experiment is converted to the corresponding value of signal to noise ratio based on the expression for the signal to noise ratio of the higher the better characteristics. The TWR and the SR obtained in the orthogonal design experiment are converted to the corresponding value of signal to noise ratio based on the expression for the signal to noise ratio of the smaller the better characteristics. The optimal combinations of machining parameters for the highest MRR, the lowest TWR and the lowest SR are obtained based on the theory and the method of the analysis on the signal to noise ratio. The results of single objective optimization are verified by the verification experiments.The significant impact factors for the signal to noise ratio of each machining evaluation index of the PMND-EDM process is found out by doing analysis of variance (ANOVA) on results of the orthogonal design experiment. The significant impact factors are peak current, pulse on time and flow rate for the signal to noise ratio of MRR, peak current for the signal to noise ratio of TWR, and pulse on time and peak current for the signal to noise ratio of SR. The mathematical expressions for the signal to noise ratio of the MRR, the TWR and the SR are established by the statistical analysis software Minitab16based on the relative theory and method of nonlinear regression analysis. The significance of the regression model is verified by test of statistical significance. The predictive effects of the established models are verified through verification experiment and the error analysis for the predictive results of the regression models. Taking into account the MRR, the TWR and the SR, multi-objective optimization is done on the PMND-EDM process based on the particle swarm optimization (PSO) algorithm. The pareto fronts of the MRR and the SR, the MRR and the TWR, and the SR and the TWR for the PMND-EDM process are obtained respectively. The process could decrease the TWR and overcome the problems accompanied with the large TWR, such as the frequent replacement of the worn tool electrode, when machined with large MRR; meanwhile, lower TWR can be obtained along with lower SR.