Study On Powder Mixed Near Dry Electrical Discharge Machining Technology

With development and innovation of traditional machining technology, they display advantage of high efficiency in hard machining materials field, traditional Electrical Discharge Machining (EDM) technology faces serious challenge, develop high efficiency, high precision, high flexibility EDM technology is an urgent task for EDM industry now.A powder mixed near dry EDM (PNDEDM) technology is presented in this paper, its dielectric mediumis gas-liquid-solid three phase fluid mixtures. A certain amount of conductive or semi conductive micro powder are filled in the liquid in advance, and mixed evenly by mix equipment, thereafter liquid-solid mixture is blended with air by dispenser, then gas-liquid-solid three phase fluid mixtures are got. Gas-liquid-solid three phase fluid mixtures are sprayed to the work piece through the thin-walled pipe tool electrode, work piece and tool electrode are connected with pulse power separately, tool electrode can rotate with the rotation of spindle, and control system keeps tool electrode and work piece in a very small gap. During EDM process, various shapes are machined through changing the position of tool electrode and work piece.During PNDEDM process, a single spark process can be divided into four periods: ionization, breakdown discharge, debris ejection and deionization. After electric field is set up between electrodes, insulation medium is ionized as electrons and positive ions, when electric field intensity is over critical electric field intensity, dielectric medium is broken down in a short time quickly, electrons move towards anode and positive ions move towards cathode in high speed under electric field force, serious spark is generated among tool electrode, powder and work piece, discharge channel is formed, part metal are melted, and part metal are vaporized, debris are flushed out of spark gap by the explosion force and high pressure of dielectric medium, melted and vaporized metals are solidified into solid particals, work piece surface forms a crater, after pulse spark is dielectric deionization, insulation is resumed. Material removal mechanisms of PNDEDM including: melting and evaporation, spalling, oxidation. Due to discharge channel is high temperature and high pressure ionization gas, surface metal of work piece and tool electrode within work zone are melted, or even vaporized, oxidation also can accelerate the material removal rate(MRR), when erode hard and brittle materials, spalling effect also can help remove material. Both liquid drop and micro powder can decrease insulation strength greatly, and micro powder is more obvious, after micro powder is polarized, bound charge are generated on sphere surface, the direction of electric field intensity induced by bound charge in the powder sphere is opposite with outside electric field direction in the dielectric ball, and not parallel with the outside electric field outside the dielectric ball, therefore actual electric field will deviate original electric field, electric field aberration happened between the electrodes, compared with dry EDM and near dry EDM, gap distance of PNDEDM is the biggest.Experiment results show that the material removal rate and surface roughness increase with the increase of pulse duration, peak current, layer depth and tool electrode movement velocity, material removal rate increases and surface roughness decreases as pulse interval, dielectric medium pressure, powder concentration or solid-liquid mixture concentration increases. Tool electrode wear decreases with the increase of pulse duration, it is almost zero under big pulse duration condition, but it increases with peak current. Material removal rate of dry EDM, near dry EDM and powder mixed near dry EDM increases in turn, and surface roughness decreases in turn. Compared with dry EDM, due to spark gap is improved greatly, debris will be flushed out easily, short circuit probability is reduced, arcing phenomenon is minimized, machining process becomes stable, thereafter material removal rate is improved. Due to bigger spark gap and disperse effect of powder, erosion crater depth becomes flatter and more shallow, erosion crater edge is smoother, due to better cooling effect of gas-liquid-solid three phase fluid dielectric, debris also can be cooled timely and effectively during EDM process, less globular attachment are observed, therefore surface quality quanlity is improved.Serious net cracks are generated under excessive pulse duration and current action time during machining cemented carbides process. Experiment results show that it is not advisable to improve material removal rate by using big pulse duration and big peak current. Narrow pulse duration and higher peak current should be employed during rough machining process, brief and instantaneous high temperature make surface heat zone shallower, crack density smaller, experiments show cracks can be avoided effectively when narrow pulse duration and small peak current are employed during finishing process. During PNDEDM process, heat impact load is the main reason induced net cracks of work piece, and it is also related closely with material properties. The single spark process of PNDEDM has been simulated using finite element method. Temperature of spark point in the center is the highest, temperature decreases with the increase of radial and axis direction. Radial stress and axis stress dominated in the stress component. Radial stress is always negative, it means it is press stress, but axis stress changed from press stress to tensile stress, this may induce surface defect.Artificial neural network architecture is optimized using multi-population genetic algorithm(MPGA), coding string including hidden neuron number information, training time information, objection function is weighting sum of training data relative error and check data relative error, the optimized architecture is more compact and prediction accuracy is higher, subjectivity and locality is minimized, basis of high quality intelligence control has been set up.