Research on improving surface quality of EDM
Because EDM can process difficult-to-cut materials, and can process special and complex shapes. Therefore, EDM technology is widely used in machinery, aerospace, electronics, instruments, light industry and other sectors. However, because there are many technical parameters in the processing process, there is a certain influence between each technical parameter, which often brings some technical questions, which directly affects the quality and efficiency of processing. Therefore, the analysis of the factors affecting the quality of the processed surface has important implications.
1 Analysis of surface quality of EDM
The surface quality of EDM mainly includes surface roughness, surface transformation layer and surface mechanical properties. The surface of EDM is mainly composed of numerous non-directional small pits and hard convex edges. The calculation of surface roughness is the same as that of machining, and the average arithmetic deviation value Ra of the microscopic flatness is used to indicate. Because the EDM removal of materials relies on the high temperature generated by the discharge between the tool and the workpiece, during the deionization period, due to the rapid cooling of the working fluid, the outer surface of the workpiece is greatly changed. In the surface metamorphosis layer, it is further divided into three layers: melting and condensation layer, heat-affected layer and micro-crack, as shown in Figure 1.
Surface mechanical properties are mainly microhardness, residual stress, fatigue resistance and other indicators. A large number of experimental studies have confirmed that after EDM, the microhardness and wear resistance of the workpiece have improved, and the degree of improvement is also different for different materials. Because in the process of EDM, the surface of the workpiece experiences an instantaneous effect of first expansion and then shortening, so that the surface of the workpiece forms residual stress, most of which is tensile stress, and a small compressive stress appears in the surface layer. Due to the effect of residual stress and possible microcracks, the fatigue resistance of the workpiece after EDM will be greatly reduced, so certain heat treatment is required to eliminate this adverse effect.
2 Influence of electrical parameters on surface roughness
EDM mainly relies on the instantaneous high temperature and high heat generated by the pulsed discharge between the tool electrode and the workpiece to remove the material. The discharge trace formed by removing the material is mainly determined by the pulse energy. The energy released by a single pulse discharge depends on the inter-electrode discharge voltage, discharge current and discharge duration (ie pulse width), and its calculation formula is:
Therefore, the electrical parameters that affect the surface roughness of EDM mainly include peak voltage, peak current and pulse width. Because under certain processing conditions, the peak voltage does not change much and generally takes a fixed value. Therefore, the surface roughness mainly depends on the peak current and pulse width.
According to the experimental research, when copper is used as the east and west electrodes and steel is used as the workpiece, the experience formulas of surface roughness, peak current and pulse width can be obtained:
2.1 Influence of peak current on surface roughness
When the pulse width must be kept, with the increase of the peak current, the energy of a single pulse is also increased, so that the surface roughness value increases, as shown in Figure 2. Therefore, in order to obtain a smaller surface roughness value, the peak current value should be minimized.
2.2 Influence of pulse width on surface roughness
When the peak current must be maintained, the pulse width is large, the single pulse energy is large, the discharge traces caused by discharge corrosion are large and deep, and the surface roughness is deteriorated, as shown in Figure 3. Therefore, when the peak current must be maintained, a smaller pulse width is generally selected to obtain better surface roughness.
3 Influence of east and west electrodes on surface roughness
3.1 East and West electrode materials and surface roughness
In the process of EDM, the materials of the east and west electrodes are different, the functions of the electrodes are different, the loss of the electrodes during the machining process is different, and the impact on the surface roughness of the workpiece is also different. Poor material quality, uneven arrangement, and electrodes containing impurities will make the surface roughness value of the processed workpiece large, which cannot meet the needs of processing. Generally, in the rough machining standard, the pulse width is larger, and the surface roughness of the copper electrode is better than that of the graphite electrode. In the finishing standard, the pulse width is small, and the surface roughness of the graphite electrode is better. EDM is to copy the surface of the electrode to the surface of the workpiece. The surface roughness of the electrode will directly affect the roughness of the machined surface. Especially in the finishing process, the east and west electrodes are generally polished.
3.2 Influence of processing polarity
During the pulse discharge process, the surfaces of the positive and negative electrodes are bombarded by negative electrons and positive ions, respectively, so the energy distributed on the surfaces of the two electrodes is different. Because the mass and inertia of the electrons are small, during the initial period of the discharge, a large speed and initial speed can be obtained, and the surface of the positive electrode can be bombarded, so that the electrode material can be rapidly melted and vaporized. However, the mass and inertia of the positive ions are relatively large, and as the discharge time continues, the energy obtained by them is relatively large, and the shelling effect on the surface of the negative electrode is strong. Therefore, when the pulse width is small, positive polarity processing should be selected; when the pulse width is large, negative polarity processing should be selected. Generally speaking, the surface roughness of positive polarity processing is better than that of negative polarity processing. In order to obtain better surface quality during finishing, positive polarity processing is selected.
4 Influence of processing area
The experimental study shows that: under the same other processing conditions, the processing area is different, and the surface roughness is very different. When the processing area is large, even with a small pulse energy, the surface roughness value of the workpiece is difficult to be less than 0.32 μm, and with the increase of the processing area, the surface roughness becomes worse. This is because in the process of processing, the east and west electrodes and the workpiece are equivalent to the two poles of the capacitor. According to the calculation formula of parallel plate capacitance:
It can be seen that with the increase of the machining area, the parasitic capacitance in EDM increases, that is, the energy storage effect of the EDM two-pole plate is enhanced. When the electrical gauge is small, the electric energy is “absorbed” by this capacitor. At this moment, it can only have a “charging” effect without causing spark discharge. Usually, only one discharge can be generated by stacking multiple pulses, which makes the processed discharge pits larger and the surface roughness value larger.
The emergence of the new technology of “mixing powder processing” solves this problem. By using the “mixing powder” processing technology, a bright surface with a roughness value of 0.05-0.1 μm can be processed on a large area. The powder mixing process is to mix a certain amount of conductive powder such as silicon or aluminum into the working fluid, so that the resistivity of the working fluid is reduced, and the latent cloth capacitance is reduced. At the same time, the discharge channel is divided into many small discharge channels, so that the discharge energy is greatly refined, and a smaller surface roughness value can be obtained. Figure 4 shows the distribution of electric field between electrodes in ordinary processing and powder mixing.
5 Influence of galvanic corrosion products on surface roughness
With the progress of processing, the electric corrosion products between the east and west electrodes and the workpiece increase. If the excessive electric corrosion products are not cleaned and dispersed from the electrodes in time, it will not only make the oil dirty and increase the viscosity, which is not conducive to taking away. The galvanic corrosion product between the electrode and the workpiece will change the composition of the interstitial medium, reduce its dielectric strength, increase the number of secondary discharges between the tool and the workpiece, and affect the inter-electrode discharge condition. The discharge point is concentrated in a certain part, which damages the deionization process and affects the stability of the discharge. In this way, the pulsed spark discharge turns the vicious cycle into a harmful and stable arc discharge state. At the same time, after the high temperature decomposition of the working fluid, it may form a carbon deposit, which will gather into coke particles there, which will burn the surface of the tool electrode and the workpiece, causing the surface to burn. Roughness value increases. In addition, the insufficient discharge of electro-erosion products can easily lead to short-circuit appearance, reduce the processing speed, and the long-term short-circuit effect increases the current in the circuit and affects the life of the pulse power supply. In order to solve this problem, it is necessary to filter and clean the working fluid frequently, strengthen the filtration cycle, and manually sort out the accumulated carbon black and metal particles in the working fluid. In the process of processing, in order to strengthen the discharge of electro-corrosion products, proper working fluid circulation should be carried out. The working fluid circulation method can be divided into two methods: oil flushing and oil pumping. The flushing or pumping method is shown in Figure 5.
The oil flushing method has strong chip removal ability, which can completely discharge the galvanic corrosion products from between the poles. However, in the process of chip removal, the galvanic corrosion products will pass through the machined area, which will cause “secondary discharge” between the tool and the workpiece, and then damage the machined surface and reduce the surface quality. With the oil pumping method, the galvanically eroded product is discharged from the surface to be machined, although it does not affect the quality of the machined surface. However, the chip evacuation is insufficient, and the combustible gas generated by the machining process simply accumulates in the dead end of the oil pumping circuit, resulting in the appearance of “shooting”. This will not only lead to safety hazards, but also aggravate the wear of the electrode, make the electrode taper, and affect the machining accuracy. Therefore, the oil flushing method is generally selected to enhance the discharge of electro-corrosion products.
The choice of oil flushing method led to another study of factors affecting surface quality. The question of how to avoid the secondary discharge caused by the discharge of galvanic corrosion products from the processed area has become a popular topic that everyone pays attention to. Especially in the EDM of tiny holes, the discharge gap between the east and west electrodes and the workpiece is very small, which leads to a very high probability of secondary discharge. will also be damaged. In order to deal with this doubt, Japanese experts plated a thin insulating layer on the surface of the electrode. This medium is vaporized in the process of processing and adheres to the surface of the processed workpiece, so that the east and west electrodes and the processed surface are covered with insulating medium, so that the processed surface will not be reprocessed due to “secondary discharge”, and its surface quality will not be damaged. . After the workpiece is processed, the workpiece is rinsed to remove the insulating medium attached to the processing surface. At present, the composition of this medium is not clear, and further in-depth study by experts in the field of data is needed. The study of the plan to avoid secondary discharge damage to the machined surface is also an essential key element to ensure the quality of the machined surface, and it has important research value.
To sum up, there are many factors that affect the surface quality of EDM, and the treatment plan for each factor is very simple. However, in the process of processing, it is necessary to summarize the influence of these factors on the processing quality and the interaction between the various factors, carry out systematic analysis and scientific allocation of them, and finally find the optimal parameter control and allocation, and then improve the appearance. quality, reduce surface roughness.
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