Modeling prediction of EDM machinability of the ho

Modeling prediction of EDM machinability of structural ceramics

Abstract: after thinking about the experimental research and modeling analysis of EDM machinability of common structural ceramics, it will be found that it provides a new analysis method for predicting the EDM machinability of structural ceramics under specific conditions

key words: structural ceramics; EDM; Machinability

1 Introduction

EDM technology applied to the processing of structural ceramic materials has developed rapidly in the past 20 years. Due to the inherent characteristics of structural ceramic materials, the process characteristics in EDM are completely different from those of metals. Since the 1980s, many scholars have been committed to the study of the external characteristics of EDM of structural ceramics. Different scholars have used different materials, obtained different process data under different conditions, analyzed and summarized the same or similar process laws, but the process laws are highly dispersed, so it is difficult to predict the feasibility of EDM for a certain structural ceramics. In this paper, experimental research and theoretical analysis are carried out around this problem, and a more scientific analysis method for predicting the machinability of structural ceramics by EDM is obtained

2 material selection in experimental design

in structural ceramics, oxide, nitride and carbide are the three main series. Alumina is a typical representative of fusible structural ceramics, and it is the most mature kind of structural ceramics in oxide series. It is abundant in the earth's crust, accounting for about 25% of the total weight of the earth's crust. It is cheap and has excellent performance. According to records, only in Boshan District, Zibo City, Shandong Province, there are hundreds of millions of tons of high-quality bauxite ore. The cheap price and relatively mature development and research make the application of alumina based composite ceramics very extensive, involving metallurgy, chemical industry, electromechanical, shipbuilding, aerospace, light industry and other fields. It can be used to manufacture turning tools, milling cutters, caliper gauges, various seals, drawing dies, drawing tower wheels, sliding plates, valves of chemical equipment, pumps, bearings of aerospace vehicles, rocket nose cones, etc

silicon nitride is a typical representative of a kind of structural ceramic materials with low temperature decomposition and upgrading, poor thermal conductivity (at room temperature) and more glass phase at grain boundaries. It is the most active and most developed type of structural ceramics in nitride series. It has superior thermal shock resistance, low friction coefficient and strong self-lubricating ability, and its application scope exceeds that of alumina. In the ceramic gas turbine program in the United States, silicon nitride is used to make rotors, stators and scroll tubes. Hot pressed silicon nitride is used as piston crown in non water cooled ceramic engine. In gas turbines in Federal Republic of Germany, hot pressed Si3N4 is used as rotor and stator, and reaction sintered Si3N4 is used as burner. In Japan, Si3N4 sintered without pressure is used to make piston covers, cylinder liners, sub combustion chambers, etc. in single cylinder diesel engines. The all ceramic engine of Isuzu automobile company in Japan also mainly adopts Si3N4 based structural ceramic materials. Ceramic parts such as high-temperature valves, bearing shells and rolling bearings developed in China also have the best performance of Si3N4 based materials

silicon carbide (commonly known as "carborundum") is a typical representative of a class of structural ceramic materials with no melting point, good thermal conductivity and less glass phase at the grain boundary. It is the most widely used class of structural ceramics in the carbide series. The hardness of silicon carbide is second only to alumina, and its thermal conductivity is very good. The theoretical thermal conductivity is 400wm* ℃, which is much higher than that of alumina and silicon nitride. Its application in the tool industry has long been known. In recent years, it is widely used to manufacture high-temperature parts (rocket engine nozzles, electrodes of MHD generators, etc.), wear-resistant parts (various mechanical seal rings, wire drawing dies, etc.), and corrosion-resistant parts (pumps, valves, nozzles, etc. for chemical industry). It is the most promising high-temperature material, and its normal temperature strength can be maintained to 1200 ℃ without significant reduction

3 Prediction and analysis of machinability

machining efficiency and electrode loss satisfying surface integrity are the decisive indicators to measure the feasibility of EDM for a certain structural ceramic material. For structural ceramic materials, the duration of peak current is the key factor affecting machining efficiency and electrode loss [1 ~ 3]

3.1 determination of optimal pulse width

whether a certain structural ceramic material can be machined on a specific EDM equipment depends on whether the optimal pulse width can be obtained. By comprehensively analyzing the curves of melting volume of various materials with peak current duration ts, the optimal TS value corresponding to high efficiency and low loss can be predicted. When TS <40ns, the maximum melting volume reaches the peak, and by 200ns, the melting volume is very small. Compared with the VM TS curve of the processed material, the TS value that makes the processed material close to and the tool material away from the extreme value of the maximum melting volume is the best value

if the VM TS curves of Al2O3, SiC and Si3N4 are analyzed, the optimized pulse width of the three materials is 0.5 ~ 2 μ S range. Al2O3 optimal pulse width 0.5 μ s. SiC, Si3N4 optimal pulse width 1 μ s。

3.2 prediction of tool electrode loss

the tool electrode loss can be predicted by using the change curve of the maximum melting volume with TS, as shown in Table 1 and table 2. The research parameters in Table 1 are shown in Table 3, and the research parameters in Table 2 are shown in Table 4. Table 1 changes in electrode loss at different pulse widths (%)

Table 2 changes in electrode loss at different peak currents (%)

Table 3 research parameters in Table 1

Table 4 research parameters in Table 2

experimental values of relative electrode loss are the ratio of electrode loss length to workpiece thickness when penetrating the workpiece. The calculated value of the model is the ratio of the melting volume of copper to the melting volume of workpiece material at the end of the pulse. Analyzing table 3 and table 4, it can be seen from the data of Al2O3 that the modeling is more accurate and can reflect the actual processing state, but the experimental and calculated data of SiC and Si3N4 are quite different. The channel expansion law is less affected by the characteristics of electrode materials, and the modeling of channel radius will not produce such a big difference. The key is whether the modeling calculation method is reasonable. In terms of material properties, the biggest difference between SiC, Si3N4 and Al2O3 is that the former two have no melting point. SiC decomposes at 2600 ℃ and Si3N4 decomposes at 1900 ℃, and the decomposition process is irreversible. Therefore, the concept of melting volume of these two materials is inappropriate. The change curve of melting volume with TS shows that the melting volume will decrease after a certain ts, which indicates that the melting isotherm inside the workpiece is shrinking towards the heat source. That is, with the expansion of the channel, the energy density decreases, and the molten material solidifies at the liquid-solid interface. For materials with melting point, the solidification law can be determined by changing the temperature into a high-quality 3D printing wire. For materials without melting point, this law is much more complex, because structural ceramics are generally a composite of several phases, and the eutectic composition and single-phase composition of the phase interface jointly determine their phase transformation law. In general, the existence of melting point free phase will hinder the contraction of melting isotherm. Therefore, the calculation method of the model is modified, and the calculated value is close to the experimental value. The correction calculation method is: the relative electrode loss is equal to the ratio of the melting volume of copper to 1.05 times the maximum melting volume of SiC and Si3N4 at the end of the pulse. The calculation results are shown in Table 5 and table 6. It can be seen that the modified modeling is more accurate and can reflect the actual processing state. The value of the maximum melting volume correction coefficient is related to the thermal diffusivity of the material. The greater the thermal diffusivity, the greater the value should be; The larger the particle size of the decomposed gasification phase, the larger the value should be. Table 5 correction results of electrode loss at different pulse width (%)

Table 6 correction results of electrode loss at different peak current (%)

4 Summary

the variation curve of maximum melting volume with pulse width obtained from modeling analysis can be used to predict the possibility of EDM of certain structural ceramic materials. It is necessary to conduct in-depth and extensive research to collect such curves under various specific conditions and compile them into atlas or database software. Generally, they are now chopped and ground into small pieces or particles, so that EDM can truly become an effective and convenient processing method for structural ceramics. (end)