High-efficiency EDM wire cutting technology for large-size and special-shaped sashes Liu Zhidong, Qiu Mingbo, Wang Wei, Tian Zongjun, Huang Yinhui (School of Mechanical and Electrical Engineering, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, China) Characterized by diodes, variable resistors, voltage regulators, resistors, etc. The contact barrier of the semiconductor and the feed end material, the interpole resistance, the dielectric discharge sustain voltage, and the working fluid resistance. A special fixture for semiconductor wire EDM is designed. The surface of the semiconductor is coated with carbon paste and the graphite block is used to reduce the contact barrier and contact resistance. The inductive contact point is blown to reduce the electrochemical reaction. The production of non-conductive passivation film ensures the continuation of processing. Finally, the N-type crucible with a resistivity of 22 mm was subjected to WEDM, the cutting efficiency was more than 100 mm2/min, and the special-shaped window was cut by numerical control. The discharge waveform was detected and the characteristics of the semiconductor wire EDM were analyzed.
Fund Project: National Natural Science Foundation of China (50975142); Jiangsu Science and Technology Support Program (BE2009161); Jiangsu Science and Technology Plan Project (BG2007004) Publ-inch problem sand line large-area sand cutting line wear will lead to cut i.net in modern warfare, The advantage of acquiring battlefield information has become the key to mastering the initiative of war. Aviation infrared technology is one of the key technologies to acquire battlefield information from the air and space. Single crystal germanium has excellent optical properties, uniform absorption, high refractive index and reflectivity, and low price compared with other infrared optical materials. It is currently made into various prisms and prisms to refract and focus infrared light. A large number of applications in the infrared system, the infrared system consumes about 45% of the total consumption of cesium. 11-31. In order to improve the resolution of the infrared system to the target, plus the information of the detected object, it is necessary to make as much as possible More infrared light is transmitted into the system. Therefore, it is necessary to use larger sash windows and special-shaped windows. Therefore, how to manufacture large-sized and special-shaped sash windows has become the key to improving the performance of military and aviation infrared systems and the level of strong airborne detection and reconnaissance.
é”— is a typical semiconductor material. So far, its processing has been plagued by researchers because its most prominent feature is high brittleness, low fracture toughness, severely affecting the quality and performance of its surface, and poor processability. 141. Cutting of tantalum usually borrows equipment used for processing semiconductor silicon materials, such as internal circular cutters, external circular cutters, sand wire saws, multi-wire saws, etc., and can only be cut in a straight line. Due to the variation of the slits such as the inner and outer circular cutting discs and the large thickness cutting due to poor cooling and easy disconnection, the multi-wire saw cutting can be cut into hundreds of pieces at a time, and the efficiency is high, but the thickness of the wafer cannot be changed. And the equipment is expensive, the domestic generally can only cut 6 inches (ie 152mm) of high-altitude materials and other problems, for the window is a small batch or even a single piece of production methods are difficult to adapt and other restrictions, the existing equipment in the country can cut The maximum size of the crucible can only reach 200mm, and the cutting efficiency is very low. At present, the efficiency of cutting with a wire saw is about 1015mm2/min. For the crucible of more than 200mm, only the band saw can be used for cutting, and then the method of flat grinding is used. The method of removing the unevenness caused by the band saw blanking consumes a large amount of tantalum material, resulting in the utilization rate of the final tantalum material is only about 50%, and in the case of special-shaped (such as fan type, racetrack type) window processing The above method is more difficult to process. Therefore, the cutting method and cutting efficiency seriously restrict the manufacture of large-size and special-shaped sash windows, which seriously restricts the development of military and aviation infrared detection and reconnaissance technology in China.
The crystal germanium resistivity is usually in the range of 0. It is suitable for processing by WEDM technology, and can achieve low loss, low cost, high thickness and profile cutting. In recent years, with the continuous expansion of the application fields of semiconductor materials such as silicon and germanium, the research of this technology has received a lot of attention (H01. Although the crystal germanium has certain conductivity, its resistivity is higher than that of metal materials. It is 3~4 orders of magnitude and has very special electrical properties. Therefore, it is difficult to directly use the ordinary WEDM process. This paper has carried out the special discharge characteristics of the high-resistance N-type crystal é”— negative wire EDM. Research, established a discharge model, and guided the actual WEDM.
1 The establishment of the discharge model 11 Single-conductivity has been found to have a unidirectional conduction characteristic in the research of electrical discharge machining of semiconductor materials, and with the increase of the resistivity and the volume of the processing object, the characteristic becomes more and more obvious. The volt-ampere characteristic curve of the crystal is as shown in the figure. When the crystal is connected to the positive pole of the power supply, the loop current increases slowly as the voltage gradually increases, and basically goes to zero. When the crystal is connected to the negative pole, the loop current increases with the voltage. It is also gradually large, so it is judged that N-type crystal é”— EDM wire cutting must adopt N-type crystal é”— volt-ampere characteristic test curve 12 WEDM principle. N-type crystal é”— é—´ 电 电 åŽŸç† åŽŸç† åŽŸç†Machining (the workpiece is connected to the negative pole of the power supply), and a DC pulse voltage is applied between the two poles. In the input end of the negative electrode material, in order to reduce the contact resistance, the surface is coated with carbon. 14. The surface of the material is firmly contacted with the surface of the material, and the inlet end is made of graphite electrode. At the positive terminal, the vertical wire of the electrode wire is fed to the right, and the middle: 1 is the discharge channel to maintain the voltage. Since the working fluid used for semiconductor processing is the same as the metal processing, the maintenance voltage can be analogous to the metal electric discharge machining, which is about 20V. In the discharge loop, it is always reverse biased; 2, 4 is the contact barrier. Since the é”— is the cathode of the N-type semiconductor é”— is the equivalent diode, the diode 2 at the discharge end is forward-biased during the negative polarity processing. The partial voltage is small and negligible, and the fixed-end diode 4 is reverse-biased, the partial pressure is large, and can not be ignored during the discharge process. When the processing voltage is large, the reverse-biased diode is often in a breakdown state, The partial pressure is the breakdown maintaining voltage; 3 is the material resistance of the crucible material, which is related to its own resistivity, volume, temperature and contact area during the discharge process, so it is represented by a variable resistor; 5, 6 is the working fluid resistance, and the electrical conductivity Compared with semiconductor materials, it has become a factor that cannot be ignored. Due to the potential difference between the two ends of the contact area and the existence of a working medium with certain conductivity, an electrochemical reaction will inevitably occur, and the result will be: 1 Electrochemical action consumes a large amount of pulse power input energy and is used in the electric discharge line. The proportion of energy cut is drastically reduced, even resulting in the inability to produce normal wire EDM.
2 Because of the contact resistance, the contact point will generate heat after energization, resulting in high temperature oxidation on the surface, resulting in further surface contact resistance.
3 The carbon paste layer coated on the surface of the N-type crucible due to the high temperature of the contact point will be separated from the matrix, so that the contact resistance is more obvious as the N-type resistivity increases, and has become the N-type crystal crucible. The technical bottleneck of normal electric wire cutting.
2 process, firstly add the wire wire guide and the wire constant tension device on the wire conveying system, the purpose is to avoid the appearance of mechanically commutated stripes on the cutting surface caused by the change of the spatial position of the electrode wire caused by the traditional HSWEDM wire reversing. Secondly, the specially developed composite working fluid 112 with good washing ability is used to ensure uniform filling of the working medium in the discharge gap during cutting, to maintain the uniformity of cooling between the poles, and to further improve the cutting efficiency and the cutting height and make it in the normal service life range. Inside the wire.
22 Fixture design é”— EDM wire cutting photo As shown, according to é”— special electrical characteristics, the following problems are mainly considered in the clamping process: (1) For the clamping problem, since é”— is a brittle material, the processing The clamping force must be properly controlled, so that the picking materials maintain a certain discharge gap under the control of the servo system to maintain normal discharge. In order to limit the current to burn out the power supply device, it is necessary to connect the current limiting resistor in series in the loop.
13 Circuit Model The semiconductor material itself has a certain bulk resistance, and when in contact with other materials, the surface has special electrical characteristics and creates a contact barrier of the contact surface. The presence of resistivity makes the bulk resistance of the EDM material unnegligible and the presence of the contact barrier can be replaced by the nature of the diode. For the N-type germanium semiconductor electrical discharge machining shown, the equivalent circuit model 1111 shown can be established. The transition from an ohmic contact to a Schottky contact, the contact resistance is so large that the WEDM can not be continued.
4Because of the existence of working fluid with certain conductivity, such as the surface coating of the carbon paste on the surface of the crucible, it will cause an electrochemical reaction between the incoming graphite block and the N-type crucible, and a non-conductive passivation film is formed on the surface of the crucible. Normal wire EDM cannot be continued. X-ray diffraction (XRD) measurements of the uncharged surface of the uncoated carbon paste layer indicate that non-conductive cerium oxide will be formed after the electrochemical reaction, and the results of the detection are as shown in the line-cutting process of the germanium by WEDM § In order to ensure the electrical contact between the neodymium and the feeding electrode to protect the carbon brush, the graphite inlet brush with a single spring is directly in contact with the crucible forming surface under the thrust of 1 phase, and the inter-electrode resistance of the crucible should be significantly changed. So in the clip to the electricity.
Pointing on the thermal phenomenon during the wire EDM process, the contact hole in the strut is connected to the compressed air as the cooling to obtain real-time cooling and enters the electric power point through the empty c liquid. Electric fire n'Fig for reliable contact with the incoming electricity, first in the é”— and then using the negatively charged wire and the embedded bow, the graphite carbon brush is used in the spring, taking into account the position that can not be cut during the cutting process. No more grams. Ge (Ge) - the application of metal in modern military (18) Qiu Mingbo, Huang Yinhui, Liu Zhidong, and so on. Research status of solar wafer manufacturing methods. Mechanical Science and Technology, 2008 (8): 1017-1020. Wang Wei, Liu Zhidong, Tian Zongjun, et al. Surface integrity study of low resistivity single crystal silicon EDM/electrolytic composite cutting. Electrical processing and mold, 2007 Liu Zhidong, Wang Wei, Tian Zongjun, and so on. Solar silicon wafer EDM high efficiency Liu Zhidong, Qiu Mingbo, Wang Wei, and so on. Research on solar wafer cutting technology. Electrical processing and mold, 2009 (3): 61-65. Liu Zhidong (966-) male, doctor, professor, doctoral tutor main research direction: special processing, semiconductor material processing (Editor: Cai Fei, Yang Dong)
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