1 Introduction
Ceramics is one of the most important inorganic non-metallic materials. Compared with metal or organic polymer materials, ceramics have features such as low density, high hardness, good insulation properties, heat resistance, wear resistance, corrosion resistance, and magnetic, optical, electrical and acoustic properties, in various fields of the national economy. It has been widely used. However, the traditional ceramic preparation process is still difficult to fully meet the precision requirements of precision parts, must be through subsequent reprocessing to meet the dimensional accuracy and surface quality requirements required for engineering applications. Due to its hard and brittle nature, ceramics are extremely poor in processability and are typically difficult to machine. The difficulty in processing has seriously hindered the application of ceramic materials with excellent performance in many fields.
This research project proposes a new ceramic plastic cutting mechanism and establishes a test system. If the project is successful, it will provide a new method of processing ceramic materials. It will also help solve the processing problems of hard and brittle materials such as semiconductor materials, powder metal sintered materials, and intermetallic compounds.
2 Overview of Domestic and Foreign Ceramic Materials Processing Methods
At present, the processing methods for ceramic materials can be divided into small depth cutting using traditional metal cutting principles and special processing using energy such as light, electricity, sound, chemistry, and ion arc.
In 1987, Kiso et al. used polycrystalline diamond tools to perform turning operations on Al2O3 and Si3N4 ceramics. As a result of machining of the tool, the cutters produced many irregular cracks and severe tool wear, resulting in a rough machining surface; the machining results showed that the ceramic material It is removed by brittle fracture instead of shear deformation. Nakasuji et al. (1990) found that when the cutting depth is extremely small, brittle material removal may occur from the brittle fracture removal to plastic shear removal when the depth of cut is large, that is, brittle materials can be removed by plastic shearing. By using a diamond cutter to perform micro-turning experiments on brittle ceramic materials, a mirror-like processed surface was obtained.
Grinding is the most widely used ceramic material processing method. Similar to the turning of ceramics, the ceramic material is removed by brittle fracture when the depth of cut is large, and cracks often exist on the subsurface layer on the ground surface. In 1987, Ito et al. found that when the depth of cut is small and micro-hours, the plastic flow may occur when the ceramic material is removed. When grinding under this process condition, the ground surface is not cracked.
Although the use of diamond micro-depth cutting or grinding can get a good surface quality, but the material removal efficiency is low, the tool wear is large, so that the processing cost of ceramic materials is greatly increased, accounting for 30% to 60% of the total cost of ceramic workpieces ( Sometimes even as high as 90%). In addition, surface/sub-surface damages resulting from turning or grinding may reduce the strength of ceramic workpieces and degrade performance. In 1997, Mochida et al reported that the strength of ceramics decreased by 10% to 20% after high-speed grinding.
The special processing methods for ceramic materials include ultrasonic machining, electric spark machining, chemical auxiliary machining, laser machining, water jet machining, plasma arc machining, acoustic emission particle processing, and combined machining of the above methods. These special processing methods greatly enrich the processing methods of ceramic materials and promote the application of ceramic materials in engineering. However, these special processing methods still have the problems of low material removal efficiency and high processing cost.
Although various ceramic processing methods have been developed at present, the common disadvantages of low efficiency and high cost have seriously hindered the wide application of ceramic materials. Therefore, the development of high-efficiency, low-cost ceramic processing technology has a very important significance.
The use of ceramic cutting technology with a large depth of cut (relative to the previous small depth of cut) is an effective way to achieve high-efficiency, low-cost processing. The large depth cutting of ceramic materials has the advantages of high processing efficiency and low cost. If the practical application of this technology can be realized, the application of ceramic materials in the industrial field will be greatly accelerated. However, the traditional metal cutting theory with ductile metal as the processing object is not suitable for the processing of hard and brittle materials. The cutting mechanism and rules of hard and brittle materials must be studied and new suitable cutting depth cutting methods must be sought.
From the cutting theory, it can be seen that the formation of a typical complete chip must undergo several stages such as elastic deformation, slipping, and slicing. Slip is the basic form of plastic deformation, indicating that the complete chip is formed in a plastic state. When a complete chip is formed, the surface of the workpiece is relatively complete and smooth, with no obvious cracks. For hard and brittle materials (such as engineering ceramics, optical glass, etc.), processing using conventional processing techniques and metal processing parameters will only result in brittle removal without significant plastic deformation, under the action of cutting forces that exceed the strength limit. The material will undergo brittle fracture. It can be seen that as long as the cutting can be achieved in the plastic state, the surface cracking can be reduced or eliminated, and the complete surface can be cut out. A good example can be obtained by using diamond with a small depth of cut turning or grinding to obtain a good surface quality. Plastic deformation of brittle materials is the starting point for solving the problem of cutting ceramic materials. Plasticity and brittleness are not absolute, and they can be transformed into each other under certain conditions (such as small depth of cut). Therefore, the key to cutting ceramic materials is to look for brittle plastic transformation conditions and promote its transformation, so that brittle materials in the plastic state to complete the cutting.
Temperature has a great influence on the plasticity of the material. Under normal circumstances, the mobility of the ceramic material's atoms increases with the increase of temperature, and it is prone to slip and plasticity. Therefore, the ceramic material can be cut in a plastic state by heating it.
As early as about 1950, Schmit, Armstrong, and Krabacher conducted a study of heating cutting, and reported that as the temperature of the material increases, the shear strength of the material decreases, thereby reducing the cutting force during cutting, and the tool life. increase. In 1966, Barrow used electric current heating technology (electric current heating technology) to generate high temperatures in the processing deformation area, and observed that the decrease in material strength will extend the tool life, and the tool and chip interface temperature will shorten the tool life, so deal with temperature and cutting process parameters Optimized to increase tool life. In 1986, Uehara and Takeshita heated Si3N4 ceramic materials through oxyacetylene flames, cutting at high temperatures, resulting in continuous chips, but the surface quality was poor. Subsequently, in order to improve the heating efficiency, plasma arcs and laser heating were introduced into the heating of ceramic materials. In 1990, Kitagawa and Mackawa used plasma arc heating to cut glass and engineering ceramics such as mullite, Si3N4, alumina, and zirconia. In the Si3N4 ceramic turning experiment, when the temperature reached 1050°C, the cutting force was reduced and a continuous chip was formed. The wear is reduced, but the surface is flawed. In 1995, Westkamper et al. used laser heating to perform grinding experiments on Si3N4 ceramics, resulting in higher material removal rates than room temperature grinding. In 1998, Rozzi et al. simulated and experimentally demonstrated the temperature field distribution in the processing zone during laser heating of Si3N4 ceramics. In 2000, Rozzi et al. used laser heating to carry out turning experiments on Si3N4 ceramics. The Si3N4 ceramics were heated to a range of 1151-1330°C for cutting tests; in the temperature range below 1151°C, brittle fracture cutting was used when the temperature was increased to 1151°C. In the above, the chips gradually became semi-continuous. When the temperature reached 1330°C, the chips became continuous and exhibited plastic deformation. However, the high temperature gradient affected the surface quality and strength. In 2004, Rebro et al. used laser heating to carry out turning experiments on mullite, and eliminated temperature gradients through progressive temperature increase methods to eliminate thermal stress, but the effect was not obvious.
Domestically, there are also reports on the research of ceramic material heating cutting. For example, Prof. Wang Yang from Harbin Institute of Technology has done very significant work on the laser-heating-assisted turning of Si3N4 and ZrO2 ceramics. He explained the use of dislocation theory in materials science to explain laser heating. The mechanism of assisted turning, the use of finite element analysis method to establish the ceramic material surface temperature field after heating, physical and mathematical model. Huazhong University of Science and Technology has made initial work on laser heating assisted cutting of ceramics and composite materials. Prof. Xuetao Xue from Shanghai Jiaotong University, Prof. Ye Bangyan from South China University of Technology, Nanjing University of Aeronautics and Astronautics, Guangxi University of Technology, and Shenyang Aeronautical University have conducted research on laser or plasma arc heating assisted cutting of workpieces.
After analyzing the ceramic heating cutting experiments at home and abroad, it can be seen that the heat transfer through the plasma arc, laser, and oxyacetylene flame is performed by both the surface and the inside. The heat must be transferred through the ceramic to reach the inside of the ceramic material, but most ceramics conduct heat. The coefficient is very low, so that a large temperature gradient is formed in the processed area of ​​the material, which easily generates large thermal stress, resulting in sub-surface damage and a decrease in material strength. Chips generated during processing also prevent the ceramic surface from absorbing heat. In addition, the plasma arc, laser, and oxyacetylene flame heating equipment are expensive and technically complicated. This is also the reason why ceramic plasma arc, laser, and oxyacetylene flame heating plastic cutting technologies are limited to laboratory research and are difficult to promote in production practice in recent years. . Therefore, finding a low-cost uniform heating technology has become the key to the practical application of ceramic heating plastic cutting technology.
Microwave is an electromagnetic wave with a frequency range of 300MHz to 3000GHz. As a new type of energy source, microwave electromagnetic energy can penetrate the dielectric material and transmit it to the interior of the lossy material. It can collide with and rub against the atoms and molecules of the object, thereby heating the object. Microwave heating has been widely used in crop drying and baking, ceramic sintering and welding, chemical synthesis and digestion because of its characteristics of internal and external heating, low thermal stress, high efficiency, high heating speed, low cost, and selectivity. , etching coating, surgical sterilization, material modification and so on. For example, microwave scalpels and microwave surgical forceps are new medical devices that apply microwave energy to surgical operations. That is, a microwave power source is connected to a surgical tool through a transmission line, so that microwave energy is transmitted along a transmission line along a cutter into a human surgical site and the human body is incised. Tissue and hemostasis; Microwave surgical cutters extend the inner conductor of the microwave coaxial antenna properly, according to the need of surgery to make a certain shape of the tool, microwave scalpel, surgical forceps with a good hemostatic effect, the knife does not carbonize, sterilization, to prevent surgical infection Features such as small tool size, flexible operation, especially suitable for tumor resection, organ repair, hemostasis and other surgical operations.
In particular, Israel’s E.Jerby et al. published an article in the famous “SCIENCE†magazine (18October2002, Vol. 298), and proposed that microwave drills (Microwave Drills) be used to drill non-conductive materials such as ceramics and glass. The principle is to use a microwave antenna to heat the ceramic in a directed manner so that the ceramic material is partially melted in the processing area, and then the microwave antenna is inserted into the melting area to form a hole. Inspired by this idea, this research project has expanded the microwave drilling method to other machining methods such as turning, milling, and planing. Instead of microwave antennas, turning tools, milling cutters, or planers are used. During preparation of ceramic materials, tools and workpieces are prepared for contact. While cutting, the microwave electromagnetic energy is directed through the tool antenna to the processed area for heating, and the temperature is controlled below the melting point of the ceramic. As long as the ceramic processed area can locally undergo brittle fracture to plastic deformation instead of melting, it can be used. The traditional shear cutting principle cuts. This method integrates the heating and cutting devices. In addition, even if microcracks or stress are generated during processing, they are eliminated due to microwave annealing of the ceramic material, ie, microwave cutting and microwave annealing can simultaneously act.
Microwave plastic cutting and plasma arc, laser, oxyacetylene flame heating cutting is very different. The plasma arc, laser, and oxyacetylene flame heating cuts are based on the Boltzmann thermal effect, that is, by increasing the local temperature in the processing zone of the ceramic, the local atomic activity of the ceramic is enhanced, and slip occurs to increase the plasticity; microwave plastic cutting is used to The changed microwave electromagnetic energy collides with and rubs against the ceramic material atoms and molecules to generate slip to increase plasticity. Heat is just a by-product of the interaction of microwaves and ceramic materials. Therefore, if the interaction between the microwave and the ceramic material can be improved to plasticize the processing area and avoid generating a large amount of heat, it will be possible to achieve low-temperature plastic cutting. Due to the separation of the traditional plasma arc, laser, oxy-acetylene flame heating device and the tool, the heating zone and the tool affect each other, and because the chip affects the heating effect, gas swarf is needed, thereby affecting the uniformity and efficiency of heating; and microwave heating can The microwave antenna is integrated with the tool and the local heating zone coincides with the cutting zone, which may increase the heating efficiency. In addition, plasma arcs, lasers, and oxyacetylene flames increase the grain size of the ceramic material as a result of heating, resulting in a larger surface roughness; and from microwave sintering, it is known that microwave heating can suppress the abnormal grain growth, and thus the surface quality after processing. better. Moreover, microwave heating devices are much cheaper than plasma arcs, lasers, and oxyacetylene flame heating devices.
In summary, the use of microwave heating cutting is expected to become a new ceramic processing method, which is expected to solve the problems of plasma arc, laser, oxyacetylene flame heating cutting thermal stress, high price, etc., and hope to improve the processing through microwave annealing. Surface quality, thus achieving high efficiency, high quality, low cost machining of ceramic materials.
In addition to ceramic materials, semiconductor materials (such as silicon, gallium arsenide, etc.), powder metal sintered materials (such as new type of sintered steel), and intermetallic compounds (such as Fe3Al, etc.) also belong to hard and brittle materials, and also have problems of difficult processing. Processing difficulties are similar to ceramic processing, and there are many commonalities. Therefore, in addition to providing new theory and new methods for ceramic processing, the study of this project can also be applied to other hard and brittle materials such as semiconductor materials, powder metal sintered materials, and intermetallic compounds.
3 microwave plastic cutting ceramic research
We have developed experimental devices for microwave generation, transmission, orientation, integration of antennas and tools, and measurement of temperature and cutting forces in ceramic cutting. The microwave circuit is mainly composed of a continuous wave operation control circuit, a pulse modulation wave operation control circuit, a microwave modulator, a microwave oscillator, a microwave output cable, an antenna, and a power supply.
The microwave source uses a 2.45GHz adjustable power magnetron whose oscillation is modulated by a microwave modulator to generate continuous or pulsed microwave oscillation power, impedance matching through an EH tuner, and waveguide connection to a coaxial antenna. The inner conductor of the coaxial antenna is made into the shape of a turning tool. By optimally designing the antenna structure, microwave radiation with relatively concentrated energy and relatively uniform intensity is formed into the workpiece processing area for heating. The ability to control the directionality of microwaves is improved by the reflector launching and antenna orientation, and the radiation of the microwaves to the operators is controlled within the safety standards.
The entire microwave device uses cooling water to dissipate heat to ensure sufficient heat dissipation and reliable operation of the microwave device. In the processing of ceramic materials, before the cutting tool contacts the workpiece and prepares for cutting, the microwave electromagnetic energy is directed through the tool antenna to be processed to perform preheating. When the temperature in the processing area reaches the processing area, the ceramic material locally undergoes a transition from brittle fracture to plastic deformation. When it is not molten, the tool can be cut into the ceramic material to perform cutting. At the same time as cutting, the heat-affected zone of the tool is preheated in the processing area and the processed area is annealed.
Cutting force is measured by a three-axis force plateau. The cutting temperature is measured by a pyrometer. Instantaneous temperature is precisely controlled by the computer, and the local melting of the ceramic is prevented by adjusting the microwave power intensity to achieve stable plastic cutting.
Due to the interaction between microwaves and ceramic materials, some changes in the selection of process parameters occur during cutting. By studying the material and geometric dimensions of the microwave tool (such as the front angle, back angle, etc.), the tool parameters suitable for the cutting of the ceramic material are selected. If the tool material is a conductor, the inner conductor of the antenna can be directly formed; if the tool material is an insulator, a metal film is formed on the handle to form a conductor. Because the cutting force is concentrated near the cutting edge of the tool, in order to protect the cutting edge, the strength of the cutting head should be increased, and smaller positive rake angles, trailing angles, and cutting edge inclination angles should be selected. To improve the surface roughness, the cutting edge should be selected when sharpening the cutting tool. Small positive values ​​for the main and auxiliary angles and the larger tip radius. The principle of orthogonal process testing was used to screen out the optimized turning parameters so as to improve the productivity and tool durability and ensure the workpiece processing quality. According to the shape of chips, the change law of ceramic materials from brittle to plastic under different microwave energy was analyzed. Under the plastic state of ceramic materials, the plastic cutting rules of different processing parameters such as depth of cut and feed rate were studied.