In recent years, the aviation, aerospace, and automotive fields have increasingly demanded the development of super light, high hardness, and high strength super materials. Because highly damping materials are effective in controlling noise and enhancing the stability of vehicles and equipment, and magnesium alloys have the best vibration absorption performance of all commercial metal materials, magnesium and magnesium alloys have attracted more and more Researcher interest.
Magnesium has low density, high specific strength and specific modulus, good thermal conductivity, and excellent damping and shock absorption properties. It has abundant reserves in the crust surface metal deposits, lagoons, and the ocean. One. However, the magnesium alloy has poor corrosion resistance, poor wear resistance, poor high temperature performance, poor creep resistance, and low strength. These defects limit the further application of magnesium alloys.
Therefore, researchers began to seek a new material with good mechanical properties and shock absorption properties, and magnesium-based composites became one of the best choices. The design of the reinforcement can effectively improve the properties of the magnesium alloy, and the prepared magnesium-based composite material can exhibit excellent properties not available in any component, such as high specific stiffness, specific strength, good dimensional stability, and excellent Shock absorption performance. In addition, magnesium-based composites also have electromagnetic shielding and hydrogen storage properties, are an excellent structural and functional material, and are one of the composite materials with great application potential in today's high-tech fields.
Since the research of magnesium-based composites was carried out in the late 1980s, magnesium-based composites have become another light-metal-based composites with broad application prospects and competitiveness after aluminum-based composites. The transportation industry and other fields have been used more and more widely.
According to the type of reinforcement, magnesium-based composites can be divided into particle-reinforced, whisker-reinforced, short-fiber-reinforced, carbon nanotube-reinforced, discontinuous fiber-reinforced, and continuous fiber-reinforced magnesium-based composites.
Cf / Mg composite materials and their performance advantages
Carbon fiber reinforced magnesium-based (Cf / Mg) composites are based on magnesium or magnesium alloys and reinforced with carbon fibers. A metal-based composite with high specific strength, high specific modulus, and good thermal stability is prepared. Cf / Mg composite materials have very high specific strength, specific rigidity, good damping performance, thermoelectric conductivity, and thermal stability. The material has light weight, high structural efficiency, good dimensional stability, and can meet the requirements of aerospace structural materials. It is an ideal material for high-tech industries such as aerospace, weapons, and electronics.
Compared with other metal materials or carbon fiber resin-based composites, Cf / Mg composites have the following advantages:
Low density
Magnesium alloy is the least dense alloy among commonly used engineering metals. Compared with other commonly used alloys, such as iron, copper, zinc, nickel, titanium, aluminum and other alloys, magnesium alloys have lower density and higher specific strength. Composites prepared from it as a matrix alloy have a lower density than composites using other alloy matrixes. Table 1 shows that the 50% volume fraction of carbon fiber is compounded with different alloys. The density of the carbon fiber composite material obtained through calculation. The density of the Cf / Mg composite material is much lower than that of other metal-based carbon fiber composite materials, and slightly higher than the resin This advantage makes it possible to greatly reduce the density of the overall metal matrix composite material while ensuring the metal properties.
Table 1 Density of carbon fiber composites based on different types of matrix
Compared with carbon fiber resin-based composites, high density has always been the soft rib of carbon fiber metal-based composites, and the low density of Cf / Mg composites makes its advantages in weight reduction become as prominent as resin-based carbon fiber composites.
2. High specific strength and specific modulus
Cf / Mg composite material is a composite material composed of carbon fiber and magnesium. Among them, carbon fiber with excellent mechanical properties is the main force in the composite material. Magnesium alloy as the matrix alloy mainly plays the role of connecting carbon fiber and transmitting load. Because carbon fibers have excellent high specific strength and high specific modulus, Cf / Mg composite materials can be endowed with the same high specific strength and modulus characteristics.
3. Thermal and electrical conductivity
Carbon fiber has good electrical and thermal conductivity. The higher the degree of graphitization of the fiber, the better its electrical and thermal conductivity. Taking carbon fiber products from Toray Corporation in Japan as an example, the thermal conductivity of Toray T300 grade high-strength medium-mode carbon fiber is 6.5W / ( m * K), and the thermal conductivity of M55J grade high-strength high-modulus carbon fiber is as high as 150W / (m * K). Table 2 shows the thermal conductivity of different materials. Through comparison, it is found that the thermal conductivity of M55J grade high-strength high-modulus carbon fibers has shown certain advantages compared with some metals, and is similar to that of magnesium.
Table 2 Thermal conductivity of different materials
4. Low expansion coefficient
Carbon fiber has a very low thermal expansion coefficient along the fiber direction, and its axial thermal expansion coefficient is negative. Therefore, adding a certain amount of carbon fiber to the magnesium matrix can not only greatly increase the strength and modulus of the material, but also significantly reduce the material. Coefficient of thermal expansion.
In terms of specific modulus and thermal stability, high-strength and high-modulus carbon fiber-reinforced magnesium-based composites are the highest among various materials. When the content of high-strength and high-mode carbon fibers reaches about 50%, the thermal expansion coefficient of carbon fiber-reinforced magnesium composites is almost zero.
5. Stable performance and high machining accuracy
Compared with resin-based carbon fiber composites, C / Mg composites do not have the problems of aging, decomposition, moisture absorption, natural degradation of performance, high fatigue resistance, and will not decompose low molecular substances in space use. Pollute the instrument and the environment.
Carbon fiber resin-based composite materials have low processing accuracy due to the effect of resin retracting. In Cf / Mg composite materials, due to the presence of alloy stiffness, their processing accuracy is much higher than that of resin-based carbon fiber composite materials. The limited situation of carbon fiber composite materials in high dimensional accuracy requirements makes carbon fiber composite materials suitable for high accuracy requirements.
Preparation method of Cf / Mg composite material
The selection of Cf / Mg composite materials is appropriate, which determines the performance, structural characteristics, application scope and prospect of composite materials to a certain extent. At present, the commonly used Cf / Mg composite material preparation processes mainly include powder metallurgy, liquid infiltration and stirring casting, and most of them are more traditional preparation methods.
Powder metallurgy
The powder metallurgy method uses the principle of powder metallurgy to mechanically mix the matrix powder and the reinforcing fibers in the proportion required by the design, and then press the compact, sinter or directly press the mixture to hot-press, hot-roll, and extrude to prepare the magnesium-based composite material. Methods. The advantages of powder metallurgy to prepare magnesium-based composite materials are mainly that the matrix pure magnesium or magnesium alloy does not have to go through the high temperature state of full melting during the preparation process, so it can avoid strong oxidation such as magnesium alloy brought by the casting method, and the occurrence of the interface between the matrix and the reinforcement. Serious reactions and other problems, and in addition, the fibers are evenly distributed in the matrix, so that the magnesium-based composite material has better comprehensive mechanical properties.
Another advantage of the powder metallurgy method is that it has no restrictions on the type of reinforcement. It can change the ratio of the reinforcement and the matrix to produce a magnesium-based composite material with a high volume fraction reinforcing phase. However, the powder metallurgy process has high manufacturing cost, complicated production equipment, and it is difficult to manufacture large-sized blanks and parts. Therefore, powder metallurgy is widely used in laboratory research, and it is difficult to obtain widespread application in actual production.
2. Liquid impregnation method
The liquid infiltration method, also known as the melt infiltration method, is a method for preparing a composite material by infiltrating liquid metal into the pores of a porous preform of a reinforcing material under certain conditions and solidifying. When liquid metal is impregnated with or without external pressure, commonly used can be divided into pressureless impregnation, pressure impregnation, vacuum impregnation and vacuum pressure impregnation.
2.1 Pressure infiltration method
Pressure impregnation, also known as extrusion casting, is a method of making reinforced particles into a preform, adding a liquid magnesium alloy, and then pressing the molten magnesium alloy into the preform to produce a composite material. The characteristics of the pressure infiltration method are: under the action of high pressure, the liquid metal can penetrate into the fiber preform better, promote the wetting between the magnesium alloy melt and the fiber, and effectively eliminate micropores, Defects such as shrinkage holes, the resulting composite material has a dense structure and more excellent mechanical properties. The method has a short preparation cycle and good product quality. The disadvantage is that the preparation process of the preform is complicated and the cost is high.
2.2 Pressureless infiltration method
The pressureless infiltration method is a method for preparing a metal matrix composite material by spontaneously impregnating a reinforcing fiber preform with a metal melt without external pressure. The pressureless infiltration method has the advantages of simple process, low cost, and controllable volume of the reinforcement. However, the pressureless infiltration method needs to be performed at a relatively high temperature, and the infiltration rate is low, the process cost is high, and there are also interfacial reactions, many micro defects, and reduced material properties.
2.3 Vacuum pressure impregnation method
The vacuum pressure impregnation method is a method for preparing a metal-based composite product by pressing a liquid metal into a pore of a preform under a combination of vacuum and high-pressure inert gas. Its characteristics are: immersion in vacuum and solidification under pressure, so the product has no casting defects such as pores, porosity and shrinkage, compact structure, mechanical properties of composite materials. In addition, the process parameters of this method are easy to control and can avoid severe interface reactions. However, the equipment is complicated, the process cycle is long, and the efficiency is low. The preparation of large-sized parts requires large equipment.
3. Stir Casting
Stir-casting is a method of adding fiber reinforcement directly to the matrix metal melt, uniformly dispersing the fibers by stirring, and then casting into ingots or castings to produce fiber-reinforced magnesium-based composite materials, which are often combined with other methods to prepare composite materials. .
The stirring casting method requires less equipment input, simple process, convenient operation, low production cost, can produce large volume composite materials, and is suitable for large-scale production. However, as-cast composites are prone to form pores. Therefore, it is necessary to take appropriate follow-up processes for the as-cast composite material, such as extruding the composite material, which can reduce the porosity, make the tissue dense, and the fiber distribution more uniform, which can help improve the mechanical properties of the composite material.
Cf / Mg composite materials in aerospace applications
Compared with the more mature carbon fiber resin-based composites, Cf / Mg composites, such as lightweight and high-strength metal-based composites, have the high-temperature properties that resin-based composites lack, while maintaining high mechanical properties. Thermal expansion, high dimensional stability, electrical and thermal conductivity, and other excellent comprehensive properties. Cf / Mg composite materials with metal advantages will overcome the use restrictions of carbon fiber composite materials in many occasions. Therefore, Cf / Mg composite materials are high-performance composites. The successful development and utilization of materials is of great significance for aerospace and other fields.
NASA uses Cf / Mg composite materials to make satellite truss structures, space power recovery system components, space station struts, space shuttle rotor engine housings, and space reflector frames.
Some components of the Hubble Space Telescope use Cf / Mg composite materials; American Metal-Based Casting Composites has prepared continuous and discontinuous high-strength high-modulus carbon fiber-reinforced magnesium-based composites and applied them to the Extra-Atmosphere Killer Vehicle (EKV) Mirror frames and measuring components.
Factors affecting the properties of Cf / Mg composites
The uniform distribution of reinforcements, good interface bonding, and fine matrix grains are the prerequisites for the excellent performance of Cf / Mg composites. The factors affecting the performance of Cf / Mg composite materials are as follows:
1. Enhanced carbon fiber performance
As the reinforcement of Cf / Mg composite material, carbon fiber is the main force in the material bearing process. The performance provided by carbon fiber directly affects the performance of composite material. Carbon fiber has poor oxidation resistance at high temperature, which is manifested by significant oxidation weight loss and reduced mechanical properties of carbon fiber. When carbon fiber and magnesium are compounded, the preheating temperature of the preform containing carbon fiber will generally exceed 500 ° C, and the actual magnesium melt temperature will not be lower than the melting point (660 ° C) during infiltration. This stage can easily cause unprotected fibers due to oxidation Weight loss results in reduced performance.
In order to reduce the performance of carbon fiber due to oxidative weight loss, the ceramic fiber coating on the surface of the carbon fiber can effectively solve this problem. The heat-resistant ceramic coating can prevent the contact between carbon fiber and oxygen and protect the carbon fiber. Ceramic coatings are mainly divided into three types: oxide coatings, carbide coatings and nitride coatings.
2.Carbon fiber / magnesium matrix interface
The interface problem between the carbon fiber and the magnesium matrix can directly affect the properties of the Cf / Mg composite material because the carbon fiber and the magnesium matrix have poor wettability. The surface modification of Cf / Mg composites is one of the most effective ways to optimize the interface. Specific methods include carbon fiber surface oxidation, fluorate modification and coating treatment. The introduction of the coating changes the original bonding state of the interface, which can effectively prevent excessive interface reactions or appropriately increase interface chemical reactions, improve interface wettability, increase interface bonding strength, and thereby improve the plastic forming ability of the composite.
Concluding remarks
Through comparative analysis of the Cf / Mg composite material preparation process, it can be found that the use of different preparation processes can mostly obtain better performance composite materials, but complex production equipment and high costs are the key factors restricting the popularization of the preparation process. Moreover, there are still some problems in the research of Cf / Mg composite materials, such as: poor fiber-matrix bonding, the inclusion of composite materials and the presence of pores, etc., will reduce the mechanical properties of composite materials.
At present, Cf / Mg composite materials developed by the United States and other countries have been widely used in the aerospace field. Only a few domestic research institutions have carried out basic research in this field. There is still a long way to go from laboratories to large-scale production. However, carbon fiber-reinforced magnesium-based composites have excellent properties, especially with high-strength and high-modulus carbon fibers as reinforcements. The Cf / Mg composites prepared have the best performance, and the coefficient of thermal expansion is almost zero. Mirrors and space station architectures have gained key applications and become ideal structural materials in the aerospace field with the highest structural efficiency, so they are bound to become one of the hot materials for future development.