Preparation of metal matrix composites

In recent years, with the continuous development and progress of science and technology, traditional materials have been unable to meet the specific strength, specific stiffness and other performance requirements of a variety of industries. The research and development of high-performance materials is an important direction of the development of new science and technology, and the emergence of composite materials to a large extent to solve the problems faced by materials, promote the development of materials. Composite material is composed of two or more different properties of materials, by physical or chemical methods, in the macroscopic (microscopic) composition of materials with new properties. Because of high specific strength and specific stiffness, research and development of metal matrix composites by many industry especially heavy industry closely, however the processing difficulty is the bottleneck of restricting its industrial applications, the question of cost control has not been fully resolved, so more applied in aerospace, military technology and other advanced technology.

In recent years, people have developed a variety of effective methods to fabricate metal-matrix composites by studying the rules of interfacial reaction between metal matrix and reinforcer and the methods of controlling the interfacial reaction, which greatly promotes the development and application of metal-matrix composites. At present, the preparation methods of metal matrix composites can be divided into two categories according to the different ways of reinforcing phase generation: second phase additive method and in situ compound method.

The second phase external method is a method to prepare composites by adding the reinforcing phase to the matrix. The common second phase extrusion methods include powder metallurgy, casting solidification and injection molding.

01. Powder metallurgy
The preparation of composite materials by powder metallurgy method refers to the method of making composite materials by mixing metal matrix and reinforcement powder uniformly and pressing it into shape, sintering at lower than metal liquidus temperature, and combining metal matrix and reinforcement powder together by atomic diffusion. The preparation of composite materials by powder metallurgy generally includes four steps: screening powder, mixing of matrix powder and reinforced particles, powder prepressing, hot pressing, and secondary processing. The common secondary processing methods of powder metallurgy are extrusion, rolling and so on. The secondary processing can realize the densification treatment of the parts and achieve the purpose of final molding. The advantages are as follows: the metal composition of the matrix and the type and size of the reinforced particles can be freely selected; The compound temperature is low, and the matrix metal is not easy to react with the strengthened particles, which gives full play to the characteristics of each raw material. The granule strengthening is uniform, the reinforcing granule adding range is large, and a variety of granules can be strengthened together.

02. Casting solidification molding method
Casting solidification molding method is a method to get metal matrix composites by heating the matrix metal to the molten state and then compounding it with the reinforcing phase. Using casting solidification molding method to prepare metal matrix composites, the cost is low, the process is simple and easy to control, the quality of the obtained parts is good, and it is widely used. The commonly used methods are stir casting and squeeze casting. Stirring casting method is a common method to prepare metal matrix composites. It mixes the molten metal matrix with reinforcement by mechanical stirring method to make composite materials. Stirred casting method has been widely concerned because of its low cost and simple technological process, its equipment investment is less, and the second processing of the parts as a superplastic deformation pretreatment process, can further improve its performance. Stirred casting method can be used to produce large volume parts, easy to carry out mass production, no matter in the high-end science and technology in the aerospace field, or in the automotive, mechanical industry and other civil fields, there are a lot of application examples. Squeeze casting is a kind of precise casting method to solidify liquid or semi-solid metal filling mold by high pressure. The preparation of metal matrix composites by liquid die forging means that the reinforcing phase is preformed, heated and poured into metal liquid or melt, the mold is pressed down and pressurized, and the metal matrix composite parts are obtained by cooling.

03. Jet molding
Jet forming method is a new metal forming process, which uses the rapid solidification technology of metal to spray the molten metal and directly obtain the metal billet after deposition. It is also called jet deposition. Spray molding was introduced in the 1980s to prepare composite materials and has developed rapidly due to its general adaptability. The use of jet molding technology to prepare metal matrix composites is to spray the molten metal by atomizing the noble gas, mix with the reinforcement transported by the noble gas at the other end, deposit on the water-cooled substrate platform, and form composite material parts.

In situ synthesis, also known as reactive synthesis technology, is a new method for preparing metal matrix composites. Different from some traditional composite fabrication processes, the in-situ composite process does not require the reinforcement phase, but uses some special reactions to generate the reinforcement phase itself in the metal matrix, thus avoiding the segregation phenomenon of the reinforcement phase in the composite. In the process of in-situ composite preparation, the reinforcement phase can be solidified and precipitated out in the form of eutectic, and the composite obtained is called directionally solidified eutectic composite. It can also be obtained through the reaction of the corresponding elements in the metal matrix, the alloy melt and the added elements, which is called the reaction self-generated composite material. As the nucleation and growth of the reinforcement phase are completed in the matrix, there is no physical or chemical incompatibility between the reinforcement phase and the matrix, and the wetting effect between the reinforcement phase and the matrix is good, the bonding strength is high, and the reinforcement phase does not need to be pretreated, which ensures its purity.

At present, more and more preparation methods of metal matrix composites are being developed. In addition to the several common methods mentioned in the paper, there are also several composite methods including dip solidification, hot dip plating and reverse solidification, lamination and so on. However, due to the limitation of its technology and cost, it has not been widely used. With the further research, it is believed that more composite preparation methods will come out, which will promote the popularization and development of metal matrix composites. Compared with traditional metal or alloy materials, metal matrix composites have higher manufacturing cost and more complicated process. For civil industrial enterprises, the high production and research cost is the biggest problem that restricts the large-scale application of metal matrix composites. Therefore, in order to achieve industrialization and large-scale, it is necessary to further study the preparation method and develop new preparation technology, so as to reduce the cost and enhance the competitiveness of MMCs in the material market.