ABSTRACT: w/cu composites have been widely used in electrical engineering, mechanical processing and electronic information because of their high conductivity and thermal conductivity, low expansion coefficient, good high temperature strength and ARC ablation resistance. In this paper, the traditional technology and new preparation technology of W-CU composites are introduced, and their applications in electrical switches, electrodes, microelectronics and military industry are reviewed. Key Words: tungsten-copper Composite Material; preparation technology; Application Development Plan Classification number: TF125.2 + 41 Document Identification Code: A article number: 1673-4971(2012)05-0040-03

The research and development of w-cu composite materials can be traced back to the 1930s, when it was widely used in high-voltage electrical switch industry because of its excellent compression strength and electrical ablation resistance. In the 1960s, w-cu composites were gradually applied in the fields of resistance welding electrode, high temperature resistant parts of aerospace and so on. Since then, with the gradual improvement of the preparation technology and the continuous expansion of the application fields, the development and application of w-cu composites have gradually matured, and in the 1990s, as an electronic packaging and heat sink material, it has received extensive attention in the field of large-scale integrated circuits and high-power electronic devices. In the twenty-one ~ (th) century, w-cu composite has been used in military and high-tech fields as material of armor-piercing ammunition cover, missile nozzle and target. W/cu composites are composed of tungsten with high melting point, high density and low expansion coefficient, and copper with high conductivity and high thermal conductivity. They are widely used in electronic, electrical, mechanical and aerospace industries, it has a broad application prospect in many high-tech fields. However, the difference in melting point between tungsten and copper is large, and the density of tungsten-copper composites prepared by the Powder metallurgy method is not high, which usually results in the lack of electrical, thermal and mechanical properties of the materials, the preparation technology and application development of w-cu composites have experienced a series of complicated development processes.

1. Preparation technology of W-CU composite materials

 1.1 the method of infiltration is to press w-powder or mixture of w-powder and a small amount of cu powder into blank, and to prepare porous tungsten framework by pre-sintering at a certain temperature, w/cu composite was prepared by melting copper and filling the matrix with the capillary force flowing through the gap of tungsten particles. Due to the difficulty in controlling the pore connectivity and size uniformity of tungsten framework, it is difficult to ensure the uniformity of copper phase distribution after molten infiltration, and the copper-rich skin after molten infiltration must be processed sequentially, not Conducive to the manufacture of complex shaped parts [6 ~ 8]. However, as one of the traditional methods to prepare w-cu composites, the materials prepared by solution infiltration method have the advantages of high density, good sintering performance, good electrical and thermal conductivity, etc. . 

1.2 The high temperature liquid phase sintering method is to mix and shape certain proportion of tungsten powder and copper powder, and sintering above the melting point of copper at a certain temperature to obtain tungsten-copper composite. Because of the poor wettability of liquid phase copper and tungsten, the sintering densification process of w-cu composites is mainly particle rearrangement, which affects the sintering densification of the final materials. However, as one of the traditional preparation methods of w-cu composites, high temperature liquid phase sintering has the advantages of simple production process, easy manipulation and control. 

1.3 the activation liquid phase sintering method is to add some active elements such as PD, NI, Co and Fe into the high temperature liquid phase sintering process to improve the sintering effect. The addition of active elements will affect the conductivity and thermal conductivity of the materials to some extent, which is unfavorable to its application in materials such as heat and electric control. However, as one of the new preparation methods of w-cu composites, compared with the high temperature liquid sintering method, the material prepared by the activated liquid sintering method has the advantages of high sintering density, high hardness, good bending strength and other mechanical properties. The comparison of some properties of W-10Cu composites prepared by two sintering methods is shown in Table 1.

1.4 function gradient method function gradient method is a method to obtain w/cu composites with specific gradient structure and properties by certain technological means. The functionally graded material prepared by this method is characterized by a continuous composition change of the intermediate layer from one end of copper with high conductivity, thermal conductivity and good plasticity to the other end of copper or tungsten copper composite material with high copper content, gradual transition to one end of tungsten-copper composites with high melting point, high hardness or high tungsten content [12,13]. At present, structural design, Thermal Stress Analysis and preparation technology are still the research focus of this kind of materials, and its application and development are mainly in the basic experimental stage. However, as one of the newly developed preparation methods of W-cu composites, the materials prepared by FGM have good mechanical properties, ablation resistance and thermal shock resistance.

 1.5 other new methods in recent years, powder preparation, forming and sintering techniques have developed rapidly with the increasing requirements of modern science and technology on the properties of materials and the shape complexity of components, new methods of preparing w-cu composites are emerging. For example, hot gas atomization, thermochemistry, injection molding, flow warm compaction, microwave sintering, laser sintering, spark plasma sintering, etc. . In addition, the method of designing and preparing special structure w-cu composite materials according to the environmental requirements of parts and components has gained wide attention and great development in recent years.

2. Application and development of w-cu composites 

2.1 w-cu composites have been gradually applied as electrical contact materials for high-voltage electrical switches since their appearance in the 1930s. The melting point of tungsten at 3387 °C is much higher than the boiling point of copper at 2593 °C, which is beneficial to the "sweating" of copper in tungsten-copper composite materials under the action of arc high temperature, so as to keep the cooling of tungsten framework in time and reduce the degree of electron emission, ensure good breaking function of electrical contacts [18,19]. In recent years, the appearance of vacuum switch has put forward new requirements for the development of w-cu composites, widened the application fields of this kind of materials, and promoted the development of the preparation technology of w-cu composites. 

2.2 The electrode materials are the key materials in the Electrical discharge machining, which will have a direct impact on the finish, machining accuracy, fine machining capability and machining stability of the machined surface, the material is required to have good density and uniformity of structure, especially for some pipe materials, long and thin bar materials and special-shaped electrodes. Although copper and some of its alloys were once the dominant electrode materials at the beginning of the development of Electrical discharge machining because of their wide sources and relatively low prices. But because of the poor resistance of copper and copper alloy to electrical discharge ablation, the electrode consumption is large and the machining accuracy is insufficient. Because of its excellent ablation resistance and high temperature mechanical properties, w/cu composites have higher electrode life, machining efficiency and machining accuracy, which is an important development direction of modern Electrical discharge machining materials.

 2.3 w/cu microelectronic composites have good electrical and thermal conductivity and excellent thermal stability, has become an indispensable key material for high power microwave devices and Very-large-scale integration substrates, inserts, connectors, and heat sinks. Its high thermal conductivity, high thermal stability and good near-net-shape, can improve the power of microelectronic devices, and is conducive to the development of miniaturization and precision devices. The Low Coefficient of thermal expansion of the tungsten copper composite and the adjustability of its Coefficient of thermal expansion make it suitable for joining silicon wafers, gallium arsenide List of semiconductor materials, and ceramic materials for tubes in microelectronic devices, the thermal fatigue damage caused by thermal stress is avoided, and the application range of w-cu composite materials in microelectronics is broadened. 2.4 The military tungsten-copper composite has excellent arc resistance, friction resistance and thermal stability due to its high conductivity and thermal conductivity, since the 1960s, it has been gradually applied in the fields of electromagnetic gun guideway, rocket missile nozzle throat liner and gas rudder. In addition, w-cu composite is gradually used as the material of armor-piercing ammunition cover in recent years. Its high density, high sound speed and good mechanical properties are helpful to improve the armor-piercing power of armor-piercing ammunition. The research in Europe and America shows that the penetration depth of tungsten-copper composite liner can be increased by 30% compared with the pure copper liner. With the development of preparation technology and application research of high performance w-cu composites, the application prospect in military industry will be more extensive. 

3. As an important Powder metallurgy material, w/cu composites have been paid more and more attention and studied deeply for a long time due to its series of excellent properties. With the development of preparation technology and the development of related application fields, w-cu composite materials have gradually been widely used in electric, microelectronics, aerospace and national defense industries, but how to realize industrial simplification, diversification and energy-saving is still the key to the development of this kind of materials on the basis of ensuring the density and performance of materials. In recent years, with the rapid development of high dispersion, nano-powder preparation technology, new pressing and sintering technology, the properties of w-cu composites have been further improved, and their application prospects in high and new fields have been effectively expanded, however, how to develop a new and easy-to-control process to achieve the real significance of industrialization is still the focus of research and development of this kind of materials.

 Source: Chinanews.com, by Liu Keming

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