| Tungsten-copper composite as a kind of function material with excellent performance has been widely used in integrated circuits, contact materials, electrical discharge machining, plasma electrode, mold temperature and other requiring thermal conductivity properties and high temperature use occasions, and has become a hotspot of current research. With the development of modern science and technology, the quality and properties of tungsten copper composite materials are put forward higher requests and material preparation process are also put forward new developmental requests. At present, the infiltration sintering is widely used in the preparation of tungsten-copper composites with high performance. Therefore, by improving the traditional sintering technology, the development of highly efficient, energy-saving and environmentally friendly new technology of infiltration sintering of tungsten-copper composites has a very important and practical significance.Microwave sintering has the advantages of clean, high efficiency, energy saving and environmental protection. In the field of material preparation, microwave sintering technology has been developed into an important technical means to prepare high-quality new materials rapidly, especially having a broad application space in the preparation of powder metallurgy materials. Therefore, combining technical features of preparing high-performance tungsten-copper composite materials by infiltration sintering method, with technical advantages of microwave sintering, the present paper puts forward using microwave heating and melting copper powder and the new preparation process of infiltration sintering tungsten-copper composites. And the melting characteristics of copper powder in microwave field, mechanism of microwave heating copper power, the preparation and related performance of tungsten-copper composites has been researched.(1) Systematic research on the heating characteristics of metallic copper powder in microwave field, make a quantitative description of the heating curve of the metal copper powder in microwave field. The changes of microscopic structure and densification in the process of heating metal copper powder to melting with microwave has been analyzed, and carry out the dynamics research on the process of microwave heating copper powder. The results shows that the microwave has a good heating characteristic to the metal copper powder, the metal copper powder can be quickly heated to melting, and microwave on the metal copper melt still has a certain heating effect. In addition, the heating up rate of the metal copper powder heated by the microwave has a linear relationship with the reciprocal of the particle size. Microwave heating efficiency and the heating rate will be higher with the decrease of the particle size. The observation of the microstructure and the result of the density measurement show that when the temperature is below 900℃, the variation of the powder sintered body density is not very, while when the temperature is above 900℃, the densification process will accelerate. And when the sintering process is below the melting point, the migration of the materials is mainly based on the internal diffusion of the particles. Through the sintering mechanism research shows that the density changes of the metallic copper heated by microwave in the sintering process agrees with integrated action mechanism and the viscous flow sintering mechanism.(2) In the present paper, the mechanism of microwave heating metal copper powder has been discussed, the temperature-rise of the dielectric properties of metallic copper powder were determined, the skin depth of metallic copper powder in the field of microwave has been characterized, the electro coupling characteristics of copper in the microwave field were described. The research about the mechanism of microwave melting metal powder was carried out. The results show that the metal copper powder has a certain dielectric properties, and with temperature rising, the copper dielectric constant and dielectric loss increased to a certain degree and then tend to stable with a relatively low trend of decreasing, which means that as the microwave heats the copper powder to a certain temperature, the type of heating through dielectric loss will be weakened. Calculation results show that the microwave has certain penetration depth and larger interaction volume on fine particles of metal copper powder, on the particle size of 2.5μm spherical copper powder, microwave irradiation volume was 28.6%, and for dendrite copper powder, of which the surface is 3-5μm diameter spherical cap structure, microwave can achieve to whole heating, with better heating effect. Considering the electro coupling characteristics of metal in the microwave field, the method of metal copper dual blocks was adopted to describe the skin effect of metal in the microwave field. The results show that under the action of microwave electromagnetic field, the metal’s surface has an accumulation of high density charge and potential difference, when microwave output power is 2kW, distance between the two dual metal copper blocks is 320μm and the diameter is lcm, the coupling potential per unit area up to 335 V/cm2, and with the effective area decreasing, the potential difference value had a parabolic increase. Therefore, when the copper metal powder has contact on the surface, conductance loss and tip discharge effect can make metal copper powder reach a high temperature quickly. In addition varying magnetic field under the action of microwave effect will generate different forms of eddy current on the surface of metal copper powder, after the temperature reached a certain value, the microwave energy loss caused by eddy current to heating the metal powder will be the main mechanism.(3) The research of related processes for W-Cu composites prepared by microwave infiltration sintering technique was carried out, and the material microstructure, pore distribution, density, hardness, thermal conductivity, electrical conductivity and thermal expansion properties all got analyzed and determined. The effects of main factors such as copper content, the temperature and time of sintering, tungsten powder particle size on the W-Cu composites microstructure and physical mechanical properties were studied, and the correlation characteristics between sintering process, organization structure and properties of materials was explored. The result shows that the metal copper coated tungsten particles well, the porosity of tungsten copper composites with 5%,8%,20% addition of copper was 3.40%,2.57%, and 1.23%, respectively. The porosity of composites decreased with copper addition increasing. Density of W-Cu composites increased with temperature increasing, when the sintering temperature was above 1200℃, densification of the composite has been basically achieved, the density of WCu20 composite reached 15.25g/cm3 and the relative density was 97.51%, after 1200℃, the density increased slowly with the further increase of temperature. The hardness of the composite decreased with copper content increasing, while it increased first and then decreased with temperature increasing, composite sintered at 1200℃ for 1 h had a good performance as the hardness was 222HBS. Physical property test results show that the thermal conductivity of composite presented two different rates of reduction. The copper content influenced the decrease rate of thermal conductivity and transition temperature. The thermal expansion coefficient increased with temperature increasing, in a temperature range of 37-40℃, the average heat expansion coefficient of WCu8 and WCu20 composites were 6.5×10-6/℃ and 8.5×10-6/℃, respectively, such numerical values are relatively low. The electrical conductivity test results show better performance, the conductivity (IACS%) of WCu5, WCu8 and WCu20 composites were 26%,33.5% and 37%, respectively, as these values exceeded the national standard.(4) Combining the process characteristics and research hotspot of tungsten-copper composites prepared by microwave infiltration sintering, to carry out a new technology that fabricated WCu20 composites by microwave infiltration sintering with copper coated tungsten powder as raw material, also to develop a new microwave hot press sintering equipment and carry out a new research of tungsten-copper composites prepared by microwave hot press sintering. Tungsten powders coated with copper by electroless plating method was carried out, the results show that at the situation of a temperature of 60℃, pH value of 12-13, with formaldehyde (53ml/L) as reducing agent, iron potassium cyanide (0.3 g/L) as stabilizer, tartaric acid potassium sodium (15g/L) and EDTA-2Na (19g/L) as compound complexing agent, the surface coating thickness was relatively uniform, which is about 0.8～1μm, owning good coating effect. WCu20 composites fabricated by microwave infiltration sintering shows uniform microstructure distribution, relatively high density, when sintering at 1100℃ for 1h, the density could be up to 15.22 g/cm3, relative density was 97.13%. WCu20 composites prepared by microwave hot press sintering shows uniform microstructure, compact structure, better mechanical properties and other advantages, under the microwave enhanced hot pressing sintering conditions, favoring the crystalline slipping of copper component, promoting the migration and rearrangement of copper, thus to quickly implement the alloy matrix uniformity and densification. The results show that at sintering pressure of 40MPa, the density of WCu20 composites was 15.42 g/cm3, the relative density was 98.65%, which exhibited a better sintering performance, with greater application space and technology promotion. |
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