| With components and micro-structure spatially changing non-uniformly, functionally graded materials (FGMs) can be designed to some kind of new composite materials with mechanical properties and functions graded changing along a specific direction, to realize that both sides of the materials have different functions. So it is significant for both theory and engineering application to study the quantitative relationship between the composition and distribution of the microstructure and macroscopic properties of FGMs, and optimize the design theory and preparation technology of FGMs.In this paper, both Cu/WCp FGMs and composite materials with different volume fraction of WC were prepared by vacuum hot pressing sintering method and pressureless sintering method, respectively. The vacuum hot pressing sintering process and the densification of Cu/WCp FGMs were also analyzed using hot pressing densification theory. Then the internal microstructure of material was observed and analyzed by SEM, the density of samples was measured using drainage method, and the electric conductivity and macro hardness of the Cu/WCp composite material were measured by eddy conductive instrument and small load Vickers hardness machine. Using Universal Testing Machine, the tensile test and three point bending test of the composite material Cu/WCp and Cu/WCp FGM were carried out, respectively. Finally, the tensile fracture section of the Cu/WCp composite material was observed and analyzed by SEM.Through the above research, the following conclusions ware obtained:1. The densification process of Cu/WCp FGM can mainly be divided into three stages:(1) Initial stage of vacuum hot pressing sintering:in this stage the density of sinter increases most rapidly, so it's the most important stage of the sintering process. (2) Intermediate stage of vacuum hot pressing sintering:it's the densification deceleration phase. (3) Final stage of vacuum hot pressing sintering:in this stage the density of sinter is approaching the ultimate.2. Compared with pressureless sintering, vacuum hot pressing sintering can not only improve the microstructure of Cu/WCp FGM, but also improve the hardness and conductivity of the composite materials Cu/WC obviously. Especially in the condition of 950℃-25MPa-120min, the relative density of Cu/WCp FGM prepared by vacuum hot pressing sintering reached 98.9%. In addition, the density, hardness, elastic modulus and tensile strength of Cu/WCp composites prepared by vacuum hot pressing sintering increase as the WC volume fraction increasing. However, the electric conductivity and Poisson's ratio decrease as the WC volume fraction increasing.3. In the prediction of elastic modulus of composite material, some progress has been made in this paper. Based on the test data of elastic modulus, the experience fitting parameter q of the famous Hapain-Tsai equation for modifying mixing ratio was optimized by statistical software, and then the value of q was determined. The elastic modulus of the Cu/WCp composite materials, in which the WC volume fraction is less than 15%, can be accurately predicted by the obtained semi empirical formula.4. When there is no crack in Cu/WCp FGM, the maximum bending load of the specimen with maximum elastic modulus bottom layer is larger than that of specimen with minimum elastic modulus bottom layer. When there is a crack in the bottom layer of the specimen, the bearing force of the specimen with a crack extending from the maximum elastic modulus layer to the minimum elastic modulus layer is smaller than that of the specimen in the opposite case.5. Though analyzing the fracture section of the tensile specimen, it's found that when the volume fraction of WC in composite material is less than 15%, the microstructure characteristic of the fracture section belongs to the most common dimple of toughness fracture of metal. Generally, there are WC particles in the bottom of the dimple. |
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