| W-Cu composite possesses unique properties of superior thermal and electronic managements, high microwave absorption capacity and etc. These features make the W-Cu composite a very attractive material which has been widely used as heavy-duty electronic contactors, circuit breakers and thermal management devices, and so on. However, due to their relatively large difference in properties between W and Cu, the preparation process of this composite requires higher requirement. W-Cu composite prepared by the conventional methods has drawbacks such as low densification, microstructure inhomogeneity and/or limited composition variation which hinders its further applications. With the rapid development of the electronic information industry and the high-tech fields of defense industry, there are several new directions of development and requirements on high performance W-Cu composites, such as (1) to explore the preparation process which can be applied to industrial production; (2) to further improve the densification and microstructure uniformity of W-Cu composites with a higher performance; (3) to develop new high-performance W-Cu composites to meet the requirements of the high-tech fields; and (4) to expand the applications of W-Cu composites. In this paper, W-Cu, W-Cu/AlN composites and W-Cu functionally graded materials with high performance were fabricated by combining mechanical alloying technique and pressureless sintering or hot-pressing technique. And the technic parameters of process, optimal design and properties of those composites were investigated and analyzed. The results of those studies would provide a theoretical basis and data base for the production and application of high-performance W-Cu composites.The mechanical alloying process of nanocrystalline W-Cu composite powder with different contents was investigated. The process characteristics, structural evolution and thermal stability of the MA W-Cu nanocrystalline powder were investigated in detail by analysing phase transformation, composition distribution, grain size, lattice parameter, microstrain, and morphology changes of the composite powder during mechanical alloying and subsequent annealing. The results showed that room temperature stable supersaturated W(Cu) solid solution formed after 30h, 40h and 60h milling of W-15Cu, W-20Cu and W-30Cu composite powder. During mechanical alloying of W-15Cu powder, the grain size decreased with increasing milling time, and the particle size first increased then decreased with increasing milling time. After milling for 30h, particles were polyhedral in shape with smooth surface, and average particle size was about 4μm. At the initial stage, the structure of W-15Cu powder was the ring-like composite layer generated by the Cu particles enwrapping around the W particles. In the middle stage, the composite particles of circle-like layers became finer and the space between the layers greatly reduced. The mixed microstructure of homogeneously dispersed W and Cu was formed. After milling for 30h, the powder was composed of homogeneous W(Cu) solid solution phase. Annealing of as-milled W-15Cu powder reduces the lattice deformation and internal strain, and incresased the lattice ordering degree. Meanwhile, Cu precipitated from the supersaturated W(Cu) solid solution at the temperature of 500℃.Liquid phase sintering, or hot pressing was used to consolidate MA nanocrystalline W-Cu powders to bulk compacts of W-15Cu, W-20Cu, W-30Cu, and bulk composites of W-Cu/AlN with different contents of AlN (0.25, 0.5, 1.0, 2.0wt.%). The microstructure, physical properties such as density, thermal conductivity, electrical resistivity and conductivity, mechanical properties such as hardness, bending strength and etc. of these materials were characterized and investigated. The effects of process parameters of high energy ball milling and sintering on the microstructure and properties of W-Cu composite, and the sintering densification mechanism of MA powders were studied. The results showed that mechanical alloying technique promoted the sintering of W-Cu composite powder, strengthened the interaction and increased the contacts between W and Cu by grain refining, enhancing the microstructure homogeneity of powder and the formation of W(Cu) solid solution. Thus, the relative density and microstructure homogeneity of W-Cu composites were effectively enhanced. Which means mechanical alloying is one of the optimal preparation processes to obtain near full dense W-Cu composites. Considering all factors, the optimal sintering process parameters were as follows: forming pressure was 350MPa, sintering temperature and time were 1200℃and 90min. The relative densities of W-15Cu, W-20Cu and W-30Cu composites were 98.42%, 99.10% and 99.34%, respectively by using the above-mentioned parameters. Finer structure bulk W-Cu composites were successfully synthesized by the means of hot pressed vacuum sintering at 1200℃for 90min under the pressure of 25MPa. The relative densities of W-Cu composites with three different compositions were 97.87%, 98.29%, and 98.94%, respectively. Addition of small amount of nano AlN particles had less negative effect on degradation of relative density. The relative density of W-Cu composites remained at about 98% when the addition of AlN was 1wt%. Nano AlN particles were even distributed in Cu phase of the matrix. The hardness of Cu in the matrix would increase due to the reinforcement effect brought about by grain refining and dispersion strengthening. The increasing strengthening particles distributed at grain boundaries with the increasing of AlN content, would lead to the decrease of bending strength of composites by affecting the combination between adjacent particles and densification of materials during sintering process. However, the thermal conductivity of composites increased with increasing AlN content.The residual thermal stress of W-Cu functionally graded materials arising from the fabrication process was analyzed using finite element method (FEM). Based on the calculation results of the residual thermal stress and stress state in graded layers, W-Cu functionally graded materials with three layers and four layers structure were designed. W-20%Cu/W-33%Cu/W-50%Cu FGM (P=2.4) and W-20%Cu/W-29.1%Cu/W-39.2%Cu/W-50%Cu FGM (P=1.4) had the minimal equivalent thermal stresses and the thermal stress were reduced by 52% and 68% respectively comparing with the non-FGM. Based on the optimization results, two kinds of W-Cu FGMs with high density and pretty microstructure were prepared by hot-pressing sintering. The thermal conductivities of W-Cu FGMs with three layers and four layers structure were 198 W·m-1K-1 and 202 W·m-1K-1 respectively. After the thermal shock test with 800℃temperature sdifference, no cracks were found at the interface of two kinds of FGM samples. The two kinds of FGM survived up to 86 and 143 thermal cycle tests respectively. Cracks were found at the interface at both ends of FGMs, which was consistent with the previous calculation results of the residual thermal stress. These results indicated the two kinds of FGMs had excellent heat resistance and exhibited good property of reducing thermal stress.
|
PDF Full Text Request