| With the rapid development of high-tech in electronics industry,aerospace and national defence industry,the demand of fine grain, high density, high performance for tungsten copper composites is becoming more and more urgent. Because of the major physical differences between W and Cu, it is more and more difficult for W-Cu composites produced by the traditional method to meet the requirements due to the coarse grains, defects such as low density and poor uniformity of the microstructure and properties. This series of requirements will surely promote the improvement of synthetic process and the development of new technology of W-Cu composites. Therefore, in order to slove the issues in the preparation, structure and performance of the high density, high performance and ultrafine tungsten copper composites, we carried out the research on the relationship between structure, processing and performance based on the properties of W-Cu composites by nano powder synthesis, fine crystal sintering densification. First, nano-sized tungsten copper composite powder contains 20~30wt.% copper was in situ synthesized by hydrothermal method-calcination-reduction process; and then the W-Cu powder was processed by low temperature liquid phase sintering and canning backward extrusion; finally, the high density, high performance and ultrafine W-Cu composites were obtained. We take W-25 wt. %Cu as an example, the influence of morphology, phase and structure of precursor powder in the water thermal environment, calcination and reduction process was analyzed. The influence factors of growth process and reduction process and mechanism of tungsten copper precursor powder in hydrothermal environment were analyzed. The densification mechanism of fine grain tungsten copper composites was expounded. The effect law and mutual relations between the structure, density, electrical conductivity and hardness were explained. The tungsten copper composites with different preparation processes and microstructure on the response mechanism of arc erosion were revealed.The research contents and main results are as follows:(1) The effects of reaction time, hydrothermal temperature, pH value and solution concentration on the morphology of precursor powder and the crystal growth mechanism were comprehensive analyzed. With the increase of hydrothermal temperature and reaction time, the crystals grow up from an amorphous state to a crystalline state and the crystal size increased. In the growth process of crystal, the crystal fist accumulated without orientation(reflected on the macro to be soft or hard agglomerate), and then growth based on the first aggregation pattern. The formation of CuWO4·2H2O crystal was divided into two stage. Fist, the Cu2WO4(OH)2 crystal separated out based on the dissolve-precipitation nucleation crystallization mechanism in the hydrothermal environment; and then WO42-,H+ or OH- ions and water molecules in the solution were adsorption and diffusion on the surface of Cu2WO4(OH)2 crystal, so the heterogeneous nucleation of CuWO4·2H2O crystals occurred. According to the theoretical calculation, the optimal reaction conditions(contains 20~30wt. % Cu) which the recovery rate of tungsten and copper elements were higher than 97% were determined. The pH value was in the range of 5.2 to 5.5 and the solution concentration was in the range of 0.52 to 0.8 mol·L-1. With the optimal reaction conditions, the spherical CuWO4·2H2O or CuWO4·2H2O and Cu2WO4(OH)2 precursors about 60 nm were synthesized when the hydrothermal temperature is 180 ℃ and the reaction time is 28 h.(2) The influence of calcining time and temperature, reduction time and temperature and material thickness on the morphology and phase of tungsten copper composite powder were studied. The hydrogen reduction process of tungsten copper oxide powder prepared by hydrothermal method mainly follows the solid phase diffusion and chemical vapor transport. Because of the chemical vapor transport of volatile phase WO2(OH)2, the formation of the tungsten encapsulated copper structure occurred, which was helpful to reduce copper particles aggregation and growth up in the process of hydrogen reduction, and improve the combination of tungsten copper particle interface. The optimal process of calcination and hydrogen reduction were determined based on the effects of calcining time and temperature(calcined at 500℃ for 120min), as well as the hydrogen reduction time, temperature(hydrogen reduction at 800℃ for 60min) and bed thickness(about 3~5 mm) on the morphology and phase change of W-Cu composite powder. And in the dry hydrogen environment, the high dispersion pure W-Cu composite powder with hundreds of nanometers was prepared.(3) The the relationship between the microstructures and density, hardness electrical conductivity was comprehensive analysised by comparing the microstructure and properties of W-Cu composites prepared by SPS sintering with low temperature vacuum sintering and canning backward extrusion. This paper expounds the densification mechanism of fine grain tungsten copper composite material. We can conclude that the copper network structure(reduce the connectedness of W particles) can effectively improve the densification and uniformity of the microstructure. However, the W-Cu compacts prepared by low temperature vacuum sintering(sintering at 1090℃) and then hot extrusion(extrusion temperature is 1090℃, extrusion ratio is 7.7), finally annealing at 900℃ have better properties. The microstructure of the W-25 wt. % Cu composites was hundreds nano-sized W particles homogeneously distributed in the Cu matrix. The relative density of W-25 wt. % Cu composites was 99.62%, the hardness was 252 HB and the electrical conductivity was 53.4% IACS. The properties of W-25 wt. % Cu composites not only meet the national standards but also reach the performance indicators of the project.(4) The hot extrusion process and canning structure were optimized, as well as the defects such as fracture and canning cracking and the low utilization rate of materials due to the uneven distribution of the W-Cu materials along the longitude were solved by the numerical simulation of the hot extrusion process of the billet and defect analysis in the canning backward extrusion process. The good coincidence of the distribution of the W-Cu materials and the loading force in the hot extrusion experiment with the numerical simulation results proved the accuracy of the model and parameter selection in numerical simulation. So the simulation results have practical significance. The results show that canning cracking was mainly due to the difference strain and flow speed between the canning and the W-Cu billet, while the flow of the W-Cu billet was affected by the structure and material of the canning. It is concluded that when the extrusion temperature was near the melting point of copper phase, the extrusion speed was in the range of 5 to 10 mm/s, extrusion ratio was in the range of 7.7 to 11, the canning thickness was 5 mm and material was AISI-1016 can not only improve the temperature and stress and strain distribution of billet and canning but also enhanced the flow uniformity of billet.(5) By comparing the material transfer, contact resistance and the changes of microstructure and phases of the fine grained W-Cu contacts prepared by the method mentioned this paper and the coarse grained tungsten W-Cu contacts prepared by infiltration method in DC resistance load conditions, the effects of preparation technology and size and distribution of the W particle of the contact on the behaviors of the contacts were discussed. The results show that the total mass loss of the fine grained W-Cu contacts was bigger, but the contact resistance was smaller and more stable and the tendency of material transfer between the electrodes was smaller. That uniform distribution, small W particles can effectively inhibit the spillage of liquid copper and fluctuation of contact resistance caused by the material transfer. Material transfer and losses of W-Cu contact materials were affected by the surface oxide film; and the oxidation film can effectively inhibit the quality loss and material transfer of the contacts. In the process of the on-off of the contacts, Cu phase with the low melting point mainly evaporated and splashed, and left pores, cracks and the spherical copper drops on the contact surface. Due to the short arcing time, the liquid W has no time to spread out. So the uneven melting layer and spiral pip morphology occurred. Meanwhile the molten W can react with Cu and oxygen ions, so the cubic CuWO4 structure was occurred. The evaporation of Cu phase can absorb a large number of arc energy, so the temperature of the contact surface reduced; W phase as a skeleton can reduce the transfer and splash of the liquid phase or reduce the erosion due to energy consumption by chemical reaction in the high temperature. |
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