| Cu-Cr alloy is a kind of material known for their high strength,good conductivity and thermal conductivity,which has been seen a great role in industrial application.Cu-Cr alloy with high Cr content,as the first-choice material,is used in the vacuum switch contacts at home and abroad.The Cu-Cr contact material prepared by conventional process showing serious Cr segregation and coarse structure,owing to the properties of Cu and Cr components in terms of the large difference of the melting point and the specific gravity as well as immiscibility in the solid state,restricts its application.The most obvious way to improve the performance is to refine Cr particles in Cu-Cr alloy.So the performance of the alloy indicators is found to be significant improvement by reducing the size of the second phase Cr particles(especially refining the Cr particles to nanometer scale),which leads to the better performance of contact materials in strength,chopping current and withstanding voltage than that of coarse grained Cu-Cr contact.However,it would directly give rise to decreasing in electrical conductivity and increasing in electrical loss to the disadvantage of the development of national saving type and intelligent power grid construction.It is found that the failure of the contact material arising from local surface failure to the whole material failure by the failure behavior analysis of the vacuum switch.Therefore,the study on the refinement and homogenization of Cr particles on the surface of Cu-Cr alloy enhances the overall performance of the contact material demonstrating significantly important.In this paper,two kinds of optimal design of the structure of the Cu-Cr alloy are carried out to improve the comprehensive performance of contact materials:one is a composite structure of Cu-Cr alloy with microcrystalline or nanocrystalline surface and coarse matrix,prepared by refining the contact surface particles,according to working situation of the electrical contact materials under the premise of guaranteed the conductivity of the Cu-Cr alloy.The other is the Cu-Cr alloy with continuous and uniform distribution of Cr phase,a new type of contact material,fabricated by electroplating combined with liquid phase sintering to solve the problem of high temperature liquid phase segregation in the sintering process.The microstructure and properties of the Cu-Cr alloys with different composite structures were analyzed.The main research results are as follows:1 A composite structure of Cu-Cr alloy with microcrystalline surface and coarse-grain matrix was prepared by high energy ball milling combined with liquid phase sintering.It was found that the ball milling process made the original powder prealloyed and increased the solid solubility between Cu and Cr.By high energy ball milling to treat m(Cu):m(Cr)=3:1 Cu and Cr mixed powder,with the prolongation of the milling time,the particle size decreased gradually,and the lattice distortion and the degree of homogenization gradually increased.The composite nanocrystalline Cu-Cr prealloyed powder can be formed after ball milling for 90 min.The average size of powder particles was 200 ?m,the grain size in powder particles was 23 nm,and the lattice distortion was 0.316.The disorder and amorphization of pre-alloyed powders were caused by lattice defects and chemical disorder.Through thermodynamic calculation,it was found that the Gibbs free energy generated by the formation of solid solution and amorphous phase was positive value indicating that there was no driving force in the formation stage,and the energy required for its metastable transition was provided by the defect energy and chemical disordered energy.The Cu-Cr25 pre-alloyed powder with different ball milling time was repressed after liquid phase sintering at different temperatures.It was found that the pre-alloyed powder could provide more sintering assisting force for the liquid phase sintering process.The smaller the particle size of the liquid phase sintering process,the greater the capillary force produced during the sintering process.The Cu-Cr25 prealloyed powder with different ball milling time was repressed after liquid phase sintering at different temperatures.It was found that the optimum conditions for preparing Cu-Cr alloy were ball milling 90 min and sintering temperature at 1180?.The conductivity of the Cu-Cr alloy was about 36.3%IACS and the hardness was about 97.4 HV.The original mechanical mixture of Cu,Cr powder and nanocrystalline Cu-Cr pre-alloyed powder was compacted by one step compaction,and then liquid phase sintering was carried out to prepare Cu-Cr alloy with microcrystalline surface and coarse-grain matrix structure,which structure was similar to that of conventional Cu-Cr alloy after aging.The Cr grain size in the microcrystalline layer was about 5?20 ?m,and the Cr grain size in the coarse grain layer was about 120 ?m.The transition boundary between the microcrystalline layer and the coarse grain layer is gentle,and the bonding force between layers was fine.Its electrical conductivity was 49.3%IACS,which fully fit the requirements of industrial application,and the hardness was 98.6 HV.2 A composite structure of Cu-Cr alloy with surface nanocrystalline and coarse-grain matrix was prepared by surface mechanical attrition treatment casting Cu-Cr25 alloy.The surface of Cu-Cr alloy formed a plastic deformation layer with a thickness of 150 um.The plastic deformation layer from the surface to the matrix showed nanocrystalline layer,sub-microncrystal layer,microcrystal layer and original coarse-grain layer four parts.The thickness of nanocrystalline layer was about 25 ?m and was composed of equiaxed and randomly distributed nanocrystals.The size of nanocrystalline grain was about 10 nm.The thickness of sub-microcrystal layer and microncrystal layer was about 125 ?m.The size of Cr particles in sub-microncrystal layer was about 200 nm and the Cr phase in the microncrystal layer was distributed with fiber shape.The surface hardness of the SMAT sample reached 577 HV,greatly enhance the original sample,but the electrical conductivity to 40.2%IACS.Calculation through the model established by the electron scattering mechanism and the size effect,the scattering factor of the alloy surface was 0.94 and the scattering factor at the distance of 20 ?m from the surface was 0.3.After low temperature annealing,Cu grains in the surface layer grown up slightly and the hardness was 475 HV which was 3 times higher than the original sample,the electrical conductivity increased to 46.5%IACS,significantly higher than the other the nano-Cu-Cr alloy material.3 A new type of Cu-Cr alloy material was prepared by electroplating and liquid phase sintering.The material has a continuous and uniform distribution of Cr skeleton,which helps to avoid the segregation of elements caused by high temperature sintering of alloy in liquid phase sintering.It was found that the copper net surface after 90 min electroplating was completely coated with a uniform continuous chromium coating.The structure of the chromium plating on the surface layer was like cellular nodular.The boundaries of the coating are clear and various in size of the cellular nodules,but the structure arrangement was dense,and the average size was about 3 um.After coating-Cr/Cu-net liquid phase sintering at different temperatures,Cr skeleton with continuous and uniform distribution was obtained in the whole sample.The size of Cr skeleton decreases slightly with the increase of sintering temperature.When the sintering temperature was 1230?,the density was the highest.It resolves the problem of material segregation caused by different density at high temperature sintering.A new method for preparing homogeneous Cu-Cr alloy is put forward.4 Further improvement of the highly uniform Cu-Cr25 alloy treated by surface mechanical attrition treatment will further enhance the performance.After SMAT,a gradient plastic deformation layer with a thickness of 70 um was formed on the surface of the sample.There is no obvious boundary between the nanocrystalline layer,the sub-microncrystalline layer and microncrystalline layer in the gradient plastic deformation layer.The nanocrystalline layer existed in the range of the surface to about 10 um depth,the size of the surface nanocrystals was 10 nm,and the fine particles of the Cr skeleton were evenly distributed on the surface of the surface of the Cu matrix,The sub-micronstalline layer and microstalline layer existed in the range of 10?70 um,and the continuous Cr skeleton in this layer occured local fragmentation.After 120 min surface mechanical grinding,the hardness of the sample increased to 188.2 HV-and the conductivity decreased by 50.2%IACS,and the hardness of the sample decreased slightly to 166 HV after low temperature annealing,and the conductivity increased to 52.1%IACS.5 The preliminary study on electroplating sintering process was carried out.Cu-Cr25 alloy and Cu-Cr 15 alloy were prepared by liquid phase sintering after Cu wire electroplating.The electrical conductivity of Cu-Cr25 alloy was 53.7%IACS and the hardness value was 108.4 HV,which was lower than which was prepared by the laboratory electroplated Cu net,but still higher than that of the conventional Cu-Cr25 alloy;the conductivity of the Cu-Cr 15 alloy was 59.7%IACS,and the hardness value was 60.3 HV.Through this method,continuous homogeneous Cu-Cr alloy with Cr content of 10%-50%can be produced. |
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