Study On Cu-Cr Nano Composites Prepared By High Energy Ball Milling And Spark Plasma Sintering

Cu-Cr composites (also called Cu-Cr alloys) are widely used as contact materials in vacuum switches. The better properties of Cu-Cr contact materials are demanded with the development of vacuum switches to high power, capacity and miniaturization. Nano composites exhibit excellent comprehensive properties and the fabrication of nano bulk materials is still the research focus in the whole world. In this paper, Cu-Cr nano composites were fabricated by high energy ball milling (HEBM) and spark plasma sintering (SPS), and the application possibility of Cu-Cr nano composites in high power vacuum switches was investigated.CuCr5and CuCr50(mass fraction) systems were taken as research objects, respectively. The formation and mechanism of Cu(Cr) solid solution and Cu-Cr nanocrystalline composite powders during HEBM were investigated, the sintering densification behavior of powders and precipitation and growth of nano Cr particles during SPS werestudied, and electrical properties of nano and nonnano CuCr contact materials were tested by simulation and assembling test. The main conclusions were as followes.Cu(Cr) solid solution can be obtained by high energy ball milling, and the HEBM processing parameters have significant effects on the forming process of supersaturated solid solution. The optimum milling process parameters include PCA addition of5%, WC-Co milling ball, ball gradation of φ10:cp6=1:2, RBP of30:1, and miling time of100h. Under this condition, the dissolution completely of Cr into Cu can be realized.The thermodynamic calculation indicates that thermodynamic barriers exist in formation of solid solutions for Cu-Cr systems at normal temperature. The free energy can increase resulting from the grain size decrease and dislocation density increase during HEBM, which causes the solid solubility of Cr in Cu extending greatly.CuCr5nano composites are fabricated successfully by spark plasma sintering Cu(Cr) supersaturated solid solution powders at sintering temperature900℃for5min with heating rate100℃/min and pressure50MPa. Cr phases precipitate from Cu(Cr) supersaturated solid solution during SPS, with the size of about70～150nm, are distributed homogeneously in the Cu matrix.The relative density of the CuCr5nano composites reaches98.7%. The microhardness and the electrical conductivity are213HV and38.8%IACS, respectively. The value of microhardness is1.58times as high as that of conventional solid solution treated CuCr5alloys, and the electrical conductivity is equivalent to that of conventional solid solution treated CuCr5alloys.The optimum milling process parameters of CuCr50powders are PC A addition of5%, ball gradation with two sizes in diameter of WC-Co balls(φ10:cp6=1:3), RBP of30:1, and miling time of60h. Under this condition, nanocrystalline CuCr50composite powders can be fabricated successfully.CuCr50nano composites are fabricated successfully by spark plasma sintering at sintering temperature900℃for5min with heating rate100℃/min and pressure50MPa.The relative density (98.8%) of the CuCr50nano composites is equivalent to that of the industrial standard(99.0%), the microhardness(282HV) is2.35times as high as that of the industrial standard, and the electrical conductivity(21%IACS) is slightly lower than that of the industrial standard (30%IACS)Under the condition of low D.C. voltage and low current (24V/10A), the average break arc duration of nano CuCr50contact material in vacuum is longer than that of nonnano CuCr50material. The cathode mass loss of nano CuCr50contact material is higher than that of nonnano CuCr50material, but its eroded surface morphology by the arc is uniform without obvious arc erosion pits. While the surface of nonnano CuCr50contact is seriously eroded by the arc in local areas, and shows an obvious erosion pit in the core part.Compared with nonnano CuCr25contact material, the chopping current property of nano CuCr25contact material is improved, but voltage withstand ability declines. The breaking capacity of nano CuCr25contact material is significantly inferior to that of nonnano CuCr25contact material under the condition of high A.C. voltage and high current (24kV/20kA). The main cause might be the fact that nano CuCr25contact material has a lower melting point, lower thermal stability, and higher saturation vapor pressure, and this is disadvantageous for breaking high current.The eroded contact surface morphology of two kinds of CuCr25materials shows clear differences after the breaking tests.The anode surface of nonnano CuCr25contact material is more seriously eroded by arc than its cathode surface. Furthermore, both the anode and the cathode surfaces of nano CuCr25contact material are seriously eroded by the arc in local areas, and show obvious cracks.The results of simulation and assembling test show that the Cu-Cr nano composites prepared under this experiment condition haven’t shown obvious advantage using as contact materials of vacuum switches. Decreasing oxygen and impurity content of the Cu-Cr nano composites and improving its nano structure are research focus on next step.