Structures And Properties Optimization Of High Strength And High Electrical Conductivity Cu-Cr-Ni-Co-Si Alloy

As the third generation of high strength and high electrical conductivity material,the Cu-Cr alloy had been widely used as high-speed rail contact conductor,integrated circuit lead frame and resistance welding electrode material because of its excellent mechanical and electrical properties.Among all Cu-Cr alloys,the Cu-Cr-Zr alloy exhibited the most outstanding comprehensive properties,and was the most widely used and promised high strength and high electrical conductivity Cu-Cr alloy.However,the chemical properties of Zr element were active and easy to burn,and it was difficult to melt under non vacuum condition.Therefore,the development of new-type high strength and high electrical conductivity Cu-Cr alloys without Zr element were becoming the goal that scientific researchers strived to pursue.In this paper,the Cu-Cr-Ni-Si,Cu-Cr-Co-Si and Cu-Cr-Ni-Co-Si alloys were prepared by adding Ni,Co and Si alloying elements through composition design on the basis of the Cu-Cr alloy under vacuum condition,and were treated by room temperature rolling and aging,cryorolling and aging,and two-step rolling and aging.The effects of different rolling and aging on microstructures,properties and precipitation behaviors of precipitates were analyzed,optimizing the alloy composition and comprehensive properties;Besides,considering the demands for grain refinement of industrialized billets,the soft contact electromagnetic continuous casting under non vacuum condition was used to control solidification structures of the Cu-Cr-Ni-Co-Si alloy with the best comprehensive properties,and the effects of different electromagnetic parameters and casting parameters on solidification structures of the alloy were explored to obtain the best composite technology parameters.The tensile properties and electrical conductivity of the alloy prepared by soft contact electromagnetic continuous casting,conventional continuous casting and vacuum melting were compared after different thermo-mechanical treatments,and the high strength and high electrical conductivity copper alloy with small grain size and meeting the requirements of industrial production was prepared.The specific results were listed as follows:By adjusting the content of Ni,Co and Si alloying elements under vacuum condition,the primary Cr₃Si and Cr₁₅Co₉Si₆ granular phases were formed in the designed Cu-1Cr-1Ni-0.4Si,Cu-1Cr-1Co-0.4Si and Cu-1Cr-1Ni-1Co-0.6Si（wt.%）as-cast alloys,the above granular phases remained stable during homogenization and solid solution,and were pinned at the grain boundary through Zener force to effectively inhibit grain growth.During aging,a large number of nano-sized Cr,Ni₂Si,Co₂Si and（Ni,Co）₂Si precipitates were precipitated from the copper matrix,thereby decreased the content of solid solution atoms in the matrix,which was beneficial to obtain the precipitation strengthening and reduce the damage to electrical conductivity.The Cu-1Cr-1Ni-1Co-0.6Si alloy exhibited the best comprehensive properties after room temperature rolling 90%deformation and aging at 440℃for 60 min,and the yield strength,tensile strength,elongation and electrical conductivity were 662.8 MPa,705.1 MPa,5.9%and 47.4%IACS,respectively.By exploring the effects of cryorolling and aging on microstructures,properties and precipitation behaviors of precipitates of the Cu-1Cr-1Ni-1Co-0.6Si alloy,it was found that the cryorolling and aging had more obvious effects on microstructures and properties compared with the room temperature rolling and aging.The increase of the cryorolling deformation could promote the formation of nano-sized（Ni,Co）₂Si and Cr precipitates,and would not lead to the coarsen of precipitates;Properly increase the aging temperature and prolong the aging time could promote a large number of precipitates precipitated from the copper matrix in the initial stage,and the size of precipitates did not increase significantly,but higher temperature or longer aging time were easy to lead to the coarsen of precipitates,which was not conducive to the precipitation strengthening.After cryorolling 90%deformation and aging at 440℃for 60 min,the comprehensive properties of the alloy were better than those of the room temperature rolling and aging,and the yield strength,tensile strength,elongation and electrical conductivity were 687.2 MPa,753.8 MPa,5.6%and48.3%IACS,respectively.Meanwhile,the strengthening mechanisms and the factors affecting the electrical conductivity of the alloy were elaborated,and precipitation transformation kinetic equations at different aging temperatures were established by the relationship between electrical conductivity and volume fraction of precipitates,and isothermal transformation kinetic curves were also drawn.The properties of the Cu-1Cr-1Ni-1Co-0.6Si alloy were investigated by adjusting the reduction and aging parameters of each pass during two-step rolling and aging,it was found that the best hardness and electrical conductivity were obtained after rolling 60%deformation+the first-step aging（480℃,120 min）+rolling 30%deformation+the second-step aging（440℃,60 min）for both two-step room temperature rolling and aging and two-step cryorolling and aging,and the tensile properties and electrical conductivity of the alloy after two-step cryorolling and aging were better than those of two-step room temperature rolling and aging.Two-step room temperature rolling and aging could obviously reduce the deformation band width and the twin layer thickness,and some precipitates were pinned at deformation bands and twin boundaries during aging,which were conducive to maintaining the stability of the deformation band and the twin boundary,thereby improved the tensile properties and electrical conductivity,and the yield strength,tensile strength,elongation and electrical conductivity were 686.4 MPa,747.2 MPa,6.2%and 50.1%IACS,respectively.Two-step cryorolling and aging had finer deformation bands,deformation twins and more precipitates compared with two-step room temperature rolling and aging,which exhibited better comprehensive properties,and the yield strength,tensile strength,elongation and electrical conductivity were 762.2 MPa,807.8 MPa,5.7%and 50.7%IACS,respectively.The Cu-1Cr-1Ni-1Co-0.6Si alloy ingots with the diameter of 100 mm were prepared by soft contact electromagnetic continuous casting under non vacuum condition.When the current intensity was 100 A,the pouring temperature was 1280℃,the drawing speed was180 mm/min,and the cooling water flow was 2 m³/h,the refining effect was the best,with the average equiaxed grain size reduced from 5833μm to 617μm,and the equiaxed grain rate increased from 29.34%to 71.13%.In addition,the tensile properties of the as-cast ingot prepared by soft contact electromagnetic continuous casting were greatly improved,and the tensile strength and elongation were 397.2 MPa and 10.3%,respectively,which were increased by 8.9%,18.4%and 13.0%,13.2%,compared with conventional continuous casting and vacuum melting,but had little effect on electrical conductivity.The tensile properties and electrical conductivity of the alloy prepared by soft contact electromagnetic continuous casting,conventional continuous casting and vacuum melting were significantly increased after solid solution,room temperature rolling,and aging,and the tensile properties and electrical conductivity were at the same level because of the similar effects of the solid solution strengthening,dislocation strengthening and precipitation strengthening.