| Cu-15Ni-8Sn alloy is a high strength conductive copper alloy with a wide range of application prospects,which has high strength,excellent thermal stress relaxation resistance,stress corrosion resistance,and non-toxic.For aerospace,electrical engineering and communications engineering,electronic,and other fields,it can be used in various types of relays,potentiometers,conductive contact springs and heavy-duty bearings.However,inverse segregation and dendrite segregation rise in the industrial production of Cu-15Ni-8Sn alloy,which brings in negative influence on the performance of the alloy.The degree of segregation of the alloy can be decreased by means of powder metallurgy,vacuum melting,injection molding,etc.However,these techniques bear on liminations of high cost,and fail to fabricate large-scale productions.Based on these reasons,this article aims to reduce segregation of Cu-15Ni-8Sn alloy and to fabricate large-scale Cu-15Ni-8Sn alloy through vertical semi-continuous casting.The main experiment contents are as follows:The effects of cooling rates on the segregation of Cu-15Ni-8Sn alloy were investigated by unidirectional solidification experiments.The high-temperature melts were poured in a 950? pre-heated bottomless graphite crucible,the crucible was put on water-cooled copper molds,room-temperature graphite,and 600? graphite,the cooling rate of the mode is 58K/S,16K/S and 5K/S at the interface.When the cooling rate of Cu-15Ni-8Sn alloy is relatively fast,the inverse segregation phenomenon is more serious while the dendrite segregation is weakened.In vertical semi-continuous casting experiment,the melt casting temperature was 1350?,with casting speed 90mm/min.0kW,5kW and 10 kW intermediate frequency magnetic fields were imposed during semi-continuous casting.The experimental results show that the intermediate frequency magnetic field has a significant effect on the as-cast grain refinement,especially for the 10 kW magnetic field.Two different phases can be observed in the as-cast microstructure of Cu-15Ni-8Sn alloy,?-matrix phase,?-rich tin phase.0k W and 10 kW intermediate frequency magnetic continuous casting samples were selected for the subsequent experiments.The solution temperature was set at 880?.For samples produced with imposing 10 kW magnetic field,the solution atoms could completely dissolve into matrix when solution treated for 1h,same results can be found in samples treated for 2h fabricated without magnetic.Fine grains make the second phase homogeneously distributed in the tissue,leading to the shortened diffusion distance of solute atoms and solution time,and improve solution treated efficiency.After solution treatment,the samples were rolled at room temperature with rolling reduction 90%.The subsequent aging treatment was performed at 400?.The tensile strength reached the maximum when age treated for 10 min,the semi-continuous casting sample fabricated with 10 k W and 0 kW magnetic fields reached 1256 MPa and 1175 MPa respectively,the increase in tensile strength corresponds to 7% with imposing magnetic field.Spinodal decomposition occures at the early stage of aging.The formation of nano-sized spinodal decomposition structure produces dispersion strengthening effect.At the same time,the new phase maintains a coherent relationship with the matrix phase,inducing a coherent enhancement effect.Therefore,the tensile properties of the alloy reached the maximum value.In the later stage of aging,discontinuous precipitation took place and resulted in the decrease in properties of Cu-15Ni-8Sn alloy.During vertical semi-continuous casting process,the imposition of intermediate frequency magnetic could refine the as-cast grain and decrease the segregation of Cu-15Ni-8Sn alloy.Besides,it could also shorten solute atom diffusion distance,leading to the improvement of solution treated efficiency.In the aging process,spinodal decomposition could improve the mechanical properties. |
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