| Because of its high strength,high conductivity and excellent deformation properties,Cu-Ag alloy is an irreplaceable winding material for non-destructive high strength magnet coils.In order to reduce the destructive effect of Joule heat and Lorentz force caused by thousands of kiloampere current loaded on the high strength magnet winding,it is required that the conductor material of the coil winding has both high strength and high conductivity.The mechanical and electrical properties of the alloy depend on its microstructure,and heat treatment is one of the important means of the microstructure.In this paper,the microstructure of Cu-6wt%Ag alloy is controlled by multi-stage aging heat treatment,and the effect of multi-stage aging heat treatment on Ag precipitates and properties of Cu-Ag alloy is studied and revealed.The DSC curves of Cu-6wt%Ag solid solution alloy with different heating rates show that the precipitation reaction temperature is 350℃-450℃,and the activation energy of precipitation reaction is 56.43±3.86KJ/mol.The equal time heat treatment of pre aging and precipitation aging for Cu-6wt%Ag alloy was carried out.The results show that at the pre aging temperature of 250℃,there are a lot of Ag desolvation phases in the microstructure of the alloy.The strength,hardness and conductivity of the alloy all reach the peak value.At the precipitation aging temperature of 400℃,there are only discontinuous precipitates in the microstructure of the alloy,and there are continuous precipitates and discontinuous precipitates at 450℃。With the increase of aging temperature,the volume fraction of discontinuous precipitates decreased and that of continuous precipitates increased.When the pre-aging temperature is 250℃+precipitation aging temperature is 400℃,the comprehensive properties of Cu-6wt%Ag alloy are the best due to the largest volume fraction of discontinuous precipitation phase in Cu matrix and the most thorough precipitation of Ag.The tensile strength and conductivity of as cast Cu-6wt%Ag alloy are 312.78 MPa and 75.7%IACS.The Cu-6wt%Ag alloy was treated with isochronous heat preservation in cooling stage。The effect of the holding temperature in cooling stage on the microstructure and comprehensive properties of the alloy was analyzed.The results show that there are continuous and discontinuous precipitates in the microstructure of the alloy.The width of Ag precipitates increases first and then decreases with the increase of holding temperature.The maximum width of precipitates is 72.33±8.89 nm at 250℃.With the increase of temperature,the inter phase distance of Ag precipitation decreases first and then increases.In the cooling stage,when the temperature is 200℃,the inter phase distance of Ag precipitation reaches the minimum value。At 150℃,the volume fraction of continuous precipitated phase reaches 38.24%.At this time,the comprehensive properties of as cast cu-6wt%Ag alloy are the best,with tensile strength of 323.49 MPa and conductivity of 79.02%IACS.Cu-6wt%Ag alloy was subjected to a three-stage aging treatment at 250℃/1h+450℃/2h+200℃/1h,and the holding time in the cooling stage was extended to 4h.The influence of holding time in the cooling stage on the microstructure and comprehensive properties of the alloy was analyzed.The results show that the width of Ag precipitates increases from 59.45 nm to 89.4 nm,the distance from 362.3 nm to 433.85 nm,and the volume fraction of discontinuous precipitates increases from 29.06%to 31.91%when the holding time is extended from 1h to 4h.Due to the increase of precipitate width and volume fraction of discontinuous precipitates,the tensile strength of the alloy increased by 31.13%,reached 244.18 MPa,and conductivity reached 71.56%IACS. |
PDF Full Text Request