Effect Of Cu Content On Microstructure And Property Of Al-Zn-Mg-Cu-Ag Alloy

7xxx(Al-Zn-Mg)alloys are widely used as structural materials in aerospace,national defense and transportation fields due to their advantages of light weight and high specific strength.In industry,methods such as changing the content of alloying elements and optimizing heat treatment are usually adopted to further improve their comprehensive properties.The results show that the addition of Cu to Al-Zn-Mg alloy can improve the aging strengthening effect and corrosion resistance.The addition of Ag can improve the Strength and plasticity of the alloy.However,the effect of compound addition of Cu and Ag with different contents on the aging precipitation behavior of Al-Zn-Mg alloy has not been reported in detail.Based on this,this paper takes Al-4.2Zn-2.8Mg-x Cu-0.4Ag(x=0,0.5,1.0,2.3 wt.%)quinary alloy as the research object,and discusses the specific effects of different Cu contents on the aging process of Al-Zn-Mg-Cu-Ag quinary alloy.By means of metallographic microscope,scanning electron microscope,transmission electron microscope and three-dimensional atomic probe,the effects of different Cu content on the microstructure,hardness,electrical conductivity and corrosion resistance of the alloy under single,double and three stages of aging were investigated.Combined with the kinetic theory,the aging hardening behavior of the four alloys was calculated,and the activation energy of diffusion at each stage was obtained.The main conclusions are as follows:(1)After solid solution,the increase of Cu not only increases the number of insoluble phases such as Al₂Cu Mg and Al₇Cu₂Fe,but also increases the hardness value of solid solution state and decreases the electrical conductivity.(2)With the increase of Cu content,the hardness and strength of the alloy increase gradually,and the elongation increases firstly and then decreases.When the Cu content is0.5wt.%,the alloy has the maximum elongation and the best corrosion performance,but when the Cu content is greater than 0.5wt.%,the corrosion resistance decreases.(3)At the early single aging stage,Cu can promote the formation of clusters,and the higher the Cu content,the more obvious the promoting effect is.In the peak aging period,the increase of Cu content will increase the number of precipitates,and refine the size of precipitates,but it has little effect on the width of PFZ.In the later aging period,the growth of precipitates will be inhibited and the size of precipitates will be refined.However,the precipitation sequence of the alloy will not be changed with the addition of Cu.The precipitation sequence is as follows:cluster?GP zone??'(T')phase??(Mg Zn₂)or(T-Al₂Zn₃Mg₃)phase.(4)In the two-stage and three-stage aging processes,the increase of Cu content will reduce the peak hardness loss of the alloy.Compared with single-stage aging treatment,three-stage aging treatment can not only improve the corrosion resistance of the alloy,but also has little effect on the hardness of the alloy.In addition,after three-stage aging treatment,the number and size of precipitates at grain boundaries became less,their distribution became discontinuous,and PFZ became narrower.(5)At the?stage of aging,the diffusion activation energies of 0Cu,0.5Cu,1.0Cu and 2.3Cu alloys are 66.38,41.27,49.26 and 50.66k J/mol,respectively.At?stage of aging,the diffusion activation energies of the four alloys are 42.61,63.77,67.75 and68.53k J/mol,respectively.