| With the rapid development of China’s aerospace,high-speed train and other industrial undertakings,the demand for 7XXX series Al-Zn-Mg-(Cu)alloy is constantly increasing,and the performance requirements are also constantly improving.As a typical aging strengthening alloy,aging process and composition have a significant influence on the strength,corrosion performance,fatigue performance and other important properties of 7XXX series aluminum alloy,and most of these influences are reflected in the macroscopic properties through the microscopic structures.Therefore,the aging process,composition,microstructure,strength,fatigue property and corrosion resistance of 7XXX series Al-Zn-Mg-(Cu)alloy need to be systematically studied.In this paper,such as 7N01 7XXX series aluminum alloy as the research object,take the method of combining the theoretical simulation and test,the aging treatment and the change of composition on the effects of the alloy microstructures and macro properties.Key research objects including the RRA treatment of the alloys,alloy fatigue failure mechanism and performance change rule.By means of thermodynamic simulation,the composition change of 7XXX series aluminum alloys has obvious influence on the relevant thermodynamic parameters,such as activity,partial molar Gibbs free energy,the type and precipitation behavior of precipitated phases.For the Al-Zn-Mg alloy 7N01,the relative content of Zn and Mg will directly affect the type and content of precipitated phases in the alloys.The composition of the alloy also affects the precipitation sequence and quenching sensitivity of the precipitated phases.Different aging treatments were applied to 7N01 alloy.The results showed that the hardness of the alloy reached its peak at 120 ℃ for 24 h after one-step aging treatment,and the electrical conductivity of the alloy continued to increase with the extension of one-step aging time.For RRA treatment,the hardness of the alloy increases first and then decreases with the extension of retrogression time under different retrogression processes.The higher the retrogression temperature is the shorter the time for the hardness to reach the peak is.The hardness of 7N01 alloy in RRA185-40(120℃/24 h +185℃/40 min +120℃/24h)state is the highest of three RRA treatments.The strength test results showed that the tensile strength of the alloy increased first and then decreased with the aging time,but the elongation did not change much.The tensile fracture morphology of the alloy is mainly dimpled fractures.The microstructures of the UA(120℃/4h)state and RRA185-40 state 7N01 alloy was studied.DSC results showed that the precipitated phases in the UA alloy was mainly GP zones,and the precipitated phases in RRA185-40 alloy was mainly η′ phase and η phase.The type and size of the precipitated phases and the degree of coherency with Al matrix are different,and the ways of the dislocations through the precipitated phases are different,which will directly affect the static strength of the alloy.The fatigue test results show that for 7N01 alloy,the fatigue strength of RRA185-40 alloy is lower than that of UA alloy when the cycling number is lower,and the opposite is true when the cycling number is higher.The observation of fatigue fracture morphology shows that the microstructure of the alloy will influence the deformation behavior of the alloy.In other words,there is an internal relation between the heat treatment microstructure,deformation behavior,fatigue behavior and fatigue performance of the alloy.The stage I mode propagation of fatigue cracks is beneficial to improve the fatigue crack propagation capability of the alloy.Considering the various properties of the alloys,120℃/24h+185℃/40min+ 120℃/24 h was selected as the best RRA treatment process. |
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