Research On Microstructure And Corrosion Preperties Of Al-Zn-Mg-Cu Alloy In Strain Ageing Process

Al-Zn-Mg-Cu alloy is widely used in the aerospace field because of its excellent specific strength.The alloy is often subjected to stress field and temperature field coupling during thermal processing.This article selects the commercial 7050 alloy rolled plate as the research object and performs aging 6h,12h,18h at 0%,3%,6%,9%,12%,18%compressive strain respectively,then the evolution of the microstructure was characterized by metallographic microscopy,transmission electron microscopy(TEM)and DSC analysis during the ageing process,uses the molecular dynamics software(LAMMPS)to calculate the diffusion kinetic parameters of solid solution atoms of the alloy under strain aging,the effect of Al-Zn-Mg-Cu alloy under different strains on the aging precipitation behavior was investigated,the difference between mechanical properties and corrosion properties of aluminum alloy specimens after different strain aging was analyzed,describes the effects of strain aging on the microstructure and corrosion properties of Al-Zn-Mg-Cu alloys at last.The main conclusions are as follows:(1)The metallographic photographs show that after compressive strain aging of Al-Zn-Mg-Cu alloy with different deformations,crystal grains in the sample are elongated and larger,and the solute atoms are largely desolved along the grain boundary to form many enrichment zones.The amount of enrichment zone in the unstrained aging alloy sample is the most,while that of the strain aging sample decreases first and then increases with the increasing of the strain.And strain aging can promote the precipitation of the precipitation phase(?'phase)in the intragranular and grain boundaries of the alloy sample through TEM and DSC analysis.At the same time,the size of the precipitate phase increases with the increasing of the strain,and the width of Precipitation Free Zone is also increasing.(2)The hardness of Al-Zn-Mg-Cu alloy increases with the increasing of the strain,and the conductivity of the alloy decreases first and then increases due to strain aging.At the same time,based on the Avrami formula,the precipitation kinetics equations for strain aging of Al-Zn-Mg-Cu alloy at 120? were deduced from the conductivity:when the strain is less than 9%,f=1-exp(-0.0141t0.5214-0.1?);when the strain is greater than 9%,f=1-exp{-(0.0141-0.01s)t0.521 }.(3)The diffusion coefficients of Zn,Mg and Cu atoms in Al-Zn-Mg-Cu alloys after different strain ageing are calculated using LAMMPS.It is found that the diffusion coefficients of all three kinds of atoms increase with the increasing strain,and the order of the diffusion coefficients of the solid-solution atoms in the alloy is Zn>Mg>Cu.The simulation results show that under strain and aging conditions,Zn and Mg atoms are much easier to cluster out than Cu atoms.The Zn and Mg atoms are the main components of the precipitated phase ?' phase.It can be known from the molecular dynamics simulation that the precipitation of ?' phase in the alloy is promoted after strain aging.(4)After strain aging,the coarse intragranular and grain boundary precipitated phases in the Al-Zn-Mg-Cu alloy are the main reasons for reducing the corrosion resistance of the alloy.The increasing of the strain will promote the growth of the intragranular and grain boundary precipitated phase in the alloy,and the coarsening of the PFZ,so that the disparity in electric potential is increased and the corrosion of the grain boundary is aggravated.The results of electrochemical parameters analysis of polarization curves and AC impedance spectra show that the corrosion resistance of alloy specimens with 3%strain aging for 6 hours is the best,and that with 18%strain aging for 18 hours is the worst.