Study On The Effect Of Stress-aging Process On Microstructure And Properties Of Al-Zn-Mg-Cu Alloy

Al-Zn-Mg-Cu alloy(7xxx series)has the characteristics of low density,high specific strength and specific stiffness,and excellent formability,so it has become one of the most important structural materials in the aerospace field,while the rapid development of modern industry brings higher demands on its mechanical and corrosion resistance properties.Aging heat treatment is an important way to improve its overall performance,in which stress-aging heat treatment process can change the diffusion rate of atoms,affect the nucleation and growth rate of crystals through the combined effect of stress and temperature fields,and then change the microstructure and performance of the alloy,so it becomes the focus of scholars at home and abroad.However,Al-Zn-Mg-Cu alloys have a high risk of stress corrosion cracking,and there are few reports on the effect of stress-aging on their stress corrosion resistance.In addition,the effect of stress-aging on the microstructure and properties of 7055 aluminum alloy(AA7055)is still unclear.In this work,the effects of different stress-aging parameters(applied stress,aging time and temperature)on the corrosion resistance(intergranular corrosion,electrochemical corrosion and stress corrosion),mechanical properties and microstructure of AA7055 were studied,and the influencing mechanism was also discussed.The main conclusions are presented as follows:(1)The mechanism of stress-aging on the corrosion resistance of AA7055 was investigated.It was discovered that large-sized and discontinuous GBPs could effectively hinder the anodic dissolution of grain boundaries and reduce the tendency of hydrogen-induced cracking,while Cu enrichment at grain boundaries could reduce the potential difference between GBPs and matrix,improving the corrosion resistance.Relatively large aging stress,long aging time or high aging temperature can promote the coarsening of GBPs and the enrichment of Cu at grain boundaries,improving the SCC resistance.(2)The mechanism of stress-aging on the mechanical properties of AA7055 was investigated.The stress field was demonstrated to stimulate dislocation proliferation and aggregation,increase the nucleation sites,and facilitate the nucleation and growth of precipitated phases.Under stress-free aging treatment,the main strengthening phases were coarse η phases,and the matrix was strengthened by the Orowan bypass mechanism.While under stress-aging treatment,the number of precipitated phases increased while the average size decreased,the main strengthening phases were fine η’ phases,and the strengthening mechanism was the shear mechanism.Prolonging aging time and increasing aging temperature coarsened the matrix precipitates,and the strengthening phase transformation order was GP Ⅱ→η’ phase → η phase.Additionally,the strengthening mechanism also changed from the shear mechanism to the Orowan bypass mechanism,resulting in a drop in the strength.