Effect Of Creep Deformation Processes On The Fatigue Crack Propagation Of 7075-T6 Alloy Sheets

Al-Zn-Mg-Cu alloy has low density,high strength,good formability and corrosion performance,and has become one of the indispensable aluminum alloy materials in aerospace and rail transportation.However,the Al-Zn-Mg-Cu alloy will generate a large residual stress during the forming and quenching process.For some components that cannot be pre-stretched,creep has become a good way to reduce internal stress.In this paper,the microstructure and residual stress were characterized by metallographic microscope,X-ray diffractometer,scanning electron microscope and other instruments,and the second phase,Influence of geometrically necessary dislocation densities at grain boundaries and within grains during fatigue crack growth.The results are as follows:(1)Creep effectively reduces the residual stress of the 7075-T6 alloy sheet.Compared with the T6 sample,the residual stress reduction of the sample with a creep curvature of R=300 mm(CA300)is the most obvious,reaching 75.1 %,The residual stress of the samples with creep curvature of R=600 mm(CA600)and 1800 mm(CA1800)was reduced by 66.2 % and33.4 %,respectively.Finally,the T7 sample has a 44.9% reduction in residual stress.The results show that the artificial obsolete effect partially reduces residual stress,but the effect of creep is more obvious,and in the range of creep curvature R=300 mm to R=1800 mm,the smaller the creep curvature is,the more obvious the residual stress reduction is.(2)The second phase,grain boundary and intragranular dislocation density have a greater impact on the fatigue crack growth.The larger second phase is prone to secondary cracks,acting as a bridge between cracks and accelerating fatigue crack growth.The angle difference between the activated slip surfaces of adjacent grains and the activated slip system is an important factor in determining the resistance of fatigue crack through grain boundaries.The samples after creep have improved grain boundary resistance and reduced fatigue crack expansion rate.The geometrically necessary dislocation density of the grains penetrated by fatigue crack is reduced,and the residual stress is well relieved.(3)In the range of creep curvature R=300 mm to R=1800 mm,reducing the creep curvature can improve the comprehensive properties of the 7075-T6 aluminum alloy sheet.The CA300 sample with the smallest creep curvature has the largest resistance to fatigue crack when it penetrates the grain boundary,and the probability of deflection within the grain is also higher.As the dislocation density increases,the dislocation density distribution is the most uniform,which further reduces the tendency of crack to propagate in a certain direction,Therefore,it has the lowest fatigue crack growth rate.