Effect Of Surface Nanocrystallization Induced By Ultrasonic Rolling On The Corrosion Behavior Of 7075 Aluminum Allo

7000 series high-strength Al alloys are widely used in the aerospace,transportation,and machinery manufacturing industries due to their low density,high specific strength,and good workability.However,7000 series Al alloys are prone to corrosion in chloride environments,which largely limits their application.Generally,the corrosion of Al alloy parts originates from the surface of these materials.Local corrosion phenomena such as pitting corrosion,intergranular corrosion,and exfoliation corrosion are sensitive to the microstructure and performance of material surfaces.Therefore,a second method for improving corrosion resistance is the modification of component surfaces,which can prolong the service life of Al alloy components.In this work,ultrasonic surface rolling process(USRP)was used to modify the surface of 7075-T6 Al alloy.The effects of different ultrasonic rolling passes on the surface state(surface roughness,surface residual stress),surface microstructure evolution and corrosion resistance of the alloy were analyzed.The relationship between the surface state,surface microstructure,and corrosion resistance of 7075-T6 Al alloy was analyzed in detail.The main research contents and conclusions are as follows:(1)In this paper,laser confocal microscopy,X-ray residual stress instrument,transmission electron microscopy(TEM),etc.were used to analyze the effects of different passes of USRP treatment on the surface state and surface microstructure evolution of aluminum alloys.The research results show that the surface state and surface microstructure of 7075 aluminum alloy components can be regulated by changing the ultrasonic rolling pass.After 3 passes of USRP treatment,the surface roughness of the aluminum alloy was significantly reduced and compressive residual stress was introduced.As the number of ultrasonic rolling passes increases to 7,the surface roughness increases,the compressive residual stress increases,and the grains in the outermost layer can be refined to the nanometer level,forming a nano-gradient structure.Large plastic deformation occurred on the surface of this sample,which led to the dissolution of its precipitates and the disappearance of its PFZ.(2)In this paper,the effects of untreated and different passes of USRP treatment on the corrosion resistance of 7075 aluminum alloy were compared through immersion experiments and electrochemical tests,and the relationship between the surface state and surface microstructure and corrosion behavior of 7075 aluminum alloy was discussed in detail.The research results show that after 3 passes and 7passes of USRP treatment,the average corrosion depth of the samples is reduced by75.39% and 83.11%,respectively,and the corrosion rate is reduced by 79.79% and92.88% compared with the untreated samples,respectively.The effects of surface roughness,residual stress,surface nanocrystallinity,and precipitates on the corrosion resistance of 7075 Al alloy after USRP treatment were studied by stress relaxation and surface roughness analysis.The results show that after 3 passes of USRP treatment,the reduction of surface roughness and the increase of surface residual compressive stress greatly improve the corrosion resistance of 7075 aluminum alloy.When the ultrasonic rolling passes are increased to 7 passes to refine the grain size of the surface of the aluminum alloy to the nanometer scale,the larger number of grain boundaries caused by surface grain nanocrystallization led to the rapid enrichment of passive elements,which formed a dense passive film on the alloy surface.At the same time,the dissolution of precipitates and the disappearance of precipitation-free zones(PFZ)reduced the corrosion caused by anodic dissolution.The two work together to further improve the corrosion resistance of the material.(3)In this chapter,the effect of microstructure changes induced by different ultrasonic rolling passes on the stress corrosion resistance of 7075 aluminum alloy was studied.The research results show that: compared with the untreated samples,the surface roughness first decreases and then increases with the increase of rolling passes,and the residual compressive stress increases with the increase of rolling passes.After9 passes of USRP treatment,the surface grain size was refined to nanoscale,forming a gradient nanostructure.Through slow strain rate tensile experiments,it was found that 7075 aluminum alloy exhibited high stress corrosion susceptibility in 3.5% Na Cl solution.After 3 passes and 9 passes of USRP treatment,the stress corrosion susceptibility index of the samples decreased by 10.0% and 13.3%,respectively,compared with the untreated samples.The results show that the stress corrosion resistance of the samples after 3-pass and 9-pass USRP treatment is significantly improved.Among them,the corrosion resistance of the UR9 sample with nanocrystalline surface layer is more significantly improved.