| Al-Zn-Mg-Cu series alloy are widely used as the structural material in the aerospace and transportation industry,due to their good comprehensive properties.The RRA(retrogression and re-aging)process allows the alloy to balance the strength and the corrosion performance.The short-term retrogression treatment of traditional RRA process limits the application of its industrial thick plate.In this paper,the influence of the retrogression process on the microstructure and the properties of the aging alloy are studied.The relationship between the microstructure and the corrosion resistance of the alloy is established by studying the intergranular corrosion and exfoliation corrosion behavior of the alloy.The hardness,strength and conductivity of 7150 aluminum alloy are studied by adjusting the retrogression temperature,hardness,conductivity and tensile properties.The results show that: After retrogression treatment of 7150 aluminum alloy at 170℃,as we extent the retrogression time,the hardness curve first reduced to the valley that is 183.1HV at 50 min,and then increased to the peak that is 194 HV at 140 min,and then reduced monotonically,The hardness of the alloy was 190 HV at470min.The lower retrogression temperature is 170℃,which results in the trend of the hardness of the alloy changing more slowly.The hardness and conductivity of the alloy increase monotonously with the extension of the retrogression time.The alloy is able to obtain the best match with hardness,strength and electrical conductivity.Microstructure observation shows that:After pre-aging treatment(T6 peak aging),the alloy precipitated intracrystal precipitation of the G.P.region and η’phase of 2.3-7.62 nm.G.P.region and the smaller η’ phase return to the solution and the larger η’ phase grows on the early retrogression stage.With prolonging the retrogression time,the re-nucleating η’ phase and the existing η’ phase continue to grow.With Continuousing retrogression treatment,precipitation of the second phase particles into the Ostwarld ripening stage.The effects of aging degree on the surface potential distribution and grain boundary precipitates are analyzed by atomic force microscopy(AFM)and transmission electron microscopy(TEM).With the increasing of the retrogression time,the continuous chain of the anodic grain boundary precipitates grows and coarsens and arranges intermittently.The decrease of Cu content in the grain boundary precipitates and the decrease of the potential difference between the residual second phase and the matrix are the main reasons for the improvement of the corrosion resistance of the alloy.The corrosion behavior of the alloy was studied by immersion method,polarization curve and electrochemical impedance spectroscopy.The exfoliation corrosion development process,the surface morphology of the intergranular corrosion and the maximum depth of the intergranular corrosion pits are observed.With the increase of the retrogression time at the same retrogression temperature,the corrosion resistance of the alloy is reduced and the RRA170-470 aging alloy exfoliating corrosion grade is the PC level.Intergranular corrosion of7150 aluminum alloy surface distributes different sizes of corrosion pits and corrosion gully.With the increase of the retrogression time at the same retrogression temperature,the maximum corrosion depth of the intergranular corrosion of the alloy decreases.The maximum corrosion pit depth of intergranular corrosion of the RRA170-290 aging alloy is 52.8μm. |
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