| Al-Zn-Mg-(Cu)alloy is a heat-treatable aluminum alloy.It is widely used in aerospace,such as high-speed rail and subway,due to its excellent mechanical and welding properties.However,the corrosion resistance of Al-Zn-Mg-(Cu)alloy is poor.With the rapid development of aerospace and transportation field,people require better corrosion resistance alloy,especially the stress cracking corrosion resistance.Therefore,people use different aging process to improve the corrosion resistance of Al-Zn-Mg-(Cu)alloy.In this paper,the influence of different aging processess on the exfoliation corrosion(EXCO)and stress corrosion cracking(SCC)resistance of 7N01 and 7075 aluminum alloy was investigated by the slow strain rate test(SSRT)and EXCO test,using Transmission electron microscope(TEM),Energy Dispersive Spectrdmeter(EDS)and scanning electron microscope(SEM).It is shown that the three-step-aged alloy has better EXCO and SCC resistance,as compared with the traditional aged alloy.The relationship between the aging system,the microstructure and the corrosion resistance of the Al-Zn-Mg-(Cu)alloy was investigated,which can provide theoretical guidance for the industrial production.The main innovation points are summarized as follows:(1)7N01 aluminum alloy:The depth of EXCO of the three-stage aging 7N01 aluminum alloy,which is only 112?m,is less than the T4,T5 and T6 aluminum alloy.The Is of the three-stage aging 7N01 aluminum alloy,which is only 8.2%,is less than the T4,T5 and T6 aluminum alloy.Therefore the three-step-aged alloy has better EXCO and SCC resistance,as compared with the T4,T5 and T6 aluminum alloy.TEM shows that the grain boundary precipitates in the T4 state alloy are mainly composed of small Guinier-Preston(GP)zone and solute-atom-clusters.The T5 state alloy has coarse grain boundary precipitates(GBP)and precipitate free zone(PFZ);T6 state alloy has fine GBP.While the three-step-aged alloy can get improved corrosion resistance because of the more discrete and coarser GBP and wider PFZ.(2)7075 aluminum alloy:The depth of EXCO as the regression time goes by when the regression temperature is 160 ? and 200 ?.The depth of exfoliation corrosion has no apprent relationship with regression time when the regression temperature is 200 ?,220 ? and 240 ?.When the regression time is 10 min,the corrosion resistance rise and then decline as the regression temperature rise from 160 ? to 240 ?.The 200 ?/10min RRA treatment alloy have the best corrosion resistance,while the T6 treatment is the worst.The grain boundary precipitates of the T6 state are fine and continuously distributed,and the Cu element in the grain boundary is uniformly distributed.The heat treatment that RRA temperature is 160 ? and 200 ? has coarser GBP and Cu are enriched in the GBP;While the RRA heat treatment with regression temperature of 240 ? can get worse corrosion resistance because of the wider and lower solute atomic content in the PFZ.(3)The analysis of the process of the stress corrosion:At the beginning of stress corrosion,all the forms of corrosion are pitting corrosion.With the progress of experiment,the pitting degree deepen and the formation of cracks priority in the second phase(where the pitting corrosion take place),there are also little cracks on the A1 matrix.Then the necking appears,a lot of crack initiation and propagation and sample fracture failure.There are no pitting at the beginning of pull experiment,the necking and crack appears with the progress of experiment,then the sample fracture failure because of the cracks.The full immersion corrosion experiment:All the forms of corrosion are pitting corrosion on the full immersion corrosion experiment.There are only a little pitting appears on the surface of specimen,then the pitting deepened as the extension of time.Therefore,in order to improve the stress corrosion resistance of materials,it's ought to reduce the number of second phase particles in the materials. |
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