Effect Of Deep Cryogenic Treatment On Microstructure And Corrosion Resistance Of Al-Zn-Mg-Cu Alloy

7075 aluminum alloy is widely used in the field of aerospace because of its excellent mechanical properties.However,due to its high sensitivity to all kinds of corrosion,such as pitting corrosion,intergranular corrosion and exfoliation corrosion,the application of 7075aluminum alloy is limited to a certain extent.Heat treatments are usually used to modify alloy structure,such as T6 treatment,retrogression and re-aging treatment.However,these processes are insufficient or difficult in operation.Deep cryogenic treatment has advantages of simple operation,low price and improving heating process.Therefore,deep cryogenic treatment is used to enhance corrosion resistance of high-strength aluminum alloy.In order to study the effect of cryogenic treatment on microstructure and corrosion resistance of 7075 alloy,the processes of deep cryogenic treatment combined with T6 treatment,cyclic deep cryogenic treatment,retrogression and re-aging treatment were carried out respectively.The mechanical and corrosion properties of the alloy were studied by hardness test,intergranular corrosion test,exfoliation corrosion,polarization curve test and impedance test.The microstructure and corrosion morphology of the alloy were observed by transmission electron microscope and scanning electron microscope.The corrosion resistance of 7075 alloy was studied by combining deep cryogenic treatment with T6 temper.The results showed that the addition of deep cryogenic treatment ensured high hardness and improvement of corrosion resistance of alloy.The optimum time for deep cryogenic treatment was 12 h,the strengthen phases were distributed evenly,and grain boundary precipitates were discontinuous.The hardness of alloy was 187.6 HV and intergranular corrosion depth was 28μm.The alloy obtained better comprehensive property.Changing cycle times of deep cryogenic treatment to further explore effect of deep cryogenic treatment on corrosion resistance of 7075 alloy.The results showed that after cycle twice of deep cryogenic treatment,the number of matrix precipitates increased significantly and dislocations were generated.The grain boundary precipitates were broken chains,and precipitate-free zone become wider.The hardness of alloy was 191.6 HV,exfoliation corrosion rate was P.The mechanical property and corrosion resistance of the alloy were significantly improved.In order to explore changes of corrosion process of 7075 alloy,deep cryogenic treatment was introduced into different stages of retrogression and re-aging treatment.The results showed that deep cryogenic treatment after retrogression changed electrochemical corrosion process of alloy.The uniform distribution of matrix precipitates on the surface reduced large-area pitting corrosion and possibility of intergranular corrosion extending into alloy.The space between adjacent grain boundary precipitates were large and reduced corrosion rate.The corrosion current and corrosion rate of the alloy was 0.006 m A·cm^-2 and 0.197 mm/a,respectively,intergranular corrosion depth was 25μm.The corrosion sensitivity of the alloy was greatly reduced.