Research On Influence Of Solution And Aging Heat Treatment On 7A99 Aluminum Alloy

As a new type high strength Al-Zn-Mg-Cu alloy,7A99 Al alloy has important application potential in national defense and military industry.Now,there are few researches on extrusion,solution and aging heat treatment of 7A99 Al alloy.The precipitation sequence during aging and its effect on mechanical properties and corrosion resistance has not been clarified,therefore,it is hard to provide the theoretical basis for the formulation of heat treatment schedule.In this work,single-stage aging,double-stage aging,regression and re-aging(RRA)were carried out for 7A99 Al alloy,and the cold-rolling was introduced before or after solution to further clarify the feasibility of improving the comprehensive mechanical properties.The results of this paper can enrich the basic theory and method of heat treatment of 7xxx Al alloy,helping to improve the mechanical properties and corrosion resistance of the alloy,and providing a new prospect and theoretical guidance for the preparation of high strength Al alloy with excellent comprehensive properties,which has important theoretical significance and engineering application value.Here are the main contents and conclusions:(1)Under-aging,peak-aging and over-aging were carried out for 7A99 Al alloy,the effects on microstructure,mechanical properties and stress corrosion cracking were studied.The results indicate that:the secondary phases were refined and dispersed along ED during extrusion process.Then,MgZn2 phases dissolved after solution,and the insoluble Al7Cu2Fe was remained.GPI zones and ?' phase mainly appeared in both under-aged and peak-aged states,stable ? with large size formed and became dominant in over-aged state as aging time was extended.The precipitation sequence of 7A99 Al during aging is as follows:solid solution?GP zones????.The hardness first increases with the prolonging aging time,then decreases when reaches the top.The peak-aged sample showed good combination of a high UTS of 622 MPa and a medium elongation of 10.2%.The under-aged and over-aged samples exhibited the relatively lower strength and higher elongation.2%pre-stretching prior to aging further improved the YS,UTS,and elongation.The under-aged and peak-aged samples were susceptible to SCC with intergranular cracking.The over-aged sample showed the best SCC resistance due to the presence of coarse and discontinuous GBPs.(2)The cold-rolling with 10%reduction before or after solution was introduced,peak-aging and two-stage aging experiments were carried out respectively,the effects of cold-rolling and double-stage aging on microstructure,mechanical properties and corrosion resistance of a novel high strength 7A99 Al alloy has been studied.The results indicate that:With the comprehensive effect of fine grain strengthening,solid solution strengthening,precipitation strengthening and dislocation strengthening,the hardness and tensile strength of the alloy can be improved when the cold rolling was before solution.The yield strength and tensile strength of"cold-rolling+solution+peak-aging" sample reach 628MPa and 665MPa respectively,and the elongation is 11.62%.After double-stage aging,the hardness and tensile strength of the sample decrease by only 3%?5%.The corrosion resistance can be improved when the cold-rolling was after solution,and the double-stage aging can further improve the corrosion resistance of the alloy.Pitting corrosion mainly originated from the second phase of high potential compared to the matrix,such as Al23CuFe4 or Al7Cu2Fe.The improved corrosion resistance of samples subjected to double-stage aging or "solution+cold-rolling+peak aging" treatment is mainly attributed to grain boundary precipitates with high Cu content,larger size and discontinuous distribution.Furthermore,the sub-grain boundaries can also release the stress concentration,which is beneficial to the further improvement of SCC resistance.Generally,the strength of alloy can be can be significantly improved when the cold-rolling was introduced before solution,and the strength loss can be reduced.The introduction of cold-rolling after solution was beneficial for the improvement of corrosion resistance.(3)The cold-rolling with 10%reduction before solution was introduced,retrogression and re-aging(RRA)experiments were carried out,effects of cold-rolling and RRA on microstructure,mechanical properties and corrosion resistance of 7A99 Al alloy has been studied.The results indicate that:The hardness and tensile strength of the samples can be summarized as follows:"peak aging>Cold-rolling+RRA>RRA".The yield strength and tensile strength of T6 sample reach 621MPa and 668MPa respectively,and the elongation is 10.5%.RRA and "Cold-rolling+RRA"samples possess the best corrosion resistance,which can be attributed to grain boundary precipitates with high Cu content,larger size and discontinuous distribution.The development of pitting corrosion and stress crack corrosion were effectively inhibited by a large number of small angle grain boundaries and sub grain boundaries,increase the corrosion potential of the alloy and reduce the corrosion current density.The "Cold-rolling+RRA" sample is worth noting that,compared with peak aging,not only the corrosion resistance was effectively improved,but also has higher tensile strength and elongation,which is an effective method to improve the comprehensive properties of 7A99 aluminum alloy.