Effect Of RRA Treatment On Microstructure And Corrosion Resistance Of Al-6Zn-2Mg-2Cu Alloy

With the rapid development of aerospace,transport and other industries,the application of ultra high strength Al-Zn-Mg-Cu alloys are more extensive,and the higher comprehensive performance of these alloys are required.Although the Al-Zn-Mg-Cu alloys have good mechanical properties,especially high specific strength and specific stiffness,the corrosion resistance of the alloy is relatively poor.However,the reasonable RRA treatment can improve the corrosion resistance and make the comprehensive performance better.The object of this research is Al-6Zn-2Mg-2Cu alloy after different aging treatments.The microstructure of the alloy was observed by optical microscopy,scanning electron microscopy and transmission electron microscopy.The various properties of the alloy were tested by hardness,conductivity and tensile experiments;exfoliation corrosion,intergranular corrosion and electrochemical corrosion experiments.Analyzing the changes of the morphology and size of grain boundary precipitatess(?),the effects of the retrogression and re-ageing treatments(RRA)on the mechanical properties and corrosion resistance of Al-6Zn-2Mg-2Cu alloy were systematically studied,so as to optimize RRA treatment.The results show that the better double-stage aging treatment of Al-6Zn-2Mg-2Cu alloy is 120?/8h+165?/14h.Compared with the alloy at T6temper,the strength and hardness of the alloy after this treatment decrease obviously,and the conductivity is 38.6%IACS,while it has good corrosion resistance.After RRA treatment,large quantities of GP zones and?'phase dispersed in the matrix,the microstructure is similar to that of alloy at T6 temper.And the grain boundary is similar to that of alloy at T7X temper,containing large and discontinuous?phase.After RRA treatment,the hardness and strength of the Al-6Zn-2Mg-2Cu alloy are similar to those of alloy at T6 temper,higher than those of alloy at T73 temper.The hardness of the alloy at RRA190?/40min treatment is 172HV which reaches the peak value;meanwhile the tensile strength and the yield strength are good,as609MPa and 557MPa respectively.After RRA190?/40min or RRA170?/120min treatments,the conductivity of the alloys are more than 38%IACS.After different aging treatments,the intergranular corrosion resistance of the Al-6Zn-2Mg-2Cu alloy from low to high is as follows:T6,RRA190?/20min,RRA170?/60min,RRA190?/40min,RRA170?/120min,RRA190?/60min,RRA170?/180min,T73.The intergranular corrosion depth of the alloy at T6treatment reaches 146?m which is the deepest,while the corrosion depth of the alloy at RRA170?/180min treatment is 53?m.After 120?/24h+190?/40min+120?/24h or 120?/24h+170?/120min+120?/24htreatments,thealloyshowsgoodcomprehensive performance.The final exfoliation corrosion grade are both EB~+,the corrosion current density is 8.902?A·cm~-22 and 8.225?A·cm~-22 respectively.