Effect Of Alloying Elements On Quench Sensitivity And Stress Corrosion Resistance Of Al-Zn-Mg-Cu Alloy

Al-Zn-Mg-Cu（7xxx）series aluminum alloys are widely used in the aerospace field because of their high specific strength,low density,and easy processing.With the rapid development of the aerospace industry,higher requirements have been put forward for the lightweight and safety of spacecraft and aircraft,which has promoted the development of aluminum alloy structural parts in the direction of large-scale and integrated.However,the thick aluminum alloy used in large structural parts has quench sensitivity,which leads to uneven mechanical properties of the components.At the same time,the aircraft will be affected by stress corrosion during service,resulting in failure of aluminum alloy structural parts.Therefore,to further expand the application of 7 series high-strength aluminum alloy in the aerospace industry,it is urgent to reduce its quench sensitivity and improve its stress corrosion resistance.This thesis used 7085 aluminum alloy as the basic alloy and designed 5 alloy components.Through the end quenching experiment,combined with characterization techniques such as metallographic microscope,scanning electron microscope,transmission electron microscope,and testing methods such as microhardness and stress corrosion,to study the effect of alloying elements on the quench sensitivity and stress corrosion resistance of Al-Zn-Mg-Cu alloy and found that:（1）On the basis of 7085 aluminum alloy,increase the content of Zn element to obtain high zinc alloy.At the quenching end,the high quenching rate results in the precipitation of high-densityη’strengthening phase after aging,and theη’strengthening phase density and alloy hardness value increase with the increase of Zn content;At the quenching end,the quenching rate is greatly reduced,so a large amount ofηphase precipitates and its density increases with the increase of Zn content,resulting in a decrease in the depth of the alloy’s quenched layer.Therefore,increasing the Zn element content will increase the quench sensitivity of the alloy.（2）On the basis of 7085 aluminum alloy,increase the Zn element and adjust the Cu element content to obtain a high-zinc-low-copper and high-zinc-high-copper alloy.At the quenching end,the low quenching rate leads to the formation of more T phases（Al Zn Mg Cu）in high copper alloys.The T phase consumes solute atoms and reduces the precipitation density of theη’strengthening phase,thereby increasing the quench sensitivity of the alloy.Increasing the Zn content while reducing the Cu content can reduce the precipitation of T phase and reduce the quench sensitivity of the alloy,and its hardenability is better than that of7085 alloy.（3）On the basis of 7085 aluminum alloy,Sc element is added to obtain an alloy containing scandium.The fine Al₃（Sc,Zr）particles dispersed in the alloy can effectively pin the grain boundaries and sub-grain boundaries,and play a role in inhibiting recrystallization and promoting grain refinement.Therefore,the scandium-containing alloy has a high-fine-grain strengthening effect and micro-hardness.At the quenching end,the low quenching rate causes grain boundaries,subgrain boundaries and Al₃（Sc,Zr）particles to provide nucleation sites for theηphase,and Al₃（Sc,Zr）particles promote the precipitation of the Y phase（Al Cu Zn）.There are obvious precipitation-free zones around this phase,resulting in the reduction of the depth of the alloy’s hardened layer,so the addition of 0.2 1 wt%Sc will increase the quench sensitivity of the alloy.（4）The aluminum alloy thick plate was rolled to 4.5 mm,and the stress corrosion resistance of the alloy was further studied.It is found that increasing the Zn content will increase the potential difference between the grain boundary precipitation phase and the matrix,and reduce the alloy’s stress corrosion resistance;increasing the Cu content will reduce the potential difference between the grain boundary precipitation phase and the matrix,and improve the alloy’s stress corrosion resistance;while adding 0.21 wt%Sc will cause the intermittent distribution of precipitated phases at grain boundaries,effectively cut off corrosion channels,and improve the alloy’s stress corrosion resistance.