The Effect Of Aging On Microstructure And Properties As Well As The Corrosion Behaviour Of The Al-Zn-Mg-Cu-Mn-Sc-Zr Alloy

Al-Zn-Mg-Cu-Mn-Sc-Zr alloy not only has high strength and plasticity,but also has excellent corrosion resistance and welding properties,which has great prospects for application in the aerospace field.With the rapid development of the aerospace industry,higher requirements are put forward for the comprehensive properties of aluminum alloy,how to solve the problem that the strength and corrosion resistance of the alloy are mutually constrained and difficult to match together has become the focus of research on high-strength and corrosion-resistant aluminium alloys.In this paper,the effects of different aging systems on the mechanical properties,microstructure and corrosion behavior of Al-Zn-Mg-Cu-Mn-Sc-Zr alloy were studied by means of room temperature tensile property,hardness,conductivity,corrosion performance test and the equipments of metallurgical microscope（OM）,scanning electron microscope（SEM）,transmission electron microscope（TEM）and electron probe microanalyzer（EPMA）.It provides some experimental basis for the alloy to obtain good comprehensive properties and industrial application.The microstructure and properties of the alloy under peak-aging（120℃/24 h）,double-stage aging（120℃/6 h+140℃/20 h）and retrogression and re-aging（120℃/24 h+160℃/30 min+120℃/24 h）were studied.The results showed that:under the peak-aging treatment,the grain interior precipitates were a large number of fine GP zones andη’phases,the alloy had the highest strength and hardness;under the double-stage aging treatment,the grain interior precipitates were mainly composed ofη’phase and stableηphase,and the alloy had the lowest strength and hardness;under the retrogression and re-aging treatment,the grain interior precipitates were GP region andη’phase with slightly larger size,and the alloy had good comprehensive properties with tensile strength and hardness of 547 MPa and 168.5 HV respectively,elongation and conductivity of 13.4%and 34.3%IACS respectively.The microstructure and properties of the alloy were studied under different retrogression temperatures（140～200℃）and time（15～45 min）.The results showed that:with the increased of retrogression temperature and the extension of time,grain interior precipitates gradually grew and changed from GP zone andη’phase toη’phase andηphase;the strength and hardness of the alloy gradually decreased.The optimum retrogression system is 160℃/30 min,under this treatment,the strength and hardness of the alloy have almost no loss,and the elongation and relative conductivity are improved.The intergranular corrosion,exfoliation corrosion,electrochemical corrosion and stress corrosion behavior of the alloy under different aging systems were studied.The results showed that:under peak-aging treatment,theηphase at the grain boundary is fine and continuous,when corrosion occurred,it became an active anodic channel,resulting in rapid destruction of the grain boundary and the worst corrosion resistance of the alloy;under double-stage aging treatment,theηphase at the grain boundary was larger and discontinuous,the precipitate free zones（PFZ）was wider,which will block the intergranular corrosion path and inhibit the anodic dissolution process,the corrosion resistance of the alloy was better;under the retrogression and re-aging treatment,the size of theηphase and the width of the PFZ were between those of the peak-aged and double-stage aged alloys,therefore,the corrosion resistance was between the peak-aged and double-stage aged alloys.With the increased of retrogression temperature and time,theηphase was coarsened and distributed discontinuously,and the PFZ become wider gradually,leading to increasing corrosion resistance of the alloy.When pitting occured in the peak-aged alloy,the anodic Al matrix located around the cathodicα-Al（Fe,Mn）Si particles was preferentially dissolved and the matrix elements were uniformly corroded,producing corrosion products such as Al（OH）₃,Zn（OH）₂and Mg（OH）₂.With the aggravation of pitting corrosion,the contact area between the matrix and the particles was greatly reduced andα-Al（Fe,Mn）Si particles gradually fell off,forming numerous pitting pits in the alloy surface.