Effect Of Cu Content On Quenching Sensitivity Of High Strength Aluminum Alloys With High Zn/Mg Ratio

7XXX aluminum alloys exhibit low density,high strength and good plasticity,and therefore they have been used as structural materials in the aerospace and rail transit industry.These alloys are often quenching sensitive,and it leads to a significant decrease in properties from the surface to the core of thick plates and limits their application.In order to decrease quenching sensitivity and improve properties,the Zn/Mg ratio of7 XXX aluminum alloy has been increased.Cu is also a key element affecting the mechanical properties,corrosion resistance and quenching sensitivity.In this work,five 7XXX aluminum alloys sheets with high Zn/Mg ratio and different Cu content(0-2.6wt%)were designed and produced.The effects of Cu content on the grain structure,quenching sensitivity relative to mechanical properties before and after alternate immersion,intergranular corrosion and exfoliation corrosion of these alloys were investigated by immersion end-quenching test.The main conclusions are as follows:(1)The mechanism of Cu content affecting the grain structure was clarified.As Cu content in alloys increases from 0wt% to 2.6wt%,the recrystallization fraction after solution treatment increases from 17.3% to62.8%,and the average size of recrystallized grains increases from 12.2μm to 36.6μm.Based on the results of electron backscattering diffraction and energy dispersive spectroscopy,Fe-rich phase and η phase with sizes larger than 0.8μm can stimulate recrystallization nucleation.With the increase of Cu content,the dislocation density and the number of Fe-rich phase and ηphase with PSN effect increase in hot-rolled sheets,which leads to higher recrystallization nucleation rate and grain growth rate;as a result,the size and area fraction of recrystallized increase.(2)The effect of Cu content on quenching sensitivity relative to mechanical properties of alloys was investigated by immersion endquenching test and C-curves,and the mechanism is elucidated based on thermodynamic and kinetic results.As quenching rate decreases from400℃/s to 70℃/s,Cu content has little effect on quenching sensitivity.However,as quenching rate decreases from 70℃/s to 4℃/s,quenching sensitivity increases with Cu content increasing.With the increase of Cu content,the driving force and nucleation sites of quenching-induced phase increases.As a result,the types of quenching-induced phase increase fromη phase to η,T,S and Y phase,and their sizes increase and more solute atoms are consumed,leading to the increase of quenching sensitivity and the decrease of the critical transition time of C-curves.(3)Based on the quantitative results of scanning Kelvin atomic force microscopy,it is found that the potential difference between(sub)grain boundary precipitates and the matrix gradually increases with the increase of Cu content or the decrease of quenching rate.As quenching rate decreases,the corrosion form changes from pitting to intergranular corrosion during immersion in intergranular corrosion solution,and the exfoliation corrosion propagation path changes from only along grain boundariess to along both grain boundaries and subgrain boundaries,and the critical quenching rate of transformation increases with the increase of Cu content.(4)The mechanism of Cu content affecting quenching sensitivity relative to intergranular corrosion and exfoliation corrosion has been discussed primarily based on the features of precipitates free zones adjacent to grain boundaries,and the potential difference between grain boundary precipitates and the matrix.As Cu content increases from 0wt%to 2.6wt%,the intergranular corrosion resistance gradually decreases,while the exfoliation corrosion resistance first decreases and then increases,reaching the lowest at Cu content of 1wt%.As quenching rate decreases from 400℃/s to 4℃/s,quenching sensitivity relative to corrosion susceptibility is the lowest for Cu-free alloy but higher and similar for Cucontaining alloys.This is because due to the decrease of quenching rate,the increased rate of the width of precipitates free zones increases and the increased rate of the potential difference between grain boundary precipitates and the matrix first increases and then decreases as Cu content increases.(5)The quantitative relationship of quenching rate,the tensile properties,loss rate of tensile properties after alternate immersion and the corrosion depth was established,and the effect of Cu content on quenching sensitivity relative to tensile properties of these alloys after alternate immersion was revealed.It is found that the quenching sensitivity in alloys after alternate immersion increases with the increase of Cu content and is higher than that without immersion,and the increased value decreases with the increase of Cu content.This is because with the decrease of quenching rate,the quenching-induced corrosion form of alloys with low Cu content changes from pitting to intergranular corrosion,while the quenchinginduced corrosion form of alloys with high Cu content is intergranular corrosion and does not change.The effect of corrosion form on tensile properties is analyzed by digital image correlation.It is found that when the region around the pit is deformed,the stress state is changed from uniaxial tension to triaxial tension,and thus the yield strength increases in alloys with pitting;while the stress state is not triaxial tension in alloys with intergranular corrosion,and the yield strength decreases.