| Al-Zn-Mg-Cu series aluminum alloys with super-strength have been widely used as main structural materials in aircraft.In order to meet the design requirements of the large-scale and integrated on aircraft structures,the development of high hardenability,super-strength,high toughness and corrosion resistance Al-Zn-Mg-Cu aluminum alloys by reducing Cu content has become the development direction of this series of alloys.However,the stress corrosion cracking（SCC）sensitivity of super-strength with low Cu content is prominent,which has become a bottleneck restricting development.Focusing on the development of high hardenability,super-strength,high toughness and corrosion resistance aluminum alloys with low Cu content,it is of great significance to systematically and deeply study the relationship between Cu content,process,microstructure and properties,seek the way to improve the SCC resistance of this alloy.Based on the fact that grain boundary precipitation state plays a leading role in SCC of Al-Zn-Mg-Cu aluminum alloys with super-strength,and its formation and evolution are not only controlled by aging,but also related to pre-deformation before aging and even pre-precipitation during quenching.The influence law and mechanism of heat treatment and pre-deformation on precipitates,mechanical properties and SCC properties of Al-Zn-Mg-xCu aluminum alloys were studied by mechanical properties and corrosion properties tests,combined with metallographic microscope（OM）,scanning electron microscopy（SEM）/transmission electron microscopy（TEM）microstructural examinations and X-ray diffraction and other characterization methods in this paper.The main contents and conclusions are as follows:（1）The influence law and mechanism of Cu content and quench-cooling rate on the microstructure,mechanical properties and corrosion resistance of Al-Zn-Mg-xCu aluminum alloy were studied.Reducing Cu content of rapidly quenched-aging alloys,the strength decreases and the elongation increases;the resistance to intergranular corrosion（IGC）and exfoliation corrosion（EXCO）enhances,SCC resistance decreases.With the decrease of quench-cooling rate,the size,spacing and Cu content of grain boundary precipitates（GBPs）increase,SCC resistance enhances,and reducing quench-cooling rate has more obvious effect on the improvement of stress corrosion resistance of alloy with the decrease of Cu content;the precipitate-free zones（PFZ）of grain boundary widens,the area fraction of GBPs increases,IGC and EXCO resistance decreases;the number of quench induced precipitates increases,the strength and elongation of alloy decreases,and with the decrease of Cu content,the loss of strength is gradually reduced due to the decrease of quench-cooling rate.The quench-cooling rate decreased from 150°C/s to 8°C/s,the ultimate tensile strength（UTS）of 0.9Cu,1.55Cu and 2.3Cu decreased by 1.4%,2.9%and 10.5%,respectively;EXCO ratings decreased from EA to EB+,EB-to EC+,EC-to ED+,respectively;the SCC propagation rate(VII)of plateau region of0.9Cu and 1.55Cu alloys decreased by about an order of magnitude,and the stress intensity threshold(KISCC)values increased by about 3.2 times and 2.6 times,respectively.The VII value for 2.3Cu alloy decreased from9.71×10-9 m/s to 3.51×10-9 m/s,the KISCC value only increased from 9.1MPa·m1/2 to 9.7 MPa·m1/2.（2）The influence law and mechanism of pre-stretching on the microstructure,mechanical properties and corrosion resistance of Al-Zn-Mg-xCu aluminum alloys were studied.When 3%pre-stretching is introduced before aging after quenching,the coarse residual crystalline phase in high Cu alloy is easy to be damaged during pre-stretching,the hardness and fracture toughness are decreased,IGC,EXCO and SCC resistance are reduced.Low Cu（0.9 wt.%～1.67 wt.%）alloys after 3%pre-stretching and aging promoted the precipitation of a fewηphases at dislocations,the strength/hardness of the alloy decreases slightly and the fracture toughness increases obviously;the area fraction of GBPs and the width of PFZ increases,IGC and EXCO resistance decreases;The formation of high-density dislocations promoted the diffusion of Cu element in the grain to the grain boundary,increased the size,spacing and Cu content of GBPs,the open circuit corrosion potential of the brittle section of liquid nitrogen grain boundary significantly positive shifted,and significantly improved the SCC resistance of low Cu alloys.Reducing the quenching rate and introducing 3%pre-stretching can synergistically improve the stress corrosion resistance of the alloy.Compared with slowly quenched samples,3%pre-stretching was more effective for fast-quenched samples on enhancing the SCC resistance.When the quenching rate is 150°C/s,the fracture toughness for 3%pre-stretched 0.9Cu and 1.67Cu alloy increased from 27.9 MPa·m1/2 to 31.1 MPa·m1/2,23.4 MPa·m1/2 to 25.7MPa·m1/2,respectively;EXCO ratings decreased from EA to EB,EB to EC+,respectively;the VII values decreased from 1.40×10-7 m/s to 3.2×10-8m/s,1.06×10-8 m/s to 3.6×10-9 m/s,the value of KISCC increased from 2.5MPa·m1/2 to 6.6 MPa·m1/2,9.3 MPa·m1/2 to 15.7 MPa·m1/2,respectively.When the quenching rate is 8°C/s,the VII values for 3%pre-stretched0.9Cu alloy decreased from 3.0×10-8 m/s to 8.7×10-9 m/s,the value of KISCCincreased from 7.3 MPa·m1/2 to 8.1 MPa·m1/2.（3）The influence law and mechanism of natural aging with cyclic tension-compression pre-deformation on the microstructure,mechanical properties and corrosion resistance of Al-Zn-Mg-xCu aluminum alloy was studied.After natural aging for 4 days and cyclic tension-compression pre-deformation induced the formation of fine and uniform precipitates near the grain boundary,which decreased the width of the PFZ at the grain boundary,promoted the diffusion of Cu atoms within the grain to the grain boundary,and formed coarse discontinuous GBPs with Cu enrichment.Cyclic tension-compression pre-deformation improves IGC,EXCO and SCC resistance of alloys under the same strength.After-10/+200 MPa cyclic tension-compression pre-deformation,the VII values for 0.9Cu and1.67Cu alloys decreased from 1.40×10-7 m/s to 5.89×10-8 m/s,1.06×10-8m/s to 3.91×10-9 m/s,the values of KISCC increased from 2.5 MPa·m1/2 to4.1 MPa·m1/2,9.3 MPa·m1/2 to 14.83 MPa·m1/2,respectively.（4）The influence law and mechanism of thermal-vibration aging on the microstructure,mechanical properties and corrosion resistance of Al-Zn-Mg-xCu aluminum alloy were studied.Compared with the traditional artificial aging,thermal-vibration aging treatment promotes the precipitation of uniform and fine intracrystalline precipitates,PFZ at the grain boundary disappears,induces the formation of coarse and discontinuous GBPs with Cu enrichment,and enhances the strength and elongation of alloys,as well as the resistance to IGC,EXCO,SCC.The UTS of 1.67 Cu alloy after thermal-vibration aging treatment increased from 630 MPa to 650 MPa,the yield strength increased from 610 MPa to638 MPa,and the elongation increased from 7.1%to 8.8%.The VII values for 0.9 Cu and 1.67 Cu alloys decreased from 1.4×10-7 m/s to 1.05×10-8m/s,1.06×10-8 m/s to 1.39×10-9 m/s,respectively,and the KISCC values increased from 2.5 MPa·m1/2 to 7.7 MPa·m1/2,9.3 MPa·m1/2 to 18.5MPa·m1/2,respectively.（5）Compared with reducing the quench-cooling rate and introducing3%pre-stretching,the effect of applying thermal vibration aging on enhancing the SCC resistance of Al-Zn-Mg-xCu alloy in normal salt solution is more obvious,and it can also enhance the IGC resistance,EXCO resistance and aging strengthening effect.By adding rare earth corrosion inhibitor can modify the passive film,the stress corrosion resistance of the alloy can be improved synergistically superposed with thermal vibration aging and 3%pre-stretching.After 3%pre-stretching and thermal vibration aging with rare earth salt solution treatment respectively,the VII values for 0.9 Cu and 1.67 Cu alloys decreased from 8.92×10-9 m/s to 3.14×10-9 m/s,1.02×10-9 m/s to 3.77×10-10 m/s,respectively,and the KISCC values increased from 8.7 MPa·m1/2 to 10.7 MPa·m1/2,17.2 MPa·m1/2to 22.8 MPa·m1/2,respectively;the SCC life extended from 48 h to 182 h,164 h to 412 h,respectively;while the SCC life of alloys superposed with the primer containing rare earth salt are extended from 347 h to 521 h and796 h to 1162 h,respectively. |
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