Research On Aging Heat Treatment And Adaptability With Hot Assembly Process Of 7a56 Aluminum Alloy Forging

Al-Zn-Mg-Cu alloy is widely used in the aerospace industry because of its high specific strength,good fracture toughness and good resistance to stress corrosion.This paper do some research based on the application background of adapting the aging heat treatment process of 7A56 aluminum alloy forgings to the hot assembly process.This paper uses thermal exposure to simulate the hot assembly process based on the study of aging precipitation behavior of 7A56 aluminum alloy forgings.The changes of microstructure and properties of the alloy during heat exposure under typical aged alloys and lightly aged pretreated alloys are studied,which provides theoretical support and experimental data for the selection and optimization of the pretreatment process of 7A56 aluminum alloy forgings in subsequent applications.The main research contents are as follows:Aging heat treatment process of 7A56 aluminum alloy forgings is studied,which lays the foundation for the subsequent adaptation with thermal assembly.It is found that the remaining second phases in the deformed structure of 7A56 aluminum alloy forgings are Mg(Zn,Cu,Al)2 phase and Fe-rich phase.The appropriate solution heat treatment is determined to be 470℃/1 h and the second phase has a good re-dissolution effect.During the single-stage aging process of the alloy at 120℃,the size of the intracrystalline precipitated phase increases,the size distribution range becomes wider with the extension of the aging time.Meanwhile,the main precipitated phases change to GPII-zones,η’ phase andη phase from GPI-zones,GPII-zones and η’phase.In the two-stage aging,as the second-stage aging temperature increases,the yield strength of the alloy can reach the peak point faster.As the second-stage aging time increases,the strength properties of the alloy show a trend of first increasing and then decreasing.When the second-stage aging temperature is 160℃,with the extension of the aging time,the size of the precipitated phase increases,and the size distribution range becomes wider.The main precipitated phase changes from the GPII zone,η’ phase and η phase to η’ phase and η phase.On this basis,the specific process parameters of the typical aging T6,T79,T76 and T74 are determined.The evolution of structure and properties of typical aged 7A56 aluminum alloy after thermal assembly process is researched.After short-term thermal exposure of samples the strength of T6 alloy increases and the main precipitated phases are still GPII zone,η’phase and η phase.As the degree of thermal exposure deepens,the hardness decreases,the electrical increases and the strength decreases significantly.The main precipitated phases transform into η’ Phase and η phase.After short-term thermal exposure,the strength of the T79 alloy decreases,and the main precipitated phases are still GPII zone,η’ phase and ηphase.After complete thermal exposure,the strength further decreases and the main precipitation are η’ phase and η phase.After complete thermal exposure,the strength of T76 and T74 alloys decreases and the main precipitated phases are η’ phase and η phase.During the thermal exposure process of typical aged alloys,the size of the precipitated phases all increase.T6 and T79 alloys have a higher tensile strength after complete heat exposure,reaching 530 MPa.The change rule of the microstructure and properties of lightly aging pretreated alloys during the heat exposure process was studied.After a short-time thermal exposure of the pretreated alloy at 90℃/6 h,the main precipitated phases change from GPI zone and GPII zone to GPII zone,η’ phase and η phase.The strength of the alloy is improved.When the pretreated alloy is completely exposed to heat,the size of the precipitated phase further increases,the main precipitated phases change into η’ phase and η phase.Meanwhile,the tensile properties decrease.When the pretreatment system is 90℃/(6,12,48)h,the strength after complete thermal exposure shows a gradually decrease with the extension of pretreatment time.The size of the precipitated phase increases during thermal exposure.After pretreatment of the alloy at 90℃/3 h,the tensile strength can reach at 550 MPa and the yield strength is 510 MPa after thermal exposure,which is significantly higher than that of T6 treated alloy.