Study On The Ageing Precipitation Behavior Of Hot Extruded Sheets Of 7055 Aluminum Alloy By Spray Deposition

Higher mechanical and corrosion resistance of lightweight Al Zn Mg Cu alloys are always required in modern aerospace industry.The spray deposition technology,which is characterized by rapid solidification,can improve the alloy element contents in the ultra-high strength 7055 aluminum alloy while effectively reduce the macro-segregation and also the hot cracking in semi-continuous casting.Therefore,the microstructure of spray deposition ingot with fine grain and without compositional segregation can be obtained.As the final heat treatment,ageing is the crucial process for improving the performance of7055 aluminum alloy.Therefore,the development of the ageing technology matching to the hot extruded sheet of spray deposited 7055 aluminum alloy is important to fully actualize the potential of fine structure and the high elements composition.This dissertation first systematically characterized the microstructure of as-deposited,as-hot extruded,and as-solid solution alloys.On this basis,the precipitation behavior and dissolution behavior of nano phase during single-stage ageing(SA),regression and re-ageing(RRA)and non-isothermal heating ageing(HA)were studied by transmission electron microscopy(TEM)and differential scanning calorimetry(DSC).Finally,based on the influence of different ageing processes on the mechanical properties and corrosion resistance of the alloy,optimized parameters corresponding to various ageing processes were obtained.Under the premise of non-isothermal temperature field during the ageing process of thick section plates,this dissertation discussed the heating ageing technology in details.The main conclusions are as follows:In single-stage ageing,the precipitation sequence of spray deposited 7055 aluminum alloy is: Supersaturated solid solution(SSSS)→ GP zone → η’ phase → η phase.The content of alloying elements such as Zn and Mg shortens the time for the alloy to reach its peak hardness.The time to reach peak hardness at 120 ℃ and 105 ℃ is 16 hours and 24 hours,respectively.The intergranular phases under the two peak ageing tempers of 120 ℃,16 h and 105 ℃,24 h are composed of GP zones and η’ phases,with the average phase sizes of 4.6nm and 5.0nm,respectively.Precipitate free zone(PFZ)cannot formed during single-stage ageing.The tensile strength,yield strength and elongation after 120 ℃,16 h ageing are 659.9MPa,596 MPa,and 13.1%,respectively.However,its exfoliation corrosion level is only ED.During the regression process,the dissolution rate and secondary hardening rate both increase with the increase of regression temperature.Taking the electrical conductivity of36% IACS as the criterion after re-ageing,the optimized regression time for 190 ℃ and210 ℃ are 10 minutes and 7 minutes,respectively.The hardness of re-ageing is 201 HV and 189 HV,respectively.An optimized RRA schedule for spray deposition of 7055 aluminum alloy is 105 ℃,24 h + 190 ℃,10 min +120 ℃,16 h.The average size of the intragranular phase under this state is about 8nm.Simultaneously,the PFZ with a width of approximately 25 nm appears.After RRA treatment,the tensile strength,yield strength,and elongation of the alloy are 684 MPa,643MPa,and 13.3%,respectively.The exfoliation corrosion level can reach EA.The precipitation sequence of spray deposited 7055 aluminum alloy during heating ageing is still SSSS → GP zone → η′ phase → η phase.During the low-temperature ageing stage from the ambient temperature to 160 ℃,the main precipitates are fine GP zones and clusters.In the temperature range from 160 ℃ to the peak ageing temperature,the low-temperature precipitates smaller than the critical size re-dissolve but the large-sized precipitates grow.New η′ nuclei corresponding to the high-temperature condition appear.The hardness first increases to its peak value and then rapidly decreases with the increase of the termination temperature,while the electrical conductivity continues to increase.The increase of heating rate increases the termination temperature corresponding to the peak hardness and shortens the ageing time.The optimized heating ageing schedule for spray deposition 7055 aluminum alloy is to raise the temperature to 200 ℃ at a heating rate of20 ℃/h,which takes only 8 hours to get an excellent comprehensive performance.The tensile strength,yield strength,and elongation corresponding to this state are 642.3MPa,616 MPa,and 11.3%,respectively.The exfoliation corrosion level can reach EB.