Research On Microstructures And Properties Control Of Al-Zn-Mg-Cu Alloy For Fasteners

Al-Zn-Mg-Cu alloy is an important material for aerospace fasteners.During use,it must bear high loads and be easily corroded by corrosive media,exposing the problem that the strength and corrosion resistance cannot meet the service requirements.In this thesis,starting from the regulation mechanism of grain structure and aging precipitation phase,the regulation of streamline structure in hot upsetting forming,the regulation of intragranular and grain boundary precipitation phase in solution treatment and isothermal/non-isothermal aging process are mainly studied.The correlation and mechanism between microstructure and mechanical properties and corrosion resistance of the alloy are explored,and the regulation technology to improve the comprehensive performance of the alloy is established.The main conclusions are as follows:（1）The hot upsetting deformation process parameters for obtaining the streamline grain structure of 7055 aluminum alloy are established:temperature 300-400℃,strain rate 0.03-1 s^-1,and the grain structure characteristics and deformation softening mechanism under different deformation conditions are revealed.Dynamic recovery is the main softening mechanism under the above process parameters.The grain structure is mainly composed of deformed grains,subgrains and a small number of recrystallized grains.There are dislocation entanglements inside the grains,and the grains are uniform and the degree of recrystallization is low.According to the processing map and microstructural observations,the degree of dynamic recrystallization increases with increasing deformation temperature（over 300℃）and decreasing strain rate(below0.1 s^-1).Deform-3D finite element simulation reveals the distribution of equivalent strain and equivalent strain rate inside the bolt head,and tracer particles show the formation process of the streamline grain structure.Metallographic observation of hot upsetting products reveals the formation of metal streamline grain structure.（2）The double-stage solid solution process for obtaining highly supersaturated solid solution and effectively eliminating undissolved phase of 7055 aluminum alloy is established:450℃/1 h+475℃/1 h.The quenching-sensitive temperature range of the alloy is 375-200℃,and the exsolution can be significantly suppressed by passing through the quenching-sensitive temperature range at a cooling rate of not less than60℃/min.When the cooling rate is less than 30℃/min,obvious exsolution occur,which reduces the driving force of subsequent aging precipitation,and the corrosion resistance of the alloy decreases with the decrease of the quenching cooling rate.Significant overburning occurs when the second-stage solid solution temperature increases to 490℃.（3）The effects of three isothermal aging processes of single-stage aging,double-stage aging and retrogressive reaging on the mechanical properties and corrosion resistance of alloys are compared and studied,and the aging process parameters for obtaining ultra-high strength and synergy of high-strength corrosion resistance are determined.The mechanical properties of single-stage aging alloy at 120℃/24 h are excellent,and the tensile strength reaches 738.4 MPa.The tensile strength of 120℃/6h+160℃/18 h,120℃/6 h+160℃/24 h double-stage aging alloys exceeds630 MPa,and the corrosion resistance is good,and the stress corrosion sensitivity index(I_SSRT)are 3.9%and 3.5%,respectively.The tensile strength of the retrogressed reaged alloy at 180℃/1 h reaches 690.4 MPa,and its corrosion resistance is better than that of the single-stage aged alloy with the same hardness level.The tensile strength of the retrogressed reaged alloy at 200℃/1 h is 571.6 MPa,the corrosion resistance is excellent,and the I_SSRT is reduced to 1.6%.（4）The effect of the heating/cooling rate and peak temperature on the microstructure and properties of the constant-rate non-isothermal aging alloy is found out,and the process parameter for obtaining good mechanical properties and corrosion resistance matching is established:the heating/cooling rate is 40℃/h,with a peak temperature of 200℃.The aging time is only 5 h,the tensile strength of the alloy reaches 674.7 MPa,and the maximum intergranular corrosion depth is as low as 31.6μm.The smaller size of the precipitated phase（R_g=48.0（?））and higher volume fraction(Q=0.00213（?）^-3)are the reasons for its good mechanical properties.When the heating/cooling rate is reduced to 10℃/h,the corrosion resistance can be further improved,and the corresponding tensile strength is 570.0 MPa.When the heating/cooling rate increases to 60℃/h,the tensile strength increases to 682.2 MPa,but the corrosion resistance decreases.When the peak temperature increases to 210℃,the size of the precipitated phase is larger（R_g=54.1（?））,the volume fraction decreases(Q=0.00115（?）^-3),and the comprehensive performance decreases.（5）A new four-stage variable-rate non-isothermal aging treatment process is established,and the mechanism of variable temperature rate transformation to regulate the size and distribution of precipitated phases in the alloy grain and grain boundary is revealed.In the initial stage（100→180℃）,the temperature is raised at a slow rate of 20℃/h,so that a large number of precipitates are nucleated,and the hardness of the alloy is significantly improved;in the high temperature stage（180→210→180℃）,60℃/h rapid rate temperature change is used to obtain discontinuously distributed GBPs and widened PFZ,and the corrosion resistance and electrical conductivity of the alloy are significantly improved,the small-sized precipitates are dissolved back,and the large-sized precipitates are fully grown but not excessively coarsened;the final stage（180→100℃）using 20℃/h slow cooling rate,secondary precipitation of precipitated phase occurs.The tensile strength of the alloy reaches about 650 MPa,the elongation reaches 12.9%,the hardness exceeds 170 HV,the electrical conductivity exceeds 38.5%IACS,the corrosion resistance is excellent,and the I_SSRT is as low as 1.9%.（6）A new variable temperature retrogression reaging process is established,and the appropriate retrogression process is determined as:heating from 120℃ to 200℃ at 8℃/min,and then cooling to 120℃ at15℃/h,which can make the hardness,tensile strength and electrical conductivity reach 176.6 HV,576.4 MPa and 39.3%IACS,respectively.The alloy has excellent corrosion resistance,with I_SSRT as low as 1.5%.The performance difference between the surface and the center of the product can be effectively reduced.As the cooling rate decreases during the variable temperature retrogression process,the average size of precipitates increases,the content of Zn,Mg,and Cu elements in GBPs increases,the PFZ is wider,and the electrical conductivity is higher.Compared with the traditional retrogressive reaging,the new variable temperature retrogressive reaging overcomes the limitation of the need for converter operation or moving products,and the mechanical properties and corrosion resistance of the manufactured alloy are both improved.