| The mechanical properties, microstructures and it’s change regularity of the super high-strength AA7050aluminum alloy at defferent heat treatment conditions were investigated using hardness analysis, room temperature tensile test, electrical conductivity measurement, High resolution electron microscopy (HRTEM),Selected area diffraction spots (SAED, X-raydiffraction(XRD), electronic microscopic analysis (SEM, TEM), Electron Probe Spectrum Analysis (EPMA), et. The heat-treating parameters were optimized. By analyzing the experimental results theoretically, several conclusions can be summarized as follows:1. The results show that the as-cast structure of the7050alloy is composed of the a (Al) Solid Solution and low melting point eutectic phase T (Al2Mg3Zn3or Al6CuMg4T(AlZnMgCu) and S (Al2CuMg), the samples have coarse grain which is mainly150um and the grain keeps equiaxed,the7050alloy should be homogenizated treatment due.2. With increase of single grade homogenization at430℃, the low melting point eutectic phase T (Al2Mg3Zn3or Al6CuMg4T(AlZnMgCu) and S (Al2CuMg) begins to dissolve, but part of high melting point eutectic phases S (Al2CuMg) can not dissolve, the reasonable scheme of single grade homogenization is430℃×18h,prolong the time of single grade homogenization had no obvious effect.3. Two-step homogenizing contrast traditional single grade homogenization shows that.The increase of homogenizing temperature beneficial to impove homogenization degree.To avoid the overheating of low melting point eutectic phase, second high temperature homogenizing must beased on sufficient low temperature homogenizing, and most of the low temperature can be decomposition. The high melting point eutectic phases S (Al2CuMg) can be dissolved by use two-step homogenizing which can not dissolved in single grade homogenization. The reasonable scheme of single grade homogenization is430℃/18h+467℃/12h.4. The Research of time factor of single grade solution shows that,with the solution time prolonging the hardness increased first, the main reason is the strengthening effect of replacement solution and gap solution which is made by increase of insolid solubility. Then the hardness decrease rapidly, because of the effect of softening effect cause by grain growth more then the effect of strengthening effect.As the time peolonged, the solution reach saturation, the size of grain growth and the hardness further decreased.5. The research of correlation interaction of time factor and temperature factor at single grade solution shows that, as the increase of temperature in single grade solution,both of the insolid solubility and distribution homogenization of solute atoms rise. But the exorbitant temperature lead to overheating, so the temperature of primary solution must lower then the melting point of eutectic phase, The size of grain growth,but the effect of strengthening effect more then the effect of softening effect cause by grain growth,compared with other temperature the secondary phases decreased significantly when treatment at470℃, show that the solute atomic fully re-dissolution. The optima single grade solution is470℃.6. The two-step strengthening solution contrast single grade solution shows that, The two step solution could effectively reduce the massive excess phase, further improve the supersaturation of solid solution,make the tissue more uniform, shows a good strengthening effect, prepare for a good tissue basis for the following aging treatment. The optimal two step solution process is470℃/40min+485℃/40min.7. The research of correlation interaction of time factor and temperature factor at single grade aging shows that, the higher of aging temperature the farst of the hardening rate,the shorter of time to reach the peak strength which is lower.The alloy has low resistance to over-agein when re-aging at temperature above145℃,and a high resistance to over-agein when re-aging at temperature no higher than135℃which is the suitable temperature. The aging hardend effect of AA7050is significantly.When aging at135℃the hardness of alloy increased at the early storage time, and then decreased later and reach mild peak12hours alter, the hardness is176HB2.5/62.5.8. The re-aging time was confirmed to be12h when the alloy achieved sub-peak state with the major strengthing phase GP zones precipatated in grain interior and fine and sequential η equilibrium phase without obvious PFZ precipitated along the grain boundaries. The results indicate that with the prolongation of the time of secondary higher temperature aging, the hardness of the alloy first decreased, but then increased sharply and arrived at the peak at2hours.The results show that the changes of the properties are closely related to the size and distribution of the precipitates and the transformation between G.P zones, η and η, which shows the importance of re-aging scheme. After two-step aging at135℃/12h+170℃/2h, the optimization of the size, proportion and distribution of main precipitates GP zones, η’ and η was realized, which resulted in the improvement of the comprehensive properties of the alloy.9. The GP zones reprecipitation at the low temperature reaging process after high temperature regression aging, manifested as the appear GP spots in the diffraction spots and as heat release wave packer which corresponding to the heat release peak temperature range in the cooling process of the rmogravimetric analysis curve.Through the optimized RRA treatment based on appropriate retrogression time, the ultimate tensile strength, the tensile yield strength, elongation and hardness of AA7050were raised to643MPa,629MPa,11.76%, and182HB... |
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