Influence Of Fe And Si Impurities On Quench Sensitivity And Aging Behavior Of Al-Zn-Mg-Cu Alloy

Al-Zn-Mg-Cu alloys are mainly used as the thick section of structural materials in the aerospace field, but they can not often meet the design requirement, due to the high quench sensitivity. The influence of Fe and Si impurities on quench sensitivity has been reported rarely. In this thesis, the influence of Fe and Si impurities on the quench sensitivity and aging behavior of Al-7.5Zn-1.5Mg-1.7Cu alloy was investigated by means of end-quenching tests, hardness test, conductivity test, room temperature mechanical test and DSC thermal analysis. The microstructures were characterized by X-ray analysis, optical microscopy (OM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The main conclusions obtained are listed as follows:(1) Fe and Si impurities are mainly existed as Al7Cu2Fe phase and Mg2Si phase. Their content and size in as-cast state and homogenized state are almost the same, but after plastic deformation they are ruptured along the deformation direction. On the one hand, with increase of impurities, the content of impurity phase increases significantly, thus reduces the solutes concentration of Cu and Mg elements in the matrix, which reduces the degree of supersaturation, which leads to lower quench sensitivity. On the other hand, the coarse impurity phases are favorable for the formation of the recrystallization nucleas, which increases the number of recrystallization grains,(sub) grain boundaries, and the nucleation positions of equilibrium phases, which leads to higher possibility of quench sensitivity.(2) Hot extrusion deformation can lead to higher quench sensitivity. After hot deformation, the grains become smaller, which increases the number of (sub)grain boundaries and the nucleation positions of equilibrium phases, which results in higher possibility of quench sensitivity as well.(3) With increases of the aging temperature, the mechanical properties reduce degree for air-cooled samples compared with the water-cooled samples increases. High aging temperature leads to the increasing number and size of equilibrium phases, and the decreasing dispersion degree of strengthening phase. The dissolution temperature of GP zones increases about23℃, which is unfavorable for the precipitation and nucleation of GP zones.(4) With increases of Fe and Si impurities content, the hardness of the alloy decreases, and the conductivity increases. The increasing impurities or the decreasing quenching rate lead to lower dispersion degree of strengthening phase, bigger grain size and precipitation free zone(PFZ) width. They reduce the hardness and increase the electrical conductivity. With increases of the impurities content, the precipitating activation energy of η phase increases. And with increase quenching rates after solid solution, the activation energy of vacancy diffusion and η phase precipitating increase. So it takes longer time for the electrical conductivity and hardness to be stable in natural aging and artificial aging process.