Effects Of Quenching Rate And Aging Process On The Inhomogeneities Of Thick Plate Of Aluminum Alloy7085

Aluminum alloy7085developed by Alcoa in2003has been widely used in aviation aerospace industry as structural materials for its high specific strength, fracture toughness, stress corrosion resistance and excellent hardenability. As the launch of large aircraft subject, the independent research and development ability in high-performance aluminum is in pressing need. While the inhomogeneity caused by quench of thick plate of aluminum alloy7085is the key factor restricting its production and application. In this thesis, the influence of aging process and quenching rate on inhomogeneities were investigated by means of hardness testing, electrical conductivity testing, tensile testing, optical microscope(OM), scanning electron microscope (SEM), transmission electron microscope (TEM) and differential scanning calorimeter (DSC). The main conclusions are as follows:(1) Influence of sample shape on inhomogeneity of temperature field in quenching is revealed. The difference between cylinder model and square column model is negligible except that the heat exchange between non-quenching surface and air in cylinder model is much more vigorously than that of square column model. With time going on, the difference between the two models becomes smaller. In short, the sample shape has little effect on inhomogeneity. And during the quenching process, the sample shape can be selected by product requirements.(2) The optimal ageing processing is established for uniform property. With the decrease of the quenching rate, the number and the size of quench-induced equilibrium η phases increased which in turns led to less and larger η’strengthening precipitates after ageing. At the same time, the time needed to reach the peak hardness gets longer with decreasing quench rate, which is related to the increasing of the vacancy diffusion activation energy. When the quench rate is1967℃/min,370℃/min,148℃/min, the vacancy diffusion activation energy is34.42kJ/mol, 49.63kJ/mol,58.44kJ/mol respectively. The higher vacancy diffusion activation energy is, the more difficult precipitation is.(3) Influence of aging on inhomogeneity is obtained. When the aging time is same and aging temperature is140℃, the inhomogeneity is the least, the difference of hardness between head and end of the sample is6%, and the hardness value is high when aged at120℃. At a certain ageing temperature, homogeneity is the best at140℃/84h, the difference of hardness between head and end of the sample is only4%, and the hardness value is just lower than that of24h. Therefore, the optimal aging processing is140℃/84h, not only the hardness value is good, but also the homogeneity is the best.(4) Inhomogeneity of thick plate of aluminum alloy7085is characterized. Properties and structures of the thick plate are inhomogeneous in thickness, the strength of center layer is the highest, reaching588MPa, and the fourth layer is the lowest,50MPa lower than center layer. Heavy recrystallization and coarse precipitates may have led to this result.