The Research Of Thermomechanical Treatment And Microstructure Properties Of 7075 Aluminum Alloy

High-strength Al-Zn-Mg-Cu alloy plays an important role in national economic development and defense equipment manufacturing.However,the traditional heat treatment process is difficult to achieve the combination of high strength,high plasticity and high uniformity,which not only reduces the service life of the material,but also limits the application potential of the material.Therefore,it is the key technology to realize the research and development of high performance aluminum alloy materials by using plastic deformation and heat treatment to control the microstructure of matrix and alloy.In this paper,semi-continuous casting 7075 aluminum alloy was used to systematically explore the microstructure evolution,precipitation behavior and conventional mechanical properties of the material under different process parameters by means of plastic deformation and heat treatment process.The corresponding relationship between crystal structure,micro-precipitation phase and mechanical properties was established.The specific research results are as follows:(1)In order to study the microstructure change of 7075 aluminum alloy during dynamic strain aging,a single-pass thermal simulation compression experiment was carried out on as-cast high-strength aluminum alloy.The true stress-strain curve was drawn according to the thermal simulation data,and the flow stress behavior of the alloy was analyzed.Then,the constitutive equation and Z parameters of the alloy material were obtained by fitting the stress-strain curve.At the same time,the microstructure evolution behavior of 7075 aluminum alloy during single-pass thermal simulation compression was analyzed.The main conclusions are as follows: Dynamic recovery is the main mechanism of dynamic softening during hot deformation.Only at higher deformation temperature and lower strain rate,dynamic recrystallization occurs obviously.When the Z parameter is high,the alloy mainly appears discontinuous dynamic recrystallization.When the Z parameter is in the middle value,the alloy mainly appears continuous dynamic recrystallization.When the Z parameter is low,the alloy mainly appears geometric dynamic recrystallization.When the deformation temperature and strain rate of the alloy are the same,the higher the strain,the more prone to dynamic recrystallization,DDRX is the first dynamic recrystallization.Higher Z parameter can promote the precipitation of precipitated phase during hot compression,but the dislocation density decreases with the decrease of Z value.At the same time,the higher the Z value,the more dynamic the precipitation.(2)In order to study the changes of microstructure and properties of7075 aluminum alloy at different stages during static strain aging,cold deformation compression test and solution treatment were carried out on homogeneous high-strength aluminum alloy.The conclusions are as follows:During the cold deformation process,the grain size is reduced to 74.23 μm when the compression amount is 40%,which is 28.1% lower than the original structure.After solution treatment,the grain size is obviously refined to 19.22μm,which is 81.5% lower than that of the original structure.With the increase of recrystallization degree,the content and strength of the main texture decreased,but the type did not change.With the increase of deformation degree,the hardness of the alloy also increases,but the hardness after solid solution is always lower than that of the deformed alloy.(3)The effects of homogenization-cold deformation-solid solution-aging process on the microstructure and properties of 7075 aluminum alloy were studied.The conclusions are as follows: After thermomechanical treatment,the matrix structure was significantly refined.When the deformation was 40% and the aging temperature was 140℃,the average grain size reached 16.28 μm.When the deformation amount is 30%,the average particle size of the precipitated phase at the aging temperature of 140°C is 9.6 nm,and the average particle size of the precipitated phase at the aging temperature of 160°C is 30.8nm.It can be judged that the main precipitation at the aging temperature of140°C is η ’,and the main precipitation at the aging temperature of 160°C is η.The reason for the coarsening of the precipitated phase is that the increase of the aging temperature accelerates the ostwald ripening process.Under this process,the hardness of the alloy reaches a peak value of 152 HV when the deformation is 30 % and the aging temperature is 140°C.(4)The effects of homogenization-cold deformation-aging process on the microstructure and properties of 7075 aluminum alloy were studied.The conclusions are as follows: After thermomechanical treatment,there are a large number of deformation bands in the matrix structure,and the grain size is slightly reduced.It was found that there were a large number of dislocation tangles and precipitated phases in the matrix.The average size of the precipitated phase is 7.9 nm,and the volume fraction of the precipitated phase is as high as 29.04%.This is because the dislocation provides the heterogeneous nucleation site of the precipitate and promotes the precipitation of the precipitated phase.Under this process,the hardness reaches a peak of 212.0 HV when the deformation is 40% and the aging temperature is 120℃.(5)The rolling verification experiment of 7075 aluminum alloy was carried out.Through the tensile curve at room temperature,it can be seen that the homogenization-cold deformation-solid solution-aging process can obtain higher fracture elongation,up to 10.0%,and the fracture type is ductile fracture;the homogenization-cold deformation-aging process can obtain higher strength,the highest tensile strength and yield strength are 456.0 MPa and 398.9 MPa,respectively,and the fracture type is brittle fracture.