Effect Of Solid Solution Cooling Path On Microstructure And Properties Of 7055 Aluminum Alloy

7055 aluminum alloy belongs to Al-Zn-Mg-Cu series alloy.It has been widely used in aerospace and high-quality transportation equipment manufacturing because of its high specific strength,high hardness and easy processing.Besides,due to its high alloying degree and other special properties,7055 aluminum alloy has gradually become one of the indispensable structural materials in related industry.The medium corrosion resistance and the strength of 7055 aluminum alloy are always contradictory.Thus,how to improve the medium corrosion resistance of 7055 aluminum alloy on the basis of the high strength has important research significance.In this thesis,the influence of solution cooling path on the microstructure and properties of 7055 aluminum alloy plate is systematically studied by using the calculation simulation and experiment.The effect of cooling pretreatment temperature on microstructure and corrosion resistance of the alloy is studied,and the key parameter of cooling pretreatment temperature in the solid solution cooling path is optimized.Through the study of cooling rate during cooling process,the influence of cooling rate on the microstructure and corrosion resistance of the alloy is clarified,and then the cooling rate is optimized.By comparing the aging precipitation behaviors of the alloys under three different heat treatment states,the relationship between the intragranular characteristics of the alloy or the micro-composition of?-Mg Zn₂ phase at the grain boundary and the comprehensive performance of the alloy is clarified,and the aging process is reasonably optimized.And the main conclusions are as follows:Firstly,through the study of the cooling pretreatment temperature,it is found thatreasonable cooling pretreatment temperature can make the Cu element segregation in the grain boundary comparing with the conventional solid solution treatment state.Moreover,the second phase with Cu element don?t precipitate near the grain boundary and within the grain,and the crystal still maintained a high saturation state,especially the CPt455(480?/1h?455?/0.5h?water quenching)sample.At the same time,CPt455(480?/1h?455?/0.5h?water quenching)sample showed strong intergranular corrosion resistance,the intergranular corrosion depth of 82.5?m and the corrosion evaluation of 3+level.Secondly,the cooling rate from solid solution to cooling pretreatment temperature is studied.It?s found that the cooling rate has a certain influence on the composition of grain boundary micro-region.When the cooling rate is 0.05?/s,the Cu content in the grain boundary microzone of the alloy slightly increases and no new phases are produced.The intergranular corrosion resistance of the sample with cooling rate of 0.05?/s is improved.The intergranular corrosion depth is 73?m and the corrosion evaluation is 3-level.Finally,the microstructure and comprehensive properties of the materials under the three aging states of Solid solution state+T6,Solid solution state+T74 and CPt455_(v=0.05)+T6 were compared.It is found that CPt455_(v=0.05)+T6 sample had high density and fine??phase in grain,and narrow PFZ,which was very close to the precipitation state in the grains of Solid solution state+T6 sample,the mechanical properties of CPt455_(v=0.05)+T6sample are similar to that of Solid solution state+T6 sample.The tensile strength is 623MPa and the elongation is 15.7%.the grain boundary precipitates of?-Mg Zn₂ phase of CPt455_(v=0.05)+T6 sample are coarse and discontinuous,which is very similar to the grain boundary precipitates of Solid solution state+T74 sample.However,the Cu/Zn ratio in the grain boundary precipitates of the CPt455_(v=0.05)+T6 sample is higher than that of the sample treated by Solid solution state+T74,which greatly improves the corrosion resistance of the alloy.The corrosion potential is the most positive(-0.870V),the self-corrosion current is the smallest(3.820×10^-5A/cm²),and the intergranular corrosion depth is shallower(68?m).Finally,the optimized heat treatment process:480?/1h?455?/0.5h?120?/24h,and the cooling rate from 480?to 455?is 0.05?/s,which improves the strength,and corrosion resistance of the material.