Effect Of Two-step Aging On Microstructure And Properties Of 7A60 Aluminum Alloy

7A60 aluminum alloy belongs to the new Al-Zn-Mg-Cu aluminum alloy.Compared with traditional Al-Zn-Mg-Cu aluminum alloy,7A60 aluminum alloy make an increase of the content of Zn.The content of impurities is reduced,and the adverse effects of impurities on the overall performance of the alloy are reduced.It is widely used in various industries.However,the as-cast 7A60 aluminum alloy is difficult to meet the national aerospace material standard due to its poor comprehensive performance.Due to its heat treatment strengthening characteristics,it is necessary to explore a suitable heat treatment system to improve the comprehensive performance of the as-cast 7A60.In this paper,by studying the effects of single-step and two-step aging processes on the hardness,tensile strength,electrical conductivity,and microstructure of as-cast7A60 aluminum alloy,I develop an aging treatment process that can be applied to large-scale production with high strength,high plasticity,and good electrical conductivity.The aging treatment makes the performance of 7A60 aluminum alloy fully meet the national standard and meets the needs of China's aerospace industry.The results show that the solid solution treatment at 470?for 2h increases the hardness of 7A60 aluminum alloy from 103.7 HV to 202.9 HV,the tensile strength from264.4MPa to 355.6MPa,and the elongation from 1.8%to 2.3%,but the conductivity decreased from 36%IACS to 30.5%IACS.The hardness and tensile strength of 7A60aluminum alloy are generally higher after aging at 135?,and peak at 12h,which is231.8HV and 568MPa respectively.The single-step aging makes the elongation of aluminum alloy increase slowly,and the electrical conductivity increases with the holding time.The two-step aging has a certain decrease in the hardness and tensile strength of the 7A60 aluminum alloy.The higher the second-step aging temperature,the faster the rate of hardness and tensile strength decreases,and the effect on the elongation is small.The conductivity gradually rises with the two-step aging.The higher the second-step aging temperature,the faster the rate of conductivity rises.After optimizing the results of the two-step aging experiment,the optimum process parameters are135°C/12h+169.3?/15.3h.Under this condition,the hardness,the tensile strength,the elongation and the conductivity of it were 165.3HV,516.2MPa,3.6%and 42.1%IACS respectively.The solution treatment increases the grain size,and the aging treatment has little effect on the grain size.The second phase precipitated during the aging process is mainly Mg Zn₂ phase.The long strip metastable?'phase is precipitated first,and the?'phase is the strengthening phase.The hardness and strength of the aluminum alloy are gradually increased with the precipitation of the?'phase.As the aging progresses,the?'phase transforms into a steady-state?phase.The?phase strengthening effect is weaker than the?'phase.So the hardness and strength are lowered.The?'phase is semi-coherent with the matrix,and the?phase is not coherent with the matrix.After the?'phase is transformed into the?phase,the lattice distortion and the free electron scattering are reduced.The electrical conductivity is increased.The two-step aging does not solve the problem of low elongation of the 7A60aluminum alloy.Therefore,a combination of high-pressure torsion and two-step aging is adopted to improve the overall performance of the 7A60 aluminum alloy.The results show that the combination of high pressure torsion,solid solution,and aging can further improve the comprehensive performance of aluminum alloy than a single method.The hardness,the tensile strength,the elongation and the conductivity of it were 207.4HV,685.6MPa,9.4%and 44.3%IACS respectively.In addition,the strengthening mechanism of solid solution and aging after high-pressure torsion were studied by SEM observation.The results show that the increase of hardness and tensile strength is mainly due to fine-grain strengthening,solid solution strengthening and second phase strengthening.The solution strengthening and the second phase strengthening have little effect on the plasticity of the aluminum alloy.After the high pressure torsion solid solution and aging,the solid solution strengthening and the second phase strengthening are performed under the action of fine grain strengthening.The elongation is greatly increased.