Effect Of Continuous Rheological Extrusion Forming And Heat Treatment On Microstructure And Properties Of 7055 Aluminum Alloy

Al-Zn-Mg-Cu alloy is the highest strength aluminum alloy at room temperature,because of its high strength,small density and good hot working performance and corrosion resistance,it is an important material in the fields of modern automobiles,aerospace,military precision instruments,etc.,and 7055 aluminum alloy is a typical representative of this ultra-high strength aluminum alloy.In order to cope with the increasingly stringent material application requirements and achieve the purpose of replacing steel materials with aluminum alloys,it is of great significance to study the effects of continuous rheological extrusion forming and heat treatment on the properties and microstructure of 7055aluminum alloys.In this paper,the self-configured 7055 aluminum alloy was prepared by continuous rheological squeeze casting process,and the optimal extrusion process parameters were determined by exploring the influence of different working roller speed,pouring temperature,cooling water flow rate and other extrusion processes on its microstructure and properties,and then the effects of double-stage solid solution and two-stage aging heat treatment on the microstructure and properties of 7055 high-strength aluminum alloy were studied,aiming to improve its comprehensive mechanical properties.At the same time,the microstructure evolution mechanism and strengthening mechanism of alloy under extrusion forming process and heat treatment process were revealed.The test results show that compared with the traditional gravity casting process,the7055 aluminum alloy wire prepared by continuous rheological extrusion can effectively improve the alloy dendrite and the segregation of components at grain boundaries,the grain refinement is obvious,the mechanical properties are significantly improved,and with the increase of the speed of the working roll,the lower the pouring temperature,the higher the cooling circulating water flow,the formed alloy grains are finer and the mechanical properties are higher.The optimal extrusion process parameters of 15 r/min working roller speed,700°C pouring temperature and cooling circulating water flow rate of 2.5 m³/h were obtained through orthogonal test test,and its tensile strength,yield strength,elongation,Vickers hardness,conductivity and grain size reached 515.7 MPa,349.2 MPa,15.3%,157.9HV,41.8%IACS,and 12.7μm,respectively,compared with the casting temperature of 740The tensile strength,yield strength,elongation,Vickers hardness and conductivity of 7055alloy prepared by gravity casting increased by 46.3%,31%,30.7%,53.1%and 20.8%,respectively,and the grain size decreased by 33.8%.Through the bipolar solution study of 7055 aluminum alloy wire prepared by continuous rheological extrusion,the optimal solution treatment process was450℃×2h+470℃×2 h,and the comprehensive mechanical properties under this process were the best,and the tensile strength,yield strength,elongation,Vickers hardness and conductivity reached 582.1 MPa,447.6 MPa,18.1%,168.5 HV and 38.7%IACS,respectively,which were higher than the tensile strength and yield strength of extruded alloy wire without solution treatment,elongation and Vickers hardness increased by 12.8%,28.1%,18.3%and 6.7%respectively.XRD analysis showed that with the increase of secondary solid solution time and secondary solid solution temperature,the diffraction peaks of Al₂Zn₃Mg₃,Mg Zn₂and Al₂Cu Mg in the original room temperature tissue gradually decreased,indicating that the second phase in the alloy decreased with the increase of solid solution temperature and solid solution time,until the fine second phase was almost all dissolved in the grain when the secondary solid solution system was 470℃×2 h,and the number and size of the coarse second phase decreased.Overburning occurs when the secondary solution temperature is 480℃.The tensile strength and hardness showed a trend of first increasing and then decreasing with the increase of holding time,while the conductivity showed a decreasing trend with the increase of the solution temperature.7055 alloy through 450℃×2h+470℃×2 h bipolar solid solution and then through single-stage aging treatment,with the increase of aging time,the second phase re-precipitated by aging is dispersed and distributed in the matrix,the precipitated phase is a fine sphericalη’phase when the aging time is 4h,the precipitated phase is composed of a large number of punctateη’phases and a small number of sphericalηphases when the aging time is 8 h,and its tensile strength trend first increases and then decreases,reaching a peak at 8 h in the aging time,at this time the tensile strength,hardness and elongation are617.3 MPA、176.1 HV、16.9%。With the increase of the secondary aging time,the number of particles in the second phase showed a downward trend,and the tensile strength,yield strength and extension first increased and then decreased,and the tensile strength and yield strength reached a peak at 16 hours of the secondary aging time.The best bipolar aging process is 120℃×8 h+160℃×16 h,and its tensile strength,yield strength,elongation,Vickers hardness and conductivity reach 639.5 MPa,475.6 MPa,20.1%,187.3 HV and47.9%IACS,respectively,which increase the tensile strength,yield strength,elongation,Vickers hardness and conductivity of 7055 aluminum alloy wire without heat treatment by24%,36.2%,31.4%,18.6%and 14.6%,respectively.