| Increasing the content of main alloying elements,combined with Sc microalloying,achieves strength of 750MPa in Al-Zn-Mg-Cu aluminum alloy.Despite the evident potential in strengthening,the extremely high content of Zn and the Sc addition can significantly influence the evolution of the microstructure and properties in this catalog of 7xxx alloys,which then can induce microstructure formation,aging response and residual stress effect that can be significantly different from that of the traditional 7xxx alloy.It is thereafter of great significance to study the evolution of microstructure and properties in new 7xxx aluminum alloy subjected to hot working and heat treatment for tapping the potential and application,this paper systematically studied the two new 7xxx series aluminum alloy organization performance and residual stress in the process of thermal evolution,two kinds of alloy constitutive equation and hot working diagram,explores the alloy extrusion sheet under different heat treatment process of organizational performance,and analyzes the 40mm thick metal plate in the evolution of residual stress in the process of heat treatment.In this paper,Gleeble 1500D thermal simulation machine is used to simulate the thermal deformation behavior of the two alloys,draw the rheological stress curve,construct the constitutive equation and the thermal processing diagram,give the appropriate thermal processing window and analyze the organizational characteristics of typical areas.The microstructure properties of the alloy under different heat treatment processes were analyzed by tensile test and TEM,and the mechanism of the differences is analyzed.The evolution of residual stress during quenching,aging and cryogenic treatment of alloy is analyzed by means of finite element simulation and contour method.The results showed that the new 7xxx series aluminum alloy experienced three stages of working hardening,dynamic softening and steady state in the process of thermal deformation.The level of rheological stress is higher in Sc alloys.According to the constitutive equations of the two alloys,the rheological behavior of the two alloys can be well predicted.By comparing the thermal processing diagrams of the two alloys,the energy dissipation coefficients of the two alloys are higher in the area where the deformation temperature is 410°C-460°C and the strain rate is 0.050s-1-0.158s-1,which is the common window suitable for processing.The instability area of Sc alloy is smaller,and the suitable processing area is larger,which is more suitable for hot processing.Recrystallization during thermal deformation needs to occur at a suitable temperature,and the addition of Sc element can significantly inhibit the recrystallization nucleation of the alloy.Al-11Zn-2.5Mg-1.5 Cu alloy reached peak aging at 130°C/24h and 140°C/16h,at which time the yield strength of the alloy exceeds 740MPa and the strong plasticity matches well,which is higher than the aging peak aging state of 120°C.With the increase of single aging temperature,the size of precipitation phase in the grain boundary of the alloy increases,and the precipitation phase at grain boundary does not change obviously.For the alloy of two-stage aging peak aging state,when the second-stage aging temperature is 145°C,the comprehensive properties of the alloy are not significantly different from that of the alloy under the single-stage aging peak aging state of140°C.When the second-order aging temperature is 155°C,the aging response of the alloy is rapid,the peak aging performance is significantly improved,and then the performance decreases rapidly.The size of the precipitation phase in the alloy crystal is slightly larger than the peak aging state at 145°C.The precipitating phase in the crystal of alloy with Sc element increases slightly,the number and size of precipitating phase at the grain boundary decrease,and the precipitate-free zone at the grain boundary becomes narrower.After cryogenic treatment,the strength and plasticity of the alloy are improved,the density of precipitation phase in the alloy increases,the density of precipitation phase at the grain boundary is slightly smaller,and the density decreases,and precipitate-free zone in the grain boundary is narrowed.After quenching at 25°C,the simulation results of the new 7xxx series aluminum alloy thick plate are consistent with the results of the contour test.The residual stress in the thickness direction of the quenched thick plate is symmetrically distributed,the surface is compressive stress,the core is tensile stress,and the inside of the plate is The residual stress amplitude is about 240 MPa.And the amplitude of the residual stress inside the 70°C quenching alloy is about 160MPa.In the two-stage aging process,the residual stress reduction of the alloy is about 40MPa at the end of the pre-aging,and the second-stage aging did not contribute to the reduction of the residual stress.After cryogenic treatment of the alloy at 120°C/8h+145°C/8h,the cryogenic treatment cannot reduce the residual stress of the alloy due to the insufficient temperature gradient.On the contrary,the strengthening of the mechanical properties of the alloy by cryogenic treatment weaken the contribution of creep to the reduction of residual stress in the aging process.In this part,the measurement error of simulation and contour method is less than 15%,so it is considered that the simulation results match well. |
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