Study On Isothermal Quenching Property Of High Zn 7 Series Aluminum Alloy With 0.4Sc And Numerical Simulation

With the development of aerospace industry,aircraft lightweight becomes more and more important in order to improve competitiveness and save costs.At the same time,in order to ensure the uniformity of thickness direction performance of large-scale integral components,it is urgent to develop high-performance aluminum alloys with high strength,high toughness and low quenching sensitivity.In this paper,the quenching sensitivity of Al-9.0Zn-2.5Mg-l.5Cu-0.15Zr-0.4Sc aluminium alloy was studied by step quenching method.The time-temperature-transformation(TTT)curve and time-temperature-property(TTP)curve of the alloy were plotted.DSC,XRD and TEM were used to analyze the microstructural changes of the alloy during isothermal process.The transformation kinetics during isothermal process was studied by Johnson-Mehl-Avrami equation.In addition,numerical simulation software was used to simulate the temperature field and stress field of the alloy model obtained by different quenching methods,and the following conclusions were obtained:(1)The TTT and TTP curves of AI-9.0Zn-2.5Mg-1.5Cu-0.15Zr-0.4Sc alloy are C-shaped,and the nose tip temperature is near 330?,and the incubation period is less than 3s.The quenching sensitivity range of Al-9.0Zn-2.5Mg-1.5Cu-0.15Zr-0.4Sc alloy is 2700C-390 0C,and the quenching sensitivity in high temperature region(>390?)is lower than that in low temperature region(<270?).After solid solution of the alloy,the cooling rate(greater than 15?/s)should be increased within the quenching sensitive interval to ensure the mechanical properties of the alloy,and the cooling rate can be appropriately reduced outside the quenching sensitive interval to reduce the internal stress of the material.(2)During isothermal holding,the supersaturated solid solution in Al-9.0Zn-2.5Mg-1.5Cu-0.15Zr-0.4Sc alloy decomposes and precipitates the second phase.The formation of the coarse second phase will lead to the decrease of the concentration of solute atoms around the alloy.During aging,the solute atoms near the second phase can be absorbed and continue to grow,and at the same time,the solute depletion near the second phase will be aggravated,and no precipitation will easily form around the coarse second phase.The precipitate-free precipitation zones(PFZ)is gradually broadened with the prolongation of isothermal holding time.(3)Compared with Al-9.0Zn-2.5Mg-L5Cu-0.15Zr-0.4Sc alloy and Al-9.0Zn-2.5Mg-1.5Cu-0.15Zr-0.2Sc alloy,with the increase of Sc content,the number of A13(Sc,Zr)particles in the alloy will increase,which can be used as the core of heterogeneous nucleation of the second phase during quenching process,and promote the heterogeneous nucleation precipitation of the second phase and increase the quenching sensitivity of the alloy.(4)Coupled with the variation of the field by numerical simulation technology,the simulation results show that the material stress state on the surface experienced from tensile to be compressed in the quenching process.While the inner stress state experienced the reverse of that.Compared with direct immersion quenching,subsection quenching(spray quenching at 455??390?+immersion quenching at 390?-270?+spray quenching ay 270??25?)can greatly reduce the internal stress of the alloy while ensuring that the alloy performance basically does not decline.