Research On Regulation Of The Microstructure And Properties Of 800 MPa Al-Zn-Mg-Cu Alloy For Air Vehicle Tail Wing And Related Mechanism

The tail wing extruded by high alloyed Al-Zn-Mg-Cu alloy can not only greatly reduce weight,but also improve manufacturing efficiency.However,the volume fraction of primary phase increases with the increasing content of alloy element,and the problems such as coarse and inhomogeneous microstructure,macro segregation and hot cracking are easily observed in the DC casting,resulting in more difficult in plastic deformation,narrowing the processing window,which eventually leads to poor formability of the material and low mechanical properties of the components.To address these problems,this paper conducts research on the design of alloy composition,the preparation of high-quality materials,and the evolution of microstructure and mechanical properties.The influence of Zn/Mg and Cu content on the microstructure and mechanical properties was investigated and the alloy composition was optimized,the mechanism of homogeneous,fine grain and low segregation in electromagnetic field aided casting was discovered,the effect of the initial microstructure on the processability was analyzed.And the research results were applied to the industrial manufacturing,a tail wing with 800 MPa is successfully manufactured.The main research contents and conclusions are as follows:（1）The influence and mechanism of Zn/Mg ration and Cu content on the microstructure and mechanical properties of high alloyed Al-Zn-Mg-Cu alloy was studied.For the alloys with constant Zn+Mg,when Zn/Mg≤1.91,the alloy is in theα（Al）+T two-phase region at 470°C,and a large number of solute elements exist as residual eutectic phases,which cannot be effectively utilized.In Al-10.0Zn-2.6Mg-x Cu-0.15Zr alloy,Cu exists inσphase in form of solid solution,resulting in the suppression of second phase dissolution due to the low diffusion rate of Cu.Based on the study of mechanical properties,the composition of an 800 MPa Al-Zn-Mg-Cu alloy was obtained.（2）A coupled multi-physical field mathematical model of electromagnetic field aided casting was established,and the effects of electromagnetic fields on the flow,heat transfer and solidification of the melt were investigated.The results indicate the flow direction of Al melt is changed by forced convection induced by electromagnetic force,which prevents the superheat Al melt from flowing into the ingot center.In addition,the flow velocity and heat transfer efficiency of Al melt increase significantly due to forced convection,the liquid fraction in the mold decreases from 0.9 to 0.8 compared with DC casting.（3）The DC casting and electromagnetic field aided casting ofΦ320mm ingot were carried out,and the differences of microstructure,elements distribution and mechanical properties between the two ingots were studied,the mechanism of fine grain and low segregation in electromagnetic field aided casting was revealed by combining with numerical simulation.Under the shear effect of electromagnetic force,the dendrite arms at the front of solidification break.These broken dendrite arms are remelted and multiplied during the flow process,which increases the nucleation rate and promotes the transformation of grains from dendrites to equiaxed grains.In addition,the melt cooled by the mold enters the center of the ingot,which increases the supercooling in the center,which is beneficial to the formation of uniform and fine grain in the whole ingot cross section.Solute-rich liquid Al is supplemented by electromagnetic force from the inlet to the edge,which reduces the negative segregation at the edge,and the solute-poor floating grain at the solidification front are brought to the center by electromagnetic force,which reduces the positive segregation at the center.（4）The evolution of microstructure during homogenization of DC ingots and electromagnetic field aided casting ingots was studied.The kinetic equations of second phase dissolution and element diffusion were established,and the time of second phase dissolution and diffusion time of solute atoms during homogenization was predicted.The influence of homogenization treatment on the precipitation behavior of Al₃Zr in the two alloys and the recrystallization behavior was investigated.The results show that the size of Al₃Zr in electromagnetic field aided casting alloy is slightly lower than that in DC casting alloy.Compared with single-stage homogenization,the size of Al₃Zr particles decreased from 13.14 nm to6.59 nm and the number density increased from 1.93×10²⁰ m^-3 to 9.32×10²⁰m^-3 after two-stage homogenization.The recrystallization is more likely to be found in electromagnetic field aided casting ingots due to smaller grain size and more sites for recrystallization.（5）The hot deformation behavior of the two ingots was studied,and the constitutive models were established.The processing map were established based on the DMM model,and the best processing window were provided.The microstructure was characterized by EBSD and TEM to investigate the deformation mechanism.The results show that electromagnetic field aided casting can significantly reduce the deformation resistance,after the application of the electromagnetic field,the hot deformation activation energy is reduced from 171.6 k J/mol to144.7 k J/mol.The processing map indicates that the optimum processing window is 400-450°C/0.01-0.001 s^-1.At high Z values,the dynamic softening mechanism is DRV and DDRX,which transforms into DRV and CDRX as the Z value decreases.（6）An industrial manufacturing scheme for tail wing was proposed based on the above elementary research,and the method of"high-temperature solid solution→aging→cold finishing"was proposed to improve the mechanical properties of tail wing.The method achieves synergistic strengthening of high-density dislocations and nano precipitates,and the tensile strength is 801.7 MPa,the yield strength is776.5 MPa,the elongation is 5.9%.