| Ultra-high strength Al-Zn-Mg-Cu alloys are widely used in aerospace,defense and military industries due to their excellent comprehensive properties.Among them,the tail fin frame component is an important load-bearing component of the projectiles and rockets weapons,and its high strength,light weight and integrated manufacturing have become the development needs of the new generation of high-end equipment.Aiming at the problem that the forming strength of the cylindrical component of the aluminum alloy tail fin frame with external longitudinal ribs is low and cannot meet the requirements of the existing performance,in this paper,the hot deformation behavior of a new high strength-ductility Al-10.0Zn-3.0Mg-2.8Cu alloy was systematically studied.The constitutive equation and hot processing map were established.The evolution law of the insoluble phase and the recrystallization mechanism during the hot deformation of the alloy were explored.The optimal deformation region in the hot processing map was optimized.Based on the guidance of hot processing map,the aluminum alloy tail fin frame with outer longitudinal ribs were successfully prepared by the repetitive upsetting extrusion-precise backward extrusion process and the thermomechanical treatment experiment was carried out.The effect of thermomechanical treatment process on the hardening behavior,grain microstructure,precipitated phase and tensile mechanical properties of the new high-strength and high-ductility Al-10.0Zn-3.0Mg-2.8Cu alloy was studied,and the optimal thermomechanical treatment process was established.The strengthening and toughening mechanism were revealed,which provided technical and theoretical guidance for the engineering application of this kind of high strength-ductility aluminum alloy.The conclusions are summarized as follows:(1)The Arrhenius constitutive model of the new high strength-ductility Al-10.0Zn-3.0Mg-2.8Cu alloy was established,and the average activation energy of the alloy was 160.43 k J/mol(380 ℃~460 ℃,0.001~1 s-1).The hot processing map of the alloy under 0.9 strain was constructed,and the optimal hot processing region was 431 ℃~456 ℃/0.004~ 0.108 s-1.Combined with the evolution of the insoluble phase,the strain rate can be optimized to around0.1 s-1.(2)Based on the above research,the forming experiment of the tail fin frame was completed,and the single-stage / double-stage pretreatment process was carried out for the component.The results show: Under single-stage pretreatment process: When the prestretching amount is 2 %,the matrix hardness value reaches the highest and the peak aging time of the alloy is 15 h.Compared with the unpretreated sample,the matrix hardness value is increased from 189.5 HB to 232.9 HB,and the grain size is refined from 31.8 μm to 12.4 μm.Under two-stage pretreatment process(60 ℃ / 9h pre-aging + pre-stretching): When the prestretching amount is 5 %,the hardness value of the matrix reached the highest and the peak aging time was 18 h.Compared with the unprestretched sample,the hardness value of the matrix increased from 215 HB to 243 HB,and the grain size was refined from 35.1 μm to 7.5μm.It can be seen that the two-stage pretreatment process has a more significant effect on the aging hardening behavior and grain characteristics of the components.(3)By comparing the effects of the two pretreatment processes on the precipitated phase,the delayed effect of the double-stage thermomechanical treatment on the coarsening of the precipitated phase was revealed.The results show :Under single-stage pretreatment process,the size of the matrix precipitated phase decreases from 5.11 μm to 4.1 μm when the predeformation increases from 0 % to 2 %.When the pre-deformation increases from 2 % to 7 %,the matrix precipitated phase coarsens and its size increases from 4.1μm to 7.24 μm.Under double-stage pretreatment process,the size of the matrix precipitates decreases from 4.53 μm to 3.94 μm,when the pre-deformation increases from 0 % to 5 %.When the pre-deformation increases to 7 %,the size of the matrix precipitates increases to 6.99 μm.Finally,the thermomechanical treatment process of the tail frame was determined as follows: solid solution(475 / 3h)+ pre-aging(60 ℃ / 9h)+ 5 % pre-stretching + aging treatment(120 ℃ / 18h).At this time,the comprehensive performance of the alloy reached the best,the ultimate tensile strength reached 719.7 MPa,the yield strength was 639.6 MPa,and the elongation was 13.9 %.(4)The strengthening and ductility mechanism of the tail fin frame after two-stage thermomechanical treatment was studied.The precipitation strengthening,dislocation strengthening and fine grain strengthening are involved in the alloy strengthening during the thermomechanical treatment of Al-10.0Zn-3.0Mg-2.8Cu alloy tail fin frame.Pre-deformation introduces dislocations,and a large number of dislocations are entangled with each other,resulting in a large amount of deformation energy storage and dislocation strengthening.The introduction of dislocation promotes the uniform and fine dispersion of precipitated phase,which increases the volume fraction of precipitated phase and realizes precipitation strengthening.On the one hand,the deformation-induced grain refinement mechanism is that pre-deformation promotes dislocation recombination and the absorption energy of small-angle grain boundaries to generate a large number of recrystallized grains.On the other hand,the strain-induced dispersed precipitation plays an important role in the pinning of dislocations and grain boundaries,achieving fine grain strengthening.The above three strengthening mechanisms work together to achieve the balance of strength and toughness of aluminum alloy tail fin frame. |
PDF Full Text Request