| High-performance Al-Zn-Mg-Cu alloy(7xxx series aluminium alloy)is an important structural material indispensable for aerospace,weaponry,rail transportation and other fields.At present,the following problems exist in the preparation of Al-Zn-Mg-Cu alloys:billet ingots prepared by traditional direct-chill casting are prone to macroscopic defects such as hot tearing and cold cracking as well as microscopic defects such as non-uniform organization during the preparation process,and Al-Zn-Mg-Cu alloys are difficult in deformation processing due to poor plasticity.Powder metallurgy process has unique advantages in the preparation of large-sized and high-performance Al-Zn-Mg-Cu alloy.However,the high Zn-containing aluminum alloys are difficult to achieve densification by pressureless sintering.And at present,the thermal processing preparation technology and theoretical research basis of Al-Zn-Mg-Cu alloy prepared by powder metallurgy are relatively weak which limits the large-scale application of high-strength Al-Zn-Mg-Cu prepared by powder metallurgy.Therefore,this paper aims to prepare high-performance 7xxx series alloys by powder metallurgy,and carries out research on powder metallurgy 7xxx series alloys via pressureless sintering process,which solves the densification problem of pressureless sintering of high Zn-containing aluminium alloys.The hot working behavior was studied by Gleeble thermal compression experiment which provided a theoretical basis for thermal deformation processing;the high-performance Al8.3Zn-2.3Mg-2.5Cu alloy was prepared by extrusion and rolling process,respectively.The research work and the conclusions are as follows:The effect of Zn element addition on formability and sintering behaviour was investigated by adding pure Zn,Al-Zn50 and pure alloy powders.It was found that the addition of Al-Zn50 as a Zn element maintained good formability while effectively reducing the expansion caused by the Kirkendall effect during sintering.By pre-dehydrating the powder,the air pressure in the closed pores during the hightemperature process can be reduced,thus significantly increasing the density of the sintered billets.The H content in sample after powder dehydration is only 0.31 ppm.An Al-8.3Zn-2.3Mg-2.5Cu sintered billet with a density of 99.1%was obtained,and the grains are isotropic equiaxed with an average grain size of 5.6 μm.Through the pressureless sintering process of powder metallurgy large-size billet ingots,the problem of poor density uniformity of large-size billet ingots was solved,and the Al-Zn-Mg-Cu aluminum alloy billet with a diameter of 650 mm with high overall density consistency was successfully prepared.The rheological behaviour of the powder metallurgy Al-8.3Zn-2.3Mg-2.5Cu alloy prepared by powder metallurgy was systematically investigated using thermal compression experiments in the range of 300-500℃ and strain rates of 0.01-1 s-1.It was found that the steady-state rheological stage was not reached at the final stage at low temperatures and high strain rates,but was reached at high temperatures and low strain rates.The thermal simulation data were used to establish the constitutive equations for the Al-8.3Zn-2.3Mg-2.5Cu alloy,where the presence of internal alumina particles prevented dynamic softening and the average thermally excited activation energy was 193.44 kJ/mol.The optimum deformation region for the Al8.3Zn-2.3Mg-2.5Cu alloy was found to be approximately 380 ℃～450 ℃/0.1 s-1～1 s-1 with a maximum power dissipation efficiency of 0.365.It provides theoretical guidance for subsequent hot working.Powder metallurgy Al-8.3Zn-2.3Mg-2.5Cu alloys with an oxygen content of 0.15%and 0.33%were prepared by blending process and high energy ball milling process,respectively.Al-8.3Zn-2.3Mg-2.5Cu alloy plates with different oxygen contents were successfully prepared by hot extrusion process.Al-8.3Zn-2.3Mg2.5Cu alloy plates with 0.15%oxygen content were studied by extrusion ratio of 4:1,9:1 and 17:1,respectively.It was found that with the increase of extrusion ratio,the recovery and recrystallization structure of the heat-treated samples gradually increased,and the tensile strength also gradually increased,which were 643 MP,677 MPa and 690 MPa,respectively.The Al-8.3Zn-2.3Mg-2.5Cu alloy with oxygen content of 0.33%was studied with 9:1 extrusion ratio.It was found that due to the increase of alumina content,the recovery and recrystallization behaviors were hindered.Compared with the 0.15%oxygen content plate of the same process,the deformed structure with a volume fraction of 28.1%remains along the extrusion direction after heat treatment,the average grain size was reduced from～5 μm to～2μm,and the yield strength was increased by 54 MPa.Its tensile strength,yield strength and elongation were 719 MPa,671 MPa and 11.5%,respectively.After calculation,the contributions of fine-grain strengthening,dislocation strengthening and precipitation strengthening to the yield strength of the alloy were 74 MPa,101 MPa and 485 MPa,respectively.Finally,the ultra-high strength and toughness powder metallurgy 7055 aluminum alloy was prepared in small batches using the best extrusion process and verified through engineering applications.The rolling process was used to study the rolling of powder metallurgical Al8.3Zn-2.3Mg-2.5Cu alloy with 0.33%oxygen content at different deformation amounts.It was found that the average grain size after heat treatment was～2.5 μm,and the tensile strength,yield strength and elongation were 702 MPa,679 MPa and 12%,respectively,after only 44.4%deformation.The plate after 77.8%deformation had more deformation structure than the former rolling state,and the static recovery behavior was dominant for the heat treatment state,but the average grain size remained at a small scale(～2.3 μm)after heat treatment.The tensile strength,yield strength and elongation were 704 MP a and 682 MPa and 12%,respectively,indicating that the powder metallurgy aluminum alloy can obtain good structure and properties under low deformation.The powder metallurgy Al-8.3Zn-2.3Mg-2.5Cu sheet was prepared using a single large deformation rolling process(77.8%deformation)and its microstructure and properties were studied.It was found that after the single large deformation rolling,the deformation storage energy inside the material increased significantly and the deformed grains dominated.After heat treatment,the deformation storage energy was rapidly released and recrystallisation was more likely to occur,so the volume fraction of recrystallized grains along the rolling direction was 50.6%at this point,but the presence of alumina inhibited the growth of recrystallised grains.The tensile strength,yield strength and elongation of the samples were 715 MPa and 691 MPa and 12.5%respectively.The ultra-high strength and toughness powder metallurgy 7055 aluminum alloy armor plate was successfully prepared using this process,and passed the mechanical performance assessment verification. |
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