| Al-Zn-Mg-Cuseries aluminum alloys have the advantages of low density,high specific strength,and good processing performance,and are widely used as structural materials in aviation,aerospace and military fields.With the rapid development of aerospace industry and the acceleration of national defense modernization,higher requirements have been put forward for the mechanical properties of ultra-high strength Al-Zn-Mg-Cualuminum alloys.Optimizing the mechanical properties of the alloys produced by conventional processes has become more difficult,and the development of new production processes plays an important role in the development of the alloys that are urgently needed for structural materials that are both cost-effective and excellent.Based on the characteristics of deep undercooling and rapid solidification,the spray formed Al-Zn-Mg-Cuseries alloys have wide applications in aeronautics and astronautics fields.Therefore,the appearance of spray forming technology provides new ideas for further improving the performance of Al-Zn-Mg-Cualuminum alloys.At present,many results have been achieved in the research of spray formed Al-Zn-Mg-Cualuminum alloys,but there are still some shortcomings in the influence of heat treatment process on the microstructure and mechanical properties of spray formed Al-Zn-Mg-Cualuminum alloys.In this paper,the industrially produced large-size spray formed Al-Zn-Mg-Cu7055aluminum alloy is used as the research object.Optical Microscope(OM),Three-dimensional X-ray Microscope,Scanning Electron Microscope(SEM),Electron Transmission Electron Microscope(TEM),Three-dimensional Atom Probe(3DAP),Electron Backscatter Diffraction Technology(EBSD),end quenching experiment,room temperature tensile test and microhardness testing technology were used to systematically study:(1)Effect of spray forming technology on micro-defects,grain structure and types of primary phase in spray formed 7055 alloy ingot;(2)Effect of homogenization treatment on the evolution of primary phase and the precipitation of Al3Zr dispersoids of spray formed 7055 alloy;(3)Effect of homogenization treatment and extrusion processing on the mechanical properties of spray formed 7055 alloy;(4)Effect of different homogenization treatment on the hardenability of spray formed 7055alloy and effect of quenching sensitivity on the mechanical properties of the alloy.The main conclusions are shown as below:(1)The main micro-defect in the spray formed 7055 alloy ingot is microporosity,which is caused by the formation of a large number of micro-pores due to the trapping of atomizing gas,metal shrinkage and insufficient liquid supply during the spray forming process.Fortunately,extrusion processing can effectively eliminate the adverse effects of microporosity.The main alloying elements and trace alloying elements in the microstructure of the ingot of spray formed 7055 alloy are uniformly distributed.Different from the coarse dendritic structure in the traditional casting 7055 alloy,the grains in the spray formed 7055 alloy ingot are small equiaxed grains.The primary phases in spray formed 7055 alloy mainly include(?)(Mg(Zn,Al,Cu)2)phase and?(Al2Cu)phase.Compared with traditional casting 7055 alloy,the S(Al2CuMg)phase and T(Al2Mg3Zn3)phase were not detected in the as-deposited 7055 alloy.The Al2Cuphase embedded in Mg(Zn,Al,Cu)2 phase are distributed on grain boundary.Moreover,a large number of Mg(Zn,Al,Cu)2 phases with different morphologies and sizes are uniformly distributed in the grain.Effect of spray forming process on the types of primary phases is realized by changing the phase transformation path during rapid solidification.(2)When spray formed 7055 alloy is not homogenized,the Mg(Zn,Al,Cu)2 phase will transform into S(Al2CuMg)phase during the alloy preheated at 420oC?450oC before extrusion processing.Adding high temperature homogenization treatment before preheat treatment can dissolve almost all Mg(Zn,Al,Cu)2 phases,avoiding the formation of S(Al2CuMg)phases.After solid solution treatment,there are still S phases in the residual phases after high temperature homogenization treatment.Because the Mg(Zn,Al,Cu)2 phase precipitated after homogenization air cooling will transform into S phase in the preheating stage.But the size of this type of S phase is smaller than the S phase transformed by the primary phase.When the spray formed 7055 alloy homogenized at 350oC/5h+470oC/24h,30oC/h heating,the number of precipitated Al3Zr dispersoids is the largest and evenly distributed in the entire grain.Therefore,the Al3Zr dispersoids under this homogenization system have the strongest Zener drag and the strongest ability to inhibit recrystallization and grain growth.Compared with traditional casting 7055 alloy,spray formed 7055 alloy has stronger recrystallization resistance.(3)Extrusion process can significantly improve the mechanical properties of spray formed 7055 alloy due to elimination of the adverse effects of microporosity.The mechanical properties of the peak-aged spray formed 7055 alloy homogenized at350oC/5h+470oC/24h,30oC/h heating,are the best.The yield strength,tensile strength and elongation are 736±4.3MPa and 784±4.9MPa,9.5±0.9%,respectively.On the one hand,homogenization treatment can reduce the amount of residual S phase after solid solution and the second phase strengthening caused by?'phases precipitated after aging contributes the most to the mechanical properties.On the other hand,a large amount of Al3Zr dispersoids precipitated during homogenization leads to the smallest grains after solution annealing of the alloy so that the grain boundary strengthening of the alloy contributes the most to the mechanical properties.(4)Different homogenization treatment has an important influence on the hardenability of aged spray formed 7055 alloy.After homogenization treatment at470oC/24h,the precipitation effect of Al3Zr dispersoids is the worst,but the hardenability is the best,and the hardening depth is 80mm.In the alloy after the homogenization treatment at 350oC/5h+470oC/24h,30oC/h,the precipitation effect of Al3Zr dispersoids is the best,but the hardenability is the worst,and the hardening depth is 45mm.Al3Zr dispersoids,grain boundaries,subgrain boundaries and dislocations have important effect on the quenching sensitivity of spray formed 7055 alloy.Because some Al3Zr dispersoids have an increased mismatch with the aluminum matrix after extrusion treatment and solution annealing,resulting in an increase in the interfacial strain energy between the particles and the matrix so that these Al3Zr dispersoids will become heterogeneous nucleation point during slow quenching,increasing the quenching sensitivity.At the same time,the interaction of Al3Zr dispersoids with dislocations,sub-grain boundaries and grain boundaries will also increase the tendency of precipitation of second-phases during slow quenching,increasing the quenching sensitivity. |
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