| 7xxx series aluminum alloys are widely used in the manufacture of lightweight structural parts in the aerospace equipment and aviation manufacturing industry and are indispensable backbone structural materials for advanced military and civil aircraft.In this paper,three typical ultra-high strength and high toughness aluminum alloys(7B50-T7751,7055-T7751 and 7A56-T7751)for upper wing bulkheads on large aircraft were used as research objects.By carrying out the quantitative characterization of microstructure refinement and its correlation with key properties,the correspondence between key properties and microstructure characteristics of three typical alloys was clarified.At the same time,the basic understanding of the development patterns of aluminum alloys for different generations of airfoil bulkheads was deepened.And the microstructure control methods to enhance the key properties of the alloys were proposed.Which can provide data support a and theoretical basis for the optimization of comprehensive performance,technology maturity enhancement and industrial application of the new generation 7A56 aluminum alloy.Based on the above objectives,the following studies were carried out in this paper.The microstructural differences of three alloys were investigated and the typical microstructural characteristics were quantitatively characterized.The average grain size of the alloys was characterized by EBSD.The types,morphology and size distribution of the insoluble phases of the alloys were characterized and counted by SEM and OM.The intracrystalline and grain boundary precipitates were characterized and counted by TEM and HRTEM.These analyses can provide basic data for the study of the correlation between microstructural characteristics and key properties of the alloys.The quantitative relationship between microstructural characteristics and tensile properties of three alloys was investigated.The quantitative model between microstructure and ultra tensile strength of the alloy was constructed by considering the four strengthening theories:UTS=??+kyd1/2+364.4f1 1/2 r1 1/2+0.4(?)+? AC02/3+BGb/?.The critical value(2.5 nm)of the radius of the precipitates cut through/bypassed by dislocations,which was optimized for the model,was given.And the model calculation results were in good agreement with the measured results.When the volume fraction of the precipitates is fixed,the dislocation hindering effect was larger when the precipitates was concentrated on the right side near the critical radius value,and the alloy will have a higher tensile strength.The quantitative relationship between the microstructural characteristics and fracture toughness of three alloys was investigated.Based on the classical prediction model of fracture toughness,the characteristics such as the shape factor and distribution of the Fe-rich phase were introduced into the classical model in combination with the finite element analysis,and a significant improvement in the accuracy of fracture toughness prediction was achieved.By analyzing the model parameters,it was found that when the shape factor of the Fe-rich phase is small and the volume fraction is small,it is beneficial to improve the fracture toughness of the alloy.The strength and toughness matching levels of thick plates of 7xxx series aluminum alloys for upper wing bulkheads were compared and analyzed.The ultimate tensile strength and fracture toughness of 7055 aluminum alloy increased by 8.0%and 4.0%compared with 7B50 aluminum alloy,respectively,and the matching of strength and toughness improved slightly.7A56 aluminum alloy decreased the ultimate tensile strength by 4.0%and increased the fracture toughness by 17.9%compared with 7055 aluminum alloy,and the matching of strength and toughness improved significantly.With the improvement of strength,fracture toughness and their matching levels of the three alloys,the stress corrosion resistance of the alloys gradually decreased,and the stress corrosion sensitivity was:7B50 aluminum alloy<7055 aluminum alloy<7A56 aluminum alloy. |
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