Study On The Correlation Between Grain Structure And Properties Of 7A56 Aluminum Alloy

7xxx aluminum alloys are the most important lightweight structural materials in aerospace manufacturing industry because of their high strength and toughness as well as good corrosion resistance.As the structural design of aircraft puts forward higher requirements on the comprehensive performance of high strength and toughness 7xxx aluminum alloys,it has become the focus and hot spot of current research in aluminum alloy industry to maximize the performance potential of the material and obtain the best match of corrosion resistance and toughness by regulating the microstructure of 7xxx aluminum alloys.The microstructure characteristics of grains,coarse compounds and precipitates are the key parameters to the performance of 7xxx aluminum alloys.The preparation of 7xxx aluminum alloys mainly includes melting and casting,homogenization heat treatment,heat deformation processing,solution quenching and ageing treatment processes.Most of the previous research work on microstructure control focused on the coarse compounds of ingots,homogeneous dispersed precipitates,quenching or aging precipitates,etc.The correlation between grain structure and properties of 7xxx aluminum alloys has been little studied and is still controversial.The evolution of grain structure characteristics of 7xxx series aluminum alloys is mainly affected by thermal deformation and solution treatment,especially directly controlled by the deformation parameters.Therefore,to provide a theoretical basis and experimental foundation for alloy organization and property matching,the high performance 7A56 aluminum alloy was selected for the study and the same ingot,solution and ageing treatment were adopted to ensure that the original grain structure,coarse compounds and precipitates of the alloy were basically the same.The effect of deformation parameters on the grain structure and mechanical properties of the alloy have been studied systematically under industrial conditions and the relationship between grain structure and mechanical properties is established.Besides,the correlation between recrystallized grain characteristics and tensile properties-fracture toughness-fatigue properties-stress corrosion resistance has been investigated in combination with appropriate solution treatment.Furthermore,the results have been verified by industrially prepared 7A56 aluminum alloy large-size forgings and plates.The main jobs and results include the following aspects.The effect of deformation rate on the evolution of grain structure and mechanical properties of 7A56 aluminum alloy under the coupling effect of deformation degree and deformation temperature was studied.Under the conditions of coupled deformation degree(40%,60%),deformation temperature(430℃,450℃)and deformation rate of 5-20 mm/s,the size of sub-grains does not differ much,and the recrystallized grains are equiaxed with a small area fraction.While the size of recrystallized grains increases first and then decreases with the increase of deformation rate,getting the largest grain size at a deformation rate of 10 mm/s.As for the mechanical properties,the deformation rate has no significant effect on the tensile strength and fracture toughness.However,the fatigue crack propagation rate decreases and then increases with the increases of the deformation rate.In summary,the alloy has the best comprehensive mechanical properties at the deformation rate of 10 mm/s.The role of deformation degree on the evolution of grain structure and mechanical properties of 7A56 aluminum alloy under the coupling effect of deformation rate and deformation temperature was investigated.Under the conditions of coupled deformation rate(5 mm/s,10 mm/s),deformation temperature(450℃)and deformation degree of 40%-90%,the size of sub-grains has no obvious change,and the recrystallized grains are equiaxed.The recrystallized grain size increases gradually with the increase of deformation degree.The recrystallization area fraction is small when the deformation degree is 40%-60%and increases significantly when the deformation reaches 90%.Under this combination of deformation parameters,the deformation degree has little influence on tensile strength and fracture toughness,while the resistance to fatigue crack propagation increases first and then decreases.The alloy possesses the best resistance to fatigue crack propagation and the perfect comprehensive mechanical properties at the deformation degree of 60%.The influence of deformation temperature on the evolution of grain structure and mechanical properties of 7A56 aluminum alloy under the coupling effect of deformation rate and deformation degree was learned.The effect of deformation temperature on the grain characteristics of alloy is closely related to the deformation degree.At a deformation rate of 10 mm/s,a deformation degree of 60%and deformation temperature of 400-450℃,the size of sub-grains does not differ much,and the recrystallized grains are equiaxed with a small area fraction.While the size of recrystallized grains increases with the increase of deformation temperature.When the temperature is as low as 300℃,the morphology of recrystallized grains changes from fine equiaxed grains to coarse grains with lath features.At the same time,the recrystallization area fraction increases significantly.At a large deformation degree(90%)and a deformation rate of 5 mm/s,the size of fine equiaxed recrystallized grains increased slightly and the area fraction increased significantly when the deformation temperature reduces from 450℃ to 400℃.Under the conditions of coupled deformation rate(10 mm/s,5 mm/s),deformation degree(60%,90%)and deformation temperature of 400-450℃,with the increase of deformation temperature,the tensile property does not change significantly,the fracture toughness increases slightly but the difference is small,while,the fatigue crack propagation rate decreases.The comprehensive mechanical properties of the alloy with deformation temperature of 300℃ and 450℃ do not differ much at the deformation degree of 60%.Based on the investigation of the correlation between the deformation parameters and the mechanical properties,the suitable interval of deformation parameters are put forward as:deformation temperature of 400℃-450℃,deformation degree of 50%-60%and deformation rate of 5 mm/s-15 mm/s.Based on these results,the effects of recrystallization morphology and degree on the mechanical properties and corrosion resistance of 7A56 aluminum alloy were investigated.When the area fractions of lath recrystallization and equiaxed recrystallization are not more than 22.34%and 13.46%,respectively,the recrystallized grains are distributed in a sporadic and non-aggregated form.The recrystallized grains with larger size can effectively hinder the crack propagation,thus improving the fracture toughness and the fatigue crack propagation resistance of the alloy.When the area fractions of lath recrystallization and equiaxed recrystallization are 34.10%and 22.40%,respectively,or above,the recrystallization will be connected into chains or nets.The larger the area fraction of the recrystallization,the more pronounced of the corresponding characteristics.As a result,the fracture toughness and fatigue crack propagation resistance of the alloy decrease with the increase of recrystallization degree.The size of the coarse lath recrystallized grains is larger than that of the plastic cycle zone at the fatigue crack tip,so the hindering effect on crack propagation is stronger than that of equiaxed recrystallized grains.Furthermore,the size and spacing of grain boundary precipitates of lath recrystallized grains are bigger than those of equiaxed recrystallized grains.At the same time,the content of Zn and Mg in the former ones are lower than that in the latter ones,while the Cu content of the former is higher than that of the latter.Therefore,the corrosion resistance of lath recrystallized grain boundaries is better than that of equiaxed recrystallized grain boundaries.In summary,the coarse lath recrystallization is more beneficial to the fracture toughness,fatigue crack propagation resistance and stress corrosion cracking resistance of the alloy than the fine equiaxed recrystallization.The grain characteristics,mechanical properties and stress corrosion resistance of the industrially prepared 7A56 aluminum alloy forgings and plates were analyzed.Both the fine equiaxed recrystallized grains with small area fraction and the large lath recrystallized grains with discrete distribution were obtained by industrial forging.And the prepared 7A56 forgings have excellent mechanical properties and stress corrosion resistance.The 7A56 aluminum alloy plates with lath recrystallized grains were prepared under industrial production conditions.The results of performance show that with the increase of recrystallization area fraction,the fracture toughness and fatigue crack propagation resistance of the alloy increase first and then decrease.When the recrystallization area fraction is 20.20%or above,the lath recrystallization forms a network,which deteriorates the mechanical properties and the stress corrosion cracking resistance of the alloy.