Study On Formability And Microstructure Properties Of Al-Cu-Li Alloy With High Strength And Damage Resistance

Since modern times,the design goal of aircraft is to pursue high reliability and long life.Therefore,aircraft materials generally require high comprehensive performance of strength and damage tolerance.Al-Cu-Li alloys not only has high specific stiffness and strength,but also has excellent damage resistance.It is regarded as a new type of high-strength and damage resistant aviation structural material and is often used on the fuselage skin of aviation aircraft.However,Al-Cu-Li alloys has poor formability at room temperature.Nowadays,a large number of researchers have proposed to improve the formability of Al-Cu-Li alloys by using the plasticizing properties of aluminum alloy at low temperature.Although some progress has been made,the inherent property of low plasticity of Al-Cu-Li alloys has not been effectively solved.By adjusting the main alloy composition and optimizing the inherent properties of the alloy,the upper limit of material formability and damage resistance can be effectively.Therefore,based on the regulation of the main components of Al-Cu-Li alloys Cu,Li and Mg,this paper studies the influence of the main components on the formability of Al-Cu-Li alloys in o state and W state at room temperature and low temperature,and explores the influence of the main components on the damage resistance of Al-Cu-Li alloys.It is hoped that the results of this work can provide a research basis for the development and industrial application of Al-Cu-Li alloys.The main research conclusions are as follows:（1）The main component content of Al-Cu-Li alloys has a significant effect on the tensile properties of alloy sheet.The tensile properties of o-state and W-state alloys at room temperature and low temperature are the same.In terms of strength,in the o-state alloy,when the content of Li remains unchanged,its strength mainly increases with the increase of Cu.The strength of W-state alloy increases with the increase of total principal component.In terms of elongation,the elongation of low Li content alloy is higher than that of high Li content alloy in o-state and W-state alloys.In T8 alloy,the strength influence law is the same as that in o state,but the elongation is mainly affected by Mg content,and the elongation of low Mg alloy is higher than that of high Mg alloy.（2）The influence of the main component elements of Al-Cu-Li alloys on the formability of o-state and W-state alloy sheets at room temperature and low temperature is consistent.In the o-state alloy,the strain hardening index and Erichsen cupping value of low Li alloy are higher than those of high Li alloy,that is,the formability of low Li alloy is higher than that of high Li alloy.When the Li content remains unchanged,the strain hardening index of the alloy increases with the decrease of Cu content,and the formability with low Cu content is better.In W-state alloy,the strain hardening index and Erichsen cupping value increase with the decrease of the total amount of alloying elements,that is,the formability of the alloy increases gradually with the decrease of the total amount of alloying elements.Compared with o-state alloy,W-state alloy is higher than o-state alloy in strain hardening index n and forming limit ie,so the formability of W-state alloy is better than o-state alloy.（3）The influence mechanism of principal components on the row forming properties of Al-Cu-Li alloys in different initial states is different.For o-state alloy,Li element is the main factor affecting its formability.Undissolved or dynamically precipitated during annealingδRelative dislocation,sub grain boundary and grain boundary have a certain pinning effect,which can inhibit the recrystallization of the alloy.Therefore,the alloy with high Li has lower recrystallization degree and high work hardening,so the formability is low.At the same time,T₂and T_Bphases coarsen rapidly during high-temperature annealing,and the coarsened T₂and T_Bphases are very easy to become the nucleation point of cracks,resulting in the early fracture of the material,and then affect the formability of the material.For W-state alloy,the total amount of alloying elements is the main factor affecting its formability.After high temperature solid solution,the recrystallization degree of the alloy is high and the dislocation density in the grain is low.Because the strength and plasticity are generally inversely proportional,the total amount of alloying elements is high,and its solid solution strengthening effect is greater,resulting in low formability.（4）The damage resistance of low Mg and low Cu/Li alloy is the best.For alloys with low Mg content,the fatigue crack growth rate decreases with the decrease of Cu/Li.The principal component has a great influence on the initial expansion zone and rapid expansion zone of Al-Cu-Li alloys,while the influence on the steady-state expansion zone is small.For the alloy with low Mg content,with the decrease of Cu/Li,the fracture toughness of the alloy is better,and its UIE value and tear strength are higher.（5）The alloy with high Mg content is easy to form coarse S phase in the crystal,which is the place of crack initiation,resulting in rapid crack propagation and low fracture toughness.At the same time,the addition of excessive Mg is easy to mix with T₁andθ＇equal Cu containing phases produce competitive relationship and inhibit T₁phase andθThe precipitation of＇phase leads to the decrease of strength and fracture toughness of the material.On the contrary,at low Mg and low Cu/Li,there are fine and dense T₁phase and a small amount of T₁phase in the crystalθ＇phase distribution is dominant,which provides the basis for the strength and toughness of the alloy.At the same time,there is no coarse phase and obvious PFZ at the grain boundary,and the strength of the grain boundary will not be lost.In conclusion,the coexistence of multiphase and the synergistic effect of intracrystal and grain boundary are the basis of high strength and damage resistance of the alloy.