| X2A66 alloy is the fourth generation of novel Al-Li alloy developed independently by China, which has many advantages, such as low density, high specific strength, good formability. It is suitable for manufacturing the plane wall panel structure, can achieve the double weight loss goal of material itself and structure design. Besides, it has the advantages of good structural integrity and high static strength, in line with the future development trend of aerospace technology, so it has wide application prospects. The influence of heat treatment process on the microstructure and properties of the alloy was studied, so as to fully excavate its potential performance, and further promote its application in the field of aerospace.The hardening behavior, room-temperature mechanical properties and microstructure evolution rules of the X2A66 Al-Li alloy were investigated by hardness test, conductivity test, optical microscopy(OM), room-temperature tensile test, differencial scanning calorimeter(DSC), scanning electron microscopy(SEM), X-ray Diffraction(XRD) and transmission electron microscope(TEM). Not only this thesis optimized the reasonable heat treatment system, but also ascertained the dynamic precipitation behavior and evolution rules of the precipitation phases in the alloy during aging process.The research of solution treatment shows that the coarse second phase particles in the alloy gradually dissolve into the alloy matrix with the increase of solution temperature and solution time in the range of 460~520? and 40~80min, and the solution effect is enhanced. But the grains will become coarse and even recrystallize with the further increase of solution temperature or time, and the strength of the alloy decreases gradually. The best solution treatment process of X2A66 alloy obtained in this experiment is 520?/80 min.Through the study of the aging behavior of X2A66 alloy, it is found that the alloy has a strong property of natural aging at room-temperature after solution treatment. The hardness of the alloy increases rapidly in the early stage of natural aging, which increases from the initial 84.5HV to 132.4HV after naturally aged for 22 h, then the hardness increases slowly, the alloy enters into a stable state after a week, and the hardness can reach 140.3 HV. The major strengthening phases of X2A66 alloy are G.P. zone, ?' phase and a small amount of ?'/?' composite phase after long-term natural aging treatment.Single-stage aging research of X2A66 alloy shows that it can obtain the highest hardness and strength while aged at 165? for 72 h, the yield strength is 549.1MPa, while the tensile strength can reach 598.8MPa, but the elongation rate is only 6.8%.The main strengthening sources during the early stage of artificial aging at 165? are G.P. zone, ?' phase and a small amount of ?'/?' composite phase, but during the middle and late stage they are ?' phase?T1 phase and a small amount of ? phase. The precipitates species of the alloy under single-stage peak aging state at different temperature are almost the same, but the grain boundary Precipitate Free Zone(PFZ) whose width is up to about 100 nm occurs when the alloy is aged at 185?. It can lead to the decrease of the fracture toughness, so causes very adverse effect on the mechanical properties of the alloy.Double-stage aging research of X2A66 alloy shows that the double-stage aging process of 165?/48h+130?/24 h can greatly improve the strength of the alloy while makes the alloy have a good plasticity, the yield strength is 525.5MPa, the tensile strength is 581.9MPa, and the elongation rate is 7.9%, a better match of strength and plasticity of the alloy is obtained. The main precipitates of X2A66 alloy after double-stage aging are still ?' phase?T1 phase and a small amount of ? phase, but the size of T1 phase is different, and it is relatively small, the quantity is increased.RRA research of X2A66 alloy shows that the main precipitates of X2A66 alloy after treated with a RRA process of 165?/32h+225?/40min+165?/24 h are almost the same as treated with other aging systems, but, a large number of small needle-like and lath-like precipitates are intermittently distributed along the grain boundaries of the alloy, the discontinuous distribution of the precipitates in the grain boundary destroys the continuity of the grain boundaries, and it will greatly improve the corrosion resistance of the alloy. After the X2A66 Al-Li alloy extruded sheet is treated with this process, the yield strength is 514.3MPa, the tensile strength is 569.0MPa, and the elongation rate is 7.1%. |
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