| With the rapid development of aerospace industry,advanced aircraft,launch vehicles and spacecraft and other equipment are becoming more demanding in terms of lightweight and comprehensive performance of their structural components,and Al-Li alloy has become one of the most promising metal structural materials in the aerospace field because of its low density,high specific stiffness and strength,good fatigue performance,excellent tensile strength and fracture toughness.At present,Al-Li alloy is commonly used in aircraft surface,shell and wing truss,but its plasticity at room temperature is poor,and it needs to be prepared by multistage rolling or extrusion process,and the deformation process has many stages,and the deformation mechanism and microstructure evolution between stages are complicated,and the following problems still exist:Al-Li alloy has significant fiber texture and strong anisotropy after deformation,which leads to uneven flow of the material in the subsequent stages of deformation and prone to cracking.During the deformation of Al-Li alloy,the solute atom agglomeration,precipitation fracture,precipitation coarsening and dynamic precipitation will interact with the dynamic recover and dynamic recrystallization,and the deformation mechanism and microstructure evolution of the material are complicated,so that the microstructure and properties cannot be guaranteed,and it is difficult to realize the controlled shape and properties of the material during hot processing.In response to the above problems,this paper investigates the hot compression test of the as-extruded AA2055 Al-Li alloy and analyzes the hot deformation mechanism and the effect precipitation and its evolution on the recrystallization behavior with the help of scanning electron microscopy(SEM),electron backscatter diffraction(EBSD)and transmission electron microscopy(TEM)and other analytical testing methods.The main work done in this paper is as follows:(1)Hot compression tests with different strain gradients were conducted on extruded 2055 Al-Li alloy with<111>//ED and<100>//ED fiber textures to investigate the deformation mechanisms of the grains with different fiber textures and their grain boundary coordination deformation mechanisms,and to reveal the deformation mechanism of as-extruded Al-Li alloy.It is found that<100>//ED fiber texture grains can activate multiple-slip simultaneously during the deformation process,and the dislocations slip to form LAGBs,which in turn excite the occurrence of CDRX.In contrast,<111>//ED fiber texture grains activate fewer slip systemsand the dislocations are easily pinned by the coarsened precipitation during the slip process.Therefore,the dislocation density in<111>//ED fiber texture grain is much higher than that of<100>//ED fiber texture grain,and the significant dislocation density difference accelerates the migration of grain boundaries between the two grains,thus promoting DDRX behavior and<110>lamellar grain nucleation growth.In addition,the precipitation at the grain boundary also reduces the dislocation and defect density by coarsening and growing.(2)Specimens with three different precipitations was prepared and then deformed in hot compression tests at different temperatures and strain rates to investigate the influence of different precipitations and their evolution on the dynamic recrystallization,and to elucidate the relationships among precipitated phases,dynamic recrystallization,grain structure and mechanical properties.The results show that the recrystallization behavior of the 2055 Al-Li alloy during deformation is mainly influenced by the Ti phase,and the number density of the precipitation and its evolution mechanism significantly affect the volume recrystallization fraction but do not change the recrystallization mechanism.When the compression test is performed at 300℃,the main recrystallization mechanism in the alloy is the DDRX mechanism,which gradually changes to the CDRX mechanism with the decrease in precipitation density.When the compression test is performed at 400 ℃,the main recrystallization mechanism is CDRX and GDRX in the lamellar grains.The 2055 Al-Li alloy,prepared by the air cooling after solution treatment,has the highest recrystallization fraction and exhibits the lowest flow stress under all compression conditions. |
