Microstructure Evolution And Mechanical Properties Of Al-Li-Cu-Mg-Zr Alloy

Due to the high specific strength,specific stiffness and other performance characteristics,aluminum-lithium alloy has the a broad application prospect in the field of aerospace.The increase of Li content can further increase the elastic modulus and decrease the density of the alloy.However,it leads to the decrease of plasticity of the alloy severely.Therefore,it is necessary to optimize and control the microstructure to improve the comprehensive mechanical properties of the Al-Li alloy with high Li content.Alloying,heat treatment and deformation processing are important ways to regulate the microstructure and mechanical properties of alloys.It is of great significance for regulating the microstructure and mechanical properties of Al-Li alloy alloys with high Li content to study the influence of alloying elements on the precipitation behavior of the second phase,the aging behavior of the alloy,the evolution of deformation microstructure,and its influence on the evolution of the microstructure and properties of the subsequent heat treatment.In this research,the means of X-ray diffraction(XRD),scanning electron microscope(SEM),electron backscatter diffraction(EBSD),transmission electron microscope(TEM),universal mechanical testing,Vickers microhardness testing and other testing methods were employed to systematically investigate the microstructure and mechanical properties of Al-Li alloys with high Li content.The main research content includes four aspects.1).The effects of Cu/Mg ratio on the microstructure and mechanical properties of Al-Li alloys with 2.8 wt.%Li and 0.2 wt.%Zr.2).The aging precipitation behavior of as-cast Al-2.8Li-1Cu-3Mg-0.2Zr alloys.3).The evolution of microstructure and mechanical properties during hot extrusion and hot rolling of Al-2.8Li-1Cu-3Mg-0.2Zr.4).The influence of deformation on the evolution of microstructure and mechanical properties in subsequent heat treatment.The mechanism of the influence of Cu/Mg ratio on the microstructure and mechanical properties of the as-cast Al-3Li-x Cu-y Mg-Zr alloys is elucidated.The major strengthening mechanisms are solid solution strengthening and second phase strengthening of the??(Al₃Li)phase.The increase in the size of the??(Al₃Li)phase is promoted with the Mg content,resulting in an increase in the strength as the Cu/Mg ratio decreases.Meanwhile,the formation of the?'(Al₃Zr)phase,the continuous precipitation phase along the grain boundaries and the precipitation-free zone(PFZ)synergistically lead to a low plasticity of the as-cast alloy.When the Cu/Mg ratio is 0.3,the comprehensive mechanical properties of the as-cast alloy were found to be preferable,with tensile strength of 281 MPa,yield strength of251 MPa,and elongation of 5.28%.During aging treatment of Al-2.8Li-1Cu-3Mg-0.2Zr alloy,the precipitation phases are dominated by S₁(A1₂Mg Li),??(Al₃Li)phases and the formation of Al₃(Zr,Li)core-shell structure when the aging heat treatment is carried out at 155°C and 185°C.The S₁(A1₂Mg Li),??(Al₃Li)and Al₃(Zr,Li)core-shell composite phase experience more obvious growth behavior with the increase of aging temperature and time,which causes that the microhardness and tensile strength of the alloy exhibit a trend of an increase followed by a decrease with the increase of aging time.Simultaneously,the plasticity of the alloy is reduced due to the remaining Cu-rich phase at the grain boundaries and the broadening of PFZs after solid solution treatment.The mechanisms for the evolution of the microstructure and properties of the as-cast Al-2.8Li-1Cu-3Mg-0.2Zr alloy under thermal deformation are elucidated.During thermal deformation,the microstructure evolution is dominated by dynamic recovery,with a small amount of dynamic recrystallization.Regarding the texture evolution,the texture during hot extrusion deformation was concentrated on the?orientation line,while the Cube{001}<100>recrystallization texture is formed at the grain boundaries.However,the texture is mainly gathered on the?orientation line during the hot rolling deformation,with a small amount distributing on the?orientation line.The main texture is Goss{011}<100>with 50%rolling deformation,whereas the texture of Goss{011}<100>weakens and new shear texture of Rotated-cube{001}<110>is formed with 70%rolling deformation.The alloy achieves a high tensile strength up to 365 MPa after hot extrusion owing to the synergistic effect of hardening and softening during heat deformation.As for the hot rolling process,the strength of the alloy increases with the increase of deformation and the elongation decreases slightly.The mechanical properties of the alloy can be further improved after subsequent heat treatment of the thermally deformed Al-2.8Li-1Cu-3Mg-0.2Zr alloy.After the solution treatment of the deformed alloy at 535°C×40 min,the static recovery,recrystallization and dissolution of grain boundary precipitation phases occur significantly in the hot extruded and70%hot-rolled alloys,while slight recovery and recrystallization occur in the 50%hot-rolled alloy.The major precipitations are S₁(A1₂Mg Li),??(Al₃Li)and Al₃(Zr,Li)core-shell composite phase during the subsequent aging at 155°C.The tensile strength of the as-extruded alloy is 492 MPa at peak state,and 480 MPa and 510 MPa at peak state for the50%and 70%hot-rolled deformation alloys,respectively.The mechanical properties of the high lithium content Al-Li-Cu-Mg-Zr alloy can be significantly improved by optimizing the relative contents of Cu and Mg as the main alloying elements and modifying the heat treatment,the thermal deformation and the post-deformation heat treatment processes.When the alloy of Al-2.8Li-1Cu-3Mg-0.2Zr is hot rolled with 70%deformation,followed by solid solution treatment at 535°C×40 min and aging treatment at155°C×20h,the tensile strength can reach 510 MPa,which makes the alloy have a broad prospect in the application of weight-saving structural components.