Research On The Effects Of Zn Element On Mechanical And Damping Capacities Of Al-Mg Alloy And Its Mechanisms

The Al-Mg series alloys are widely used in aviation,aerospace,national defense and military industries due to their low density,excellent corrosion resistance and welding performance.However,the application of the alloys is greatly restricted because of their low mechanical properties.The author’s research group have found that increasing the Mg content can not only improve the mechanical properties of the alloy,but also reduce the stacking fault energy（SFE）of the alloy,expand the width of the extended dislocation,and significantly improve the damping performance of the alloy.However,increasing the Mg content will also result in a significant decrease in the corrosion resistance and hot formability of the alloy.It’s reported the corrosion resistance of such Al-Mg alloy alloy can be improved by Zn addtion,but effect of Zn addtion on the mechanical and damping properties of the alloy is still unclear.Based on the background,the Al-9.2Mg alloy with good damping and mechanical properties is taken as the object and Zn is used as the alloying element to design and parepare six alloy compositions of Al-9.2Mg-x Zn（x=0～1.5,wt.%）alloy in this thesis.The alloy plate was prepared by homogenization treatment,high strain rate hot rolling（HSRR）and HSRR hot rolling+cold rolling.The microstructures and secondary phase particles are characterized by metallographic microscope（OM）,scanning electron microscope（SEM）,transmission electron microscope（TEM）and X-Ray diffractometer（XRD）.The uniaxial tensile test and DMA damping test are respectively adopted to characterize the room temperatural mechanical and damping properties of the alloys.Effects of Zn content on microstructure,mechanical properties and damping behaviors of the Al-9.2Mg-x Zn alloys in the as-cast,as-homogenized,hot-rolled and hot-rolled+cold-rolled states are studied,and the related mechanisms are discussed.The main research contents and conclusions of the paper are:（1）Effect of the Zn content on the microstructure,mechanical properties and damping properties of the Al-9.2Mg-x Zn alloys in the cast and homogenization states are clarified.The results show that the as-cast Al-9.2Mg-x Zn alloy has a typical dendritic structure,and the increase of Zn content can promote the precipitation of Al₃Mg₂ phase.Under low strain amplitude,the damping performance of the alloy decreases with the increase of Zn content;under high strain amplitude,the damping performance of the alloy generally shows the opposite trend,and the damping performance of Al-9.2Mg-1.2Zn alloy is the best(The damping value Q^-1 reaches0.0168).The homogenization treatment can completely dissolve the Zn element in the matrix.With the increase content of Zn element,the degree of lattice distortion((a-a _Al)/a _Al)of the alloy increases.The strength of the alloy is promoted,while the plasticity is first improved and then reduced due to the Zn addition.The Q^-1 of the alloy also increases with the Zn content.The best comprehensive properties at room temperature is found in the Al-9.2Mg-1.2Zn alloy,with the ultimate tensile strength（UTS）,yield tensile strength（YS）,elongation（El）and damping values(Q^-1)of407±2MPa,196±1MPa,45±2%and 0.0164,respectively.In addition,there are two damping peaks（P₂ and P₄）in the temperature damping spectrum of the as-homogenized Al-9.2Mg alloy,while four damping peaks（P₁,P₂,P₃ and P₄）are detected in the Zn-containing alloys.The damping performances of Zn-containing alloys are significantly better than that of the Al-9.2Mg alloy within the temperature range of 260～400℃.（2）Effect of Zn content on dynamic recrysta3llization（DRX）behavior,microstructure,mechanical properties and damping properties of the Al-9.2Mg-x Zn alloy during high-strain-rate hot rolling are clarified.The results show that the influence of Zn content is significant.On the one aspect,the addition of Zn can promote the appearence of the deformation bands（DBs）during HSRR and enhance the DRX behavior;Meanwhile,more precipitates are developed and thus the DRX process is restricted,resulting the first enhanced and then weakened DRX behavior.Among them,the most complete dynamic recrystallization microstructure is exhibited in the Al-9.2Mg-0.5Zn sheet,with dynamic recrystallization volume fraction,high-angle grain boundary ratio,average grain boundary angle,texture strength,and dislocation density of 91.5%,91.1%,37.6°,2.71 and 7.3×10¹³m^-2,respectively;The yield tensile strength increases gradually,while the elongation firstly increases and then decreases.The Al-9.2Mg-0.5Zn shows the excellent plasticity,which can be ascribed to its low dislocation density,weak texture and strong work hardening ability.The damping performance of the hot-rolled sheet increases first and then decreases,which is influenced by its texture strength,moveable dislocation density and precipitate number.The most excellent comprehensive mechanical and damping properties are detected in the Al-9.2Mg-0.5Zn alloy,whose ultimate tensile strength,yield tensile strength,elongation and damping values are 434±4MPa,228±2MPa,48±2%and 0.0184,respectively.（3）Effect of Zn content on microstructure,mechanical properties and damping performance of the cold-rolled Al-9.2Mg-x Zn sheets after high strain rate hot rolling are clarified.The results show that the surface layer of Al-9.2Mg-x Zn alloys presents fine equiaxed microstructures,and the dislocation density increases with the increase of Zn content.Medium to high mechanical properties are found in the hot-rolled+cold-rolled Al-9.2Mg-x Zn sheets.The ultimate tensile strength is between 530～570MPa,the yield tensile strength is between 430～445MPa,and the elongation is higher than 10%.The ultimate tensile strength is improved about 20～35MPa by addition of Zn.The hightened strength of the hot-rolled Al-9.2Mg-x Zn sheets though cold rolling results from the high density of dislocations.The alloy damping performance increases first and then decreases,which is due to the movable dislocations.The best comprehensive mechanical and damping properties is obtained in the Al-9.2Mg-1.0Zn sheet,with the ultimate tensile strength,yield tensile strength,elongation and damping value of554±7MPa,445±4MPa,10.6±0.3%and 0.0168,respectively.