Preparation Of Dual-Phase Mg-Li Base Materials And Their Electromagnetic Shielding And Mechanical Properties

Magnesium-lithium（Mg-Li）alloy,as the metallic engineering material with the smallest density,is widely used in aerospace,automotive,3C and other fields because of its high specific strength,high specific stiffness,excellent plastic deformation ability and so on.However,Mg-Li alloys have not been fully developed as EMI shielding materials.At present,there are very few studies on the EMI shielding properties of Mg-Li alloys,and the Mg-Li alloy can hardly meet the requirements of both mechanical properties and electromagnetic interference（EMI）shielding properties in the field of aerospace and national defense equipment.Therefore,theα-Mg/β-Li dual-phase Mg-Li alloy was used as the research object in this work,and a series of preparation processes including alloying,heat treatment,plastic deformation and composite processing were adopted.The EMI shielding effectiveness（SE）and mechanical properties of Mg-Li alloy was investigated through microstructure characterization,EMI shielding and mechanical property testing from the aspects of matrix phase structure,texture,second phase,roll-bonding interface and composite reinforcement.High property Mg-Li matrix materials integrated with structural（high strength and ductility,light weight）and functional（high EMI SE）properties were obtained.First,three binary alloys,α-Mg single-phase,α-Mg+β-Li dual-phase andβ-Li single-phase,were selected as the research objects.The electrical conductivity and EMI SE of the three Mg-Li binary alloys with different crystal structures were researched.Then,the influence of the texture and microstructure caused by the rolling strain on EMI SE and mechanical properties of dual-phase Mg-9Li binary alloy was investigated.Subsequently,Through alloying,heat treatment and rolling,the EMI SE and mechanical properties of Mg-9Li-3Al-1Zn alloy and Mg-8Li-6Y-2Zn alloy were regulated by the type,quantity,morphology,orientation distribution and other factors of the second phase.Mg-9Li-3Al-1Zn sheet was them processed by accumulative roll bonding（ARB）.The improvement of electrical conductivity and EMI SE of dual phase Mg-Li alloy by ARB process was researched.Finally,two kinds of Mg-9Li matrix composites were prepared by composite processing technology.One is anti-electromagnetic interference Ni_0.4Zn_0.4Co_0.2Fe₂O₄（NZCF）/Mg-9Li composites prepared by multi-layer composite rolling（MCR）,in which Mg-9Li alloy serves as the reflecting layer and NZCF particles serve as the absorbing layer.The other is the MWCNTs/Mg-9Li-3Zn-0.5Gd composite with high electrical conductivity,excellent EMI SE,and moderate strength and elongation prepared by the methods of heat treatment,electrophoretic deposition（EPD）and ARB.The increase of Li content in Mg-Li binary alloys leads to the increase of the interference of solute atoms to the movement of free electrons,which reduces the electrical conductivity of Mg-Li alloy.Sinceα-Mg has better electrical conductivity thanβ-Li,the impedance mismatch betweenα-Mg andβ-Li increases the absorption loss inside the material,resulting in a relatively superior EMI SE of dual-phase Mg-Li binary alloy.The α-Mg phase andβ-Li phase in the dual-phase Mg-Li binary alloy are alternately arranged by rolling,and theα-Mg phase shows（0002）basal texture and theβ-Li phase shows（110）texture.The electrical conductivity along the c-axis in hexagonal close-packed（HCP）Mg alloys is greater than that along the a-axis.The body-centered cubic（BCC）structure has the same electrical conductivity in all directions because the electrons moving in all directions have the same mean free path.Therefore,changing the grains orientation of theα-Mg andβ-Li can not only maximizes the impedance mismatch between theα-Mg andβ-Li,but also the impedance mismatch between alloy surface and free space,which greatly increases the loss efficiency of EM waves.The Mg-9Li-3Al-1Zn alloy processed by solution-rolling-aging has subgrains and a large number of nano-scale soft Al Li phase and Mg Li₂Al strengthening phase.The increase of the content of the second phase can effectively improve the electrical conductivity and increase the number of effective reflection interfaces of EM waves,thereby further improving the EMI SE of the alloy.In terms of mechanical properties,the combined effect of subgrain,soft Al Li phase and Mg Li₂Al strengthening phase makes the as-aged alloy with a good balance of strength and plasticity.After heat treatment（500°C for 6 h）,the original irregular network-like（Mg,Zn）₂₄Y₅phase in Mg-8Li-6Y-2Zn alloy is transformed into the long strip-like 18R-LPSO phase,which significantly improves the strength and ductility of the alloy.In addition,many nano-scale（Mg,Zn）₂₄Y₅ phases are also formed in the matrix,thereby increasing the number of effective reflection interfaces for EM waves.The subsequent hot rolling process breaks the LPSO phase,and the broken LPSO phase can promote the dynamic recrystallization,which has a good effect of grain refinement strengthening.At the same time,some nano-scale（Mg,Zn）₂₄Y₅ phases are precipitated during the hot rolling process,and the dispersed precipitation phases can not only play a role in dispersion strengthening,but also greatly improve the electrical conductivity of the alloy.The roll-bonding interface of ARB sheet has multiple absorption and reflection effects on EM waves,which can effectively improve the EMI SE of Mg-9Li-3Al-1Zn sheet.The EMI SE provided by the roll-bonding interface in the multi-layer superposed dual-phase Mg-Li alloy sheet is n-1 times higher than the single-layer sheet（n is the number of sheets）.The electrical conductivity and magnetic property of the NZCF/Mg-9Li composite can be regulated and controlled by changing the content of the nano-scale NZCF magnetic particles.The EMI SE of the 6NZCF/Mg-9Li composite is greatly improved by utilizing the reflection loss of EM waves on the surface of Mg-9Li alloy and the absorption loss between NZCF magnetic particles,and the EMI SE was improved by 42%compared to the EMI SE of pure Mg in the literature.After 4 layers and 5 cycles of ARB process,high electrical conductivity（7.60 MS/m）,EMI SE（89～100 d B）,strength（ultimate tensile strength of 280 MPa）and plasticity（17.8%）of MWCNTs/Mg-9Li-3Zn-0.5Gd composite is produced due to the uniform distribution of MWCNTs and excellent bonding of layer interfaces.