Effect Of Hot Rolling On Microstructure, Mechanical Properties And Corrosion Behavior Of Mg-Li Alloy

Due to the increasingly urgent demand for light weight and energy saving in industry,Mg-Li alloy,as the lightest known metal structural material,has high specific strength,high specific stiffness,good electromagnetic shielding properties,low temperature toughness,and has a wide range of prospective applications in aerospace,biomaterials and automotive manufacturing.At present,the absolute strength of Mg-Li alloys is still less than traditional lightweight alloys and the corrosion resistance is poor.These shortcomings limit its application in national production.Therefore,the development of high-strength Mg-Li alloys to improve the strength and corrosion resistance has important practical significance.In this work,Mg-7.3Li-4Al-3Ca and Mg-6.3Li-3.5Al-3Y alloys are hot rolled at 350℃,the rolling reduction of Mg-Li-Al-Ca alloy is 15%,30%,45%and 60%,and that of Mg-Li-Al-Y alloy is 15%,30%,45%,60%and 75%,respectively.The mechanism of microstructure evolution during hot rolling deformation is clarified,and the effects of rolling pressure on the microstructure and mechanical properties of the two alloys are studied.In addition,corrosion weight loss and electrochemical tests are carried out on the two alloys,the corrosion rate,corrosion voltage and corrosion current density are obtained.The influence mechanism of hot rolling on corrosion resistance of the two alloys is also investigated.Finally,the corresponding relationship between the microstructure and comprehensive properties of the alloy after hot rolling is established.The following results are obtained:The matrix of as-cast Mg-Li-Al-Ca alloy is composed of theα-Mg andβ-Li phase.The Al₂Ca phase is distributed as a bulk in the matrix and the（Mg,Al）₂Ca phase is distributed as a eutectic at the grain boundaries.After hot rolling,the grains gradually elongate and become thinner.Under the combined effect of rolling stress and dynamic recrystallisation,the alloy shows a morphology in which elongated deformed grains and dynamically recrystallised grains co-exist.（Mg,Al）₂Ca phase is partially decomposed after hot rolling,and Al₂Ca phase is broken due to rolling stress.The mechanical properties of the rolled alloy are significantly improved,and the mechanical properties of the sample are the best when the rolling reduction is 60%,with the yield strength,tensile strength and elongation reaching 175 MPa,218 MPa and 24.8%,respectively.The mechanical properties of the alloy after hot rolling are improved by dynamic recrystallization resulting in grain refinement,solid solution strengthening formed by decomposition of（Mg,Al）₂Ca phase and enhanced precipitation strengthening by broken Al₂Ca phase.The as-cast Mg-Li-Al-Y alloy consists ofα-Mg phase,β-Li phase,AlLi phase and Al₂Y phase.The alloy matrix has a large amount of continuous light greyα-Mg phase present,dark greyβ-Li phase distributed at grain boundaries,a small amount of filamentous AlLi phase distributed in the grain intervals and a great quantity of bright white Al₂Y phase particles agglomerated in the matrix.After hot rolling,the alloy matrix is compressed into lamellae,the AlLi phase decomposes and the Al₂Y phase breaks up and promote dynamic recrystallisation by particle-stimulated nucleation mechanisms.The tensile strength of the sample increases with the increase of rolling reduction.it has the best comprehensive mechanical properties when the rolling reduction is 75%,with the yield strength,tensile strength and elongation reaching 186MPa,249 MPa and 25.1%,respectively.The grain refinement,dislocation strengthening and precipitation strengthening caused by rolling have significant contribution to the strength improvement of the alloy.The corrosion resistance of Mg-Li-Al-Ca alloy increases with the increase of rolling reduction,while the corrosion resistance of Mg-Li-Al-Y alloy increases first and then decreases with the increase of rolling reduction.The increase in the area fraction ofβ-Li phase and the decomposition of precipitated phase can improve the corrosion resistance of the alloy,while an increase in the contact area ofα-Mg phase toβ-Li phase and an improvement in the dislocation density have a negative impact on the corrosion resistance of the alloy.Therefore,the influence of hot rolling on the corrosion resistance of the alloy is determined by these two competing factors.