Research On The Superplaticity Of Magnesium-lithium Alloy

Magnesium-lithium alloy will have a broad prospect of applications in aviation and spaceflight,military industry,automobile industry,3C industry,medical device fileds etc for its numerous advantages such as super light,high specific strength,high specific stiffness,good toughness at high or low temperature,good damping property and shock absorption,good conductivity of heat,good electromagnetic shielding property,good mechanical property,anti-high-energy electron penetrability,easy recycle and so on.However,the research on magnesium-lithium alloy in our country is in the initial stage,and its low formability has become a serious problem nowadays.So the basic researchs about magnesium-lithium alloy are needed to be explored,and we need to investigate the superplasticity in order to improve its formability.Besides,as a new severe plastic deformation(SPD)forming process that makes materials with excellent performance,friction stir processing(FSP)has received wide attention.So the thesis still studied FSP magnesium-lithium alloy for being a reference about FSP that applied to the magnesium-lithium alloy.This thesis analyses the superplasticity emphatically,which based on the rolled Mg-7.28Li-2.19Al-0.091Y sheet with 2.0 mm thickness and FSP Mg-8.41 Li-1.80Al-1.77Zn alloy.At the same time,we research the mechanical properties at room temperature and micro structure of the two kinds of magnesium-lithium alloy mentioned above.The main researches and conclusions are summarized as follows:(1)Mg-7.28Li-2.19Al-0.091Y and Mg-8.41Li-1.80Al-1.77Zn alloy with two phases were prepared.The microstructure of the alloys was investigated by TEM,XRD,SEM and OM.(2)The room temperature mechanical properties of the alloy were studied.The rolled Mg-7.28Li-2.19Al-0.091Y alloy has the tensile strength of 300MPa and the elongation of 15.7%;the rolled annealed Mg-7.28Li-2.19Al-0.091Y alloy has the tensile strength of 210MPa and the elongation of 26.4%;the as-cast Mg-8.41Li-1.80Al-1.77Zn alloy has the tensile strength of 217MPa and the elongation of 32.3%;the hot-rolled Mg-8.41Li-1.80Al-1.77Zn alloy has the tensile strength of 264MPa and the elongation of 29.3%;the FSP Mg-8.41Li-1.80Al-1.77Zn alloy has the tensile strength of 229MPa and the elongation of 34.3%.(3)In order to study superplasticity of two kinds of magnesium-lithium alloy,the high temperature tesile test was conducted at the temperature from 473K to 623K and the strain rate from 1.67 × 10-4s-1 to 1.67 × 10-2s-1.The maximum elongation of the Mg-7.28Li-2.19Al-0.091 Y alloy obtained at the condition of 623K,5.0 × 10-4s-1 was 265.7%;the maximum elongation of the Mg-8.41Li-1.80Al-1.77Zn alloy obtained at the condition of 523K,1.67 ×10-3s-1 was 265.7%.(4)The strain rate sensitivity index values m at different tensile condition were calculat-ed.The evolutions of microstructures,cavities,second-phases during high temperature tesile test were studied.(5)The thermal deformation constitutive equation and the dynamaic recrystallization model of Mg-7.28Li-2.19Al-0.091Y alloy were built.The models agree well with factual data.(6)The constitutive equation that described the superpalstic deformation mechansium of Mg-7.28Li-2.19Al-0.091Y alloy was built,and the deformation activation energy was obtained.The superpalstic deformation mechansium was analyzed based on the values of stress exponent n.(7)The study on the kinetics of stir zone grain growth of FSP Mg-8.41Li-1.80Al-1.77Zn alloy was carried out by isothermal and isochronal annealing treatments.Based on the experimental data on grain growth by annealing treatments,the grain growth kinetic model and the activation energy were figured out.