Research On The Multidirectional Forging Process And Superplasticity Of Magnesium-lithium Alloy

With the increasing requirements of lightweight materials,magnesium alloys,a lightweight engineering structure material,have been attracted extensive interests of researchers.Magnesium-lithium alloys,as the lightest metal structure materials,possess many virtues such as high specific stiffness,high specific strength,excellent damping capabilities,good electromagnetic shielding properties and good mechanical property.Mg-Li alloys have a wide application potentials in the fields of aerospace,automobile manufacturing and electronic products.In recent years,metallic superplasticity forming has become more widely used in industrial production,this technology can be utilized to manufacture complex shape,large deformation workpiece,however superplasticity usually requires material initial organization has fine grain size.Multidirectional forging technology,as a representative process of severe plastic deformation,can be used to produce fine or ultrafine grained material.Therefore,in order to improve the properties of Mg-Li alloys and expand its application field,it is very important to study multidirectional forging process and superplasticity of Mg-Li alloys.This paper analyses the multidirectional forging process and superplasticity of Mg-Li alloys,which based on the new dual phase Mg-8.14Li-1.27Al-0.43Zn-0.2Ce alloy and Mg-10.2Li-2.1Al-2.23Zn-0.2Sr alloy.At the same time,we research the microstructure and mechanical properties of the two kinds of Mg-Li alloy mentioned above.The main researches and conclusions are summarized as follows:(1)Mg-8.14Li-1.27Al-0.43Zn-0.2Ce alloy processed by multidirectional forging under decreasing temperature conditions.The room temperature mechanical properties of the each pass alloy was studied,the ultimate tensile strength and elongation of the Mg-8.14Li-1.27Al-0.43Zn-0.2Ce alloy reachs the maximum after six passes multi-directional forging,it has the ultimate tensile strength of 220MPa and the elongation of 32.6%.(2)The multidirectional forging and rolling Mg-10.2Li-2.1Al-2.23Zn-0.2Sr alloy has the ultimate tensile strength of 242MPa and the elongation of 23.59%;the annealed Mg-10.2Li-2.1Al-2.23Zn-0.2Sr alloy has the ultimate tensile strength of 161MPa and the elongation of 40.37%.This result show that the properties of Mg-Li alloys can be improved by the plastic processing of multidirectional forging and rolling.(3)In order to study superplasticity of Mg-10.2Li-2.1Al-2.23Zn-0.2Sr alloy,the high temperature tensile 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 obtained at the condition of 623K,1.67×10-3s-1 was 712.1%.Based on the true stress-strain curves,the influences of temperatures and strain rates on flow stress were studied.Finally,the evolutions of microstructures and cavities during superplasticity deformation were studied.(4)The constitutive equation of thermal deformation and superplastic deformation was built,besides the values of stress exponent(n),grain size exponent(p)and deformation activation energy(Q)were calculated.