Research On The Multidirectional Forging Process And Superplasticity Of Mg-4.4Li-0.46Al-2.5Zn-0.74Y Alloy

Magnesium lithium alloy,as an ultra-light metal structural material,is widely used in aviation,3C,transportation and other fields with high specific strength,specific stiffness,excellent electromagnetic shielding properties and excellent damping thermal conductivity properties.However,the ?-phase Mg-Li alloy still belongs to the hexagonal close-packed structure,which has the disadvantages of low strength and poor deformation.By adding elements such as Al,Zn and Y,the strength of the alloy is improved.At the same time,the process of large plastic deformation like multidirectional forging,can make the grain refinement and improve material properties.Therefore,the plasticity of alloy is improved by alloying and multidirectional forging,and its superplastic behavior at high temperature is studied,it is very significant to expand its application field.This paper focuses on the multidirectional forging and high-temperature superplasticity,which based on the new single phase Mg-4.4Li-0.46Al-2.5Zn-0.74Y alloy.And the alloy microstructure and mechanical properties of various stages of the process are also analyzed.The main contents and conclusions of this paper are summarized as follows:(1)Through multi-directional forging process,one,three,six and nine passes forgings of alloy were obtained,and the microstructure of each pass was observed.The results showed that the grain size of the sixth pass was the smallest and the grain size of the ninth pass has a tendency to grow.At the same time,the tensile test were conducted at room temperature,and the elongation reached the maximum of 38.77%at the sixth pass,while the maximum tensile strength of the ninth pass was ?b=204.87MPa.The true stress-strain curve at room temperature showed obvious PLC effect.After rolling,the grains becomes banded,and the average bandwidth is 6.7?m and the tensile strength is 247MPa.(2)The high temperature tensile test was conducted at the temperature from 473K to 623K and the strain rate from 1.67×1-4s-1 to 1.67×10-2s-1.At the condition of 623K,1.67×10-4s-1,the maximum elongation obtained was 240%.Combined with the microstructure of chucks,deformed parts and the stress-strain curve,the evolution and the mechanism of hardening and softening was analyzed during the tensile process.(3)The thermal deformation constitutive equations of alloy were constructed,including hyperbolic sinusoidal constitutive model,dislocation density constitutive model and Laasraoui constitutive model.The theoretical value of dislocation density constitutive model can not be a reasonable response to the experimental results The Laasraoui constitutive model is a good fit between the theoretical peak stress and the actual peak stress.(4)The constitutive equation of superplastic deformation was established,including the values of stress exponent(n)and deformation activation energy(Q),and the normalized curve of alloy was made.