The Superplastic Characteristics And Forming Of LZ91 Mg-Li Alloy

Magnesium alloys are prevailing with the development of the modern aviation industry which leads to a growing demand for lightweight structural materials.Mg-Li alloys,the lightest in the structural materials and known as ultra-light magnesium alloy,are potential candidate due to their high specific strength,good low-temperature toughness,excellent damping capacity,and thermal conductivity.Superplasticity,the ability of certain materials to undergo very large tensile strains,is widely used in industry field.The superplastic characteristics,microstructure evolution and forming process of LZ91 Mg-Li alloys were studied in this paper.The optimal superplastic forming temperature and strain rate were obtained through tensile test.Based on true stress-true strain curves,the strain rate sensitivity,m,was calculated,and the constitutive equation was created in this study.The microstructure and dislocation evolution of the alloy after tension test at different strains were investigated.Based on tensile test results at elevated temperature,different superplastic forming(SPF)schemes of box-component were simulated by MSC.MARC;through the finite element simulation,it could be found that the SPF process of the box-component was feasible.The narrow-mouth box of 5A90 Al-Li alloy was manufactured,showing that the thickness distribution of the component was uniform and the thinnest part was the round corner.Tensile tests showed that the optimal elongation of tensile specimens was 812.6% at the temperature of 300oC and the strain rate of 5×10-4s-1.The measured strain rate sensitivity is 0.65,and the effect of the high strain rate sensitivity,m,is to inhibit catastrophic necking,and is beneficial for superplastic deformation.The constitutive equation that describes the flow stress as a function of the strain rate and deformation temperature for LZ91 mg-li alloy was established in this paper.The grain coarsening behavior was observed with the increased deformation temperature and low strain rate.It was found that the dislocation propagation rate was very slow and the density of dislocations is very low in the course of tension,and the low dislocation density was not sufficient for dynamic recrystallization.These two factors caused strain hardening phenomenon.The finite element simulation software was used to simulate different forming schemes of narrow-mouth box component,and finally the solid powder bulging process was selected.The effect of friction on the thickness distribution of the final component was discussed in this paper to optimize the forming process.Finally,the pressure-time curve in the course of forming was obtained.The segmental mold was adopted to manufacture the narrow mouth box-component at 250oC.The tensile strength and elongation of the alloy decreased after the superplastic forming.