Study On Hot Deformation Behavior And Extrusion Technology Of The As-Cast Mg-Li-Al-Zn-Si Alloy

Magnesium alloys have excellent properties such as low specific gravity,high specific strength and easy recovery.Mg-Li alloys not only have these advantages,but also have lower density.The addition of Li element can greatly improve the plastic forming ability,so it has great potential in aerospace,weapon industry and 3C industry.In this paper,the hot deformation behavior of Mg-10.13Li-2.83Zn-2.78Al-0.13Si as-cast alloy was studied by isothermal hot compression test,and the constitutive equation was established.Based on the DMM model,the processing map of the experimental alloy was constructed,and the microstructural evolution rules of the safe zone and the unstable zone were analyzed.The technological parameters of extrusion experiment for hollow profile were designed based on the processing map,and the effects of extrusion process on the formability,microstructure and mechanical properties were analyzed.The main conclusions are as follows:As-cast experimental alloys consist of ?-Mg phase,?-Li phase,LiAl phase,Li2MgAl phase and Mg2Si phase.Zn is mainly dissolved in the alloys.The shape of a phase is block or strip with distinct edges and corners,and it is evenly distributed in the matrix of ? phase.After holding at 300 ? for 18h,the precipitated phases decrease and the a phase tends to be rounded,which indicates that the homogenization effect is better.The flow stress curves of the experimental alloy all have the typical characteristics of dynamic softening.The flow stress peaks at a very small strain,then begins to decrease,followed by steady-state flow.Except for the strain rate of 10s-1,the curves rise when the strain is about 0.6,mainly due to the coordinated deformation of the a phase is more difficult and the friction force increases.The hyperbolic sinusoidal function can well describe the hot deformation behavior of the alloy.The average stress index n was 3.79961,and the average thermal deformation activation energy Q was 88.188737kJ·mol-1.The dominant deformation mechanism is dislocation climbing when hot deformation occurs below 300 ?,and the dominant deformation mechanism is dislocation viscous slip when deformation occurs above 300?.The processing map shows that the safety zone occurring at the temperature of 250 ?-300?,the strain rate of 0.001s-1-0.003s-1 and the temperature of 280?-300?,the strain rate of 0.003s-1-0.017s-1,their peak efficiency are about 45%.The instability region was also identified at the temperatures ranging from 150 to 220? and strain rates from 0.1 s-1 to 3s-1.No defects,such as adiabatic shear bands or cracking,were observed in this region.However,the strong deformation of a phase will block the movement of dislocation,which is easy to cause local stress concentration.Based on the safety zone of the processing map,the profile without forming defects can be obtained by sectional mould extrusion,and the welding quality is good.With the increase of die outlet temperature,the strength of the alloy decreases,and the elongation increases first and then decreases,all of which are ductile fracture.The best comprehensive mechanical properties of profiles were obtained at 280 ?/0.5mm·s-1.