Research On The Forming Process And Recrystallization Behavior Of Magnesium Alloy Produced By CVCDE

At present,magnesium alloys have attracted more and more attention not only in aerospace and military applications,but also in automotive and 3C industries because of their low density,good recycling performance and excellent physical properties.However,magnesium alloys have hexagonal close-packed structure,and the number of movable slip systems at room temperature is limited,which leads to their low ductility and poor machinability.In order to widen the application field of magnesium alloys,it is urgent to develop new technologies that can significantly improve the ductility and machinability of magnesium alloys.In the extrusion process,a series of defects in the processed ingot structure are eliminated due to the triaxial compressive stress and the strong shear deformation,resulting in fine grain structure and excellent mechanical properties.Therefore,extrusion process has gradually become one of the main processing methods at room temperature for low plastic magnesium alloy components.The precise control of shape and performance by which extrusion process is adopted has become one of the hot research directions in the field of extrusion forming technology.Continuous variable cross-section direct extrusion(CVCDE)process is a new extrusion method which can increase cumulative strain in a single pass.By precise design of die cavity structure,the purpose of customizing the structure and properties of extruded products can be realized.In the process of CVCDE,continuous change of the cavity passage can make the billet continue to be subjected to intense shear stress,which can significantly refine the grain.At the same time,the coarse second phase particles can also be sheared and broken,and the distribution of the microstructure is more fine and uniform,which is helpful to improve the comprehensive performance.Therefore,CVCDE shows excellent potential of refining grains and improving performance.In this paper,two interim dies were taken as a sample to investigate the microstructure and deformation characteristics of AZ31 magnesium alloy at different forming temperatures by means of DEFORM simulation software,optical microscope(OM),scanning electron microscopy(SEM),transmission electron microscopy(TEM)and electron backscattering(EBSD)technology.The results showed that the temperature rise of the material decreased and the flow uniformity increased in the extrusion process,but the shear stress at the corner of the core die decreased.So the average grain size increased with the increase of forming temperature.BesiThe percentage of high angle grain boundaries in the deformed microstructures obviously increased,which indicated that dynamic recrystallization was more adequate.Yield strength and elongation both increased first and then decreased,which was due to the higher dislocation density in the grains at 350 oC,but the weakening of texture.The recrystallization behavior of AZ31 magnesium alloy during CVCDE was studied.The results showed that the grain size of AZ31 magnesium alloy decreased obviously and the uniformity and roundness of AZ31 magnesium alloy increased remarkably when it flowed through continuous variable channel during extrusion.As the material flowed downward in the cavity,the dynamic recrystallization behavior became stronger and the proportion of recrystallized grains increased.The mixed texture of {0001}<10-10> deformation texture and {0001}<11-20> recrystallization texture was formed by CVCDE.During extrusion,the value of Schmid factor which characterized grain orientation decreased and the orientation of most grains tended to hard orientation.In conclusion,the CVCDE process can significantly refine the grain and improve the comprehensive mechanical properties of magnesium alloy,which provides a new idea for the development of high performance magnesium alloy extrusion products.