Study On Ultrafine Grain Refinement Mechanism Of Rare Earth Magnesium Alloy By Rotary Extrusion

Magnesium alloys have become an important research direction for lightweight in the aerospace field in China due to the high storage capacity,high specific strength,high specific stiffness and low density.Among them,rare earth magnesium alloys containing LPSO phase have excellent plastic deformation ability,corrosion resistance and comprehensive mechanical properties,and have great development potential.As-cast rare earth magnesium alloy has many defects,which greatly limits the application range of rare earth magnesium alloy.Plastic deformation is used to improve the properties of the alloy,and severe plastic deformation process has a significant effect on the improvement of the formability and mechanical properties of magnesium alloy.Compared with the traditional severe plastic deformation process,the rotary extrusion deformation process can more easily achieve a large amount of strain accumulation through continuous shearing,thereby reducing the production process and further reducing the requirements for equipment load.Therefore,in this study,VW124 A alloy was used as the research object,and compression deformation,LES-Rotational Extrusion deformation and LEL-Rotational Extrusion deformation experiments were carried out on Gleeble-3500 thermal simulation testing machine.Optical microscope,scanning electron microscope,X-ray diffraction,electron backscatter diffraction and field emission highresolution transmission electron microscope Talos F200 s were used to observe the microstructure characteristics during rotary extrusion deformation.Combined with the microhardness tester of UHL VMH-002 VD,the mechanical properties of the deformed samples were tested.The following conclusions are drawn :(1)Compared with the compression deformation,the rotary extrusion deformation is easier to realize ultra-fine crystallization due to the existence of shear stress.The LEL-Rotary Extrusion deformation and LEL-Rotary Extrusion deformation samples show a gradient structure in the axial direction,and the minimum grain size of the LEL-Rotary Extrusion deformation samples can reach 300 nm.The phase transformation process during LELrotating extrusion deformation is as follows: the lamellar LPSO phase and the bulk LPSO phase are elongated and broken to form fine lamellar and short rod-like LPSO phases,and a large number of bulk Mg5(Gd,Y,Zn)phases are precipitated at the same time.Then a large amount of strain induces nano-precipitates with a size of about 20～30 nm are precipitated massively and accompanied by the disappearance of bulk Mg5(Gd,Y,Zn).(2)In the process of LEL-Rotating Extrusion deformation,the coarse equiaxed grains are refined to nanoscale grain size under the joint action of various mechanisms : the coarse original grains are refined to fine grain structure with grain size less than 2μm under the joint action of PSN mechanism,CDRX mechanism and DDRX mechanism;with the assistance of nano-precipitates,strain-induced grain boundary migration leads to grain boundary bowing out and the formation of nanoscale grains.Under the combined action of uniformly distributed nano-precipitates and ultrafine-grained structure,the hardness of the alloy reaches 132 HV.(3)In the process of rotary extrusion deformation,the deformation temperature is the most important factor affecting the grain size.In the process of LEL-rotary extrusion deformation,with the increase of temperature,the process of LPSO phase breaking and grain refinement becomes slower.Due to the increase of atomic diffusion ability,the bulk phase in the fine grain zone is dissolved to reduce its volume fraction,the second phase size in the ultrafine grain zone is more uniform and fine.In addition,the flow velocity of metal in the alloy increases with the increase of deformation temperature,the local shear stress increases due to the difference of metal flow velocity between the central region and the edge region.The volume fraction of dynamic recrystallization grains in the central region of the alloy increases,and the distribution of dynamic recrystallization grains in the alloy is more uniform.