Research On Deformation Characteristics Of Magnesium Alloy Cylindrical Parts By Rotating Extrusion

Aerospace,national defense industry,transportation and other fields all focus on the equipment lightweight,and magnesium（Mg）alloys play an important role in lightweight.General plastic processing methods have limited improvement on the comprehensive mechanical properties of Mg alloys.In recent years,the severe plastic deformation（SPD）method has become a research hotspot.The purpose of grain refinement is achieved through the combination of tension,compression,torsion and multi-pass deformation,and then the formability and strength of Mg alloys are improved.However,the SPD technique is only used to prepare small-sized rod,block and sheet samples with multi-passes.Therefore,it is of great significance to find a new method for strengthening and toughening large-sized components via a single pass.Continuous shear plastic forming of large components can be achieved via single pass by rotating extrusion（RE）method,and the shear deformation of the RE process mainly comes from the shear structure at the bottom of the punch.Therefore,the design of a reasonable shear structure is very important for the RE process.In this paper,the groove shear structure is optimized by finite element simulation（FEM）,and the"non-penetrating asymmetric bending U-shaped"convex shear structure is designed according to the FEM results.ZK60 Mg alloy and Mg-Gd-Y-Zn-Zr alloy were selected to form cylindrical parts by the RE method.The deformation characteristics of RE were preliminarily determined by analyzing the microstructure distribution,texture evolution and mechanical properties in different areas of the cylinder wall,and the strengthening and toughening mechanism of different alloys by RE was analyzed.AZ80 Mg alloy and convex groove structure were selected to form cylindrical parts at different rotational speeds,and the effects of low and high rotational speeds on the deformation uniformity of the alloy were explored.The plastic deformation mechanism of RE was compared and analyzed through the above experiments.（1）Compared with the backward extrusion（BE）method,the circumferential shear stress in the deformation area is increased,and the overall cumulative strain by increasing the shear strain was also increased during the RE process.The addition of shear stress increased the shear stress in the slip direction,so as to reduce the extrusion load.The FEM optimization results of the number of grooves（area ratio）,groove depth and cone top angle of the punch showed that the increase of the number of grooves would increase the shear deformation times and circumferential flow distance of the metal at the bottom of the punch,which can effectively eliminate deformation dead zone in BE process.If the groove depth is too deep,the metal will form a velocity difference in the flow process along the Y and Z directions,and the metal in the inflow area and outflow area of the groove will form a fold when the velocity difference along the Y and Z directions exceeded 4 mm/s.The influence of cone apex angle on RE deformation mainly lay in changing the speed of metal flow from the bottom of the punch to the barrel wall."Non-penetrating asymmetric bending U-shaped"convex groove structure could effectively buffer the influence of metal circumferential flow at the bottom of punch on axial flow,and can also solve the problem of slow fluctuation after metal reduction.（2）The continuously accumulated shear strain could greatly increase the proportion of dynamic recrystallization（DRX）,and the shear stress had an obvious effect on grain refinement and second phase crushing.Compared with the cylindrical parts were formed during the BE and RE process,the grain size of ZK60 alloy is refined by 85.2%and the fine grain area of the cylinder wall is increased by nearly 3 times,and the ultimate tensile strength（UTS）,yield strength（YS）and failure elongation（EL）reached～313 MPa,～279 MPa and～12.1%.The grain size of Mg-Gd-Y-Zn-Zr alloy is refined by 31.5%at 420℃,and the mechanical properties of UTS,YS and EL were～350 Mpa,～258 Mpa and～17%respectively.Mg Zn and Mg Zn₂ phases in ZK60 alloy and Mg₅（Gd,Y,Zn）phases in Mg-RE alloy were uniformly dispersed,and these granular phases could improve the strength of the alloys due to the dislocation bypass mechanism.The hardness of LPSO phases in Mg-Gd-Y-Zn-Zr alloy was higher than that of the second phases of other alloys.Continuous shear stress could increase the degree of twist,tear and mechanical crushing of LPSO phases,and the change of rotating shear deformation on LPSO phases was very obvious.In addition,the Schmidt factor（SF）values of（0001）<11-20>basal slip was reduced,which made it difficult to activate the basal slip and improve the YS of the alloy by the RE method.（3）The"non-penetrating asymmetric bending U-shaped"convex groove shear structure could effectively solve the problem of folding layers on the inner wall surface during the RE process.The microstructure results of AZ80 magnesium alloy cylindrical parts were formed at different rotational speeds showed that the range of fine grain zone along the wall thickness of low and high rotational speeds is～11.3 mm and～10.8 mm,respectively,which is 2.6 times higher than that of BE.The proportion of dynamic recrystallization at low rotational speed is larger and the grain size of dynamic recrystallization is smaller than that at high rotational speed.From the perspective of deformation texture and grain orientation,the rotational speed affects the rotation degree of grain c-axis around extrusion direction（ED）,and[10-10]and[2-1-10]double cylindrical orientation was presented in the middle wall region.The change of orientation caused by RE results in the improvement of circumferential mechanical properties.The longitudinal and circumferential tensile strengths at high rotational speeds are 293 MPa and 303 MPa,respectively.（4）Continuous shear deformation could bring high strain in a short time,which promoted the high proportion of continuous dynamic recrystallization（CDRX）during the RE process.The discontinuous dynamic recrystallization（DDRX）was the main mechanism during BE process and the early stage of the RE process,while the continuous dynamic recrystallization is the main mechanism in the later stage of the RE process.In terms of the coordinated twin deformation mechanism,the twin orientation in the BE process is the base plane,which can coordinate the slip of the base plane,while the orientation of twins from basal plane to cylinder plane during RE.The continuous shear strain changes the grain orientation and reduces the（0001）<11-20>basal plane slip Schmid factor.This change reduces the axial texture strength and increases the annular texture strength,thus weakening the anisotropy of the alloy.In conclusion,the"non-penetrating asymmetric bending U-shaped"shear structure is suitable for rotating extrusion deformation,which is characterized by high strain,significant grain refinement,second phase dispersion distribution and effective weaken of alloy anisotropy.It is a new method for strengthening and toughening the alloy via a single pass.