Microstructure Evolution Of AZ80 Magnesium Alloy Processed By Rotating Backward Extrusion

Magnesium(Mg)alloy,the lightest metal structural material with high specific strength and stiffness,excellent damping performance and easy recycling,is conducive to the realization of components lightweight,and it is widely used in aerospace,defense,military and transportation fields.Mg alloy components prepared by casting or traditional plastic deformation methods have disadvantage of uneven microstructure and poor mechanical properties,which limit their industrial application.Severe plastic deformation(SPD)technology is to increase the amount of accumulative strain to achieve grain refinement and texture weakening,thereby promoting the improvement of mechanical properties and eliminate the anisotropy.Although the traditional SPD methods can significantly refine the grains,most of the processes are complicated,high production and only block samples with regular shape can be prepared.As a new SPD method,the rotary extrusion process can not only effectively refine the grains and weaken the texture,but also can produce high-performance components with only a single deformation pass.But the microstructure evolution of the alloy and its influence on the mechanical properties are still unclear.Therefore,this paper will take AZ80 Mg alloy as the research object,and combine the finite element numerical(FEM)simulation to explore the metal flow behavior during the rotating backward extrusion(RBE)process.The microstructure evolution of studied alloy under RBE process is systematically investigated by some characteristic means such as optical microscope(OM),X-ray diffraction technology(XRD),scanning electron microscopy(SEM)and electron backscatter diffraction(EBSD).The mechanical properties of the alloy are measured by microhardness at room temperature.By revealing the deformation mechanism of Mg alloy via RBE process,it is expected to provide technical guidance for the preparation of high-performance components using RBE technology.The main research contents and conclusions are as follows:The metal flow behavior of AZ80 Mg alloy during RBE process showed that the rotation of the open punch in the RBE could cause the billet to produce shear deformation,and promote the metal to complete the cyclic accumulative deformation of "press-in and press-out",which improved the cumulative strain amount.The average equivalent strain value was increased from0.99 in the conventional backward extrusion(CBE)to 8.65 of RBE at 30 circles.At the same time,the RBE could reduce the forming load,and the maximum forming load was changed from 13.1k N of CBE to 9.0k N of RBE with a drop of 31.3%.In addition,increasing the number of revolutions could also significantly increase the amount of strain,promote metal flow and reduce the forming load,when the revolutions exceeded the critical value,the increase in the strain would be significantly reduced.The microstructure and properties evolution of AZ80 Mg alloy processed by CBE and RBE methods were compared and analyzed at 653 K.Compared with CBE,RBE process could significantly refine grains,increase the proportion of dynamic recrystallization(DRX)and aggravate the fragmentation and refinement of the second phase.The largest decrease in grain size was about 88.9%,and the maximum increase in DRX ratio could reach 126%;discontinuous dynamic recrystallization(DDRX)was the main deformation mechanism of the RBE process.RBE could promote the opening of the <c+a> pyramidal slip system,and promote the deflection of the c* axis of grain around the extrusion direction(ED),thereby causing the texture to be weakened.In addition,the microhardness value of RBE was higher than that of CBE,with a maximum increase of about 13.4%,and fine-grain strengthening was the main strengthening mechanism.The evolution of microstructure and properties of AZ80 Mg alloy under different deformation parameters showed that reducing the deformation temperature or increasing the rotating revolutions could significantly refine the grains and increase the proportion of DRX.At 573 K and 100 cycles,the grain size in the cup bottom were refined to 2.5?m.At the same temperature,increasing the revolutions could promote the fragmentation and re-dissolution of the eutectic phase(EP),which was conducive to the precipitation of the dynamic precipitation(DP)phase,and the DP phase distributed along the grain boundary could inhibit its growth,thereby further promoting the microstructure refinement.Due to the new DRXed grains with random grain orientation,the texture was weakened.As the number of revolutions increased,the angle of grain deflection gradually increased,and the texture weakening effect was gradually aggravating.At 573 K and 100 cycles,the c* axis of the grains regrouped to the ED,resulting in a re-enhancement of the texture;at 573 K,the enhancement of the [2-1-10] prismatic component was mainly related to the activation of the prismatic slip systems within the deformed grains and the generation of CDRXed grains with a special 30°[0001] grain boundaries.In addition,increasing the number of revolutions was conducive to the fine-grained strengthening,while reducing the deformation temperature was contributed to the second phase strengthening,both of which would promote the increase of the microhardness.The region of cup bottom and wall had the largest microhardness values of 96.3HV and 95.3HV,respectively under conditions of 573 K and 100 circles.The influence of the open punch on the microstructure and mechanical properties of AZ80 Mg alloy cups was explored.The results showed that increasing the number of grooves was beneficial to the increase of accumulative strain and the reduction of forming load.At 573 K and 3 circles,the six-groove sample had the maximum average equivalent strain value of 1.8,and the eight-groove sample had the smallest forming load of 15 k N.The increase in the number of grooves could promote the grain refinement ability and the DRX proportion.The six-groove samples had the smallest grain size of 4.9?m and the largest DRX proportion of 96.5%.The microstructure of the eight-groove sample was the most uniform along the axial direction of the wall.In addition,the increase in the proportion of DRX would increase the grain deflection angle,and weaken the texture strength.The cup wall of the six-groove sample had the largest grain deflection angle of 60°.Due to the effect of fine-grain strengthening,the microhardness value of the alloy would increase with the increase of the number of grooves,and the six-groove sample had the maximum microhardness value of 98.8HV.