Study On Microstructure And Mechanical Properties Of Magnesium Alloy Sheet Produced By New Asymmetric Extrusion Process

Magnesium(Mg)alloys,as the lightest structural metal,have recently attained an increasing interest in electronics,automobile and aerospace industries owing to their high specific strength and stiffness,low density,and good damping properties.However,Mg alloys exhibit poor formability and limited ductility at ambient temperature because of their hexagonal close-packed(HCP)crystal structure,which restricts the widespread use of Mg alloys.Texture control and grain refinement has been shown to effectively improve the mechanical properties of Mg alloys.According to Hall–Petch relationship,grain refinement can increase the strength and ductility of Mg alloys.Texture weakening can also improve the ductility and formability of Mg alloys at room temperature.Extrusion technology has becomes a research hot spot in recent years owing to its economic and productive advantages,especially for sheets fabricated by using only onepass process.However,conventional wrought alloys exhibited strong basal texture after conventional extrusion(CE)process,and this strong basal texture resulted in poor formability because it restricts the activation of more basal slip during deformation.Normal gradient extrusion(NGE),transverse gradient extrusion(TGE),threedimensionall arc extrusion(3DAE)processes were proposed to fabricate Mg alloy sheets,whereas AZ31 and VK20 Mg alloys were selected as the main research objects.Microstructural characteristic and its evolution during the extrusion processes were analyzed and compared by optical microscopy(OM),X-ray diffraction(XRD),scanning electron microscope(SEM)and electron backscatter diffraction(EBSD)and so on.A finite element model(FEM)was employed using DEFORM software to investigate and compare the flow behavior of extrusion processes during extrusion.Plastic deformation ability of Mg alloy sheets was evaluated by room temperature stretching and Erichsen tests.The main research contents and conclusions of this paper were listed below:(1)The effect of NGE process on the microstructure of AZ31 Mg alloy sheet along thickness direction was analyzed.Meanwhile,the extrusion process was optimized to obtaine homogeneous microstructure and weak tilted basal texture.During the NGE process,the formation of gradient strain was beneficial to the generation of asymmetric shear strain,which eventually contributed to the formation of a weak tilted texture and grain refinement.Compared with the CE AZ31 Mg alloy sheet,the NGE AZ31 Mg alloy sheet possessed relatively higher Schmidt factor(SF)for basal slip at room temperature,irrespective of tensile direction.As a result,the NGE process could improve the plastic deformation ability of the extruded AZ31 Mg alloy sheet which exhibited lower yield strength and higher elongation.(2)The microstructure evolutions for AZ31 Mg alloy during the TGE process were analysed.The difference in microstructural evolution for the TGE and CE AZ31 sheets during tensile and Erichsen tests were also invextigated.For the TGE process,flow velocity gradient was generated along the TD during extrusion which was in favor of the basal poles of AZ31 sheet deviate from the normal direction(ND).Moreover,because of the introduced additional flow velocity along TD,the basal poles for the extruded AZ31 sheet tilted from the ED to TD.As the inclined angle of the extrusion die increases,both the deviation degree between the basal poles of extruded AZ31 sheet and the ND(resulted from the enhanced flow velocity gradient)and tilt range from the ED to TD(resulted from the increased additional introduced flow velocity along the TD)were increased.As a result,the TGE AZ31 Mg alloy sheet possessed excellent mechanical properties.Especially,when used the extrusion die with the inclined angle of 52°,the basal poles at center region of the AZ31 Mg alloy sheet were tilted toward the ED by about±70°,whereas the 1/4 edge region of the AZ31 Mg alloy sheet were tilted from ED to TD about 55°-63°.The elongation of TGE AZ31 Mg alloy sheet was as high as 41% and the yield strength decreaced to 89.6 Mpa.The Erichsen(IE)value of TGE AZ31 Mg alloy sheet reached 6.71 mm,increased by about 159% than that of CE AZ31 Mg alloy sheet(2.59 mm).(3)The effects of CE and TGE process on microstructure,mechanical and formability properties of VK20 alloy sheet were studied.For the TGE process,the amplitude of the base pole in the center region of the extruded VK20 Mg alloy sheet that deflecting to the ED direction increases,and the base pole in the 1/4 edge region deflects to the TD along the ED when used the extrusion dies with larger inclination angle.Compared with the CE VK20 Mg alloy sheet,the yield strength decreases and the elongation increases for the TD sample of the TGE VK20 Mg alloy sheet.Improving the planar anisotropy of as-extruded VK20 Mg alloy sheet was achieved by the TGE process.When used the extrusion die with inclined angle of 45°,the base pole in the cener region of the extruded sheet deflected to ED about ±62°,and the base pole in the 1/4 edge region deflected to the TD along the ED.The maximum elongation of the TGE VK20 Mg alloy sheet reacheed 45.6%,the yield strength was as low as 98.9Mpa.Consequently,the asextruded TGE-45 VK20 Mg alloy sheet exhibited excellent formability(5.49mm),and its IE value was improved by approximately 47% in comparioson to the extruded CE sheet(3.72).The improvement of formability was mainly due to the change of texture characteristics of the extruded VK20 Mg alloy sheet,which was more conducive to the basal slip and the activation of the twinning,and the anisotropy was improved.(4)The microstructures and mechanical properties of as-extruded AZ31 sheets processed by 3DAE proceses,including three-dimensional symmetrical arc extrusion(3DSAE)and three-dimensional asymmetric arc extrusion(3DAAE)processes,were investigated and made a comparison to the CE AZ31 sheet.Besides,the microstructural and texture evolutions during extrusion were analyzed.During the 3DAE processes,additional flow velocity along transverse direction(TD)was introduced,which consequently contributed to obtain a homogenous and fine microstructure and a weakened tilted texture.New texture component was also developed visibly along the TD.Furthermore,owing to the introduction of extra asymmetric shear deformation during 3DAAE process,the as-extruded AZ31 sheet exhibited the lowest basal pole intensity.Therefore,the as-extruded 3DAAE sheet showed low yield strength,but high elongation.In particular,the yield strength of TD sample for the as-extruded 3DAAE sheet decreased by 64.1 MPa compared with that of as-extruded CE sheet because of the introduction of new texture components along TD.Consequently,the as-extruded 3DAAE sheet exhibited excellent formability,and its IE value was improved by approximately 38% compared with that of the as-extruded CE sheet.