| As the lightest metal structure material,magnesium(Mg)alloys have been attracted much attention from the aerospace,national defense,automobile and electronical industries due to their high specific strength,high specific stiffness,great shock absorption and extraordinary damping capacity.Magnesium alloys are regared as “green metal structure materials in 21 th century”.Magnesium alloys possess a closed-pack hexagonal(HCP)structure,which lead to limited number of slip activated;traditional rolled or extruded magnesium alloy sheets generally exhibit a strong(0002)basal texture.Due to the two reasons,magnesium alloy sheets exhibit poor ductility at room temperature.From the point of microstructure control,this work aims to improve the ductily of AZ31 B magnesium alloy sheets by texture modification and grain refining.Two parts contents are included in this paper.Firstly,this work applies a new novel method of continuous bending(CB)to modify the basal texture of AZ31 B magnesium alloy sheets and then the effects of texture evolution on mechanical properties are systematically analyzed;Secondly,a new technique,named accumulate extrusion bonding(AEB)is proposed to refine the microstructure and improve the mechanical properties of magnesium alloys.The microstructure and texture evolution during deformation have been characterized using optical micrograph(OM),electron backscatter diffraction(EBSD),scanning electron microscope(SEM)and X-ray diffraction(XRD).The plastic deformation capacity was examined by the tensile tests and Erichsen tests at room temperature.The main research contens and results are obtained as follows:(1)The microstructures alloy were observed in the different locations to study the microstructure evolution of AZ31 B magnesium sheets during CB process.The results show that: no twins are found during CB process,which is mainly due to the inhibitory effect on twinning by fine-grain microstructure.The pyramidal <c+a> slips dominate the deformation in the inner region,while the prismatic slip dominate the deformation in the outer region.During CB process,pyramidal <c+a> slips and prismatic slips alternately dominate the deformation.After annealing,abnormal grain growth occurred in the surface part;and the depth of abnormal growth grains layer increased with the strain increasing.The CB processed AZ31 magneisum alloy sheets exhibit a RD-tilt and double-peak basal texture,and the maximum basal texture intensities are greatly weakened.(2)The effects of bending angle,accumulated strain in a single pass and different bending passes during CB process and annealing on the mechanical properties and formability of AZ31 B magnesium alloy sheets are studied.The results show that: compared with the as-received sheets,the plasticity of AZ31 B magnesium alloy sheets processed by CB process and annealing was significantly improved at room temperature.The fracture elongation was increased from 16.5 % to 19.2 %;and the formabilty was also greatly improved with the Erichsen value of 5.4 mm,increased by ~ 135 % comparing to the as-received sample.The improvement of formability can be attributed to the smaller r-value and larger n-value,which is related to the basal texture modification.(3)AZ31B magnesium alloy sheets were produced by accumulated extrusion bonding(AEB)process at 200?.The interface bonded successfully with a good bonding quality.{10-12} tensile twinning dominated the deformation at the initial stage of extrusion.As the strain increasing,dynamic recrystallization(DRX)occurred.A fully DRXed microstructure was observed in the sizing band and the average grain size of DRXed grains is refined to ~ 1.4 ?m.Grain growth was observed after the extruded sheet exited from extrusion die;AZ31B magnesium alloy sheets processed by AEB exhibited a double-peak texture with the basal poles inclined from ND towards ED.Compared with as-received sample,AEB processed samples exhibited better comprehensive mechanical properties.The yield strength(YS)and ultimate tensile strength(UTS)increased from 145 MPa and 336 MPa to 169 MPa and 385 MPa,respectively.The fracture elongation(FE)increased from 24.9 % to 28.0 %.For the second pass extruded sample,the microstructure evolution and mechanical properties showed a similar characteristic to that of first pass sample.(4)During AEB process at 150? and 200?,{10-12} tensile twinning dominate the deformation at the initial stage of extrusion,and DRX was completed at the sizing band;while at 250?,some DRXed grains are found along oringianl grain boundaries and DRX was completed at the cone die.The average grain size of DRXed grains is ~0.9 ?m at 150?,and increased with the extrusion tempemrature increasing.Compared with the as-received sample,the magnesium alloy sheet processed by AEB at 150? exhibits the best mechanical properties with the YS of 178 MPa,UTS of 391 MPa and FE of 31.5 %.(5)AEB process was carried out at 200? to study the effects of deformation direction and the oringal orientation of AZ31 B magnesium alloy sheets on the microstructure and mechanical properties.The results show that:(1)the samples with the angles of 0° and 90° betwin TD and sizing band exhibit a double-peak texture with the basal poles inclined towards ED,while the basal texture of sample with the angle of 45° inclines to ED and the basal texture components in the TD became more disperse.0°,45° and 90° samples processed by AEB show a similar microstructure and mechanical properties.(2)Compared with the RED sample,the microstructure of TED sample was finner,which led to a higher YS and FE.It is can be seen that larger grain layer and smaller grain layer alternately distributed in the microstructure of RTED sample.This grain size gradient dietribution is beneficial to improve the ductility without decreasing strength.(6)In this work,we use the water-cooling and artificial-cooling to optimize the AEB peocess.The results show that:(1)AZ31B magnesium alloy sheets was performed by AEB process with water-cooling at 150?.The average grain size was decreased to ~2.5?m,which lead to a higher YS with the value of 184 MPa.After annealing at 200?,the microstrucuture changed little,while the FE increased obviously.The grains grew with the annealing temperature increasing,which led to a lower YS and FE.(2)AZ31B magnesium alloy sheets was performed by AEB process with artificial-cooling at 250?,which led to a finner DRXed grains microstructure in the sizing band.The YS and UTS were 171 MPa and 382 MPa,respectively;and the FE increased from 19.9 % to 30.1 %.(3)The magnesium alloy sheets processed by AEB with water-cooling and artificial cooling have the fineset microstructure with the average grain size of ~ 2.3 ?m.The YS and UTS increased to 192 MPa and 412 MPa,respectively;and FE increased to 30.3%. |
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