Textural Evolution And Mechanical Properties Of Wrought Magnesium Alloy AZ31

Magnesium alloys are the lightest metallic structural materials with high specific stiffness and strength,good electromagnetic shielding capability,good damping,therefore,they are very attractive in various applications in automotive,communication,electronics,and aerospace industries.Due to their hexagonal close packed crystal structure and limited deformation mechanisms at room temperature:{0001} < 11(?)0 > basal slip and {10(?)2} <10(?)1> twinning; magnesium alloys usually have poor formability at room temperature.At present,most magnesium alloy products are limited to forming in cast;especially die casting,which limits the application scope of magnesium alloy in a great extent.Comprehensive mechanical properties of cast magnesium alloys can be improved by grain refining during hot deformation (such as extrusion,rolling).However,primary processing such as conventional hot rolling and hot extrusion generally gives rise to a strong basal texture,and this leads to a very limited ductility near the room temperature.Improved room formability is the foundation and necessity of the wide applications of wrought magnesium alloys.Therefore,enhancing the room temperature formability by texture controlling and grian refineing during hot deformation became the research focus in the development of wrought magnesium alloys.Aiming at some aspects mentioned above,research works were focused on following aspects:(1)Evolution of {0002} basal texture during hot rolling of cast pure Mg;(2)Evolution of texture in AZ31 magnesium alloy sheet during DSR and following annealing; (3)Examination of mechanical properties,characteristics of microstructure and texture in AZ31 magnesium alloy fabricated by asymmetric hot extrusion;(4)Effect of grain size and texture on compression behavior of AZ31 magnesium alloy at room temperature.The primary conclusions were as follows:Dynamic recrystallization,accompanying grain refinement and formation of {0002} basal texture,occurred during hot rolling of cast pure magnesium at 400℃.With the increasing of reduction,the amount of twins was reduced while grains were refined step by step.As the rolling was conducted,grain orientation of cast pure magnesium became orientation with {0002} basal texture,and the intensity of the basal texture increased with the inceasing of hot rolling reduction.When the rolling reduction achieved to 78%, microstructure of pure magnesium became homogeneous and typical {0002} basal texture was formed.The present study shows that the nucleation of DRXed grains was related to the specific DRX mechanism associated with twinning.Grains with basal texture orientation with low dislocation density and energy of distortion were not favorable for basal slips,which were not easy to recrystallize.With the increasing of the amount of grains with basal texture orientation,intensive {0002} basal texture formed in the hot rolled pure magnesium sheet.The main texture components of hot extruded AZ31 magnesium alloy sheets were (10(?)7)[0(?)72](90°,15°,0°) and((?)(?)26)[(?)02(?)](60°,10°,30°) textures.After cold differential speed rolling(DSR),the texture components remained while the intensity of texture changed. Compared with the texture intensity of the layer closed to the higher speed roller,the intensity of texture closed to the lower speed roller changed in a larger degree.Cold DSRed AZ31 magnesium alloy sheets were annealed at different temperature.After annealing,there was no change in texture types,but intensity had changed.(10(?)7)[0(?)72]and((?)(?)26)[(?)02(?)] textures were weakened,but was not sensitive to the annealing temperature.The weakening of texture during annealing was dependent to the recrystallization process.As for cold DSRed AZ31 magnesium alloy with 16%reduction,microstructure and texture intensity became stable at 300℃and above,indicating that the recrystallization completed at temperature 300℃or higher.Compared with conventional rolling,yield stress and ultimate tensile stress in the annealed DSRed AZ31 magnesium alloy sheet were almost the same to the conventional rolling,while the room elongation of annealed DSRed AZ31 magnesium sheet was improved.Cast AZ31 magnesium alloy was asymmetrically hot extruded at 400℃,grains were refined from 75μm to～4μm.Asymmetrically extruded AZ31 sample showed a fine grained microstructure and some grain size gradient.Average grain size of top surface(with chamfer) was smallest(2.50μm);grain size of bottom surface was largest(3.67μm),while average grain size of center was larger than that of top surface and smaller than that of bottom surface (3.13μm).There also exists an inhomogeneous distribution of textures throughout thickness in AZ31 magnesium alloy sheet produced by asymmetric hot extrusion.Asymmetric extrusion AZ31 sample showed the basal plane inclined by～15°from the normal direction toward the extrusion direction in the top surface,and weakened and scattered basal texture in the center layer,and strong basal texture in the bottom surface of the sheet.Ductility can be improved by inclined and weakened basal texture in top surface,while yielding stress was brought down. Three point bending test results indicated that mechanical properties of top surface were excellent.FEM analysis indicated that the shear strain was introduced by the asymmetrical die and,which changed grid distorations during asymmetric hot extrusion and special texture characteristics were formed.Point tracking results showed that the gradient of grain size was ascribed to the gradient of strain rate in the thickness direction during asymmetric hot extrusion. Hall-Petch relationship of hot extruded AZ31 magnesium alloy during compression and tension were:σ_0.2=22+390d^-1/2 andσ_0.2=80+303d^-1/2,respectively.Besides basal slip,non-basal slip and twinning activated during deformation of hot extruded AZ31 magnesium alloy at room temperature.Since the existing sharp fiber texture favored activating of {10(?)2} <10(?)1> twin, while loaded in the extrusion direction during compression,basal slip and {10(?)2} twinning were the main deformation mechanism,and grain orientations were alerted by the {10(?)2} twinning.