Investigation Of Texture Modification On Formability Of Magnesium Alloy Sheet

Magnesium alloys possess Hexagonal-Close-Packed(HCP)structure and the symmetry of the unit cell is quite low,so the number of the available activated slip systems is strongly limited.Even more worse,wrought Mg alloys formed during primary deformation process such as rolling or extrusion often present strong basal texture,which further leading to poor room temperature formability and significantly restrict their widely applications.Based on drawing forming process with a multidirectional stress state,this study proposed a new texture modification viewpoint which considering both the texture weakening and texture symmetry,to improve the room temperature formability of the Mg alloy sheets.The microstructures,grain orientations and the related deformation behaviors of four Mg sheets with different initial textures were analyzed in depth,and the parameters of the "texture weakening +texture symmetry" modification process based on different texture features were presented respectively.In addition,in view of the asymmetric distribution of strain in the inner and outer region during Mg bending deformation,this study proposed a new texture modification viewpoint which based on building a symmetric selection of plastic deformation mechanisms in the two regions during bending,to improve the final bending formability.In the work,five different textures were introduced by pre-straining process to analyze the influence of initial texture distribution on the selection of deformation mechanisms in the inner and outer regions during bending as well as their influence on the final bendability.The main research contents and results are listed as follows:? Specific pre-stretching process was carried out on an AZ31 sheet which had a number of soft orientations for basal slip,extension twins could be introduced in these grains with high SF for basal slip.The formation of multiple orthogonal orientation gradients based on the twin product and parent matrix could weaken the basal texture during recrystallization annealing,which could improve the ductility and room temperature formability of the AZ31 sheet effectively.? Pre-straining process was carried out on AZ31 sheet and LAZ331 sheet which both possess asymmetric texture distribution around ND.When the tensile direction was parallel to the titled direction of the texture component,the titled texture component could be consumed by the {1012} twins and orthogonal orientation gradient was formed in the(0002)pole figure.After annealing,symmetric weak basal texture could be introduced,which could effectively improve the planar isotropy of mechanical properties,room temperature formability and corrosion resistance of the sheet.? The effect of rolling paths on the microstructure and texture of the AZ31 sheet and LAZ331 sheet possess asymmetric texture was investigated respectively.When the rolling direction was perpendicular to the titled direction of the texture component,non-basal slip could be activated in the asymmetric textured grains,which was beneficial to the rollability of the sheet while provided no positive effect on the texture symmetry modification.However,when the rolling direction was parallel to the titled direction of the texture component,only extension twin mechanism could be activated in the asymmetric textured matrix and the twin products could consume the asymmetric texture component during rolling.As a result,a weak symmetric texture was introduced after annealing,which could effectively improve the planar isotropy and room temperature formability of the sheet.?“Pre-compression+annealing" and "pre-stretching+annealing" processes were both carried out on c-axis//ND textured AZ31 sheet to achieve texture modification.Orthogonal four-peak texture was introduced successfully in the(0002)pole figure and the result indicated that this type of orthogonal four-peak texture would provide considerable SF for basal slip during deformation along different directions and thus enhanced the ductility and the multidirectional formability of the sheet.Especially for the Erichsen value,which increased from 2.6mm to 5.6mm and the increasing rate exceeded 100%.? Five kinds of texture features were introduced in the AZ31 sheet by"pre-compression + annealing" process to evaluate the texture distribution on the bendability during Mg bending deformation.The deformation mechanism activated in the inner and outer regions in terms of the different textured specimens was analyzed in detail.The results indicated that the initial texture distribution had significant effect on the deformation behavior in the two regions as well as on the final bendability.Tilting the basal texture was confirmed as an effective way to enhance the room temperature bending formability.?The minimum bending radius of the c-axis//TD textured specimen could decrease to 1.21mm and 1.58mm during bending along 0° and 45°,respectively,significantly lower than that(3.51mm,4.15mm)of the c-axis//ND textured as-extruded sheet.Meanwhile,their final bending angle reached to 64.8° and 85.6°under the bending parameter:angle of bottom bending die ? = 60°,radius of the upper bending die r = 4mm,obviously smaller than that(124.5°,137.7°)of the as-extruded sheet.Such a texture modification could promote the selection symmetry of the deformation mechanism between the inner and outer regions during Mg bending,and effectively improved the room temperature bendability of the AZ31 sheet.