Study On Torsional Large Deformation Of Extrusion Magnesium Alloy Based On Microscopic Crystal Plastic Model

Due to low density(1/4 of steel and 1/6 of aluminum)and good mechanical properties,magnesium and its alloys have attracted more and more attention.Magnesium alloys have good biocompatibility,high strength and stiffness and high stability.Magnesium alloys are widely used in transportation,manufacture,aerospace and other fields.However,high plastic formability is required to achieve the desired component shape and configuration in practical applications.The plastic formability of magnesium and its alloys is low showing strong anisotropy in mechanical behavior which restricts the development and application of magnesium alloys.Uniaxial tensile deformation can obtain positive stress state but it can not explain the mechanical properties under shear stress.During torsion test,the sample is subjected to pure shear force and the axial deformation is uniform until the material is broken.The torsion experiment provides an ideal experimental method for studying the large strain behavior caused by shear stress.It is of great practical significance to study the properties of large torsional deformation to improve the plastic formability.A preload-torsional deformation experiment was carried out to obtain the stress-strain curves of torsion under different pre-deformation to AZ31 magnesium alloys.Microstructure observation and XRD experiments were used to determine the evolution of texture.By means of the viscoplastic self-consistent(VPSC-PTR)model,the effect of initial texture is explored on torsional deformation.The mechanical properties of magnesium alloys are studied by experiment and model calculation.The main results include:(1)AZ31 magnesium alloy increases the preserving load stress before torsion and the yield stress decreases along extrusion direction.The effect of initial texture and evolutionary texture are explored by studying the torsional deformation of magnesium alloy under different textures.(2)Texture seriously affects torsional mechanical properties.Metallographic observation and XRD experiments were used to explore the evolution of texture.It is found that the preload affect the number of slips and twins.The experimental results show that the texture of torsional deformation is changed to varying degrees.The decrease of texture strength is mainly related to the micro mechanism of slip and twin.(3)The numerical simulation results of VPSC are analyzed.It is found that the base slip and prismatic slip are the main deformation mechanisms during torsional deformation and twins produce at small strains.Preload affects the active of slip and twin system.Different initial texture and slip system are the main reasons for different torsional yield strength.