Grain Refinement And Weak Anisotropy Mechanism Of VW134A Alloy Under Complex Stress

Magnesium alloy is the lightest metal structural material with low density,high specific strength and good damping resistance.In particular,the excellent mechanical properties of rare earth magnesium alloy at ambient and high temperatures make it have a wide application prospect in aerospace,weapons and equipment fields.The close-packed hexagonal structure of rare earth magnesium alloy results in fewer slip systems and poor plastic working ability.Conventional forward extrusion can significantly increase the plastic forming capacity of the alloy by applying three-way compressive stress,which is a common processing method for bar and various profiles.However,the conventional extruded bar and sheet have strong basal texture and strong mechanical anisotropy,and edge crack is easy to appear.This greatly limits its application in the main load-bearing members.In this paper,high performance magnesium alloy sheet was fabricated by rotary forward extrusion and the mechanisms of grain refinement and anisotropy weakening were studied.VW134A alloy was taken as the research object.Based on the simple shear principle of compression-shear(CS)and compression-torsion(CT),one direction compression & simple shear(1-CS),two directions compression & simple shear(2-CS),three directions compression & simple shear(3-CS)tests were designed.The microstructure evolution and mechanical properties of the materials were characterized by optical microscope,scanning electron microscope,X-ray diffractometer,electron backscatter diffraction and transmission electron microscope combined with microhardness and tensile test.The effect of hydrostatic pressure on grain refinement mechanism was studied.On this basis,compared with the CT test,the grain refinement mechanism under strong shear stress was analyzed.Based on the above,the principle sample of rare-earth magnesium alloy sheet was formed by rotating forward extrusion using Gleeble-3500 thermal simulation torsion unit.The flow stress and the microstructure evolution of VW134 A alloy under the conditions of strong shear stress and high hydrostatic pressure were studied.The grain refinement and anisotropy weakening mechanisms of rare-earth magnesium alloy under complex stress were revealed.Specific research results are as follows:The simple shear tests of CS and CT show,the stress-strain curve under CS condition is work hardening type,while it is dynamic recrystallization type under CT condition.The forming load increases significantly with the increase of the equivalent strain rate under both conditions,and the metal deformation severity is also increased.The stress triaxiality first increased and then gradually decreased with the increase of strain.The grain is significantly refined,and the average grain size reaches 6.55μm.The PSN mechanism of β phase precipitated in both conditions promotes DRX nucleation process,and its pinning effect limits the further grain growth.The research shows,with the increase of the equivalent strain rate,the decrease of dynamic recrystallization fraction is related to the insufficient nucleation time of grains.On the basis of CS test,hydrostatic pressure is changed by increasing compressive stress in different directions.The 1-CS,2-CS and 3-CS tests were carried out to study the deformation behavior of VW134 A alloy under different hydrostatic pressures.With the increase of hydrostatic pressure,deformation resistance and forming load are increased,and the stress triaxiality also increases.The hydrostatic pressure has a weak effect on the DRX critical strain and nucleation.There is no significant difference in grain size with different hydrostatic pressure.However,the hydrostatic pressure has a significant effect on the evolution of the second phases.The bending deformation energy of lamellar LPSO phase increases with the increase of hydrostatic pressure,and the solute diffusion and deconstruction of LPSO phase occur under the action of hydrostatic pressure.The boundary of lamellar LPSO phase becomes fuzzy under low hydrostatic pressure,and the deconstruction begins.The bulk LPSO phase is deconstructed under higher hydrostatic pressure.Based on the Glebble-3500 thermal torsion unit,a rotary forward extrusion(RFE)die of principle sample was designed.The accumulation of equivalent shear strain can be realized by RFE process,and the forming load can be reduced by 83.6% compared with traditional forward extrusion(CFE).Under the complex stress condition of strong shear stress and high hydrostatic pressure,with the increase of revolution,the peak stress of the stress-strain curve decreases gradually,and the equivalent stress and hydrostatic pressure in the deformation zone also decrease gradually,while the grain size decreases first and then increases.RFE25 sample has the best grain refinement with an average grain size of 4.86μm.The results show that the decrease of grain size is related to the DRX process promoted by strong shear stress,and the increase of grain size is related to the second phase.High hydrostatic pressure promotes the fracture and dissolution of LPSO phase,and the pinning effect of the second phase almost disappears,resulting in the growth of grain size.Under complex shear stress condition,pyramidal slip is easily activated and the texture is deflected greatly.The(0001) basal texture strength increases with the increase of the equivalent strain rate,and the texture distribution is more dispersed.1010 and 1010 are generated at high strain rate.The hydrostatic pressure under strong stress has little effect on the basal texture strength of α-Mg phase.The activation of <c+a> dislocation,as the main source of non-basal slip,shows the same distribution characteristics under different hydrostatic pressure.The c axis of most grains can be continuously rotated in the transverse direction(TD)during RFE process,and the basal plane is gradually parallel to the extrusion direction,forming a more random texture type and significantly reducing the texture strength.The results shows,the strong shear strain accumulated by RFE process can significantly refine the grain and weaken the basal texture.Under high hydrostatic pressure,the LPSO phase has a large degree of kinking and brokening,and the morphological differences in different directions are greatly weakened,so the anisotropy of the extruded sheet is greatly weakened.In the process of RFE forming large sheet,the equivalent stress and strain are also different in all directions due to the great difference in velocity of billet in all directions at the square die hole.The storage pool and discharge slope structure are introduced to improve the equivalent strain uniformity.The results show,the equivalent strain uniformity is the best when the discharge slope height is 5mm and the distance from the exit position is 17.5mm,and the range of deformation dead zone is the smallest.The maximum difference of axial and transverse tensile strength is less than 6%,the maximum difference of axial and transverse elongation is less than 10%,and the mechanical anisotropy is almost negligible.The tensile strength and elongation of the edge and the middle is almost equal,showing good uniformity of mechanical properties.