Investigation On The Effect Of Direct Extrusion And Bending-Shear Deformation On Mechanical Properties Of Magnesium Alloys

Magnesium(Mg)alloys is the lightest metal structure material in the world.Due to its high specific strength and stiffness and good damping performance,it has been widely used in the fields of electronics,automobile and aerospace in recent years.However,Mg and its alloys are hexagonal close-packed(HCP)crystal structure with poor elongation and plastic formability at room temperature.As grain refinement can greatly improve the strength and elongation of Mg alloys,and texture weakening can improve the elongation and formability of Mg alloys at room temperature.Therefore,the mechanical properties of Mg alloys can be effectively improved by microstructure regulation and texture weakening.The required thickness of Mg alloy plate can be obtained by the traditional extrusion process,but the Mg alloy plate of traditional extrusion has a relatively strong substrate texture,resulting in poor plasticity and formability at room temperature and poor mechanical properties of the material.In recent years,severe plastic deformation(SPD)techniques have been developed to improve the mechanical properties of Mg alloys by refining grains.Because grain refinement can greatly improve the strength and elongation of Mg alloys,and texture weakening can improve the elongation and formability of Mg alloys at room temperature.The mechanical properties of Mg alloys can be effectively improved through microstructure regulation and texture weakening.In this paper,we take the AZ31 Mg alloy of basal texture type of traditional extrusion and the high-strength AQ80 Mg alloy for aviation and as research objects,and adopt a new type of extrusion process,namely the(Direct Extrusion and Bending Shearing(DEBS))to prepare Mg alloy sheet.Microstructural characteristic and its evolution during the extrusion processes were analyzed and compared by optical microscopy(OM),X-ray diffraction(XRD),scanning electron microscope(SEM)and electron backscatter diffraction(EBSD)and so on.The plastic deformation ability of Mg alloy sheet was tested by room temperature tensile and hardness experiments.The main research contents and conclusions of the paper are as follows:1.The effects of direct extrusion and bending shear deformation on the microstructure and mechanical properties of as-cast AZ31 Mg alloy at different extrusion temperatures along the extrusion direction were analyzed.At the extrusion temperatures of 290?and 370?,the coarse grains of as-cast AZ31 Mg alloy of 300?m can be refined to 3.2?m and 5.1?m after one-pass extrusion.At 290?,the yield strength R_m,tensile strength R_el and elongation A_gt of DEBS Mg sheet are 203MPa,290MPa and 27.7%,respectively.At 370?,the yield strength R_m,tensile strength R_el and elongation A_gtof DEBS Mg sheet are 220 MPa,300 MPa and 25.7%respectively,Compared with the traditional extrusion,the mechanical properties of Mg alloy are improved more obviously.2.The mechanism of grain refinement and microstructure regulation of AZ31 Mg alloy during DEBS extrusion was revealed.Due to the process of bending-shearing deformation in the direct extrusion area,severe strain can be accumulated and a large number of twins can be produced,which can promote the dynamic recrystallization.At the same time,shear strain was introduced into the bending shear region to further refine the coarse grains.The bending-shear structure can also weaken the strong basal texture formed during the direct extrusion of AZ31 Mg alloy,promote the opening of non-basal slip system,and further weaken the strong basal texture formed by the direct extrusion through the bending-shear deformation.3.The effect of DEBS deformation process on as-cast and forged AQ80 Mg alloys was analyzed.It is found that this process can refine the grain of AQ80 Mg alloy.After extrusion at extrusion temperatures of 290?,310?,340?,and 370?,the grain sizes were refined to 1.1?m,2.5?m,4.8?m,and 5.3?m,respectively.The grain size of the forged Mg alloy extruded at 330?and 370?is 1.8?m and 4.2?m,respectively.4.A large amount of fine second phase Mg₁₇Al₁₂ will precipitate during the extrusion process of AQ80 Mg alloy.The second phase is dispersedly distributed at the grain boundaries and a small amount is distributed in the grains.The precipitation of the second phase provides nucleation points for dynamic recrystallization,and prevents further growth of dynamic recrystallized grains,which can promote the grain refining effect of the AQ80 Mg alloy during extrusion.Precipitation of the second phase is more obvious at higher temperatures.