Study On Microstructure And Mechanical Properties Of AZ31 Magnesium Alloy Sheet Fabricated By Extrusion-angle Extrusion

In view of the current mainstream preparation methods of magnesium alloy sheets,there are problems such as strong base surface texture,poor plasticity at room temperature and large anisotropy.In this study,by optimizing the traditional forward extrusion process of magnesium alloy sheets,a shear stress was introduced during the extrusion process,a new magnesium alloy sheet preparation process was designed,extrusion-angle extrusion(EX-AE).It can not only refine the grain structure of magnesium alloy sheet,but also adjust the texture of the base surface of the sheet.The metal finite element simulation system DEFORM-3D was used to simulate the rheological behavior and effective stress-strain distribution of the magnesium alloy sheet during the EX-AE extrusion process.EX-AE sheets were prepared using extruded rods and as-cast AZ31 magnesium alloys as raw materials.The microstructure and texture evolution of the sheets were observed using optical microscope(OM)and electron backscatter diffraction(EBSD)techniques,respectively.Image-Pro Plus software was used to calculate the grain size of the sheet.The tensile properties were used to analyze the mechanical properties and anisotropy of the sheet.Draw the following main conclusions:(1)The rheological behavior and effective stress-strain distribution of magnesium alloy sheets was simulated during the EX-AE extrusion process.The flow velocity in the thickness direction of the sheet at the first bending channel of the die and the shearing step shows a significant difference.The difference in flow rate causes a difference in effective strain,which ultimately results in a"rolling"effect in the thickness direction of the sheet.At the sheet metal forming entrance of the mold and the shearing step,due to the greater degree of material deformation,there is a stress concentration phenomenon.The two reference points P1 and P2 recorded in the thickness direction of the sheet finally formed a deviation of 32°.According to the strain characteristics,the total strain formula of one pass EX-AE extrusion was derived,and the formula calculated?e=5.42.(2)The effect of EX-AE process on the microstructure of extruded bar material of AZ31 magnesium alloy was studied.The structure of the sheet after EX-AE extrusion is fine,uniform and completely dynamic recrystallized grains.The lower the extrusion temperature,the smaller the average grain size of the sheet structure.At different temperatures(350?,400?and 450?),the average grain size of the sheets prepared by the EX-AE process can be refined to 2.2?m,3.1?m and 5.8?m respectively;the EX-AE process is in the extrusion process The introduction of a new flow velocity V_TDresults in a 45°angle between the c-axis of the grain and the TD direction.The higher the extrusion temperature,the more deflection of the grains,the greater the degree of deflection.At 450?,the grain c-axis is parallel to the extrusion direction.After EX-AE extrusion,the texture strength of the sheet was 12.37(350?),8.27(400?)and 8.58(450?).(3)The effect of EX-AE on the mechanical properties of the extruded bar made of AZ31 magnesium alloy was studied.The average mechanical properties of EX-AE extruded sheets in three directions(ED,45°and TD)show that the EX-AE process can significantly improve the mechanical properties and the anisotropy of the sheet.With the extrusion temperature The lower the slab's yield strength and tensile strength,the greater the elongation remains.Co MPared with the original bar,the sheet prepared by EX-AE extrusion at 350?has a yield strength increased by 112%(246 MPa vs 116 MPa),a tensile strength increased by 34%(369.3 MPa vs 276.5 MPa),and elongation An increase of14%(19.1%vs 16.8%).The sheet prepared by extrusion at 400?has a yield strength of 236.3 MPa,a tensile strength of 340.3 MPa and an elongation of19.3%.When the extrusion temperature is 450?,the yield strength of the sheet is 212.3 MPa,the tensile strength is 333 MPa and the elongation is 19.3%.(4)The effect of EX-AE on the microstructure evolution of the as-cast AZ31 magnesium alloy was studied.The structure of the as-cast AZ31magnesium alloy is coarse grains of 200?m or more.As the degree of deformation increases,the structure of the sheet gradually evolves from coarse incomplete dynamic recrystallization to complete dynamic recrystallization.After EX-AE extrusion,the average grain size of the sheet can be refined to 2.0?m(350?),3.2?m(400?)and 6.7?m(450?).The higher the extrusion temperature,the greater the degree of deflection of the grains,and the distribution of the orientation of the basal plane is also more divergent.After EX-AE extrusion,the texture strength of the base surface of the sheet was 11.0(350?),11.24(400?)and 7.94(450?),respectively.(5)The effect of EX-AE on the mechanical properties of the as-cast AZ31magnesium alloy was studied.As the extrusion temperature decreases,the yield strength and The tensile strength increases and the elongation decreases.Co MPared with the original cast sheet,the yield strength of the sheet prepared at350?was increased by 152%(231.5 MPa vs 92 MPa),the tensile strength was increased by 47%(360 MPa vs 245 MPa),and the elongation increased by 43%(21.1%vs 14.8%).When the extrusion temperature is 400?,the yield strength of the sheet is 195.3 MPa,the tensile strength is 340.5 MPa,and the elongation is 21.9%.The yield strength of the sheet extruded at 450?is 165.8 MPa,the tensile strength is 315.8 MPa,and the elongation is 23.6%.In addition,the anisotropy of tensile properties of AZ31 magnesium alloy sheet prepared by EX-AE process is small.