The Twinning Behavior Of AZ31 Magnesium Alloy Sheet During Corrugated Wide Limited Alignment Process And Its Effect On Formability

Magnesium alloy has been widely used in mechanical,communication and aerospace fields due to its low density,high specific strength and specific stiffness,and good electromagnetic shielding performance.However,the application of magnesium alloys is limited,due to its poor formability at room temperature.The AZ31 magnesium alloy sheet with basal texture was selected for the present research.Through the corrugated wide limit alignment(CWLA)process,the original AZ31 magnesium alloy sheet was compressed along the rolling direction,resulting in the transverse shortened and thickness thickened,in order to induce the tensile twins and weaken the basal texture of the original sheet.The microstructure evolution of the sheet after CWLA deformation and post-annealing process was observed and analyzed.The mechanical properties were tested by universal tensile testing machine and the formability was evaluated by cupping test machine at room temperature.The mechanical properties,formability,microstructure,and plastic deformation mechanism of AZ31 magnesium alloy sheet were investigated,the microstructure evolution of AZ31 magnesium alloy plate during CWLA treatment and post-annealing was revealed,and the corresponding relationship between microstructure and formability of AZ31 magnesium alloy sheet was established.In this thesis,AZ31 magnesium alloy sheet was subjected to CWLA deformation at 250℃,300℃ and 350℃,and the deformed sheet was annealed under different conditions.Microstructure analysis showed that a large number of lenticular {10(?)2} tensile twins appeared after CWLA.In addition,the tensile twins made the crystal grains with the c-axis parallel to the normal direction(ND)significantly deflected toward the rolling direction(RD),making the basal texture of the original hot-rolled sheet significantly weakened and the RD deflection texture strengthened.There were one or two twin variants in AZ31 magnesium alloy sheet during CWLA.When there were two twin variants,they were always in adjacent position.The results of uniaxial tensile test at room temperature indicated that the existence of tensile twins significantly changed the mechanical behavior of samples.Especially when it was stretched along the RD direction,a de-twinning mechanism with softening effect would be produced,which causing reduced yield strength(YS).Tensile twins would also hinder dislocations movement,produce stress concentration,cause work-hardening,resulting in increased ultimate tensile strength(UTS)and decreased fracture elongation(FE).After deformed at 350℃,the plasticity of the plate was improved,the YS was 169 MPa,the UTS was 303 MPa,the FE was 16.7%,and the IE value at room temperature reached 4.34 mm,which was about 95%higher than that of the original plate.The microstructure of the annealed sheet showed that the structure of the twin lamellae would not change when the deformed samples were annealed at a lower temperature(annealing temperature ≤200℃),and the tensile twins generated during the deformation process were basically retained.With the increase in annealing temperature,the parallel twin lamellas in the grains would grow up and fuse with each other,and finally swallowed the matrix grains.At the same time,the fine recrystallized grains would also increase.The recrystallization mechanism was mainly strain induced {10(?)2} tensile twins migration in the annealed samples deformed at 250℃ and 300℃,and the texture component of RD texture gradually increased.However,in the annealed samples deformed at 350℃,most of the recrystallized grains had basal plane orientation,but there were relatively dispersed and gathered near the basal plane,which weakens the original RD deflection texture.The results of uniaxial tensile tests and cupping tests at room temperature showed that the formability of annealed samples deformed at 250℃and 300℃ was improved,which was caused by the strong RD-deflected texture.In the annealed samples after deformed at 350℃,the formability was also improved to a certain extent due to the relatively dispersed basal texture and the RD texture of a certain components.The magnesium alloy sheet had the best plasticity after CWLA deformation at 300℃ and annealed at 300℃ for 30 min,with the YS of 124 MPa,the UTS 285 MPa,the FE of 19.1%,and the IE value of 4.84 mm.