| Magnesium alloys are the lightest commercially structural alloys and are widely used in aerospace, automotive, communication and other industries. However, Mg alloys have poor ductility at room temperature due to its hcp structure and limited slip systems. Commonly used extruded and rolled magnesium alloy sheets have strong basal texture and anisotropy, which limits its widely spread. With the aim of improving the microstructures, mechanical properties and formability of commercial magnesium alloy sheets, in this work, the texture evolution of Mg alloy sheets during tensile test and compression were simulated based on the Crystal Plastictity Finite Element Method(CPFEM). According to the simulation results, two techniques were used to improve the microstructures of Mg alloy sheets, such as pre-streching at large strains and precompression deformation, also, the corresponding microstructures, texrure evolutions, mechanical properties and formability were investigated. The forming limit diagrams(FLD) of AZ31 B alloy sheets at room temperature were obtained based on three theories, such as M-K theory, Swift theory and Hill theory, and the effects of n-value and rvalue on the FLD were also investigated. The main research contents and results are as follows:① The texture evolution of AZ31 B alloy sheets during the tensile test and compression were simulated based on CPFEM. After pre-streching process, the intensity of basal texture became stronger, but its spread became more dispersive. The tilted direction of basal texture depends on the direction of tensile test, namely the tilted direction is perpendicular to the direction of tensile test and its spread become more dispersive along the direction. The c-axes of grains of AZ31 B alloy sheets tilted along tranverse direction(TD) during the compression along TD, which results in the weakened basal texture. The tilted direction of c-axes of grains also depends on the direction of compression, the c-axes of grains are always parallel to the compression direction.② The formability of AZ31 alloy sheets can be improved by pre-streching at large strains followed by recrystallization annealing. The basal texure intensity is significantly weakened, the c-axes of grains of SA sheets tilt from ND towards to ED, and its spread becomes more dispersive. As the pre-strecthing strains increase, the formability of SA sheets becomes better and better. Compared with the as-received sheets, the Erichsen value of SA17% sheet is increased form 2.5mm to 5.6mm by ~124%. The im-provement of formability could be explained by the lower yield strength and r-value, higher n-value.③ The improvement of mechanical properties of SA sheets depends on the direction of pre-streching. Three different pre-stretching routes were used to improve the mechanical properties of AZ31 B alloy sheets, the basal texture intensity of all the three SA sheets was weakened, the tilted direction of basal texture is always perpendicular to the direction of pre-stretching, which results in the improvement of mechanical properties along the tilted direction, namely lower yield strength and r-value and higher nvalue, and the improvement of mechanical properties along the pre-streching direction is not obvious.④ The effects of strain rates of pre-stretching on the formability of AZ31 B alloy sheets were investigated. As the strain rates increase, the formability of SA sheets was improved gradually. At the strain rate of 0.01s-1, the Erichsen value is 5.4mm. However, the IE value decreases as the strain rates exceed 0.01s-1. Due to the different strain rate sensitive of slip and twinning, the deformed microstructures vary as the strain rates increase, which results in the different basal texture and formability.⑤ The pre-compression process was used to improve the mechanical properties of AZ31 B magnesium alloy sheets. After pre-compression along TD and annealing process, due to the tensile twins and subsequent annealing, the c-axes of PCA sheets are parallel to the TD, which results in the easy activation of basal slip, <c+a> slip and tensile twinning. Compared with the as-received sheets, the Erichsen value of PCA4.8% sheet was increased from 2.35 mm to 4.55 mm by ~94%, which results from the lower yield strength, yield ratio and r-value and higher n-value. As the pre-compression strains exceed 4.8%, the improvement of formability of PCA sheets is not obvious.⑥ The effects of different pre-compression routes on the microstructures and mechanical properties of PCA sheets were investigated. The results exhibit that the improvement of mechanical properties depends on the pre-compression direction. After annealing, the c-axes of grains of PCA sheets parallel to the pre-compression direction, the mechanical properties of PCA sheets along 45° can be significantly improved by pre-compression along RD and TD, and the mechanical properties of PCA sheets along both RD and TD can be improved by pre-compression along 45°. The formability can be significantly improved by pre-compression along the direction perpendicular to the spread direction of basal texture.⑦ The pre-compressed sheets were annealed at 250°C for 2h to remove the dislo-cation and while keeping twin structures, and were annealed at 450°C for 1h to obtain recrystallized structure. Both annealing treatments could weaken the basal texture, make c-axes of grains tilt, decrease yield strength, yield ratio and r-value, increase n-value. But the Erichsen value of PCA sheets with twin structures is only 2.57 mm, and that of PCA sheet with recrystallized structures is 5.28 mm.⑧ The pre-compression along TD at 150°C was used to improve the microstructures and mechanical properties of AZ31 B magnesium alloy sheets. After precompression and annealing process, due to c-axes of recrystallized grains parallel to the TD, the basal texture was weakened, which results in the low yield strength, yield ratio and r-value, and high n-value. Compared with the as-received sheets, the Erichsen value of PCA7.5% sheet was increased from 2.35 mm to 5.28 mm by ~125%.⑨ The forming limit diagrams(FLD) of AZ31 B magnesium alloy sheet at room temperature were obtained based on M-K theory, Swift theory and Hill theory, respectively. The FLD based on M-K theory could predict the forming limit of Mg alloy sheet at tension-tension strain conditions, while that based on Swift theory could predict the forming limit at tension-compression strain conditions. The FLD based on Hill theory is not suitable for computation of forming limit of Mg alloy sheet. The forming limit of AZ31 B magnesium alloy sheet at room temperature couled be improved by increasing n-value and decreasing r-value. |
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