| The development of new materials with the aim of reducing energy consumption, protecting environment and achieving sustainable human development is one of main ways to solve the depletion of natural resources and deterioration of environment, and has become a major concern of mankind. Magnesium alloys, as the lightest engineering metal material, have many advantages such as high specific strength and specific stiffness, excellent thermal conductivity, strong electromagnetic shielding and good environmental compatibility, and have broad application prospects in automotive, communications, electronics and other fieldsMagnesium alloys show low ductility at room temperature because of their typical hexagonal close-packed crystal structure. However, the plastic formability of magnesium alloy would be significantly improved with the increase of temperature. The application of warm and hot stamping technology for magnesium alloy sheet can not only make full use of material’s excellent performance, environmental performance and meet the need for thin and lightweight product, but also greatly improve production efficiency and product qualified rate. So the research and development of forming technology for magnesium alloy sheet has been a hotspot in recent years.Flow stress and constitutive equation of magnesium alloy at high temperatures are indispensable conditions for numerical simulation of hot stamping, which reflect the relationship between flow stress and strain, strain rate, temperature and other factors. Since available basic performance data of magnesium alloy in the present is still very lack, in this paper, uniaxial tensile tests of AZ31B magnesium alloy sheet were conducted at room and high temperatures, true stress-true strain curves at different temperatures and strain rates were obtained. Studies show that the tensile flow curves of AZ31B magnesium alloy sheet at room temperature are not sensitive to deformation velocity and controlled by heating effect. An obvious softening phenomenon was observed during hot tensile deformation of AZ31B magnesium alloy sheet. And as the temperature increases and/or strain rate decreases, the softening effect is more notable, which shows that dynamic recrystallization occurs. Through the analysis of the true stress-strain curves, two mathematical models of flow stress were developed, namely the model based on the Fields-Backofen equation, and the model based on a natural exponential function whose exponent is a quadratic function. The research indicates that, at the first stage of the deformation, two kinds of model are in good agreement with the experimental value. But the model built based on Fields-Backofen equation is inaccurate to describe the softening behavior after the peak stress. While the exponential model can accurately reflect the work hardening and strain softening effect of AZ31B magnesium alloy sheet at high temperatures. Therefore, the new model built in this paper is more appropriate for the prediction of flow stress of AZ31B magnesium alloys at high temperatures.Major causes of failure in the forming process of Magnesium alloy sheet is not stress but ultimate strain. The ultimate strain can be determined through bulge test. In this paper, the forming limit diagrams of AZ31B magnesium alloy sheet at the temperatures of150℃,200℃and300℃were determined experimentally by conducting hemispherical punch stretching tests. The effects of temperature on forming properties of AZ31B magnesium alloy sheets were discussed. The research indicates that, The forming limit diagrams of AZ31B magnesium alloy sheet are affected by temperatures. When temperature is below100℃, the FLC is almost concentrated to a point. A complete forming limit diagram can be obtained at150℃, but the location is low. When the temperature increases to300℃, the location of forming limit curve rise and the bulging performance is improved significantly. As we all know, experimental determination of forming limit diagram is a time-consuming and laborious work and only can obtain results at given temperature, the mathematical model of the forming limit diagrams for AZ31B magnesium alloy sheet at different temperatures was established on the basis of analysis of the experimental data. It can be used for predicting the locus of failure in stamping for AZ31B magnesium alloy sheet at high temperatures. It also provides an important safety judgment criterion in simulation of AZ31B magnesium alloy sheet metal forming processes.Limit dome height is one of the important parameters reflecting the forming property of sheet metal. In this paper, the effect of blank thickness, forming temperature, forming speed, lubrication conditions on the forming properties of AZ31B magnesium alloy sheet was analyzed by experimental method and forming optimum conditions was obtained. That is to say, sheet metal with a thickness of0.6mm shows the best forming performance under the conditions of the temperature of250℃, with MoS2lubricating, a pre-forming of10mm dome height and heat holding time of1.0h. The limit dome height reaches42mm with complete recrystallization.It is well known that friction is inevitable during the forming process of sheet metal and friction coefficient is necessary for controlling sheet metal forming process and numerical simulation. In this paper, a kind of new method for determining the friction coefficient was put forward by the combination of the finite element simulation and experiment. Friction coefficient between magnesium alloy sheet and the punch at different temperatures during hemispherical rigid punch bulging test process is estimated. The rationality of the simulation results are verified by forming limit diagram obtained by experiment. The research indicates that, simulation results are in good agreement with the experimental ones. and so it provides a simple, feasible new idea for the determination of the friction coefficient of the contact position between two surfaces in engineering.Deep drawing is a common sheet metal stamping process. In this paper, drawing tests of AZ31B magnesium alloy sheet were conducted using Swift drawing experiment device at room and high temperature, the effect law of the process parameters such as forming temperature, blank-holder force, die clearance on the forming performance of AZ31B magnesium alloy sheet are obtained. The research indicates that, the forming property of AZ31B magnesium alloy sheet was poor at room temperature, but it is significantly improved at elevated temperatures. The preferred forming temperature was in the range of210℃-240℃. At lower temperature, die clearance desirable is1.0to1.05times thickness of the material. And good drawing parts can be obtained with gap1.1times thickness of the material at high temperature. The blank-holder force calculated using the empirical formula is not suitable for AZ31B magnesium alloy sheet. BHF acting on the specimen should be controlled less than1kN.Springback is the main factor influencing the precision of parts during the forming process of sheet metal. The effect of forming temperature, lubrication condition, pressure holding time, the minimum bend radius on the deformation and springback of AZ31B magnesium alloy sheet was explored by V shape bending correction experiment with90°. The results indicate that the higher the temperature is, the better bending performance of magnesium alloy sheet is and the smaller springback and bending radius are. The springback angle decreases rapidly with the holding time in the initial stage, no longer changes when increased to2minutes. The effect of the lubricant on bending springback is less at heating condition. The minimum bending radius should be greater than2.0mm and less than8mm at room and high temperatures for AZ31B magnesium alloy sheet. At lower temperature, a large value is selected, but when the temperature is higher, a smaller critical value can be taken.Finite element numerical simulation has become main aided analysis method in modern industrial design. Hot forming bahavior of AZ31B magnesium alloy sheet is investigated by finite element method. The effect of virtual stamping velocity on calculation precision and the effect of the blank-holder force, original blank size and punch fillet radius on the deep draw ing performance of AZ31B magnesium alloy sheet are discussed. The results indicate that the kinetic energy of the system and the hourglass energy are far lower than the internal energy of the system at a virtual stamping velocity of5000mm/s, which can ensure the precision of calculation. But considering the low actual deformation rate of magnesium alloy sheet, virtual stamping velocity values should not be too high and it is suggested to be less than2000mm/s. The reasonable selection of blank-holder device and blank-holder force plays a critical role in sheet metal forming. Different sizes of sheet metal have different reflect on the blank-holder force. For small size specimen, because of sheet metal prematurely from the blank holder, instability wrinkling phenomenon appears in the last stage of deformation, and the smaller initial blank-holder force is, the more powerful wrinkling is. For large size specimen, when the blank-holder force is less than500N, severe wrinkling in flange zone of the specimen happens due to lack of blank-holder force, finally results in fracture in the straight wall zone of cylindrical cup close to the entrance of the die. When the blank-holder force is more than1000N, the specimen is pulled crack in the cylinder straight wall zone, and a uniform wrinkle in flange zone is also formed. For deep drawing conditions in this paper under a good blank-holder status, the maximum blank size is70mm at200℃. With the increase of the punch fillet radius, the punch stroke increases, and deep drawability is significantly improved.When punch fillet radius changes from2mm to4mm, punch stroke increases rapidly. When punch fillet radius is larger than4mm, the punch stroke increases at decreased rate. This slows that punch fillet radius should be between4mm and10mm for AZ31B magnesium alloy sheet at200℃. The results of numerical simulation are basically consistent with that of thermal drawing experiment. It indicated that flow curves, forming limit curves and friction coefficient obtained by inverse finite element method were reasonable, which ensures the accuracy and reliability of numerical simulation results. |
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