Research On Warm-Hot Deformation Behavior And Mechanism Of AZ31 Magnesium Alloy Sheet

As one of the lightest structure materials, magnesium alloys are widely used in automobile, spaceflight, electronic fields for their high specific stiffness and specific strength, strong capacity of the antivibration, excellent electromagnetic shielding, easy recycling and so on. However, magnesium alloys have poor plastic formability caused by their limited slip systems. The hexagonal close-packed structure restricts their formability and application fields. Magnesium alloys attract more attentions for their abundant resources and high application potential. Up to now, considerable advances have been achieved in the study of their deformation mechanism and forming processes of magnesium alloys.The research work on the formability of the magnesium alloy under the warm-hot temperature condition is still continuing, especially about the rheological/mechanical property and forming limit diagram (FLD) prediction aspects. The constitutive equations for flow stress of the magnesium alloy are simple and they decouple the combined effect of temperature and strain rate. The parameters of constitutive equations are obtained mostly by direct method. The FLD of magnesium alloy is obtained mainly through experimental methodology. This method is not only consuming time and energy of the researchers but also have high requirements on the qualities of the researchers and equipments. Moreover, the obtained result is also not so easy to use in the application. The FE M-K model has been used for FLD prediction combining the influence of plastic instability model, constitutive equations. Unfortunately, the research on FLDs based on FE model is less. In general, studies on the formability of magnesium have been carried out under high temperature and low strain rate. Researchers already have found that the good mechanical property of magnesium alloy can be obtained in higher strain-rate condition. However, the drawing process parameters and forming mechanism under crank press are unknown. Indeed, magnesium alloy can be easily deformed at room temperature coupled with pulse current. The influence of pulse current on the electroplasticity of magnesium alloy is discussed in this study. The electroplastic mechanism of magnesium alloy with pulse current effect needs to be further studied.The warm-hot forming behavior and electroplastic forming mechanism of AZ31 magnesium alloy are studied here in order to solve the problems above. The main contents and the conclusions of this thesis are shown as follows:(1) Based on the data of the uniaxial tension test under different temperatures and strain rates, the optimum parameters of different type constitutive equations are obtained with the optimization method. The constitutive equations coupled with the temperature and strain rate are constructed, including Ludwick model, Voce model and H-V model. The rheology behavior of AZ31 is predicted with the constructed material model. The predicted results have a good agreement with the experimental data under certain conditions.(2) Based on the FE M-K model and experiment data, the imperfection value f0 can be obtained using inverse analysis method. Through user-defined subroutine (Uhard), different constitutive equations can be implemented and the FLDs of AZ31 alloy can be determined. Comparing the simulated results and input experiment values, the influence of related parameters on FE M-K prediction is studied. It shows that f0 value is important for the predicted FLDs. Owing to different hardening characters, the constitutive equations have the different effect on FE M-K FLD prediction. The choices on the constitutive equations, definition of their parameters and the initial imperfection value fo are very important.(3) Process parameters, including the temperature, punch radii, clearance between punch and die, have a great influence on the deep drawing formability of AZ31 sheet with the crank press device. Orthogonal test is carried out to study the drawing formability of AZ31. The result is analyzed with the range analysis method. The obtained optimum test temperature ranges from 240℃ to 310℃, and the soapsuds is taken as the lubricant. The die clearance is 1.10t (t is sheet thinkness), and punch radius is 8 mm. The ME20 and ZE10 sheets can be deformed successfully with the optimum results. Their forming mechanism is studied using the optical microscope, x-ray diffraction and scanning electron microscope methods. It is found that dynamic recrystallization occurs rapidly during drawing process. This causes finer grains and weaker basal texture. As a result, magnesium alloy has good formability under these conditions.(4) The effect of the process parameters and the current parameters on the mechanical properties of AZ31 is studied through tensile test with the pulse current effect. It is found that the temperature and voltage are the main factors on AZ31 tensile property. The flow stress decreases and the electroplastic effect increases with the increasing temperature and voltage. The electroplastic forming mechanism is studied with the microstructure analysis and fracture profile observation. The pulse current can not only accelerate the occurrence of dynamic recrystallization but also affects the precipitation and solution of the second particles. This will bring different effects on the mechanical property of AZ31. Higher EP effect leads to the bigger dynamic recrystallization grains, and the formability of AZ31 alloy suffers significant deterioration.(5) Primary research on the critical condition of dynamic recrystallization with pulse current is carried out. The critical conditions for the temperature and voltage obtained with one-parameter method. It is found that the high temperature and voltage are benefit for the occurrence of dynamic recrystallization. The critical condition for the voltage is verified with the metallography. The dynamic recrystallization grains can be found when the critical stress is satisfied but absent when the critical stress can not be reached. This observation confirms that the one-parameter method can be used for the analysis about the critical condition of dynamic recrystallization with current.