Study On Thermal Deformation Behavior And Simulation Of Microstructure Evolution For AZ61 Magnesium Alloy

With the arrival of the Age of lightweight,magnesium alloy is more and more favored,and it has a wide application prospect in the aerospace,automotive and 3C areas.Its plastic deformation ability is poor because of its crystal structure of close-packed hexagonal.Hence,the die-castings are more common in the current products.Sometimes die casting does not meet the requirements of high-performance parts.The microstructure of fine grain and good comprehensive performance can be obtained by plastic deformation,and the parts performance is better than die casting.Therefore,the study of plastic deformation of magnesium alloy is of great significance to the development of magnesium alloy.The macroscopic mechanical properties of metals are depended on the microstructure.At present,the most commonly used research method is to establish an appropriate model for forecasting the microstructure during thermal deformation by studying the microstructure evolution and dynamic recrystallization behavior in the process of thermal deformation.As an interdisciplinary field,the cellular automata has introduced the curvature-driven mechanism,thermodynamic driving mechanism and the energy dissipation mechanism,which can reflect the physical process of grain boundary migration more truly.It is one of the most popular simulation methods.The traditional research method is to establish the dynamic model and constitutive model based on the stress-strain curve.These models are the basis of studying dynamic recrystallization,which can provide the basic material parameters for simulation model.In this paper,the thermal deformation behavior of extruded and cast AZ61 magnesium alloys was studied through establishment of the flow stress modeling,dynamic recrystallization kinetics model.The dynamic recrystallization CA model was established by introducing the grain topological deformation mechanism,and the microstructure evolution of the extruded AZ61 magnesium alloy was simulated.The optimum heat treatment method was used to optimize the heat treatment of AZ61 magnesium alloy,and after homogeneous structure was obtained,hot extrusion was carried out to obtain extruded magnesium alloy.The stress-strain curves of AZ61 magnesium alloy in cast and extruded state were tested by Gleeble-1500 simulation test under different deformation conditions.The effects of strain on the parameters such as ?,Q,n and lnA were studied by establishing flow stress and modeling.The AZ61 magnesium alloy in different states has the same thermal deformation behavior,but the dynamic softening of extruded magnesium alloy is obvious than the cast.Through the dynamic recrystallization kinetics,it is found that the occurrence of recrystallization in the extrusion state is easy to form.Compared with cast magnesium alloy,the extruded magnesium alloy's grain size is smaller and the dynamic recrystallization is easier.Besides,the cast magnesium alloy is more prone to twins in the early stage of deformation.Based on the principle of cellular automata,the initial grain growth model was established by combined with curvature drive mechanism and the energy dissipation mechanism.The dynamic recrystallization model of the cellular automata was established based on the dynamic recrystallization theory,and the grain topological deformation technique was introduced to consider the effect on the grain shape during the deformation.The programming of the model program was completed by using MATLAB,and thermal deformation simulation was carried out.The results show that the simulation results of the initial grain growth model can accurately reflect the characteristics during grain growth,and the dynamic recrystallization CA model can accurately reproduce the influence of deformation parameters on dynamic recrystallization.The microscopic morphology obtained by stimulation is similar to that of the experimental microstructure.The obtained stress and strain curve and the dynamic recrystallization kinetics curve can reflect the characteristics of the curve accurately.The obtained peak stress,steady state stress,average grain size and so on are in good agreement with the experimental data.So the established model has a certain accuracy,the established cellular automata model can be used for dynamic recrystallization simulation.The change of activation energy is introduced,which affects the recrystallization behavior by affecting the dynamic recrystallization nucleation rate.The results show that the activation energy is more realistic.