Numerial And Experimental Research On The Microstructure Evolution Of Magnesium Alloy AZ31 During Extrusion Process

Magnesium alloy is promising metal in automotive and aerospace applications because of its low density, high specific strength and stiffness and recyclability. Due to the close-packed hexagonal structure, the formability of magnesium alloy is poor at room temperature, while it is much better at warm condition. Grain reorientation caused by slip and twinning can induce the formation of texture during deformation. Besides, dynamic recrystallization is easily activated during warm forming process of magnesium alloy. The experimental research shows that grain size and texture evolution during deformation of magnesium alloy has significant influence on its material properties. Hence, it is necessary to gain a better understanding of microstructure evolution during forming process from the point of view of predicting material properties.Integrated computational materials engineering (ICME) is a new field of study that is evolving within the global materials profession. It enables concurrent analysis of manufacturing, design, and materials within a holistic system. The forming process and microstructure evolution during forming process are simulated with the input of the initial microstructure distribution. ICME entails integration of information across length scales and forming processes for all relevant materials phenomena. In fully mature form, integrated computational materials engineering offers a solution to the industrial need to quickly develop durable components at the lowest cost. It also has important potential for accelerating the development of new materials. In this paper, microstructure evolution during extrusion process is studied with ICME methodology. A systematic research is conducted to investigate the influence of extrusion process on average grain size and texture distribution. This work lays a solid foundation for predicting material properties.Crystal plasticity has been widely applied to predict texture evolution recently. To find out the one which fits for magnesium alloy AZ31, comparison between different models and simulation methods has been carried out. A rate dependent single crystal model is built for slip dominated metals. The model is validated by modeling earing behavior during stamping of aluminum sheet. Based on the single crystal model, Taylor model and elasto-plastic self consistent (EPSC) model are built respectively, the effect of twinning is also incorporated into the polycrystalline plasticity model. Comparison between material point simulation and crystal plasticity based finite element method (CPFEM) is conducted by simulating texture evolution during compression of AZ31 alloy. The research lays the foundation for numerical simulation of magnesium alloy.To gain a better understanding of microstructure evolution during hot deformation of casting magnesium alloy AZ31, the isothermal compressions have been carried out at different deformation conditions (different temperatures, strain rates and strain). The sampling locations on the casting billet do not have obvious effect on the stress-strain curves. The microstructure distribution is measured by optical microscopy, and the influence of strain on the average recrystallized grain size is studied. The evolution of macro-texture distribution is measured by X-ray diffraction, and electron backscattered diffraction is employed to studied the deformed texture and recrystallized texture.The dynamic recrystallization kinetics is formulated as a function of strain, temperature and strain rate based on the modified Avrami equation. The dynamic recrystallization kinetics is then implemented into crystal plasticity model. The initial critical resolved shear stress and hardening parameters are obtained by fitting the stress-strain curve. The coupled model is employed to predict stress-strain curve, texture and averaged recrystallized grain size evolution during compression, the model is validated by comparing with experimental results.Direct extrusions of casting AZ31 alloy are carried out at different conditions (different extrusion ratios, extrusion temperatures and velocities), component extrusion test is conducted based on the dimension of an automotive part (roof rail). The microstructure and texture distribution of extruded samples are measured by optical microscopy, X-ray diffraction and electron backscattered diffraction respectively. Tensile tests are conducted at room temperature to reveal the influence of extrusion on the material properties. The experimental yield strength can not be solely described by average grain size. The grain size and orientation in the extruded rods are characterized by electron backscattered diffraction, and Hall-Petch equation is applied to each individual grain with the input from electron backscattered diffraction results (individual grain size and orientation). The yield strengths of tensile sample (polycrystalline aggregate) and individual grain are related by Taylor assumption. The predicted yield strength shows the same trend as experiment results.The integrated simulation of extrusion process is carried out based on the coupled method of dynamic recrystallization kinetics and crystal plasticity model, while the effect of temperature variation during extrusion process is also considered. Texture evolution during direct extrusion and component extrusion are simulated, the predicted results are in qualitative agreement with experimental results. The yield strength of extruded component is calculated with the predicted texture and grain size, the predicted yield strength is similar with the experimental value. Hence, investigating extrusion process of magnesium alloy with ICME methodology is possible to reveal its microstructure evolution. It can also provide theory basis of devising extrusion process design, and promote the extrusion technique of magnesium alloy.