Study On Microstructure Analysis And Numerical Simulation Of Equal Channel Angular Pressing For ZK60 Magnesium Alloys

In recent years, magnesium and its alloys are the lightest applicable structural material. They have many good properties such as specific strength, specific stiffness, good thermal conductivity, ease of machining and facilitate recycling. Therefore, it has been widely used in the area of aerospace, automotive and electronic industries. However, it also has the poor ductility and plastic deformation for the hexagonal close-packed at room temperature, which limited its extensive application. It is well known that fine-grain strengthening mechanism can not only enhance the strength but also improve the plastic of material. Therefore, the study on the method of grain refinement has important meaning in theory and application.As one of the methods to obtain ultra-fine grain structure, Equal-channel Angular Pressing (ECAP) has been widely studied. Many metals and alloys can be effective enhanced by the technology. As the plastic of cast magnesium is poor, the pass of successfully extrusion is limited, and the cracks of appear during ECAP deformation. Consequently, a hot extruding was designed before ECAP processing to make the deformation of ECAP easier. By mean of severe plastic deformation of ECAP, the material can be remained uniform deformation, defect-free and no dead area, and the microstructure can be changed into equiaxed crystals. The technological parameters such as process route, back pressure methods, structure of die, temperature and velocity of extrusion were determined.The first pass of ECAP processing was simulated by using a finite element software DEFORM-3D. The influence of velocity of material flow, temperature distribution and effective stress on extrusion part was analyzed. The evolution of microstructure and the process of dynamic recrystallization were also simulated. In this way, the dynamic process of microstructure before and after experiment was obtained. The optical microscopic analysis of the samples produced by casting, hot-extruding, one pass ECAP was taken, and the mechanism of microstructure changes during ECAP processing was studied. The SEM was employed to analyzed the extruding part after 4 passes ECAP processing, and the precipitates in the stereoscan photograph were subjected to energy spectrum analysis, in order to determine its chemical composition and the differences to the substrate metal. The changes of grain structure before and after experiment were analyzed by a combination methods of finite element simulation and experiment. The results of simulation and experiment show that there is a good fit between the two.