| Magnesium alloys have low density, high specific strength, high specific stiffness and many other outstanding advantages. Therefore, they have very essential applied value in the fields of aerospace, automobile and communication. However, conventional magnesium alloys have poor workability at room and low temperature due to their hexagonal close-packed(HCP) crystal structure and limited slip systems. The application of as-cast magnesium alloys has also been restricted owing to the limitations of casting process. The emergence of seimi-solid technology overcomes the shortcomings of casting and forging, which has become a practical industrial technology to achieve magnesium alloy products with high performance.Preparation of semi-solid belliets is the key to the success of semi-solid forming technology. The strain-induced melt activation(SIMA) method has become the semi-solid billet preparation method with industrial application prospect. In view of the drawbacks of current SIMA methods, such as serious shape change and inhomogeneous microstructure of pre-deformed belliets, a new SIMA method which is based on the repetitive upsetting-extrusion(RUE) processing is proposed for preparing semi-solid magnesium alloy billets. The main advantages of the new process lies in that it is simple and convenient in operation, easy to obtain severe plastic deformation, capable of maintaining the original billet shape and size, and high in material utilization ratio. Firstly, the RUE process was adopted to achieve large strain-induced deformation of magnesium alloy materials. Through the process, the second phases were fully broken, and the matrix phases were also refined with a certain degree. Then fine and homogeneous semi-solid structure with higher spheroidization degree would be obtained through reasonable isothermal annealing treatment.This paper focused on the new SIMA method of RUE process for preparing semi-solid metal billets. Take AZ61 and AZ91 D alloys as the research objects, the fabrication of semi-solid billets and thixoforming properties were investigated. Based on the flow stress data of the isothermal compression tests, the constitutive equations were established, the processing maps were developed to provide safety criterion for hot processing, and the dynamic recrystallization kinetics models were proposed. Through FEM method and experiment research, the microstructure evolution patterns of RUE process were revealed. By microstructure examination during solid-liquid isothermal treatment process, the main influence factors and mechanisms of semi-solid grain spheroidization were clarified. Furthermore, the matching relationship between RUE deformation process parameters and the isothermal treatment parameters was established, and the control principles and methods for semi-solid microstructure of RUE-ed magnesium alloys were proposed. Seimi-solid thixotropic compression tests were performed for RUE-ed magnesium alloys. The characteristics of flow stress curves and liquid phase flow were investigated, and the influence of deformation conditions on flow stresses and compressed microstructures were discussed. On the basis of semi-solid thixoforming and the investigation on the mechanical properties and microstructure, the influence of semi-solid billet preparation and processing conditions of thixoforming on the microstructure and mechanical properties of the final products was revealed.In the present investigation, a practical technology was proposed to prepare high-quality semi-solid billet of magnesium alloy. Issues related to basic science were solved, key technical difficulties were broken through, and the process regularity was mastered. It is of great significance to promote the application of this process and optimize parameters for the process. Meanwhile, it is extremely important for enriching semi-solid processing methods and theories, promoting the progress of semi-solid processing technology, expanding application areas and promoting the industrial development of magnesium alloys. |
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