Research On Semi-Solid Microstructure Of The AZ91D Magnesium Alloy Fabricated By Coupled Large Deformation-Isothermalmethod

Semi-solid forming (SSF) is a novel and promising technique in the metal forming today. The key to SSF is fabricating the semi-solid materials with fine, uniform and spherical microstructure. Strain Induced Melt Activation (SIMA) is an ideal technique for solving the pollution problem with the significant advantages of simplicity and low equipment cost in the fabricating process. In this paper, the AZ91D alloy has been fabricated by severe plastic deformation that include Equal Channel Angular Extrusion (ECAE) and forward extrusion, and then isothermal heat treatment in semi-solid temperature range. The fine, uniform and spherical microstructures can be obtained by controlling the process parameters such as deformation, isothermal temperature and holding time.First, ECAE numerical simulation and analysis were conducted using DEFORM-3D in this paper. The effective strain distribution under both friction conditions for one pass and four passes was also investigated. The deformation distribution in each shear plane was described by introducing the non-uniformity parameter. The results show that the non-uniformity is the least at BC route. We also obtain that the different stress field, the variable flow velocity of steady deformation zone and the friction force are the main three factors resulting in non-uniformity.The prepared AZ91D specimen was produced by forward extrusion (ration=7) and ECAE with BC route and then conducted isothermal heat treatment in semi-solid temperature range. In order to provide scientific foundation for determining the SIMA process parameters, the distribution of grain size and roundness has been investigated. The results show that the semisolid AZ91D alloy microstructure produced by four passes ECAE was the best in both grain size and roundness. It can be gained the best semisolid microstructure at 570℃holding 10 to 15 minutes for forward extrusion and one pass ECAE,but only 10 minutes for four passes ECAE. Recrystallization grain growth mechanism has been put forward according to the microstructure evolution of the AZ91D alloy during the SIMA process.In the end, integrating the advantages of the artificial neural network (ANN)...