Tensile experiment of AZ91D magnesium alloy was carried out and serrated flow was apparent throughout the deformation history. Dynamic strain aging (DSA) occurs when the AZ91D magnesium alloy treated by solid solution treatment has been deformed at a set range of strain rates (1.11×10-4 s-1 to1.67×10-3 s-1) and a certain range temperatures (248 K to 423 K). The critical plastic strain εc was observed to increase with increasing strain rates but decrease with increasing temperature. The diffusing activation energy of solute atoms during the DSA occurring in AZ91D magnesium alloy is 140.8 kJ/mol by calculating, which is correspondence match with the diffusing activation energy of Al solute atoms in Mg matrix. Therefore, the micro-mechanism of DSA in the alloy is believed that the Al atoms in solid solution gather around dislocations to form Cottrell solute atmospheres by vacant diffusion and then pin the moving dislocations.
Three groups of commercial 5052 Al alloy sheets were subjected to groove pressing (GP) at room temperature using parallel GP, 180° cross GP and 90° cross GP, respectively. Mechanical properties and fracture modes of as-annealed and GPed samples were investigated. The microhardness of the samples processed by parallel GP increases by a factor of about 1.6 compared to the as-annealed state, and that of the samples processed by cross GP is higher. The ultimate tensile strength (UTS) increases significantly after GP, while the elongation decreases. But they are strongly dependence on the number of GP passes and the pressing modes. Besides, fracture surface morphology demonstrates that the fracture mode is ductile even after four passes. With increasing the number of GP pass, the amount of small dimples increases, and the dimples become shallow and more uniform.
In this study AZ31 sheet with a thickness of 1.2mm and diameter of 52mm was simulated to press into a dish by a finite element method(FEM) software, which to obtain better processing of plastics forming of magnesium alloy by varying die parameters. In order to find the way of development on drawing property and to formulate the rational stamping processing, simulations have been applied on the maximum principal stress various with round radius of dent die and round radius of punch and die gap. Simulation results show that: to obtain a dish of 29mm diameter, a sheet of AZ31 magnesium with a thickness of 1.2mm and diameter of 52mm has been drawn, the fracture occurring at the corner of dish wall bottom. the ability of drawing varies with the round radius of dent die, which better radius is 3.8 mm. In the same way better round radius of punch is 3.0 mm, while better half gap is 1.8mm. Experiments also show that high diameter ratio has been increased with the various of die parameters and forming ability of material has been developed. It is reliable of simulation of finite element method.
Equal channel angular pressing (ECAP) was applied to a commercial AZ61 magnesium alloy for up to 8 passes at temperatures as low as 473K. Microstructures and mechanical properties of as-received and ECAP deformed samples were investigated. The microstructure was initially not uniform with a “bimodal” grain size distribution but became increasingly homogeneous with further ECAP passes and the average grain size was considerably reduced from over 26 μm to below 5 μm. The ultimate tensile strength (UTS) decreases clearly after one pass, but increases significantly up to two passes, and then continuously slowly decreases up to six passes, and again increases slightly up to eight passes. In contrast, the uniform elongation increased significantly up to 3 passes, followed by considerable decrease up to 8 passes. These observations may be attributed to combined effects of grain refinement and texture development.
Instable plasticity, which is normally a consequence of interactions between diffusing solute atoms and dislocations, is observed in AZ91D magnesium at room temperature. Experimental measurements of different heat treatment of AZ91D magnesium were taken to investigate different influence on PLC effect by the presence and structural state of second phase in this study. Tensile specimens were tested at room temperature at a crosshead speed of 0.6 mm/min. The results indicate that some number of large,coherent precipitates prevents serrated flow by way of trapping vacancies, but small precipitates content do not supress it.
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