Technical And Theoretical Research Rheological On Forming Of AZ91D Alloy Under High Shear Rate

Coarse grain structure and the precipitation of (3-phase which formed an uneven network are difficult to meet the need of high performance, potential advantages and characteristics structural material in the conventional magnesium alloy casting.High shear rate has been introduced for the production of slurry preparation. This work aims to achieve the precise control of casting process and product quality, the improvement of the structure and property, the formation of an efficient, clean, fine processing technology. Therefore, solidification behaviour of AZ91D Mg-alloy using a twin-screw slurry maker was investigated to understand the effect of he nucleation and growth behaviour under high shear rate, which provide the theoretical basis for high-performance parts. Main contents and results are as follows.The slurry of AZ91D Mg-alloy was successfully prepared under high shear rate. Compared with the microstructures under low shear rate, it appeared that the microstructures were homogenous spherical and fine grains. The influences of shearing temperature, rotation speed and shearing time on the size and morphology of grains were investigated.The results show that the solidification structure evolution is determined by shearing temperature, rotation speed and shearing time. High shear rate is beneficial to the formation of fine globular grains at the fixed shearing temperature and shearing time level.The distribution of temperature field and boundary concentration was investigated to analyze the characteristics of grain refinement and globalization under high shear rate,. First, the computational fluid dynamics and thermodynamics were undertaken to investigate the effect of high shear on the temperature field of the melt.The results show it is crucial to achieve a uniform temperature field under high shear rate, temperature gradient of the melt decreases with the increasing of rotation speed, which is favorable to nucleate in the entire volume of the melt.Concentration gradient in front of solid-liquid interface, Interface concentration, effective distribution coefficient were undertaken to investigate the effect of high shear rate on the boundary concentration of the melt.The results show high shear rate is crucial to decrease concentration gradient in front of solid liquid interface and alloy undercooling, increase Interface stability, and support the formation of ideal globular microstructures.Based on experimental results and the distribution of temperature field and boundary concentration under high shear rate, the nucleation behavior and the grain growth under high shear rate were analyzed.The temperature field is extremely uniform and below its liquidus throughout the entire alloy melt, nucleation will occur in the entire volume of the liquid and each nucleus will survive. The probability of overlap of diffusion fields of the adjacent grains increases, which leads to the decrease of concentration gradient in front of solid-liquid interface and increase of their interface stability. In addition, the distribution of grains and spacing among them are uniform because of high shear rate. Under this situation, the growth will be limited and stable, and ideal globular microstructures can be obtained.Besides, a mathematical model between the solidification velocity and rotation speed and the solidification structure evolution was developed.The morphology of solidification structure is strongly affected by the solidification velocity and rotation speed.With decreasing solidification velocity or increasing rotation speed, shape factor of solidification structure increases.Compared with experimental results, it was found that mathematical model showed a very good agreement with experimental results,the spheroidal interface stability was investigated under high shear rate,it is found that high shear rate can greatly improve the interface stability of single grain.Finally,the rheo-diecasting under high shear rate was investigated to produce the casting. In order to get the good performance of magnesium alloy parts, injection velocity was optimized by numerical simulation and experimental results. Compared with the ordinary high pressure diecasting (HPDC),the rheo-diecasting under high shear rate based on optimum parameters had a uniform microstructure and enhanced mechanical properties. In addition, it could be effectively improved mechanical properties by heat treatment schedules.