A novel two-stage reheating process with new alloy design has been developed to improve the microstructure morphology of semi-solid Al-Si casting aluminum alloy for thixoforming. The process consists of first reheating the material to the liquidus temperature, holding for 5 minutes, and then lowering to the predetermined two-stage reheating temperature between 843 -863 K and holding for 10 minutes. The experimentally-obtained grain diameter, roundness, and the amount of liquid trapped within the solid phase were characterized, along with the microstructure obtained using the traditional feedstock reheating process. The Wilcox test (with α=0.05) was then applied to statistically analyze the measured differences in the microstructures obtained using the two different processing routes. It was found that a refined near-spherical structure with uniform globule size, higher sphericity, lower coarsening rate constant, and less entrapped liquid was obtained via the new two-stage reheating process in comparison with the microstructure obtained using the traditional feedstock reheating process.
The constitutive equation of semi-solid 6061 alloy was established according to the thermal simulation compression tests of semi-solid billet of 6061 alloy prepared by near-liquids semi-continuous casting method. Parameters of Bingham model, such as apparent viscosity, had been investigated based on the constitutive equation of semi-solid 6061 alloy. The software ANYCASTING was applied to simulate and analyze the semi-solid die-casting processes of semi-solid 6061 alloy. A well-designed runner and gating system of automobile aluminum alloy part were presented and the casting defect were investigated here utilizing these parameters by simulation. The results show that a well-designed runner and gating system is very important to secure good quality die castings through providing a homogenous mould filling pattern. Most defects of semi-solid casting products can be founded in the final solidification of the parts of casting.
Parameters of Bingham model had been verified according to the thermal simulation compression tests of semi-solid 6061 aluminum and the equations presented by Stefan. The effects of the mold filling temperature of semi-solid slurry of 6061 aluminum during thixo-diecasting were analyzed. The results showed that such conditions of the mold filling temperature about 625°C, the injection speed changing to 2 m/s from 0.4 m/s when 60% of die cavity had been filled, were ideal to gain solidification in sequence and beneficial to get fully filled semi-solid parts with clear contour. The thixo-diecasting experiments were performed under above optimized conditions.
Temperature is an important factor affecting the changes in the amount of gas exchanged between eggs and the surrounding during storage. The effect of temperature changes on the release of carbon dioxide (CO2) from eggs was studied using eggs from the same breed. The experimental samples were divided into three groups, and placed in a storage box at 4, 25, or 35°C with 65% relative humidity for 20 days, and a breathalyser was used to measure the amount of CO2 released by the eggs. The FLUENT software was used to simulate the diffusion at different temperatures. The conclusion from the present work was that as the temperature decreased, the amount and speed of CO2 released by the eggs also decreased. The simulation results show that the diffusion of CO2 released by egg respiration can be divided into the following sequential stages: initial, descending, mixing, and turbulence. When the storage temperature was 4°C, the CO2 cloud of eggs was the lowest, followed by 35°C, and finally 25°C. The results show that the direction of gas diffusion was mainly affected by temperature, diffusion volume, and diffusion velocity, and that temperature mainly affected vertical gas diffusion. The present work thus provides a theoretical basis for egg respiration related research.
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