Filling ability refers to the ability of molten metal to flow through the casting system to fill the entire space, forming a clear outline and a complete casting. During the boosting phase, the pressure of the upper and lower tanks increases, which not only enhances the interface heat transfer during the filling process, but also increases the interface heat transfer during the solidification process, thereby reducing the initial solidification temperature of the molten metal away from the runner and making the sheet tend to sequence. solidification. The molten metal flows in a pure liquid state before the heat of overheating is exhausted. When the temperature is below the liquidus, crystals are precipitated in the liquid stream, which proceeds downstream and grows. The front of the liquid flow is continuously in contact with the cold-shaped wall, the cooling is the fastest, and the number of crystal grains is the largest, so that the viscosity of the molten metal increases and the flow velocity slows down. When the number of crystal grains reaches a certain critical value, a continuous network is formed to form a resistance, and the liquid When the flow pressure cannot overcome this resistance, the flow stops.
Under the action of the metal liquid, the front of the liquid stream crystallizes and then forms a network. As the liquid flow continues to advance, the density of the network continues to increase, and the pressure required to overcome the network resistance also increases. If the pressure does not overcome the network resistance, the molten metal stops flowing and forms insufficient pouring.
At the beginning of boosting, the external pressure may be greater than the network resistance, but if the boosting speed is much less than the network resistance increase rate, the network resistance may catch up and exceed the external pressure before filling the cavity, and the molten metal stops flowing. Therefore, to fill the cavity, it must be ensured that the boosting speed is not less than the network resistance before filling the cavity. This requires that the boosting time be as short as possible with a constant holding pressure. Only then can the molten metal fill the cavity.
For thin-walled castings, the thinner the solidification time, the shorter the dendrites in the thin-walled part. When the dendrites show a high strength, the liquid phase cannot flow, making it difficult to fill, and the castings are prone to shrinkage. Loose or even insufficient defects. As long as the strength is lower than this external pressure, the dendrites will be crushed by the liquid flow, the flow channel will remain unblocked, and the molten metal can continue to flow between the dendrites, which is conducive to the filling of thin-walled castings, and is not easy to produce shrinkage cavities and shrinkage. And poorly poured. Therefore, the faster the boosting speed, the higher the quality of the thin-walled casting.
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