Study on heat transfer coefficient of directional solidification interface in Squeeze Casting

Trovant et al. Found that the peak value of the heat transfer coefficient of the casting die interface appears when the liquid metal on the surface of the casting begins to solidify, that is, when the surface temperature of the casting drops to the liquidus, the heat transfer coefficient of the casting die interface reaches the peak value, and then it begins to decline soon. At the same time, they believe that the rapid decrease of the heat transfer coefficient at the casting die interface after reaching the peak value is due to the partial solidification shell on the casting surface. As the solidification proceeds, the heat transfer coefficient of the casting die interface decreases rapidly when the gap between the casting and the die begins to increase. The heat transfer between the casting and the die is mainly the heat conduction and radiation in the gap layer. In the process of metal mold casting, the gap layer contains not only air, but also coating and some oxides, which makes it very difficult to accurately calculate the thermal resistance of the gap layer. An approximate method is to assume that the gap layer is an air layer. Trovant et al. And Coates et al. Found that there is an inverse relationship between the heat transfer coefficient of the casting die interface and the thickness of the interface gap layer.

For directional solidification, in order to obtain columnar crystals with specific orientation, it is necessary to form a specific direction temperature gradient in the solidified and non solidified metal melts by forced means during solidification process, so that the melt solidifies in the opposite direction of heat flow. This heat transfer characteristic also determines that in the process of studying the heat transfer at the interface of directional solidification, each temperature measuring point should also follow the set direction Good heat flow direction. Researchers measured the temperature distribution inside the casting and chilling plate through experiments, and then calculated the heat transfer coefficient of directional solidification interface through inverse algorithm. A large number of studies show that the change of heat transfer coefficient of directional solidification interface with time approximately conforms to the exponential law, which can be basically divided into three stages

(1) At the beginning stage, the alloy melt is poured into the mold, and a high enough liquid column is formed after a short period of time. Therefore, in the first stage, a stable metal solidification thin layer is formed on the chilling plate. Due to the static pressure generated by the liquid metal, it can be considered that the casting is in close contact with the chilling surface, and the interface heat transfer coefficient is the largest at the beginning, and then the interface heat transfer coefficient drops sharply .

(2) In the second stage, with the solidification process, a casting with enough thickness is formed, and the interface between the casting and the chilling surface can no longer maintain close contact due to solidification shrinkage. At this time, the heat transfer coefficient of the interface still continues to decline, but the decline rate is obviously slower.

(3) In the third stage, the solidification is nearly completed, and the interfacial heat transfer coefficient decreases, but it is not obvious.

Griffiths et al. Found that the interface gap layer formed during directional solidification has a great influence on the interface heat transfer, and the interface heat transfer coefficient obtained by experiment is consistent with that calculated by gap layer. Griffiths et al. Also systematically studied the influence of casting and mold orientation on the interfacial heat transfer. The results showed that the interfacial heat transfer coefficient was significantly higher when the casting was located above the mold than when the casting was located below the mold. In the process of directional solidification, there are many factors that affect the interfacial heat transfer coefficient. Ferreira and others systematically studied the effects of melt superheat, solute concentration and growth direction on the interfacial heat transfer coefficient.

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