Casting processes demand a rapid supply of molten liquid metal or alloys to the target molds. The melting procedure, which has been used, entails adding the ingots to an existing melt. Then there occurs heat transfer at the solid-liquid interface. Heat in the liquid phase must be transferred to the solid ingots. The transfer mechanism includes natural convection and conduction via the solid-liquid interface to the ingot’s surface.
At first, the temperature of the solid material has to be raised from the starting temperature to its melting point. When melting occurs, heat of fusion needs to be added, and finally, the temperature of the resulting liquid as well as the melt around the liquid has to be raised to the initial temperature. Therefore, duration of the procedure will count on flow conditions, temperature, and material characteristics.
A casting shop needs to determine the duration needed to melt an ingot of a selected mass and shape in a melt of its own materials. However, the recorded temperature of the fed ingots is lower than the liquidus temperature. The solid-liquid interface of the ingot moves in all directions causing a decrease in the surface area with time.
German scientists, André Ditze from Engineering Office MetuRec, and Christiane Scharf at Helmholtz Centre Dresden Rossendorf were able to establish a relationship between the dimensionless Nusselt, Prandtl, Grashof, subsequent the Rayleigh number, and Stefan numbers that could explain the important parameters to optimize the melting conditions of pure metals, zinc, tin and lead, and magnesium and aluminum alloys as well as of ice. Their work is published in International Journal of Thermal Sciences.
The authors investigated the melting characteristics of ice, pure metals, lead, tin and zinc, and of aluminum and magnesium alloys. Given the geometry of a specific ingot, nearby temperature, and the initial temperature, they were able to compute the dimensionless Nusselt, Prandtl, Grashof and Stefan numbers. The researchers noted that melting magnesium alloy ingots pre-warmed to 200 °C took more time as compared to melting ingots with the same size and at an initial temperature of 20 °C at the same mean temperature.
Therefore when melting times are compared to constant mean temperature during the melting process, the computed melting time is longer when pre-warmed materials are used. The driving force of melting could explain this. With small temperature changes, heat flow is observed to be low as well. If the initial temperature is closer to the mean temperature, heat flow nears zero and the melting time increases.
The authors successfully computed the time required for melting and this depended on the mean temperature near the selected ingot and the geometry of the ingot. However, analyses with indirect and direct heating of the melt indicated that only the mean temperature was important.
André Ditze, Christiane Scharf. Experimental measurements in melting ingots in the melt of the same material. International Journal of Thermal Sciences 112 (2017) 211-221.Go To International Journal of Thermal Sciences