After the molten metal is poured into the sand coated iron mold, “casting, sand coated layer and iron mold” form an unstable heat exchange system. In order to simplify the problem, the heat exchange system is assumed to be a semi infinite wall thickness casting and a finite thickness sand coating and iron mold; It is assumed that the temperature field of each component in the system is distributed according to the law of straight line. When the sand layer is within the normal thickness, the cooling rate of the casting mainly depends on the thickness of the sand layer. That is, the cooling rate increases with the decrease of sand layer thickness. Therefore, the cooling rate of the casting can be controlled by changing the thickness and thermophysical properties of the sand coating.
In addition to the casting wall thickness, sand coating thickness, iron mold thickness and other factors, the factors affecting the cooling rate of iron mold sand coated castings include casting material, pouring temperature, iron mold material and mold temperature. The composition and thermophysical parameters of the heat transfer system of iron mold sand coated casting composed of casting, sand coating and iron mold need to be considered in numerical simulation. In magmasofl, the cooked physical parameters of each component of iron mold sand coated casting heat transfer system can be set respectively. Generally, after the selection of materials, the thermophysical parameters are given in magma database and will not be changed. However, the interfacial heat transfer coefficient (HTC) between components must be determined in combination with the actual situation.
The heat transfer coefficient qualitatively and quantitatively describes the heat transfer process of iron mold sand coated casting at the interface between two adjacent materials. The larger the heat transfer coefficient, the faster the heat transfer. Internal heat exchange occurs between various materials in the whole casting system. In the database, the heat exchange between adjacent material groups is divided into constant, temperature dependent, time dependent and cooling channel modes.
Because the actual measured temperature value is affected by the wall thickness of the casting, the thickness of the sand coating, the thickness of the iron mold, and even the position of the measured point (if it is close to the gating system, the temperature is high). Therefore, if the simulation effect can reflect the real cooling environment of the casting as much as possible, it is unnecessary to consider that the heat transfer coefficient of iron mold sand coated casting changes with time or temperature. In the simulation, one method can ignore the sand layer and take the relatively low value of the interfacial heat transfer coefficient between the casting and the iron mold to approximate the thermal insulation effect of the sand layer; Or use useri to customize the sand coating material as shell.sand, and the interface heat transfer coefficient between the casting and the sand coating takes a higher value, because the actual sand coating layer is thin, and the heat transfer process of iron mold sand coating casting is much faster than that of general sand mold. There is little difference between the simulation results of the two methods. The second method was selected in the experiment.