The thermophysical parameters used in numerical simulation of solidification process of sand castings mainly include specific heat CP, latent heat l and thermal conductivity of sand castings and mold materials λ， And density ρ。 Among them except ρ Can be roughly regarded as constants, CP, l λ All change with temperature. There are many factors affecting the thermophysical properties of materials, and the data measured by various researchers are inconsistent, especially the CP and CP of modeling materials λ Due to the influence of process factors such as compactness and molding sand formula, it changes greatly. If this change is reflected in the numerical simulation equation, it will affect the simulation accuracy.
When the temperature variation range is small, the average value of the thermophysical properties of the material can be substituted into the calculation. If the temperature changes violently and the change of material thermophysical properties with temperature is not considered, there will be a large deviation between the calculated results and the actual situation. However, when the temperature of the material changes greatly, the thermophysical property value of the material must be regarded as a function of temperature. At present, the methods for testing the thermophysical property value of sand mold can be divided into three types: steady-state method, unsteady state method and pouring method.
1) Steady state method
Early sand mold casting workers measured the thermal conductivity of sand mold at different constant temperatures with the help of more mature steady-state method. By establishing a constant temperature field in the sample that does not change with time, and then measuring the heat flow rate and temperature gradient per unit area of the sample, the thermal conductivity of the sample is obtained. The test equipment of this method is expensive and complex, the test cycle is long, and the temperature is not easy to control. It is generally only used for the accurate analysis of inorganic materials, but not suitable for the thermophysical property test of a wide variety of molding sand, Therefore, few scholars adopt it.
2) Unsteady state method
The unsteady state method uses the unsteady state heat conduction differential equation. The value obtained is the relationship between temperature and time, and the thermal diffusivity of the material can be obtained. At present, the method of determining the thermal conductivity by measuring the thermal diffusivity has been widely used. Due to the short experimental time of measuring the thermal diffusivity by the unsteady state method, the test work is greatly convenient and the error caused by heat exchange can be reduced.
3) Pouring method
Pouring method is essentially an unsteady engineering test method. The difference between pouring method and unsteady method is only that the heating method is different. The principle is to measure the temperature change of the mold after liquid metal pouring, and calculate the thermophysical property value of the mold through the treatment of the measured temperature curve. Due to the comprehensive consideration of the effects of volatiles such as water and resin in the mold under the actual sand mold casting conditions, as well as metal gas, metal mold interface reaction and other factors on the thermophysical parameters of the mold, which is closest to the actual production conditions, this method is more accurate.
The pouring method is used to test the temperature curve of several points in the sand mold. When the test device is close to one-dimensional heat conduction, the thermophysical property value of sand mold is obtained by inverse calculation method.