In the process of investment casting, solidification refers to the process that high-temperature liquid metal changes from liquid to solid by continuously transferring heat to the outside world and reducing its temperature. Heat conduction, heat convection and heat radiation are three basic forms of heat transfer in nature. In the process of investment casting, energy is mainly transferred between shell and casting through heat conduction, while heat transfer between external environment and investment casting is mainly carried out through radiation and convective heat transfer.
(1) Heat conduction
In the process of investment casting, after the metal liquid fills the mold cavity, the cooling and solidification process includes heat conduction between many media. Among them, the heat transfer and heat conduction process between the mold shell and the investment casting is unstable. Its general form is:
Is the specific heat pressure, CP; ρ Is the fluid density; λ Is the thermal conductivity; T is the temperature; Q is the heat source phase.
(2) Thermal convection
Heat convection originates from fluid movement. When the relative movement of each part of the fluid with different temperature occurs, the heat is brought by the fluid from one place to another. In the solidification process of investment casting, heat exchange will also occur because the temperatures between pouring liquid and mold shell, mold shell and external environment, external environment and pouring liquid are different. At present, when studying convective heat transfer, Newton cooling formula is mainly used to describe this heat transfer mode. Its general form is:
Where TW is the wall temperature (k); TF is the fluid temperature (k); α Is the convective heat transfer coefficient.
(3) Radiant heat
Any object whose absolute temperature is not zero will emit electromagnetic waves to the outside world, which will cause the electrons in the object to vibrate. With this process, heat transfer and thermal radiation occur between objects. The thermal radiation process can be described by Stefen Boltzman law, and its mathematical expression is:
Where, σ 0 is Stefen Boltzman constant; ε Is the radiation blackness of the object, ε< 1； TS is the absolute temperature of the surface of the investment casting.