Liquid metal cooling method

In order to further improve the temperature gradient in directional solidification, the researchers developed a liquid metal cooling method based on the rapid cooling method. This method uses liquid metal to cool castings, that is, the extracted castings are immersed in liquid metal with high thermal conductivity, high boiling point and low melting point (Sn is used in the mainstream) (see Figure 1(b)) to increase the cooling effect. Liquid metal cooling method can improve the cooling rate of castings and the temperature gradient of the solid-liquid interface, up to 200 K/cm, and can maintain a stable temperature gradient, so that the crystallization process is stable, so that the dendrite spacing can be significantly reduced, and the probability of various solidification defects can be reduced. However, the liquid metal cooling method also has some limitations, such as: the equipment required by the method is complex, and it is not simple enough in practical operation; The cooling medium Sn is a harmful element, and when the casting is immersed in low-melting metal such as Sn, Sn liquid is easy to penetrate and pollute the casting. In recent years, people have optimized the process from the aspects of shell preparation, and improved the shortcomings of liquid metal cooling process, which has been applied to the production of single crystal turbine blades for aero engines and large size single crystal turbine blades for ground gas turbines.

In addition, new ways of increasing the temperature gradient are constantly being explored, such as: Gas cooled casting directional solidification technology, electromagnetic constrained forming directional solidification technology, supercooling directional solidification, SDS), laser rapid solidification (LRM), fluidized bed quenching directional solidification technology, two-dimensional directional solidification technology (bidirectional solidification, BDS), thin shell casting directional solidification technology. However, these new technologies are still immature and have not been applied in directional solidification of gas turbine blades.

Liquid metal spray enhanced cooling method

In order to overcome the problems such as casting may be contaminated by liquid metal coolant and casting defects easily formed by liquid metal cooling method, our research group developed liquid metal spray cooling (LMSC) directional solidification technology, and developed industrial directional solidification equipment. The design structure and object of LMSC directional solidification furnace are shown in Figure 2. LMSC technology is based on LMC technology from the original way of directly dipping shell and casting into liquid metal liquid for cooling, to the use of liquid metal liquid for spray cooling of shell and casting. The technology has the characteristics of strong heat dissipation, uniform cooling and good heat insulation between the insulation zone and the cooling zone. LMSC technology not only retains the advantages of strong cooling capacity of LMC technology, but also solves the disadvantages of LMC technology. Due to the controllable flow rate of the liquid metal liquid sprayed, combined with the adjustment of the withdrawal rate, columnar crystals or single crystals with good structure and smaller dendrite spacing can be obtained, which can reduce or even avoid the formation of solidification defects in superalloy. LMSC directional solidification technology is very important for the development and industrial production of superalloys.