What is called directional solidification?

In the investment casting shell to establish a specific direction of the temperature gradient, so that the molten alloy along the opposite direction of the heat flow in accordance with the requirements of the crystallization orientation of a casting process. Directional solidification technology is the most outstanding achievements in the aerospace industry. Since 1965, when Pratt & Whitney Aeronautics adopted the directional solidification technology for high-temperature alloys, this technology has been applied in many countries. Directional solidification allows the production of thin-walled hollow turbine blades with excellent thermal shock resistance, long fatigue life, good creep resistance and medium-temperature plasticity. The application of this technology can increase the operating temperature of the turbine blade by 10 to 30 [2oc], and the turbine inlet temperature by 20 to 60 [2oc], thus increasing the thrust and reliability of the engine and prolonging its service life.

Ordinary castings are generally composed of polycrystalline without a certain crystallization direction. In the high temperature fatigue and creep process, perpendicular to the main stress of the transverse grain boundary is often the main part of the crack generation and expansion, but also the turbine blade high temperature operation of the weak link. The directional solidification technique can be used to obtain the unidirectional growth of columnar crystals with the same growth direction as the direction of the principal stress). Directional solidification improves the resistance to high-temperature creep and fatigue due to the elimination of transverse grain boundaries. Directional solidification casting organization is divided into columnar, single crystal and directional **** crystal 3 kinds.

Directional solidification of castings requires two conditions: first, the heat flow in a single direction and perpendicular to the growth of the solid-liquid interface; second, the crystal growth ahead of the melt in the absence of a stable crystalline core. For this reason, process measures must be taken to avoid lateral heat dissipation and to create a large temperature gradient in the melt near the solid-liquid interface. This is essential to ensure that the oriented columnar and single crystals grow straight and in the correct orientation. To increase the temperature gradient in the alloy as a starting point, directional solidification techniques have evolved from power reduction and rapid solidification to liquid metal cooling.