The so-called heating agent method is to pour molten metal into the mold with one side wall insulated, bottom cooled and top covered with heating agent. A top-down temperature gradient is established between the metal liquid and the solidified metal, which makes the casting solidify from top to bottom and realizes unidirectional solidification. Because the temperature gradient obtained by this method is small and difficult to control, the solidification structure is coarse and the casting performance is poor. Therefore, this method is not suitable for the production of large-scale and high-quality castings. However, the process is simple and the cost is low, so it can be used to manufacture small batch parts.
1. Power reduction method (PD method)
The heater of the holding furnace is divided into several groups, and the holding furnace is heated in sections. When the molten metal is placed in the holding furnace, when the casting is cooled from the bottom, the heater is closed from the bottom to the top, and the metal solidifies gradually from the bottom to the top, so as to realize directional solidification in the casting. By choosing the appropriate heating device, the cooling rate can be increased, but the temperature gradient decreases gradually during the solidification process, which results in the shorter columnar crystal area and the less ideal structure. In addition, the equipment is relatively complex and energy consumption is large, which limits the application of this method.
2. High speed solidification method (HRS method)
In order to improve the slow cooling speed of power reduction method after the heater is turned off, a new directional solidification technology, rapid solidification method, has been developed on the basis of Bridgman crystal growth technology. The characteristic of this method is that the casting is moved out of the furnace at a certain speed or the furnace is moved away from the casting, and the air cooling method is adopted, and the furnace keeps heating state. This method can avoid the influence of furnace and use air to cool, so it can obtain higher temperature gradient and cooling speed. The distance between columnar crystals is longer, the structure is fine, straight and even, which can improve the performance of castings and has certain application in production.
3. Liquid metal cooling method (LMC method)
The HRS method is cooled by radiation heat transfer, and the temperature gradient and cooling rate obtained are very limited. In order to obtain higher temperature gradient and growth rate. On the basis of HRS method, a new directional solidification technology, LMC method, is formed by partially immersing the drawn casting into the liquid metal with high boiling point, low melting point and large heat capacity, which has high thermal conductivity. This method can improve the cooling rate of casting and the temperature gradient of solid-liquid interface, and keep the temperature gradient at the front of the interface stable in a large range of growth speed. The crystallization can be carried out in a relatively stable state, and a relatively long unidirectional columnar crystal can be obtained.
The commonly used liquid metals are GA in alloy, GA in Sn alloy, and Sn liquid. The former two have low melting point but high price, so they are only suitable for laboratory use. The melting point of Sn liquid is a little higher (232 ℃), but it is suitable for industrial application because of its cheaper price and better cooling effect. This method has been used in the production of Aeroengine Blades in the United States, the former Soviet Union and other countries.
On 2018-08-08 11:45:46
