Solidification of precision castings

The solidification of precision castings is more important for castings. Generally, the solidification methods of castings are divided into two methods: layer-by-layer solidification and paste solidification according to the properties of metal materials; according to the structure of castings, they are divided into two methods: sequential solidification and simultaneous solidification. Here I would like to remind everyone that this layer-by-layer solidification and paste solidification are properties of materials, which are related to the solidification interval of metal materials, or the crystallization interval. The solidification region refers to the coexistence region of liquid-solid two phases in the molten metal; the so-called crystallization region is the temperature difference between the liquidus and the solidus of the molten metal. The temperature range determines how the alloy solidifies. This is the inherent property of the material. When the material is selected, its solidification method is also selected and determined. The latter two solidification methods have nothing to do with the metal material, and are mainly determined by the structure of the casting. In order to obtain a dense casting, according to the structure of the casting, find a way from the casting process to force the casting to adopt a certain solidification method. So, what does layer-by-layer solidification mean? As the name suggests, it is layer-by-layer solidification. That is to say, the solidification interval of the molten metal is a line. The solidification of pure metals or eutectic alloys crystallized at a constant temperature at a certain instant is layer-by-layer solidification. Simultaneous solidification, as the name implies, means that the entire casting is solidified at the same time. The solidification zone of the casting is very wide, and even during a certain period of solidification, the solidification zone runs through the entire section of the casting. So, what is sequential solidification and simultaneous solidification?

1. The so-called sequential solidification is to artificially change the sequence of solidification, so that the castings are solidified according to the process design requirements. Casting first, then gate, and finally riser, so that the shrinkage cavity in the casting is transferred to the riser, and then a dense casting is formed; at the same time, various methods are adopted for solidification to reduce the temperature difference of each part of the casting, so that each part of the casting is in the solidifies at the same time (regardless of its size and structure).

2. Some people may understand that the material solidified layer by layer should adopt the principle of sequential solidification; and the material solidified in paste form should adopt the principle of simultaneous solidification. Is this statement correct? No! That's what we talked about above. Layer-by-layer solidification and pasty solidification are properties of the metal material itself, and you have no choice. If the customer determines the casting material, this one is equal to his decision. The sequential solidification and simultaneous solidification are determined by you, the craftsman. I don't know if I'm saying this clearly, but I think quite a few people have confused this issue, so it's worth reiterating here. In other words, the castings designed with layer-by-layer solidification materials can adopt the principle of simultaneous solidification according to the structure; while the castings designed with mushy solidification materials can adopt the principle of sequential solidification.

3. In general, gray iron, low carbon steel, industrial copper, aluminum, aluminum-silicon alloy, etc. are all materials that solidify layer by layer. The ductile iron, high carbon steel, aluminum-copper alloy, aluminum-magnesium alloy, magnesium alloy, etc. all belong to the paste solidification method. There is another problem that needs attention, that is, the temperature difference of the casting section, or the temperature gradient. When the temperature gradient is large, the paste-like solidified material may also become layer-by-layer solidification.