Solidification of precision castings

Solidification of precision castings

The solidification of precision castings is crucial for castings. The solidification methods of general castings are divided into two types based on the properties of the metal material: layer by layer solidification and paste solidification; According to the structure of the casting, it can be divided into two methods: sequential solidification and simultaneous solidification. Here, I would like to remind everyone that layer by layer solidification and paste solidification are properties of materials, which are related to the solidification range or crystallization range of metal materials. The solidification zone refers to the coexistence area of liquid and solid phases in the metal liquid; This so-called crystallization interval is the temperature difference between the liquidus and solidus of the metal liquid. The temperature range determines the solidification mode of the alloy. This is an inherent property of the material, and once the material is chosen, its solidification mode is also determined. The latter two solidification methods are independent of the metal material and mainly determined based on the structure of the casting. In order to obtain dense castings, based on the structure of the castings, a method is found from the casting process to force the castings to adopt a certain solidification method. So, what does layer by layer solidification mean, as the name suggests, is to solidify layer by layer. That is to say, the solidification range of the molten metal is a line. The solidification of pure metals or eutectic alloys crystallized at a constant temperature at a certain moment is layer by layer solidification. Simultaneous solidification, as the name suggests, refers to the solidification of all parts of the casting at the same time. The solidification area of the casting is very wide, and even during a certain period of solidification, the solidification area runs through the entire cross-section of the casting. So, what are sequential solidification and simultaneous solidification? Today, precision casting manufacturer Tianyao Metal will talk to you about the solidification of precision castings.

1、 The so-called sequential solidification refers to artificially changing the order of solidification, allowing the casting to solidify according to the process design requirements. First, the casting, then the gate, and finally the riser, to transfer the shrinkage cavity in the casting to the riser, thereby forming a dense casting; Meanwhile, solidification is achieved by adopting various methods to reduce the temperature difference between different parts of the casting, so that each part of the casting solidifies at the same time (regardless of its size and structure).

2、 Some people may understand it as a material that solidifies layer by layer, which means that the principle of sequential solidification should be adopted; The principle of simultaneous solidification should be adopted for materials with paste like solidification. Is this statement correct? Not right! This is what we discussed above. Layer by layer solidification and paste solidification are inherent properties of metal materials, and you have no choice. If the customer has determined the material of the casting, this is equivalent to him having already confirmed it. And sequential solidification and simultaneous solidification are determined by yourself, that is, the process personnel. I'm not sure if you're clear about what I'm saying, but I think a considerable number of people have confused this issue, so it's necessary to reiterate it here. In other words, castings designed with layer by layer solidified materials can adopt the principle of simultaneous solidification based on their structure; The principle of sequential solidification can be adopted for castings designed for materials with paste solidification.

3、 In general, materials such as gray iron, low-carbon steel, industrial copper, aluminum, and aluminum silicon alloys are solidified layer by layer. And ductile iron, high carbon steel, aluminum copper alloy, aluminum magnesium alloy, magnesium alloy, etc. all belong to the paste solidification method. There is another issue that needs everyone's attention, which is the temperature difference or temperature gradient of the casting cross-section. When the temperature gradient is large, the paste solidified material may also change to a layer by layer solidification method.

So, for castings, which solidification method should be used for sequential and simultaneous solidification methods?

Firstly, it depends on the requirements of the casting. If the casting requires density, the principle of sequential solidification should be adopted; If the requirement for density is not high, then the principle of simultaneous solidification should be adopted.

Secondly, for alloys with large solidification shrinkage and without producing a large amount of crystalline skeleton that hinders shrinkage, such as cast steel, ductile iron, etc., they tend to undergo directional solidification; However, with large solidification shrinkage, there is a tendency to produce crystalline frameworks that hinder shrinkage, which can easily lead to the formation of dispersed and loose alloys such as tin bronze, or alloys with small solidification shrinkage, such as gray iron, tend to adopt simultaneous solidification. Castings that are prone to hot cracking and deformation can adopt the principle of simultaneous solidification, while for products with significant thickness differences, the principle of sequential solidification should be adopted. For castings or certain parts of castings that are subjected to high temperature and pressure, do not allow leakage, or are subjected to significant force and must be absolutely safe for use; The parts on the casting that require higher requirements and processing should be solidified in sequence. Although a certain part of the casting is subjected to a large force, other requirements are not high, and it is not a hot spot; When the wall thickness is uniform, the principle of simultaneous solidification should be adopted. Furthermore, it can be seen from the above that a casting may not necessarily adopt one solidification method, but can also adopt two solidification methods according to the needs of the product. In areas with uniform wall thickness, the principle of simultaneous solidification is adopted, while in areas with significant differences in wall thickness, the principle of sequential solidification is adopted.

The advantages and disadvantages of adopting the principles of simultaneous solidification and sequential solidification: simultaneous solidification is due to the fact that the temperature of each part of the casting is basically the same, the cooling is uniform, the shrinkage stress of the casting is small, and the casting is not prone to deformation and cracks. The disadvantage is that castings with large liquid shrinkage often produce axial shrinkage porosity, insufficient density, and reduced plasticity of the castings; Sequential solidification is opposite to simultaneous solidification, as the temperature difference between different parts of the casting is significant, which can cause significant thermal stress and easily lead to deformation or cracking of the casting. Moreover, the use of risers increases the consumption of molten metal and the workload of cutting risers. But the advantage is that the casting has good internal quality and density.

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