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The exact class of a manufacturing technique is dependent on the starting form of metal or manufacturing material. But with the developing manufacturing techniques, it is very hard to define the exact starting form of material. So the classification of these manufacturing techniques can be tough. Semi-solid metal casting process is a one these metal casting technologies. But do not stuck on the name of ‘casting’ on it. In casting processes, generally fully molten materials are used. But you understand from its name, semi-solid metal is used in here.
Also in some manufacturing methods, different kinds of manufacturing disciplines are combined. This fact is valid for semi-soid metal casting method also. At the end of this article, you will understand that semi-solid metal casting method is the combination die casting and metal injection process.
Semi-solid metal is generally said for metal alloys that are avaliable in market. As you know that, metal alloys has no exact completely melting temperature like pure metals. There is a transition from solid to liquid phase for metal alloys. In this transition phase, their situation is called as semi-solid metal. Take a look at the liquidus and solidus curves for metal alloys to undertand the logic.
Metal alloys are heated up to temperatures between solidus and liquidus temperatures to obtain semi-solid metal. To obtain semi-solid metal, a small ratio of metal must be solidified. In this solidification mechanism, generally dendritic growth occur. This is a microstructural mechanism of solidification. Dendritic structure is not desirable thing in semi-solid metal casting method. Solid globule granule inside liquid phase instead of dendritic microstructure is desired in semi-solid metal casting.
To obtain the globule microstructure, some shaking and stirring mechanisms are used to shake and stir the semi-solid metal.
After this process, semi-solid metal is injected inside mold via screw mechanism. It is just like metal injection molding.
As in squeeze casting method, both ferrous and non-ferrous metals are cast in semi-solid metal casting processes. But because of their low melting temperature, non-ferrous metals are desired.
Magnesium alloys are the common materials in semi-solid metal casting processes. Magnesium granules are pushed inside mold cavity via rotating screw mechanism. During this pushing activity of magnesium, granules are heated to obtain semi-solid phase. Stirring effect that originated from rotating screw is also lead to the needed globular solidification microstructure inside cast magnesium material. This process has a different name called ‘thixomolding‘.
In the casting of aluminum alloys, another different method is used in semi-solid casting. In the casting of aluminum alloys, the starting form of them is solid billets which have not dendritic microstructure inside them. These billets are heated up to a semi-solid temperatures to obtain slurry to fill the mold cavities with that. Die casting equipments are used to fill the mold cavities. And this special process is called as ‘rheocasting‘.
The most important advantage of semi-solid metal casting process is the zero or very low porosity inside the cast part, which leads to a very good mechanical properties compared with the parts that produced by other casting processes.
Because the of used equipments to cast metal, very complex part geometries and details can be obtained with a very good tolerance control over dimensions in semi-solid casting method.
In some parts such as engine casings, thin part sections can be obtained in the desired characteristics.
These are the general characteristics of semi-solid metal casting process. Do not forget to leave your comments and questions about semi-solid metal casting process below.
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