Vacuum casting and atmospheric casting in investment casting

Vacuum casting and atmospheric casting in investment casting

In investment casting, vacuum casting and atmospheric casting belong to the category of gravity casting, but the atmosphere during casting is different. Today, investment casting manufacturer Tianyao Metal will introduce the differences between these two pouring methods.

1、 Atmospheric pouring. What we usually see is atmospheric pouring. When the metal liquid comes into direct contact with the atmosphere, it will oxidize and produce oxide inclusions; During pouring, molten metal may be entrained with foreign gases, which may cause invasive gas pores if they cannot be discharged; Before pouring, the metal liquid must be refined, and different metals have different refining priorities, such as the refining of aluminum alloy and steel liquid. Aluminum alloy refining mainly involves removing hydrogen and oxide inclusions, while steel involves deoxidation, removal of hydrogen, nitrogen, and oxide inclusions; The pouring temperature of molten metal is relatively high because under atmospheric conditions, the heat dissipation speed of the metal is very fast; The fluidity of metal liquid is related to the superheat of the metal liquid itself, the temperature of metal materials and molds (including sand molds, metal molds, ceramic shells, and gypsum molds), and air resistance; The design of the pouring system must fully consider the influence of metal materials and atmospheric conditions. For example, aluminum alloy casting prioritizes bottom pouring systems.

2、 Vacuum pouring. Vacuum pouring refers to the pouring of molten metal at a pressure of several to tens of pascals, and it is done inside a pressure vessel. Compared to atmospheric casting, vacuum casting also comes into contact with air, but the air is very thin, so the probability of metal liquid oxidation producing oxide inclusions is much lower; In a vacuum state, the probability of generating entangled pores is also very low due to the thin air; The metal liquid will also be refined before pouring, but in comparison, in the vacuum state, due to the low pressure inside the vacuum furnace, the power of processes such as degassing and deoxidation will tend to increase, and the refining of the metal liquid in the vacuum furnace will become more favorable; Due to the increase in vacuum degree, the melting point of the metal is relatively reduced, and the pouring temperature of the metal liquid in vacuum state will correspondingly decrease, that is, the degree of overheating will be relatively reduced; The fluidity of metal liquid is relatively enhanced; Relatively speaking, the design of the pouring system under vacuum may consider fewer factors.

From the above situation, it can be seen that under the same conditions, such as the same materials and castings, atmospheric casting requires more complex casting process design and careful consideration compared to vacuum casting in order to obtain castings of the same quality as vacuum casting. In other words, vacuum casting simplifies the casting process relatively. In fact, in both atmospheric and vacuum pouring situations, the flow of molten metal relies on the conversion of gravitational potential energy into flow kinetic energy, and turbulence will also occur. The main reason is that the cross-sectional area of the gate area decreases and increases, causing turbulence. However, under atmospheric conditions, gas and oxide inclusions can be drawn into the mold cavity, resulting in casting defects. The probability of occurrence in a vacuum state is much lower.

Therefore, atmospheric pouring has a relatively wide range of applications and can be applied to castings with low or slightly higher requirements. However, vacuum casting has a relatively narrow range and is generally only suitable for high-quality and high demand castings. For example: aerospace castings. After all, its cost is very high. So, is vacuum casting defect free casting? The answer is definite: no. Vacuum pouring only simplifies the design of the pouring process, but it does not mean that there is no need for design or careful design. We often see aluminum castings produced by vacuum casting, which also have pores and inclusions. Why is this? Vacuum casting does not mean no air, and good refining effect of metal liquid does not mean perfect; When foreign materials and mold shells enter the vacuum furnace, they will also carry air. So, vacuum furnace casting can also produce pores, and the same goes for inclusions. It can only be said that it will decrease. Of course, pouring the same set of mold shells in different states will immediately show the effect. The design and operation of both are indeed like this. So we should learn from all atmospheric casting enterprises, and if their casting quality level is very high, it should be an example for us to learn from.

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