What Is Vacuum Casting?

Vacuum casting is a casting process in which metal is smelted, poured, and crystallized in a vacuum chamber. Vacuum casting can minimize the gas content in the metal and prevent metal oxidation. This method can produce highly demanding special alloy steel castings and highly oxidizable titanium alloy castings. Vacuum casting is expensive, and it is generally used for smelting alloys and high-quality castings with high vapor pressure and easy volatility loss. The mold should not contain volatiles. ^[1]

Vacuum casting is generally divided into

Vacuum suction casting is

Vacuum die-casting is a process method in which a certain degree of vacuum is established in a die-casting mold cavity, and the vacuum degree is generally in the range of 50 to 80 kPa. ^[5]

Vacuum casting process principle

The vacuum die-casting device is shown in FIG. 3. There are two main forms of vacuum die casting process.

image 3

(1) The entire die-casting mold is closed with a vacuum cover. As shown in Figure 4. When the mold is closed, the vacuum cover closes the entire die casting mold. After the liquid metal is poured into the pressure chamber, the pressure chamber is sealed with an injection punch, and then the vacuum valve is opened, and the air in the vacuum cover is extracted. Die-casting. This method has a large amount of air extraction each time, and is not suitable for molds with a hydraulic core pulling mechanism. ^[5]

Figure 4

(2) Vacuum by means of the parting surface. In this method, the exhaust groove of the die-casting mold is passed into a general exhaust groove with a large cross-sectional area, and the overall exhaust groove is connected to a vacuum system, as shown in FIG. 5. When die-casting, when the injection punch seals the gate, the stroke switch 6 automatically opens the vacuum valve 5 to evacuate, and the die-casting is performed when the vacuum degree reaches the requirement. The role of the hydraulic cylinder 4 is to close the main exhaust tank to prevent the alloy liquid from entering the vacuum system. This method is simple and easy to implement, with less air extraction volume, and convenient mold manufacturing and maintenance. ^[5]

Figure 5

Vacuum casting process features

(1) Eliminate or significantly reduce the porosity defects of the casting, increase the density of the structure, enhance the mechanical properties, and improve the surface quality.

(2) The casting can be heat treated to obtain better microstructure and mechanical properties.

(3) The filling back pressure is significantly reduced, and the wall thickness of the die-casting casting is smaller. For example, when ordinary die-cast zinc alloy, the average wall thickness of the casting is 1.5mm, and the minimum wall thickness is 0.8mm, while when vacuum-die-casting the zinc alloy, the average wall thickness of the casting can be 0.8mm, and the minimum wall thickness can be 0.5mm.

(4) The pouring system and the overflow system can be appropriately reduced to save alloy materials.

(5) The sealing structure is more complicated, it is more difficult to manufacture and install, and the cost is higher. ^[5]