Vacuum precision casting is a casting process in which metals are melted, poured and crystallized in a vacuum chamber. It can reduce the gas content in the metal and prevent metal oxidation and trachoma. Demanding castings can be produced in this way. The cost of vacuum casting is high, and it is generally used for high-quality castings, and the mold should not contain volatiles. Vacuum precision casting is a casting method in which the casting gate (or riser) is inserted into the metal liquid, and then the casting is vacuumed, so that there is negative pressure in the casting cavity. Under the action of negative pressure, the metal liquid rises along the gate (or riser) and is filled with the mold and solidified to form the casting.
The process principle of vacuum precision casting is to put the investment shell in the sealed chamber, the sealed chamber is lowered, the straight sprue is immersed in the liquid metal, and then the vacuum pump is started to pump the sealed chamber into a vacuum, and the liquid is cast at the same time. After the casting in the mold shell solidifies, the vacuum state is lifted, the residual metal liquid in the runner returns to the furnace, and the vacuum casting is obtained by sand cleaning. According to the different pressure difference established by the vacuum casting process, vacuum precision casting can be divided into direct casting method and special vacuum method:
Direct casting process
Taking CLA method as an example, the shell made by ordinary investment process is first placed in the sealed chamber, the sealed chamber is lowered, and the straight runner is inserted into the liquid metal. Start the vacuum pump to vacuum the sealing chamber and fill the liquid metal at the same time. After the liquid metal in the shell is solidified, the sealed chamber is connected to the atmosphere, the vacuum is eliminated, and the liquid metal that has not yet solidified in the runner flows back to the crucible. Then take out the shell sand to get the casting. In addition to CLA method, there is also tilting and inversion vacuum precision casting, that is, after the filling of the mold is completed, the cast is inverted, and the remaining metal liquid in the straight gate provides supplementary shrinkage for the solidification of the casting. This method is easy to realize continuous production and has high production efficiency.
Special vacuum casting method
This kind of vacuum precision casting method is mainly based on the casting process requirements or alloy types, and some specialized operations are added in the casting process. For example, to enhance the feeding capacity of vacuum precision casting. Vacuum the sealing chamber on the completion; After the metal liquid is filled with the cavity, the lower sealing chamber is then filled with compressed gas to improve the pressure difference between the upper and lower sealing chambers, thus enhancing the metal liquid feeding ability during the crystallization and solidification of the casting. The process diagram is shown in Figure 2. There is also vacuum precision casting with inert gas protection, the method is mainly used for the production of superalloy and easily oxidized alloy vacuum melting and casting casting method (also known as CLV method), the method is to melt the metal under vacuum, to the vacuum melting chamber and the casting chamber at the same time through inert gas, and make them maintain the same air pressure. Insert the mold runner or riser into the metal liquid, and then reduce the pressure of the casting chamber to cast. After holding for a certain time, the metal liquid in the straight runner flows back to the crucible after pressure relief.
Process features: (1) High yield, good casting quality, stable liquid metal filling, less oxidation slag inclusion and spatter, reduce the porosity and slag inclusion of the casting defects, improve the yield. In addition, the casting can be cast at a lower pouring temperature, so that the casting grain is refined and the mechanical properties are improved. (2) Good filling performance. In precision casting, the back pressure in the mold cavity is small and the filling speed is adjustable, so the filling ability is strong, and the thin part of the casting can reach 0.3mm. (3) Improve the utilization rate of metal liquid and process yield. (4) Simplify the process and reduce the cost. (5) Easy to realize mechanization, high labor productivity. Compared with the ordinary investment casting process, each module can be more assembled wax mold, generally can increase the output of 85% ~ 135%.
Vacuum precision casting is suitable for investment mold, ceramic mold, shell mold, gypsum mold, graphite mold and sand mold casting, can be used for the production of aluminum alloy, magnesium alloy, titanium alloy, stainless steel and high temperature and heat resistant alloy with an average wall thickness of less than 5mm complex thin wall casting, and a single casting weight of less than 100kg.
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