Vacuum casting can be broadly distinguished into vacuum suction casting, vacuum low pressure casting and vacuum differential pressure casting:
The mold is placed in a closed container and the air inside the mold is extracted to create a certain negative pressure inside the mold, which causes the metal liquid to be sucked into the mold cavity. When the inner sprue of the casting solidifies, the negative pressure is removed and the unconsolidated liquid metal in the vertical sprue flows back into the molten pool. The advantage is that the filling capacity of the alloy liquid is improved, the minimum wall thickness of the casting can be 0.2mm, and the casting area is 300mm², and the defects such as porosity and slag are reduced. Suitable for the production of thin and fine small precision castings, cast steel (including stainless steel) parts in particular. Its advantages are:
1. Facilitate the exclusion of gas in the casting mold, inhibit the generation of turbulence and roll gas, overcome the disadvantages of low pressure casting and differential pressure casting, so that the filling capacity of the metal liquid is significantly improved.
2. Due to the improvement of filling capacity in vacuum casting, the casting temperature of metal liquid can be 20~30℃ lower than that in gravity casting.
3. It is possible to cast castings with large difference in wall thickness, thin wall and high quality requirements.
4. By choosing the suitable rate of change of vacuum degree and controlling the speed of metal liquid entering the mold cavity, a smooth filling effect can be obtained.
In the process of pressurization and filling, vacuum is pumped into the mold, and after filling, the pressure is maintained so that the casting crystallizes and solidifies under constant pressure, and the casting is fully shrunken, thus the casting is dense and the mechanical properties are improved. This method is mostly used for precision casting of Al, Mg alloy castings. Its advantages are as follows:
1. The existence of negative vacuum pressure, can make the mold sand to get tight, increase the strength of the casting mold.
2. Moderately increase the vacuum level, so that the filling time becomes shorter, not only the filling speed is accelerated and smooth.
3. Vacuum low pressure casting can speed up the flow of aluminum alloy at low temperature, avoiding the phenomenon of aluminum alloy suction caused by high temperature casting, which increases the pinhole defects of the casting.
4. Good casting formability, conducive to the formation of clear contours, smooth surface castings and large thin-walled castings.
5. Dense casting organization, high mechanical properties. 6.
6. Generally, no riser is needed, so that the yield of metal liquid is greatly improved, usually up to 90%.
7. Good working environment, simple equipment, help mechanization and automation.
The main thing is to seal the resistance holding furnace and the casting mold all together, and pass compressed air with higher pressure such as 500kPa inside the sealing cover, then the metal liquid will not rise because the pressure inside the casting mold and crucible is equal, and then supplement the pressure of 50kPa on the metal liquid surface, and the metal liquid will rise to fill the mold cavity. This method is especially suitable for the production of complex thin-walled castings, but the equipment is large and troublesome to operate, so it is only applied when special requirements are met. Its advantages are as follows:
1. Compared with the traditional gravity casting and vacuum suction casting, vacuum differential pressure casting in 1mm thick thin specimens embodies a very good filling capacity.
2. The casting has a denser crystalline organization and mechanical properties, in the casting strength performance, compared with vacuum suction casting and gravity casting about 20% ~ 25%, elongation is increased by about 50%.
3. Vacuum die casting can effectively reduce the porosity of die casting, and can improve the density of casting by more than 1%.
