Principle and technology of thin wall casting by vacuum differential pressure casting

In order to meet the requirements of lightweight and integration of components, a large number of large-scale complex thin-walled aluminum alloy castings are widely used in the field of aerospace. This kind of products have higher and higher requirements for castings, which put forward the requirements of large size, thin-walled, precision, complexity and high quality for the production of castings. This kind of casting is difficult to be formed by traditional casting technology. Vacuum differential pressure casting is an advanced counter gravity precision forming technology. The filling pressure and solidification pressure are controllable, and the filling under vacuum condition has superior filling and solidification conditions. It is one of the most feasible methods to produce high-quality complex thin wall castings. Researchers at home and abroad have done a lot of research work on thin-walled parts of vacuum differential pressure casting, and achieved remarkable results. In this paper, the research progress of vacuum differential pressure casting equipment, thin-walled casting filling and solidification in recent years is introduced, and the existing problems and future development direction are put forward.

Vacuum differential pressure casting is improved on the basis of the principle of differential pressure casting. It combines the advantages of low pressure and high pressure casting technology. Its principle is shown in the figure. The equipment first makes the upper tank mold and the lower tank metal liquid in the sealed state, and vacuumizes them. When filling the mold, a controllable filling pressure is applied to the metal liquid level of the lower tank, while the upper tank mold remains in the vacuum state, and the metal liquid in the crucible fills the mold smoothly along the riser under the effect of the pressure difference between the upper tank and the lower tank; At the end of mold filling, increase the controllable pressure on the upper and lower tanks at the same time and keep a certain pressure difference between the metal liquid in the lower tank and the mold cavity of the upper tank, so that the metal liquid can be solidified under pressure. After the casting is completely solidified, remove the pressure, and the non solidified metal liquid in the lifting pipe flows back to the crucible. The traditional process curve is shown in the figure.

Liquid metal 2. Crucible 3L liquid pipe 4. Clapboard 5. Sample 6. Sand mold 7. Upper tank 8. Vacuum pump 9. Air tank 10. Lower tank 11. Base

(1) The process of vacuum differential pressure casting is a complex dynamic process. There are many factors affecting the filling process, such as the change of cavity section, the drop of liquid metal level, the time lag of regulating valve, etc., so it is difficult to establish an accurate mathematical model for precise control, while the precise control of thin-walled casting process is particularly important, Therefore, it is necessary to further develop the precise control system of vacuum differential pressure casting, especially the intelligent control system such as fuzzy expert control and fuzzy neural network control.

(2) At present, the control system of vacuum differential pressure casting is carried out under the fixed pressure in the filling and solidification stages, and the research shows that changing the pressure in the filling and solidification stages is more conducive to improving the forming quality of thin-walled parts, so the research and development of precise control of the pressure change in the filling and solidification stages is also a development direction.

(3) The filling behavior and solidification feeding behavior of vacuum differential pressure casting thin-walled parts have always been a research hotspot. Because of its special structure, there is a sudden change of filling section, which will have a negative impact on filling and solidification. Therefore, according to the casting structure, the filling and solidification process will be classified to obtain a more stable filling flow field and improve the feeding capacity of liquid metal, It will be one of the research directions in the future to fill and solidify thin-walled parts by stages in vacuum differential pressure casting.