In order to eliminate the shortcomings of, improve the quality of die and expand the application range of sand casting technology, scientists have developed the counter gravity sand casting technology. In counter gravity casting, the alloy liquid is filled and solidified from bottom to top in the direction opposite to gravity. It has the characteristics of stable filling, controllable filling rate, reasonable distribution of temperature field, solidification under pressure and conducive to solidification and feeding of castings. The mechanical properties of castings are good, the structure is compact, and the casting defects are few, so it has been widely used. According to different process, the counter gravity casting can be divided into low pressure casting, differential pressure casting and pressure regulating casting.
During the Second World War, Birmingham cast aluminum company of England successfully used low-pressure casting technology to manufacture aircraft air-cooled engine cylinder block castings for the first time in large scale, which quickly attracted the attention of the sand casting industry around the world. In 1961, on the basis of low pressure casting, Sofia Foundry Research Institute of Bulgaria developed a differential pressurewith the characteristics of low pressure casting and autoclave casting, which is used to manufacture large, complex and thin-walled parts. Soon, Bulgaria’s differential pressure casting machine export countries all over the world, including the United States, Britain, France, Russia, Australia and other developed countries. With the continuous development of science and technology, as well as the increasingly high quality requirements for sand castings, differential pressure casting technology and equipment are also constantly improved and updated. At present, the differential pressure casting technology has been mature. Compared with the low pressure casting, the main advantages of the differential pressure casting are as follows.
(1) Reduce the defects of porosity and pinhole. The pressure of differential pressure casting is 4 ~ 5 times of that of low pressure casting, so the volume of gas discharged from the mold and core will be reduced by 1 / 4 ~ 1 / 5 in the process of casting, which will reduce the chance of forming invasive pores. The main reason for the difficulty in exhausting air from low pressure casting mold is that low pressure casting can only control the gas pressure in the crucible, but not the atmosphere in which the mold is located. Different from differential pressure casting, it can control the pressure of upper and lower pressure tanks at the same time.
(2) Improvement of surface roughness of castings. When the liquid metal fills the mold, there will be a layer of gas film in the groove or rough part of the mold wall. The gas pressure of this part acts on the liquid metal, which prevents the complete reproduction of the shape of the mold wall on the casting. The density of the gas film is large in the differential pressure casting, so when heated and expanded, the greater the reverse pressure of the gas is, the stronger the ability to prevent the liquid metal from invading the pit is, so the roughness of the casting surface is smaller.
(3) The feeding pressure of differential pressure casting is 4 ~ 5 times of that of low pressure casting. The absolute pressure of the upper tank is about 0.7MPa in differential pressure casting, while that of the crucible is about 0.15Mpa in low pressure casting, which is quite different. As a result, the differential pressure casting has a strong feeding capacity, which can fill the shrinkage cavity and microcrack in time, and to a certain extent “press fit” eliminates some hidden dangers of hot cracking.
(4) Differential pressure casting can reduce the solidification time by 20% ~ 25%, correspondingly, it can also reduce the deterioration phenomenon in the solidification period, and its grains are also refined.
In recent years, Zeng Jianmin and Zhou Yaohe of Northwest Polytechnic University of China have developed the pressure regulating casting technology on the basis of differential pressure casting. The biggest difference between the pressure regulating casting and the differential pressure casting is that they can not only realize the control of positive pressure, but also the control of negative pressure, which requires higher control accuracy of the control system. Northwest Polytechnic University has mastered the core technology of counter gravity casting equipment manufacturing, and is in the leading position in the development and production of differential pressure and pressure regulating casting equipment in China.
In recent years, based on the original sand casting technology, Shenyang Foundry Research Institute has applied differential pressure casting and pressure regulating casting to the production of complex thin-walled precision aluminum alloy castings, and has built a first-class production line of aluminum alloy sand differential pressure casting and pressure regulating casting in China. The mechanical properties and internal quality of the produced aluminum alloy cabin meet the requirements of American military standard ams-a-21180, the dimension accuracy reaches CT6 (precision casting grade) at the highest level, and the thickness of the thinnest part is only 3 mm. In 2008, Shenyang Foundry Research Institute developed and produced a large-scale thin-wall high-strength casting with an overall wall thickness of only 2 mm. The material is d357 (American brand, the same composition as ZL114A in China, and the Mg and impurity content requirements are more stringent). The tensile strength, yield strength and elongation of the casting body can reach 350 MPa, 300 MPa and 5%, respectively. In addition, Northwest University of technology, Beijing Institute of aeronautical materials, capital Aerospace Machinery Co., Ltd. have also developed relevant technologies for the development and production of sand aluminum alloy ultra-thin wall castings.