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2023.1.28
The application of metal mold casting technology in the field of mechanical manufacturing is expanding, among which aluminum alloy casting is the most widely used. Therefore, this paper mainly discusses and briefly describes the metal mold structure and gating system of Al-Si alloy castings.
Aluminum alloy gravity casting has its inherent characteristics. The metal mold has fast cooling speed and strong chilling effect on the casting. The casting has fine grain, compact structure, high comprehensive mechanical properties, accurate size, smooth surface, and improved quality. The uncertain factors affecting the casting quality have been reduced, especially suitable for mass production. The metal mold with good structure can minimize the machining allowance and riser size, while the process yield and blank utilization rate are higher than that of ordinary sand mold gravity casting, making the cost of casting relatively lower, and improving the environmental pollution of ordinary sand mold casting and the working conditions of workers. To sum up, although there are many casting processes for aluminum alloy casting at present, the metal mold gravity casting process still has specific advantages due to its flexibility, versatility and low cost, and occupies a certain position.
1. Pouring system of metal mold
On the basis of the good structure of the metal mold cavity, the correctness of the gating system design will have an important impact on the quality of the casting and the process yield.
The design principles of the gating system are as follows:
(1) The heat distribution in the mold is reasonable, which is convenient for directional solidification, so that the casting can be fully fed.
(2) The gating system should be as short as possible, simple and without losing its functional integrity.
(3) The molten metal shall be smoothly injected into the mold cavity through the pouring system without impact, eddy current and splash, effectively preventing the secondary oxidation of the molten metal.
(4) During the pouring process, it shall be convenient for cavity exhaust and skimming.
(5) On the premise of ensuring quality, maximize the process yield and create favorable conditions for the casting cleaning process.
(6) It shall be an open gating system.
In any form of casting process, as long as it is gravity casting, its directional solidification is manifested by crystal solidification from bottom to top, and the riser set above is fully used to feed the casting. Due to the different shapes and structures of various castings, the crystallization and solidification of molten metal in the mold cavity is a complex process. For castings with large wall thickness difference, there are both directional solidification and simultaneous solidification in this process. However, the design of the gating system should strictly follow the principle of gravity casting directional solidification, so that the heat distribution in the mold cavity conforms to the condensation sequence of the casting from bottom to top.
The aluminum alloy has high thermal conductivity and small volume heat capacity. The temperature of molten metal will drop rapidly during the flow process. The aluminum liquid is active and easy to absorb and oxidize. If the oxide film formed on the surface is always complete during pouring, it has a good protection effect on the aluminum liquid below and prevents secondary oxidation. If the pouring is unstable or not stable enough, the oxide film will break, and the oxide slag on the surface will be drawn into the aluminum liquid below, It is difficult to float to the surface again, and all kinds of gases are not easy to precipitate. Because the density of aluminum and aluminum oxide is similar, the slag inclusion in the casting is caused, the shrinkage rate is large, and the casting defects such as shrinkage cavity, shrinkage porosity, pinhole and slag inclusion are easy to occur. Therefore, the smooth pouring of aluminum alloy castings is particularly important.
In actual production, it is difficult to organically combine the advantages of the traditional metal mold gravity casting gating system. If we focus on the smoothness of mold filling (bottom pouring), we can not get a reasonable cavity heat distribution; Focusing on the reasonable heat distribution of the cavity (top pouring) will lose the smoothness of pouring. Therefore, the traditional metal mold gravity casting process has long caused the unfavorable situation of low process yield and blank utilization rate.
2. Several traditional forms of aluminum alloy gravity casting gating system
There are: top injection type, middle injection type, bottom injection type and gap type.
(1) The top-pouring gating system is short and simple, the heat loss of molten metal is small, the amount of metal occupied by the system is small, and there is a reasonable cavity heat distribution, which is conducive to the directional solidification and feeding of castings. However, the mold filling is unstable and easy to splash. With the increase of the casting height, the impact phenomenon is serious, which is unfavorable for exhaust, and the casting is easy to cause slag inclusion and porosity. Therefore, the top-pouring pouring system is basically not used in the gravity casting of aluminum alloy.
(2) The impact phenomenon of the filling of the middle pouring system is better than that of the top pouring system, but the heat distribution of the system to the cavity is not as reasonable as that of the top pouring system. It is generally used in the production of small castings with a height of less than 100 mm.
(3) Under the action of gravity, the molten metal of the bottom pouring system enters the mold cavity through the system and fills the mold steadily from bottom to top, which is beneficial to exhaust and skim, but the heat distribution of the mold cavity is extremely unreasonable, and the directional condensation sequence required for gravity casting cannot be achieved. The casting cannot be fully shrunk. At present, this method is widely used in small and medium-sized aluminum alloy castings with larger risers. In addition to the above disadvantages, the bottom pouring system is also designed to reduce the impact. The straight runner is often designed to be serpentine and inclined and curved. In most cases, the slag collecting ladle is added to replace the transverse runner, which results in the long route of molten metal flowing through, large heat loss, complex structure and high manufacturing cost. The heat distribution of the cavity can only make the casting solidify from top to bottom, which is the opposite of the condensation sequence of gravity casting. Castings are easy to cause cold shuts, blocked feeding, shrinkage holes at the middle and bottom, and pinholes at the top. Once problems occur, it is very difficult to adjust the mold. The common methods are to increase the pouring temperature, increase subsidies, and increase the riser. In addition to the difficult quality assurance, it also causes a large amount of metal consumption. The process yield is generally less than 50%, and the average utilization rate of blank is only 65% – 70%.
(4) In theory, the gap gating system is always in the top filling state, but its filling is stable and the exhaust is good, which can effectively prevent the secondary oxidation of molten aluminum. The gap gating system also has a certain skimming function. The heat distribution of the cavity is reasonable compared with the bottom gating system, which is conducive to the shrinkage of the casting. But in fact, its feeding performance is not very prominent, and the size of the riser is not much smaller than that of the bottom pouring system.
Application of stepped gap gating system
The improved ladder-shaped gap gating system is suitable for aluminum alloy castings of various shapes and certain height (except for disc type). The casting weight ranges from 0.5kg to nearly 100kg, and the high solid and hollow thin-walled castings are particularly suitable. For castings less than 200 mm, the common vertical pouring method is used. For castings with large height, the horizontal pouring method has been used in the past to reduce the pouring height. However, there are many disadvantages, such as long pouring route, poor quality of the upper part of the casting, and multiple risers are set on the upper part. In addition to wasting a lot of metal, it also increases the difficulty of cleaning.
Application of stepped gap gating system.
We use the stepped gap pouring system to lower the pouring height of castings with large height by using the reverse inclined pouring method. Generally, before pouring, the inclined pouring machine tilts the mold by about 45 °, and the molten metal is injected into the mold cavity through the pouring system. Each inner runner is placed under the mold cavity to form an inclined layered bottom pouring environment. Generally, when the mold is half filled, the inclined pouring machine slowly flattens the mold and restores the vertical pouring state, During this process, with the mold slowly lifted, the pouring should not be stopped and the flow should not be cut off. When the liquid level reaches half of the riser, the pouring speed should be reduced to increase the feeding time below. For large size castings, in order to speed up the pouring speed, two symmetrical pouring systems can be designed on the parting surface. When pouring at an inclined angle, the two systems are poured at the same time, and the inner runner is placed on the two sides of the mold cavity respectively, forming an inclined layered pouring environment. The metal liquid is injected into the mold cavity evenly from the left and right corresponding inner runners at the same time, and the specific operation is the same as the above. Of course, the time and speed of mold lifting are also different due to the difference of casting structure and shape, but the smoothness of mold filling must be considered.