When performing low-pressure casting of aluminum alloy wheels, the surface area of ​​aluminum alloy is prone to thermal shrinkage due to heat, that is, easy to deform, and bubbles will be generated after thermal shrinkage, causing the mold to be scrapped and cannot be used. Since this happens frequently, it brings great losses to the mold factory and seriously affects the economic benefits of the factory, so we must seriously study and solve this problem. After research, we found that changing the cooling method of the lower mold can cause the surface area of ​​the casting to be cooled in order, thereby reducing the disadvantages caused by instant cooling, so that no bubbles are generated, the surface area of ​​the casting is smooth, and the product is qualified.

1 Process flow of aluminum alloy wheel production
The production process of aluminum alloy wheels is relatively complicated, including: raw material preparation, smelting, material inspection, casting, application of radiographic flaw detection technology, intersection, heat treatment, material performance inspection, shot peening, cutting treatment, deburring treatment, cleaning, Dynamic balance inspection, air tightness inspection, surface grinding, cleaning, pretreatment before painting, painting and drying, fine turning, deburring, pretreatment before painting, performance and appearance inspection, packaging and storage. Casting technology has very strict requirements on the surface of the car, and no impurities are allowed on the surface. This has always been a major difficulty in the low-pressure casting industry. If the quality of each process can be controlled well, it will be achieved in the casting industry. Certain achievements ensure the quality of the wheel surface. In the current technological development, we mainly adopt low-pressure wheel casting technology, which occupies 80% of the wheel technology. Let's analyze the cooling method of low-pressure casting of aluminum alloy wheels.

2 Low-pressure casting cooling method for aluminum alloy wheels
In the actual production process, the low-pressure mold is filled from the center of the hub, and the aluminum alloy solution flows through the spokes to the hub. Then it will fill the entire wheel hub. Cooling from the outside of the spokes to the center, we generally call this cooling method sequential cooling. In fact, the channels of the gating system are the spokes. To fill the entire hub, the spokes must pass through. Therefore, the order of solidification of the hub is extremely important.

The improvement of the mold will be a more feasible and scientific cooling method, which will be explained below to study the different effects of different cooling methods on casting. The cooling mold is divided into an upper mold and a lower mold. The lower mold is equipped with six-ring cooling air ducts. One end of the air duct is the cross section, and the other five air ducts are responsible for cooling the spokes. The cooling of this kind of mold is the surface of the automobile, and the spokes are wider. In order to ensure the quality of cooling, the cooling is performed in sequence as much as possible, so the cooling air pipes are arranged relatively tightly.

The arrangement of the air ducts is tight, but the air ducts with intervals in the middle are arranged in several rings. In order to reduce the temperature of the hub quickly, we correspond the branch pipes on the air ducts to the cooling and exhaustion. Can quickly reduce the temperature of the wheel hub. In the surrounding environment of the hub, the temperature situation is that the temperature changes from strong to weak from the inside to the outside along the hub. In this case, the aluminum alloy solution will shrink to the corresponding point of the cooling air. This is based on the theory of sequential feeding. According to this principle, the position of the principle cooling point will be shifted and its surface The phenomenon of cold shrinkage occurs. Such point cooling will cause uneven heat and cold on the surface of the casting, thereby forming uneven spots. This cooling method makes the pass rate of castings very low, only 5% ten. This method will greatly increase the cost of production due to the lower qualification rate of the product. Therefore, ordinary enterprises will not consider using this method for production, and should improve this method.

Through the above analysis, we should improve the cooling method of the mold. The research shows that changing the point cooling to the surface cooling is a good choice. The function of the surface cooling is to make the front side of the spokes cool relatively uniformly, and local overcooling is rare. Status. This method of mold cooling has certain advantages. It keeps the air duct next to the mold pouring. This device eliminates three air ducts and replaces the cancelled air duct with a cooling insert. The cooling insert corresponds to the back of the spokes of the lower mold, and a duct corresponds to the underside of the spokes and the inserts.
For cooling, in order to achieve sequential cooling from the outside to the inside of the lower mold, the cooling air must flow from the outside to the center of the lower mold. Such sequential condensation is surface condensation. This cooling method is obviously different from the single point cooling of the previous scheme.
After the cooling method of the mold is changed, a cooling air duct corresponds to the insert at the spokes and the gap between the back cavity of the lower mold. The cooling is performed from the outside to the center, and the cooling intensity is gradually weakened instead of enhanced. This kind of mold manufacturing The method conforms to the theory of casting sequence solidification and has strong operability. According to statistics from experienced companies, a follow-up survey of 200 cast products found that there was no obvious heat shrinkage on the surface of the 200 products. After the front of the spoke was processed, its white spots disappeared. In this way
The pass rate of the produced castings has reached more than 96%, the production cost has been greatly reduced, and the production efficiency of the enterprise has been greatly improved. It is not feasible to use low-pressure molds to fill the center of the hub for cooling. After coating the surface of the product cooled in this way, there is no bubble phenomenon. This method is loved by people and has been quickly promoted.

3 Conclusion
In short, during low pressure casting of aluminum alloy wheels, the surface is prone to heat shrinkage, resulting in bubble points, resulting in unqualified products and waste of materials. Therefore, the heat shrinkage and bubbling on the surface of the wheel can be treated scientifically and reasonably by cooling. Surface cooling can ensure that the castings solidify in a certain order and promote the improvement of casting quality. Of course, this technology is not yet mature, and further research and improvement are needed to continuously improve the qualification rate of castings.

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