At this stage, the most important aluminum alloy wheel casting technologies include gravity casting, low pressure casting and forging. The following content describes gravity, low pressure and forging in detail to further clarify the relevant content and lay a foundation for the subsequent key technology and quality control recommendations.
1. Gravity casting
Gravity casting is a traditional casting technology, “molten metal is injected into the mold under the action of the earth’s gravity”, specifically including sand casting, investment casting and metal mold casting. Among them, metal mold casting is the main way of casting aluminum alloy wheel hub of automobile, and it is also gravity casting in narrow sense.
2. Low pressure casting
Low-pressure casting is the most commonly used and common new casting technology at present. The casting mold is placed above the sealed crucible, and compressed air is injected into it, which creates low pressure (about 0.06 MPa – 0.15 MPa) on the molten metal surface, so that it can be solidified from the riser to the hub cavity and controlled. After removing the air pressure, the remaining aluminum alloy melt at the sprue sleeve or in the riser will flow back to the crucible again. After the blank is completely cooled, the upper mold and the lower mold of the mold are separated and raised to the specified height, and then it is taken out. For the applied air pressure, it is necessary to set the corresponding parameters in advance to ensure the density of the hub microstructure and good mechanical properties. This method can greatly save materials and reduce the scrap rate.
The design of gating system should be paid attention to in the low pressure casting die structure of aluminum alloy hub to avoid oxidation slag inclusion or drilling defects caused by the oxidation of alloy melt. For example, according to the characteristics of hub center hole and symmetrical thin-walled die-casting parts, the center gate mode shall be properly selected. In addition, attention should also be paid to the design of exhaust system, cooling system and ejecting mechanism. Because the cavity is closed, the exhaust problem during punching should not be ignored to avoid affecting the aluminum liquid punching. In the design of the cooling system, attention should be paid to the positions of the upper die splitter cone, the mounting plate, and the places where the rim and spoke are connected, and water cooling pipes should be set to achieve the purpose of rapidly cooling the aluminum liquid.
After the hub mold is formed, the mold surface needs to be subject to anti-corrosion treatment, such as die-casting mold steel H13, which has not strong corrosion resistance to aluminum liquid after quenching, tempering and nitriding treatment. However, after the ceramic coating is prepared on its surface, its corrosion resistance has been significantly improved and the service life of the mold has been improved as a whole.
3. Forging
Forging is the process type with the highest level of hub casting technology at present. Generally, the forging machinery is used to exert pressure on the metal blank to make its plastic deformation have mechanical properties, corresponding shape and size, and finally form the forging. Although the forging process has good casting effects, such as optimizing the microstructure, preserving the metal flow line, etc., the production equipment required by this process technology is highly sophisticated, and the yield is relatively low, and the manufacturing cost is high. At present, in addition to some high-end models that will use this method to forge the wheel hub, the low-pressure casting method is widely used, with relatively low cost, excellent performance, and simple manufacturing process.