Aluminum alloy wheels are more suitable for passenger cars than steel wheels. Currently, their manufacturing process can be basically divided into three types. The first type is casting, and most car manufacturers choose to use casting technology. The second type is forging, which is commonly used in high-end sports cars, high-performance cars, and high-end modification markets. The third type is more unique, which is the MAT spinning technology first put into use by Enkei Company in Japan. Currently, this technology is not as widely used in China as the first two types.

1. Gravity casting method

Simply put, gravity casting mainly relies on the gravity of aluminum water itself to fill the mold, which is an early casting method.

This method has low cost, simple process, and high production efficiency. However, during the pouring process, inclusions are easily drawn into the casting, and sometimes gases are also drawn in, forming porosity defects. Wheel hubs produced by gravity casting are prone to shrinkage and looseness, and have poor internal quality. In addition, the limitation of aluminum liquid flowability may also lead to a low yield rate for wheel hubs with complex shapes. Therefore, the automotive wheel hub manufacturing industry rarely uses this process anymore.

2. Low pressure casting method

Low pressure casting is a process in which aluminum liquid is filled into a mold under pressure and solidifies and crystallizes under pressure. In the same situation, compared with gravity casting, the internal structure of low-pressure casting wheel hubs is denser and has higher strength. In addition, low-pressure casting utilizes pressure filling and replenishment to greatly simplify the pouring system structure, resulting in a metal liquid recovery rate of up to 90%. At present, low-pressure casting has become the preferred process for aluminum wheel hub production, and most domestic aluminum alloy wheel hub manufacturing enterprises adopt this process for production. However, the low-pressure casting method also has its drawbacks: longer casting time, longer feeding and mold changing time, and higher equipment investment.

3. Forging method

Hot forging → RM forging → Cold spinning → Heat treatment → Machine work → Shot blast → Surface finishing

Forging is the transformation from solid to solid, forming the wheel hub style through methods such as tapping, pressing, forging, etc. This process does not undergo liquid phase changes, it is all solid transformation. So its mechanical properties are higher than those of casting, with advantages such as high strength, good corrosion resistance, and precise dimensions. The grain flow direction is consistent with the direction of force, so the strength, toughness, and fatigue strength are significantly better than those of cast aluminum wheels. At the same time, the typical elongation rate of forged aluminum wheels is 12% to 17%, which can effectively absorb road vibrations and stresses. In addition, the forged aluminum wheel hub has no pores on its surface, making it highly capable of surface treatment.

However, the biggest disadvantage of forging aluminum wheels is that there are multiple production processes and the production cost is much higher than that of casting. Although forged wheels have better performance, car manufacturers still mainly use cast wheels on most vehicles, and only a few luxury cars are equipped with forged wheels. However, the leading domestic wheel hub manufacturing enterprise CITIC Dicastal has successfully entered the passenger car forged wheel hub production line and compressed the cost of forged wheel hubs to a thousand yuan, and has started supplying original wheel hubs to domestic joint venture factories.

4. Squeeze casting method

Squeeze casting, also known as liquid forging, is a process that combines the characteristics of casting and forging. It involves pouring a certain amount of liquid metal directly into an open metal mold, applying a certain pressure to the liquid metal through a punch, filling, forming, and crystallizing it, and producing a certain amount of plastic deformation during the crystallization process. Advantages: Smooth filling, direct crystallization and solidification of metal under pressure, so the casting will not produce casting defects such as porosity, shrinkage, and looseness, and the structure is dense. The mechanical properties are higher than those of low pressure die-casting parts and the investment is much lower than that of low-pressure casting method. Disadvantage: Like traditional forged products, milling is required to complete the shape of the spokes. A considerable number of aluminum wheels for automobiles in Japan are produced using extrusion casting technology, with the entire process from pouring molten metal to removing the castings controlled by computers, and the degree of automation is very high. Currently, countries around the world consider squeeze casting as one of the directions for producing aluminum wheels for automobiles.

5. Special Forming: MAT Spinning Technology

MAT spinning technology was first introduced by Enkei Corporation in Japan, and strictly speaking, it can be considered as a type of casting. It refers to the use of specialized equipment to perform rotational pressure treatment on the stressed area after the wheel rim is cast as a whole, causing changes in the molecular arrangement inside the metal at the treated position. The specific dividing surface presents a higher density fiber like structure compared to general casting products, thereby changing the overall metal mechanics process. The quality, strength, and extensibility of wheels manufactured by MAT spinning technology are close to those of forged wheels, and now it is easier to produce forged wheels. Overall, MAT spinning technology can not only relatively ensure the manufacturing cost of wheel hubs, but also enable cast wheel hubs to achieve similar weight and strength as forged wheel hubs. However, the domestic technology is not mature and the cost is high, so it is not widely used.