Car wheels are a key component of vehicles, directly related to driving safety. Thus, the quality and performance of car wheels must be guaranteed. A critical factor affecting wheel quality and performance is the material used. There are many materials for manufacturing car wheels; with the development of related technologies and the emergence of new materials, wheel materials are constantly updated, and more high-performance materials are applied in wheel production. Different materials correspond to different manufacturing processes.
In recent years, car wheels have become increasingly lightweight. Therefore, the selection of wheel materials tends to prioritize those that are light, high-performance, high-quality in surface finish, recyclable, and cost-effective. Generally, current car wheel materials fall into three categories: steel materials, alloy materials, and composite materials, with a variety of processing and manufacturing processes available.
Steel was the earliest material used for car wheels; steel disc wheels already appeared in 1905. The main advantages of steel for wheels are high strength, wear resistance, and good heat dissipation. Although more lightweight wheel materials have emerged with technological development, steel wheels still hold a certain market share due to their simple manufacturing process, good performance, and low cost.
Steel Materials
1. Carbon Steel
Formed steel wheels typically use carbon steel, which consists of a carbon steel disc welded to a cylindrical rim. Drilling holes in the wheel can effectively improve brake disc ventilation without reducing resistance to external forces. The biggest advantage of carbon steel is its low cost, and it can meet the needs of general-performance vehicles, but it is not suitable for certain types of cars. Specifically, first, carbon steel has low heat resistance, so carbon steel wheels cannot cool brake discs over a large area, making them unsuitable for high-performance, high-safety vehicles. Second, carbon steel has low strength; to meet requirements, the thickness of the disc and rim must be increased, which adds to the wheel weight—contradicting the current trend of lightweight wheels. Increased weight also raises wheel inertia, further increasing brake resistance and control system resistance for wheels without suspension support, ultimately making the vehicle’s direction harder to control. Third, carbon steel wheels are difficult to decorate with exquisite patterns on the surface, lacking aesthetics, so they are not suitable for luxury cars and sports cars.
2. Ductile Iron
Ductile iron has excellent comprehensive mechanical properties, so it is often used to make car wheels. However, ductile iron also shares some shortcomings of carbon steel; its production process is difficult, and its shape is hard to control, so its practical application is greatly limited.
3. Other Steel Materials
Some alloy steels are also commonly used in wheel manufacturing. These alloy steels generally have high strength, high plastic toughness, good weldability, and formability. In practical application, they are usually welded from formed rims and discs, with weight reduced as much as possible. In recent years, foreign countries have also developed technologies for cold-forming steel plates into rims and discs, and their welding methods have been updated.
Alloy Materials
1. Aluminum Alloy
Advantages of Aluminum Alloy Wheels
Aluminum alloy wheels are favored by the industry mainly due to the following advantages:
(1) Light weight: Aluminum alloy wheels are lighter than traditional steel wheels, usually 30% to 40% lighter. Thus, their rotational inertia is lower, and the vehicle’s acceleration is better. They require less energy for braking, making them more energy-efficient.
(2) Low runout and vibration: Aluminum alloy wheels have high precision and good separation from tires, which can reduce vehicle runout and vibration during driving, maintain a more stable vehicle center of gravity, and improve balance performance—thus enhancing driving performance.
(3) Good heat dissipation: The heat source of vehicle wheels mainly comes from brakes and friction between tires and the road. When the vehicle is driving at high speed, wheel temperature rises significantly, increasing the risk of tire blowouts. Aluminum alloy wheels have a high heat dissipation coefficient, and their surface design is more conducive to heat dissipation. Even with continuous braking, wheel temperature will not rise excessively, effectively reducing the incidence of tire blowouts.
Manufacturing Processes of Aluminum Alloy Wheels
Currently, the main processes used in aluminum alloy wheel production are as follows:
(1) Casting process: Aluminum alloy wheels often have a wide variety of designs, strong adaptability, and low cost, making them very suitable for casting. Statistics show that over 80% of aluminum alloy wheels worldwide are cast wheels. Casting processes can be specifically divided into gravity casting, squeeze casting, low-pressure casting, and pressure casting.
(2) Forging process: The advantages of forging include small processing volume, high dimensional accuracy, high strength, and good corrosion resistance. Forged aluminum alloy wheels are usually lightweight; the stress direction is consistent with the grain flow direction, giving them high toughness and fatigue strength. Each wheel has the same mechanical properties, with a typical elongation of 12% to 17%. They have strong resistance to impact, shear, and tensile loads, no surface pores, easy handling, and uniform coating for good aesthetics. However, their manufacturing process is more complex and costs are higher.
(3) Semi-solid die forging: This process is between solid and liquid forming. Similar to forging, it has small processing volume, dense internal structure, smooth surface, good mechanical properties, few pores, and long mold life. Although it is still in the initial stage in China, it has broad development prospects in the future.
2. Magnesium Alloy
The main advantages of magnesium alloy materials are: yield strength similar to aluminum alloy, but lower elastic modulus than aluminum alloy and steel—under the same stress, it can consume more deformation energy, enabling shock absorption, noise reduction, and resistance to large impact and vibration loads; good processability and castability, strong machinability, easy to process; good electromagnetic shielding, thermal conductivity, and wear resistance; stable dimensions and shrinkage rate, high precision of castings; recyclable, meeting the requirements of sustainable development, environmental protection, and energy conservation. Therefore, magnesium alloy materials have gradually been widely used in car wheel manufacturing. Car wheels made of magnesium alloy have advantages such as good shock resistance, high specific strength, no sparking during friction, good flexibility under external forces, high freedom in cross-sectional design, and good machinability. Their manufacturing process is similar to that of aluminum alloy wheels.
3. Composite Materials
Composite materials are modern new materials, which can be used to manufacture various parts such as car bodies, wheels, engine frames, and drive shafts. In terms of performance, composite materials have great advantages over traditional materials—for example, high specific strength and specific modulus, resulting in lighter and more rigid parts; good chemical stability; good friction reduction, wear resistance, and self-lubrication; high toughness, thermal shock resistance, electrical conductivity, thermal conductivity, and heat resistance. In addition, some composite materials have additional properties such as radiation resistance, ablation resistance, and creep resistance. These properties greatly improve the quality of car wheels while meeting the requirements of lightweight and energy conservation. There are many manufacturing methods for composite materials; in addition to traditional casting, there is powder metallurgy: mixing metal powder with reinforcing particles, then manufacturing through degassing, hot pressing consolidation, rolling, extrusion, and other steps; in-situ reaction synthesis: synthesizing composite materials with higher strength and better performance through chemical reactions between elements or compounds, with simple process and low cost.
In summary, with the continuous update of car wheel materials, their quality, performance, and energy-saving effect are constantly improving. Under the current trend of vehicle lightweighting, it is necessary to select wheel materials reasonably and continuously improve the corresponding manufacturing processes to produce car wheels that better meet people’s needs.