Usually distinguish between steel wheels and alloy wheels. The rims of light alloy is used only aluminum and magnesium. Magnesium wheels in conventional cars are extremely rare due to their high cost, and most importantly, low corrosion resistance.
Wheels: steel and aluminum
The main advantage of cast aluminum wheels over steel - it is possible to obtain diverse design, high dimensional accuracy, and optimal static and dynamic mechanical characteristics.
Losing weight aluminum wheels over steel is one of their advantages, but often not decisive. In some cases, weight cast aluminum wheels is equal to or only slightly lighter than standard steel with a simple design,.
Aluminum rims: cast or forged
Most aluminum wheels are cast or forged. A small number of disks, mainly for the "elite" and sports cars, manufactured from several constituent parts and mixed technologies involving casting methods, shoeing, punching, pressing, rolling, welding and other.
Cast aluminum wheels
When it comes to the cast disks, it usually refers only ALUMINUMe alloy wheels. Steel wheels are made not by casting, and stamping, and magnesium alloy wheels are used only for reasons of exclusive cars, the above.
Casting aluminum wheels
For the production of aluminum wheels using various casting methods. the method aluminum casting It depends on the quality of the cast wheel rim, external and internal. The choice of casting method is mainly determined by the quality of the cast microstructure (e.g., porosity), applicable types of aluminum alloys and heat treatment regimes. All this determines not only the strength and reliability of the wheels, but also affect the level of the quality of their appearance.
The basic methods of casting wheels
The main methods of casting, which are used in the manufacture of wheels, are as follows:
- low pressure casting is the main method;
- chill casting - used less often;
- Casting with counterpressure - even rarer.
Sometimes applied technology, combining casting forging and stamping.
Casting into the chill wheel
Generally speaking, injection molding, when the metal is injected into the mold, It is more preferred, than simple pouring into it under gravity. However, gravity die casting is still quite relevant casting process for the production of wheels. Gravity die casting is cheaper, it is used mainly when, when not chasing weight loss, and want to get the original design. As the process in filling the mold relies only on gravity, then the casting structure usually has more defects (for example, porosity), something and, which is obtained by injection molding. Therefore discs, cast in a metal mold, usually have more weight to, to provide the desired strength.
Casting wheels, low pressure
Most of the alloy wheels are made by low pressure casting (picture 2). The low pressure casting method uses a relatively low pressure (about 2 bar) for, to achieve the rapid filling of the mold and to obtain a more dense microstructure, a, Consequently, and better mechanical properties, compared with gravity die casting. In addition, this technology also provides slightly higher performance (figure 3).
Other methods of casting wheels
In addition to the classical method of low-pressure casting used numerous technological options, which are optimized for the production of wheels. for instance, get even lighter and stronger wheels, when special equipment is used, which provides a high pressure casting.
An interesting new development is the patented method of «air inside technology» company BBC. Its basic idea is the use of a structure of hollow chambers rim and spokes instead of a solid metal. The result is a lighter wheel with better dynamics and driving comfort. This technology includes the addition casting operations such as molding and welding.
Quality Control alloy wheels
Each cast disk is exposed to X-ray inspection, and then usually subjected to a heat treatment and machining. Thereafter, the disc surface is subjected to a special preparation for painting and the paint or applied protective coating. Then samples of statistical sampling drives are the three-dimensional size control, check the dynamic balance, tests for fatigue and impact strength.
aluminum alloys requirements for wheels
The materials for the wheels to meet a series of requirements, which may conflict with each other.
The casting alloy must have good casting properties:
- perfect filling of the mold,
- no metal sticking to the mold,
- Minimal tendency to hot cracking and shrinkage.
The material should have
- high ability to withstand mechanical shocks (ductility, impact strength).
Rim material should have
- high corrosion resistance in both normal, and a saline atmosphere.
ROM material must be
- high fatigue strength.
Aluminum-silicon alloys for wheels
In accordance with these requirements for the manufacture of rims used hypoeutectic aluminum-silicon alloys with a silicon content of 7 to 12 %. These alloys also have various additive amount of magnesium to provide good combinations of strength and ductility. Moreover, These alloys have a low content of iron and other impurities.
Aluminum alloy AlSi11Mg
Up to 80 years in Germany and Italy, applied close to the eutectic composition AlSi11Mg alloy containing silicon 11-12 %. This alloy has very good casting properties, especially with regard to the filling of molds and minimal shrinkage. On the other hand, the chemical composition of the aluminum alloy it does not provide a sufficiently high strength and the fatigue limit, which would allow to further reduce the weight of the wheel rim.
Aluminum alloy AlSi7Mg0.3 (A356)
Currently, the standard for the manufacture of alloy wheel rims is cast aluminum alloy AlSi7Mg0,3, which is more commonly known as Alloy A356, with an additional modifying strontium. For the first time, this alloy is used for the manufacture of wheel rims in France, wherein the heat treatment was not applied.
However, the advantage of this alloy is exactly AlSi7Mg0,3, that it is thermally reinforcing, and it allows you to provide additional strength to the disks. In the United States and Japan, this alloy from the beginning applied with heat treatment T6, that is, in the state after quenching and artificial aging.
Thermal hardening aluminum wheels
The graphs pattern 6 shows the dependence of the strength properties of the alloy AlSi7Mg, modified sodium, the magnesium content. AlSi7Mg0,3 strength characteristics of the alloy gives the best combination of fatigue strength and elongation. With an increase in the magnesium content fatigue strength does not increase substantially, elongation drops significantly.
Similar studies were performed for various silicon contents. established, that with increasing content of silicon decreases ductility of the alloy, especially at low curing speed in places thickenings. Nonetheless, alloys with a silicon content 11-12 % continue to apply in cases, Increased flowability when molten aluminum is needed.
Of great importance for the fatigue strength of the alloy has AlSi7Mg level of porosity in the casting. The figure 7 shows the dependence of the fatigue strength of the aluminum alloy AlSi7Mg0,3 maximum pore size in the test sample material.
Aluminium Automotive Manual – European Aluminium Association – 2011