Alloy wheels - this is how they are made
Light alloy rims - colloquially referred to as aluminum rims, are extremely popular with car buyers and drivers. Almost every vehicle in Germany is driven with aluminum rims in summer. Various manufacturers produce light alloy wheels directly for equipping new cars or for retrofitting for the tuning market. The aluminum rims not only differ in size and design, the manufacturing process is also different. The production of aluminum rims can be divided into three types: casting, rolling, forging.
Conventional manufacturing: the casting process
To put it very simply, the casting process is a process that is somewhat reminiscent of baking a cake. For every rim design there is a mold into which the liquid aluminum is poured. At the beginning there is the liquefaction of the material. The aluminum is not processed in a pure form, but in a special alloy. As a rule, it is an aluminum-silicon alloy. The solid material is in a melting furnace at a temperature of approx. Liquefied 1000 degrees. Before the casting, the first quality check is carried out, in which the material composition is checked for deviations using spectral analysis. If the result is satisfactory, the liquid aluminum alloy is transported to the casting machine, where it is pressed and shaped at a temperature of 700 degrees Celsius using the low pressure process at 1,2 bar. In order to process the rim further, it is first cooled in a complex compressed air water cooling system. The ideal material structure can only be guaranteed by precisely controlling the cooling process. After the alloy has solidified, the next quality control takes place. The wheel is X-rayed and examined for cracks, pores and tiny voids in the material. Only flawless rims are released for further processing. The scrap is sorted out and melted down again. In the further manufacturing process, the wheel is heat-treated again at over 500 degrees Celsius for several hours. This process is called solution annealing and, together with the quenching and relaxation process, in which the material rests for several hours at a temperature of 150 degrees Celsius, forms the basis for maintaining the final strength of the rim. In the subsequent mechanical processing, the cutting machine removes unnecessary material by turning off the rim base on the inside and outside. The rim gets its final shape from the subsequent drilling of the valve, spherical cap and wheel hub. These steps are of course carried out with CAD support and are carried out down to the thousandth of a millimeter and then checked using 3D scanning. During the subsequent unbalance test, the even mass distribution of the wheel is checked. The so-called finishing includes grinding the design side and deburring the wheel. These steps are not carried out fully automatically. This is where the expertise of the manufacturing company's employees is required. In the further process, the grinding dust is washed off and the airtightness of the rim bed is ensured by a leak test. Then the wheel can be painted. The application of base powder and colored lacquer leads to a flawless surface.
Rolling rims - the flowforming process
Strictly speaking, the flowforming process is an additional process within the casting process. The English term can be translated as 'forming a rim in motion' and is also known as ironing or rolling. The liquid aluminum alloy is not poured into the final shape as in the conventional casting process, but into a narrower rim base with a significantly higher wall thickness. After the solution heat treatment, quenching and relaxation, the wheel is heated again to 300 degrees Celsius and clamped on a cylinder that is slightly conical in shape. The wheel and cylinder are made to rotate. In the meantime, three rotating rollers transfer their pressure of up to 15 tons to the rim base and bring it into the desired shape. The material is deformed and compressed at the same time. Colloquially, this process is called rolling, as the smaller blank gets its final shape through rolling. The ironing or rolling leads to an increase in the strength of the material. With the same stability, certain areas of the rim can be made more filigree than in the conventional casting process. Another advantage is the lower weight of the flowforming rim. After ironing, as with the conventional cast wheel, mechanical processing is required in which the rim base is turned off on the outside and inside. All further production steps up to painting correspond to those in the manufacture of the cast wheel.
Forged wheels - pressed into shape with great force
The so-called forged wheel cannot do without a casting process. First, a cylindrical blank is cast from a silicon-free aluminum alloy. The cast cylinder has a height of about 60 cm and a diameter of 30 cm and corresponds approximately to the mass of the desired rim. In the first pressing process, the block is pressed into a round disc with the aid of a hydraulic press. A pressure of approx. 4000 tons is exerted on the material. A second press gives the disc its later design with a contact pressure of approx. 7000 tons by pressing in the contours. Ventilation holes, wheel hubs and screw connection points are created by another pressing tool. While the rim has been cold-formed up to this point, the rim disc must be heated for further processing so that it can be brought into a three-dimensional shape in a rolling process. The rim is shaped and pulled out under enormous pressure. This step-by-step process is known as 'forging'. The finishing in the form of drilling, turning and grinding is followed by surface treatment. As a rule, the painting is carried out in the form of the corrosion-resistant powder coating process.
The material structure, which is highly compressed in the forging process, gives the rim an extremely high density and a perfect surface that has no inclusions. With the same strength, the forged wheel can be manufactured with thinner wall thicknesses than a wheel manufactured in the casting or flow forming process and has a lower weight. This results in a reduction in the unsprung masses in the vehicle, which contributes significantly to increasing the driving dynamics of the vehicle.
Advantages and disadvantages of different manufacturing processes
The forging process is the most complex manufacturing process and, accordingly, the light alloy rims that are forged are significantly more expensive than conventional cast wheels or flowforming rims. Due to the strong compression during the pressing process, they are much stronger and more resilient. With the same load capacity, a smaller wall thickness can be achieved. This leads to a lower dead weight and consequently to better driving dynamics. The surface quality of forged rims is very high quality, since the aluminum rims have no inclusions. Forged wheels are very resistant to environmental influences and damage. Should damage nevertheless occur, these aluminum rims cannot be repaired and reworked like cast or rolled rims. However, in the forging process, the processing options of the pressing tools are limited, so that the range of shapes is not as varied as with cast or rolled aluminum rims.
Cast aluminum rims, on the other hand, can be produced in a large number of variants. It is crucial for the design that the blanks can be removed from the mold. The conventional casting process is the cheapest in production. Therefore, most new cars are equipped with these alloy wheels. Rolled rims manufactured in flow forming can be made with less wall thickness while maintaining strength. In this way, attractive designs can be produced at a slightly lower weight.
Basically, the three manufacturing processes stand side by side, they only serve different markets. The material composition and the expertise of the manufacturer are decisive for the quality of the aluminum rim.
The subject of aluminum rims - this is how they are made is an interesting issue for every automobile and motorsport enthusiast that provides valuable insights.