When it comes to producing light-alloy wheels, Mercedes-Benz only trusts the world’s best and most prestigious suppliers. These companies have a high level of know-how and produce wheels in accordance with strict Mercedes specifications. Numerous test procedures, both on current production as well as in the laboratory, help to guarantee a high standard of quality. These procedures include monitoring the raw materials at the time of delivery and during subsequent processing, 100-percent x-ray examination, leakage tests, continual monitoring of strict geometric data and concentricity during machining, as well as monitoring the painting process.
100-percent x-ray inspection during ongoing production
After casting, all of the wheels pass through a fully automatic x-ray facility. This system screens the untreated blanks in sealed chambers and analyses the resulting images in real time. Casting defects such as cavities (air pockets) or pores which may lead to reduced stability of the wheel are reliably identified. Any wheels affected by such issues are removed from the plant and these parts are subsequently melted down again.
Mercedes-Benz wheels are produced exclusively from warm-stored Gk-AlSi7 aluminium alloy. This material attains its exceptionally high stability by means of a three-stage heat treatment process which follows the casting process. Prior to quenching in the water bath, the wheel blanks are first brought to a temperature of around 530 degrees for “solution annealing”, followed by warm storage for several hours at around 150 degrees. Due to this process the aluminium is able to achieve its final strength.
A smooth process – flowforming
In conjunction with cast, one-piece aluminium wheels, “flowforming” is a relatively recent machining method. Basically wheels which have been machined using the flowforming method offer the best of both worlds. Not only does the designer retain the high degree of freedom over the design which a cast wheel provides, but a flowformed wheel also enjoys the advantages afforded by forged wheels, which are expensive and complex to produce: high strength combined with optimized component weight.
The flowforming production process: as with the wheel production process described above, first of all a blank is cast which has a very narrow wheel rim base but with significantly greater wall thickness. After being heated up to around 350 degrees, this blank is then stretched on a cylinder which tapers slightly towards a cone at the top. The blank and cylinder are rotated while three rolling heads, which are also rotating, are pressed against the blank from the outside in a downward motion and at high pressure – around 120 tons. This forces the metal into the desired shape over the cylinder, hence “flowforming”, and at the same time compresses it. A wheel rim base created in this way has a similar structure to a forged wheel, offering the highest degree of stability with the lowest possible weight. Subsequent heat treatment of the flowformed wheel then gives it its final strength.
Perfectly formed: machining
All wheel blanks are subsequently transformed into their final form by means of several machining stages. In this production area, where the production process now calls for reliable dimensional accuracy and a high degree of precision, only computer-controlled mechanical treatment machinery is used which achieves production tolerances of a few hundredths of a millimetre.
Integrated measuring equipment in the individual production centres monitors the quality of the machining during the ongoing production process. If a particular machining step comes close to the strict tolerance limits, the machine corrects itself automatically. In addition, Mercedes-Benz demands 100-percent independent checks: as such, the centre alignment is measured separately while another measuring station reliably detects unacceptable imbalances. In the last test run before reaching the paint shop, all of the wheels are subjected to an automated leak test. In this test the wheel is clamped between two rubber-coated steel plates and pressurized with the inert gas helium. Helium detectors outside the wheel are able to pick up the smallest quantities of escaping gas and are therefore able to reliably detect any leaks. Any wheels which fail the test are removed from the production process. Since inert gas has a significantly smaller molecular structure than air, even the smallest of leaks can be reliably detected.
In-process inspection in the works laboratories
The ongoing production process is of course also precisely monitored in the manufacturer’s laboratories in accordance with Mercedes-Benz specifications. The examinations carried out by the laboratory technicians include:
Incoming inspection of the supplied raw material, GK-AlSi7, in a spectrometer
Tensile strength test using specially prepared wheel samples
Microscopic examination of the metal microstructure of the finished light-alloy wheel
Dimensional Inspection using a 3D coordinate measuring machine
Direct involvement of the laboratory in the production process and standardized communication strategies enable any production issues to be quickly corrected if necessary.