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. Just how wheels are produced in accordance with the high quality standards demanded by Mercedes-Benz is best described by looking at one such supplier as an example.
The process initially heats up in the casting facility
Primary alloys are always used as the basic raw material for light-alloy wheels, supplemented with metal waste from the company’s own production processes, such as machining shavings and defective wheels from current production. The roaring fires of the furnaces must be maintained within a narrow temperature margin of around 775 degrees. After tapping the molten aluminium alloy and subsequent cleaning of the melted mass with special additives, workers transport the so-called transport pan to the actual casting machines into which they transfer the molten metal.
Aluminium cast wheels for Mercedes-Benz are produced exclusively using the low-pressure gravity die casting process. This works similar to a traditional espresso maker which is placed on a hotplate, in which excess pressure in the lower container forces the liquid into the top through a rising pipe. In the case of the espresso maker, it is water which flows into the upper container through the filter, but in the case of the casting machine it is the molten aluminium mass which is forced into the upper mould. Here too the temperature must be maintained within a narrow margin, so that the metal can flow through the mould and solidify in a precisely defined sequence. Both casting properties and the solidification process are accurately simulated during the development phase of a new wheel. After solidification, the multipiece mould is opened and the wheel blank is moved on to the next stage in a fully automated 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. Thanks 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 optimised 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. The following areas of the wheels are worked into shape here in turning, milling and drilling centres:
• Wheel rim base; the part of the wheel on which the tyre subsequently sits
• Outer rim flange; end profile around the circumference of the wheel
• Wheel contact surface; this is where the wheel contacts the wheel hub of the vehicle
• Centre alignment; the correct dimension guarantees that the wheel sits centrally on the wheel hub
Clamp terminal; groove in the centre alignment into which the hub cover is subsequently inserted
• Wheel bolt holes; all Mercedes-Benz wheels have a spherical contact area (hemispherical crown) between the wheel hole and wheel bolt head
• Valve hole; precise alignment is extremely important for the tyre pressure monitoring sensors, which are fitted with the valve
• The so-called “brake contour” on the inner side of the wheel disc and the rim, which guarantees the necessary clearance for the braking system.
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 paintshop, 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 pressurised 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.
Brilliant results in the paintshop
In the first stage carried out by the paintshop production department, the wheels are put through a huge washing machine which is almost a hundred metres long. This is where the wheels are washed and degreased. After this, a base coating is applied which acts as a bonding agent between the bare metal and the subsequent coat and at the same time also provides initial protection against corrosion. The next powder coating, applied electrostatically, is baked on in an oven and is the thickest layer applied to the wheels. It helps to even out any slight roughness of the wheel surface. Next, specially trained staff members use their fine touch in special light boxes to check the results of the first coat and correct any unevenness. If it is not possible to do this then any wheels which are below standard are removed from the production process. Painting robots then apply the coloured base coat in clean-room conditions and – as a final sealer – the clear coat. The wheels are then approved for dispatch in a final test station.
Machine turning for two-tone wheels
Mercedes-Benz and Mercedes-Benz Accessories offer particularly attractive two-tone wheels which feature an interesting contrast between the bare-look metal areas and the paint finish. The production of these wheels requires a considerable amount of additional effort since special coatings are applied to provide reliable protection against corrosion.
After applying the coating described above, the system removes these wheels from the production process and transports them to special turning machines fitted with diamond cutting tools which remove the coating layers in specifically defined areas of the wheel – mostly on the spokes or the outer wheel rim base. This so-called “high-sheen polishing” process creates metallic high-gloss areas which contrast against the remaining coating. Of course these areas must be resealed as part of a new coating process. This has a three-layer structure: after being pretreated once more, the wheel then receives a transparent anti-corrosion coating, specially developed for Mercedes-Benz wheels, followed by various clear coatings to build up the final glossy finish.
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 the following:
• 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
• Inspection of the dimensions in a 3D measuring machine
Direct involvement of the laboratory in the production process and standardised communication strategies enable any production issues to be quickly corrected if necessary.
Source: Daimler AG