Light-alloy wheels from Mercedes-Benz and Mercedes-Benz Accessories - PART IV


Stuttgart, Germany, Apr 13, 2010

Revealing insights: X-rays, computed tomography and metallography

An important part of the development process for new light-alloy wheels are examinations which look at the internal microstructure of the wheel, right down to atom level. While the examinations conducted on the various test systems and facilities give an indication of whether the performance of a new wheel is able to meet the high standards required by Mercedes-Benz, supplementary examinations using reflected-light or scanning electron microscopes, and both x-ray and computed tomography systems, provide an insight into why some wheel prototypes are more resilient than others.

One example: as already described, in the rotary bending fatigue test the wheels are subjected to a load until the material breaks. This test therefore sheds light on how long a light-alloy wheel lasts under standard conditions, and provides initial indications of specific weak spots based on the position of crack initiation and the subsequent path of the fracture. What the system cannot indicate, however, is the reason for this breakage. The causes of these weak spots may be of a geometric nature, or could be down to insufficient basic properties of the material which has been used. Irregularities resulting from the casting process may also need to be taken into account. This is where the latest examination methods come into play to aid with the analysis: metal microscopy, scanning electron microscope (REM), x-ray systems and computed tomography (CT) provide detailed insights which highly-specialised engineers are able to use to optimise the wheels. Different methods are used depending on the objective of the test: while metal microscopy and the raster electron microscope (REM) can mainly be used to establish metallographic causes, x-ray systems and CT technology meanwhile are useful for identifying cavities (voids), gas pores or foreign inclusions.

Causal investigation at micro-level resolutions

Gaining an insight into the microscopic world of a light-alloy wheel starts with the reflected-light microscope, which has a magnification ranging from 20 to 1000 times. A sample weighing just a few grams is taken from the wheel under investigation and finely ground and polished. Using a subsequent contrasting technique, the microstructure of the crystalline metal alloy is made visible. The resulting two-dimensional image of this area enables conclusions to be drawn regarding the raw material used and the processing quality during the casting process. If irregularities are revealed at this point, the wheel supplier is notified of the issue and requested to rectify it before production start-up.

After the test runs, the failure points can only be assessed under the raster electron microscope because this system is able to provide a three-dimensional image at a significantly higher resolution down to micro-level (1µ = 0.001 mm). As such, the specialists at the Stuttgart-Untertürkheim plant are able analyse the failure points very precisely to determine whether the crack formation resulted from a casting flaw, or from an adverse phase formation in the metal microstructure. A crack emanating from an intact metal microstructure may indicate a structural weak spot. Once again, in such a case the relevant development areas or the supplier are notified about how these weak spots need to be eliminated.

Leading role in industrial computed tomography

While it is always necessary to destroy the test wheel in order to produce samples for examination under reflected-light or scanning electron microscopes, non-destructive testing on the other hand is also used to supplement the development phase and makes an important contribution to checking subsequent series production at suppliers. Non-destructive analysis may be necessary for example when a light-alloy wheel needs to be kept available for further tests after being examined. At the Daimler Mettingen plant, wheels are inspected at the acceptance stage using x-ray systems, and specialists use the x-ray images to identify cavities and imperfections larger than 0.3 of a millimetre.

If even more detailed non-destructive examinations are necessary then computed tomography (CT) is used. This is a process used in the medical field and was introduced by Daimler AG for industrial use – the first automotive company to do so – as early as 1995. The company now plays a leading role in this field. The Stuttgart-Untertürkheim plant currently has the most modern CT system for industrial use in the world. Boasting a resolution of 5 µ, this system was fully developed and independently built by Daimler, reflecting the company's high level of competence in this field for which in the meantime it has become renowned throughout the entire industry. Thanks to this system, the tomography specialists have often been able to provide assistance "outside of their usual field", for example in the case of EADS, for the optimisation of the Ariane rockets.

Unlike x-ray examination, which only provides a two-dimensional image, CT produces three-dimensional images. They are created by x-raying the wheels in layers. The three-dimensional image is generated by a computer cluster with a main memory of 54 gigabytes, using up to 2880 individual x-ray layer images of the wheel taken both horizontally and vertically by the CT system in a lead-lined room.

As a result, the position and size of weak spots caused during the casting process, such as cavities or gas pockets, can be accurately determined. Used in conjunction with the tests conducted on the test benches, these CT assessments provide evidence of where the smallest imperfections might have a big impact on performance. These component areas can then be enhanced during the technical casting and production processes. In addition, the knowledge gained can be carried over into future wheel development.

Precise specifications for series production

The production and materials engineering department located at the Daimler headquarters in Stuttgart-Untertürkheim plays an important role in planning and monitoring external wheel production. The specialists who work there specify the basic metal alloys to be used, which must not only have good casting qualities but at the same time also conform to the requirements for adjusting the mechanical properties that are required. The casting process, using low-pressure gravity die casting (see section on production), must also be carried out under optimum conditions during subsequent production. Other important elements are the 100‑percent x-ray test which Mercedes-Benz requires to be carried out during the ongoing production process, as well as precise heat-treatment for adjustment of the mechanical properties.

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