Material savings thanks to correct corrosion protection

The subject of lightweight construction has occupied a wide range of industries in recent years. This includes the automotive industry. In order to make cars lighter, more efficient and more environmentally friendly, new materials, forms or solutions are constantly being sought. Zinc flake systems provide long-term corrosion protection for automotive parts and help to reduce the overall weight of a vehicle.

Suspension parts in cars like chassis control armsare usually oversized. Meaning they are produced from the outset with a risk premium that already includes damage caused by potential corrosion. The daily work and development of long-lasting corrosion protection technologies drove Dörken MKS to the question of how high the theoretical weight reduction of components can be through wall thickness reduction. The ACS (Automotive Center Südwestfalen) was therefore commissioned to investigate this on a simplified example of a chassis control arm.

Simulation tests for sheet thickness reduction

The results of the study of ACS are impressive: Theoretically, up to 30% of the mass can be reduced. How was this figure determined? Using various experiements such as the Leonard Euler buckling test.

The simplified control arm created for the purposes of the study: a U-section with a thickness of 2.6 mm (length 300 mm).

In the first step, the finite element method was used to determine how the simplified model (in the form of a steel U-profile) behaves with the current sheet thickness of 2.6 mm under buckling load. This means that the U-profile is compressed in its longitudinal direction in the simulative model construction until it buckles. The U-profile buckled at a force of 49.2 kN. The OEMs' specifications – depending on the vehicle model – are only approx. 35 kN. The over-sizing was already evident here. For this reason, the second step was to investigate up to what plate thickness the U-profile can be reduced under a given buckling load of at least 35 kN without neglecting the required buckling strength. The simulation showed that the sheet thickness of the U-profile could be reduced to approx. 2 mm – with full compliance with the required buckling strength. This means that the mass can be minimised by up to 30 %.

Compliance with mechanical properties

Within the scope of the study, the influence of corrosion on the mechanical properties was also investigated with the aid of a 3-point bending test (based on DIN EN ISO 14125).

The minimum metal thickness of 1.95 mm needed to withstand the buckling load was evaluated in different three-point bending tests.

The bending test showed hardly any deviations between the test specimen with safety pad and the 2 mm thick U-profile. If it were not for the risk of corrosion.

First, the minimum bending stiffness of the profile was calculated. The reference sheet thickness was the 2 mm thick profile, which yielded a bending stiffness of 3.1 kN/mm. Then the profiles were coated: the 2 mm thick U-profile with a high-performance zinc flake basecoat and topcoat from Dörken MKS, the 2.6 mm thick U-profile with a phosphate and subsequent cathodic dip coating surface. The test specimens were then subjected to a stone impact (in accordance with EN ISO 20567) in the buckling area and three corrosion stresses were applied: the salt spray test (in accordance with DIN EN ISO 9227), the accelerated corrosion test II (ACT II) and the VDA test (in accordance with VDA 233-102).

Zinc flake coating: Low wall thicknesses with the same component performance

The results were clear: The 2 mm wall thickness test specimen coated with the zinc flake system performed without red rust infestation in all three corrosion tests. 1,000 hours in the salt spray test and six cycles each in the climate change tests were achieved. At the same time – both loaded and unloaded – a bending stiffness between 3.2 and 3.4 kN/mm was achieved in the 3-point bending test. By way of comparison: The 2.6 mm thick U-profile showed recognisable red rust infestation at the same times and an unloaded bending stiffness of approx. 4 kN/mm and values between 3.8 and 3.9 kN/mm after loading.

Local maximum tension measurements in a genuine control arm under a tensile load.

Weight saving due to effective coating system

The ACS study commissioned by Dörken MKS shows: The additional 0.6 mm, which is planned in practice as a safety component, can be saved with the aid of effective corrosion protection. It became clear that this could also reduce the buckling strength. An effective corrosion protection thus fulfils both the different corrosion properties and the mechanical properties and can lead to an overall saving of around 30%. More than promising for the future of lightweight construction!