A lightweight and economical two-point steel suspension arm
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Salzgitter Mannesmann Research, using a two-point suspension arm as an example, shows how lightweight and cost-efficient chassis parts can be produced from steel materials. The developed component can be produced up to 54 % cheaper that the original aluminum suspension arm.
Steel Over Aluminum
In recent years, lightweight materials have increasingly taken over shares of classic in-car steel applications, for example forged aluminum for suspension arms in the premium mid-size segment. The current transition in the automotive industry, spearheaded by electrification, demands extending the range of electric vehicles, as well as cost savings to make financing more feasible. Recent studies suggest that it is less expensive to allow for a larger battery to increase the range and to forgo the extensive use of lightweight materials for cost reasons. Although this increases overall vehicle weight, researchers at the University of Duisburg-Essen, Germany, were able to demonstrate in studies  that the energy requirements resulting from energy recuperation in electric vehicles remained almost unchanged, despite greater payload. The designers have focused on steel-based, and hence more cost-efficient, lightweight design. Accordingly, the possibility of replacing forged aluminum with higher-strength steels is investigated using a two-point suspension arm made from aluminum, focusing on the cost advantages of a steel structure while keeping component weight in mind. Numerical optimization methods are used for the steel-based structure.
It is less expensive to allow for a larger battery and forgo the use of lightweight materials.
The study involves classic reverse engineering of a series-production, two-point suspension arm made of aluminum. The components used to perform the experimental and numerical investigations are procured through after-sales channels. They are then used to derive the component requirements, which, together with geometry characteristics generated from the digitalized two-point suspension arm, are used as boundary conditions in an optimization program. Various concept variants are generated on this basis. In a final step, the component concepts are compared and a cost analysis is performed against the forged aluminum suspension arm.
Determining Component Properties
Since no concrete component requirements exist for the aluminum suspension arm under consideration, the relevant data is determined via classic reverse engineering. Literature  reveals two crucial requirements for the design of suspension arms in particular. First, fatigue strength mirroring stresses in daily vehicle operation needs to be taken into account, and second, cases of improper use, e.g. when driving over a curb, also need to be considered. Tensile and compressive tests are performed accordingly on the aluminum suspension arm, where components are subjected to stresses until failure without considering the rubber mountings. Furthermore, numerical investigations of the digitalized components are conducted using the material map of the assumed wrought alloy.
Steel Sheet Structure
The aim is to use a single-piece steel sheet solution made from high-strength steel.
Steel components are 28 to 34 % heavier than the aluminum suspension arm.
To further reduce mass, the results of free-size optimization, where the optimization tool can assign a sheet thickness separately for each element, are used to underpin additional consideration of local reinforcements, such as bonded and soldered patches, additively produced steel parts and tailor-welded blanks (TWBs). Here, it transpires that using TWBs helps optimize the performance of a steel structure in numerical investigations. Compared with basic solutions featuring a constant sheet thickness of 1.6 mm, the steel-sheet structure, made from a TWB with a basic sheet thickness of 1.5mm and a planned sheet thickness of 1.7 mm in the center of the component, withstands the force level over a displacement of the bearing points under pressure load at the level of the aluminum suspension arm. At the same time, the TWB concept, weighing 480 g, or 28.5 % more than the aluminum suspension arm, represents the lightest steel structure.
In order to develop a weight-neutral steel counterpart to the two-point handlebar made of forged aluminium under consideration, the requirements with regard to maximum buckling load and the application of force when reaching the fatigue limit must be adapted. This would allow the use of smaller sheet thicknesses. Under the given load assumptions, the steel components are 28 to 34 % heavier than the aluminium handlebars.Conversely, if component cost is considered, the steel structures come out on top. The price calculation for the aluminum suspension arm is based on the cost structure of hot-forged components , according to which 30 to 50% of the component cost is accounted for by the material and 30 to 50 % by production, plus additional costs such as for setup and tooling, which are excluded here for the sake of simplicity. As only a raw material price for wrought alloys is available, a customary fraction of the mass is added to the component weight to cover processing, resulting in an estimated material cost of 50%. It is further assumed that it is a simple forged part, meaning the production cost only accounts for 30%. The production costs thus correspond to 60 % of the material costs.
A similar process is used for the steel sheet structures. The material costs are estimated to comprise 70 % of the service weight, while production accounts for 30%. The TWB variant also incurs additional cost for the welded plate and both steel sheet variants include additional costs due to protection against corrosion through galvanization and coating with cathodic dip painting. All of which means that the price of the forged component and the steel sheet structure do not constitute the final price of a suspension arm. Instead, what they represent is the estimated price of the finished metal component without rubber mounting and without additional overhead costs.
A comparison of the cost calculations shows potential savings of up to 54 %.
The high rigidity and formability of modern steel materials provide development engineers with a reliable and relatively inexpensive structural material that allows them to respond to changing technological challenges. The high modulus of elasticity of steel with 210 GPa compared to aluminium with 70GPa enables very flexurally stiff components with low wall thicknesses, which can absorb a considerable additional battery weight to achieve the necessary range. Steel is thus an established and cost-effective lightweight design material for chassis-related applications.
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