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Lightweight Design worldwide

, Volume 11, Issue 2, pp 30–35 | Cite as

A lightweight and economical two-point steel suspension arm

  • Matti Teschner
  • Michael Braun
Design Chassis
  • 164 Downloads

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 [1] 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 [2] 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.

The experimental and numerical investigations are compared in Figure 1. The positive correlation between the force paths confirms the assumption about the material and shows that the two-point suspension arm reacts more critically to compressive loads due to its curved form. The component fails significantly earlier (34 kN) than when subjected to tensile loading (77 kN). Accordingly, special attention is paid to the compressive load case (buckling) in the assessment. Determining the wrought alloy (EN AW-6082) used — and hence the associated fatigue strength — it is possible to calculate the load horizons and component rigidities needed for the aluminum suspension arm to reach the fatigue strength value under tensile and compressive loads [3, 4, 5] using finite element (FE) analysis. To this end, the aluminum suspension arm is pushed into a bearing seat until the strains produced correspond to those of fatigue strength. Under a buckling load, this occurs at a displacement of 0.6 mm, or when a force of 9.6 kN is applied.
Figure 1

Comparison of experimental and numerical compressive and tensile experiments (© Salzgitter)

Steel Sheet Structure

To minimize costs, the aim is to use a solution featuring a single piece of high-strength steel sheet (SZBS800/CR570Y780T-CP-GI). These steels are characterized by high yield and tensile strength alongside good formability, paving the way for producing even geometrically complex components. Such steel sheet structures must meet requirements relating to fatigue strength as well as the minimum buckling load of the aluminum suspension arm. Numerical optimization methods are used to take advantage of the potential of stronger steel materials and minimize component weight. For this purpose, a reference component is produced with length, bearing spacing and required upper-edge curvature copied directly from the aluminum suspension arm. The height and width of the reference geometry are extended, as shown in Figure 2, otherwise the limited assembly space would render a steel-sheet structure infeasible.
Figure 2

Derivation of reference geometry based on the digitalized two-point suspension arm (© Salzgitter)

The aim is to use a single-piece steel sheet solution made from high-strength steel.

Optimization

The first step involves producing concept variants based on sheet thicknesses of 1 and 2 mm using topology and topography optimization in combination with restrictions such as bead depth and width, as well as direction and minimum thickness of the bionic structures. These variants are to be seen as geometry proposals that achieve the optimization goal under the prescribed boundary conditions. The next step involves realizing these geometry proposals in actual structures, whereupon the most effective variant is selected by assessing the fatigue strength in vehicle operation, based on FEanalysis and behavior under improper use in accordance with the tensile and compressive tests performed on the forged aluminum suspension arm. Figure 3 shows that variant b (green line), weighing approx. 600g and resulting from a combination of topology and topography optimization, exhibitsoptimal performance for further consideration.
Figure 3

Assessment of numerically optimized steel sheet concepts (© Salzgitter)

Steel components are 28 to 34 % heavier than the aluminum suspension arm.

To align component behavior under improper use more precisely with the force path of the aluminum suspension arm and to further minimize weight, the size is optimized to determine the optimum sheet thickness, where sheet thickness is the only available variable. According to this, a sheet thickness of 1.56 mm is sufficient to achieve the required buckling load of 34 kN, even if the component exhibits slightly higher stiffness in the elastic range. For practical reasons, a sheet thickness of 1.6 mm is selected. The course of the force path in the case of improper use is plotted in Figure 5, allowing a 20 % weight saving, leading to an overall weight of around 500 g. Moreover, the consideration of the steel sheet structure under a buckling load of 9.6 kN shows that the concept also meets the requirement of 310 MPa fatigue strength as calculated for a number of load cycles with N = 106 and using strain-controlled Wöhler fatigue tests.
Figure 4

Von Mises equivalent stress under a buckling load of 9.6 kN (© Salzgitter)

Figure 5

Force paths of the aluminum suspension arm and the steel variant (© Salzgitter)

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.

Producibility Assessment

A producibility assessment is conducted to ensure the manufacture of the developed component concept is feasible. For this purpose, the component model is returned to the state of a flat sheet in a one-step analysis using the material CR570Y780T-CP-GI with a sheet thickness of 1.6 mm. Here, in addition to thinning out, major and minor form changes were determined by approximation. The major form change is plotted over the minor change in the forming limit diagram, resulting in the cloud of points shown in Figure 6. Used alongside the forming limit curve of the material under consideration, this allows statements to be made about the producibility of components. As shown, the data points are in the uncritical range and producibility appears realistic. However, given the proximity of some ranges of the forming limit curve, forming simulations that take a closer look at the production process are indispensable for further investigations.
Figure 6

Forming limit diagram used to estimate the producibility of the CR570Y780T-CP-GI steel structure with 1.6 mm sheet thickness (© Salzgitter)

Cost Comparison

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 [6], 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 %.

As the comparison of the three cost estimates in Figure 7 shows, 1.20 euros for a steel suspension arm contrast with 2.61 euros for a forged aluminum suspension arm, meaning a potential saving of up to 54 % — particularly when considering electric vehicles, where the focus is currently less on lightweight design owing to recuperative energy systems, as shown by researchers at the University of Duisburg-Essen. Steel thus offers significant cost benefits over aluminum with moderate weight changes.
Figure 7

Consideration of the costs of the steel sheet structures compared with the forged suspension arm (© Salzgitter)

Conclusion

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.

References

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    Vetter, P.: Leichtbau fällt nicht schwer ins Gewicht. In: Welt am Sonntag 49 (2017)Google Scholar
  2. [2]
    Ersoy, M.; Gies, S.: Fahrwerkhandbuch. Grundlagen— Fahrdynamik — Fahrverhalten— Komponenten— Elektronische Systeme — Fahrerassistenz — Autonomes Fahren — Perspektiven. 5. Auflage. Wiesbaden: Springer Vieweg, 2017Google Scholar
  3. [3]
    Ostermann, F.: Anwendungstechnologie Aluminium. Berlin: Springer Vieweg, 2014CrossRefGoogle Scholar
  4. [4]
    FA. SAPA: Werkstoffdatenblatt EN AW-6082. Finspang, Schweden, 2016Google Scholar
  5. [5]
    Honsel AG. Handbuch der Knetwerkstoffe, 2008Google Scholar
  6. [6]
    Herbertz, R.; Hermanns, H.; Labs, R.: Massivumformung kurz und bündig. Hagen: Industrieverband Massivumformung e. V., 2015Google Scholar

Copyright information

© Springer Fachmedien Wiesbaden 2018

Authors and Affiliations

  • Matti Teschner
    • 1
  • Michael Braun
    • 1
  1. 1.Salzgitter Mannesmann Forschung GmbHSalzgitterGermany

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