The SUPA-Wheel project aims at producing aluminium wheels with polymer inserts, focusing on sustainability and advanced alloy development.

The project SUPA-Wheel (Sustainable Production of Aluminium Wheels) focuses on the development of aluminium wheels with polymer inserts that meet stringent technical, environmental, and economic requirements. It is funded by the German Federal Ministry for Economic Affairs and Energy (BMWE) and coordinated by Dortmund University of Applied Sciences and Arts. The project consortium includes Trimet Aluminium SE, Borbet GmbH, Jordan Spritzgusstechnik GmbH & Co. KG, and the Fraunhofer IGCV. Running from January 2023 to June 2026, the project integrates alloy development, casting process optimization, simulation, and life-cycle assessment to develop circular, lightweight automotive wheels.

Secondary casting alloy for the automotive industry

The primary objective is to develop a secondary aluminium-based casting alloy that can be produced on an industrial scale with the highest possible recycled content, while meeting automotive standards for strength, ductility, fatigue resistance, and corrosion performance. To achieve this ambitious goal, a Design of Experiments (DoE) approach is used to systematically vary and evaluate the concentrations of Fe, Cu, Zn, and other trace elements across 30 alloy compositions. These are assessed using suitable correlation models with respect to mechanical properties, corrosion susceptibility, and CO₂ footprint.

Based on the generated data, an optimized alloy composition is derived and subsequently processed and validated via low-pressure die casting on both a pilot and production scale. Following controlled casting, the real component undergoes a T6 heat treatment process consisting of solution annealing, quenching, artificial aging, and thermal post-treatment for coating applications. These steps induce precipitation hardening, promote the formation of Mg₂ Si and other strengthening phases, and simultaneously control the morphology, size, and distribution of intermetallic compounds.

For each of the alloys, phase fractions of the Fe-, Cu- and Zn-containing phases were determined using equilibrium calculations. The thermodynamic approach, respectively the procedure followed in the project, is illustrated in Fig. 1.

The alloys and their specifics

Al₉Fe₂Si₂ and Al₁₈Fe₂Mg₇Si₁₀, both described as needle-/plate-like and Al₁₅(Mn,Fe)₃Si₂ present in Chinese script form are the three main equilibrium Fe-containing phases that were predicted across the alloy range considered. For several alloy variants there is an interaction between Al₉Fe₂Si₂ and Al₁₈Fe₂Mg₇Si₁₀ during cooling, while Al₁₅(Mn,Fe)₃Si₂ remains unaffected [2]. Simulations for Cu-containing equilibrium phases indicate an almost constant presence of Al₅Cu₂Mg₈Si₆, whereas Al₂Cu, only occurs within the highly Cu-alloyed versions. However, there is no formation of Zn-rich phases, as the majority of Zn is dissolved in the solid solution [2].

Based on insights gained from thermodynamic simulations, conclusions can be drawn regarding the alloy-specific microstructure to be expected, which is a key factor determining the alloy’s performance. This microstructure can be analyzed in terms of phase types, relative phase fractions, and crystallographic orientations using X-ray diffraction (XRD) and scanning electron microscopy (SEM). For this purpose, Trimet Aluminium SE uses, amongst other tools, a newly installed XRD system with a large sample chamber that allows the analysis of entire components. This significantly reduces sample preparation effort and enables rapid assessment of phase evolution, precipitation kinetics, and heat treatment efficiency. Such capability is particularly relevant when processing secondary aluminium, as elevated Fe and Cu contents promote the above-mentioned formation of brittle intermetallic phases.

Aluminium alloy design

An illustrative example of this challenge is shown in Fig. 2, which compares the microstructures of (A) a standard primary A356 alloy and (B) an A356 alloy containing 0.35 wt.% Fe, both after T6 heat treatment. In (B), Fe-containing phases with needle-like (2D) resp. plate-like (3D) morphologies are clearly visible, which can be identified as Al₉Fe₂Si₂ and Al₁₅(Mn,Fe)₃Si₂. These phases have a pronounced influence on the mechanical properties [2, 8]. The average Fe-needle length is considered an indicator of key alloy performance characteristics such as ductility, toughness, and fatigue strength [2, 8].

Fig. 2: Comparison of (A) primary and (B) secondary-based alloy in T6 condition.

In addition to targeted alloy design, controlled solidification, directional cooling, and optimized heat treatment are used to mitigate any deleterious effects through promoting uniform precipitation of strengthening phases – a key factor influencing both static and dynamic mechanical properties [2]. The influence of micro segregation and eutectic structures is also systematically investigated. During solidification, Fe, Cu, and Zn tend to segregate, leading to the formation of localized intermetallic clusters that act as critical sites for crack initiation, corrosion, and fatigue.

Improving alloy composition and cooling rates

By combining Design of Experiment (DoE), microstructural characterization, and thermodynamic modelling, alloy compositions and cooling rates are optimized to minimize undesirable phase formation and to maintain mechanical performance comparable to that of primary alloys. Precipitation hardening is precisely controlled to ensure a homogeneous Mg₂Si distribution, thereby improving tensile strength and fatigue resistance without compromising elongation at fracture. Elevated Fe levels are compensated by controlled Mn additions to reduce coarse Fe-rich phases, while Cu content is adjusted to enhance the aging response. Mechanical testing focuses primarily on ductility to ensure sufficient energy absorption under critical loading conditions, while Linear Sweep Voltammetry (LSV) is used to evaluate susceptibility to pitting and crevice corrosion [1, 2]. The resulting alloy exhibits an optimized combination of strength, ductility, and corrosion resistance.

Optimization with simulation

The simulation framework is fully integrated into the development process. Using finite element method (FEM), a wheel model can be evaluated for stress distributions, load paths, and critical deformation zones under bending, torsion, and impact conditions. Fig. 3 illustrates computational fluid dynamics (CFD) simulations that support aerodynamic optimization, reduction of air drag, and improvement of brake cooling performance. Topology optimization is done to identify material-efficient geometries that balance weight reduction, stiffness, and fatigue resistance. In addition, polymer inserts contribute to smoother airflow and enhanced brake ventilation. Thermal simulations are used to predict temperature gradients within the wheel, thereby aiding material selection and geometry design [3–5]. Correlations between microstructural features and FEM data enable the prediction of local mechanical responses and support the development of optimized wheel designs [2].

Fig. 3: Flow model using the example of a generic car body model and a wheel without inserts.

Sustainability at all levels

The recycling concept is central to all activities, aiming for optimal control of the CO₂ footprint. The target alloy must be designed to be compatible with the chemistry of commercially available recycled materials to close material loops sustainably [6]. Casting parameters and heat treatments are adjusted to preserve mechanical, corrosion, and acoustic performance. The polymer inserts are recyclable and thermally stable, minimizing environmental impact while ensuring functional and aerodynamic performance. Fig. 4 shows a design proposal for an alloy-specific wheel concept.

Fig. 4: Design draft of an aluminium wheel with polymer inserts.

Conclusion

SUPA-Wheel demonstrates a holistic, data-driven methodology for the sustainable development of automotive wheels. By integrating alloy composition, heat treatment, mechanical testing, FEM/CFD simulations, recycling processes, and life-cycle assessment, the project illustrates how metallurgical innovation, digital simulation, and circular economy principles can work together to create environmentally responsible next-generation automotive components [1–8]. By Tobias Beyer, Leiter Forschung & Entwicklung der Trimet Aluminium SE

References

[1] Hottenroth, H. et al.: Carbon Footprints für Produkte - Handbuch für die betriebliche Praxis kleiner und mittlerer Unternehmen. Institute for Industrial Ecology, Pforzheim University of Applied Sciences, Pforzheim (2013)

[2] Beyer, T. et al.: Influence of Increased Fe, Cu, and Zn Concentrations on Phase Formation in Aluminum A356 (AlSi7Mg0.3) Alloy. The Minerals, Metals & Materials Society, Trimet Aluminum SE, Fraunhofer IGCV, Orlando (2024)

[3] Material Economics: Europe's Missing Plastics - Taking Stock of EU Plastics Circularity. Commissioned by Agora Industry. (2022)

[4] Umweltbundesamt: Development of dismantling and recycling standards for rotor blades. Karlsruhe Institute of Technology. Dessau-Roßlau (2022)

[5] Berg, H., Brandt, A.: Investigation of Aerodynamic Wheel Design. Department of Mechanics and Maritime Sciences, Chalmers University of Technology, Gothenburg (2018)

[6] Kleinhans R. et al.: Opportunities and Possibilities for the direct use of scrap in aluminum foundries; Fraunhofer IGCV; Livarski Vestnik 71/2024, pp. 193–209. Ljubljana (2024)

[7] Kaufman L.; Bernstein H.: Computer Calculation of Phase Diagrams. Academic Press NY. ISBN 0-12-402050-X (1970)

[8] Siemund A., et al.: Correlation of Thermodynamic Calculations and Mechanical Properties of an Al-Si Cast Alloy, The Minerals, Metals & Materials Society, Trimet Aluminum SE, Orlando (2024)

Source：INTERNATIONAL ALUMINIUM JOURNAL