Recycled cast alloy for aluminium wheels

So far, the use of recycled materials in aluminium wheels has been limited in order to meet these strict requirements, with stringent restrictions on alloy components and their quantities (e.g. iron, copper). With this in mind, AMAG and AUDI engaged in a multi-year collaborative project to develop an AlSiMg alloy that

• makes it possible to use up to 70% recycled materials,
• fulfills strict technical and corrosionrelated requirements,
• uses heavily contaminated post-consumer scrap and reduces the material’s carbon footprint.

Extensive testing of the new alloy’s strength, corrosion behavior, castability and long-term stress resistance was conducted on the road to reach series maturity. While the alloy satisfied all specified requirements, the project partners were especially cautious regarding corrosion requirements. After the recycled cast alloy developed by AMAG and AUDI and cast by the RONAL GROUP was used to produce coated wheels, it has now satisfied the high requirements of machinepolished wheels (Figure 1). This significant milestone shows that the favorable properties of primary alloys can also be achieved with recycled alloys. The new alloy class opens up new possibilities for the use of recycled alloys and fulfillment of sustainability requirements without the need to compromise on any key properties.

Following changes to the legal framework, customers are now more open in discussing recycling-friendly alloy design. They are prepared to consider new developments and scrutinize old specifications to ascertain whether long-standing requirements actually add value - or belong to an outdated, historical context. From AMAG’s perspective, this is the right path to sustainable, resource-friendly production of automotive components. In the automotive industry, demand for CO₂-reduced aluminium components has increased significantly in recent years. Quite apart from the high mechanical requirements, aluminium wheels are subject to further requirements in relation to their corrosion resistance, visual appearance and ability to accommodate sophisticated designs. AMAG has developed deep expertise in material science and value-adding recycling over the course of decades. AMAG is ideally equipped to meet these customer requirements by combining the methods available to process mixed scrap with sensor-based sorting systems (e.g. X-ray transmission, LIBS, etc.). Following years of development, AMAG successfully developed an AlSiMg alloy with an extended tolerance range, which ultimately may contain over 70% recycled material. This has also enabled AMAG to markedly expand its input material portfolio for scrap.

With AUDI as the OEM and RONAL GROUP as the wheel manufacturer, the successful completion of the alloy development project for coated wheels, which focused on visible safety-related components, demonstrated that wheels made from a high proportion of recycled materials can satisfy existing specifications by adjusting the alloy’s chemical composition and production processes. These wheels were coated, in part to serve as winter wheels, and were mass-marketed. A fully coated (black) wheel was launched initially to gain initial real-world experience. If the material proved itself in this context, the plan was to advance to a machine-polished wheel - a real acid test of corrosion requirements, given the increased visibility and customer attention on these products. The alloy’s composition was therefore further adapted for machine-polished wheels, in particular to guarantee even better corrosion performance. Above all, this relates to common contaminating elements in scrap, such as Fe, Cu and Zn. However, Mn, Cr, Ni and other elements were also examined to determine their influence on the visual results of comprehensive corrosion testing. For example, the addition of Mn can reduce the formation of plate-like α-AlFeSi phases or convert it into less critical, block-like ß-AlFeSi phases. However, further addition of Mn increases the proportion of intermetallic phases that make the matrix more brittle and impair the material’s ductility.

Similar correlations occur with Mg, Zn and Cu. For example, copper can form Al₂Cu, a phase with a particularly high potential difference to the aluminium matrix, which results in a high corrosion potential. However, when kept in the right ratio to other elements, it is broadly possible to suppress this phase and convert the resulting main phase. The same goes for the AlZn₂ phase. Other phases that (can) occur, such as the Q/Q’ phase, the T phase and the ɳ/ɳ’ phase, have a far lower potential difference to the aluminium base matrix and therefore significantly reduce corrosive attack. It is, therefore, important to achieve the correct ratio of Mg, Cu and Zn to minimize corrosion as far as possible. The aging behavior of Al-Si-Mg alloys with Cu and Zn can often be highly complex due to the emergence of numerous intermediate phases. Alloy composition must therefore be geared to the classic T6 heat treatment in the wheel production process and adapted as required.

Scanning Kelvin probe force microscopy (SKPFM) can provide indications of which phase mix for a given composition has the lowest corrosion potential. This is shown in Figures 2 and 3, with a primary AlSi7Mg used as a reference (REF) material. Potential mapping with SKPFM square fields measuring 90 x 90 qm² are scanned within the image areas captured with the SEM. The tapping mode is used with a scanning rate of 0.5 Hz and a scan angle of 90°. This produces a topographical image and provides potential mapping. The most important parameter derived from the current density/potential curve to highlight a difference in the examined samples’ corrosion behavior is the width of the passive area. It is calculated as the difference between the open corrosion potential (Ecorr) and the pit potential (Epit). This is followed by the pit potential (Epit) itself, the repassivation potential (Epassiv) and the corrosion current (Icorr). The solutions applied also influence the result. Figures 2 and 3 show the influence of the solution when other parameters remain constant.

This example shows that there is not a single most effective solution: instead, all influencing factors must be considered to achieve an optimal result that is satisfactory overall. After finding the best tolerance interval of an AlSi7Rec alloy for machine-polished alloys, this alloy was cast with AlSi7Mg under real conditions in an established series wheel geometry. It was rigorously tested and the results were compared against reference figures. We are pleased to report that AlSi7Rec has also passed the tests for machine-polished wheels. As a result, AMAG is now able to offer an innovative and sustainable aluminium cast alloy, ensuring that aluminium wheels satisfy both current and future requirements while meeting stringent sustainability requirements.

This expertise in alloy composition and subsequent performance is AMAG’s USP, because the company has built up its experience over decades as both a supplier and alloy developer. As a result, AMAG is ideally positioned to accommodate wideranging customer requirements and contribute to its customers’ success. AMAG ticks all the boxes:

• Over 40 years’ experience in aluminium recycling; long-term closed-loop partnerships for cast alloys
• State-of-the-art smelting and casting technology
• State-of-the-art sorting and separation technologies for aluminium scrap
• Center for Material Innovation (CMI) with labs featuring ultra-modern equipment for microscopic, mechanical, surface-related, chemical and environmental testing
• Extensive network of renowned universities and research institutions
• Continuous research (dissertations, projects, incl. in partnership with customers) into recycling and recycled alloys
• Support with the production process and required logistics related to scrap

Of course, AMAG is also happy to offer an all-in-one solution that includes taking back scrap materials. After all, aluminium recycling makes it possible to close material cycles in industrial production processes. AMAG works closely with its customers to develop efficiently structured and standardized processes.

The aim is to keep aluminium products in the material cycle to preserve their value and make full use of aluminium’s excellent recyclability.