Rheocasting thrives with Secondary Alloys

The aluminium content in vehicles is rising from platform to platform, especially in EVs. That makes alloy strategy a major lever for the total carbon footprint of the car. Automotive manufacturers want to increase the share of secondary aluminium, but they still require the same performance from structural castings: high elongation, fatigue resistance, crash reliability, and stable processing. The difficulty is that these requirements have traditionally been tied to narrow alloy specifications with very low impurity limits, especially for iron and copper. That restricts the use of recycled metal. And the consequence is massive. The CO₂ footprint gap between primary and recycled aluminium is enormous. If a vehicle contains 150 to 200 kilograms of aluminium, the question is not whether secondary aluminium is attractive. The real question is how to use it in demanding structural castings without losing the required properties.

Why does HPDC limit higher recycling contents?

In high-pressure die casting, castability and silicon content are tightly linked. When foundries need reliable filling, long flow lengths, and stable production, they tend to stay in alloy ranges with relatively high silicon because silicon improves fluidity.

But that advantage comes at a cost. Higher silicon increases the eutectic fraction in the microstructure, and eutectic phases are comparatively brittle. As the eutectic fraction rises, elongation falls. That creates the core deadlock. Secondary aluminium introduces more impurities, and those impurities also reduce ductility. If the process already “consumes” ductility by relying on a high-silicon, eutectic-rich alloy for castability, there is very little room left to tolerate the impurity burden of recycled metal.

Once silicon is reduced, the alloy moves farther from the eutectic point, the solidification interval widens, and it becomes more challenging to avoid defects. So lower-silicon alloys may promise better elongation, but in liquid HPDC, they are often harder to cast complex structures. That is exactly why the industry gets trapped between two goals: more recycled metal on one side, and castability on the other, which keeps pushing back toward higher silicon.

Rheocasting changes the Castability Equation

Rheocasting changes this at the root because castability no longer depends on the silicon content. In Rheocasting, the castability is defined by the flow behavior of a semi-solid slurry. Under shear, that slurry behaves thixotropically. That means, it becomes highly flowable when shear force is applied. This allows the material to move through the cavity with excellent filling behavior.

That changes the role of silicon completely. In liquid die casting, silicon is needed to secure castability. In Rheocasting, the semi-solid slurry provides castability through its flow behavior. As a result, demanding castings can be produced with much lower silicon content, therefore reducing the brittle eutectic phases.

Lower Silicon means more Elongation

As Rheocasting makes low-silicon alloys castable, the microstructure improves immediately. Lower silicon means less eutectic phase and a higher share of primary alpha aluminium. That gives the casting a more ductile matrix with a much better basis for elongation.

This matters because the extra elongation can be used in two ways. It can be taken directly as a property gain, or it can serve as a reserve to compensate for the ductility loss caused by impurities generated by the secondary aluminium. That is the real sustainability benefit of Rheocasting. It does not remove impurities from scrap, but it creates a much more favorable microstructural starting point for handling them.

Rheocasting opens the door for Secondary Alloys

Rheocasting breaks the traditional link between silicon content and castability. Because castability now comes from thixotropic semi-solid flow, silicon no longer has to carry the burden of making the part fill. That allows the alloy to move into a lower-silicon range. Lower silicon means less eutectic, more alpha phase, and higher elongation. Higher elongation creates the margin needed to tolerate the impurity burden of secondary aluminium.

This is why Rheocasting is more than an alternative casting route. It is a practical way to combine castability, structural performance, and high recycled content in the same part. As aluminium use per vehicle continues to rise, Rheocasting becomes a powerful route to lower-carbon structural castings.

Learn more about alloys and how to utilize their properties in Rheocasting in the Rheocasting Masterclass or schedule your Free Consultation directly below this article.