Low Carbon Footprint and more Design Freedom

In transitioning to more sustainable aluminum casting, the choice of alloy is one of the most powerful levers, and Rheocasting opens doors that are limited in conventional high-pressure die-casting. In HPDC, the key factors in castability are the size of the solidification window. This forces an alloy change for near-eutectic or eutectic alloys. Design owners can utilize this to their advantage and tailor their alloys for improved functionality.

The global average carbon footprint of primary aluminum sits around 16 kg CO₂e per kg Al, with “best” primary alloys reaching ~4 kg CO₂e per kg. Many conventional high-performance casting alloys (e.g., AlSi10MnMg) emit significant amounts of CO₂ solely from their silicon content (~9 kg CO₂ per kg Si), which can contribute around 0.9 kg CO₂ per kg of alloy.

One of the key advantages of Rheocasting is that it uncouples the dependence on high silicon content to ensure castability. This enables the use of low-silicon alloys that are from hard to cast to uncastable with HPDC.

Rethink Structural Casting Alloys

The trend in structural castings is to move away from the two-step heat treatment, which causes high deformation in large, thin-walled castings, such as Gigacastings. To achieve this, the eutectic phase must be reduced; therefore, the silicon content must be reduced. That is how the successor to AlSi10MnMg, AlSi7MnMg, was born.

Still, many foundries struggle with the new alloy, as achieving a good castability with the upper limit of silicon content requires a delicate balance. However, reaching the elongation limits after a 5-day natural aging process becomes a challenge, often on a razor-thin margin.

Because Rheocasting’s semi-solid slurry inherently improves feeding and reduces porosity issues, foundries can now lower the silicon levels to the lower end of the specification, providing the casting with more elongation for crash-relevance.

Additionally, with this reduction in silicon content, Rheocasting provides tolerance for higher impurity levels from scrap streams, allowing for a greater inclusion of recycled or secondary aluminum. To fully utilize this potential, the current specifications require wider ranges of these impurities to increase the amount of post-consumer scrap on a large scale.

New Applications with new Alloys

When rethinking the current alloy specification, why not fill the white space between 2 and 7 percent silicon in the AlSi phase diagram with life? Two percent is the upper limit for wrought alloys, and 7 percent is the lower limit of HPDC casting alloys.

In thermal management applications, every disturbance of the aluminum lattice causes a reduction in thermal conductivity. So, using a high-silicon, high-castability alloy limits power-hungry electronics. So, reducing the silicon content is the logical step. However, with the reduction of castability in liquid HPDC, the fin density has to be reduced, and more efficient layouts cannot be cast.

Rheocasting has enabled alloys with low silicon content, such as AlSi2.5Fe, to achieve conductivity comparable to that of pure aluminum (180–195 W/m·K) while maintaining excellent castability. And the kicker is that this AlSi2.5Fe alloy is the alloy you receive from melting down various scrap sources before adding anything to it. That is why such an alloy achieves a carbon footprint of 0.36 kg CO₂ per kg Al.

Conclusion

Rheocasting offers greater alloy design flexibility, enabling lower silicon content and increased use of recycled materials. Additionally, by adjusting the specifications, Rheocasting alloys can achieve functional performance that was previously unattainable. And all of this drives down lifetime carbon emissions per casting.

Therefore, when we aim to achieve carbon neutrality, we must modify certain aspects of our processes. Rheocasting gifts you a giant leap forward. What is stopping you from learning more about what Rheocasting can do for your product spectrum? Sign up for the Rheocasting Masterclass to find out.