What's the Impact of the Solid Fraction in Rheocasting?

Rheocasting is often introduced as a special variant of high-pressure die casting, sometimes even sold as the ultimate solution to solve all problems. This impression is misleading. Rheocasting is not magic, not a trick, and certainly not a guarantee for better parts by default. It is a fundamentally different approach to high-pressure die casting, and at the centre of this difference lies one decisive parameter: the solid fraction.

To understand why solid fraction matters, it helps to start with the basics. In conventional casting, molten aluminium is pushed into a die while it is fully liquid. In Rheocasting, the metal is processed in a semi-solid state. This means that, at the moment of casting, the alloy consists of both liquid and solid phases. The proportion of solid phase within this mixture is called the solid fraction.

The Amount of Solid Fraction is Key

For beginners, this point is crucial. Rheocasting does not begin with a machine or a special device; it begins with a defined material condition. If the metal is almost fully liquid, the process behaves like conventional die casting, regardless of what name is used. If the metal contains a significant amount of solid particles, its behaviour changes completely. Flow, filling, solidification, and defect formation are all directly influenced by the solid fraction.

A very low solid fraction, so a solid fraction below 25% means that the melt needs to flow quickly and, therefore, turbulently. Air entrapment, oxide formation, and porosity remain similar to standard die casting. In such cases, the expected benefits of Rheocasting are oversold or don’t appear at all. This is one of the main reasons why Rheocasting is sometimes declared a failure. The process was evaluated, but the defining parameter was never truly adjusted.

As the solid fraction exceeds 35%, the material’s behaviour changes. The semi-solid slurry becomes thixotropic and begins to flow in a laminar fashion. When pouring this slurry into the shot sleeve, it barely flows on its own due to the high viscosity. However, during casting, when the plunger pushes the metal into the narrow cavity, shear forces are generated.

In a thixotropic slurry, viscosity changes with applied shear forces. The higher the shear forces, the better the slurry flows. The plunger, in combination with the cavity, is the perfect shear force generator, allowing excellent filling of thick and thin sections as well as long flow lengths.   

Long Flow Length and great Feedability

Another advantage of Rheocasting is how globulites serve as crystallization points for the melt. As the melt solidifies onto these globulites, latent heat is redistributed back into the remaining melt, like in a pocket warmer. This process ensures that the latent heat is utilized efficiently within the melt, unlike in HPDC, where the solidification process only transfers heat into the steel die.

Consider the analogy of a pocket warmer: when the liquid inside crystallizes, it releases latent heat, keeping your hands warm. Similarly, in Rheocasting, the latent heat released during solidification keeps the remaining melt hot, making it ideal for long flow lengths typical in Gigacastings. Unlike liquid metals, which cool quickly and fail to recombine, Rheocasting keeps the melt hotter, enhancing feeding and reducing defects.

Rheocasting is not a Free Lunch

Too little solid fraction and the process loses its semi-solid character. Too high a solid fraction makes the slurry difficult to process, leading to poor filling and feeding problems. Rheocasting, therefore, ideally operates within a process window between 35 and 45% solid fraction. Within that range, the slurry can be poured into a conventional shot sleeve, yet still behaves thixotropically during casting. More on the process control in next week’s article.

Another common misunderstanding is the belief that existing die designs can simply be reused. As you now know, the semi-solid slurry does not behave like a fully liquid melt. Gates, overflows, and filling strategies that work well in high-pressure die casting may separate the liquid and solid phases in the slurry, thereby reintroducing turbulence. Solid fraction and tool design are therefore inseparably linked.

Additionally, typical HPDC alloys, such as AlSi10MnMg, ADC12, and many other near-eutectic alloys, are not usable in Rheocasting. The solidification window is too small, and the achievable primary alpha phase, which determines the solid fraction, is too low. So, besides the tool adaptations, there also needs to be an alloy change. But that is a bigger advantage than you might think for developing new applications.

Conclusion

For beginners, the most important takeaway is this: a solid fraction is not a secondary detail and is not an optional optimisation parameter. It defines whether Rheocasting is actually taking place or not. If the solid fraction is unknown, uncontrolled, or unsuitable, the process cannot be evaluated fairly, and its potential advantages cannot be realised.

If you have more questions about Rheocasting, you can schedule a Free Consultation Call below this article to get the answers you’re looking for directly.