Rheocasting Explained

In this article, Rheocasting is explained so you can understand this casting method, which uses a semi-solid slurry of metal rather than fully liquid metal. Imagine you have molten metal, but instead of it being entirely liquid, part of it has already solidified into small, globular particles. You still pour it into the shot sleeve, but this time, you’re dealing with a thixotropic mixture, much like ketchup, that can flow more easily under shear stress and then revert to a thicker state when the stress is removed.

Conventional HPDC uses fully liquid metal, which solidifies rapidly upon contact with the colder die surface. This can create issues, especially with large or complex parts where metal fronts meet before they are completely fluid. Rheocasting, in contrast, introduces partially solidified metal particles into the melt. Solidification on these particles releases latent heat into the surrounding liquid, which helps keep the slurry at a more uniform temperature for a longer period. This improved temperature stability allows for better flow through the cavity, reduces porosity, and can enhance mechanical properties.

A slurry in rheocasting is simply a mix of liquid and solid metal. When the metal begins to solidify inside the melt, globular particles form. Unlike long, dendritic structures, these globules reduce the melt’s viscosity under shear, making it more flowable. Again, think of ketchup: the more you shake it, the easier it flows. But once you stop, it thickens. The same principle applies to semi-solid metal; it becomes easier to fill the die at lower speeds, reducing turbulence and entrapped air.

Critical Overview of Rheocasting Technologies

Over the past five decades, 68 different Rheocasting technologies have been patented. 67 of them use a temperature-controlled cooling process to create the slurry. While this may sound straightforward, controlling the temperature to the exact fraction of a degree needed is incredibly challenging. Even tiny shifts in melt chemistry, such as a small percentage change in silicon, iron, magnesium, manganese, or strontium, alter the melt’s liquidus temperature. Since standard foundry operations see frequent chemistry variations, replicating the same slurry conditions becomes nearly impossible when basing the process solely on temperature control.

The Comptech RheoMetal Process, on the other hand, works by melting an aluminium “ice cube” called an EEM (Enthalpy Exchange Mass). This ice cube is produced from the same melt as the shot weight, ensuring it shares an identical chemistry. Because the melt composition remains consistent from start to finish, there is no need to adjust for slight chemistry fluctuations. As a result, RheoMetal can reliably deliver a homogeneous and consistent slurry without relying on temperature measurements.

Rheocasting Is Cool

Rheocasting slurries start at a lower overall temperature than fully liquid melt. Yet, they maintain that temperature long enough to fill the mould more gently, with reduced turbulence. The secret is again tied to latent heat; as the remaining liquid solidifies onto the existing globules, it gives off energy that warms the surrounding metal. This process helps avoid abrupt temperature drops that can hinder flow.

Lower injection speed, more laminar flow, and better control over the filling process translate into parts with lower porosity and superior mechanical properties. For foundries, this means higher yields and fewer defects throughout a production run, even for extremely large or complex castings.

Follow for more info about the applications the RheoMetal Rheocasting can access and bring profitable new castings to your foundry. Or book your in-house training with the Rheocasting Workshop from Casting-Campus GmbH.