What's the Difference between GISS and Rheometal?

When talking to people about Semi-Solid casting, it feels like the wild west. Everybody has a story or opinion about why it is the future or why it will never work. But it’s just a lot of talking and not much experience in these discussions. That is why I want to describe the two most common Rheocasting methods. These are GISS and Rheometal. To be as neutral as possible, I let ChatGPT write the description of these two processes. The explanations afterwards are again based on the knowledge and experience of working with Rheocasting daily.

GISS – Process

GISS (Gas-Induced Superheated Slurry) prepares the metal by dipping a graphite diffusor and bubbling inert gas into the ladle to create a superheated slurry with a small, controlled solid fraction (5 – 15%). The nuclei in the solid fraction claim to calm the flow, cut turbulence, and feed faster, more uniform solidification. According to Gissco, porosity and distortion drop while pressure-tightness and thin-wall capability rise. Usually, the melt temp is reduced from around 670 to 620 °C. The same is true for the gate speed. GISSCO further claims broader alloy flexibility, less die soldering/erosion, longer die life, and lower scrap, i.e., quality gains and lower cost without redesigning existing tooling.

Rheometal – Process

The Comptech RheoMetal makes a slurry by casting a solid EEM (Enthalpy Exchange Material) from the same alloy and stirring it into the ladle; the EEM absorbs heat and melts, producing a high-solid-fraction slurry (30–45%). This process is mass/enthalpy-controlled rather than temperature-critical, which improves repeatability. The thixotropic slurry fills more laminarily, cutting entrainment/shrinkage porosity; pressure-tight parts often need no impregnation. Reported capabilities include very thin sections and tall fins, plus higher thermal conductivity up to ~200 W/m·K. It’s an HPDC melt-preparation add-on, typically enabling smaller presses and marketed tool-life gains.

Where’s the difference?

On first glance, just bubbling gas in the melt and getting new properties sounds great. Too great. There are several issues in this process, which make GISS a headache in series production. First of all, the diffusers are finicky. The graphite wets, clogs with aluminium, and erodes. So, the flow of bubbles is never consistent, and a replacement diffuser comes with a hefty price tag. At the same time, the whole method hinges on hitting a razor-thin temperature window (around 0.1 Kelvin) while your melt chemistry and, therefore, the liquidus temperature, keep shifting with Si, Fe, Mg, Mn, Sr, etc. In practice, you’re trying to cool 5–50 kg of metal evenly within about a tenth of a degree and hold it there shot after shot. That’s not realistic, so your solid fraction deviates or isn’t even there.

The difference in flow behaviour of liquid metal and 15 to 20% solid fraction is the same in a casting. The laminar filling speed at these solid-fraction slurries is so low that it would solidify before it hits the gate. One thing is true: the solid fraction that is in the slurry reduces the solidification shrinkage in the part. That can be the difference between passing and not passing of a casting lot. However, you need to check every part, as you cannot rely on the process stability.

To achieve the desired calm, laminar fill, you need to maintain a level of 35% solid fraction or higher, and reduce the metal speeds. That is not feasible with the GISS process. That’s why the enthalpy-controlled RheoMetal route is production-proof. You drop in an EEM (ice cube) that is made from the same alloy. So, chemistry and heat equilibrium are baked in. The result is a stable, higher solid fraction without chasing tenths of a degree or babysitting a fragile diffuser. Fewer surprises at the press, more consistent tightness and microstructure, less rework, and a process that actually scales to profitable series production.

I have already worked on several projects, where the foundry or OEM tried GISS first because of the simplicity of the description and bogus marketing claims. The result was that the GISS system became an ornament in storage. After the Comptech Slurrymaker was installed, the tool was adjusted, and suddenly, the promised properties could be achieved and delivered successfully to the customer.

Conclusion

A seemingly simple process description leads to a massively complex and uncontrollable process with the GISS. The prerequisites and the process explanation for the Comptech Rheometal process are more complicated. But at the end, only this process delivers high-quality parts consistently.

This is why I built the Rheocasting Masterclass to teach the ins and outs of the Rheocasting process. Learn how Rheocasting works and how to utilise it profitably. Secure your spot in the Rheocasting Masterclass today!