Process Control matters in Rheocasting

Process stability is the backbone of any successful casting method. The whole foundry gets tracked by its OEE. Despite numerous Rheocasting methods patented over the past 30 years, most have struggled to move beyond experimental labs due to inherent instability in process control. Today, only two significant technologies remain: GISS and RheoMetal. The difference between their effectiveness lies in how each method controls slurry characteristics.

Challenges with Temperature-Controlled Rheocasting

GISS technologies employ temperature-controlled slurry preparation by bubbling gas into the liquid melt. While seemingly intuitive, temperature control faces a fundamental problem: precision. Why is precision so critical?

Slurries, a mixture of solid and liquid phases of metal, depend drastically on temperature. The difference between 20 and 40% of the solid fraction is as little as 0.8 Kelvin. Imagine the complexity of consistently cooling a batch weighing between 5 and 50 kg to within one-tenth of a degree! This accuracy level is beyond the capabilities of even the most sophisticated control systems, making slurry consistency challenging to achieve.

Furthermore, temperature-controlled methods are particularly vulnerable to variations in melt chemistry. Elements like silicon, iron, magnesium, manganese, and strontium significantly influence liquidus temperatures. Small chemical variations are inevitable in industrial foundries, leading to constant slurry parameter adjustments. Such a system is not feasible for series production, resulting in part quality fluctuations that no foundry can sustainably tolerate. And many have learned that the hard way. The tragic point is that many think that is evidence that Rheocasting does not work. It is the opposite. Rheocasting works well if you use the Rheometal process.

The Advantage of Enthalpy-Controlled Rheocasting

RheoMetal’s approach stands out because it uses an enthalpy-controlled process. Unlike temperature-controlled methods, RheoMetal employs an aluminium “ice cube” known as Enthalpy Exchange Mass (EEM). This EEM is produced from the same melt used for casting, ensuring identical chemistry and solidification properties. Consequently, typical chemical variations occurring in foundries do not affect slurry consistency. This enthalpy-controlled method results in stable slurry production and consistent casting quality.

Apart from slurry preparation methods, solid fraction, the percentage of solid particles within the slurry, is another crucial process parameter. Solid fractions of 10 to 20% hardly differ in deployment from conventional HPDC. You get the benefit of having a cooler melt, but you don’t need a Rheocasting system for that. However, very high fractions (above 50%) require extensive tool modifications, and the high eutectic content produces microporosity, undermining part quality.

The optimal solid fraction for robust Rheocasting lies between 35% and 45%. This range enables straightforward ladle pouring without necessitating tool modification. It ensures a uniformly distributed eutectic phase, which significantly reduces porosity. Still, it allows for high enough laminar filling speeds, minimizing air entrapment and subsequent casting defects.

Interested in mastering Rheocasting and enhancing your foundry’s capabilities? Take a look at the Rheocasting Workshop from Casting-Campus GmbH. Schedule a Free Consultation Call down below to inquire about more insights.