Solid Fraction Matters in Rheocasting

After the Giga-/Megacasting trend, Rheocasting is the second trend passed around the industry. As for the Megacastings, there is validity to them as they reduce the body-in-white (BIW) production time significantly.

People tell me they tried Rheocasting, and it never worked. And they are probably right. The first slurry preparation method started in the 1970s. Many companies and people tried it and created ways of making the slurry. So, if you talk to most people about Rheocasting, they answer in unison, “It doesn´t work”. They are right in their claim. Yet I showcase working series production castings; how can that be?

It comes down to two things. One controls your slurry preparation process; another is the solid fraction (including a tool change from HPDC).

The most important topic for a production process is its robustness to variations. A Rheocasting process that is temperature-controlled isn´t robust. Each new batch forces you to adjust the process as the temperature window is within a tenth of a degree. The Comptech Rheocasting process is the only one working enthalpy-controlled; therefore, it is not susceptible to liquidus point changes.

The second topic is the solid fraction. Here are four different categories. Everything under 10% solid fraction is basically liquid HPDC casting. Then, there is the next bracket of 10 to 30% solid fraction. It adds some features, like a longer flow length, to the mix. Then, there is the high solid fraction above 50%. It requires heavy modifications to the DCM, producing a lot of microporosity in the eutectic phase. The sweet spot is the 30 to 50% tier. Let me explain why:

It is the sweet spot, as it can be poured with a ladle system. So, there is no modification to the shot unit that is necessary. The eutectic phase is well-distributed and can be fed during solidification. The most significant advantage is that the speed of laminar fill increases with an increasing solid fraction. You can see the effect in the picture. It is taken from the award-winning Rivian Hinge Pillar. The 20% solid fraction produced a turbulent melt front that absorbed air. After a T6 heat treatment, you can see the blisters and scrap the part. As for the 40% solid fraction, the speed of laminar filling is higher. You cannot see any porosity or blisters on the casting.

As you can see, there is more to Rheocasting than just the headline. In HPDC, it doesn´t matter too much which brand you buy. In Rheocasting, knowing which slurry preparation method you use is crucial. So, if you hear someone tell you Rheocasting doesn´t work, ask them what method they tried, what their solid fraction was, and if they modified the tool. You will quickly discover what they did and why it didn´t work.

The Rheocasting Strategy Development is perfect for you when you want to implement Rheocasting properly and expand your product portfolio. You heard correctly: Rheocasting doesn´t replace HPDC. It extends the accessible part spectrum and reaches beyond the limitations of liquid HPDC. Especially in a declining market, Comptech Rheocasting is the way out of the crisis!