High Solid Fractions overcome the limitations of HPDC

You might ask yourself why I should even care about a solid fraction of a new technology when HPDC has been around for decades. The answer is that most foundries won’t care at all. But for your product portfolio, it can be the key to new, profitable business opportunities. HPDC has run into a few limitations.

• Gigacastings require high, crash-relevant mechanical properties after 1.5 to 2.5 meters of flow in the cavity.
  
  
• Due to the EU F-Gas Ban, compressor housings have to contain 130 bars of CO2 operating pressure. That is impossible to achieve in HPDC.
  
  
• Powerful electronics need housings with high thermal conductivities. So, you need to be as close to pure aluminium as possible. In HPDC, the castability drops when you reduce the carbon footprint.
  
  
• High-quality, low-iron, and impurity-free post-consumer scraps are rare and more expensive than primary alloys. HPDC needs low iron alloys to achieve ductile properties, which rules out a lot of sustainability efforts.

For Rheocasting, these limitations do not exist if you are careful when selecting the right slurry-making technology, as there are 87 of them. When following this channel, you already know that only one is enthalpy-controlled, and all the others are temperature-controlled.

The solid fraction has a major impact on the flow behaviour of the slurry in the cavity. As the properties of the slurry are thixotropic, the flow behaviour depends on the applied shear forces. But you need to reach 35% solid fraction or higher to be able to get a laminar filling.

In the picture below, you can see four different solid fractions in the simulation, staged from 0% to 50%. Round obstacles are also implemented in the flow of the slurry to showcase its behaviour during the filling process. Focus on these obstacles when analyzing the different pictures.

There is basically no difference in the filling behaviour between the 0% (liquid HPDC) and 15% solid fractions. You still produce the same filling issues and will never get a leak-tight casting because you entrap air behind the obstacle.

At 35% and higher solid fractions, the flow encloses the obstacle in the slurry (melt) front. This flow pattern allows for a directed laminar flow and outstanding casting quality throughout the casting.

Next week, you will learn why the AlSi10MnMg and the AlSi12Fe are terrible Rheocasting alloys and how the silicon content of the alloy also impacts casting quality. If you cannot wait, schedule a Free Consultation Call to see how Rheocasting can improve and expand your product portfolio!