Rheocasting slurries behave differently...

… and we can use that to our advantage. The image above shows the comparison. On the left side, a perpendicular rip is filled in traditional HPDC with a liquid melt. On the right side, the semi-solid slurry fills the rip.

The images are from the 2022 NADCA paper by Martin Hartlieb, Dr. Per Jansson, Jean-Claude Tawil, and S. Bergeron. They also gave an excellent explanation that I couldn’t phrase better. That’s why I’m citing their text below:

Prior work in both Thixocasting and Rheocasting had indicated that vacuum is much less important when semi-solid metal is injected into a die than when liquid metal is injected. The explanation seems quite plausible: the laminar flow of the melt will allow the metal front to simply “push the air out” of the cavity and will not entrap any air during the filling process. While this is certainly true (in theory), a different phenomenon is observed with semi-solid metal flow (compared to liquid metal):

Liquid metal, when injected at high speed into a cavity, will fill every angle and every corner of the cavity very quickly and will freely move (and splash) in all directions. A semi-solid slurry however will not do that! It has the tendency to always simply “go straight” in the cavity, which can mean that any perpendicular feature might initially be “ignored” by the metal front and only filled later once the metal front experiences resistance on its straightforward path.

Any such feature (e.g. a rib perpendicular to the main casting body) will, therefore, only be filled completely when the metal has already blocked the air evacuation path. Then the remaining air in this feature (rib) will be compressed, which will likely lead to porosity or surface defects/incomplete fill. This would destroy the tremendous advantage of Rhoecasting for such structural castings to fill even very thin and tall ribs that in liquid die casting could not fill. While in liquid die casting, single droplets of aluminium can enter a thin rib and quickly solidify, and then preventing the rest of the metal front to fill the rib, the laminar metal front of Rheocating with its globular microstructure will easily fill even thin and tall ribs quite easily (if no air is building up counter-pressure).

This was one paragraph of this great paper with lot`s of insights into Rheocasting. It also shows a series production Rheocasting part for Rivian. Schedule a free consultation below to learn more about what Rheocasting can do for your foundry.