These HPDC fuel system or battery castings are quite challenging parts. They usually have higher wall thicknesses than conventional structural castings. Despite the higher wall thickness, these castings must be leak-tight under extreme circumstances. The leakage allowance is often measured with Helium at high pressures and has minimal acceptance limits.

The tragic story about that measurement of the leakage rate is that it can only be measured at the end of the production chain. So, all process steps have to be done already. These steps are casting, stamping, heat treatment, straightening, and machining.

Suppose the leakage rate is above the acceptable limit. There is one step additional repair step. The parts can be impregnated in some areas with a polymer called Dichtol. It is a time-consuming manual process. This labor and the polymer are expensive and can eat up the profit margin. 

Still, not all parts can be saved like that. In many areas of the casting, impregnation is not allowed. These parts experience the whole production chain and must return to the melting furnace. A lot of added value and production capacity is lost that way.

Many of the issues come from feeding and shrinkage problems in conventional HPDC. The solidification shrinkage from the liquid to the solid state in aluminum is 6,6%. In HPDC, you need to compensate for the whole 6,6%. In Rheocasting, there are already solid globulites in the slurry. So the shrinkage is lower, so there are fewer shrinkage issues to feed.

In HPDC, a dendritic microstructure is typical. These dendrites introduce microporosity into the microstructure and physically block the shrinkage's feeding. This leads to porosity pathways through the part, which results in high leakage rates.

In Rheocasting, dendrites are broken up during the slurry-making process, and spherical globulites are formed. The mixture of solid and liquid phases with its thixotropic properties has excellent flow behavior. It also helps to prevent these porosity pathways. Therefore, a leak-tight battery casting can be cast with Rheocasting.

See the images of the CT scans from a fuel system part below. You can clearly see the excellent low porosity levels of Rheocasting.

Thank you, Comptech AB, for providing the images!