Leak-free Battery Castings

The EU wants to ban all combustion-engine cars starting in 2035, strengthening the trend towards fully electric vehicles. Battery technologies are constantly developing. Still, batteries are large and integral to the car body structure. Many of these large batteries, have cast aluminium parts, currently made in HPDC.

These HPDC battery castings are quite challenging parts. They are large in size and usually have higher wall thicknesses than conventional structural castings. Despite the higher wall thickness, these castings need to be leak-tight. The allowance for leakage is often measured with helium 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, machining.

If the leakage rate is above the acceptable limit, there is one additional repair step. In some areas, the parts can be impregnated with a polymer called Dichtol. It is a time-consuming, manual process. This labor and the polymer are expensive and can eat up the whole 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 are lost that way.

A lot of the issues come from feeding and shrinkage problems in conventional HPDC. The solidification shrinkage from the liquid to the solid state in aluminium 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 common. These dendrites introduce microporosity into the microstructure and also physically block the feeding of the shrinkage. This leads to porosity pathways through the part, which results in high leakage rates.

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

See the images of the CT scans below. The bottom picture clearly shows the improvements of Rheocasting over the HPDC one.

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