Rheocasting for Gigacastings

In the article about the Hot Tearing in Gigacastings, I mentioned that these issues form after long flow lengths. Gigacastings, like Tesla and Volvo Cars are casting, feature long flow lengths above 2 meters. The major issue is the feeding behaviour at the end of the filling. Then, with the shrinkage from liquid to solid, the casting cracks.

Switching from HPDC to Rheocasting is an excellent way to improve the quality of the parts and the hot tearing issue. I’ll explain it in the following paragraphs and in the picture above.

1. In HPDC, a dendritic microstructure is common. Especially at long flow lengths, the dendrites block some of the feeding potential. The longer the flow length, the more critical this effect becomes. The results are the hot tears mentioned.
   During the slurry-making process, dendrites are broken up, and spherical globulites are formed. The mixture of solid and liquid phases, with their thixotropic properties, has excellent flow behavior.
   
   
2. Another advantage of Rheocasting is that the globulites inside the slurry are a perfect crystallization point for the melt to solidify on. With every bit of melt solidifying onto the globulites, the latent heat is distributed back into the melt. The slurry basically becomes a pocket warmer.
   In HPDC, the dendrites grow in size, and solidification moves from the surface to the core. Thus, the heat is transferred into the steel die instead of the melt.
   
   
3. The solidification shrinkage in aluminium from the liquid to the solid state 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, and therefore, there are fewer shrinkage issues to feed.
   
   

But be aware: There are two mechanisms for making a slurry for recasting. One method is thermally controlled, like GISS or SEED. That one does not work for a series production. The 60 to 120 kg of melt has to be cooled homogeneously within seconds to a tenth of a degree. This has also been adjusted with every new batch of metal in the dosing furnace.

The other version is mass-controlled. That is the Comptech RheoMetal process. The final slurry temperature and the chemical composition variations do not matter for reaching the same slurry quality every time. This is achieved by casting aluminium ice cubes, also called enthalpy exchange mass (EEM), that are stirred in the melt. The energy needed for melting the EEM prepares the slurry to the perfect condition every time.

Do you want to utilize the Rheocasting for your product portfolio? The Rheocasting Expert on Demand is the perfect solution for you. Schedule a free consultation to get more information tailored to your situation.