Can Structural Castings be improved?

Structural castings are the home turf of the AlSi10MnMg. This universal alloy has been used for decades with its excellent castability. Most castings, from small brackets to large longitudinal members, are made from this alloy.

The processing is always the same. After melting and casting, the part goes into the heat treatment line. A two-step heat treatment is required to achieve the high ductility necessary for crashworthiness. Alloys close to the eutectic point form many pointy silicon needles during solidification. They need to be rounded in the heat treatment, which requires a minimum of 30 minutes above 440 degrees for a thin-walled casting.

After the solutionizing step, the part is quenched with air or water. During the quenching, the part’s residual stresses deform it. These stresses come from casting, cooling, trimming, or mounting inside the heat treatment rack. Afterwards, this requires complex multi-dimensional straightening.

Latest HPDC Trends

As parts grew bigger and bigger, culminating in Gigacasting, many OEMs started to use lower silicon, naturally ageing alloys, like the AlSi7MnMg. This brings several issues in the HPDC process. In HPDC, the castability is higher, with a smaller solidification range. So, the closer you are to the eutectic point, the better the castability.

That alloy is further away from that point and has a larger solidification interval. That requires longer feeding times through the gate to avoid shrinkage porosity. Another issue is that melt fronts with different temperatures often cannot recombine and leave bi-films inside the casting. You’ll find them with luck in a tensile test and pretty quickly in a leakage test (if that is a requirement, e.g. battery castings)

In addition, the AlSi7MnMg alloy tends to stick and solder onto the die steel at high metal speeds. Thus, it’s not ideal for a large casting that needs to be filled quickly.

The Solution?

In Rheocasting, the silicon content does not define castability as long as the solidification interval is large enough. So, the AlSi10MnMg is not castable in Rheocasting. The AlSi7MnMg works fine. That solves the castability issue. Also, because of the thixotropic properties, you can make the part slower to avoid the soldering problems, especially after adjusting the ingate for the Rheocasting process. Another advantage is that you have a more laminar and consistent cavity filling, which avoids many quality issues of the HPDC part.

The kicker? You can fit the same casting on a smaller machine, reducing operating costs. In addition, due to the lower slurry temperature and the slower filling, erosion is drastically lower. This will be reflected in longer tool lifetimes!

Doesn’t that sound great? All you have to do is buy a slurrymaker, modify the ingate and overflows to the Rheocasting process and adjust the chemical composition within the specification of the AlSi7MnMg. You get a wonderfully quality headache-free casting!

In the Rheocasting Workshop from Casting-Campus GmbH, you can learn more about using Rheocasting to enhance your existing portfolio and access new markets. Schedule your Free Consultation down below to inquire about more details.