Do we use the right alloys for Gigacasting?

I remember the discussions at the Euroguss 2020 vividly. Everybody was laughing at Tesla for their idea to cast a whole rear floor in one shot. “If that were possible, we would already do it”, was the prevailing opinion. Tesla didn’t care; they continued to develop it. Then the Euroguss 2022 came around, and the same people were now talking about the terrible casting quality, all the cracks and pores.

Euroguss 2024 showed a totally different picture; the same people smack-talking the Tesla Gigacasting were now going around and trying to collect information on how they managed to do it, as now the whole Chinese EV industry had adopted the technology. The hybris was gone.

Are today’s Gigacastings perfect?

The improvement of these Gigacastings will continue to be a development project for the next 10 to 15 years. The results shown by Tesla and many Chinese OEMs are far beyond what was previously thought possible. Still, the castings are not perfect, even though perfect has different definitions for different OEMs.

The main focus is on the alloy used for the casting, as it is the defining factor for the mechanical properties, the tool lifetime, and the carbon footprint of the structure. The biggest issue is around how to work with gradually deteriorating properties, depending on the flow length. Especially, the elongation tanks at the end of the filling, when the melt is already cold. But in precisely these areas, the most riveting, welding, and glueing operations have to be done. This means the interphases which introduce the load into the casting during a crash are the weakest.

Alloy requirements for Gigacastings

The world of structural castings has been and remains dominated by the AlSi10MnMg. A fantastic alloy that doesn’t care about metal speed changes and solidifies quickly as it is close to the eutectic point. Which brings us to the weak spot of this alloy. It requires a two-step heat treatment to form a high amount of the brittle eutectic phase.

Such a heat treatment leads to massive distortions during the quenching process. For a large thin-walled casting, this is a no-go! So, you now see many large structural castings, not only Gigacastings made out of the AlSi7MnMg, which is basically a silicon-reduced version of the AlSi10MnMg. This reduces the amount of eutectic phase but increases the solidification interval. This results in a natural ageing alloy with way worse castability.

The elongation, in particular, is negatively impacted by a bad casting quality, which leads to a sharp drop in elongation. This effect basically requires all gigacasting foundries to use near-primary alloys, as the impurities from the secondary scrap route reduce the elongation even further. These strict specifications lead to only a minimal amount of external scrap in the used alloys. But when you consider the high amount of aluminium in the new vehicles and the sustainability efforts are limited by the usable alloys.

Higher Quality and more Sustainable Gigacastings

By reducing the silicon content from 7% to 4 or 5%, the eutectic phase is further reduced to leave more room for impurities from the secondary alloys. It also gives you a higher elongation to start with and a larger solidification interval to better feed shrinkages.

The only downside? It is impossible to cast that alloy in conventional liquid high-pressure Die-Casting.

The castability of Rheocasting is not defined by the silicon content, unlike HPDC. Rheocasting works perfectly with these low-silicon alloys, even with 100% post-consumer scrap alloys. The thixotropic filling properties of the slurry allow you to fill laminarly and feed it for a long time. This allows for way more homogeneous properties throughout the casting, especially impacting the end of the filling. The result is a higher elongation, a longer tool lifetime, and a lower carbon footprint. If you’re new to the game, you can even reduce the machine size you buy and reduce your investment amount drastically!

Conclusion

Changing the alloy might seem like a small thing to do, but it can solve many of the problems with large structural castings by just changing the melt processing. The only thing companies need to do is add a new specification that locks out conventional HPDC and specifies a specific Rheocasting process.

The Rheocasting technology is there and ready. So, what is stopping you from reaching your quality and sustainability goals? Let’s start the conversation when you want to utilise the advantages of Rheocasting! Schedule a Free Consultation Call below.