How does Rheocasting make Gigacasting more Efficient?

The question of whether gigacastings are viable for body-in-white constructions has long died down. But as we heard at the China Diecasting, a new challenge has emerged: achieving consistent quality at an acceptable cost.

Every rejected Gigacasting is expensive. Solving quality issues by simply increasing machine size and metal pressure further increases investment and operating costs. Rheocasting with a 30–40% solid fraction offers another approach by expanding the process window and improving castability.

In most structural castings, switching to Rheocasting isn’t easy, as the alloy needs to be changed as well. But for gigacastings, this is simpler, as most of these castings already use an AlSi7-type alloy. So, they already have large enough solidification windows to stay within the same specification.

Long Flow Length means Longer Feeding

One of the most interesting properties of Rheocasting is its long flow length. This is particularly important for Gigacastings, where aluminium travels more than two meters from the gate.

The high solid fraction reduces heat transfer between the slurry and the die. Although the slurry enters the cavity at a lower temperature, it loses its remaining thermal energy more slowly. The surface area of the globilites is far greater than the tool surface. Therefore, most solidification happens on top of these globules, and the latent heat goes back into the remaining melt. It is like casting a pocket warmer.

But reaching the end of the cavity is not the real advantage. Conventional high pressure die-casting can fill the cavity. The real advantage is that the remaining liquid stays available to feed sections farther from the gate, helping mitigate shrinkage porosity. If the slurry enters the die with around 35% solid fraction, approx. 50% solidification shrinkage has already occurred during the slurry-making process. Therefore, less liquid metal remains to shrink inside the casting, and the rest can be fed from the plunger more effectively. And this is exactly what large battery structures need.

A Larger Process Window for Leak-Tight Battery Cases

Starting at 30% of solid fraction, the slurry also develops sufficient internal strength to stabilize the filling front. This stable melt front allows laminar fill of the cavity, and oxides as well as residue on the tool surface get pushed out of the casting into the overflow system. To utilize this effect, changes in the gating and overflow systems are necessary. The ideal gate in Rheocasting has no impact on the slurry flow. In HPDC, a gate gets thinner than the part to spray the liquid metal into the tool. In Rheocasting, the same tool separates solid from liquid particles within the slurry and hinders hot-spot feeding as the gate freezes first.

But changing the gate geometry is particularly valuable for battery cases. A two-meter-long casting will be completely filled in HPDC and still fail a leak test because of shrinkage or interconnected porosity. Rheocasting cannot guarantee leak tightness on its own. Vacuum, venting, overflows, melt quality, and homogeneous slurry preparation remain a key part of the process. But Rheocasting reduces the dependence on high metal pressure to compensate for an increasingly difficult feeding situation.

Lower Pressure means Smaller Machine

This is where the technical advantage becomes an economic one. The locking force of a die-casting machine depends directly on projected area and metal pressure. Gigacastings already have enormous projected areas, so high intensification pressures quickly drive machine sizes upward. 

Rheocasting provides sufficient filling and feeding at lower metal pressures, so the required locking force decreases automatically. Depending on geometry and alloy, it can produce castings on machines significantly smaller than those required for conventional HPDC. But instead of switching machines, for gigacasting Rheocastings offers a more interesting possibility: keep the same machine and cast a larger part.

A battery case or structural casting that would otherwise require the next machine size could now be produced within the existing cell. That avoids not only a more expensive casting machine, but also one that would be underutilized just like the one you already have. This saves you CAPEX and brings OEE to an acceptable level. Lower pressures and lower casting temperatures reduce thermal loading on the die and extend the tool lifetime. This further reduces operating costs.

Crash Performance depends on Elongation

So far, the conversation has focused only on the production side of the gigacastings. However, the casting has to prove its abilities in a crash. The structure has to take as much load as possible to reduce impact on passengers. Oftentimes the furthest section from the gate is the first touch point in a collision. And as usual, the worst properties are to be found on the furthest end of the filling. Elongation drops are not uncommon there, and, therefore, elongation is a great validator of casting quality.

Better feedability in Rheocasting increases elongation directly, but this is often not enough. Most gigacastings use the upper end of their silicon range, between 7.5 and 8.5%, to ensure good castability. But a higher amount of the brittle eutectic phase limits the achievable elongation and the possibility of incorporating secondary alloys.

In Rheocasting, the castability isn’t defined by the silicon content. Reducing the silicon content to 4 or 5% enables higher elongation and scrap content with minimal impact on strength. It also saves a few percent in alloy costs and allows for a lower carbon footprint. This small change in the alloy multiplies all the previous effects, resulting in high-quality castings and a smooth-running car production line.

Gigacasting needs Efficiency, Not Just Size

The next step in Gigacasting should therefore not simply be another increase in machine size. 

Rheocasting with 30–40% solid fraction offers the possibility of widening the process window. A long flow length keeps the remaining liquid available to feed further into the casting, while the solid fraction reduces the shrinkage that still needs to be fed. Low silicon alloys increase elongation and crash performance.

The result can be more consistent quality and leak-tight battery structures at lower metal pressures. This means something very tangible for Gigacasting: larger castings on the same machine, lower investment, and lower operating costs; exactly what foundries need!