Rheocasting is not comparable to HPDC

Rheocasting offers exceptionally long flow lengths, enabling excellent feedability. This can be used to produce porosity-free, leak-tight castings with outstanding weldability. Thick and thin regions in the same casting can be filled perfectly. Automatically, that triggers the question of why Rheocasting is not in series production everywhere.

Rheocasting in Series Production

Manipulating melts during solidification is nothing new. It was discovered by Prof. Fleming at MIT in the early 1970s. Since then, more than 20 variants have been developed and patented, and 87 remain active.

Then why is Rheocasting so rare to see in series production? The answer lies in its applications. High-pressure die-casting (HPDC) performs well in most applications. Lower porosity, or even higher properties, don’t give an advantage to the application. Additionally, typical HPDC alloys such as AlSi10MnMg and ADC12 are unsuitable for Rheocasting due to their high eutectic content. So, to switch over such a production, an alloy change needs to come first. And all that hassle for no real advantage to the application kills initiatives fast.

We’re now seeing the first applications that require properties beyond what HPDC can deliver. We now have castings with flow lengths exceeding 2 meters that require good crash and weld performance at the end of filling. We’re now seeing compressor housings that require 180 bar helium tests and don’t allow for impregnation. We’re seeing electronic housings that require heat conductivities beyond 175 W/mK.

All these applications require Rheocasting to be achievable and are now running in series production worldwide.

Comparison kills Execution

Rheocasting is, for many, a new technology. Many new technologies in the past never progressed beyond the test phase. So, it needs to be checked what Rheocasting can do. To provide a reference point, an existing tool is used to enable a 1:1 comparison with HPDC. You can infer which process the tool is optimized for.

Rheocasting results vary significantly depending on the preparation process, solid fraction, thermal management, venting, and alloy. There are 4 distinct regions, depending on the solid fraction.

Below 25% of Solid Fraction

Low-solid-fraction processes typically operate at 5-20% solid fraction. Typical examples of processes in this range include GISS (Gas-Induced Superheated Slurry) and Meltec (Aluminium Vacuum Robotic Dosing Unit). These processes typically apply to all alloys that are processable in HPDC. There are also the first signs of semi-solid behaviour, such as a small reduction in porosity and clamping force.

However, the differences to the HPDC process are small. The cavity is still filled with turbulence, and the expected quality level remains the same. In most applications where these processes are installed, conventional HPDC with proper thermal management, spray, and vacuum systems would deliver significantly higher results. Because these processes bear a hidden threat.

Between 25% and 35% of Solid Fraction

This is where Rheocasting shows its ugly face and forces the conclusion that all seems to be just marketing. I call this the messy middle!

As the solid fraction increases, the speed at which you can fill the cavity laminarly increases as well. In this range, there are certain areas where it actually fills laminarly and other areas where it fills turbulently. However, this result is not consistent. As the typical thin HPDC ingate systems squeeze out the slurry like a sponge. The liquid phase travels into the cavity, and the solid phase stays in front of the gate. Which causes severe defects throughout the entire casting.

Also consider that nearly all Rheocasting processes are controlled by temperature. The liquidus point changes caused by normal alloy composition changes in the melt shop are way higher than the temperature difference between a 20 and 40% solid fraction slurry. That is just 0.8°C. So, it is highly likely that many parts on certain days will end up in the messy middle and then later in the melt shop again.

Between 35% and 45% of Solid Fraction

This is the ideal process window for Rheocasting. The slurry is still pourable in a normal shot sleeve. The speed of laminar filling is high enough to fill casting laminarly. The thixotropic properties during the filling allow for excellent feedability and a significant machine size reduction in the range of 20 to 30%. The Comptech RheoMetal process is here the prominent example. It is the only Rheocasting process that is not temperature-controlled; therefore, the slurry remains in the same condition every time.

BUT, these improvements aren’t just given. To achieve outstanding results, modifications to the tool are needed. It starts with the gating and overflow systems. The next steps involve the vacuum and venting systems, and finally the spray process. And lastly, there is the alloy change. An AlSi10MnMg doesn’t work here, but an AlSi7MnMg does. Without these adaptions Rheocasting doesn’t work. And that is why these 1:1 comparisons with HPDC fail 100% of the time.

Above 45% of Solid Fraction

With a higher solid fraction, the laminar fill speed increases further. However, a modification is needed to the shot sleeve and plunger system. Additionally the viscosity of the slurry increases exponentially. This requires larger machine sizes for the same part in HPDC (if it is possible to cast in HPDC). Additionally, recombination of slurry fronts becomes increasingly difficult, and only a small subset of alloys permits such high alpha-phase content.

However, high-solid-fraction processes such as SEED or SAG are not particularly sensitive to temperature variations because the filling behaviour at 40 or 60% solid fraction is relatively similar. Therefore, there are many applications for leak-tight castings in that solid fraction range.

Summary

It is a noble idea to compare high-pressure die-casting and Rheocasting side by side. However, if no tooling and alloy adaptations are needed, the advantages of Rheocasting are too small, or the quality is even worse. To achieve truly outstanding Rheocasting results, adaptations are needed to avoid a 1:1 comparison.

If you want to leverage the advantages of Rheocasting for your product portfolio, the Rheocasting Expert on Demand service guides you from ideation through the adaptation process into series production. Schedule your Free Consultation Call down below to start the conversation.