Excerpt from the August 2015 issue of Foundry Management & Technology by Pascal Côté, Giovanni Pucella, and Steve Midson
The Swirled Enthalpy Equilibration Device (SEED) represents a new possibility for high-volume, high-productivity, thin-walled casting production. Production process advantages:
- Minimal residual porosity
- Increased die-filling speed
- Extended tool life
- Minimal turbulence
Semi-solid metalcasting (SSM) was developed in the 1970s and became a commercial process for thixocasting (billet re-heating) in the 1990s, and for rheocasting (generating the semi-solid slurry directly from the liquid) in the 2000s. However, despite significant technical and economic advantages, as pointed out recently by Jorstad(1), SSM has never advanced to become the major metalcasting process anticipated by so many early investigators. Certainly, there have been a number of obstacles to the commercialization of SSM, such as the original thixocasting process being relatively expensive, early rheocasting processes not being fully competitive, along with improvements made to already-facilitated competing casting processes (squeeze casting, conventional die casting, and high vacuum die casting.)
We agree with Jorstad and suggest that semi-solid casting still has much to offer. Certainly, large-volume buyers of castings, such as automotive companies, are under pressure to reduce casting weight to improve fuel economy and reduce CO2 emissions. So, our purpose here is to re-examine semi-solid casting and review the benefits of the process.
There are many advantages to producing castings in uncoated metal molds, including the production of excellent surface finishes, close dimensional tolerances, and probably most important, fine-scale microstructures (due to rapid heat extraction).
However, the high cooling rates experienced in metal molds also present processing difficulties, as the liquid metal needs to fill the cavity quickly before the onset of solidification. To produce high-quality castings, Campbell(2) suggests that cavity-filling speeds should not be greater than 0.5 m/sec, thereby establishing a limit on the minimum wall thickness and the maximum casting size that can be produced by gravity pouring or low-pressure processes.
In contrast, high-pressure die casting ignores Campbell’s rule on the maximum cavity-filling speed and sprays the liquid metal into the die at extremely high rates (30-50 m/sec). While this allows the die casting process to produce extremely thin-walled castings (as thin as 1-2 mm), the turbulent die-filling process tends to trap large quantities of air, resulting in high levels of detrimental residual porosity in the castings.
Semi-solid metalcasting maintains all the advantages of die casting while eliminating all (or most) of the residual porosity, thereby producing high-quality, thin-walled castings with excellent mechanical properties. Instead of using a fully liquid metal to produce the castings, semi-solid casting uses a high-viscosity feed material that is about 50% solid and 50% liquid. The fluid behavior of semi-solid materials is displayed in Figure 1, which shows the impact of solid content on apparent viscosity. While low solid contents have only a small impact on viscosity, increasing the solid content above 40% or so significantly increases viscosity.