Semi-solid casting is a near-net shape casting process that is capable of producing extremely high-quality castings. It differs from all other casting processes as it does not use fully liquid metal to produce the castings, instead using a feed material that is preferably about 50% solid and 50% liquid. The high solid fraction of the feed material results in a slurry that acts like a highly viscous liquid, and while a semi-solid slug is rigid enough to maintain a cylindrical shape, it is still soft enough to be cut by a spatula.
Semi-solid casting is a modified die casting process that uses a high-pressure die casting machine to inject this viscous semi-solid slurry into a hardened, reusable steel die. Semi-solid casting maintains many of the advantages of conventional die casting such as the production of complex, near-net shape components with thin walls, cosmetic surfaces, and close dimensional tolerances. However, while conventional die castings often suffer from high levels of residual porosity (up to 1% porosity, or more) that limits their usage in structural applications, the high viscous feed material used with semi-solid casting provides controlled filling of the die cavity that essentially eliminates residual porosity and other defects. Semi-solid castings, therefore, can have exceptionally high component integrity, which provides excellent mechanical properties and allows them to be used in structural, pressure-tight, and welded applications.
Numerous methods have been proposed for the production of semi-solid castings, but many of these processes treat the liquid metal in a turbulent manner that can introduce detrimental oxides and entrapped gasses during the production of the castings. To produce high-quality components capable of meeting the quality requirements for safety-critical and pressure-tight applications, it is crucial that such defects be avoided, and so a semi-solid casting process must be used that treats the metal gently. The SEED (Swirled Enthalpy Equilibrium Device) process was designed to prevent the formation of defects that can be pervasive in other casting processes.
In the first step of the SEED process, a tilted crucible is gently filled with fully liquid aluminum. In the second step, the crucible is gently swirled to uniformly distribute the solid particles. Within a minute or so, the alloy has cooled to the appropriate solid fraction, and the crucible is transferred to a high-pressure die casting machine, where the slug is ejected from the crucible into the shot sleeve. Finally, the slug is injected into the die using a controlled injection process.
Once the die cavity is filled, the plunger continues to apply high pressure throughout the solidification process, so that solidification shrinkage can be completely eliminated. A commercial SEED system contains several crucible systems, to match the high production rate achievable with the semi-solid casting machine.
Performance and applications
Semi-solid casting utilizes many of the same alloys as conventional casting processes, including A356 (Al-7%Si-0.35%Mg) and 357 (Al-7%Si-0.55%Mg). Typical mechanical properties of components semi-solid cast from these alloys are listed in Table 1. Of special interest are the extremely high elongation values achievable with semi-solid casting, which is due to the low levels of defects such as porosity and oxide inclusions.
Semi-solid castings also exhibit extremely good fatigue properties, primarily as a consequence of their low defect (porosity and oxide) content. Data measured by Cummins comparing the fatigue behavior of semi-solid cast 319S-T6 against both wrought and cast alloys reveals that not only is fatigue strength of the semi-solid cast 319S significantly better than other casting alloys, it is comparable to components produced by forging. A turbocharger impeller is a good example of a complex geometry part used in a fatigue application.
As semi-solid castings contain extremely low levels of residual porosity, they can also be used in applications that require high levels of pressure tightness—5000 psi (345 bar) or higher. Examples of semi-solid castings used in pressure-tight applications include fuel rails and master brake cylinders.
Conventional high-pressure die casting is generally considered the lowest-cost method for producing complex, 3D shaped aluminum components. Due to the similarity of the process equipment, it is expected that semi-solid casting should be capable of similar low-cost production. This is confirmed by recently published data, which presented a detailed comparison of the manufacturing costs associated with several casting processes (Table 2). The results of this analysis confirm that semi-solid casting is only slightly more expensive than conventional die casting, but significantly lower cost compared with competing foundry processes.
As described above, semi-solid aluminum castings have been used in a range of applications and markets. Commercial components range in size from a few ounces to as large as 20 lb (9.0 kg) or more, and can be used in both automotive and non-automotive applications.
Pascal Côté of Québec-based STAS, Steve Midson of Denver-based The Midson Group, and Giovanni Pucella of STAS wrote this article for SAE Magazines.