Implementation of Metal Casting Best Practices - EERE - U.S. ...

V-6 blocks, the gating system was changed from a bottom pouring to a top pouring arrangement.
This reduced leaks by about nine times from the previous level. Aluminum Casting Facility-1
attributes this success to the efforts of the Lost Foam Consortium, Flow Simulations, and UAB,
which were cost shared by ITP.
Semi-Solid Metalcasting
Aluminum Casting Facility-1 also has a large high-pressure die casting (HPDC) process that has
benefited from the research on semi-solid metalcasting (SSM) performed by the Worcester
Polytechnic Institute (WPI) with funding from ITP. For this facility, the time required to make a
casting, or “cycle time,” is the cost driver in its high-pressure die casting operation. As noted by
this facility, the cycle time is a function of part size and weight. For example, for large parts,
water channels are incorporated into the die steel to facilitate faster cooling and shorter cycle
times. Very large parts, however, require a higher cycle time to allow the heat of fusion to be
released, even with water cooling channels in the die. According to Aluminum Casting Facility-1,
the SSM casting process developed by WPI is a deviation from the popular HPDC process and
can reduce cycle time.
The SSM casting process takes advantage of the thixotropic behavior of semi-solid aluminum
alloy. This means that the metal can be handled like a solid when at rest, but flows like a liquid
similar to toothpaste or gelatin when force is applied. The billets can be transferred easily to the
shot sleeve of a die-casting machine with a robotic arm. The plunger of the die-casting machine
can then impose a high rate of shear that will cause the semi-solid alloy to flow like a liquid,
filling the mold smoothly without entrapping gas. In the HPDC process, molten metal sprays into
the die, which can lead to entrapped gases and high porosity levels in the casting. Because of the
semi-solid state of the aluminum in the SSM process and the lower temperatures it employs in
the casting, shrinkage is minimized and less energy is consumed in heating the billets as
compared to heating the metal to a liquid state. 62
Developed in the 1970s at MIT and termed rheocasting, the SSM process did not immediately
catch on because heating the specialized billets put a size constraint on the billet to avoid sagging.
This meant that the cast part could not be much larger than one pound. At this small size, cycle
time reductions were difficult to achieve and casting costs were significantly greater than for
HPDC. Although SSM offered superior mechanical properties, that benefit was not great enough
to compensate for the size constraint.
According to Aluminum Casting Facility-1, this changed when new processes were developed to
form billets that were not restricted by size or weight limitations. These new methods formed the
50% solid and 50% liquid SSM billet on cooling rather than the traditional 18-stage induction
heating process. The new processes developed billets through magnetic stirring, insertion of cold
fingers, grain refinement, mixing of liquids, or other means, all of which produced copious
amounts of nucleation just below the liquid state, which is referred to as a “slurry on demand”
(SOD) processes.
Aluminum Casting Facility-1 began to work with WPI to gain insight into the SSM process and
to explore the possibilities of implementing it in its production. The first element the facility put
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