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

location of shrinkage that had been an on-going and significant problem at the plant. Because of
this successful working relationship, Aluminum Casting Facility-1 can now predict the location
of shrinkage and has begun validating the model with other castings.
The suppression of hydrogen porosity has led to a shrinkage value that is a full percentage higher
than of alloys that solidify at less than one atmosphere of pressure. For example, if the normal
shrinkage of an alloy is expected to be 6%, when solidification occurs under 10 atmospheres of
pressure, 7% shrinkage is expected. Because of this, the higher shrinkage value and the critical
feeding distance for the alloy are major considerations when designing the casting for
pressurized lost foam casting. This process may require that the gates stay open longer, which
may only necessitate enlarging the gate or may require a major design change to the interior of
the casting.
According to Aluminum Casting Facility-1, the feeding failure could result in interior porosity in
several ways. First, hydrostatic tensile failure in the interior of the casting can occur because of
unclean metal that contains oxides or inclusions. Another defect may be surface-connected
porosity due to a puncture of the skin, which can result from low silicon content in the alloy.
Another cause may be a distortion of the shell or casting wall due to high hydrostatic tension If
10 atmospheres of isostatic pressure is applied during solidification to lower the hydrostatic
tension in the solidifying liquid, none of these failures are likely. This is because pressurized lost
foam lowers the hydrostatic tension in the solidifying liquid and thus can avoid exceeding the
critical hydrostatic tension value.
Application of modeling tools is necessary to fully capitalize on pressurized lost foam casting.
According to personnel at Aluminum Casting Facility-1, it is extremely important that the
“filling” event be modeled to define the “cooling curve” at various critical locations throughout
the casting and identify and eliminate potential hot spots. In that regard, Aluminum Casting
Facility-1 worked with UAB and Flow Science to validate Flow 3D Model.
Aluminum Casting Facility-1 had a 60-pound aluminum casting of a 3.0L V-6 lost foam block
that had a high level of porosity between the cylinder bores that caused over 50% of the blocks
produced to leak. Several people attempted to model this casting in order to predict the porosity
failure, yet none were successful. Aluminum Casting Facility-1 worked with UAB and Flow
Science to validate Flow 3D on the 3.0L block to predict the location of shrinkage and convince
the casting community and the facility’s engineering department that modeling in lost foam had a
future. In this investigation, the assessment team determined that the A356 parameters in the
program were grossly wrong and needed revision. After that revision, the model accurately
predicted the location of the shrinkage and porosity. This was a major accomplishment for
Aluminum Casting Facility-1 because had been attempting such prediction for over five years.
Because of this accomplishment, the engineering department at Aluminum Casting Facility-1 has
taken over the responsibility for modeling castings for design correctness and for proper gating
system using the Flow 3D model. For heads gated into the face of the engine head, modeling
indicates that 22 gates freeze off early and solidification is uncontrolled from the bottom up.
However, for the engine heads gated into the six spark plug holder, the gates stay open and
solidification is horizontally controlled from the face back to the sprue. In the case of the 3.0 L
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