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Flow and Solidification Studies |
Fundamental studies of
molten metal flow in permanent molds have been conducted
as part of a recent DOE/ AFS project. The experimental
data is used to verify flow and solidification computer
simulation models. Different gating methods employed in
vertical permanent molds and the effect of filters on
the flow have been evaluated. Both real-time X-Ray
studies with molten metal and water simulations have
been employed.
A project concerning the gating of aluminum castings
in permanent molds was recently completed. The main goal
of the project was to improve the quality of aluminum
castings produced in permanent molds. The approach taken
was determine how the vertical type gating systems used
for permanent mold castings can be designed to fill the
mold cavity with a minimum of damage to the quality of
the resulting casting. It is evident that somewhat
different systems are preferred for different shapes and
sizes of aluminum castings. The main problems caused by
improper gating are entrained aluminum oxide films and
entrapped gas. The project highlighted the
characteristic features of gating systems used in
permanent mold aluminum foundries and recommended gating
procedures designed to avoid common defects. The study
also provided direct evidence on the filling pattern and
heat flow behavior in permanent mold castings.
Equipment and procedures for real-time X-Ray radiography
of molten aluminum flow into permanent molds have been
developed. Other studies have been conducted using water
flow and the behavior of liquid aluminum in sand mold
using real time photography. This investigation utilized
a strong graphite mold where it is possible to observe
the flow pattern with X-Rays. The gating included
systems that are choked at the base of a rounded
vertical sprue and enter vertical gating systems with a
variety of different ingate sprues into the bottom of a
mold cavity.
When a bottom ingate vertical sprue and bottom
horizontal runner are employed, a smooth filling pattern
is obtained with a large runner tapered at the blank end
and a large bottom ingate. Good filling patterns were
obtained in a horizontal ceramic foam filter located on
the ingate on top of the horizontal runner entering into
the mold cavity. When a vertical riser was used on the
side of the mold cavity connected to the mold cavity
through a web (3mm thick) a smooth filling pattern was
obtained when the bottom of the mold cavity was slightly
higher than the bottom of the web. A calculation of the
actual velocity could be determined for these systems
and this velocity was reduced to nineteen inches per
second for all of the gating systems listed above.
Studies have shown this as a critical velocity; higher
velocities will cause excessive oxide and air
entrainement.
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