April - June 1998
Aluminium
Components for Automobiles:
Monitoring
the Solidification of Metal Castings
[e-lab Home Page] [Energy Lab Home Page] [MIT Home Page] .
Aluminium
Components for Automobiles:
|
|
April - June 1998
Aluminium
Components for Automobiles:
Monitoring
the Solidification of Metal Castings
[e-lab Home Page] [Energy Lab Home Page] [MIT Home Page] .
Aluminium
Components for Automobiles:
|
o
meet the needs of today's auto industry, manufacturers have begun producing
aluminum components by pouring the metal into molds when it is already
partially solidified. The lightweight precision parts produced cost less
than those made by forging and machining solids and are stronger and more
reliable than those made by conventional casting of liquids. "Semisolid
processing" became possible 25 years ago, when an MIT graduate student
found that stirring molten metal while it solidifies produces a semisolid
that can be cast even when it is solid enough to be handled by robotic
equipment. Now the MIT researchers are using industrially important aluminum
alloys and new experimental techniques to clarify exactly why the process
works and how to make it work better. As expected, the more solid the material
is, the shorter the distance it flows before stopping--not good behavior
for filling a mold. However, partially solidified samples flow farther
if they have been formed by cooling the molten metal slowly and stirring
it vigorously. Microstructural analysis shows why. Slow cooling and long
stirring produce particles that are large and spherical, while fast cooling
and short stirring produce small, jagged particles that tend to stick together.
As a result, the larger particles slide by each other more easily than
the smaller ones do, producing a semisolid with less resistance to flow.
High-speed videos show that the semisolid advances smoothly and evenly,
producing parts with a consistent microstructure, smooth finish, high reliability,
and constant properties.
f manufacturers
of metals could see exactly how their molten metals solidify, they could
control their operations to make their products faster and better. Energy
Laboratory researchers have invented a novel way to monitor the solidification
process directly without affecting the metal object being formed. They
use computed tomography, or CT, the technique used in taking brain scans.
A small linear accelerator fires high-energy X-rays along many pathways
through a solidifying metal object. How much of the X-ray beam makes it
through the object depends on the density it encounters. The solid and
liquid forms of a metal differ substantially in density, so a CT image
of the object based on the density data shows which areas are solid and
which are liquid. This method has yielded minute-to-minute images of an
aluminum sample as it solidifies during an hour. The results are consistent
with temperature measurements taken within the solidifying sample. The
researchers are now developing a new analytical technique that should reduce
the scanning time to 1-3 seconds--fast enough for real-time process control
in industry. The MIT team is starting to work with important industrial
alloys of aluminum and is getting ready for field tests in a commercial
casting plant.