REFERENCES
25
Law of Resistance in Parallel Channels,’’ Transactions of the Royal Society of London
, Vol. A174, 1883, p. 935. 2. Flemings, M., ‘‘Behavior of Metal Alloys in the Semisolid State,’’ Metallur-
gical Transactions, Vol. 22B, June 1991, p. 269.
3. Gaskell, D., An Introduction to Transport Phenomena in Materials Engi- neering,
Macmillan, New York, NY, 1992. 4. Keeney, M., J. Courtois, R. Evans, G. Farrior, C. Kyonka, A. Koch, K. Young.
‘‘Semisolid Metal Casting and Forging,’’ in Stefanescu, D. editor, Metals Handbook,
9th ed., Vol. 15, Casting, ASM International, Materials Park, OH, 1988, p. 327.
5. Young, K., ‘‘Semi-solid Metal Cast Automotive Components: New Markets for Die Casting,’’ Paper Cleveland T93-131, North American Die Casting
Association, Rosemont, IL, 1993. 6. Siegert, K. and R. Leiber. ‘‘Thixoforming of Aluminum,’’ SAE Paper Number
980456, Society of Automotive Engineers, Warrendale, PA, 1998. 7. Alexandrou, A., G. Burgos, and V. Entov ‘‘Semisolid Metal Processing: A
New Paradigm in Automotive Part Design,’’ SAE Paper Number 2000-01- 0676, Society of Automotive Engineers, Warrendale, PA, 2000.
HIGH INTEGRITY DIE CASTING PROCESSES
29
3
VACUUM DIE CASTING
3.1 VACUUM DIE CASTING DEFINED
Entrapped gas is a major source of porosity in conventional die castings. Vacuum die casting is characterized by the use of a con-
trolled vacuum to extract gases from the die cavities, runner sys- tem, and shot sleeve during processing.
1
This high integrity process stretches the capabilities of conventional die casting while
preserving its economic benefits. Numerous metal casting processes have utilized vacuum sys-
tems to assist in the removal of unwanted gases. These processes include permanent-mold casting, lost-foam casting, plaster mold
casting, and investment casting. The constraint in the evolution of vacuum die casting has been the development of a reliable vacuum
shut-off valve. Vacuum die casting is compatible with other high integrity processes, including squeeze casting and semi-solid met-
alworking.
The integrity of vacuum die cast components is much improved in comparison to conventional die casting. This is due to reduced
porosity levels made possible by the minimization of entrapped gas, as discussed in the following section.
3.2 MANAGING GASES IN THE DIE
Gas porosity can originate from many sources, including the phys- ical entrapment of gas during die filling, the decomposition of
manufacturing lubricants, and the evolution of gases dissolved in
30
VACUUM DIE CASTING
the liquid alloy.
2
The vacuum die casting process minimizes gas entrapment by removing gases from the cavity generated by two
of these mechanisms. Both air in the die cavity and gases gener- ated by the decomposition of lubricants can be removed using the
vacuum die casting process.
In conventional die casting, gases are typically vented from the die. However, the amount of gas that must vent from the dies is
much greater than that of just the die cavity. All gases in the runner system must be vented as well as any volume of the shot
sleeve not filled with metal. When examining the volume of gas that must be evacuated from the die combined with the short cycle
times of conventional die casting, one finds that it is virtually impossible for all gases to exit the die before die fill is complete,
as presented in the following sample calculation.
Example Calculation 3.2
Using the conventional die casting process, a component is man- ufactured with a four-cavity die in a commercially available alu-
minum alloy. Each casting weighs 0.5 kg. The runner system with all four components attached weighs 4 kg and includes a 5.0-cm-
thick biscuit that is 7.5 cm in diameter. The length of the shot sleeve is 100 cm, and the length from the cover die end to the
edge of the pore hole is 75 cm. Within the die are eight vents that are 3 cm in length and 0.1 mm in width. Given that the density
of aluminum is 2.7 gcm
3
, answer the following questions: 1. Assuming no porosity, what is the volume of the runner
system with all four components attached? 2. What is the volume of the shot sleeve from the pour hole to
the die? 3. What is the volume of the entire system, including the four
component cavities, the runner system, and the shot sleeve? 4. What percent of the shot sleeve volume is filled with metal
for each shot? 5. What percent of the system volume is filled with metal for
each shot? 6. How much gas must be managed in this example?
7. For a 1-sec shot, how fast must the gases flow through the vents?
