Die-casting and Precision Machining

1.4 STRATEGIES TO IMPROVE DIE CASTING
CAPABILITIES
Several efforts have proven successful in stretching the capabilities
of conventional die casting while preserving short cycle times and
providing dimensional stability and other beneficial characteristics.
In these efforts, three strategies have extended the capabilities
of the al die casting process:
1. eliminating or reducing the amount of entrapped gases,
2. eliminating or reducing the amount of solidification shrinkage,
and zinc die casting
3. altering the microstructure of the metal.
The first two strategies noted affect each of the major quantities
that contribute to porosity as defined in Equation 1.1. The third
strategy addresses the mechanical properties by modifying the
fundamental structure of the die cast machining components
1.5 HIGH INTEGRITY DIE CASTING PROCESSES
Three high integrity die casting  processes have been successfully
developed and deployed for commercial use in high volume production.
These processes are vacuum die casting , squeeze casting,
and semi-solid metalworking (SSM). al die casting

Vacuum die casting utilizes a controlled vacuum to extract
gases from the die cavities and runner system during metal injection.
This process works to minimize the quantities of Entrained and
Lube as defined in Equation 1.3. Porosity due to entrapped gases
is virtually eliminated.al die casting
Squeeze casting is characterized by the use of a large gate area
and planar filling of the metal front within the die cavity. As with
vacuum die casting, this process works to minimize the quantities
of Entrained and Lube as noted in Equation 1.3. The mechanism,
however, is much different. Planar filling allows gases to escape
from the die, as vents remain open throughout metal injection.
Furthermore, the large gate area allows metal intensification pressure
to be maintained throughout solidification, reducing the magnitude
of V* as defined in Equation 1.2. Both porosity from
entrapped gas and solidification shrinkage are reduced by using
squeeze casting.
Semi-solid metalworking is the most complex of the high integrity
die casting processes. During semi-solid metalworking a
partially liquid–partially solid metal mixture is injected into the
die cavity. The fill front is planar, minimizing gas entrapment, as
in squeeze casting. Moreover, solidification shrinkage is greatly
reduced, as a significant portion of the metal injected into the die
cavity is already solid. Semi-solid metalworking addresses both
sides of the porosity relationship defined in Equation 1.1.
In addition to reducing porosity, a unique microstructure is generated
during semi-solid metalworking. The mechanical properties
inherent to this microstructure are superior to those created in
conventionally die cast components.al die casting
Products produced using high integrity die casting processes
have little or no porosity. Moreover, the mechanical properties are
much improved in comparison to conventional die cast components.
This is due to reduced levels of porosity, the viability of
subsequent heat treating, and formation of microstructures not
possible with the conventional zinc die casting process.

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