Die-casting and Precision Machining

1.3 PROBLEMS WITH CONVENTIONAL DIE CASTING
Conventional die casting is utilized to produce many products in
the current global market. Unfortunately, conventional die casting
has a major limitation that is preventing its use on a broader scale.
A potential defect, commonly found in conventionally die cast
components, is porosity.zinc die casting
Porosity often limits the use of the conventional die casting
process in favor of products fabricated by other means. Pressure
vessels must be leak tight. Conventional die casting often are
unable to meet this requirement. Moreover, the detection of porosity
is difficult. In some cases, an ‘‘as-produced’’ component is
acceptable. al die casting, Subsequent machining, however, cuts into porosity
hidden within the component, compromising the integrity of the
product.
Porosity is attributed to two main sources: solidification shrinkage
and gas entrapment. al die casting Most alloys have a higher density in their
solid state as compared to their density in the liquid state. As a
result, shrinkage porosity forms during solidification. Due to the
turbulent manner in which metal enters and fills the die cavity,zinc die casting
gas often becomes entrapped in the metal, resulting in porosity.
Porosity also affects the mechanical properties of conventionally
die cast components. In structural applications, porosity can
act as a stress concentrator creating an initiation site for cracks.
Numerous studies have documented how porosity in die castings
varies with several operating conditions.3–8 A method has
been developed for quantifying the porosity in die cast components.
9 The total porosity contained in a component is defined
using the equation al die casting
%P
(solidification shrinkage)  (gas contribution) (1.1)
which can be further defined as where
%P
percent porosity,


solidification shrinkage factor in percent,
V*
volume of liquid in casting cavity that is not supplied
liquid during solidification in cubic centimeters,
Vc
volume of the al die casting cavity in cubic centimeters,
T
temperature of the gas in the casting cavity in degrees
Kelvin,
P
pressure applied to the gas during solidification in atmospheres,

fraction of the gas that does not report to the solidification
shrinkage pores,

liquid alloy density at the melting temperature in grams
per cubic centimeter,zinc die casting

quantity of the gas contained in the casting at standard
temperature and pressure conditions (273 K at 1 atm)
in cubic centimeters per 100 g of alloy, and
*
solubility limit of gas in the solid at the solidus temperature
at standard temperature and pressure conditions
in cubic centimeters per 100 g of alloy.zinc die casting
The first portion of Equation 1.2 is a relationship for porosity due
to solidification shrinkage. The second portion of Equation 1.2
describes the porosity due to gas entrapment. The total gas contained
in the die casting includes gas from physical entrapment, gas
from lubricant decomposition, and gas dissolved in the alloy. This
relationship can also be described mathematically,

     (1.3) Entrained Lube Soluble gas
Each of the gas contributions in Equation 1.3 is expressed in cubic
centimeters at standard temperature and pressure conditions per
100 g of alloy.
In addition to porosity, the microstructures inherent with the
conventional al die casting cannot meet the mechanical requirements
needed for many applications. Subsequent heat treating, which can
alter the microstructure, is rarely possible due to defects that
emerge during thermal processing, such as blistering.
Regardless of the limitations found in conventional zinc die casting
components, demands exist for high integrity products. In many
cases, product engineers and designers turn to investment casting,
forging, injection molding, and assembled fabrications to meet
necessary requirements. Typically, these processes are more costly
than conventional die casting in both processing time and raw
material costs.