Page 267 - Manufacturing Engineering and Technology - Kalpakjian, Serope : Schmid, Steven R.

2 Chapter 10 Fundamentals of Metal Casting

Surface Tension. A high surface tension of the liquid metal reduces fluidity.
Because of this, oxide films on the surface of the molten metal have a significant
adverse effect on fluidity. For example, an oxide film on the surface of pure molten
aluminum triples the surface tension.

lnclusions. Because they are insoluble, inclusions can have a significant adverse
effect on fluidity. This effect can be verified by observing the viscosity of a liquid
(such as oil) with and without sand particles in it; the liquid with sand in it has a
higher viscosity and, hence, lower fluidity.

Solidification Pattern of the Alloy. The manner in which solidification takes place
(Section 10.2) can influence fluidity. Moreover, fluidity is inversely proportional to
the freezing range: The shorter the range (as in pure metals and eutectics), the higher
the fluidity. Conversely, alloys with long freezing ranges (such as solid-solution
alloys) have lower fluidity.
The following casting parameters influence fluidity and also influence the fluid
flow and thermal characteristics of the system:

Mold Design. The design and dimensions of the sprue, runners, and risers all influ-
ence fluidity.

Mold Material and its Surface Characteristics. The higher the thermal conduc-
tivity of the mold and the rougher its surfaces, the lower the fluidity of the molten
metal. Although heating the mold improves fluidity, it slows down solidification
of the metal. Thus, the casting develops coarse grains and hence has lower
strength.
Degree of Superheat. Superheat (defined as the increment of temperature of an
alloy above its melting point) improves fluidity by delaying solidification. The
pouring temperature often is specified instead of the degree of superheat, because it
is specified more easily.

Rate of Pouring. The slower the rate of pouring molten metal into the mold, the
lower the fluidity because of the higher rate of cooling when poured slowly.
Pouring cup Heat Transfer. This factor directly affects the viscosity of the liquid metal

Sprue (see below).
Although complex, the term castability generally is used to describe the
Fluidity index ease with which a metal can be cast to produce a part with good quality.
Castability includes not only fluidity, but the nature of casting practices
as well.

l0.4.l Tests for Fluidity
Several tests have been developed to quantify fluidity, although none is
accepted universally. In one such common test, the molten metal is made
to flow along a channel that is at room temperature (Fig, 10.9); the dis-
FIGURE l0.9 A test method for
fluidity using a spiral mold. The fluidity tance the metal flows before it solidifies and stops flowing is a measure of
index is the length of the solidified its fluidity. Obviously, this length is a function of the thermal properties
metal in the spiral passage. The greater of the metal and the mold, as well as of the design of the channel. Still,
the length of the solidified metal, the such fluidity tests are useful and simulate casting situations to a reason-
greater is the metal’s fluidity. able degree.