Page 263 - Manufacturing Engineering and Technology - Kalpakjian, Serope : Schmid, Steven R.

242 Chapter 10 Fundamentals of Metal Casting

The compositions of dendrites and the liquid metal are
given by the phase diagram of the particular alloy. When the
alloy is cooled very slowly, each dendrite develops a uniform
composition. However, under the normally faster cooling rates
encountered in practice, cored dendrites are formed. Cored
dendrites have a surface composition different from that at
their centers, a difference referred to as a concentration gradi-
ent. The surface of the dendrite has a higher concentration of
alloying elements than does its core, due to solute rejection
(H) (la)
from the core toward the surface during solidification of the
FIGURE |0.1 Schematic illustration of cast dendrite (microsegregation). The darker shading in the inter-
structures in (a) plane front, single phase, and dendritic liquid near the dendrite roots shown in Fig. 10.6 in-
(b) plane front, two phase. Source: Courtesy of dicates that these regions have a higher solute concentration;
D. Apelian. microsegregation in these regions is much more pronounced
than in others.
There are several types of segregation. In contrast to microsegregation,
macrosegregation involves differences in composition throughout the casting itself.
In situations where the solidification front moves away from the surface of a cast-
ing as a plane (Fig. 10.7), lower melting-point constituents in the solidifying alloy
are driven toward the center (normal segregation). Consequently, such a casting
has a higher concentration of alloying elements at its center than at its surfaces. In
dendritic structures such as those found in solid-solution alloys (Fig. 10.2b), the
opposite occurs; that is, the center of the casting has a lower concentration of alloy-
ing elements (inverse segregation) than does its surface. The reason is that liquid
metal (having a higher concentration of alloying elements) enters the cavities devel-
oped from solidification shrinkage in the dendrite arms, which have solidified

SOOHCI.
Another form of segregation is due to gravity. Gravity segregation describes
the process whereby higher density inclusions or compounds sink and lighter ele-
ments (such as antimony in an antimony-lead alloy) float to the surface.
A typical cast structure of a solid-solution alloy with an inner zone of
equiaxed grains is shown in Fig. 10.2b. This inner zone can be extended throughout
the casting, as shown in Fig. 1O.2c, by adding an inoculant (nucleating agent) to the
alloy. The inoculant induces nucleation of the grains throughout the liquid metal
(heterogeneous nucleation).
Because of the presence of thermal gradients in a solidifying mass of liquid
metal, and due to gravity and the resultant density differences, convection has a
strong influence on the structures developed. Convection promotes the formation of
an outer chill zone, refines grain size, and accelerates the transition from columnar
to equiaxed grains. The structure shown in Fig. 10.6b also can be obtained by
increasing convection within the liquid metal, whereby dendrite arms separate
(dendrite multiplication). Conversely, reducing or eliminating convection results in
coarser and longer columnar dendritic grains.
The dendrite arms are not particularly strong and can be broken up by agita-
tion or mechanical vibration in the early stages of solidification (as in semisolid
metal forming and rheocasting, described in Section 11.4.7). This process results in
finer grain size, with equiaxed nondendritic grains distributed more uniformly
throughout the casting (Fig. 10.6c). A side benefit is the thixotropic behavior of
alloys (that is, the viscosity decreases when the liquid metal is agitated), leading to
improved castability. Another form of semisolid metal forming is thixotropic
casting, where a solid billet is heated to the semisolid state and then injected into a
die-casting mold (Section 11.4.5). The heating is usually by convection in a furnace,
but can be enhanced by the use of mechanical or electromagnetic methods.