Page 262 - Manufacturing Engineering and Technology - Kalpakjian, Serope : Schmid, Steven R.

Section 10.2 Solidification of Metals

the alloy system. When the volume fraction of the minor phase of the alloy is less
than about 25%, the structure generally becomes fibrous. These conditions are par-
ticularly important for cast irons.
For alloys, a short freezing range generally involves a temperature difference of
less than 50°C, and a long freezing range greater than 110°C. Ferrous castings gen-
erally have narrow mushy zones, whereas aluminum and magnesium alloys have
wide mushy zones. Consequently, these alloys are in a mushy state throughout most
of the solidification process.

Effects of Cooling Rates. Slow cooling rates (on the order of 102 K/s) or long local
solidification times result in coarse dendritic structures with large spacing between
dendrite arms. For higher cooling rates (on the order of 104 K/s) or short local solid-
ification times, the structure becomes finer with smaller dendrite arm spacing. For
still higher cooling rates (on the order of from 106 to 108 K/s) the structures devel-
oped are amorphous, as described in Section 6.14.
The structures developed and the resulting grain size influence the properties
of the casting. As grain size decreases, the strength and the ductility of the cast alloy
increase, microporosity (interdendritic shrinkage voids) in the casting decreases, and
the tendency for the casting to crack (hot tearing, see Fig. 10.12) during solidifica-
tion decreases. Lack of uniformity in grain size and grain distribution results in cast-
ings with anisotropic properties.

A criterion describing the kinetics of the liquid-solid interface is the ratio G/R 3
where G is the thermal gradient and R is the rate at which the interface moves.
Typical values for G range from 102 to 103 K/m and for R range from 10T3 to
10`4 m/s. Dendritic-type structures (Fig. 10.6a and b) typically have a G/R ratio in
the range from 105 to 107, whereas ratios of 1010 to 1012 produce a plane-front,
nondendritic liquid-solid interface (Fig. 10.7).

I0.2.3 Structure-property Relationships

Because all castings are expected to possess certain properties to meet design and
service requirements, the relationships between properties and the structures devel-
oped during solidification are important aspects of casting. This section describes
these relationships in terms of dendrite morphology and the concentration of alloy-
ing elements in various regions within the metal.

-Liquid -Liquid -Liquid

(G) (D) (C)

FIGURE l0.6 Schematic illustration of three basic types of cast structures: (a) columnar
dendritic; (b) equiaxed dendritic; and (c) equiaxed nondendritic. Source: Courtesy of
D. Apelian.