Page 306 - Manufacturing Engineering and Technology - Kalpakjian, Serope : Schmid, Steven R.

Section 11.5 Casting Techniques for Single crystal Components 2

I l.5 Casting Techniques for Single-crystal Components

The characteristics of single-crystal and polycrystalline structures in metals were
described in Section 1.3. This section describes the techniques used to cast single-
crystal components (such as gas turbine blades), which generally are made of nickel-
based superalloys and used in the hot stages of the engine. The procedures involved
also can be used for other alloys and components.

Conventional Casting of Turbine Blades. The conventional-casting process uses a
ceramic mold. The molten metal is poured into the mold and begins to solidify at the
ceramic walls. The grain structure developed is polycrystalline, similar to that
shown in Fig. 1O.2c. However, the presence of grain boundaries makes this structure
susceptible to creep and cracking along the boundaries under the centrifugal forces
and elevated temperatures commonly encountered in an operating gas turbine.

Directionally Solidified Blades. The directional-soIidi]?cation process (Fig. 11.25a)
was first developed in 1960. The ceramic mold is preheated by radiant heating, and
the mold is supported by a water-cooled chill plate. After the metal is poured into the
mold, the chill-plate assembly is lowered slowly. Crystals begin to grow at the chill-
plate surface and on upward, like the columnar grains shown in Fig. 10.3. The blade
thus is solidified directionally, with longitudinal, but no transverse, grain boundaries.
Consequently, the blade is stronger in the direction of centrifugal forces developed in
the gas turbine.

Single-crystal Blades. In crystal growing, developed in 1967, the mold has a con-
striction in the shape of a corkscrew or helix (Figs. 11.25 b and c). The cross section
is so small that it allows only one crystal to fit through. The mechanism of crystal
growth is such that only the most favorably oriented crystals are able to grow (a sit-
uation similar to that shown in Fig. 1O.3) through the helix, because all others are
intercepted by the walls of the helical passage.
As the assembly is lowered slowly, a single crystal grows upward through the
constriction and begins to grow in the mold. Strict control of the rate of movement
is important. The resultant casting is a single-crystal blade. Although these blades

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(H) (D) (C)
FIGURE ll.25 Methods of casting turbine blades: (a) directional solidification; (b) method
to produce a single-crystal blade; and (C) a single-crystal blade with the constriction portion
still attached. Source: (a) and (b) After B.H. Kear, (c) Courtesy of ASM International.