Page 295 - 04. Subyek Engineering Materials - Manufacturing, Engineering and Technology SI 6th Edition - Serope Kalpakjian, Stephen Schmid (2009)

274 Chapter 11 Metal-Casting Processes and Equipment

up and the casting is removed. A number of pat-
terns can be joined to make one mold, called a tree
(Fig. 11.13), significantly increasing the produc-
tion rate. For small parts, the tree can be inserted
into a permeable flask and filled with a liquid slur-
ry investment. The investment then is placed into a
chamber and evacuated (to remove the air bubbles
in it) until the mold solidifies. The flask usually is
placed in a vacuum-casting machine, so that
molten metal is drawn into the permeable mold
FIGURE I l.l4 Investment casting of an integrally cast rotor for a and onto the part, producing fine detail.
gas turbine. (a) Wax pattern assembly. (b) Ceramic shell around wax Although the mold materials and labor in-
pattern. (c) Wax is melted out and the mold is filled, under a vacuum, volved make the lost-wax process costly, it is suit-
with molten superalloy. (d) The cast rotor, produced to net or near- able for casting high-melting-point alloys with
net shape. Source: Courtesy of Howmet Corporation.
good surface finish and close dimensional toler-
ances; few or no finishing operations, which oth-
erwise would add significantly to the total cost of
the casting, are required. The process is capable of producing intricate shapes, with
parts weighing from 1 g to 35 kg, from a wide variety of ferrous and nonferrous
metals and alloys. Recent advances include the casting of titanium aircraft-engine
and structural airframe components with wall thicknesses on the order of 1.5 mm,
thus competing with previously used sheet-metal structures.

Ceramic-shell Investment Casting. A variation of the investment-casting process is
ceramic-shell casting. It uses the same type of wax or plastic pattern, which is dipped
first in ethyl silicate gel and subsequently into a fluidized bed (see Section 4.12) of
fine-grained fused silica or zircon flour. The pattern is then dipped into coarser
grained silica to build up additional coatings and develop a proper thickness so that
the pattern can withstand the thermal shock due to pouring. The rest of the procedure
is similar to investment casting. The process is economical and is used extensively for
the precision casting of steels and high-temperature alloys.
The sequence of operations involved in making a turbine disk by this method
is shown in Fig. 11.14. If ceramic cores are used in the casting, they are removed by
leaching with caustic solutions under high pressure and temperature. The molten
metal may be poured in a vacuum to extract evolved gases and reduce oxidation,
thus improving the casting quality. To further reduce microporosity, the castings
made by this (as well as other processes) are subjected to hot isostatic pressing.
Aluminum castings, for example, are subjected to a gas pressure up to 100 MPa at
500°C.

EXAMPLE |.l Investment-cast Superalloy Components for Gas Turbines
I
Since the 19 60s, investment-cast superalloys have been pouring techniques, and the cooling rate of the casting
replacing wrought counterparts in high-performance (see Section 1O.2). In contrast, note the coarse-grained
gas turbines. The microstructure of an integrally structure in the lower half of Fig. 11.15 showing the
investment-cast gas-turbine rotor is shown in the upper same type of rotor cast conventionally. This rotor has
half of Fig. 11.15. Note the fine, uniform equiaxed inferior properties compared with the fine-grained
grains throughout the rotor cross section. Casting pro- rotor. Due to developments in these processes, the pro-
cedures include the use of a nucleant addition to the portion of cast parts to other parts in aircraft engines
molten metal, as well as close control of its superheat, has increased from 20% to about 45% by weight.