Cores for use in precision investment casting

Details Cores for use in precision investment casting Cores for use in precision investment casting

2001-10-24

2003-10-28

Elve, M. Alexandra (Department: 1725)

Metal founding

Means to shape metallic material

United particle type shaping surface

C164S397000, C148S404000

Reexamination Certificate

active

06637500

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to investment casting cores, and in particular to investment casting cores which are formed at least in part from refractory metals.
2. Background Information
Investment casting is a commonly used technique for forming metallic components having complex geometries, especially hollow components, and is used in the fabrication of superalloy gas turbine engine components. The invention will be described in respect to the production of superalloy castings, however it will be understood that the invention is not so limited.
Gas turbine engines are widely used in aircraft propulsion, electric power generation, and ship propulsion. In all gas turbine engine applications, efficiency is a prime objective.
Improved gas turbine engine efficiency can be obtained by operating at higher temperatures, however current operating temperatures are at such a level that, in the turbine section, the superalloy materials used have limited mechanical properties. Consequently, it is a general practice to provide air cooling for components in the hottest portions of gas turbine engines, typically in the turbine section. Cooling is provided by flowing relatively cool air from the compressor section of the engine through passages in the turbine components to be cooled. It will be appreciated that cooling comes with an associated cost in engine efficiency, consequently, there is a strong desire to provide enhanced specific cooling, maximizing the amount of cooling benefit obtained from a given amount of cooling air.
Referring to
FIG. 1
, a gas turbine engine
10
includes a compressor
12
, a combustor
14
, and a turbine
16
. Air
18
flows axially through the sections
12
,
14
, and
16
of the engine
10
. As is well known in the art, air
18
, compressed in the compressor
12
, is mixed with fuel which is burned in the combustor
14
and expanded in the turbine
16
, thereby rotating the turbine
16
and driving the compressor
12
.
Both the compressor
12
and the turbine
16
are comprised of rotating and stationary airfoils
20
,
22
, respectively. The airfoils, especially those disposed in the turbine
16
, are subjected to repetitive thermal cycling under widely ranging temperatures and pressures. To avoid thermal damage to the airfoils, each airfoil
20
includes internal cooling.
Referring to
FIG. 2
, the airfoil
20
includes a leading edge
26
and a trailing edge
28
extending from a root end
30
to a tip
32
thereof and a platform
34
. A leading edge cooling passage
40
is formed within the leading edge
26
of the airfoil
20
having radially extending, connected channels
42
-
44
and a leading edge inlet
46
, formed within the platform
34
and in fluid communication with the channel
42
. A plurality of leading edge crossover holes
48
formed within a leading edge passage wall
50
separating the channel
44
from a leading edge exhaust passage
52
, allow the cooling air from the channel
44
to flow into the leading edge exhaust passage
52
. A trailing edge cooling passage
56
is formed within the trailing edge
28
of the airfoil
20
having radially extending connected channels
58
-
60
and a trailing edge inlet
62
formed within the platform
34
and in fluid communication with the channel
58
. A first plurality of trailing edge crossover holes
66
is formed within a first trailing edge wall
68
and a second plurality of trailing edge crossover holes
72
is formed within a second trailing edge wall
74
to allow cooling air from channel
58
to flow through an intermediate passage
78
to a plurality of trailing edge slots
80
.
A ceramic core
120
, as depicted in
FIGS. 3 and 4
, is used in the manufacturing process of the airfoils
20
and defines the hollow cavities therein. A ceramic core leading edge
126
and a ceramic core trailing edge
128
correspond to the leading edge
26
and trailing edge
28
in the airfoil
20
, respectively. A ceramic core root
130
and a tip
132
correspond to the airfoil root
30
and tip
32
, respectively. Ceramic core passages
140
,
156
with channels
142
-
144
,
158
-
160
, and inlets
146
,
162
respectively, correspond to passages
40
,
56
with channels
42
-
44
,
58
-
60
and inlets
46
,
62
, of the airfoil, respectively. Passages
52
and
78
of the airfoil correspond to channels
152
and
178
in the ceramic core. Pluralities of fingers
148
,
166
,
172
in the core
120
correspond to the plurality of crossover holes
48
,
66
,
72
in the airfoil
20
, respectively. A core tip
190
is attached to the core passages
140
,
156
by means of fingers
182
-
185
, to stabilize the core
120
at the tip
132
. An external ceramic handle
194
is attached at the core trailing edge
128
for handling purposes. A core extension
196
defines a cooling passage at the root to the airfoil
20
. Centerlines
197
-
199
extend radially through each row of fingers
148
,
166
,
172
, respectively.
While turbine blades and vanes are some of the most important components that are cooled, other components such as combustion chambers and blade outer air seals also require cooling, and the invention has application to all cooled turbine hardware, and in fact to all complex cast articles.
Currently cores such as that shown in
FIGS. 3 and 4
are fabricated from ceramic materials but such ceramic cores are fragile, especially the advanced cores used to fabricate small intricate cooling passages in advanced hardware. Current ceramic cores are prone to warpage and fracture during fabrication and during casting. In some advanced experimental blade designs casting yields of less than 10% are achieved, principally because of core failure.
Conventional ceramic cores are produced by a molding process using a ceramic slurry and a shaped die; both injection molding and transfer-molding techniques may be employed. The pattern material is most commonly wax although plastics, low melting-point metals, and organic compounds such as urea, have also been employed. The shell mold is formed using a colloidal silica binder to bind together ceramic particles which may be alumina, silica, zirconia and alumina silicates.
The investment casting process to produce a turbine blade, using a ceramic core, will be explained briefly here. A ceramic core having the geometry desired for the internal cooling passages is placed in a metal die whose walls surround but are generally spaced away from the core. The die is filled with a disposable pattern material such as wax. The die is removed leaving the ceramic core embedded in a wax pattern. The outer shell mold is then formed about the wax pattern by dipping the pattern in a ceramic slurry and then applying larger, dry ceramic particles to the slurry. This process is termed stuccoing. The stuccoed wax pattern, containing the core, is then dried and the stuccoing process repeated to provide the desired shell mold wall thickness. At this point the mold is thoroughly dried and heated to an elevated temperature to remove the wax material and strengthen the ceramic material.
The result is a ceramic mold containing a ceramic core which in combination define a mold cavity. It will be understood that the exterior of the core defines the passageway to be formed in the casting and the interior of the shell mold defines the external dimensions of the superalloy casting to be made. The core and shell may also define casting portions such as gates and risers which are necessary for the casting process but are not a part of the finished cast component.
After the removal of the wax, molten superalloy material is poured into the cavity defined by the shell mold and core assembly and solidified. The mold and core are than removed from the superalloy casting by a combination of mechanical and chemical means.
As previously noted, the currently used ceramic cores limit casting designs because of their fragility and because cores with dimensions of less than about 0.012-0.015 inches cannot currently be produced with acceptable casting

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