Etching a substrate: processes – Etching to produce porous or perforated article
Reexamination Certificate
1998-11-12
2001-04-10
Gulakowski, Randy (Department: 1746)
Etching a substrate: processes
Etching to produce porous or perforated article
C216S017000, C216S018000, C216S039000, C029S889721
Reexamination Certificate
active
06214248
ABSTRACT:
BACKGROUND OF THE INVENTION
Higher operating temperatures for gas turbine engines are continuously sought in order to increase their efficiency. However, as operating temperatures increase, the high temperature durability of the components of the engine must correspondingly increase. Significant advances in high temperature capabilities have been achieved through formulation of iron, nickel and cobalt-based superalloys, though components formed from such alloys often cannot withstand long service exposures if located in certain sections of a gas turbine engine, such as the turbine, combustor or augmentor.
A common solution is to provide internal cooling of turbine, combustor and augmentor components, at times in combination with a thermal barrier coating. Airfoils of gas turbine engine blades and vanes often require a complex cooling scheme in which cooling air flows through cooling channels within the airfoil and is then discharged through carefully configured cooling holes at the airfoil surface. Convection cooling occurs within the airfoil from heat transfer to the cooling air as it flows through the cooling channels. In addition, fine internal orifices can be provided to direct cooling air flow directly against inner surfaces of the airfoil to achieve what is referred to as impingement cooling, while film cooling is often accomplished at the airfoil surface by configuring the cooling holes to discharge the cooling air flow across the airfoil surface so that the surface is protected from direct contact with the surrounding hot gases within the engine.
In the past, cooling channels have typically been integrally formed with the airfoil casting using relatively complicated cores and casting techniques. More recently, U.S. Pat. Nos. 5,626,462 and 5,640,767, both to Jackson et al. and commonly assigned with the present invention, teach a method of forming a double-walled airfoil by depositing an airfoil skin over a separately-formed inner support wall (e.g., a spar) having surface grooves filled with a sacrificial material. After the airfoil skin is formed, preferably by such methods as electron-beam physical vapor deposition (EBPVD) or plasma spraying, the sacrificial material is removed to yield a double-walled airfoil with cooling channels that circulate cooling air against the interior surface of the airfoil skin.
The sacrificial material must be carefully selected to withstand the skin deposition temperature, typically about 1000 to 1200° C., and have adequate thermal conductivity to prevent overheating of the deposition (spar) surface. In addition, the sacrificial material must be deposited to completely fill the surface grooves of the spar, and have a coefficient of thermal expansion (CTE) that is at least as high as that of the spar to prevent shrinkage of the sacrificial material away from the groove wall during deposition of the airfoil skin. If incomplete fill or shrinkage occurs, a gap will be present between the sacrificial material and the groove wall during skin deposition, which, if sufficiently large, leads to an unacceptable surface defect in the airfoil skin. Airfoil skins deposited by EBPVD are particularly sensitive to surface discontinuities due to the atom-by-atom manner in which the coating is built up. For example, gaps of less than up to about 2 mils (about 50 Fm) can be tolerated by plasma-sprayed airfoil skins, while gaps should not be larger than about 0.5 mil (about 13 Fm) for airfoil skins deposited by EBPVD.
For the above reason, sacrificial materials in the form of a solid shaped to fit a surface groove of a spar are unacceptable, because of the inherent likelihood of a gap between the solid and the groove wall. Consequently, and as shown in
FIGS. 1 through 3
, Jackson et al. teach that a sacrificial material
12
is deposited in the surface groove
14
of a spar
10
in excess amounts, with the excess being removed by machining or another suitable technique so that the surface of the sacrificial material
12
is flush with the surrounding surface of the spar
10
. Depending on its composition, the sacrificial material
12
can be removed after deposition of the airfoil skin
16
by melting/extraction, chemical etching, pyrolysis or another suitable method.
As is evident from
FIG. 3
, though a sacrificial material
12
having the desired physical properties outlined above can be deposited to completely fill the groove
14
, a problem arises because the groove
14
inevitably has rounded corners and edges, particularly if formed by casting or machining. As a result, even if the sacrificial material
12
completely fills the groove
14
and does not shrink to form a gap during deposition of the airfoil skin
16
, removal of the material
12
leaves a sharp (<90°) notch
18
between the skin
16
and the wall of the groove
14
. Cracks will tend to initiate from this notch
18
, and thereafter propagate between the airfoil skin
16
and spar
10
.
In view of the above, it can be seen that it would be desirable if a method were available that could ensure adequate filling of the groove prior to deposition of the airfoil skin, while also eliminating notches that could serve as the source of cracks between the skin and spar.
BRIEF SUMMARY OF THE INVENTION
According to the present invention, there is provided a method of forming an internal channel in an article, and particularly a cooling channel in an air-cooled component, such as a blade, vane, shroud, combustor or duct of a gas turbine engine. The method generally entails forming a substrate to have a groove recessed in its surface. A solid member is then placed in the groove, with the solid member being sized and configured to only partially fill the groove so that a void remains in the groove. The void is then filled with a particulate fill material so that the groove is completely filled. A layer is then deposited on the surface of the substrate and over the solid member and the fill material in the groove, after which at least the solid member is removed from the groove to form a channel in the substrate beneath the layer.
In one embodiment of the invention, the solid member and fill material are consolidated so that the fill material forms a solid mass that is bonded to the substrate, and only the solid member is removed from the groove so that the solid mass remains and forms a wall of the channel. In an alternative embodiment, the groove is formed so that its edge at the surface of the substrate projects over the groove, and the solid member and fill material are both removed from the groove to form the channel. With each embodiment, the tendency is greatly reduced for a sharp notch to be present within the channel between the substrate and layer. In the first embodiment, the fill material fills the notch formed between the layer and the rounded edge of the groove, while in the second embodiment the groove is configured to eliminate the notch. Accordingly, with each embodiment, removal of a sacrificial filler (i.e., the solid member and optionally the fill material) from the groove does not leave a sharp notch from which cracks are likely to initiate and propagate between the layer and substrate.
In another embodiment, a channel is formed in an article by steps of forming a substrate having a surface and a groove recessed in the surface, placing a solid member in the groove so as to partially fill the groove and leave a void therein, filling the void with a fill material so that the groove is completely filled, depositing a layer on the surface of the substrate and over the solid member and the fill material in the groove and then removing at least the solid member from the groove so as to form a channel in the substrate and beneath the layer. The fill material can be removed from the groove after the depositing step. The groove can be formed to have an edge at the surface of the substrate, the edge projecting over the groove.
Other objects and advantages of this invention will be better appreciated from the following detailed description.
REFERENCES:
patent: 4956037 (1990-09-01), Vi
Browning Janel Koca
Jackson Melvin Robert
Ahmed Shamim
General Electric Company
Gulakowski Randy
Johnson Noreen C.
Stoner Douglas E.
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