U.S. patent application number 11/589731 was filed with the patent office on 2008-05-01 for investment casting process and apparatus to facilitate superior grain structure in a ds turbine bucket with shroud.
This patent application is currently assigned to General Electric Company. Invention is credited to Robert Alan Brittingham, Stephen Daniel Graham.
Application Number | 20080099177 11/589731 |
Document ID | / |
Family ID | 39265117 |
Filed Date | 2008-05-01 |
United States Patent
Application |
20080099177 |
Kind Code |
A1 |
Graham; Stephen Daniel ; et
al. |
May 1, 2008 |
Investment casting process and apparatus to facilitate superior
grain structure in a DS turbine bucket with shroud
Abstract
An investment casting process that enables directionally
solidified tip shrouded turbine blades or buckets to have a
continuous grain structure that extends through the tip shroud in
addition to increasing the quantity of grains in the root of the
part.
Inventors: |
Graham; Stephen Daniel;
(West Union, SC) ; Brittingham; Robert Alan;
(Piedmont, SC) |
Correspondence
Address: |
NIXON & VANDERHYE P.C.
901 NORTH GLEBE ROAD, 11TH FLOOR
ARLINGTON
VA
22203
US
|
Assignee: |
General Electric Company
Schenectady
NY
|
Family ID: |
39265117 |
Appl. No.: |
11/589731 |
Filed: |
October 31, 2006 |
Current U.S.
Class: |
164/122.1 ;
164/516 |
Current CPC
Class: |
B22D 27/045
20130101 |
Class at
Publication: |
164/122.1 ;
164/516 |
International
Class: |
B22D 27/04 20060101
B22D027/04 |
Claims
1. An investment casting process for forming a directionally
solidified blade comprising: providing a blade mold, said mold
being oriented so that a portion of a mold cavity thereof for
forming a base of said blade is at a base thereof and a portion of
the mold cavity for forming a tip of the blade is at a vertically
upper end thereof; providing a heat removal feature to extend below
said mold; flowing molten metal into said mold cavity of said mold;
and allowing said blade to solidify from the base upwardly.
2. The investment casting process of claim 1, wherein said molton
metal is fed to a base of said mold cavity.
3. The investment casting process of claim 1, further comprising
providing a sprue adjacent to said blade mold and providing a
feeder tube for flowing molten metal from said sprue to said mold
cavity.
4. The investment casting process of claim 3, wherein said molten
metal flows through said feeder tube from said sprue into the base
of said mold cavity.
5. The investment casting process of claim 3, wherein said sprue is
vertically oriented, adjacent and parallel to said blade mold.
6. The investment casting process of claim 5, wherein said molten
metal flows through said feeder tube from said sprue into the base
of said mold cavity.
7. The investment casting process of claim 5, wherein said flowing
molten metal comprises flowing molten metal into the vertically
upper end of the sprue before said flowing through said feeder
tube.
8. The investment casting process of claim 1, wherein the mold is
configured for forming a turbine bucket, the base of said mold
cavity is configured to form a dovetail/shank of the turbine bucket
and the vertically upper end of said mold cavity is configured to
form a tip shroud of the turbine bucket.
9. An investment casting assembly for molding a directionally
solidified blade comprising: a blade mold, said mold being oriented
so that a portion of a mold cavity thereof for forming a base of
said blade is at a base thereof and a portion of the mold cavity
for forming a tip of the blade is at a vertically upper end
thereof; a heat removal feature disposed to extend below said mold;
and a plumbing system for flowing molten metal into said mold
cavity of said mold, whereby the molded blade will solidify from
the base upwardly.
10. The investment casting assembly of claim 9, wherein said
plumbing system comprises a sprue adjacent to said blade mold and a
feeder tube for flowing molten metal from said sprue to said mold
cavity.
11. The investment casting assembly of claim 10, wherein said
feeder tube extends from said sprue to the base of said blade
mold.
12. The investment casting assembly of claim 10, wherein said sprue
is substantially vertically oriented, adjacent and parallel to said
blade mold.
13. The investment casting assembly of claim 12, wherein said
feeder tube extends from said sprue to the base of said blade
mold.
14. The investment casting assembly of claim 12, further comprising
a pour cup at the vertically upper end of the sprue for receiving
molten metal.
15. The investment casting assembly of claim 9, wherein the mold is
configured for forming a turbine bucket, the base of said mold
cavity is configured to form a dovetail/shank of the turbine bucket
and the vertically upper end of said mold cavity is configured to
form a tip shroud of the turbine bucket.
Description
BACKGROUND OF THE INVENTION
[0001] A gas turbine is typically comprised of a compressor section
that produces compressed air. Fuel is mixed with a portion of the
compressed air and burned in one or more combustors, thereby
producing hot compressed gas. The hot compressed gas is expanded in
a turbine section to produce rotating shaft power. The turbine
section is typically comprised of a plurality of alternating rows
of stationary vanes (nozzles) and rotating blades (buckets). Each
of the rotating blades has an airfoil portion and a root portion by
which it is affixed to a rotor.
[0002] On many rotating airfoils, integral tip shrouds are used on
the radially outer end of the blade to create an outer surface of
the passage through which the hot gases must pass. Having the
shroud as a part of the airfoil results in an increase in
performance for the engine. As such, it is desirable for the entire
outer surface to be covered by the tip shrouds. However, integral
shrouds on rotating airfoils are highly stressed parts due to the
mechanical forces applied via the rotational speed. The high
temperature environment coupled with the high stresses makes it a
challenge to design a shroud that will effectively perform over the
entire useful life of the remainder of the blade. One weak area of
the shroud is the fillet between the airfoil and tip shroud. One
possibility for resolving this challenge is to reduce the stress
applied to the tip shroud fillet. One common method is to scallop
or remove a portion of the overhanging shroud, thus reducing the
load applied. However, physically removing tip shroud coverage
results in a detriment to engine performance.
BRIEF DESCRIPTION OF THE INVENTION
[0003] The present invention relates to a blade for a turbine, e.g.
aircraft engine, gas turbine, steam turbine, etc. More
specifically, the present invention relates to an investment
casting process that enables directionally solidified tip shrouded
turbine blades or buckets to have a continuous grain structure that
extends through the tip shroud in addition to increasing the
quantity of grains in the root of the part. The invention may be
readily applied to land-based turbine buckets or aircraft engine
turbine blades.
[0004] Thus, the invention may be embodied in an investment casting
process for forming a directionally solidified blade comprising:
providing a blade mold, said mold being oriented so that a portion
of a mold cavity thereof for forming a base of said blade is at a
base thereof and a portion of the mold cavity for forming a tip of
the blade is at a vertically upper end thereof; providing a heat
removal feature to extend below said mold; flowing molten metal
into said mold cavity of said mold; and allowing said blade to
solidify from the base upwardly.
[0005] The invention may also be embodied in an investment casting
assembly for molding a directionally solidified blade comprising: a
blade mold, said mold being oriented so that a portion of a mold
cavity thereof for forming a base of said blade is at a base
thereof and a portion of the mold cavity for forming a tip of the
blade is at a vertically upper end thereof; a heat removal feature
disposed to extend below said mold; and a plumbing system for
flowing molten metal into said mold cavity of said mold, whereby
the molded blade will solidify from the base upwardly.
BRIEF DESCRIPTION OF THE DRAWINGS
[0006] These and other objects and advantages of this invention,
will be more completely understood and appreciated by careful study
of the following more detailed description of the presently
preferred exemplary embodiments of the invention taken in
conjunction with the accompanying drawings, in which:
[0007] FIG. 1 is a schematic perspective view of a turbine blade
with tip shroud;
[0008] FIG. 2 is a schematic plan view of conventional tip shrouds,
illustrating shroud scalloping;
[0009] FIG. 3 is a schematic illustration of a typical turbine
blade casting arrangement; and
[0010] FIG. 4 is a schematic illustration of a turbine blade
casting arrangement according to the process of an example
embodiment of the invention.
DETAILED DESCRIPTION OF THE INVENTION
[0011] A typical blade with cooling passages exiting at the blade
tip to flow over the tip shroud is schematically illustrated in
FIG. 1. As schematically illustrated therein, each turbine blade 10
is comprised of an airfoil portion 12 and a root portion 14. The
airfoil portion has a leading edge and a trailing edge. A generally
concave pressure surface and a generally convex suction surface
extend between the leading and trailing edges on opposing sides of
the airfoil. In the illustrated example, the blade root 14 is
comprised of a shank 16 and a dovetail 18 to engage a corresponding
dovetail groove on the rotor to secure the blade to the rotor.
[0012] As shown in FIGS. 1 and 2, a shroud 20 is formed at the tip
of the airfoil 12 and extends outwardly from the airfoil. The
shroud thus has radially inward and radially outward facing
surfaces and is exposed to the hot compressed gas flowing through
the turbine section. Each shroud has bearing surfaces 22,24 over
which it contacts a shroud of an adjacent blade thereby restraining
blade vibration. Furthermore, one or more baffles 26 typically
extend radially outward from the shroud to prevent leakage of hot
gas around the respective blade row. In some conventional bucket
blade structures, a plurality of cooling air passages extend
radially outwardly through the blade into the blade tip. In other
conventional bucket blade structures serpentine passages are
defined in the airfoil. As shown in FIG. 2, the radial cooling air
passages, conventionally terminate at air discharge holes 28 that
allow the cooling air to discharge at the radially outward surface
of the shroud.
[0013] Directionally solidified (DS) turbine blades/buckets (FIG.
1) are desired in some applications because of the superior
mechanical properties exhibited by DS grain structure compared to
equiaxed grain structures. Typically, DS grains will grow in the
desired direction normal to the chill plate and parallel to the
withdrawal direction, but only up to a point. The DS grains will
not grow around 90 degree angles or corners. Beyond that, e.g.,
through radiuses where platforms or tip shrouds connect with
airfoils, the grain structure in those surfaces which are more or
less perpendicular to the airfoil and the grain growth direction
usually end up being equiaxed or something like it. Thus, the grain
structure in regions of the turbine blades/buckets such as portions
of the tip shroud will not have the desired or required mechanical
properties. This invention provides a process whereby a DS grain
structure can be obtained from the airfoil through the tip shroud
of certain blades/buckets in addition to increasing the quantity of
grains in the root of the part.
[0014] Before DS and Single Crystal (SC) casting techniques became
productionized, investment cast turbine blades and buckets were
cast with an equiaxed grain structure. Because of the geometry of
these components; i.e. heavier cross-sections in the root and shank
ends, tapering to thinner cross-sections at the tip end, gates to
allow the molten metal to enter and fill the molds were placed on
the heavier ends. These components are usually cast in a tip down
attitude in part to take advantage of gravity in the filling and
feeding processes required to produce sound castings. The Tip-Down
attitude allowed a natural filling and feeding to take place where
the metal remained molten longest at the gated end and remained
available to feed the casting as it shrank volumetrically on
cooling. These structures have different mechanical properties in
different crystallographic directions.
[0015] The Bridgeman process enabled investment castings to be
produced with a controlled crystallographic orientation, so that
the superior properties of a specific crystallographic orientation
could be utilized. In the DS process, grains nucleate on a chill
plate and their growth is controlled by the direction and method of
heat extraction. The grains grow normal to the chill plate. They
can grow at an angle, but generally they will not grow around
corners, i.e., they usually will stop growing as they become
parallel to the chill plate (or perpendicular to the withdrawal
direction). The grains will also stop growing in the desired
direction if/when their growth is interrupted by a surface of the
mold that intersects the growth direction (e.g., a tip shroud or a
platform).
[0016] Since conventional DS castings are produced in the Tip-Down
attitude, grain growth begins at the outermost surface of the tip
shroud of the bucket. As the grains grow towards the airfoil, those
grains that enter the tip shroud from the chill plate (outside of
the casting), encounter the airfoil gas path surface of the part
(the mold material) and are stopped from growing further. These
grains are truncated DS grains, and have the appearance of equiaxed
grains when the part is etched. They are not really equiaxed, but
are short sections of DS grains, and the properties in these areas
are most likely comparable to the transverse properties of the DS
grain structure.
[0017] The number of grains in the entire structure is limited by
the smallest cross-section through which the grains are permitted
to grow. Referring to FIG. 3, in the typical casting process of a
turbine bucket or aircraft engine blade, as grain growth progresses
from the tip shroud 120, a smaller cross-section through the ever
increasing cross-section of the airfoil 112, and then through the
platform and the still larger cross-section of the shank and the
dovetail, shown generally at 114, the number of grains that grew
through the small tip shroud 120 does not increase, and in fact may
decrease as the larger, faster-growing grains absorb and crowd out
the smaller-growing grains. Thus, the limited number of grains
present is required to fill the increased cross-section and volume
of the part and the dovetail will therefore have fewer grains than
the tip shroud. There will be many grains throughout the airfoil
that do not extend to or through the dovetail. These grains will be
held together only by the strength that bonds them to the adjacent
grain. It is desirable to have as many grains as possible held at
the dovetail, so that the strength of the component is that of the
crystallographic grains, rather than the grain boundary
strength.
[0018] Thus, in the prior art utilizing the Bridgeman process, the
components are aligned with the tip shroud end 120 of the component
attached to the heat removal feature 130 (which may for example be
a chill plate) as shown in FIG. 3. More specifically, molten metal
132 is poured into a pour cup 134 and then through an appropriate
plumbing system which feeds the molten metal ultimately into the
part 110. A heat removal feature, for example a chill plate, 130 is
disposed to extend horizontally below the mold and is provided as a
grain nucleation starter. In the illustrated example the plumbing
system is comprised of a vertically oriented sprue 136 which is
simply a hollow tube, and a feeder tube 138 that extends from the
sprue 136 to the mold.
[0019] As illustrated in FIG. 3, the part mold is configured so
that the tip shroud 120 is disposed adjacent the heat removal
feature, the airfoil 112 extends vertically from the tip shroud to
the shank/dovetail mold section 114, and grain growth is in the
direction towards the shank/dovetail. These parts are cast in
vacuum; i.e. there is no air present and no need for air vents.
[0020] An objective of the invention is to create a grain structure
in a bucket tip shroud that is more desirable than the result of
the prior art. For example, the invention will allow the bucket
grain to grow around the tip shroud fillet, producing superior
mechanical properties in this highly stressed region of the part.
Given the superior properties afforded by the invention, the
turbine bucket can be designed to operate at a higher temperature
or for a longer duration. In the example of the tip shroud fillet,
this invention may eliminate the need to scallop the tip shroud,
resulting in superior engine performance. Another objective of the
invention is to increase the number of grains in the dovetail
(where the part is affixed to the turbine wheel). In the prior art
process, the number of grains that extend through the part from the
tip shroud 120 to the dovetail 114 is limited by the number of
grains that can grow through the airfoil 112 and the footprint of
that airfoil where it is attached to the platform/shank. This small
footprint, or window, restricted the number of grains of the
desired orientation and properties that will reach into the
dovetail.
[0021] The invention orients the mold for the part 210 such that
the root 214 is down so that solidification and grain initiation
and growth begins at the opposite end of the component as compared
to the prior art and the solidification front moves from the
dovetail 214 towards the tip shroud 220; approximately 180 degrees
opposite from the solidification and grain growth pattern of the
prior art. Thus, referring to the example embodiment depicted in
FIG. 4, as in the conventional molding process, molten metal 232 is
poured into a pour cup 234 before flowing through the plumbing
system into the part 210. A heat removal feature, e.g. a chill
plate, 230 is disposed to extend horizontally below the mold as a
grain nucleation starter. In the illustrated example embodiment the
plumbing system is comprised of a vertically oriented sprue 236
which is simply a hollow tube, and a feeder tube 238 that extends
from the sprue 236 to the mold. As an alternative, alloy may be
introduced into the part cavity through a non-vertical sprue, and
the feeder tube may be disposed in another location, either near
the heat removal feature 230 or higher along the part mold.
[0022] In contrast to the conventional process described above with
reference to FIG. 3, In accordance with embodiments of the
invention, the mold is configured so that the shank/dovetail mold
section 214 is disposed adjacent the heat removal feature 230, the
airfoil 212 extends vertically upwardly from the shank/dovetail to
the tip shroud 220, and grain growth is in the direction towards
the tip shroud 220.
[0023] In accordance with the process of the invention, the larger
area of contact with the heat removal feature 230 initiates a
larger number of properly oriented grains that then grow through
the part from the lager cross-section into the diminishing
cross-section. It is desirable to have more correctly aligned DS
grains with their superior properties in the highly stressed
dovetail region 214. The same grains that will be initiated and
held in the dovetail will extend through the length of the airfoil
212. An increase in the quantity of through-going grains will
result in an increase in the stress-carrying capability in the
component.
[0024] While the invention has been described in connection with
what is presently considered to be the most practical and preferred
embodiment, it is to be understood that the invention is not to be
limited to the disclosed embodiment, but on the contrary, is
intended to cover various modifications and equivalent arrangements
included within the spirit and scope of the appended claims.
* * * * *