Investment casting process and apparatus to facilitate superior grain structure in a DS turbine bucket with shroud
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.
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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.
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 INVENTIONThe 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.
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.
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.
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:
A typical blade with cooling passages exiting at the blade tip to flow over the tip shroud is schematically illustrated in
As shown in
Directionally solidified (DS) turbine blades/buckets (
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.
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).
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.
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
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
As illustrated in
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.
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
In contrast to the conventional process described above with reference to
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.
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.
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.
Type: Application
Filed: Oct 31, 2006
Publication Date: May 1, 2008
Applicant: General Electric Company (Schenectady, NY)
Inventors: Stephen Daniel Graham (West Union, SC), Robert Alan Brittingham (Piedmont, SC)
Application Number: 11/589,731
International Classification: B22D 27/04 (20060101);