Metallic cellular structure

Details Metallic cellular structure Metallic cellular structure

2000-11-27

2002-11-26

Knight, Anthony (Department: 3676)

Seal for a joint or juncture

Seal between relatively movable parts

Close proximity seal

C277S415000, C415S173400, C415S174400

Reexamination Certificate

active

06485025

ABSTRACT:

BACKGROUND OF THE INVENTION
(i) Field of the Invention
The present invention relates to abradable gas turbine seals and, more particularly, to novel turbine seals having cellular metallic structures suitable for operation in an oxidizing and/or carburizing gas environment at high temperatures.
(ii) Description of the Related Art
Cellular structures made from thin sheet metal or metal foil are attractive for use in space and aerospace applications, in particular in jet engines, because they provide a high stiffness to weight ratio and high mechanical energy absorption capabilities and acoustic damping while being light in weight. As a direct consequence of their low density they are also readily abradable, which is a characteristic favorable for usage in jet engines, and also in stationary gas turbines used for power generation, to reduce the gap between the stationary shroud and rotating blade components, thereby improving efficiency of the gas turbine cycle. While the cellular structure used as an abradable seal must be soft enough to allow the tips or knife edges of a rotating blade to cut into it without causing damage to the blade and the structure carrying the abradable cellular structure, it must be strong enough to withstand static and high frequency vibratory loads, abrasion, cyclic thermal stresses and cyclic oxidation and/or carburization attack, all occurring at high temperatures. Furthermore, the seal must not crack under thermal shock and low cycle fatigue loading. In other words, the cellular structure used as a gas path seal must be resistant to hot gas corrosion attack and must have excellent structural integrity and long term dimensional stability to withstand the mechanical and thermal loads imposed on it for many cycles and over lengthy periods of time.
Conventional abradable cellular gas path seals are manufactured from highly alloyed, austenitic stainless steels and nickel-base alloys and are provided having regular hexagonal cells formed by corrugating ribbons of sheet or foil and welding the corrugated sheets together where abutting walls of adjacent corrugated ribbons meet to form a double wall, i.e. a node.
U.S. Pat. No. 3,867,061 issued Feb. 18, 1975 typifies a conventional prior art honeycomb shroud for rotor blades for turbines in which the honeycomb cell walls are made from nickel-base heat-resistant alloys and the honeycomb strips are brazed or resistance welded to a back-up ring.
U.S. Pat. No. 4,063,742 issued Dec. 20, 1977 discloses another embodiment of prior art abradable fluid seal for use in gas turbines consisting of a conventional honeycomb made by conventional honeycomb equipment in which abutting three-sided semi-hexagonal strips each having a pair of flat slanted sides and a flat crest of equal length standing on edge (“soe”) are resistance welded together at the crests.
Honeycomb structures typically are fabricated by building up the structure layer by layer to result in a three dimensional body having a height determined by the width to which the ribbon had been slit prior to corrugation. The length of the structure is parallel to the plane of the double walls or nodes and the width is represented by the direction of layer build up. Such a cellular body is brazed to face sheets to form a sandwich skin or brazed to backplates of a rind or ring segments to form a seal, the contacting surface of the cellular body being a “soe” surface. Such brazing does not only join the cellular structure body to the face sheet or backplate but also contributes significantly to the stiffness of the cellular structure itself. This is due to the fact that the brazing alloy, in a liquid state and due to capillary action, rises up the gap formed by the two neighbouring walls of the node, thereby wetting the abutting surfaces of said node walls and, after resolidification of the braze filler metal, forms a stiffened cellular structure. The braze flow up the nodal walls is referred to as “wicking”. Such wicking is essential to provide a brazed cellular structure with good mechanical behaviour at high temperatures to resist combined thermal and mechanical loads.
The most commonly used method to provide a turbine engine seal segment or ring is to sandwich a braze filler metal foil, tape or brazing powder between the abutting surfaces of the “soe” cellular structure and the backplate surface, and brazing this assembly together. The liquid braze metal must travel up the full depth of the nodes to impart improved structural strength. The exposed surface at which the rotor blade tip or knife edge rubs is also the surface where the most severe combination of mechanical load, wear and temperature occurs and therefore requires excellent structural integrity. If inadequate wicking success is achieved during brazing, there is a twofold drawback: not only is the cellular structure unstiffened, with the consequence of low shape stability, which may cause premature failure of the seal body in service, but also the bulk of the braze filler metal remains at the backplate surface and penetrates both the backplate material and the cellular foil alloy structure by diffusion while in the liquid state to a much larger extent than would be the case if the braze alloy flowed up the nodes. This has the effect of significantly altering the chemical and mechanical characteristics of the backplate and the foil metal alloy, at least locally at their juncture.
Because oxidation resistance and carburization resistance at high temperatures are required, sheet or foil metals having a good hot gas corrosion resistance must be used for the manufacture of turbine seals. The resistance of metals to oxidation and carburization is based on the formation of surface oxide layers which protect the underlying metal from further attack. The nickel base alloys and highly alloyed, austenitic stainless steels used in conventional abradable cellular seals rely on the formation of chromia (Cr
2
O
3
) or mixed Cr
2
O
3
/NiO oxides to provide such protection. At very high temperatures or in combustion gas atmospheres flowing at high speeds, both found in turbine engines, this type of protection is unstable due to further oxidation of the Cr
2
O
3
to volatile CrO
3
as described by James L. Smialek and Gerald E. Meier in Superalloys II by Chester T. Sims et al. (eds.), John Wiley & Sons, Inc. (1987). The same authors, in the same handbook, describe that much better protection is achieved with alumina (Al
2
O
3
) which is formed on metals having a high Al concentration and being further enhanced by high chromium (Cr) contents and the addition of rare earth metals, such as yttrium (Y), zirkonium (Zr), cerium (Ce), hafnium (Hf), ytterbium (Yb), praseodymium (Pr), neodymium (Nd), samarium (Sm) or lanthanum (La) leading to so called MCrAlX alloys with X representing the rare earth metal addition and M being the major alloy constituent selected from the group of Ni, Fe or Co or combinations thereof. If yttrium is chosen as the main rare earth addition, then the resulting alloys are referred to as MCrAlY alloys. MCrAlY alloys are disclosed in U.S. Pat. No. 5,116,690 by W. J. Brindley et al., issued May 26, 1992. Other patents such as U.S. Pat. No. 4,034,142 issued Jul. 5, 1977 to R. J. Hecht and U.S. Pat. No. 4,503,122 issued Mar. 5, 1985 to A. R. Nicholls describe similar MCrAlY alloys with excellent self protection against hot gas attack. All the aforementioned patents describe the use of MCrAlY alloys as overlay coatings and not as a structural material in He form of foil to provide a welded cellular structure.
It is difficult to obtain MCrAlY alloys in thin sheet or foil form because they are hard and difficult to roll which is the effect of the high aluminium concentration, typically in the range 2-6% by weight, with 6″7% representing the upper limit to retain workability. If available in thin sheet or foil form the MCrAlY materials are difficult to corrugate and to form into a cellular structure such as described above. In particular, these materials are difficult to form into a corrugated ribbon, if

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