Corrosion-resistant abradable covering

Abstract

A corrosion-resistant abradable covering to adhesively bond to a wear portion of a structural part, the covering including at least a polyester-based matrix and an aluminum alloy filler that are molded together.

Claims

1. An abradable covering for adhesively bonding to a wear portion of a structural part, the covering comprising: at least a polyester-based matrix and an aluminum alloy filler that are molded together, wherein the molding is performed by sintering or hot injection into a mold, wherein the abradable covering contains 25% to 55% polyester, a remainder of the abradable covering composition being constituted by an aluminum alloy having 11% to 13% silicon, and wherein the matrix is constituted by an aromatic polyester of liquid crystal polymer type.

2. A covering according to claim 1, wherein the aluminum alloy is of 4000 or 5000 series.

3. A structural part including at least one wear portion having an abradable covering according to claim 1.

4. A part according to claim 3, the part being made of a metal material or of a composite material.

5. A part according to claim 3, the part corresponding to a compressor ring of an aeroengine, the compressor ring including a wear portion on an inside surface thereof.

6. A turboprop comprising at least one structural part according to claim 3.

7. An aircraft comprising at least one turboprop according to claim 6.

8. A covering according to claim 1, wherein the aluminum alloy is free of copper.

9. A method of making an abradable covering on a structural part having a wear portion, the method comprising: molding a composition comprising at least a polyester powder and an aluminum alloy filler to form the abradable covering; and adhesively bonding the abradable covering on the wear portion of the structural part, wherein the molding is performed by sintering or hot injection into a mold, wherein the abradable covering contains 25% to 55% polyester, a remainder of the abradable covering composition being constituted by an aluminum alloy having 11% to 13% silicon, and wherein the matrix is constituted by an aromatic polyester of liquid crystal polymer type.

10. A method according to claim 9, wherein the molding is performed at a temperature in a range of 180 C. to 400 C.

11. A method according to claim 9, wherein the aluminum alloy is of 4000 or 5000 series.

12. A method according to claim 9, wherein the abradable covering is made as a plurality of abradable covering sectors that are adhesively bonded adjacent to one another on the wear portion of the structural part to form the abradable covering.

13. A method according to claim 9, wherein the abradable covering is adhesively bonded on the wear portion of the structural part at a temperature that is less than or equal to 180 C.

14. A method according to claim 9, wherein the abradable covering is cooled prior to being adhesively bonded on the wear portion of the structural part.

15. A method according to claim 9, wherein the aluminum alloy is free of copper.

Description

BRIEF DESCRIPTION OF THE DRAWING

(1) Other characteristics and advantages of the invention appear from the following description of particular embodiments of the invention given as non-limiting examples, and with reference to the accompanying drawing, in which:

(2) FIG. 1 is a diagrammatic perspective view showing an abradable covering being mounted on a turbine ring in an embodiment of the invention; and

(3) FIG. 2 is a section view of the FIG. 1 ring mounted around a set of rotary blades.

DETAILED DESCRIPTION OF AN EMBODIMENT

(4) The abradable covering of the invention is made from a composition or mixture comprising a polyester powder and an aluminum alloy, in particular an alloy of aluminum and silicon (AlSi), likewise in the form of a powder of or solid particles. The composition comprises 25% to 55% of polyester, with the remainder or balance being constituted by the aluminum alloy. With an AlSi alloy, the alloy has 11% to 13% silicon. By way of example, such a composition is available in the form of powders for thermal spraying such as the following powders: Metco 601NS and Durabrade 1605 (60% AlSi alloy with 12% Si and 40% polyester) sold by the supplier Sulzer-Metco. The polyester used in the composition of the abradable covering of the invention is preferably selected from aromatic polyesters of the liquid crystal polymer class, which are polyesters that have little reactivity, that are inert, and that strongly resist fire, such as for example the polymers sold under the trademarks Ekonol and Xydar, or under the reference CGZ-351. The aluminum alloy is preferably a copper-free AlSi or AlMgSi alloy in the 4000 or 5000 series that is particularly good at withstanding saline corrosion.

(5) In accordance with the invention, the abradable covering of the invention is not formed by thermal spraying as in the prior art, but by molding so as to obtain a matrix of polyester instead of a matrix of AlSi alloy.

(6) The wear portion of the path onto which the abradable covering is to be adhesively bonded is molded to shape at a temperature lying in the range 180 C. to 400 C. The molding may be performed by sintering or by hot injection.

(7) Sintering presents the advantage of enabling temperature to be controlled accurately during molding, and consequently enabling the abradable nature of the abradable covering to be better adjusted. The sintering/compression time depends on the shape of the mold and on the thickness of the abradable covering to be formed. The sintering pressure is not less than 50 megapascals (MPa).

(8) Hot injection is adjusted mainly as a function of the fluidity of the composition to be injected. The parameters for controlling hot injection are as follows:

(9) the temperature of injection;

(10) the pressure of injection; and

(11) the duration of injection.

(12) These parameters depend on the nature of the material that is to be injected and also on the shape of the molding tooling, and in particular on the number of injection points.

(13) The abradable covering may be molded as a single piece, or else as a plurality of sectors that are assembled together during adhesive bonding. Molding as a single piece requires a specific mold to be prepared having the shape of the entire wear portion of the part, but subsequently facilitates assembly during adhesive bonding, whereas molding in sectors simplifies the design of the mold, but requires accuracy in assembling the sectors together during adhesive bonding.

(14) When molding in sectors, it is possible to optimize the shapes of the connection portion between sectors so as to make them easier to align, such as a shape that is smooth or chamfered. In addition, the connection portions of each sector may be more elaborate in shape, such as for example a shape with crenellations and/or chevrons or an undulating shape, in order to increase the cohesion of mechanical strength and reduce any leakage between sectors.

(15) Once the abradable covering has been molded to the desired shape, it is adhesively bonded onto the wear portion of the structural part. Mounting the covering by adhesive bonding may be performed at ambient temperature, with the structural part and the abradable covering being at the same temperature. This makes it possible to obtain this an abradable covering that is not under any stress on the part, as contrasted to when the covering is deposited on the part by thermal spraying as in the prior art. With thermal spraying, the covering formed in that way is under tension on the part because of the differences between the coefficients of thermal expansion of the covering and of the part or substrate onto which it is sprayed.

(16) Furthermore, by the method of the invention, the covering may even be adhesively bonded on the part while the covering is in compression, with this being done by cooling the molded covering before bonding it onto the part, when the covering presents a coefficient of thermal expansion that is greater than that of the material of the part.

(17) When the bonding is performed with adhesive in the form of a liquid or a film that requires heat treatment, generally at a maximum temperature of 180 C., the molded abradable covering may be put into place on the structural part at ambient temperature. Nevertheless, the element(s) making up the abradable covering is/are preferably cooled prior to being put into place and adhesively bonded, e.g. by spending time in a deep freeze or by using a refrigerating gas such as CO.sub.2, in order to make the element(s) contract. Thus, it is easier to put the elements making up the abradable covering into place on the structural part because of the greater clearance between the element(s) of the abradable covering and the structural part. On heating, the element(s) of the abradable covering expand and move correctly into final position on the part for adhesive bonding. Under such circumstances, the abradable covering is in compression on the structural part.

(18) The adhesive bonding is performed using support tooling enabling dynamic pressure points and wetting to be applied so as to position the molded covering or the molded covering sectors accurately in position on the wear portion of the part.

(19) For bonding, an adhesive is used in the form of a liquid or a film capable of operating continuously at a temperature of at least 140 C., e.g. an adhesive of 180 C. class such as the adhesives sold under the references FM 475, FM 34, Meltbond 329, and Redux 322.

(20) After the covering has been bonded onto the part, it may optionally be machined in order to adjust it to match the defined final dimensions.

(21) FIG. 1 shows a low pressure compressor ring 10, e.g. for a turbine engine diffuser. As shown in FIG. 2, the ring 10 surrounds a rotary assembly made up of a plurality of blades 20, with the clearance between the inside surface of the ring and the tips 21 of the blades being zero or almost zero.

(22) The ring 10 comprises a structural metal support 11 having on the inside a wear portion 110 defined by two rims 11a and 11b. The abradable covering 12 is adhesively bonded to the wear portion 110. More precisely, in the presently-described example, the abradable covering 12 is made up of a plurality of sectors 120 that are bonded end to end onto the wear portion so as to form an abradable covering that is continuous on the inside of the ring. The ends of each of the sectors 120 are provided with respective connection portions 120a and 120b presenting a stepped shape so as to enable the sectors to be assembled together while being mutually engaged.

(23) Nevertheless, the abradable covering could be molded as a single piece and adhesively bonded directly onto the ring, which is also a single piece, without any need to sectorize the covering or the ring.

(24) In the presently-described example, the support 11 is made of a metal material such as titanium, steel, or aluminum.

(25) The structural part (ring) on which the abradable covering is bonded may also be made of composite material, i.e. a part comprising fiber reinforcement that is densified by a matrix. In particular, it may be made out of an organic matrix composite material, the fiber reinforcement being made from a fiber structure obtained by weaving, assembly, knitting, etc. using fibers such as glass fibers, carbon fibers, or metal filaments. The fiber structure is then densified with a matrix, which may in particular be a thermoplastic or a thermosetting resin.

(26) The present invention is not limited to forming abradable coverings for a compressor ring. It applies to making any type of abradable covering that is to be used in a corrosive environment.