Abstract:
The invention relates to a composition for an abradable seal of a turbomachine, the composition comprising an aluminium base, nickel powder, polyester powder. The invention also relates to an outer casing of a low-pressure compressor of an axial-flow turbomachine with an abradable seal surrounding an annular row of rotor blades. The seal comprises a rounded support covered with a layer of abradable material comprising a metallic phase mainly made of aluminium and with nickel in a lesser proportion. The abradable material additionally comprises from 25% to 55% of additive, such as polyester, methyl methacrylate, hexagonal boron nitride, calcium fluoride. The support is segmented, and forms an organic matrix composite outer casing of the compressor. The invention also proposes a process for producing an abradable seal by plasma spraying an Al—Ni-polyester powder.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit, under 35 U.S.C. §119, of EP 14194526.1, filed Nov. 24, 2014, the disclosure of which is incorporated herein by reference in its entirety. 
       FIELD 
       [0002]    The invention relates to the field of axial-flow turbomachine seals made of abradable material. More specifically, the invention tackles the subject of seals made of aluminium-based abradable material for an axial-flow turbomachine compressor for aircraft. 
       BACKGROUND 
       [0003]    An abradable material is known for ensuring dynamic sealing in a turbomachine, e.g., in a compressor of the axial-flow turbomachine. Such material is applied as a layer to an outer casing and is capable of eroding in the event of contact with the tips of rotor blades without damaging them. The mechanical integrity is preserved despite speeds of rotation of greater than 8000 rpm, and a relative speed between a blade tip and its seal substantially equal to the speed of sound. Such a material is also used for ensuring sealing under inner shrouds, or on a drive shaft bearing. 
         [0004]    In this way, it is possible to design a compressor while reducing the operating clearance between the tips of rotor blades and the inner surface of the casing. The optimization can also take into account the expansion, the centrifugal force; and certain operating risks such as ingestions and pumping phenomena. An abradable material that forms a casing coating generally comprises aluminium, silica, polyester. 
         [0005]    Document EP 1 010 861 A2 discloses a composition of a layer of abradable material for an axial-flow turbomachine compressor. The composition comprises a mixture of aluminium powder and silica powder, the aluminium being in the majority. In addition, the powder has a polymer powder with methyl methacrylate. This composition is applied by plasma spraying onto a wall of an axial-flow turbomachine compressor, the wall receiving beforehand a bonding layer with a majority amount of nickel, and aluminium in a lesser proportion. This composition makes it possible to produce an abradable seal with a requirement of corrosion resistance, however this corrosion resistance still warrants being improved in order to optimize the resistance and the efficiency of the turbomachine. 
       SUMMARY 
       [0006]    The objective of the invention is to solve at least one of the problems posed by the prior art. More specifically, one objective of the invention is to improve the corrosion resistance of an axial-flow turbomachine seal made of abradable material. Another objective of the invention is to optimize both the durability of the abradable material, the friable nature of the abradable material and the sealing provided by the abradable material. 
         [0007]    In various embodiments, the present disclosure discloses a seal for an axial-flow turbomachine, e.g., for a compressor, e.g., a low-pressure compressor, of an axial-flow turbomachine. In various embodiments, the seal comprises a rounded support, at least one coating made of abradable material with a metallic phase mainly or predominantly comprising aluminium, e.g., aluminium powder, the metallic phase additionally comprises nickel, e.g., a nickel powder, the abradable coating being applied to the rounded support, and being structured and operable to cooperate by abrasion with a rotor element of the axial-flow turbomachine. 
         [0008]    Another subject of the invention is a composition for an abradable coating of a seal of an axial-flow turbomachine, e.g., for application to a rounded support by plasma spraying, the composition comprising metal, e.g., a metallic powder, with mainly aluminium, an organic or mineral filler, such as powder, in various embodiments the metal additionally comprises nickel, e.g., nickel powder. 
         [0009]    According to various embodiments of the invention, the metal comprises, by weight, between 20% and 45%, e.g., between 35% and 45% of nickel, e.g., of nickel powder. 
         [0010]    According to various embodiments of the invention, the metal comprises, by weight, between 55% and 80% of aluminium, e.g., of aluminium powder. 
         [0011]    According to various embodiments of the invention, the composition comprises, by weight, between 5% and 50% of filler, e.g., between 15% and 25% of filler. In various embodiments, the filler is a polymer such as polyester or methyl methacrylate, or hexagonal boron nitride, or calcium fluoride, e.g., as powder(s). 
         [0012]    According to various embodiments of the invention, the combination of aluminium and nickel represents more than 80% of the weight of the metal of the composition, for example, more than 90%, e.g., more than 95%. 
         [0013]    Another subject of the invention is a seal of an axial-flow turbomachine, e.g., of a compressor of an axial-flow turbomachine, wherein the seal comprises a rounded support, at least one coating made of abradable material covering the rounded support, and which is intended to cooperate by abrasion with a rotor element of the axial-flow turbomachine. In various embodiments the composition of the abradable coating is in accordance with the invention, as described herein. 
         [0014]    According to various embodiments of the invention, in the abradable coating, the metal forms a matrix which is combined with the filler. In various embodiments in the abradable coating, the volume occupied by the metallic matrix is the majority volume. 
         [0015]    According to various embodiments of the invention, the rounded support is an outer shroud, the abradable coating being on the inside of the outer shroud, or alternatively the rounded support is an outer casing, the abradable coating being positioned on the inside of the outer casing. 
         [0016]    According to various embodiments of the invention, the support is made of an organic matrix composite material with carbon fibres and/or glass fibres, the fibres being arranged as a stack of optionally woven fibrous plies. 
         [0017]    According to various embodiments of the invention, the support comprises attachment means, such as orifices and/or at least one annular attachment tab, the means being positioned upstream and/or downstream of the abradable coating. 
         [0018]    According to various embodiments of the invention, the support has an annular groove with an upstream edge and a downstream edge, the abradable coating covering the annular groove from the upstream edge to the downstream edge. 
         [0019]    According to various embodiments of the invention, the abradable coating forms a layer having a thickness of greater than 0.30 mm, for example, greater than 2.00 mm, e.g., greater than 5.00 mm. 
         [0020]    According to various embodiments of the invention, the coating forms a uniform layer over its thickness and/or over its main area. 
         [0021]    According to various embodiments of the invention, the rounded support forms a circle or a portion of a circle. The expression “portion of a circle” can be understood to mean an angular portion of a circle. 
         [0022]    According to various embodiments of the invention, the abradable coating has a compactness of greater than 80%, for example, of greater than 95%, e.g., of greater than 99%. 
         [0023]    According to various embodiments of the invention, in the metal of the abradable coating, the volume of aluminium is the majority volume. 
         [0024]    According to various embodiments of the invention, the abradable coating forms a leak-tight barrier so as to protect the support from chemical attacks of the turbomachine. 
         [0025]    According to various embodiments of the invention, the seal comprises an interlayer positioned between the support and the abradable coating, e.g., the interlayer is a metal strip. 
         [0026]    Another subject of the invention is a process for producing an abradable seal of an axial-flow turbomachine, e.g., of a compressor of an axial-flow turbomachine, the seal comprising a rounded support and an abradable coating applied to the support, the process comprising the following steps: (a) provision or manufacture of a rounded support; (d) application of an abradable composition to the rounded support, the composition comprising metallic powder that mainly comprises aluminium powder. In various embodiments, during the application step (d), the metallic powder additionally comprises nickel powder. IN various embodiments the composition is in accordance with the invention, as described herein. 
         [0027]    According to various embodiments of the invention, during the application step (d), the composition is applied by plasma spraying to the support; e.g., the composition additionally comprises an organic powder such as a polymer, or a mineral powder. 
         [0028]    Another subject of the invention is a turbomachine comprising an abradable seal produced according to a process, wherein the abradable seal is in accordance with the invention, as described herein, and/or the process is in accordance with the invention, as described herein. 
         [0029]    According to various embodiments of the invention, the turbomachine comprises a low-pressure compressor with at least one row of rotor blades, the seal being a seal of the low-pressure compressor which surrounds the at least one row of rotor blades. 
         [0030]    According to various embodiments of the invention, the turbomachine comprises at least one annular row of stator blades with platforms attached to the support, the abradable coating being in contact with the platforms of blades. In various embodiments, the abradable coating forms a leak-tight barrier that protects the support starting from, axially, the platforms. Such a seal provides two-fold sealing, for example, dynamic sealing and static sealing. 
         [0031]    According to various embodiments of the invention, the support is an inner shroud connected to the inner ends of a row of stator blades, the abradable coating being applied to the inside of the inner shroud. 
         [0032]    According to various embodiments of the invention, the compressor comprises heating elements capable of heating the abradable material coating, and/or the compressor comprises flow bleed ports. 
         [0033]    According to various embodiments of the invention, the inner surface of the abradable seal guides an annular flow, generally an axial annular flow, of the turbomachine. 
         [0034]    Generally, the advantageous embodiments of each subject of the invention are also applicable to the other subjects of the invention. 
         [0035]    The performance offered by the invention limits releases of the filler, such as polymer, into the turbomachine so as not to disrupt the combustion therein. The release of particles that obstruct the ducts and the mechanisms, or that abrade the latter, is restricted. These advantages fall within a context of improving safety. 
         [0036]    The invention is particularly relevant within the context of a low-pressure compressor since the air which enters therein can be at −50° C., and at +170° C. at the compressor outlet. The composition proposed is suitable for low temperatures as well as for intermediate temperatures, for the temperature variations of the primary flow as a function of the flight phases of an aircraft over a range of more than 200° C. The invention additionally makes it possible to preserve the thermal resistance of the abradable material. 
         [0037]    The Al—Ni-polyester composition offers a certain resistance to corrosion, e.g., faced with a salt spray as tests have proven. The resistance can be observed over a wide range of temperatures, for example from −50° C. to more than 150° C. This resistance also respects the sealing over time without prematurely degrading the rotor blades. These advantages space out the maintenance operations of the turbomachine which enables a substantial saving to be made. 
     
    
     
       DRAWINGS 
         [0038]      FIG. 1  represents an axial-flow turbomachine according to various embodiment of the invention. 
           [0039]      FIG. 2  is a diagram of a turbomachine compressor according to various embodiment of the invention. 
           [0040]      FIG. 3  outlines a turbomachine abradable seal according to various embodiment of the invention. 
           [0041]      FIG. 4  illustrates a flowchart of a process for producing a turbomachine abradable seal according to various embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0042]    In the description which follows, the terms interior or inner and exterior or outer refer to a positioning relative to the axis of rotation of an axial-flow turbomachine. The axial direction corresponds to the direction along the axis of rotation of the turbomachine. 
         [0043]      FIG. 1  represents an axial-flow turbomachine in a simplified manner. In this specific case it is exemplarily illustrated as a turbofan. The turbofan  2  comprises a first compression stage, referred to as low-pressure compressor  4 , a second compression stage, referred to as high-pressure compressor  6 , a combustion chamber  8  and one or more turbine stages  10 . In operation, the mechanical power of the turbine  10  transmitted by the central shaft to the rotor  12  moves the two compressors  4  and  6 . The latter comprise several rows of rotor blades combined with rows of stator blades. The rotation of the rotor about its axis of rotation  14  thus makes it possible to generate an air stream and to gradually compress the latter up to the inlet of the combustion chamber  8 . One or more gear reduction means can increase the speed of rotation transmitted to the compressors. 
         [0044]    An inlet fan commonly denoted by fan or blower  16  is coupled to the rotor  12  and generates an air flow which is split into a primary flow  18  that passes through the aforementioned various stages of the turbomachine, and a secondary flow  20  that passes through an annular duct (partially represented) along the machine in order to then rejoin the primary flow at the outlet of the turbine. The secondary flow can be accelerated so as to generate a thrust reaction. The primary flow  18  and secondary flow  20  are annular flows, they are channelled by the casing of the turbomachine. For this purpose, the casing has cylindrical walls or shrouds which can be inner and outer walls or shrouds. 
         [0045]      FIG. 2  is a cross-sectional view of a compressor of an axial-flow turbomachine such as that from  FIG. 1 . The compressor can be a low-pressure compressor  4 . A portion of the blower  16  and the splitter  22  for separating the primary flow  18  and the secondary flow  20  can be seen therein. The rotor  12  comprises several rows of rotor blades  24 , in this particular case three. 
         [0046]    The low-pressure compressor  4  comprises several straighteners, in this particular case four, which each contain a row of stator blades  26 . The straighteners are combined with the blower  16  or with a row of rotor blades  24  in order to straighten the air flow, so as to convert the speed of the flow into static pressure. The stator blades  26  extend essentially radially from an outer casing  28  of the compressor, and can be attached thereto with the aid of a bolt  30  that radially extends the platforms  32  of stator blades  26 . 
         [0047]    The outer casing  28  can comprise an annular wall  34  and annular attachment flanges  36  that axially delimit the wall  34 . The outer casing  28  can be formed of two half-shells. The wall  34  forms a sleeve and has an ogive shape, and its rotational profile is curved and mainly extends axially; its radius varies. The wall  34  serves as support for attachment to the attachment platforms  32  of the stator blades  26 , and as support for the application of coatings of abradable materials  38  that ensure dynamic sealing, so as to form abradable seals  39  around the rotor blades  24 . The abradable coatings  38  form uniform annular layers, such as circular strips, the thicknesses of which are greater than 2.00 mm. Dynamic sealing is understood to be a limitation of the flow between the abradable material and a rotor blade that turns during the operation of the turbomachine. 
         [0048]    The annular attachment flanges  36  extend radially outwards. The annular flanges  36  upstream and downstream of the casing  28  make it possible to attach the compressor  4  to the intermediate blower casing  40 , but also enable the attachment of the splitter  22 . The attachment flanges  36  can comprise axial attachment orifices (not represented), and tubular seatings. The abradable coatings  38  can be positioned axially between the annular attachment flanges  36 . 
         [0049]    The casing  28 , e.g., its wall  34 , can be made from an organic matrix composite material. The composite material can comprise an epoxy resin and a preform with a stack of three-dimensionally woven carbon fibrous plies. Alternatively, the casing can be made of metal, such as a titanium or aluminium alloy. According to an alternative of the invention, the casing can be formed from several outer shrouds, added and attached axially to one another, for example using radial annular flanges. 
         [0050]    The rows of stator blades  26  can support inner shrouds  42  connected to the inner ends of the stator blades  26 . The inner surfaces of the inner shrouds  42  can support layers of abradable materials  44  so as to form seals  45  around annular strips of the rotor  12 , or lips, to combat recirculations under the inner shrouds  42 . 
         [0051]    The term “coating” can be understood to mean the skin of the structure of the seal ( 39 ;  45 ), over which the air flows and which is subjected to aerodynamic forces. The coating can form the outer or inner surface of the seal ( 39 ;  45 ). 
         [0052]      FIG. 3  represents an abradable seal  39  of a compressor such as that from  FIG. 2 . Represented therein is a wall  34  of a casing  28 , or support  28 , an abradable coating  38  which is applied thereto, and a tip of a rotor blade  24  between two stator blades  26 . 
         [0053]    The abradable coating  38  extends from one platform  32  of a blade  26  to the next, which belongs to a neighbouring row positioned upstream or downstream. The abradable coating  38  is flush with the inner surfaces of the platforms  32 . The platforms  32  of stator blades  26  form circular shoulders  46  that axially delimit the abradable coatings  38 . Alternatively, the wall has radial annular grooves formed in its thickness, which are filled in by the abradable coatings, over the entire depths thereof. The abradable coatings cover the entire inner surface of the wall between the platforms of the stator blades. The combination of the platforms  32  and the abradable coatings  38  forms a generally leak-tight barrier all along the casing  28 , or at least the wall  34 , in various embodiments, except for grooves between the platforms  32  of one and the same annular row. 
         [0054]    The abradable coating  38  can be applied directly on its support  34 . Or else, the seal  39  can comprise an interlayer between the support and the abradable coating. The interlayer can be a strip, such as a sheet of steel, or a sheet of nickel. The strip can be perforated and/or cut. 
         [0055]    The abradable coating  38  has an inner surface in contact with the primary flow  18 . Its surface guides and delimits the primary flow  18  during the compression thereof. The abradable coating  38  can comprise at least two mixed phases, namely a metallic phase and optionally a filler phase, such as a mineral phase and/or an organic filler, so as to form a composite. The materials of the abradable coating can be granular, or some can be granular and others can fill in the spaces between the grains thereof. 
         [0056]    The metallic phase of the abradable coating  38  mainly comprises aluminium. The metallic phase of the coating is based on aluminium. That is, among the metals of the abradable material, the one having the largest weight is aluminium. The predominance of aluminium optimizes the weight of the seal  39 . The metallic phase of the abradable coating  38  also comprises nickel, in a proportion by weight lower than that of the aluminium. The metallic phase can comprise between 20% and 45% nickel, and between 55% and 80% aluminium. In addition, the metallic phase can, in various instances, comprise iron, copper, zinc, manganese, magnesium, impurities, these components each representing between 1% and 0.1% of the weight of the metallic phase. 
         [0057]    The filler of the abradable coating  38  can comprise polymer, such as polyester or methyl methacrylate. The filler can also comprise hexagonal boron nitride or calcium fluoride. The weight of the filler can represent between 5% and 50%, for example, between 15% and 25%, e.g., 20% of the weight of the coating  38 . The metallic phase can represent the majority of the volume of the abradable coating. Thus, the metallic phase can form therein a matrix that receives the filler or additive. In various instances, the abradable coating can be formed of grains of metal powders, the intergranular spaces of which are filled in by the filler. The empty space in the abradable coating is less than 1%, e.g., less than 0.1%. 
         [0058]      FIG. 4  represents a flowchart of a process for producing an abradable seal of an axial-flow turbomachine as presented in  FIG. 3 . The seal can be used on a compressor, e.g., a low-pressure compressor. 
         [0059]    The process comprises the following steps, optionally carried out in the following order:
   (a) provision  100  of a rounded support, such as a compressor outer casing,   (b) provision  102  of stator blades with platforms;   (c) attachment  104  of the blades by their platforms to the rounded support forming annular rows;   (d) application  106  of an abradable composition to the rounded support between the annular rows of platforms so as to cover the support between the rows of blade platforms;   (e) assembly  108  of the support around a rotor of the turbomachine, for example an annular row of rotor blades.   
 
         [0065]    At the start of the step (d) of application  106 , the composition has a metallic phase with mainly aluminium, for example in powder form. The metallic phase can also comprise nickel and a filler, e.g., both as powders. The composition of the powder can correspond to the chemical composition of the abradable coating presented above. At the end of the step (d) of application  106 , at least one or each compound of the composition remains in powder form, or at least one of the compounds has melted, or every compound has melted. 
         [0066]    Optionally, some grains or every type of grain are essentially solid, and their own voids represent less than 5% of their material, for example., less than 1%, e.g., less than 0.20%. Each grain can have a homogeneous material. In various embodiments, one type of grain is hollow, for example the aluminium or nickel grains. 
         [0067]    During the step (d) of application  106 , the composition can be applied to the support by plasma spraying. Such a technique is well known to a person skilled in the art, and it can be carried out in a manner similar to that disclosed in document EP 1 010 861 A2. The filler powder can be introduced into the plasma jet downstream of the metallic powders. Other techniques can be envisaged. Alternatively, the composition can be applied to the support by sintering, optionally with prolonged heating. In this alternative, certain grains can keep their initial shapes. 
         [0068]    The steps (b) provision  102  of stator blades, (c) attachment  104  of the blades, (e) assembly  108  of the support are entirely optional according to the invention. Indeed, the abradable composition can be applied to a support free of blades and their platforms. In this case, the step (d) of application  106  can be carried out in a groove formed in the thickness of a shroud.