Method for treating the surface of a protective shield for a leading edge of a blade

Abstract

The invention relates to a method for treating the surface of a steel protective shield (14) for a leading edge (12a) of a blade (11) for an aircraft turbine engine (1), characterized in that it comprises the following chronological steps: (c) stripping the protective shield (14) in an electrolyte bath (18); (e) depositing an adhesion layer on the protective shield (14).

Description

[0001] DESCRIPTION

[0002] TITLE: METHOD FOR TREATING THE SURFACE OF A PROTECTIVE SHIELD FOR A LEADING EDGE OF A BLADE

[0003] Technical field of the invention

[0004] The invention relates to the field of surface treatment methods for steel protective shields for the leading edge of blades for aircraft turbomachines.

[0005] Technical background

[0006] An aircraft turbomachine generally comprises, from upstream to downstream, a fan, a low-pressure compressor, a high-pressure compressor, a combustion chamber, a high-pressure turbine, a low-pressure turbine and a gas exhaust nozzle. The rotor of the high-pressure compressor is connected to the rotor of the high-pressure turbine by a high-pressure shaft and the rotor of the low-pressure compressor is connected to the rotor of the low-pressure turbine by a low-pressure shaft.

[0007] Fans, compressors, and turbines are equipped with blades regularly distributed over a hub. A blade typically comprises a blade possibly connected to a fixing foot to connect the blade to the hub. The blade has an aerodynamic shape comprising a pressure face and an extrados face, the faces being connected by a leading edge and a trailing edge. In order to reduce the weight of blades, particularly fan blades, blades made of composite material have been proposed. The composite material is, for example, an organic matrix composite (OMC) typically comprising a polymer matrix chosen from epoxy resins, for example, and reinforcing fibers embedded in the matrix. During aircraft flight, the blades, and particularly the leading edge of the blades, can be subjected to impacts and wear that significantly degrade the composite material of the blades.To protect the blades, it has therefore been proposed to arrange a protective shield on the leading edge of the blades. The protective shield comprises a lower surface fin and an upper surface fin connected by a central nose. The nose extends along the leading edge while the lower surface fin extends on the lower surface of the blade and the upper surface fin extends on the upper surface of the blade. The protective shield is typically arranged on the leading edge by gluing. The protective shield is made of titanium alloy which has good impact and wear properties. However, the use of titanium alloy is not entirely satisfactory.

[0008] In this context, it has been proposed to replace the titanium alloy protective shield with a steel protective shield, particularly stainless steel. Steel has the advantage of being less expensive and having better conductivity properties than titanium alloy. This is particularly advantageous in the implementation of de-icing systems that require conductive materials. Furthermore, a steel protective shield has the advantage of being easier to form than a titanium alloy protective shield. However, such a steel protective shield includes a passive layer on the surface which tends to reduce the bonding performance of the protective shield to the leading edge of the blades.

[0009] There is therefore a need to provide a protective shield for a leading edge of a blade for an aircraft turbomachine, the bonding properties of which are improved, while preserving the mechanical properties of this protective shield.

[0010] Summary of the invention

[0011] To this end, the invention proposes a method for surface treatment of a protective shield for a leading edge of a blade for an aircraft turbomachine which is remarkable in that it comprises the following chronological steps:

[0012] (c) stripping the protective shield in an electrolyte bath,

[0013] (e) deposit an adhesion layer on the protective shield.

[0014] By "adhesion layer" is meant in the present invention any layer likely to promote adhesion between the protective shield and the leading edge of the blade.

[0015] The adhesion layer is, for example, a layer of glue.

[0016] According to the invention, the pickling is carried out in an electrolyte bath. In other words, the pickling according to the invention is an electrochemical pickling. Thanks to such pickling, the bonding properties of the protective shield are improved.

[0017] Indeed, electrochemical stripping can be carried out over a long period of time, particularly for more than three minutes, which ensures uniform stripping of the entire protective shield, even when it is large. This ensures a surface treatment compatible with the given specifications.

[0018] Also, it was found that such electrochemical pickling allows the formation of nano-pores in the austenitic grains of the steel and the formation of a micro-porous surface by a preferential attack of the ferrite at the austenitic grain boundaries of the steel which are favorable to bonding. Indeed, the combination of such a surface and these nano-pores makes it possible to increase the specific surface promoting the bonding performance of the protective shield. Therefore, it was demonstrated that, thanks to electrochemical pickling, the bonding performance of the protective shield is improved. Also, it was highlighted that despite this attack of the ferrite, the stress concentrations in the protective shield are limited, thus preserving the mechanical properties of the protective shield such as fatigue resistance.The invention may comprise one or more of the following features, taken in isolation from each other or in combination with each other:.

[0019] - the electrolyte bath includes nitric acid,

[0020] - the volume concentration of nitric acid is between 10% and 50%,

[0021] - the electrolyte bath is crossed by an electric current with a density of between 0.1 A / dm 2 and 10 A / dm 2 ,

[0022] - step (c) is carried out for a duration greater than 3 min, preferably a duration between 5 min and 20 min,

[0023] - the volume concentration of nitric acid is 25% and the electric current density is 0.5 A / dm 2 and the duration of step (c) is 15 min,

[0024] - the temperature of the electrolyte bath is between 20°C and 100°C, preferably between 30°C and 70°C,

[0025] - it includes, before step (c), the following step (a): degreasing the protective shield,

[0026] - it includes, between steps (c) and (e), the following step (d): drying the protective shield,

[0027] - the protective shield is made of stainless steel, advantageously of the austenitic type such as the AISI321 alloy,

[0028] - the etching step is configured to attack the ferrite at the austenitic grain boundaries and the austenitic grains to create nano-pores in the austenitic grains,

[0029] - at the end of the stripping stage, the protective shield has a roughness Rz of less than 4 pm,

[0030] - the protective shield has an elongated dihedral shape and comprises a first lateral fin and a second lateral fin connected to the first lateral fin by a central nose.

[0031] The invention also relates to a method of manufacturing a blade for an aircraft turbomachine, comprising the following steps: - providing a blade having a leading edge and a trailing edge connected by an intrados face and an extrados face,

[0032] - provide a steel protective shield,

[0033] - carry out a surface treatment of the protective shield according to any of the above characteristics,

[0034] - glue the protective shield to the blade, especially the leading edge.

[0035] Brief description of the figures

[0036] Other characteristics and advantages will emerge from the following description of a non-limiting embodiment of the invention with reference to the appended drawings in which: Figure 1 is a schematic representation in axial section of a half-turbomachine of an aircraft; Figure 2 is a schematic representation in perspective of a blade equipping the turbomachine of Figure 1; Figure 3 is a cross-sectional view of a protective shield fixed to the leading edge of the blade of Figure 2; Figure 4 is a block diagram illustrating a method according to an embodiment of the invention; Figure 5 is a diagram of an electrolytic unit implemented in step (c) of the method according to the invention; Figure 6a is a scanning electron microscopy image (x 1000) of a protective shield treated by the method of the invention;Figure 6b is another scanning electron microscopy image (x 1000) of a protective shield treated by the method of the invention; Figure 6c is an optical microscopy image of a part subjected to electrochemical etching according to first conditions; Figure 6d is an optical microscopy image of a part subjected to electrochemical etching according to second conditions.;

[0037] Detailed description of the invention An aircraft turbomachine 1 is for example shown in FIG. 1.

[0038] The turbomachine 1 extends along a longitudinal axis A. It comprises from upstream to downstream in the direction of flow of the gases F along the longitudinal axis A, a fan 2, at least one compressor such as a low pressure compressor 3 and a high pressure compressor 4, a combustion chamber 5, at least one turbine 6 such as a high pressure turbine and a low pressure turbine, and a nozzle (not shown).

[0039] The rotor of the low pressure turbine is connected to the fan 2 and to the rotor of the low pressure compressor 3 by a low pressure shaft 7. The rotor of the high pressure turbine is connected to the rotor of the high pressure compressor 4 by a high pressure shaft 8 arranged coaxially around the low pressure shaft 7.

[0040] The turbomachine 1 also optionally comprises a nacelle secured to a fan casing 9 surrounding the fan 2.

[0041] The turbomachine 1 further comprises a rectifier 10. The rectifier 10 makes it possible to straighten the flow at the outlet of a rotor located upstream in order to provide maximum thrust at the outlet of the turbomachine 1. In the particular example of FIG. 1, the rectifier 10 is located downstream of the fan 2. The rectifier 10 is for example arranged between the low-pressure compressor 3 and the high-pressure compressor 4 and inside the fan casing 9.

[0042] The blower 2 allows the suction of an air flow dividing into a primary flow F1 and a secondary flow F2. The primary flow F1 passes through a primary vein of the turbomachine 1 while the secondary flow F2 is directed towards a secondary vein surrounding the primary vein.

[0043] The primary flow F1 is compressed within the low-pressure compressor 3 and then the high-pressure compressor 4. The compressed air is then mixed with a fuel and burned within the combustion chamber 5. The gases formed by the combustion pass through the high-pressure turbine and the low-pressure turbine. The gases finally escape through the nozzle, the cross-section of which allows the acceleration of these gases to generate propulsion. The secondary flow F2 passes through the rectifier 10, which accelerates the circulation speed of the secondary flow F2 to generate propulsion.

[0044] The fan 2 and the rectifier 10 are equipped with a set of blades 11. The blades 11 are movable or fixed in rotation about the longitudinal axis A. The blades 11 of the fan 2 are movable in rotation about the longitudinal axis A while the blades of the rectifier 10, also called OGV (for "Outlet Guided Vanes" in English) are fixed in rotation about the longitudinal axis A. The blades 11 extend radially relative to the longitudinal axis A. As better seen in Figure 2, each blade 11 comprises a blade 12 and a protective shield 14 according to the invention.

[0045] The blade 12 extends along an elongation axis X. The elongation axis X of the blade 12 extends radially relative to the longitudinal axis A of the turbomachine 1 after mounting the blade 11 on the turbomachine 1. The blade 12 has an aerodynamic profile. The blade 12 thus comprises an extrados face 12e and a intrados face 12i connected by a leading edge 12a and a trailing edge 12b. The blade 12 thus extends along a transverse axis Y between the leading edge 12a and the trailing edge 12b. The transverse axis Y is perpendicular to the elongation axis X. The blade 12 further extends longitudinally along the elongation axis X between a first end and a second end opposite the first end.

[0046] The blade 12 is made of a composite material. The composite material is, for example, an organic matrix composite (OMC). The composite material comprises a polymer matrix and a fiber reinforcement embedded in the matrix. The matrix is, for example, a thermoplastic or thermosetting polymer matrix. The thermosetting material is, for example, an epoxy polymer. The fiber reinforcement comprises fibers which are, for example, carbon fibers or glass fibers. The fibers are organized, for example, in the form of a fiber preform. The blade 11 further comprises a root 13. The root 13 is in particular connected to the second end of the blade 12. It is intended to be fixed to a hub (not shown) centered on the longitudinal axis A of the turbomachine 1.

[0047] The protective shield 14 is arranged on the blade 12. The protective shield 14 advantageously extends over the leading edge 12a and even more advantageously along the entire length of the leading edge 12a. The protective shield 14 has an elongated dihedral shape. It is intended to protect the leading edge 12a from external impacts and wear. As best seen in FIG. 3, the protective shield 14 has a V-shaped or U-shaped cross-section. The protective shield 14 comprises a first lateral fin 14a and a second lateral fin 14b connected to the first lateral fin 14a by a central nose 14j. The first and second lateral fins 14a, 14b define between them a cavity in which the leading edge 12a is arranged.The first lateral fin 14a has a first free longitudinal end and the second lateral fin 14b has a second free longitudinal end which are opposite the central portion 14j. The longitudinal ends extend respectively on the intrados face 12i and the extrados face 12e of the blade 12. Each lateral fin 14a, 14b has a first edge and a second edge opposite the first edge along the elongation axis X. The edges extend transversely relative to the longitudinal ends. Advantageously, the thickness of the protective shield 14 is variable. For example, the thickness of the central nose 14j is greater than the thicknesses of the first and second lateral fins 14a, 14b. Advantageously, the thickness of the first and second lateral fins 14a, 14b decreases towards the trailing edge 12b of the blade 12.The first and second side fins 14a, 14b are tapered towards the trailing edge 12b of the blade 12.

[0048] The protective shield 14 is made of steel. The steel is advantageously a stainless steel. The stainless steel is advantageously of the austenitic type such as the AISI321 alloy. The protective shield 14 comprises an adhesion layer (not shown) to optimize its attachment to the leading edge 12a. The adhesion layer is for example a bonding primer, such as an epoxy resin.

[0049] The protective shield 14 is therefore fixed to the leading edge 12a by gluing. A layer of glue 15 is arranged between the protective shield 14 and the blade 12.

[0050] In order to ensure the bonding performance of the protective shield 14, the protective shield 14 is subjected to a surface treatment.

[0051] According to the invention and with reference to Figure 4, the surface treatment comprises the following chronological steps:

[0052] (a) optionally, degrease the protective shield 14,

[0053] (r) optionally, rinse the protective shield 14,

[0054] (b) optionally, pre-stripping the protective shield 14,

[0055] (r) optionally, rinse the protective shield 14,

[0056] (c) stripping the protective shield in an electrolyte bath 18,

[0057] (r) optionally, rinse the protective shield 14,

[0058] (d) optionally, drying the protective shield 14, and

[0059] (e) depositing the adhesion layer on the protective shield 14.

[0060] In the following description, where applicable, the baths are topped up with water up to 100% of their volume.

[0061] Advantageously, in step (c), the protective shield 14 is immersed in the electrolyte bath 18. The electrolyte bath 18 is an aqueous bath. The electrolyte bath 18 comprises nitric acid. Preferably, the electrolyte bath 18 comprises nitric acid as the only acid. Preferably, the electrolyte bath 18 is free of hydrofluoric acid. Advantageously, the nitric acid is for example in aqueous solution at a mass concentration of 68% in the solution and has a density of 1.41. The volume concentration of the nitric acid in the electrolyte bath 18 or of the solution in the electrolyte bath 18 is between 10% and 50%. Advantageously, the temperature of the electrolyte bath 18 is between 20°C and 100°C, preferably between 30°C and 70°C. Advantageously, the electrolyte bath 18 is traversed by an electric current with a density of between 0.1 A / dm 2 and 10 A / dm 2Advantageously, the protective shield 14 is electrochemically stripped in step (c) for a duration greater than 3 min, preferably a duration of between 5 min and 20 min.

[0062] Electrochemical etching improves the bonding performance of the protective shield 14 and its mechanical properties. Indeed, Figures 6a and 6b are scanning electron microscope images at a magnification of 1000. The images show that the electrochemical etching step results in the formation of nanopores 142 in the austenitic gains 140. The nanopores 142 promote the bonding of the protective shield 14 to the leading edge 12a. Also, a preferential attack of the ferrite at the austenitic grain boundaries 140 is observed, forming a microporous surface 141 also favorable to the bonding of the protective shield 14.

[0063] Preferably, the etching step is configured to attack the ferrite at the austenitic grain boundaries and the austenitic grains 140 to create nano-pores 142 in the austenitic grains 140.

[0064] Furthermore, the duration of the electrochemical stripping step (c) is particularly advantageous since it allows immersion of the protective shield 14 for a sufficient time to obtain homogeneous stripping over the entire protective shield 14 despite its large dimensions.

[0065] Particularly preferably, the volume concentration of nitric acid (for example 68%) in the electrolyte bath 18 is 25%, the density of the electric current is 0.5 A / dm 2 and the duration of step (c) is 15 min.

[0066] These preferred conditions of the electrochemical stripping step (c) make it possible to limit the stress concentrations and to achieve stress concentrations similar to those generated by the surface imperfections of the protective shield 14 such as machining grooves, cracks. This makes it possible to improve the fatigue resistance of the protective shield 14.

[0067] Preferably, at the end of the stripping step, the protective shield 14 has a roughness Rz of less than 4 pm.

[0068] The rinsing steps (r) are advantageously carried out with demineralized water.

[0069] According to a first exemplary embodiment, drying step (d) is carried out using compressed air.

[0070] According to another exemplary embodiment, drying step (d) is carried out in an oven.

[0071] According to a first exemplary embodiment, step (e) of depositing the adhesion layer comprises a first sub-step of depositing the bonding primer and then a sub-step of polymerizing the bonding primer. The polymerization sub-step is carried out for example at a temperature between 100°C and 200°C.

[0072] The method according to the invention can be implemented in an installation (not shown) for surface treatment of the protective shield 14. The installation comprises an electrolytic unit 16 comprising an electrolytic tank 17 shown for example in FIG. 5 and implemented in step (c). The electrolytic tank 17 comprises the electrolyte bath 18. The electrolytic unit 16 further comprises at least one cathode 19 arranged in the electrolyte bath 18 and an anode formed by the protective shield 14, also arranged in the electrolyte bath 18. Advantageously, at least two cathodes 19 are arranged in the electrolyte bath 18. The electrolytic unit 16 further comprises a generator 19 connected to the cathode and to the protective shield 14. The generator 19 is advantageously a direct electric current generator.

[0073] Optionally, the installation further comprises a drying unit implemented in step (d). According to a first example, the drying unit comprises a compressed air projection device or, according to a second example, an oven. The installation further comprises a unit for depositing the adhesion layer implemented in step (e). The unit for depositing the adhesion layer comprises, for example, a device for applying the bonding primer, such as a brush or a gun.

[0074] A method of manufacturing the blade 11 will now be described. The method comprises the following steps:

[0075] - provide blade 12,

[0076] - provide the 14 steel protective shield,

[0077] - carry out the surface treatment of the protective shield 14 as described above,

[0078] - glue the protective shield 14 on the leading edge 12a.

[0079] EXAMPLE

[0080] Example 1: Microscope analysis

[0081] Test specimens P1, P2 made of austenitic stainless steel AISI321 were subjected to electrochemical pickling according to the invention in a bath under the conditions presented in Table 1.

[0082] Nitric acid is initially in aqueous solution at a mass concentration of 68% and has a density of 1.41.

[0083] Figures 6c and 6d are optical microscopy images at x100 magnification of specimens P1 and P2 respectively. The optical microscope is the ZEISS microscope.

[0084] It is observed that the depth D of the intragranular attack of the steel is 4 pm on specimen P1 and is only 2 pm on specimen P2.

[0085] It is therefore concluded that the electrochemical etching conditions of specimen P2 make it possible to reduce the depth of the intergranular attack, thus reducing the stress concentrations in the steel. The fatigue resistance of the protective shield is therefore improved under these conditions.

[0086] [Table 1] Conditions of the electrochemical stripping step

Claims

CLAIMS 1. Method for surface treatment of a protective shield (14) made of steel for a leading edge (12a) of a blade (11) for an aircraft turbomachine (1), characterized in that it comprises the following chronological steps: (c) etching the protective shield (14) in an electrolyte bath (18), (e) depositing an adhesion layer on the protective shield (14).

2. Method according to any one of the preceding claims, characterized in that the electrolyte bath (18) comprises nitric acid.

3. Method according to the preceding claim, characterized in that the volume concentration of nitric acid is between 10% and 50%.

4. Method according to any one of the preceding claims, characterized in that the electrolyte bath (18) is traversed by an electric current with a density of between 0.1 A / dm 2 and 10 A / dm 2 .

5. Method according to any one of the preceding claims, characterized in that step (c) is carried out for a duration greater than 3 min, preferably a duration of between 5 min and 20 min.

6. Method according to claims 1 to 5, characterized in that the volume concentration of nitric acid is 25% and the density of the electric current is 0.5 A / dm 2 and that the duration of step (c) is 15 min.

7. Method according to any one of the preceding claims, characterized in that the temperature of the electrolyte bath (18) is between 20°C and 100°C, preferably between 30°C and 70°C.

8. Method according to any one of the preceding claims, characterized in that it comprises, before step (c), the following step: (a) degreasing the protective shield (14).

9. Method according to any one of the preceding claims, characterized in that it comprises, between steps (c) and (e), the following step: (d) dry the protective shield (14).

10. Method according to any one of the preceding claims, characterized in that the protective shield (14) is made of stainless steel, advantageously of the austenitic type such as the AISI321 alloy.

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