Aircraft turbomachine blade repair method

Abstract

This nickel-based alloy repair process for aircraft turbomachine blades (3, 3', 4) comprises successive steps of: - descaling, - nickel plating, and - post-treatment for surface smoothing. Figure for the abbreviation: [Fig 1]

Description

Title of the invention: Method for repairing aircraft turbomachine blades technical field

[0001] The present invention relates to the repair and corrosion reinforcement of aircraft turbomachine blades. Previous techniques

[0002] The reliability and robustness of aircraft turbomachinery and the maximization of operating time are major challenges for the aeronautical industry, particularly with the increase in flight-hour contracts.

[0003] During the operational phases, the blades of aircraft turbomachinery can develop degradations related to oxidation, corrosion and / or pollution (erosion due to the presence of sand, interaction with fine particles, etc.).

[0004] These degradations weaken the mechanical strength and considerably reduce the lifespan of the parts, thus potentially generating premature removals compared to an expected lifespan.

[0005] Traditionally, to address these issues, turbomachines are periodically disassembled into parts in order to clean and repair blades with defects.

[0006] Chemical baths are generally used to remove impurities present on the surface and in the material of blades with defects.

[0007] However, these chemical baths are difficult to control, as they constitute exothermic processes which can generate greater pickling with increasing temperature.

[0008] Generally, chemical baths are followed by sandblasting, the intensity of which can vary, in order to obtain a smooth surface, i.e. by limiting the value of the roughness of the blade surface below a maximum acceptable value.

[0009] However, the combination of chemical baths and sandblasting can lead to the removal of base metal from the blades, thus reducing the bearing area, sometimes unacceptably.

[0010] It remains possible to fill these original material losses by building up new material that is brazed and diffused onto the surface of the blade, in order to restore the blade to its nominal dimensions. However, the mechanical characteristics of a brazed and diffused material do not allow for the production of load-bearing sections capable of withstanding mechanical stresses effectively and sufficiently. Description of the invention

[0011] In view of the foregoing, the object of the invention is to propose a method of repairing blades allowing them to be restored to their nominal dimensions with good mechanical strength.

[0012] The invention relates to a method for repairing aircraft turbomachine blades using a nickel-based alloy. The method comprises successive steps of: - Carbon cleaning, - nickel plating, and - Post-processing for surface smoothing.

[0013] Preferably, the process includes a deoxidation step carried out before the descaling step.

[0014] According to an advantageous feature, the process includes a destabilization step of the oxides followed by a bleaching step carried out after the descaling step and before the nickel plating step.

[0015] For example, the nickel plating step is carried out by a chemical bath.

[0016] For example, the nickel plating step is carried out by electrolysis in a bath.

[0017] According to one feature, the chemical bath is composed of an aqueous solution comprising nickel ions and a reducing agent.

[0018] According to another feature, the chemical bath further comprises the addition of at least one corrosion-resistant material to enhance the corrosion resistance of the blade. For example, the use of chromium or platinum provides excellent results against corrosion.

[0019] For example, the chemical bath further comprises a filler metal selected from hafnium, tantalum, molybdenum or zirconium.

[0020] According to another feature, the surface smoothing post-treatment step includes sandblasting or polishing.

[0021] According to another aspect, the invention relates to a blade repaired by the implementation of a repair process as described above. Brief description of the drawings

[0022] Other objects, features and advantages of the invention will become apparent from the following description, given solely by way of non-limiting example, and made with reference to the accompanying drawings in which:

[0023] [Fig-1] is a partial longitudinal cross-sectional view of a low-pressure turbine of an aircraft turbomachine; and

[0024] [Fig.2] illustrates a flowchart of a blade repair method according to the invention. Detailed description of at least one embodiment

[0025] Fig. 1 partially represents in longitudinal section a low-pressure turbine 1 of an aircraft turbomachine, representative of turbine architectures comprising moving blades or rotors, and fixed blades or distributors.

[0026] The process according to the invention proposes a nickel-based alloy repair of the surface of the blades by restoring their bearing section as well as a reinforcement of the corrosion resistance of the blades.

[0027] It is understood that the present invention applies to any other aircraft turbomachine assembly equipped with movable and / or fixed blades.

[0028] Figure 1 more precisely represents five stages of a low-pressure turbine 1. Regarding the orientation with respect to the general direction of gas flow through the turbine, the first stage of the turbine is located upstream and the last stage of the turbine is located downstream.

[0029] The first stage comprises a movable wheel formed of a rotor disk 2 on which a plurality of movable blades 3 are radially mounted. The other stages each comprise a distributor formed of a plurality of fixed blades 4 and a movable wheel placed behind the distributor and formed of a rotor disk 2' on which a plurality of movable blades 3' are radially mounted.

[0030] The discs 2, 2' are fixed to each other by means of bolted links 5, making all the discs fixed in rotation around the longitudinal axis XX of the turbomachine.

[0031] Each disk 2, 2' has at least one annular flange 6 which extends towards the disks 2, 2' of the adjacent floors.

[0032] We will now describe with reference to [Fig.2] a nickel-based alloy repair method for aircraft turbomachine blades, according to the invention.

[0033] The process begins with a deoxidation step 7 intended to remove coked oil and iron oxides such as rust or Fe(OH3)). The deoxidation step 7 is an acidic chemical surface treatment that removes the oxides without attacking the base metal or by limiting this attack (presence of a corrosion inhibitor in the acid).

[0034] Step 7 can be carried out cold or hot. If carried out hot, the operation is significantly accelerated. The acid attacks not only the adhering oxide but also the metal, releasing hydrogen, some of which is absorbed by the metal. To remedy this problem, a pickling inhibitor is added to the bath, which slows the action of the acid on the metal without hindering the dissolution of the oxides.

[0035] The process continues with a descaling step 8, which promotes the attack of certain persistent oxides such as discoloration or surface oxides. The descaling step 8 is primarily aimed at eliminating oxides formed during the combustion of fats and oils in internal combustion engine parts. The operation Descaling allows the renovation of mechanical parts without altering the metallographic quality of the metal and the dimensions of the part and is generally carried out in a highly alkaline environment and at high temperature.

[0036] In the following step 9, very stable oxides such as Cr2O3, NiO, NiAl2O4 are destabilized by modifying their degree of valence.

[0037] Next, a bleaching step 10 is carried out in which the destabilized oxides are loosened and the persistent residues are removed. This bleaching step 10 reveals the existence of surface defects in the blade's base metal, such as underlying cracks. The bleaching step 10 involves an alkaline etch that removes a thin layer of material, generally between 5 and 10 µm, as well as the persistent oxides.

[0038] The process continues with a nickel plating step 11. Preferably, the nickel plating step is carried out by a chemical bath, for example, by immersing the part in a liquid bath that results in a metallic deposit being deposited by an autocatalytic chemical process. The chemical nickel plating bath consists of an aqueous solution containing nickel ions and a reducing agent. A layer of nickel then forms on the surface of the blade, which acts as a catalyst for the chemical reaction. For example, the reducing agent is hypophosphite or a boron compound. In the case of hypophosphite baths, a basic medium is used, with a temperature ranging from 70°C to 100°C and salt concentrations ranging from 10 to 25 g / L. In the case of baths using boron compounds, either borohydrides are used in an alkaline medium, or dimethylamine-borane in a neutral or slightly acidic medium, with a temperature ranging from 70°C to 100°C.

[0039] Preferably, the chemical bath also includes an addition of at least one corrosion-resistant material to enhance the corrosion resistance of the blade. For example, this addition could be chromium or platinum.

[0040] Depending on the required mechanical characteristics of the blade, the chemical bath may further include a filler metal such as hafnium, tantalum, molybdenum, or zirconium. Such a filler metal improves the mechanical characteristics of the part. For example, such a filler metal fills dislocations in the treated part and delays crack propagation.

[0041] Alternatively, the nickel plating step may be carried out by electrolysis in a bath. For example, nickel sulfate baths or Watts baths, nickel chloride baths, or nickel sulfamate baths may be used.

[0042] In the case of sulfate baths, the concentration of nickel sulfate ranges from 225 g / L to 400 g / L and the concentration of nickel chloride ranges from 30 g / L to 60 g / L, for a temperature between 45°C and 55°C and a current density of approximately 1 A / dm². Yield varies between 93%, corresponding to a pH of 2, and 99%, corresponding to a pH of 5.

[0043] In the case of chloride baths, an all-chloride bath or, preferably, a pre-nickel plating bath with chloride, also known as a Wood's bath, can be used. For example, in a pre-nickel plating bath, the nickel chloride concentration ranges from 30 g / L to 250 g / L, at a temperature between 20°C and 30°C and a current density between 1 and 2 A / dm². The yield varies between 99%, corresponding to a pH of 2, and 99.5%, corresponding to a pH of 5.

[0044] In the case of sulfamate baths, the nickel sulfamate concentration is on the order of 650 g / L, for a temperature between 40°C and 60°C and a current density between 1 and 20 A / dm². The efficiency varies between 95.5%, corresponding to a pH of 2.5, and 99.5%, corresponding to a pH of 4.

[0045] It should be noted that the pH and temperature of the bath used influence the process yield. For an electrolytic bath, it is possible to lower the pH value by using boric acid. For a chemical bath, it is possible to raise the pH value by using sodium hydroxide.

[0046] The process ends with a surface smoothing post-treatment step 12 to obtain a blade surface with a predetermined maximum roughness. For example, the surface smoothing post-treatment is mechanical, including sandblasting or grinding. For example, the predetermined maximum roughness is on the order of 3.2 pm.

[0047] Alternatively, the process may comprise only successive descaling, nickel plating, and surface post-treatment steps. Alternatively, it is possible to omit the surface post-treatment step.

Claims

Demands

1. A nickel-based alloy repair method for aircraft turbomachine blades (3, 3', 4) characterized in that it comprises successive steps of: - descaling carried out in a strongly alkaline medium, corresponding to a pH greater than 8, - nickel plating, and - post-treatment of surface smoothing.

2. A process according to claim 1 comprising a deoxidation step carried out before the descaling step.

3. A process according to claim 1 or 2 comprising an oxide destabilization step followed by a bleaching step carried out after the descaling step and before the nickel plating step.

4. A method according to any one of claims 1 to 3, wherein the nickel plating step is carried out by a chemical bath.

5. A method according to any one of claims 1 to 3, wherein the nickel plating step is carried out by electrolysis in a bath.

6. A method according to claim 4, wherein the chemical bath is composed of an aqueous solution comprising nickel ions and a reducing agent.

7. A method according to claim 6, wherein the chemical bath further comprises an addition of at least one corrosion-resistant material so as to enhance the corrosion resistance of the blade (3, 3', 4).

8. A method according to claim 6 or 7, wherein the chemical bath further comprises a filler metal selected from hafnium, tantalum, molybdenum or zirconium.

9. A method according to any one of claims 1 to 8, wherein the surface smoothing post-treatment step comprises sandblasting or polishing.

10. Blade (3, 3', 4) repaired by implementing a repair process according to any one of claims 1 to 9.

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