SURFACE TREATMENT PROCESS FOR A PART MADE OF TITANIUM OR A TITANIUM-BASED ALLOY

Abstract

The invention relates to a method for surface treatment of a part (13) made of titanium or a titanium-based alloy comprising a step of removing a scale layer and an alpha-case layer by immersing the part (13) made of titanium or a titanium alloy in an ultrasonic bath of aqueous solution containing sodium hydroxide and sodium nitrate. Figure 3

Description

Title of the invention: METHOD FOR SURFACE TREATMENT OF A PART MADE OF TITANIUM OR A TITANIUM-BASED ALLOY

[0001] The present invention relates to a method for surface treatment of a part made of titanium or a titanium-based alloy. Titanium has many advantageous physicochemical properties, such as an excellent mechanical strength / density ratio and good corrosion resistance.

[0002] The invention finds a particularly advantageous, but not exclusive, application with parts made of titanium or a titanium-based alloy used in aeronautics. In particular, the invention is implemented with parts made of titanium or a titanium-based alloy having a complex geometry, such as blade reinforcement parts intended to cover the leading edges of the blades of a fan. These parts have a function of protecting the blade in the event of impact with projectiles, in particular birds, likely to enter the engine during flight of the aircraft.

[0003] This type of part is obtained by cutting and shaping an elongated rod. After being degreased using a cleaning solution, the part is covered with a layer of boron nitride. Boron nitride has high thermal conductivity, high insulation, low dielectric property, great lightness and excellent chemical stability. The layer of boron nitride makes it possible to limit an oxidation phenomenon occurring during heating by forming a chemical barrier with the external atmosphere.

[0004] After being covered with the boron nitride layer, the part is shaped by low-temperature forging. During this forging step, the part is shaped so as to fold its edges to define a groove for receiving the fan blade. After the forging step, the edge nitride layer is removed by water jet blasting.

[0005] However, the heat treatment results in the appearance on the surface of the part of a layer of calamine, a mixture of iron oxides which is deposited as a crust on the part, as well as an oxygen-rich and brittle "alpha-case" layer a few tens of microns thick. This layer of material is the cause of the formation of cracks and considerably reduces the life of the part. The dissolution of the calamine layer and the alpha-case layer is therefore carried out on any part made of titanium or a titanium alloy which has undergone heat treatment.

[0006] To this end, the part is immersed in a molten salt bath of the Kolene type (name commercial), at a temperature of around 400 degrees. Then, a sulfuric acid bath removes the deposit of the crystallized salt layer covering the part at the end of the previous step. This sulfuric acid bath is followed by a fluonitric acid bath.

[0007] This method, however, presents a safety problem, insofar as there is a risk of explosion in the event that the molten salt comes into contact with water during the immersion step in the bath. In addition, it can be difficult to remove the scale and alpha case layers in all areas of a complex-shaped part, such as a blade reinforcement.

[0008] The invention aims to effectively remedy the aforementioned drawbacks by proposing a method for surface treatment of a part made of titanium or a titanium-based alloy comprising a step of removing a layer of calamine and an alpha-case layer by immersing the part made of titanium or a titanium alloy in an ultrasonic bath of aqueous solution containing sodium hydroxide and sodium nitrate.

[0009] The invention thus makes it possible, by using a bath of aqueous solution based on alkaline salts in aqueous solution, to eliminate the risks of explosion linked to the use of molten salts of the Kolene type (trade name). In addition, the invention can easily be implemented with parts with complex geometry due to the activation of the surface of the titanium part thanks to ultrasound which renews the liquid in the bath around the part. The invention also makes it possible to save on the step of removing by acid bath the layer of crystallized salt which is created around the part following immersion in the molten salt bath.

[0010] According to one implementation of the invention, a sodium hydroxide concentration of the aqueous solution is between 25% and 50% by volume.

[0011] According to one implementation of the invention, a sodium nitrate concentration of the aqueous solution is between 2.5% and 10% by volume.

[0012] According to one implementation of the invention, a duration of immersion of the part in the ultrasonic bath is between 5 and 100 minutes.

[0013] According to one implementation of the invention, the ultrasonic bath is configured at a frequency between 20 kHz and 400 kHz.

[0014] The invention also relates to a method for manufacturing a part made of titanium or a titanium alloy comprising successively: - a degreasing step, - a step of depositing a layer of boron nitride, - a step of shaping the part by forging, - a step of stripping the layer of boron nitride, - a step of removing a layer of scale and an alpha-case layer by placing implementation of the surface treatment process as previously defined, and - a step of machining the part by chemical bath.

[0015] According to one implementation of the invention, the step of depositing the boron nitride layer is carried out by electrostatic deposition of a boron nitride powder on an external face of the part.

[0016] According to one implementation of the invention, the shaping step is carried out by forging at a low temperature between 400 degrees Celsius and 800 degrees Celsius.

[0017] According to one implementation of the invention, the step of stripping the boron nitride layer is carried out using a water jet.

[0018] According to one implementation of the invention, the chemical bath is a sulfuric acid bath followed by a fluonitric acid bath.

[0019] According to one implementation of the invention, the part made of titanium or a titanium alloy is a blade reinforcement.

[0020] According to one implementation of the invention, the shaping step consists of folding back lateral edges of the blade reinforcement to define a groove for receiving a fan blade.

[0021] The present invention will be better understood and other characteristics and advantages will become apparent upon reading the detailed description which follows, comprising embodiments given by way of illustration with reference to the appended figures, presented as non-limiting examples, which may serve to complete the understanding of the present invention and the description of its embodiment and, where appropriate, contribute to its definition, in which:

[0022] [Fig-1] [Fig.l] is a front view of a turbomachine fan comprising a blade reinforcement obtained by a manufacturing method according to the invention;

[0023] [Fig.2] [Fig.2] is a cross-sectional view of a blade reinforcement obtained by a manufacturing method according to the invention;

[0024] [Fig.3] [Fig.3] is a diagram of the different stages of the manufacturing process of a blade reinforcement according to the present invention.

[0025] It should be noted that, in the figures, the structural and / or functional elements common to the different embodiments have the same references. Thus, unless otherwise stated, such elements have identical structural, dimensional and material properties.

[0026] [Fig. 1] shows a turbomachine fan 10 comprising a ring 11 with axis X and a plurality of blades 12 extending radially towards the outside of the ring 11 along their axis Y. The blades 12 are distributed angularly in a regular manner along a circumference of the ring 11.

[0027] The fan 10 comprises blade reinforcements 13 intended to cover the leading edges of the blades 12 of the fan 10. These parts 13 have a function of protection of the blades 12 in the event of impact with projectiles, in particular birds, during flight of the aircraft. As can be seen in [Fig.2], a blade reinforcement 13 has two lateral edges 15 defining, with the rest of the part, a groove 16 for receiving the blade 12 of the fan 10.

[0028] The method of manufacturing a reinforcement is described below, with reference to [Fig. 3]. blade 13 made of titanium or a titanium alloy.

[0029] This method comprises a step 100 of degreasing the blade reinforcement 13 to rid the part 13 of the deposits which foul it. The degreasing step 100 is carried out by immersion in a bath of aqueous solution based on potash. The total duration of this bath and the rinsing phase is of the order of approximately 50 minutes. By "of the order of" is meant a variation of plus or minus 10% compared to the indicated value.

[0030] The method comprises a step 101 of depositing a layer of boron nitride. Boron nitride has high thermal conductivity, high insulation, low dielectric property, great lightness and excellent chemical stability. The layer of boron nitride makes it possible to limit an oxidation phenomenon occurring during heating by forming a chemical barrier with the external atmosphere. Step 101 of depositing the layer of boron nitride is carried out by electrostatic deposition of a boron nitride powder on an external face of the blade reinforcement 13.

[0031] The method also comprises a step 102 of shaping the part by forging. The forging step 102 is carried out at a low temperature of between 400 degrees Celsius and 800 degrees Celsius and preferably of the order of 750 degrees Celsius. During this step 102, the lateral edges 15 of the blade reinforcement 13 are folded back to define the groove 16 for receiving the blade 12 of the fan 10.

[0032] After cooling the part 13, the method comprises a step 103 of stripping the boron nitride layer. The stripping of the boron nitride layer is preferably carried out by means of a water jet.

[0033] The method then comprises a step 104 of removing a layer of scale and an alpha-case layer by immersing the blade reinforcement 13 in an ultrasonic bath of aqueous solution containing sodium hydroxide and sodium nitrate. Immersing the part 13 in such a bath makes it possible to activate the surface of the part thanks to the ultrasound which renews the liquid in the bath around the blade reinforcement 13. The invention can thus be implemented with parts of complex geometry, insofar as the bath is active even in areas that are difficult to access, such as the bottom of the blade reinforcement 13. The entire part 13 can thus be cleaned of its layer of scale and its layer of alpha-case.

[0034] Advantageously, a sodium hydroxide concentration of the aqueous solution is between 25% and 50% by volume. A sodium nitrate concentration of the aqueous solution is between 2.5% and 10% by volume. A product such as Ardrox 1631 (trade name) can be used to obtain this solution. The concentrations of sodium hydroxide and sodium nitrate can be adapted according to the number of parts immersed in the bath. The greater the number of parts, the higher the concentrations of sodium hydroxide and sodium nitrate. Conversely, the lower the number of parts, the lower the concentrations of sodium hydroxide and sodium nitrate.

[0035] An immersion time of a blade reinforcement 13 in the ultrasonic bath is between 5 and 100 minutes. The ultrasonic bath is configured at a frequency between 20 kHz and 400 kHz.

[0036] The method also comprises a final step 105 of machining the blade reinforcement 13 by chemical bath, in particular a sulfuric acid bath followed by a fluo-nitric acid bath. The chemical machining step 105 is implemented to remove a predetermined thickness of material, in particular of the order of 0.6 mm. Alternatively, the machining step 105 may be a mechanical machining step.

[0037] The invention can be implemented for the surface treatment and / or the manufacture of parts made of titanium or a titanium-based alloy other than blade reinforcements 12.

[0038] Of course, the various features, variants and / or embodiments of the present invention may be combined with each other in various combinations to the extent that they are not incompatible or mutually exclusive.

[0039] Furthermore, the invention is not limited to the embodiments described above and provided solely by way of example. It encompasses various modifications, alternative forms and other variants that may be envisaged by those skilled in the art within the scope of the present invention and in particular all combinations of the different operating modes described above, which may be taken separately or in combination.

Claims

Claims

1. Method for manufacturing a part made of titanium or a titanium alloy characterized in that it successively comprises: - a step (100) of degreasing, - a step (101) of depositing a layer of boron nitride, - a step (102) of shaping the part by forging, - a step (103) of stripping the layer of boron nitride, - a step (104) of removing a layer of calamine and an alpha-case layer by immersing the part (13) made of titanium or a titanium alloy in an ultrasonic bath of aqueous solution containing sodium hydroxide and sodium nitrate, and - a step (105) of machining the part by chemical bath.

2. A method according to claim 1, characterized in that a sodium hydroxide concentration of the aqueous solution is between 25% and 50% by volume.

3. Method according to claim 1 or 2, characterized in that a sodium nitrate concentration of the aqueous solution is between 2.5% and 10% by volume.

4. Method according to any one of claims 1 to 3, characterized in that a duration of immersion of the part in the ultrasonic bath is between 5 and 100 minutes.

5. Method according to any one of claims 1 to 4, characterized in that the ultrasonic bath is configured at a frequency between 20 kHz and 400 kHz.

6. Method according to any one of claims 1 to 5, characterized in that the step (101) of depositing the boron nitride layer is carried out by electrostatic deposition of a boron nitride powder on an external face of the part.

7. Method according to any one of claims 1 to 6, characterized in that the shaping step (102) is carried out by forging at a low temperature between 400 degrees Celsius and 800 degrees Celsius.

8. Method according to any one of claims 1 to 7, characterized in that the step (103) of stripping the boron nitride layer is carried out by means of a water jet.

9. Method according to any one of claims 1 to 8, characterized in that the chemical bath is a sulfuric acid bath followed by a fluonitric acid bath.

10. Method according to any one of claims 1 to 9, characterized in that the part made of titanium or a titanium alloy is a blade reinforcement (13).

11. Method according to claim 10, characterized in that the shaping step (102) consists of folding back lateral edges (15) of the blade reinforcement (13) to define a groove (16) for receiving a blade (12) of the fan (10).

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