METHOD FOR MANUFACTURING A PART MADE OF A MONOCRYSTALLINE SUPERALLOY

Abstract

The invention concerns a method for manufacturing an aircraft part, the part comprising a monocrystalline nickel-based superalloy substrate, the method consecutively implementing the steps of moulding the part at a moulding temperature greater than the melting temperature of the superalloy, and cooling the part, such that the monocrystalline superalloy has a γ phase and a γ phase, solution heat treatment of the part at a first temperature between the solves temperature of the γ′ phase and the melting temperature of the superalloy, homogenising the crystalline structure or the part, cooling the part to ambient temperature, first tempering and second tempering.

Claims

1. Manufacturing method for an aircraft part, the part comprising a monocrystalline nickel-based superalloy substrate, the method successively implementing the following steps: moulding the part at a moulding temperature greater than the melting temperature of the superalloy and cooling the part, so that the monocrystalline superalloy has a γ phase and a γ′ phase, solution heat treatment of the part at a first temperature T.sub.1 comprised between the solvus temperature of the γ′ phase and the melting temperature of the superalloy, cooling the part, homogenization of the crystalline structure of the part, cooling the part, first tempering and second tempering.

2. Method according to claim 1, wherein the superalloy is predominantly composed of nickel and has a mass fraction of chromium comprised between 7% and 9%, of cobalt comprised between 5.5% and 7.5%, of aluminium comprised between 4% and 6%, of titanium comprised between 1% and 2%, of tantalum comprised between 7% and 9%, of molybdenum comprised between 1% and 3% and of tungsten comprised between 4.5% and 6.5%, the superalloy also comprising carbon and zirconium.

3. Method according to claim 1, wherein the superalloy is predominantly composed of nickel and preferentially has a mass fraction of chromium comprised between 2.5% and 4.5%, of cobalt comprised between 9% and 11%, of aluminium comprised between 4.5% and 6.5%, of titanium comprised between 0.5% and 1%, of tantalum comprised between 7% and 9%, of molybdenum comprised between 0.3% and 1%, of tungsten comprised between 5% and 7%, and of rhenium comprised between 4% and 5.5%.

4. Method according to claim 1, wherein the moulding step is followed by a demoulding step, and wherein the step of homogenization of the crystalline structure of the part is implemented after the demoulding step.

5. Method according to claim 1, wherein the homogenization of the crystalline structure of the part is implemented by a heat treatment of the part at a second temperature T.sub.2 greater than the solvus temperature of the γ′ phase and strictly less than the first temperature T.sub.1.

6. Method according to claim 5, wherein the homogenization is implemented by the heat treatment of the part at a second temperature T.sub.2 for at least 10 minutes, especially for 20 minutes, and preferentially for one hour.

7. Method according to claim 5, wherein the second temperature T.sub.2 is strictly comprised between 1280° C. and 1350° C.

8. Method according to claim 1, wherein the homogenization of the crystalline structure of the part is implemented by a heat treatment of the part at a third temperature T.sub.3 comprised between 800° C. and 1000° C., a tensile stress being applied to the part during the heat treatment at temperature T.sub.3 so as to cause plastic deformation of the part.

9. Method according to claim 8, wherein the tensile stress is applied so that the deformation rate is less than 10.sup.−3 s.sup.−1 at any point of the part.

10. Method according to claim 8, wherein the application of the tensile stress is oriented in a tensile direction, and is removed as soon as the length of the part in the tensile direction is greater than 1.008 times the initial length of the part in the tensile direction.

11. Method according to claim 1, wherein the first tempering is implemented at a fourth temperature T.sub.4 comprised between 1000° C. and 1200° C. for at least 3 hours, and the second tempering is implemented at a fifth temperature T.sub.5 comprised between 800° C. and 900° C. for at least 10 hours.

12. Method according to claim 6, wherein the second temperature T.sub.2 is strictly comprised between 1280° C. and 1350° C.

Description

DESCRIPTION OF THE FIGURES

[0033] Other characteristics, objectives and advantages of the invention will appear from the following description, which is purely illustrative and non-limiting and should be read with regard to the attached drawings, in which:

[0034] FIG. 1 illustrates a manufacturing method for the nickel-based superalloy AM1 of the prior art,

[0035] FIG. 2 illustrates a manufacturing method for the nickel-based superalloy AM1 of the prior art,

[0036] FIG. 3 is a photomicrograph of a section of the superalloy according to the prior art,

[0037] FIG. 4 is a photomicrograph of a section of the superalloy of the prior art, the scale bar corresponding to a length of 100 μm,

[0038] FIG. 5 illustrates a manufacturing method for the nickel-based superalloy AM1 according to one embodiment of the invention wherein homogenization of the crystalline structure of the part is implemented by a heat treatment of the part at a second temperature T.sub.2 greater than the solvus temperature of the γ′ phase and strictly less than the first temperature T.sub.1, the scale bar corresponding to a length of 10 μm,

[0039] FIG. 6 illustrates a manufacturing method for the nickel-based superalloy AM1 according to one embodiment of the invention wherein the homogenization of the crystalline structure of the part is implemented by a heat treatment of the part at a temperature T.sub.3 comprised between 800° C. and 1000° C., a tensile stress being applied to the part during the heat treatment at temperature T.sub.3 so as to cause plastic deformation of the part,

[0040] FIG. 7 is a photomicrograph of a section of the superalloy according to one embodiment of the invention, the scale bar corresponding to a length of 1 μm,

[0041] FIG. 8 is a photomicrograph of a section of the superalloy according to one embodiment of the invention, the scale bar corresponding to a length of 100 μm,

[0042] FIG. 9 is a photomicrograph of a section of the superalloy according to one embodiment of the invention, the scale bar corresponding to a length of 10 μm,

[0043] FIG. 10 illustrates the creep of the parts according to different embodiments of the invention.

[0044] Throughout the figures, similar elements bear identical references.

Definitions

[0045] “Superalloy” means an alloy exhibiting very good resistance to oxidation, corrosion, creep and cyclic stresses (especially mechanical or thermal stresses) at high temperature and high pressure.

[0046] A superalloy can have a biphasic microstructure comprising a first phase (called “γ phase”) forming a matrix and a second phase (called “γ′ phase”) forming precipitates hardening in the matrix. The coexistence of these two phases is designated by γ-γ′ phase.

[0047] The “base” of the superalloy designates the main metal component of the matrix. In most cases, superalloys comprise a cobalt or nickel base. The superalloy base is preferentially a nickel base.

[0048] “Nickel-based superalloys” have the advantage of offering a good compromise between resistance to oxidation and resistance to breakage at high temperature and weight, which justifies their use in the hottest parts of turbojets.

[0049] Nickel-based superalloys are made up of a γ phase (or matrix) of the face-centered γ-Ni cubic austenitic type, optionally containing α-substitution additives in solid solution (Co, Cr, W, Mo, Re), and a γ′ phase (or precipitates) of the γ′-Ni.sub.3X type, with X=Al, Ti or Ta. The γ′ phase has an L12 ordered structure, derived from the face-centered cubic structure, consistent with the matrix, i.e., having an atomic lattice very close to the latter.

[0050] By its ordered nature, the γ′ phase has the remarkable property of having mechanical resistance that increases with temperature up to approximately 800° C. The very strong consistency between the γ and γ′ phases gives a very high hot mechanical strength for nickel-based superalloys, which itself depends on the γ/γ′ ratio and on the size of the hardening precipitates.

[0051] The term “mass fraction” designates the ratio of the mass of an element or a group of elements to the total mass.

DETAILED DESCRIPTION OF THE INVENTION

[0052] The aircraft part comprises a monocrystalline nickel-based superalloy substrate. The superalloy chosen can be predominantly composed of nickel and preferentially have a mass fraction of chromium comprised between 7% and 9%, of cobalt comprised between 5.5% and 7.5%, of aluminium comprised between 4% and 6%, of titanium comprised between 1% and 2%, of tantalum comprised between 7% and 9%, of molybdenum comprised between 1% and 3% and of tungsten comprised between 4.5% and 6.5%, the superalloy also comprising carbon and zirconium, Especially, the superalloy called “AM1” (registered trademark) can be chosen.

[0053] Other nickel-based superalloys can also be used for the manufacture of the substrate, especially the superalloy called “CMSX-4Plus” (registered trademark). The superalloy can be predominantly composed of nickel and preferentially has a mass fraction of chromium comprised between 2.5% and 4.5%, of cobalt comprised between 9% and 11%, of aluminium comprised between 4.5% and 6.5%, of titanium comprised between 0.5% and 1%, of tantalum comprised between 7% and 9%, of molybdenum comprised between 0.3% and 1%, of tungsten comprised between 5% and 7%, and of rhenium comprised between 4% and 5.5%.

[0054] In reference to FIG. 5, a method for manufacturing a part according to one embodiment of the invention comprises a step of moulding the part at a temperature greater than the melting temperature of the superalloy.

[0055] The method comprises, after the moulding step, a solution heat treatment S1 of the part. The part is put into solution at a first temperature T.sub.1. Temperature T.sub.1 is comprised between the solvus temperature of the γ′ phase and the melting temperature of the superalloy. Solution heat treatment makes it possible to diffuse the elements of the superalloy in the substrate of the part. The concentration of the different elements in the substrate is therefore homogenized.

[0056] The part is then cooled to room temperature at a controlled speed.

[0057] The part can then be demoulded. For example, it is possible to break the mould using vibration. Demoulding can lead to a high local concentration of stresses on the part, these stresses leading to a plastic deformation D1.

[0058] The part can undergo plastic deformation D1 by other means, such as assembling the part to another part or handling or moving the part. In particular, the plastic deformation D1 can be unintentional.

[0059] A step of homogenization S2 of the crystalline structure of the part is implemented following the plastic deformation(s) undergone by the part. In reference to FIG. 5, the homogenization S2 can be implemented by a heat treatment of the part at a second temperature T.sub.2 greater than the solvus temperature of the γ′ phase and strictly less than the first temperature T.sub.1. Thus, the γ′ phase, during the homogenization S2, can be dissolved in the matrix in the γ phase, leading to annihilation of the dislocations caused by plastic deformation. Thus, the local stresses internal to the substrate can be reduced.

[0060] Indeed, the temperature corresponding to the solvus of the superalloy decreases after the solution heat treatment S1. Thus, the upper bound of the second temperature T.sub.2 allows preventing recrystallization of the substrate during the homogenization S2. Moreover, the temperature is sufficiently high to decrease the internal stresses caused by plastic deformation effectively. Thus, the lower bound of the temperature T.sub.2 allows preventing recrystallization of the substrate during one or more later tempering treatments and during the homogenization S2.

[0061] The second temperature T.sub.2 is preferentially strictly comprised between 1280° C. and 1350° C., especially between 1280° C. and 1300° C., and preferentially between 1285° C. and 1295° C. Especially, when the nickel-based superalloy used for the manufacture of substrate is “CMSX-4Plus”, the second temperature T.sub.2 can be comprised between 1330° C. and 1335° C.

[0062] The homogenization S2 is implemented by the heat treatment of the part at a second temperature T.sub.2 for at least 10 minutes, especially for 20 minutes, and preferentially for one hour.

[0063] Thus, the treatment time for the homogenization S2 is adapted to the reaction kinetics of the homogenization S2 in the substrate of the part.

[0064] In reference to FIG. 6, the homogenization S2 of the crystalline structure of the part can be implemented by a heat treatment of the part at a third temperature T.sub.3 comprised between 800° C. and 1000° C., a tensile stress being applied to the part during the heat treatment at the third temperature T.sub.3 so as to cause plastic deformation of the part. In this case, the plastic deformation is intentional. The combined effect of the heat treatment implemented at the third temperature and the tensile stress makes it possible to produce homogeneous dislocations at the interfaces of the γ matrix and the γ′ precipitates. Thus, the effect of the plastic deformation in the microstructure is no longer visible after the homogenization S2.

[0065] In reference to FIG. 7, the homogenization S2 makes it possible to eliminate the trace of localized stresses in the substrate. The microstructure of the superalloy directly after the homogenization S2 has cuboid γ′ precipitates.

[0066] After the homogenization S2, the part is cooled to room temperature.

[0067] A first tempering R1 at a fourth temperature T.sub.4 comprised between 1000° C. and 1200° C. for at least 3 hours, and a second tempering R2 at a fifth temperature T.sub.5 comprised between 800° C. and 900° C. for at least 10 hours are then implemented. These treatments make it possible to optimize the size, morphology and distribution of γ′ precipitates, as well as the volume fraction thereof.

[0068] The tensile stress is preferentially applied to the part so that the deformation rate is less than 10.sup.−3 s.sup.−1 at any point of the part.

[0069] Thus, it is possible to prevent the appearance of slip bands in the microstructure of the substrate.

[0070] In reference to FIG. 8 and FIG. 9, when the homogenization S2 comprises a heat treatment of the part at a third temperature T.sub.3 comprised between 800° C. and 1000° C. and a tensile stress, the shape of the γ′ precipitates after the tempering treatments is different from the known cuboid form of the superalloy AM1.

[0071] FIG. 10 illustrates a creep test. Curve (a) is a measurement of the elongation of a known part that has not undergone plastic deformation. Curve (b) is a measurement of the elongation of a known part that has undergone plastic deformation. Curve (c) is a measurement of the elongation of a part manufactured by a method according to one embodiment of the invention, the method comprising a step of homogenization S2 of the crystalline structure of the part implemented by a heat treatment of the part at a second temperature T.sub.2 greater than the solvus temperature of the γ′ phase and strictly less than the first temperature T.sub.1. Curve (d) is a measurement of the elongation of a part manufactured by a method according to the invention, the method comprising a step of homogenization S2 of the crystalline structure implemented by a heat treatment of the part at a third temperature T.sub.3 comprised between 800° C. and 1000° C., a tensile stress being applied to the part during the heat treatment at temperature T.sub.3 so as to cause plastic deformation of the part. The plastic deformation of the part corresponding to curve (b) reduces the service life of the part due to recrystallization during creep. The creep duration of the part corresponding to curve (c) is greater than that of the part corresponding to model (a). The creep duration of the part corresponding to curve (d) is 85% of that of the part corresponding to model (a).