Patent ID: 12241151

Throughout the figures, similar elements bear identical references.

DEFINITIONS

“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.

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.

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.

“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.

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′-Ni3X 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.

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.

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

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.

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%.

In reference toFIG.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.

The method comprises, after the moulding step, a solution heat treatment S1of the part. The part is put into solution at a first temperature T1. Temperature T1is 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.

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

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.

The part can undergo plastic deformation D1by other means, such as assembling the part to another part or handling or moving the part. In particular, the plastic deformation D1can be unintentional.

A step of homogenization S2of the crystalline structure of the part is implemented following the plastic deformation(s) undergone by the part. In reference toFIG.5, the homogenization S2can be implemented by a heat treatment of the part at a second temperature T2greater than the solvus temperature of the γ′ phase and strictly less than the first temperature T1. 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.

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 T2allows 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 T2allows preventing recrystallization of the substrate during one or more later tempering treatments and during the homogenization S2.

The second temperature T2is 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 T2can be comprised between 1330° C. and 1335° C.

The homogenization S2is implemented by the heat treatment of the part at a second temperature T2for at least 10 minutes, especially for 20 minutes, and preferentially for one hour.

Thus, the treatment time for the homogenization S2is adapted to the reaction kinetics of the homogenization S2in the substrate of the part.

In reference toFIG.6, the homogenization S2of the crystalline structure of the part can be implemented by a heat treatment of the part at a third temperature T3comprised between 800° C. and 1000° C., a tensile stress being applied to the part during the heat treatment at the third temperature T3so 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.

In reference toFIG.7, the homogenization S2makes it possible to eliminate the trace of localized stresses in the substrate. The microstructure of the superalloy directly after the homogenization S2has cuboid γ′ precipitates.

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

A first tempering R1at a fourth temperature T4comprised between 1000° C. and 1200° C. for at least 3 hours, and a second tempering R2at a fifth temperature T5comprised 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.

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

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

In reference toFIG.8andFIG.9, when the homogenization S2comprises a heat treatment of the part at a third temperature T3comprised 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.

FIG.10illustrates 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 S2of the crystalline structure of the part implemented by a heat treatment of the part at a second temperature T2greater than the solvus temperature of the γ′ phase and strictly less than the first temperature T1. 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 S2of the crystalline structure implemented by a heat treatment of the part at a third temperature T3comprised between 800° C. and 1000° C., a tensile stress being applied to the part during the heat treatment at temperature T3so 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).