METHOD FOR PRODUCING A PART MADE OF A Y/Y' NICKEL-BASED ALLOY BY HOT FORGING

Description

FIELD OF THE INVENTION

The present invention relates to the field of producing parts made of a γ/γ′ alloy by hot forging tools.

It finds an advantageous application in the field of aeronautics, in particular for the production of turboshaft engine parts, such as high-pressure turbine or low-pressure turbine discs, or else high-pressure compressor discs.

STATE OF THE ART

The new nickel-based alloys with a γ/γ′ microstructure (such as AD7300 from Aubert & Duval or Rene65 chosen by Safran Aircraft Engines for some of its engines) are conventionally used in aeronautics for parts subjected to extreme stresses in service.

Such alloys are, for example, chosen by turboshaft producers as components of high-pressure turbine discs or low-pressure turbine discs, or for high-pressure compressor discs on the latest generations of aircraft engines.

More generally, they are now considered for any turboshaft engine part for which it is desired to increase the operating temperature and resistance to mechanical stress at these temperatures.

The forging method for these parts is a sequence of hot deformation steps (crushing of the billet, blank stamping, finish stamping, etc.) and heat treatments. Hot deformations are generally carried out on hydraulic presses with conventional hot-forging tools heated up to 650° C./680° C.

Due to their high content of hardening phase γ′, these alloys have a very narrow temperature forgeability range. During press treatments, the parts are heated to temperatures between 1040° C. and 1070° C., with a skin temperature for the part above 800° C. throughout the forging.

Two types of major issues must be mastered when producing parts with these alloys.

During forging operations, cooling during transfer between the heating furnace and the press or during contact with colder hot-forging tools during forging, leads to the appearance of surface cracks which must be eliminated after forging by finalization operations.

Furthermore, these alloys are sensitive to the phenomenon of abnormal grain growth. In very specific forging conditions, this phenomenon appears and results in, locally, a very significant coarsening of the grain. This phenomenon—which must be absolutely avoided—is a cause of rejection of the part because the mechanical properties, in particular fatigue resistance, of the part in these areas are prohibitive with respect to the required specifications.

Various studies have been carried out on these subjects. The treatment of these issues generally leads to very restrictive forging conditions.

Typically, this can lead to imposing minimal deformations throughout the part during forging operations.

Also, the proposed solutions generally require a greater material implementation weight, on the one hand to anticipate the loss of material during the finalization operations which aim at eliminating forging cracks and on the other hand, due to the addition of material in certain areas, to be able to respect the minimum deformation criterion in order to avoid the phenomenon of abnormal grain growth.

A consequence is an additional machining operation before the heat treatment operation. Since the raw forging parts are more massive, there is a risk that the heat treatment will not generate the required mechanical properties.

Indeed, on these γ/γ′ alloys, the static mechanical properties are strongly dependent on the cooling rate during heat treatment. An increase in the massiveness causes a decrease in this rate and therefore in the mechanical properties.

The elimination of cracks on raw forged parts, either by finishing or by machining before heat treatment, is necessarily accompanied by specific checks to ensure their total elimination. Indeed, the presence of residual surface cracks before heat treatment can lead to their propagation in the part during quenching. These additional checks generate additional costs.

DISCLOSURE OF THE INVENTION

A purpose of the invention is to propose a technique for producing γ/γ′ alloy parts with a hot press tool which allows to overcome the problems of abnormal grain growth and cracks, without having the complexity and the weight disadvantages of the solutions proposed to date.

In particular, one purpose of the invention is to propose a press forging technique which allows to reduce the amount of material and to optimize the weight of the part, while allowing to obtain parts with very few cracks, free from abnormal grain growth areas.

The present invention proposes to carry out forgings with a new design of forging range at a lower temperature compared to current ranges.

It was indeed found by the inventors that it was possible to eliminate the phenomenon of abnormal grain growth on this type of alloy by carrying out forging at a lower temperature. This reduction in temperature allows to leave the abnormal growth range and therefore to no longer risk degrading the microstructure of the part during forging.

For this purpose, according to one aspect, the invention proposes a method for producing a part made of a nickel-based alloy with a γ/γ′ microstructure in which at least one hot-forging step is performed, which method is characterized in that the temperature at which the part is heated is maintained within a temperature range lower than the abnormal grain growth temperature range of the alloy, the hot-forging tool being maintained at a temperature lower than the temperature of the part, the difference between this temperature and the temperature at which the part is heated being less than 325° C., preferably less than 250° C. and more preferably less than 150° C.

Abnormal grain growth temperature of the alloy means here and throughout this text, the temperature below which a small deformation in a conventional forging speed range does not generate abnormal grain growth, that is to say a burst grain size at least 2 times greater than the nominal grain size. In this regard, reference can advantageously be made to the thesis of Marie Agathe Charpagne:

• • “Evolutions de microstructure au cours du forgeage de l'alliage Rene65”—PSL Research University—Mines Paris Tech—https://pastel.archives-ouvertes.fr/tel-01764932

A snapshot of burst grains in a fine-grained matrix is shown in FIG. 1. This snapshot is from the thesis mentioned above (p. 151). The burst grains correspond to the white areas in the center.

Thus, forging is carried out with parts at low temperature, so as not to enter the area of abnormal grain growth which degrades the microstructure.

This change in heating temperature is further accompanied by a change in the forging tool temperatures, the hot tool temperature (higher) allowing, by reducing thermal losses during tool/part contact, to keep the part within its forgeability range and not generate cracks.

The alloy may be Udimet720™.

In preferred embodiments, the alloy is of the Rene65 or AD730® type.

At least one blank forging or finishing step is performed at a temperature at which the part is heated equal to the solvus temperature γ′ of the alloy minus 80° C. (+/−10° C.) or lower, in particular 80° C. or lower, the hot-forging temperature being higher than 700° C., preferably 750° C. and less than 900° C., or even 850° C.

In a preferred embodiment, the difference compared with the temperature at which the part is heated is then less than 150° C.

The heating temperature can advantageously be comprised between 1000° C. and 1025° C.

For example, the temperature at which the part is heated is 1025° C. or lower (in the case where the alloy is Rene65 or AD730® in particular, whose solvus temperatures γ′ are respectively 1105° C. and 1110° C., the solvus temperature γ′ of the Udimet720™ in turn being 1155° C.).

The invention further relates to an aircraft turbine engine part, produced using a production method as proposed, in particular a high-pressure turbine disc part, a low-pressure turbine disc, or a high-pressure compressor disc.

It also relates to an aircraft turbine engine including such a part.

DESCRIPTION OF THE FIGURES

Other features, purposes and advantages of the invention will emerge from the description which follows, which is purely illustrative and non-limiting, and which must be read in conjunction with the appended drawings in which:

FIG. 1 is a snapshot of burst grains;

FIG. 2a illustrates a turbine disc produced using a production method in accordance with the state of the art;

FIG. 2b illustrates a turbine disc produced according to a production method in accordance with the invention;

FIG. 3 schematically illustrates an example of a turbojet engine structure.

DETAILED DESCRIPTION OF THE INVENTION

Nickel-Based Superalloys with γ-γ′ Microstructure

The alloy used for the production of the part is a nickel-based superalloy with γ-γ′ microstructure.

Nickel-based superalloys are typically composed of a γ phase (or matrix) of the y-Ni face-centered cubic austenitic type, possibly containing substitution additives in solid solution α(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 ordered L12 structure, derived from the face-centered cubic structure, consistent with the matrix, that is to say having an atomic mesh very close thereto.

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

The chosen superalloy may be mainly composed of nickel and preferably have a mass fraction of chromium, cobalt, aluminum, titanium, molybdenum, and in particular preferably between 15 and 17% of chromium, between 8 and 15.5% of cobalt, between 1.5 and 4% of aluminum, between 3 and 5.2% of titanium, between 2 and 4% of molybdenum, between 2 and 4.2% of Tungsten.

The superalloy can also comprise carbon, zirconium, iron, etc.

Typically, as an example, the production alloy may be AD7300, Rene65.

An example of mass composition is as follows (AD730®):

• • Cr: 15% to 17%,
  • Co: 8% to 10%,
  • Mo: 2.5% to 3.5%,
  • W: 2.3% to 3.3%,
  • Nb: 0.8% to 1.4%,
  • Ti: 3.2% to 3.8%,
  • Al: 2% to 2.6%,
  • B: 0.005% to 0.025%,
  • Zr: 0.01% to 0.05%,
  • Fe: 3% to 5%
  • C: 0.005% to 0.02%,
  • Mn<0.5%,

Another example of mass composition is still (Rene65)

• • Cr: 15.5% to 16.5%,
  • Co: 12.5% to 13.5%,
  • Al: 1.95% to 2.3%,
  • Ti: 3.55% to 3.9%,
  • Mo: 3.8% to 4.2%,
  • W: 3.8% to 4.2%,
  • Nb: 0.6% to 0.8%,
  • B: 0.012% to 0.02%,
  • Zr: 0.03% to 0.06%,
  • C: 0.005% to 0.011%
  • Mn<0.1%,
  • Fe<1.2%
  • Ta: traces (1000 ppm max).

Other compositions are of course possible. The alloy can for example also be Udimet720™ whose composition is as follows:

Udimet 720

Hot-Forging Tools

Different hot-forging tools can be provided depending on the operations considered: crushing of the billet, stamping of the blank, finish stamping, etc.

Typically, the hot-forging tool is mounted on a press with press speeds comprised between 0.5 and 20 mm/s and pressures between 2000 and 60000 T.

A possible press is, for example, a hot die type forging press of the type described in patent application FR2880827.

A hot heating system maintains the temperature of the hot-forging tool so that its contact surface with the part is permanently above 750° C. Several systems can be provided for this purpose: heating by heating rods immersed in the hot-forging tool, induction heating via a peripheral heating system, heating by peripheral electrical resistors.

Examples of Forging

Solvus temperatures γ′ are as follows: Rene65, or Udimet720™

• • Udimet720™: 1155° C.
  • AD730®: 1110° C.
  • Rene65: 1105° C.

In the case of alloys of the Rene65 and AD730® type, in particular (but also in the case of Udimet720™), forging can take place under the following conditions:

Blank Forgings and Finishing:

• • Heating temperature of the part: between 1000° C. and 1040° C.
  • Hot-forging temperature: between 750° C. and 900° C.
  • Maintaining tools at temperature under the press

Preferably, the temperature at which the part is heated is more particularly comprised between 1000° C. and 1025° C.

Preferably, the hot-forging temperature is more particularly comprised between 800° C. and 900° C.

Alternatively, the hot-forging temperature is more particularly comprised between 750° C. and 850° C.

Other upstream forgings can be carried out under different conditions:

• • Heating temperature: between 1040° C. and 1060° C.
  • Temperature of the stamping tool: between 400° C. and 650C. In the case of rolled blanks, this rolling operation is carried out in a conventional manner, the finish forging steps are then carried out under the conditions indicated above.

Examples of Parts

The production method is used, for example, for the production of an aircraft turbine engine part, in particular a high-pressure turbine or low-pressure turbine part, or else a high-pressure compressor part.

Different tests were able to be carried out for the production of different types of parts.

Example 1

In particular, a test campaign was carried out for the production of crowns by circular blank rolling, then low-deformation stamping at a temperature comprised between 1000° C. and 1025° C. with a Rene65 type alloy.

The crowns obtained do not have any burst grains.

As illustrated in FIGS. 2a and 2b, significant differences in terms of crack formation were observed depending on the hot-forging temperature at the contact surface with the part (heated flat pile hot-forging tool).

For a hot-forging temperature of 650° C. (FIG. 2a), deep cracks could be observed (up to 1.5 mm) over a large portion of the circumference.

For a hot-forging temperature of 750° C. or higher (typically, 850° C. and higher), the crown does not have a crack (FIG. 2b).

Example 2

Tests were also conducted on reduced-scale slugs heated to between 1000° C. and 1025° C. and forged in two low-deformation operations, with hot tools at contact surface temperatures maintained between 800° C. and 900° C.

The slugs were found to have no burst grains and also no cracks.

Turbojet Engine

The dual-flow turbojet engine 1 of FIG. 3 extends along an axis A-A and includes a flow path for a primary flow or primary flow path 2 comprising, from upstream to downstream in the direction of circulation of the gas flow within the turbomachine, a low-pressure compressor 3, a high-pressure compressor 4, a combustion chamber 5, a high-pressure turbine 6 and a low-pressure turbine 7.

The discs of the low-pressure compressor 3 are for example made of titanium alloy, while all or part of the discs of the high-pressure and low-pressure turbines and the discs of the last stages of the high-pressure compressor can be produced according to a production method of the type described above.