EP3223981B1 - Process for manufacturing three-dimensional parts made of aluminium-titanium alloy - Google Patents

Description

Background of the invention

The present invention relates to the general field of manufacturing processes for three-dimensional parts based on metal alloys.

More particularly, titanium-based alloys are used for parts intended to be subjected to significant thermomechanical stresses and corrosive atmospheres. These alloys make it possible to reduce the mass of these parts and their use is therefore advantageous for reasons of cost and/or energy efficiency, as is the case for example in the aeronautical field.

The manufacture of titanium-based metal alloy parts is traditionally done by processes involving foundry or the electron beam melting technique or “Electron Beam Melting” (EBM). The manufacture of parts of complex geometry, such as a turbomachine blade, is difficult and requires significant processing and machining steps subsequent to the application of the aforementioned development processes. In particular, additional machining steps often result in a high scrap rate, which increases production costs.

In order to control these costs and obtain a part of precise shape requiring less machining after development, it is desirable to have a process which makes it possible to manufacture complex parts in a titanium-based alloy which do not present these disadvantages. .

We know the metal injection molding process, or MIM (“Metal Injection Molding”), which makes it possible to obtain metal parts of precise shapes which do not require heavy and expensive machining after their development.

The documents US 6,350,407 And Lindberg et al. "Influence of sintering atmosphere on the tensile properties of MIM-processed Ti 45Al5Nb0.2B " describe examples of metal injection molding processes.

Such a method comprises a step of preparing an injection composition based on metal powder (for example a metal alloy) and at least one binder (for example a thermoplastic resin), a step of injecting the injection composition in a cavity of a mold for producing a blank of the part, a step of selective elimination of the binder present in the blank or debinding, for example using a solvent at a controlled temperature, and a step of sintering the metal powder in order to densify it.

However, titanium-based alloy parts produced by traditional MIM processes often exhibit inhomogeneous mechanical properties and relatively significant oxidation, which reduces their service life.

Object and summary of the invention

The inventors noticed during tests that the inhomogeneity of mechanical properties or the relatively significant oxidation of parts obtained by a traditional MIM process was mainly due to changes in the chemical composition of the alloy occurring during manufacturing. of the room. More precisely, the Inventors observed that this modification of the chemistry of the part occurs during the sintering stage of the alloy powder and that it is mainly due to the evaporation of addition elements. In addition, most known MIM processes recommend applying a reduced pressure in the sintering chamber, and the evaporation of the addition elements is higher as the pressure in the chamber is reduced.

The present invention aims to overcome the disadvantages of the MIM processes of the prior art by proposing a method of manufacturing a sintered three-dimensional part comprising a titanium-based alloy which makes it possible to compensate for undesirable modifications in the chemistry of the alloy and to obtain, consequently, parts of complex geometry presenting homogeneous mechanical properties.

This goal is achieved thanks to a method of manufacturing a sintered three-dimensional part according to claim 1.

Controlling the pressure during the first sintering step is necessary because it is necessary to ensure the densification of the part at a high temperature, while avoiding a significant change in the chemistry of the preform following the first sintering step. Also, by setting a first pressure greater than or equal to 1 mbar, this first pressure is greater than the saturated vapor pressure of the addition elements at the sintering temperature, which limits their evaporation and therefore changes in the chemistry of the part following the first sintering step.

The first pressure can be greater than or equal to 10 mbar. The first pressure can be applied for a duration of, for example, between 1 hour and 24 hours.

The method further comprises, after the first sintering step, a second sintering step during which a second pressure is imposed, the second pressure being lower than the first pressure, the duration of application of the second pressure being chosen so that the content mass of aluminum and/or chromium in a 200 µm thick layer located on the surface of the preform does not vary by more than 5% in relative value following the second sintering step.

The second pressure is less than 1 mbar. For example, the second pressure may be less than or equal to 10 ^-1 mbar, less than or equal to 10 ^-2 mbar, or even less than or equal to 10 ^-3 mbar. The second pressure is applied for a period of less than 5 hours, for example between 10 minutes and 5 hours.

Thus, by carrying out such a second sintering step in which the second pressure applied is lower than the first pressure, the porosity of the preform obtained after the first sintering step is further reduced due to the evacuation of the gas present in the porosity. . However, even if the conditions of the second sintering stage are optimal for evacuating the gas from the porosity, they are also favorable to the evaporation of the addition elements within the alloy which can lead to a modification of its chemistry, particularly on the surface of the preform. It is therefore desirable to limit the duration of this second sintering step. This limitation of duration is possible in the present invention because the densification of the preform has already been advanced during the first sintering step without affecting its chemistry. The duration of the second sintering step can then be significantly reduced so as not to unduly affect the chemistry of the alloy while being useful for evacuating the gas present in the porosity of the preform and thus improving the densification obtained.

The duration of application of the second pressure is determined so that the mass contents of addition elements (such as aluminum and/or chromium) on the surface of the preform do not vary by more than 5% in relative value. following the second sintering stage.

Here we mean by mass content of an addition element on the surface of the preform, the mass proportion of an element in a layer of thickness of the order of 200 μm located on the surface of the preform.

By relative variation in the mass content of a given element is meant the relative variation between the mass content of said element before the first sintering step and after the second sintering step. For example, if the aluminum mass content was 30% before the first sintering stage, and it is 28.5% after the second sintering stage, the relative variation in the aluminum mass content following the first two stages sintering is (30-28.5)/30=5%.

These surface mass contents are determined on samples of the preform before sintering and after sintering by destructive or semi-destructive chemical analyses, in particular by: plasma torch spectrometry (ICP), energy dispersive analysis (EDX), analysis wavelength dispersive (WDS) or X-ray fluorescence spectrometry (XRF).

Preferably, the method further comprises, after the second sintering step, a third sintering step during which a third pressure is imposed, the third pressure being greater at the second pressure, and which may for example be greater than or equal to 1 mbar.

The third sintering step makes it possible to complete the densification of the part, for example if too many addition elements have evaporated and the desired densification is not achieved. The duration of this third step therefore depends on the progress of the densification of the preform at the end of the second sintering step. The duration of this third step can be, for example, between 10 minutes and 10 hours.

The invention also relates to the manufacturing process described above in which the manufactured part is a turbomachine blade.

According to one aspect of the invention, the aluminum mass content of the titanium-based alloy powder is greater than 10% before the first sintering step.

Preferably, the titanium-based alloy powder has the following mass contents of elements before the first sintering step: between 32% and 33.5% aluminum, between 4.5% and 5.1% niobium , and between 2.4% and 2.7% chromium.

Alternatively, the titanium-based alloy powder has the following mass contents of elements before the first sintering step: between 28.12% and 29.12% aluminum, between 8.56% and 9.56% aluminum. niobium, and between 1.84% and 2.84% molybdenum.

Alternatively, the titanium-based alloy powder has the following mass contents of elements before the first sintering step: between 5.4% and 6.6% aluminum, and between 3.6% and 4.4%. % vanadium.

Brief description of the drawings

• la figure 1 est un ordinogramme représentant les principales étapes d'un procédé selon un mode de réalisation de l'invention,
• la figure 2 est une vue très schématique d'un moule d'injection, et
• la figure 3 est une vue très schématique d'une aube de turbomachine pouvant être fabriquée par un procédé selon l'invention.

Other characteristics and advantages of the present invention will emerge from the description given below, with reference to the appended drawings which illustrate an exemplary embodiment devoid of any limiting character. In the figures:

• there figure 1 is a flowchart representing the main steps of a process according to one embodiment of the invention,
• there figure 2 is a very schematic view of an injection mold, and
• there Figure 3 is a very schematic view of a turbomachine blade that can be manufactured by a process according to the invention.

Detailed description of the invention

The invention will now be described in its application to the manufacture of sintered three-dimensional parts made of titanium-based alloys.

In a manner well known in itself, one of the steps of a MIM process consists of injecting under pressure into a cavity of a mold an injection composition comprising a powder of a metal alloy and a binder.

The alloy powder may preferably be a titanium and aluminum alloy powder. The alloys described above can be used.

The powder is preferably in the form of substantially spherical grains. The powder preferably has a grain size (d ₉₀ ) less than or equal to 150 µm. In other words, if we consider the size distribution of the grains making up the powder, 90% of the grains have a size less than or equal to 150 µm.

The binder may, in a manner known per se, comprise a compound chosen from: paraffins, thermoplastic resins, agar gel, cellulose, polyethylene, polyethylene glycol, polypropylene, stearic acid, polyoxymethylene, etc. . and their mixtures.

In reference to the figure 1 , a mode of implementing a method according to the invention, comprises the following steps.

An injection composition is prepared (step E10) from an alloy powder as described above and a binder.

The injection composition can typically consist of alloy powder between 50% and 70% by volume, and 30% to 50% by volume of binder.

The injection composition can first be mixed at a temperature between 150°C and 200°C under a neutral atmosphere for example, and will be injected at this temperature.

As illustrated very schematically on the figure 2 , the injection mold 1 generally consists of two parts 14, 16 forming a cavity 12 having the shape of the part to be manufactured. The injection mold advantageously has several injection points 18a, 18b, 18c which allow injection into several parts of the cavity 12 of the mold 1.

Typically, injection is carried out at pressures which can vary from 400 bars to 800 bars.

The injection is then carried out (step E20) in the injection mold 1 which is temperature regulated, between 30°C and 70°C for example, so that the injection composition becomes plastic to form a blank of the piece to be made. The blank thus produced is said to be in a “green” or plastic state.

It is advantageous to do the injection in a cavity of the mold in which a vacuum has been created, in order to facilitate the injection and to ensure the homogeneity of the blank which will be molded.

The blank is then demolded (step E30), and possibly machined in the green state (step E40) to remove burrs or carrots from the injection points which could have appeared during demoulding.

The next step consists of selectively eliminating the binder present in the blank thus formed.

The step of selective elimination of the binder (step E50), also called "debinding", makes it possible to obtain a powder which has the shape of the part to be manufactured from a blank of the part in the green state.

Selective removal of the binder may include dissolving the binder by treatment with a solvent.

The selective removal of the binder can be carried out entirely or finalized thermally. In this case, it can be carried out in a sintering chamber so as not to move the powder between the step of selective elimination of the binder present in the blank and the first sintering step.

Prior to introducing the powder into the sintering chamber, the sintering chamber was purged and decontaminated by cycles pumping under vacuum, for example under reduced pressure of argon or dihydrogen. Indeed, it is necessary to be under a neutral or reducing atmosphere during sintering to avoid oxidation of the elements present in the alloy.

The sintering step (step E60) is carried out in a sintering chamber, in which a sintering temperature is imposed gradually. In a manner known per se, the sintering temperature is of the order of 80% to 90% of the solidus temperature of the alloy present in the powder to be sintered and ramps from 0.10°C/minute to 20° C/minute allows you to gradually reach this temperature.

In accordance with the invention, a first sintering step (step E601) is carried out by subjecting the powder to a first pressure, of neutral or reducing atmosphere (under argon or dihydrogen for example), greater than or equal to 1 mbar, for example greater than or equal to 10 mbar.

The evaporation of addition compounds such as chromium and/or aluminum is negligible throughout the duration of the first sintering stage in which this first pressure is applied. Thus, during this step the densification of the preform is carried out while avoiding a modification of the chemistry of the powder on the surface of the preform by evaporation of the addition elements.

Partial sintering is carried out during the first sintering step and then a second sintering step is carried out.

During this second sintering step, the preform is subjected to a second pressure, lower than the first, which is imposed in the sintering chamber for a determined duration (step E602).

The purpose of this second pressure is to evacuate the gas present in the porosity of the preform to increase its densification. However, as explained above, the duration of application of the second pressure is limited in order to minimize the evaporation of the addition elements such as aluminum and/or chromium from the surface of the preform. In other words, during the second sintering step, a treatment is carried out to evacuate the gas present in the porosity. generated during sintering without significantly affecting the composition of the preform, particularly on its surface.

By evaporation on the surface of the preform is meant the evaporation of the addition elements in a layer of characteristic thickness (generally of the order of 200 μm) on the surface of the preform.

For example, if a very low second pressure value is chosen, the evacuation of the gas present in the porosity will be more effective and the densification faster, but the evaporation of the addition elements on the surface of the preform will be even more important.

Alternatively, if a higher second pressure value is applied, the evacuation of the gas present in the porosity will be longer and the densification more limited, but the evaporation of the addition elements on the surface of the preform will be less.

Thus, the duration of application of the second pressure will be adapted to minimize the relative variation in the mass content of aluminum and/or chromium on the surface of the preform following the second sintering step, preferably to less than 5%, more preferably less than 3%, even more preferably less than 1%. In other words, the mass content of aluminum and/or chromium on the surface of the preform does not preferentially vary by more than 5% in relative value following the second sintering step, more preferably by 3%, even more preferably 1%.

After the second sintering step, it is possible to carry out a third sintering step (step E603) during which a third pressure greater than the second pressure is imposed. This third pressure can for example be greater than or equal to 1 mbar.

After the first sintering step (step E601), or after the second or third sintering step if necessary (steps E602 and E603), the preform is cooled by temperature reduction ramps, for example by 0.1° C/minute at 60°C/minute, in order to optimize the microstructure of the part.

The final part is obtained from the preform which will have undergone finishing treatments (step E70), known per se, such as hot isostatic compression to finalize the densification of the part, additional heat treatments to optimize the microstructure, surface treatments by machining or polishing, etc.

The method of the invention is particularly suitable for the manufacture of a blade 2 of a turbomachine, comprising for example a foot 22, a blade 24 and a head 26, like that illustrated very schematically on the Figure 3 .

First example (excluding the invention)

The first example describes a method of manufacturing a blade 2 made of titanium alloy of the TiAl6-V4 type by a method according to the invention.

We first have a commercial powder of a grade 23 titanium alloy (TiAl6-V4) having substantially spherical grains with a d ₉₀ of 45 µm.

We also have a binder consisting in particular of paraffin wax, poly(ethylene-vinyl acetate) and stearic acid.

The injection composition is produced (step E10) by mixing the alloy powder with the binder under Argon, at a temperature of 120° C. for 2 hours.

The injection composition is injected into the cavity 12 of the injection mold 1 (step E20).

The blank of blade 2 in the green state is then demolded (step E30) and machined in the green state (step E40) to remove the burrs due to the injection.

Then, the blade blank is placed in a hexane bath at 40°C for 10 hours to remove the binder by dissolution (step E50).

The step of selective elimination of the binder continues in a sintering chamber, in which the blank partially removed from the binder will have been placed, carrying out heat treatments to eliminate the last traces of binder.

The sintering step (step E60) is initiated by raising the temperature in the sintering chamber to 1350°C.

The pressure inside the enclosure is then adjusted to 10 mbar for 2 hours to carry out a first sintering step (step E601).

The preform is cooled then extracted from the sintering chamber to undergo conventional finishing treatments (step E70).

Second example

The second example describes a method of manufacturing a blade 2 of titanium alloy of the TiAl 48-2-2 type by another method according to the invention.

We first have a commercial powder of a titanium alloy of chemical composition as described in Table 1, having substantially spherical grains with a d ₉₀ of 25 µm. <b>Table 1 -</b> Chemical composition (in % by weight) of the alloy Ti Al No. Cr Fe Base 32.0-33.0 4.50-5.10 2.40-2.70 0.10 VS NOT _{H2 O2} If 0.015 0.02 0.01 0.04-0.13 0.025

We also have a binder mainly made up of polyethylene and polyethylene glycol.

The injection composition is produced (step E10) by mixing the alloy powder with the binder, at a temperature of 170°C.

The injection composition is injected into the cavity 12 of the injection mold 1 (step E20) regulated at 40°C and in which a vacuum has been created.

The blank of blade 2 in the green state is then demolded (step E30) and machined in the green state (step E40) to remove the burrs due to the injection.

Then, the blade blank is placed in a water bath at 75°C for 24 hours to remove the binder by dissolution (step E50).

The step of selective elimination of the binder continues in a sintering chamber in which the blank partially removed from the binder will have been placed, carrying out heat treatments to eliminate the last traces of binder.

The sintering step (step E60) is initiated by raising the temperature in the sintering chamber to 1410°C.

The pressure inside the enclosure is adjusted to 1 mbar for 6 hours to carry out a first sintering step (step E601).

After the first sintering step, a second sintering step is carried out (step E602) by lowering the pressure to 10 ^-1 mbar in the enclosure for 30 minutes.

The preform is cooled then extracted from the sintering chamber to undergo conventional finishing treatments (step E70).

Claims (8)

1. A method of fabricating a sintered three-dimensional part comprising a titanium-based alloy, the method comprising the following steps:

   · preparing an injection composition comprising a binder and a powder of a titanium-based alloy including aluminum and/or chromium as alloying addition element(s) (step E10);

   · injecting the injection composition into a cavity (12) of a mold (1) so as to obtain a blank for the part to be made (step E20);

   · selectively eliminating the binder present in the blank (step E50);

   · a first step of sintering the powder of titanium-based alloy (step E601), the powder being subjected during the first sintering step to a first pressure that is higher than or equal to 1 mbar in order to obtain a preform of the part made of sintered alloy powder; and

   · a second sintering step, performed after the first sintering step, during which a second pressure is imposed (step E602), the second pressure being lower than the first pressure, the duration for which the second pressure is applied being selected so that the content by weight of aluminum and/or chromium in a layer having a thickness of 200 µm situated at the surface of the preform does not vary by more than 5% in relative value as a result of the second sintering step, wherein the second pressure is lower than 1 mbar, and wherein the second pressure is applied during less than 5 hours.
2. A method according to claim 1, further including, after the second sintering step, a third sintering step during which a third pressure is imposed (step E603), the third pressure being higher than the second pressure.
3. A method according to claim 2, wherein the third pressure is higher than or equal to 1 mbar.
4. A method according to any one of claims 1 to 3, wherein the part obtained is a turbine engine blade (2).
5. A method according to any one of claims 1 to 4, wherein the content by weight of aluminum in the alloy powder is greater than 10% prior to the first sintering step.
6. A method according to any one of claims 1 to 5, wherein the alloy powder prior to the first sintering step presents the following contents by weight of the following elements:

   · 32% to 33.5% aluminum;

   · 4.5% to 5.1% niobium; and

   · 2.4% to 2.7% chromium.
7. A method according to any one of claims 1 to 5, wherein the alloy powder prior to the first sintering step presents the following contents by weight of the following elements:

   · 28.12% to 29.12% aluminum;

   · 8.56% to 9.56% niobium; and

   · 1.84% to 2.84% molybdenum.
8. A method according to any one of claims 1 to 4, wherein the alloy powder prior to the first sintering step presents the following contents by weight of the following elements:

   · 5.4% to 6.6% aluminum; and

   · 3.6% to 4.4% vanadium.

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