US20170321303A1

US20170321303A1 - A method of fabricating three-dimensional parts out of an alloy of aluminum and titanium

Info

Publication number: US20170321303A1
Application number: US15/529,011
Authority: US
Inventors: Guillaume Fribourg; Jean-François Castagne; Jean-Claude Bihr; Clément GILLOT
Original assignee: Safran Aircraft Engines SAS
Current assignee: Safran Aircraft Engines SAS
Priority date: 2014-11-25
Filing date: 2015-11-24
Publication date: 2017-11-09
Also published as: EP3223981B1; CN107002178A; EP3223981A1; WO2016083724A1; FR3028784B1; CN107002178B; FR3028784A1

Abstract

A method of fabricating a sintered three-dimensional part, the method including: preparing an injection composition including a binder and a powder of a titanium-based alloy including aluminum and/or chromium; injecting the composition into a cavity of a mold to obtain a blank; eliminating the binder present in the blank; a first step of sintering the powder, the powder subjected to a first pressure higher than or equal to 1 mbar to obtain a preform of the part; and a second sintering step during which a second pressure, which is lower than the first pressure, is imposed, 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 due to the second sintering step.

Description

BACKGROUND OF THE INVENTION

The present invention relates to the general field of methods of fabricating three-dimensional parts based on metal alloys.
More particularly, alloys based on titanium are used for parts that are to be subjected to high thermo-mechanical stresses and to corrosive atmospheres. These alloys serve to reduce the weight of such parts and consequently they are advantageous to use for reasons of cost and/or energy efficiency, as applies for example in the field of aviation.
Parts made of titanium-based metal alloy are traditionally fabricated by methods involving in particular casting or electron beam melting (EBM). Parts that are of complex shape, such as a turbine engine blade, are difficult to fabricate and require major treatment and machining steps after applying the above-mentioned production methods. In particular, additional machining steps often lead to a high reject rate, which increases production costs.
In order to keep costs under control and obtain a part of accurate shape that requires less machining after production, it is desirable to have a method that makes it possible to fabricate parts of complex shape out of a titanium-based alloy without presenting those drawbacks.
The metal injection molding (MIM) method is known and enables metal parts to be obtained that are of accurate shape without requiring large amounts of expensive machining after they have been produced.
Such a method comprises a step of preparing an injection composition based on metal powder (e.g. of a metal alloy) together with at least one binder (e.g. a thermoplastic resin), a step of injecting the injection composition into a cavity of a mold in order to make a blank of the part, a step of selectively eliminating the binder present in the blank, referred to as “debinding”, e.g. by using a solvent at a controlled temperature, and a step of sintering the metal powder in order to densify it.
Nevertheless, parts made of titanium-based alloys using traditional MIM methods often present mechanical properties that are non-uniform, together with relatively large amounts of oxidation, thereby shortening their lifetime.

OBJECT AND SUMMARY OF THE INVENTION

As a result of testing parts obtained by a traditional MIM method, the inventors have observed that the non-uniformity of their mechanical properties or the relatively large amounts of oxidation are due mainly to modifications to the chemical composition of the alloy that take place while the part is being fabricated. More precisely, the inventors have observed that this modification to the chemistry of the part occurs during the step of sintering the alloy powder, and that it is due mainly to alloying addition elements evaporating off. In addition, most known MIM methods recommend applying low pressure in the sintering enclosure, with the evaporation of addition elements increasing with reduction of pressure in the enclosure.
The present invention seeks to overcome drawbacks of prior art MIM methods by proposing a method of fabricating a sintered three-dimensional part comprising a titanium-based alloy that makes it possible to mitigate the undesirable modifications to the chemistry of the alloy and consequently to obtain parts of complex shape that present mechanical properties that are uniform.
This object is achieved by 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);
- injecting the injection composition into a cavity of a mold so as to obtain a blank for the part to be made;
- selectively eliminating the binder present in the blank; and
- a first step of sintering the powder of titanium-based alloy, the powder being subjected during the first sintering step to a first pressure that is higher than or equal to 1 millibar (mbar) in order to obtain a preform of the part made of sintered alloy powder.

Controlling pressure during the first sintering step is necessary since it is necessary to ensure that the part is densified at high temperature while avoiding any significant modification to the chemistry of the preform as a result of the first sintering step. Thus by setting the first pressure to be higher than or equal to 1 mbar, this first pressure is higher than the saturated vapor pressure of the addition elements at the sintering temperature, thereby limiting evaporation thereof and thus limiting any modification to the chemistry of the part as a result of the first sintering step.
The first pressure may be higher than or equal to 10 mbar. The first pressure may be applied for a duration lying in the range 1 hour to 24 hours, for example. After the first sintering step, the method also includes a second sintering step during which a second pressure is imposed, 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 micrometers (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.
Preferably, the second pressure is lower than 1 mbar. For example, the second pressure may be lower than or equal to 10⁻¹mbar, lower than or equal to 10⁻²mbar, or indeed lower than or equal to 10⁻³mbar. The second pressure may be applied for a duration of less than 5 hours, e.g. a duration lying in the range 10 minutes to 5 hours.
Thus, by performing such a second sintering step in which the applied second pressure is lower than the first pressure, the porosity of the preform obtained after the first sintering step is further reduced by evacuating the gas present in the pores. Nevertheless, even though the conditions of the second sintering step are good for evacuating gas from the pores, they are also favorable to evaporating addition elements from within the alloy, thereby modifying its chemistry, in particular at the surface of the preform. It is therefore desirable to limit the duration of this second sintering step. This limited duration is possible in the present invention since the densification of the preform is already well advanced during the first sintering step that does not affect its chemistry. The duration of the second sintering step can then be significantly shortened so as to avoid excessively affecting the chemistry of the alloy, while still being useful for evacuating the gas present in the pores of the preform, thereby improving the resulting densification.
The duration for which the second pressure is applied is determined so that the content by weight of addition elements (such as aluminum and/or chromium) at the surface of the preform does not vary relatively by more than 5% in relative value by the end of the second sintering step.
The content by weight of an addition element at the surface of the preform is used herein to mean the proportion by weight of an element in a layer of thickness of the order of 200 μm situated at the surface of the preform.
The relative variation in the mass content of a given element means the relative variation between the mass content of said element before the first sintering step and the mass content of said element after the second sintering step. For example, if the mass content of aluminum was 30% before the first sintering step, and if it becomes 28.5% after the second sintering step, then the relative variation in the mass content of aluminum after the first two sintering steps is (30-28.5)/30=5%.
These mass contents at the surface are determined on samples of the preform before sintering and after sintering by chemical analyses that may be destructive or semi-destructive, and in particular by: inductively coupled plasma-mass spectrometry (ICP), energy dispersive X-ray spectroscopy (EDX), wavelength dispersive spectroscopy (WDS), or X-ray fluorescence spectrometry (XRF).
Preferably, after the second sintering step, the method further includes a third sintering step during which a third pressure is imposed, the third pressure being higher than the second pressure, and possibly being higher than or equal to 1 mbar, for example.
The third sintering step makes it possible to finish off densifying the part, e.g. if too much of the addition elements has evaporated and the desired densification has not been achieved. The duration of this third step thus depends on how far densification of the preform has advanced by the end of the second step. The duration of this third step may lie in the range 10 minutes to 10 hours, for example.
The invention also provides the above-described fabrication method in which the fabricated part is a turbine engine blade.
In an aspect of the invention, the content by weight of aluminum in the titanium-based alloy powder is greater than 10% prior to the first sintering step.
Preferably, the titanium-based 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.
Alternatively, the titanium-based 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.
Also alternatively, the titanium-based 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.

BRIEF DESCRIPTION OF THE DRAWINGS

Other characteristics and advantages of the present invention appear from the following description given with reference to the accompanying drawings, which show an implementation having no limiting character. In the figures:

FIG. 1 is a flow chart showing the main steps of a method in an implementation of the invention;

FIG. 2 is a highly diagrammatic view of an injection mold; and

FIG. 3 is a highly diagrammatic view of a turbine engine blade suitable for being fabricated by a method of the invention.

DETAILED DESCRIPTION OF THE INVENTION

The invention is described below in its application to fabricating sintered three-dimensional parts made out of titanium-based alloy.
In well-known manner, one of the steps of a MIM method consists in injecting an injection composition under pressure into a mold cavity, the composition comprising a metal alloy powder and a binder.
The alloy powder may preferably be a powder of a titanium and aluminum alloy. It is possible to use the alloys described above.
The powder is preferably in the form of substantially spherical grains. The powder preferably possesses a grain size (d₉₀) that is less than or equal to 150 μm. In other words, if consideration is given to the distribution of the sizes of the grains making up the powder, 90% of the grains have a size that is less than or equal to 150 μm.
In known manner, the binder may comprise a compound selected from: paraffins; thermoplastic resins; agar gel; cellulose; polyethylene; polyethylene glycol, polypropylene; stearic acid; polyoxymethylene; etc.; and mixtures thereof.
With reference to FIG. 1, an implementation of a method in accordance with the invention comprises the following steps.
An injection composition is prepared (step E10) from a powder of an alloy as described above together with a binder.
The injection composition may typically be constituted by 50% to 70% by volume of alloy powder with 30% to 50% by volume of binder.
The injection composition may initially be mixed at a temperature lying in the range 150° C. to 200° C. under an inert atmosphere for example, and it is subsequently injected at this temperature.
As shown very diagrammatically in FIG. 2, the injection mold 1 is generally constituted by two portions 14 and 16 forming a cavity 12 having the shape of the part to be fabricated. The injection mold advantageously has a plurality of injection points 18 a, 18 b, 18 c that enable injection to take place into a plurality of portions of the cavity 12 of the mold 1.
Typically, injection is performed at pressures that may lie in the range 400 bar to 800 bar.
Injection is then performed (step E20) into the injection mold 1, which itself is at a regulated temperature in the range 30° C. to 70° C. for example, such that the injection composition becomes plastic in order to form a blank of the part that is to be made. The blank as made in this way is in a state that is said to be “green” or plastic.
It is advantageous to inject into a cavity of the mold that has been evacuated, so as to facilitate injection and ensure that the blank that is molded is uniform.
Thereafter, the blank is unmolded (step E30) and optionally machined while in the green state (step E40) in order to eliminate flash or injection-point sprues that might appear during unmolding.
The following step consists in selectively eliminating the binder present in the blank as formed in this way.
The step of selectively eliminating the binder (step E50), also known as “debinding”, serves to obtain powder having the shape of the part to be fabricated from a blank of the part in the green state.
Selective elimination of the binder may consist in dissolving the binder by treatment with a solvent.
Heat treatment may be used for selectively eliminating the binder, either in full, or else to finish off elimination. Under such circumstances, it may be performed in a sintering enclosure so as to avoid moving the powder between the step of selectively eliminating the binder present in the blank and the first sintering step.
Prior to inserting the powder into the sintering enclosure, the sintering enclosure is purged and decontaminated by vacuum pumping cycles, e.g. under a low pressure of argon or of dihydrogen.
Specifically, it is necessary to be in an inert atmosphere or a reducing atmosphere during sintering in order to avoid oxidizing the elements present in the alloy.
The sintering step (step E60) is performed in a sintering enclosure in which a sintering temperature is progressively imposed. In known manner, the sintering temperature is of the order of 80% to 90% of the solidus temperature of the alloy present in the powder for sintering, and temperature ramps of 0.10° C./minute to 20° C./minute enable this temperature to be reached progressively.
In accordance with the invention, a first sintering step (step E601) is performed by subjecting the powder to a first pressure of inert or reducing atmosphere (e.g. under argon or dihydrogen), that is higher than or equal to 1 mbar, e.g. higher than or equal to 10 mbar.
The evaporation of alloy addition elements such as chromium and/or aluminum is negligible throughout the duration of the first sintering step during which this first pressure is applied. Thus, this step of densifying the preform is carried out while avoiding any modification to the chemistry of the powder at the surface of the preform as a result of additional elements evaporating.
In a variant, the sintering performed during the first sintering step is partial, and subsequently a second sintering step is performed.
During this second sintering step, the preform is subjected to a second pressure, lower than the first, which is imposed in the sintering enclosure for a determined duration (step E602).
The purpose of this second pressure is to evacuate the gas present in the pores of the preform in order to increase its densification. Nevertheless, as explained above, the duration for which the second pressure is applied is limited in order to minimize the evacuation from the surface of the preform of addition elements such as aluminum and/or chromium. In other words, during the second sintering step, processing is performed to evacuate the gas present in the pores generated during sintering, but without significantly affecting the composition of the preform, in particular at its surface.
Evaporation “at the surface” of the preform means evaporation of additional elements from a layer of characteristic thickness at the surface of the preform (thickness generally of the order of 200 μm).
For example, if a very low value is selected for the second pressure, then the gas present in the pores will be evacuated more effectively and densification will take place more quickly, however the evaporation of addition elements at the surface of the preform will be correspondingly greater.
Alternatively, if a higher second pressure value is applied, then the gas present in the pores takes longer to be evacuated and densification will be more limited, however the evaporation of addition elements at the surface of the preform is smaller.
Thus, the duration for which the second pressure is applied is adapted to minimize the relative variation in the content by weight of aluminum and/or chromium at the surface of the preform after the second sintering step, preferably to less than 5%, more preferably to less than 3%, still more preferably to less than 1%. In other words, the content by weight of aluminum and/or chromium at the surface of the preform preferably does not vary by more than 5% in relative value after the second sintering step, more preferably by not more than 3%, still more preferably by not more than 1%.
After the second sintering step, it is possible to perform a third sintering step (step E603) during which a third pressure higher than the second pressure is imposed. By way of example, this third pressure may be greater than or equal to 1 mbar.
After the second or third sintering steps (steps E602 and E603), if any, the preform is cooled with temperature reduction rates lying for example in the range 0.1° C./minute to 60° C./minute, in order to optimize the microstructure of the part.
The final part is obtained from the preform that has been subjected to finishing treatments (step E70) that are themselves known, such as hot isostatic compression in order to finalize the densification of the part, additional heat treatment for optimizing its microstructure, surface treatments of machining or polishing, etc.
The method of the invention is particularly adapted to fabricating a turbine engine blade 2, e.g. comprising a root 22, an airfoil 24, and a tip 26, as shown very diagrammatically in FIG. 3.

First Example

The first example describes a method of fabricating a blade 2 out of titanium alloy of TiAl6-V4 type using a method of the invention.
Firstly, a commercial powder of a grade 23 titanium alloy (TiAl6-V4) is obtained having substantially spherical grains with d₉₀of 45 μm.
A binder is also obtained constituted in particular by paraffin wax, poly(ethylene-co-vinyl acetate), and stearic acid.
The injection composition is made (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 the blade 2 while in the green state is then unmolded (step E30) and machined while in the green state (step E40) to eliminate flash due to injection.
Thereafter, the blank of the blade is placed in a bath of hexane at 40° C. for 10 hours in order to eliminate the binder by dissolution (step E50).
The step of selectively eliminating the binder is continued in a sintering enclosure, in which the blank is placed after partially eliminating the binder, by performing heat treatments in order to eliminate the last traces of binder.
The sintering step (step E60) is started by raising the temperature in the sintering enclosure up to 1350° C.
The pressure inside the enclosure is then adjusted to 10 mbar for 2 hours in order to perform a first sintering step (step E601).
The preform is cooled and then extracted from the sintering enclosure in order to be subjected to conventional finishing treatments (step E70).

Second Example

The second example describes a method of fabricating a blade 2 out of titanium alloy of TiAl 48-2-2 type by another method of the invention.
Initially, a commercial powder is obtained of a titanium alloy having the chemical composition as set out in Table 1, with substantially spherical grains having d₉₀of 25 μm.

TABLE 1

Chemical composition of the alloy (% by weight)

Ti	Al	Nb	Cr	Fe

Base	32.0-33.0	4.50-5.10	2.40-2.70	0.10

C	N	H₂	O₂	Si

0.015	0.02	0.01	0.04-0.13	0.025

A binder is also obtained constituted mainly by polyethylene and polyethylene glycol.
The injection composition is made (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 that has been evacuated.
The blank of the blade 2 while in the green state is then unmolded (step E30) and it is machined in the green state (step E40) in order to eliminate the flash due to injection.
Thereafter, the blade blank is placed in a bath of water at 75° C. for 24 hours in order to eliminate the binder by dissolution (step E50).
The step of selectively eliminating the binder is continued in a sintering enclosure in which the blank from which the binder has been partially eliminated is placed, by performing heat treatments in order to eliminate the last traces of binder.
The sintering step (step E60) is started by raising the temperature in the sintering enclosure up to 1410° C.
The pressure inside the enclosure is adjusted to 1 mbar for 6 hours in order to perform a first sintering step (step E601).
After the first sintering step, a second sintering step is performed (step E602) while lowering the pressure to 10⁻¹mbar in the enclosure for 30 minutes.
The preform is cooled and then extracted from the sintering enclosure in order to be subjected to conventional finishing treatments (step E70).

Claims

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

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);

injecting the injection composition into a cavity of a mold so as to obtain a blank for the part to be made;

selectively eliminating the binder present in the blank;

a first step of sintering the powder of titanium-based alloy, 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, 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.

2. A method according to claim 1, wherein the second pressure is lower than 1 mbar.

3. A method according to claim 1, further including, after the second sintering step, a third sintering step during which a third pressure is imposed, the third pressure being higher than the second pressure.

4. A method according to claim 3, wherein the third pressure is higher than or equal to 1 mbar.

5. A method according to claim 1, wherein the part obtained is a turbine engine blade.

6. A method according to claim 1, wherein the content by weight of aluminum in the alloy powder is greater than 10% prior to the first sintering step.

7. A method according to claim 1, 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.

8. A method according to claim 1, 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.

9. A method according to claim 1, 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.