METAL POWDER FOR METAL INJECTION MOLDING METHOD

Abstract

The invention relates to a metal powder for a metal injection moulding method, the metal powder being formed of a cobalt-based alloy comprising: —between 23.00 wt % and 24.25 wt% chromium; —between 9.00 wt % and 11.00 wt % nickel; —between 6.50 wt % and 7.50 wt % tungsten; —between 3.00 wt % and 4.00 wt % tantalum; —between 0.45 wt% and 0.60 wt % carbon; —between 0.30 wt % and 0.50 wt % zirconium; —between 0.15 wt % and 0.25 wt % titanium; —at most 2.00 wt % iron; —at most 0.30 wt % silicon; —at most 0.10 wt % manganese; —at most 0.10 wt % copper; —at most 0.015 wt % sulphur; —at most 0.015 wt % phosphorus; —at most 0.010 wt % boron; —at most 200 ppm oxygen; —at most 200 ppm nitrogen; —at most 100 ppm hydrogen.

Description

FIELD OF THE INVENTION

The present invention concerns the field of metal injection molding and more particularly metal powder for the implementation of a metal injection molding method.

STATE OF THE ART

The metal injection molding method consists of producing more or less complex metal parts by injecting a mixture of metal powder and a polymer binder. This method makes it possible to manufacture parts for turbomachines.

There are various commercially available materials obtained by metal injection molding, such as Inconel 718 alloy. However, this material is limited in temperature to a maximum of 650° C., which is too low a temperature to be used, for example, in a turbine or a combustion chamber of a turbomachine. There is also Hastelloy X alloy, which resists temperatures up to 950° C., but is mechanically limited and can therefore only be used to produce parts subject to low mechanical loads. There is also IN738 alloy which has good mechanical strength and a good average oxidation/corrosion resistance up to 1000° C.

Unfortunately, all these alloys have maximum operating temperatures below 1050° C., or even 1000° C. They therefore cannot be used for the manufacture of certain aeronautical parts, especially turbomachine parts subjected to high mechanical stresses and temperatures above 1000° C., such as fuel injection systems, combustion chambers, turbine nozzle liners or sealed sectors of turbine blades.

There is therefore a need for solutions that enable parts to be obtained, from a metal injection molding method, which have average resistance to traction, fatigue and creep, and very good resistance to oxidation/corrosion at least up to 1000° C., or even up to 1050° C.

DISCLOSURE OF THE INVENTION

An object of the invention is therefore to provide a metal powder for a metal injection molding method, which makes it possible to obtain a part having average resistance to traction, fatigue and creep, and very good resistance to oxidation/corrosion at least up to 1000° C., or even up to 1050° C.

Another object of the invention is to provide a metal injection molding method for obtaining a part having the abovementioned characteristics.

According to a first aspect, a metal powder is proposed for a metal injection molding method, the metal powder being formed of a cobalt-based alloy comprising:

• • between 23.00% and 24.25% by weight of chromium,
  • between 9.00% and 11.00% by weight of nickel,
  • between 6.50% and 7.50% by weight of tungsten,
  • between 3.00% and 4.00% by weight of tantalum,
  • between 0.45% and 0.60% by weight of carbon, preferably between 0.45% and 0.55%, excluding 0.55%, by weight of carbon,
  • between 0.30% and 0.50% by weight of zirconium,
  • between 0.15% and 0.25% by weight of titanium,
  • at most 2.00% by weight of iron,
  • at most 0.30% by weight of silicon,
  • at most 0.10% by weight of manganese,
  • at most 0.10% by weight of copper,
  • at most 0.015% by weight of sulfur,
  • at most 0.015% by weight of phosphorus,
  • at most 0.010% by weight of boron,
  • at most 200 ppm of oxygen,
  • at most 200 ppm of nitrogen,
  • at most 100 ppm of hydrogen.

According to advantageous and non-limiting characteristics, taken alone or in any combination:

the powder has a particle diameter distribution such that the D10 value is comprised between 3 μm and 10 μm;

• • the powder has a particle diameter distribution such that the D50 value is comprised between 10 μm and 20 μm; the powder has a particle diameter distribution such that the D90 value is comprised between 20 μm and 40 μm.

According to a second aspect, there is proposed a method for the manufacture of a powder described above, by atomization.

According to a third aspect, a method is proposed for metal injection molding from a powder presented above, comprising the steps of:

• • a) feeding an injection molding machine with a primary mixture for molding a so-called green part, said primary mixture comprising the powder and at least one polymer binder,
  • b) debinding the green part to obtain a so-called brown part,
  • c) sintering the brown part to obtain a so-called sintered part,
  • d) obtaining a so-called final part, the final part corresponding to the sintered part or corresponding to the sintered part after this part has undergone one or more heat treatments.

According to advantageous and non-limiting characteristics, taken alone or in any combination:

• • the debinding step is a step of debinding the solvent with water or of the catalytic type;
  • the injection molding method further comprises a thermal debinding step;
  • the injection molding method further comprises a step of treatment by hot isostatic pressing of the sintered part;
  • the injection molding method further comprises a step of quenching the sintered part;
  • the powder is obtained from the manufacturing method according to the second aspect. According to a fourth aspect, a part obtained by the injection molding method described above is proposed.

According to advantageous and non-limiting characteristics: the part is made of a cobalt-based alloy comprising:

• • between 23.00% and 24.25% by weight of chromium,
  • between 9.00% and 11.00% by weight of nickel,
  • between 6.50% and 7.50% by weight of tungsten,
  • between 3.00% and 4.00% by weight of tantalum,
  • between 0.55% and 0.65% by weight of carbon,
  • between 0.30% and 0.50% by weight of zirconium,
  • between 0.15% and 0.25% by weight of titanium,
  • at most 2.00% by weight of iron,
  • at most 0.30% by weight of silicon,
  • at most 0.10% by weight of manganese,
  • at most 0.10% by weight of copper,
  • at most 0.015% by weight of sulfur,
  • at most 0.015% by weight of phosphorus,
  • at most 0.010% by weight of boron,
  • at most 450 ppm of oxygen,
  • at most 300 ppm of nitrogen,
  • at most 125 ppm of hydrogen.

According to a fifth aspect, a turbomachine is proposed comprising at least one part described above.

DESCRIPTION OF THE FIGURES

Other characteristics and advantages of the present invention will become apparent upon reading the following description of a preferred embodiment. This description will be given with reference to the attached figures, in which:

FIG. 1 shows the steps of the metal injection molding method.

DETAILED DESCRIPTION OF THE INVENTION

Powder

A metal powder for a metal injection molding method is proposed. The metal powder is made of a cobalt-based alloy. In other words, the metal powder consists predominantly, by weight, of cobalt.

The metal powder alloy further comprises:

• • between 23.00% and 24.25% by weight of chromium,
  • between 9.00% and 11.00% by weight of nickel,
  • between 6.50% and 7.50% by weight of tungsten,
  • between 3.00% and 4.00% by weight of tantalum,
  • between 0.45% and 0.60% by weight of carbon, preferably between 0.45% and 0.55%, excluding 0.55%, by weight of carbon,
  • between 0.30% and 0.50% by weight of zirconium,
  • between 0.15% and 0.25% by weight of titanium,
  • at most 2.00% by weight of iron,
  • at most 0.30% by weight of silicon,
  • at most 0.10% by weight of manganese,
  • at most 0.10% by weight of copper,
  • at most 0.015% by weight of sulfur,
  • at most 0.015% by weight of phosphorus,
  • at most 0.010% by weight of boron,
  • at most 200 ppm of oxygen,
  • at most 200 ppm of nitrogen,
  • at most 100 ppm of hydrogen.

The metal powder may also comprise other elements, called contaminating elements, in minimal quantities. Preferably, the metal powder comprises at most 50 ppm of the same contaminating element. Also preferably, the sum of the proportions of the contaminating elements of the metal powder is at most 500 ppm.

The remainder of the composition detailed above is cobalt.

It will be understood that the “proportions” or “contents” of each element of the metal powder, expressed as percentages, are expressed by mass (i.e. mass of said element over the total mass of the metal powder).

By “between”, it is understood that the lower and upper bounds are included. For example, “between 23.00% and 24.25% by weight of chromium” means that 23.00% and 24.25 % are included as possible proportions by weight of chromium in the metal powder.

When “at most” is specified, it is understood that the proportion of the corresponding element may be zero, i.e., the element may be absent from the metal powder. For example, “at most 2.00% by weight of iron” is understood to mean that the metal powder may comprise 0.00% by weight of iron, that is to say that it does not comprise or substantially does not comprise iron. As a second example, “at most 200 ppm of oxygen” is understood to mean that the metal powder may comprise 0 ppm of oxygen, that is to say that it does not comprise or substantially does not comprise oxygen.

By “ppm” is meant “parts per million”, 1 ppm corresponding to 1 mg/kg. Therefore, for example, “200 ppm of oxygen” means that 1 kg of metal powder comprises 200 mg (i.e. 0.2 g) of oxygen. In other words, “200 ppm of oxygen” means that the metal powder contains 200 millionths by weight of oxygen, therefore 0.02% by weight of oxygen.

As explained, the metal powder is a cobalt-based alloy. Consequently, the cobalt content in the metal powder corresponds to 100% minus the sum of the contents of the other constituent elements of the metal powder presented above. Thus, the metal powder comprises by weight at least 49.31% of cobalt and at most 57.60% of cobalt.

This specific metal powder makes it possible, by using it in a metal injection molding method, to obtain parts having an average resistance to traction, fatigue and creep and very good resistance to oxidation/corrosion at least up to 1000° C., or even up to 1050° C. This metal powder therefore has the advantage of allowing the manufacture of specific aeronautical parts subjected to high mechanical stresses and high temperatures (greater than or equal to 1000° C., or even 1050° C.).

Preferably, the method for manufacturing the metal powder is atomization. Also preferably, the metal powder is obtained by atomizing an alloy whose composition is close to that of Mar-M 509 alloy, which is based on cobalt. The main components of Mar-M 509 alloy are iron (0%-2% by weight), nickel (9%-11% by weight) and cobalt. In addition, Mar-M 509 alloy may include titanium (0%-0.4% by weight), chromium (22%-25% by weight), zirconium (0.3%-0.7% by weight), tantalum (3%-4% by weight), tungsten (6%-8% by weight), trace carbon (0.55%-0.65% by weight) and other elements in a quantity of less than 50 ppm. However, Mar-M 509 is a typical foundry alloy and is therefore not used in powder form. The composition of the alloy forming the metal powder according to the invention has been optimized for the injection molding method.

Thus, the maximum oxygen and nitrogen contents of the alloy forming the powder according to the invention are higher than those of Mar-M 509 alloy, which are less than 50 ppm, but remain acceptable in terms of the final properties of the material. In return, the carbon content is lower than that of Mar-M 509 alloy, which is comprised between 0.55 and 0.65% in the foundry, to compensate for the fact that at the end of the injection molding method, the carbon content will have increased due to binder residues.

Consequently, the powder according to the invention advantageously comprises between 0.45% and 0.55%, 0.55% being excluded, by weight of carbon. In other words, the carbon content of the powder is advantageously greater than or equal to 0.45% and strictly less than 0.55% by weight.

According to a particular arrangement, 10% of the metal powder particles have a diameter smaller than a value (D10) comprised between 3 μm and 10 μm, 50% of the particles have a diameter smaller than a value (D50) comprised between 10 μm and 20 μm and 90% of the particles have a diameter smaller than a value (D90) comprised between 20 μm and 40 μm. The particle size parameters D10, D50 and D90 are measured by laser particle size analysis according to ISO 13320 or ASTM B822.

This specific particle size makes it possible to combine an optimal density of the powder when used in a MIM manufacturing method with good injection, while having an optimal rheology from feedstock to injection and promoting sintering. This reduces the risk of cracking during injection and sintering and provides good dimensional stability since sintering provides a consistent density.

In particular, the D10 value specifies the dimensions of the finest particles of powder intended to fill the empty spaces between the particles of larger size. These dimensions are optimized to be small enough to fill voids well and promote sintering.

The D50 value represents the mean value of the particles.

The D90 value indicates the maximum particle size, so the upper limit of D90 ensures particles large enough not to add too much friction to the injection and break the parts during injection. The combination of the three dimension parameters D10, D50 and D90 corresponds to a Gaussian distribution of particle sizes presenting the best compromise for injection and sintering.

Method

A method of injection molding metal from the metal powder described above is proposed.

The metal injection molding technique, also known by the abbreviation “MIM”, allows the manufacture of large series of metal parts, especially metal parts of complex shapes, from a metal powder. Indeed, parts can be injected at a high rate and debinding can be carried out in batches comprising a plurality of parts.

Moreover, the parts obtained have an excellent surface finish and have fine dimensional tolerances.

The metal injection molding method makes it possible to obtain parts, especially aeronautical parts such as fuel injection systems, combustion chambers, turbine nozzle liners or sealed sectors of turbine blades.

FIG. 1 shows the steps of the metal injection molding method.

According to a certain embodiment, the molding method first comprises a prior step of mixing the metal powder with at least one polymer binder to obtain a so-called primary mixture. During the mixing step, the metal powder particles are advantageously coated with the binder or binders. In the specific terminology of metal injection molding, the primary mixture is referred to as feedstock.

The primary mixture is in the form of granules and preferably comprises between 55% and 75% of metal powder and therefore, respectively, between 45% and 25% of binder. The binder is a polymer, preferably thermoplastic, and may for example be polyethylene or polyethylene glycol. Preferably, the metal powder is mixed with polyethylene and polyethylene glycol.

The granules of the primary mixture preferably have a diameter greater than 1 mm and less than 5 mm.

Preferably, the hot fluidity of the primary mixture is greater than 60 cm³/10 min and less than 85 cm³/10 min.

The method may also comprise a granulation step to obtain the primary mixture. In other words, the metal powder is mixed with the binder(s) and is granulated to obtain the primary mixture.

The method comprises a step a) of feeding an injection press with primary mixture for molding a so-called green part. The injection press is a typical press for implementing a metal injection molding method. The primary mixture is heated in the injection press to a temperature sufficient for the binder(s) to melt without melting the metal elements. The heating temperature depends on the geometry of the green part that will be formed. Preferably, the heating temperature is comprised between 170° C. and 200° C. This temperature will make it possible to obtain a green part without porosity. The heated primary mixture is injected into a mold corresponding to the shape of the green part to be obtained. After cooling and solidifying the binder, the green part can be extracted from the mold.

The method then comprises a step b) of debinding the green part to obtain a so-called brown part. Debinding makes it possible to remove the majority of the binder present in the green part.

The debinding can be a debinding of the solvent type with water or of the catalytic type.

Water solvent type debinding consists of bathing the green part in water so that the binder dissolves. For example, the green part can be bathed in a bath of demineralized water with a temperature comprised between 20° C. and 150° C. for a period comprised between 100 and 300 hours, the water being stirred.

Catalytic debinding consists of placing the green part in an oven so that the binder is vaporized and then burned by injecting acid vapor into the oven. For example, the green part can be placed in an oven at a temperature comprised between 100° C. and 150° C., for a period comprised between 2 hours and 10 hours in an atmosphere of nitrogen introduced in a flow comprised between 60 and 100 L/min with introduction of nitric acid vapor in a flow comprised between 2 and 5 ml/min.

At the end of debinding step b), a majority of the binder(s) of the green part is removed, preferably at least 95% of the binder, and the brown part obtained is therefore porous.

Advantageously, the method comprises a thermal debinding step making it possible to eliminate the remainder of the binder in the brown part. For example, thermal debinding may comprise two consecutive stages, namely a first stage according to which the brown part is subjected to a temperature comprised between 450° C. and 550° C. for a period comprised between 150 and 300 minutes under 200 to 500 mbar of argon and a second stage according to which the brown part is subjected to a temperature comprised between 550° C. and 650° C. for a period comprised between 150 and 300 minutes under an atmosphere of 200 to 500 mbar of argon.

The method then comprises a step c) of sintering the brown part (or, as applicable, the brown part which has undergone thermal debinding) to obtain a so-called sintered part. During sintering, the brown part is heated to a temperature close to the melting temperature of the constituent metals of the brown part (i.e., the constituent metals of the metal powder used) but below this melting temperature.

Sintering causes homothetic reduction, or shrinkage, of the part because the particles of metal powder bond together by diffusion, thus causing densification of the brown part.

Preferably, during sintering, the brown part is subjected to a temperature comprised between 1200° C. and 1300° C. for a period comprised between 4 and 8 hours under an atmosphere of 20 to 50 mbar of argon.

These sintering parameters are easily applied industrially and make it possible to obtain a sintered part with good dimensional strength and a good quality microstructure, in which defects, for example microcracks or porosity, are minimized.

Advantageously, the sintered part is made of a cobalt-based alloy which comprises:

• • between 23.00% and 24.25% by weight of chromium,
  • between 9.00% and 11.00% by weight of nickel,
  • between 6.50% and 7.50% by weight of tungsten,
  • between 3.00% and 4.00% by weight of tantalum,
  • between 0.55% and 0.65% by weight of carbon,
  • between 0.30% and 0.50% by weight of zirconium,
  • between 0.15% and 0.25% by weight of titanium,
  • at most 2.00% by weight of iron,
  • at most 0.30% by weight of silicon,
  • at most 0.10% by weight of manganese,
  • at most 0.10% by weight of copper,
  • at most 0.015% by weight of sulfur,
  • at most 0.015% by weight of phosphorus,
  • at most 0.010% by weight of boron,
  • at most 450 ppm of oxygen,
  • at most 300 ppm of nitrogen,
  • at most 125 ppm of hydrogen.

The sintered part obtained thus has good mechanical properties and resistance to temperatures greater than or equal to 1000° C., or even 1050° C.

Compared to powder, the sintered part has a higher content of carbon, oxygen and nitrogen because these elements are binder residues that permeate the alloy.

The sintered part has a microstructure (i.e., the size of the metallurgical particles) between 3 and 9 ASTM (ASTM Standard E112). 3 ASTM corresponds to 127 μm and 9 ASTM corresponds to 15.9 μm. The microstructure of the part obtained is therefore fine, which allows improved resistance to fatigue, as well as tensile strength and elasticity.

The sintered part may be used directly or may undergo various treatments depending on the desired end application.

The method thus comprises a step d) of obtaining a part called the final part. The final part corresponds to the sintered part or corresponds to the sintered part after this part has undergone one or more heat treatments. The heat treatments are presented below.

Advantageously, the method comprises a step of treatment by hot isostatic pressing of the sintered part. This compacting step makes it possible to densify the sintered part. Indeed, this step makes it possible to reduce as much as possible the residual porosities from sintering; the proportion of porosities thus goes from less than 6% to less than 0.4%. Indeed, it makes it possible to fill the pores, thus obtaining a sound part with fewer dimensional dispersions and improved mechanical properties.

The hot isostatic pressing treatment consists, for example, of subjecting the sintered part to a temperature of 1260° C.±20% for a period of 3 hours±1 hour under an atmosphere at 1020 bars±10% argon with cooling of the air type.

The method may also or alternatively comprise a step of quenching the sintered part. This quenching step makes it possible to homogenize the microstructure and precipitation of the sintered part.

The quenching may, for example, consist of subjecting the sintered part to 1260° C.±20% for a period of 3 hours±1 hour under an argon atmosphere with cooling of the air type. The heat treatments presented (hot isostatic pressing or quenching) especially allow good precipitation and recrystallization without modification of the particle size.

The final part obtained thus has good mechanical properties and resistance to temperatures greater or equal to 1000° C. or even 1050° C. The final part is sound, that is to say it has an adequate microstructure for optimal mechanical resistance to traction, creep and fatigue and has optimal resistance to oxidation and corrosion. More precisely, the final part obtained has a mechanical tensile strength of 300 MPa at 950° C. and 1300 MPa at 20° C.±50 MPa. The final part also has a conventional yield strength of 290 MPa at 950° C. and 750 MPa at 20° C.±50 MPa. In addition, the final part has a creep resistance at 900° C. of 140 MPa for more than 20 h.

The final part can especially be used in a turbomachine and can, for example, be part of a turbine.

The invention is not limited to the embodiment described and shown in the attached figure. Modifications remain possible, especially from the viewpoint of the constitution of the various technical characteristics or by substitution of technical equivalents, without thereby departing from the general teaching.

Claims

1. A metal powder for a metal injection molding method, the metal powder being formed of a cobalt-based alloy comprising:

between 23.00% and 24.25% by weight of chromium,

between 9.00% and 11.00% by weight of nickel,

between 6.50 % and 7.50% by weight of tungsten,

between 3.00% and 4.00% by weight of tantalum,

between 0.45% and 0.60% by weight of carbon,

between 0.30% and 0.50% by weight of zirconium,

between 0.15% and 0.25% by weight of titanium,

at most 2.00% by weight of iron,

at most 0.30% by weight of silicon,

at most 0.10% by weight of manganese,

at most 0.10% by weight of copper,

at most 0.015% by weight of sulfur,

at most 0.015% by weight of phosphorus,

at most 0.010% by weight of boron,

at most 200 ppm of oxygen,

at most 200 ppm of nitrogen,

at most 100 ppm of hydrogen.

2. The metal powder according to claim 1, having a particle diameter distribution such that the D10 value is comprised between 3 μm and 10 μm.

3. The metal powder according to claim 1, having a particle diameter distribution such that the D50 value is comprised between 10 μm and μm.

4. The metal powder according to claim 1, having a particle diameter distribution such that the D90 value is comprised between 20 μm and μm.

5. A method of manufacturing a powder according to claim 1 by atomization.

6. A method of powder metal injection molding using a powder according to claim 4, comprising

feeding an injection molding machine with a primary mixture for molding a so-called green part, said primary mixture comprising the powder and at least one polymer binder,

debinding the green part to obtain a so-called brown part,

sintering the brown part to obtain a so-called sintered part,

obtaining a final part, the final part corresponding to the sintered part or corresponding to the sintered part after this part has undergone one or more heat treatments.

7. The method according to claim 6, wherein the debinding is either solvent debinding with water or catalytic debinding.

8. The method according to claim 6, further comprising a thermal debinding.

9. The method according to claim 6, further comprising treating the sintered part by hot isostatic pressing.

10. The method according to claim 6, further comprising quenching the sintered part.

11. The method according to claim 6 wherein the powder is obtained by atomization.