Metal powder for additive manufacturing process

Abstract

The invention relates to a metal powder for use in an additive manufacturing method, the metal powder comprising an alloy, the alloy comprises, by weight, 20% to 24% of chromium, 20% to 24% of nickel, 13.00% to 16.00% of tungsten, 0.02% to 0.12% of lanthanum, 0.05% to 0.15% of carbon, 0.20% to 0.50% of silicon, up to 1.25% of manganese, up to 3.00% of iron, up to 0.015% of sulfur, up to 0.020% of phosphorus, up to 0.0001% of bismuth, up to 0.0010% of silver, up to 0.0010% of lead, up to 0.015% of boron, up to 0.0250% of oxygen, and the balance of iron and unavoidable impurities. Up to 0.0200% nitrogen and less than 0.050% in total of other elements, the balance being cobalt.

Description

TECHNICAL FIELD

[0001] The present invention relates generally to the field of metal powders for additive manufacturing. More specifically, the present invention proposes a cobalt-based alloy for implementing an additive manufacturing method and a method for manufacturing a metal part intended for high temperature applications. BACKGROUND

[0002] Many alloys are known for implementing powder bed additive manufacturing methods, in particular for laser beam melting (LBM). An alloy for implementing a powder bed additive manufacturing method refers to a powder comprising a metal alloy. The powder is intended to be fused by a laser and then solidified during the implementation of the powder bed additive manufacturing method to form a part.

[0003] Currently, there are many powders comprising a metal alloy. However, most of the existing powders do not allow obtaining a material suitable for aerospace applications, such as for turbine, after laser beam melting in an additive manufacturing method. In particular, many powders do not allow obtaining a material resistant to temperatures above 650°C. However, a maximum resistance temperature of 650°C is too low for use in a turbine. Other materials, such as alloys IN738 and IN939, are resistant to higher temperatures, but are insufficient in mechanical properties for manufacturing aerospace parts. Moreover, these alloys are also limited to use at temperatures lower than 1000°C or even 950°C.

[0004] However, additive manufacturing methods allow designing parts with complex geometries, which are adapted to form optimized parts, such as for fuel injection systems, combustion chambers, turbine nozzle liner or turbine blade sectors. Therefore, there is a need to provide a powder which allows manufacturing parts resistant to high temperatures and having mechanical properties adapted to such applications. SUMMARY

[0005] It is an object of the present invention to provide a powder comprising a metal alloy for a powder bed additive manufacturing method, which allows obtaining a material which maintains its tensile and creep properties at temperatures of at least up to 1050°C and which exhibits high oxidation and corrosion resistance, and which is weldable.

[0006] To this end, the application proposes a metal powder for an additive manufacturing method, said metal powder comprising an alloy comprising, by weight, 20 to 24% of chromium, 20 to 24% of nickel, 13.00 to 16.00% of tungsten, 0.02 to 0.12% of lanthanum, 0.05 to 0.15% of carbon, 0.20 to 0.50% of silicon, at most 1.25% of manganese, at most 3.00% of iron, at most 0.015% of sulfur, at most 0.020% of phosphorus, at most 0.0001% of bismuth, at most 0.0010% of silver, at most 0.0010% of lead, at most 0.015% of boron, at most 0.0250% of oxygen, at most 0.0200% of nitrogen and less than 0.050% of other elements in total, the balance being cobalt.

[0007] Advantageously, the powder comprises a plurality of particles having a particle size for which 10% of the particles have a diameter less than the D10 value of 10 pm to 25 pm.

[0008] Preferably, the powder comprises a plurality of particles having a particle size for which 50% of the particles have a diameter less than the D50 value of 25 pm to 40 pm.

[0009] Advantageously, the powder comprises a plurality of particles having a particle size for which 90% of the particles have a diameter less than the D90 value of 40 pm to 70 pm.

[0010] The application also relates to a method for manufacturing a metal powder as described above, comprising in succession the following steps:

[0011] o mixing raw materials in elemental form or in pre-alloyed form,

[0012] o melting the resulting mixture,

[0013] o atomizing the molten mixture with a gas, preferably argon or nitrogen,

[0014] o sieving the resulting powder to obtain a predetermined particle size,

[0015] o recovering the resulting powder.

[0016] Advantageously, the method alternately comprises at least one step of forming a layer of a metal powder as described above and at least one step of selectively melting a portion of said layer by a scanning laser beam.

[0017] Preferably, the additive manufacturing is carried out by powder bed laser melting. The laser beam can have a power of 150 W to 300 W and / or a diameter of 50 pm to 200 pm and / or a displacement speed of 900 mm / s to 1300 mm / s. The scanning step can be carried out in scanning bands having a width of 2 mm to 15 mm, the overlap of said bands being 0.05 mm to 0.15 mm.

[0018] Advantageously, the orientation axes of the respective bands of two adjacent layers form an angle of 67° ± 5°.

[0019] Advantageously, the thickness of each respective layer is comprised between 20 pm and 60 pm.

[0020] Preferably, the method is carried out under an argon and / or nitrogen atmosphere.

[0021] The present application also relates to a method for manufacturing a metal part comprising the additive manufacturing method as described above and the step of heat treatment at a temperature comprised between 1200 °C and 1240 °C for a duration comprised between 5 minutes and 15 minutes and the step of cooling to room temperature.

[0022] The present application also relates to a material obtained from the powder as described above according to the method for manufacturing a metal part as described above.

[0023] Advantageously, the material comprises an alloy comprising by weight 20% to 24% of chromium, 20% to 24% of nickel, 13.00% to 16.00% of tungsten, 0.02% to 0.12% of lanthanum, 0.05% to 0.15% of carbon, 0.20% to 0.50% of silicon, at most 1.25% of manganese, at most 3.00% of iron, at most 0.015% of sulfur, at most 0.020% of phosphorus, at most 0.0001% of bismuth, at most 0.0010% of silver, at most 0.0010% of lead, at most 0.015% of boron, at most 0.0350% of oxygen, at most 0.0250% of nitrogen and less than 0.050% of other elements in total, the remainder being cobalt.

[0024] The present application also relates to a turbomachine metal part made of the material as described above.

[0025] The present application also relates to a turbomachine comprising at least one part as described above. BRIEF DESCRIPTION OF DRAWINGS

[0026] Other features and advantages of the present application will appear from the following detailed description, made with reference to the drawings in which:

[0027] Figure 1 is a flowchart representing the steps of a method for manufacturing a metal part from a powder according to the present application;

[0028] Figure 2 is an image of a metallographic structure of a material obtained by the method according to the present application. DETAILED DESCRIPTION

[0029] Metal powder

[0030] The present invention proposes a metal powder for additive manufacturing methods, in particular powder bed methods, such as laser beam melting (LBM). Alternatively, the additive manufacturing method can be other methods, such as powder bed methods by electron melting or powder spray methods. The metal powder comprises a cobalt-based alloy comprising at least the following elements: chromium, nickel, tungsten, lanthanum, carbon and silicon. The alloy can further comprise manganese, iron, sulphur, phosphorus, bismuth, silver, lead, boron, oxygen and nitrogen.

[0031] In the rest of the present description, the "level" or "content" will be expressed in mass (i.e. the mass of the element with respect to the total mass of the alloy).

[0032] The amounts of the various chemical elements present in the powder are shown in the following table 1, with cobalt making up the balance of the alloy weight. The powder can contain impurities in the form of other elements, in an amount less than 0.005% for each respective element and less than 0.050% for the total of the other elements. With these levels, a sufficient purity is obtained to implement the additive manufacturing method and to ensure the mechanical properties and temperature resistance required for aerospace applications.

[0033]

[0034] In particular, the oxygen and nitrogen levels are suitable for a powder implemented in an additive manufacturing method. Suitable levels are those which allow obtaining a material with the required thermal and mechanical properties after melting the powder with a laser beam. Indeed, the wrought metal alloy HA188 is known, which comprises a substantially identical list of elements, but with different oxygen and nitrogen levels and which are not suitable for additive manufacturing methods.

[0035] Preferably, the oxygen content is between 0.005% and 0.025% by weight. Advantageously, the nitrogen content is between 0.005% and 0.020% by weight. Such oxygen and / or nitrogen contents allow significantly improving the properties of the material obtained by additive manufacturing method from such a powder.

[0036] Limiting the oxygen and nitrogen levels in these preferred ranges makes it possible to increase the R m / R p02 value by 10-15%, where R m is the tensile limit of the material, R pO2 is the yield point of the material and the fatigue of such a material is increased by 5%. The elongation A% corresponds to the elongation A% of a material with less than 50 ppm of oxygen and nitrogen content.

[0037] With oxygen and / or nitrogen contents higher than the ranges given, a loss of elongation and a loss of fatigue properties are observed.

[0038] Advantageously, the powder has a particle size suitable for implementation in a powder bed additive manufacturing method. A suitable particle size refers to a particle size that allows easy deposition of a layer of powder in the bed, for example by supplying a predetermined amount onto the powder bed and spreading the powder with a doctor blade to form a layer having a uniform thickness and density. Too large particles would lead to fusion defects, resulting in a material having too large porosities and mechanical weaknesses. Too fine particles require more fusion energy and would lead to cracks, which are also not favorable for the mechanical strength of the material.

[0039] Thus, advantageously, the particle geometry is substantially spherical, that is to say without sharp angles. This geometry ensures fluid flow during deposition of a layer on the powder bed and ensures uniform fusion of the powder forming the part. Furthermore, the spherical particle geometry allows the particle size of the powder to be selected with good precision by one or more sieving steps.

[0040] According to a particular setting, 10% of the particles have a diameter smaller than the D10 value between 10 pm and 25 pm, 50% of the particles have a diameter smaller than the D50 value between 25 pm and 40 pm, and 90% of the particles have a diameter smaller than the D90 value between 40 pm and 70 pm. The particle size parameters D10, D50 and D90 are measured by laser granulometry according to the standard ISO 13320 or ASTM B822.

[0041] This particular particle size, when used in a powder bed additive manufacturing method, allows the best powder compactness to be combined, while having the best fluidity and minimizing fusion limitations. This reduces the risk of cracking of the material obtained by the additive manufacturing method during the manufacturing, the cooling and the subsequent steps, for example welding of the obtained part.

[0042] In particular, the D10 value designates the size of the finest powder particles used to fill the interstitial spaces between the larger grains. These sizes are optimized to be small enough to properly fill the interstitial spaces and large enough to prevent these particles from being expelled by the air flow present in the additive manufacturing machine, as well as to prevent them from being expelled or trapped in the filters present in various machines and thus to clog these filters.

[0043] The D50 value corresponds to the median of the particle size.

[0044] The D90 value represents the maximum size of the particles, the upper limit of D90 thus ensuring the limitation of fusion defects due to particles that are too large.

[0045] The combination of the three size parameters D10, D50 and D90 corresponds to a Gaussian distribution of the particle size, with sufficient uniformity for the layer deposition and the fusion, thus enabling uniform fusion of the powder.

[0046] Manufacture of the powder

[0047] Preferably, the powder is obtained by a method of gas atomization in an atomization tower. First, a mixture of raw materials in powder or granular form is prepared. The raw materials can be in the form of pure elements and / or pre-alloys comprising a portion of the elements to be incorporated into the powder. The metal mixture is heated above its melting temperature and subjected to a gas jet. Typically, the gas is argon or nitrogen. Preferably, the gas is argon, to avoid variations in the level of nitrogen in the alloy to be manufactured. Under the mechanical load of the gas jet, the molten metal forms spherical droplets which, when they cool, form powder particles. Sieving can then be carried out to remove particles having an unsatisfactory geometry, and the powder particles are classified according to their particle size.

[0048] This method ensures a substantially spherical morphology, and each particle is free of sharp angles. At the same time, this obtaining technique limits the risk of contamination of the powder with foreign bodies and residues in the manufacturing equipment. It also allows densification melting to be carried out and particles having a low porosity to be obtained, which is advantageous for obtaining uniform parts having a low porosity level in additive manufacturing methods. Typically, the powder particles have a density greater than or equal to 99.9%.

[0049] Additive manufacturing method

[0050] According to a second aspect, the application relates to a powder bed additive manufacturing method, in particular a laser beam melting method.

[0051] In a known manner, in an LBM type additive manufacturing method, a first layer of particles of starting powder is deposited. Subsequently, by irradiation with a laser beam, a portion of the first layer is melted to form a portion of the metallurgical product. Other powder layers are then successively formed on the surface of the powder and of the metallurgical product portion formed in the preceding step, and by irradiation with a laser beam, a portion of the second powder layer is consolidated so as to be welded to the metallurgical product portion. Thus, the object is formed successively layer by layer. At the end of the method, the part is allowed to cool, then it can be extracted by removing the non-consolidated powder.

[0052] The laser beam power and the scanning speed are chosen according to the thickness of each layer, the size and geometry of the metal part to be manufactured, and the specific case of the additive manufacturing machine. The laser beam generally has a Gaussian power distribution. Preferably, the laser beam has a power of 150 W to 300 W, which allows the powder to be locally melted without causing excessive heating of the powder in the vicinity of the part. According to 1 / e 2 (maximum 13.5%) and / or D4s (diameter of the second moment) standard measures the beam diameter, which is very close to the 1 / e 2The standards specify the minimum requirements for the quality of the material to be manufactured. According to one of these standards, the diameter is generally between 50 pm and 200 pm to achieve the precision of the part to be manufactured and a satisfactory production speed. The speed of displacement of the beam is generally between 900 mm / s and 1300 mm / s. Such a speed allows a good compromise to be obtained between the production speed of the part and the precision of the geometry of the part to be manufactured.

[0053] The laser beam scanning method is generally performed using scanning bands. These bands comprise a plurality of parallel laser beam channels along an orientation axis X. The bands generally have a width of between 2 and 15 mm. These bands are made with an overlap of two adjacent channels in the peripheral zone, advantageously between 0.05 and 0.15 mm. This ensures good mechanical strength along the edges of each band.

[0054] In order to avoid the structuring effects due to the scanning, the bands of adjacent layers are angularly offset. Advantageously, the orientation axes X of the bands of two adjacent powder layers form an angle which allows the orientation of the bands in each successive layer to be changed. In order to obtain good homogeneity of the resulting material, it is necessary to orient the bands so that the orientation axes of two successive layers form a significant angle. At the same time, it is sought to prevent the orientations from being repeated in layers close to one another and to prevent the orientations from varying over as many superimposed layers as possible. To this end, an offset angle which is not a 360° divisor is chosen. Preferably, an angle of 67° is chosen, with a tolerance of about ± 5° between the orientation axes of two successive layers. Such an angle prevents the main orientation of the microstructure. In particular, the angular offset prevents the appearance of aligned overlapping regions. In the case of fusion defects, aligned fusion regions can lead to mechanically weakened regions, since these defects will be aligned over the macroscopic thickness of the material to be manufactured.

[0055] After deposition and consolidation of the last layer of powder and, if necessary, cooling of the part, the manufactured part can be extracted from the remaining powder, which can be reused for the manufacture of other parts.

[0056] Post-treatment of the part obtained by additive manufacturing

[0057] The billet obtained by laser fusion comprises micro-cracks distributed throughout the structure of the alloy. These micro-cracks are generated by local mechanical and thermal stresses during the additive manufacturing method and are generally unavoidable.

[0058] In order to use such a part for aerospace applications, it is necessary to eliminate the micro-cracks to improve the mechanical properties of the part. Generally, a heat treatment is used to eliminate the micro-cracks and to improve the metallurgical quality of the part.

[0059] A quenching heat treatment at a temperature of 1200°C to 1240°C, preferably close to 1220°C, for 10 minutes ± 5 minutes is generally used to improve the metallurgical quality of the parts resulting from the additive manufacturing method described above. The temperature rise is about 10°C / min for illustrative purposes and not limitation.

[0060] This treatment leads to the dissolution of the alloy and the precipitation of some elements, in particular carbides at the grain boundaries. This carbide increases the mechanical strength and hardness of the material. The duration is adjusted to achieve a slight precipitation and a stable post-treatment state. During the treatment, in addition to the precipitation of carbides, the elements re-diffuse in the matrix without melting other elements or particles.

[0061] The treatment is followed by cooling to room temperature, for example air cooling. Room temperature means a temperature within the normal temperature of a residential premises, that is to say approximately between 15°C and 30°C. The rapid cooling allows to fix the chemical elements that can diffuse at high temperature in their place and to increase the yield strength of the alloy.

[0062] Figure 1 The main steps of the manufacturing method are illustrated in the form of a flowchart. During the additive manufacturing step FA, the steps of depositing a layer of powder FA1 and the consolidation step FA2 are repeated in a cycle. After completion of the additive manufacturing FA, a quenching heat treatment T1 is performed.

[0063] The resulting part

[0064] During the additive manufacturing, the levels of oxygen and nitrogen elements are modified due to the contact of the powder particles with the atmosphere in the additive manufacturing machine.

[0065] Table 2 below shows the amounts of various chemical elements present in the metal parts manufactured by LBM.

[0066]

[0067] Figure 2 Metallographic structures of the parts after the quenching heat treatment in a plane parallel to the layers deposited during the additive manufacturing are illustrated. These images are optical microscope photographs of the parts after mechanical polishing and chemical etching in the corresponding plane. This material is free of cracks. The grain size in the parts ranges from 3 to 11 ASTM, with an average of 5 ASTM, which corresponds to a metallurgical quality suitable for parts exposed to the thermal and mechanical conditions of an aircraft turbine. For reference, 3 ASTM corresponds to 127 pm, 5 ASTM corresponds to 63.5 pm and 11 ASTM corresponds to 7.9 pm.

[0068] The material after heat treatment has a Vickers hardness of 250 HV to 380 HV. For example, the hardness can be measured with a type HV 0.3, that is to say with a mass of 0.3 g. The material has a temperature resistance of at least 1050 °C, in particular a very good oxidation resistance and corrosion resistance, when used in the vicinity of the combustion chamber of a turbine. The material has a tensile, fatigue and creep strength which meets the requirements of turbine components, in particular components for turbine injection systems which are frequently exposed to high temperatures and corrosive environments, and components such as retaining rings, sealing sectors, decorative components and turbine nozzle bushings. Furthermore, the components can be welded with good weld quality.

Claims

1. A metal powder for use in an additive manufacturing process, said metal powder comprising an alloy comprising, by weight, 20 to 24% of chromium, 20 to 24% of nickel, 13.00 to 16.00% of tungsten, 0.02 to 0.12% of lanthanum, 0.05 to 0.15% of carbon, 0.20 to 0.50% of silicon, up to 1.25% of manganese, up to 3.00% of iron, up to 0.015% of sulfur, up to 0.020% of phosphorus, up to 0.0001% of bismuth, up to 0.0010% of silver, up to 0.0010% of lead, up to 0.015% of boron, up to 0.0250% of oxygen, up to 0.0200% of nitrogen and less than 0.050% of other elements in total, the balance being cobalt.

2. The metal powder according to claim 1, said powder comprising a plurality of particles having a particle size wherein 10% of the particles have a diameter smaller than the D10 value of 10 to 25 pm.

3. The metal powder according to claim 1 or 2, said powder comprising a plurality of particles having a particle size wherein 50% of the particles have a diameter smaller than the D50 value of 25 to 40 pm.

4. The metal powder according to any one of the preceding claims, said powder comprising a plurality of particles having a particle size wherein 90% of the particles have a diameter smaller than the D90 value of 40 to 70 pm.

5. A method for manufacturing a metal powder according to any one of claims 1 to 4, comprising in sequence the following steps: o mixing raw materials in elemental form or in pre-alloyed form, o melting the resulting mixture, o atomizing the molten mixture with a gas, preferably argon or nitrogen, o sieving the resulting powder to obtain a predetermined particle size, o recovering the resulting powder.

6. A method for additive manufacturing of a metal part, said method comprising alternately at least one step of forming a layer of a metal powder according to any one of claims 1 to 4 and at least one step of selectively melting a portion of said layer by a scanning laser beam. Said additive manufacturing is carried out by powder bed laser melting. Said laser beam has a power of 150 to 300 W and / or a diameter of 50 to 200 pm and / or a displacement speed of 900 to 1300 mm / s. Said scanning step is carried out in scanning bands having a width of 2 to 15 mm, the overlap of said bands being 0.05 to 0.15 mm. The orientation axes of the respective bands of two adjacent layers form an angle of 67° ± 5°. The thickness of each respective layer is 20 to 60 pm.

12. The additive manufacturing method according to any one of claims 6 to 11, said method being carried out under an argon and / or nitrogen atmosphere.

7. The additive manufacturing method of claim 6, wherein, 13. A method for manufacturing a metal part, said method comprising an additive manufacturing method according to any one of claims 6 to 12 and a step of heat treatment at a temperature of 1200 to 1240 °C for a duration of 5 to 15 minutes and a step of cooling to room temperature.

8. The additive manufacturing method of claim 7, wherein, ​ 9. The additive manufacturing method according to any one of claims 6 to 8, wherein, ​ 10. The additive manufacturing method of claim 9, wherein, ​ 11. The additive manufacturing method according to any one of claims 7 to 10, wherein, ​ ​ ​ 14. The material obtained from the powder of any one of claims 1 to 4 according to the method of any one of claims 6 to 13.

15. The material of claim 14, comprising an alloy comprising, by weight, 20% to 24% chromium, 20% to 24% nickel, 13.00% to 16.00% tungsten, 0.02% to 0.12% lanthanum, 0.05% to 0.15% carbon, and 0.20% to 0.50% silicon, up to 1.25% manganese, up to 3.00% iron, up to 0.015% sulfur, up to 0.020% phosphorus, up to 0.0001% bismuth, up to 0.0010% silver, up to 0.0010% lead, up to 0.015% boron, up to 0.0350% oxygen, up to 0.0250% nitrogen, and other elements totaling less than 0.050%, with the balance being cobalt.

16. A turbomachine metal component made of the material of claim 15.

17. A turbomachine comprising at least one component of claim 16.

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