Metal powder for additive manufacturing processes

Abstract

The invention relates to a metal powder for an additive manufacturing process, the metal powder comprising a cobalt-based alloy containing by weight between 20% and 24% chromium, between 20% and 24% nickel, between 13.00% and 16.00% tungsten, between 0.02% and 0.12% lanthanum, between 0.05% and 0.15% carbon, between 0.20% and 0.50% silicon, a maximum of 1.25% manganese, a maximum of 3.00% iron, a maximum of 0.015% sulfur, a maximum of 0.020% phosphorus, a maximum of 0.0001% bismuth, a maximum of 0.0010% silver, a maximum of 0.0010% lead, a maximum of 0.015% boron, a maximum of 0.0250% oxygen, and a maximum of 0.0200% nitrogen. Figure for the abbreviation: Fig 2

Description

Title of the invention: Metal powder for additive manufacturing process Technical field

[0001] The present invention relates generally to the field of metal powders for additive manufacturing. More particularly, it proposes a cobalt-based alloy for implementing an additive manufacturing process and a process for manufacturing a metal part intended for a high-temperature application. State of the art

[0002] Many alloys are known for the implementation of powder bed additive manufacturing processes, in particular for laser beam melting (LBM). By alloy for the implementation of a powder bed additive manufacturing process is meant a powder comprising a metal alloy. The powder is intended to be melted by a laser and then solidified during the implementation of a powder bed additive manufacturing process, to form a part.

[0003] Currently, many powders comprising metal alloys exist. However, most existing powders do not allow obtaining - after fusion by a laser beam in an additive manufacturing process - a material suitable for use in aeronautical applications such as use in a turbomachine. In particular, many powders do not allow obtaining a material resistant to a temperature of more than 650°C. However, a maximum resistance temperature of 650°C is much too low for use in a turbomachine. Other materials, for example IN738 and IN939 alloys, are resistant to a higher temperature, but have insufficient mechanical properties for the manufacture of aeronautical parts. In addition, these alloys are also limited to use at temperatures below 1000°C or even 950°C.

[0004] However, additive manufacturing processes make it possible to design parts with a complex geometry suitable for forming optimized parts, for example for fuel injection systems, combustion chambers, turbine distributor liners, or turbine blade sectors. It is therefore desirable to provide a powder making it possible to manufacture such a part having high temperature resistance and mechanical properties suitable for such an application. Statement of the invention

[0005] An aim of the invention is to provide a powder comprising a metal alloy for a powder bed additive manufacturing process, which makes it possible to obtain a material retaining its tensile strength and finishing characteristics and exhibiting high resistance to oxidation and corrosion, at least up to a temperature of 1050°C, and which is weldable.

[0006] To this end, the invention provides a metal powder for an additive manufacturing process, the metal powder comprising a cobalt-based alloy comprising by weight between 20% and 24% of chromium, between 20% and 24% of nickel, between 13.00% and 16.00% tungsten, between 0.02% and 0.12% of lanthanum, between 0.05% and 0.15% of carbon, between 0.20% and 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% oxygen, maximum 0.0200% nitrogen.

[0007] Advantageously, the powder comprises a plurality of grains having a particle size distribution in which at least 10% of the grains have a diameter of between 10 μm and 25 μm.

[0008] Preferably, the powder comprises a plurality of grains having a particle size distribution in which at least 50% of the grains have a diameter of between 25 μm and 40 μm.

[0009] Advantageously, the powder comprises a plurality of grains having a particle size distribution in which at least 90% of the grains have a diameter of between 40 pm and 70 pm.

[0010] The invention also relates to a method for manufacturing a metal powder as described above, successively comprising the following steps: • mixing of elementary or pre-alloyed raw materials, • melting of the mixture obtained, • atomization of the molten mixture by a gas, preferably by argon or nitrogen, • sieving of the powder obtained so as to obtain a predefined particle size, • recovery of the powder obtained.

[0011] Advantageously, the method comprises alternately at least one step of forming a layer of a metal powder as described above and at least one step of selective melting of a portion of said layer by scanning with a laser beam.

[0012] Preferably, the additive manufacturing is carried out by laser fusion on a powder bed. The laser beam may have a power between 150 W and 300 W, a diameter between 50 pm and 200 pm, and a moving speed between 900 mm / s and 1300 mm / s. The scanning step may be carried out according to scanning bands having a width between 2 and 15 mm, the overlap of the bands being between 0.05 and 0.15 mm.

[0013] Advantageously, the orientation axes of the respective strips of two adjacent layers form an angle of 67°±5°.

[0014] Advantageously, the thickness of each respective layer is between 20 pm and 60 pm.

[0015] Preferably, the process is carried out under an argon and / or nitrogen atmosphere.

[0016] The invention also relates to a method for manufacturing a metal part, comprising an additive manufacturing method as described above and a heat treatment step at a temperature of between 1200 and 1240°C for a duration of between 5 minutes and 15 minutes and a cooling step to room temperature.

[0017] The invention also relates to a material obtained according to a method of manufacturing a metal part as described above from a powder as described above.

[0018] Advantageously, the material comprises a cobalt-based alloy comprising by weight between 20% and 24% of chromium, between 20% and 24% of nickel, between 13.00% and 16.00% tungsten, between 0.02% and 0.12% of lanthanum, between 0.05% and 0.15% of carbon, between 0.20% and 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% nitrogen.

[0019] The invention also relates to a metal turbomachine part made of a material as described above.

[0020] The invention also relates to a turbomachine comprising at least one part as described above. Brief description of the figures

[0021] Other characteristics and advantages of the invention will emerge from the detailed description which follows, with reference to the appended drawings, in which:

[0022] [Fig. 1] is a flowchart representing the steps of a method for manufacturing a metal part from a powder according to the invention.

[0023] [Fig.2] is an image of the metallurgical structure of the material obtained by the process according to the invention. Detailed description of embodiments Metal powder

[0024] The invention provides a metal powder for an additive manufacturing process, in particular a powder bed process such as laser beam fusion LBM. Alternatively, the additive manufacturing process may be another process, for example a powder bed process by electron fusion or a powder spraying process. The metal powder comprises a cobalt-based alloy comprising at least the elements chromium, nickel, tungsten, lanthanum, carbon, and silicon. The alloy may further comprise manganese, iron, sulfur, phosphorus, bismuth, silver, lead, boron, oxygen, and nitrogen.

[0025] In the remainder of this description, the “rates” or “contents” will be expressed in mass terms (i.e. mass of said element over the total mass of the alloy).

[0026] Table 1 below shows the quantities of the different chemical elements present in the powder, with cobalt making up the remainder of the alloy weight. The powder may contain impurities in the form of other elements at a rate of less than 0.005% for each respective element, and a rate of less than 0.050% of other elements in total. These rates provide sufficient purity to carry out an additive manufacturing process and to ensure the mechanical properties and temperature resistance required for an aeronautical application.

[0027] [Tables 1] Element [wt%] Co Cr Ni w La c SI Fe Minimum Base 20.00 20.00 13.00 0.02 0.05 0.20 - Maximum 24.00 24.00 16.00 0.12 0.15 0.50 3.00 Element [wt%] SP Bi Ag Pb BNO Minimum - - - - - - - - Maximum 0.015 0.020 0.0001 0.0010 0.0010 0.015 0.0200 0.0250

[0028] In particular, the oxygen and nitrogen levels are suitable for a powder to be used in an additive manufacturing process. A suitable level is understood to mean a level that makes it possible to obtain a material having the required thermal and mechanical characteristics after fusion of the powder by a laser beam. Indeed, the HA188 forging metal alloy is known, which contains substantially the same list of elements, but its oxygen and nitrogen levels are different and not suitable for an additive manufacturing process.

[0029] Advantageously, the powder has a particle size suitable for a implemented in a powder bed additive manufacturing process. By suitable particle size, we mean a particle size that allows easy deposition of a layer of powder in a bed, for example by supplying a predefined quantity onto the powder bed and spreading the powder by a scraper to form a layer with a uniform thickness and density. Grains that are too large can cause fusion defects resulting in a material with excessively large pores and mechanical weaknesses. Grains that are too fine require greater fusion energy and can cause cracks that are also detrimental to the mechanical strength of the material.

[0030] Thus, advantageously the geometry of the grains is essentially spherical, that is to say without sharp angles. This geometry makes it possible to ensure a fluid flow when depositing a layer on a bed of powder and to ensure a homogeneous fusion of the powder forming the part. Furthermore, a spherical geometry of the grains makes it possible to choose the particle size of the powder with good precision by one or more sieving steps.

[0031] According to a particular arrangement, 10% of the grains (value D10) have a diameter between 10 pm and 25 pm, 50% of the grains (value D50) have a diameter between 25 pm and 40 pm and 90% of the grains (value D90) have a diameter between 40 pm and 70 pm.

[0032] This specific particle size allows for the combination of optimal compactness of the powder when used in a powder bed additive manufacturing process, while having optimal flowability and minimizing melting stresses. This reduces the risk of cracking of the material obtained by an additive manufacturing process during manufacturing, cooling and subsequent steps, for example welding of the resulting parts.

[0033] In particular, the value D10 specifies the dimensions of the finest powder grains intended to fill the empty spaces between the larger grains. These dimensions are optimized to be small enough to fill the empty spaces well, and large enough to prevent these grains from being evacuated by the gas flows present in the additive manufacturing machine, and from being evacuated or trapped in the filters present in the various machines and from clogging these filters as a result.

[0034] The value of D50 represents the value of the average of the grains.

[0035] The value D90 indicates the maximum grain size, the upper limit of D90 therefore ensures a limitation of fusion defects due to grains having an excessively large size.

[0036] The combination of the three dimension parameters D10, D50 and D90 corresponds to a Gaussian distribution of grain sizes exhibiting sufficient homogeneity for the deposition of layers and fusion, thus allowing a homogeneous fusion of the powder. Powder manufacturing

[0037] Preferably, the powder is obtained via a gas atomization process in an atomization tower. The process begins with the preparation of a mixture of raw materials in the form of powder or granules. The raw materials may 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 modifying the nitrogen content in the alloy to be manufactured. Under the mechanical stress of the gas jet, the molten metal forms spherical droplets which form powder grains as they cool. Sieving can then be carried out to eliminate grains of unsatisfactory geometry and to sort the powder grains according to their particle size.

[0038] This process ensures the essentially spherical morphology and without sharp angles of each grain. At the same time, this production technique limits the risk of pollution of the powder by foreign bodies and residues in the manufacturing equipment. It also makes it possible to achieve a dense fusion and obtain grains with little porosity, which is advantageous for obtaining a homogeneous part with a low porosity rate in the additive manufacturing process. Typically, the powder grains have a density greater than or equal to 99.9%. Additive manufacturing process

[0039] According to a second aspect, the invention relates to a powder bed additive manufacturing method, in particular a laser beam melting method.

[0040] In a known manner, in an additive manufacturing process of the LBM type, a first layer of particles of the starting powder is deposited. Subsequently, a portion of the first layer is melted by irradiating it with a laser beam to form a portion of the metallurgical product. Other layers of powder are then repeatedly formed on the surface of the powder and of the portion of the metallurgical product formed in the previous step, and a portion of the second layer of powder is consolidated by irradiating it with the laser beam so as to weld it onto the portion of the metallurgical product. An object is thus formed successively layer by layer. At the end of the process, the part is allowed to cool and the part can then be extracted by removing the unconsolidated powder.

[0041] The power of the laser beam and the scanning speed are chosen according to the thickness of each layer, the dimensions and the geometry of the metal part to be manufacture and the specificities of the additive manufacturing machine. Preferably, the laser beam has a power between 150 and 300W, which allows the powder to be melted locally without causing excessive heating of the powder near the part. The beam diameter is typically between 50pm and 200pm to achieve precision of the part to be manufactured and a satisfactory production speed. The beam movement speed is typically between 900 mm / s and 1300 mm / s. Such a speed allows for a good compromise between the production speed of the part and the precision of the geometry of the part to be manufactured.

[0042] The laser beam scanning process is often carried out according to scanning strips. These strips comprise a plurality of parallel laser beam passes along an X orientation axis. The strips typically have a width between 2 and 15 mm. The strips are produced with an overlap of two adjacent passes in a peripheral zone, advantageously between 0.05 and 0.15 mm. This ensures good mechanical strength along the edges of each strip.

[0043] In order to avoid structuring effects due to 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 making it possible to modify the orientation of the bands in each successive layer. To obtain good homogeneity of the material obtained, 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 avoid an orientation being repeated in layers close to each other and the orientations varying over a maximum number of superimposed layers. For this purpose, an offset angle is chosen that is not a divisor of 360°. Preferably, an angle of 67° is chosen with a tolerance of approximately ±5° between the orientation axes between two successive layers. Such an angle makes it possible to avoid a dominant orientation of the microstructure.In particular, the angular offset helps to avoid the appearance of aligned overlap zones. Aligned fusion zones can cause mechanically weakened zones in the event of fusion defects, because these defects will be aligned over a macroscopic thickness of the material to be manufactured.

[0044] After the deposition and consolidation of the last layer of powder and, if necessary, the cooling of the part, the manufactured part can be extracted from the remaining powder which can be reused for the manufacture of other parts.

[0045] Post-processing of the part obtained by additive manufacturing

[0046] A raw part obtained by laser fusion has micro-cracks distributed throughout the structure of the alloy. These micro-cracks are generated by mechanical stresses local thermal and heat changes during the additive manufacturing process and are often unavoidable.

[0047] For the use of such a part for aeronautical applications, it is necessary to eliminate microcracks in order to improve the mechanical properties of the part. Generally, heat treatments are used to eliminate microcracks and improve the metallurgical quality of the part.

[0048] A quenching heat treatment is typically used at a temperature between 1200°C and 1240°C, preferably close to 1220°C, for 10 minutes 5 minutes to improve the metallurgical quality of a part resulting from the additive manufacturing process described above. By way of illustration and not limitation, the temperature rise rate is approximately 10°C / min.

[0049] This treatment causes the alloy to be dissolved and certain elements to precipitate, particularly carbides at the grain boundaries. Such carbides increase the mechanical strength and hardness of the material. The duration is adapted to obtain slight precipitation and a stable post-treatment state. During the treatment, in addition to the precipitation of the carbides, a rediffusion of the elements within the matrix is ​​caused, without however fusing other elements or grains.

[0050] . This treatment is followed by cooling to room temperature, for example at air. Ambient temperature means a temperature within the normal range of residential premises, i.e. approximately between 15°C and 30°C. Rapid cooling allows the chemical elements that may have diffused at high temperatures to be fixed in their position and improves the elastic resistance of the alloy.

[0051] [Fig.l] illustrates the main steps of the manufacturing process in the form of a flowchart. During the FA step of additive manufacturing, a FAI step of depositing a layer of powder and a FA2 consolidation step are cyclically repeated. After finalizing the FA additive manufacturing, the T1 heat treatment of quenching is carried out. Part obtained

[0052] During additive manufacturing, the level of the elements oxygen and nitrogen is modified due to the contact of the powder grains with the atmosphere in the additive manufacturing machine.

[0053] Table 2 below indicates the quantities of the different chemical elements present in the metal part manufactured by LBM.

[0054] [Tables2] Element [wt%] Co Cr Ni w La c SI Fe Minimum Base 20.00 20.00 13.00 0.02 0.05 0.20 - Maximum 24.00 24.00 16.00 0.12 0.15 0.50 3.00 Element [wt%] SP Bi Ag Pb BNO Minimum - - - - - - - - Maximum 0.015 0.020 0.0001 0.0010 0.0010 0.015 0.0250 0.0350

[0055] [Fig.2] illustrates the metallurgical structure of a part after a quenching heat treatment in a plane parallel to the layers deposited during additive manufacturing. The images are optical microscopy photographs of a part obtained after mechanical polishing and chemical etching in the respective plane. The material shows no cracking. The grain size in the part is between 3 and 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 turbomachine. As a guide, 3 ASTM corresponds to 127 pm, 5 ASTM to 63.5 pm and 11 ASTM to 7.9 pm.

[0056] The Vickers hardness of the material after heat treatment is between 250 and 380 HV. It can be measured for example as a hardness of type HV0.3, i.e. with a mass of 0.3g. The material has a temperature resistance of at least up to 1050°C for use near a combustion chamber in a turbomachine, and in particular very good resistance to oxidation and corrosion. The material has tensile, fatigue and fining strength meeting the requirements for turbomachine parts, in particular parts for turbomachine injection systems which are often exposed to high temperatures and a corrosive environment, and parts such as retaining rings, sealed sectors, trim parts, and turbine nozzle liners. In addition, the parts can be welded with good weld quality.

Claims

Claims

1. A method of manufacturing a metal powder for an additive manufacturing process, the metal powder comprising a cobalt-based alloy comprising by weight between 20% and 24% chromium, between 20% and 24% nickel, between 13.00% and 16.00% tungsten, between 0.02% and 0.12% lanthanum, between 0.05% and 0.15% carbon, between 0.20% and 0.50% silicon, at most 1.25% manganese, at most 3.00% iron, at most 0.015% sulfur, at most 0.020% phosphorus, at most 0.0001% bismuth, at most 0.0010% silver, at most 0.0010% lead, at most 0.015% boron, at most 0.0250% oxygen, maximum 0.0200% nitrogen, said process successively comprising the following steps: • mixing of elementary or pre-alloyed raw materials, • melting of the mixture obtained, • atomization of the molten mixture with argon, • sieving of the powder obtained so as to obtain a predefined particle size,• recovery of the powder obtained.,

2. The method of claim 1, wherein the powder comprises a plurality of grains having a particle size distribution in which at least 10% of the grains have a diameter between 10 pm and 25 pm.

3. A method according to claim 1 or claim 2, wherein the powder comprises a plurality of grains having a particle size distribution in which at least 50% of the grains have a diameter of between 25 pm and 40 pm.

4. A method according to any preceding claim, wherein the powder comprises a plurality of grains having a particle size distribution in which at least 90% of the grains have a diameter of between 40 pm and 70 pm.

5. A method of additive manufacturing of a metal part, the method comprising alternating at least one step of forming a layer of a metal powder manufactured by a method according to any one of claims 1 to 4 and at least one melting step selective removal of a portion of said layer by scanning a laser beam.

6. The additive manufacturing method of claim 5, wherein the additive manufacturing is performed by laser powder bed fusion.

7. An additive manufacturing method according to claim 6, wherein the laser beam has a power of between 150 W and 300 W, a diameter of between 50 pm and 200 pm, and a displacement speed of between 900 mm / s and 1300 mm / s.

8. Additive manufacturing method according to any one of claims 5 to 7, wherein the scanning step is carried out according to scanning bands having a width of between 2 and 15 mm, the overlap of the bands being between 0.05 and 0.15 mm.

9. An additive manufacturing method according to claim 8, wherein the orientation axes of the respective strips of two adjacent layers form an angle of 67°±5°.

10. An additive manufacturing method according to any one of claims 6 to 9, wherein the thickness of each respective layer is between 20pm and 60pm.

11. Additive manufacturing method according to any one of claims 5 to 10, said method being carried out under an argon and / or nitrogen atmosphere.

12. A method of manufacturing a metal part, comprising an additive manufacturing method according to any one of claims 5 to 11 and a heat treatment step at a temperature between 1200 and 1240°C for a duration between 5 minutes and 15 minutes and a cooling step to room temperature.

13. Material obtained by a process according to any one of claims 5 to 12 from a powder manufactured by a process according to any one of claims 1 to 4.

14. Material according to claim 13, comprising a cobalt-based alloy comprising by weight between 20% and 24% of chromium, between 20% and 24% of nickel, between 13.00% and 16.00% tungsten, between 0.02% and 0.12% of lanthanum, between 0.05% and 0.15% of carbon, between 0.20% and 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 0.0001% of zinc ...

15. maximum 0.0010% lead, maximum 0.015% boron, maximum 0.0350% oxygen, maximum 0.0250% nitrogen. Metal turbomachine part made of a material according to claim 14.

16. Turbomachine comprising at least one part according to claim 15.

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