Metal powder for additive manufacturing method

Abstract

The invention relates to a metal powder for an additive manufacturing method, the metal powder comprising an alloy comprising, by weight, between 23% and 24.5% of chromium, between 9% and 11% of nickel, between 6.5% and 7.5% of tungsten, between 3% and 4% of tantalum, between 0.55% and 0.65% of carbon, between 0.3% and 0.5% of zirconium, between 0.15% and 0.25% of titanium, at most 2% of iron, at most 0.3% of silicon, at most 0.1% of manganese, at most 0.1% of copper, at most 0.015% of sulfur, at most 0.015% of phosphorus, at most 0.01% of boron, at most 0.025% of oxygen, at most 0.020% of nitrogen and at most 0.010% of hydrogen and less than 0.050% of other elements in total, the remainder being cobalt.

Description

[0001] METAL POWDER FOR ADDITIVE MANUFACTURING PROCESS

[0002] FIELD OF THE INVENTION

[0003] 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.

[0004] STATE OF THE ART

[0005] Many alloys are known for implementing powder bed additive manufacturing processes, particularly for laser beam melting (LBM). An alloy for implementing a powder bed additive manufacturing process is understood to mean 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.

[0006] Currently, many powders containing metal alloys exist. However, most existing powders do not allow the production of a material suitable for use in aeronautical applications such as turbomachinery after fusion by a laser beam in an additive manufacturing process. In particular, many powders do not allow the production of a material resistant to temperatures above 650°C. However, a maximum resistance temperature of 650°C is far too low for use in a turbomachinery. Other materials, for example IN738 and IN939 alloys, are resistant to higher temperatures, 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.

[0007] 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 nozzle 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. DISCLOSURE OF THE INVENTION

[0008] An aim of the invention is to provide a powder comprising a metal alloy for an additive manufacturing process, in particular a powder bed laser fusion process, which makes it possible to obtain a material retaining its tensile strength and creep characteristics and exhibiting high resistance to oxidation and corrosion, at least up to a temperature of 1050°C, and which is weldable.

[0009] To this end, the invention provides a metal powder for an additive manufacturing process, the metal powder comprising an alloy comprising by weight between 23% and 24.5% chromium, between 9% and 11% nickel, between 6.5% and 7.5% tungsten, between 3% and 4% tantalum, between 0.55% and 0.65% carbon, between 0.3% and 0.5% zirconium, between 0.15% and 0.25% titanium, at most 2% iron, at most 0.3% silicon, at most 0.1% manganese, at most 0.1% copper, at most 0.015% sulfur, at most 0.015% phosphorus, at most 0.01% boron, at most 0.025% oxygen, at most 0.020% nitrogen and at most 0.010% hydrogen and less than 0.050% other elements in total, the remainder being cobalt.

[0010] Advantageously, the powder comprises a plurality of grains having a particle size distribution in which 10% of the grains have a diameter less than a D10 value of between 10 pm and 25 pm.

[0011] Preferably, the powder comprises a plurality of grains having a particle size distribution in which 50% of the grains have a diameter less than a D50 value of between 25 pm and 40 pm.

[0012] Advantageously, the powder comprises a plurality of grains having a particle size distribution in which 90% of the grains have a diameter less than a D90 value of between 40 pm and 70 pm.

[0013] The invention also relates to a method for manufacturing a metal powder as described above, the method successively comprising the following steps:

[0014] • mixture of elementary or pre-alloyed raw materials,

[0015] • fusion of the mixture obtained,

[0016] • atomization of the molten mixture by a gas, preferably by argon or nitrogen,

[0017] • sieving of the powder obtained so as to obtain a predefined particle size,

[0018] • recovery of the powder obtained.

[0019] Advantageously, the method comprises 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 selective melting of a portion of said layer by scanning with a laser beam.

[0020] Preferably, the additive manufacturing is carried out by laser powder bed fusion. The laser beam may have a power of between 150 W and 300 W and / or a diameter of between 50 pm and 200 pm and / or a displacement speed of between 900 mm / s and 1300 mm / s. The scanning step may be 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.

[0021] Advantageously, the orientation axes of the respective strips of two adjacent layers form an angle of 67°±5°.

[0022] Advantageously, the thickness of each respective layer is between 20pm and 60pm.

[0023] Preferably, the process is carried out under an argon and / or nitrogen atmosphere.

[0024] The invention also relates to a method for manufacturing a metal part, comprising an additive manufacturing method as described above and a first step of heat treatment at a temperature of between 1200 and 1240°C for a duration of between 5 hours and 30 minutes and 6 hours and 30 minutes and a first step of cooling to room temperature.

[0025] Advantageously, the method for manufacturing a metal part further comprises a second step of heat treatment at a temperature between 920°C and 960°C for a duration between 23 and 25 hours and a second step of cooling to room temperature.

[0026] 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.

[0027] Advantageously, the material comprises an alloy comprising by weight between 23% and 24.5% of chromium, between 9% and 11% of nickel, between 6.5% and 7.5% of tungsten, between 3% and 4% of tantalum, between 0.55% and 0.65% of carbon, between 0.3% and 0.5% of zirconium, between 0.15% and 0.25% of titanium, at most 2% of iron, at most 0.3% of silicon, at most 0.1% of manganese, at most 0.1% of copper, at most 0.015% of sulfur, at most 0.015% of phosphorus, at most 0.01% of boron, at most 0.03% of oxygen, at most 0.03% of nitrogen and at most 0.0125% of hydrogen and less than 0.050% other elements in total, the remainder being cobalt.

[0028] The invention also relates to a metal turbomachine part made of a material as described above. The invention also relates to a turbomachine comprising at least one part as described above.

[0029] BRIEF DESCRIPTION OF THE FIGURES

[0030] Other characteristics and advantages of the invention will emerge from the detailed description which follows, with reference to the appended drawings, in which:

[0031] Figure 1 is a flowchart representing the steps of a method for manufacturing a metal part from a powder according to the invention.

[0032] Figure 2 is an image according to a first plane of the material obtained by the method according to the invention.

[0033] Figure 3 is an image along a second plane of the material obtained by the method according to the invention.

[0034] DETAILED DESCRIPTION OF EMBODIMENTS

[0035] Metal powder

[0036] 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 an electron fusion powder bed process or a powder spray process. The metal powder comprises a cobalt-based alloy comprising at least the elements chromium, nickel, tungsten, tantalum, carbon, zirconium, and titanium. The alloy may further comprise iron, silicon, manganese, copper, sulfur, phosphorus, boron, oxygen, nitrogen, and hydrogen.

[0037] 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).

[0038] Table 1 below shows the quantities of the different chemical elements present in the powder. 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 necessary for an aeronautical application.

[0039] 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 Mar-M-509 forging metal alloy is known, which contains substantially the same list of elements, but its oxygen, nitrogen and hydrogen levels are different and not suitable for an additive manufacturing process.

[0040] 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 make it possible to significantly improve the properties of the material obtained by an additive manufacturing process from such a powder.

[0041] Restricting oxygen and nitrogen levels within these preferred ranges can achieve a 10-15% gain in R value. m / R P o2, Rm being the breaking limit and R p o2 the elastic limit of the material, and a 5% gain in fatigue on such a material. The elongation A% is equivalent to that of a material obtained with an oxygen and nitrogen content of less than 50 ppm.

[0042] In the case of oxygen and / or nitrogen contents higher than the ranges presented, a loss of elongation and a loss of fatigue properties is observed.

[0043] Advantageously, the powder has a particle size suitable for use in a powder bed additive manufacturing process. By suitable particle size is meant a particle size allowing 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 having a homogeneous 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.

[0044] Thus, advantageously the geometry of the grains is essentially spherical, that is to say without sharp angles. This geometry ensures a fluid flow when depositing a layer on a powder bed and ensures 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 stages.

[0045] According to a particular arrangement, 10% of the grains have a diameter less than a D10 value between 10pm and 25pm, 50% of the grains have a diameter less than a D50 value between 25pm and 40pm and 90% of the grains have a diameter less than a D90 value between 40pm and 70pm. The particle size parameters D10, D50 and D90 are measured by laser particle size measurement according to ISO 13320 or ASTM B822.

[0046] This specific particle size allows for optimal powder compactness when used in a powder bed additive manufacturing process, while also achieving 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, such as welding the resulting parts.

[0047] In particular, the D10 value specifies the dimensions of the finest powder grains intended to fill the void spaces between larger grains. These dimensions are optimized to be small enough to fill the void spaces well, and large enough to prevent these grains from being evacuated by the gas flows present in the additive manufacturing machine, and their evacuation or trapping in the filters present in the various machines and the obstruction of these filters as a result.

[0048] The value of D50 corresponds to the median value of the grain diameter.

[0049] The D90 value indicates the maximum grain size, the upper limit of D90 therefore ensures a limitation of fusion defects due to grains being too large.

[0050] The combination of the three dimension parameters D10, D50 and D90 corresponds to a Gaussian distribution of grain sizes with sufficient homogeneity for layer deposition and fusion, thus allowing homogeneous fusion of the powder.

[0051] Powder manufacturing

[0052] Preferably, the powder is obtained via a gas atomization process in an atomization tower. The process begins by preparing a mixture of raw materials in powder or granule form. The raw materials may be in the form of pure elements and / or pre-alloys comprising some 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 changing 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 upon cooling. A screening process can then be carried out to eliminate grains with an unsatisfactory geometry and to sort the powder grains according to their particle size.

[0053] This process ensures that each grain has a predominantly spherical morphology with no sharp angles. At the same time, this production technique limits the risk of powder contamination by foreign bodies and residues in the manufacturing equipment. It also allows for dense fusion and produces grains with low porosity, which is advantageous for obtaining a homogeneous part with a low porosity rate in the additive manufacturing process. Typically, powder grains have a density greater than or equal to 99.9%.

[0054] Additive manufacturing process

[0055] According to a second aspect, the invention relates to a powder bed additive manufacturing method, in particular a laser beam melting method.

[0056] 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 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 can then be extracted by removing the unconsolidated powder.

[0057] The laser beam power and scanning speed are chosen according to the thickness of each layer, the dimensions and geometry of the metal part to be manufactured and the specificities of the additive manufacturing machine. The laser beam typically has a Gaussian power profile. 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 measured according to 1 / e standards 2 (13.5% of the maximum) and / or D4o (quadratic moment diameter) which is very close to the standard 1 / e 2for a Gaussian profile. The diameter is typically between 50pm and 200pm according to one of these standards to achieve a 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 to obtain a good compromise between the production speed of the part and the precision of the geometry of the part to be manufactured.

[0058] The laser beam scanning process is often performed using scanning strips. These strips comprise a plurality of parallel laser beam passes along an X-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.

[0059] 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 allowing the orientation of the bands in each successive layer to be modified. 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.

[0060] 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.

[0061] Post-processing of the part obtained by additive manufacturing

[0062] A raw part obtained by laser fusion has micro-cracks distributed throughout the alloy structure. These micro-cracks are generated by local mechanical and thermal stresses during the additive manufacturing process and are often unavoidable. When using such a part for aeronautical applications, it is necessary to remove the micro-cracks in order to improve the mechanical properties of the part. Generally, heat treatments are used to remove micro-cracks and improve the metallurgical quality of the part.

[0063] A quenching heat treatment is typically used at a temperature between 1200°C and 1240°C, preferably close to 1220°C, for 6 hours ±30 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.

[0064] This treatment causes the alloy to dissolve 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 adjusted to achieve slight precipitation and a stable post-treatment state. During treatment, in addition to the precipitation of carbides, a rediffusion of the elements within the matrix is ​​caused, without fusing other elements or grains.

[0065] This treatment is followed by cooling to room temperature, for example in air. Room temperature is understood to be 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 temperature to be fixed in their position and improves the elastic resistance of the alloy.

[0066] A second heat treatment can then be applied to eliminate the remaining mechanical stresses. Such a stress-relieving treatment is typically carried out at a temperature between 920°C and 960°C for 24 hours ± 1 hour, with subsequent air cooling. Stress-relieving treatment is optional and is mainly applied to parts with significant thickness, or to parts that have undergone welding that can cause further mechanical and thermal stresses. Such a treatment provides stress relief for such stresses, as well as stresses occurring during melting due to atomic rearrangement. At the same time, the occurrence of precipitation or grain coarsening is prevented. The metallurgical structure therefore remains in a metastable state.

[0067] Figure 1 illustrates the main steps of the manufacturing process in the form of a flowchart. During the FA stage of additive manufacturing, a FA1 step of depositing a layer of powder and a FA2 step of consolidation are cyclically repeated. After finalizing the FA additive manufacturing, the T1 heat treatment of quenching is carried out. Subsequently, an optional T2 heat treatment of stress relief can be carried out.

[0068] Part obtained

[0069] During additive manufacturing, the levels of the elements oxygen, nitrogen and hydrogen are changed due to the contact of the powder grains with the atmosphere in the additive manufacturing machine.

[0070] Table 2 below shows the quantities of the different chemical elements present in the metal part manufactured by LBM.

[0071] Figure 2 illustrates the metallurgical structure of a part after a quenching heat treatment in a plane parallel to the layers deposited during additive manufacturing. Figure 3 is a photograph of the metallurgical structure of such a part in a plane perpendicular to a layer 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 4 and 10 ASTM with an average of 6 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.

[0072] The material has a temperature resistance up to 1050°C for use near a combustion chamber in a turbomachine. The material has very good mechanical stability meeting the requirements for turbomachine parts exposed to high mechanical stress. Such stress during use corresponds to a tensile strength Rm of 300MPa at 950°C and a tensile strength Rm of 1300MPa at 20°C with a tolerance of approximately + / -50MPa. The required value of the elastic limit Rp02 is 290MPa at 950°C and 750MPa at 20°C with a tolerance of + / -50MPa.

[0073] Regarding creep resistance, the rupture time at 900°C at 140MPa must be greater than 20 hours. In addition, the parts can be welded with good weld quality.

Claims

CLAIMS 1. Metal powder for an additive manufacturing process, the metal powder comprising an alloy comprising by weight between 23% and 24.5% chromium, between 9% and 11% nickel, between 6.5% and 7.5% tungsten, between 3% and 4% tantalum, between 0.55% and 0.65% carbon, between 0.3% and 0.5% zirconium, between 0.15% and 0.25% titanium, at most 2% iron, at most 0.3% silicon, at most 0.1% manganese, at most 0.1% copper, at most 0.015% sulfur, at most 0.015% phosphorus, at most 0.01% boron, at most 0.025% oxygen, at most 0.020% nitrogen and at most 0.010% hydrogen and less than 0.050% of other elements in total, the remainder being cobalt.

2. Metal powder according to claim 1, comprising a plurality of grains having a particle size distribution in which 10% of the grains have a diameter less than a D10 value of between 10 pm and 25 pm.

3. Metal powder according to claim 1 or claim 2, comprising a plurality of grains having a particle size distribution in which 50% of the grains have a diameter less than a D50 value of between 25 pm and 40 pm.

4. Metal powder according to any one of the preceding claims, comprising a plurality of grains having a particle size distribution in which 90% of the grains have a diameter less than a D90 value of between 40 pm and 70 pm.

5. Method for manufacturing a metal powder according to any one of claims 1 to 4, successively comprising the following steps: o mixing of elementary or pre-alloyed raw materials, o melting of the mixture obtained, o atomization of the molten mixture by a gas, preferably by argon or nitrogen, o sieving of the powder obtained so as to obtain a predefined particle size, o recovery of the powder obtained.

6. Method for additive manufacturing of a metal part, the 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 selective melting of a portion of said layer by scanning with a laser beam.

7. Additive manufacturing method according to claim 6, in which the additive manufacturing is carried out by laser fusion on a powder bed.

8. Additive manufacturing method according to claim 7, wherein the laser beam has a power of between 150 W and 300 W and / or a diameter of between 50 pm and 200 pm and / or a movement speed of between 900 mm / s and 1300 mm / s.

9. Additive manufacturing method according to any one of claims 6 to 8, in which 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.

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

11. Additive manufacturing method according to any one of claims 7 to 10, wherein the thickness of each respective layer is between 20 pm and 60 pm.

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

13. Method for manufacturing a metal part, comprising an additive manufacturing method according to any one of claims 6 to 12 and a first step of heat treatment at a temperature between 1200 and 1240°C for a duration between 5 hours and 30 minutes and 6 hours and 30 minutes and a first step of cooling to room temperature.

14. Manufacturing method according to claim 13, further comprising a second step of heat treatment at a temperature between 920°C and 960°C for a period of between 23 and 25 hours and a second step of cooling to room temperature.

15. Material obtained according to a method according to any one of claims 6 to 14 from a powder comprising an alloy comprising by weight between 23% and 24.5% of chromium, between 9% and 11% of nickel, between 6.5% and 7.5% tungsten, between 3% and 4% of tantalum, between 0.55% and 0.65% of carbon, between 0.3% and 0.5% of zirconium, between 0.15% and 0.25% of titanium, at most 2% of iron, at most 0.3% of silicon, at most 0.1% of manganese, at most 0.1% of copper, at most 0.015% of sulfur, at most 0.015% of phosphorus, at most 0.01% of boron, at most 0.03% of oxygen, at most 0.03% of nitrogen and at most 0.0125% hydrogen and less than 0.050% other elements in total, the remainder being cobalt.

16. Metal part of a turbomachine made of a material according to claim 15.

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

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