Powder for manufacturing a part by selective laser melting

Abstract

The invention relates to a powder for manufacturing a part by selective laser melting, the powder being prepared by atomisation of an initial cobalt alloy, the cobalt alloy including 22% to 45% by weight of cobalt, 20% to 35% by weight of nickel, and 18% to 32% by weight of iron, the powder having the following particle size distribution: D10 size of less than 25 μm, D50 size of 20 μm to 50 μm, and D90 size of less than 80 μm. Figure for the abstract:

Description

[0001] POWDER FOR MANUFACTURING A PART BY SELECTIVE LASER MELTING

[0002] FIELD OF THE INVENTION

[0003] The present invention relates to a powder for manufacturing a part by selective laser melting and to a method for manufacturing a part using said powder. The invention also relates to a part obtained by said method and to an aircraft comprising one or more parts obtained according to said method.

[0004] TECHNOLOGICAL BACKGROUND

[0005] Aircraft propulsion systems include metal alloy parts, such as turbine blades and nozzles, which are subjected to extreme operating conditions, including very high temperatures of up to 1000°C. These metal alloy parts must therefore have mechanical properties that enable them to withstand these extreme operating conditions, such as sufficient tensile strength and good resistance to fatigue, creep, oxidation and corrosion.

[0006] These parts can, for example, be manufactured using forging or casting processes for metals or alloys. However, such processes do not allow the production of parts with complex shapes. Furthermore, parts obtained using a forging process have insufficient mechanical properties.

[0007] To manufacture metal parts with complex shapes, it is possible to use a selective laser melting process known as "Laser Beam Melting" (LBM). Such a process consists of melting a powder bed of a metal alloy using a laser beam to obtain a solid part. This process can, for example, be carried out using the Inconel 718 alloy or Hastelloy X.

[0008] However, parts made from these alloys have unsatisfactory mechanical properties, including excessively high expansion coefficients at the operating temperatures of an aircraft propulsion system. This results in significant clearances between these parts and other parts of the propulsion system, leading in particular to wear of the parts.

[0009] STATEMENT OF THE INVENTION An objective of the invention is to manufacture metal alloy parts for aircraft propulsion systems having complex shapes and possessing optimized mechanical properties, in particular at the operating temperatures of said propulsion system.

[0010] To this end, the invention has as its first subject a powder for manufacturing a part by selective laser melting, said powder being prepared by atomization of an initial cobalt alloy, said cobalt alloy comprising 22% to 45% by mass of cobalt, 20% to 35% by mass of nickel, and 18% to 32% by mass of iron, said powder having the following particle size distribution: size D10 less than 25 pm, size D50 20 pm to 50 pm, and size D90 less than 80 pm.

[0011] This powder makes it possible, in particular due to its particle size distribution, to manufacture a solid part having optimized mechanical properties. Advantageously, these optimized mechanical properties are retained even at very high temperatures of up to 750°C, preferably up to 650°C. The solid part obtained from this powder may in particular have a lower coefficient of expansion compared to parts usually manufactured by selective laser melting from, for example, Hastelloy X or Inconel 718. The solid part may also have optimized tensile strength and good resistance to fatigue, creep and / or oxidation and corrosion.

[0012] The initial alloy may further comprise 2.0% to 4.0% by mass of chromium, 2.0% to 4.0% by mass of niobium, and 4.0% to 7.0% by mass of aluminum.

[0013] The initial alloy may for example include, in mass percentages:

[0014] - 2.5 to 3.5% chromium,

[0015] - 26.0 to 30.0% nickel,

[0016] - 24.0 to 27.0% iron,

[0017] - 2.5 to 3.5% niobium,

[0018] - 5.0 to 6.0% aluminum and

[0019] - 33.5 to 42.5% cobalt.

[0020] The second subject of the invention is a method for manufacturing a part, made of metal alloy, said method comprising the following steps:

[0021] - S1: preparing a powder by atomization of an initial cobalt alloy, said cobalt alloy comprising 22% to 45% by mass of cobalt, 20% to 35% by mass of nickel, and 18% to 32% by mass of iron, said powder having the following particle size distribution: size D10 less than 25 pm, size D50 ranging from 20 pm to 50 pm, and size D90 less than 80 pm;

[0022] - S2: fuse the powder by selective laser fusion to obtain a solid part.

[0023] This process makes it possible to manufacture a solid part that can have a complex shape and have optimized mechanical properties. Advantageously, these optimized mechanical properties are retained even at very high temperatures of up to 750°C, preferably up to 650°C. The solid part obtained by this process can in particular have a lower coefficient of expansion compared to parts usually manufactured by selective laser melting from, for example, Hastelloy X or Inconel 718. The solid part can also have optimized tensile strength and good resistance to fatigue, creep and / or oxidation and corrosion.

[0024] In the present description, the mass percentage corresponds to the mass percentage of a compound relative to the total mass of the composition comprising this compound. For example, the mass percentage of cobalt in the initial alloy corresponds to the mass percentage of cobalt relative to the total mass of the initial alloy.

[0025] In this description, "solid part" means a solid part made from a single piece.

[0026] Preferably, the powder may have the following particle size distribution: D10 size ranging from 10 pm to 25 pm, D50 size ranging from 25 pm to 40 pm, and D90 size ranging from 40 pm to 70 pm;

[0027] According to a preferred embodiment, the preparation of the powder comprises atomizing the powder and then sorting the powder so as to obtain the desired particle size distribution. Sorting is carried out for example by sieving or classifying. Powder particles not having the desired size are eliminated.

[0028] Atomization is a process that allows good powder quality to be obtained, particularly optimized sphericity.

[0029] Preparation of the powder

[0030] The powder is prepared by atomization, preferably gas atomization, of a liquid initial alloy to obtain a powder. Sorting is then carried out in order to obtain the desired particle size distribution. The particle size distribution of the powder described above makes it possible to obtain good compactness of the powder bed as well as optimized flowability of the powder. Furthermore, this particle size makes it possible to limit the melting stresses, thus limiting the cracking of the alloy. In particular, this particle size makes it possible to limit the residual stresses which are the stresses which occur when the material solidifies, and therefore contracts, and thus pulls on either side of the material. Cracks can then occur in the material.

[0031] Cracking can therefore occur during solidification of the alloy but also during heat treatment which allows the stresses to be rehomogenized.

[0032] In addition, the calibrated grain size ensures that the part manufactured with the parameters used according to the process described above has optimized mechanical strength. In particular, the manufactured part is dense and has few or no defects.

[0033] By "size D50" or "median" is meant the size for which the cumulative function F(D) is equal to 50%. By "size D10" is meant the size for which the cumulative function F(D) is equal to 10%. By "size D90" is meant the size for which the cumulative function F(D) is equal to 90%. The cumulative function F(D) is defined according to the following relation:

[0034] [Math. 1] in which: f(D) is the particle size distribution in number, and D, is a size class.

[0035] Sizes D10, D50 and D90 can be determined according to ISO 13320 or ASTM B822.

[0036] The powder preferably comprises particles of substantially spherical shape. In the present description, when the particles are of substantially spherical shape, their size is given in equivalent sphere diameter. The shape of the powder particles can be determined according to ISO 13320 or ASTM B822.

[0037] The initial alloy comprises 22% to 45% by mass of cobalt, preferably 25% to 42% by mass of cobalt, and even more preferably 27% to 41% by mass of cobalt.

[0038] The initial alloy further comprises 20% to 35% by mass of nickel, preferably 23% to 34% by mass of nickel, and even more preferably 25% to 31% by mass of nickel. The initial alloy further comprises 18% to 32% by mass of iron, preferably 20% to 31% by mass of iron, and even more preferably 23% to 28% by mass of iron.

[0039] The initial alloy may further comprise 2.0% to 4.0% by mass of chromium and preferably 2.2% to 3.2% by mass of chromium.

[0040] The initial alloy may further comprise 2.0% to 4.0% by mass of niobium and preferably 2.2% to 3.2% by mass of niobium.

[0041] The initial alloy may further comprise 4.0% to 7.0% by mass of aluminum, preferably 4.5% to 6.5% by mass of aluminum, and even more preferably 4.8% to 6.2% by mass of aluminum.

[0042] The alloy may also include trace amounts of compounds selected from manganese, silicone, titanium, copper, and a mixture thereof. These compounds are present in the alloy in an amount less than or equal to 1% by mass, and preferably less than or equal to 0.7% by mass.

[0043] The initial alloy may further comprise unavoidable impurities chosen from carbon, phosphorus, sulfur, chromium, boron, tantalum, oxygen, nitrogen, hydrogen, and a mixture thereof. These unavoidable impurities are present in the alloy in an amount less than or equal to 0.1% by mass, and preferably less than or equal to 0.07% by mass.

[0044] The initial alloy used can for example be Inconel 783. The composition of Inconel 783 is as follows in mass percentages:

[0045] 2.5 to 3.5% chromium

[0046] 26.0 to 30.0% nickel

[0047] - 24.0 to 27.0% iron

[0048] 2.5 to 3.5% niobium

[0049] 5.0 to 6.0% aluminum remaining amount of cobalt

[0050] - 0.003 to 0.012% boron

[0051] 0.03% maximum (max) carbon

[0052] 0.50% max manganese

[0053] 0.50% max manganese

[0054] 0.015% max phosphorus

[0055] 0.005% max sulfur

[0056] 0.1 to 0.4% titanium

[0057] 0.50% max copper

[0058] The initial alloy may not contain oxygen, nitrogen, and / or hydrogen. The powder obtained by atomizing the initial alloy may have a composition identical to that of the initial alloy. An identical composition means that the mass percentage of each element in the initial powder varies by ± 2% from the mass percentage of the same element in the initial alloy.

[0059] The composition of the powder may differ from the composition of the initial alloy in particular by the presence of oxygen, nitrogen and / or hydrogen, or the increase in their mass percentage if the initial alloy already included oxygen, nitrogen and / or hydrogen. The mass presence of oxygen, nitrogen and / or hydrogen, or the increase in their mass percentage may in particular be due to the atomization process.

[0060] Selective laser powder melting

[0061] Selective laser melting is performed on the powder described above.

[0062] The fusion is carried out with a laser power ranging from 100 watts to 1000 watts, preferably from 120 watts to 350 watts, and even more preferably from 150 watts to 300 watts.

[0063] The laser speed for performing the fusion may range from 700 mm / s to 4600 mm / s, preferably from 800 mm / s to 150 mm / s, and even more preferably from 900 mm / s to 1300 mm / s.

[0064] The laser power and speed are chosen to provide sufficient energy to achieve fusion, while limiting residual stresses and cracking. This optimizes the selective laser melting process, enabling the desired solid part to be obtained efficiently.

[0065] The thickness of the powder layers to be fused may range from 10 μm to 100 μm, preferably from 15 μm to 60 μm, and even more preferably from 20 μm to 40 μm. The individual powder layers fused one after the other during the laser melting process may have the same or different thicknesses, preferably identical.

[0066] When melting, the laser line width may range from 40 pm to 300 pm, preferably from 45 pm to 250 pm, and even more preferably from 50 pm to 200 pm.

[0067] Furthermore, the width of the strips during fusion can range from 0.5 mm to 300 mm, preferably from 1.2 mm to 22 mm, and even more preferably from 2 mm to 15 mm.

[0068] The overlap between bands may range from 0.02 mm to 0.20 mm, preferably from 0.03 mm to 0.17 mm, and even more preferably from 0.05 mm to 0.15 mm. Preferably, the orientation of the bands is changed between each layer of fused powder. Preferably, the orientation is changed by an angle of between 60° and 70°.

[0069] Furthermore, the overlap between the fusion lines may range from 0.010 mm to 0.800 mm, preferably from 0.015 mm to 0.170 mm, and even more preferably from 0.020 mm to 0.150 mm.

[0070] Preferably, the selective laser melting step is carried out under an inert gas atmosphere, for example nitrogen or argon. The inert atmosphere makes it possible in particular to avoid traces of humidity and oxygen which could affect the mechanical properties of the part.

[0071] The melting parameters just described make it possible to obtain a solid raw part with the desired shape.

[0072] In a step S3, the raw solid part can then be subjected to a heat treatment, preferably a heat treatment under vacuum or alternatively under a partial pressure of an inert gas, for example under nitrogen.

[0073] This heat treatment step S3 is carried out at a first temperature T1 ranging from 900°C to 1300°C, preferably from 1000°C to 1200°C, and even more preferably from 1100°C to 1150°C.

[0074] During the heat treatment step S3, the solid part is subjected to the first temperature T1 for an initial duration ranging from 30 min to 4 h, preferably from 45 min to 1 h 30.

[0075] Following heat treatment step S3, an intermediate solid part is obtained.

[0076] The S3 heat treatment step advantageously allows the relaxation of residual stresses present in the part, the homogenization of the local chemistry of the part material and the re-solution of harmful phases. Furthermore, during cooling following the heat treatment, the desired hardening phases precipitate in the right size and number.

[0077] The intermediate solid part is then subjected to an S4 tempering step which can be carried out according to two possible alternatives.

[0078] According to a first alternative, the intermediate solid part is subjected to an aging step S4a which consists of subjecting the intermediate solid part to a second temperature T2 ranging from 700°C to 1000°C, preferably ranging from 820°C to 880°C, and even more preferably ranging from 830°C to 850°C.

[0079] During this first alternative of the S4a tempering step, the solid part is subjected to the second temperature T2 for a second duration ranging from 30 min to 8 h, preferably for a duration ranging from 1 h to 5 h, and even more preferably for a duration ranging from 2 h to 4 h.

[0080] According to a second alternative, the intermediate solid part is subjected to a second alternative of tempering step S4b which consists of subjecting the intermediate solid part to a third temperature T3 ranging from 670°C to 770°C for a third duration then to a fourth temperature T4 ranging from 570 to 670°C for a fourth duration.

[0081] Preferably, the third temperature T3 can range from 680°C to 750°C, and even more preferably from 700°C to 730°C. The third temperature T3 is kept constant during the third duration which can range from 6h to 20h, preferably from 7h to 19h, and even more preferably from 7h30 to 8h30.

[0082] Preferably, the fourth temperature T4 can range from 590°C to 735°C, and even more preferably from 605°C to 730°C. The fourth temperature T4 is kept constant during the third duration which can range from 6h to 10h, preferably from 7h to 9h, and even more preferably from 7h30 to 8h30.

[0083] Preferably, the cooling between the third temperature T3 and the fourth temperature T4 is carried out in air at a cooling rate of 50°C ± 20°C per hour. Such rapid cooling makes it possible to avoid the precipitation of harmful phases and makes it possible to harden the alloy by precipitating the hardening phases.

[0084] Following one of the alternatives of the tempering step, a final solid part is obtained, said part having the following mechanical characteristics: a hardness greater than or equal to 20 HRC (according to the Rockwell C scale), preferably greater than or equal to 25 HRC; this hardness is determined according to the ISO 6508 standard; a grain size ranging from 1 to 15 according to the ÀSTM standard, preferably ranging from 3 to 11 according to the ÀSTM standard; the grain size is determined according to the ÀSTM E112 standard; a tensile strength Rm greater than or equal to 900 MPa (newton per square millimeter), preferably greater than or equal to 1100 MPa at 20°C; the tensile strength is determined according to the ISO EN-2001 standard; a yield strength Rp0.2 greater than or equal to 600 MPa, preferably greater than or equal to 700 MPa at 20°C; the yield strength is determined according to ISO EN-2001;an elongation at break greater than or equal to 8%, preferably greater than or equal to 10% at 20°C; the elongation at break is determined according to ISO EN-2001; and / or an average coefficient of linear expansion at 200°C ranging from 9 to 11 x 10'; 6 / C°, and preferably 9.5 to 10.5 x 10' 6 / C°, and an average coefficient of linear expansion at 600°C ranging from 11 to 13 x 10' 6 / C°, and preferably 11.5 to 12.5 x 10' 6 / C°; the average coefficient of expansion is determined according to standard NF EN ISO 17562 with a dilatometer.

[0085] The solid part obtained according to the method of the invention has improved mechanical properties compared to parts manufactured according to the methods of the prior art, whether by forging, casting or selective laser melting. Advantageously, these mechanical properties are retained even at extreme temperatures and conditions, such as when the part is used in an aircraft propulsion system.

[0086] The solid part manufactured according to the method described above can for example be chosen from a part of the fuel injection system, a combustion chamber, a turbine distributor sleeve, a turbine casing and sealed sectors of a turbine blade without macro cracks, a macro crack being in particular a crack whose dimension is greater than 300 μm.

[0087] DETAILED DESCRIPTION OF AN EXAMPLE OF IMPLEMENTATION

[0088] A turbine casing with a diameter ranging from 300 mm to 1500 mm is manufactured according to an exemplary embodiment of the method according to an exemplary embodiment of the invention.

[0089] For this, a powder is prepared during step S1 by atomization from Inconel 783.

[0090] The powder obtained has a composition identical to that of the initial alloy and also contains 0.025% by mass of oxygen, 0.02% by mass of nitrogen and 0.001% by mass of dihydrogen.

[0091] The powder is then used in a selective laser melting step S2.

[0092] Step S2 is carried out under an argon atmosphere and the following parameters are applied:

[0093] - laser power = 300 W;

[0094] - laser speed = 1200 mm / s;

[0095] - thickness of powder layers = 40 pm;

[0096] - laser line width = 150 pm;

[0097] - strip width = 2 mm; and

[0098] - angle of modification of orientation of the bands from one layer to another = 67°. A raw solid part is then obtained. This part has a composition identical to that of the initial alloy but also contains 0.035% by mass of oxygen, 0.025% by mass of nitrogen and 0.005% by mass of dihydrogen.

[0099] In step S3, the raw solid part is subjected to an air quenching heat treatment.

[0100] This heat treatment step S3 is carried out at a first temperature T1 between 1107°C and 1121°C. Step S3 is carried out in air according to the AMS 2750 standard for a duration of 1 hour. Then the tempering step S4a is applied.

[0101] The final solid part is then obtained and has the following characteristics: a hardness greater than 25 HRC; a grain size ranging from 3 to 11 according to the ASTM standard with an average of 5 ASTM; a tensile strength Rm of 1100 MPa at 20°C; an elastic limit Rp0.2 greater than 700 MPa at 20°C; an elongation at break greater than 10% at 20°C; and an average coefficient of linear expansion at 200°C of 10 ± 0.3, and an average coefficient of linear expansion at 600°C of 12 ± 0.3.

[0102] The comparative solid part obtained using this process has optimized properties compared to a part manufactured using the same process but with Inconel 718 as the initial alloy. In particular, the comparative solid part has an average coefficient of linear expansion at 200°C of 13 ± 0.3, and an average coefficient of linear expansion at 600°C of 15.5 x 10' 6 / C° ± 0.3 x 10' 6 / C° .

[0103] The method according to one embodiment of the invention therefore made it possible to improve the average coefficient of linear expansion by approximately 30% compared to a selective laser melting method according to the prior art.

Claims

CLAIMS 1. Powder for manufacturing a part by selective laser melting, said powder being prepared by atomization of an initial cobalt alloy, said cobalt alloy comprising 22% to 45% by mass of cobalt, 20% to 35% by mass of nickel, and 18% to 32% by mass of iron, said powder having the following particle size distribution: D10 size less than 25 pm, D50 size 20 pm to 50 pm, and D90 size less than 80 pm.

2. The powder of claim 1, wherein the alloy further comprises 2.0% to 4.0% by mass of chromium, 2.0% to 4.0% by mass of niobium, and 4.0% to 7.0% by mass of aluminum.

3. Powder according to claim 1 or 2, in which the cobalt alloy comprises, in mass percentages: - 2.5 to 3.5% chromium, - 26.0 to 30.0% nickel, - 24.0 to 27.0% iron, - 2.5 to 3.5% niobium, - 5.0 to 6.0% aluminum and - 33.5 to 42.5% cobalt.

4. Powder according to any one of the preceding claims, in which the cobalt alloy is Inconel 783.

5. Method of manufacturing a part, said method comprising the following steps: - S1: preparing a powder according to any one of claims 1 to 4; - S2: fuse the powder by selective laser fusion to obtain a solid part.

6. Method according to claim 5, further comprising a step S3 of heat treatment of the solid part at a first temperature T1 ranging from 900° C to 1300° C.

7. Method according to claim 5 or 6, further comprising a first alternative tempering step S4a consisting of subjecting the solid part to a second temperature T2 ranging from 700°C to 1000°C.

8. Method according to claim 5 or 6, further comprising a second alternative tempering step S4b consisting of subjecting the part to a third temperature T3 ranging from 670°C to 770°C for a third duration and then to a fourth temperature T4 ranging from 570 to 670°C for a fourth duration.

9. A method according to any one of claims 5 to 8, wherein step S2 is carried out at a laser power ranging from 100 watts to 1000 watts.

10. Method according to any one of claims 5 to 9, in which step S2 is carried out at a laser speed ranging from 700 mm / s to 1300 mm / s.

11. Solid part obtained by the method according to any one of claims 5 to 10, comprising: - a hardness greater than or equal to 20 HRC, preferably greater than or equal to 25 HRC, and / or - a grain size ranging from 1 to 15 ÅSTM, preferably ranging from 3 to 11 ÅSTM, and / or - a tensile strength greater than or equal to 900 MPa, preferably greater than or equal to 1100 MPa.

12. Aircraft comprising one or more solid parts according to claim 11 or obtained by the method according to any one of claims 5 to 10, the part or parts being chosen from a part of the fuel injection system, a combustion chamber, a turbine distributor sleeve, a turbine casing and sealed sectors of a turbine blade without macro cracks.

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