Refractory metal powders for additive manufacturing and methods for producing the same

Abstract

The present invention relates to refractory metal powders for additive manufacturing, methods for their production, and their use in additive manufacturing.

Description

[Technical Field]

[0001] The present invention relates to a refractory metal powder for additive manufacturing, a method for its preparation, and its use in additive manufacturing. [Background technology]

[0002] The term "additive manufacturing" encompasses all manufacturing methods in which, in contrast to subtractive methods, a workpiece is built by automatically adding volume elements or layers directly from digital 3D data, or in which additional volume elements are applied to an existing workpiece, thus producing a three-dimensional object. Layer-by-layer construction is typically performed under computer control from one or more liquid or solid materials. The construction of the three-dimensional object is achieved through different physical and chemical hardening and melting processes, resulting in the production of a tangible product without specific tools that memorize the respective geometric shape of the product.

[0003] Although the processing of plastics, synthetic resins, and ceramics to produce complex models, specimens, and prototypes has found widespread application in industry and research, the number of suitable metals remains limited. This is due, among other things, to the fact that metals must meet certain properties in order to be usable in additive manufacturing. Metals can be in solid form, such as rods or wires, or in powder form.

[0004] All manufacturing processes that start with powdered metals have certain requirements that the processed metal must meet, including, among other things, high flowability and high bulk and tapped densities, in order to achieve uniform and well-defined application in the manufacturing process and, on the other hand, to obtain compact workpieces.

[0005] Metal powders that meet the above-mentioned requirement profile typically have a high degree of sphericity, which can currently only be achieved by energy-intensive melting and nozzle injection methods.

[0006] Thus, WO 2020 / 220143 describes a metal powder for additive manufacturing prepared by plasma treatment, which comprises spherical particles with a particle size distribution of 0 to 1000 μm and has a flowability of up to 20 seconds.

[0007] US Patent Application Publication No. 2022 / 0023941 discloses a metal powder for 3D impressions comprising a metal selected from the group consisting of tantalum, titanium, niobium, and alloys of tantalum, titanium, and niobium, wherein the particles of the metal powder have an average aspect ratio Ψ of 0.7 to 1. A and Ψ A =x Feret min / x Feret max is.

[0008] DE 102019130941 A1 relates to a method for processing metal powders for additive manufacturing, comprising the following steps: a) at least two metal powders with different physical properties are provided, at least one powder being a residual powder from additive manufacturing and at least one other powder being a new powder; b) the at least two powders are introduced and subsequently mixed into one powder mixture; and c) the powder mixture is subsequently dried and then separated into fractions, wherein in a first step an ultrafine fraction is separated from the residual fraction by sifting and a coarse fraction is separated from the remaining fraction by screening, or wherein a coarse fraction and at least one fine fraction are separated by screening in a screening device, the remaining fraction being a processed metal powder for additive manufacturing, which processed powder can be homogenized by the previous method steps and sent for further use.

[0009] In the paper "Manipulation and characterization of novel titanium powder precursor fur additive manufacturing applications" published in The Minerals, Metals & Materials Society, 2015, Vol. 67, No. 3, YY, Sun et al. describe spherical Ti powder for additive manufacturing obtained from a precursor powder that is not suitable for use in additive manufacturing due to its large particle size and other morphologies. [Prior art documents] [Patent documents]

[0010] [Patent Document 1] International Publication No. 2020 / 220143 [Patent Document 2] US Patent Application Publication No. 2022 / 0023941 [Patent Document 3] German Patent Application Publication No. 102019130941 [Non-patent literature]

[0011] [Non-Patent Document 1] Sun et al., “Manipulation and characterization of novel titanium powder precursor fur additive manufacturing applications”, The Minerals, Metals & Materials Society, 2015, Vol.67, No.3, YY Summary of the Invention [Problem to be solved by the invention]

[0012] Melting metals with very high melting temperatures, as is typical for refractory metals, results in very high specific energy demands and therefore high production costs. While the prior art already has some solutions for low-melting refractory metals such as titanium, niobium, and tantalum, satisfactory solutions for high-melting refractory metals are still needed, among which tungsten has the highest melting point and is further characterized by its extraordinary strength, which makes mechanical processing more difficult but at the same time makes tungsten an attractive material.

[0013] In view of the above, it is an object of the present invention, on the one hand, to provide a refractory metal powder, in particular tungsten, that is suitable for use in additive manufacturing, and a further object of the present invention, to provide a more sustainable method for producing such powder compared to methods known from the prior art. [Means for solving the problem]

[0014] Within the scope of the present invention, it has surprisingly been found that powders that do not exhibit the sphericity that is said to be essential in the prior art can also be used in additive manufacturing. It has therefore surprisingly been found that powders with particles whose shape deviates from the ideal spherical shape that is usually required are also suitable for additive manufacturing. [Effects of the Invention]

[0015] Unless otherwise stated, particle size distributions determined by laser diffraction relate to the mass distribution of the particles. The following applies: D10: 10% of the mass of the powder has a particle size smaller than the specified value, or 90% of the mass of the powder has a particle size larger than the specified value. D50: 50% of the mass of the powder has a particle size smaller than a specified value or 50% of the mass of the powder has a particle size larger than a specified value. D90: 90% of the mass of the powder has a particle size smaller than a specified value, or 10% of the mass of the powder has a particle size larger than a specified value.

[0016] Within the scope of this application, the terms "grain" and "particle" are used interchangeably, as are the terms "grain size" and "particle size."

[0017] In particular, "non-spherical particles" as used herein means particles having a cylindrical or ellipsoidal shape.

[0018] The term "fraction" as used herein refers to a separated portion of a powder obtained by a separation process, where at least two fractions are formed in each separation process. Such separation can be carried out by characteristic features such as mass, density, size, color, or shape. DETAILED DESCRIPTION OF THE INVENTION

[0019] In a first aspect, the present invention provides a non-spherical refractory metal powder selected from the group consisting of vanadium, chromium, molybdenum and tungsten, and alloys thereof, for additive manufacturing of three-dimensional workpieces, said powder consisting of non-spherical particles, each particle having a diameter of A n Dimension A n The particle size can be determined by optical methods, for example, using scanning electron micrographs.

[0020] The term "dimension" as used in the present invention means any distance between two points on the surface of a particle, with the connecting line between the two points passing through the center of gravity of the particle. While spherical particles have the same distance from the surface to the center in all spatial dimensions, any number of dimensions can be determined for all particles of the refractory metal powder according to the present invention, at least two of which, preferably at least three, and more preferably at least four, are different from each other. While A1 = A2 for spherical particles, the powder according to the present invention is particularly characterized by having at least two distances where A1 ≠ A2. For further explanation, reference is made to FIG. 1. In a particularly preferred embodiment of the refractory metal powder according to the present invention, the smallest dimension A of the particle ismin and maximum dimension A max differ by a factor of f(A), where f(A) has a value of greater than 1.001 to 5. In a further preferred embodiment, the particles have a maximum dimension A max and minimum dimension A min Ratio to (A max / A min ) is less than 5.

[0021] Within the scope of the present invention, it has surprisingly been found that powders consisting of non-spherical particles are characterized by a particular distribution function of their particle sizes, since the model of spheres with the same volume, on which particle size distributions are usually based, does not apply here.

[0022] In a preferred embodiment, the refractory metal powder of the present invention is characterized by a particle size distribution determined as follows. screening the refractory metal powder using n screens, where n≧2 and the screens have different mesh sizes; F(x i )=n+1 fractions, i +1)>x i In the case of coarse particle fraction F(x i +1) and F(x i )=x i In the case of the medium grain fraction F(x i ) and F(x i -1) <x i The fine particle fraction F(x i -1), and x i represents the mesh size of the screen, and 1.1≦a i x if ≦1.5 i+1 / x i =a i Step The medium particle fraction F(x) obtained using laser diffraction i determining the particle size distribution of The fraction F(x) determined by laser diffraction i ) particle size distribution D90 value is the screen x i +1 mesh size and xi +1>x i is.

[0023] Screening can be carried out in accordance with DIN 66165, for example using sieve columns equipped with standard test sieves such as those described in DIN ISO 3310 or ASTM B214.

[0024] Determination by laser diffraction can be carried out in accordance with DIN ISO 13320.

[0025] In a preferred embodiment, a i is the ratio of all x i+1 / x i are essentially the same for each value, with the deviation between the values preferably being less than 5%, more preferably less than 3%.

[0026] For spherical particles, the position at which the particle falls through the mesh of the screen does not matter, but this is not the case for non-spherical particles, which have a longitudinal dimension that is larger than the mesh size of the screen. Without being bound by theory, this effect can be attributed to the fraction of non-spherical particles, F(x i ) distribution is smaller for coarser screens x i+1 The particle size distribution of each fraction is determined by laser diffraction.

[0027] The screening of the powder according to the invention within the scope of determining its particle size distribution is preferably carried out by means of a screen with a mesh size x i In particular, x i+1 =ai*x i The screen is run using a formula: i <1.43. In particular, for powders designed for use in additive manufacturing, screens with a mesh size of up to 125 μm have proven to be particularly suitable. Therefore, the following mesh size x i ​Preferred is an embodiment in which a screen having x i :32μm(+ / -5μm); x i +1: 45 μm (+ / - 5 μm); preferably a1 = 1.40 x i +2: 63 μm (+ / - 8 μm); preferably a2 = 1.40 x i +3: 90 μm (+ / - 12 μm); preferably a3=1.42 x i +4: 125 μm (+ / - 20 μm); preferably a4=1.38 The value in parentheses is the tolerance describing the allowable deviation from the nominal mesh size.

[0028] The particle size distribution of spherical particles usually follows a Gaussian normal distribution. In contrast, it has surprisingly been found that the particle size distribution of the refractory metal powder according to the invention deviates from this distribution. Therefore, embodiments in which the powder has a particle size distribution corresponding to a log-normal distribution are preferred.

[0029] The present invention relates to a refractory metal that has proven to be highly corrosion-resistant due to passivation. The refractory metal powder according to the present invention is selected from the group consisting of vanadium, chromium, molybdenum, and tungsten, and alloys thereof. More preferably, the refractory metal is tungsten and / or a tungsten alloy. Even more preferably, the refractory metal is tungsten, preferably obtained by reduction from tungsten oxide.

[0030] The refractory metal powder according to the invention is particularly adapted to the requirements of additive manufacturing, with a flowability of 9 seconds or less proving to be particularly suitable within the scope of powder printing methods. Within the scope of the present invention, it has surprisingly been found that this requirement is also met by the refractory metal powder according to the invention, despite its non-spherical shape, which was previously thought not to be indicative of use in additive manufacturing. Therefore, preferred are embodiments in which the refractory metal powder according to the invention has a flowability of 9 seconds or less, preferably 6 seconds or less, as determined according to ASTM B213.

[0031] Further properties important for the suitability of metal powders in additive manufacturing include bulk density and tapped density. Surprisingly, these properties have been found to be achieved by the refractory metal powders according to the present invention, which could not have been predicted because the non-spherical shape of the particles actually deviates from the preferred spherical particle morphology.

[0032] In a preferred embodiment, the refractory metal powder according to the present invention has a bulk density of at least 40%, preferably greater than 45%, more preferably greater than 50% of the theoretical density of the refractory metal, as determined according to ASTM B329.

[0033] More preferably, the refractory metal powder according to the invention has a tap density of at least 45%, preferably greater than 50%, more preferably greater than 56% of the theoretical density of the refractory metal, as determined according to ASTM B 527. Thus, the powder according to the invention has both a bulk density and a tap density comparable to conventional powders having spherical particles.

[0034] Bulk and tapped densities are stated as a ratio to the theoretical density of the refractory metal. In the exemplary case where the refractory metal is tungsten metal, it is 19.25 g / cm 3 The theoretical density of refractory metals is known to those skilled in the art and can be obtained from the corresponding references.

[0035] The powder according to the present invention differs from prior art powders in particular by the non-spherical shape of its particles. The sphericity of a solid is defined as the ratio of the surface of a sphere having the same volume to the surface of the solid, with a sphericity of 1 representing a sphere. Therefore, preferred embodiments are those in which the refractory metal powder has a sphericity coefficient of less than 0.95, preferably between 0.65 and 0.9. Sphericity may be determined statically according to ISO 13322-1 or dynamically according to ISO 13322-2. Within the scope of the present invention, refractory metal powders having a "semi-rounded" or "rounded" particle shape (see FIG. 2) according to Power's classification in Power, M.C., Journal of Sedimentary Petrology, 1953, Volume 23, page 118, have proven particularly advantageous. Refractory metal powders have a native oxide passivation on the surface, which makes the metal powder relatively corrosion resistant, but can introduce defects into workpieces produced by additive manufacturing due to incorporated foreign matter. Therefore, preferred embodiments are those in which the refractory metal powder has an oxygen content of up to 1000 ppm, preferably up to 500 ppm, as determined by the LECO method.

[0036] Refractory metal powders for additive manufacturing characterized by high sphericity, as described in the prior art, have the disadvantage that they can only be prepared by plasma spheronization or other energy-intensive atomization methods, in which the metal is melted or partially melted and atomized, which involves a significant expenditure of energy, mainly due to the high melting point of the refractory metal. In contrast, it has surprisingly been found that the powders according to the invention can be prepared by simple mechanical methods. These methods have the advantage, in particular, that the metal does not need to be heated above its melting point. The invention therefore relates to a method for producing the powder according to the invention, comprising the following steps: i) providing a starting refractory metal powder selected from the group consisting of vanadium, chromium, molybdenum and tungsten, and alloys thereof, having a particle size distribution D10 of at least 10 μm and D90 of less than 1000 μm as determined by laser diffraction; ii) mechanically treating the starting refractory metal powder to obtain the refractory metal powder according to the invention; The present invention further relates to a method, including:

[0037] The general profile of requirements for metal powders in additive manufacturing provides for a minimum particle size of 10 μm. If the starting powder already has a corresponding particle size distribution, it has proven advantageous here that the first step of the method according to the invention is to provide a starting refractory metal powder with a particle size distribution D10 of at least 10 μm, with D90 being less than 1000 μm.

[0038] Preferably, the particles of the starting refractory metal powder have a compacted, non-porous structure in addition to the particle size distribution described above. Thus, in a preferred embodiment, the starting refractory metal powder preferably has a density of at least 80%, more preferably at least 90%, and especially at least 95% of the theoretical density of the refractory metal. Theoretical densities of materials are known to those skilled in the art and can be obtained from the corresponding collection of tables.

[0039] Within the scope of the method according to the present invention, the starting refractory metal powder is subjected to a mechanical treatment, which surprisingly provides a powder having the necessary properties for use in additive manufacturing, such as bulk density and tapped density, with low energy consumption compared to that of melt-spheroidization. Without being bound by theory, it is assumed that the mechanical treatment of the starting powder results in smoothing of the particle surface, thereby achieving advantageous properties. Therefore, the mechanical treatment is preferably selected so as not to result in crushing of the particles.

[0040] In this connection, mechanical treatment in a ball mill or jet mill has proven to be particularly advantageous.

[0041] Therefore, preferred embodiments of the method according to the present invention involve mechanical processing of the starting refractory metal powder in a ball mill. It has proven advantageous to select the size and number of grinding balls accordingly to limit the formation of platelets by compacting the particles. Therefore, preferred embodiments involve grinding balls with a maximum size of 8 mm, preferably 4 mm or less, where the values refer to the diameter of the grinding balls. Furthermore, the mass of the grinding balls must be in an appropriate ratio to the mass of the powder being processed. In this regard, preferred embodiments of the method according to the present invention involve a ratio of the weight of the starting refractory metal powder to the weight of the grinding balls of 1:1 to 1:5. Both measures limit the energy input and thus help to obtain powder particles with a shape suitable for additive manufacturing. The grinding balls may be made of iron-based materials, steel, Fe-Co, Ni alloys, ceramics, or carbides. The mechanical processing in the ball mill is preferably carried out for a period of 3 to 7 hours, preferably 4 to 6 hours, and particularly 4.5 to 5.5 hours. After this period, the powder particles have the appropriate shape to be able to use the powder for additive manufacturing. In the mechanical treatment, the revolutions per minute of the ball mill is preferably selected to be lower than the critical revolutions per minute, which depends on its diameter and is calculated according to:

[0042]

number

[0043] Alternatively, the mechanical treatment may be carried out in a jet mill.Therefore, an alternative embodiment of the process according to the invention is preferred, in which the mechanical treatment of the starting refractory metal is carried out in a jet mill.

[0044] According to the jet mill principle, particles are ground in a gas stream without the use of mechanical tools, such as high-speed rotors. The grinding gas is introduced into the grinding chamber through a nozzle and is highly accelerated, causing each particle to be carried away and collide with each other. Because particle processing is primarily autogenous, this method is particularly suitable for the grinding of very hard materials. Surprisingly, within the scope of the present invention, it has been found that particles suitable for additive manufacturing can be produced using a conventional jet mill.

[0045] According to the present invention, the mechanical treatment is preferably carried out under a grinding pressure of less than 8 bar. Preferably, a grinding gas having a grinding gas temperature of 10 to 40°C is used, the temperature being related to the gas under the grinding pressure. Surprisingly, it has been found that the grinding time can be shortened by using a jet mill compared to a ball mill. Therefore, preferred embodiments are those in which the mechanical treatment is carried out for a period of 0.5 to 3 hours, preferably 0.5 to 1.5 hours.

[0046] Mechanical processing of powders as provided in accordance with the present invention can already produce refractory metal powders that have some of the properties required for use in additive manufacturing, particularly with regard to bulk density and tapped density.

[0047] In a preferred embodiment, the obtained refractory metal powder may be subjected to further classification, for example to obtain a particle size in a desired range. Thus, in a preferred embodiment, the method according to the invention provides a step of fractionating the refractory metal powder.

[0048] The fractionation of the material to be ground is preferably carried out by screening, in particular by sieve columns. Alternatively, fractionation may be carried out by using other mechanical classification methods. For this purpose, devices such as gravity sieves, fluidized beds, zigzag sieves, cyclones, centrifuges, spiral sieves or dynamic air sieves, or modifications of such basic principles, can be used.

[0049] If the mechanical processing of the starting refractory metal is carried out in a jet mill, it can be advantageously combined with fractionation by sifting. In this case, an embodiment in which the jet mill has a dynamic air sifter is preferred. Preferably, a classic sifter wheel is used, set at a high rotational speed so that the separation boundary it establishes is finer than the smallest particle size of the particles being processed. In this case, the grinding gas can exit the mill, but the particles remain in the grinding space. Only the very fine fraction is removed from the mill together with the grinding gas flow and can be separated and removed as a by-product in a filter.

[0050] Jet mills are typically operated in continuous mode. However, within the scope of the present invention, it has been found that the jet mill can be advantageously operated in batch mode in this case. The mill can be automatically filled, and the powder can be processed over the specified time period and removed from the device in a separate step. The plant can then be refilled and another process can begin. The discharge of the processed particles from the grinding space can be carried out in various ways. In a preferred embodiment, the grinding material is blown out of the grinding space, together with the grinding gas flow, through a flap or valve that opens after a predetermined time into a separate filter. In an alternative preferred embodiment, the processed refractory metal powder is separated and removed by reducing the revolutions per minute of a sifter and cyclone located between the grinding device and the filter. After the discharge process, the sifter revolutions per minute can be reset to its original high value. This procedure can then be started anew and repeated multiple times until the desired amount is completely processed.

[0051] The method according to the invention offers the advantage that the particle size distribution of the metal powder to be processed can be adjusted as required and according to the intended processing method.

[0052] The refractory metal powder used as the starting powder has a particle size distribution D10 of at least 10 μm. In a preferred embodiment, the starting refractory metal powder has a particle size distribution within the limits of 10 to 250 μm, preferably 10 to 150 μm, as determined by laser diffraction.

[0053] The refractory metal powder according to the invention is provided in particular for use as a material in additive manufacturing. The invention therefore further relates to the use of the refractory metal powder according to the invention in additive manufacturing.

[0054] The advantages of the present invention will be further illustrated by the figures and the following examples, which should not be understood as limiting the concept of the present invention in any way. [Brief explanation of the drawings]

[0055] [Example]

[0056] Tungsten metal powder with a starting particle size distribution D10 of 39.2 μm and D90 of 151 μm was processed as described in Example 1 (ball mill) and Example 2 (jet mill). The resulting powder was passed through the screens listed in Table 1 to separate the individual fractions.

[0057] [Table 1]

[0058] The obtained fractions, F1:<45μm>32μm F2:<63μm>45μm F3:<90μm>63μm F4:<125μm>90μm These were measured using laser diffraction on a Malvern Panalytical Mastersizer 3000 to determine the D10, D50, D90 and D95 values of the particle size distribution, respectively.

[0059] 1. Ball Mill 20 kg of tungsten metal powder was ground in a ball mill (26 cm outer diameter, 45 cm length) with 40 kg of 4 mm carbide balls. The mill revolutions per minute was 40 rpm for a duration of 5 hours. The material was then screened to less than 63 μm. The results are summarized in Table 2.

[0060] [Table 2]

[0061] 2. Jet Mill In the second example, tungsten metal powder was processed in a jet mill. For this purpose, an Alpine AFG 100 jet mill was used. The grinding pressure of the grinding gas (nitrogen at 20 °C) was set at 5 bar. The mill was equipped with two lateral nozzles with 1.9 mm holes and a bottom nozzle with a 3 mm hole. The sifter wheel had a rotation speed of 3000 rpm during rounding, set so that only the very fine fraction formed (approximately 1-2% of the starting amount of fine dust) could leave the grinding chamber. The rounding process was carried out for 1 hour using 7.5 kg of tungsten metal powder. The powder was subsequently fractionated by screening. The results are summarized in Table 3.

[0062] [Table 3]

[0063] Figure 3 shows the frequency distribution as measured by laser diffraction, with the dotted lines indicating the mesh size of the screen used. As can be seen, the particle size distribution of the produced fractions is significantly broader than expected for screening. Thus, each particle size distribution established by laser diffraction extends significantly beyond the mesh size of the coarser screen. For spherical particles with the same volume, each distribution should lie exactly between the dotted lines. This deviation is therefore due to the non-spherical shape of the particles of the powder according to the invention. We can also see the parallel shift of the distribution curve resulting from a series of selected screens (see coefficient a).

[0064] Figures 4a and 4b show the particle size distributions of conventional tungsten metal powders having spherical particles compared to the particle size distribution of tungsten metal powders according to the present invention. Figure 4a shows the particle size distribution of conventional spherical particles fractionated using a fine sieve of 32 μm and a coarse sieve of 45 μm. Figure 4b shows the particle size distribution of non-spherical particles fractionated according to the present invention using a fine sieve of 32 μm and a coarse sieve of 45 μm.

[0065] Figure 5 shows a scanning electron micrograph of a tungsten metal powder according to the present invention, in which the non-spherical shape of the particles is clearly visible.

[0066] FIG. 6 shows the tungsten metal powder prepared according to Example 1 in a ball mill.

[0067] FIG. 7 shows tungsten metal powder prepared according to Example 2 in a jet mill. Within the scope of the sustainability discussion, the issue of energy consumption plays an important role. Table 4 shows a survey of the specific energy consumption of electrical energy of the method according to the invention compared to the melting and spheronization of tungsten metal powder.

[0068] [Table 4]

[0069] The specific energy consumption represents an essential factor in production costs and ecologically sustainable production. The essential drawback of the conventional method of melt spheroidization is clearly seen in its very high energy consumption. In particular, the processing of tungsten, a metal with a very high melting temperature, requires a large amount of energy to produce powder with good flowability, as required in additive manufacturing. In contrast, the method according to the present invention represents a simple and energy-efficient alternative.

Claims

1. Non-spherical refractory metal powder selected from the group consisting of vanadium, chromium, molybdenum, and tungsten, and their alloys, for additive manufacturing of three-dimensional workpieces, wherein the powder consists of non-spherical particles, and each particle is A ｎ It has dimensions, and the dimension A ｎ A non-spherical refractory metal powder characterized in that at least two of the elements are different from each other.

2. The aforementioned powder is processed in the following steps: - A step of screening the refractory metal powder using i screens, wherein i ≥ 2 and the screens have different mesh sizes. ・F(x ｉ ) = i + 1 division obtaining step, where, in the case of F(x ｉ + 1) > x ｉ , the coarse fraction F(x ｉ + 1), in the case of F(x ｉ ) = x ｉ , the medium fraction F(x ｉ ), and in the case of F(x ｉ - 1) < x ｉ , the fine fraction F(x ｉ - 1), and x ｉ represents the mesh size of the screen, and when 1.1 ≤ a ｉ ≤ 1.5, x ｉ＋１ / x ｉ = a ｉ , step - The medium-grain fraction F(x) obtained using laser diffraction ｉ ) Step to determine the particle size distribution It has a particle size distribution determined as follows, The D90 values ​​of the particle size distribution of the fraction F(xi) determined by laser diffraction are, respectively, screen x ｉ＋１ Larger than the mesh size, x ｉ +1 > x ｉ The refractory metal powder according to claim 1.

3. The mesh size x of the screen ｉ A series of screens in which each differs by a certain coefficient, in particular, x ｉ＋１ = a ｉ *x ｉ And in the formula, 1.38 < a ｉ The refractory metal powder according to claim 2, characterized in that a screen having a coefficient of <1.43 is used.

4. The following mesh size x ｉ : x ｉ :32μm+ / -5μm、 x ｉ＋１ :45μm+ / -8μm、 x ｉ＋２ :63μm+ / -12μm、 x ｉ＋３ :90μm+ / -20μm、 x ｉ＋４ :125μm+ / -20μm The refractory metal powder according to claim 2, characterized in that a screen having the same properties is used for fractionation.

5. The refractory metal powder according to claim 1, characterized in that the powder has a particle size distribution corresponding to a log-normal distribution.

6. The refractory metal powder according to claim 1, characterized in that the powder has a fluidity of 9 seconds or less, preferably 6 seconds or less, as determined by ASTM B213.

7. The refractory metal powder according to claim 1, characterized in that the powder has a bulk density of at least 40%, preferably more than 45%, and more preferably more than 50% of the theoretical density of the refractory metal, as determined according to ASTM B329.

8. The refractory metal powder according to claim 1, characterized in that the powder has a tap density of at least 45%, preferably more than 50%, and more preferably more than 56% of the theoretical density of the refractory metal, as determined according to ASTM B527.

9. The refractory metal powder according to claim 1, characterized in that the powder has a sphericity coefficient of less than 0.95, preferably 0.65 to 0.9, as determined by image analysis.

10. A method for preparing refractory metal powder according to at least one of claims 1 to 9, The following steps: i) Providing a starting refractory metal powder selected from the group consisting of vanadium, chromium, molybdenum, and tungsten, and their alloys, having a particle size distribution D10 of at least 10 μm and D90 smaller than 1000 μm, respectively, as determined by laser diffraction; ii) The step of mechanically processing the starting refractory metal powder to obtain the refractory metal powder, Methods that include...

11. The method according to claim 10, characterized in that step b) is carried out in a ball mill, the diameter of the grinding balls is preferably not more than 8 mm, more preferably not more than 4 mm, and / or the weight ratio of refractory metal powder to grinding balls is preferably 1:1 to 1:

5.

12. The method according to claim 10, characterized in that step b) is performed in a jet mill and the pressure of the grinding gas preferably does not exceed 8 bar.

13. The method according to claim 10, characterized in that the starting refractory metal powder has a particle size distribution of 10 to 250 μm, preferably 10 to 150 μm, as determined by laser diffraction.

14. Use of refractory metal powder according to at least one of claims 1 to 9 in additive manufacturing.

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