Refractory metal powder for additive manufacturing and method for the production thereof

Abstract

The present invention relates to a refractory metal powder for additive manufacturing, a method for its production, and to its use in additive manufacturing.

Description

[0001] Refractory metal powders for additive manufacturing and processes for their production

[0002] The present invention relates to a refractory metal powder for additive manufacturing, a process for its production and its use in additive manufacturing.

[0003] TECHNICAL BACKGROUND

[0004] Additive manufacturing encompasses all manufacturing processes in which a component—in contrast to subtractive processes—is automatically constructed by adding volume elements or layers directly from digital 3D data, or by applying additional volume elements to an existing workpiece, thus creating three-dimensional objects. The layer-by-layer construction is usually computer-controlled from one or more liquid or solid materials. The construction of the three-dimensional object is achieved through various physical and chemical curing and melting processes, allowing the manufacture of a specific product without special tools that store the specific geometry of the product.

[0005] While the processing of plastics, synthetic resins, and ceramics for the production of complex models, samples, and prototypes is widely used in industry and research, the number of suitable metals remains limited. This is due, among other things, to the fact that the metals must meet certain properties to be used in additive manufacturing processes. The metal can be in solid form, such as rod or wire, or in powder form.

[0006] All manufacturing processes that start with a metal powder place a number of requirements on the metal being processed. These include, among other things, good flowability and a high bulk and tap density in order to achieve a uniform and defined application during the manufacturing process and to produce a dense component.

[0007] Metal powders that meet the aforementioned requirements typically exhibit high sphericity, which can currently only be achieved through energy-intensive melting and atomization processes. WO 2020 / 220143 describes a metal powder produced by plasma treatment for additive manufacturing processes that comprises spherical particles with a particle size distribution of 0 to 1000 pm and exhibits a flowability of up to 20 s.

[0008] US 2022 / 0023941 discloses a metal powder for 3D printing 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 from 0.7 to 1, with = ,XFerret min / XFerret max.

[0009] DE 10 2019 130 941 relates to a method for preparing a metallic powder for additive manufacturing, comprising the following steps: a) at least two metallic powders differing in their physical properties are provided, wherein at least one powder is residual powder from additive manufacturing and at least one further powder is new powder; b) the at least two powders are dosed and then mixed to form a powder mixture;c) the powder mixture is subsequently dried and then separated into fractions, wherein in a first step a very fine fraction is separated by sifting and from the residual fraction a coarse grain fraction is separated from a remaining fraction by sieving or a coarse grain fraction and at least one fine grain fraction are separated by sieving in a sieving device, wherein the remaining fraction is the processed metallic powder for additive manufacturing, wherein the processed powder has been homogenised by the preceding process steps and can be used further.;

[0010] In their article "Manipulation and characterization of novel titanium powder precursor for additive manufacturing applications", published in The Minerals, Metals & Materials Society, 2015, Vol. 67, No. 3, YY Sun et al. describe spherical Ti powders 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 morphology.

[0011] TECHNICAL PROBLEM

[0012] Melting metals with very high melting points, such as those typical for refractory metals, results in very high specific energy requirements and, consequently, high manufacturing costs. Although the state of the art already offers some approaches for lower-melting refractory metals such as titanium, niobium, and tantalum, there is still a lack of satisfactory solutions for higher-melting refractory metals. Among these, tungsten has the highest melting point and is also characterized by its unusual strength, which makes it difficult to machine but simultaneously an attractive material.

[0013] Against this background, the present invention seeks to provide a refractory metal powder, in particular tungsten, that is suitable for use in additive manufacturing. Furthermore, the present invention seeks to provide a more sustainable process for producing such powders than is known in the prior art.

[0014] TECHNICAL SOLUTION

[0015] Within the scope of the present invention, it was surprisingly discovered that powders that do not exhibit the sphericity proclaimed to be essential in the prior art can be used in additive manufacturing processes. Thus, it has surprisingly been shown that powders with particles whose shape deviates from the ideal spherical shape typically desired are also suitable for additive manufacturing.

[0016] GENERAL REMARKS

[0017] Unless otherwise stated, the particle size distribution determined by laser diffraction refers to the mass distribution of the particles. The following applies:

[0018] DIO: 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.

[0019] D50: 50% of the mass of the powder has a particle size smaller than the specified value, or 50% of the mass of the powder has a particle size larger than the specified value.

[0020] D90: 90% of the powder mass has a particle size smaller than the specified value, or 10% of the powder mass has a particle size larger than the specified value. In the context of this application, the terms grain and particle, as well as grain size and particle size, are used synonymously.

[0021] Non-spherical particles in the sense of the present invention are understood to mean in particular those which have a cylindrical or ellipsoidal shape.

[0022] For the purposes of the present invention, a fraction is understood to mean the separated portion of a powder obtained by a separation process, whereby at least two fractions are formed during a separation process. The separation can be based on a characteristic feature such as mass, density, size, color, or shape.

[0023] DETAILED DESCRIPTION OF THE INVENTION

[0024] A first aspect of the present invention is a non-spherical refractory metal powder selected from the group consisting of vanadium, chromium, molybdenum, and tungsten, as well as alloys thereof, for the additive manufacturing of three-dimensional components. The powder consists of non-spherical particles, each particle having dimensions An, wherein at least two of the dimensions An are different from each other. The dimensions of the particles can be determined, for example, using optical methods on SEM images.

[0025] For the purposes of the present invention, extension is understood to mean any distance between two points on the surface of the particle whose connecting line passes 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 extensions can be determined for all particles of the refractory metal powder according to the present invention, of which at least two, preferably at least 3, particularly preferably at least 4, are different from one another. While for a spherical particle Ai = A2, the powder according to the invention is characterized in particular by having at least two distances for which Ai * l 2 applies. For further clarification, reference is made to Figure 1.In a particularly preferred embodiment of the refractory metal powder according to the invention, the minimum expansion Amin and the maximum expansion Amax of a particle differ by the factor f(A), where f(A) assumes a value of >1.001 to 5. In a further preferred embodiment, the particles have a ratio of maximum expansion Amax to minimum expansion Amin (Amax / Amin) of <5.

[0026] In the context of the present invention, it was surprisingly found that powders consisting of non-spherical particles are characterized by a special distribution function of their grain size, since the model of equal-volume spheres usually used for grain size distribution does not apply here.

[0027] In a preferred embodiment, the refractory metal powder of the present invention is characterized by a grain size distribution determined as follows:

[0028] • sieving the refractory metal powder using n sieves, where n is >2, and where the sieves have a different mesh size;

[0029] • Obtaining F(xi) = n + 1 fractions, with a coarse grain fraction F(xi+ 1) with F(xi+ 1) > xi; a medium grain fraction F(xi) with F(xi) = Xi and a fine grain fraction F(xi-l) with F(xi-l) < Xi, where Xi is the mesh size of the sieve and where Xi+i / xi = ai with 1.1 < ai <1.5;

[0030] • Determining the grain size distribution of the obtained medium grain fraction F(xi) by means of laser diffraction, whereby the D90 value of the grain size distribution of the fraction F(xi) determined by means of laser diffraction is in each case larger than the mesh size of the sieve Xi+1, whereby Xi+ 1 > Xi .

[0031] Sieving can be carried out according to DIN 66165, whereby, for example, sieve towers with standardized test sieves as described in DIN ISO 3310 or ASTM B214 can be used.

[0032] The determination by laser diffraction can be carried out according to DIN ISO 13320.

[0033] In a preferred embodiment, ai is substantially the same for all ratios xi+i / xi, the deviation between the values ​​preferably being less than 5%, particularly preferably less than 3%.

[0034] While for spherical particles it is irrelevant in which position a particle falls through the mesh of the sieve, this is not the case for non-spherical particles. Here, the longitudinal expansion is greater than the mesh size of the sieve. Without being bound to a specific theory, it is assumed that this effect leads to the distribution of the fraction F(xi) for non-spherical particles having a proportion that is coarser than the mesh size of the coarser sieve Xi+i. The particle size distribution of the fractions is determined using laser diffraction.

[0035] The sieving of the powder according to the invention in the context of determining its particle size distribution is preferably carried out using sieves of a series in which the sieve meshes Xi of the sieves differ by a constant factor, in particular with sieves for which xi+i = ai*xi applies; where 1.38 <ai <1,43. Insbesondere für Pulver, die auf die Verwendung in der additiven Fertigung ausgelegt sind, haben sich Siebe mit einer Maschenweite bis zu 125 |jm als besonders geeignet erwiesen. Daher ist eine Ausführungsform bevorzugt, in der für die Siebung Siebe mit folgenden Maschenweiten Xi verwendet werden: xi: 32 pm (+ / - 5 |jm);

[0036] Xi+i: 45 |jm (+ / - 5 |jm); preferably with ai = 1.40

[0037] Xi+2: 63 |jm (+ / - 8 |jm); preferably with a2 = 1.40

[0038] Xi+3: 90 |jm (+ / - 12 pm); preferably with as = 1.42

[0039] Xi+4: 125 pm (+ / - 20 pm); preferably with a4 = 1.38

[0040] The values ​​given in brackets are tolerances and describe the permissible deviations from the nominal mesh size.

[0041] The particle size distribution of spherical particles generally follows a Gaussian normal distribution. In contrast, it was surprisingly found that the particle size distribution of the refractory metal powder according to the invention deviates from this distribution. Therefore, an embodiment in which the powder has a grain distribution corresponding to a logarithmic normal distribution is preferred.

[0042] The present invention relates to refractory metals that have proven highly corrosion-resistant due to their passivation. The refractory metal powder according to the invention is selected from the group consisting of vanadium, chromium, molybdenum, and tungsten, as well as alloys thereof. The refractory metal is particularly preferably tungsten and / or a tungsten alloy. The refractory metal is most preferably tungsten, which is preferably obtained by reduction from tungsten oxide.

[0043] The refractory metal powder according to the invention is particularly tailored to the requirements of additive manufacturing, with flow rates of no more than 9 s having proven suitable, particularly in powder printing processes. Within the scope of the present invention, it was surprisingly found that this requirement is also met by the refractory metal powder according to the invention, despite its non-spherical shape, which until now was considered rather preclusive for use in additive manufacturing. Therefore, an embodiment is preferred in which the refractory metal powder of the present invention has a flow rate of no more than 9 s, preferably no more than 6 s, as determined by ASTM B213.

[0044] Other properties that determine the suitability of a metal powder for additive manufacturing are bulk density and tap density. Surprisingly, these properties were also achieved by the refractory metal powder according to the invention, which was unexpected due to the non-spherical shape of its particles, which deviates from the normally preferred spherical particle shape.

[0045] In a preferred embodiment, the refractory metal powder according to the invention has a bulk density of at least 40%, preferably greater than 45%, particularly preferably greater than 50% of the theoretical density of the refractory metal, determined according to ASTM B329.

[0046] Furthermore, the refractory metal powder preferably has a tap density of at least 45%, preferably greater than 50%, and particularly preferably greater than 56% of the theoretical density of the refractory metal, determined according to ASTM B527. The powder according to the invention thus has both a bulk density and a tap density comparable to those of conventional powders with spherical particles.

[0047] The bulk and tapped densities are given in reference to the theoretical density of the refractory metal. For the example case of tungsten metal, a density of 19.25 g / cm 3 as a 100% reference. The theoretical densities of refractory metals are known to those skilled in the art and can be found in relevant reference works.

[0048] The powder according to the invention differs from those of the prior art in particular in the non-spherical shape of its particles. The sphericity of a body is defined as the ratio of the surface area of ​​a sphere of equal volume to the surface area of ​​the body, with a sphericity of 1 describing a sphere. Therefore, an embodiment is preferred in which the refractory metal powder has a sphericity factor of <0.95, preferably from 0.65 to 0.9. The sphericity can be determined statically according to ISO 13322-1 or dynamically according to ISO 13322-2. Refractory metal powders that have a particle shape of "sub-rounded" or "rounded" according to the classification by Power in: Power, MC, Journal of Sedimentary Petrology, 1953, Volume 23, page 118 (see Figure 2).

[0049] Refractory metal powders exhibit a natural oxide passivation on their surface, which makes them relatively corrosion-resistant, but which can lead to defects in the component produced by additive manufacturing due to the foreign elements introduced. Therefore, a preferred embodiment is one in which the refractory metal powder has an oxygen content of a maximum of 1000 ppm, preferably a maximum of 500 ppm, as determined using the LECO method.

[0050] Refractory metal powders for additive manufacturing, which are characterized by high sphericity, as described in the prior art, have the disadvantage that they can only be produced by plasma spherodization or other energy-intensive atomization processes, in which the metal is melted or partially melted and sprayed, which is associated with considerable energy expenditure, especially given the high melting points of the refractory metals. In contrast, it was surprisingly found that the powder according to the invention can be produced using simple mechanical processes. These processes have the particular advantage that the metal does not have to be heated above its melting point.Therefore, a further subject of the present invention is a process for producing the powder according to the invention, which comprises the following steps: i) providing a starting refractory metal powder selected from the group consisting of vanadium, chromium, molybdenum and tungsten, as well as alloys thereof, with a grain size distribution DIO of at least 10 pm and D90 of less than 1000 pm, determined by laser diffraction; ii) mechanically treating the starting refractory metal powder to obtain the refractory metal powder according to the invention.

[0051] The general requirement profile for metal powder in additive manufacturing stipulates a minimum grain size of 10 pm. It has proven advantageous for the starting powder to already have a corresponding grain size distribution, so that a first step of the process according to the invention is the provision of a starting refractory metal powder with a grain size distribution D10 of at least 10 pm, with the D90 being less than 1000 pm.

[0052] In addition to the specified grain size distribution, the particles of the starting refractory metal powder preferably have a compact, non-porous structure. In a preferred embodiment, the starting refractory metal powder therefore preferably has a density of at least 80% of the theoretical density, particularly preferably at least 90%, and especially at least 95% of the theoretical density of the refractory metal. The theoretical densities of the materials are known to those skilled in the art and can be found in corresponding tables.

[0053] In the process according to the invention, a starting refractory metal powder is subjected to a mechanical treatment. This mechanical treatment surprisingly allows the powder to acquire the properties necessary for use in additive manufacturing, such as bulk density and tap density, with comparatively low energy expenditure compared to melt spherodization. Without being bound to a particular theory, it is assumed that the mechanical treatment of the starting powder smoothes the surfaces of the particles, thereby achieving the advantageous properties. Therefore, the mechanical treatment is preferably selected so that it does not result in particle comminution.

[0054] In this context, mechanical treatment in a ball mill or jet mill has proven particularly advantageous. Therefore, an embodiment of the process according to the invention is preferred in which the mechanical treatment of the starting refractory metal powder is carried out in a ball mill. In order to limit the formation of platelets due to compression of the particles, it has proven advantageous to select the size and number of grinding balls accordingly. Therefore, an embodiment is preferred in which the size of the grinding balls is at most 8 mm, preferably 4 mm or less, where this specification refers to the diameter of the grinding balls. Furthermore, the mass of the grinding balls should be proportional to the amount of powder to be treated. In this context, an embodiment of the process according to the invention is preferred in which the weights of starting refractory metal powder and the grinding balls are in a ratio of 1:1 to 1:5.Both measures serve to limit the energy input and thus obtain powder particles in a shape suitable for additive manufacturing. The grinding balls can be made of iron-based materials, steel, Fe-Co, Ni alloys, ceramic or hard metal. The mechanical treatment in the ball mill is preferably carried out for a period of 3 to 7 hours, preferably 4 to 6 and in particular 4.5 to 5.5 hours. After this time, the powder particles have a suitable shape to enable the powder to be used for additive manufacturing. The speed of the ball mill during the mechanical treatment is preferably selected so that it is lower than the critical speed. The critical speed of the ball mill depends on its diameter and is calculated as follows.

[0055] Alternatively, the mechanical treatment can also 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.

[0056] According to the principle of a jet mill, particles are ground in a gas stream without the use of mechanical tools such as high-speed rotors. The grinding gas is introduced into a grinding chamber through nozzles and accelerated to such an extent that the individual particles are entrained and collide with one another. Since the particle treatment is primarily autogenous, this method is particularly suitable for comminuting very hard materials. It has been surprisingly found that conventional jet mills can be used within the scope of the present invention to produce suitable particles for additive manufacturing.

[0057] According to the invention, the mechanical treatment preferably takes place at a grinding pressure of less than 8 bar. Preferably, a grinding gas with a grinding gas temperature of 10 to 40 °C is used, where the temperature refers to the gas under grinding pressure. Surprisingly, it has been found that the use of a jet mill can shorten the grinding time compared to a ball mill. Therefore, an embodiment in which the mechanical treatment is carried out for a period of 0.5 to 3 hours, preferably 0.5 to 1.5 hours, is preferred.

[0058] By means of the mechanical treatment of the powder provided according to the invention, refractory metal powders can be obtained which have a number of the properties required for use in additive manufacturing, in particular with regard to bulk and tap density.

[0059] In a preferred embodiment, the resulting refractory metal powder can be further subjected to classification, for example, to obtain desired particle size ranges. Therefore, in a preferred embodiment, the process according to the present invention provides a step of fractionating the refractory metal powder.

[0060] The fractionation of the ground material is preferably carried out by screening, especially using a sieve tower. Alternatively, fractionation can also be carried out using other mechanical classification methods. For this purpose, equipment such as gravity classifiers, fluidized beds, zigzag classifiers, cyclones, centrifugal separators, spiral classifiers, or dynamic air classifiers, or modifications of these basic principles, can be used.

[0061] If the mechanical treatment of the starting refractory metal is carried out in a jet mill, this can advantageously be combined with fractionation by classifying. A preferred embodiment is one in which the jet mill has a dynamic air classifier. Conventional classifier wheels are preferably used, the speed of which is set so high that the resulting cutoff is finer than the smallest grain size of the particles to be treated. In this case, the grinding gas can leave the mill, but the particles remain in the grinding chamber. Only the very fines are removed from the mill with the grinding gas stream and can be separated as a by-product in a filter.

[0062] A jet mill is generally operated continuously. However, within the scope of the present invention, it has been shown that the jet mill advantageously operates in batch mode. The mill can be filled automatically, and the powder treated for the aforementioned time and then removed from the apparatus in a subsequent step. The system can then be refilled, and further treatment can begin. The treated particles can be discharged from the grinding chamber in various ways. In a preferred embodiment, the material to be ground is blown from the grinding chamber into a separate filter with the grinding gas stream via a flap or valve that opens after a predetermined time. In an alternative, preferred embodiment, the treated refractory metal powder is separated by reducing the classifier speed and using a cyclone located between the grinding apparatus and the filter.After the discharge process, the classifier speed can be reset to its original high value. The process can then be started again and repeated until the desired amount has been completely processed.

[0063] The process according to the invention offers the advantage that the particle size distribution of the metal powder to be processed can be adjusted depending on requirements and the intended processing method.

[0064] The refractory metal powder used as the starting powder has a grain size distribution DIO of at least 10 pm. In a preferred embodiment, the starting refractory metal powder has a grain size distribution in the range of 10 to 250 pm, preferably 10 to 150 pm, determined by laser diffraction.

[0065] The refractory metal powder according to the invention is particularly intended for use as a material in additive manufacturing. Therefore, a further subject of the present invention is the use of a refractory metal powder according to the present invention in additive manufacturing. The advantages of the present invention will be further illustrated by the figures and the following examples, which are in no way to be understood as limiting the inventive concept.

[0066] EXAMPLES / FIGURES

[0067] A tungsten metal powder with an initial particle size distribution DIO of 39.2 pm and D90 of 151 pm was processed as described in Examples 1 (ball mill) and 2 (jet mill).

[0068] The powder obtained was divided into individual fractions using the sieves listed in Table 1.

[0069] Table 1 :

[0070] They receive fractions

[0071] Fri: < 45 pm > 32 pm

[0072] F2 < 63 pm > 45 pm

[0073] Fri < 90 pm > 63 pm

[0074] F4 < 125 pm > 90 pm were measured to determine their DIO, D50, D90, and D95 ​​values ​​of the grain size distribution using laser diffraction on a Malvern Panalytical Mastersizer 3000. 1. Ball mill

[0075] In a ball mill (outer diameter = 26 cm and length = 45 cm) with 40 kg of 4 mm carbide balls, 20 kg of tungsten metal powder were ground. The mill speed was 40 rpm for a duration of 5 hours. The material was then sieved to a particle size of < 63 μm. The results are summarized in Table 2.

[0076] Table 2:

[0077] 2. Jet mill

[0078] In a second example, the tungsten metal powder was processed in a jet mill. A jet mill from Alpine, type AFG 100, was used. The grinding pressure of the grinding gas (nitrogen (20°C)) was set to 5 bar. The mill was equipped with two side nozzles with a 1.9 mm bore and a bottom nozzle with a 3 mm bore. The classifier wheel had a speed of 3000 rpm during rounding and was adjusted so that only the resulting fines (fine dust approximately 1 to 2% based on the input quantity) could leave the grinding chamber. The rounding process took place with 7.5 kg of tungsten metal powder over a period of 1 hour. The powder was then fractionated by sieving. The results are summarized in Table 3.

[0079] Table 3:

[0080]

[0081] Figure 3 shows the frequency distribution measured by laser diffraction, with the dashed lines indicating the mesh size of the sieves used. As can be seen, the grain size distribution of the fractions produced is significantly broader than would be expected from sieving. Thus, the grain size distribution determined by laser diffraction extends significantly beyond the mesh size of the coarser sieve. In the case of particles in the form of spheres with equal volume, the distribution should lie exactly between the dashed lines. The deviation is therefore attributed to the non-spherical shape of the particles of the powder according to the invention. Also evident is the parallel shift of the distribution curves, which results from the series of sieves selected (compare factor a).

[0082] Figures 4a and 4b show the grain size distribution of a conventional tungsten metal powder with spherical particles compared to the grain size distribution of a tungsten metal powder according to the invention. Figure 4a shows the grain size distribution of fractionated, conventional, spherical particles through a fine sieve of 32 pm; a coarse sieve of 45 pm. Figure 4b shows the grain size distribution of fractionated non-spherical particles according to the present invention through a fine sieve of 32 pm; a coarse sieve of 45 pm.

[0083] Figure 5 shows an SEM image of a tungsten metal powder according to the invention. The non-spherical shape of the particles is clearly visible.

[0084] Figure 6 shows a tungsten metal powder produced in a ball mill according to Example 1. Figure 7 shows a tungsten metal powder produced in a jet mill according to Example 2.

[0085] In the context of the discussion of sustainability, the issue of energy consumption plays a crucial role. Table 4 provides an overview of the specific electrical energy requirements for the process according to the invention compared to melt spherodization for tungsten metal powder.

[0086] Table 4:

[0087] Specific energy consumption represents a significant factor in manufacturing costs and ecologically sustainable production. The very high energy consumption clearly demonstrates the major disadvantage of the conventional melt spherodization process. Especially when processing tungsten, a metal with a very high melting temperature, large amounts of energy are required to produce the free-flowing powders required in additive manufacturing. In contrast, the process according to the invention presents a simple and energy-efficient alternative.

Claims

Patent claims:

1. Non-spherical refractory metal powder selected from the group consisting of vanadium, chromium, molybdenum and tungsten and alloys thereof for the additive manufacturing of three-dimensional components, characterized in that the powder consists of non-spherical particles, each particle having An dimensions, at least 2 of the dimensions An being different from one another.

2. Refractory metal powder according to claim 1, characterized in that the powder has a grain size distribution determined as follows: • sieving the refractory metal powder using i sieves, where i is >2, and where the sieves have a different mesh size; • Obtaining F(xi) = i + 1 fractions, with a coarse grain fraction F(xi+ 1) with F(xi+ 1) > xi; a medium grain fraction F(xi) with F(xi) = Xi and a fine grain fraction F(xi-l) with F(xi-l) < Xi, where Xi is the mesh size of the sieve and where Xi+i / xi = ai with 1.1 < ai <1.5; • Determining the grain size distribution of the obtained medium grain fraction F(xi) by means of laser diffraction, whereby the D90 value of the grain size distribution of the fraction F(xi) determined by means of laser diffraction is in each case larger than the mesh size of the sieve Xi+1, whereby Xi+1 > Xi.

3. Refractory metal powder according to claim 2, characterized in that sieves of a series in which the sieve meshes Xi of the sieves differ by a constant factor, in particular with sieves for which Xi+1 = ai*xi; where 1.38 <ai <1,43, verwendet werden.

4. Refractory metal powder according to at least one of claims 2 or 3, characterized in that sieves with the following mesh sizes xi are used for the fractionation: xi: 32 pm + / - 5 pm; Xi+i: 45 pm + / - 8 pm; Xi+2: 63 pm + / - 12 pm; Xi+3: 90 |jm + / - 20 m; Xi+4: 125 μm + / - 20 μm. Refractory metal powder according to at least one of the preceding claims, characterized in that the powder has a grain size distribution corresponding to a log-normal distribution. Refractory metal powder according to at least one of the preceding claims, characterized in that the powder has a flowability of no more than 9 s, preferably no more than 6 s, determined by ASTM B213. Refractory metal powder according to at least one of the preceding claims, characterized in that the powder has a bulk density of at least 40%, preferably greater than 45%, particularly preferably greater than 50% of the theoretical density of the refractory metal, determined according to ASTM B329.Refractory metal powder according to at least one of the preceding claims, characterized in that the powder has a tap density of at least 45%, preferably greater than 50%, particularly preferably greater than 56% of the theoretical density of the refractory metal, determined according to ASTM B527. Refractory metal powder according to at least one of the preceding claims, characterized in that the powder has a sphericity factor of <0.95, preferably 0.65 to 0.9, determined by image analysis. Method for producing a refractory metal powder according to at least one of claims 1 to 9, 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 with a grain size distribution D10 of at least 10 pm and D90 of less than 1000 pm, each determined by laser diffraction;. ii) mechanically treating the starting refractory metal powder to obtain the refractory metal powder. Process according to claim 10, characterized in that step b) is carried out in a ball mill, wherein the diameter of the grinding balls preferably does not exceed 8 mm at most, particularly preferably 4 mm and / or the weight ratio of refractory metal powder to grinding ball is preferably 1:1 to 1:

5. Process according to claim 10, characterized in that step b) is carried out in a jet mill, wherein the pressure of the grinding gas preferably does not exceed 8 bar. Process according to at least one of claims 10 to 12, characterized in that the starting refractory metal powder has a grain size distribution of 10 to 250 pm, preferably 10 to 150 pm, determined by laser diffraction. Use of a refractory metal powder according to at least one of claims 1 to 9 in additive manufacturing.< / ai>

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