Granular mixture for additive manufacturing

Abstract

The present invention relates to a granular mixture for additive manufacturing, its use for producing three-dimensional components by additive manufacturing, and a kit for producing three-dimensional components by additive manufacturing.

Description

[0001] Granule mixture for additive manufacturing

[0002] The present invention relates to a hard metal granulate mixture for additive manufacturing, its use for producing three-dimensional components by means of additive manufacturing and a kit for producing three-dimensional components by means of additive manufacturing.

[0003] In the traditional manufacturing process for ceramic and hard metal components, green bodies with densities of up to 55% of the theoretical density are typically first produced, which are then densely sintered under vacuum or a protective atmosphere. The most common forming process for simple components is uniaxial pressing. Cylindrical components can be manufactured by extrusion, while processes such as powder injection molding are suitable for more complex components.

[0004] Although this already makes it possible to provide the component with internal structures such as cooling channels, the production of components with more complex geometries cannot be achieved using conventional methods at all or only with a high degree of rework.

[0005] An alternative to traditional manufacturing is offered by additive manufacturing processes, in which material is applied layer by layer to create three-dimensional workpieces. Compared to conventional abrasive processes, this layer-by-layer additive buildup allows for a high degree of flexibility and design freedom, for example, in the production of prototypes, but also in series production. Additive manufacturing processes offer the possibility of producing the component with its full properties either directly through additive buildup or first creating a green body, which is then sintered into the desired component in further process steps, similar to traditional powder metallurgy processes.

[0006] Due to its similarity to existing cemented carbide production processes, binder jetting has attracted particular attention. In binder jetting, a powdered starting material is applied layer by layer and bonded with a liquid binder at selected points to create the component. However, binder jetting requires flowable powders to create a uniform powder bed. The current state of the art offers various approaches to providing flowable cemented carbide powders that tend to agglomerate, particularly in the small grain size range, for example, through the use of spheroidized granules.

[0007] For example, US 2016 / 0375493 describes a method for producing a component in which a sintered hard metal powder comprising tungsten carbide and a metallic binder phase is provided, the powder is formed into a green body by one or more additive manufacturing methods, and the green body is sintered to obtain a component with a density of more than 90% of the theoretical density, wherein the green body had a density of less than 50% of the theoretical density before sintering.

[0008] US 11,065,863 describes cemented carbide powders for use in additive manufacturing processes, which contain cemented carbide particles with a density of at least 6 g / cm 3 and have a bimodal or multimodal granule size distribution.

[0009] WO 2015 / 162206 describes a process for producing a powder from dense and spherically shaped cermet or cemented carbide granules, the process comprising the steps of: (a) producing spherically shaped granules comprising metal, hard components, and an organic binder, (b) mixing the spherically shaped granules with a sintering inhibitor powder to form a mixture of spherically shaped granules and the sintering inhibitor powder, (c) loading the mixture of spherically shaped granules and sintering inhibitor powder into a furnace chamber, (d) heat-treating the mixture obtained in step (b) in the furnace chamber at a sintering temperature to remove the organic binder from the spherically shaped granules and to sinter the hard components with the metal in each spherically shaped granule, thereby forming a mixture of sintered, dense, spherically shaped granules and the sintering inhibitor powder,(e) discharging the mixture of sintered, dense, spherically shaped granules and the sinter inhibitor powder from the furnace chamber, and (f) separating the sinter inhibitor powder from the sintered, dense, spherically shaped granules, thereby forming a powder of dense and spherically shaped cermet or cemented carbide granules. WO 2017 / 178319 discloses a powder mixture for the three-dimensional printing of a cemented carbide or cermet body, wherein the powder mixture comprises 65-85 wt.% porous cemented carbide and / or cermet granules with an average D50 size of 10-35 μm and 15-35 wt.% dense cemented carbide and / or cermet granules with an average D50 size of 3-10 μm.

[0010] WO 2021 / 072173 describes the production of cemented carbide bodies using powder bed processes, using powders obtained by compacting spherical granules containing tungsten carbide and a metallic binder phase.

[0011] US 2020 / 0346365 describes a powder mixture for use in additive manufacturing methods for components, consisting of sintered hard metal particles that have an at least bimodal particle size distribution, with some of the particles having a D50 value of 25 pm to 50 pm and another part of the particles having a D50 value of less than 10 pm. The bulk density of the powder mixture is 3.5 g / cm 3 up to 8 g / cm 3 .

[0012] US 2005 / 126334 describes hybrid cemented carbide composites consisting of a disperse and continuous cemented carbide phase, where the contiguity ratio of the disperse phase is less than or equal to 0.48.

[0013] CN 107 557 639 describes a cemented carbide with a three-phase structure, which is characterized by the fact that it is composed of three components with different proportions of a binder phase.

[0014] US 2010 / 044115 describes cemented carbide materials that consist of a disperse and a continuous cemented carbide phase and form a hybrid cemented carbide.

[0015] T. Rieger et al. describe in their article "Vat Photopolymerization of Cemented Carbide Specimen", published in Materials 2021, 14, 7631, their investigations on Vat Photopolymerization using photosensitive WC-12 Co (wt%) slips.

[0016] Common granules, such as those obtained by spray drying, generally have a morphology that is too porous to be sufficiently compacted in a subsequent sintering process after additive manufacturing using binder jetting, so that components with a high density are difficult or even impossible to obtain in this way.

[0017] There is therefore still a need for hard metal granules that can be used in additive manufacturing processes to produce dense components, preferably in binder jetting or comparable processes.

[0018] Against this background, it was surprisingly found within the scope of the present invention that this need can be addressed by a granulate mixture based on hard metals.

[0019] Therefore, a first subject matter of the present invention is a granulate mixture for additive manufacturing comprising at least one first granulate A and at least one second granulate B, each comprising at least one hard material, wherein at least one of the granulates A or B further comprises at least one binder metal and wherein the granules each have a different content of hard material.

[0020] Granules in the sense of the present invention are understood to mean an agglomerated solid consisting of a large number of primary grains.

[0021] The present invention is particularly concerned with the production of three-dimensional components made of hard metals. The hard material contained in the granulate mixture according to the invention is accordingly preferably selected from the group consisting of the carbides of the metals Ta, Ti, Nb, Cr, Hf, V, Mo, Zr, and W, as well as mixtures thereof. Tungsten carbide (WC) is particularly preferably used as the hard material.

[0022] The granulate mixture according to the invention enables individual adjustment of the binder metal content, thereby achieving advantageous sintering behavior. Therefore, an embodiment is preferred in which both the at least first granulate A and the at least second granulate B contain a binder metal, wherein the binder metal content in the granules differs. Without being bound to a particular theory, it is assumed that the different contents of hard material and binder metal achieve an inhomogeneous distribution of the binder metal in the granulate mixture, which can be exploited during sintering to close cavities in the component. The granulate mixture according to the invention is characterized in particular by the different hard material contents of the granules of the mixture.In a preferred embodiment, the difference in hard metal content between the granules is at least 1%, preferably at least 5%, particularly preferably at least 10%.

[0023] The binder metal is preferably selected from the group consisting of Cr, Mo, Fe, Co and Ni as well as mixtures and alloys thereof, with Co being particularly preferred as the binder metal.

[0024] The ratio of the granules in the mixture according to the invention can be adjusted as needed, particularly with regard to the desired binder metal content in the granule mixture. A preferred embodiment of the present invention is characterized in that the mass ratio of the at least first granule A to the at least second granule B is 1:99 to 99:1, preferably 10:90 to 90:10, in particular 1:5 to 5:1.

[0025] Preferred embodiments of the present invention include granulate mixtures in which only one of the granules further comprises a binder metal, i.e., one of the granules contains only hard material, as well as granulate mixtures in which both granules contain both hard material and binder metal. Thus, an embodiment is preferred in which the hard material content in at least the first granulate A is 75 to 95 mass %, preferably 80 to 90 mass %, in each case based on the total mass of the granulate A.

[0026] In a further preferred embodiment, the content of hard material in the at least second granulate B is 90 to 100 mass%, preferably 93 to 97 mass%, in each case based on the total mass of the granulate B.

[0027] At least one of the granules of the granulate mixture according to the invention comprises, in addition to the hard material, at least one binder metal. The binder metal content is preferably 5 to 25 mass%, particularly preferably 10 to 20 mass%, based on the total mass of the granules.

[0028] In a particularly preferred embodiment, the granulate mixture according to the invention comprises a first granulate with a hard material content of 75 to 95 mass %, preferably 80 to 90 mass % and a binder metal content of 5 to 25 mass %, preferably 10 to 20 mass %, and a second granulate with a hard material content of 90 to 100 mass %, preferably 93 to 97 mass % and a binder metal content of 0 to 10 mass %, preferably 3 to 7 mass %, wherein the mass fractions each relate to the total mass of the granulate, with the proviso that the hard material content of the individual granules is different in each case. Without being bound to a specific theory, it is assumed that the different contents of hard material and binder metal in the granules lead to an inhomogeneous distribution of the binder metal in the granule mixture, which leads to the movement of the molten metal during sintering, which in turn causes cavities in the component to be closed.

[0029] One prior art approach to improving the flowability of cemented carbide powders is to use powders with a bimodal or multimodal granule size distribution. In contrast, it was surprisingly discovered within the scope of the present invention that advantageous flow and sintering behavior of the powder can also be achieved with a granule size distribution that is as homogeneous as possible. Therefore, an embodiment is preferred in which the size distribution of the granules of the granule mixture according to the invention differs from one another by no more than 35%, preferably no more than 20%, based on the D50 value of the granule size distribution.

[0030] In the context of the present invention, the granule size can be determined, for example, by laser diffraction according to DIN ISO 13320:2020. Unless otherwise stated, the granule size distribution determined by laser diffraction refers to the mass distribution of the granules. The following applies:

[0031] D10: 10% of the powder mass has a granule size smaller than the specified value, or 90% of the powder mass has a granule size larger than the specified value.

[0032] D50: 50% of the mass of the powder has a granule size smaller than the specified value, or 50% of the mass of the powder has a granule size larger than the specified value.

[0033] D90: 90% of the mass of the powder has a granule size smaller than the specified value, or 10% of the mass of the powder has a granule size larger than the specified value.

[0034] In a preferred embodiment, the granulate mixture according to the invention is characterized in that the difference in the D50 values ​​of the hard material grain size distribution in the respective granules is preferably no more than 20%. The grain size distribution can be determined, for example, by chord length analysis according to ISO 4499-2 / 3 or EBSD measurement (electron backscatter diffraction) using a scanning electron microscope.

[0035] In order to achieve high green densities from the outset, it has proven advantageous to use appropriate starting powders. Within the scope of the present invention, it has surprisingly been found that a further improvement in the green density could be achieved by using pretreated granules. Granules with low porosity, such as those obtained by thermal compaction of spray-dried granules, have proven particularly advantageous. Therefore, an embodiment is preferred in which the granules of the granule mixture according to the invention are spray-dried, thermally compacted granules. Thermal compaction is preferably carried out by sintering, microwave, or plasma treatment.

[0036] In a preferred embodiment, the granules of the granule mixture according to the invention are obtained by spray drying, thermal densification and subsequent fractionation, wherein the fractionation is preferably carried out in such a way that granules with a granule size distribution D50 of 10 to 35 pm, preferably 15 to 25 pm, are subsequently obtained and / or with a granule size distribution D90 of 25 to 50 pm, preferably < 50 pm.

[0037] Particularly preferred granules are those having a porosity of 0 to 40 vol.%, preferably 20 to 35 vol.%, whereby the porosity can be determined, for example, by means of gas adsorption measurements according to DIN ISO 9277:2014.

[0038] The granules used in the granulate mixture according to the invention preferably have a BET surface area of ​​0.01 to 1 m 2 / g, particularly preferably 0.1 to 0.5 m 2 / g on.

[0039] Furthermore, an embodiment in which the granulate mixture has a bulk density of 30 to 50% of the theoretical density, determined according to ASTM B329, is preferred; the theoretical density can be taken from corresponding tables. The granulate mixture preferably has a tapped density of at least 35%, preferably greater than 40%, particularly preferably greater than 46% of the theoretical density, determined according to ASTM B527. Within the scope of the present invention, it has been shown that components with high densities can be produced using granulates having such a property profile.

[0040] Granules are generally defined as macroscopic particles composed of many small primary particles, also known as granules. The primary particles are held together by adhesive forces, such as those created by sintering bridges. These adhesive forces can be broken down again; a certain granule strength has proven advantageous for the application of such granules, as this strength can have a positive effect on the properties of the final component. Therefore, an embodiment of the present invention is preferred in which the granules of the granule according to the invention have a compressive strength of more than 400 MPa. This granule strength can be determined, for example, by means of a compression test.

[0041] Further components can be added to the granulate mixture according to the invention as required. Particularly preferred further components are carbides of metals from transition groups 4 to 6 of the Periodic Table. The further components can be granulated or powdered. The further components can be used, for example, to advantageously influence the sintering behavior of the granulate mixture according to the invention.

[0042] The granulate mixture according to the invention was developed especially for use in additive manufacturing techniques. Therefore, a further subject of the present invention is the use of a granulate mixture according to the invention for the production of three-dimensional components using additive manufacturing techniques, preferably using binder jetting or powder bed fusion. It has surprisingly been found that the use according to the invention leads to components with a high density.

[0043] The present invention further provides a method for producing a three-dimensional component using the granulate mixture according to the invention. The method comprises the following steps: a) providing a granulate mixture according to the present invention; b) printing the granulate mixture into a three-dimensional green body; and c) sintering the green body to obtain the three-dimensional component.

[0044] Printing is preferably done using binder jetting.

[0045] Within the scope of the process according to the invention, the granulate mixture according to the invention can be printed together with a binder. Therefore, the process preferably includes a step of debinding the green body prior to sintering.

[0046] A further subject of the present invention is a kit for producing a three-dimensional component by means of additive manufacturing processes comprising at least one first granulate A and at least one second granulate B, each comprising at least one hard material, wherein at least one of the granulates A or B further comprises at least one binder metal and wherein the granules each have a different content of hard material.

[0047] The present invention is described in more detail with reference to the following examples and figures, which are in no way to be understood as a limitation of the inventive concept.

[0048] Examples

[0049] A spray-dried WC / Co powder was sintered at 1000 to 1200 °C and then classified and sieved to achieve a D50 granule size distribution of 20 pm. In this way, two granules, A and B, were produced and processed into a granule mixture according to the invention. The composition is shown in Table 1:

[0050] The granulate mixture was printed into a sample using binder jetting, debindered, and sintered. The resulting sample exhibited a density of more than 99% of its theoretical density. Figure 1 shows an SEM image of a WC / Co granulate mixture according to the invention with an inhomogeneous distribution of the binder metal (dark gray).

[0051] Figure 2 shows a micrograph of a sample produced by sintering or an additive process using the granulate mixture according to the invention. The high density of the sample is clearly visible.

Claims

Patent claims 1. Granule mixture for the additive manufacturing of hard metal components comprising at least one first granulate A and at least one second granulate B, each comprising at least one hard material, wherein at least one of the granulates A or B further comprises at least one binder metal and wherein the granules each have a different content of hard material.

2. Granule mixture according to claim 1, characterized in that the hard material contained in the granulate mixture is selected from the group consisting of the carbides of the metals Ta, Ti, Nb, Cr, Hf, V, Mo, Zr and W and mixtures thereof.

3. Granule mixture according to at least one of the preceding claims, characterized in that the at least first granulate A and the at least second granulate B comprise a binder metal, wherein the content of binder metal in the granules is different.

4. Granule mixture according to at least one of the preceding claims, characterized in that the binder metal is selected from the group consisting of Cr, Mo, Fe, Co and Ni and mixtures and alloys thereof.

5. Granule mixture according to at least one of the preceding claims, characterized in that the content of hard material in the at least first granulate A is 75 to 95 mass%, preferably 80 to 90 mass%, in each case based on the total mass of the granulate A.

6. Granule mixture according to at least one of the preceding claims, characterized in that the content of hard material in the at least second granulate B is 90 to 100 mass%, preferably 93 to 97 mass%, in each case based on the total mass of the granulate B.

7. Granule mixture according to at least one of the preceding claims, characterized in that the granule size distributions differ from each other by not more than 35%, preferably not more than 20%, based on the D50 value of the granule size distributions, determined according to DIN ISO 13320:2020. Granule mixture according to at least one of the preceding claims, characterized in that the granules are spray-dried, thermally compacted granules. Granule mixture according to at least one of the preceding claims, characterized in that the granules have a porosity of 0 to 40 vol.%, preferably 20 to 35 vol.%. Granule mixture according to at least one of the preceding claims, characterized in that the granules have a BET surface area of ​​0.01 to 1 m 2 / g, particularly preferably 0.1 to 0.5 m 2 / g, determined according to DIN ISO 9277:2014. Granule mixture according to at least one of the preceding claims, characterized in that the granulate mixture has a bulk density of 30 to 50% of the theoretical density, determined according to ASTM B329. Granule mixture according to at least one of the preceding claims, characterized in that the granulate mixture has a tap density of at least 35%, preferably greater than 40%, particularly preferably greater than 46% of the theoretical density, determined according to ASTM B527. Use of a granulate mixture according to at least one of claims 1 to 12 for the production of three-dimensional components by means of additive manufacturing techniques, preferably by means of binder jetting.A method for producing a three-dimensional component, comprising the following steps: a) providing a granulate mixture according to at least one of claims 1 to 12; b) printing the granulate mixture to form a three-dimensional green body; and c) sintering the green body to obtain the three-dimensional component. A kit for producing a three-dimensional component using an additive manufacturing process, comprising at least one first granulate A and. at least one second granulate B, each comprising at least one hard material, wherein at least one of the granulates A or B further comprises at least one binder metal and wherein the granules each have a different content of hard material.

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