Process for the production of hard metal bodies

Abstract

Method for producing hard metal bodies that have a hard metal microstructure at room temperature, which is free of tungsten dicarbide and eta phase, in which LPBF, SLM, DMLS or SLS as a laser-based additive manufacturing process with energy input via laser powers of 20 W to 55 W and scan speeds of 20 mm / s to 75 mm / s and track spacings of 30 µm to 205 µm and layer thicknesses of 20 µm to 45 µm at each point of energy input to achieve a temperature of 800 °C to a maximum of < 1800 °C from hard metal granules as starting materials made of WC as a ceramic hard material with Co as a metallic binder phase and an additive of Cr3C2 and the hard metal granules have a porosity of > 0 vol.% to 40 vol.%.-%, a hard metal green body with a density of at least 50% and at most 70% of the theoretical density of the hard metal body is produced, and which is subsequently subjected to sintering at temperatures up to a maximum of 1600 °C, by means of vacuum sintering at temperatures of 1200 to 1600 °C and at partial pressures of 100 to 90000 Pa or gas pressure sintering at temperatures of 1380 to 1600 °C and pressures of 5 to 10 MPa until a density of the hard metal body of ≥ 98% to 99.9% of the theoretical density.

Description

[0001] The invention relates to the fields of hard metal materials and ceramic and / or powder metallurgical process engineering and concerns a method for the production of hard metal bodies, such as can be used for the production of wear parts or tools with hard metals.

[0002] The production of cemented carbide bodies, which in their green state contain cemented carbide starting powders in addition to organic binders, by means of pressing, extrusion, or MIM / CIM followed by sintering, is known in the art. Cemented carbide components with various compositions can be produced in this way. Binder metal contents (e.g., cobalt, iron, and / or nickel) of 0 to ≤ 32 vol.% are achievable.

[0003] These known manufacturing processes have limitations regarding the geometry of the components to be produced, which cannot be eliminated with these technologies.

[0004] For design freedom in the production of complex hard metal components, the use of additive manufacturing processes is necessary. In such manufacturing processes, the components are produced according to a 3D model generated by a computer, in which the 3D model is essentially sliced ​​into thin sheets, and then the component is manufactured sheet by sheet.

[0005] One such additive manufacturing process is laser sintering, in which a higher strength of the green body can be achieved with a local direct energy input (Y. Xiong et al: Powder Metallurgy Vol. 53, Iss. 1, 2010; T. Gläser, Investigations on laser sintering of tungsten carbide-cobalt, Dissertation 2010; Generative manufacturing of extrusion tools made of hard metal-GENIAL (BMBF)).

[0006] Also well-known is the additive manufacturing of tungsten carbide-cobalt cemented carbides using the Laser Powder Bed Fusion (LPBF) process, which enables the creation of highly customizable shapes and the integration of functional properties into cemented carbide tools. This process also allows for significant resource and production time savings. In this method, cemented carbide powder is introduced into a fixture in the form of a powder bed, and the desired areas of the powder are compacted by very short exposure times of a laser beam. During these brief exposure times, a liquid phase forms in the area of ​​the laser beam due to local melting, resulting in significantly higher sintering temperatures.The laser beam is guided by computer using a complete 3-dimensional description of the finished workpiece beforehand, so that only the areas of the later finished workpiece are melted and densified (T. Schubert et al: 35th Hagen Symposium Powder Metallurgy, H. Kolaska, H. Danninger, D. Biermann (Eds.), Heimdall Verlag, Dortmund, 163 - 176, (2016).).

[0007] Pre-sintered and partially compacted hard metal granules can be used as hard metal powder (Faisal, NH et al: J. Therm. Spray Tech. (2011) 20, 1071; SM Nahvi et al: Surface and Coatings Techn., (2016) 286, 95-102; G. Bolelli et al: Surface and Coatings Techn. (2012) 206, 4079-4094).

[0008] Regarding the microstructure formation in the production of hard metals, for example from WC-Co, conventional production ideally results in a hard metal microstructure consisting of WC grains in a cobalt-rich matrix with dissolved tungsten and carbon.

[0009] Since higher temperatures are reached locally during the LPBF process, it must be noted that WC decomposes at temperatures > 2735 °C.

[0010] Likewise, the process must counteract evaporation of the liquid phase and decarburization by higher cobalt contents in order to prevent embrittlement of the hard metal structure due to the formation of the η-phase (T. Schubert et al: 35th Hagen Symposium Powder Metallurgy, H. Kolaska, H. Danninger, D. Biermann (Eds.), Heimdall Verlag, Dortmund, 163 - 176, (2016).).

[0011] It is also known from other publications that WC-Co hard metals are very difficult to produce using laser-based additive manufacturing processes such as Selective Laser Melting (SLM), and only with very high laser powers and high Co contents with sufficient densification. This, in turn, leads to WC decomposition, which is undesirable (E. Uhlmann et al: Procedia CIRP 35 (2015) 8-15; T. Gläser, Dissertation RTH Aachen, 2010, Summary).

[0012] Furthermore, according to Gläser, T. et al: Genial. Joint results report on the BMBF joint project, funding reference 02PU2220, September 1, 2009, page 2 (http: / / publica.fraunhofer.de / documents / N-162064.html), the production of hard metal bodies using laser sintering or 3D printing is known.

[0013] From US 2016 / 0 375 493 A1, a process for the production of sintered products is known in which a hard metal body is produced by means of isostatic pressing, isostatic hot pressing, binder jetting, vacuum sintering or sintering under a hydrogen or argon atmosphere.

[0014] According to AT 015 102 U1, a method for producing a hard metal body layer by layer by alternately applying a hard metal powder layer by layer and selectively, locally hardening the applied hard metal powder by the action of a directed energy beam, wherein the hard metal powder has particles with a sintered WC binder metal structure.

[0015] Also known according to DE 10 2015 203 873 A1 is a 3D printing process and a powder mixture for 3D printing. In the 3D printing process, a sintering component is mixed with a surface coating agent and produced by laser sintering or laser melting in a selective laser sintering or laser melting process to a powder mixture that is used for the 3D printing process.

[0016] According to Pötschke, J. et al, Congress & Exhibition. Proceedings, 1-5 October 2017, Milan. ISBN 978-1-899072-49-1. http: / / publica.fraunhofer.de / eprints / urn_nbn_de_0011-n-5378546.pdf, additive manufacturing processes for hard metals are known, with the investigations concerning a powder-based 3D binder jetting and a selective laser sintering and also a suspension-based thermoplastic 3D printing process.

[0017] A disadvantage of known laser-based additive manufacturing processes is the insufficient densification, resulting in a large number of microstructural defects. These defects can only be partially remedied by subsequent thermal treatment under pressure (post-densification), and in many cases, only partially. Furthermore, the microstructure is characterized by defects such as free carbon, pure tungsten, tungsten dicarbide, and various η-phases, leading to a significant deterioration of properties (e.g., flexural strength). (T. Gläser, Investigations on the Laser Sintering of Tungsten Carbide-Cobalt, Dissertation 2010; Additive Manufacturing of Extrusion Tools from Hard Metal-GENIAL (BMBF)) The occurrence of cracks in the microstructure due to the different coefficients of thermal expansion of the microstructural constituents during the cooling of the hard metals, and the loss of cobalt through evaporation, is also highly detrimental.

[0018] The object of the present invention is to provide a method for producing hard metal bodies with which hard metal bodies with a density of ≥ 98% can be produced, which have a hard metal microstructure at room temperature which consists almost entirely of WC particles in a cobalt-rich matrix with dissolved tungsten and carbon and is free from the undesired phases of free carbon, pure W, tungsten dicarbide and various η-phases.

[0019] The problem is solved by the invention specified in the claims. Advantageous embodiments are the subject of the dependent claims.

[0020] In the inventive process for producing hard metal bodies having a hard metal microstructure at room temperature that is free of tungsten dicarbide and eta phase, LPBF, SLM, DMLS or SLS is used as a laser-based additive manufacturing process with energy input via laser powers of 20 W to 55 W and scan speeds of 20 mm / s to 75 mm / s and track spacings of 30 µm to 205 µm and layer thicknesses of 20 µm to 45 µm at each point of energy input to achieve a temperature of 800 °C to a maximum of < 1800 °C. The starting materials are hard metal granules made of WC as a ceramic hard material with Co as a metallic binder phase and an additive of Cr3C2, and the hard metal granules have a porosity of > 0 vol.% to 40 vol.%.-%, a hard metal green body with a density of at least 50% and at most 70% of the theoretical density of the hard metal body is produced, and is subsequently subjected to sintering at temperatures up to a maximum of 1600 °C by means of vacuum sintering at temperatures of 1200 to 1600 °C, and at partial pressures of 100 to 90000 Pa or gas pressure sintering at temperatures of 1380 to 1600 °C and pressures of 5 to 10 MPa until a density of the hard metal body of > 98% to 99.9% of the theoretical density.

[0021] It is also advantageous if vacuum sintering is carried out at pressures of 200 to 90000 Pa.

[0022] It is also advantageous to use hard metal granules as starting materials for the laser-based additive manufacturing process with granule sizes from 2 µm to 90 µm.

[0023] It is also advantageous to use partially compacted and / or fully compacted pre-sintered hard metal granules as starting materials for the laser-based additive manufacturing process.

[0024] It is also advantageous if partially compacted hard metal granules are used as starting materials for the laser-based additive manufacturing process, which have a bulk density of 25 to 55% of the theoretical density and both monomodal and bimodal or multimodal particle size distributions.

[0025] With the solution according to the invention, it is possible for the first time to specify a method for producing hard metal bodies with which hard metal bodies with a density of > 98% can be produced, which have a hard metal microstructure at room temperature that consists almost entirely of WC particles in a cobalt-rich matrix with dissolved tungsten and carbon and is free from the undesired phases of free carbon, pure W, tungsten dicarbide and various η-phases.

[0026] This is achieved through a process that uses laser or electron beam-based additive manufacturing processes, such as LPBF, SLM, DMLS or SLS.

[0027] It is well known that the production of hard metal bodies using laser-based additive manufacturing processes requires high laser power and small track spacing and / or low laser scanning speeds to achieve the lowest possible overall porosity and high density. This leads to very high temperatures of > 2000 °C and above in the area of ​​laser energy input. As a result, WC decomposes above 2735 °C, disrupting the resulting hard metal microstructure and negatively impacting the properties of the finished hard metal body.

[0028] The inventive method produces hard metal bodies using laser-based additive manufacturing processes, which are a hard metal green body and may have a density of at least 50% and a maximum of 70% of the theoretical density of the hard metal body.

[0029] According to the invention, the conditions of the laser-based additive manufacturing process are adjusted such that the energy input via the laser at any point of energy input results in a temperature of only 800 °C to a maximum of < 1800 °C. These temperatures can be measured directly and approximately using pyrometer measurements, and indirectly by analyzing the laser-sintered green bodies using X-ray diffraction, which can detect tungsten dicarbide phases after a temperature > 2735 °C. Furthermore, to verify that only temperatures up to 1800 °C are applied by the laser, X-ray fluorescence measurement can be used to determine the cobalt content in the starting powder and in the pre-sintered green body. The cobalt loss during laser-based additive manufacturing of the green body should be no more than 10% of the initial cobalt content.

[0030] This can be achieved through lower laser power and / or larger track spacing and / or higher scan speeds and / or smaller layer thickness and / or shorter laser pulses than are known according to the state of the art.

[0031] Advantageously, this is achieved with laser powers of 20 W to 55 W, track spacings of 30 µm to 205 µm, scan speeds of 20 mm / s to 75 mm / s, and layer thicknesses of 20 µm to 45 µm. The laser power can be applied continuously or, for better control of the energy input, in pulses.

[0032] On the other hand, the conditions of the laser-based additive manufacturing process must be adjusted according to the invention such that energy input via laser is achieved at every point of energy input to realize a temperature of at least 800 °C, whereby the hard metal green body has a density of at least 50% of the theoretical density of the hard metal body. This ensures sufficient mechanical stability of the green body for further processing.

[0033] The hard metal green bodies produced in this way are in no case intended for use as hard metal bodies for the known applications of dense hard metal bodies, but according to the invention these hard metal green bodies must subsequently be subjected to further densification by means of vacuum sintering or gas pressure sintering, also called SinterHIP, at temperatures up to a maximum of 1600°C, in the case of vacuum sintering at partial pressures of 100 to 90000 Pa or in the case of gas pressure sintering at pressures of 5 to 10 MPa, up to a density of the hard metal body of ≥ 98 % to 99.9 % of the theoretical density.

[0034] Advantageously, gas pressure sintering can be carried out at temperatures of 1380 to 1600 °C and pressures of 5 to 10 MPa.

[0035] Vacuum sintering is also advantageously carried out at temperatures of 1200 to 1600 °C and pressures of 200 to 90000 Pa.

[0036] The result is an essentially dense hard metal body that can be used for the known applications of dense hard metal bodies.

[0037] According to the invention, pre-sintered hard metal powders or hard metal granules can be used as starting materials for the laser-based additive manufacturing process.

[0038] When using pre-sintered cemented carbide granules, these advantageously exhibit a closed porosity of > 0 vol.% to 40 vol.% and / or average granule sizes of 2 µm to 90 µm. The pre-sintered cemented carbide granules can have a bulk density of 25 to 55% of the theoretical density and exhibit monomodal, bimodal, or multimodal particle size distributions.

[0039] Hard metals made of WC as a ceramic hard material with Co as a metallic binder phase and an additive of Cr3C2 can be used.

[0040] According to the invention, partially compacted and / or compacted sintered hard metal granules can be used as starting materials for the laser-based additive manufacturing process.

[0041] Partially compacted cemented carbide granules are cemented carbides mixed with a binder metal and processed into granules as a green compact. These cemented carbide granules are then sintered in the absence of oxygen, during which the binder metal is heated to the liquid phase, partially filling the spaces between the cemented carbide particles and partially encasing them. Partially compacted cemented carbide granules of this type exhibit a porous structure. Subsequently, or directly in a single step, the partially compacted cemented carbide granules can be further compacted, resulting in a homogeneous, virtually pore-free structure.

[0042] Granulation and partial compaction can be achieved, for example, via spraying / fluidized bed processes or mechanical agglomeration / granulation. Further compaction of the hard metal granules can be achieved by subsequent partial or complete sintering of the granules and, advantageously, subsequent deagglomeration.

[0043] Of particular importance according to the invention is that the two-stage production of the hard metal bodies results, on the one hand, in the production of a hard metal green body that does not fall apart during or after debinding, and on the other hand, in the subsequent sintering, in the production of complexly shaped, dense hard metal bodies.

[0044] It is also important according to the invention that the remaining space in partially compacted hard metal granules is available for necessary rearrangement processes in the hard metal green body, so that further compaction of the hard metal green body is achieved in the sintering process.

[0045] The two-stage process according to the invention for the production of hard metal bodies utilizes, on the one hand, the advantage of laser-based additive manufacturing processes with the higher degrees of freedom in the design of shaped bodies, with simultaneously lower to low energy input and preservation of the hard metal structure, and, on the other hand, a dense hard metal body can be produced for which a lower energy input is also required during sintering in gas pressure sintering process or vacuum sintering process.

[0046] Furthermore, it is particularly advantageous that the hard metal bodies produced using the inventive method have microstructures that do not have interfering phases such as W2C, W and / or eta phases, and / or that the Co loss, especially in the case of SLS, is < 10% compared to the initial cobalt content.

[0047] The invention will now be explained in more detail using several exemplary embodiments. Example 1

[0048] Partially compacted hard metal granules were produced by a mixed milling process using WC, Co, and 2 wt% organic binder (in this case, paraffin) in heptane, followed by spray granulation and sintering at 1030 °C. The granules were then deagglomerated and sieved into fractions ≤ 90 µm. Subsequently, fractions ≥ 10 µm and ≤ 32 µm were obtained using conventional screening techniques.

[0049] The partially compacted hard metal granules consisted of 12 wt% WC with an initial grain size of 0.75 µm dFSSS .

[0050] The measured bulk density was 4.8 g / cm³. 3 , this corresponds to a density of approximately 35% of the theoretical density.

[0051] The green bodies were produced using SLS with the following parameters to create test specimens with the target geometry of 12*12*7 mm. 3 Laser power 55 W, scan speed 75 mm / s, track spacing 205 µm, layer thickness 45 µm.

[0052] The green body had a density of 63% and therefore sufficient strength to be handled for subsequent sintering.

[0053] The hard metal green body was then sintered in a gas pressure sintering furnace at 1400 °C and 10 MPa for a holding time of 75 minutes.

[0054] The resulting hard metal component was characterized after sintering, and its composition and porosity were checked. The density was 14.35 g / cm³. 399.9% of the theoretical density and the porosity determined on light microscopic sections was A04-B02-C00. Example 2

[0055] Partially compacted hard metal granules were produced by a mixed milling process of WC, Co, Cr3C2, and 5 wt% organic binder (in this case, PVB) in ethanol, followed by spray granulation and sintering at 1070 °C. The partially compacted granules were then sieved into fractions ≤ 90 µm. Subsequently, fractions ≥ 15 µm and ≤ 5 µm were obtained using conventional screening techniques.

[0056] The partially compacted hard metal granules consisted of 12 wt% WC with an initial grain size of 0.3 µm d FSSS .

[0057] The measured bulk density was 5.2 g / cm³. 3 , this corresponds to a density of approximately 40% of the theoretical density.

[0058] The green bodies were produced using SLS with the following parameters to create test specimens with the target geometry of 12*12*7 mm. 3 Laser power 20 W, pulse frequency 120 kHz, pulse length 250 ns, scan speed 20 mm / s, track spacing 30 µm, layer thickness 20 µm.

[0059] The green body had a density of 53% and therefore sufficient strength to be handled for subsequent sintering.

[0060] The hard metal components were then sintered in a gas pressure sintering furnace at 1380 °C and 8.7 MPa for a holding time of 65 minutes.

[0061] The resulting hard metal component was characterized after sintering, and its composition and porosity were checked. The density was 14.20 g / cm³. 3 99.8% of the theoretical density and the porosity determined on light microscopic sections was A04-B04-C00. Example 3

[0062] Partially compacted hard metal granules were produced by a mixed milling process using WC, Co, and 2 wt% organic binder (in this case, paraffin) in heptane, followed by spray granulation and sintering at 1050 °C. The granules were then deagglomerated and sieved into fractions ≤ 90 µm. Subsequently, fractions ≥ 5 µm and ≤ 20 µm were obtained using conventional screening techniques.

[0063] The partially compacted hard metal granules consisted of 13 wt% WC with an initial grain size of 0.95 µm d FSSS .

[0064] The measured bulk density was 5.3 g / cm³. 3 , this corresponds to a density of approximately 40% of the theoretical density.

[0065] The green bodies were produced using SLS with the following parameters to create test specimens with the target geometry of 12*12*7 mm. 3 Laser power 20 W, pulse frequency 120 kHz, pulse length 250 ns, scan speed 23 mm / s, track spacing 30 µm, layer thickness 25 µm.

[0066] The green body had a density of 58% and therefore sufficient strength to be handled for subsequent sintering.

[0067] The hard metal components were then sintered in a vacuum sintering furnace at 1450 °C and 5000 Pa with a holding time of 95 minutes.

[0068] The resulting hard metal component was characterized after sintering, and its composition and porosity were checked. The density was 14.11 g / cm³. 3 99.9% of the theoretical density and the porosity determined on light microscopic sections was A04-B00-C00.

Claims

[1] A method for producing hard metal bodies having a hard metal microstructure at room temperature that is free of tungsten dicarbide and eta phase, using LPBF, SLM, DMLS or SLS as a laser-based additive manufacturing process with energy input via laser powers of 20 W to 55 W and scan speeds of 20 mm / s to 75 mm / s and track spacings of 30 µm to 205 µm and layer thicknesses of 20 µm to 45 µm at each location of energy input to achieve a temperature of 800 °C to a maximum of < 1800 °C from hard metal granules as starting materials made of WC as a ceramic hard material with Co as a metallic binder phase and an additive of Cr3C2 and the hard metal granules having a porosity of > 0 vol.% to 40 vol.%.-%, a hard metal green body with a density of at least 50% and at most 70% of the theoretical density of the hard metal body is produced, and which is subsequently subjected to sintering at temperatures up to a maximum of 1600 °C, by means of vacuum sintering at temperatures of 1200 to 1600 °C and at partial pressures of 100 to 90000 Pa or gas pressure sintering at temperatures of 1380 to 1600 °C and pressures of 5 to 10 MPa until a density of the hard metal body of ≥ 98% to 99.9% of the theoretical density. [2] Method according to claim 1, wherein the vacuum sintering is carried out at pressures of 200 to 90000 Pa. [3] Method according to claim 1, wherein hard metal granules are used as starting materials for the laser-based additive manufacturing process with granule sizes from 2 µm to 90 µm. [4] Method according to claim 1, wherein partially compacted and / or fully compacted pre-sintered hard metal granules are used as starting materials for the laser-based additive manufacturing process. [5] The method of claim 1, wherein the starting materials for the laser-based additive manufacturing process are partially compacted hard metal granules which have a bulk density of 25 to 55% of the theoretical density and both monomodal and bimodal or multimodal particle size distributions.