Additive manufacturing of metal matrix composites

Abstract

Method for producing a shaped body from a silicon matrix composite material, wherein the shaped body is built up step by step from several layers and subsequently infiltrated with silicon, the molded body is built up using powder bed printing, also known as binder jetting, with a layer thickness of 50 to 250 µm, where SiC is used as the ceramic powder and a binder or binder-hardener system in liquid state, characterized in that the ceramic powder has a multimodal particle size distribution, the main fraction of which has a mean particle size measured by laser diffraction in the range of 40-55 µm and the percent difference (D90-D10) / D50 of the total particle size distribution is between 0.5 and 1.0 and wherein the ceramic powder comprises at least one further fraction whose mean particle sizes are 3-5 µm and / or 10-15 µm.

Description

[0001] The present invention relates to the production of shaped bodies made of metal matrix composites (MMC) by a special additive manufacturing process, in particular a process known as powder bed printing or binder jetting.

[0002] Metal matrix composites are known in themselves and are usually produced via a multi-stage process in which an open-porous preform is first produced from a first material, for example a ceramic material, which is then infiltrated with another material, for example a metallic material.

[0003] EP 2 998 282 A1 describes a process for producing reaction-bonded silicon carbide (RBSiC) using selective laser sintering (SLS) for manufacturing components with excellent properties despite increased component size and complexity. US 2012 / 0 237 745 A1 discloses a process for manufacturing a ceramic or glass-ceramic article, which describes the layer-by-layer construction of the ceramic or glass-ceramic article by applying a layer to a surface, heating at least one area of ​​this layer above the melting point of the layer material, and cooling the area so that the layer bonds to the surface in that area. WO 2017 / 077 137 A9 discloses a process for the economical production of metallic components using additive manufacturing, which exhibits high flexibility with regard to the achievable component geometries.

[0004] US 2010 / 0279007A1 describes a process for manufacturing near-net-shape products, such as silicon-infiltrated silicon carbide composites. A mixture of a build-up material and a binder is prepared and applied to a surface to create a layer. An activator liquid is then applied to at least one selected area of ​​the layer to bond the binder to the build-up material. These steps can be repeated to produce a porous body, which is then heat-treated to obtain a porous green body with a porosity of approximately 30% to approximately 70%. The green body is then infiltrated with a molten material, such as molten silicon.

[0005] From DE 10 2013 017 193 A1, a method for producing a shaped body from a silicon-infiltrated silicon carbide composite material is known, wherein the shaped body is built up stepwise from several layers and subsequently infiltrated with silicon, wherein the build-up of the shaped body is carried out by powder bed printing “binder jetting” with a layer thickness of 100 to 500 µm, wherein a ceramic powder SiC and a binder in the liquid state are used, wherein the ceramic powder has a mean grain size of 70 to 200 µm and a grain size distribution (D90-D10) / D50 between 0.5 and 1.0.

[0006] From the publication YUN, BAI et al.: Effect of Bimodal Powder Mixture on Powder Packing Density and Sintered Density in Binder Jetting of Metals, International Solid Freeform Fabrication Symposium, October 10, 2015; a method for producing a molded body via powder bed printing (binder jetting) using a bimodal powder mixture and a liquid binder is known. By adding a powder fraction with a smaller particle size to a powder fraction with a larger particle size, the bulk density of the powder is advantageously increased.

[0007] Conventional methods for manufacturing metal matrix composites (MMCs), especially SiSiC, have the disadvantage that achieving the final contour requires complex processes, and a significant portion of the preform material must be discarded, particularly with complex geometries. Therefore, manufacturing methods using additive manufacturing processes, such as selective laser sintering (SLS), have recently been proposed. The layer-by-layer process creates steps on inclined or curved surfaces and edges, negatively impacting surface quality. Furthermore, undesirable material deposits on the component surface are unavoidable, especially with SLS. Consequently, even these manufactured parts require post-processing to remove surface roughness and protrusions.

[0008] The object of the present invention was therefore to provide a method for the production of metal matrix composites (MMC) which does not have the disadvantages of methods known from the prior art.

[0009] The problem is solved by using a special additive manufacturing process, in particular a process known as powder bed printing or binder jetting, for the production of metal matrix composites (MMC), where the MMC powder used has a specific grain size or grain size distribution. This enables the near-net-shape production of shaped parts or components, largely eliminating the need for complex post-processing. The smaller the layer thickness from which the shaped parts are built, the finer the printed structures become.

[0010] Generally, preforms are first produced, e.g., for MMC (metal-ceramic composites), which consist of a ceramic framework that is subsequently infiltrated with metal. Typical examples are the production of SiSiC or cemented carbide.

[0011] According to the invention, the metal matrix composites (MMCs) can generally be produced as follows: For the bottom layer, a bed of ceramic powder, e.g., consisting of a ceramic powder selected from the group consisting of Al₂O₃, AlN, C, mullite, SiC, Si₃N₄, ZrO₂, or mixtures thereof, preferably SiC, is transferred into a homogeneous layer of defined height, e.g., using a doctor blade. The thickness of the individual layer is 50 to 250 µm, preferably 80 to 150 µm.

[0012] To fix the bottom layer required for the molded part, a liquid fixative is applied according to the required area using program control. For the purposes of this invention, a fixative is defined as an organic and / or inorganic material or mixture with which the ceramic powder is held or fixed in the layer, for example, a binder or binder-hardener system. Application can be carried out, for example, via a printhead. The fixative should preferably wet the powder thoroughly to ensure homogeneous distribution of the organic matter. Consolidation of the printed areas in the layer can be achieved either by removing the volatile components of the fixative or by thermal and / or light-induced crosslinking (e.g., with an IR or UV lamp). After consolidation, another layer of ceramic powder is applied over the bottom layer using a doctor blade.The fixing component is applied and consolidated according to the required next layer of the molded part. This process is repeated until the molded part is built up according to the layer model.

[0013] A problem arises when using binders as a fixing component, where cross-linking is caused by heat (e.g., phenolic resin resoles), because cross-linking also occurs at room temperature – albeit at a significantly slower rate. This can lead to the nozzles in the print head becoming clogged due to unwanted hardening of the binder, for example, caused by heat generation during the process, rendering them unusable. The print head is a very expensive individual component in 3D printing systems.

[0014] One solution is to use separate binder-hardener systems as the fixing component (for example, phenolic resin novolacs or two-component epoxy resin systems). For such binders to cure, a hardener component must be added. This allows the individual components to be stored for extended periods without cross-linking.

[0015] The separation of binder and hardener can be maintained until the very end using the following procedure: The powder or granules to be processed are pre-coated with the hardener-free binder (e.g., the phenolic resin Novolak) in a separate process step. In the production facility, for example, a 3D printer, the hardener or hardener solution is only printed onto the pre-coated powder / granules afterward. The component is formed only at the points where both components meet, through solidification of the powder bed, e.g., by heat treatment.

[0016] The reverse approach is also possible: pre-coating the powder or granules with the hardener component and printing the hardener-free binder in the production plant, for example a 3D printer.

[0017] After the component is fully formed, any loose powder remaining on or within the mold is removed, for example, using a suction or blowing system, leaving only the binder-bonded, near-net-shape molded body. The removed material can be reused. Powder pre-coated with binder or hardener can also be removed in this way and later recycled.

[0018] A preferred embodiment relates to the additive manufacturing of SiSiC components. Due to its powder bed structure, the preform has a porosity of approximately 50 vol%. If the pores are filled with silicon, the resulting material has a correspondingly high silicon content and insufficient properties. Therefore, in SiSiC 3D printing, a binder is preferably used that forms a carbon residue after pyrolysis ("coking"). This carbon reacts with silicon to form secondary silicon carbide. This somewhat reduces the porosity of the preform.

[0019] Improved material properties can be achieved with a significantly reduced silicon content. For this purpose, the preform, produced as described above, is subsequently impregnated: during impregnation, the cavities of the preform are filled. The impregnation medium can be an organic compound that exhibits a high carbon yield after coking, a particle suspension, or a mixture thereof. The particles in the suspension consist of carbon black, graphite, ceramic powders (e.g., SiC) with particle sizes from 0.1 µm to 5 µm, or nanoparticles with particle sizes smaller than 100 nm, or mixtures of these particles.

[0020] The coking process is then carried out at approximately 600 to 1500 °C under a protective gas atmosphere (usually N2, but also noble gases such as Ar or He) using non-oxidizing agents. After coking, an open pore system should be present, which is necessary for infiltration (if the pores are too narrow or the carbon content too high, complete, crack-free silicification is no longer possible).

[0021] Multiple impregnation and coking processes are possible to increase the carbon content and improve strength. The silicon content in the component is then preferably less than 40 vol%, and particularly preferably less than 35 vol%, based on the total volume of the component. Shaping machining steps as in the conventional process are not necessary but are possible. The grain size of the primary SiC, the pressing density, and the carbon content determine the material properties.

[0022] In another variant, the subsequent impregnation process can be completely omitted. If suitable fixing components are used for the manufacturing process, which leave a sufficiently high carbon residue after coking, the silicon content in the SiSiC component can be significantly reduced, in particular to less than 40 vol%, preferably less than 35 vol%, based on the total volume of the component.

[0023] The SiSiC components produced according to the invention, with a grain size of 25 to 60 µm, preferably 40 to 55 µm, exhibit a 4-point flexural strength of more than 200 MPa, preferably more than 250 MPa. The SiSiC components preferably have a structure in which the layered structure is no longer visually discernible at a vertical cut edge of the component, for example, using light microscopy or scanning electron microscopy (cf. Fig. 1).

[0024] The ceramic powder used according to the invention is selected from the group consisting of Al₂O₃, AlN, C, mullite, SiC, Si₃N₄, ZrO₂, or mixtures thereof, preferably SiC. The ceramic powder has a mean particle size in the range of 25 to 60 µm, preferably 40 to 55 µm. The particle size is measured by laser diffraction (Mastersizer 2000 from Malvern). By using powders in this particle size range, small layer heights can be achieved during printing, resulting in a particularly good surface quality of the printed component. Furthermore, this leads to significantly higher component strength during subsequent sintering compared to a coarser powder larger than 60 µm according to the prior art. Very fine powders with a mean particle size of less than 40 µm can exhibit insufficient flowability and result in an insufficiently homogeneous powder bed during powder bed printing.insufficiently homogeneous material.

[0025] To process fine powders in powder bed fusion printing, a narrow particle size distribution is necessary, which can be quantified by the percentage difference between D90 and D10: the value (D90-D10) / D50 lies between 0.5 and 1.0, preferably between 0.6 and 0.9. The powder preferably exhibits very high flowability, which can be characterized by the so-called discharge time. The discharge time from a discharge hopper with an angle of 60° and an opening of 3.6 mm is less than 10 seconds for the powder. The bulk density is preferably > 45% of the theoretical density, particularly preferably > 48% of the theoretical density. For SiC, the bulk density is preferably > 1450 g / l, particularly preferably > 1500 g / l.

[0026] The powders can also be present in a multimodal particle size distribution, in particular a bimodal or trimodal particle size distribution, i.e., consisting of fractions of powders with different particle sizes. The packing density can be optimized by having an even finer fraction fill the gaps in the next coarser powder fraction. To achieve a high packing density, the particle size distribution can be optimized such that the fine powder fills the gaps in the bed of the coarser powder. The particle size distribution mentioned above is also advantageous in this case.

[0027] In a preferred embodiment, the ceramic powder with a mean particle size in the range of 25 to 60 µm, preferably 40 to 55 µm, can constitute a fraction, preferably the main fraction, and be mixed with one or more further fraction(s), for example, with a mean particle size of 3 to 5 µm and / or 10 to 15 µm. The proportion of the further fraction(s) in the total powder quantity can then be, for example, 40% by mass to achieve the highest possible packing density. However, when optimizing the powder mixtures, it must be ensured that sufficiently large pore channels are present for subsequent metal infiltration. Furthermore, the powder mixture must have sufficient flowability.

[0028] Various primary SiC compositions are possible in the production of SiSiC components, e.g., 80% SiC and 20% C or 98% SiC and 2% C, preferably up to 85% SiC. Alternatively, a preform can be initially built up entirely from carbon and then completely or partially converted to SiSiC by infiltration. The carbon content of the primary SiC determines the content of the secondary SiC. Several modifications are possible with this preferred embodiment. For example, the SiC powder can be mixed with carbon, such as carbon black or graphite, before the preforms are produced, so that the carbon content required for the secondary SiC is already present.

[0029] The fixing component of the present invention can be a binder or a mixture of binder and hardener. The binder used in the process according to the invention can be cured by thermal and / or light-induced crosslinking, for example, by large-area or localized irradiation with an IR or UV lamp. UV-curing components used in the present invention can be radically or cationically curing UV systems or a mixture of both, e.g., acrylates, epoxides, enol ethers, vinyls. The thermally curing binder can be a component that is dried and / or reacted by means of a heat source. Exemplary components are phenolic resins, furan resins, epoxy resins, graphite resins, starch, sugar, or cellulose solutions. The binder can also be inorganic, such as water glass. Preferred binders are phenolic resins.

[0030] The fixing component for the process according to the invention can also be a two-component mixture, a binder-hardener mixture. The material hardens upon application of the second reaction component. Binder-hardener mixtures can consist of epoxy resins, polyurethanes, novolacs, or methyl methacrylate-based binders with a corresponding hardener. Preferred binder-hardener mixtures are phenolic resin novolacs with, for example, hexamethylenetetramine as the hardener.

[0031] The amount of binder (pure active substance, i.e. excluding solvents) in the printed molded body is 2 to 30%, preferably 5 to 10% by mass, based on the total mass of the printed molded body.

[0032] The infiltration of the preformed body can be carried out with an element selected from the group consisting of Ag, Al, Au, B, Co, Cr, Cu, Fe, Mg, Mo, Mn, Nb, Si, Sn, Ta, Ti, V, W, Zr, or a mixture or alloy thereof. In a preferred embodiment of the present invention, the infiltration is carried out with silicon.

[0033] In the SiSiC process, near-net-shape parts are placed in a siliconizing furnace. There, the parts are brought into contact with liquid silicon in a vacuum or protective gas atmosphere. The silicon penetrates the cavities of the silicon-coated parts via capillary action and ideally fills them completely.

[0034] The carbon reacts completely or partially with the silicon to form secondary SiC. Bridges form between primary and secondary SiC, which increases the strength. After infiltration, it can be assumed that the component dimensions have not changed. The densities achieved with the inventive process are in the range of 2.7 to 3.1 g / cm³. 3 preferably 2.9 to 3.1 g / cm³ 3 depending on the free silicon content or residual carbon content. The SiC content in the finished, infiltrated component is up to 92% by mass.

[0035] In some cases, for example in the production of MMCs excluding SiSiC components, it may be advantageous to produce a more porous material. For this purpose, the powder or granules can be mixed with pore-forming substances (porosity agents), such as polymethyl methacrylate (PMMA), polystyrene, polypropylene, polyethylene, polyurethane, polyamide, or other substances that decompose thermally without leaving residue. Such a more porous surface can offer tribological advantages.

[0036] In special cases, only one functional surface or layer should exhibit porosity. For this purpose, during the layer-by-layer application of the powder, a mixture of ceramic powder and pore-forming agent is applied only to the desired layers. This mixture must either be pre-mixed and fed from a second storage container, or mixed by a controllable mixing device from a ceramic powder supply and a pore-forming agent container shortly before the layer-by-layer application.

[0037] Furthermore, a pore-forming substance can be deposited via a dispenser for targeted pore creation. It is also possible to create porosity in only selected areas. This would allow for the production of any desired pore pattern, which could, for example, be the result of a finite element analysis.

[0038] Sometimes continuous channels, channel systems, or arbitrarily shaped recesses are required that are difficult or impossible to produce using conventional methods. Metal infiltration can fill these cavities. Particularly with SiSiC materials, the volume expansion during the solidification of the silicon molten metal can cause silicon beads to form unpredictably within the cavities, leading to shape deviations, narrowing of openings, or complete blockage.

[0039] The unwanted filling of cavities and the formation of silicon beads can be prevented by coating the molded part with a release agent such as boron nitride, graphite, molybdenum, carbon black, silicon nitride, or yttrium silicates. This can be achieved by printing a release agent slurry around the contour that should remain free of silicon beads using an additional printhead. This release agent slurry consists of water or an organic solvent, the release agent (such as boron nitride, silicon nitride, or carbon), and a binder as a third component. Alternatively, the release agent, such as boron nitride or silicon nitride, can be applied as a powder layer, similar to the methods mentioned above, and fixed by printing a binder component. After removing the loose powder via the blowing or suction system, the molded part remains coated with the release agent. No silicon beads form in these areas during the siliconization process.The release agent can be removed after silicification, e.g. by suction, blowing, or washing. Example:

[0040] SiC powder: The starting material was the particle size fraction F240 (D50 44.5 µm, manufacturer St. Gobain); through appropriate post-treatment of the powder in the laboratory, flowability and bulk density of the powder were improved. The post-treated powder exhibited a settling time of 8 s (untreated raw material 12 s) and a bulk density of 1547 g / l (untreated raw material 1464 g / l). The post-treated powder also exhibited a mean particle size D50 of 54 µm and a particle size distribution (D90-D10) / D50 of 0.80.

[0041] Printing on an Exone commercial M-Flex system using Exone's standard binder. Layer height 100 µm. Test bars 7 mm x 8 mm x 65 mm. Impregnation of the printed component with an aqueous carbon black suspension (30% solids content). Subsequently, pyrolysis (N₂, 600°C) and infiltration with silicon (1600°C). The silicon content after each impregnation process is shown in Table 1. Table 1: Si content in vol% (calculated from density) Without potions 51,0 1x soaking 32,1 2x soaking 23,9

[0042] The layered structure is no longer visible in the siliconized component; the microstructure is homogeneous. The component's 4-point flexural strength is 248 MPa.

[0043] Fig. Figure 1 shows the structure of a test specimen from this example after subsequent 2-fold impregnation; the individual, applied 100µm-thick layers run horizontally in this image, but are not visually discernible.

Claims

[1] Method for producing a shaped body from a silicon matrix composite material, wherein the shaped body is built up step by step from several layers and subsequently infiltrated with silicon, the molded body is built up using powder bed printing, also known as binder jetting, with a layer thickness of 50 to 250 µm, where SiC is used as the ceramic powder and a binder or binder-hardener system in liquid state, characterized by , that the ceramic powder has a multimodal particle size distribution, the main fraction of which has a mean particle size measured by laser diffraction in the range of 40-55 µm and the percent difference (D90-D10) / D50 of the total particle size distribution is between 0.5 and 1.0 and wherein the ceramic powder comprises at least one further fraction whose mean particle sizes are 3-5 µm and / or 10-15 µm. [2] Method according to claim 1, wherein the main fraction constitutes 60% by mass of the total powder quantity. [3] Method according to one of the preceding claims, wherein the discharge time from a discharge funnel with an angle of 60° and an opening of 3.6 mm for the powder is less than 10 seconds. [4] Method according to claim 1, wherein the percentage difference (D90-D10) / D50 of the total particle size distribution is between 0.6 and 0.

9. [5] Method according to any of the preceding claims, wherein the binder is a UV-curing component selected from a radically or cationically curing UV component or a mixture of both, a heat-curing component, or an inorganic-based binder. [6] Method according to any of the foregoing claims, characterized by, that when using a binder-hardener system, the application of binder and hardener is separated, in that the ceramic powder to be processed is pre-coated with the hardener-free binder or the binder-free hardener in a separate process step, and the hardener or the binder is only applied to the pre-coated ceramic powder afterwards in the production plant, whereby hardening only occurs at the points where hardener and binder meet. [7] Method according to any of the foregoing claims, characterized by that the layer thickness is 80 to 150 µm.

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