WO2018206250A1 - Additive manufacturing of metal matrix composite materials - Google Patents

Abstract

The present invention relates to the production of molded bodies from metal matrix composites (MMC) by means of a special additive manufacturing method, in particular a method which is referred to as powder bed printing or also as binder jetting.

Description

 Additive manufacturing of metal matrix composites

The present invention is the production of shaped bodies of metal matrix composite materials (MMC) by a special additive manufacturing process, in particular a process which is referred to as powder bed printing or as binder jetting.

Metal matrix composite materials are known per se and are usually produced by a multi-stage process in which initially an open-porous preform (preform) of a first material, such as a ceramic material is produced, which in the course with another material, for example a metallic material is infiltrated.

The conventional methods for the production of metal matrix composites (MMC), in particular SiSiC, have the disadvantages that complex methods have to be used to achieve the final contour, whereby a considerable proportion of the preform material has to be discarded, especially with complex geometries. Therefore, manufacturing methods have recently been proposed in which shaped bodies of metal matrix composite materials (MMC), in particular SiSiC, are produced by means of additive manufacturing processes, for example SLS. Due to the process-related layer structure, sloping or curved surfaces and edges create steps that adversely affect the surface quality. In addition, especially with SLS unwanted material deposits on the component surface can not be avoided. Therefore, moldings produced in this way must also be reworked to rid the surface of roughness and supernatants.

The object of the present invention was thus to provide a process for the production of metal matrix composite materials (MMC), which does not have the disadvantages of the processes known from the prior art.

The object is achieved by the use of a special additive manufacturing process, in particular a process that is used as powder bed printing, or also referred to as binder jetting, for the production of metal matrix composite materials (MMC), wherein the MMC powder used has a certain grain size or a determined grain size distribution. This makes it possible to produce shaped bodies or components close to final contours and, thus, avoids costly reworking methods for the most part. The smaller the layer thickness from which the shaped bodies are constructed, the finer the printed structures become.

Basically, preforms z. B. for MMC (metal-ceramic composites) made, which consist of a ceramic framework, which is then infiltrated with metal. Typical examples are the production of SiSiC or carbide.

According to the invention, the metal matrix composite materials (MMC) can generally be produced in the following manner: For the lowermost layer, a ceramic powder spillage, for example, is produced in a mold corresponding to the shaped article to be produced. B. consisting of a ceramic powder selected from the group consisting of Al2O3, AIN, C, mullite, SiC, Si3N ₄ , ZrO2 or mixtures thereof, preferably SiC, z. B. transferred with a doctor blade in a homogeneous position defined height. The thickness of each layer is 50 to 250μιτι, preferably 80 to 150 μιτι.

In order to fix the lowermost layer necessary for the shaped body, a liquid fixing component is applied programmatically in accordance with the necessary area. In the context of the present invention, the term fixing component refers to organic and / or inorganic materials or mixtures with which the ceramic powder is held or fixed in the layer position, for example binder or binder-hardener systems. The application can be done for example via a print head. The fixing component should preferably wet the powder well, so that the organics are distributed homogeneously. A consolidation of the printed areas in the layer can be done either by removing the volatile components of the fixing component or by thermal and / or light-induced crosslinking (eg with an IR or UV lamp). After consolidation, another layer is spread over the bottom layer Applied ceramic powder with a squeegee. According to the required next layer of the molded article, the fixing component is applied and consolidated. The process is repeated until the molding is constructed according to the layer model.

When using binders as fixing component in which the crosslinking is caused by the action of heat (for example phenolic resin resoles), it is problematic that the crosslinking also takes place at room temperature - albeit at a much slower rate. This may result in the problem that the nozzles in the printhead become clogged by unwanted hardening of the binder, for example by heat generation in the process, and thus become unusable. The printhead is a very expensive single component in 3D printing equipment.

One solution is the use of separate binder-hardener systems as fixing component (for example, phenolic resin novolaks or 2-component epoxy resin systems). To cure such binders, a hardener component must be added to them. The individual components can be stored for a long time, without it comes to a networking.

The separation of binder and hardener can be maintained until the end by the following procedure: The powder or granules to be processed are precoated with the hardener-free binder (for example the phenolic resin novolak) in a separate process step. In the production plant, for example an SD printer, the hardener or the hardener solution is only then printed on the precoated powder / granules. Only at the points where both components meet does the component form by solidification of the powder bed, e.g. through a heat treatment.

The reverse procedure is also possible: precoating the powder or granules with the hardener component and printing the hardener-free binder in the production plant, for example a 3D printer. After the complete construction of the component, loose powder still on or in the shaped body is removed, for example with a suction or blowing system, so that only the shaped body solidified with binder remains in the final contour. The removed material can be reused. The binder precoated with binder or hardener can be removed and later recycled.

A preferred embodiment relates to the additive manufacturing of SiSiC components. Due to the structure of a powder bed, the preform contains a porosity of about 50 vol%. When the pores are filled with silicon, a material with a correspondingly high silicon content and unsatisfactory properties is obtained. For SiSiC, a binder is therefore preferred for 3D printing, which forms a carbon residue after pyrolysis ("coking"), which reacts with silicon to form secondary silicon carbide, thereby reducing the porosity of the preform somewhat.

Better material properties can be obtained with a significantly reduced silicon content. For this purpose, the shaped body (preform) produced as described above is subsequently impregnated: during impregnation, the cavities of the shaped body are filled. The impregnation medium may be an organics having a high carbon yield after coking or a particle suspension or a mixture thereof. The particles in the suspension consist of carbon black, graphite, ceramic powders (for example SiC) with particle sizes of from 0.1 μm to 5 μm or of nano-particles with particle sizes of less than 100 nm or mixtures of these particles.

The coking is then carried out at about 600 to 1500 ° C under inert gas (usually N ₂ but also noble gases such as Ar or He) non-oxidizing. After coking, there should be an open pore system necessary for infiltration (if the pores are too narrow or the C content too high, then complete, crack-free silicification is no longer possible).

Multiple soaking and coking is possible to increase the carbon content and increase the strength. The silicon content in the component is thereafter preferably less than 40% by volume, more preferably less than 35% by volume, based on the total volume of the component. Shaping processing steps as in the conventional method are not necessary but possible. The grain size of the primary SiC, the compaction density and the carbon content determine the material properties.

In another variant, the subsequent impregnation can be completely dispensed with. If suitable fixation components are used for the production process, which form a sufficiently high carbon residue after coking, the silicon content in the SiSiC component can be significantly reduced, in particular to less than 40% by volume, preferably less than 35% by volume, based on the total volume of the component ,

The SiSiC components produced according to the invention with a particle size of 25 to 60 μm, preferably 40 to 55 μm, have a 4-point bending strength of more than 200 MPa, preferably more than 250 MPa. The SiSiC components preferably have a structure in which the structure of individual layers in the microstructure on a vertical cutting edge of the component is no longer visually recognizable, for example by means of light microscopy or scanning electron microscopy (see FIG. 1).

The ceramic powder used according to the invention is selected from the group consisting of Al 2 O 3, AlN, C, mullite, SiC, Si 3 N ₄ , ZrO 2 or mixtures thereof, preferably SiC. The ceramic powder has a mean particle size in the range of 25 to 60 μιτι, preferably 40 to 55 μιτι on. The measurement of the grain size is done by laser diffraction (Mastersizer 2000 Malvern). Through the use of powders in this particle size range, on the one hand, small layer heights during printing and thus a particularly good surface quality of the printed component can be achieved. On the other hand, in a subsequent sintering treatment, a significantly higher component strength is achieved in comparison with a coarser powder greater than 60 μm in accordance with the prior art. Very fine powders having an average particle size of less than 40 μm can have insufficient flowability and lead to powder bed printing during processing to a sufficiently homogeneous powder bed or not sufficiently homogeneous material.

In order to be able to process fine powders in powder bed 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 is between 0.5 and 1.0, preferably between 0 , 6 and 0.9. The powder preferably has a very high flowability, which can be characterized by the so-called flow time. The flow time from an outlet funnel 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, more preferably> 48% of the theoretical density. For SiC, the bulk density is preferably> 1450 g / l, more preferably> 1500 g / l.

The powders can also be used in a multimodal particle size distribution, in particular a bimodal or trimodal particle size distribution, i. from fractions of powders of different particle sizes. The packing density can be optimized by an even finer fraction fills the gaps of the next coarser powder fraction. To achieve a high packing density, the particle size distribution can be optimized so that the fine powder fills the gaps in the bed of coarser powder. The above-mentioned particle size distribution is also advantageous.

In a preferred embodiment, the ceramic powder having a mean particle size in the range of 25 to 60μηη, preferably 40 to 55 μιτι represent a fraction, preferably the main fraction and with one or more further fraction (s), for example with a mean grain size of 3 to 5 μιτι and / or 10 to 15 μιτι, are mixed. The proportion of the further fraction (s) in the total amount of powder can then be, for example, 40% by mass in order to achieve the highest possible packing density. When optimizing the powder mixtures, however, it must be taken into account that, if necessary, sufficiently large pore channels are present for subsequent metal infiltration. In addition, a sufficient flowability of the powder mixture must be given. In the production of SiSiC components, different starting compositions of primary SiC are possible, e.g. B. 80% SiC and 20% C or 98% SiC and 2% C, preferably up to 85% SiC. On the other hand, it is also possible that a shaped body is initially built only from carbon and then completely or partially converted by infiltration in SiSiC. The C content of the primary SiC determines the content of the secondary SiC. Various modifications are possible in this preferred embodiment. For example, carbon, for example soot or graphite, can be added to the SiC powder before the preforms are produced, so that the carbon content necessary for the secondary SiC has already been introduced.

The fixing component of the present invention may 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 spot irradiation with an IR or UV lamp. UV-curable components used in the present invention may be radically or cationically curable UV systems or a mixture of both, e.g. As acrylates, epoxides, enol ethers, vinyls. The thermosetting binder may be a component which is dried and / or reacted via a heat source. Exemplary components are phenolic resins, furan resins, epoxy resins, graphite resins, starch, sugar or cellulose solutions. The binder may also be inorganic based, e.g. Water glass. Preferred binders are phenolic resins.

The fixing component for the process according to the invention may also be a mixture of two components, a binder-hardener mixture. By applying the second reaction component, the material hardens. As binder-hardener mixtures mixtures can be used, which consist of epoxy resins, polyurethanes, novolaks or binders based on methyl methacrylate with a corresponding hardener. Preferred binder-hardener mixtures are phenolic resin novolaks with, for example, hexamethylenetetramine as hardener, The amount of binder (pure active substance, ie less solvent contained) in the printed body is 2 to 30%, preferably 5 to 10% by mass, based on the total mass printed body.

The infiltration of the preformed body (preform) may 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 is selected, or a mixture or alloys thereof are carried out. In a preferred embodiment of the present invention, the infiltration is carried out with silicon.

For SiSiC, the near-net shape parts are placed in a siliconizing furnace. There, the parts are brought into contact with liquid silicon in vacuo or in a protective gas atmosphere. The silicon penetrates via capillary forces into the cavities of the coked parts and fills them in the ideal case completely.

The carbon reacts wholly or partially with the silicon to secondary SiC. Bridges are formed between primary and secondary SiC, which increases strength. After infiltration, it can be assumed that the component dimensions have not changed. In the process according to the invention, the densities are in the range from 2.7 to 3.1 g / cm ³ , preferably 2.9 to 3.1 g / cm ^3, depending on the free Si content or residual carbon content. The SiC content in the finished-infiltrated component is up to 92% by weight.

In some cases, for example, in the MMC production exclusive of the SiSiC components, it may offer to produce a pore-richer material. For the powder or granules with pore-forming substances (porosity), z. As polymethylmethacrylate (PMMA), polystyrene, polypropylene, polyethylene, polyurethane, polyamide or other substances that can be thermally decompose residue-free, are added. Such a pore-richer surface can have tribological advantages.

In special cases, only one functional surface or functional layer should have a porosity. For this purpose, the layered order of the powder, only in the desired layers applied a mixture of ceramic powder and porosity. This mixture must either be previously mixed and conveyed from a second reservoir, or mixed by a controllable mixing device of a ceramic powder reservoir and a Porosierungsmittelbehälter just before the layered order.

In addition, a pore-forming substance can be deposited via a dispenser for targeted pore generation. It is also possible that only selected areas are porosiert. This would also arbitrary pore patterns produced, the z. B. could be the result of a finite element calculation.

Sometimes continuous channels, channel systems or arbitrarily shaped recesses are desired, which are difficult or impossible to produce with the usual methods. In metal infiltration, these cavities can be filled. In the case of SiSiC materials in particular, volume expansion during solidification of the silicon melt can give rise to silicon beads which form unpredictably in the cavities and thus produce shape deviations, constrict openings or completely occlude them.

The unwanted filling of cavities and also the formation of silicon beads can be prevented by adding the moldings with a release agent such. As boron nitride, graphite, molybdenum, carbon black, silicon nitride, yttrium silicates surrounds. For this purpose, a separating agent slip around the contour, which should remain free of silicon beads, can be printed via an additional print head. This release agent slip consists of water or an organic solvent and the release agent such as boron nitride, silicon nitride or carbon and as a third component a binder. Alternatively, the release agent, such as boron nitride or silicon nitride can be applied as a powder layer similar to the aforementioned methods and fixed by printing a binder component. After removal of the loose powder via the blowing or suction system then the release agent-coated molded body remains. In siliconizing, no silicon beads are formed in the regions. The release agent may, after the siliconizing z. B. be removed by suction, blowing, washing. Example:

 SiC powder: Starting material was the particle size fraction F240 (D50 44.5μηη, manufacturer StGobain); By a corresponding post-treatment of the powder in the laboratory flowability and bulk density of the powder were improved. The after-treated powder had a flow 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 had a mean particle size D50 of 54 μιτι and a particle size distribution of (D90-D10) / D50 of 0.80.

Print on Exone's commercial M-Flex line with Exone standard binders. Layer height Ι ΟΟμιτι. Test rods 7mmx8mmx65mm. Impregnating the printed component with an aqueous carbon black suspension (solids content 30%). Then pyrolysis (N2, 600 ° C) and infiltration with silicon (1600 ° C). The Si content after each impregnation process is shown in Table 1.

Table 1 :

On the siliconized component of the layered structure is no longer visible; the texture is homogeneous. The 4-point bending strength of the component is 248 MPa.

Fig. 1 shows a structure of a test piece of this example followed by 2-fold impregnation; the individual, applied Ι ΟΟμηη-thick layers run horizontally in this image, but are not visually recognizable.

Claims

claims A method for producing a shaped body from a metal matrix composite material, characterized in that the shaped body step by step of several layers, preferably via in processes, which is referred to as powder bed printing or as binder jetting, is constructed with a layer thickness of 50 to 250 μιτι, wherein a ceramic powder from the group consisting of Al ₂ O ₃ , AIN, C, mullite, SiC, Si ₃ N ₄ , ZrO ₂ or mixtures thereof, and a fixing component are used and wherein the ceramic powder has a mean particle size of 25 to 60 μιτι, preferably 40 to 55 μιτι or is present in a multimodal particle size distribution, wherein a fraction has an average particle size in the range of 25 to ΘΟμιτι, preferably 40 to 55 μιτι. A method according to claim 1, characterized in that the fixing component consists of a binder or a mixture of binder and hardener. The method of claim 2, wherein the binder is a UV-curable component, preferably a free-radically or cationically curing UV component or a mixture of both, for example acrylates, epoxies, enol ethers, vinyls, a thermosetting component, for example phenolic resins, furan resins, epoxy resins Graphite resins, starch, sugar or cellulose solutions, or an inorganic-based binder, for example water glass; wherein the binder-hardener mixture consists of epoxy resins, polyurethanes, novolaks or binders based on methyl methacrylate with a corresponding hardener. A method according to claim 2, characterized in that when using a binder-hardener system, the application of binder and hardener is separated, in which precoated the ceramic powder to be processed with the hardener-free binder or the binder-free hardener in a separate process step and the hardener or binder, For example, in the form of a solution, only then be applied to the pre-coated ceramic powder in the manufacturing plant, whereby the curing occurs only at the points where hardener and binder meet. 5. The method according to any one of the preceding claims, characterized in that the layer thickness is 80 to 150 μιτι. 6. The method according to any one of the preceding claims, characterized in that the value (D90-D10) / D50 for the ceramic powder is between 0.5 and 1, 0. 7. The method according to any one of the preceding claims, characterized in that the ceramic powder has a bulk density of> 45% of the theoretical density. 8. The method according to any one of the preceding claims, characterized in that the ceramic powder is SiC. 9. component produced by the method according to one of claims 1 to 8, in which the structure of individual layers in the microstructure on a vertical cutting edge of the component is no longer visually recognizable, 10. Component according to claim 9, characterized in that the component is a SiSiC component and the silicon content is less than 40% by volume, preferably less than 35% by volume.