EP1685263B1 - Method for producing a component provided with a metal matrix and a fibre or particle reinforcement - Google Patents

Abstract

A method is disclosed for producing a reinforced component from a composite material (MMC). A metallic matrix material is reinforced by embedded fibers or particles, for which purpose a semi-finished product is prepared that comprises fibers and particles or both, together with a metallic matrix material. Forming is effected by thixoforming in a mold at a temperature above the solidus temperature and below the liquidus temperature of the metallic matrix material. The method allows to manufacture near-net-shaped products with excellent mechanical properties.

Description

• Herstellen eines Halbzeuges, in dem die Fasern oder Partikel und der metallische Matrixwerkstoff enthalten sind und
• Thixoumformen des Halbzeuges in einem Werkzeug bei einer Temperatur oberhalb der Solidustemperatur und unterhalb der Liquidustemperatur des metallischen Matrixwerkstoffes.

The invention relates to a method for producing a component from a composite material (MMC) with a metallic matrix material, which is reinforced by embedded fibers or particles, comprising the following steps:

• Producing a semifinished product in which the fibers or particles and the metallic matrix material are contained, and
• Thixotropic shaping of the semifinished product in a tool at a temperature above the solidus temperature and below the liquidus temperature of the metallic matrix material.

Such a method is known from US-A-6 151 198 known.

In the known method, particle-reinforced semifinished products are produced with a thixoformable metal matrix and then converted by thixoforming into parts with near-net shape shape.

A similar procedure is from the US-A-6,135,195 known.

Out SW Youn et al., "Mechanical characteristics evaluation of hollow shape part with metal matrix composites fabricated by thixoforging process" in Journal of Materials Processing Technology, Vol. 130, 2002, pp. 574-580, XP002277297 Further, the thixoforming of SiC-reinforced semifinished products is known.

The need to save primary energy sources and reduce emissions is making light metal in automotive and aerospace technology more and more important. The use of MMC materials (metal matrix composites) or the incorporation of high-strength reinforcing fibers can increase the lack of high-temperature strength and rigidity of these materials, which is the case for many applications. In addition, short production cycles and the production of close-to-net shape components ("net shape forming") should reduce manufacturing costs.

In the prior art, the incorporation of fibrous reinforcing components leads to a significant increase in manufacturing costs. In addition, the lack of resistance of the candidate fiber types compared to the chemically aggressive molten metal in the conventional manufacturing processes from the liquid phase prevents optimal utilization of fiber strength. The conventional production routes from the liquid phase for MMC materials are usually Druckinfiltrationsprozesse in which the melt is forced by a piston in the porous fiber preform to overcome the capillary resistance ("squeeze casting"). This die casting process can also be carried out under vacuum ("Vacural method"). To realize complex geometries and to reduce the mechanical stress of this fibrous preform during the infiltration process, this can be immersed in an evacuated autoclave in the melt and then pressurized with gas pressure ("gas pressure method"). In addition, of course, the powder metallurgical shaping route is basically known by sintering and subsequent pressing, if necessary by hot pressing or hot isostatic pressing (HIP). sintering process However, they are very time-consuming and costly and do not allow net-shape molding of complex component geometries. In addition, in sintering processes, considerable residual porosities usually occur, which can adversely affect the properties.

In the manufacture of aluminum-based fiber-reinforced light metals, melt metallurgical and liquid phase impregnations and die casting techniques are generally used. The long reaction and contact times of molten metallic phases with the reinforcing fibers and the molds or surfaces cause undesirable damage. For example, dissolution and elimination processes as well as chemical interface reactions occur. This leads to difficulties due to adhesion during separation from the mold, due to heat transfer-related microstructures with an overall very complex process control. As a result, a complex system technology is required, which works with comparatively long cycle times for the shaping. In all cases, working with liquid, chemically highly active light metal melts (usually aluminum), so that an additional protection of the fibers used by a suitable protective layer is essential. Especially when aluminum is to be reinforced with carbon fibers, the formation of an aluminum carbide (Al ₃ C ₄ ) at the fiber-matrix interface is undesirable because this carbide phase causes premature failure of the composite due to its pronounced brittle susceptibility and lack of corrosion resistance. A fiber coating can basically be dispensed with if the light metal matrix is combined in the solid phase with the reinforcing component ("diffusion bonding"). In this forming process Laminates made of fiber fabrics and metal sheets are processed by hot pressing. However, the cost-effectiveness of this method is questioned by very high cycle times. A summary of the common processes for making fiber reinforced aluminum composites can be found in Talat Lecture 1402, Froyne, L., Verlinden, B., University of Leuven (Belgium), 1994, EAA European Aluminum Association ,

The Thixoumformverfahren is basically known in the aforementioned prior art for the transformation of special particle-reinforced alloys, especially aluminum-based. With the known methods, however, it is not possible to produce components which have a near-net shape and which have high strength and can be adapted to specific loads. Also, the production of semi-finished products in the prior art is not disclosed.

It is therefore an object of the invention to enable the production of components made of a composite material with metallic matrix, which is reinforced by embedded fibers or particles (MMC), which can be achieved with high strength and a good adaptability to different requirements. The method should also allow the most cost-effective and energy-saving production of components that are molded close to production (net-shape-molding).

This object is achieved by a method according to claim 1.

According to the invention, within a specially selected temperature range above the solidus line and below the liquidus line, a two-phase or multi-phase region in which a liquid phase fraction is present is utilized. In this hot working in the partially solidified state, a pronounced solidification interval of the alloy to be formed is required, ie a temperature range in which both solid and liquid phase components are present next to one another (cf. Fig. 1 ). In this temperature range, application of pressure in a tool can achieve near-net-shape reshaping (thixotropy = shear-rate erosion behavior).

The use of thixo-forming for the production of composite materials reinforced by embedded fibers allows significantly improved properties to be achieved.

Since coated fiber composites are used for producing the semifinished product, particularly advantageous properties of the parts produced therefrom result. In particular, a better homogeneity results in comparison to individual layers of fiber composites and matrix material.

Another advantage is the significantly lower temperature compared to the use of melt processes or melt infiltration processes and the significantly reduced shaping time. In this way it is possible to dispense with the coating of the stored fibers or particles, since the embedded fibers are hardly damaged by the short melting process at a relatively low temperature.

Compared to squeeze casting, an alternating layer structure of the semifinished product allows significantly shorter infiltration paths of the melt to be achieved. As a result of the shortened process times and the relatively short contact of the required tool with the melt, significantly improved tool life results, which leads to significant cost savings, for example compared to the squeeze casting. It can achieve fast process times in the range of milliseconds compared to several seconds for squeeze casting and several hours for diffusion bonding. Furthermore, the shaping in a temperature range below the squeeze casting (about 100 K lower) achieves energy savings of a corresponding size. Due to the reduced liquid phase content during shaping, a shortening of the solidification time in the tool and thus a reduction of the cycle time is made possible. Due to a suitable preconditioning of the matrix material, the entire melting operation (melting furnace, alloying furnace, degassing, alloying control, casting furnace) can be dispensed with. Furthermore, owing to the modified tool concept, significantly less circuit material accumulates, which must be remelted before re-introduction into the production process.

Due to lack of air pockets in the Thixoumformung, especially in Thixoschmieden, it is also possible, the Strength of the matrix material by a suitable heat treatment, in particular by solution annealing and aging to increase.

Furthermore, it is possible with Thixoschmieden to join the component cohesively with other components.

In the context of this application, the term "semifinished product" is understood to mean the preform, which is subsequently formed by thixoforming within the tool to form the component.

This semifinished product may be a prepreg consisting of a single or preferably of several laminate layers. In the context of this application, semifinished products are therefore always understood to mean the generic term which includes a prepreg and other preforms prepared in another way, which are manufactured inside the tool by thixotropic forming of the component.

According to a first variant of the method, a prepreg is produced by laminating layers of coated fiber composites and matrix material in the form of sheets.

In this particularly simple process variant, layers of coated fiber composites are preferably joined together alternately with the suitable metal sheets by lamination in order to provide the desired shape of the semifinished product. As a result of the Thixoumformung provides the provision of the matrix material in the form of sheets, sufficiently short flow paths and yet to ensure even wetting of the trapped fibers or particles.

In an advantageous development of this process variant, the sheets are provided in the form of cold-rolled sheets. In the case of heating, due to the dislocation density induced by the cold rolling process, a fine-grained recrystallization with a globular structure occurs, which has a favorable effect on the resulting properties of the component.

In an expedient development of the invention, several coated fiber composites are laminated to form a prepreg.

In this way, a near-net-shape geometry can be approximated and targeted, targeted characteristics can be achieved.

The coating of the fiber composite with the metallic matrix material can be carried out according to a first process variant by a screen printing process.

Another variant of the method for coating a fiber composite with the metallic matrix material consists in an application by electrostatic charging.

Another method variant for coating a fiber composite with a metallic matrix material consists in an electrophoretic deposition (EPD) of an aqueous suspension with the assistance of an electric field.

Here, the fiber structure to be coated is connected electrically conductively as an electrode and the charge-carrying metal powder particles are driven by the electric field deposited on the fiber or fabric surface as a uniform layer. Such process control is particularly useful with fiber composites that are inherently electrically conductive, such as C fibers. However, other fibers which are not readily electrically conductive can also be processed using suitable intermediate layers.

With such a method, metal particles are preferably used which have a particle size with a diameter between 10 nm and 100 microns, preferably with sizes between 100 nm and 10 microns. Through the use of surface-active liquid or dissolved excipients, targeted charge distributions of the solid particles in the suspension can be set, which permit a concentration and field strength-dependent mass transport for layer deposition. In this way, the properties of the subsequently produced component can be varied within wide limits and adapted to the respective requirements.

According to a further variant of the invention, a Fa serverbund is coated by thermal spraying with the metallic matrix material. Here is basically atmospheric Plasma spraying (APS), electric wire spraying, wire flame spraying or high-speed flame spraying in question. However, the thermal spraying is preferably carried out by electric arc wire spraying or powder plasma spraying, in particular by atmospheric plasma spraying.

Fiber composites, which are provided with thermally sprayed metal layers, offer the advantage of a finer grained structure compared to the use of alternating layers of sheets and fiber composites. While the grain sizes of thixotropically deformable Al-Si sheets are in the range of 2 to 20 μm, the dimensions of the individual phases in the thermally sprayed AlSi layer structure are in the sub-micron range due to the high cooling rate during layer application. As a result, an improved impregnability of the metallic phase in the fiber skeleton can be achieved in the thixotropic deformation.

The volume ratio of matrix material to fibers is preferably set between 0.3 and 8.0, in particular between 0.8 and 3.0, during thermal spraying.

In all variants of the method of the invention, it is ensured that, during the production of the semifinished product, layers for the metallic matrix and the incorporated reinforcing layers are heated only to a low temperature. Even with the coating by thermal spraying can be ensured by a suitable process management that the semi-finished product is heated to a temperature of at most 300 ° C only a few seconds. In this case, this period of time can be at most five seconds or at most two seconds or even be confined below. These measures according to the invention ensures that the fiber composites are not degraded or chemically attacked during the production of semifinished products.

For this purpose, the fiber composite is preferably cooled during thermal spraying, in particular using liquid carbon dioxide.

By means of targeted cooling measures even short-term heating of the fiber composite during the coating can be limited to temperatures that are significantly lower than 300 ° C. and preferably in the range of about 100 to 200 ° C. maximum.

According to a preferred development of the invention, the fiber composite is held under tension during thermal spraying on a carrier device.

In this way, differences can be compensated, which are due to the significantly greater thermal expansion of the metallic matrix material compared to the embedded fibers. In particular, composites can be produced in this way, in which stored long fibers are subjected to tensile stress under load, without the matrix itself being claimed in an undesired manner beforehand.

For the purpose of coating on an industrial scale, the carrier device can promote fiber composite in allow continuously or in clock mode in a coating layer for thermal spray coating.

For this purpose, the fiber composite can be guided via a winding device.

Furthermore, it is possible to first coat the fiber composite when using such a winding device on a first side and then to coat on the opposite side.

According to a preferred embodiment of the invention, a spray distance of 50 to 200 mm is maintained between the surface of the fiber composite and the nozzle exit during the coating by plasma spraying, while in a coating by electric arc spraying a spray distance of 80 to 300 mm is maintained.

By using such process parameters, it is possible to achieve particularly favorable coatings of fiber composites with which, on the one hand, a uniform coating is achieved and, on the other hand, excessive thermal stress on the fiber composites is avoided.

According to a further process variant of the invention, a mixture of the matrix material and of short fibers is granulated or pelletized.

In this case, preference is given to using fibers having a length of between 0.5 and 20 mm, preferably between 2 and 6 mm.

The volume ratio of matrix material to fibers in this case is preferably between 0.3 and 5, preferably between 1 and 2.

According to a further process variant, a mixture of the matrix material and powdered particles is granulated or pelletized.

The production of granules or pellets using short fibers or powdery particles can be advantageously used to produce special graded layers or to produce, for example, bearing materials.

According to a further embodiment of the invention, the mixture produced by granulation or pelletizing is applied to a fiber composite by a suitable coating method. For this purpose, in turn, the thermal spraying, a screen printing or other previously mentioned method can be used.

In order to enable targeted improvements in the strength of the components produced according to the invention, preference is given to at least one layer having a fiber composite of long fibers for the production a prepreg used. Long fibers in this context are understood to mean a fiber length of at least 1 mm or an aspect ratio (ratio of length to diameter) of the fiber of at least 50, preferably of at least 100, particularly preferably of at least 150.

It is understood that the layer sequence in the lamination of prepregs can be varied in a suitable manner in order to influence the properties of the component to be produced in a targeted manner. For example, fibrous composites of long fibers coated with matrix material can be laminated together in a suitable manner. At the same time layers of matrix material in the form of sheets or foils can be inserted. Furthermore, intermediate layers of granules or pellets can be inserted. Finally, the combination of coated fiber composites with preforms made from pelletizing or granulating blends of matrix material and short fibers or particles is also conceivable.

According to a further advantageous embodiment of the invention, a prepreg produced by laminating is provided with an outer layer of matrix material.

In this way it can be achieved that the surface of the manufactured component is largely free of embedded fibers or particles.

The fiber composites can be used in a variety of forms to ensure certain properties of the manufactured component. Thus, fiber composites can be used as a clutch of unidirectional long fibers (UD), it can spin nonwovens ("unwoven") or woven fiber composites in the form of 2D fiber composites, 3D fiber composites, in the form of knitted or crocheted used.

In this case, furthermore, the semifinished product can be produced from graded layers, in order to achieve a specific influencing of the properties at particularly stressed areas of the component or in preferred stress directions. In this way, the ratio of matrix material to fibers can be selectively changed over the component cross-section.

This is particularly advantageous in order to be able to produce structural components that are subject to particularly high thermal or mechanical stress locally, such as pistons or the like.

In principle, all materials which permit thixotropic transformation are suitable as the matrix material.

For this purpose, aluminum alloys or copper alloys are preferably used, in particular alloys which consist of the main components aluminum, magnesium and copper or which consist of the main components copper and tin or zinc.

In this context, as the matrix material alloys are preferred which consist of alloys of the type AlMg4.5Mn0.4 (AA 5182), of the type AlMgSi1 (EN AW-6082), of the type AlSi7Mg (EN AW-356, EN-AW-357), of the type AlSi3 (AA 208, AA 296), of the type AlSi12 (AA 336, AA 384), of the type CuZn40A12 or of the type CuSn13, 5A10.3.

The use of copper alloys focuses on the wear-reducing effect of reinforcing fibers. With these materials, in particular advantageous bearing materials can be produced, for which purpose in particular a combination with carbon fibers or particles in a near-graphite modification is suitable, since this allows emergency running properties to be achieved.

According to a further variant of the method, the metallic matrix material itself is reinforced by embedded particles which are preferably formed as oxide ceramics, as carbides, as nitrides, as metals or alloys or as tribologically active substances.

Fiber reinforcement of the MMC according to the invention fibers are used, which consist of carbon, silicon carbide, alumina, mullite. Also, modifications of these fibers with nitrogen, titanium, boron, carbon or silicon and their compounds are conceivable.

Although in principle no coating of the fibers used is necessary as a consequence of the shortened process times and the reduced process temperatures, coated fibers can also be used on their surface according to a further process variant, in particular fibers provided with diffusion barrier or protective layers or fibers provided with adhesion promoter layers.

In this way, the properties of the component to be manufactured can be adapted to the respective requirements to an even greater extent. For example, by using Adhesives, the interfacial adhesion between embedded fibers and the metallic matrix phase can be improved. At the same time, fibers which are less compatible with the matrix phase used can also be incorporated.

Silicon carbide, silicon nitride, titanium carbide, titanium nitride, carbon or mixed phases or compounds thereof are particularly suitable for coating the fibers.

According to the invention fibers are preferably used with a diameter between 0.5 and 150 .mu.m, more preferably between 5 and 20 microns.

As already mentioned, the fibers can be used both as long fibers or continuous fibers and in the form of short fibers ("chopped fibers").

As already mentioned, in the thixotropic shaping according to the invention the semifinished product is heated to a specific temperature interval as a function of the matrix material used, within which the matrix material has a defined liquid phase fraction.

If, for example, the AlSi7Mg alloy is used, the semifinished product is heated to a temperature between 574 and 584 ° C. for thixoforming, whereby a liquid phase fraction of between 43 and 51% by volume is established. If, for example, the alloy AlMgSi1 is used, heating to a temperature between 635.degree. C. and 645.degree. C. takes place for thixoforming, a liquid phase proportion of between about 15 and 35% being established. When using the alloy CuZn40Al2, heating takes place over a temperature range between 871 ° C. and 875 ° C., with a liquid phase fraction of between about 20 and 40%.

The Thixoumformung is preferably done as Thixoschmieden within a suitable tool (die) with controlled ram speed and pressing force. Tappet speed and pressing force are adapted to the process. Tappet speeds up to 800 mm / s are possible. The impact velocity of the tool upper part on the workpiece is preferably set between 10 mm / s and 300 mm / s depending on the used fiber matrix ratio, the component complexity and the component volume.

In order to prevent premature solidification of the metallic material, the tool is preferably heated to temperatures between 100 ° C and 400 ° C.

In particular, by increasing the ram speed after setting up a particularly rapid infiltration can be achieved, in the range of well below one second under high pressure. This results in a short contact time between metallic melt and fiber material, which leads to a reduced chemical attack of the melt on the fibers and thus leads to improved properties of the manufactured component.

Furthermore, it is possible to perform the Thixoumformung under inert gas atmosphere to reduce the oxide formation.

For Thixoumformung the semifinished product is precompressed in a mold according to a first process variant. The mold used here is preferably the tool used later for thixoforming.

The precompressed semi-finished product can now be preheated outside the tool, which can be done inductively, in a convection oven with inert gas atmosphere, with infrared radiators or with lasers, is then introduced in the preheated state in the tool and then converted thixotropic, in particular thixogeschmiedet.

The preferred matrix materials allow the semifinished product to be heated up to the temperature necessary for the thixotropic forming while the semifinished product still has sufficient strength, which permits handling of the semifinished product for insertion into the mold, which can happen automatically, for example. Only when the pressure is subsequently applied by thixoforging over the stamp does the semifinished product lose its shear strength, so that the material is formed in a very short time.

According to a variant of the method, the semifinished products within the tool are heated to a temperature above the solidus line, but below the liquidus line of the matrix material. In this case, the layered material can be brought by a slight pressure into contact with the mold wall. This improves the heat transfer and thus reduces the heating time. Immediately thereafter, the thixotropic transformation takes place, preferably by forging.

In both variants of the method, a controlled cooling of the thixotropically deformed component preferably takes place within the tool in order to achieve directional solidification of the metallic matrix.

According to the invention, composite materials produced in this way can preferably be used as production-shaped (net-shaped) high-strength structural components with high specific rigidity or with a high specific modulus of elasticity. In addition, there is an advantageous use as bearing materials.

• Fig. 1 ein Phasendiagramm Al-Si mit ausgewählten Bereichen für eine thixotrope Umformung für die Knetlegierung AlMgSil und die Gusslegierung AlSi7Mg;
• Fig. 2 ein Prepreg, das aus einem Schichtenverbund von alternierenden Schichten aus Faserschichten und Blechen (Folien) besteht;
• Fig. 3 ein Prepreg, das aus einer Folge von mit metallischem Matrixwerkstoff beschichteten Faserverbunden besteht;
• Fig. 4 ein globolithisches Gefüge einer Al-Si-Legierung;
• Fig. 5 einen Querschnitt durch einen mit einer Metallmatrix infiltrierten Al-Si-Verbundwerkstoff mit eingelagerten C-Fasern nach einem Thixo-Schmiedevorgang und
• Fig. 6 eine schematische Darstellung einer Wickelanlage zur thermischen Spritzbeschichtung von Fasergeweben im industriellen Maßstab.

Further features and advantages of the invention will become apparent from the following description of preferred embodiments with reference to the drawings. Show it:

• Fig. 1 a phase diagram Al-Si with selected areas for a thixotropic transformation for the wrought alloy AlMgSil and the casting alloy AlSi7Mg;
• Fig. 2 a prepreg consisting of a layer composite of alternating layers of fibrous layers and sheets (foils);
• Fig. 3 a prepreg consisting of a series of metallic matrix material-coated fiber composites;
• Fig. 4 a globolithic microstructure of an Al-Si alloy;
• Fig. 5 a cross section through an infiltrated with a metal matrix Al-Si composite material with embedded C-fibers after a Thixo-forging process and
• Fig. 6 a schematic representation of a winding system for the thermal spray coating of fiber fabrics on an industrial scale.

In Fig. 1 is a phase diagram of the preferred eutectic system Al-Si (with magnesium additives) shown. The solid content in the dark hatched area is α (Al). Alloys suitable for thixotropic transformation are wrought alloy AlMgSi1 and casting alloy AlSi7Mg. For a thixotropic transformation, a narrow temperature range is required which lies between the solidus line and the liquidus line for the wrought alloy AlMgSi1 and Fig. 1 is characterized by the rectangular drawn temperature range, which is between 635 and 645 ° C. In this area, the liquid content (L) is 15 to 35%. For the thixotropic transformation of the AlSi7Mg casting alloy, on the other hand, there is a temperature range that lies just above the eutectics and is again denoted by a rectangle. This temperature window is between 574 ° C and 584 ° C. This results in a liquid content (L) of about 43 to 51%.

Such relationships for the thixotropic transformation of light metal alloys are known in principle (see Siegert, K .; Messmer, G .; Baur, J .; Wolf, A .: "Thixoforging of Aluminum Components", in: Proceedings for the 7th Saxon Symposium Forming technology, Chemnitz, 2000 ; Baur. J., Messmer, G .: Automated Thixoforging Unit, in: Proceeedings to the 7th Int. Conf. On Semi-Solid Processing of Alloys and Composites, Tsukuba, Japan, September 24-28, 2002 ).

The Thixoumformung of such light metal materials has been developed in particular for the production of molded parts, which have a production-related shaping. In the temperature range at which the Thixoumformung takes place, the semifinished product is still present with sufficient strength that allows handling of the semi-finished product (also called "bolt" in thixoforging). The alloys in question ideally have a globolithic structure which forms a solid-phase skeleton and thus ensures good handleability of the heated semifinished product when it is placed in the tool (the mold) (cf. Fig. 4 ). This solid-phase skeleton breaks up under shear stress and thus ensures a reduction of the yield stress, so that high flow paths in the tool and a near-net shape forming are ensured.

According to the invention MMCs are now prepared with a metallic matrix material, which is reinforced by embedded fibers or particles, by preparing a semi-finished product in which the fibers or particles are contained in a metallic matrix material and the semifinished product by thixotropic forming in a tool at a temperature is formed above the solidus temperature and below the liquidus temperature of the metallic matrix material.

In principle, a loose combination of fibers on the one hand and metallic matrix material on the other hand is sufficient to ++ Thixoumformvorgang with a short forming time to produce a high-quality, virtually non-porous composite material.

Basically, several process lines are possible for this purpose.

In a first method line, which is in Fig. 2 is shown schematically, however, is not within the scope of the invention, a semi-finished product designated generally by the numeral 10 is produced by laminating of alternating sheet metal and fiber fabric layers. A fiber composite 12 consists for example of carbon fibers, which are arranged in the form of a fabric. To produce the semifinished product 10, fiber composite layers 12 are laminated alternately with thin metal sheets 14 of the matrix material, for example AlSi7Mg. Depending on the desired volume ratio between fibers and matrix material, the thicknesses d _{1 of} the fiber composites 12 and d _{2 of} the sheets 14 are adjusted. Expediently, the semifinished product 10 is enclosed by an outer layer 18 of matrix material, in order to ensure that as far as possible no fibers reach the surface of the component during the subsequent thixoforming. Such a semifinished product or prepreg is placed in a suitable die form and reshaped therein in the appropriate temperature range by thixoforging using a plunger.

In this case, precompacting can first be carried out within the tool in a still cold state, and then the semifinished product 10 outside the die mold can be preheated to the temperature necessary for thixotropic deformation. This can be carried out inductively or alternatively in a circulating air oven with protective gas atmosphere or with high-power infrared radiators or by means of laser by rapid heating of the precompressed semifinished product. Thereafter, a quick transfer takes place in the suitably preheated Gesenkform (100 to 400 ° C). This process can be done manually, semi or fully automated. Then the thixo forging process is carried out. For this purpose, the ram meets with process-specifically adapted ram speed of up to 800 mm / s on the surface of the semi-finished product. The impact velocity of the ram on the semi-finished product is preferably set between 10 mm / s and 300 mm / s depending on the fiber-matrix ratio, the component complexity and the component volume. In order to prevent premature solidification of the metallic matrix material, the plunger is expediently preheated accordingly.

Alternatively, the heating of the semifinished product can also take place within the die tooling. In this case, the layered semifinished product can be brought into contact with the tool wall by a slight pressure, which improves the heat transfer and reduces the heating time. Immediately thereafter, the thixotropic transformation takes place by thixoforging.

When sheets (foils) are used, preferably cold-rolled sheets are used, since the high dislocation density during subsequent rewarming in the course of the thixo-forming results in fine-grained recrystallization with a globular structure.

A second process variant according to the invention for the production of a semifinished product consists in the coating of individual fiber composites, which are then laminated by superimposing into a prepreg and expediently again surrounded by an outer metal layer or film layer of matrix material.

This variant of the method is described below with reference to Fig. 3 and 6 explained in more detail.

A third alternative, which does not lie within the scope of the invention, for producing the semifinished product consists in producing a mixture of short fibers or pulverulent reinforcing particles with metallic matrix powders. This is followed by shaping by dry pressing or further processing, again by application to a fiber composite by a coating process.

In the second variant of the method, application by electrostatic charging, application by screen printing or electrophoretic deposition (EPD) to the fiber structure is generally suitable for coating the fiber composite.

By means of EPD, an electrically conductive or electrically rendered fiber structure is connected as an electrode, and the charge-carrying metal powder particles are driven by an electric field deposited on the fiber or fabric surface as a uniform layer. The metal particles have a particle size with diameters between 10 nm and 100 μm, preferably with sizes between 100 nm and 10 μm. Through the use of surface-active liquid or dissolved excipients, a targeted charge distribution of the solid particles in the suspension can be set, which allows a concentration and field strength-dependent mass transfer for layer deposition.

A particularly preferred coating method is the coating by thermal spraying. This is in particular Electro-arc wire spraying and powder plasma spraying, preferably atmospheric plasma spraying (APS) in the foreground.

During the thermal spray coating of the fibers, a temperature increase of the fiber composite is largely avoided by targeted cooling measures, so that temperatures of at most 100 ° C can be maintained in the rule, and thus damage to the fibers is excluded. For this purpose can be cooled locally by means of coolant injectors and other suitable devices locally, preferably using compressed air and possibly of liquid carbon dioxide (CO ₂ ).

Furthermore, it is preferred in this process variant, long or continuous fibers or fiber composites produced therefrom (scrim, fabric or knitted fabric) stretched or zugvorgespannt on a suitable support device 30 (see. Fig. 6 ) and applying one or more layers of matrix metal by thermal spraying. With a winding system 30 according to Fig. 6 can be unwound continuously or intermittently from a roll 34, which contains fiber composite layers to be coated, in a coating plane (left-hand curved surface in FIG Fig. 6 ) and then rewind onto a roll 32 after coating. With suitable dimensioning and coating thickness, in this case firstly a coating can take place on a first upper side and then a coating on the opposite surface.

A particular advantage lies in the targeted bias of the fabric in the coating plane, which can also be made controllable in order to ensure an equal, permanent bias. As a result, thermally and convectively generated stresses in the fiber structure are mechanically compensated. For coating, a 5-axis robotic motion system can be used which moves an arc or plasma torch in a manner such that NC controlled motion sequences can be retrieved from previously stored programs for consistent coating and reproducibly and processably realized , In this case, spray gaps of 50 to 200 mm between nozzle exit and fiber surface are preferably used between 100 and 140 mm in APS and between 120 and 160 mm in electric arc spraying.

As in Fig. 3 As shown, such layers 22 of fiber composites coated on both sides with matrix metal 24 are laminated together to form a prepreg, and preferably encased externally by a sheet metal 18 of matrix metal so as to produce a semifinished product 20. The coating thickness d ₃ can be suitably controlled in the previous coating operation. For this purpose, single or multiple layers can be applied and the layer thickness, of course, be influenced by the speed of transit or residence time and the other spray parameters. If larger layers of matrix metal are desired, individual metal sheets or foils can also be interposed.

In the third process variant by granulation or pelletizing, mixtures of matrix metal powders and short fibers or reinforcing particle powders are produced. These granulated or pelletized mixtures can then be compacted into a green body by cold pressing, which is then used as a semi-finished product. If the matrix metal has sufficient ductility, cold pressing can take place without pressing aids. If the ductility of the metal matrix material is lower, suitable binder aids (eg paraffin) which volatilise easily during later heating are added.

Another variant of the method consists in the application of the mixtures produced by granulation or pelletizing by a suitable coating method on fiber composites, which in turn can be done for example by thermal spraying.

As already mentioned, graded layers can be used to produce special component properties which are adapted to specific local thermal or mechanical stresses, for which purpose the entire spectrum in the processing of fiber composites can be utilized.

The particularly favorable distribution that results in an infiltration process by thixoforging in such a composite is out Fig. 5 to see.

Claims (15)

1. Method for producing a component from a composite material (MMC) having a metallic matrix material which is reinforced by embedded fibers or particles, the method comprising the steps of:

   - providing a semi-finished product (10; 20) in which the fibers (16) or particles and the metallic matrix material (14; 24) are contained, and

   - thixoforming the semi-finished product (10; 20) within a tool at a temperature above the solidus temperature and below the liquidus temperature of the metallic matrix material (14; 24);

   characterized in that the step of providing a semi-finished product (10; 20) comprises a coating of a fibrous structure with the metallic matrix material or with a granulatized or palletized mixture of a matrix material and short fibers or powdered particles, using a process which is selected from the group which is formed by:

   - coating a fibrous structure by thermal spraying, preferably by atmospheric plasma spraying (APS), wire flame spraying, or by electric arc spraying;

   - coating the fibrous structure with the metallic matrix material by a screen printing process;

   - coating the fibrous structure with the metallic matrix material by electrostatic charging;

   - coating the fibrous structure with the metallic matrix material by electrophoretic deposition from an aqueous suspension, at the aid of an electric field.
2. Method according to claim 1, characterized in that a succession of layers (12, 14; 22) is laminated to form a prepreg (10; 20) which contains fiber composite structures (22) coated with matrix material, or with mixtures of matrix material and short fibers or particles, as well as metal sheets of matrix material (14) or mixtures of matrix material and short fibers or particles.
3. Method according to claim 2, characterized in that the mixtures of matrix material (14) and short fibers or particles are prepared by granulatizing or pelletizing.
4. Method according to any of the preceding claims, characterized in that the proportion by volume of matrix material and fibers is adjusted to between 0.3 and 8.0, preferably to between 0.8 and 3.0.
5. Method according to any of the preceding claims, characterized in that during production of the semi-finished product the fibrous structure is heated up to a temperature of maximally 300° Celsius for a period of maximally 5 seconds, preferably of maximally 2 seconds.
6. Method according to any of the preceding claims, characterized in that the fibrous structure is maintained under tensile stress during thermal spraying on a carrier device (30).
7. Method according to any of claims 3 to 6, characterized in that during the step of pelletizing or granulatizing fibers are used having a length between 0.5 and 20 mm, preferably between 2 and 6 mm, and wherein preferably a proportion by volume of matrix material and fibers between 0.3 and 5, especially preferred between 1 and 2, is used.
8. Method according to any of the preceding claims, characterized in that at least one layer comprising a fibrous structure of long fibers having a length of at least one millimeter, or of long fibers having an aspect ratio (ratio between length and diameter) of at least 50, preferably of at least 100, especially preferred of at least 150, is used for preparing the prepreg.
9. Method according to any of the preceding claims, characterized in that an aluminium alloy or a copper alloy is used as the matrix material, in particular an alloy is used made of the main constituents aluminium, magnesium and silicon, or made of the main constituents copper, and tin or zinc, preferably an alloy is used which is selected from the group formed by alloys of the type AlMg4.5Mn0.4, AlMgSil, AlSi7Mg, AlSi3, AlSil2, CuZn40A12, and CuSn13.5A10.3.
10. Method according to any of the preceding claims, characterized in that the metallic matrix material is reinforced by embedded particles, which are preferably configured as oxide ceramics, carbides, nitrides, metals or alloys or tribologically active materials.
11. Method according to any of the preceding claims, characterized in that fibers are used that consist of carbon, silicon carbide, aluminium oxide, mullite or that contain modifications of these fibers with nitrogen, titanium, boron, carbon or silicon or compounds thereof, wherein the fibers are preferably coated on their surface, in particular with diffusion barrier layers and protective layers or primer layers.
12. Method according to claim 11, characterized in that fibers are used that are coated with silicon carbide, silicon nitride, titanium carbide, titanium nitride, carbon or mixed phases or compounds thereof.
13. Method according to any of the preceding claims, characterized in that the semi-finished product is heated up for the thixoforming process to a temperature of between 574 and 584° Celsius when using the alloy AlSi7Mg, to a temperature of between 635 and 645° Celsius when using the alloy AlMgSi1, and to a temperature of between 871 and 875° Celsius when using the alloy CuZn40A12.
14. Method according to any of the preceding claims, characterized in that the tool is preheated, preferably to a temperature of between 100 and 400° Celsius.
15. Method according to any of the preceding claims, characterized in that following the thixotropic transformation the component is cooled down within the tool to a temperature below the solidus temperature.

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