Method and device for producing a reinforced component by thixoforming

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

RELATED APPLICATIONS

This application is a continuation application of copending International Patent Application PCT/EP2003/012352 filed on Nov. 5, 2003, published in German, which is fully incorporated by reference herewith.

BACKGROUND OF THE INVENTION

The present invention relates to a method for producing a component from a composite material (MMC) using a metallic matrix material which is reinforced by fibers or particles.

The invention further relates to such a composite material and to the use of such a composite material.

The need to economize on primary sources of energy and to reduce emissions gives light-metal materials ever increasing importance in the automotive industry and in aeronautical and space technologies. The use of MMC materials (metallic matrix composites) and/or embedded high-strength reinforcing fibers is capable of increasing the resistance to high temperatures and the rigidity of such materials, which is insufficient for many applications. In addition, it is desired to reduce the production costs by short production cycles and net shape forming of components.

According to the prior art, embedding fibrous reinforcing components leads to a significant increase in production costs. In addition, insufficient resistance of the types of fibers suited for this purpose to the chemically aggressive melts prevents the strength of the fibers to be optimally utilized in conventional production processes from the liquid phase. Conventional production routes for MMC materials, from the liquid phase, mostly consist in squeeze infiltration processes where a capillary resistance is overcome by squeezing the melt into the porous fiber preform using a piston (“squeeze casting”). The squeeze casting process may be carried out also with vacuum support (“vacural process”). For realizing complex geometries and reducing the mechanical stress acting on such fiber pre-forms during the infiltration process, the preform may be dipped into the melt in an evacuated autoclave and may then be exposed to a gas pressure (“gas pressure process”). In addition, powder-metallurgical forming by sintering, followed by a pressing operation, for example by hot pressing or hot isostatic pressing (HIP), is of course also known. However, sintering methods are very time-consuming and costly and do not allow net shape forming of components of complex geometries. And in most of the cases, considerable residual porosity is encountered in the case of sintering methods, which may have negative effects on the properties.

For producing fiber-reinforced aluminum-based light metals, one normally makes use of molten-metal and liquid-phase impregnation and pressure-casting techniques. The long reaction and contact times of molten metal phases with the reinforcing fibers and the molding tools or mold surfaces lead, however, to undesirable damaging effects. For example, solution and separation processes and chemical interface reactions are encountered that lead to problems such as adhesion of the part to the mold as the part is removed from the mold, structural transformation by heat transmission and, generally, to an extremely complex process sequence. This results in complex production systems with comparatively long cycle times for the molding operation. All processes make use of liquid, chemically highly active light-metal melts (mostly aluminum) so that additional protection for the fibers used, in the form of a suitable protective coat, is inevitable. When aluminum is to be reinforced by carbon fibers, the formation of an aluminum carbide layer (Al₃C₄) at the fiber-matrix interface is especially undesirable as given its marked proneness to brittle fracture and its insufficient corrosion resistance such carbide phase will lead to premature failure of the bond. Coating of the fibers can generally be dispensed with if the light-metal matrix is united with the reinforcing component in the solid phase (“diffusion bonding”). This transforming process is used to process laminates of a fibrous fabric and metal sheets by a hot-pressing process. The economic efficiency of that process is, however, prejudiced by extremely high cycle times. A summary of the commonly used processes for producing fiber-reinforced aluminum composite materials is provided in Talat Lecture 1402, Froyne, L., Verlinden, B., University of Leuven (Belgium), 1994, EAA European Aluminium Association.

SUMMARY OF THE INVENTION

It is a first object of the present invention to disclose a method of producing product from a metallic matrix composite material (MMC) which avoids the drawbacks that arise in the prior art.

It is a second object of the invention to disclose a method of producing a near-net-shaped reinforced product from a metallic matrix composite material which limits damage of any reinforcing material caused by melting of the metallic matrix material.

It is a third object of the invention to disclose a method of producing a product from a metallic matrix composite material that is cost-effective and energy-saving.

It is a forth object of the invention to disclose a method of producing a MMC product by thixoforming.

According to the invention these and other objects are achieved by a method for producing a component from a MMC using a metallic matrix material which is reinforced by embedded fibers or particles, comprising the steps of

producing a semi-finished product containing the fibers or particles and the metallic matrix material; and

thixoforming the semi-finished product in a mold at a temperature above solidus temperature and below liquidus temperature of the metallic matrix material.

Thixoforming has been generally known in the art for transforming special alloys, especially aluminum-based or copper-based alloys, where use is made of a two-phase or a multi-phase range, in which a liquid-phase content exists, in an especially selected temperature range above the solidus line and below the liquidus line. For purposes of that hot transformation in a partially solidified condition, a marked solidification interval is required for the alloy to be transformed, i.e. a temperature range in which both solid-phase and liquid-phase contents are present one beside the other (compare FIG. 1). In that temperature range, near net shape transformation can be reached by application of pressure in a mold (thixotropy-shear rate destabilizing behavior). Ideally, an alloy suited for thixoforming comprises a globolithic structure, which forms a solid-phase framework that ensures good maneuverability of the heated semi-finished product as it is placed in a pressing mold. That solid-phase framework breaks up the shear load, thereby reducing the yield stress so that the flow distances in the mold are only small and near net shape transformation can be reached.

By utilizing the thixoforming process, which is known as such for light-metal alloys, for the production of composite materials that are reinforced by embedded fibers or particles, clearly improved properties can be achieved.

A first advantage consists in the substantially lower temperature and the resulting reduced forming time, compared with melting processes or melt infiltration processes. It is possible in this way to do without coating the embedded fibers or particles, as the embedded fibers will be hardly damaged by the short melting process at a relatively low temperature.

Compared with a squeeze-casting process, clearly shorter infiltration paths of the melt can be achieved by a structure of alternating layers in the semi-finished products. As a result of the shorter process times and the relatively short contact between the required tool and the melt clearly improved mold times are achieved, which leads to considerable cost savings compared, for example, with the squeeze casting process. It is now possible to achieve rapid process times in the millisecond range, compared with several seconds in the squeeze casting process and several hours in the diffusion bonding process. By carrying out the forming process in a temperature range lower than that used for the squeeze casting process (lower by approximately 100 K) energy savings in a corresponding order are achieved. The reduced liquid phase content during the forming process leads to shorter solidifying times in the mold and, thus, to reduced cycle times. Due to suitable preconditioning of the metallic matrix the entire melting system (melting furnace, alloying furnace, degassing, alloy control, casting furnace) can be omitted. And due to the modified mold concept there is a clearly reduced amount of material to be recycled, which must be molten again before it can be used once more in the production process.

As no air inclusions are encountered in thixoforming, especially in thixoforging, it is further possible to improve the strength of the metallic matrix by a suitable heat-treatment, especially by solution annealing and settling.

Further, thixoforging processes offer the possibility to join the component to other components by a material bond.

In the context of the present invention the term “semi-finished product” is understood to describe the preform which is then formed into the final component by thixoforming in a mold.

A semi-finished product may be a prepreg consisting of a single or, preferably, a plurality of laminate layers, or a perform produced from a granulated product by a pressing process. The term semi-finished product when used in the context of the present invention is therefore always used as a generic term, including both a prepreg and other preforms prepared in other ways, that are to be formed into a component by thixoforming in a mold.

According to a first variant of the process a prepreg is produced by laminating layers of fibrous structures and metallic matrix materials in the form of metal sheets.

This especially simple variant of the process consists in joining layers of fibrous structures with the suitable sheets by, preferably, alternating lamination in order to obtain the desired form of the semi-finished product. Due to the thixoforming process, providing the matrix material in the form of metal sheets will be sufficient to guarantee short flow distances and yet uniform wetting of the embedded fibers or particles.

According to an advantageous further development of that variant of the process, the metal sheets are provided in the form of cold-rolled sheets. During the heating process, the dislocation density induced by the cold-rolling process leads to fine-grained recrystallization with a globular structure, which has a favorable effect on the resulting properties of the component.

According to another variant of the process according to the invention, a fibrous structure is coated with a metallic matrix material.

Such a process sequence permits improved homogeneity to be achieved, compared with the use of individual layers of fibrous structures and matrix materials.

According to a convenient further improvement of that embodiment of the invention, a plurality of coated fibrous structures are laminated to form a prepreg.

This permits a near-net-shape geometry to be approximated, and selectively determined property characteristics to be achieved.

According to a first variant of the process coating the fibrous structure with a metallic matrix material may be effected by screen printing.

Another variant of the process for coating a fibrous structure with the metallic matrix material consists in applying the coating by electrostatic charging.

Another variant of the process for coating a fibrous structure with a metallic matrix material consists in electrophoretic deposition (EPD) of the material from an aqueous suspension, supported by an electric field.

In this case, the fibrous structure to be coated is arranged as an electrically conductive electrode, and the charge-carrying metal powder particles are deposited on the fiber and/or fabric surface as a uniform layer under the effect of the electric field. Such a process sequence is suited especially for fibrous structures that are electrically conductive by nature, such as C fibers, though other fibers, which are not electrically conductive by nature, may also be processed if suitable intermediate layers are used.

Such a process preferably uses metal particles having a grain size of between 10 nm and 100 82 m in diameter, preferably a grain size of between 100 nm and 10 μm. The use of liquid or dissolved surfactants permits electric charge distributions of the solid particles in the suspension to be adjusted to ensure concentration and field-strength controlled substance transport for the deposition of layers. It is possible in this way to vary the properties of the component, that is to be produced from that material later, within very broad limits and to adapt them to the requirements of the particular case.

According to another variant of the invention, a fibrous structure is coated with a metallic matrix material by thermal spray-coating. Processes that can be generally envisaged in this connection are atmospheric plasma spraying (APS), arc wire spraying, wire flame spraying or high-speed flame spraying. Preferably, thermal spraying is effected by electric arc wire spraying or powder plasma spraying, especially by atmospheric plasma spraying.

Compared with the use of alternating layers of sheet metal and fibrous structures, fibrous structures provided with thermally sprayed metal layers offer the advantage of a finer-grained structure. While the grain sizes of thixotropically transformable Al—Si sheets, for example, are in the range of 2 to 20 μm, the dimensions of the different phases in thermally sprayed AlSi layer structures are in the sub-micrometer range, due to the high cooling-down speed during application of the layer. Thixotropic transformation therefore permits improved impregnation of the metallic phase into the fibrous framework.

The proportions by volume of matrix material and fibers is preferably adjusted in the thermal spraying process to between 0.3 and 8.0, especially to between 0.8 and 3.0.

All process variants of the invention guarantee that when producing the semi-finished products from layers the metallic matrix and the embedded reinforcing layers are heated up to a low temperature only. Even when applying the coat by thermal spraying, it can be ensured by suitable process control that the semi-finished products will be heated up to a temperature of maximally 300° Celsius for a few seconds only. That period of time may even be reduced to a maximum of five seconds or to a maximum of 2 seconds or even to a shorter period of time. These features guarantee, according to the invention, that the fibrous structure will not be degraded or chemically attacked during production of the semi-finished product.

Preferably, the fibrous structure is cooled during thermal spraying, especially using liquid carbon dioxide.

By applying suitable cooling measures, it is possible to limit the heating-up temperatures of the fibrous structure during the coating process to temperatures clearly below 300° Celsius, preferably to temperatures in the range of 100 to 200° Celsius, and this even for short times.

According to a preferred further development of the invention, the fibrous structure is kept under tensile stress on a carrier device during the thermal spraying process.

It is thus possible to compensate any differences resulting from the considerably greater expansion of the metallic matrix material compared with the embedded fibers. It is especially possible in this way to produce fibrous structures where embedded long fibers are subjected to tensile stress when loaded, while the matrix as such is not stressed before in an undesirable way.

For carrying out a coating operation at an industrial scale, the carrier device may allow the fibrous structure to be conveyed to a coating plane for thermal spray coating either continuously or intermittently.

The fibrous structure may be delivered for this purpose via a winding system.

If such a winding system is used, the fibrous structure may first be coated on one surface and then be coated on the opposite surface.

According to a preferred further development of the invention, a spray distance of 50 to 200 mm is maintained between the surface of the fibrous structure and the orifice in plasma spray coating, while a spray distance of 80 to 300 mm is maintained in electric arc spraying.

Using such process parameters, coating of fibrous structures can be carried out in an especially favorable way so that on the one hand uniform coating is achieved and on the other hand aggressive thermal stresses acting on the fibrous structures are avoided.

According to another variant of the process according to the invention, a mixture of matrix material and chopped fibers is granulated or pelletized.

Preferably, fibers having a length of between 0.5 and 20 mm, preferably between 2 and 6 mm, are used.

The proportions by volume of matrix material and fibers are, preferably, between 0.3 and 5, preferably between 1 and 2.

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

The production of granulated products or pellets using chopped fibers or pulverized particles can be utilized with advantage for producing special graded layers or for producing bearing materials, for example.

According to a first variant of the process, the granulated or pelletized mixtures can be transformed to the semi-finished product by cold pressing. If the matrix material used is sufficiently ductile, the cold pressing process can be carried out without the addition of binders. If, however, the material lacks in sufficient ductility, it will be convenient to add suitable pressing aids, such as paraffin.

According to another variant of the process, the mixture obtained by the granulating or pelletizing process is applied on a fibrous structure by a suitable coating process. This again may consist of thermal spraying, a screen-printing process or any other of the before-mentioned processes.

In order to allow the strength of the components to be produced according to the invention to be improved in a selective way it is preferred to use at least one layer with a long-fiber composite structure in the production of a prepreg. The term long fibers as used in this context describes fibers having a length of at least 1 mm or an aspect ratio (length-to-diameter ratio) of the fiber of at least 50, preferably at least 100, more preferably of at least 150.

It is understood that for laminating prepregs the succession of layers may be varied in a suitable way to selectively influence the properties of the component to be produced. For example, fibrous structures consisting of long fibers, that are coated with a matrix material, can be laminated together in a suitable way. At the same time, layers of a matrix material in the form of metal sheets or films can be inserted. And there is also the possibility to insert intermediate layers of granulated products of pellets. Finally, a combination of coated fibrous structures with preforms, produced from pelletized or granulated mixtures of matrix material and chopped fibers or particles, is likewise imaginable.

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

It can be ensured in this way that the surface of the component so produced is substantially free from embedded fibers or particles.

The fibrous structures may be used in the most different forms in order to guarantee specific properties of the component to be produced. For example, fibrous structures may be used in the form of oriented fiber arrangements consisting of unidirectionally oriented long fibers (UD), or unwoven or woven fibrous structures may be used in the form of 2D fiber composites, 3D fiber composites, in the form of textured or knitted fabrics.

The semi-finished product may be produced for this purpose from graded layers in order to selectively influence the properties at heavily stressed locations of the component or in preferential directions of stress. It is possible in this way to vary the proportion of matrix materials and fibers selectively over the cross-section of the component.

This feature is particularly advantageous for producing components, that are subjected to especially high thermal and/or mechanical local stresses, such as pistons or the like.

Generally, all materials that allow thixotropic reforming are suited as matrix materials.

Preferably, the materials used for this purpose are aluminum alloys or copper alloys, especially alloys composed of aluminum, magnesium or copper as main components, or composed of copper and tin or zinc as main components.

The matrix materials preferred in this connection are alloys of the type AlMg4.5Mn0.4 (AA 5182), of the type AlMgSil (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.

When copper alloys are used, the wear-reducing effect of reinforcing fibers will be the main aspect. Such materials are especially well suited for the production of advantageous bearing materials, the use of a combination with carbon fibers or particles, modified to approximate graphite, being particularly well suited for this purpose because anti-seizure performance can be achieved in this way.

According to another variant of the process, the metallic matrix material as such is reinforced by embedded particles consisting preferably of oxide ceramics, carbides, nitrides, metals or alloys or tribologically effective substances.

For realizing the fiber reinforcement of the MMC, the invention uses fibers consisting of carbon, silicon carbide, aluminum oxide, mullite. And modifications of those fibers with hydrogen, titanium, boron, carbon or silicon, and compounds thereof, are likewise imaginable.

Although due to the shorter process times and the reduced process temperatures coating of the fibers used generally will not be required, the invention also provides the possibility, according to another variant of the process, to use fibers with coated surfaces, especially fibers provided with diffusion barrier or protective layers, or fibers with primer layers.

It is thus possible to adapt the properties of the component to be produced much more effectively to the particular requirements. The use of primers, for example, may improve interface adhesion between the embedded fibers and the metallic matrix phase. At the same time, it is possible to embed fibers which as such are less compatible with the matrix phase employed.

Especially well suited for coating of the fibers are silicon carbide, silicon nitride, titanium carbide, titanium nitride, carbon or mixed phases or compounds thereof.

According to the invention the fibers used preferably have diameters of between 0.5 and 150 μm, more preferably between 5 and 20 μm.

As has been mentioned before, the fibers may be used both in the form of long fibers or endless fibers, and in the form of chopped fibers.

It has been mentioned before that during the thixoforming process, the semi-finished product is heated up to a given temperature interval, depending on the particular matrix material used, in which the matrix material comprises a defined liquid phase content.

When using the alloy AlSi7Mg, for example, the semi-finished product is heated up to a temperature of between 574 and 584° Celsius for thixoforming, in which case a liquid phase content of between 43 and 51 percent by volume will be reached. When using the alloy AlMgSil, for example, the semi-finished product is heated up to a temperature of between 635 and 645° Celsius for thixoforming, in which case a liquid phase content of between approximately 15 and approximately 35 percent by volume will be reached. When using the alloy CuZn40A12, for example, the semi-finished product is heated up to a temperature of between 871 and 875° Celsius for thixoforming, in which case a liquid phase content of between approximately 20 and approximately 40 percent by volume will be reached.

Thixoforming is preferably carried out as thixoforging in a suitable die at controlled ram velocity and pressing force. The ram velocity and pressing force are adjusted in this case according to the particular requirements of the process. Ram velocities of up to 800 mm/s are possible. The velocity of impact of the upper die on the workpiece is preferably adjusted to a rate of between 10 mm/s and 300 mm/s as a function of the fiber-matrix ratio used, the complexity of the component and the volume of the component.

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

Especially by increasing the ram velocity after contact, rapid infiltration can be achieved at high pressure in a time range clearly below 1 second. One thus obtains a short contact time between the metallic melt and the fibrous structure, which results in the fibers being subjected to reduced chemical attack by the melt and, thus, in improved properties of the component so produced.

Further, it is possible, for reducing the formation of oxides, to carry out the thixoforming process in a protective-gas atmosphere.

According to a first variant of the process, the semi-finished product is pre-compacted in a mold for the thixoforming process. The mold used for this purpose is preferably the same that will be used later for thixoforming.

The pre-compacted semi-finished product may now be heated up outside the mold, for example inductively, in a circulating-air furnace, in a protective-gas atmosphere, by infrared radiators or with the aid of lasers, and is then placed in the mold in preheated condition for being thixotropically transformed, especially by thixoforging.

The preferred matrix materials permit the semi-finished product to be heated up to the temperature necessary for thixotropic transforming, while still ensuring sufficient stability of the semi-finished product to allow handling of the semi-finished product when placing it in the mold, which may be effected automatically, for example. The shear strength of the semi-finished product gets lost only by the pressure applied by the ram during the thixoforging process so that the material can be removed from the mold in a minimum of time.

According to one variant of the process, the semi-finished products are heated up in the mold to a temperature above the solidus line, but below the liquidus line of the matrix material. The layered material can be urged into contact with the mold wall by slight pressure in this case. This will improve heat transmission and, thus, reduce the heating-up time. Thixotropic transforming is then carried out immediately thereafter, preferably by forging.

With both variants of the process, the thixotropically transformed component is preferably cooled down in a controlled manner in the mold in order to achieve oriented solidification of the metallic matrix.

Composite materials produced in this way can be used according to the invention preferably as net-shaped highly resistant construction components having high rigidity per unit or a high specific modulus of elasticity. In addition, such materials can be used with advantage as bearing materials.

It is understood that the features of the invention mentioned above and those yet to be explained below can be used not only in the respective combinations indicated, but also in other combinations or in isolation, without leaving the scope of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

Further features and advantages of the invention will become apparent from certain preferred embodiments of the invention which will be described hereafter with reference to the drawings in which:

FIG. 1 shows an Al-Si phase diagram with selected ranges for thixotropic transformation of the wrought alloy AlMgSi1 and the casting alloy AlSi7Mg;

FIG. 2 shows a prepreg consisting of a layered composite material composed of alternating layers of fibers and metal sheets (films);

FIG. 3 shows a prepreg consisting of a sequence of fibrous structures coated with a metallic matrix material;

FIG. 4 shows a globolithic structure of an Al—Si alloy;

FIG. 5 shows a cross-section of an Al—Si composite material, infiltrated with a metallic matrix, with embedded C fibers after a thixoforging process; and

FIG. 6 shows a diagrammatic representation of a winding system for thermal spray coating of fiber fabrics at an industrial scale.

PREFERRED EMBODIMENTS OF THE INVENTION

FIG. 1 shows a phase diagram of the preferred eutectic system Al—Si (with admixtures of magnesium). The solid content in the dark shaded area is α (Al). Wrought alloy Al—SiMi1 and casting alloy AlSi7Mg are shown as alloys suited for thixotropic transformation. The thixotropic transforming process requires a narrow temperature range, which is defined, for the wrought alloy AlMgSil, by the solidus line and the liquidus line and which is represented in FIG. 1 by the rectangle marking the temperature range between 635 and 645° Celsius. The liquid content (L) obtained in this range is between 15 and 35%. In contrast, the temperature range required for thixotropically transforming the casting alloy AlSi7Mg is indicated immediately above the eutectic line and is again represented by a rectangle. That temperature window ranges from 574° Celsius to 584° Celsius, and the liquidity content (L) obtained in this window is between approximately 43 and approximately 51%.

Such interrelations are generally known in connection with the thixotropic transformation of light-metal alloys (compare Siegert, K.; Messmer, G.; Baur, J.; Wolf, A.: “Thixoschmieden von Aluminiumbauteilen” (Thixoforging of aluminium components), in: Tagungsband zur 7. Sächsischen Fachtagung Umformtechnik, Chemnitz, 2000: Baur, J., Messmer, G.: “Automated Thixoforging Unit, in: Proceedings to the 7. Int. Conf. on Semi-Solid Processing of Alloys and Composites, Tsu-kuba, Japan, Sep. 24-28, 2002).

The thixoforming process for such light-metal materials was developed especially for the production of net-shaped components. In the temperature range in which thixoforming occurs the respective semi-finished products still have sufficient strength to allow handling of the semi-finished products (also known as “studs” in thixoforging technology). Ideally, the respective alloys have a globolithic structure that forms a solid-phase framework thereby guaranteeing good handling properties of the heated semi-finished products as the latter are being placed in the pressing die (compare FIG. 4). The solid-phase framework breaks up under shear load, thereby reducing the yield stress, so that high flow distances in the mold and near net shape transformation are guaranteed.

According to the invention, one now produces MMCs-with a metallic matrix material, reinforced by embedded fibers or particles, by preparing a semi-finished product that contains the fibers or particles in the metallic matrix material, and forming the semi-finished product by a thixoforming process in a mold at a temperature above the solidus temperature and below the liquidus temperature of the metallic matrix material.

For the above purpose, a loose bond between the fibers on the one hand and the metallic matrix material on the other hand will be sufficient to produce a high-quality composite material, which is practically free from pores, by a thixoforming process in a short transforming time.

Basically, three process lines are possible to obtain the desired product.

According to a first process line, which is illustrated diagrammatically in FIG. 2, a semi-finished product indicated generally by reference numeral 10 is produced by lamination from alternating layers of sheet metal and fibrous fabric. A fibrous structure 12 consists, for example, of carbon fibers arranged in the form of a fabric. For producing the semi-finished product 10, fibrous composite layers 12 alternating with thin metal sheets 14 of the matrix material, for example of AlSi7Mg, are laminated. The thickness d, of the fibrous structure 12 and d₂ of the metal sheets 13 is adjusted depending on the desired proportions by volume of the fibers and the matrix material. Conveniently, the semi-finished product 10 is enclosed by an outer layer 18 of the matrix material to avoid that any fibers will get to the surface of the component during the subsequent thixoforming process. A semi-finished product or prepreg of that type is then placed in a suitable die and is transformed in the latter by thixoforming in a suitable temperature range using a ram.

During that process an initial pre-compacting step may be carried out in the die, in cold condition of the semi-finished product 10, followed by the step of heating up the semi-finished product to the temperature required for thixoforming. This can be achieved by heating up the pre-compacted semi-finished product rapidly, either inductively or alternatively in a recirculating-air furnace in a protective-gas atmosphere, or by high-power infrared radiators or by laser. That step is then followed by rapid transfer to the conveniently preheated die (100 to 400° Celsius). This operation may be carried out manually, semi- or fully-automatically. Thereafter, the thixoforging operation is carried out by the ram striking the surface of the semi-finished product at a ram velocity of up to 800 mm/s, adapted to the specific requirements of the process. The impact velocity of the ram is adjusted, preferably, to between 10 mm/s and 300 mm/s, depending on the fiber-to-matrix ratio, the complexity of the component and the volume of the component. In order to prevent premature solidification of the metallic matrix material the ram conveniently is also preheated in a suitable way.

Alternatively, heating-up of the semi-finished product may be effected also in the die. For this purpose, the layered semi-finished product may be urged into contact with the wall of the die at slight pressure whereby heat transmission can be improved and the heating-up time can be reduced. Immediately thereafter, the thixotropic transforming operation is then effected by thixoforging.

If metal sheets (films) are used, cold-rolled sheets are preferred as the high relocation density will later, as the material is re-heated for thixoforming, lead to fine-grained recrystallization with globular structure.

A second variant of the process for producing a semi-finished product consists in coating individual fibrous structures which are then arranged one on top of the other to laminate a prepreg, and are again conveniently enclosed by an outer metal sheet or film layer of matrix material. This variant of the process will be described hereafter in more detail with reference to FIGS. 3 and 6.

A third alternative of producing the semi-finished product consists in preparing a mixture of chopped fibers or pulverized reinforcing particles and metallic matrix powder. The mixture is then formed by dry pressing, or is further processed by applying it on a fibrous structure using a coating process.

In the case of the second variant of the process, application by electrostatic charging, application by screen printing processes or electrophoretic deposition (EPD) on the fibrous structure are generally well suited for coating the fibrous structure.

In the case of the EPD process, an electrically conductive layer, or one that has been made electrically conductive, is arranged as an electrode, and the charge-carrying metal powder particles are deposited on the fiber and/or fabric surface as a uniform layer under the effect of the electric field. The metal particles in this case have a grain size of between 10 nm and 100 μm in diameter, preferably a grain size of between 100 nm and 10 μm. The use of liquid or dissolved surfactants permits the electric charge distribution of the solid particles in the suspension to be adjusted to ensure concentration and field-strength controlled substance transport for the deposition of layers.

An especially preferred coating method is the thermal spray-coating method. Among these processes, the electric arc wire spraying and powder plasma spraying processes, preferably the atmospheric plasma spraying (APS) processes, are of main importance.

During thermal spray coating of the fibers, a rise in temperature of the fibrous structure is largely prevented by selective cooling measures so that as a rule temperatures of maximally 100° Celsius can be maintained, and any damage to the fibers can be excluded. Cooling can be effected locally by the simultaneous use of coolant injectors and other suitable devices, preferably using air and, if necessary, liquid carbon dioxide (CO₂).

Moreover, it is preferred in the case of that variant of the process to apply long or endless fibers, or fibrous structures produced from there from (oriented arrangements, fabrics or knitted structures) on a suitable carrier device 30, in oriented or prestressed fashion (compare FIG. 6), or to apply one or more layers of the matrix metal by thermal spraying. Using a winding system 30—illustrated in FIG. 6—the fibrous structure can be wound off continuously or intermittently from a roll 34 containing the fibrous structure layers to be coated, and can then be coated in a coating plane (curved surface on the left side of FIG. 6) and be wound up again on a roll 32 following the coating process. If suitable dimensions and coating thicknesses are selected, a first coating may be initially applied on a first upper surface, hereafter the opposite surface may be coated as well.

A particular advantage resides in the option to selectively prestress the fabrics in the coating plane, which prestress additionally may be controllable to guarantee uniform permanent prestressing. This then mechanically compensates for thermal and convective stresses in the microstructure. Coating may be effected using a 5-axes-robot-aided motion system adapted to move an arc or plasma burner so that NC-controlled motion sequences can be retrieved from previously stored programs and can be realized in a reproducible, process-stable way for the purpose of achieving uniform coating results. The spray distances between orifice and fiber surface are 50 to 200 mm, preferably between 100 and 140 mm for APS, and between 120 and 160 mm for electric arc spraying applications.

As illustrated in FIG. 3, such layers 22 of fibrous structures, coated with a matrix metal 24 on both sides, are laminated to form a prepreg and are again preferably enclosed on their outsides by a metal sheet or a film 18 of matrix metal to produce a semi-finished product 20. The coating thickness d₃ of the preceding coating process can be suitably controlled. This can be done by applying one or more layers, or by influencing the layer thickness by varying the actuating speed and/or the dwelling time or the remaining spraying parameters. If larger layers of matrix metal are desired, individual metal sheets and/or films may be inserted additionally.

In the case of the third variant of the process, using granulating and/or pelletizing operations, mixtures of matrix metal powders and chopped fibers or reinforcing particle powders are produced. Such granulated or pelletized mixtures can then by compacted by cold pressing to form a green compact which is then used as a semi-finished product. If the matrix metal has sufficient ductility, cold pressing may be effected without any pressing aids. If the ductility of the metal matrix material is low, suitable binder aids, such as paraffin, may be used which will evaporate readily during the subsequent heating-up process.

Another variant of the process consists in applying the mixtures, formed by granulating or pelletizing, on fibrous structures using a suitable coating process, for example a thermal spraying process.

As has been mentioned before, special component properties, adapted to particular local thermal or mechanical stresses, can be obtained by the use of graded layers, for which purpose the entire spectrum available in processing fibrous composite materials may be utilized.

A particularly favorable distribution obtained in such a composite material by an infiltration process resulting from a thixoforging operation can be derived from FIG. 5.

Claims

1. A method for producing a component from a composite material (MMC), comprising the steps of:

providing a fibrous structure;

consolidating said fibrous structure with a metallic matrix material to a semi-finished product;

transferring said semi-finished product into a mold;

heating said semi-finished product to a temperature above a solidus temperature and below a liquidus temperature of the metallic matrix material;

thixoforming said semi-finished product to form a product; and

cooling said product down to room temperature.

2. The method of claim 1, wherein said semi-finished product is produced by a process selected from the group formed by:

laminating layers of fibrous structures and metal sheets to form a prepreg;

coating a fibrous structure with a metallic matrix material;

laminating a plurality of coated fibrous structures to form a prepreg;

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

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

coating a fibrous structure with the metallic matrix material by electrophoretic deposition from an aqueous suspension, at the aid of an electric field; and

coating a fibrous structure with the metallic matrix material by thermal spraying.

3. The method of claim 2, wherein said fibrous structure is coated by a thermal spraying process selected from the group formed by:

atmospheric plasma spraying, wire flame spraying and electric arc spraying.

4. The method of claim 1, wherein a proportion by volume between said metallic matrix material and a reminder of said semi-finished product is selected to be in the range of 0.8 to 3.0.

5. The method of claim 2, wherein the fibrous structure during thermal spraying is maintained under a tensile stress.

6. A method for producing a component from a composite material (MMC), comprising the steps of:

mixing a metallic matrix material with a reinforcement material by a method selected from the group formed by granulating and pelletizing;

transferring a mixture obtained from said mixing step into a mold;

pressing said mixture within said mold to form a semi-finished product;

transferring said semi-finished product into a die;

heating said semi-finished product to a temperature above a solidus temperature and below a liquidus temperature of the metallic matrix material;

thixoforming said semi-finished product within said die to form a product; and

cooling said product down to room temperature.

7. The method of claim 6, wherein said reinforcement material is formed by chopped fibers.

8. The method of claim 7, wherein said chopped fibers have a length of between 0.5 and 20 mm.

9. The method of claim 6, wherein a proportion by volume between said metallic matrix material and a reminder of said semi-finished product is selected to be in the range of 0.8 to 3.0.

10. The method of claim 6, wherein said reinforcement material is selected from the group formed by:

pulverized particles, oxide ceramics and carbides.

11. The method of claim 1, wherein said fibrous structure is prepared with at least one layer made from long fibers, said long fibers being defined by a characteristic selected from the group formed by:

a length of at least one millimeter; and

an aspect ratio (length-to-diameter ratio) of at least 50.

12. The method of claim 11, wherein a succession of layers is laminated to form a prepreg, wherein said succession of layers comprises at least two layers selected from the group formed by:

a fibrous structure comprising long fibers;

a fibrous structure coated with a metallic matrix material;

a metallic matrix material layer;

a mixture of metallic matrix material and chopped fibers;

a mixture of metallic matrix material and reinforcement particles.

13. The method of claim 6, further comprising the step of providing at least one graded layer within said first mold.

14. The method of claim 1, wherein said metallic matrix material is selected from the group formed by:

an aluminum alloy;

a copper alloy;

an alloy comprising aluminum, magnesium and silicon as main components;

an alloy comprising copper and tin as main components;

an alloy comprising zinc as main component.

15. The method of claim 1, wherein said metallic matrix material is selected from the group formed by:

an alloy of the type AlMg4.5Mn0.4;

an alloy of the type AlMgSil,

an alloy of the type AlSi7Mg,

an alloy of the type AlSi3,

an alloy of the type AlSi12,

an alloy of the type CuZn40A12 and

an alloy of the type CuSn13.5A10.3.

16. The method of claim 1, wherein said metallic matrix material comprises embedded particles, said embedded particles being selected from the group formed by oxide ceramics, carbides, nitrides, metals, alloys and tribologically active materials.

17. The method of claim 1, wherein said fibrous structure comprises fibers selected from the group formed by:

carbon;

silicon carbide;

aluminum oxide;

mullite;

a modification of a carbon fiber with at least one component selected from the group formed by nitrogen, titanium, boron, carbon and silicon;

a modification of a silicon carbide fiber with at least one component selected from the group formed by nitrogen, titanium, boron;

a modification of an aluminum oxide fiber with at least one component selected from the group formed by nitrogen, titanium, boron, carbon and silicon;

a modification of a mullite fiber with at least one component selected from the group formed by nitrogen, titanium, boron, carbon and silicon.

18. The method of claim 1, wherein said fibrous structure comprises fibers selected from the group formed by:

a fiber coated on its surface;

a fiber provided with a diffusion barrier;

a fiber provided with a protective layer;

a fiber provided with a primer layer.

19. The method of claim 1, wherein said fibrous structure comprises fibers having a diameter of between 5 and 20 μm.

20. A method for producing a component from a composite material, comprising the steps of:

preparing a mixture from a metallic matrix material and a reinforcement material;

providing a fibrous structure;

applying said mixture by a coating process onto said fibrous structure and forming a semi-finished product there from;

transferring said semi-finished product into a die;

heating said semi-finished product to a temperature above a solidus temperature and below a liquidus temperature of the metallic matrix material;

thixoforming said semi-finished product to form a product; and

cooling said product down to room temperature.