Casting device and casting process for the production of metal matrix composite materials

Abstract

The present invention relates to a casting apparatus for producing metal matrix composite materials, which comprises a metal melting device for producing a molten metal, a particle feeding device for feeding solid particles to the molten metal and a mixing zone having at least one mechanical and / or electromagnetic stirrer which for distributing the solid particles in the molten metal. The invention further relates to a casting method for producing metal matrix composite material, in which a molten metal is produced, to which are fed solid particles which are distributed in the molten metal by at least one mechanical or electromagnetic stirrer, whilst the molten metal with the solid particle is kept in a continuous flow. According to the invention the mixing zone has an inclined runner having at least one mixing well, the associated stirrer being provided in and / or on the mixing well, or the molten metal with the solid particles flows into at least one inclined runner and is mixed thoroughly by the associated stirrer in at least one inclined mixing well forming a depression in the runner.

Description

[0001] The present invention relates to a casting device for the production of metal matrix composite materials, comprising a metal melting device for generating a metal melt, a particle feeding device for feeding solid particles to the metal melt and a mixing zone comprising at least one mechanical and / or electromagnetic stirrer for distributing the solid particles in the metal melt.

[0002] The invention further relates to a casting process for the production of metal matrix composite materials, in which a metal melt is produced, solid particles are added to the metal melt, and in a mixing step the solid particles are distributed in the metal melt by stirring through at least one mechanical and / or electromagnetic stirrer, while the metal melt with the solid particles is kept in a continuous flow.

[0003] Metal matrix composite (MMC) materials are metals or metal alloys containing solid particles. Compared to their unreinforced counterparts, these particle-reinforced metals or metal alloys offer significantly higher wear resistance and increased strength, particularly at high temperatures.

[0004] The solid particles used in MMC materials can be, for example, ceramic particles. These solid particles can consist of metal oxide(s), preferably aluminum oxide, metal nitride(s), metal carbide(s), preferably silicon carbide, metal silicon cide(s), and / or glass. Metal matrix composite materials, especially aluminum matrix composites (AMC), were first considered commercially in the early 1970s. The primary motivation stemmed from the need for higher-performance lightweight materials for aerospace and military applications. Research led to the first practical manufacturing processes by the end of the 1980s.

[0005] Nevertheless, only a few commercial applications of MMC have been developed so far, as the insufficient robustness of established MMC production processes, despite elaborate process control, leads to fluctuating material qualities and additional post-processing costs for MMC products, resulting in high MMC material costs to date. Therefore, despite their great application potential, MMC materials are only found in niche applications or high-end technologies. Currently, it is not foreseeable that any of the globally operating suppliers of MMC materials will overcome the hurdle of the necessary cost-benefit ratio for mass production.

[0006] MMC materials are currently manufactured by stir casting, sintering, melt infiltration or spray compaction.

[0007] Only stir casting has been developed to industrial maturity so far, and AMC ingots produced with this method are the most commonly used AMC precursors on the market. AMC ingots are remelted in foundries and cast into finished products.

[0008] Such a stir casting process is described in the generic publication DE 692 23 950 T2. In this process, for example, an aluminum alloy is first melted in a melting furnace. Oxides, particles, dissolved gas, and other impurities are removed from the melt by blowing an argon-containing gas through it. The melt is then pumped from the top into a first cylindrical mixing vessel. Ceramic particles are also introduced into the first mixing vessel and stirred into the melt until the ceramic particles are sufficiently wetted by the aluminum alloy. A dispersing impeller, immersed in the melt, is used for stirring.

[0009] The resulting mixed composite material is drawn off from the bottom of the first mixing vessel and conveyed via a pipe into a second cylindrical mixing vessel. In this second vessel, the composite material flows upwards, unlike in the first, and is then mixed again using a stirring paddle. The mixed composite material is then transferred from the second mixing vessel to a holding oven.

[0010] As an alternative to vertically oriented mixing vessels, the German patent application DE 692 23 950 T2 further proposes an elongated tubular mixing vessel with a horizontal cylinder axis. In such a horizontal mixer, an induction coil is wound around the outside of the mixing vessel and / or one or more agitator vanes project vertically from above into the interior of the mixing vessel in order to mix the molten metal and the particulate material that continuously flows through the mixing vessel from one side to the other during operation.

[0011] In the publication US 2012 / 0043050 A1, a stirrer is also used when sprinkling particles into a molten metal. Here, at least one pair of rollers is provided at a tapered end of the melt line, with which the cooling melt is rolled into a plate, thereby refining the particles.

[0012] The device described in US 6,015,528 A for producing metal matrix composite materials also features a stirrer that extends from above into a mixture of molten metal and solid particles and is intended to prevent segregation of the particles by their settling or rising before the melt solidifies.

[0013] Instead of stirrers, the use of ultrasonic devices for mixing a flowing melt containing solid particles has also been proposed. However, such methods are limited to small volumes.

[0014] From German patent application DE 1 194 152 A, for example, a method for the fine dispersion of insoluble substances in molten metal by means of mechanical vibrations is known, in which the acoustic stress on the walls of the vessel containing the molten metal remains low. To achieve this, the substances to be dispersed are brought into direct contact with a sound-radiating surface located within the molten metal. For this purpose, the substances to be dispersed are pressed or sintered into rods or formed as powder-filled tubes, which are guided or pressed against the sound-radiating surface.

[0015] The only commercially available AMC material produced using a stir molding process is based on an AlSi9-10Mg0.5 matrix with SiC reinforcement of 10 or 20 vol.% at a fixed SiC particle size of approximately 13 µm.

[0016] The known stir molding processes cannot yet be used to produce the AMC materials required for specific applications on an industrial scale. Currently, these processes only allow the production of 150 to 160 liters of AMC melt in a single batch. The stirring process alone takes one to two hours per batch. Furthermore, the stir molding process described in German patent application DE 692 23 950 T2 is energy-intensive and therefore economically and ecologically problematic.

[0017] Furthermore, the known stir molding processes exhibit insufficient process flexibility with regard to matrix and reinforcing materials and their contents, require excessively long process times to ensure a homogeneous particle distribution, and are limited with regard to the maximum achievable reinforcing content.

[0018] Particularly due to the low reproducibility of stir-molded MMC semi-finished products, alternative MMC production methods such as sintering, spray compaction or melt infiltration are used to increase material quality and process robustness, but these methods are associated with a significant increase in the cost of the manufactured MMC materials.

[0019] In the sintering or powder metallurgy process for the production of AMC, a powder mixture of ceramic particles and aluminum powders is processed. This allows for hard material content of up to 50%.

[0020] The product resulting from sintering or powder metallurgy is a high-quality, solid AMC semi-finished product, but without potential undercuts or intricate contours, which generally necessitates extensive mechanical post-processing. The superior properties of powder-metallurgically produced AMC compared to melt-metallurgically manufactured AMC are, however, associated with higher raw material costs and greater effort in pretreatment and the manufacturing process. For these reasons, powder-metallurgically produced AMC semi-finished products are primarily used in demanding high-tech applications in the aerospace and defense industries. Large-scale production of AMC semi-finished products using powder metallurgy with correspondingly low material costs is not yet feasible.

[0021] In spray compaction, ceramic particles are incorporated into a stream of molten aluminum particles using a gas jet. Currently, this process can be used to produce bolt-shaped semi-finished products up to 2 m high with a maximum outer diameter of 300 mm.

[0022] Spray compaction can only achieve a particle reinforcement content of approximately 20 vol.% in an MMC material. Furthermore, in addition to high manufacturing costs, the limited manufacturability of complex geometries severely restricts the mass production of this technology.

[0023] In melt infiltration, the ceramic reinforcement phase is prepared as a porous foam and then infiltrated with molten aluminum. Besides particles, fibers and foams with high reinforcement content can also be processed into near-net-shape components.

[0024] Due to high process and semi-finished product costs, melt infiltration is only suitable for demanding components with high added value. Large-scale production is currently not feasible.

[0025] Furthermore, numerous devices and methods for casting molten metal are known from the prior art in which no particles are introduced into the molten metal. For example, German patent application DE 199 62 471 A1 describes a device for casting molten metal in which a casting trough, mounted on a support structure so as to be tiltable and rotatable, is arranged between a storage vessel and a casting mold.

[0026] It is therefore the object of the present invention to provide a casting device and a casting method of the type mentioned above, with which large quantities of MMC materials with a high particle fill level and homogeneous particle distribution can be produced at comparatively low costs.

[0027] The object of the invention is achieved, firstly, by a casting device for the production of metal-matrix composite materials, which has a metal melting device for generating a metal melt, a particle feeding device for feeding solid particles to the metal melt, and a mixing zone having at least one mechanical and / or electromagnetic stirrer for distributing the solid particles in the metal melt, wherein the mixing zone has at least one casting trough inclined in a casting and flow direction of the casting device with at least one mixing basin forming a depression in the casting trough, wherein the at least one stirrer is arranged in and / or on the at least one mixing basin.

[0028] At least one of the pouring channels provides a forced guide for the molten metal-solid particle dispersion. The pouring channel has, for example, a C-, U-, or V-shaped cross-section.

[0029] The inclination of the at least one casting channel, through which the molten material injected with the solid particles—i.e., the composite melt—can flow, results in a continuous material flow in the casting device according to the invention. Due to the inclination of the casting channel, in which the at least one mixing basin is formed, its outlet is lower than its inlet, thus ensuring a continuous outflow of the composite melt from the respective mixing basin in the direction of the inclination. Therefore, an MMC material can be produced continuously with the casting device according to the invention without the need to integrate complex and energy-intensive pumping technologies into the process line. Thus, the respective MMC material can be produced very efficiently.

[0030] The casting device according to the invention enables a stir casting process in which the molten metal containing the solid particles is in a continuous but slow flow, ensuring reliable wetting and homogeneous distribution of the solid particles in the molten metal, even with large particle quantities. Accordingly, the casting device according to the invention allows for the continuous production of high-quality MMC materials with a high particle fill level, for example, with a particle fill level of at least 20 vol.%.

[0031] In the casting device according to the invention, at least one mixing basin is located in a flow path formed by the casting trough for the molten metal-solid particle dispersion. That is, the molten metal-solid particle dispersion is forced into the at least one mixing basin. The at least one mixing basin is formed by a depression in the casting trough. The molten metal-solid particle dispersion remains in the at least one mixing basin for a longer period than it would on a correspondingly long, straight section of the casting trough.

[0032] In the at least one mixing tank, where the molten metal has a longer residence time than in the casting trough, the solid particles are even better wetted by the molten metal. This wetting is further enhanced in the at least one mixing tank by the forces exerted on the flowing molten metal and the solid particles contained within it by the at least one stirrer. The interaction between the molten metal and the stirring action of the at least one stirrer results in particularly good wetting of the solid particles in a comparatively very short process time.

[0033] If at least one stirrer is a mechanical stirrer, shear forces are exerted on the metal molten-solid particle dispersion by it, which promotes the wetting of the solid particles with the molten metal.

[0034] If the at least one stirrer is an electromagnetic stirrer with one or more coils, the magnetic field it generates, depending on the specific arrangement, creates a turbulence in the molten metal-solid particle dispersion, which accelerates wetting. The at least one electromagnetic stirrer ensures contactless mixing of the particle-laden molten metal and promotes wetting.

[0035] With at least one stirrer, significantly larger shear zones can be formed in the molten metal compared to ultrasonic treatments when using a mechanical operating principle, which particularly promotes the wetting of the solid particles.

[0036] Furthermore, the use of at least one stirrer makes it advantageous to remove gas inclusions from the molten metal, regardless of the stirring principle used.

[0037] The present invention is not limited with regard to the design of the respective stirrer. If at least one mechanical stirrer is used, the use of a stirrer with impeller blades is recommended, for example with two, three or more than three impeller blades. The angular orientation of the impeller blades is variable.

[0038] Preferably, at least one stirrer is designed to be height-adjustable, so that it / they can be removed from melt residues in the respective mixing tank, for example at the end of the process.

[0039] At least one mechanical stirrer is typically electrically operated.

[0040] More than one agitator can be arranged in or on each mixing tank. For example, a mechanical agitator can be located inside the mixing tank and an electromagnetic agitator on the outside. It is also possible for the mixing tank to be long enough to accommodate several agitators arranged in series within or on its side.

[0041] The inclination of the casting trough and the at least one mixing basin is generally variable and depends, for example, on a starting value of the viscosity of the molten metal solid particle dispersion.

[0042] The casting device according to the invention can process a wide variety of liquid metals / metal alloys and solid particles, both in terms of the respective material and particle size, into MMC materials. For this purpose, the at least one casting trough and the at least one mixing basin can be dimensioned accordingly.

[0043] In a preferred embodiment of the casting device according to the invention, at least one slide is arranged in the at least one mixing basin, the slide being adjustable vertically in its distance to a bottom of the respective mixing basin.

[0044] The distance of the lower edge of the respective slide to the bottom of the respective mixing basin, which determines the passage height or cross-sectional area for the molten metal solid particle dispersion, can be easily changed and thus controlled by the vertical mobility of the slide.

[0045] To prevent premature (partial) solidification of the molten metal-solid particle dispersion due to cooling and thereby keep the molten metal-solid particle dispersion permanently in flow, it is advantageous if at least one heater is arranged at the casting trough and / or at the at least one mixing basin.

[0046] In an advantageous embodiment of the casting device according to the invention, at least one cover is applied to the at least one casting trough and / or the at least one mixing basin. The at least one cover prevents splashing of the molten metal from the at least one casting trough and / or the at least one mixing basin. If no cover is used, other precautions should be taken to protect users of the casting device according to the invention from escaping hot molten metal.

[0047] In some applications of the casting device according to the invention, such as when a mold change is required, it may be advantageous for the casting device to have a buffer tank into which at least one casting channel opens. To compensate for settling movements of the solid particles in the molten metal, i.e., sedimentation, it is then advantageous if a stirrer or an ultrasonic sonotrode is arranged in or on the buffer tank.

[0048] The problem is further solved by a casting process for the production of metal-matrix composite materials, in which a metal melt is produced, solid particles are added to the metal melt, and in a mixing step the solid particles are distributed in the metal melt by at least one mechanical and / or electromagnetic stirrer, while the metal melt with the solid particles is kept in a continuous flow, in which the metal melt with the solid particles flows during the mixing step in at least one casting trough inclined in a casting and flow direction and is mixed in at least one mixing basin forming a depression in the casting trough by the at least one stirrer.

[0049] In the casting process according to the invention, continuous molten metal production and solid particle feeding take place. Compared to prior art batch processes, the casting process according to the invention therefore enables the continuous casting of MMC materials with significantly increased solid particle content. The continuity of the process allows for the efficient production of MMC melts or solid-containing molten metals.

[0050] For example, the casting process according to the invention has no idle time due to filling or emptying processes.

[0051] In the casting process according to the invention, by mechanically or electromagnetically mixing the continuously flowing molten metal solid particle dispersion in at least one mixing basin with a dispersion residence time increased compared to the casting trough, a high solid particle content can be homogeneously incorporated into the molten metal.

[0052] The at least one mixing basin, into which the flowing molten metal-solid particle dispersion is directed in the casting process according to the invention, forms an intensive mixing zone for the at least one stirrer. This results in a possible maximization of the mixing effect, which is accompanied by a significant reduction in process time.

[0053] In the casting process according to the invention, the molten metal is produced under normal atmosphere or in a vacuum. The molten metal is conveyed as needed via a piping system with a defined pressure differential into a process chamber. The piping system is preferably heatable or heated.

[0054] The solid particles can be located in a storage container or silo which is coupled to the process chamber and in which preferably the same pressure conditions prevail as in the process chamber.

[0055] The solid particles can be conveyed into the piping system or directly into the casting trough, for example, by means of at least one screw conveyor or a vibrator. The conveying section along which the solid particles are fed into the piping system or the casting trough is preferably heated or can be heated.

[0056] After the solid particles are added to the molten metal, the molten metal injected with the solid particles flows downwards in the at least one inclined casting channel due to gravity.

[0057] In the casting process according to the invention, the molten metal is stirred with the solid particles introduced therein to ensure a homogeneous distribution and wetting of the solid particles in the molten metal and simultaneous degassing of the molten metal. The flowing molten metal with the solid particles introduced therein can be stirred both mechanically and electromagnetically.

[0058] Preferably, the geometry of the at least one casting trough and the at least one mixing basin is designed such that the residence time of the molten metal containing the solid particles in the at least one mixing basin is maximized. Nevertheless, the molten metal-solid particle dispersion remains in a continuous flow at all times. In the present invention, "continuous flow" means that the molten metal-solid particle dispersion never comes to a standstill during the mixing step; however, the velocity at which the molten metal-solid particle dispersion flows during the mixing step is lower in the mixing basin(s) than it would be in the at least one casting trough.

[0059] This ensures that, in the casting process according to the invention, the solid particles are continuously wetted by the molten metal material, the molten metal is degassed, and the molten metal-solid particle dispersion is homogenized.

[0060] In the casting process according to the invention, the dosing of the solid particles, the injection or feeding of the solid articles to the molten metal, as well as the mixing step in which the solid particles are wetted with the molten metal, can take place both in a vacuum and under a protective gas atmosphere.

[0061] The metal molten solid particle dispersion produced in the mixing step can, for example, be directed into a buffer container in which there is no longer a continuous flow of molten metal.

[0062] Furthermore, it is possible to finally pour the metal molten-solid particle dispersion produced in the mixing step into at least one, preferably heated, permanent mold, in which the metal molten-solid particle dispersion subsequently solidifies into a solid MMC material or an MMC semi-finished product suitable for further processing or a near-net-shape product made of MMC material.

[0063] Alternatively, the generated, homogenized metal melt solid particle dispersion can be introduced in the molten state into a die casting machine in which near-net-shape components made of MMC material can be manufactured.

[0064] Furthermore, it is possible to fill the metal molten solid particle dispersion produced in the mixing step into a suitable transport system that is actively or passively heated.

[0065] In a preferred embodiment of the casting process according to the invention, the molten metal containing the solid particles is continuously mixed in the mixing step by several stirrers arranged one behind the other in the casting and flow direction. This allows the degree of wetting and the homogeneity of the solid particle incorporation into the molten metal to be gradually increased. The individual stirrers can mix the composite melt at the same or different speeds.

[0066] In an advantageous embodiment of the casting process according to the invention, the volume of material in the respective mixing basin is controlled by adjusting the height of at least one slide arranged in the at least one mixing basin.

[0067] To minimize thermal energy loss during the mixing step, components of the casting apparatus used that come into contact with the melt, such as the casting trough and / or the at least one mixing basin, are preferably heated and / or at least partially made of heat-insulating material in the casting process according to the invention. This prevents solidification during the process; instead, the molten metal-solid particle dispersion remains molten until the end of the process.

[0068] A preferred embodiment of the casting device according to the invention is described below with reference to Figure 1 explained.

[0069] This shows Figure 1This is a purely schematic representation, not to scale, of only a section of a casting device according to the invention. The casting device includes a metal melting unit (not shown) in which, for example, an aluminum alloy is melted. At least one [unclear] is connected to the metal melting unit. Figure 1 not shown, metal melting line, through which the molten metal produced in the metal melting device is conveyed to a particle feeder. With the metal melting line, which also belongs to the casting device, in Figure 1 Solid particles are added to the molten metal via a particle feeding device (not shown).

[0070] A mixing zone 1 of the casting device adjoins the section of the casting device that includes the particle feed device. One possible embodiment of this mixing zone 1 is shown in Figure 1 schematically represented.

[0071] The mixing zone 1 has a pouring trough 2. The pouring trough 2 is inclined at an angle α to a horizontal mounting surface 7 of the pouring device in a pouring and flow direction A of the pouring device. Accordingly, a molten metal containing solid particles, directed into the pouring trough 2, flows down the pouring trough 2 due to gravity.

[0072] In the illustrated embodiment, several mixing basins 3 in the form of depressions are incorporated into the casting trough 2. The bottoms 31 of the mixing basins 3 are also inclined at an angle α to the horizontal support surface 7 of the casting device. This inclination of the bottoms 31 of the mixing basins 3 facilitates the flow of the composite melt within and out of the respective mixing basin 3. Alternatively, the bottoms 31 can also be oriented horizontally, i.e., parallel to the support surface 7 of the casting device.

[0073] Each of the mixing basins 3 contains a stirrer 4. In the illustrated embodiment, the stirrers 4 are preferably height-adjustable mechanical stirrers, but in other embodiments of the invention they can also be electromagnetic stirrers.

[0074] The mechanical stirrers 4 each have stirring blades 41 which, when the mixing vessel 3 is filled, are in mechanical contact with the molten metal-solid particle dispersion contained therein. Rotating the stirrers 4 introduces shear forces into the molten metal-solid particle dispersion via the stirring blades 41. These forces result in a homogeneous distribution of the solid particles within the dispersion, good wetting, and thus good adhesion of the solid particles to the molten metal.

[0075] In the illustrated embodiment, each of the stirrers 4 is height-adjustable, as schematically shown by the double arrow.

[0076] During the mixing process, the molten metal-solid particle dispersion is constantly in motion. For example, the flow rate of the molten metal-solid particle dispersion in the casting trough can be 2 to 10 l / min.

[0077] If the molten metal solid particle dispersion in the respective mixing basin 3 exceeds its overflow 32, it flows further along the casting trough 2.

[0078] In the illustrated embodiment, a heater 6 is arranged on the casting trough 2.

[0079] In the illustrated embodiment, each of the mixing basins 3 is further equipped with a slide valve 5. The width of each slide valve 5 corresponds to the inner width of the respective mixing basin 3. However, instead of one slide valve 5, several slide valves 5 can also be provided per mixing basin 3.

[0080] Each of the slides 5 is height-adjustable, as schematically shown by the double arrow, so that a distance a between a lower edge of the respective slide 5 to a bottom 31 of the respective mixing basin 3 and thus a flow cross-section for the metal molten solid particle dispersion flowing into the respective mixing basin 3 can be changed and thus controlled by means of the slide 5.

Claims

1. Casting device for producing metal matrix composite materials comprising a metal melting unit for producing a metal melt, a particle feeding unit for supplying solid particles to the metal melt, and a mixing zone (1) comprising at least one mechanical and / or electromagnetic stirrer (4) for distributing the solid particles in the metal melt, characterized in that the mixing zone (1) comprises at least one casting gutter (2) inclined in a casting and flow direction (A) of the casting device with at least one mixing basin (3) forming a depression in the casting gutter (2), wherein at least one of the at least one stirrer (4) is arranged in and / or at each of the at least one mixing basin (3).

2. Casting device according to claim 1, characterized in that at least one pusher (5) is arranged in the at least one mixing basin (3), which pusher (5) is adjustable vertically with respect to its distance (a) from a bottom (31) of the respective mixing basin (3).

3. Casting device according to one of the preceding claims, characterized in that at least one heater (6) is arranged at the casting gutter (2) and / or at the at least one mixing basin (3).

4. Casting device according to any one of the preceding claims, characterized in that at least one cover is applied to the at least one casting gutter (2) and / or the at least one mixing basin (3).

5. Casting device according to any one of the preceding claims, characterized in that the at least one casting gutter (2) opens into a buffer container, in and / or at which a stirrer or an ultrasonic sonotrode is arranged.

6. Casting method for producing metal matrix composite materials, in which a metal melt is produced, solid particles are supplied to the metal melt, and in a mixing step the solid particles are distributed in the metal melt by at least one mechanical and / or electromagnetic stirrer (4), while the metal melt with the solid particles is maintained in a continuous flow, characterized in that during the mixing step the metal melt with the solid particles flows in at least one casting gutter (2) inclined in a casting and flow direction (A) and is mixed by the at least one stirrer (4) in at least one mixing basin (3) which forms a depression in the casting gutter (2).

7. Casting method according to claim 6, characterized in that in the mixing step the metal melt provided with the solid particles is continuously mixed by a plurality of the at least one stirrer (4), which are arranged one behind the other in the casting and flow direction (A).

8. Casting method according to claim 6 or 7, characterized in that a material volume in the respective mixing basin (3) is controlled by adjusting the height of at least one pusher (5) arranged in the at least one mixing basin (3).

9. Casting method according to any one of claims 6 to 8, characterized in that the casting gutter (2) and / or the at least one mixing basin (3) is / are heated.

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