DE102021108933B4 - Casting device and casting method for the production of metal matrix composite materials - Google Patents

Abstract

Casting device for the production of metal matrix composite materials, which has 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 (1) having at least one mechanical and/or electromagnetic stirrer (4) for distributing the solid particles in the molten metal, characterized in that the mixing zone (1) has at least one pouring channel (2) inclined in a pouring and flow direction (A) of the casting device with at least one mixing basin (3) forming a sink in the pouring channel (2), at least one of the at least one stirrer (4) being arranged in and/or on the at least one mixing tank (3).

Description

The present invention relates to a casting device for the production of metal matrix composite materials, which has 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 for distributing the solid particles in has the molten metal.

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

Metal matrix composite materials (MMC) are metals or metal alloys containing solid particles. Such particle-reinforced metals or metal alloys offer significantly higher wear resistance and increased strength, in particular increased heat resistance, compared to their unreinforced variants.

The solid particles used in MMC materials can be ceramic particles, for example. The solid particles can consist of metal oxide(s), preferably aluminum oxide, metal nitride(s), metal carbide(s), preferably silicon carbide, metal silicide(s), and/or glass. Metal matrix composites, particularly aluminum matrix composites (AMC), were first considered commercially in the early 1970's. The motivation came primarily from the need for more efficient lightweight materials for aerospace and military applications. At the end of the 1980s, research led to the first practicable manufacturing processes.

Despite this, only a few commercial applications of MMC have been developed to date, since insufficient robustness of the established MMC production processes, despite complex process control, leads to fluctuating material qualities and additional post-processing costs for the MMC products and thus to high MMC material costs. Therefore, despite their great application potential, MMC materials can only be found in niche applications or high-end technologies. At the moment it is not foreseeable that one of the global suppliers of MMC materials will overcome the hurdle of the necessary cost-benefit ratio for large-scale production.

MMC materials are currently manufactured by stir casting, sintering, melt infiltration or spray compacting.

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

In the generic publication DE 692 23 950 T2 such a stirred casting process is described. In doing so, e.g. B. an aluminum alloy first melted in a furnace. Oxides, particles, dissolved gas and other impurities are removed from the melt by bubbling an argon-containing gas through the melt. The melt is then pumped from above into a first cylindrical mixing tank. In addition, ceramic particles are introduced into the first mixing tank and stirred into the melt until the ceramic particles are sufficiently wetted with the aluminum alloy. A dispersing impeller immersed in the melt is used for stirring.

The mixed composite material thus produced is removed from the bottom of the first mixing vessel and fed via a line below into a second cylindrical mixing vessel, where the flow of the composite material is upwards in contrast to the first mixing vessel and in which mixing again takes place with a stirring blade. The composite material mixed therein is then fed from the second mixing vessel to a holding furnace.

The publication proposes an alternative to the vertically aligned mixing tanks DE 692 23 950 T2 also proposes an elongate tubular mixing vessel with a horizontal cylinder axis. In such a horizontal mixer, an induction coil is wrapped around the exterior of the mixing vessel and/or one or more impeller blades project vertically from above into the interior of the mixing vessel to, in use, agitate the molten metal and particulate material which circulates the mixing vessel from side to side flow continuously to mix.

In the pamphlet U.S. 2012/0043050 A1 a stirrer is also used when sprinkling particles into a molten metal. At least one pair of rollers is provided here at a tapering end of the melt line in which the cooling melt is rolled into a sheet, through which process the particles become finer.

Also in print US 6,015,528A the device described for producing metal matrix composite materials has a stirrer which protrudes from above into a mixture of molten metal and solid particles and which is intended to prevent segregation of the particles by their settling or rising before the melt solidifies.

Instead of stirrers, the use of ultrasonic devices for mixing a flowing melt with solid particles introduced therein has also been proposed. However, such methods are only limited to small volumes.

From the pamphlet DE 1 194 152 A For example, a method is known for finely dispersing insoluble substances in molten metal by means of mechanical vibrations, in which the acoustic stress on the walls of the vessel in which the molten metal is located remains low. In order 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 designed as tubes filled with powder, which are guided or pressed against the sound-emitting surface.

The only commercially available AMC material produced using a stir casting process is based on an AlSi9-10Mg0.5 matrix with SiC reinforcement of 10 or 20% by volume with a fixed SiC particle size of around 13 µm.

The AMC materials required for specific applications have hitherto not been able to be produced on an industrial scale using the known stir-casting processes. Currently, these processes only enable the production of 150 to 160 liters of AMC melt in one batch. The stirring process alone takes one to two hours per batch. In addition, it is in the publication DE 692 23 950 T2 The agitated casting process described above is energy-intensive and therefore economically and ecologically critical.

The known stir casting methods also have insufficient process flexibility with regard to matrix and reinforcement materials and their contents, require too long process times to ensure homogeneous particle distribution and are limited in terms of the maximum achievable proportion of reinforcement.

In particular due to the low reproducibility of stir-cast MMC semi-finished products, alternative MMC production variants such as sintering, spray compacting or melt infiltration are used to increase the material quality and process robustness, but these are associated with a significant increase in costs for the MMC materials produced.

During sintering or powder metallurgy to produce AMC, a powder mixture of ceramic particles and aluminum powder is processed. This allows hard material proportions of up to 50% to be set.

The product resulting from sintering or powder metallurgy is a high-quality, solid AMC semi-finished product, but without any possible undercuts or filigree contours, which usually requires extensive mechanical post-processing. However, the properties of AMC produced by powder metallurgy, which are superior to those produced by melting metallurgy, are associated with higher input material costs and greater effort in pretreatment and in the production process. For these reasons, powder-metallurgically manufactured AMC semi-finished products are primarily used in demanding high-tech applications in the aerospace and defense industries. A powder-metallurgical large-scale implementation of AMC semi-finished products with correspondingly low material costs is not feasible to this day.

In spray compacting, ceramic particles are integrated into an aluminum melt particle jet using a gas jet. This process can currently be used to produce bolt-shaped semi-finished products up to 2 m high with a maximum outer diameter of 300 mm.

With spray compacting, a maximum reinforcement proportion of particles in an MMC material of about 20% by volume can be achieved. In addition to the high production costs, the limited manufacturability of complicated geometries severely limits the large-scale use of this technology.

With melt infiltration, the ceramic reinforcement phase is prepared as a porous foam body and then infiltrated with aluminum melt. In addition to particles, fibers and foam bodies with a high proportion of reinforcement can also be processed into near-net-shape components.

Due to the high process and semi-finished product costs, melt infiltration is only suitable for demanding components with high added value. A large-scale implementation is currently not feasible.

Furthermore, a large number of devices and methods for casting molten metal are known from the prior art, in which no particles are introduced into the molten metal. So comes e.g. B. in the pamphlet DE 199 62 471 A1 a device for casting molten metal is used, in which a pouring channel mounted on a supporting structure such that it can be tilted and rotated is arranged between a storage vessel and a casting mold

It is therefore the object of the present invention to provide a casting device and a casting method of the type specified at the outset, with which large quantities of MMC materials with a high degree of particle filling and homogeneous particle distribution can be produced at comparatively low costs.

The object is achieved according to the invention on the one hand by a casting device for the production of metal matrix composite materials, which has a metal melting device for producing a metal melt, a particle feed device for feeding solid particles to the metal melt and a mixing zone having at least one mechanical and/or electromagnetic stirrer for distribution the solid particles in the molten metal, wherein the mixing zone has at least one pouring channel inclined in a pouring and flow direction of the casting device with at least one mixing basin forming a depression in the pouring channel, wherein in and/or on the at least one mixing basin in each case the at least one Stirrer is arranged.

The at least one launder forms a restricted guide for the molten metal/solid particle dispersion. The trough has, for example, a C-, U- or V-shaped cross-section.

Due to the inclination of the at least one casting channel, through which the molten material injected with the solid particles, ie the composite melt, can flow along the casting channel, a continuous flow of material results in the casting device according to the invention. Due to the inclination of the launder, in which the at least one mixing basin is formed, its outlet is lower than its inlet, as a result of which there is a continuous outflow of the composite melt from the respective mixing basin in the direction of inclination. An MMC material can therefore be produced continuously with the casting device according to the invention, without complex and energy-intensive pump technologies having to be integrated into the process line. The respective MMC material can thus be produced very efficiently.

The casting device according to the invention enables stirred casting, in which the molten metal with the solid particles introduced therein is in a continuous but slow flow, in which reliable wetting and homogeneous distribution of the solid particles in the molten metal can take place even with large amounts of particles. Correspondingly, with the casting device according to the invention, high-quality MMC materials with a high particle filling level, for example with a particle filling level of at least 20% by volume, can be produced continuously.

In the case of the casting device according to the invention, the at least one mixing basin is located in a flow path for the molten metal-solid-particle dispersion formed by the casting channel. This means that the molten metal/solid particle dispersion is forced into the at least one mixing tank. The at least one mixing basin is formed by a depression in the casting channel. In the at least one mixing tank, the molten metal-solid particle dispersion is longer than on a correspondingly long, straight section of the casting channel.

Since the molten metal remains in the at least one mixing basin longer than in the launder, the solid particles are even better wetted with the material of the molten metal than in the launder. The wetting is intensified in the at least one mixing tank in that forces are exerted and/or applied by the at least one stirrer to the molten metal in the flow therein with the solid particles introduced therein. It is precisely the interaction between the metal melt that is not standing still and the stirring effect of the at least one stirrer that leads to particularly good wetting of the solid particles in a comparatively very short process time.

If the at least one stirrer is a mechanical stirrer, shearing forces are exerted on the molten metal/solid particle dispersion as a result of this shearing force, which promotes the wetting of the solid particles with the molten metal.

If the at least one stirrer is an electromagnetic stirrer with one or more coils, the at least one generated magnetic field causes turbulence in the molten metal-solid-particle dispersion, depending on the respective arrangement accelerated wetting. With the at least one electromagnetic stirrer, the metal melt mixed with particles is mixed without contact and the wetting is promoted.

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

The at least one stirrer can also advantageously be used to remove gas inclusions from the molten metal, independently of the active stirring principle used.

The present invention is not restricted with regard to the configuration of the respective stirrer. If at least one mechanical stirrer is used, it is advisable to use a stirrer with blades, for example with two, three or more than three blades. The angular orientation of the blades is variable.

The at least one stirrer is preferably designed to be adjustable in height, so that it/they can be removed from melt residues in the respective mixing basin at the end of the process, for example.

The at least one mechanical stirrer is typically electrically operated.

More than one stirrer can also be arranged in or on the respective mixing tank. For example, a mechanical stirrer can be arranged in the respective mixing tank and an electromagnetic stirrer can be arranged on the outside of the mixing tank. It is also possible for the respective mixing tank to be designed so long that several stirrers arranged one after the other are provided in or on it.

The inclination of the casting channel and the at least one mixing basin is basically variable and is based, for example, on a starting value for the viscosity of the molten metal-solid-particle dispersion.

With the casting device according to the invention, a wide variety of liquid metals/metal alloys and solid particles can be processed into MMC materials, both with regard to the respective material and with regard to the respective particle size. For this purpose, the at least one casting channel and the at least one mixing basin can be dimensioned accordingly.

In a preferred embodiment of the pouring device according to the invention, at least one slide whose distance from a bottom of the respective mixing basin can be adjusted vertically is arranged in the at least one mixing basin.

The distance between a lower edge of the respective slide and the bottom of the respective mixing tank, through which a passage height or a passage cross section for the molten metal-solid particle dispersion is determined, can be easily changed and thus controlled by the vertical mobility of the slide.

In order to prevent premature (partial) solidification of the molten metal-solid-particle dispersion as a result of its cooling and thereby to keep the molten metal-solid-particle dispersion permanently in flow, it is advantageous if at the casting channel and/or at the at least one mixing basin at least one heater is arranged.

In an advantageous embodiment of the pouring device according to the invention, at least one cover is placed on the at least one pouring channel and/or on the at least one mixing basin. The at least one cover can prevent the molten metal from splashing out of the at least one launder and/or out of the at least one mixing basin. If no lid is used, other precautions should be taken to protect users of the casting device according to the invention from escaping hot melt.

In some applications of the casting device according to the invention, such as. B. when a mold change is provided, it may be useful that the casting device has a buffer tank, into which the at least one casting channel opens. In order to compensate for sinking movements of the solid particles in the molten metal, ie sedimentation, it is advantageous if a stirrer or an ultrasonic sonotrode is arranged in or on the buffer container.

The object is also achieved by a casting process for the production of metal matrix composite materials, in which a molten metal is produced, solid particles are fed to the molten metal and, in a mixing step, the solid particles are distributed in the molten metal by at least one mechanical and/or electromagnetic stirrer , while the molten metal with the solid particles is kept in a continuous flow, in which the molten metal with the solid particles during the mixing step flows in at least one runner inclined in a pouring and flowing direction and in at least one sink in the runner forming mixing tank is mixed by the at least one stirrer.

In the casting method according to the invention, a continuous production of molten metal and supply of solid particles takes place. The casting method according to the invention therefore enables continuous casting of MMC materials with significantly increased filling levels of solid particles compared to the batch processes known in the prior art. Due to the continuity in the process, MMC melts or metal melts containing solids can be efficiently produced.

For example, the casting method according to the invention has no idle times due to filling or emptying processes.

By mechanically or electromagnetically mixing the continuously flowing molten metal/solid particle dispersion in at least one mixing tank with a longer dispersion residence time than the launder in the casting process according to the invention, a high solid particle content can be incorporated homogeneously into the molten metal.

The at least one mixing tank, into which the flowing molten metal-solid particle dispersion is guided in a targeted manner in the casting method 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.

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 into a process chamber as required via a line system with a defined pressure difference. The line system is preferably heatable or heated.

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

The solid particles can be conveyed into the line system or directly into the casting channel, for example by means of at least one screw or a vibrator. The conveying section along which the solid particles are fed to the line system or the casting channel is preferably heatable or heated.

After the solid particles have been fed 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.

In the casting method according to the invention, the molten metal with the solid particles introduced therein is stirred for 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.

The geometry of the at least one launder and of the at least one mixing basin is preferably designed in such a way that the dwell time of the molten metal that is passing through and contains the solid particles is maximized in the at least one mixing basin. Nevertheless, the molten metal-solid particle dispersion remains in a continuous flow at all times. A "continuous flow" in the present invention means that the molten metal-solid particle dispersion does not stop at any time during the mixing step, but the respective speed at which the molten metal-solid particle dispersion flows during the mixing step is in the / the Mixing basin less than usual in the at least one launder.

In the casting method according to the invention, this ensures continuous wetting of the solid particles by the material of the molten metal, degassing of the molten metal and homogenization of the molten metal/solid particle dispersion.

In the casting method according to the invention, metering of the solid particles, injection or feeding of the solid articles to the molten metal and the mixing step, in which the solid particles are wetted with the molten metal, can take place both in a vacuum and in an inert gas atmosphere.

The metal melt/solid particle dispersion produced in the mixing step can, for example, be fed into a buffer container in which there is no longer a continuous flow of melt.

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

Alternatively, the generated, homogenized molten metal-solid-particle dispersion can be fed into a die-casting machine in the molten state, in which near-net-shape components can be manufactured from MMC material.

In addition, it is possible to fill the molten metal/solid particle dispersion produced in the mixing step into a suitable, actively or passively heated transport system.

In a preferred embodiment of the casting method according to the invention, the molten metal provided with the solid particles is continuously mixed in a flowing manner in the mixing step by a plurality of stirrers arranged one behind the other in the direction of casting and flow. In this way, the degree of wetting and the homogeneity of the solid particle incorporation into the molten metal can be gradually increased. The respective stirrers can mix the composite melt at the same speed or at different speeds.

In an advantageous embodiment of the casting method according to the invention, a material volume in the respective mixing basin is controlled by adjusting the height of at least one slider arranged in the at least one mixing basin.

In order to lose as little thermal energy as possible during the mixing step, components of the casting device used that come into contact with the melt, such as the casting channel and/or the at least one mixing basin, are preferably heated and/or at least partially made of heat-insulating material in the casting method according to the invention. As a result, there are no solidification processes during the process, instead the molten metal/solid particle dispersion remains molten until the end of the process.

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

while showing 1 purely schematic and not true to scale, only a section of a casting device according to the invention. The casting device has a metal melting device, not shown, in which, for example, an aluminum alloy is melted. At least one in 1 molten metal line, not shown, with which the molten metal produced in the metal melting device is conducted to a particle feed device. With the in 1 not shown particle supply solid particles are added to the molten metal.

A mixing zone 1 of the casting device follows the section of the casting device which has the particle feed device. A possible embodiment of this mixing zone 1 is in 1 shown schematically.

The mixing zone 1 has a pouring channel 2 . The casting channel 2 is inclined in a casting and flow direction A of the casting device at an angle of inclination α to a horizontal installation surface 7 of the casting device. Correspondingly, a molten metal conducted into the casting trough 2 with solid particles introduced therein flows down the casting trough 2 due to gravity.

In the exemplary embodiment shown, a plurality of mixing basins 3 in the form of depressions are introduced into the casting channel 2 . In the exemplary embodiment shown, the floors 31 of the mixing tanks 3 are also inclined at the angle of inclination α relative to the horizontal installation surface 7 of the pouring device. The inclination of the bottoms 31 of the mixing tanks 3 favors the flow of the composite melt inside and out of the respective mixing tank 3. Alternatively, the bottoms 31 can also be horizontal, ie. H. be aligned parallel to the installation surface 7 of the casting device.

A stirrer 4 is located in each of the mixing tanks 3. In the exemplary embodiment shown, the stirrers 4 are preferably height-adjustable mechanical stirrers, but in other embodiments of the invention they can also be electromagnetic stirrers.

The mechanical stirrers 4 each have stirring blades 41 which, when the mixing basin 3 is full, are in mechanical contact with the molten metal-solid-particle dispersion located therein. By turning the stirrer 4, shear forces are introduced into the molten metal/solid particle dispersion by means of the stirring blades 41, which result in a homogeneous distribution of the solid particles in the molten metal/solid particle dispersion and in good wetting and thus in a good connection of the solid particles to the Lead material of the molten metal.

In the exemplary embodiment shown, each of the stirrers 4 is adjustable in height, as is shown schematically by the double arrow.

During the mixing step that takes place, the molten metal-solid-particle- Dispersion always in fluent movement. For example, the flow rate of the molten metal/solid particle dispersion in the casting trough 2 can be 10 l/min.

If the molten metal-solid particle dispersion in the respective mixing basin 3 exceeds its overflow 32, it continues to flow along the casting channel 2.

In the embodiment shown, a heater 6 is arranged on the casting channel 2 .

In the exemplary embodiment shown, there is also a slider 5 in each of the mixing basins 3. The width of each of the sliders 5 corresponds to an inside width of the respective mixing basin 3. Instead of one slider 5, however, several sliders 5 per mixing basin 3 can also be provided.

As shown schematically by the double arrow, each of the slides 5 is adjustable in height so that there is a distance a between a lower edge of the respective slide 5 and a bottom 31 of the respective mixing tank 3 and thus a flow cross section for the molten metal flowing into the respective mixing tank 3 -Solid particle dispersion is changeable and thus controllable by means of the slide 5.

Claims (9)

Casting device for the production of metal matrix composite materials, which has 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 (1) having at least one mechanical and/or electromagnetic stirrer (4) for distributing the solid particles in the molten metal, characterized in that the mixing zone (1) has at least one pouring channel (2) inclined in a pouring and flow direction (A) of the casting device with at least one mixing basin (3) forming a depression in the pouring channel (2), at least one of the at least one stirrer (4) being arranged in and/or on the at least one mixing tank (3). pouring device claim 1 , characterized in that in the at least one mixing basin (3) at least one vertically adjustable slide (5) in its distance (a) to a bottom (31) of the respective mixing basin (3) is arranged. Pouring device according to one of the preceding claims, characterized in that at least one heater (6) is arranged on the pouring channel (2) and/or on the at least one mixing basin (3). Pouring device according to one of the preceding claims, characterized in that at least one cover is applied to the at least one pouring channel (2) and/or to the at least one mixing basin (3). Casting device according to one of the preceding claims, characterized in that the at least one casting channel (2) opens into a buffer container in and/or on which a stirrer or an ultrasonic sonotrode is arranged. Casting process for the production of metal matrix composite materials, in which a molten metal is produced, solid particles are fed to the molten metal and, in a mixing step, the solid particles are distributed in the molten metal by at least one mechanical and/or electromagnetic stirrer (4), while the Molten metal with the solid particles is kept in a continuous flow, characterized in that during the mixing step the molten metal with the solid particles flows in at least one pouring channel (2) inclined in a pouring and flow direction (A) and in at least one sink in the pouring channel (2) forming mixing tank (3) is mixed by the at least one stirrer (4). casting process claim 6 , characterized in that the metal melt provided with the solid particles is continuously mixed in the mixing step by a plurality of the at least one stirrer (4) which are arranged one behind the other in the casting and flow direction (A). casting process 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 slide (5) arranged in the at least one mixing basin (3). Casting method according to one of Claims 6 until 8th , characterized in that the casting channel (2) and/or the at least one mixing basin (3) is/are heated.

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