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

Description

[0001] The present invention relates to a casting apparatus for producing metal-matrix composite materials, which comprises a melt line inclined in a flow direction of the casting apparatus, each having a flow path for a molten metal, and a particle feed device for feeding solid particles to the molten metal. The invention further relates to a casting method for producing metal-matrix composite materials, in which solid particles are fed into a molten metal while the molten metal flows down a melt line in a continuous flow.

[0002] Metal matrix composites (MMCs) are metals or metal alloys containing solid particles. Such particle-reinforced metals or metal alloys offer significantly higher wear resistance and / or increased strength, particularly increased heat resistance, compared to their unreinforced counterparts.

[0003] The solid particles used in MMC materials can be, for example, ceramic particles. 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 composite materials, particularly aluminum matrix composite materials (AMC), were first considered commercially in the early 1970s. The motivation came primarily from the need for higher-performance lightweight materials for aerospace and military applications. By the late 1980s, research had led to the first practical manufacturing processes.

[0004] Nevertheless, only a few commercial applications of MMC have been developed to date. The lack of robustness of established MMC production processes, despite complex process control, leads to fluctuating material qualities and additional post-processing costs for MMC products, thus resulting in high MMC material costs. 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 active suppliers of MMC materials will overcome the hurdle of the required cost-benefit ratio for large-scale production.

[0005] Currently, only a few solutions exist for introducing particles into a completely liquid molten metal. The problems arise from the oxide layer that forms on the molten metal, which prevents the particles from sinking in, the mixing of surface oxides into the molten metal, and the uneven distribution and wetting of the particles in the molten metal.

[0006] The idea of ​​adding the particles to a prepared melt under vacuum and dispersing them using a stirrer originates from US Pat. No. 4,786,467 B1. The stirrer, with its multiple stacked stirring arms, ensures that the shear forces are sufficiently high for wetting.

[0007] In the stirring technology described in US Pat. No. 6,547,850 B1, the particles are introduced into a crucible below the melt surface via a hollow stirrer and simultaneously set in rotation by the stirrer. This overcomes the challenge of the particles breaking through the oxide layer, which forms even in a vacuum. The rotation also results in a homogeneous distribution and wetting of the particles in the molten metal.

[0008] US Pat. No. 6,253,831 B1 also describes the addition of particles to a melt contained in a crucible, although in this case only part of the overall system, i.e., the crucible and the mixing unit, is operated in a vacuum. The mixing of melt and particles is achieved through a combination of ultrasonic treatment and electromagnetic stirring using induction coils.

[0009] In US Pat. No. 7,509,993 B1, a matrix alloy is first melted in a crucible, followed by the addition of nanoparticles. To embed these nanoparticles into the matrix, the melt is treated using vibration or ultrasound. This not only achieves homogeneous distribution and wetting, but also assists cooling, thus transforming the composite melt into a partially solidified, mushy to doughy, state. The partially solidified MMC melt is subsequently used for thixoforming to directly produce components with improved mechanical properties through a combination of primary and final forming.

[0010] The known technologies listed above are all batch solutions based on discontinuous process control and have weaknesses in their reproducibility or are difficult to modify for continuous process control.

[0011] Since the respective batch technologies usually operate under vacuum, a sealed container is required. When refilling material, the vacuum must be released, more raw material added, then the melt container evacuated again and the vacuum restored.

[0012] DE 692 23 950 T2 describes a stir casting process for the continuous production of metal matrix composite material. This process involves multiple mixing stages. In each of the mixing stages, the molten metal is mixed with the particulate material introduced therein, for example, using a dispersion impeller to achieve sufficient shear of the molten metal against the particulate material until the particles are sufficiently wetted with the metal. However, this process is very complex.

[0013] Another continuous process for producing composite melts is described in EP 3 586 999 A1. It focuses on inline ultrasonic treatment for the homogeneous distribution and wetting of the solid additive in a metal melt. This ultrasonic treatment also serves to lower the melt temperature for further processing of the semi-liquid composite material in die-casting machines.

[0014] This technology has the disadvantage that the ultrasonic action on the melt containing the particles only occurs when the melt flows past the respective sonotrodes. Due to the limited exposure time and the locally restricted effective area, the desired homogeneous mixing of the entire material can only be achieved with this technology if the ultrasonic treatment is carried out at a high frequency and / or is very time-consuming.

[0015] If such a process takes place in a normal atmosphere, oxidation of the melt surface must also be expected, which makes the introduction of solids more difficult and can cause the oxidation products of the molten metal to be mixed into the melt during ultrasonic treatment.

[0016] The disadvantages of batch solutions associated with the vacuum used are eliminated in continuous processes. Instead, raw material can be replenished at any time. However, this occurs at the expense of contamination in the near-surface melt zone.

[0017] Furthermore, the state of the art involves splitting the respective melt stream into several partial melt streams using a cascade-shaped channel system. The aim is to significantly increase the surface area of ​​the partial melt streams compared to the original total melt stream and to allow solid particles to trickle into them. However, this can result in very thin melt films that can break off, or in trickles, allowing the solid particles to impinge on areas where no melt is present at all. Furthermore, the increase in the melt surface area greatly increases the oxide content relative to the melt volume, which has a negative impact on both the process control of the liquid melt and the properties of the solidified material. Even under vacuum conditions, the proportion of oxide inclusions in the MMC melt is very high with this technology.The cascade principle also places high demands on the surface quality of the runners and also on their geometric design, which depends on the melt as well as the process temperatures.

[0018] In addition, there is a state-of-the-art technology for producing composite metal powder. In this case, the particles are added onto or into a very thin melt stream, which is subsequently atomized to produce the powdered composite material.

[0019] It is therefore the object of the present invention to provide a casting device and a casting method which enable a continuous production of metal matrix composite materials with high homogeneity and wetting of the solid particles introduced into the metal matrix with low technological expenditure.

[0020] This object is achieved, on the one hand, by a casting device for producing metal-matrix composite materials, which has a melt line inclined in a flow direction of the casting device with a flow path for a metal melt and a particle feed device for feeding solid particles to the metal melt, wherein the particle feed device is designed as a particle feed shaft extending at least to a bottom of the flow path with a particle outlet window formed in a shaft casing of the particle feed shaft.

[0021] In the present invention, the particle feed chute performs two functions.

[0022] Firstly, the particle feed chute extending into the flow path divides the melt flowing in the melt line into two partial streams. These partial streams flow past the particle feed chute on both sides and rejoin after passing through the particle feed chute. The two partial streams continue to flow in the melt line, whose cross-section is preferably constant. Accordingly, there is no significant increase in the surface area of ​​the partial streams compared to the previous total melt flow, and thus no increased oxide formation on the melt surface, which, as explained above, would not only complicate the introduction of particles into the melt but would also negatively affect the properties of the subsequently solidified material.

[0023] On the other hand, the solid particles trickle into the molten metal through the particle outlet window of the particle feed chute in an area of ​​the melt line where the two partial flows merge again.

[0024] The flow conditions resulting from the confluence between the partial streams literally pull the solid particles into the molten metal, resulting in a particularly good injection of the solid particles below the melt surface.

[0025] Raw material and solid particles can be continuously fed into the casting device according to the invention. Furthermore, a metal matrix composite material can be continuously produced using the casting device according to the invention.

[0026] The casting device according to the invention is also very robust against process fluctuations and varying requirements. All its components are easy to maintain and service.

[0027] The present invention further allows the molten metal to be heated before and / or in the melt line, which facilitates the continuous process control by the molten metal always being kept in the liquid state, and / or the solid particles to be heated before and / or in the particle feed shaft, which can prevent the solid particles introduced into the molten metal from partially cooling the molten metal and leading to premature solidification and the formation of clumps in the molten metal.

[0028] In a preferred embodiment of the casting device according to the invention, the melt line is designed as a melt line pipe, the particle feed shaft is guided to the flow path through a jacket passage formed opposite the flow path in a pipe jacket of the melt line pipe, and the particle outlet window is arranged within the melt line pipe.

[0029] This structural design of the casting device according to the invention is easy to implement and creates the ideal conditions for continuous particle introduction into the molten metal flowing continuously in the melt line. Due to the closed design of the melt line as a melt line pipe and the direct branching of the particle feed shaft into the melt line pipe, there is no exposed melt surface during particle introduction, thus avoiding the aforementioned disadvantages of the prior art.

[0030] In principle, in the present invention, a melt line profile that is not closed around its circumference can also be used for the melt line instead of the melt line pipe.

[0031] The particle feed chute, which is inserted laterally into the melt line pipe, is preferably welded to the melt line pipe. This creates a closed particle feed system that enables loss-free, targeted, and clean particle feed to the molten metal.

[0032] The angle between the melt pipe and the particle feed chute can influence the speed and thus the quantity per unit time at which the solid particles are introduced into the molten metal.

[0033] The particle feed chute is preferably designed as a particle feed tube. The particle feed tube can have either a round or a rectangular cross-section. The tubular shape of the particle feed chute, thanks to its closed circumference, enables easy filling and loss-free feeding of the solid particles to and into the melt line, largely independent of ambient conditions.

[0034] Particularly preferably, the particle feed pipe has a smaller pipe cross-section than the melt line pipe.

[0035] When the word "tube" is used in the description of the present invention, e.g., in the term "melt line tube," this is not limited to round tube cross-sections, but can also have an angular, such as rectangular, tube cross-section. Furthermore, the respective tube can have at least one kink and / or at least one bend.

[0036] The solid particles can be easily guided inside the particle feed chute in the direction of the particle outlet opening if a transverse floor opening into the particle outlet opening is formed in the particle feed chute.

[0037] The transverse floor is preferably inclined in a particle exit direction. This can be achieved by arranging the transverse floor at an angle in the particle feed chute, resulting in an elliptical shape when the particle feed chute has a round internal cross-section. If the installation angle of the particle feed chute acting as a melt splitter is large enough, the transverse floor can also be positioned straight in the particle feed chute and thus be circular when the particle feed chute has a round internal cross-section.

[0038] The transverse floor preferably extends over the entire internal cross-section of the particle feed chute.

[0039] The transverse floor can, for example, be designed as a plate.

[0040] In a special embodiment of the casting device according to the invention, the melt line splits in the flow direction upstream of the particle feed chute into two melt sub-lines, which are rejoined at the particle feed chute. This results in two Y-shaped paths of the molten metal at the particle feed chute, which create flows in the molten metal that further improve the injection of the solid particles into the molten metal. This embodiment is therefore referred to as a double-Y design.

[0041] At the point at which they trickle into the molten metal, the solid particles are located relatively centrally in the melt flow across the melt's cross-section. Downstream inline treatment of the particle-laden molten metal can compensate for this localized distribution. The solid particles are thus homogeneously distributed and wetted.

[0042] In an advantageous embodiment of the casting apparatus according to the invention, an agitator extending into the flow path, whose drive shaft is arranged in the particle feed shaft, is used as a means for such inline treatment. The agitator can reduce the viscosity of the molten metal containing the solid particles for subsequent process steps.

[0043] The object is further achieved by a casting method for producing metal matrix composite materials, in which solid particles are fed to a metal melt while the metal melt flows down a melt line in a continuous flow, wherein the metal melt flowing down along a flow path of the melt line is divided into two partial flows, each flowing laterally past the particle feed shaft, by a particle feed shaft projecting into the flow path and dividing the flow path, and where the partial flows merge again after flowing past the particle feed shaft, the solid particles are trickled into the converging partial flows from a particle outlet window of the particle feed shaft arranged above the flow path.

[0044] The casting process according to the invention ensures a high degree of MMC melt quality. The casting process according to the invention also enables continuous melting under vacuum conditions.

[0045] In an advantageous embodiment of the casting method according to the invention, the solid particles are guided to the particle exit window along a transverse floor formed in the particle feed shaft and inclined in a particle exit direction.

[0046] Preferably, before passing through the particle feed chute, the molten metal flows into two melt sub-lines of the melt line, which are rejoined at the particle feed chute.

[0047] A preferred embodiment of the present invention is explained in more detail below with reference to figures, wherein Figure 1 schematically shows a side view of a particle feed section of an embodiment of the casting device according to the invention; Figure 2 schematically shows the particle feed section of Figure 1 in a perspective view; and Figure 3 schematically shows a particle feed section of another embodiment of the casting apparatus according to the invention.

[0048] The Figures 1, 2 and 3 illustrate the present invention by means of schematic views of a particle feed section of two different embodiments of a casting device according to the invention.

[0049] In front of the respective particle feed section, the casting device has at least one component not shown here, such as a metal melting and / or heat-holding device for producing and / or keeping a metal melt warm.

[0050] Optionally, the casting device may comprise at least one component not shown in the present figures after the respective particle feed section, such as a mixing zone with at least one mechanical and / or electromagnetic stirrer for distributing solid particles applied to or introduced into the molten metal in the particle feed section.

[0051] The particle feed section shown in each case comprises a melt line 1 supplied with molten metal from the metal melting device and a particle feed shaft 2. In the embodiments shown, the melt line 1 is tubular, i.e., designed as a melt line pipe or in the form of two tubular melt sub-lines 14, 15 that initially diverge and then converge again. The particle feed shaft 2 is also tubular in the embodiments shown, i.e., designed as a particle feed pipe.

[0052] The melt line 1 is inclined in a respective flow direction A, A' of the casting device. Accordingly, a molten metal flows down a flow path 11 formed in the melt line 1 in the flow direction A, A'.

[0053] A through-opening is formed in a casing of the melt line 1, which in the illustrated embodiments is a tubular casing 12, through which the particle feed chute 2 is guided into the interior of the melt line 1. The particle feed chute 2 extends to an inner wall of the melt line 1 opposite the through-opening, i.e., through the flow path 11 in which the molten metal flows. The flow path 11 is divided by the particle feed chute 2 into two partial streams, which flow past the particle feed chute 2 on either side.

[0054] In the exemplary embodiments shown, the particle feed chute 2 is aligned at an angle of 90° or 45° to a rotation axis of the melt line 1, so that its inclination B is aligned at an angle of 90° or 45° to the inclination of the melt line 1 corresponding to the flow direction A, A'. In other embodiments of the present invention, however, the particle feed chute 2 can also be positioned, for example, perpendicular to the inclined melt line 1, so that an angle in a range of 30 to 60°, e.g., 45°, is formed between the central axis of the particle feed chute 2 and the respective flow direction A, A' in the melt line 1.

[0055] The particle feed shaft 2 has an open particle exit window 22 in its shaft casing 21. Furthermore, a transverse floor 23 opening into the particle exit window 22 is arranged in the particle feed shaft 2. The transverse floor 23 is inclined in a particle exit direction C toward the particle exit window 22.

[0056] The casting device according to the invention operates, for example, according to the following casting process: In the metal melting and / or heat-holding device of the casting device, a metal melt is first generated and / or kept hot. Optionally, the metal melting and / or heat-holding device can be refined and / or modified in the metal melting and / or heat-holding device. The metal melting and / or heat-holding device, like the preferably provided process chamber of the casting device according to the invention, in which the particle feed section is located, is operated in a vacuum, i.e., at approximately 10 -4 < to 1 mbar, or in a protective gas atmosphere.

[0057] In preferred embodiments of the casting apparatus according to the invention, a supply line, which can be locked by a vacuum-tight valve device, is provided between the metal melting and / or heat-holding device and the process chamber in which the particle feed section is located. A vacuum melt container is coupled to this supply line.

[0058] Simple embodiments of the present invention can also be designed so that the particle feed section is not located in a separate process chamber.

[0059] After an equal pressure level has been reached in the entire casting device, the valve device located between the metal melting and / or heat-holding device and the process chamber in which the particle feed section is located is opened and a valve device located between this process chamber and a pouring chamber adjoining the process chamber is closed.

[0060] The molten metal produced by the metal melting and / or holding device is fed into the vacuum melting vessel.

[0061] In one embodiment of the invention, the melt line 1, which is introduced laterally into the process chamber in a vacuum-tight manner from the outside, can protrude from above, for example. In this embodiment, a first section of the melt line 1 forms a riser tube. The lower end of the riser tube is always located below the melt surface of the molten metal. An increase in pressure in the metal melting and / or heat-holding device causes a rise in the melt in the riser tube and thus in the melt line 1.

[0062] When a critical fill level is reached between the melt surface and the lower edge of the riser pipe, melt line 1 is filled from an additional melt tank by gravity. It is also possible to operate the additional melt tank under normal atmosphere and use the resulting pressure difference to fill the molten metal into the vacuum melt tank via the riser pipe.

[0063] However, the present invention is independent of the method of supplying the molten metal to the melt line 1. In particular, the invention is not dependent on whether a riser pipe, as described above, is used. The invention can also be used, for example, in gravity casting.

[0064] The melt line 1 is heated directly or indirectly with at least one heating element so that the molten metal in the melt line 1 cannot solidify at any time.

[0065] Another section of the melt line 1 is inclined downwards in the respective flow direction A, A', i.e., at an angle of approximately 30 to 60°. As a result, the molten metal flows continuously, i.e., without interruption, in the respective flow direction A, A' within the melt line 1, along the flow path 11 formed on the inside of the melt line 1.

[0066] Insofar as, as in the embodiment of Figure 3 , the melt line 1 branches into melt sub-lines 14, 15, the flow path 11 also branches accordingly.

[0067] The molten metal, which initially flows as a single stream, is divided into two partial streams by the particle feed chute 2 where it extends into the flow path 11 and divides it. Preferably, the particle feed chute 2 extends centrally into the flow path 11, so that the total flow of molten metal is evenly divided into the two partial streams. In the embodiments of the Figures 1 and 2 The total flow is divided into two partial flows in a Y-like manner.

[0068] In the embodiment of Figure 3 the particle feed shaft 2 protrudes into the flow path 11 where the two melt lines 14, 15 are brought together again.

[0069] The partial flows flow past the particle feed chute 2 on both sides and flow together again after passing the particle feed chute 2.

[0070] When solid particles are introduced into the particle feed chute 2, which can be done, for example, by means of a shaking unit or a vibration unit connected to a storage silo, they initially fall or slide supported by the inclination B of the particle feed chute 2 onto the transverse floor 23 located in the particle feed chute 2. On the transverse floor 23, the solid particles slide diagonally downwards in the particle exit direction C towards the particle exit window 22 according to the inclination of the transverse floor 23.

[0071] The solid particles then trickle down from the particle exit window 22 onto the reconverging partial streams of the molten metal. Preferably, the solid particles impact the molten metal precisely where the two partial streams reconverge, i.e., where a gap initially closes between the partial streams. This traps the solid particles beneath the melt surface.

[0072] The inclination B or the angle of inclination of the particle feed chute 2 determines the point at which the two partial flows rejoin.

[0073] By adjusting the cross-sections of the melt line 1 and the particle feed shaft 2 in the area of ​​its introduction into the flow path 11, almost any melt quantity can be provided with solid particles.

[0074] The guidance of the molten metal in the pipe system described above ensures that there is no melt surface open to an oxygen-containing environment and, accordingly, unwanted oxide formation due to the low oxygen content in or on the molten metal in the process chamber is largely prevented.

[0075] After the solid particles are injected into the molten metal, the MMC melt, now enriched with solid particles, flows through a mixing zone of the casting device to homogenize the solid particles and wetting them. To support mixing and wetting, an agitator, such as a two- to four-blade agitator with material-specific blade pitch, can be integrated into melt line 1 after the partial streams have been combined to reduce the viscosity of the MMC melt.

[0076] Finally, the MMC melt is poured into a casting mold to solidify there or fed into a heatable collecting container to collect it and then feed it to a further processing facility, such as a pressure die or continuous casting plant.

[0077] In the above-described embodiment of the casting device according to the invention, heating elements are preferably located on all components that come into contact with the molten metal or the MMC melt, except for the casting mold, in order not to expose the hot melt with the solid particles to the risk of premature solidification.

Claims

1. Casting apparatus for producing metal matrix composite materials comprising a melt channel (1) which is inclined in a flow direction (A, A') of the casting apparatus with a flow pathway (11) for a metal melt, and a particle feed device for adding solid particles to the metal melt, characterized in that the particle feed device is formed as a particle feed chute (2) that extends at least to a base of the flow pathway (11) and has a particle exit window (22) formed in a chute casing (21) of the particle feed chute (2).

2. Casting apparatus according to claim 1, characterized in that the melt channel (1) is formed as a melt channel pipe, the particle feed chute (2) is guided through a casing feedthrough (13) formed opposite the flow pathway (11) in a pipe jacket (12) of the melt channel pipe to the flow pathway (11), and the particle exit window (22) is arranged inside the melt channel pipe.

3. Casting apparatus according to one of the preceding claims, characterized in that the particle feed chute (2) is formed as a particle feed pipe.

4. Casting apparatus according to claims 2 and 3, characterized in that the particle feed pipe has a smaller tube cross-section than the melt channel pipe.

5. Casting apparatus according to one of the preceding claims, characterized in that a transverse floor (23) opening into the particle exit opening (22) is formed in the particle feed chute (2).

6. Casting apparatus according to one of the preceding claims, characterized in that the casting apparatus comprises an agitator projecting into the flow pathway (11), the drive shaft of which is arranged in the particle feed chute (2).

7. Casting apparatus according to one of the preceding claims, characterized in that the melt channel (1), before the metal melt passes through the particle feed chute (2), splits into two melt sub-channels (14, 15) which are combined together again on the particle feed chute (2).

8. Casting method for producing metal matrix composite materials, in which solid particles are added to a metal melt while the metal melt is flowing in a continuous flow down a melt channel (1), characterized in that the metal melt flowing down along a flow pathway (11) of the melt channel (1) is divided into two partial streams which flow around a particle feed chute (2) by a particle feed chute (2) that projects into the flow pathway (11) and thus divides the flow pathway (11), and at the point where the partial streams combine again after flowing around the particle feed chute (2), the solid particles are trickled into the confluence of the partial streams via a particle exit window (22) in the particle feed chute (2) located above the flow pathway (11).

9. Casting method according to claim 8, characterized in that the solid particles are guided along a transverse floor (23) formed in the particle feed chute (2) to the particle exit window (22).

10. Casting method according to claim 8 or 9, characterized in that the metal melt, before it passes the particle feed chute (2), flows into two melt sub-channels (14, 15) of the melt channel (1) which are combined together again at the particle feed chute (2).