A DEVICE AND METHOD FOR HIGH SHEAR LIQUID METAL TREATMENT

Abstract

A high shear liquid metal treatment device for treating metal includes a barrel, a rotor shaft, rotor fans, and stator plates. The barrel has a longitudinal axis that extends between an upper end and a lower end, and an opening at its upper and lower ends. The rotor shaft is mounted centrally through, and parallel to the longitudinal axis. The rotor fans are mounted along an axial length of the shaft. The stator plates are formed on an inner surface of the barrel and are located between adjacent rotor fans. Each stator plate has at least one passage formed therethrough to allow fluid to pass through the plate; and upper and lower surfaces of each stator plate are formed to be within the minimum distance of an adjacent rotor fan. The minimum distance is between 10 μm and 10 mm. The device allows improved treatment of liquid and semi-liquid metals during processing.

Claims

1. A high shear liquid metal treatment device comprising: a barrel having a longitudinal axis extending between a first end and a second end, and the barrel having respective openings at the first end and the second end; a rotor shaft mounted centrally through the longitudinal axis and parallel to the longitudinal axis; a plurality of rotor fans mounted along an axial length of the rotor shaft and within the barrel, each rotor fan formed such that its outer end is within a minimum distance of an internal wall of the barrel; and a plurality of stator plates formed on an inner surface of the barrel, the plurality of stator plates being located between adjacent rotor fans, each of the plurality of stator plates extending from an inner surface substantially to the rotor shaft, each of the plurality of stator plates having at least one passage formed therethrough to allow fluid to pass through the plurality of stator plates; and upper and lower surfaces of each of the plurality of stator plates are formed to be within a minimum distance of an adjacent rotor fan; wherein the minimum distance of the adjacent rotor fan is between 10 μm and 10 mm.

2. The high shear liquid metal treatment device of claim 1, wherein the barrel has a decreasing diameter from the first end to the second end.

3. The high shear liquid metal treatment device of claim 1 wherein a diameter of the barrel at the first end and a diameter of the barrel at the second end are substantially similar and the diameter of the barrel varies therebetween.

4. The high shear liquid metal treatment device of claim 1, further comprising a reservoir formed at the first end.

5. The high shear liquid metal treatment device of claim 4, wherein the reservoir comprises internal baffles positioned to prevent swirling of liquid metal contained therein.

6. The high shear liquid metal treatment device of claim 1, wherein the plurality of stator plates are substantially circular and are formed of two halves of a circular plate.

7. The high shear liquid metal treatment device of claim 1, wherein the plurality of stator plates are discs having at least one hole formed therethrough to allow fluid to pass through at least one of the plurality of stator plates.

8. The high shear liquid metal treatment device of claim 7, wherein a diameter of the at least one hole is between 0.5 mm and 10 mm.

9. The high shear liquid metal treatment device of claim 7, wherein each of the plurality of stator plates has a plurality of holes formed therethrough.

10. The high shear liquid metal treatment device of claim 7, wherein the diameter of the at least one hole formed through the plurality of stator plates reduces along the longitudinal axis of the barrel.

11. The high shear liquid metal treatment device of claim 1, wherein one or more of the plurality of stator plates comprises a ring of blades.

12. The high shear liquid metal treatment device of claim 1, further comprising a motor connected to the rotor shaft to rotate the rotor fans.

13. The high shear liquid metal treatment device of claim 1, wherein the device is substantially formed of materials with a melting point of not less than 200° C.

14. The high shear liquid metal treatment device of claim 1, wherein the device is substantially formed of materials with a melting point of not less than 600° C.

15. The high shear liquid metal treatment device claim 1, wherein the device is substantially formed of materials with a melting point of not less than 1000° C.

16. The high shear liquid metal treatment device of claim 1, wherein the barrel is formed of two halves that are bolted together and wherein the two halves are sealed using a flange.

17. The high shear liquid metal treatment device of claim 1, wherein the first end is located above the second end such that passage of fluid from the first end to the second end is aided by gravity.

18. The high shear liquid metal treatment device of claim 1, wherein the rotor fans are formed such that when the rotor shaft is rotated, the rotor fans may operate to draw fluid from the first end to the second end.

19. The high shear liquid metal treatment device of claim 1, wherein the barrel is encased in a protective housing.

20. A method of treating molten material comprising: rotating a plurality of rotor fans to draw molten material into a liquid metal treatment device though a first end of a barrel, wherein the molten material passes through the barrel from the first end to a second end whilst the plurality of rotor fans rotate at a speed between 1 rpm and 50,000 rpm.

Description

DRAWINGS

[0092] FIG. 1 comprises schematic illustrations of a first embodiment of a device according to the present invention and its component parts;

[0093] FIG. 2 is a schematic illustration of a second embodiment of a device according to the present invention;

[0094] FIG. 3 is a schematic illustration of a liquid metal conditioning process using the device of FIG. 1;

[0095] FIG. 4 is a schematic illustration of a liquid metal degassing process using the device shown of FIG. 1;

[0096] FIG. 5 is a schematic illustration of a direct chill (DC) casting process integrating a conventional DC casting process with the device of FIG. 1; and

[0097] FIG. 6 shows schematic illustrations of various rotor fans and stator plates of embodiments of the device of the present invention.

[0098] An embodiment of a device 1 according to the present invention and its component parts is schematically illustrated in FIG. 1. The device 1 comprises a barrel 2 having an upper end 3 and a lower end 4 and a longitudinal axis extending therebetween. The diameter of the barrel 2 decreases at a constant rate between its upper end 3 and its lower end 4 such that the barrel 2 is an inverted truncated cone.

[0099] A rotor shaft 5 extends through the barrel 2 between the upper and lower ends 3, 4 along the longitudinal axis. Three rotor fans 6, 7, 8 are mounted on the rotor shaft 5. Three stator plates 9, 10, 11 are mounted on an internal wall of the barrel 2 and extend from the internal wall to the rotor shaft 5. A reservoir 12 is formed at the upper end 3 of the barrel 2 above the upper rotor fan 6. The reservoir 12 contains a baffle 13 to prevent liquid swirling within the reservoir and has a plate 15 mounted at its upper end. The plate 15 forms the upper end of the reservoir 12 and has an opening 16 formed therein to allow liquid metal to enter the reservoir. A bush 14 is mounted on the rotor shaft 5 near its upper end.

[0100] Details of each rotor fan 6, 7, 8 are shown in FIG. 1. The upper rotor 6 consists of sixteen substantially flat rotor blades, the middle rotor fan 7 consists of eight substantially flat rotor blades, and the lower rotor fan 8 consists of four substantially flat rotor blades. The rotor blades of each fan are aligned with the rotor shaft 5 and are equally circumferentially spaced about the rotor fan 6, 7, 8. The rotor fans 6, 7, 8 are formed such that the radially outer end of each blade is positioned within a minimum distance of the internal wall of the barrel 2 and such that the upper and lower surfaces of each blade are positioned within the minimum distance of the adjacent stator plates 9, 10, 11. The minimum distance is less than 10 mm. It will be readily understood that, as FIG. 1 is a schematic diagram, the gap between the stator plates 6, 7, 8 and the rotor fans 9, 10, 11 is exaggerated in the Figure.

[0101] FIG. 1 also shows the details of the stator plates 9, 10, 11. The stator plates comprise substantially flat plates with a plurality of holes 17 formed therethrough. The holes allow liquid metal to pass through the plates 9, 10, 11. FIG. 1 also shows details of the baffle 13. The baffle 13 comprises a plate with a plurality of holes formed therethrough a number of vertical blades extending from a surface of the baffle 13 to prevent liquid swirling within the reservoir. As shown in the lower left corner of FIG. 1, the barrel 2 and the stator plates 9, 10, 11 are formed in two halves that are then secured together.

[0102] In use, liquid metal is provided into the device 1 through the hole 16 in the upper plate 15. This liquid metal enters the reservoir 12 and then passes through the baffle 13 and the upper stator plate 9 and enters the barrel 2. The liquid metal can then pass through the device 1 before leaving the barrel 2 at its lower end 4. During its passage through the device 1 the rotor shaft 5, and thereby the rotor fans 5 are rotated at a speed between 1 rpm and 50,000 rpm. This acts to shear the metal between the rotor blades and the internal wall of the barrel or between the rotor blades and the stator plates 9, 10, 11. As the rotor blades are within the minimum distance of both the internal wall and the stator plates 9, 10, 11 the liquid metal is subject to high shear and is processed.

[0103] An alternative embodiment of a device 1 according to the present invention is shown in FIG. 2. The device 1 of FIG. 2 is similar to and operates according to the same principles as the device of FIG. 1, as such the same components of the device 1 are labelled using the same reference numerals where appropriate and will not be explained in detail except for where there are significant structural differences.

[0104] The device 1 of FIG. 2 differs from the device 1 of FIG. 1 in that the barrel 2 is substantially cylindrical and has a constant diameter along its longitudinal axis. As a result each of the stator plates 9, 10, 11 are identical to one another and each of the rotor fans 6, 7, 8 are identical to one another. Further, the stator plates 9, 10, 11 are formed of a plurality of equally circumferentially spaced blades with passages formed between adjacent blades. The blades are flat and are at an angle to the longitudinal axis of the barrel 2. The rotor fans 6, 7, 8 are formed in a similar manner although they comprise fewer blades and the passages between the blades are larger as a result. Both the rotor fans 6, 7, 8 and the stator fans 9, 10, 11 have a radially outer ring that acts to support the blades. The blades of the rotor fans 6, 7, 8 are formed to draw liquid metal through the barrel 2 when the device 1 is in operation.

[0105] FIGS. 4, 5, and 6 show potential applications of a device 1 according to the embodiment of FIG. 1. In these Figures the device 1 is schematically represented by a triangle. FIG. 4 is a schematic illustration of a liquid metal conditioning process using the device 1. FIG. 5 is a schematic illustration of a liquid metal degassing process using the device 1. FIG. 6 is a schematic illustration of a direct chill casting process using the device 1. The skilled person will readily understand the conventional manner in which each of these processes are typically carried out so that will not be repeated here. Rather, the implementation of the use of the device 1 of the present invention will be explained with reference to each of the relevant processes.

[0106] In the process shown in FIG. 4 the device 1 is fixed on an adjustable platform 22 and the rotor shaft 5 is driven by a motor (not shown). The position of the device 1 is controlled such that it is partially immersed in liquid metal 21 contained in a crucible 20 by adjusting the position of the platform. The crucible 20 is heated to keep the liquid metal 21 at a desired temperature.

[0107] During operation, liquid metal 21 is drawn into the device through its upper end by the rotation of the rotor fans and is subject to high shear. The liquid metal 21 then exits the device 1 from its lower end. The passing of the liquid metal 21 through the device 1 by the action of the rotor fans results in a macroscopic flow pattern in the crucible as indicated by the arrows in the Figure. This macroscopic flow delivers the liquid metal 21 to the device 1 such all the liquid metal in the crucible 20 will be subjected to repeated high shear treatment. In addition the macroscopic flow also promotes spacial uniformity of both melt temperature and chemical composition.

[0108] This high shear treatment disperses oxide clusters, oxide films and any other metallic or non-metallic inclusions present in the liquid metal 21. The macroscopic flow distributes dispersed particles uniformly throughout the liquid metal 21. It should be pointed out that the macroscopic flow in the crucible 20 will be weak near the surface of the liquid metal 21, and consequently, the macroscopic flow will maintain a relatively undisturbed melt surface, avoiding the possible entrapment of gas, dross or any other potential contaminants in the liquid metal 21. This makes the conditioned liquid metals particularly suitable for manufacturing high quality castings.

[0109] The process of FIG. 4 can also disperse exogenous solid particles into the liquid metal 21. Exogenous solid particles can be grain refiner particles, ceramic particles for metal matrix composites (MMCs) or nano particles for production of nano metal matrix composites (NMMCs). The device 1 will disperse the solid particles, distribute the dispersed solid particles uniformly in the liquid metal 20, and force the solid particles to be wetted by the liquid metal 21.

[0110] The process of FIG. 4, can be used to treat liquid metals either above the alloy liquidus to condition liquid metal or below the alloy liquidus to make semi-solid slurry. When treating liquid metal 21 above liquidus, the process can increase potential nucleation sites by dispersing oxide films and/or clusters into individual particles, improving the wettability and spacial distribution in the liquid metal. This is very helpful for grain refinement without addition of any chemical grain refiners. This is referred to as physical grain refinement. When treating the metals below their liquidus, the process can provide semisolid slurry with solid particles of fine size and a narrow size distribution. In addition, the said apparatus and method can provide high quality semi-solid slurry in large quantities.

[0111] Liquid metal 21 conditioned by the process of FIG. 4, treated either above or below the alloy liquidus, can be supplied batch-wise or continuously to a specific casting process, for example high pressure die casting, low pressure die casting, gravity die casting, sand casting, investment casting, direct chill casting, twin roll casting, or any other casting process which requires liquid or semi-solid metal as a feedstock.

[0112] In the process shown in FIG. 5 is identical to the process of FIG. 4 with the exception that tubes 26 for inputting gas into the liquid metal 21 are formed through the platform 22 such that an end of each tube is located immediately above the device 1. For the purpose of degassing the liquid metal 21, inert gas, such as argon, nitrogen or the like, is introduced into the liquid metal through the tubes 26 such that it enters the liquid metal 21 immediately above the device.

[0113] During operation of the process both the liquid metal 21 and the gas are drawn through the device 1 in the same manner as the process of FIG. 4. This subjects the liquid metal 21 and the gas to high shear and produces a macroscopic flow of the liquid metal 21. This disperses large inert gas bubbles into much smaller inert gas bubbles. Further, the macroscopic flow can distribute the inert gas bubbles uniformly throughout the liquid metal 21 in the crucible 20, creating significantly increased gas/liquid interfacial area. The dissolved gas in the liquid metal 21 will diffuse to the inert gas bubbles due to the much lower partial pressure in the inert gas than in the liquid metal 21. Due to their buoyancy, and with the assistance of the macroscopic flow, the inert gas bubble containing the dissolved gas will escape from the melt surface of the liquid metal 21, resulting in significantly reduced gas contents in the liquid metal.

[0114] When degassing using the process of FIG. 5, the size of the inert bubbles in the liquid metal can be controlled by varying the specific embodiment of the device 1 that is used. In particular the following parameters will affect the size of the inert bubbles: the minimum distance of the device 1, the size and shape of the passages in the stator plates, the speed at which the rotor fans and rotor shaft are rotated, the number of rotor fans and stator plates, the size, shape and construction of the rotor fans, and the size and shape of the barrel.

[0115] The process of FIG. 5 can also be used to prepare metal matrix composites (MMCs) by replacing the input inert gas with ceramic powders such as silicon carbide, aluminium oxide or the like. The high shearing applied by the device 1 of the present invention can improve the uniformity and the wettability of the particles, which is very important for preparing high quality MMC materials.

[0116] The process of FIG. 5 can also be used to prepare in situ metal matrix composites (MMCs) by changing the input inert gas to a reactive gas to form reinforcing particles in situ. One example is introducing oxygen to liquid aluminium alloy to prepare alumina particle reinforced aluminium MMCs.

[0117] The process of FIG. 5 can also be used to mix immiscible metals by changing the input inert gas to a liquid metal which is immiscible with the liquid metal 21 in the crucible 20. The process can disperse and distribute the immiscible metallic liquids uniformly.

[0118] The process of FIG. 5 can also be modified by using a hollow rotor shaft 5 to introduce the inert gas, the ceramic particles, the immiscible liquid metals or the like to the liquid metal 21 for the purpose of degassing, preparing MMCs, mixing immiscible metallic liquids or the like.

[0119] FIG. 6 shows a schematic diagram of a direct integration of a conventional direct chill (DC) casting process with the device 1 of the present invention, forming a high shear DC casting process. The high shear device 1 is fixed on an adjustable platform (not shown) for positioning. It is assumed that the features of a conventional DC casting process will be well-known to a person skilled in the art so they will not be repeated here. The device 1 is submerged into the sump of the DC caster. The preferred location of the bottom of the device 1 is 0-300 mm above the mushy zone.

[0120] During DC casting, liquid metal is continuously supplied to the DC mould through a feed tube and continuously sheared by the device 1 of the present invention. Liquid metal containing rejected solute elements and solid particles in the mushy zone is sucked into the device from the solidification front, subjected to intensive shearing and then forced out. The intensively sheared melt generates a macroscopic flow pattern in the sump of the DC caster in the same manner as the processes described above. The macroscopic flow pattern causes the homogenisation of temperature and chemical composition in the liquid metal around the device 1. This creates a unique solidification condition in the sump of the DC caster, resulting in a cast ingot with a fine and uniform microstructure, uniform chemical composition and reduced/eliminated cast defects.

[0121] FIG. 7 shows a number of stator plates 9, 10, 11 and rotor fans 6, 7, 8 that may form part of a device according to the present invention. The stator plates 9, 10, 11 and rotor fans 6, 7, 8 are substantially the same as those of the device 1 shown in FIG. 1 but further comprise a peripheral ring 40 that is formed round their outer radial edges. This outer ring 40 provides structural reinforcement for the stator plates 9, 10, 11 and rotor fans that may be necessary in some embodiments of the invention.