WO2021146769A1

WO2021146769A1 - Apparatus and method for preparing metal matrix composites

Info

Publication number: WO2021146769A1
Application number: PCT/AU2021/050030
Authority: WO
Inventors: Daniel Dong Liang; Shiqin Yan; Michael Kellam
Original assignee: Commonwealth Scientific And Industrial Research Organisation
Priority date: 2020-01-23
Filing date: 2021-01-19
Publication date: 2021-07-29
Also published as: AU2021210817A1; CN115298501A

Abstract

An apparatus for mixing a metallic melt containing non-metallic particles, the apparatus comprising: a vessel for containing the metallic melt, the vessel including a sidewall and a bottom; and an impeller including: a rotor configured for submergence at or below the top surface of the metallic melt, the rotor rotatable about a substantially vertical axis, the rotor including at least two annularly spaced apart blades extending radially outwardly of the vertical axis and configured to be operated at a rate of at least 200 revolutions per minute in the metallic melt; and a surface plate which extends over and outward of the outer radius of the rotor and is positioned level with or submerged just below a top surface of the metallic melt.

Description

APPARATUS AND METHOD FOR PREPARING METAL MATRIX COMPOSITES

TECHNICAL FIELD

[001] The present invention generally relates to an apparatus and method that can be used to prepare metal matrix composite materials. The invention is particularly applicable to the formation of non-ferrous metal matrix composites, for example aluminium alloy matrix composites and it will be convenient to hereinafter disclose the invention in relation to that exemplary application. However, it is to be appreciated that the invention is not limited to that application and could be used in forming a variety of metal matrix composites using other non-ferrous alloys, for example magnesium or zinc-based metal matrix composites, which incorporate non-metallic particles that do not react in the alloy melt.

BACKGROUND OF THE INVENTION

[002] The following discussion of the background to the invention is intended to facilitate an understanding of the invention. However, it should be appreciated that the discussion is not an acknowledgement or admission that any of the material referred to was published, known or part of the common general knowledge as at the priority date of the application.

[003] Metal matrix composites (MMC) are a composite material composed of a metal matrix embedded with a reinforcing material formed from a non-metallic material such as fibres, particulates, powder or the like, or a different metal. The reinforcing material imparts strength, stiffness and other desirable properties to the composite, while the metal matrix protects the reinforcing material and transfers load within the composite.

[004] MMCs are formed by dispersing the reinforcing material throughout the metal matrix. One method that has gained wide acceptance for forming MMCs is stir casting which essentially involves mixing the reinforcing material into a molten metal melt using a rotating impeller. For example, forming an aluminium alloy MMC using stir casting techniques typically involves the following process steps: • melting the Al alloy in a high temperature vessel, for example a crucible contained in a furnace, to form a metal melt;

• feeding a reinforcing material, for example ceramic particles, into a downward drawing vortex formed in the metal melt using a melt-stirring system which includes a rotating impeller, thereby forming a melt mixture,

• continuingly stirring the melt mixture using the impeller for a period of time to fully wet and distribute the reinforcing material throughout the melt mixture; and casting the melt mixture.

[005] Conventional melt-stirring systems for manufacturing MMCs use impellers placed well below the melt surface, usually close to the bottom of the furnace. These arrangements rely on shearing forces, either between turbulent flows generated by the rotating blades or between the impeller surface moving closely to another stationery surface, to break up any agglomerated ceramic powders and particles within the melt mixture. Examples of these conventional melt-stirring technologies include:

[006] United States Patents Nos. 4,786,467 and 4,865,806 (Duralcan technologies) disclose a melt-stirring system that uses vortex mixing to mix ceramic powder into an aluminium alloy melt at a temperature above the liquidus temperature of the metal. The system uses a furnace heated crucible containing molten aluminium alloy and a motor driven paddle-style rotating impeller made of graphite or coated steel. The vortex formed by the rotating impeller is used to draw the ceramic particles into the melt and then is operated continuously to disperse the particle clusters within the melt. The furnace and crucible therein are required to be operated under vacuum to prevent oxidation of the melt surface and the melt surface in the vortex and allow any trapped air to escape from the bottom of the crucible. However, the mixing efficiency of this system is low due to prolonged mixing times required to break up agglomerates and produce an even distribution of reinforcing material within the melt mixture. Moreover, the cost of establishing a vacuum environment in the furnace is high. [007] United States patents No. 6,106,588 (MC-21) and US64914923 teach a MMC melt-stirring system which avoids the use of a vacuum by injecting an insert gas (argon or nitrogen) above the melt to reduce oxidation of the melt surface. Reinforcing material (ceramic powder) is introduced into the melt below the impeller using a hollow feeding shaft. The melt is held in a semi-solid state to achieve higher shear forces during mixing due to the higher viscosity of the melt compared to a liquid state. The higher viscosity of the semi-solid state melt generates higher shear forces to break up any agglomerates in the reinforcing material. However, there is a limitation of the amount of ceramic powder that can be added to a very high viscous semi-solid state. Moreover, the entrapped gas is difficult to be removed from a metallic melt in a semi-solid state, which tends to produce porous metal matrix composites. The semi-solid state melt also results in a high wear rate on the impellers leading to high repair and maintenance costs.

[008] It would therefore be desirable to provide a new or alternate apparatus and process for preparing MMCs.

SUMMARY OF THE INVENTION

[009] The present invention provides an improvement to the production of MMCs, in particular the melt-stirring system used to produce MMCs. MMCs produced utilising the present invention can be used for high-performance structural and functional applications.

[010] A first aspect of the present invention provides an apparatus for mixing a metallic melt containing non-metallic particles (a melt-stirring system), the apparatus comprising: a vessel for containing the metallic melt, the vessel including a sidewall and a bottom; and an impeller including:

• a rotor configured for submergence at or below the top surface of the metallic melt, the rotor rotatable about a substantially vertical axis, the rotor including at least two annularly spaced apart blades extending radially outwardly of the vertical axis and configured to be operated at a rate of at least 200 revolutions per minute in the metallic melt; and • a surface plate which extends over and outward of the outer radius of the rotor and is positioned level with or submerged just below a top surface of the metallic melt.

[011] The present invention provides a new mixing impeller system for use in a stir casting process of metal matrix composites. This impeller mixing system is used to disperse non-metallic particles uniformly within the metallic melt and minimise disturbance at the melt surface. Mixing also assists in fully wetting the non-metallic particles with the metal matrix in the metallic melt.

[012] It should be appreciated that “positioned level with” means that the surface plate is positioned at the same level as or with the top surface of the metallic melt, being in contact with or at least partially immersed within the metallic melt. Here the surface plate could be resting on the top surface of the metallic melt, with the bottom surface of the surface plate in contact with the top surface of the metallic melt or may be at least partially immersed within the metallic melt at the top surface of the metallic melt.

[013] It should also be appreciated that “submerged just below a top surface of the metallic melt” means that the surface plate is positioned submerged in the metallic melt but is proximate to the top surface relative to the height of the metallic melt within the vessel. Here only a comparatively small volume (typically less than 1%, preferably less than 0.5%, yet more preferably less than 0.1%) of metallic melt would be located above the upper surface of the surface plate relative to the total volume of the metallic melt within the vessel.

[014] Thus, in embodiments the surface plate may extend over and outward of the outer radius of the rotor and is positioned level with or submerged below and proximate to the top surface of the metallic melt.

[015] The role of the surface plate is to minimise disturbance at the melt surface while the rotor rotates (typically at high speed) and, in some embodiments when the rotor is close to the top surface of the metallic melt. The surface plate reduces agitation induced by the rotor on the top surface of the metallic melt. The surface plate achieves this by being positioned level with or submerged just below the top surface of the metallic melt and covering the top surface of the metallic melt. The dimension of the surface plate should therefore be preferably selected to cover as much the top surface of the metallic melt as possible. In embodiments, the surface plate is sized to substantially cover the top surface of the metallic melt. For example, the surface plate can be sized to have substantially the same diameter as the vessel at the top surface of the metallic melt. The use of the surface plate in this position therefore mitigates and more preferably prevents the generation of surface waves when positioned close to the top surface of the metallic melt.

[016] The surface plate is preferably positioned to be in direct communication with the top surface of the metallic melt. This positioning mitigates and more preferably prevents the generation of surface waves and also preferably positions the surface plate to minimise oxygen uptake at the surface of the metallic melt. In this respect, the surface plate is preferably positioned to provide a low to zero oxygen environment thereunder. As indicated above, the surface plate can also be configured to cover the majority of the metallic melt surface shielding the metallic melt surface from interaction with the atmosphere air. In this respect, the surface plate is also preferably sized to provide a low to zero oxygen environment thereunder.

[017] The surface plate can have any suitable configuration. In embodiments, wherein the surface plate comprises a disc, the surface plate may comprise a substantially planar or flat plate. In other forms, the surface plate includes a downwards curvature relative to the center of the surface plate towards the outer rim or edge of the plate. This downward curvature (especially at the rim) assists in directing the melt flow away from the melt surface. The latter is considered to be beneficial to prevent the melt flowing over the top of the surface plate by directing the flow towards the wall. The upward flowing of the melt over the surface plate could lead to formation of dross consisting of ceramic powders (i.e. loss of the ceramic additions) as well as to creation of oxide defects in the resultant metal matrix composite. [018] It should also be understood that the substantially vertical axis referred to in the impeller configuration relates to the axis of the vessel containing the rotor and is relative to the orientation of that vessel.

[019] The rotor is configured to rotate at a reasonably fast speed (at least 200 revolutions per minute (rpm), preferably at least 500 rpm) in order to create sufficient impact and shear force when directing the metallic melt from flowing up the vertical axis and radially outwardly through each radial aperture and impacting the structure of the stator framing each radial aperture. The speed that the rotor is configured to rotate can vary depending on the application. In embodiments, the rotor is configured to be operated at a rate of from 200 to 4000 rpm in the metallic melt, preferably 500 to 3000 rpm, more preferably 500 to 2000 rpm. In embodiments, the rotor is configured to be operated at a rate of at least 1000 rpm, preferably 500 to 3500 rpm, and more preferably 500 to 2500 rpm. In particular embodiments, the rotor is configured to rotate at 800 to 2500 rpm, preferably about 1500 rpm.

[020] Mixing is conducted for a duration sufficient to wet the molten metallic alloy to the non-metallic particles and to distribute the particles throughout the molten metallic alloy. The rotor is therefore preferably configured to be operated for at least 10 minutes, preferably at least 15 minutes to mix the non-metallic particles throughout the metallic melt.

[021] The surface plate is preferably configured to be separate, preferably operatively separate to the rotor. In this respect, the surface plate is preferably configured as a separate component to the rotor and is not functionally or operatively interconnected with the rotation of the rotor about the vertical axis. Moreover, the surface plate is preferably configured to be rotationally stationary relative to the rotor. This can be achieved by the surface plate being fixed in position within the apparatus about the vertical axis, allowing the rotor to rotate as a separate component to the surface plate. The surface plate is configured as a stationary surface plate which extends over the top of the rotor. In embodiments, the surface plate comprises a planar sheet, preferably shaped as disc. The disc preferably has a large enough diameter to cover the free surface of the melt as much as possible to prevent the disturbance of the melt surface and thus reducing the generation of the oxide impurities.

[022] In some embodiments, the impeller can further include a stator that includes a housing having a top and an annular side enclosing the rotor, the annular side including at least one radial aperture relative to the vertical axis, wherein the impeller is configured to draw the metallic melt into the rotor and then direct that metallic melt radially outwardly from the rotor into the stator to be directed onto and through each radial aperture of the stator.

[023] The use of a stator provides a newly-designed impact mixing impeller system (IMIS system) for use in a stir casting process of metal matrix composites. As is explained in more detail below, use of a stator can assist in breaking up any agglomerates of the non-metallic particles that are within the metallic melt.

[024] The impeller is preferably positioned within the metallic melt with the rotor positioned submerged at or below the surface of the metallic metal, preferably just below the surface of the metallic metal. When adopted the stator is preferably positioned in an upper portion of the vessel, typically at the surface of the metallic melt or just below the surface of the metallic melt. When the stator is positioned at the surface of the metallic melt, the top of the stator is typically positioned level with or above the top surface of the metallic melt, i.e. the same level with the top of the stator exposed to the atmosphere.

[025] This embodiment of the impeller is designed to draw the metallic melt through the rotor and to impact with the stator. Here, the metallic melt below and/or above the rotor is drawn into the rotor and then spanned radially outwardly from the rotor into the stator. The metallic melt is typically drawn into the rotor from below, and not the top surface of the metallic melt. That radial outward flow from the rotor is directed onto and through each radial aperture of the stator, where the metallic melt flow (molten metal containing non-metallic particles) impacts the components of the stator framing each radial aperture. The high speed of the rotor is able to break up and/or comminute any material agglomerations in the non-metallic particles within that portion of the metallic mixture. The impeller therefore functions to produce a well-mixed mixture of the non-metallic particles throughout the metallic melt and to break up any agglomerates in the non-metallic particles in the metallic melt.

[026] The stator can be separate to the surface plate and is optionally submerged below the surface plate and thus immersed within the metallic melt. In these embodiments, the stator and rotor are both submerged in the metallic melt between the top surface of the metallic melt and the bottom of the vessel, for example between 1/3 and 2/3 the height of the molten metal within the vessel - i.e. the overall height from the top surface of the metallic melt to the bottom of the vessel. In some embodiments, the rotor is positioned to be between 1/5 and 3/5 the overall height from the top surface of the metallic melt to the bottom of the vessel. In some embodiments, the rotor is positioned to be between 1/10 and 1/4 the overall height from the top surface of the metallic melt to the bottom of the vessel. In some embodiments, the rotor is positioned to be no more than 2/3 (less than or equal to 2/3), preferably no more than 1/3, and more preferably no more than 1/6 of the overall height from the top surface of the metallic melt to the bottom of the vessel. In some embodiments, the rotor is positioned to be no more than 1/4, preferably 1/5, more preferably no more than 1/5, and yet more preferably no more than 1/10 of the overall height from the top surface of the metallic melt to the bottom of the vessel. In preferred embodiments, the stator and rotor are submerged in the metallic melt close to, or at, the top surface of the metallic melt.

[027] In other embodiments, the surface plate comprises a top wall (top plate) of the stator. In these embodiments, the impeller is positioned immersed within the metallic melt, with the surface plate level with or submerged just below the top surface of the metallic melt.

[028] In some embodiments, the surface plate can have the same diameter as the stator. However, it should be appreciated that the diameter of the surface plate can be significantly larger than the stator diameter, particularly in those embodiments where the furnace has a much larger diameter than the stator. [029] The stator is preferably configured to be stationary relative to the rotor, and more particularly rotatably stational relative to the rotor relative to and about the vertical axis. This can be achieved by the stator being fixed in position within the apparatus about the vertical axis, allowing the rotor to rotate within the stationary housing provided by the stator. The stator can have any suitable configuration. In embodiments, the stator includes a surface plate which extends over and outward of the outer radius of the rotor (typically defined by the outer edge or tip of the blades of the rotor). Again, the surface plate is configured as a stationary surface plate which extends over the top of the rotor. In embodiments, the surface plate comprises a planar sheet, preferably shaped as disc. The disc preferably has a large enough diameter to cover the free surface of the melt as much as possible to prevent the disturbance of the melt surface and thus reducing the generation of the oxide impurities. Extending beneath the surface plate is an annular side which includes at least one radial aperture. Each radial aperture comprises a radially orientated opening in the annular side of the stator.

[030] It should be appreciated that the gap between the stationary stator and rotor parts needs to be sufficiently large not to jam (i.e. if the gap is too small) or but if too large it is not effective in breaking-up of particle agglomerations.

[031] The stator preferably includes two or more radial apertures annularly spaced apart around the vertical axis, and more preferably multiple radial apertures annularly spaced apart around the vertical axis. Each radial aperture may comprise an opening in a side (sidewall) of the stator. However, it is preferable to configure each radial aperture as a radial passage or channel adapted to direct flow outwardly from the rotor. Each radial passage/ channel can be formed by at least two spaced apart radially extending members that also axially extend between the surface plate and a base plate of the stator. The radially extending members form guide vanes between the surface plate and based plate which radially direct flow from the rotor radially outwardly of the impeller. The radial passages/ channels from the rotor to the metallic melt enable the metallic melt to be directed radially outwardly of the rotor and the vertical axis through rotation of the rotor. It should be appreciated that the metallic melt impacts these guide vanes in the course of directing the flow from the rotor. The thickness of radially extending members also plays a role in breaking up particle agglomerations or clusters through a shearing action between the ends of these members and the blades of the rotor.

[032] The base plate of the stator typically includes an opening below the rotor to allow the metallic melt to be drawn into the impeller and through to the rotor. To achieve this, the base plate of the stator may comprise an annulus, preferably a substantially planar annulus shaped plate which radially extends from the outer radius of the rotor to the outer side of the stator. The base plate is spaced apart from the surface plate by the radially extending members.

[033] The rotor of the impeller can have any suitable construction and configuration which creates a pattern of flow, a swirl flow, in the metallic melt of the vessel. In embodiments, the rotor comprises a cylindrically shaped hub attached to a drive shaft. Each of the blades of the rotor is attached to the hub and spaced apart around the circumference of the hub. The rotor typically includes at least two annularly spaced apart blades, and more preferably multiple annularly spaced apart blades. In one particular embodiment, the rotor includes four annularly spaced apart blades. In another embodiment, the rotor includes eight annularly spaced apart blades. Each of the blades of the rotor are preferably attached to one end of a rotating shaft. Where the impeller includes a stator, the blades are preferably located within the stator. Each blade of the rotor preferably has the same length and configuration. The rotor preferably comprises a radial flow impeller. In some embodiments, each blade comprises a flat blade i.e. have a flat blade configuration. However, in other embodiments the blades may include a curved portion or be curved. The blades are smaller (typically at least 50% less in diameter) compared to impellers used in prior MMC stir casting processes (for example as outline in the background of the invention section). In embodiments, the ratio of outer rotor diameter to vessel diameter is 1 :3 to 1 :5, preferably 1 :4. In embodiments, the ratio of rotor diameter to stator diameter is 1 :2 to 1 :3.

[034] The surface plate, stator and rotor of the impeller can be formed of any suitable material capable of withstanding the temperature, mechanical requirements, and erosive effects from high-speed rotation within a ceramic particle-filled composite matrix within a metallic melt. In embodiments, the surface plate, stator and rotor are formed from a ceramic material. Suitable ceramic materials include nitrides, silicides, oxides, and carbides. Particularly preferred ceramics include silicon carbide, aluminium oxide, boron carbide, silicon nitride, and boron nitride. In particular embodiments, surface plate, stator and rotor are formed from S13N4 alloys such as a SiAION ceramic. It should be appreciated that the materials selection and design for these components of the impeller immersed in the melt can be optimised for example by using, for example anti-impact and/or wear resistant materials at the edge and sides of the radial apertures including the radially extending members.

[035] The impeller can be configured to be located at any suitable location within the metallic melt. The impeller is configured to be located in the metallic melt with the surface plate of the impeller positioned level with (i.e. at the level of the top surface of the metallic melt) or just below the top surface of the metallic melt. Where the surface plate comprises a top plate (top side) of the stator, the top plate of the stator can be positioned level with (i.e. at the level of the top surface of the metallic melt) or just below the top surface of the metallic melt. The use of the surface plate in this position mitigates and more preferably prevents the generation of surface waves when positioned close to the top surface of the metallic melt. The surface plate of the stator avoids agitation of the top surface of the metallic melt generated by the rotor. The radial apertures direct the flow radially outwardly rather than towards the top surface of the metallic melt. In other embodiments, the impeller is configured to be submerged below the liquid surface by a distance that is approximately one-third of the height of the liquid - i.e. the overall height from the top surface of the metallic melt to the bottom of the vessel.

[036] In some embodiments, the impeller is operated to produce a swirl flow pattern. Where the impeller is positioned in an upper portion of the vessel, for example at the surface of the metallic melt or just below the surface of the metallic melt, the impeller is configured to produce (a) an inner, upward flow region located along said vertical axis, (b) a transition flow region located around the rotor in which liquid moves radially outwardly through the at least one radial aperture of the stator toward the vessel sidewall, and (c) an outer, downward flow region. In embodiments, the downward flow region is located along the sidewall.

[037] It should be understood that the created swirl flow in the metallic melt of the vessel comprises a stable swirling flow through the vessel characterised by (i) an outer annular region of moderate rotational flow around said vertical axis adjacent to the containing wall of the vessel moving from the upper end toward the lower end (i.e. a downwards flow region) so as to maintain a continuous flow of liquid over the containing sidewall of the vessel, (ii) a transition flow region located in and around the impeller in which liquid moves radially outwardly toward the vessel sidewall, and (iii) an inner core region of rapid rotational flow around said axis about the central region of the vessel moving from the lower end toward the upper end (i.e. an upwards flow region) and extending from substantially adjacent the lower end of the vessel to the impeller. It should be appreciated that the transition flow region results from the rotor changing the flow direction during operation, and also through impact of the metallic melt mixture with the stator walls.

[038] A benefit of the swirl flow pattern is that the metal surface is relatively undisturbed compared to the prior art arrangement which has a vortex enhancing the gas-metal melt interface. This flow pattern disturbs the top surface less than the downward vortex flow of the prior art. The horizontal transition flow forced by the stationary stator top advantageously assists with providing an undisturbed top surface. Additionally, this swirl flow pattern reduces oxidation within the melt compared to prior art arrangements and avoids the need for high-capital equipment such as vacuumed furnace.

[039] In embodiments, the flow pattern produced by the impeller can also be influenced by the configuration, typically the curvature of the surface plate. It is preferred that the impeller is operated to alter the flow above the surface plate, such that the flow is directed away from the melt surface to prevent the disturbance of the melt surface. In embodiments, the surface plate of the stator has a curved shape, preferably a convex curved shape to influence the flow pattern to have a downward direction. However, it is preferable for the flow pattern to have the top flow being parallel to the melt surface. Thus, in other embodiments, the surface plate is planar or flat.

[040] The metallic material and non-metallic particles preferably comprise metals/metal alloys and non-metallic particles that can co-exist in the metallic melts. The metallic material preferably comprises a non-ferrous metal or metal alloy and is preferably selected from: aluminium or an aluminium alloy; or magnesium or a magnesium alloy or zinc or a zinc alloy. Preferably, the metallic material is aluminium or an aluminium alloy.

[041] The non-metallic particles are preferably a ceramic or crystalline particulate material including powders. In embodiments, the non-metallic particles comprise a metal oxide, metal nitride, metal carbide, or metal silicide, preferably in the form of a refractory ceramic. Examples include silicon carbide, silicon dioxide, aluminium oxide, boron carbide, silicon nitride, boron nitride, titanium carbide, tantalum carbide, titanium boride (titanium diboride), tungsten carbide as well as fly ash or the like. The most preferred composite material is an aluminium matrix composite (AMC), for example silicon carbide or aluminium oxide particulate reinforcing material in an aluminium alloy matrix.

[042] The non-metallic particles can have any suitable particle size range desired to be include in the MMC composition. In embodiments, the particle size range is from 8 microns to 100 microns. In other embodiments, the non-metallic particle comprises sub-micro particles.

[043] The present invention enables the production of MMCs with a high loading level of non-metallic particles (>20 % volume fraction and more particularly >30% volume fraction). The metallic melt therefore can include a composition in which the non-metallic particles comprises from 5 to 40 % volume of the metallic melt, preferably at least 20 % volume of the metallic melt, preferably from 5 to 40 % volume of the metallic melt, preferably 20 to 40 vol. %, more preferably greater than 30 vol. %. In some embodiments, the non-metallic particles comprise between 30 and 40% volume of the metallic melt. The present invention has particular application in producing aluminium alloy matrix composites (AMCs) with a high level (>20% volume fraction and more particularly >30% volume fraction) of ceramic particle non-metallic reinforcing material content.

[044] The apparatus can include a feeder for feeding non-metallic particles into the metallic melt. That feeder typically includes a feeding conduit which either feeds the non-metallic particles onto the surface of the melt or may comprise a submerged feeder for example a submerged feeding lance or the like which injects the non-metallic particles into the metallic melt. In embodiments, the non- metallic (reinforcing) particles are fed onto the surface of the metallic melt through a top feeder. The particles are drawn into the melt through a simple rotor which generates vortex but has little break-up action. Once the non-metallic particles enter the metallic melt under the melt surface (but not dispersed), then the apparatus and process of the present invention is used to disperse the particles clusters.

[045] The metallic melt is preferably a molten liquid melt during operation of the impeller in order to mix the non-metallic particles therethrough. In order to form a molten liquid melt, the metallic melt preferably comprises a molten metal mixture with the non-metallic particles that is maintained in a temperature range from about the liquidus temperature of the metal to 100 ^SC above the liquidus temperature, and preferably about 80 to 100 ^SC above the liquidus temperature. In some embodiments, the temperature range is at least 20 ^eC above the liquidus temperature of the metal or the metallic alloy of the metallic melt.

[046] In some embodiments, at least one inert gas is applied over at least the top surface of the metallic melt. Suitable inert gases include nitrogen or argon.

[047] In particular embodiments, the present invention has application to vessels that have a height equal to or greater than the diameter of the vessel. The present invention has been found to provide satisfactory mixing in vessels having heights from 1 to 3 times the diameter. In some embodiments, the ratio of the vessel sidewall height to the vessel diameter is at least 2. However, it should be appreciated that the process can be adapted to work with any size of commercial furnace. The vessel preferably includes an upper end and a lower end and comprises a generally cylindrical containing sidewall extending between the upper end and a lower end. The vessel preferably has a circular cross-section. In a number of embodiments, the vessel comprises a crucible.

[048] The present invention also relates to a stir casting apparatus for forming metal matrix composites comprising an apparatus for mixing a metallic melt containing particulates according to the first aspect of the present invention.

[049] A second aspect of the present invention provides a method of mixing non- metallic particles into molten metal, comprising: mixing a metallic melt comprising a mixture of non-metallic particles and a molten metal or metal alloy held in a vessel with an impeller at a temperature above the liquidus of the metal or metal alloy, the vessel including a sidewall and a bottom, the impeller including:

• a rotor positioned at or below the surface of the metallic melt, the rotor rotating about a substantially vertical axis, the rotor including at least two annularly spaced apart blades extending radially outwardly of the vertical axis and is rotated at a rate of at least 200 revolutions per minute in the metallic melt; and

• a surface plate which extends over and outward of the outer radius of the rotor and is positioned level with or submerged just below a top surface of the metallic melt.

[050] This aspect of the present invention provides a melt-mixing method for forming a well-mixed metal and non-metallic particle melt mixture. Again, the role of the surface plate is to minimise disturbance at the top surface of the metallic melt while the rotor rotates, typically at a location close to the top surface of the metallic melt.

[051] It should be appreciated that the method of this second aspect of the present invention can be performed using an apparatus according to the first aspect of the present invention and therefore the description of the feature of the first aspect equally apply to this second aspect. [052] In exemplary embodiments, the impeller further includes a stator that consists of a housing having a top and an annular side enclosing the rotor, the annular side includes at least one radial aperture relative to the vertical axis In these embodiments, the rotor is operated to draw the metallic melt inwards (into and through the rotor) and then radially outwards from the rotor into the stator and then directed onto and through each radial aperture of the stator.

[053] As explained above, the radial outward flow of the metallic metal from the rotor is directed onto and through each radial aperture of the stator, where the flow impacts the components of the stator framing the aperture. The high speed of the rotor is able to comminute any material agglomerations in the non-metallic particles within that portion of the metallic mixture. This embodiment of the present invention therefore performs high impact mixing at a temperature at or above the liquidus temperature of the metal, enabling a high-level of non-metallic particles (for example ceramic particles) loading (>20 vol% or higher, preferably >30 vol%) to be readily mixed uniformly using the method. The resulting high impact mixing also removes entrapped air in the melt, and hence can produce dense metal matrix composites (denser than existing mixing techniques used in conventional stir casting processes).

[054] The rotor can be operated to produce a swirl flow pattern in the metallic melt. In these embodiments, the rotor is operated to produce a flow pattern comprising (a) an inner, upward flow region located along said vertical axis, (b) a transition flow region located around the rotor in which liquid moves radially outwardly through the at least one radial aperture of the stator toward the vessel sidewall, and (c) an outer, downward flow region

[055] During the mixing step, the rotor is rotated at a rate of at least 500 revolutions per minute, and preferably from 200 to 4000 revolutions per minute in the metallic melt. In embodiments, the rotor is rotated at a rate of at least 1000 rpm, preferably 500 to 3000 rpm, and more preferably 500 to 2000 rpm. In particular embodiments, the rotor rotated at a rate of at 800 to 2500 rpm, preferably about 1500 rpm. Mixing is conducted for a time sufficient to wet the molten metallic alloy to the particles and to distribute the particles throughout the molten metallic alloy. In embodiments, mixing is conducted for at least 5 minutes, preferably at least 10 minutes, preferably at least 15 minutes. In embodiments, mixing time is typically 10 to 15 minutes.

[056] The impeller can be located in any suitable location within the metallic melt. In embodiments, the impeller is located in the metallic melt with the surface plate of the impeller positioned level with (i.e. at the level of the top surface of the metallic melt) or submerged just below the top surface of the metallic melt. The rotor may be located close to the surface plate or may be submerged below the liquid surface of the metallic melt i.e. the overall height from the top surface of the metallic melt to the bottom of the vessel. In some embodiments, the rotor is positioned to be between 1/5 and 3/5 the overall height from the top surface of the metallic melt to the bottom of the vessel. In some embodiments, the rotor is submerged below the liquid surface of the metallic melt by a distance that is approximately one-third of the height of the metallic melt in the vessel. In yet other embodiments, the rotor can be placed towards the lower part of a deep vessel to ensure efficient mixing, for example, between 4/5 and 5/6 the overall height from the top surface of the metallic melt to the bottom of the vessel. In some embodiments, the rotor is positioned to be no more than 2/3 (less than or equal to 2/3), preferably no more than 1/3, and yet more preferably no more than 1/6 of the overall height from the top surface of the metallic melt to the bottom of the vessel. In some embodiments, the rotor is positioned to be no more than 1/4, preferably no more than 1/5, more preferably no more than 1/6, and yet more preferably no more than 1/10 of the overall height from the top surface of the metallic melt to the bottom of the vessel. In preferred embodiments, the stator and rotor are submerged in the metallic melt close to, or at, the top surface of the metallic melt.

[057] The surface plate is preferably configured to be operatively separate to the rotor. As indicated above the surface plate can be configured as a separate component to the rotor and is not functionally or operatively interconnected with the rotation of the rotor about the vertical axis. Moreover, the surface plate is preferably rotationally stationary relative to the rotor. This can be achieved by the surface plate being fixed in position within the apparatus about the vertical axis, allowing the rotor to rotate as a separate component to the surface plate. The surface plate is configured as a stationary surface plate which extends over the top of the rotor.

[058] The metallic material can be selected from: aluminium or an aluminium alloy; magnesium or a magnesium alloy; or zinc or a zinc alloy. The non-metallic particles preferable comprise a ceramic or a crystalline particulate material. In embodiments, the non-metallic particles comprise a metal oxide, metal nitride, metal carbide, or metal silicide, preferably in the form of a refractory ceramic. Examples include silicon carbide, silicon dioxide, aluminium oxide, boron carbide, silicon nitride, boron nitride, tungsten carbide and fly ash. The non-metallic particles preferably comprise from 5 to 40 % volume of the metallic melt, preferably 20 to 40 vol. %, more preferably greater than 30 vol. %.

[059] The metallic melt preferably comprises a molten metal mixture with the non-metallic particles that is maintained in a temperature range from about the liquidus temperature of the metal to 100 ²C above the liquidus temperature, and preferably about 80 to 100 ²C above the liquidus temperature. In some embodiments, the temperature range is at least 20 ²C above the liquidus temperature of the metal or the metallic alloy or the metallic melt.

[060] The feed material for the vessel is either an existing composite with poor particle distribution or a melt mixture (molten metal or metal alloy with non- metallic particles) mixed in a short period of time using a traditional vortex stirrer. Thus, in some embodiments the method may include a step of forming the metallic melt. Here, the method includes the preliminary step of feeding non- metallic particles into molten metal held in a vessel to form a melt mixture.

[061] In some embodiments, at least one inert gas is applied over at least the top surface of the metallic melt. Suitable inert gases include nitrogen or argon.

[062] A third aspect of the present invention provides a method for preparing a composite of a metallic alloy reinforced with particles of a non-metallic material, comprising: mixing non-metallic particles into molten metal according to the method of the second aspect of the present invention; and casting the resulting mixture.

[063] During the mixing and casting steps of the second and third aspects of the present invention, the molten metal should not be heated to a temperature that is too high, or there may be an undesirable reaction between the particulate and the molten metal which degrades the strength of the particulate and the properties of the finished composite. The maximum temperature is therefore chosen so that a significant degree of reaction does not occur between the particles and the metallic melt in the time required to complete processing. The maximum temperature of the melt is from the liquidus temperature of the metal or metal alloy of the metallic melt to 100 ^eC above the liquidus temperature. In embodiments, the maximum temperature of the melt is at least 20 ^SC above the liquidus temperature, and more preferably about 80 to 100 ^SC above the liquidus temperature, depending on the composition of the metallic melt. For example, the maximum temperature is found to be about 20 ^SC. above the liquidus for metallic alloys containing volatile, reactive alloying elements, about 70 ^QC above the liquidus for most common metallic alloys, and about 100 ^SC to about 125 ^eC above the liquidus for metallic alloys containing alloying elements that promote resistance to reaction.

[064] A fourth aspect of the present invention provides a composite material prepared using method according to the third aspect of the present invention.

[065] The composite material made by the method of the invention has a cast microstructure of the metallic matrix with the non-metallic particles, typically a particulate, distributed generally evenly throughout the cast volume. The non- metallic particles are well bonded to the matrix due to the thorough mixing and wetting processes created by swirl flow mixing. The cast composite is particularly suitable for processing by primary forming operations such as rolling and extruding to useful shapes. The properties of the cast or cast and formed composites are dependent on the metal and non-metallic particles properties, but generally have a high stiffness and strength, and acceptable ductility and toughness.

[066] A further aspect of the present invention provides an apparatus for mixing a metallic melt containing non-metallic particles (a melt-stirring system), the apparatus comprising: a vessel for containing the metallic melt, the vessel including a sidewall and a bottom; and an impeller including:

• a rotor configured for submergence at or below the metallic melt surface, the rotor rotatable about a substantially vertical axis, the rotor including at least two annularly spaced apart blades extending radially outwardly of the vertical axis and configured to be operated at a rate of at least 200 revolutions per minute in the metallic melt; and

• a stator including a housing having a top and an annular side enclosing the rotor, the annular side including at least one radial aperture relative to the vertical axis; wherein the impeller is configured to draw the metallic melt into the rotor and then direct that metallic melt radially outwardly from the rotor into the stator to be directed onto and through each radial aperture of the stator.

[067] This aspect of the present invention provides a newly-designed impact mixing impeller system (IMIS system) for use in a stir casting process of metal matrix composites. This impeller mixing system is used to disperse non-metallic particles uniformly within the metallic melt and to break up and/or comminute any particle agglomerations in the non-metallic particles in that metallic melt. Mixing also assists in fully wetting the non-metallic particles with the metal matrix in the metallic melt. It should be appreciated that this aspect of the present invention shares the same features as the first aspect of the present invention where the impeller includes a stator. The feature described in relation to that embodiment equally apply to this further aspect of the present invention.

[068] The advantages of the apparatus and method of the present invention in comparison with the existing stir casting technologies are as follows: • Enables the production of AMCs with a high loading level of ceramic powders and particles (>20 vol%, preferably >30 vol%), which can be difficult using conventional melt-mixing technologies.

• Reduces the operational time for achieving an even/ uniform distribution of non-metallic particles throughout the metallic melt through impact mixing of the impeller enabling the break up any agglomerated non-metallic particles/ powders and releasing the trapped gases more effectively.

• Minimises the requirements for controlling the oxygen level above the metallic melt, resulting in simplifying the process set-up and operations (especially for the fully liquid melt processing, which can be done without vacuum in the invention).

• Eliminates the need to use a vacuum melting furnace, reducing the capital expenditure and energy footprint of the process.

[069] Furthermore, it should be understood that the apparatus and method of the present invention enables the production of quality metal matrix composites with a high ceramic addition of >20 vol%, preferably >30 vol% in a cost-effective and efficient operation by achieving one or more of the following performances: a) Releasing the trapped air from the melts quickly, to prevent the formation of porosity defects; b) Dispersing agglomeration of ceramic powders of the ceramic addition using a sufficient high impellor speed, while not disrupting the melt surface excessively forming dross defects; c) Obviating the need for a vacuum environment, which is also a key for reducing the operational time and cost; d) Reducing wear on the impellors compared to existing stir casting technologies; and e) Operating at a duration comparable or less than existing stir casting technologies, assisting in controlling operational costs.

BRIEF DESCRIPTION OF THE DRAWINGS

[070] The present invention will now be described with reference to the figures of the accompanying drawings, which illustrate particular preferred embodiments of the present invention, wherein: [071] Figure 1 is a schematic front cross-sectional view of an impact mixing impeller apparatus according to an embodiment of the present invention.

[072] Figure 2 provides a top cross-sectional view of the impact mixing impeller used in the apparatus shown in Figure 1 .

[073] Figure 3 provides a schematic diagram of the flow paths within the vessel when the impact mixing impeller apparatus shown in Figure 1 is in operation.

[074] Figure 4 provides two views of a first embodiment of the mixing impeller used in the apparatus shown in Figure 1 which includes a stator (thus forming an impact mixing impeller) showing: (A) a bottom perspective view of the impeller assembly; and (B) a front cross-sectional view of the impeller assembly.

[075] Figure 5 illustrates a second embodiment of the mixing impeller used in the apparatus shown in Figure 1 which includes a surface plate and rotor combination showing: (A) a front cross-sectional view of the impeller assembly; (B) two different cross-sectional shapes of the surface plate of the impeller assembly; and (C) a bottom face photograph of the impeller assembly.

[076] Figure 6 illustrates the impeller configuration used for adding non-metallic particles to a molten metal melt prior to mixing using the mixing apparatus of the present invention.

[077] Figure 7 provides optical microscopy images of formed metal matrix composites showing (A) Prior art metal matrix composite formed using the Duralcan technologies; (B) Metal matrix composite formed using general vortex only stirring of 30 min or more, showing a significant amount of agglomerates; and (C) a metal matrix composite formed using the process and apparatus of the present invention, showing uniform particle distribution.

[078] Figure 8 illustrates two optical microscopy images above show SiC particle distribution in the Al alloy matrix formed by vortex mixing (A) without baffle; and (B) with a baffle. [079] Figure 9 provide optical microscopy images comparing the ceramic distributions (A) before and (B) after mixing operation with the impeller configuration shown in Figure 5.

[080] Figure 10 illustrates optical microscopy images comparing mixing runs where the SiC particle distribution in the Al alloy matrix mixed by (A) an impeller without a stator as shown in Figure 5; (B) an impeller with the stator as shown in Figure 4; and (C) an impeller with the stator as shown in Figure 4 (higher magnification).

DETAILED DESCRIPTION

[081] The present invention provides an improved melt-mixing process and apparatus for preparing a metal matrix composite material through the incorporating non-metallic reinforcing material into a molten mass of a metal matrix material (MMC). The improvement stems from the provision and use of a surface plate in mixing the non-metallic reinforcing material into the molten metal. The surface plate is positioned level with or submerged just below a top surface of the metallic melt and covering the top surface of the metallic melt. The use of the surface plate in this position therefore avoids agitation of the top surface by the rotor, thereby mitigates and more preferably prevents the generation of surface waves at the top surface of the metallic melt.

[082] In some embodiments, the impeller includes a stator which incorporates the surface plate. In addition to the above advantages, the use of a rotor- in -stator impeller more effectively breaks up agglomerated particulates in the non-metallic reinforcing material (such as ceramic powders and particles) fed and mixed into the metallic melt. The rotor in stator impeller system leads to a reduction in the operational time and improvement of distribution of the non-metallic reinforcing material in the metallic melt.

[083] It should be appreciated that the melt-mixing apparatus and associated method of the present invention form part of a stir casting process and apparatus used to form an MMC. Stir casting is a type of casting process in which a mechanical stirrer is used to mix reinforcing material in a metal matrix material. The process typically takes place in a furnace, for example a bottom pouring furnace or other high temperature vessel which is used to heat and melt the materials. A stirrer, or an impeller type configuration, is used to mix the reinforcing material introduced in the melt. In the process, the metal matrix material is melted in a high temperature vessel to form a molten liquid (typically just below, at or just above the liquidus temperature depending on the specific process) and the stirrer is started to create fluid flow within the molten liquid in the vessel and operated for certain time period. The reinforcing material is then fed into the molten liquid. The stirring process is continued for a certain time period after complete feeding of the reinforcing material to fully distribute the reinforcing material throughout this molten mixture and ensure that the reinforcing material is fully wetted by the molten metal. The molten mixture is then cast, for example by being poured in a preheated mold and allowed to solidify.

[084] The characteristics of the stirrer in such stir casting processes plays a vital role over the final microstructure and mechanical properties of the cast composites as it controls the distribution of reinforcing material within the metal matrix. Optimum mechanical properties can be attained by the uniform distribution of reinforcing material throughout the metallic melt, thereby providing uniform dispersion of the reinforcing material throughout the cast composite.

[085] The present invention uses a specially configured impeller to create a swirl flow mixing pattern within the vessel along with a flow impact zone for breaking up any particle agglomerations. The swirl flow pattern provides a vertical/ axial velocity component for solids suspension, and a large horizontal velocity to increase the total velocity over the vessel wall to enhance mixing throughout the vessel. In this set-up, the impeller is used to draw in the melt composition (metal plus non-metallic particulates) along the vertical axis of the vessel and discharges the melt composition radially outwardly with a large tangential velocity component, while also imparting a large swirl velocity (see for example Figures 1 and 3, explained in more detail below). As the melt composition reaches the vessel wall it changes direction and spirals down along the vessel wall. Upon reaching the vessel bottom, the melt composition spirals towards the axis of the vessel. In doing so, the swirl velocity increases due to conservation of angular momentum which requires that the product of tangential velocity and radius remains constant. The fast-swirling melt composition then rises along the vessel axis and enters the rotating rotor of the impeller where the impeller imparts a radial motion to the melt composition.

[086] A typical mixing apparatus 100 according to one embodiment the present invention is illustrated in Figures 1 to 5. The illustrated mixing apparatus 100 includes a high temperature vessel 102, typically a crucible, configured to contain a metallic melt, and a stirrer/impeller assembly 104. The vessel 102 may include means for heating the vessel 102 (not shown). Vessel 102 includes a vessel sidewall 120 and a vessel bottom 124 and defines a vessel height 128 and a vessel diameter 130. Vessel sidewall 120 includes a vessel sidewall inside surface 122. Whilst not shown, a feeder can be attached with the vessel 102 and used to feed the non-metallic particles into a metallic melt 161 held in the vessel 102.

[087] The impeller assembly 104 comprises an agitator which includes an impeller shaft 142, a mechanical drive and agitator motor (not illustrated), and an impeller 300. The agitator motor is typically a variable speed motor allowing the rotation speed of a rotor 310 to be controlled to spin at rotation speeds of 200 revolutions per minute or more, and more typically 1000 to 3000 RPM. The mechanical drive may be any mechanical drive known in the pertinent art that may be adapted to rotate the impeller shaft 142 and the rotor of the impeller to the desired speed, such as a gear box, a belt drive, hydraulic drive and the like. The mechanical drive is coupled to the upper end of impeller shaft 142.

[088] As best shown in Figure 1 , a melt liquid 160 within vessel 102 includes a melt surface 162, an upward flow region 164, a transition flow region 166, and a downward flow region 168. The melt liquid 160 comprises a metallic melt 161 mixed with non-metallic particles (not illustrated but should be understood to make up particles within the metallic melt 161 ). Selection of the metal and non- metallic reinforcing material is discussed above in more detail. This mixture is heated and held at a temperature between the liquidus temperature of the metal and around 100 ^eC above that liquidus temperature. [089] The illustrated vessel 102 is cylindrical in shape with a circular cross section, and it may have any vessel height 128 and any vessel diameter 130. The particular dimensions may be chosen according to well-known design principles depending on the properties of the particular metal melt, non-metal material, and casting configuration. The vessel sidewall 120 and vessel bottom 124 may be made of any material suitable for the high temperature application. The vessel is formed around vertical axis X-X which runs through the center of the vessel 102. Vessel 102 may be of any volume that is appropriate for use as a molten metal stir casting apparatus. The volume of the vessel 102 is typically between 0.02 and 0.5 m³. However, it should be appreciated that the process can be adapted to work with any size of commercial furnace.

[090] The vessel bottom 124 may be of any shape and will typically be defined by the casting configuration of the vessel 102. For example, a bottom casting vessel will include a casting outlet and likely a sloped or angled base to enable the molten metal to egress from the vessel 102 for casting.

[091] Figures 4 and 5 illustrate two different embodiments of impellers 300 and 300A that can be used in the impeller assembly 104.

[092] A first impeller embodiment (stator impeller embodiment) used in the impeller assembly 104 is illustrated in Figures 2 and 4. As shown in these embodiments, the impeller 300 includes a rotor 310 which is rotatable about vertical axis X-X, the rotor 310 including a number of annularly spaced apart blades 315 which extend radially outwardly of the vertical axis X-X; and stator 305 enclosing the rotor 310. The impeller 300 is preferably configured to produce (a) an inner, upward flow region located along said vertical axis, (b) a transition flow region located around the rotor in which liquid moves radially outwardly through the at least one radial aperture toward the vessel sidewall, and (c) an outer, downward flow region typically located along the sidewall. This swirl flow pattern is produced from the cooperative operation and configuration of the rotor 310 and stator 305 of the impeller 300. [093] The stator 305 is configured to be stationary relative to the rotor 310, being fixed in position within the vessel 100 about the vertical axis X-X. The illustrated stator 305 comprises a disc shaped top surface plate 320 which extends over and outward of the outer radius R of the rotor 310. Whilst the surface plate 320 is illustrated as having the same diameter as the stator 305, it should be appreciated that the diameter of the surface plate 320 can be significantly larger than the stator diameter, especially when the furnace has a much larger diameter than the stator 305. The stator 305 also includes a disc shaped base plate 322 which includes an opening 323 below the rotor 310 to allow the metallic melt 161 to be drawn into the impeller 300 and through to the rotor 310. The base plate 322 is spaced apart from the surface plate 320 by a series of annularly spaced apart radially extending members 325. The radially extending members 325 define radial passages/ channels 327 between the surface plate 320 and base plate 322 configured to direct flow outwardly from the rotor 310. The radially extending members 325 essentially form guide vanes which radially direct flow from the rotor 310 radially outwardly of the impeller 300. That flow exits the stator 305 through one of the radial apertures 328 formed at the exit of each radial passage 327. It should be appreciated that the shape, configuration, size and positions of the radially extending members 325 can be optimised with the aid of fluid dynamic modelling. The radial apertures 328 are annularly spaced apart around the vertical axis and position around the annular side of the stator 305.

[094] The rotor 310 of the impeller 300 includes at least two annularly spaced apart blades 315 attached to a cylindrical hub 326 (Figure 4) attached the lower end of impeller shaft 142. The rotor 310 is located within the stator 305. Each blade 310 is connected to or integrally formed with the hub 326 and annularly spaced apart approximately at equidistant radial locations about impeller shaft 142. Each blade 315 of the rotor 310 preferably has the same length and configuration, and in the illustrated embodiment comprises a flat blade. The rotor 310 can have any number of blades. However, any radial flow impeller blade design would be suitable. In the embodiment shown, it can have four spaced apart blades (Figure 4), or eight spaced apart blades (Figure 2). The rotor 310 is configured to rotate at a reasonably fast speed (200 to 4000 revolutions per minute) in order to create sufficient impact and shear force when directing the metallic melt 161 from flowing up the vertical axis and radially outwardly through the radial aperture and impacting the structure of the stator 305 framing each radial aperture 327. Rotor blades 315 may be contained in a one-piece assembly for attachment to the lower end of impeller shaft 142, or they may be individually attached to the lower end of impeller shaft 142.

[095] The surface plate 320, stator 305 and rotor 310 can be formed of any suitable material capable of withstanding the temperature and mechanical requirements within the metallic melt 161 . In embodiments, the stator 305 and rotor 310 are formed from a ceramic material. Suitable ceramic materials include nitrides, silicides, oxides, and carbides. It should be appreciated that the materials selection and design for these components of the impeller immersed in the metallic melt 161 can be optimised by using, e.g. anti-impact and/or wear resistant materials at the edge and sides of the radial apertures 327 including the radially extending members 325.

[096] The melt surface/ level 162 is the highest point that melt liquid 160 reaches in vessel 102. The impeller 300 may be positioned within the metallic melt 161 to any suitable depth, depending on the desired flow characteristics of melt liquid 160 in vessel 102. In the illustrated embodiment, the impeller 300 is configured to be located in the metallic melt 161 with the surface plate 320 of the stator 305 positioned level with or just below the melt surface 162 and the rotor 310 submerged within the metallic melt 161. The use of a stator 305 in these embodiments impedes and more preferably prevents the generation of surface waves when positioned close to the melt surface 162 of the metallic melt 161. The surface plate 320 of the stator 305 avoids agitation of the melt surface 162 by the rotor 310. The radial apertures 327 direct the flow radially outwardly rather than towards the melt surface 162. In existing stir casting technologies, the disturbance to the melt surface 162 is significant. Measures for the minimisation of oxidation of the melt surface, such as vacuuming and insert gas covering, are critical to reducing the formation of defects including oxides inclusions, gas entrapment, and porosities. In the system of the present invention (Figures 1 to 3), the top disc remains stationary while the blades rotate underneath the disc, thus preventing the disturburbance to the melt surface 162, and minimising the formation of the trapped oxides, air and minimising other defects e.g. porosity associated with them. This eliminates the need to use a vacuum melting furnace, reducing the capital outlay and energy footprint of the process.

[097] A second impeller embodiment (top plate impeller embodiment) used in the impeller assembly 104 is illustrated in Figure 5. This embodiment of the impeller 300A again includes a rotor 310A which is rotatable about vertical axis X-X and includes a number of annularly spaced apart blades 315 which extend radially outwardly of the vertical axis X-X. The rotor 300A has the same configuration as rotor 300 taught in relation to the impeller 300 shown in Figure 4, and the described configuration equally applies to this rotor 300A. This embodiment of the impeller 300A does not include a full stator, but rather just the top surface plate 320A component of the previous embodiment. That surface plate 320A extends over and outward of the outer radius of the rotor 310A and is positioned level with or submerged just below a top surface of the metallic melt 162 (similar to the position shown in Figure 1 for surface plate 320 of impeller 300). The role of the surface plate 320A is to minimise disturbance at the melt surface while the rotor 300A runs at high speed and close to the top surface of the metallic melt 162.

[098] The surface plate 320A is configured to be stationary relative to the rotor 310A, being fixed in position within the vessel 100 about the vertical axis X-X. The surface plate 320A is sized to cover as much the top surface of the metallic melt 162 as possible and is preferably sized to substantially cover the top surface of the metallic melt. For example, the surface plate 320A can be sized to have substantially the same diameter as the vessel 102 at the top surface of the metallic melt 162.

[099] The illustrated surface plate 320A comprises a flat disc. The surface plate 320A may comprise a substantially planar or flat plate. As shown in Figure 5B, the surface plate 320B may also be configured with a downwards curvature relative to the center of the surface plate 320B which curves downwardly towards the outer rim or edge of the surface plate 320B. This downward curvature (especially at the rim) assists in directing the melt flow away from the melt surface. [100] Again, the impeller configuration 300A is preferably configured to produce (a) an inner, upward flow region located along said vertical axis, (b) a transition flow region located around the rotor in which liquid moves radially outwardly toward the vessel sidewall, and (c) an outer, downward flow region typically located along the sidewall. This swirl flow pattern is produced from the cooperative operation and configuration of the rotor 310A and top plate 320A of the impeller 300A.

[101] The surface plate 320A, 320B and rotor 310A can be formed of any suitable material capable of withstanding the temperature and mechanical requirements within the metallic melt 161. In embodiments, the surface plate 320A, 320B and rotor 310A are formed from a ceramic material. Suitable ceramic materials include nitrides, silicides, oxides, and carbides.

[102] In operation, the mixing apparatus 100 typically comprises a stir casting arrangement. Stir casting typically follows the following process steps:

1 . melting the metal or metal alloy in the vessel 102 to form a metal melt 161 ;

2. optionally preheating the reinforcing material;

3. feeding the reinforcing material into the metal melt using the feeding arrangement (as noted above either a surface feeder or submerged feeder) to form a melt mixture. In many embodiments, the rotor 310, 310A of the impeller 300, 300A operated prior to feeding to create fluid flow throughout the metal melt to assist feeding and mixing of the fed non-metallic particles;

4. Continuingly mixing the melt mixture using the impeller 300, 300A for a period of time to break up any agglomerated ceramic particles and distribute the reinforcing material throughout the melt mixture; and

5. Casting the melt mixture into casting moulds.

[103] Mixing in step 4 using the impeller 300, 300A is conducted for a time sufficient to fully wet the molten metallic alloy to the particles and to distribute the particles throughout the molten metallic alloy to homogenize the distribution of particles in the subsequently cast MMC. The rotor 310, 310B is therefore preferably configured to operate for at least 10 minutes, preferably at least 15 minutes to mix the non-metallic particles throughout the metallic melt 161 .

[104] The rotor 310, 310A is configured to rotate at a reasonably fast speed (at least 200 revolutions per minute, preferably at least 500 rpm). For the impeller 300 shown in Figures 2 and 4, the rotational speed must be fast enough to create sufficient impact and shear force when directing the melt liquid 160 from flowing up the vertical axis X-X and radially outwardly through each radial aperture 327 and impacting the structure of the stator 305 framing each radial aperture 327. The speed that the rotor 310, 310A is configured to rotate can vary depending on the application. It should however be appreciated that these operational parameters, especially, the rotation speed and duration can be optimised for particular melt compositions and conditions.

[105] During mixing, the melt mixture liquid 160 includes an upper flow region 164, a transition flow region 166, and a downward flow region 168.

[106] The upward flow region 164 may have both an axial (upward, substantially along the axis X-X of impeller shaft 142) and tangential (rotating substantially about the axis X-X of impeller shaft 142) velocity component to its motion. Melt liquid 160 moves through upward flow region 164 towards the blades 315, 315A of the rotor 310, 310A. In one preferred embodiment, the velocity of the center of upward flow region 164 is higher than at the outer edges of upward flow region 164, in both the axial component and the tangential component of the velocity. The relationship between the velocities of various portions of the upward flow region 164 may vary depending on the dimensions of vessel 102 and impeller assembly 104, as well as the rotational speed of the rotor 310, 310A. Importantly, the surface plate 320, 320A functions to minimise disturbance at the melt surface while the rotor runs at high speed and close to the top surface of the metallic melt 162.

[107] The transition flow region 166 may have axial, tangential, and radial (moving from the center of vessel 102 towards the vessel sidewall 120) velocity components. As can be seen in Figure 3, melt liquid 160 may have velocity components in an arc, moving upwards towards melt surface 162, outwards towards vessel sidewall 120, and/or downwards towards the base 124.

[108] The downward flow region 168 may have axial, tangential, and radial velocity components to its motion. In one preferred embodiment, the velocity of the center of downward flow region 168 is higher than at the outer edges of downward flow region 168, in both the axial component and the tangential component of the velocity. The relationship between the velocities of various portions of the downward flow region 168 may vary depending on the dimensions of vessel 102 and impeller assembly 104, as well as the rotational speed of blades 315, 315A. The entire downward flow region 168 may move in a fast, tangential motion, moving about the impeller shaft axis X-X, while at the same time moving downward.

[109] The rotation of impeller assembly 104 may produce three velocity components of flow in the fluid 160: axial, radial, and tangential. The radial flow velocity component is caused by the impeller rotation, and this flow may move the fluid 160 through the transition flow region 166, towards the vessel sidewall 120. The axial flow velocity component may help to move the fluid 160 from the vessel bottom 124, through the upward flow region 164, towards the blades 315, 315A. The tangential flow velocity component causes rotation of the entire body of fluid 160 in vessel 102, about a central vertical axis that is substantially coincident with the impeller shaft 142 rotational axis (central vertical axis) X-X. The non-metallic particles typically comprising non-metallic powder 106 are carried throughout upward flow region 164, transition flow region 166 and downward flow region 168 with generally the same velocity vectors as the portions of melt liquid 160 in which they are mixed.

[110] The motion of fluid 160 may reach a steady state condition, in which the tangential flow motion that is induced by the impeller 300 produces an upward tornado-like effect in upward flow region 164. In this situation, the tangential angular velocity of the fluid 160 in upward flow region 164 may be greater than the tangential angular velocity in the downward flow region 168 at the vessel sidewall 120. Also, the fluid in upward flow region 164 may have an axial velocity component that exceeds the axial velocity component in downward flow region 168.

[111] As well as imparting a swirl flow pattern within the metallic melt 161 as described above, the stator 305 of impeller 300 illustrated in Figure 2 and 4 also functions as an impact comminution device. Here the fast-swirling melt composition rises along the vessel axis X-X in upward flow region 164 and enters the rotating rotor 310 of the impeller 300 (in the transition flow region 166). The impeller 300 imparts a radial motion to the melt composition, forcing the flow through the radial passages 327 of the stator 305 at high velocity. During this process, the fast rotation of the rotor 310 drives high velocity impacts of the melt composition with the leading edge and the walls of these radial passages 327. The melt composition and non-metallic particles therein continuously bounce between the walls of the radial passages 327 and finally exits the radial passages 327 in the form of separate rather than agglomerated powders and particles. This high velocity impact causes comminution of any particulate agglomerations within the melt composition. A large impact force is much more effective in breaking up the agglomerated ceramic powders and particles than a smaller shearing force.

[112] The final mixture is mixed to have a well-distributed particle distribution (up to 40 vol% particles) with excellent flow to fill a casting mould.

[113] Once the melt liquid 160 is sufficiently mixed to produce a uniform distribution of the non-metallic particles within the molten metal 161 , the composition can be cast as set out in step 5 above. This can be achieved using any suitable casting technique, for example by being poured in preheated mold and allowed to solidify. A permanent mold, sand mold or a lost-wax mold can be used for pouring the mixed metallic melt mixture (melt liquid 160).

[114] Both the vessel 102 and the reinforcing material particle supply and feeder (not illustrated) may be kept at atmospheric or near-atmospheric pressure during the MMC manufacturing process. Whilst not illustrated, it should be appreciated that in some embodiments, at least one inert gas is applied over at least the top surface of the metallic melt 162 to act as a protective barrier to oxidation. Suitable inert gases include nitrogen or argon. Thus, the vessel is preferably sealable so that little or no leakage of cover gas occurs.

[115] The mixing process of the present invention does not create any turbulent flow at the melt surface through the use of surface plate 320, 320A and can operate at ambient pressure with a minimum amount of inert gas to protect the melt surface 162.

[116] In addition, the use of a stator 305 of the first embodiment of the impeller 300 assists in breaking agglomerated ceramic powders and particles. The stator 305 is preferably located near the melt surface 162, and thus can also assist in helping any trapped air between ceramic powder and particles to escape into the atmosphere without travelling through the bulk of the metallic melt 161 .

[117] Whilst not wishing to be limited to any one theory, it should be understood that the position, configuration and operation of the impeller 300 and 300A within vessel 102 provide a number of unique advantages to embodiments of the mixing apparatus 100 of the present invention.

[118] Firstly, in many embodiments including those illustrated in Figures 1 and 3, the impeller 300, 300A is positioned at the top of the vessel 102 at just below the molten melt surface 162. Whenever the impellor 300, 300A and connected shaft 142 rotate in the melt liquid 160, they induce air to be trapped into the melt. By placing the impellor 300, 300A close to the melt surface 162 and decreasing the length of the shaft 142 immersed in the melt liquid 160, this limits the air that can be trapped in the smaller volume of the melt close to the melt surface 162, which minimises the amount of the air therein and allows the air to be escape back to the atmosphere, leading to lessened porosity defects in the solidified product.

[119] Furthermore, the reinforcing material fed into the metallic melt 161 may include agglomerations which are comminuted, as detailed above. Those agglomerations are generally not densely packed even in the metallic melt 161 and may have a large amount of the air trapped therein. When the agglomerations are broken up by the impellor 300, 300A positioned close to the metallic melt surface 162, the trapped air can easily escape from the metallic melt 161 into the atmosphere with a very short travel distance through the metallic melt 161. In addition, with a smaller volume metallic melt 161 involved in the interaction with both trapped and atmosphere air, oxidation of the metallic melt 161 by the air is reduced, resulting in fewer oxide inclusion defects.

[120] Finally, the short length of a shaft 142 immersed into melt liquid 160 minimises wearing and corrosion of the shaft 142, which significantly reduce the operation for repairing and replacing the shaft 142.

[121] Secondly, the positioning of the surface plate 320, 320A at level or just below the molten metallic melt surface 162 (also positioned close to the rotor 310, 310A), effectively inhibits the formation of vortex and the disturbance to the metallic melt surface 162 by the rotating rotor 310, 310A, protecting the metallic melt 161 from the oxidation and trapping airs. The surface plate 320, 320A also covers the majority of the metallic melt surface 162 in the mixing vessel 102 shielding the metallic melt surface 162 from interaction with the atmosphere air. Consequently, much less oxide inclusion and porosity defects are generated in operation without vacuum.

[122] More importantly, the surface plate 320, 320A prevents the added ceramic powders to float up on the top of the metallic melt surface 162 and being lost as parts of the dross, especially, when 30% or more ceramic powders are introduced into the metallic melt 161 . So, the use of the surface plate 320, 320A is essential to produce high-ceramic content metal matrix composites.

[123] Furthermore, with a lower tendency to form defects, the rotor 310, 310A can be preferably operated at a rotation speed of 1000 RPM or more to break up the agglomeration of the ceramic powders without excessive defects in the solidified product, compared with that of conventional stir casting rotation rate of less than 800 RPM.

[124] Thirdly, in the embodiments that include a stator 305, the impeller 300 (see for example Figure 4) provides an advantageous impact-mixing mechanism with the aid of a stator 305 built with channels, instead of a shear-mixing mechanism. To break-up the agglomerates of ceramic powders in the metallic melt 161 , this invention utilises impacting of the agglomerates onto walls of channels in a tubular stator, as well as onto the surface plate 320, 320A and the furnace crucible sidewall, with the rotor speeds of for example at about 1 ,000 RPM.

[125] In the process, the metallic melt 161 flows upward along the vertical axis of rotor 310, 310A toward a small rotating rotor of 70 mm in diameter, the rotor propels the metallic melt 161 horizontally to a relatively-large disc-like stator 305 (for example 170 mm diameter) that is built with radial passages/ channels 327 between the surface plate 320 and base plate 322 which include radially extending members 325 aligned vertically to the rotor axes on which particles in the melt liquid 160 can impact while travelling outwardly through radial passages 327 toward the vessel sidewall 120, and finally impacts on the sidewall 120 moving downwardly along sidewall 120 to the vessel bottom 124.

[126] The channels 327 play key role in the impact-mixing mechanism as they initiate a series of impacts to break up the agglomeration of the ceramic powders.

[127] The impeller configurations of the present invention can achieve the strongest impact between the metallic melt 161 and the sidewalls 120, by minimising the resistance to the metallic melt 161 flow (e.g. shearing the metallic melt 161 between rotor blades and the stator internal wall) and maximising the area of the channel internal walls.

[128] The mixing apparatus, method and/or system of the present invention perform very well, especially, for mixing high-level ceramic powders, >20 vol%, preferably 30 vol% or greater, in the metallic melt 161. With the minimised metallic melt shearing, the present invention overcomes the problem of jamming and wearing the impellors, which are often encounter in the conventional and other prior art processes and become a more severe problem with the increase in the ceramic content.

[129] The invention can be readily used commercially without significant further development for the Al matrix composite materials. The same process is readily applicable to processing other nonferrous alloys (e.g. Mg and Zn). EXAMPLES

[130] The following example demonstrates one embodiment of the invention relating to aluminium alloy matrix composites (AMCs). It should be appreciated that the invention is not limited to that application and could be used in forming a variety of non-ferrous MMCs, for example magnesium or zinc based MMCs using a similar apparatus and experimental procedure.

EXAMPLE 1 - ALUMINIUM ALLOY MATRIX COMPOSITES (AMCS)

[131] The apparatus 100 illustrated and described in relation to Figures 1 to 6 was used to mix and then subsequently cast aluminium alloy matrix composites following the experimental procedure outlined below:

1.1 Materials:

[132] The following materials were used:

• Aluminium alloy - metal matrix, for example A357; and

• Silicon carbide (SiC) powder - reinforcing material - having size distribution of d5o: 22 microns. The silicon carbide powder is added to provide a 30 vol% mixture within the metallic melt mixture.

1 .2 Experimental Procedure:

[133] The following experimental procedure was followed:

1 . Aluminium alloy A357 is heated to around 680 to 720 ^QC in vessel 502 (a crucible) placed in a furnace 503 to form a liquid aluminium alloy melt.

2. Silicon carbide (SiC) powder is added to the melt using an impeller 500 , shown in Figure 6 to form a downwardly directed vortex, to mix the particles within the liquid aluminium melt. The impeller 500 includes a two bladed rotor 510 and baffles 550 (Figure 6). The baffles 550 are positioned near the melt surface to prevent excessive surface disturbance, while the rotation of the rotor 510 is below the baffles 550 to avoid forming dross and oxide-related defects. Here simple rotor mixing was conducted at 800-1000 rpm for a duration of about 30 to 50 min generating vortex to draw the particles into the melt, but little agglomeration break-up actions. 3. The two bladed impeller is removed.

4. The inventive impeller 300 is placed on the surface of the aluminium alloy melt for 5 minutes to warm up the impeller;

5. Protective inert gases (N2 or argon) are turned on to flow over the surface of the aluminium melt;

6. The impeller 300 is immersed to a position with the surface plate 320 located just beneath the surface of the aluminium melt;

7. Rotation of the rotor 310 in the impeller 300 is switched on at a pre determined rotation speed (800 to 2500 rpm) for a given duration;

8. After the predetermined duration, rotation of the rotor 310 is switched off and the impeller 300 is removed from the vessel 300 in the melting furnace;

9. The protective gas flow is turned off; and

10. The molten mixture held in the vessel 102 is poured into a die to cast the required MMC product. In these experiments, a 6 mm thick AMC plate is cast.

1.3 Experimental Results

[134] A comparative experimental run was also conducted using the same experimental setup, but with the mixing impeller substituted with a vortex generating impeller. This impeller was run for 30 minutes to mix the silicon carbide powder into the aluminium melt. Here simple rotor mixing was conducted, generating vortex to draw the particles into the melt, but little agglomeration break-up actions.

[135] The results of the castings of one experimental run are shown in Figures 7(B) and 7(C). A comparative prior art casting is also provided in Figure 7(A). In this respect, Figure 7 compares the optical microscopy images showing the morphology of an aluminium MMC cast as 6 mm thick AMC plates which are formed by:

(A) Prior art metal matrix composite formed using the Duralcan technologies, as set out in United States Patents Nos. 4,786,467 and 4,865,806 discussed in the background of the invention section;

(B) General vortex only stirring of 30 min with a significant amount of agglomerates; and (C) The process of the present invention mixing with uniform particle distribution using a rotor speed of 1400 rpm. The optical microscopy image shown in Figure 5(C) illustrate a greatly improved particle distribution by using the system of the present invention.

[136] Firstly, comparing the particle distribution of the casting formed using the Duralcan technologies (Figure 7(A)) with the casting formed using the method of the present invention (Figure 7(C)), it can be seen that both methods produce a good distribution of reinforcing particles, with minimal agglomerations. The method of the present invention therefore has comparable results to the best prior art MMC formation technique.

[137] Secondly, comparing the particle distribution of the casting formed using the comparative vortex stirring method (Figure 7(B)) with the casting formed using the method of the present invention (Figure 7(C)), it can be seen that the casting of the present invention has a more even reinforcement particle distribution and significantly less agglomerated particles. Assuming the black areas are agglomerated particles, using image/phase analysis, the particle break-up rate (volume percentage of particle break-up over total volume of particles added) is 63% before applying the impeller 300 and 92% after applying the impeller apparatus/configuration of the current invention, respectively.

EXAMPLE 2 - SURFACE PLATE COMPARITIVE TRIALS

[138] Two different embodiments of the apparatus 100 illustrated and described in relation to Figures 1 to 5 were used to mix and then subsequently cast aluminium alloy matrix composites. The two embodiments of apparatus 100 used two different impeller configurations for mixing a metallic melt containing silicon carbide (SiC) powder ceramic particles, namely (A) A surface plate covered rotor as illustrated in Figure 5; and (B) A rotor enclosed a stator as illustrated in Figure 4.

2.1 Materials:

[139] The following materials were used:

• Aluminium alloy - metal matrix, for example A357; and • Silicon carbide (SiC) powder - reinforcing material - having size distribution of d5o: 22 microns. The silicon carbide powder is added to provide a 30 vol% mixture within the metallic melt mixture.

2.2 Experimental Procedure:

[140] Each experiment followed the following general steps to produce metal matrix composite (MMCs) sheet:

(1) melting Al alloy ingots in a furnace to produce a molten Al alloy melt;

(2) introducing ceramic powders into the molten Al alloy melt;

(3) mixing the ceramic powder into the molten Al alloy melt; and

(4) casting the mixed melt into a metal mould to produce the desired MMC sheets. Each step followed the experimental procedure outlined in Example 1 , where for a first run mixing step 3 was conducted using an impeller which includes a surface plate and rotor as illustrated in Figure 5; and for a second run mixing step 3 was conducted using an impeller which includes a rotor enclosed within a stator as illustrated in Figure 4.

[141 ] For the surface plate covered rotor runs, mixing (3) step commenced after the introduction of the ceramic particles into the melt. At this mixing step, a surface plate is placed at the position just below the melt surface and the rotor is lifted close to the bottom surface of the surface plate. With the surface plate, the rotation speed of the rotor is operated at 1200-2000 rpm for 15 to 30 min.

[142] For the stator runs, mixing (3) step again commenced after the introduction of the ceramic particles into the melt. At this mixing step, the surface plate of the stator is placed at the position just below the melt surface and the rotor positioned enclosed within the rotor.

2.3 Experimental Results

[143] Figure 8 illustrates two optical microscopy images showing the SiC particle distribution in the Al alloy matrix by (A) vortex mixing without baffle 550 and (B) vortex mixing with baffle 550 (Figure 7). The general vortex mixing without baffle can only mix at about 300-500 rpm to avoid entrapping excessive air, and hence the microstructure shown in Figure 8(A) has significant large agglomerates. However, mixing speed with baffles can be increased up to 1000 rpm, which generates high shear force to break up more agglomerates as shown in Figure 8(B).

[144] Figure 9 provide optical microscopy images comparing the ceramic distributions (A) before and (B) after mixing operation with a flat surface plate. Comparison of these micrographs indicate a significant reduction in the size of the agglomerates, which assist downstream friction stir forming (FSF) operation. The micrograph of Figure 9(B) also indicates the presence of the oxide defects (marked by red arrows) confirming the importance to further modify the surface plate for minimising the disturbance to the melt surface by the forced flows. The inventors consider that the quality of this microstructure would be adequate for some downstream processing such as FSF that do not require a perfect distribution of the ceramic particles.

[145] Figure 10 illustrates optical microscopy images comparing mixing runs where the SiC particle distribution in the Al alloy matrix mixed by (A) an impeller without a stator; (B) an impeller with the stator; and (C) an impeller with the stator (higher magnification). A comparison of Figures 10(A) and 10(B) show that the size of agglomerates was further reduced with a stator.

[146] Furthermore, comparing the micrograph shown in Figure 9(B) with the micrograph shown in Figure 10(C) (i.e. each micrographs of high magnification produced without (Figure 9(B)) and with (Figure 10(C)) a stator, it can be seen that the presence of the oxide defects is significantly reduced by using a stator.

[147] The MMCs fabricated using an impeller that includes a stator (Figure 4) are suitable for the down-stream processing that needs more uniform distribution of the ceramic particles and fewer oxide effects, to provide higher performance.

[148] Those skilled in the art will appreciate that the invention described herein is susceptible to variations and modifications other than those specifically described. It is understood that the invention includes all such variations and modifications which fall within the spirit and scope of the present invention. [149] Where the terms "comprise", "comprises", "comprised" or "comprising" are used in this specification (including the claims) they are to be interpreted as specifying the presence of the stated features, integers, steps or components, but not precluding the presence of one or more other feature, integer, step, component or group thereof.

Claims

1 . An apparatus for mixing a metallic melt containing non-metallic particles, the apparatus comprising: a vessel for containing the metallic melt, the vessel including a sidewall and a bottom; and an impeller including:

• a rotor configured for submergence at or below the top surface of the metallic melt, the rotor rotatable about a substantially vertical axis, the rotor including at least two annularly spaced apart blades extending radially outwardly of the vertical axis and configured to be operated at a rate of at least 200 revolutions per minute in the metallic melt; and

2. The apparatus according to claim 1 , wherein the surface plate is sized to substantially cover the top surface of the metallic melt.

3. The apparatus according to claim 1 or 2, wherein the surface plate is sized to have substantially the same diameter as the vessel at the top surface of the metallic melt.

4. The apparatus according to any preceding claim, wherein the surface plate comprises a disc.

5. The apparatus according to any one of claims 1 to 4, wherein the surface plate comprises a substantially planar or flat plate.

6. The apparatus according to any one of claims 1 to 4, wherein the surface plate includes a downwards curvature relative to the center of the surface plate towards the outer rim or edge of the plate.

7. The apparatus according to any preceding claim, wherein the surface plate is positioned to be in direct communication with top surface of the metallic melt.

8. The apparatus according to any preceding claim, wherein the surface plate is positioned to provide a low to zero oxygen environment thereunder.

9. The apparatus according to any preceding claim, wherein the rotor is configured to be operated at a rate of from 200 to 4000 revolutions per minute in the metallic melt, preferably 500 to 2000 revolutions per minute in the metallic melt, more preferably 1000 to 2000 rpm, and yet more preferably about 1500 rpm.

10. The apparatus according to any preceding claim, wherein the rotor is positioned to be no more than 2/3, preferably no more than 1/3, and more preferably no more than 1/6 of the overall height from the top surface of the metallic melt to the bottom of the vessel.

11 . The apparatus according to any preceding claim, wherein the surface plate is separate to the rotor.

12. The apparatus according to any preceding claim, wherein the surface plate is configured to be rotationally stationary relative to the rotor.

13. The apparatus according to any preceding claim, further including a stator including a housing having a top and an annular side enclosing the rotor, the annular side including at least one radial aperture relative to the vertical axis, wherein the impeller is configured to draw the metallic melt into the rotor and then direct that metallic melt radially outwardly from the rotor into the stator to be directed onto and through each radial aperture of the stator.

14. The apparatus according to claim 13, wherein the stator is configured to be stationary relative to the rotor.

15. The apparatus according to claim 13 or 14, wherein the stator includes at least two radial apertures annularly spaced apart around the vertical axis.

16. The apparatus according to claim 15, wherein each radial aperture comprises a radial passage configured to direct flow outwardly from the rotor.

17. The apparatus according to claim 16, wherein each radial passage is formed at least two spaced apart radially extending members which extend between the surface plate and a base plate of the stator.

18. The apparatus according to claim 17, wherein the base plate includes an opening below the rotor to allow the metallic melt to be drawn into the impeller to the rotor.

19. The apparatus according to any preceding claim, wherein the surface plate comprises a top wall of the stator.

20. The apparatus according to any preceding claim, wherein the rotor comprises a radial flow impeller.

21. The apparatus according to any preceding claim, wherein the rotor includes at least two annularly spaced apart blades.

22. The apparatus according to claim 21 , wherein each blade of the rotor has the same length and configuration.

23. The apparatus according to claim 21 or 22, wherein the blades of the rotor have a flat blade configuration.

24. The apparatus according to any preceding claim, wherein the rotor is configured to be operated for at least 10 minutes, preferably at least 15 minutes to mix the non-metallic particles throughout the metallic melt.

25. The apparatus according to any preceding claim, wherein the impeller is configured to produce a flow pattern comprising (a) an inner, upward flow region located along said vertical axis, (b) a transition flow region located around the rotor in which liquid moves radially outwardly through the at least one radial aperture of the stator toward the vessel sidewall, and (c) an outer, downward flow region.

26. The apparatus according to any preceding claim, wherein the metallic material is selected from: aluminium or an aluminium alloy; magnesium or a magnesium alloy; or zinc or a zinc alloy.

27. The apparatus according to any preceding claim, wherein the non-metallic particles comprise a ceramic or a crystalline particulate material.

28. The apparatus according to any preceding claim, wherein the non-metallic particles comprise a metal oxide, metal nitride, metal carbide, or metal silicide.

29. The apparatus according to any preceding claim, wherein the non-metallic particles is selected from the group consisting of silicon carbide, silicon dioxide, aluminium oxide, boron carbide, silicon nitride, boron nitride, tungsten carbide and fly ash.

30. The apparatus according to any preceding claim, wherein the non-metallic particles comprises from 5 to 40 % volume of the metallic melt, preferably at least 20 % volume of the metallic melt, more preferably between 30 and 40% volume of the metallic melt.

31 . The apparatus according to any preceding claim, wherein the metallic melt comprises a molten metal mixture with the non-metallic particles maintained in a temperature range from about the liquidus temperature of the metal to about 100 ^eC above the liquidus temperature, preferably about 80 to 100 ^eC above the liquidus temperature.

32. The apparatus according to any preceding claim, further including a feeder configured to feed non-metallic particulate material into the metallic melt.

33. The apparatus according to any preceding claim, wherein the vessel includes an upper end and a lower end and comprises a generally cylindrical containing sidewall extending between an upper end and a lower end.

34. The apparatus according to any preceding claim, wherein the ratio of the vessel sidewall height to the vessel diameter is from 1 to 3.

35. A stir casting apparatus for forming metal matrix composites comprising an apparatus for mixing a metallic melt containing particulates according to any one of the preceding claims.

36. A method of mixing non-metallic particles into molten metal, comprising: mixing a metallic melt comprising a mixture of non-metallic particles and molten metal or metal alloy held in a vessel with an impeller at a temperature above the liquidus of the metal or metal alloy, the vessel includes a sidewall and a bottom, the impeller including:

37. The method according to claim 36 using an apparatus according to any one of claims 1 to 35.

38. The method according to claim 36 or 37, wherein the impeller further includes a stator that includes a housing having a top and an annular side enclosing the rotor, the annular side includes at least one radial aperture relative to the vertical axis; and wherein the rotor is operated to draw the metallic melt into the rotor and then direct that metallic melt radially outwardly from the rotor into the stator to be directed onto and through each radial aperture of the stator.

39. The method according to claim 36, 37 or 38, wherein the rotor is operated to produce a flow pattern comprising (a) an inner, upward flow region located along said vertical axis, (b) a transition flow region located around the rotor in which liquid moves radially outwardly through the at least one radial aperture of the stator toward the vessel sidewall, and (c) an outer, downward flow region.

40. The method according to any one of claims 36 to 39, wherein the rotor is rotated at a rate of from 500 to 4000 revolutions per minute in the metallic melt, preferably 1000 to 2000 rpm, and more preferably about 1500 rpm.

41. The method according to any one of claims 36 to 40, wherein mixing is conducted for at least 5 minutes, preferably at least 10 minutes.

42. The method according to any one of claims 36 to 41 , wherein the impeller is located in the metallic melt with the surface plate of the impeller positioned level with or submerged just below the top surface of the metallic melt.

43. The apparatus according to any preceding claim, wherein the surface plate is operatively separate to the rotor.

44. The apparatus according to any preceding claim, wherein the surface plate is rotationally stationary relative to the rotor.

45. The method according to any one of claims 38 to 44, wherein the metallic material is selected from: aluminium or an aluminium alloy; magnesium or a magnesium alloy; or zinc or a zinc alloy.

46. The method according to any one of claims 38 to 45, wherein the non- metallic particles comprise a ceramic or a crystalline particulate material.

47. The method according to any one of claims 38 to 46, wherein the non- metallic particles comprise a metal oxide, metal nitride, metal carbide, or metal silicide.

48. The method according to any one of claims 38 to 47, wherein the non- metallic particles is selected from the group consisting of silicon carbide, silicon dioxide, aluminium oxide, boron carbide, silicon nitride, boron nitride, tungsten carbide and fly ash.

49. The method according to any one of claims 38 to 48, wherein the non- metallic particles comprise from 5 to 40 % volume of the metallic melt.

50. The method according to any one of claims 38 to 49, wherein the metallic melt comprises a molten metal mixture with the non-metallic particles maintained in a temperature range of from about the liquidus temperature of the metal to about 100 ^eC above the liquidus temperature, more preferably from 20 to 100 degrees above the liquidus temperature.

51 . The method according to any one of claims 38 to 50, wherein at least one inert gas is applied over at least the top surface of the metallic melt.

52. A method for preparing a composite of a metallic alloy reinforced with particles of a non-metallic material, comprising: mixing non-metallic particles into molten metal according to the method of any one of claims 38 to 51 ; and casting the resulting mixture.

53. A composite material prepared using method according to claim 52.