Advanced material for molten metal processing equipment

A molten metal processing apparatus selected from a pump, a degasser, a flux injector, and a scrap submergence device constructed to include at least one element comprised of C/C composite.

BACKGROUND

The present exemplary embodiment relates to molten metal processing. It finds particular application in conjunction with molten metal pumps, submergence devices, degassing equipment, and the like, and will be described with particular reference thereto. However, it is to be appreciated that the present exemplary embodiment is also amenable to other like applications.

Aluminum is the third most abundant element (after oxygen and silicon), and the most abundant metal, in Earth's crust. It makes up about 8% by weight of the Earth's solid surface. Aluminum is remarkable for the metal's low density and for its ability to resist corrosion due to the phenomenon of passivation. Components made from aluminum and its alloys are vital to the world's production of structural materials. Aluminum is particularly valuable because of its further advantageous ability to be readily recycled.

Aluminum is typically either melted and cast into a finished product, or cast into a billet for transport and eventual remelting and casting into the desired end product. Special handling equipment has been developed to facilitate the melting, processing, and transporting of molten aluminum.

Although the present disclosure has been associated with aluminum, it is noted that the equipment described herein may be equally suitable for use with other motel metals (and their salts), including zinc, magnesium, and nickel, as examples.

The process of molten metal handling and recycling is complex. It requires equipment for melting the metal, pumps for molten metal circulation, devices for submerging scrap metal pieces, devices for removal of impurities (e.g. filtering and degassing), devices for introduction of flux and other alloying agents, and devices for transport of the molten metal.

In a typical melting operation, a melting furnace is provided with an enclosed hearth and a connected open side well. A pump or other molten metal flow inducing apparatus is positioned in the side well and causes molten metal to circulate within the hearth. The side well may include a pump well and a melting bay which may be further divided into a charge well and a dross well. Metal may be melted by the introduction of solid bars to the main hearth and/or by the addition of metal pieces to the side well.

The charge well can be utilized to melt metal scrap. Various pieces of equipment have been developed to help submerge the scrap pieces, and are referred to herein as scrap submergence devices. The dross well can be utilized to remove contaminants. Moreover, scrap metal is usually contaminated with organic and inorganic contaminants. Organic contaminants most commonly consist of remnants of various types of oils, coatings, or paints and the like. The inorganic contaminants may include dust particles, pigments, minor amounts of various scrap metals other than the principal metal, and the like. Aluminum scrap will also normally contain varying amounts of metal oxides. The majority of the contaminants will float to the top of the bath of molten metal or form slag or slag-like skin of inorganic contaminants on the molten metal which can be skimmed off of the metal in accordance with well-established techniques.

In the processing of molten metals, for example aluminum or zinc, one commonly employed piece of equipment is a circulation pump for creating molten metal flow in a furnace. In addition, it is often necessary to pump molten metal from one vessel to another. When the molten metal needs to be removed from a vessel by elevating it over a containment wall, a so-called transfer pump is often used. These can include the traditional style of transfer pump shown, for example, in U.S. Pat. No. 5,947,705 (herein incorporated by reference) or an overflow transfer system of the type shown in U.S. Published Application US 2013/0101424 (herein incorporated by reference) or a launder/ladle transfer system of the type shown in U.S. Pat. No. 8,337,746 (herein incorporated by reference).

Most typical of this situation is where the transfer pump is placed in the charge well of a molten metal furnace to remove molten metal from the furnace, perhaps for introduction to a ladle and from there to die casters. In the aluminum recycling industry, the removal of magnesium has become a particular focus. The ability to remove magnesium from molten aluminum is made possible by a favorable chemical reaction between magnesium and chlorine. A gas injection pump can be used for this purpose.

Degassing apparatus may be used for increasing the quality of the molten metal prior to the execution of a casting operation. In such a degassing operation, a large quantity of finely bubbled inert gas such as argon gas or nitrogen gas is introduced into the molten metal, so that dissolved gas and nonmetallic inclusions are entrapped or caught by the bubbles of the inert gas, which are floated for removal. Typically, the inert gas is injected into the molten metal by means of a rotating shaft and impeller assembly disposed below the surface of the molten metal. In addition, apparatus exist for the introduction of flux, typically chlorine and/or chlorine salts, into molten metal. These apparatus can include rotating impeller/shaft combinations through which inert gas and flux can be introduced. U.S. Pat. Nos. 3,767,382 and 8,025,712 are examples of flux injectors and the disclosure of each is herein incorporated by reference.

As the skilled artisan will appreciate, the environment in which the molten metal processing equipment operates is extraordinarily harsh. For example, aluminum and magnesium melt at above 1200° F. Accordingly, not many materials function in these types of molten metals. Furthermore, the density of these liquids can provide significant mechanical stress on the equipment used to move the molten metal. In addition, the zone in which the equipment transitions from the molten metal to the surrounding atmosphere is a high temperature highly oxidative environment that renders many materials unsuitable for use. Accordingly, to date, the primary materials used to construct molten metal processing equipment, at least the elements operating below the melt line, have been graphite, silicon nitride and silicon carbide. Each of these materials suffers from shortcomings such as machinability, strength, susceptibility to thermal shock and high cost.

BRIEF DESCRIPTION

The present disclosure is directed to the concept of using an alternative material in the construction of various molten metal processing equipment components and further, representative examples of improved components that can be constructed therefrom.

According to a first embodiment, a molten metal processing apparatus selected from a pump, a degasser, a flux injector, and a scrap submergence device is provided. The apparatus is constructed to include at least one element comprised of C/C composite.

According to a further exemplary embodiment, the present disclosure is directed to an apparatus such as a molten metal pump, degasser, flux injector, and/or scrap submergence device. The apparatus can include a motor, a shaft engaging the motor at a first end and an impeller at a second end, wherein at least one component intended to be disposed below, or transition through, a molten metal surface is comprised of a C/C composite material.

According to a further embodiment, a method of processing a molten metal is provided. The method includes the steps of (i) impregnating a carbon fiber body with a resin; (ii) heating the body of step (i) to form a C/C composite; (iii) machining the C/C composite of step (ii) to form a component of a molten metal pump, degasser or scrap submergence device; and, (iv) operating a pump, degasser or scrap submergence device including the component of step (iii) in the processing of molten metal. Furthermore, it may be desirable to include an optional oxidation resistance treatment following one of steps (ii) or (iii).

DETAILED DESCRIPTION

According to the present disclosure, it is contemplated that various components of molten metal processing equipment are partially or fully constructed of a carbon-carbon composite material (hereinafter C/C composite). C/C composites can be expensive to produce but provide high strength-to-weight ratio and rigidity. C/C composites can also be impregnated with an oxidation resistant chemical of the type commonly used with graphite components such as a solution including a phosphate based oxidation retardant (see U.S. Pat. No. 4,439,491, as an example, the disclosure of which is herein incorporated by reference). This is beneficial relative to, for example, high density graphite which is not easily impregnated. C/C composites offer excellent combinations of thermal conductivity and stiffness. Also, C/C composites offer low density, high stiffness, low coefficient of thermal expansion, zero to little outgassing, and a unique high temperature capability.

C/C composites have thermal stability, high resistance to thermal shock due to high thermal conductivity, and low thermal expansion behavior, i.e., low thermal expansion coefficient. These materials are also characterized as having high toughness, strength and stiffness in high temperature applications. C/C composites may comprise carbon or graphite reinforcements mixed or contacted with matrix precursors to form a “green” composite, which is then carbonized to form the C/C composite. C/C composites may also comprise carbon or graphite reinforcements in which the matrix is introduced fully or in part by chemical vapor infiltration (CVI) or chemical vapor reaction (CVR).

C/C composites may be made from fibrous materials such as carbon fibers or carbon fiber precursors. In the course of manufacturing the C/C composites, these fibrous materials are generally mixed with binders. One type of such C/C composites are made with chopped fibers mixed with pitch-based thermoplastic binder in powder form. The mixture is placed in a mold where it is compacted and heated to form a preform, and the resulting preform is carbonized by heating.

C/C composites are commercially available from such companies as Amoco, DuPont, Hercules, Celanese and others, and can take the form of fiber, chopped fiber, cloth or fabric, or chopped cloth or fabric which are referred to as molding compounds. C/C composites may also take the form of continuous filament yarn, chopped yarn, or tape made from continuous filaments and which are referred to as unidirectional arrays of fibers. Yarns may be woven in desired shapes by braiding or by multidirectional weaving. The yarn, cloth and/or tape may be wrapped or wound around a mandrel to form a variety of shapes and reinforcement orientations. The fibers may be wrapped in the dry state or they may be impregnated with the desired matrix precursor prior to wrapping, winding, or stacking to form what is commonly known as a prepreg. Such prepreg and woven structure reinforcements are commercially available from various sources including Fiberite, Hexcel and Cytec. The carbon fiber reinforcements can be prepared from precursors such as polyacrylonitrile (PAN), rayon or pitch.

Matrix precursors which may be used to form C/C composites include liquid sources such as phenolic resins and pitch, and gaseous sources, including hydrocarbons such as methane, ethane, propane, and the like. Representative phenolics include, but are not limited to, phenolics sold under the commercially available trade designations USP39 and 91LD, such as supplied by Stuart-Ironsides of Willowbrook, Ill.

The C/C composites may be fabricated by a variety of techniques. Conventionally, resin impregnated carbon fibers are autoclaved or press-molded into the desired shape on a tool or in a die. The molded parts are heat-treated in an inert environment to temperatures from approximately 1300° F. (700° C.) to 5250° F. (2900° C.) in order to convert the organic phases to carbon. The carbonized parts are then densified by carbon chemical vapor infiltration or by multiple cycle reimpregnations with resins as described above. Other fabrication methods include hot pressing and the chemical vapor infiltration of dry preforms. Methods of fabrication of C/C composites which may be used in carrying out some of the steps necessary in the fabrication method are described in U.S. Pat. Nos. 3,174,895 and 3,462,289, which are herein incorporated by reference.

Once the general shape of the C/C composite article is fabricated, the piece can be readily machined to precise tolerances, on the order of about 0.1 mm or less. Accordingly, given the strength and machinability of C/C composites, in addition to the shaping possible in the initial fabrication process, C/C composites can be formed into highly precise shapes for components by machining. In this regard, the C/C composites of the present disclosure may provide fabrication advantageous relative to ceramic which has casting precision limitations and strength advantages relative to graphite.

The C/C composites of the present description can have low friction characteristics at high temperatures by the inclusion of a controlled amount of boron, for example. C/C composites of this type may be particularly useful as a bearing ring in a molten metal pump.

An aluminum recycling furnace is described in U.S. Pat. No. 6,217,823 herein incorporated by reference. Referring now toFIG. 1, an aluminum recycling furnace100is depicted. Furnace100includes a main hearth component120which is heated, for example, with gas or oil burners or by any other means known in the art. Adjacent, and in fluid communication with the hearth120, is the primary recycling area comprised of a pump well140, a charge well160and a dross well180. Although not shown, the wall of the hearth120opens to the pump well140, which opens to the charge well160, which opens to the dross well180, which in turn opens to the hearth120to allow the circulation pattern shown by the arrows. The pump well receives a molten metal pump. The molten metal pump circulates molten metal from the hearth120to the charge well160where scrap chips of the metal to be processed are deposited onto the surface of the melt. Molten metal from the charge well160flows into the dross well180where impurities in the form of dross are skimmed from the surface before the melt flows back into the hearth120.

Referring now toFIG. 2, a molten metal circulation pump200within a pump well201of recycling furnace203is shown. This type of pump is more fully described in U.S. Pat. No. 6,887,425, herein incorporated by reference. Pump200includes a plurality of posts205attached to a base207and suspended from a motor mount209. An impeller (not shown) is disposed within base207and connected to motor210via a shaft and coupling (not shown). Pump200circulates molten metal from pump well201into charge well211and dross well213. The pump depicted inFIG. 2, is commonly referred to as a circulation pump.

In accord with the present disclosure and more fully described within the following discussion of various molten metal pump apparatus, it is envisioned that the below melt line (ML) components of the pump may be constructed wholly or in part of C/C composite materials. Similarly, in view of excellent oxidation resistance achieved by chemical treatment, components at or near the ML may also be constructed of C/C composite materials. These components include the base housing, the shaft, the impeller, one or more bearing rings, and/or pump posts or sleeves.

In certain molten metal processing operations, a gas injection pump of the type depicted in U.S. Pat. No. 5,993,728, herein incorporated by reference, may be employed. Moreover, in working with certain molten metals, it may be necessary to perform gas injection to remove undesired impurities. Referring now toFIG. 3, a typical gas injection pump301is depicted. Pump301includes a hanger assembly302used for lifting and positioning of the pump as necessary within a furnace. A motor303is supported by a motor mount304, itself supported by a support plate306. The motor303is connected via a coupling assembly308to a rotatable shaft310secured to an impeller312. A base assembly314is attached to motor mount304by a plurality of posts316. The impeller312is rotatable within a pumping chamber318and it's rotation draws molten metal319into the pumping chamber318through an inlet320and discharges the molten metal through an outlet passage322. Bearing rings pairs321and323are disposed cooperatively on the impeller312and in the wall of pumping chamber318. A further bearing ring325can be disposed in the top of pumping chamber318and opposed to a top radial edge of impeller312.

A reactive gas (such as chlorine) is provided to a gas injection tube324supported by a clamping mechanism326attached to the support plate306. The submerged end of the gas injection tube324is connected via a tube plug328to the outlet passage322. In addition to C/C composite elements identified inFIG. 2, the gas injection tube324and tube plug328may be constructed of a C/C composite material. Accordingly, components of pump301that may be advantageously wholly or partially constructed from a C/C composite material include base assembly314, impeller312, posts316, shaft310, gas injection tube324, tube plug328, and one or more bearing rings321,323and325.

In addition to situations where molten metal is circulated by a circulation pump or circulated and treated by a gas injection pump, there are circumstances where molten metal is removed from a furnace and transferred remotely for further processing. An exemplary transfer pump is described in U.S. Pat. No. 5,947,705, herein incorporated by reference.

A typical transfer pump401as shown inFIG. 4includes a motor411attached to a rotatable shaft413by a coupling assembly415. The shaft413is attached at its lower end to a rotatable impeller417which rotates within the pumping chamber418within base419. A bearing ring421is provided in the lower region of base419in a facing orientation with a bearing ring423disposed in a lower annular edge of the impeller417. A further bearing ring424can be disposed in an upper region of base419, facing an upper annular edge of impeller417to allow proper rotation of the impeller. The motor411is supported and connected to the base assembly419by a pair of posts425(only one of which is visible) which are attached to a motor mount platform429.

A riser tube451has a first end disposed within an outlet453in the base419and is secured in a motor mount opening460via a coupling adaptor465. An upper end of the riser tube451includes a flange455to which an elbow (not shown) can be attached. The elbow engages transfer piping that allows molten metal to be moved to a remote location. In addition to the pump components articulated above which are suitable for construction from C/C composite materials, the transfer pump riser assembly may be constructed therefrom.

With respect toFIG. 5, an impeller including a C/C composite component is depicted. The impeller501is a generally cylindrical shaped body of graphite or ceramic and includes an upper face502having a recess504to accommodate a shaft. The upper face502also includes inlets505to passages506which extend downwardly from the upper face and outwardly through a sidewall508, to an outlet509. A bearing ring510of a ceramic, such as silicon carbide or C/C composite material, is provided surrounding the outer edge of a lower face512. A C/C composite disc513, is secured to the top surface502of the impeller501to improve the wear characteristics of the device (the disk513is shown both removed and attached inFIG. 5).

Of course, the shape of the impeller and/or the protective top plate is not limited to a cylindrical shape. Rather, the use of a protective top or bottom plate of C/C composite material with any shape impeller, including bird cage, vaned, triangular or any polygonal shape, is contemplated. Furthermore, it is contemplated that the entire impeller body may be constructed of a C/C composite material.

With reference toFIGS. 6 and 7, a molten metal overflow transfer pump630is depicted in association with a furnace628. The pump assembly is more fully described in U.S. Patent Publication 2013/0101424, which is herein incorporated by reference. Pump630is suspended via metallic framing632which rests on the walls of the furnace bay634. A motor635rotates a shaft636and the appended impeller638. A refractory body640forms an elongated generally cylindrical pump chamber or tube641. The refractory body can be formed, for example, from fused silica, silicon carbide, C/C composite material or combinations thereof. Body640includes an inlet643which receives impeller638. Impeller638can be constructed wholly or partially of a C/C composite material. Preferably, bearing rings (not shown) are provided to facilitate even wear and rotation of the impeller638therein. The bearing rings can be composed of C/C composite material.

In operation, molten metal is drawn into the impeller through the inlet643and forced upwardly within tube641in the shape of a forced (“equilibrium”) vortex. At a top of the tube641a volute shaped chamber642is provided to direct the molten metal vortex created by rotation of the impeller outwardly into trough644. Trough644can be joined/mated with additional trough members or tubing to direct the molten metal to its desired location such as a casting apparatus, a ladle or other mechanism as known to those skilled in the art. The trough can be formed or coated with a C/C composite material.

Although centrifugal pumps operate satisfactorily to pump molten metal, they have never found acceptance as a means to fill molten metal molds. Rather, this task has been left to electromagnetic pumps, pressurized furnaces and ladeling. Known centrifugal pumps generally control a flow rate and pressure of molten metal by modulating the rotational rate of the impeller. However, this control mechanism experiences erratic control of the flow rate and pressure of molten metal when attempting to transfer molten metal into a mold such as a form mold. The erratic control of the flow of molten metal into the form mold is especially prevalent when attempting to fill a form mold for a complicated or intricately formed tool or part. A centrifugal pump capable of filling mold forms has been described in U.S. Published Application 2014/0044520, which is herein incorporated by reference.

With reference toFIG. 8, a mold pump assembly810is illustrated. The assembly includes an elongated shaft816having a cylindrical shape having a rotational axis that is generally perpendicular to the base member820. The elongated shaft has a first end828that is adapted to attach to a motor (not shown) by a coupling (not shown) and a second end830that is connected to an impeller822. The impeller822is rotably positioned within the pump chamber818such that operation of the motor/coupling rotates the elongated shaft816which rotates the impeller822within the pump chamber818.

The base member820defines the pump chamber818that receives the impeller822. The base member820is configured to structurally receive one or more refractory posts (not shown) within passages831. Each passage831is adapted to receive a metal rod disposed within a refractory sheath component of the refractory post to rigidly attach to a motor mount (not shown). The motor mount supports the motor above the molten metal.

The impeller822is configured with a first radial edge832that is axially spaced from a second radial edge834. The first and second radial edges832,834are located peripherally about the circumference of the impeller822. The pump chamber818includes a bearing assembly835having a first bearing ring836axially spaced from a second bearing ring838. The first and second radial edges832and834face the bearing rings836and838, respectively. The radial edges can be comprised of a silicon carbide bearing ring. The remainder of the impeller body823can be comprised of a C/C composite material. The first radial edge832is facially aligned with the first bearing ring836and the second radial edge834is facially aligned with the second bearing ring838. The bearing rings are made of a material, such as silicon carbide, having frictional bearing properties at high temperatures to prevent cyclic failure due to high frictional forces. The bearings are adapted to support the rotation of the impeller822within the base member such that the pump assembly810is at least substantially prevented from vibrating.

The use of a C/C composite body has been particularly advantageous in a mold pump assembly where precise dosing of molten metal quantities to a specified mold volume and shape is required. In this regard, the historical use of a graphite main body has been found to develop wear on the radial surface, particularly where the graphite material engages the silicon carbide bearing ring. Such wear can result unpredictable molten metal flow and pressure at a selected motor RPM over time.

The rotation of the impeller822draws molten metal, into the inlet848and into the chamber818such that continued rotation of the impeller822causes molten metal to be forced out of the pump chamber818to an outlet (not shown) of the base member820which communicates with a mold. Although the illustrated pump includes a C/C composite material as the main body of the impeller, it is contemplated that any of the elements intended to be disposed in the molten metal may be constructed from a C/C composite material, including bearing rings.

For example,FIG. 9depicts an alternative impeller shaft arrangement for a mold pump. The arrangement is comprised of a shaft916having a motor mount end928and an impeller mount end930. The shaft can be comprised of graphite, ceramic or C/C composite material or combinations thereof. The shaft could also include a sleeve of C/C composite material. The impeller922can be constructed completely of a C/C composite material. In this manner, it is feasible to optionally eliminate bearing rings.

FIG. 10schematically illustrates a conventional structure of a continuous degassing apparatus. However, the use of C/C composite elements in batch degassing is equally applicable. In order to improve the dispersion of the gas throughout the molten metal, rotating injectors are commonly used which provide shearing action of the gas bubbles and intimate stirring/mixing of the process gas with the liquid metal. The degassing apparatus receives molten metal1009continuously through an inlet1002. The upper opening of a degassing container1001is covered by a lid1003and, at the downstream side, a partition1004extends downwardly in the direction so that it crosses the flow of the metal1009for preventing floating substances (suspended matter) including oxides etc., which can form dross, from being flown into the subsequent treatment process. Namely, the partition1004extends downwardly, so that a relatively narrowed passageway of a predetermined flow area is formed between the bottom end of the partition1004and the inner bottom wall of the container1001. Such an arrangement of the partition1004can obtain a maximized residence time of molten metal at the treating chamber1008upstream from the partition1004, so that a prolonged duration of time of a degassing operation can be achieved. A rotary gas-diffusing device1005is inserted through an aperture made in the lid1003and is located in the molten metal in the degassing container1001. The gas-diffusing device1005has an impeller1010mounted to rotatable shaft1012located (immersed) in the molten metal while being subjected to a rotating movement, so that the inert gas is ejected from the lower part of the gas-diffusing device1005, while a finely bubbled inert gas is diffused into the molten metal. A burner1006can be included to maintain a desired temperature. Impeller1010and optionally shaft1012can be constructed of a C/C composite material. An exemplary impeller is described in U.S. Pat. No. 8,178,036, herein incorporated by reference. Similarly, a flux injector apparatus of the type depicted in U.S. Pat. Nos. 3,767,382 and 8,025,712 can benefit from constructing various components (e.g. shaft and rotor) from a C/C composite material.

With reference toFIG. 11, an impeller1120constructed from C/C composite material is illustrated. The impeller is in the form of a rectangular prism having a face1124, a face1126, and side walls1128,1130,1132,1134. The impeller1120includes a gas discharge outlet1136opening through the face1124. The gas discharge outlet1136constitutes a portion of a threaded opening1138that extends through the impeller1120and which opens through the faces1124and1126. The shaft (not shown) includes a longitudinally extending bore that opens through the ends of the outlet1136. The faces1124,1126are approximately parallel with each other as are the side walls1128,1132and the side walls1130,1134. The faces1124,1126and the side walls1128,1130,1132,1134are planar surfaces which define sharp, right-angled corners1139. It also is possible that the impeller1120could be triangular, pentagonal, or otherwise polygonal in plan view.

A plurality of grooves1152,1154,1156,1158,1160,1162,1164,1166,1168,1170,1172,1174extend radially outwardly from the hub1150. Each groove extends from the hub to a respective side wall and the respective groove is open at the side wall.

The grooves1152. . .1174extend into the body of the impeller1120from the face1124and have a surface that is spaced from and generally parallel to the face1126. The grooves1152. . .1174include longitudinal axes L (which is also a symmetrical axis) that are aligned with each other and that extend from one side to the opposed side The axes are colinear with the radius of the threaded opening1138(i.e. extend through the center of the threaded opening).

FIG. 12illustrates an alternative impeller1220wherein the main body1222is constructed of graphite and the corners1239receive insert1241constructed of C/C composite material. Moreover, the corners may constitute high wear surfaces which can benefit by longer lived C/C composite yet total unit cost is held relatively low by inclusion of a relatively lower cost graphite main body.

The C/C composite parts can be secured to the graphite, ceramic or other C/C composite elements of the molten metal processing equipment by mechanical means, adhesive means (cement for example), or by means of a reactive-bonding joint interlayer. The interlayer can be formed of fine particles of carbide-forming metallic ingredients and carbon. The metals included in the compounds may be selected from the group consisting of W, Ti, Si, Ta, Nb, Zr, Hf, V, Cr, and Mo. Tungsten is the preferred metallic ingredient in the joint compound. The reactive-bonding layer may also contain one or more refractory compounds as a filler material. Representative refractory compounds include TiB2, BN, B4C, SiC, TiC, MoSi2, WSi2. A bonding layer can comprise a slurry made from, for example, 10 grams of tungsten powder and 0.5 grams of carbon powder and 12 milliliters of methanol. The parts to be joined with the bonding layer are heated in an argon atmosphere and under a compressive pressure of 5 megapascals to a temperature of 1450-1580° C. for a period of from 10-30 minutes. The method includes the steps of: providing a first C/C composite piece and a second piece, wherein the second piece has a surface that is complementary to a surface of the C/C composite piece; providing a layer of a mixture of metal powder and carbon powder on the first complementary mating surface; arranging the second C/C composite piece on the powder layer such that the second complimentary mating surface is matched to the first complementary mating surface, thereby forming a construct of the first C/C composite piece, the powder layer, and the second piece; placing the construct into a press and applying pressure to the construct to press together the two pieces joined at their complementary surfaces; and applying an electrical current to the powder in the construct to initiate an oxidation-reduction reaction, thereby bonding the pieces together.

With reference toFIG. 13, it is noted that the traditional shapes of molten metal impellers (including pump impellers, scrap submergence impellers and degassing impellers) have been constrained by the strength of graphite and/or the machineability of ceramic. Accordingly, by employing C/C composite in the manufacture of the impeller, it is envisioned that increased efficiency designs are achievable. For example, the configuration ofFIG. 13constructed entirely of C/C composite material is machineable and can possess sufficient strength to operate in a molten metal environment. Advantageously, the design provides a central hub1301defining bore1302and surrounded by relatively large fluid receiving slots1303defined by thin vanes1305. Vanes1305can be forward or rearwardly curved as desired to increase flow rate or pressure.

Turning now toFIG. 14, the use of C/C composite material in combination with graphite is demonstrated. Particularly, a graphite hub1401defining a shaft receiving bore1402is provided. A plurality of C/C composite vanes1405extend from hub1401and define fluid receiving slots1403. Hub1401further includes a plurality of cut-outs1407configured to receive an end1409of each vane1405. The vane ends1409can be cemented or powder bonded to the graphite hub1401within each cut out1407.

Molten metal scrap, particularly aluminum, can be difficult to submerge based on a variety of characteristics such as the size of the scrap particles and the presence of oil or other organic material on its surface. More specifically, piece size and organic content can strongly influence the buoyancy of the material and adversely affect the ability of the scrap submergence system to submerge the scrap. In this regard, scrap which is not submerged and floats on the top will typically not melt, and may in fact burn. Accordingly, rapid submergence of scrap particles is an essential characteristic of any system.

A variety of apparatus have been used in the melting bay (specifically in the charge well) to facilitate the submergence of the scrap metal below the surface of the molten metal bath. One system is a mechanical system constructed primarily of a rotor which creates a molten metal flow from the top surface. Examples of these devices are shown in U.S. Pat. Nos. 3,873,305; 3,997,336; 4,128,415; 4,930,986; and 5,310,412, the disclosure of which are herein incorporated by reference. The various components of these apparatus may benefit from construction from C/C composite material.

Referring toFIG. 15, a conveyor1546is disposed adjacent the charge well1518, forwardly of the front wall1522. Particles1548of scrap metal are conveyed by the conveyor1546for discharge into the charge well1518. A mixing apparatus1510includes a drive motor and support1550. The drive motor and support1550are disposed above the charge well1518. A coupling1552projects from the underside of the drive motor and support1550. A vertically oriented, elongate shaft1554projects downwardly from the underside of the coupling1552. An impeller1556is rigidly secured to the shaft1554at a location remote from the coupling1552. Impeller1556is disposed within the molten metal1542. The impeller1556or portions thereof and optionally the shaft1554or portions thereof can be made of C/C composite material.

As the invention has been described, it will be apparent to those skilled in the art that the same may be varied in many ways without departing from the intended spirit and scope of the invention, and any and all such modifications are intended to be included within the scope of the appended claims.