WO2000000311A1

WO2000000311A1 - Microwave processing system for metals

Info

Publication number: WO2000000311A1
Application number: PCT/US1999/014418
Authority: WO
Inventors: Izak Parviz Nazarian; Ira Maroofian; Hanna Dodiuk-Kenig; Shmuel Kenig
Original assignee: Hpm Stadco, Inc.
Priority date: 1998-06-26
Filing date: 1999-06-25
Publication date: 2000-01-06
Also published as: AU4832299A

Abstract

High volumetric, steady state flow rates of metal compositions can be rapidly melted, or otherwise heated to a desired degree, by conveying the metal composition (106) through the length of a microwave chamber (140) containing a microwave susceptor (150) in thermal contact with the flow. Microwave energy is propagated in the cavity to heat the microwave susceptor. The resultant thermal energy is then transferred to the metal composition. Because metal compositions tend to be excellent heat conductors, the transfer of thermal energy from the susceptor throughout the volume of the metal composition occurs quickly. Indeed, enough thermal energy can be generated to easily melt nonferrous metal alloys in connection with steady state die casting operations. Microwave melting would occur so rapidly, that significant reductions in cycle time would be achieved. Additionally, the use of microwave energy for melting is economically advantageous, because microwave heating is extremely energy efficient.

Description

MICROWAVE PROCESSING SYSTEM FOR METALS

FIELD OF THE INVENTION

The present invention relates to metal processing systems and methods for heating a metal composition and then transporting the heated metal composition to a point of use involving metalworking of the heated metal composition More specifically, the present invention relates to metal processing systems and methods in which microwave energy is used to accomplish heating of the metal composition

BACKGROUND OF THE INVENTION

Articles formed from metal compositions are found everywhere and are used in an incredibly wide range of applications Not surprisingly, the many different kinds of metal articles are formed using a wide range of metalworking techniques These include sand casting, die casting, wrought ironworking, forging, soldering and brazing, welding, various heat treatments, surface finishing, grinding, drilling, sawing, lathe turning, planing, milling, metal spinning, cold forming, extrusion, powder sintering, water jet cutting, chemical machining, electrical discharge machining, electro chemical machining, electron beam machining, laser machining, ultrasonic machining, high energy rate forming, explosive forming, magnetic forming, and the like

These and other metalworking techniques are generally described in Walker, Modern Metalworking, The Goodheart-Willcox Company, Ine (1993), incorporated herein by reference

In many of these techniques, it is often desirable to heat treat the metal composition in order to modify the physical and/or chemical characteristics of the metal composition in preparation for metalworking For example, in a typical die casting operation, which is one of the most important production processes in the metalworking industry, the metal composition (typically a nonferrous alloy) is heated up to a temperature at which the composition can be fluidly transported into the cavity of a die casting mold In the mold, the composition is allowed to cool and solidify, after which the mold is opened and the resultant die casting is removed There are generally two heating approaches by which metal compositions can be rendered fluidly flowable, depending upon whether the material being processed is thixotropic or nonthixotropic First, both nonthixotropic and thixotropic metal compositions can be rendered fluidly flowable by heating the material up to a temperature above the melting point so that the heated metal is in a liquid state Alternatively, if the metal composition is a thixotropic composition, the material can be rendered fluidly flowable by heating the material to a temperature above the sohdus temperature but below the quidus temperature such that the material is in a thixotropic, semiso d state In the thixotropic, semisohd state, application of shearing forces causes the material to flow like a true liquid, yet the material behaves like a solid when the shearing force is removed Thixotropic metal compositions are described in more detail in TJ S Pat Nos 4,694,882 and 4,694,881 See also Frederick, P S , et al , "Injection Molding Magnesium Alloys", Advanced Materials & Processes Ine Metal Progress, Appendix C (October 1988)

As used hereinafter, the terms "melted", "molten", "fluid", "fluidly", or the like when used in connection with a metal composition shall be deemed to collectively refer to either a metal composition in the liquid state or the thixotropic, semisohd state, as the case may be, unless expressly noted otherwise

With respect to die casting, cycle time refers collectively to the period of time it takes to first heat a given charge of the metal composition up to a temperature at which the metal composition can be caused to flow, then to fluidly transport the heated metal composition into a mold, then to allow the metal composition to solidify in the mold to form the die casting, and finally to open the mold and remove the die casting Faster cycle times are generally desired, because a higher output of die castings can be produced per unit of time One factor affecting cycle time concerns the technique which is used to accomplish heating With some heating approaches, the time required to heat the metal composition up to the desired temperature for casting is relatively long, thus adversely affecting cycle time Additionally, some heating approaches do not use energy efficiently, increasing the amount of energy required, and therefore the expense, for heating as compared to a process that is more energy efficient Some heating approaches also require complex and/or expensive machinery, thus further increasing the costs associated with polymer processing Heating delays adversely affect not just die casting cycle time, but also the cycle time of all metalworking techniques involving heating

It would be desirable, therefore, to provide an approach by which metal compositions can be heated more quickly so that cycle time could be reduced It would also be desirable if such an approach used energy more efficiently and required less complex, less expensive machinery so that the costs of metal processing could be reduced

Microwave radiation is a type of electromagnetic radiation characterized by a wavelength greater than radio waves but shorter than infrared radiation Microwaves generally are characterized by a wavelength in the range from about 1 mm (about 300 GHz) to about 50 cm (about 0 6 GHz) The use of microwave radiation to generate heat is becoming increasingly more prevalent in both consumer and industrial applications For example, microwave radiation currently is widely used as a means for heating food As another example, Assignee's copending application U S Serial No

08/938,237, filed September 26, 1997, in the names of Parviz Nazarian et al describes polymer processing systems that efficiently and effectively use microwave energy to melt polymer compositions in connection with polymer injection molding and extrusion operations

Microwave radiation for heating can be propagated in either single mode or multimode fashion The mode of the radiation refers to the specific electromagnetic field pattern that develops inside a microwave cavity

The mode pattern is governed primarily by the internal geometry of the cavity and the wavelength of the electromagnetic energy which propagates within the cavity A multi-mode cavity generally refers to a microwave cavity that is relatively large compared to the wavelength of microwave energy, such as, for example, a household microwave oven A multi-mode microwave cavity generally contains multiple mode patterns which tend to be somewhat random The electric field strength throughout a multi-mode cavity, therefore, is typically random and difficult to control When materials are heated inside a multi-mode cavity, heating uniformity can be improved by constant motion, agitation, or stirring of the material In contrast, a single mode microwave cavity refers to a smaller cavity that is capable of supporting only a single well-defined mode pattern which tends to be very regular and predictable

Microwave radiation has not been effectively used in the past, if it has been used at all, to heat metal compositions in connection with metalworking operations such as die casting, sand casting, or the like In particular, the present inventors are not presently aware of any commercially available die casting machine that relies upon microwave radiation to melt a metal composition under steady state conditions The lack of reliance upon microwave energy in metalworking is believed to be due to the significant engineering challenges that still would have to be overcome before microwave heating technology could be incorporated into industrial metalworking operations Specifically, metal compositions generally do not absorb microwaves and so cannot be directly heated using microwave radiation Of course, it is known to use a microwave susceptor (l e , a material that generates heat when the material absorbs microwave radiation) to heat surrounding media indirectly, but no system satisfactorily embodying this approach has yet been developed for heating steady state flows of metal materials, perhaps due at least in part to the high volume demands of industrial processes On one hand, the metal to be melted or heated must be in thermal contact with the microwave susceptor long enough for the desired degree of heating to occur, yet high volumetric flow rates of metal material must be sustained if the process is to be practically and/or economically feasible

Thus, there is a strong need for technological advancement allowing microwave energy to be effectively used to support metalworking operations

SUMMARY OF THE INVENTION

The present invention advantageously provides metal processing systems and methods that use microwave energy to achieve extremely rapid, efficient heating of metal compositions The present invention is based in part upon the appreciation that high volumetric, steady state flow rates of solid metal compositions, typically in the form of pellets, flakes, or the like, can be rapidly melted, or otherwise heated to a desired degree, by conveying the metal composition through the length of a microwave chamber containing a microwave susceptor in thermal contact with the flow Microwave energy is propagated in the cavity to heat the microwave susceptor The resultant thermal energy is then transferred to the metal composition either through a heat transfer mechanism such as conduction, convection, and/or radiation Because metal compositions tend to be excellent heat conductors, the transfer of thermal energy from the susceptor throughout the volume of the metal composition occurs quickly Indeed, enough thermal energy can be generated to easily melt nonferrous metal alloys in connection with steady state die casting operations Thus, it can be appreciated that thermal contact may involve direct physical contact between the metal and the susceptor However, because the heated susceptor radiates infrared (IR) energy, the metal need not directly physically contact the susceptor in order for the metal and susceptor to be in thermal contact with each other Microwave melting would occur so rapidly, that significant reductions in cycle time would be achieved by the present invention Additionally, the use of microwave energy for melting is economically advantageous, because microwave heating is extremely energy efficient

Accordingly, in one aspect, the present invention relates to a method of using microwave energy to process a solid, metal composition A microwave susceptor is heated with microwave energy A charge comprising the solid, metal composition is caused to thermally contact the heated microwave susceptor Preferably, sufficient thermal energy is transferred to melt the solid, metal composition The heated charge is then transported to a point of use at which a metalworking operation is carried out

In another aspect, the present invention provides an apparatus that uses microwave energy to process a charge comprising a metal composition The apparatus comprises an electrically conductive housing defining a microwave chamber having an inlet for receiving the metal composition and an outlet for discharging the metal composition A microwave susceptor is positioned in the microwave chamber in a manner such that thermal contact can be established between the microwave susceptor and an amount of the charge being conveyed through the microwave chamber A microwave energy source is operationally coupled to the microwave chamber such that, while a charge of the composition is in the microwave chamber, the microwave susceptor can be irradiated with microwave energy under conditions effective to heat the metal composition

In another aspect, the present invention provides a method of melting a metal composition Microwave radiation is propagated in a microwave chamber A charge comprising the metal composition is caused to flow through the chamber from a chamber inlet to a chamber outlet The charge thermally contacts a microwave susceptor that is heated by the microwave radiation The charge is heated and melts as a result of the thermal contact In preferred embodiments, the microwave chamber is configured so that the microwave radiation propagates in single mode fashion In such embodiments, residence time, 1 e , the period during which the metal and the susceptor are in thermal contact, is easily controlled merely by adjusting the length of the single mode microwave chamber to ensure that the desired level of heating is accomplished To increase residence time, for example, the length of the cavity can be increased without requiring any drop in material flow rate

Preferred embodiments of the present invention may involve separate heating and transport stages that can be carried out at pressures independent of the other stage Advantageously, this approach allows microwave heating to occur in one chamber under a first, relatively low pressure, after which the heated metal can be pressurized in a separate chamber to a second, relatively high pressure more suitable for extrusion, die casting, or the like This greatly simplifies the structure and construction of the apparatus, because the microwave heating chamber need not be constructed to withstand the high transport pressures

In preferred embodiments of the invention suitable for die casting operations, the molten metal composition is conveyed using a suitable motivating means, such as a screw, piston, or the like, to the cavity of the die mold Preferably, the motivating means is a piston, or the like, rather than a screw which is more conventionally used but much more expensive and complex BRIEF DESCRIPTION OF THE DRAWINGS

The above-mentioned and other features and advantages of this invention, and the manner of attaining them, will become more apparent and the invention will be better understood by reference to the following description of an embodiment of the invention taken in conjunction with the accompanying drawings, wherein

Fig 1 is a schematic representation of a system for heating metal compositions according to the present invention, Fig 2 is a schematic representation of the system of Fig 1 further including a closed-loop, temperature control system, and

Fig 3 is a side sectional view of one embodiment of a die casting machine according to the present invention

Fig 4 is a side sectional view of an alternative embodiment of a die casting machine of the present invention

DETAILED DESCRIPTION OF THE INVENTION

The various aspects of the present invention will now be described with reference to the particular embodiments of the present invention shown in Figs 1 through 3 However, the embodiments disclosed below are not intended to be exhaustive or to limit the invention to the precise forms disclosed in the following detailed description

Fig 1 shows a schematic representation of a metal processing system 10 of the present invention in which supply 12 comprising a metal composition is heat treated with thermal energy derived from microwave source 14 Ordinarrly, metal compositions do not absorb microwave energy Consequently, merely irradiating a metal composition with microwave energy would not be effective to heat the composition However, in accordance with the principles of the present invention embodied in system 10, microwave energy from microwave source 14 is used to heat microwave susceptor 16 positioned inside single mode microwave chamber 17 The position of microwave susceptor 16 is in no way critical and it can be placed anywhere so long as heat developed in susceptor 16 can be transferred to charge 18 For example, when microwave chamber 12 is cylindrical, susceptor 16 can be a cylinder (not shown) placed along the central axis of chamber 17, or a liner (as illustrated) lining the walls 19 of chamber 17, or an annular shaped member extending along the axis of chamber 17, or the like Microwave susceptor 16 absorbs at least a portion of the microwave energy and is heated as result Charge 18 of the metal composition is then caused to thermally contact heated microwave susceptor 16 Because metal compositions generally are extremely thermally conductive, thermal contact causes thermal energy from microwave susceptor 16 to be rapidly transferred to charge 18 The metal composition is heated as a result

In the practice of the present invention, a "microwave susceptor" is any material, or combination of materials, that (1) absorbs microwaves, and (2) can survive the high temperatures associated with metal processing Representative examples of suitable susceptor materials include ceramic-based carbides and nitrides such as silicon carbide, boron carbide, silicon nitride, boron nitride, combinations of these, and the like

The amount of thermal energy transferred from microwave susceptor 16 to charge 18, and hence the degree to which charge 18 is heated, can be easily increased or decreased by increasing or decreasing the microwave energy output of microwave source 14, respectively Depending upon the amount of thermal energy developed in microwave susceptor 16 and thereafter transferred to charge 18, the metal composition can be heated to varying degrees as desired in order to change the physical state and/or physical characteristics of the metal composition At low power settings, for instance, the metal composition can be lightly heat treated At higher power settings, enough thermal energy can be developed to fully liquify the metal composition Thus, system 10 can accomplish a wide range of heat treating operations including annealing, hardening, case hardening, surface hardening, stress relieving, melting, conversion to a thixotropic, semisohd state in which the metal composition can be caused to flow upon application of shear forces, tempering, combinations of these, and the like

After being heated as a result of thermal contact with microwave susceptor 16, charge 18 can then be transported to point of use 20, which may involve further metalworking operations such as surface finishing, machining, shaping, spinning, storage, casting into a die, cold working, cold forming, extrusion, soldering, brazing, sintering, incorporation into metal/polymer composites and/or metal/ceramic composites, or the like Because system 10 allows metal compositions to be melted so efficiently and quickly, system 10 is particularly suitable for use in die casting operations in which the metal composition is first melted, and then cast into one or more cavities of one or more single cavity dies, multiple cavity dies (I e , dies comprising a plurality of identical mold cavities), and/or combination dies (1 e , dies comprising a plurality of different mold cavities) Indeed, the principles of the present invention may be applied to melt or otherwise soften metal compositions for a wide variety of die casting operations including those involving plunger type die casting machines, air injection die casting machines, cold chamber die casting machines, or the like Advantageously, system 10 may be used to process any metal composition, or combination of metal compositions, including ferrous or nonferrous metals which may be pure metals, metal alloys, lntermetalhc compositions, combinations of these, and the like With respect to die casting, for example, the metal composition typically may be any nonferrous alloy such as an alloy based upon zinc, aluminum, magnesium, copper, lead, tin, combinations of these, or the like The metal composition to be processed may be in any convenient form that allows the desired degree of heat treatment to be carried out including sheet form, bar form, fiber form, pellet form, flake form , shaving form , granule form, or the like For die casting operations pellets, flakes, granules, shavings, and/or other particulate forms are preferred Microwave chamber 17 is generally defined by walls 19 formed from electrically conductive mateπal(s) that electrically shield the cavity, thereby substantially preventing microwaves from escaping from microwave chamber 17 Representative examples of such materials are well-known in the art and include corrosion-resistant metals, metal alloys, lntermetalhc compositions, combinations of these, and the like Preferred examples of such materials include aluminum, stainless steel, copper, die-cast zinc alloys, combinations of these, and the like

Microwave chamber 17 can be either a single mode microwave chamber or a multi-mode microwave chamber, as desired The term "mode" refers to the specific electromagnetic field pattern that develops inside a microwave chamber The mode pattern is governed primarily by the internal geometry of the cavity and the wavelength of the electromagnetic energy which propagates within the cavity A multi-mode cavity generally refers to a microwave chamber that is relatively large compared to the wavelength of microwave energy, such as, for example, a household microwave oven A multi-mode microwave chamber generally contains multiple mode patterns which tend to be somewhat random The electric field strength throughout a multi-mode cavity, therefore, is typically random and difficult to control When materials are heated in a multi-mode cavity, heating uniformity can be improved by constant motion, agitation, or stirring of the material In contrast, a single mode microwave chamber refers to a smaller cavity that is capable of supporting only a single well-defined mode pattern which tends to be very regular and predictable In the practice of the present invention, a single mode microwave chamber is preferred as being more efficient, particularly for use in a continuous process

Microwave chamber 17 can be provided with any suitable shape and dimensions The precise configuration of microwave chamber 17 will depend upon a variety of factors including, for example, the frequency of the microwave energy, the residence time required to accomplish melting, whether cavity 17 is intended for multimode or single mode operation, whether cavity 17 is intended for batch and continuous processing, and the like Preferred microwave cavities 17 of the invention, nonetheless, are provided with a cylindrical shape, because cylindπcally-shaped cavities can be relatively easily provided with dimensions effective to resonate at the frequency of the microwave energy supplied by microwave source 14 so as to promote even energy distribution in microwave chamber 17 Depending upon the mode of operation, microwaves can be propagated along the longitudinal axis of such cylindπcally-shaped cavities, generally perpendicular to such axis, or at an angle to such axis, as desired

In particularly preferred embodiments of the present invention, microwave chamber 17 is cylindrical in shape and is configured to provide a single mode pattern which places the electric field parallel to the axis of the cavity The material being heated is then conveyed through the cavity in line with the axis and electric field in order to achieve the maximum heating efficiency Even more preferably, in order to further optimize heating performance, a cylindπcally-shaped, single mode microwave chamber of the present invention has the so-called TM₀₂₀ mode configuration in which a peak electric field exists at the center axis of the cavity with another peak forming an annulus about the center axis The electric fields in both peaks thus are parallel to the center axis of the cylindrical cavity

As an option, at least a portion and preferably substantially all of the interior surfaces 21 of the electrically conductive walls defining microwave chamber 17 are sufficiently reflective so that at least a portion of the radiant heat energy generated during polymer melting is reflected back into microwave chamber 17 in order to promote more effective melting The interior surfaces of the cavity walls are preferably as reflective as practical circumstances allow Although a surface cannot be too reflective from a technical perspective, there is a level of reflectivity beyond which the incremental improvement in performance offered by additional improvement in reflectivity characteristics may not justify the extra cost of attaining such improvement Advantageously, using the combination of both microwave energy and reflected radiant energy provides much better melting performance than using microwave energy alone Any technique known in the art can be used to provide the interior surfaces 21 of the microwave chamber walls 19 with the desired level of reflective characteristics As one approach, for example, the interior surfaces 21 of electrically conductive cavity walls 19 formed from aluminum, stainless-steel, copper, or the like, can be polished in order to enhance the reflective characteristics of such surfaces As an alternative approach, instead of polishing the interior surfaces 21 of the walls 19, the interior surfaces 21 can be coated with an intrinsically reflective material such as gold, silver, nickel, or the like It should be understood, however, that polished interior surfaces 21 make microwave heating more efficient, but polishing is not mandatory because microwave susceptors are so effectively absorbed and become heated using microwave energy

Materials intrinsically characterized by low wall losses within the intended operating regime may not require any kind of polishing treatment to provide good melting performance For example, a microwave chamber formed from aluminum provides excellent performance without resort to a polishing treatment On the other hand, a surface treatment may be more desirable for materials having relatively high wall losses in the intended operating regime For example, stainless steel, which is stronger than aluminum and better for cavities subjected to high internal pressures, nonetheless contributes to higher wall losses than aluminum Therefore, a surface treatment involving polishing and/or applying a finish of nickel, gold, or platinum may be more desirable for stainless steel cavities

In the practice of the present invention, the reflective characteristics of the interior surfaces 21 of microwave chamber 17 can be quantitatively defined in terms of emissivity Emissivity refers to the ratio of the radiation emitted by a surface as compared to the radiation emitted by a black body at the same temperature Materials with lower emissivity are more reflective than materials with higher emissivity For purposes of the present invention, the interior surfaces 21 of microwave chamber 17 preferably have an emissivity of less than 0 1, more preferably 0 05 or less

Microwave energy for melting is supplied to microwave chamber 17 by microwave source 14 through waveguide 22 In the practice of the present invention, microwave energy refers to electromagnetic radiation characterized by a wavelength greater than radio waves but shorter than infrared radiation Preferred microwaves are characterized by a wavelength of about 1 mm (about 300 GHz) to about 50 cm (about 0 6 GHz) More preferred microwaves have a wavelength such that the frequency of the microwaves is in the range from about 0 9 GHz to about 5 GHz, preferably about 0 915 GHz or about 2 45 GHz The most common factors to be considered when selecting the most practical microwave source 14 are operating frequency, power requirement, output waveform and cost In most cases size and weight are of secondary importance, but these factors should not be overlooked if space is limited or ease of maintenance is critical With respect to operating frequency, microwave source 14 may be tunable so that microwave source 14 is capable of generating a range of microwave frequencies Alternatively, microwave source 14 may be of a "universal" type that generates microwaves characterized by only a single frequency Use of a "universal" type microwave source 14 is preferred because microwave sources that generate either 2 45 GHz or 0 915 GHz microwaves, respectively, are widely available at economic prices from a number of commercial sources Microwave sources operating at 2 45 GHz are more preferred

Microwave source 14 should have an appropriate power output such that microwave energy source 14 is capable of radiating microwave energy at a power level sufficient to achieve the desired degree of heat treatment at the rate at which the metal composιtιon(s) is to be processed Generally, an available power output level in the range from about 0 5 kW to about 500 kW would be suitable in the practice of the present invention In order to provide the flexibility to process a wide variety of metal composites in a variety of metalworking applications, it is preferred that the power output level of microwave energy source 14 be controllably variable over such a wide range As a specific example, 2 45 GHz microwave sources are most commonly available with power output ranging from 500 Watts up to 150 kW The output waveform from microwave source 14 is not particularly critical in the present invention because microwave susceptors so readily absorb and are heated by microwave energy Accordingly, any suitable output waveform may be used so long as the metal composition can be processed at desired flow rates Thus, the output waveform may be pulsed, low ripple, or the like For example, less expensive generators utilizing power supplies commonly found in household microwave ovens have a pulsed waveform where the pulse rate is equal to the power line frequency (60 Hz in the US) and an output spectral bandwidth of approximately 5 MHz In contrast, high performance generators utilizing switch mode power supplies have extremely low ripple, or CW, waveforms and typical output spectral bandwidths of approximately 250 kHz A useful rule of thumb is to use a microwave source having a spectral bandwidth no more than half the coupling bandwidth of the load being heated The use of any generator with a broader spectral output will result in reduced coupling efficiency and/or operational instability Therefore, low ripple generators, which have such capabilities, are preferred

Waveguide 22 is typically a pipehke structure that may have any suitable cross-section for carrying microwaves from microwave source 14 to microwave chamber 17 Preferred waveguides 20 have either a square, rectangular, or circular cross-section Like the walls used to define microwave chamber 17, waveguide 22 is generally formed from an electrically conductive material such as a corrosion-resistant metal, a metal alloy, an lntermetalhc composition, combinations of these, and the like Preferred waveguides comprise aluminum, stainless steel, and/or copper Waveguide 22 may be flexible if desirable or necessary to allow for tolerance build-ups between the respective mounting positions of the applicator and microwave generator Flexible waveguide 22 can also be used where movement between mounting positions is required However, flexible waveguide 22 could be subject to fatigue failure due to repeated working of its metallic structure Caution should be exercised during the design phase to limit the amount of flexure sufficiently to prevent any portion of the metallic structure from reaching its yield point while flexing

Almost all microwave power delivery systems require a device which is used to match the impedance of the load to that of the waveguide and thus the microwave generator Without this kind of device, the amount of microwave power coupled to the load may be partially reduced The most common form of this device is a waveguide stub tuner, but other types of devices such as irises are also used Waveguide tuners are popular for their convenience in adjusting the match while microwave power is being delivered

Tuners are available for either manual or automatic operation Manual tuners are adjusted by turning one or more stubs, or threaded rods, into the waveguide while the operator observes a power meter which monitors the amount of microwave power reflected from the load Tuning is accomplished when reflected power is minimized Automatic tuners operate essentially the same way, except that the stubs are driven by motors and sophisticated electronics are used to monitor reflected power and adjust the stubs accordingly

Most commercially available tuners, whether manual or automatic, have three or four stubs for more versatile impedance matching In certain cases it may be possible to accomplish all of the tuning requirements using only a single stub If this is the case, a significant amount of cost can be reduced for this components requirement However, the ability to use only a single stub can only be determined experimentally on the actual configuration of equipment for which it is desired

Another component typically used by most industrial microwave heating systems is a waveguide isolator which is used to protect the magnetron (the device which actually produces the microwave energy) from reflected power The isolator includes a waveguide circulator, which directs the reflected power away from the magnetron and a dummy load which absorbs and dissipates the reflected power Often these two elements exist as separate components which work together, but they are also available from some manufacturers incorporated together as a single component

A means to measure reflected power for tuning purposes is also desirable Waveguide power couplers and meters are available as separate components which can be incorporated into the heating system, but they are also available as a feature of the isolator

Other miscellaneous waveguide components may also be desirable depending on the configuration of the equipment onto which they are to be installed These components typically include short sections of rigid waveguide with one or more elbows to direct the microwave energy around corners Almost any configuration is possible

In some applications, it may be desirable to be able to accurately monitor and control the temperature of microwave susceptor 16 For example, when the metal composition is a thixotropic material to be processed in a semisohd state, it may be desirable to monitor temperature and control susceptor to ensure that the alloy is heated to a temperature that is above the so dus temperature, but below the hquidus temperature

Temperature control may be accomplished using any suitable open-loop or closed-loop process control technique For instance, Fig 2 is a schematic diagram in which system 10 of Fig 1 includes components that allow the temperature of microwave susceptor 16 and/or the output stream of processed metal to be controlled using a closed-loop, feedback control system, generally designated as 24 in Fig 2 As one component of control system 24, temperature sensor 26 is thermally coupled to system 10 in a manner such that temperature sensor 26 generates output signals indicative of the temperature of microwave susceptor 16 and/or the metal output stream (as shown) Temperature sensor 26 may be any conventionally known temperature sensing device capable of sensing temperature while withstanding the elevated temperatures associated with metal processing Several kinds of temperature sensors capable of operating in elevated temperature regimes are known, including a pyrometer that determines temperature from the character of IR radiation emitted by an object The sensor preferably is of a type capable of measuring the temperature of the metal output stream The electrical signals generated by temperature sensor 26 are provided as input to controller 28, which is another component of control system 24 Controller 28 then applies a suitable process control methodology to generate a control signal which is then transmitted to microwave source 14 in order to adjust the power output of microwave source 14 depending upon the degree to which the temperature of microwave susceptor 16 deviates from the desired temperature Although any suitable control methodology may be used, techniques of proportional/integral/deπvative ("PID") control are preferred PID control, and principles of process control generally, are described in Coughanowr and Koppel, Process Systems Analysis and Control, McGraw-Hill Book Company (1965), and F G Shinskey, Process Control

Systems (1988) Controller 28 may be formed from any combination of hardware, software, and the like, effective to enable controller 28 to derive the output control signal from the temperature sensor input A variety of commercially available hardware-based, software-based, and hardware/software-based devices suitable for use as controller 28 are commercially available, and any of these may be purchased and incorporated into system 10

Control system 24 desirably may also include conventional driver circuit 30 and noise filter 32 to process the temperature signals acquired from temperature sensor 26 before the data reaches controller 28 Driver circuit 30 is used to adjust the amplitude of the detected temperature signals, if desired, and/or to convert the detected data into an alternate form more suitable for further processing Noise filter 32 reduces the noise content, l e , increases the signal to noise ratio, of the temperature signal to enhance its processabihty by controller 28

Fig 3 shows one specific embodiment of a metal processing system of the present invention that embodies the principles of the present invention described above with respect to Fig 1 For purposes of illustration, the illustrated system of Fig 3 is in the form of cold chamber die casting machine 100 Machine 100 generally comprises supply subassembly 102, including hopper 103 containing supply 106 comprising a metal composition to be die cast Supply 106 preferably is in particulate form, e g , flakes, granules, pellets, or the like, to facilitate feeding into melting unit subassembly 104 If supply 106 includes a material, such as a magnesium alloy, that is easily oxidized, hopper 103 may be sealed with cover 107 to define hopper chamber 108 within which a protective atmosphere may be developed around the material to minimize oxidation Argon, nitrogen, carbon dioxide, and/or helium would be suitable inert gases to be used to develop such a protective atmosphere Optionally, supply 106 may be preheated while it is in hopper 103, although this is not required In fact, because supply 106 is so easily melted upon microwave heating in accordance with the present invention, it is more energy efficient to feed supply 106 into melting unit subassembly 104 at ambient temperature

Melting unit subassembly 104 includes electrically conductive, cylindrical housing 112 formed from sidewall 1 14, bottom 1 16, top 1 18, and interior partition 120 Interior partition 120 divides housing 1 12 into a feed zone 122 and a melting zone 124 The center region of interior partition 120 is fitted with perforated plate 126 that comprises a plurality of apertures permitting charge 125 of supply 106 to pass from feed zone 122 into melting zone 124 Preferably, interior partition 120 and perforated plate 126 are formed from an electrically conductive material to provide electric shielding at the top of melting zone 124 As an option, a cooling jacket (not shown) may be provided on the exterior of housing 112 in order to carry away excess heat generated during melting operations Feed zone 122 includes rotatable feed screw 128 operationally supported in housing 1 12 and stationary bushing member 130, which defines the inner diameter of feed zone 122 Bushing member 130 preferably is formed from hardened steel Bushing member 130 is fixedly attached to housing 1 12 by any suitable technique including welding, riveting, bonding with an adhesive, press fitting, and the like Charge 125 of supply 106 is gravity fed from hopper 103 into helical chamber 136 through entry port 117 Helical chamber 136 is defined by interior surface 131 of bushing member 130, feed member threads 138, and center member 134 Rotation of feed screw 128 motivates charge 125 through perforated plate 126 and into melting zone 124 The feed rate can be controlled easily by adjusting the rotational speed of feed screw 128 Generally, faster rotation of feed screw 128 provides higher feed rates Preferably, feed screw 128 is rotated at a rate so that a substantially continuous, steady state flow of charge 125 through melting unit subassembly 104 can be maintained Melting zone 124 includes cyhndrically shaped chamber 140 operationally coupled to microwave source 137 by waveguide 139 Although only one microwave source 137 and waveguide 139 is shown, one or more microwave sources in combination with one or more waveguides coupled to chamber 140 at one or more positions could also be used In the preferred embodiment shown, chamber 140 not only functions as a passage for the metal composition to be conveyed through melting unit subassembly 104, but chamber 140 also functions as a single mode microwave chamber Thus, both microwaves and the metal composition are conveyed along a path substantially aligned with the longitudinal axis of chamber 140 In preferred embodiments, at least a portion of the interior surfaces 141, 145, and/or 147 of housing 1 12 that define chamber 140 are sufficiently reflective (e g , characterized by an emissivity of less than 0 1, preferably 0 05) so that not only microwaves, but also some of the radiant energy generated during melting operations, are reflected back into chamber 140 in order to enhance melting performance

Chamber 140 generally has a diameter that is determined by the distance between the interior surfaces 141 of chamber 140 The diameter of chamber 140 as measured between the interior surfaces 141 is preferably equal to an integer multiple of the wavelength of the microwaves being used for processing Providing chamber 140 with such a diameter helps ensure that the microwaves will resonate inside chamber 140 in single mode fashion to achieve uniform energy distribution and efficient melting performance Preferably, such diameter is 2 to 5 times, and more preferably 2 to 3 times, the wavelength of the microwaves For example, for 2 45 GHz microwaves, the diameter of chamber 140 is preferably 10 75 cm Principles for determining suitable geometry characteristics of a microwave chamber such as chamber 140 are described, for example, in Metaxas, "Industrial Microwave Heating", Peter Peregπnus Ltd , London (1983)

The length of chamber 140 is determined by the distance between plate 126 at the entrance to chamber 140 and perforated plate 144 positioned at the exit from chamber 140 Chamber 140 may be provided with any suitable length as desired to ensure that the metal composition residence time in chamber 140 is long enough for melting to be accomplished The preferred length of chamber 140 will depend upon factors including the thermal properties of the material being processed and the characteristics, e g , field strength, of the power supply For example, the field strength should be high enough to effect reasonable rates of heating but should not be so high as to result in breakdown and electric discharge (arcing) inside chamber 140 To achieve this goal, preferred electric fields are on the order of about 375 kV/m for power levels on the order of 6 kW Given this field strength, and for an apparatus in which 2 45 GHz microwave energy is being used at a power output level of 6 kW, chamber 140 typically may have a length in the range from 20 cm to 100 cm, preferably about 40 cm to 50 cm, for processing a wide range of different metal compositions See generally Metaxas, "Industrial Microwave Heating", Peter Peregπnus Ltd , London (1983) at page 72

A microwave susceptor in the form of liner 150 is positioned in chamber 140 Advantageously, liner 150 absorbs microwave radiation and is heated as a result This thermal energy is rapidly transferred to charge 125 in thermal contact with liner 150 This, in turn, causes charge 125 to melt or, alternatively, to form a thixotropic, semisohd composition in those applications in which an alloy such as a magnesium alloy is being processed under conditions in which the thixotropic, semisohd state is formed Liner 150 is provided proximal to sidewall 114, but does not line all of bottom 1 16 No liner is needed proximal to such portions of bottom 1 16, because charge

125 is typically fully melted, or rendered semisohd as the case may be, by the time the material reaches the bottom portion of chamber 140 This positioning is not critical, and liner 150 may be positioned in chamber 140 any other suitable way For example, as one alternative liner 150 may be a cylinder positioned along the longitudinal axis of chamber 140 in such a way that metal would be conveyed through an annular volume between such a susceptor and the walls of chamber 140 As another alternative, the susceptor may be in the form of a rotatable screw having flights of a suitable diameter to facilitate transport of material through chamber 140 The thickness of the walls of liner 150 will depend upon a variety of factors including the diameter of chamber 140, the material from which liner 150 is to be formed, the power output setting of microwave source 137, the frequency of the microwave radiation generated by microwave source 137, and the like In preferred embodiments of the invention operating at 2 45

Hz and 6 kW, and in which chamber 140 is a single mode cavity having a diameter of 4 39 cm and liner 150 is formed from silicon carbide, providing liner 150 with a wall thickness in the range from 0 9 cm to 1 1 cm, preferably 0 95 to 1 05 cm, would be suitable In order to be able to optimize heating performance for differing materials and processing conditions when using the embodiments of the invention shown in Fig 3, it may be desirable that liner 150 is removable so that different sized parts can be inserted into place in chamber 140 as needed This allows heating performance to be optimized for any kind of metal composition being processed On the other hand, for manufacturing practicability, it may be desirable to use a nonremovable liner 150 having characteristics that generally provide good heating performance for a range of materials over a range of operating conditions

In embodiments of the invention including a liner 150, liner 150 may be rotatable about the axis of chamber 140 to assist with material transport or mixing Liner 150 may comprise flights and/or threads, or the like, in order to facilitate this transport action

The rotational speed of feed screw 128 generally determines the pressure in chamber 140 Desirably, melting occurs in chamber 140 at a relatively low pressure that is effective to develop sufficient force to transport charge through chamber 140 It is particularly desirable to match the flow rate of material with the heating rate so that material enters chamber 140 through plate 126 at substantially the same rate that material leaves chamber 140 through plate 144 Melting preferably is carried out in chamber 140 under a relatively low pressure in the range from about 2 kg/cm² to about 40 kg/cm², preferably 2 kg/cm² to 20 kg/cm² In the practice of the present invention, the pressure applied to material in chamber 140 is deemed to be the pressure of the material at the exit port of chamber 140, l e , at plate 144

After melting, the material is transferred from melting unit subassembly 104 to transport unit 151 through conduit 149 Transport of the material between melting unit subassembly 104 and transport unit 106 may be accomplished using any suitable motivating means As shown in Fig 3, pumping device 152 is used to accomplish such transport Pumping device 152 performs at least three important functions Firstly, pumping device 152 helps control the flow rate of molten material transferred to transport unit 151

Additionally, pumping device helps homogenize the molten material Finally, in embodiments in which the material being processed is a thixotropic, semisohd material, pumping device 152 provides shearing action which facilitates formation and transport of the thixotropic, semisohd phase Although pumping device 152 is preferred, other devices, such as a check valve, could also be used

Transport unit 151 includes cylindrical housing 153 enclosing metering chamber 154 for holding a metered amount of material 160 Transport unit 151 is further provided with piston 156 which is capable of reciprocating movement inside housing 153 in directions along the longitudinal axis of transport unit 151 Piston 156 is disposed so that movement of piston 156 in the direction of arrow 158 forces material through passage 162 and into the cavity (not shown) of die (also not shown) in a conventional manner Because metering chamber 154 is separate from and independently pressuπzable relative to chamber 140, relatively high pressures suitable for die casting can be developed in metering chamber 154 without affecting the performance of melting operations For example, relatively high pressures in the range from 10 kg/cm² to 2000 kg/cm², preferably 500 kg/cm² to 1000 kg/cm², can be easily developed in metering chamber 154 while lower pressures are maintained in chamber 140 In some modes of operation, the energy field established in chamber 140 may have a peak power region not only around the outer periphery of chamber 140 proximal and/or within liner 150, but also may have another peak power region located centrally along the axis of chamber 140 To take advantage of this additional power region, an additional or microwave susceptor in the form of a core member (not shown in Fig 3) positioned along the axis of chamber 140 can be used to absorb such energy and then transfer the energy in the form of heat to the material being processed The material being processed would thus be transported in the annular region between such a core member and liner 150 This concept is shown in Fig 4

Fig 4 shows metal processing apparatus 200 that is generally similar to metal processing apparatus 100 of Fig 3, except that apparatus 200 further includes an additional microwave susceptor in the form of core member 202 positioned in chamber 140 A All corresponding features of Fig 4 also found in Fig 3 bear the same identification number as the corresponding parts of Fig 3 except that the identification numbers of Fig 4 also include the suffix "A"

Core member 202 extends from plate 126A to plate 144A Material being processed is transported through chamber in annular fashion between liner 150A and core member 202 Use of core member 202, which is formed from a microwave susceptor material, further optimizes the heating performance of apparatus 200 by absorbing microwave energy resonating along the central axis and is heated as a result This heat energy is then transferred to the material being processed Thus, core member 202 allows the microwave energy generated along the central axis of chamber 140A to be used much more efficiently for heating purposes When chamber 140A is a TM020 mode cavity operating at 2 45 GHz and having an inside diameter of 17 2 cm, a preferred liner 150A is made from SiC and has a thickness in the range from about 1 to 3 cm, and core member 202 preferably has a radius in the range from 1 cm to about 2 cm While this invention has been described with respect to preferred embodiments, the present invention can be further modified within the spirit and scope of this disclosure This application is therefore intended to cover any variations, uses, or adaptations of the invention using its general principles Further, this application is intended to cover such departures from the present disclosure as come within known or customary practice in the art to which this invention pertains and which fall within the limits of the appended claims

Claims

WHAT IS CLAIMED IS:

1 A method of using microwave energy to process a solid, metal composition, comprising the steps of

(a) heating a susceptor with microwave energy,

(b) causing a charge comprising the solid, metal composition to thermally contact the heated microwave susceptor under conditions such that the solid, metal composition is heated and at least partially melted, and

(c) fluidly transporting the heated charge to a point of use involving a metalworking operation

2 The method of claim 1, wherein the solid, metal composition is a solidified, thixotropic alloy prior to step (b), step (b) comprises heating the alloy to a temperature at which the alloy is in a thixotropic, semisohd state, and step (c) comprises fluidly transporting the alloy while the alloy is in the thixotropic, semisohd state

3 The method of claim 1 , wherein step (b) comprises

(I) heating the metal composition to a temperature above the hquidus temperature of the metal composition to form a melt, and

(n) while agitating the metal composition, cooling the melt to a temperature below the hquidus temperature of the metal composition but above the sohdus temperature of the metal composition, said cooling and agitating occurring under conditions to form a thixotropic composition

4 The method of claim 1, wherein step (b) comprises heating the metal composition to a temperature above the melting point of the metal composition to form a liquid phase 5 The method of claim 1, wherein step (b) comprises

(l) heating the metal composition up to, but not over, a processing temperature in a temperature range at which the metal composition is a semisohd, and (n) while the metal composition is maintained at said processing temperature, agitating the metal composition under conditions such that a thixotropic alloy is formed

6 The method of claim 1, wherein the microwave susceptor is positioned in a chamber having an inlet for receiving a charge to be heated and an outlet for discharging the charge after heating, and wherein step (a) comprises propagating microwave radiation in the chamber in single mode fashion

7 The method of claim 1, wherein the microwave susceptor is positioned in a multimode microwave chamber

8 The method of claim 6, wherein steps (a) and (b) occur while transporting the charge through the cavity under substantially steady state conditions

9 The method of claim 1, wherein step (c) comprises actuating a piston to cause fluid transport of the composition from a metering chamber to the point of use

10 The method of claim 1 , wherein the point of use is a cavity of a die casting mold

11 The method of claim 1 , wherein the point of use is an extruder die

12 The method of claim 1, wherein steps (a) and (b) are carried out in a chamber substantially defined by an electrically conductive housing, and wherein the microwave susceptor is provided as a lining on at least a portion of an interior surface of the housing so that the charge transported into the chamber does not directly contact said housing portion

13 The method of claim 12, wherein the lining is a cylindrical sleeve, and step (b) comprises transporting the charge through the sleeve

14 The method of claim 1, wherein the microwave susceptor comprises a ceramic-based carbide, a ceramic-based nitride, and combinations thereof

15 The method of claim 12, wherein a core member is positioned in the cavity in a manner effective to define an annularly shaped passage for the charge to be transported through the cavity, and wherein the core member comprises a microwave susceptor material

16 An apparatus that uses microwave energy to process a charge comprising a metal composition, said apparatus comprising

(a) an electrically conductive housing defining a microwave chamber having an inlet for receiving the metal composition and an outlet for discharging the metal composition,

(b) a microwave susceptor positioned in the microwave chamber in a manner such that thermal contact can be established between the microwave susceptor and an amount of the charge being conveyed through the microwave chamber, and

(c) a microwave energy source operationally coupled to the microwave chamber such that, while a charge of the composition is in the microwave chamber, the microwave susceptor can be irradiated with microwave energy under conditions effective to heat the metal composition 17 The apparatus of claim 16, further comprising a mixing unit downstream from the microwave chamber, wherein an amount of the charge within said mixing unit is subjected to shearing forces

18 The apparatus of claim 16, wherein the microwave chamber is a single mode microwave chamber

19 The apparatus of claim 16, wherein the microwave chamber is a single mode microwave chamber in which the microwave energy propagates in the cavity along a path extending substantially between the inlet and the outlet

20 The apparatus of claim 16, wherein the cavity is a multimode microwave chamber

21 The apparatus of claim 16, further comprising a transport mechanism that comprises a metering chamber for receiving the heated composition and a piston operationally coupled with the metering chamber such that movement of the piston is capable of causing transport of an amount of the composition from the metering chamber to a point of use involving a metalworking operation

22 The apparatus of claim 21 , wherein the point of use is a cavity of die casting mold positioned in fluid communication with the metering chamber

23 The apparatus of claim 21, wherein the point of use is an extruder die positioned in fluid communication with the metering chamber

24 The apparatus of claim 21, wherein the apparatus further comprises a lining covering at least a portion of an interior surface of the microwave chamber, wherein the microwave susceptor forms at least a portion of said lining 25 The apparatus of claim 21, wherein the microwave susceptor comprises a ceramic-based carbide, a ceramic-based nitride, and combinations thereof

26 The apparatus of claim 20, further comprising a ceramic-based carbide, a ceramic-based nitride, and combinations thereof

27 A method of melting a metal composition, comprising the steps of

(a) propagating microwave radiation in a microwave chamber in single mode fashion,

(b) causing a charge comprising the metal composition to flow through the chamber from a chamber inlet to a chamber outlet,

(c) during step (b), causing the charge to thermally contact a microwave susceptor, said microwave susceptor being heated by the microwave radiation whereby the charge is heated and melts as a result of the thermal contact