Classifications

• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B22—CASTING; POWDER METALLURGY
  • B22D—CASTING OF METALS; CASTING OF OTHER SUBSTANCES BY THE SAME PROCESSES OR DEVICES
  • B22D19/00—Casting in, on, or around objects which form part of the product
  • B22D19/14—Casting in, on, or around objects which form part of the product the objects being filamentary or particulate in form

Definitions

• This invention relates to the production of metal matrix composites using electromagnetic body forces to drive molten metal into a reinforcing material.
• Processes for casting metal matrix composites currently use applied pressure (i) to overcome capillary forces at the infiltration front of the liquid metal matrix material as it advances into the reinforcement material and (ii) to minimize processing times and hence both costs and the extent of chemical reaction between matrix and reinforcement materials in reactive systems.
• Metal pressurization is obtained by mechanical means, via a piston (as in squeeze casting) or pressurized gas (as in the Cray process) .
• Many pressure infiltration devices have thus been designed, such as the squeeze casting presses presently in use for fabricating the mass marketed metal matrix component, an aluminum Toyota diesel engine piston selectively reinforced with a alumina fibers.
• a new method and apparatus for driving molten metal into a preformed reinforcing phase is described, using electromagnetic body forces. While electromagnetically induced body forces have been used in other materials processing operations such as electroforming solid metals, such forces are used here for the first time to induce flow of liquid metal into a reinforcement material such as particles, fibers, or a preform to produce composite materials.
• sufficiently strong electric and magnetic fields interact to create an electromagnetic body force in a liquid metal. This force can be used to propel the liquid metal in a chosen direction.
• the use of such a force is an efficient method for the production of metal matrix composites.
• the electric and magnetic fields can be produced by a current discharge through a coil of conducting material placed in the vicinity of the liquid metal which is to form the matrix of the composite.
• This current creates a transient magnetic field B within a certain thickness of the metal, which in turn creates a transient eddy current j in the molten metal.
• the invention features, in one aspect, a method for the production of metal matrix composites, including placing a substantially liquid metal in the vicinity of a reinforcement material and the source of an inactive transient magnetic field, sufficient, when activated, to produce an electromagnetic body force within the metal through the interaction of the transient magnetic field and eddy currents induced by the transient field within the metal, and activating the transient magnetic field, thereby propelling the substantially liquid metal into the reinforcement material.
• the activating step is repeated; quantities of the liquid metal and the reinforcement material are continuously provided, including the additional step of withdrawing from the vicinity of the source of the transient magnetic field the reinforcement material into which metal has been propelled;
• the metal includes at least one of or includes an alloy comprising aluminum, nickel, cobalt, copper, beryllium, lead, tin, zinc, magnesium, titanium, or iron;
• the reinforcement material includes a ceramic;
• the reinforcement material includes fibers, whiskers, particles platelets, or rods;
• the reinforcement material is shaped into a preform;
• the reinforcement material includes at least one of silicon carbide, boron, tungsten, carbon, silicon nitride, boron carbide, silicon oxide, aluminum oxide, titanium, or steel;
• the propelling step additionally includes subjecting the substantially liquid metal to an electrical field;
• the transient magnetic field is produced by a discharge coil through which electric current is passed; the frequency and damping constant of the repeatedly activated transient magnetic field are tailored to the geometry of the discharge coil, reinforcement material
• the invention features a method for the production of metal matrix composites, including placing a quantity of substantially solid metal into a heat resistant vessel, heating the metal until substantially liquid, immersing in the metal a preform of a reinforcement material, placing the heat resistant vessel containing the metal and the reinforcement material in the proximity of the source of an inactive transient magnetic field, sufficient, when activated, to produce an electromagnetic body force within the metal through the interaction of the transient magnetic field and eddy currents induced by the transient field within the metal, and activating the transient magnetic field, thereby propelling the metal into the reinforcement material.
• the invention features a method for the production of metal matrix composites, including placing a reinforcement material into a heat resistant vessel, substantially surrounding the reinforcement material with a quantity of substantially solid metal, heating the metal until substantially liquid, placing the heat resistant vessel in the proximity of the source of an inactive transient magnetic field, sufficient, when activated, to produce an electromagnetic body force within the metal through the interaction of the transient magnetic field and eddy currents induced by the transient field within the metal, and activating the transient magnetic field, thereby propelling the metal into the reinforcement material.
• the invention features a method for the continuous production of a metal matrix composite including the steps of conveying substantially liquid metal into an infiltration region, conveying a reinforcement material into the infiltration region and into the vicinity of the liquid metal, infiltrating the reinforcement material with the liquid metal by subjecting the liquid metal to a magnetic field, and conveying the infiltrated composite out of the infiltration region.
• the reinforcement material includes particles, fibers, whiskers, or rods; the particulates includes silicon carbide particles; the fibers comprise carbon fibers; the fibers conveyed into the infiltration region are uniaxially oriented and are maintained in this uniaxial orientation during the infiltrating step.
• the invention features apparatus for producing metal matrix composites using electromagnetic body forces, including an infiltration zone having adjoining liquid metal and reinforcement material subzones, and an electromagnetic field source, capable of being activated and deactivated, adjacent to the liquid metal subzone of the infiltration zone, that produces a transient magnetic field and associated eddy currents within the metal, the electromagnetic field source oriented so as to propel the metal into the reinforcement material subzone of the infiltration zone.
• the electromagnetic field source surrounds the infiltration zone;
• the electromagnetic field source includes a discharge coil;
• the discharge coil includes a spiral coil adjacent to one side of the infiltration zone;
• the apparatus additionally includes at least one capacitor bank and a triggering circuit through which the capacitor bank discharges current through the discharge coil;
• the apparatus additionally includes a flux concentrator coupled to the discharge coil;
• the flux concentrator includes copper or graphite;
• the infiltration zone is defined by a heat resistant crucible;
• the reinforcement material is a preform and the crucible additionally includes apparatus for lowering and raising a preform into and out of the infiltration zone;
• the apparatus for lowering and raising the preform into and out of the infiltration zone includes a bobbin centered within the crucible;
• the bobbin guides the flow of metal propelled by the electromagnetic field source in a direction radial to the central axis of the crucible;
• the infiltration zone is defined by a heat resistant tube; and
• the apparatus additionally includes conveying apparatus to convey
• the invention features apparatus for producing metal matrix composites using electromagnetic body forces, including an infiltration zone having adjoining liquid metal and reinforcement material subzones, heating apparatus surrounding the infiltration zone able to maintain metal placed within the liquid metal subzone of the infiltration zone in a liquid state, and an electromagnetic field source, capable of being activated and deactivated, adjacent to the liquid metal subzone of the infiltration zone, that produces a transient magnetic field and associated eddy currents within the metal, the discharge coil oriented so as to propel the metal into the reinforcement material subzone of the infiltration zone.
• the heating apparatus includes a thermostatically controlled heating element surrounding the electromagnetic field source.
• Electromagnetic body forces literally propel the metal into the reinforcement material. No additional apparati are required to push the metal into the reinforcement material, rendering unnecessary other pressure-inducing devices and pressure- resistant or pressure-containing vessels.
• Fig. 1 is a cross-section of an apparatus for producing cylindrical or tubular metal matrix composites
• Fig. 2 is a cross-section of an apparatus for producing planar metal matrix composites
• Fig. 3 is a micrograph of a composite produced according to the invention.
• Fig. 4 is a micrograph of the composite of Fig. 3 at higher magnification.
• Fig. 5 compares the magnetic flux density of a search coil to that of a damped sinusoid.
• Fig. 6 show the flux profiles in a furnace for various discharge voltages (apparatus 1) .
• Fig. 7 is a stress-strain curve in compression for a 24 volume percent Saffil preform at 673° K.
• Fig. 8 shows the distance a preform is infiltrated over the course of a typical discharge.
• Fig. 9 shows the cumulative infiltration distance after each of nine 3 kHz discharges.
• Fig. 10 shows the cumulative infiltration distance after each of fifty 5.6 kHz discharges.
• Fig. 11 shows predicted infiltration distance after five discharges with a 3 tesla peak.
• Metal matrix composites may be formed in the devices shown in Figs. 1 and 2. Common to both is the function of the electrical components that generate the electromagnetic body forces. The arrangement of those components differ in each, however, as do the arrangement of the heating components, as their arrangement is determined by the geometry of the composite to be produced.
• copper discharge coil 22 is 0.25 inch copper wire electrically connected through any conventional triggering circuit to a bank of capacitors (not shown) with a total capacity of 640 microfarads and a power supply able to produce 4.5 kilovolts (not shown).
• the coil 22 is arranged as a solenoid and is equipped with copper flux concentrator 21 for concentrating the magnetic field produced by the energized discharge coil 22.
• the height of the inner radius of concentrator 21 is one third the height of the outer radius, and as a result, increases the flux some 300% in the infiltration zone defined by the inner height.
• the Magneform apparatus uses several ignitron tubes.
• Insulated chamber 2 includes an insulating top 12, insulating base 32, and wall 30. Insulation 28 surrounds heating elements 26. Discharge coil 22, encased within refractory cement 24, encircles flux concentrator 21.
• a preform 18 of a reinforcement material such as silica bonded Saffil alumina fibers is inserted into crucible 20. To ensure the preform 18 remains precisely centered within the crucible 20, it was mounted on a bobbin 19 as shown in the apparatus of Fig. 1.
• the infiltrated composite can be withdrawn from the crucible while the matrix is still molten, and the flanges of the bobbin help to constrain the metal flow to a radial direction, minimizing axial flow.
• Several bobbin designs were used, all of which were functionally identical, varying only in the materials chosen, the central rod being either of steel or high density alumina.
• the fiber preforms were infiltrated along their plane of pressing.
• Crucible 20 is preferably of a heat-resistant ceramic such as alumina.
• molten aluminum 16 is poured around a preform 18 placed within the crucible, covering it.
• Insulating plug 14 caps the preform/melt mixture to prevent stray metal flow during infiltration.
• the copped crucible is then lowered into the central cavity 5 of chamber 2 through opening 7 by crucible lifting mechanism 34.
• the required amount of aluminum was first added to each crucible, and placed in a holding furnace (not shown) set at 973° K with an alumina plug.
• the preforms, already mounted in their bobbins, were loaded into the furnace once the aluminum had begun to melt in the crucibles. Once the metal was fully molten, the preform-bobbin assembly was immersed in the melt, and allowed to equilibrate for 5 minutes.
• the crucible with its preform was then withdrawn from the holding furnace and lowered into the infiltrating furnace, which had been preheated to 973 K. At this point the ceramic plug was pushed into the top of the crucible so as to rest upon the melt surface.
• Discharging the charged capacitors through discharge coil 22 creates a very high pulse of current in the coil which in turn creates a correspondingly high transient magnetic field inside crucible 20 via the concentrator.
• Fields of about 2 to 10 tesla were used, although higher or lower strength fields may be used depending on the other system variables.
• Currents of from about 20,000 to about 50,000 amperes were used, although this too may be higher or lower in other systems.
• the penetration depth of the magnetic field into the molten metal is preferably no more than the total thickness of the layer of metal. Increasing the frequency of the current reduces the penetration depth of the field, and is one way to adapt the apparatus to various possible geometries.
• the transient magnetic field induces electrical currents in the molten metal 16 which interact with the magnetic field produced by coil 22. This interaction produces a net body force on molten metal 16 around preform 18, forcing it away from coil 22 and flux concentrator 21 and into the preform.
• Multiple discharges assure penetration of the liquid to the desired depth within the preform. Capillary and frictional forces that oppose the infiltration of the liquid are insufficient to prevent substantial infiltration of the preform.
• Room temperature coils were also used. The procedure is identical to that above, except that the crucible should be returned to the holding furnace within 30 seconds of its transfer to the discharge coil, since it was determined that freezing of the melt began 45-60 seconds after the hot crucible was introduced into the cold concentrator. The number of discharges possible during this 30 seconds time interval varied between 3 and 8, depending upon the voltage of the discharge. Several of the samples thus had to be reheated more than once to obtain the required number of discharges.
• the current produced by the apparatus has the character of an exponentially decaying sinusoid after the first half-cycle.
• a typical flux profile is shown in Fig. 5.
• the voltage to which the capacitors are charged can be varied, and is one of the main process parameters, since this determines the intensity of the magnetic pulse.
• the discharges can be repeated as soon as the capacitors have recharged (two to five seconds in the laboratory apparatus) .
• Other characteristics of the pulse such as its frequency and damping constant, depend upon the capacitance, inductance, and resistance of the electrical circuit, which are largely determined by the design of the coil and capacitance of the energy modules.
• the geometry of the process is flexible, since the coil, the crucible, and the preform need not be cylindrical. With a quantitative understanding of its kinetics, infiltration lengths can also be accurately controlled.
• a search coil was designed that produces a voltage signal proportional to the time derivative of magnetic flux density.
• This search coil was calibrated against a RFL Model 912 Gaussmeter (RFL Industries Inc. , Boonton, New Jersey) .
• This allowed measurement of the peak magnetic field intensity, B Q assuming the first peak of the magnetic field is a sinusoid (Fig. 5) .
• B in combination with output from the resistor, yields the curve of magnetic field versus time.
• B 0 was calibrated as a function of peak intensity of current measured by the transducer for each set of coil and concentrator used.
• FIGs. 3 and 4 The micrograph of Fig. 3, taken at 1 00 x magnification, and of Fig. 4, at lOOOx magnification, show negligible residual porosity in a preform having 24% volume percent reinforcing phase.
• the samples shown in Figs. 3 and 4 are of an aluminum infiltrated Saffil alumina preform.
• the 16 mm diameter, 5 cm long cylindrical preform had 4% silica added as a binder, and had fibers 3 ⁇ m in diameter.
• Samples produced are completely infiltrated to within a distance of about 300 ⁇ m from the infiltration front. Nearer the infiltration front, porosity gradually increases, leading to a relatively sharp infiltration front. Molten aluminum does not dewet SaffilTM preforms spontaneously once these are infiltrated. Therefore, provided an elevated value of pressure was experienced by the metal at any region of the composite during infiltration, that region will remain fully infiltrated.
• the low porosity found in the preforms is a result of the relatively high pressures generated by the Lorentz forces (up to 6 MPa) , and, at low frequencies, of the occurrence of a reversal in the Lorentz force, which induces elevated pressures near the infiltration front.
• the process may be controlled by varying the current through the coils, as well as the number, duration, and frequency of the pulses. Optimum conditions will vary with preform shape and size, fiber or particle size, and matrix metal composition. The geometry of each of the coil, crucible, melt, and preform may also be varied to optimize infiltration with varied reinforcement and matrix materials and geometries. Process Parameters
• Fig. 8 shows graphically how infiltration distance varies during a typical discharge of 2.1 kHz and 3 tesla peak, damping factor of 0.5 mS. It is seen that as the body force builds up, no infiltration is predicted until the body force is sufficiently large to overcome the capillary forces. At this point, there is a rapid acceleration of the melt into the preform during which the fluid friction forces build up to slow the flow. The melt advances until, as the Lorentz forces fall again, it is brought to a halt by the combined action of fluid friction and capillary forces. When the Lorentz force becomes negative, the melt progresses backwards appreciably, even though the magnitude of the negative forces are much lower than the forward forces at other parts of the discharge cycle. This is because capillary forces were assumed not to impede backward metal flow.
• Fig. 9 shows cumulative infiltration depth for one to nine discharges for peak flux intensities of 2 , 3, and 4 tesla, at 2.1 kHz discharge frequency and a damping constant of 0.5 mS.
• the model predicts that the infiltration increment from the first few discharges is more than for subsequent discharges. This is clearly because earlier discharges have lower fluid friction forces to overcome due to the shorter infiltrated length. Calculations show, however, that after the first few discharges, the infiltration depth increment per discharge becomes nearly constant, only decreasing by a very small amount as infiltration progresses, Fig. 10.
• Fig. 9 also demonstrates that there is an optimum discharge intensity for a given frequency. This effect is due to preform compression — if the discharge is too intense the preform compresses to such an extent that the increased fiber volume fraction lowers preform permeability so the gain in propelling force is more than negated by the increased capillary and fluid friction forces.
• Fig. 11 shows the cumulative infiltration predicted after 5 discharges with 3 tesla peak, for a wide range of frequencies having identical relative damping coefficients.
• the penetration depth is so large, in relation to the melt ring thickness, that the Lorentz forces generated are insufficient to overcome capillary forces, and so zero infiltration is predicted.
• the body force is higher, its duration is much shorter.
• Inertia is then more of a limitation to infiltration, and the higher velocities lead to greater fluid friction losses in the infiltrated portion of the liquid composite.
• the process of this embodiment though discontinuous in the sense that the motive coil current is generally not continuous even in a batch mode, is easily adapted to a continuous casting process by using a repeated pulsed current.
• the infiltration zone may be partially open and need not be adapted to retain pressure. Since they remain accessible during the pressurization stage of the process, metal and reinforcement may be continuously fed into the infiltration zone to be retrieved by continuously casting the resulting infiltrated composite.
• the unsealed process zone also permits very short cycle times since there is no need to retrieve any pistons, vent pressurized gas, and open pressure-tight vessels.
• Fig. 1 depicts an embodiment where preforms are infiltrated in a batch mode
• that apparatus may be easily adapted to continuously cast a metal matrix composite.
• the ceramic crucible would be replaced by a ceramic tube.
• the Fig. 1 apparatus would be open along its central axis not only at the top as shown, but also at the bottom to accommodate a continuous length of rod or tube preform.
• a chill zone at the discharge end would solidify the composite before it exited the apparatus and was recovered.
• the reinforcing phase preform e.g. a rod or a hollow cylinder, would be fed through the apparatus and infiltrated with liquid metal as it passed through the infiltration zone within the flux concentrator's concentrated magnetic field.
• any flux concentrator that may be used, and the heating components can be modified depending on the type and geometry of the composite to be produced. While the cylindrical or tubular composite produced by the apparatus of Fig. 1 used a solenoid-type coil, a planar composite would use a flat "pancake" spiral coil. Fig. 2. Such a configuration would allow infiltration from one side of a flat, essentially two-dimensional preform, such as one with woven continuous fibers.
• Fig. 2 shows such an apparatus, with a furnace and coil adapted to make planar composites. Heating elements 62 within insulating walls 64 would keep the temperature of ceramic crucible 54 above the melting point of the metal 60. After placing metal 60 and a flat preform 52 into the crucible, refractory plug 50 would cap the crucible to prevent splashing. The flat, spiral discharge coil 56, embedded in refractory cement 58, would then be energized and propel liquid metal 60 into the preform.
• the composite produced by an apparatus of the type in Fig. 2 could be chilled from the side opposite the infiltration side (here, the refractory plug side) , which would lead to more rapid solidification of the matrix.
• refractory plug 50 could serve as a chill, and the preform would be positioned flush with the underside of the plug/chill prior to infiltration.
• the reduced exposure of the fibers to high melt temperatures would reduce possible fiber degradation, leading to improved composite microstructure and properties.
• a continuous casting version of the planar embodiment of Fig. 2 is also possible. As with the continuous version of the cylindrical embodiment, the ends of the insulating walls would be opened to permit entry of the preform and recovery of the finished composite. Pulsed treatment of the materials within the infiltration zone and the movement of the materials into and out of the infiltration zone would continuously cast a composite.
• silicon carbide particles were packed into a cylindrical cavity drilled into an aluminum slug. This was placed into a ceramic crucible and heated until the aluminum was molten. Since wetting between silicon carbide particles and molten aluminum is poor, the metal did not spontaneously infiltrate the particles. The crucible was then placed into the central cavity formed by a discharge coil. The capacitor banks discharged 3 kV, 9 times, into the coil, at which point mechanical problems caused a pause of several hours before another 8 discharges were carried out. The metal remained molten at all times.
• Micrographic analysis of the product showed that the particles in the composite had undergone substantial undesirable reaction with the metal because of the pause between the two groups of discharges. Nevertheless, the composite was substantially homogeneous, with only a few large pores scattered throughout. Given more refined reaction conditions, for example, adjusted melt temperature, discharge number, and discharge strength, it is believed that the large-scale porosity can be eliminated.
• substantially homogeneous composites can be made using electromagnetic body forces. No infiltration front was present within the composite produced by this embodiment and no entrained gas was evident. A substantially uniform product was produced.
• Preliminary work with this embodiment demonstrates that substantially homogeneous metal matrix composites may be produced in more rapid and economical fashion using electromagnetic body forces. These composites may either be cast from the crucible or continuously cast from an opening in the bottom of the crucible to produce ingots having homogeneously dispersed reinforcement particles. The ingots may be further processed into any desireable products.
• a 15 mm diameter bundle of carbon fibers held together with circumferential tows of carbon fibers and wrapped around a threaded steel rod were placed in the center of a crucible.
• the metal was liquefied.
• the crucible was then placed within the cavity of the discharge coil and subjected to multiple discharges. The electromagnetic body forces propelled the metal into the tows, infiltrating them.
• the metal matrix composite thus produced will have anisotropic properties, having its greatest strength lying parallel to the axis of the fibers within the composite.
• Composites with parallel reinforcement fibers can be cast into short lengths from the crucible, or continuously cast into longer rods.
• the body force is created by an induced magnetic field. The invention is not limited to such embodiments, however.
• a molten metal could be subjected to a separately applied electric field such as via electrodes immersed into the metal. If this occurred while the metal was within a magnetic field, the interacting fields would thus produce electromagnetic body forces that would propel the molten metal.

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• 1991
  • 1991-06-06 WO PCT/US1991/003994 patent/WO1992021458A1/en not_active Application Discontinuation
  • 1991-06-06 EP EP19910915508 patent/EP0587560A4/de not_active Withdrawn
  • 1991-06-06 JP JP3514422A patent/JPH06507347A/ja active Pending

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