Classifications

• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B22—CASTING; POWDER METALLURGY
  • B22D—CASTING OF METALS; CASTING OF OTHER SUBSTANCES BY THE SAME PROCESSES OR DEVICES
  • B22D27/00—Treating the metal in the mould while it is molten or ductile ; Pressure or vacuum casting
  • B22D27/02—Use of electric or magnetic effects
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B22—CASTING; POWDER METALLURGY
  • B22D—CASTING OF METALS; CASTING OF OTHER SUBSTANCES BY THE SAME PROCESSES OR DEVICES
  • B22D11/00—Continuous casting of metals, i.e. casting in indefinite lengths
  • B22D11/001—Continuous casting of metals, i.e. casting in indefinite lengths of specific alloys
  • B22D11/003—Aluminium alloys
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B22—CASTING; POWDER METALLURGY
  • B22D—CASTING OF METALS; CASTING OF OTHER SUBSTANCES BY THE SAME PROCESSES OR DEVICES
  • B22D11/00—Continuous casting of metals, i.e. casting in indefinite lengths
  • B22D11/10—Supplying or treating molten metal
  • B22D11/11—Treating the molten metal
  • B22D11/114—Treating the molten metal by using agitating or vibrating means
  • B22D11/115—Treating the molten metal by using agitating or vibrating means by using magnetic fields
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B22—CASTING; POWDER METALLURGY
  • B22D—CASTING OF METALS; CASTING OF OTHER SUBSTANCES BY THE SAME PROCESSES OR DEVICES
  • B22D11/00—Continuous casting of metals, i.e. casting in indefinite lengths
  • B22D11/16—Controlling or regulating processes or operations

Definitions

• the present exemplary embodiment relates to a method of refining the microstructure of metal castings, such as those formed of light metals including aluminum, magnesium and titanium and their alloys.
• the electromagnetic casting process described in detail herein is primarily designed for castings containing light metals
• Casting of metal is one of the oldest manufacturing processes, where liquid metal is poured into a mold to produce parts.
• Traditional casting involves the pouring of metal into a permanent or non-permanent mold, including runners or gating systems and risers allowing for sufficient pressure such that trapped gas escapes and the liquid metal completely fills the mold.
• the microstructure and the physical properties of the molded metal can be influenced during the solidification process using various treatments.
• a widespread practice is to chill the casting mold with a cooling system, e.g. Direct Chill or DC casting, permanent active/passive cooled molds or others to remove the thermal energy from the mold and increase the speed of the solidification process.
• the speed of the solidification affects the microstructure by increasing the crystallization speed which restricts the time for grains to grow, thereby generating a finer microstructure with better physical properties.
• grain refiners can act as a nucleation grain, increasing the number of nuclei and forming a larger number of crystals during solidification which have less space to grow. In this manner a finer grain structure can be achieved in the finished casting. Unfortunately, grain refiners can be detrimental in certain applications.
• Casting technology has also contemplated the use of an electromagnetic field to contain a body of metal being cast. It is known from French Patent No. 1 509 962, herein incorporated by reference, that steel or aluminum ingots can be produced by electromagnetic casting.
• the procedure disclosed comprises generating an alternating electromagnetic field around a column of metal in a molten condition, by means of an angular inductor.
• the magnetic field provides a means of inducing electromagnetic pressure within the primary casting area to prevent the molten metal from spreading and thus impart a certain geometry to the metal.
• a suitable cooling agent it then solidifies, following the form imposed by the field.
• the articles produced are generally ingots which have a better surface condition and which, in some cases, may be used directly in dimensional transformation operations without the need to have recourse to particular surface treatments, such as for example a scalping operation.
• cold crucibles and/or contactless solidification systems can provide superior chemical cleanliness suitable for use with high purity metal testing and production.
• this requires a very high energy process.
• the liquid metal is held in a confined condition by applying an electromagnetic field which is generated by means of an annular inductor supplied with an alternating current at a frequency which is generally between 500 and 5000 Hertz.
• the interaction of the electromagnetic field with the eddy currents induced in the molten metal generates the electromagnetic forces, which control the cross-sectional shape of the solidifying metal to the same general shape as the inductor.
• the radial force components generated by the electromagnetic field prevent any significant lateral movement of molten metal and thus allow for no contact between the molten metal and the inductor.
• Radjai et al. used DC magnets and AC currents to successfully refine Mg, Al and grey iron.
• Mizutani et al. has employed a similar method and for grain refinement of Al alloys and bulk metallic glasses.
• Greenwich University and Valdis Bojarevics (2015- 2016) papers discuss ultrasonic refinement by EM vibration using high frequency and immersed coils.
• a process for the electromagnetic casting of metals employs an electromagnetic confinement field on the molten metal in the course of solidification.
• the process further includes applying a single phase stationary field to the metal, wherein the field is applied by a low frequency induction coil placed at only one or two sides of the metal.
• the low frequency induction coil will operate in about the range of 0.1-240 Hz or 0.1-120Hz. It is further contemplated that the process could use a coil having a vertical axis is aligned with a vertical orientation of an associated casting table. It may be desirable to provide a coil that is shaped and positioned such that the associated electromagnetic field can penetrate and induce a current in all sections of the casting. It may be desirable to use only a single coil.
• the field may also be desirable that the field satisfy at least one of (a) less than 2 Tesia or less than 1 Tesia or less than 0.5 Tesia and (b) 6-60 Hz.
• the coil operates at a power of less than 500 amps or less than 250 amps or less than 0.8kA.
• a single plate 30 turn coil operating at less than 500 amps with a 10OmTesla field may be suitable.
• the present disclosure further contemplates the adjustment of power, current and/or frequency during the solidification process (optionally dependent on metal phase).
• FIGURE 1 is a schematic illustration of a first representative electromagnetic die casting configuration
• FIGURE 2 is a schematic illustration of a second representative electromagnetic die casting configuration
• FIGURE 3 is a schematic illustration of a third representative electromagnetic die casting configuration
• FIGURE 4 is a top cross section view of the configuration of FIG. 3;
• FIGURE 5 is a side cross section view of the configuration of FIG. 3;
• FIGURE 6 is a schematic illustration of alourth representative electromagnetic die casting configuration
• FIGURE 7 is a perspective view of a round single coil
• FIGURE 8 is a top view of a round double coil:
• FIGURE 9 is a schematic illustration of the pancake coil testing set up
• FIGURE 10 is a schematic illustration of the round coil testing set up
• FIGURE 11 is a schematic illustration of a representative EM vibration model
• FIGURE 12 is a schematic illustration of a representative EM continuous casting vibration model
• FIGURE 13 is a schematic illustration of a representative strong EM vibration model
• FIGURE 14 is a schematic illustration of a representative strong EM vibration continuous casting model
• FIGURE 15 is a schematic illustration of a EM pressure model.
• electromagnetic solidification refers to the solidification of a metal or metal alloy at or below the solidification temperature during the exposure to an alternating or static magnetic field.
• electromagnetic refining refers to the effect of the electromagnetic field on the solidification process, by introducing kinetic and thermal energy to refine the microstructure of the casting.
• One goal of the present disclosure is application of an electromagnetic field during casting and solidification of aluminum to refine the microstructure with the direct increase of the mechanical stability of the casting. To achieve this, the relevant influencing variables and effects of independent input variables have been evaluated.
• the process does not induce significant velocity in the metal (limited stirring).
• the single phase allows EM flux at a portion/volume of interest without having huge coil packs, traveling magnetic fields and strong flow fields to manage. It also allows the design of coils, molds and solidification rates by a variable frequency, giving high current densities at the solidification front by the variable frequency and giving high current densities at the solidification front without significant mixing. Variability is advantageous because the electrical conductivity of solid and liquid aluminum is significantly different.
• Important design parameters of the present disclosure include power, field and geometry. As there is evidence that velocity or vibrations influence the microstructure of the solidifying metal, the parameters that alter this influence are separated into 3 groups:
• Field contains the frequency, the phase architecture, the position of the coil, the crucible geometry and material, the used alloy, the coil shape and geometry and the shielding;
• Geometry contains the crucible geometry and material, wall thickness, casting shape, coatings for controlled cooling (with higher or lower heat conductivity)
• Aluminum and its alloys are the prime choice for the manufacture of automobile parts. Different manufacturers are using various casting processes to produce the casts, e.g. engine blocks and cylinder heads. The most popular processes are die casting, precision sand casting, lost foam casting, and investment casting. The castings can be conducted as direct chill casting (DC), pressurized casting (counter pressure casting PCP as low and high pressure casting) and modified casting.
• DC direct chill casting
• PCP counter pressure casting
• modified casting One commonly used aluminum alloy in cast houses is an aluminum-silicon alloy, which provides good fluidity, strength, ductility, good wear and corrosion resistance.
• the present disclosure finds particular usefulness with aluminum alloys.
• aluminum alloys demonstrate a solidification process wherein a "mushy" zone occurs along the solidification front.
• the low power single phase system of the present disclosure creates just enough EM flux to cause localized vibration and breaking of large dendrites along the solidification front.
• the power to the inductor coil can be reduced when metal at any location in the casting apparatus' reaches its solidification temperature (e.g. between 550 ° C and 660 ° C for aluminum alloy). It is further contemplated that the power to the coil will be substantially continuously reduced commensurate to the quantity of solidified metal in the casting.
• the fatigue resistance and the reliability of aluminum alloys are directly affected by the casting process.
• the defects altering the fatigue are first pores, due to shrinkage or gas, and second exogenous inclusions and/or second phase particles, such as intermetallic inclusions and precipitates.
• the distribution and the size of these obstacles/imperfections/precipitates are introduced by the chemical composition and the geometry of the casting, wherein traditional casting modifies these effects by altering the solidification rate and the casting pressure.
• Fatigue cracks nucleate and grow from existing defects.
• Microstructural refining of the castings, such as secondary dendrite arm spacing is traditionally dependent on the heat transfer rates within the metal and within the mold during the solidification stage.
• Microstructural refining can also occur by external forces.
• the role of fluid flow during the solidification of aluminum is a complex and important topic. Both, the micro- and macrostructure and micro- and macro segregation are affected.
• the source of the fluid flow can be either natural or forced convection. Natural convection is driven by variations in density and thermal energy occurring during the solidification process due to differences in temperature and/or chemical composition, whilst the forced convection can arise from mechanical or electromagnetic stirring.
• the fluid flow can occur in both the bulk liquid and the liquid portion of the semi solid metal.
• EM stirring can be classified in linear induction machine (LM) stirring where 2 to 3 phases of current induce a traveling magnetic field generating a strong directed propulsion within the liquid.
• a weak stirring occurs if a single-phase current is applied to an inductor in proximity to the liquid metal, allowing the flux to travel through the liquid metal.
• the magnetic flux still induces a predominant flow field circulating the metal within the affected area, with lower velocities primarily generating strong electromechanical vibrations.
• the power of the EM flux in the affected area should be less than a level which would create bulk flow via drag forces in the metal casting.
• the effect of the electromagnetic field may be most efficient if applied during the early stage of solidification. Without being bound by theory, it is believed that additional kinetic energy in the melt helps nucleation by decreasing critical radius for nucleation, leading to higher nucleation rate and grain refinement, but also disrupts the growing dendrites forming the dendritic arm spacing structural (DAS) growth during solidification. This generates additional nucleation grains within the proximity of the solidification front.
• DAS dendritic arm spacing structural
• EMC Electromagnetic Modified Casting
• EMR Electromagnetic Modified Refining
• the conductivity is strongly dependent on the state of a metal.
• the conductivity of liquid aluminum is, depending on the purity of the used metal and the amount of alloying elements, approximately 30% lower than of solid aluminum. In other words, the current will choose the path of least resistance and conduct mainly in the solidified sections of the aluminum.
• Pure Al has a 25 ° C conductivity of 36.9 micro Si/m, the conductivity decreases to 9.4 micro Si/m at 650 ° C and when liquid it reduces drastically to 4.1 micro Si/m. This is a conductivity of approximately 44% relative to when solid.
• the electromotive force induced by the electromagnetic flux can be experienced as strong vibrations and a velocity gradient.
• the electromotive forces generate a drag by predominating in one direction, depending on the inductor geometry and the phase of the electric current.
• the penetration depth defined by the applied frequency of polarization of the current, defines the depth the EM flux interacts within the metal, inducing vibrations and velocity within the volume.
• the present disclosure contemplates an AC field design generally providing a max B field of 1-2 Tesla.
• a connector for the AC current can be selected from different conductive metal types such as steel or copper.
• the design may include designated mold sections for the AC current supply and/or a copper finger as connectors. It is contemplated that a single or double plate inductor coil with a single phase AC current would be employed.
• An exemplary current can be in the range of about 400A.
• An exemplary DC field can be on the order of about 0.5 Tesla.
• EMC can be achieved by using an induction device which uses alternating current (AC) EM flux which penetrates into the liquid metal.
• the penetration depth can be adjusted by changing the applied frequency and the field strength of the current applied.
• resistive force alternating current
• Lorentz force is induced twice in alternating directions.
• the electromotive forces initiate a velocity and vibrations within the metal.
• the coil is advantageously positioned in order to generate Lorentz forces, which act to cause the solidifying metal, that change with the applied frequency and current, enhancing the refining action.
• the microstructure of the casting is refined depending on the physical properties applied to the coil. The casting requires less post treatment after electromagnetic casting, generating a clear economic advantage.
• a low frequency induction coil (0.1-120 Hz) is placed at one side of a casting with its axis aligned in the vertical direction of the casting crucible.
• the coil can be shaped and positioned in such a way that the EM field can penetrate and induce a current in all sections of the casting when using the lowest contemplated frequency for the solidification process.
• DC direct current
• PWM pulse width modulation
• DC coils could be used with a pulsed current from a Ac/Ac inverter or a DC/AC inverter applicable for single phase use, but also for example by a standard DC drive which can be pulsed at a controllable rate. This imposes a frequency of changing current, while the polarization remains always the same direction, hence the coil experiences a DC current, generating magnetic flux during the electrification.
• one feature of the present disclosure is the exposure of the casting during solidification to a relatively low power. This has in places in this disclosure been expressed as a system which does not create bulk flow in the molten metal being solidified.
• An alternative effect of the desired low power EM grain refinement process is only limited, if any, addition of induced heat to the metal casting during solidification.
• FIGURES 1- 4 Several conceptual ideas of the present disclosure are shown in FIGURES 1- 4 for AC and DC systems.
• Figure 1 a schematic illustration of a DC EM die casting apparatus is depicted using AC current applied directly through the metal.
• the casting metal which can be contained in any manner known in the art, such as a sand mold, is bound radially by a DC coil (a pancake inductor coil on one or two sides is also viable).
• An AC connection is provided at each end of the mold.
• the DC coil is replaced with a pair of DC Helmholtz coils.
• FIG. 3-5 a schematic illustration of a single AC plate inductor coil positioned at one end of the casting mold is shown.
• the 12-turn pancake coil shaped of, for example, rectangular 6.5 mm copper tubes with 1 mm walls and a 14 cm 0.
• a magnetic shield surrounding the coil can be provided.
• FIG. 6 A similar concept is shown in Figure 6 wherein a pair of AC plate inductor coils are offset relative to one another within a soft magnetic iron core shield. This provides an AC field with two phases (two coils on a lamination leg). The coils are overlapping on two legs each, with the center leg being surrounded by both coils. This generates a traveling magnetic wave when the phases are applied.
• This concept provides two different AC phases with a low frequency inducing minor velocity stream, combining the grain refining resulting from electromagnetic forces with a minor stirring action (less than bulk flow) decreasing the segregating of the applied casting alloy during solidification.
• FIG. 7 illustrates a round single coil with 16 turns.
• a stronger EM field was evaluated using a double round coil (31 turns) with SiOa sleeves as shown in Figure 8.
• the relative positioning of a pancake coil (e.g. Figures 4 and 5) to the casting mold is illustrated in Figure 9.
• Figure 10 provides a schematic illustration of the testing setup used with the double round coil (shown) and the single round coil.
• the outer radius of the crucible was 10 cm at the top diameter with a capacity of approx. 0.8 I.
• the metal was heated and melted in a resistance furnace using an air atmosphere. The solidification was not forced, the crucibles were preheated.
• the metal temperature was set to be 800 ° C in the resistance furnace and was measured before electrifying the coils.
• the crucible was placed on a preheated sand bed.
• the trials with the round coils allowed the crucible to be placed within the coil, while the pancake coil was placed on top of the crucible.
• the coils were powered by a 50 Hz single-phase variable power supply giving up to 400 A at 40 V. Each coil was evaluated three times using pure aluminum.
• the graph shows the r distribution of the magnetic flux density at the surface of the metal level of the crucible, as shown in the small sketch at the right side of the image, following the arrow pointing to the right.
• the different lines represent different material characteristics and the effect of the magnetic shield on the EM flux density.
• the different lines represent different material characteristics and the effect of the magnetic shield on the EM flux density at the z-axis.
• the peak velocity in the experiments has been calculated to be 5.9 cm/s at the wall region. There is a curl in the upper section accelerating the liquid aluminum with approx. 4.5 cm/s towards the wall. The curl in the center is moving from bottom upwards, with a similar velocity. In steady state, the velocity within the crucible homogenizes to two curls opposing each other, inducing a downward flow in a center and an upward flow in the wall regions.
• the magnetic flux density to drive the velocity has its peak value of -22 mT at the metal surface of the crucible.
• Table 2 Electrical data taken with the variable power supply at 100 A.
• the resultant castings were sectioned, polished, etched and visually inspected.
• the aluminum grains of an untreated sample can be spotted with the naked eye, as the grains are several mm in diameter.
• the solidification front initiated from the outer shell towards the center of the metal sample, following the temperature gradients.
• a dendritic growth structure was visible in the gas cavity (resulting from shrinkage and different solubility of hydrogen and other gases in the aluminum alloy) in the bottom of the sample.
• the 3D structure of the DAS was observed, revealing symmetrical pyramids growing into the hollow space.
• the visual inspections after etching revealed that the experimental protocol changed the cast aluminum microstructure by applying a weak field to the aluminum during solidification.
• the round single coil was electrified with 100 A, with 0.48kVar giving a maximum alternating magnetic field of 15mT at the metal crucible interface, exponentially decaying towards the center of the crucible.
• the grain structure changes and the normal solidification structure of a pure aluminum alloy without EM treatment were eliminated.
• the DAS were interrupted, but still grew, while smaller grains were observed.
• the gas cavity became smaller and was moved from the bottom to the center of the metal sample, which can be correlated to a weak induced velocity field.
• the surface of the cavity was smoother and the 3D-structures of the DAS are smaller compared to a comparative sample without weak EM treatment.
• the strongest magnetic field tested was generated by the double coil.
• the round double coil was electrified after the 800 C hot metal filled crucible was placed within the coil.
• the excitation current was 100 A, with 1.1 kVar, giving a maximum alternating magnetic field of 28 mT at the metal crucible interface.
• Visual inspection of the cast aluminum demonstrated a refined grain structure, particularly at the lower section of the casting.
• the gas cavity moved from the bottom to the top of the metal casting, connecting with the surface and allowing the gas to be removed during the solidification and shrinking.
• variable magnetic field generated by a single phase AC induction coil in the range of 15 to 28 mT for the 0.8L size crucible is highly beneficial. Moreover, it appears that the penetration depth is sufficient to allow the EM vibrations to contribute to the solidification structure.
• the magnetic field in Tesla represents the relative interaction possible between the metal being cast and the applied EM field, while the frequency being applied represents the volume of the metal being cast which is affected.
• a variable magnetic field in the range of 15mT to 1T for example 200mT
• a variable magnetic field in the range of 5mT to 250mT for example as 60mT may be beneficial.
• the present experiments further demonstrate that a minor induced velocity is beneficial and allows the metal to solidify without lowering the solidification temperature significantly.
• a velocity within the casting of an 800 C. and below aluminum alloy in the range of between about greater than 0 and 12 cm/s may be desirable depending on the geometry and the alloy used.
• the single coil demonstrated a maximal velocity of approximately 4 cm/s at the crucible surface.
• the double coil provided a crucible surface velocity in the range of 8 to 10 cm/s.
• the magnetic field strength will also reduce, as there is less change of current per time. Nonetheless, the EM penetration depth will increase and thereby increase the volume the interaction of the magnetic flux and the metal can take place Hence, a lower frequency increases the volume, while it reduces the force distribution over this volume.
• One interesting aspect of the disclosure is that during solidification there is variation of conductivity, changes in penetration depth with current flowing the path of lowest resistance, and changes in the generation of eddy currents. Without being bound by theory, it is believed that by varying the power and/or frequency throughout solidification to focus the energy in the region of interest, EM interaction can support the refining of the microstructure without bulk flow.
• the present disclosure contemplates several EM casting configurations.
• the systems disclosed are intended to provide EM vibration alone or in combination with EM pressure velocity.
• the systems may have the ability to provide varied frequency throughout the solidification process such that the frequency and/or intensity is modified based on the metal conductivity and alloy composition. This allows the system to be tailored to induce EM vibrations and EM pressure primarily at the solidification front.
• This disclosure further contemplates the combined use of AC and DC currents applied on different coils to provide Lorentz forces at the region(s) of interest within the casting during solidification, such as the moving solidification front.
• FIG. 11 an EM vibration casting model is depicted.
• AC/DC coils are provided at the top and bottom regions of the casting.
• Current outlets are provided in the top surface of the casting and a current inlet at the bottom surface of the casting.
• the current will prefer the solid fraction of the metal, same as the magnetic field, due the differences in conductivity within the metal from liquid to solid.
• an extrusion, wire, rod, of wire and/or rods at a relatively high production rate as the grain refinement stage will be contactless and therefore without wear.
• the commercial embodiment may include several more (e.g. up to lOcoils or more) and/or thixo casting can be formed using a similar arrangement making the design suitable for production.
• any of the coil configurations disclosed herein are believed suitable for both single mold and continuous casting processes.
• FIG. 15 an EM pressure model is depicted.
• a counter pressurized mold is employed in combination with a first DC or low frequency AC coil and an AC coil.
• This is an illustration of a standard CPC (Counter Pressure Casting) or PCP (Pressurized Casting Process) used for car rims, pistons, semi-forged high quality parts, or other high quality demanding products.
• the pressure is usually created by a mechanical pump within the metal bath and/or by a vacuum within the mold (counter pressure casting).
• the induction coils could enhance the grain structure, macro- segregations and homogeneity while the pressurized casting is further reducing the shrinkage and the porosity.

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• Engineering & Computer Science (AREA)
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• Continuous Casting (AREA)
• Manufacture And Refinement Of Metals (AREA)
• General Induction Heating (AREA)

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• 2018
  • 2018-05-24 RU RU2019141258A patent/RU2019141258A/ru not_active Application Discontinuation
  • 2018-05-24 JP JP2019564943A patent/JP2020521637A/ja active Pending
  • 2018-05-24 US US16/616,648 patent/US20210162491A1/en not_active Abandoned
  • 2018-05-24 WO PCT/US2018/034389 patent/WO2018218022A1/en unknown
  • 2018-05-24 CN CN201880049580.XA patent/CN110944769A/zh active Pending
  • 2018-05-24 KR KR1020197037625A patent/KR20200000848A/ko not_active Application Discontinuation
  • 2018-05-24 CA CA3064757A patent/CA3064757A1/en active Pending
  • 2018-05-24 EP EP18806563.5A patent/EP3630388A4/en not_active Withdrawn

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