EP3630388A1 - Electromagnetic modified metal casting process - Google Patents

Abstract

A process for the electromagnetic refining of light metals being cast is provided. The process includes applying a single phase weak stationary field to the metal with a low frequency induction coil during solidification.

Description

ELECTROMAGNETIC MODIFIED METAL CASTING PROCESS

This application claims the benefit of Provisional Application No. 62/510,472, filed May 24, 2017, the disclosure of which is herein incorporated by reference.

BACKGROUND

[0001] 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

[0002] 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.

[0003] 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.

[0004] Another way to enhance the microstructure is to add grain refiners to the metal prior to casting. The grain refiner 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.

[0005] 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. When the metal, confined in that manner, is subjected to a cooling effect by a suitable cooling agent, it then solidifies, following the form imposed by the field. Unlike the conventional casting process, solidification does not occur in contact with the walls of a mold, but outside of any contact with a solid material. Under those circumstances, 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. Advantageously, cold crucibles and/or contactless solidification systems can provide superior chemical cleanliness suitable for use with high purity metal testing and production. Of course, this requires a very high energy process. For example, 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. These process suffer from a drawback in that they are batch processes not generally suitable to large scale industrial applications.

[0006] In US Patent Number 3,985,179 (the disclosure of which is herein incorporated by reference), Goodrich et al. disclose a method using an electromagnetic casting apparatus wherein a ring-type inductor generates an electromagnetic field having a flux density which diminishes in intensity towards the top of the inductor to more efficiently control the shape of the molten metal within the inductor for use with light metals. In US Patent Number 4,004,631 (the disclosure of which is herein incorporated by reference) a cooling jacket was added. Goodrich uses this technology to control the shape of the solidifying metal by electromagnetic forces to reduce the wear on the refractory and hence increase their lifetime. When molten metal is fed to the inner peripheral area of the inductor, 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.

[0007] Using electromagnetic fields to control the shape of a billet or slab during the casting of aluminum alloys is shown in US 4,307,772 (the disclosure of which is herein incorporated by reference) and in US Re. 32,529 (the disclosure of which is herein incorporated by reference). Using electromagnetic levitation Hull et al. shows the possibility of horizontal casting of thin sheets as described in US Patent 4,741 ,383 (the disclosure of which is herein incorporated by reference). Each of these techniques use a high power alternating electromagnetic field to maintain the shape of the casting by induction of eddy currents and Lorentz forces.

[0008]

[0009] Electromagnetic stirring during solidification has been applied to direct chill (DC) casting of wrought alloys through the CREM (Casting, Refining, ElectroMagnetic) process, developed by Charles Vives in the late 1980's. In US Patent 4,530,404 (the disclosure of which is herein incorporated by reference), Vives discusses the effect of the electromagnetic field on the structure of the metal. Within this document Vives describes the effect of the alternating current frequency on the microstructure and the stirring effects of the liquid metal. Vives teaches that as the power input to the stirrer was increased, the subsequent grain size in the ingot was reduced. In some alloys a grain size smaller than that associated with the use of a grain-refining master alloy was obtained.

[0010] 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.

[0011] However, these publications all teach a high force approach, using multiple phase approaches and frequencies of at least 50 Hz at very high magnetic field strength. Therefore the investigations all focus on the direction of the EM field. The present disclosure contemplates a single phase relatively low variableforce and variable low frequency approach instead. Moreover, using micro-movements, resonance effects, increasing the kinetic energy, altering the critical radii of nucleation grains and the disturbance of the growing DAS structure instead of inducing a massive bulk flow with significant inductive heating has been found to provide unexpected benefits.

BRIEF DESCRIPTION

[0012] According to one embodiment, a process for the electromagnetic casting of metals is provided. The process 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.

[0013] It is contemplated that 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.

[0014] It 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. In certain embodiments the coil operates at a power of less than 500 amps or less than 250 amps or less than 0.8kA. For example, a single plate 30 turn coil operating at less than 500 amps with a 10OmTesla field may be suitable.

[0015] The present disclosure further contemplates the adjustment of power, current and/or frequency during the solidification process (optionally dependent on metal phase).

BRIEF DESCRIPTION OF THE DRAWINGS

[0016] FIGURE 1 is a schematic illustration of a first representative electromagnetic die casting configuration;

[0017] FIGURE 2 is a schematic illustration of a second representative electromagnetic die casting configuration;

[0018] FIGURE 3 is a schematic illustration of a third representative electromagnetic die casting configuration; [0019] FIGURE 4 is a top cross section view of the configuration of FIG. 3;

[0020] FIGURE 5 is a side cross section view of the configuration of FIG. 3;

[0021] FIGURE 6 is a schematic illustration of alourth representative electromagnetic die casting configuration;

[0022] FIGURE 7 is a perspective view of a round single coil;

[0023] FIGURE 8 is a top view of a round double coil:

[0024] FIGURE 9 is a schematic illustration of the pancake coil testing set up;

[0025] FIGURE 10 is a schematic illustration of the round coil testing set up;

[0026] FIGURE 11 is a schematic illustration of a representative EM vibration model;

[0027] FIGURE 12 is a schematic illustration of a representative EM continuous casting vibration model;

[0028] FIGURE 13 is a schematic illustration of a representative strong EM vibration model;

[0029] FIGURE 14 is a schematic illustration of a representative strong EM vibration continuous casting model; and

[0030] FIGURE 15 is a schematic illustration of a EM pressure model.

DETAILED DESCRIPTION

[0031] As used herein, the term "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.

[0032] As used herein, the term "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.

[0033] 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.

[0034] In one embodiment it has been found that using a single phase improves control of the induced Lorentz forces and eddy currents. In this manner, 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:

• Power: contains the current, voltage, power factor, capacitor

requirements, inductance and resistance of the coil, cooling

requirements, the coil shape and geometry and the shielding;

• 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; and

• Geometry: contains the crucible geometry and material, wall thickness, casting shape, coatings for controlled cooling (with higher or lower heat conductivity)

[0035] As a positive side effect the reduction of porosity, reduction of macro segregation, homogeneity of chemical composition, and refined microstructure can be expected.

[0036] 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. One commonly used aluminum alloy in cast houses is an aluminum-silicon alloy, which provides good fluidity, strength, ductility, good wear and corrosion resistance.

[0037] The present disclosure finds particular usefulness with aluminum alloys. Particularly, 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. In this regard, and recognizing that solid metal is a better conductor of EM energy, 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.

[0038] 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.

[0039] 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.

[0040] 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.

[0041] Electromagnetism is one of the four fundamental interactions that exist in nature. The focus of the electromagnetism is based on the interaction of particles with an electric charge. When a system is exposed to a change of electric current a magnetic flux is induced. The magnetic flux and the electric currents are proportional. This is defined by Amperes circuit law. [0042] To be able to define the reaction force that is within the magnetic flux Faraday's law can be used, which is defined as the curl of the electric field being equal to the rate of change of the magnetic field. The magnetic flux is dependent on the frequency and the amount of the applied current and the kind of induction device used. When we know the current density J and the magnetic flux B, the reaction force F can then be found by using the simple equation: FL=JXB. The force FL is called Lorentz force and is the volume force acting on the fluid.

[0043] Since the applied current changes with time, oscillating with the number of cycles given by the frequency, the Lorentz force also changes its direction twice as often as the frequency of the current.

[0044] 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. In general, 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.

[0045] 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. The secondary DAS are expected to be partially suppressed by the EM vibrations and the induced velocity gradients at the solidification front.

[0046] The concept of the Electromagnetic Modified Casting (EMC) and Electromagnetic Modified Refining (EMR) is not only implementing the use of EM stirring effects, but is based on a combined effect of induced bulk flow, electromechanical vibrations, electromagnetic pressure and the change of conductivity of aluminum during solidification. As solidified aluminum has a higher electrical conductivity, current will prefer to conduct along the solid fraction of the solidification front and the already solidified metal. The current then induces EM fields within the solid fraction of a solidifying metal. These fields will induce Lorentz forces and hereby vibrations at the solidification front, which induce a EM pressure to disturb and break dendrites (disconnected dendrites can act as a nucleation site), increasing the energy within growing grains forming dislocations and grain boundaries, generating nucleation sites, as a result reducing the grain size, the dendritic arm spacing (DAS) and suppressing the secondary dendritic arm spacing (SDAS). Additionally, detaching dendritic structures from the solidifying grains and diluting them within the melt might generate particles acting as nucleation sites/grains.

[0047] By using a single phase of an AC current a steady state condition can emerge for every frequency applied. Changing the phase and/or designing/optimizing the inductors can alter the vibration and flow patterns. The interaction with the solidifying and mushy metal during the solidification stage can be adjusted by increasing/decreasing the frequency and the current depending on the stage of the solidification within the casting. In this regard, a process wherein the frequency is (decreased) during metal solidification may be beneficial. Similarly, a process where current is (increased) during solidification may be beneficial.

[0048] 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.

[0049] The existing literature focusses on the reduction of solidification time to reduce the DAS and the SDAS. A fine dendritic structure and a reduced DAS are leading to a shorter periodicity of micro segregation. There is convincing evidence that ultrasonic vibrations and electromagnetic vibrations alter the DAS and the SDAS towards a finer structure,

[0050] 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.

[0051] 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.

[0052] 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. As the flux penetrates it interacts with the conductive medium inducing a counteracting current (resistance force), which in return induces a Lorentz force, generating a drag force. Remembering the nature of EM flux being always a closed loop, for every phase, the Lorentz force is induced twice in alternating directions. The electromotive forces initiate a velocity and vibrations within the metal.

[0053] In one embodiment, 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. Once solidified, 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.

[0054] Due to the lack of space around the castings, safety issues with water in proximity to liquid aluminum and to be able to change the casting, while using the same coil, a coil geometry with a magnetic flux going "top-down" or "bottom-up" are viable options. A coil in the lid of the casting table is another viable example where the coil geometry and shielding are restricted in space but close to the casting. [0055] Turning now to detailed coil design considerations, it should be remembered that parallel wires, carrying the same current in the same phase, such as occurring in pancake coils generate strong attraction to each other, while generating a magnetic flux perpendicular to the pancake shape, depending on the phase and the geometry. However, although pancake shape is referenced in this paragraph, the present disclosure also contemplates round coils, plate coils, and solenoidal coils, for example.

[0056] In perhaps the simplest configuration of the disclosure, 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.

[0057] Alternatively, by turning a direct current (DC) applied to a solenoid or an inductor of any geometry on and off it generates a magnetic field during the change of current, while the inductor is charged, similar to an AC simulation. This is also known as a pulse width modulation (PWM). As one example, 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.

[0058] Regardless of the source of the EM field, as articulated previously, 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. Moreover, it may be desirable that less than 10% or 5% or 1 % or V % of power supplied to the elements of the system, depending on the applied frequency, and used to generate inductive forces be converted to heat when the system is being powered to refine the grain structure in the solidifying casting.

[0059] Several conceptual ideas of the present disclosure are shown in FIGURES 1- 4 for AC and DC systems. [0060] In 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. In Figure 2, the DC coil is replaced with a pair of DC Helmholtz coils.

[0061] In Figures 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. In certain instances, it may be desirable to provide a sleeve, e.g. (S1O2), around the individual coils.

[0062] 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.

[0063] EXAMPLES

[0064] A round single coil with S1O2 sleeves as shown in Figure 7 was used to investigate the weak linear EM fields during the solidification of molten aluminum. Figure 7 illustrates a round single coil with 16 turns. In certain testing, 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.

[0065] Electrical insulation and physical integrity of the coil was provided by glass fiber sleeves (S1O2) and high-temperature glass fiber tape. Utilization of fiber tape, epoxy resin, Si-polymer rubber, or other supportive materials are also contemplated. The coils were water cooled (the disclosure also contemplates glycol, two phase oils and fog coating or alternatives for cooling) to remove the heat from eddy current, resistive heat of the coil and the radiation heat from the hot metal.

[0066] Exemplary dimensions of the single, double and pancake coils are provided in the following Table.

[0067]

TABLE

[0068] 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.

[0069] When the required temperature was reached 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.

[0070] 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.

[0071] Experiments have demonstrated that a 7-turn pancake coil was able to generate a strong magnetic field in the center of a casting with an EM flux generating vibrations and inducing a minor flow field sufficient to stir the metal below the coil. The peak velocity in the sample was calculated to be 3.4 cm/s at the wall region. There was a curl in the upper section accelerating the liquid aluminum with approx. 2.5 cm/s towards the wall. The curl in the center was moving from the bottom upwards. The used frequency of 50 Hz allowed a penetration depth of approx. 30 mm such that Lorentz forces extended the full depth of the crucible. The magnetic flux density to drive the velocity was modelled to have a peak value of ~11 mT at the metal surface within the crucible. [0072] Various shielding materials were evaluated with respect to flux on the surface of

[0073] 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.

[0074] The effect of the shielding material on the magnetic flux strength at the center of the metal crucible (z axis following the arrow pointing down) is shown in the graph below. The frequency of 50 Hz allowed for a penetration depth of approximately 30mm, evidencing that Lorentz forces can be created within the first three penetration depths and at a frothing at full depth of 100mm of the crucible.

[0075] The graph shows the z-axis distribution of the magnetic flux density at the center (r=o) of the metal in the crucible, as shown in the small sketch at the right side of the image, indicated by the arrow pointing down reflecting the center of the crucible magnetic flux intensity using the pancake coil setup. The different lines represent different material characteristics and the effect of the magnetic shield on the EM flux density at the z-axis.

[0076] Shielding was found to provide a beneficial increase in flux. Silicon steel, such as the GO (Grain Oriented) 3wt% Si, laminated or (FM-B) Si-Fe gave almost 250% more flux at the center of the crucible. The soft magnetic ferritic iron shield gave an extra field strength of 180%, with the remaining flux being dissipated via inductive heating of the shielding metal.

[0077] 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.

[0078] According to a further set of experiments to evaluate the flux impact on grain structure, a single coil (see Figure 7) providing a peak magnetic flux density of 15 mT and a double round coil (see Figure 8) providing a peak magnetic flux density of 28mT, when electrified with 100 A, were evaluated in the configuration of Figure 10. The pancake coil was electrically tested. The electrical data is shown in Table 2.

[0079] Table 2: Electrical data taken with the variable power supply at 100 A.

[0080] Each of the single and double round coils were tested in association with casting pure aluminum that was heated to 800 C, removed from the furnace and solidified in a sand bed.

[0081 ] 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.

[0082] 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. In addition, 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.

[0083] 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.

[0084] The testing suggests that a 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. Generally speaking, 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. Broadly speaking, for a typical casting such as an engine block a variable magnetic field in the range of 15mT to 1T, for example 200mT, may be beneficial and for a casting such as a wheel a variable magnetic field in the range of 5mT to 250mT, for example as 60mT may be beneficial.

[0085] 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.

[0086] When looking at one single induction system and when reducing the frequency, 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.

[0087] 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.

[0088] Referring now to Figure 11 , an EM vibration casting model is depicted. In this configuration, 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. This embodiment provides the advantages of larg applied geometries as the coils can be at very low frequency or DC, giving maximum penetration depth (B) while the AC currents are directly applied to the casting (J), generating Lorentz forces by the formula FL = JxB at the area where most current passes. 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. This will induce Lorentz forces within the growing DAS and SDAS, but also at the interface of the solidification front. With continuing reference to Figure 12, a similar configuration can be used in association with a continuous casting process such as employed with wire, rod and/or thixo castings. [0089] Referring now to Figure 13, a strong EM vibration model is depicted. In this configuration, a DC/low frequency AC coil is placed adjacent the top and bottom surfaces of the casting and an AC/DC coil placed intermediate the DC/low frequency AC coil(s). This model provides similar advantages as the design of Figures 11 and 12. It gives the additional advantage of a contactless induction of the AC field, further reducing the wear and/or interaction of the current connectors. With continuing reference to Figure 14, 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. It should be noted that although a set of three coils is illustrated, 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. In fact, it is noted that any of the coil configurations disclosed herein are believed suitable for both single mold and continuous casting processes.

[0090] Turning now to Figure 15, an EM pressure model is depicted. Particularly, 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.

[0091 ] The exemplary embodiment has been described with reference to the preferred embodiments. Obviously, modifications and alterations will occur to others upon reading and understanding the preceding detailed description. It is intended that the exemplary embodiment be construed as including all such modifications and alterations insofar as they come within the scope of the appended claims or the equivalents thereof.

Claims

CLAIMS:

1. A process for the electromagnetic refining of metals be cast wherein an electromagnetic confinement field acts on molten metal in the course of solidification, the process including applying a single phase magnetic field to the metal, said field being applied by a low frequency induction coil placed at only one side of the metal.

2. The process of claim 1 wherein said low frequency comprises 0.1-120 Hz.

3. The process of claim 2 wherein said frequency is quasi-sinusoidal.

4. The process of claim 1 wherein said low frequency comprises pulsed DC.

5. The process of claim 1 wherein the coil is shaped and positioned such that the associated electromagnetic field can penetrate and induce a current in all sections of the casting.

6. The process of claim 1 wherein said field satisfies at least one of less than 2 Tesla and 6-60 Hz.

7. The process of claim 1 wherein said coil operates at a power of less than 800 amps.

8. A process for electromagnetic casting of light metals, wherein an electromagnetic field acts on a molten metal in the course of solidification, said electromagnetic field being provided by an induction coil, wherein the induction coil provides a field of less than about 2 Tesla during the solidification.

9. The process of claim 8, wherein an applied current is an alternating current of single-phase.

10. The process of claim 8, wherein the induction coil is a single layer coil or a multi-layer coil.

11. The process of claim 8, wherein the frequency and/or power and/or current is modified during the process of solidification.

12. The process of claim 8, wherein the applied frequencies comprise 0.1-120 Hz or even 240Hz.

13. The process of claim 8, wherein the coil is shaped and positioned such that the associated electromagnetic field can penetrate and induce a current in all sections of the casting within the first two penetration depths for the lowest applied frequency.

14. The process of claim 6, wherein the coil operates at current of less than

800 A.

15. The process of claim 6, wherein a surface velocity of the molten metal being solidified is between about 0 and 12 cm/s during at least a portion of the process.

16. The process of claim 8 being a continuous casting.

17. The process of claim 8 wherein the light metal being cast comprises an alloy and wherein the electromagnetic field is maintained but reduced during the solidification process.

18. The method of claim 8 wherein the electromagnetic field is at least substantially stationary.

19. The method of claim 8 wherein the electromagnetic field is sinusoidal or quasi-sinusoidal.

20. The method of claim 8 wherein the electromagnetic field is pulsed DC.

21. The method of claim 8 wherein an inverter is used.

22. The method of claim 8 wherein a power supplied to a system creating the electromagnetic field induces less than a 10% increase in a temperature of the metal being cast.

23. The method of claim 8 wherein frequency to the coil is maintained but decreased during solidification.

24. The method of claim 8 wherein current to the coil is increased during solidification.