WO2011058568A1

WO2011058568A1 - Treating and stirring metal parts cast in non-conductive mold

Info

Publication number: WO2011058568A1
Application number: PCT/IL2010/000954
Authority: WO
Inventors: Ran Rosen; Ginat R. Muginstein
Original assignee: Netanya Plasmatec Ltd.
Priority date: 2009-11-16
Filing date: 2010-11-16
Publication date: 2011-05-19

Abstract

Apparatus and process for improving cast metals and alloys quality in desired locations. At least one first polarity electrode and at least one second polarity electrode are placed in a metallic cast situated in a non conductive mold, the electrodes capable of inducing changing electric fields within a cast, that may cause metal flow and stirring of the molten metal in desired areas, typically close to the first polarity electrodes. Stirring of molten metal in desired areas of a cast during solidification may improve the microstructure and porosity level in the treated areas, thus may improve mechanical properties of the cast part, such as elongation, fatigue and strength.

Description

TREATING AND STIRRING METAL PARTS CAST IN

NON-CONDUCTIVE MOLD

FIELD OF THE INVENTION

[001] This invention relates to the field of casting shaped parts in nonconductive molds such as sand mold, investment casting and in particular lost wax and lost foam casting. The invention also relates to the process of metal stirring by electric field and magnetic field during the solidification stage of the cast.

BACKGROUND OF THE INVENTION

[002] Casting shaped parts in nonconductive (non-metallic) molds, such as sand molds and lost foam casting have several disadvantages; high amount of porosity, porosity size and coarse microstructure, both are the result of long solidification time, due to the poor heat conductivity of the nonconductive mold. However, both casting techniques have many advantages; molds are cheap and simple to construct, and both are suitable for ferrous and non ferrous alloys. In addition lost foam casting provides unlimited casting geometry to the design engineers, unlike Gravity Die Casting and sand casting. Those characteristics make lost foam casting very appealing to the automotive industry, especially for casting complicated parts such as cylinder heads. It allows the engine design engineers to construct the cylinder head geometry based only on efficiency and environmental considerations and not on the casting technology limitations.

[003] The lost foam pattern, typically made of polystyrene, can be shaped into substantially any desirable geometry. The lost foam pattern may be coated with refractory coating embedded in a loose sand mold. Molten metal, usually Al-Si alloy, ductile iron or steel, is poured and evaporates the pattern so that the metal replaces the polystyrene pattern. This technology creates major quality problems, such as high porosity size and ratio and coarse microstructure of the part. High porosity size reduces the fatigue resistance and high porosity ratio reduces the elongation of the part. Good fatigue resistance and elongation ability are critical for high performance automotive parts especially cylinder heads, turbocharger, suspension parts, wheels etc.

[004] There is a major effort among lost foam foundries to overcome the porosity problems. However casting, for example, diesel engine cylinder heads having porosity of less than 0.5 mm porosity size, 1% porosity ratio and elongation higher than 3%, still remains a challenge.

[005] In US 4,448,235 Bishop presents a two-layer pattern coating; one with low permeability and the other with high permeability. The two-layer pattern coating is aimed at increasing the metal's filling and gas evaporation rates, and as a result reduce porosity levels. In US 4,520,858 Ryntz et al. suggest attaching chillers in desired areas to the fugitive pattern prior to embedding it in the loose sand. The chillers locally accelerate the cooling rate of the metal, thus reducing the porosity and refining the microstructure in the casting at these locations. To further increase the effectiveness of the chillers, Pederson et al in US 7,360,577 suggest creating a 0.5 mm gap filled with molten metal between the chiller and the part. The major disadvantage of these methods is the need to bond each and every chiller to the pattern manually by vaporizable adhesive agent, and then manually disassemble them from the casting. This imposes a difficulty in the operation of fully automatic lines, like lost foam casting lines. It also limits the geometry of the casting to suit the chillers.

[006] Another way to reduce the porosity is by utilizing the pattern's foam. In US 5,960,851 Donahue offers a method for preparing a polymeric pattern having heat of fusion of less than 60 Joules per gram, especially for hypereutectic aluminum silicon alloys. This method eliminates the "fold" defects due to the fugitive gases escaping during the pouring phase, but it does not reduce porosity. US patent 7,025,109 of Ward et al also aims to reduce casting defects due to back filling. It teaches a method and apparatus for controlling the dispersion of molten metal in a mold cavity, by localized densification of the foam. The embedded dense portions placed in desired areas in the foam cavity may help to control the filling speed of the metal during pouring.

[007] Grant in US 5,058,653 suggests applying isostatic gas pressure after filling and before the metal is 40% solid by weight. The gas pressure should cause the pours to shrink. However this is a very complicated and expansive process and the need of complete sealing of the moving container mold makes it not feasible commercially.

[008] In US 7,347,905 Donahue et al. suggest optimization of the chemical composition of Al-Si alloys used for lost foam casting under isostatic pressure of 10 ATM, to gain a reduction in the porosity. The key elements beside Si are strontium 0.05-0.3% weight, and and maximum of 0.2% weight Fe. This method although giving very good results has the same limitation of US 5,058,653, i.e. it is complicated and expansive.

[009] The lost foam casting foundries need a technology that can elevate the casting quality, but still be economical and simple to apply. Stirring process can be very helpful. Effective stirring will create flow in the metal during the filling and solidifying phases and thus may reduce porosity size and increase the soundness of the cast part. It will also refine the microstructure of the alloy, and thus improve its mechanical properties. There are several ways to create stirring in metal during solidification, e.g. Ultra Sonic (US), Mechanical Vibration (MV), Electromagnetic Stirring (EMS), and introducing changing electric currents into the cast, forcing the liquid metal to be a part of the electric circuit. US and MV are not feasible for lost foam, because they can brake the coating and damage the casting. In addition stirring large casting such as cylinder heads by US requires huge amount of energy which makes it not economical.

[0010] EMS may be achieved by placing very large magnetic coils around the polystyrene cluster inside the container, to create the re quired magnetic field. However, a relative movement between the pattern and the magnet coils is needed in order to create the required magnetic forces to stir the metal, which is very hard to achieve.

[001 1] A more feasible way to electrically and magnetically stir metal in a complex shape is by introducing changing electric currents into a casting. The changing currents will induce changing magnetic fields and stirring. There are several techniques for inducing changing currents in liquid metal. One possibility is to use a plasma generator such as that described in US 6,169,265 by Dvoskin et al. This apparatus uses a positive polarity (Plus) electrode to create a moving arc on top of a liquid metal, while the liquid metal is connected to the negative polarity (Minus) electrode. The moving arc can create changing currents in the solidifying metal. Another way to create a moving arc on top of a liquid metal is to use a small conductive coil around the stationary electrode. In both cases the electrodes that create the arc do not touch the metal. Additional way is to mechanically move an electrode, preferably in a circular path. SUMMARY OF THE INVENTION

[0012] According to embodiments of the invention a process for improving cast metals and alloys quality in at least one desired location in a cast, wherein said metals and alloys may be cast in a nonconductive mold, may comprise: placing at least one first polarity electrode close to said the at least one desired location of said cast, pouring molten metal into said non conductive mold, placing at least one second polarity electrode so that it may be electrically connectable with said cast, and applying electrical current to an electrical circuit comprising the electrodes and the molten metal, to induce changing electrical fields and stirring forces within said molten metal during solidification.

[0013] According to embodiments of the invention the non conductive mold may be selectable from a list comprising: loose sand mold, sand mold and ceramic mold. The non conductive mold may comprise a pattern; the pattern may be made of a material selectable from a list comprising: polystyrene, wax, plastic or frozen mercury.

[0014] According to embodiments of the invention the process may further comprise determining at least one set of current and voltage levels to be applied to the electrical circuit during casting prior to applying said electrical current and setting time duration for each of said at least one set, and applying electrical current and voltage conforming with said current and voltage levels.

[0015] According to embodiments of the invention the second polarity electrode may be electrically connectable with a riser of the cast. Optionaly, the riser may be the pouring downsprue. Alternatively, the riser may not be the pouring riser.

[0016] According to embodiments of the invention the energy of said electrical current may not be sufficient for melting solidifying metal.

[0017] According to embodiments of the invention, one of said electrodes may be an electrode adapted to create a moving arc on top of said molten metal, the electrode is selectable from a list comprising: plasma generator, slotted electrode, gas driven moving electrode, and electrode having a conductive coil. Alternatively, the process may further comprise mechanically moving the electrode while applying said electrical current. Alternatively, the second polarity electrode may be immersed in the molten metal and the changing electric fields may be induced by introducing AC currents. [0018] According to embodiments of the invention, an apparatus for improving metals and alloys quality in at least one desired location in a cast, the metals and alloys may be cast in a nonconductive mold, may comprise: at least one first polarity electrode to be placed close to the at least one desired location of the cast, at least one second polarity electrode to be to be electrically cormectable with said cast, and a power supply to apply electrical current to an electrical circuit comprising the electrodes and the molten metal, to induce changing electrical fields and stirring forces within the molten metal during solidification.

[0019] According to embodiments of the invention, the apparatus may further comprise a controller to control the power supply. The controller may be configured to control at least one set of current and voltage levels to be applied to the electrical circuit, during casting. The controller may be further configured to control time duration for each of said set of current and voltage levels.

BRIEF DESCRIPTION OF THE DRAWINGS [0020] The subject matter regarded as the invention is particularly pointed out and distinctly claimed in the concluding portion of the specification. The invention, however, both as to organization and method of operation, together with objects, features, and advantages thereof, may best be understood by reference to the following detailed description when read with the accompanying drawings in which:

[0021] Fig. 1A is a schematic diagram illustrating apparatus for stirring solidifying metal in a non conductive mold according to embodiments of the present invention;

[0022] Fig. IB is a schematic diagram illustrating apparatus for stirring solidifying metal in lost foam casting according to embodiments of the present invention;

[0023] Fig. 2A is a flowchart illustration of a method for treating metals and alloys solidifying in a non conductive mold, according to embodiments of the present invention;

[0024] Fig. 2B is a flowchart illustration of a method for lost foam casting according to embodiments of the present invention;

[0025] Figs. 3-4 depict exemplary double cylinders head lost foam pattern and electrodes arrangement according to embodiments of the present invention; [0026] Figs. 5-6 depict pattern and electrodes arrangement embedded in a loose sand mold, according to embodiments of the present invention; and

[0027] Figs. 7-10 present results of an experiment done with an apparatus according to embodiments of the present invention.

[0028] It will be appreciated that for simplicity and clarity of illustration, elements shown in the figures have not necessarily been drawn to scale. For example, the dimensions of some of the elements may be exaggerated relative to other elements for clarity. Further, where considered appropriate, reference numerals may be repeated among the figures to indicate corresponding or analogous elements.

DETAILED DESCRIPTION OF THE PRESENT INVENTION

[0029] In the following detailed description, numerous specific details are set forth in order to provide a thorough understanding of the invention. However, it will be understood by those skilled in the art that the present invention may be practiced without these specific details. In other instances, well-known methods, procedures, and components have not been described in detail so as not to obscure the present invention.

[0030] Although embodiments of the invention are not limited in this regard, the terms "plurality" and "a plurality" as used herein may include, for example, "multiple" or "two or more". The terms "plurality" or "a plurality" may be used throughout the specification to describe two or more components, devices, elements, units, parameters, or the like. Unless explicitly stated, the method embodiments described herein are not constrained to a particular order or sequence. Additionally, some of the described method embodiments or elements thereof can occur or be performed at the same point in time.

[0031] An apparatus and method for improving the quality of metals and alloys cast in non conductive mold, in accordance to some embodiment of the invention, may include at least two polarity electrodes connected to the poles of a DC or an AC power supply. At least one first polarity electrode may be placed close to or in desired area wherein the quality of the casting needs to be improved (referred to herein after as "desired locations") and the at least one second polarity electrode may be placed in any location in the mold other than the desired location. For example, the second polarity electrode may be placed in or at or closely above a riser attached to the mold. Throughout the specification the description presents connection of second polarity electrode to a riser, however, it is noted noted that the second polarity electrode may be connected in any other way or location as long as this connection enables the completion of an electrical circuit from a power source through the electrodes and the solidifying metal in the mold via the first polarity electrode back to the power source. In will further be noted that any one of the first and or the second polarity electrodes may be connected so that galvanic path is made with the solidifying metal, or placed close to the solidifying metal to enable the creation of an electrical arc. The electrodes capable of inducing changing electric fields within the cast, which may cause metal flow and stirring of the molten metal in desired areas, typically close to the first and/or second polarity electrodes (herein after also treated areas). Stirring molten metal in desired areas of a cast during solidification may improve the microstructure and porosity level in the treated areas, thus may improve mechanical properties of the cast part, such as elongation, fatigue and strength.

[0032] Reference is made to Fig. 1A depicting a high-level diagram of an exemplary apparatus 100 for stirring and treating solidifying metal in a non-conductive mold, according to embodiments of the present invention. According to embodiments of the present invention, apparatus 100 may comprise mold 190, power supply 1 10, second polarity electrode 140 and first polarity electrode 170. Mold 190 may be made of any non-conductive material suitable for holding molten metal during solidification. For example, mold 190 may be a sand mold, loose sand mold or a ceramic mold. Mold 190 may further include at least one riser 150. The term riser according the some embodiments of the invention is typically relates to substantially vertical channel, , located in desired areas on top or on the side of mold 190, aimed at adding excess material and hydrostatic pressure to compensate for a reduction in volume during the solidification of the metal in the mold. Riser 150 may be coated with a refractory coating. A second polarity electrode 140 may be placed in, on top or closely above riser 150. Alternatively electrode 140 may be partially immersed in the molten metal in riser 150. Electrode 140 may be connected to a first pole of power supply 110. At least one first polarity electrode 170 may be connected to one or more desired areas in mold 130 and connected to the second pole of power supply 110. Alternatively, more than one second polarity electrodes 140 may be used. For example, another electrode 140 may be placed in in or on or closely above riser 180. A controller 160 may be connected to or embedded in power supply 1 10.

[0033] Reference is made to Fig. IB which illustrates apparatus 105 for stirring and treating solidifying metal in lost foam casting mold, according to embodiments of the present invention. According to embodiments of the present invention, apparatus 105 may comprise mold 195, power supply 1 10 and electrodes 140 and 170. Mold 195 may comprise pattern 130, typically made of polystyrene, placed in flask 120, typically filled with loose sand, and having a riser for pouring molten metal or dowiisprue 155. The term downsprue relates to a riser through which the molten metal enters mold 190. Downsprue 155 may be coated with a refractory coating. Flask 120 may be a large bucket that may be filled with sand and pattern 130 during casting. Flask 120 may be conductive or non conductive. A second polarity electrode 140 may be placed in, on top or closely above downsprue 155 and connected to a first pole of power supply 1 10 and a first polarity electrode 170 may be connected to one or more desired areas in pattern 130 and connected to the second pole of power supply 1 10. Alternatively, second polarity electrode 140 may be placed in or on or closely above the top of risers other than downsprue 155 located directly on a pattern 130, such as riser 180. A controller 160 may be connected to or embedded in power supply 110.

[0034] Electrodes 140 and 170 may be any object made from conductive material that may be used to induce currents in the metal of the cast during solidification. An electrode may be made, for example, from graphite, steel, copper based alloys, aluminum based alloys and/or tungsten. Second polarity electrode 140 may have tubular shape with or without slots, rectangular shape, or any cross section suitable to induce current with changing current densities and changing electric fields in the molten metal. It is to be noted that the electrodes of the current invention are not limited to any shape or material. Optionally, part of first polarity electrode 170 may be cast together with the cast part. At least one part of first polarity electrode 170 may constructed from polystyrene pattern and attached or glued to pattern 130, in desired areas prior to casting. When molten metal is poured into the mold, molten metal may replace the pattern of first polarity electrode 170 and may become part of first polarity electrode 170. Other portions of first polarity electrode 170 may be constructed from conductive materials that may not melt during casting. Optionally, if flask 120 is made of conductive material, it may be connected to first polarity electrode 170 and to power supply 1 10 and become part of the electrical circuit.

[0035] According to embodiments of the present invention, electrodes 140 and 170 may be configured to create changing electric fields in the molten metal. The term "changing electric fields" describe changing current densities in the molten metal therefore changing the electric fields in a unit volume. Changing electric fields in the molten metal may be achieved by creating a moving electric arc between second polarity electrode 140 and the molten metal that creates changing electric fields (changing current density per volume unit) in the metal. Changing electric fields may induce changing magnetic fields and thus may create intensive flow in the molten metal. A moving electric arc may be created by closing an AC or DC electrical circuit through power supply 1 10, second polarity electrode 140, the molten metal and first polarity electrode 170 while mechanically moving second polarity electrode 140. Alternatively, second polarity electrode 140 may be stationary and the arc may be forced to move by for example, magnetic coils placed around second polarity electrode 140, flow of gas, Lorenz force in slotted electrode or any other electromagnetic means. Other option may be to use a finger shaped electrode for second polarity electrode 140 in the upper part of the downsprue 150 and to create changing currents either by mechanical movement or electromagnetic means, such as coil around the downsprue 150. For example, a stationary electrode for moving plasma arc, as described in US 6,169,265, or an electrode having gas, for example, Argon, tubs in its peripheral area that when applied during the ignition of the arc, cause the arc to move in the peripheral section of the electrode as described in US 7,243,701, of the applicant of the present invention, may be used as second polarity electrode 140. Alternatively, second polarity electrode 140 may be immersed in the molten metal and changing magnetic fields may be induced by electromagnetic means, such as coil around the downsprue 150 (not shown) or by introducing AC currents. It should be noted that embodiments of the invention are not limited to any particular method for creating changing electric fields in the molten metal. The methods described herein are given by a way of example only.

[0036] It should be noted that according to embodiments of the present invention, second polarity electrode 140 may represent an arrangement of several second polarity electrodes and first polarity electrode 170 may represent an arrangement of several first polarity electrode electrodes. Additionally pattern 130 may comprise a plurality of risers 150 and/or downsprues 155. Each second polarity electrode 140 may be placed over, in or on top, of a common or separate riser or downsprue. Each of first polarity electrodes 170 may be placed in a different location of mold 190 or pattern 130. It has been shown experimentally that maximal stirring may be achieved in the proximity of first polarity electrodes 170, for example, see the results presented in Figs. 7 and 8. Stirring molten metal or alloy in sufficient intensity may reduce the size of coarse grains and dendrites that are typically created during casting, especially in non conductive molds. It may produce a vortex and molten metal flow that may fill macro porosity and micro porosity, formed in the casting due to cooling shrinkage, thus improving the overall quality of the resultant part. The term macro porosity relates to porosity size of 1 mm in diameter and up and the term micro porosity relates to porosity size of less than 1 mm in diameter. As stirring may be related to reduction in porosity and improvement of mechanical characteristics of the cast part, first polarity electrodes 170 may be placed near regions where improved mechanical characteristics may be desired. For example Al-7%Si alloys cast in conventional lost foam or sand molds casting processes typically have porosity size of 0.5-1 mm and porosity ratios of 1-2%. Using apparatuses 100 and 105 during lost foam or sand molds casting processes may result in a decrease in porosity size to, for example, less than 0.5 mm, or 0.4 mm or 0.3 mm or 0.2 mm in diameter and less than 1% or 0.75% or 0.5% in porosity ratio.

[0037] As the molten metal solidifies its conductivity increases. Therefore, electrical current flow may substantially concentrate in the solidifying part of the cast. Additionally, the electrical current typically concentrates in proximity to first polarity electrodes 170, thus inducing substantially maximal changing electric field and maximal stirring in proximity to first polarity electrodes 170.

[0038] Power supply 110 according to embodiments of the present invention may be any apparatus capable of producing and / or providing DC or AC power sufficient for introducing changing electric fields in the molten metal. For example, Power supply 1 10 may be any DC power source capable of driving power in the range of 0.5Kw - 300Kw or any AC power source capable of driving current in the range of 10 A - 6000 A. For example, a DC power source may be one of the following models: Miller - XMT 304 CC/CV, XMT 456 CC/CV, Sub Arc 650 or Lincoln - DC - 600, DC- 655, and an AC power source may be one of the following models: Lincoln - AC/DC 1000 SD, AC -1200 SD or ESAB I- TAF 801/1251 AC. Controller 160 may be connected to or embedded within power supply 110. Controller 160 may control and/or regulate and/or monitor and/or supervise power supply 110 as well as electrodes 140 and 170, as needed. Controller 160 may be a generic off-the-shelf controller such as SIMENES - Simatic S7 or may be specially designed. Controller 160 may be a programmable logic controller (PLC). Controller 160 may comprise a processor, microprocessor, microcontroller, microchip or the like. Controller 160 may control the current and voltage levels supplied to each of electrodes 140 and 180 separately or for two or more electrodes simultaneously. Additionally, controller 160 may control the time durations of the current and voltage supplied by power supply 110. Additionally controller 160 may, for example, control an electric motor that may provide three dimensional movement to second polarity electrode 140, in some particular cases the movement may be vertical, horizontal (in one or two dimensions) or a combination of both. Optionally the movement of second polarity electrode 140 may be done by a robotic unit (not shown); optionally the robotic unit may be configured to move a plurality of second polarity electrodes 140 or even the whole apparatus. Controller 160 may further control the output currents and voltage of power supply 1 10.

[0039] Reference is now made to Fig. 2 A which is a flowchart illustration of a method for treating metals and alloys solidifying in a non conductive mold according to embodiments of the present invention. At block 210 a non-conductive mold, having a desired shape, and further including at least one downsprue or riser may be created. The interior part of the mold may be coated with a refractory coating. At block 220 at least one first polarity electrode may be placed in one of a group of desired locations in the mold. Optionally, the placement of the first polarity electrode or electrodes may be done during the assembly of the mold, e.g. during the folding of sand molds. As stirring may be related to reduction in porosity and improved mechanical characteristics of the cast part, the electrodes may be placed near regions were improved mechanical characteristics are desired i.e. in desired locations. At block 240, at least one second polarity electrode may may be placed over, in or on top of a riser of the pattern or the mold, for example, the downsprue. At block 250, molten metal may be poured into the mold. At block 260, electrical currents are induced between the electrodes and the molten metal so as to create changing electric fields in the molten metal, thus causing stirring in the molten metal during the solidification phase.

[0040] Optionally at least one sets of electrical currents is induced, each for a predetermined time duration. The current and voltage levels and the time duration of the set may be determined in an optimization process done prior to the casting process. The optimization process may take into consideration several parameters, for example, the total casting weight, the geometrical complexity of the casting shape, the chemical composition of the alloy, the number of risers and the number and locations of the areas in the casting wherein quality improvement is needed. The optimization process may require several steps wherein the current and voltage levels are re-determined according to the metallurgical results and/or mechanical properties obtained by the prior current- voltage set. For example; optimization process done to a cylinder head casting, weighing 4-20 Kg, cast from Al-Si alloy in lost foam casting may result in two current-voltage sets. A first set may have current levels ranging between 100-1500 Amps, or 200- 1000 Amps or 300-700Amps, and voltage levels of 28-45 volts. The first set may be applied for 120- 180 seconds. A second set may have current levels ranging between 50 -1000 Amps, or 100-800 Amps or 100-300Amps, and voltage levels of 28-45 volts. The second set may be applied for 60-120 seconds. According to embodiments of the invention the energy levels applied to induce stirring may be substantially not sufficient for melting of the metal in the cast.

[0041] The apparatuses and method disclosed above with reference to Figs. 1 and 2 can be applied on any nonconductive mold such as, for example, ceramic molds used for investment casting (also known as lost wax casting) or sand mold casting and not limited to the exemplary embodiment disclosed. The pattern may be made by any suitable material known in the art, for example, of polystyrene, wax, plastic or frozen mercury.

[0042] In some embodiments the non conductive mold may be a sand casting mold wherein the at least one second polarity electrode may be placed either on top of the pouring riser or downsprue, or in the case of bottom pouring, on the top of one or more of the risers that may be placed on top of the mold. The at least one second polarity electrode may be connected either to or at the at least one riser or to the mold, in such a way that there will be an electrical contact between the molten metal and the electrode. This may be done by placing conductive metal pieces in the mold prior to folding the mold. The metal pieces may be placed in desired areas.

[0043] Reference is now made to Fig. 2B which is a flowchart illustration of a method for lost foam casting according to embodiments of the present invention. At block 205 a pattern, typically made of polystyrene, having the internal shape and size of the cast part, and further including at least one downsprue or riser may be created. The interior part of the pattern may be coated with a refractory coating. At block 215 at least one first polarity electrode may be placed in one of a group of desired locations in the pattern. Optionally, the placement of the first polarity electrode or electrodes may be done during assembly of the pattern. For example, after gluing all polystyrene pieces of the pattern together, additional polystyrene parts may be glued in desired areas to later be replaced by molten metal and become the first polarity electrodes. Optionally, the additional polystyrene parts may be glued prior to coating of the pattern. As stirring may be related to reduction in porosity and improved mechanical characteristics of the cast part, the electrodes may be placed near regions were improved mechanical characteristics are desired. The pattern may then be placed, together with the first polarity electrodes, in a filled with flask loose sand to create a mold as indicated in block 225. Optionally, as indicated in block 235, the flask may be made of conductive material and the first polarity electrodes may be connected to the flask. Thus, the flask may become part of the electrical circuit. At block 245, at least one second polarity electrode may be placed over, in or on top of a riser of the pattern or the mold, for example, the downsprue. At block 255, molten metal may be poured into the mold. At block 265, electrical currents may be induced between the electrodes and the molten metal so as to create changing electric fields in the molten metal, thus causing stirring in the molten metal during the solidification phase.

[0044] Embodiments of the invention are not limited to any particular casting method utilizing non-conductive mold. Although some examples were made throughout this description of embodiments of the present invention using the method of lost foam casting, the description is provided to illustrate a few exemplary principles of the invention, and is not intended to limit the scope of the invention to any particular casting method. In addition, embodiments of the invention are not limited to any casting shape or part, alloy or metal and may be utilized with any form and/or shape solidifying metals may take while solidifying in a mold. Additionally, embodiments of the invention may be applied to various types of metal and alloys, including, but not limited to, Al-Si alloy (e.g. A356), cast iron (e.g. ductile iron), steel (e.g. 4130 and 4340), copper alloys, magnesium alloys and the like.

Examples

[0045] Reference is now made to Figs. 3 and 4 which depict ah exemplary double cylinder head block lost foam pattern 20 and electrodes arrangement according to embodiments of the present invention. In the embodiment depicted in Fig. 3 a single second polarity electrode 10 is placed on top of downsprue 50 and a plurality of first polarity electrodes 40 are placed in areas in pattern 20 next to where an improved quality of the cast is required. First polarity electrodes 40 may be connected, via connection 41, to a power source (not shown). In the embodiment depicted in Fig. 4 two second polarity electrodes 10 are placed on top of risers 60 and a single first polarity electrode 30 is placed in an area in pattern 20 next to where an improved quality of the cast may be required. In both arrangements second polarity electrodes 10 may be connected to a positive pole of a DC power source and first polarity electrodes 30 and 40 may be connected to a negative pole of a DC power source (not shown). Alternatively, second polarity electrodes 10 may be connected to a negative pole of a DC power source and the first polarity electrodes 30 and 40 may be connected to a positive pole of a DC power source (not shown). Optionally, electrodes 10, 30 and 40 may be connected to the poles of an AC power source. The power source may be connected to a controller (not shown) that may monitor the power levels to be applied, for example by controlling at least one of the current, the voltage and time duration of the provided power. The controller may comprise a PLC controller configured to determine the current and the voltage required in the process.

[0046] Reference is now made to Fig. 5 which depicts pattern 20 and the electrodes arrangement of Fig. 3, embedded in a loose sand mold 80, according to embodiments of the present invention. Shown are downsprue 50 and connecting point 35 connected to electrode 30 that in turn is connected to the plurality of first polarity electrodes 40 (both electrodes 30 and 40 are shown in Fig. 3). After embedding pattern 20 in loose sand mold 80, molten alloy, such as, but not limited to Al-Si, may be poured through downsprue 50 and the fugitive pattern 20 may burn out. As the pouring ends, a second polarity electrode (not shown) may be placed above downsprue 50.

[0047] Reference is now made to Fig. 6 which depicts pattern 20 and the electrodes arrangement of Fig. 4 embedded in a loose sand mold 80, according to embodiments of the present invention. Shown are downsprue 50, risers 60, connecting point 35 and clumping means 70 for connecting first polarity electrode 30 to a power supply (not shown). After embedding pattern 20 in loose sand mold 80, molten alloy, such as, but not limited to Al-Si, may be poured through downsprue 50 and the fugitive pattern 20 may burn out. Second polarity electrode 15 may be placed on top of downsprue 50 and as the pouring ends, a current voltage sets of, for example, current levels of 50 - 1500 Amps (Amperes), or 100-lOOOAmps or 300-900Amps, and voltage levels of 28-45 Volts may be applied by a power supply (not shown) for tome durations of, for example, 10-500 seconds, and controlled by a controller (not shown). Second polarity electrode 15 may be similar to the plasma arc generator electrode presented in US 6,169,265, and may create circulating electric arc to induce changing electric fields within the molten metal.

[0048] Reference is now made to Figs. 7-9 which present results of an experiment done with the apparatus shown in Figs. 4 and 6 according to embodiments of the invention. In the experiment Al-Si molten alloy was poured through downsprue 50 to pattern 20 of a cylinder head, second polarity electrode similar to the plasma arc generator electrode presented in US 6,169,265, was placed on top of risers 60, a single first polarity electrode was placed in a desired area. 1000 Amps and 32 volts were activated during the solidification phase. A control cylinder head was cast in conventional, known in the art lost foam casting. Conventional, known in the art lost foam casting refers to lost foam casting without the use of second polarity electrodes and first polarity electrodes, and without inducing changing electromagnetic stirring forces. The final weight of the cast cylinder heads was approximately 25 kg.

[0049] Fig 7 shows optical microscopy micrographs of cross sections taken at different locations in the combustion chamber area of two lost foam cast cylinder heads. The combustion chamber is shown in Fig. 4 as the area located near downsprue 50 in cluster 25. Scale is presented on the micrographs as a dark horizontal bar the length of which is indicated underneath the bar. Micrographs 790, 700 and 710 presented on the left hand side of Fig. 7 were taken from a cylinder head cast by lost foam casting while applying stirring method according to embodiments of the present invention as described with reference to Fig. 6 (referred to herein after as "stirred part"), and micrographs 795, 705 and 715 were taken at substantially same locations, respectively, but from a cylinder head cast in conventional, prior art lost foam casting (referred to herein after as "control part"). Micrographs no. 790 and 795 were taken from the top part of the combustion chambers, micrographs 700 and 705 from the middle part of the combustion chambers, and micrographs 710 and 715 from the bottom part of the combustion chambers. Visual comparison of the microstructure and Dendrite Arm Spacing (DAS) between the two cylinder heads may reveal finer DAS and eutectic structure of the stirred part than the control part in all three locations. The most significant refining is presented in micrographs 710 and 715, as this cross section was taken close to the connection of the first polarity electrode.

[0050] Fig. 8 shows a bar diagram of DAS measurements done on samples from cylinder heads cast by lost foam casting while applying stirring method according to embodiment of the present invention as described with reference to Fig. 6 (referred to herein after as "stirred"), and cylinder head cast in conventional, prior art lost foam casting (referred to herein after as "control") The averaged stirred DAS values are between 35-50 μηι, while the averaged DAS values of controls are between 62-70 μη . According to embodiments of the invention, the stirring during solidification may apply shear forces on the growing dendrites, thus breaking the dendrite tips, that in return become new nucleation sites for finer and smaller grains and dendrites. The vertical solid line associated with each bar in Fig. 8 represents standard deviation.

[0051] Fig. 9 shows micrographs of cross sections taken at different locations in the two cylinder heads of Fig. 7 for comparison of the porosity ratio. Micrographs 920 and 930 presented on the left hand side of Fig. 9 were taken from the stirred part and micrographs 925 and 935 presented on the right hand side of Fig. 9 were taken from the control part. Visual comparison of the porosity size (presented as dark stains in bright areas) between the two cylinder heads reveals smaller pores 940, 942 in the stirred part than the control part 960, 962. A computer calculation of porosity ratio from optical micrographs from the stirred part resulted in 0.2-0.3% porosity ratio in comparison to 0.8-1.2% in the control part. Such reduction in porosity ratio may increase the elongation by 30%.

[0052] Reference is now made to Fig. 10 which presents micrographs of cross sections taken from two small cylinder heads. Scale is presented on the micrographs. Micrograph 1040 presented on the left hand side of Fig. 10 was taken from a cylinder head cast by lost foam casting while applying stirring method according to the embodiments of the present invention as described with reference to Fig. 6 (referred to herein after as "stirred part"), and micrograph 1045 was taken at substantially same location, but from a cylinder head cast in conventional, prior art lost foam casting (referred to herein after as "control part"). The second polarity electrode used in this experiment is similar to the plasma arc generator electrode presented in US 6,169,265. Inspection of the porosity size after machining in the stirred part in comparison to the control part shows reduction in porosity size from 0.5-0.9 mm to 0.15-0.25 mm. Such reduction in porosity size may increase the elongation by 30% - 150%.

[0053] While certain features of the invention have been illustrated and described herein, many modifications, substitutions, changes, and equivalents will now occur to those of ordinary skill in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention.

Claims

CLAIMS [0054] What is claimed is:

1. A process for improving cast metals and alloys quality in at least one desired location in a cast, said metals and alloys are cast in a nonconductive mold, the process comprising: placing at least one first polarity electrode close to said the at least one desired location of said cast;

pouring molten metal into said non conductive mold;

placing at least one second polarity electrode so that it is electrically connectable with said cast; and

applying electrical current to an electrical circuit comprising the electrodes and the molten metal, to induce changing electrical fields and stirring within said molten metal during solidification.

2. The process of claim 1, wherein said non conductive mold is selectable from a list comprising: loose sand mold, sand mold and ceramic mold.

3. The process of claim 1, wherein said non conductive mold comprises a pattern, said pattern is made of a material selectable from a list comprising: polystyrene, wax, plastic or frozen mercury.

4. The process of claim 1 , further comprising determining at least one set of current and voltage levels to be applied to the electrical circuit during casting prior to applying said electrical current.

5. The process of claim 4, further comprising setting time duration for each of said at least one set.

6. The process of claim 5, wherein applying electrical current comprises applying said at least one set of current and voltage levels.

7. The process of claims 1, wherein said second polarity electrode is electrically connectable with a riser of said cast.

8. The process of claims 7, wherein said at least one riser is the pouring downsprue.

9. The process of claim 7, wherein said at least one riser is not the pouring riser.

10. The process of claim 1 wherein the energy of said electrical current is not sufficient for melting solidified metal.

1 1. The process of claim 1 , wherein said second polarity electrode is an electrode adapted to create a moving arc on top of said molten metal.

12. The process of claim 10, wherein the electrode adapted to create a moving arc on top of said molten metal is selectable from a list comprising: plasma generator, slotted electrode and electrode having a conductive coil.

13. The process of claim 1, further comprising mechanically moving said second polarity electrode while applying said electrical current.

14. The process of claim 1, wherein said second polarity electrode is immersed in said molten metal and said changing electric fields are induced by introducing AC currents.

15. An apparatus for improving metals and alloys quality in at least one desired location in a cast, said metals and alloys are cast in a nonconductive mold, the apparatus comprising: at least one first polarity electrode to be placed close to said at least one desired location of said cast;

at least one second polarity electrode to be electrically connectable with said cast; and

a power supply to apply electrical current to an electrical circuit comprising the electrodes and the molten metal, to induce changing electrical fields and stirring within said molten metal during solidification.

The apparatus of claim 14, further comprising a controller to control said power supply.

The apparatus of claim 15, wherein the controller is configured to control at least one set of current and voltage levels to be applied to the electrical circuit, during casting.

18. The apparatus of claim 16, wherein the controller is further configured to control time duration for each of said set of current and voltage levels.

19. The apparatus of claim 14, wherein said non conductive mold is selectable from a list comprising: sand mold and ceramic mold.

20. The apparatus of claim 14, wherein said non conductive mold comprises a pattern, said pattern is made of a material selectable from a list comprising: polystyrene, wax, plastic or frozen mercury.

21. The apparatus of claim 14, wherein said second polarity electrode is an electrode adapted to create a moving arc on top of said molten metal

22. The apparatus of claim 20, wherein said electrode adapted to create a moving arc on top of said molten metal is selectable from a list comprising: plasma generator, slotted electrode and electrode having a conductive coil.

23. The apparatus of claim 14, further comprising mechanically moving said second polarity electrode while applying said electrical current.

24. The apparatus of claim 14, further comprising a conductive coil to be placed around said riser.

25. The apparatus of claim 14, wherein said second electrode is immersed in said molten metal and said changing magnetic fields are induced by introducing AC currents.