Method for magnetic flux compensation in a directional solidification furnace utilizing a stationary secondary coil

A process for directional solidification of a cast part comprises energizing a primary inductive coil coupled to a chamber having a mold containing a material; energizing a primary inductive coil within the chamber to heat the mold via radiation from a susceptor, wherein the resultant electromagnetic field partially leaks through the susceptor coupled to the chamber between the primary inductive coil and the mold; determining a magnetic flux profile of the electromagnetic field; sensing a magnetic flux leakage past the susceptor within the chamber; generating a control field from a secondary compensation coil coupled to the chamber, wherein the control field controls the magnetic flux experienced by the cast part; and casting the material within the mold under the controlled degree of flux leakage.

BACKGROUND

The present disclosure is directed to a method and device for directional solidification of a cast part. More particularly, this disclosure relates to a directional solidification casting process that controls a magnetic field to provide a desired microstructure.

A directional solidification (DS) casting process is utilized to orient crystal structure within a cast part. The desired orientation is provided by moving a mold from a hot zone within a furnace into a cooler zone at a desired rate. As the mold moves into the cooler zone, the molten material solidifies along a solidification front traveling in one direction.

Mixing of the molten material at the solidification front within the cast component is known to be deleterious to the quality of single crystal castings. Such mixing can be induced in the molten metal material by a magnetic field generated from an energized coil encircling the furnace cavity. Typically, an induction withdrawal furnace utilizes such an electric coil that produces energy required for maintaining the metal in a molten state. A susceptor is utilized to transduce an electromagnetic field produced by the electric coil into radiant heat transferred to the casting mold.

The susceptor is usually a graphite cylinder located internal to the induction coil and external to the mold. The susceptor is heated by induction coils and radiates heat toward the mold to maintain metal in a molten state, and is intended to isolate the magnetic field from the hot zone of the furnace.

Casting single crystal gas turbine parts can experience less than 100% yields. Some defects that occur during the casting process are separately nucleated grains, freckels, porosity, mis-oriented boundaries, and others. The causes of these defects are not always known, but have been empirically determined to be influenced by the geometry of the part and the relative orientation of the part and the mold in the furnace. It is hypothesized that remnant magnetic field in the interior of the susceptor may be detrimental to the production of the desired microstructure in a cast part. Calculations have been made estimating the significance for a given production furnace design.

It has been recognized that the leakage of the magnetic field into the furnace hot zone could directly influence the solidification process during casting.

SUMMARY

In accordance with the present disclosure, there is provided a process for directional solidification of a cast part comprising energizing a primary inductive coil coupled to a chamber having a mold containing a material; energizing a primary inductive coil within the chamber to heat the mold, wherein the resultant electromagnetic field partially leaks through a susceptor coupled to the chamber between the primary inductive coil and the mold; determining a magnetic flux profile of the leaked magnetic field; sensing a magnetic flux leakage past the susceptor within the chamber; generating a control field from a secondary compensation coil proximate the chamber, wherein said control field adjusts said magnetic flux that has leaked past the susceptor; and casting the material within the mold under the controlled degree of flux leakage.

In another and alternative embodiment, the magnetic flux leakage comprises a portion of the electromagnetic field passing through the mold that is not blocked by the susceptor.

In another and alternative embodiment, the control field is increased or decreased to control inductive stirring in the casting material to produce a predetermined microstructure.

In another and alternative embodiment, the control field controls the primary induction coil magnetic flux leakage into the mold.

In another and alternative embodiment, the process further comprises generating a control signal, the control signal being responsive to at least one of a flux sensor input and a flux set point input.

In another and alternative embodiment, the control signal is sent to a power amplifier that generates the electrical power sent to the secondary compensation coil for generating the control field.

In another and alternative embodiment, the secondary compensation coil is fixed relative to the susceptor.

In accordance with the present disclosure, there is provided an induction furnace assembly comprising a chamber containing a mold; a primary inductive coil coupled to the chamber; a susceptor surrounding the chamber between the primary inductive coil and the mold; and at least one secondary compensation coil fixed to the chamber between the susceptor and the mold; the at least one secondary compensation coil configured to generate a control field configured to control a magnetic flux leakage past the susceptor from the primary induction coil.

In another and alternative embodiment, a controller is coupled to at least one flux sensor located within the chamber, wherein the controller is configured to generate a control signal responsive to an input from at least one of a flux sensor and a flux set point.

In another and alternative embodiment, a power amplifier is coupled to the controller and the secondary compensation coil, wherein the power amplifier generates electrical power responsive to the control signal to the at least one secondary compensation coil to generate the control field.

In another and alternative embodiment, the magnetic flux leakage is sensed by at least one flux sensor at a predetermined location within the chamber.

In another and alternative embodiment, the controller comprises a set point comparator.

In another and alternative embodiment, the at least one secondary compensation coil is coupled to a control system configured to control material casting.

In accordance with the present disclosure, there is provided a process for directional solidification of a cast part comprising generating an electromagnetic field from a primary inductive coil coupled to a chamber of an induction furnace, wherein the electromagnetic field includes a magnetic field leakage that passes a susceptor coupled to the chamber between the primary inductive coil and a mold; controlling the magnetic field leakage entering the mold inside the chamber by use of an applied magnetic control field generated by at least one secondary compensation coil fixed between the susceptor and the mold in the chamber; and casting a part within the mold from a molten material

In another and alternative embodiment, the casting step further comprises at least one of increasing and decreasing the applied magnetic field to control a stirring in the casting material to produce a predetermined microstructure.

In another and alternative embodiment, the process further comprises generating a control signal, the control signal being responsive to at least one of a flux sensor input and a flux set point input.

In another and alternative embodiment, the process further comprises transmission of electrical power to the at least one secondary compensation coil to generate the control field, responsive to the control signal.

In another and alternative embodiment, the process further comprises sensing the magnetic field leakage past the susceptor within the chamber with at least one flux sensor.

Other details of the method and device for directional solidification of a cast part are set forth in the following detailed description and the accompanying drawings wherein like reference numerals depict like elements.

DETAILED DESCRIPTION

Referring toFIG. 1, an exemplary induction furnace assembly10includes a chamber12that includes an opening14through which a mold16is received and withdrawn. The chamber12is isolated from the external environment by insulated walls18. A primary inductive coil20generates an electromagnetic field which is converted into heat by the susceptor. Heat, indicated by arrows22, serves to heat a material24within the mold16to a desired temperature.

The exemplary furnace assembly10includes a susceptor26that partially absorbs the electromagnetic field (schematically shown at28) that is generated by the primary inductive coil20. The susceptor26is a wall that surrounds the chamber12. The susceptor26is fabricated from material such as graphite that absorbs the electromagnetic field28produced by the primary inductive coil20. The susceptor26can also provide for the translation of energy from the electromagnetic field into heat energy, as indicated at arrows22to further maintain a temperature within the mold16. In the disclosed example, molten metal material24is disposed in the mold16which in turn is supported on a support30. The support30includes a chill plate32that both supports the mold16and includes cooling features to aid in cooling and directional solidification of the molten material24.

The primary inductive coil20receives electrical energy from an electric power source schematically indicated at34. This electrical energy is provided at a desired current level determined to provide sufficient power and energy to create the desired temperature within the chamber12that maintains the metal24in a molten state.

The primary inductive coil20comprises a plurality of electrically conductive hollow tubes35. The plurality of tubes35also provide for the circulation of a fluid that is generated by a pump36that supplies fluid from a fluid source38to flow through the tubes35.

In operation, the furnace10is brought up to a desired temperature by providing a sufficient current from the electric power source34to the primary inductive coil20. Water supplied from the pump36and fluid source38is pumped through the plurality of tubes35that make up the inductive coil20. The heat22created by the partial conversion of the electromagnetic field by the susceptor26heats the core furnace zone in the chamber12to a desired temperature. Once a desired temperature is reached, molten material, metal24is poured into the mold16. The mold16defines the external shape and features of the completed cast article.

In the exemplary directional solidification casting process utilized, after the molten material24is poured into the mold16within the chamber12the material24is maintained at a desired temperature in a molten state. The support30and chill plate32are then lowered from the opening14out of the hot chamber12through a baffle. The mold16is lowered from the chamber12at a desired rate to cool the molten material24in a controlled manner to produce desired columnar structure or single crystal. The controlled cooling produces a solidification front within the molten material24.

In many applications, the completed cast part is desired to include a specific grain structure. The size, orientation, and structure of grains within the completed cast part provide desired material characteristics and performance, such as for example material fatigue performance. The exemplary furnace assembly10includes the susceptor26with a constant thickness to block an amount of the electromagnetic field28. The portion of electromagnetic field28that passes the susceptor26, that is, magnetic flux leakage44, has potential to generate a certain amount of magnetic stirring within the molten metal material24.

The generated electromagnetic field28not absorbed by the susceptor has a potential to produce currents within the molten metal material24that interact with the molten metal material24to provide stirring and mixing and may inhibit defect-free single crystal growth. In a standard induction furnace, the susceptor26is sized to include a thickness that is thick enough to shield the electromagnetic field within the hot zone of the chamber12. However, it has been discovered that a certain amount of electromagnetic field28may leak past the susceptor26. This electromagnetic field28leakage may be unwanted and detrimental to proper grain structure formation.

The exemplary furnace10includes a secondary compensation coil40fixed to the chamber12. The secondary compensation coil40is configured to generate a control field42. The control field42can be a secondary field to control the local electromagnetic field at the solidification front. The control field42can cancel magnetic flux leakage44from the primary induction coil20or constructively increase the flux experienced by the mold16. The control field42can be generated depending on the magnetic flux leakage44at predetermined locations, such as proximate the mold16, within the chamber12, within the mold16, and the like. The magnetic flux leakage44can include the portions of the electromagnetic field28passing through the mold16that are not blocked by the susceptor26.

The secondary compensation coil40is a fixed coil relative to the susceptor26. The secondary compensation coil40can be coupled to a power amplifier46. The power amplifier46can be coupled to flux sensors48. The flux sensors48can transmit data to a controller50as part of a control system52shown in more detail atFIG. 2. In this way stirring can be better controlled or eliminated within the molten material to produce castings with desired microstructure.

As shown inFIG. 2, the control system52can include a plurality of magnetic flux sensors48positioned in predetermined locations for detection of the magnetic flux leakage44. A flux set point54can be set based on empirical data, physics-based modeling, materials being cast, a property of the primary inductive coil20, a property of the susceptor26, the chamber12and the like. The flux set point54can be part of a proportional, differential, integral controller50that is designed to null out residual electromagnetic field, or tailor a response such that magnetic stirring is controlled to desired set point. The actual control schedule may be derived through a combination of empirical setting data or by thermal fluid analysis of the melt. Alternatively, the control schedule response to the flux sensor48may be tailored to produce no stirring or complete stirring, where again the actual controller signal58may be derived empirically or supported by thermal fluid analysis. The flux sensor(s)48and flux set point54provide inputs56to the controller50. In an exemplary embodiment, the controller50can comprise a null point comparator. The controller50receives the inputs56from the flux sensor(s)48and flux set point54and generates a control signal58to the power amplifier46. In an exemplary embodiment, the control signal58can comprise an error signal generated by the null point comparator. The power amplifier46then generates the electrical power to produce the frequency and amplitude to the secondary compensation coil40during the solidification process for control of the solidification of the metal24. The secondary compensation coil40generates the control field42.

In an exemplary embodiment, the control field42can be utilized to “control to nullify.” The electromagnetic control field42from the secondary compensation coil40can be created so that the control field42is partially or wholly out of phase with the electromagnetic field28. The control system52can generate an appropriate control signal58to the secondary compensation coil40to nullify the magnetic flux leakage44experienced by the mold16to a range of about 0-200 Gauss range, 10 Gauss resolution, and 2 Gauss accuracy.

In an exemplary embodiment, the control field42can be utilized to “control to amplify.” The electromagnetic control field42from the secondary compensation coil40can be created so that it is partially or wholly in phase with primary electromagnetic field28. The control system52can generate an appropriate control signal58to the secondary compensation coil40to amplify the magnetic flux leakage44experienced by the mold16to a range of about 100-50,000 Gauss.

An exemplary process map is illustrated atFIG. 3. The process for controlled solidification behavior100can include, at step110, determining a desired magnetic flux profile at a selected location in the chamber12. At step112, the magnetic flux is sensed at a predetermined location where flux control is desired. At step114the flux measurement can be compared to a flux set point. At step116a control signal can be generated by the controller50. At step118, a control field42can be generated by the secondary compensation coil40. The amount, frequency and amplitude of electrical power can be used to drive the secondary compensation coil40to generate the control field42during solidification of the metal24to control the electromagnetic field28that influences the solidification of the metal24. In another exemplary embodiment, physics-based models can be utilized to actively control the power amplifier46and thus, generate the control field42to control the magnetic flux leakage44.

It is desirable to control the magnetic stirring within the molten material24as the mold16leaves the hot chamber12and begins to solidify to produce the desired micro-structure within the completed cast part. The electromagnetic control field42can be increased or decreased to control the stirring in order to produce desired microstructure.

Accordingly, the disclosed exemplary inductive furnace assembly provides for the control of magnetic flux leakage and resultant stirring based on a fixed secondary compensation coil proximate the mold that in turn produce the desired microstructures with the cast part.

There has been provided a method and device for directional solidification of a cast part. While the method and device for directional solidification of a cast part has been described in the context of specific embodiments thereof, other unforeseen alternatives, modifications, and variations may become apparent to those skilled in the art having read the foregoing description. Accordingly, it is intended to embrace those alternatives, modifications, and variations which fall within the broad scope of the appended claims.