Abstract:
An induction heated furnace assembly for producing a directionally solidified casting includes a susceptor that tailors strength of the magnetic field within the chamber to provide a desired grain structure in a completed cast part. The susceptor proportionally blocks portions of the magnetic field to provide different levels of magnetic stirring within the molten material at different locations within the furnace assembly stirring induced by the magnetic field is controlled and varied throughout the furnace assembly to create the desired grain structures in the completed cast article.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    This disclosure generally relates to a method and device for directional solidification of a cast part. More particularly, this disclosure relates to a directional solidification casting process that varies magnetic stirring to provide a desired grain structure. 
         [0002]    A directional solidification (DS) casting process is utilized to orientate grain 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 in one direction. 
         [0003]    Mixing of the molten material within the furnace is known to produce a desired grain size. Such mixing can be induced in the molten metal material by a magnetic field generated from a coil encircling the furnace cavity. Typically, an induction furnace utilizes an electric coil that produces heat required for maintaining the metal in a molten state. Insulation is utilized to retain heat within the furnace cavity and a susceptor is utilized to block any magnetic field produced by the electric coil. When magnetic mixing is desired the susceptor is eliminated. 
         [0004]    Disadvantageously, a minimum level of current is required to produce the heat required to maintain the metal in a molten state. The current level also controls the strength of the magnetic field. However, the levels required to maintain heat may not provide the desired strength of the magnetic field. Accordingly, it is desirable to design and develop a method and device for controlling the strength of the magnetic field acting on the molten material separate from the heating function of the inductive coil. 
       SUMMARY OF THE INVENTION 
       [0005]    A disclosed induction heated furnace assembly for producing a directionally solidified casting includes a susceptor that tailors strength of the magnetic field within the chamber to provide a desired grain structure in a completed cast part. 
         [0006]    The example susceptor proportionally blocks portions of the magnetic field to provide different levels of magnetic stirring within the molten material within the mold. Stirring induced by the magnetic field is reduced in a direction towards the opening of the chamber through which a cast article is removed in the directional solidification process to create the desired grain structures in the completed cast article. 
         [0007]    These and other features of the present invention can be best understood from the following specification and drawings, the following of which is a brief description. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]      FIG. 1  is a schematic illustration of an example inductive furnace with a mold disposed within the furnace. 
           [0009]      FIG. 2  is a schematic illustration of the example inductive furnace with the mold partially withdrawn from the furnace. 
           [0010]      FIG. 3  is a schematic illustration of another example inductive furnace including a plurality of openings in an example susceptor. 
           [0011]      FIG. 4  is a schematic illustration of another example inductive furnace including a single opening in an example susceptor. 
           [0012]      FIG. 5  is a schematic illustration of another example inductive furnace including another example susceptor. 
           [0013]      FIG. 6  is a schematic illustration of another example inductive furnace that includes an example inductive coil with a variable configuration. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0014]    Referring to  FIG. 1 , an example induction furnace assembly  10  includes a chamber  12  that includes an opening  14  through which a mold  32  is received and withdrawn. The chamber  12  is isolated from the external environment by insulated walls  16 . An inductive coil  18  generates heat, indicated by arrows  50 , to maintain metal  34  within the mold  32  at a desired temperature. 
         [0015]    The example furnace assembly includes a susceptor  22  that blocks a portion of a magnetic field (schematically shown at  52 ) that is generated by the inductive coil  18 . The example susceptor  22  is a wall that surrounds the chamber  12  and is made of a graphite material. The susceptor  22  is fabricated from material such as graphite that blocks the penetration of the magnetic field  52  produced by the inductive coil  18 . The susceptor  22  can also provide for the translation of energy from the magnetic field into heat energy, as indicated at arrows  50  to further maintain a temperature within the mold  32 . In the disclosed example, molten metal material  34  is disposed in the mold  32  and supported on a support  38 . The example support  38  includes a chill plate  40  that both supports the mold  32  and includes cooling features to aid in cooling the molten material  34 . 
         [0016]    The inductive coil  18  receives electrical energy from an electric power source schematically indicated at  44 . This electrical energy is provided at a desired current level determined to provide sufficient power and energy to create the desired temperature within the chamber  12  that maintains the molten metal  34  in a molten state. 
         [0017]    The example inductive coil  18  comprises a plurality of electrically conductive hollow tubes  20 . The plurality of tubes  20  also provide for the circulation of a fluid that is generated by a pump  46  that supplies fluid from a fluid source  48  to flow through the tubes  20 . 
         [0018]    In the example a directional solidification casting process is utilized where molten material is poured into the mold  32  within the chamber  12  at a desired temperature to maintain the molten material in a molten state. The support  38  is then lowered through the opening  14  out of the hot chamber  12 . The mold  32  is lowered from the chamber  12  at a desired rate to cool the molten material in a controlled manner to produce desired columnar structure. The controlled cooling produces a solidification front within the molten material  34 . 
         [0019]    In many applications, the completed cast part is desired to include a specific grain structure and size. The size and structure of grains within the completed cast part provide desired material characteristics and performance, such as for example material fatigue performance. In many applications, the finer the grain size the more favorable the performance of the completed cast article. The example furnace assembly  10  includes the susceptor  22  with a varying thickness to block a proportionate amount of the magnetic field  52 . The proportional blocking of the magnetic field  52  generates a proportional amount of magnetic stirring within the molten metal material  34 . 
         [0020]    The generated magnetic field  52  produces currents within the molten metal material that interact with the molten metal material  34  to provide stirring and mixing to break up large grain nuclei to form smaller grain structures. In a standard induction furnace, the susceptor is sized to include a thickness that is thick enough to completely eliminate the generation of any magnetic field within the hot zone of the chamber  12 . The example furnace  10  includes a susceptor of a varying thickness such that it can vary the strength of the magnetic field  52  depending on the position of the mold  32  within the chamber  12 . In this way a variable stirring can be induced within the molten material to break up the larger grain structures to form smaller and more desirable grains in a completed part. 
         [0021]    The example susceptor  22  includes a first thickness  28  disposed at a portion closest to the opening  14 . The susceptor  22  also includes a second thickness  30  that is disposed at an end opposite the opening  14 . The second thickness  30  is much less then the first thickness  28  to allow the largest portion of the magnetic field  52  to pass into and create stirring. 
         [0022]    The example structure of the susceptor  22  provides for the generation of a strongest magnetic field point indicated by  54  and a weakest magnetic field point indicated at  56 . Note that points  54  and  56  represent an area or region within the chamber  12  where the magnetic field  52  is at a greatest or weakest strength. 
         [0023]    The example Susceptor  22  includes the wall that is sloped at an angle  62  between the first thickness  28  and the second thickness  30 . The example susceptor  22  is disposed at a constant angle that provides a uniform increase in thickness in a direction towards the opening  14 . The steady increase in the thickness of the susceptor  22  in a direction towards the opening  14  provides for the steady decrease in magnetic field strength generated within the chamber  12 . The decrease in the magnetic field strength towards the opening  14  produces a decrease in stirring and mixing encountered within the molten material  34 . 
         [0024]    It is desirable to decrease the magnetic stirring within the molten material  34  as the mold  32  leaves the hot chamber  12  along the solidification front to produce the desired grain structure within the completed cast part. 
         [0025]    Referring to  FIG. 2 , with continued reference to  FIG. 1 , the example furnace  10  is illustrated with the mold  32  partially removed from the hot chamber  12 . As the mold  32  is removed, a solidification front  58  is formed within the molten material  34 . Mixing at the solidification front does not provide the desired fine grain structure and can disrupt any desired columnar structures, and therefore in some instances it can be desirable to reduce the amount of magnetic mixing along the solidification front  58 . The example induction furnace  10  reduces the magnitude of the magnetic field  52  in a direction towards the opening  14  such that as the mold  32  is withdrawn from the hot chamber  12 , mixing is slowly reduced until such mixing is completely stopped at a point where the solidification front  58  is formed. 
         [0026]    As is schematically shown, the molten material  34  remains above the solidification front  58  and a solidified portion of the desired cast part  60  extends downward from the solidification front  58 . The solidification front  58  remains substantially stationary relative to the opening  14  as the mold  32  is moved downwardly and out of the chamber  12 . 
         [0027]    This process may also be utilized in concert with a single crystal seed  36  or can use other directional solidification processes to create the desired grain structure. The amount of magnetic stirring can be tailored to provide varying amounts of mixing to induce formation of the desired grain structure in a completed part. 
         [0028]    In operation, the furnace  10  is brought up to a desired temperature by providing a sufficient current from the electric power source  44  to the inductive coil  18 . Water supplied from the pump  46  and fluid source  48  is pumped through the plurality of tubes  20  that make up the inductive coil  18 . The heat  50  created by the inductive coil  12  and also created by a partial conversion of the magnetic field by the susceptor  22  heats the chamber  12  to a desired temperature. Once a desired temperature is reached, molten material  34  is poured into the mold  32 . The mold  32  defines the external shape and features of the completed cast article. In this example, a seed  36  is placed within the mold  32  to further orientate the desired grain structure of the completed cast article. 
         [0029]    The mold  32  is placed on a support  38 . The support  38  is movable in a direction axially into and out of the chamber  12 . Support  38  also includes the chill plate  40  that is supplied with a coolant to maintain a desired cooling temperature to encourage cooling in a uniform manner. With the mold in the chamber  12 , molten material  34  is filled within the mold  32 . Once molten material  34  is received within the mold  32 , the magnetic field  52  generates a mixing and stirring motion within the molten material  34 . This mixing and stirring is governed by the strength of the magnetic field  52 . 
         [0030]    The shape and thickness of the example susceptor  22  governs the strength of the magnetic field  52  by blocking a desired portion of that magnetic field generated by the inductive coil  18 . In this example, the susceptor  22  includes the increasing thickness to proportionally block a greater amount of the magnetic field  52  in a direction toward the opening  14 . At the top most portion of the chamber  12 , where the magnetic field  52  is at the greatest strength the susceptor  22  is at its smallest thickness  30 . As appreciated, the susceptor thicknesses can be adapted to provide the specific magnetic field and stirring properties required to provide the desired grain structure in the completed cast article. 
         [0031]    Referring to  FIG. 3 , another example furnace assembly  70  includes a susceptor  72  that includes a plurality of openings  74 . The plurality of openings provides for a portion of the magnetic field  52  generated by the inductive coils  18  to enter the chamber  12 . Accordingly, the strength of the magnetic field  52  is proportionally controlled by the number and area of the openings within the susceptor  72 . 
         [0032]    In this example, the susceptor  72  includes at least three zones of openings. In a first zone  76 , a large number of openings  74  are provided to allow the generation and strength of the magnetic field  52  to be at its greatest part. In this example, that zone is provided at the top most part of the chamber  12 . 
         [0033]    A second or intermediate zone  78  is disposed between the first zone  76  and a third zone  80 . This second zone  78  provides an intermediate level or strength of a magnetic field to find an intermediate mixing. The third zone  80  blocks a greater portion of the magnetic field  52  to provide the least amount of mixing and blocks most of the magnetic field  52  at a point where the magnetic field  52  within the chamber  12  is at its weakest as is indicated by  56 . The openings  74  are disposed through the entire thickness of the susceptor  72  and provide for the proportional control of the magnitude of the strength of the magnetic field that is encountered within the chamber  12 . 
         [0034]    Referring to  FIG. 4 , another example susceptor  90  is provided for another furnace assembly  88 . The susceptor  90  of this example includes a large opening  96 . The large opening  96  includes an area  100  that decreases in a direction towards the opening  14 . In this example, the opening  96  is triangular shaped having the base or largest width portion disposed at a top most portion of the chamber  12 . The sides  104  are disposed at an angle  102  that decreases to a point  98  and smallest area  94  in a direction towards the opening  14  such that the magnetic field  52  that is blocked from entering the chamber  12  increases in a direction towards the opening  14  and withdrawal of the mold from the furnace assembly  88 . 
         [0035]    The example opening  96  includes the sides  104  disposed at a decreasing angle  102 . This angle  102  is a uniform and constant to provide a proportional reduction in magnetic strength in a direction towards the opening  14 . This decrease in the opening  96  provides for a change in area from a largest area  96  at the top most portion to a smaller area  94  at the bottom most portion. Decreasing area  100  of the opening  96  provides for the controlled reduction in the magnetic field  52  that is utilized for stirring molten material within the example furnace assembly  10 . 
         [0036]    Referring to  FIG. 5 , another furnace assembly  105  is illustrated and includes the opening  96 . The opening  96  also includes a decreasing area  100 . However, the opening  96  differs from the previous example in that the side  106  is at a non-uniform angle. This non-uniform angle is utilized to tailor the strength of the magnetic field  52  as it decreases from a greatest amount of magnetic strength to a least amount of magnetic strength. As appreciated, the shape of the opening  96  can be modified to tailor the magnetic field strength and thereby the amount of mixing of the molten material. 
         [0037]    As appreciated, the several different embodiments of the example inductive furnace assemblies all provide proportionate blocking of the magnetic field  52  to tailor the strength of the magnetic field based on a position of the mold to further tailor mixing and stirring of the molten material of the cast part. Other areas and shapes of opening can be utilized to block portions of the magnetic field that are to generate the desired stirring that provides the desired final grain structure in the cast article. 
         [0038]    Referring to  FIG. 6 , another example induction furnace assembly  110  includes an inductive coil  112  that has a variable number of turns to tailor the strength of the magnetic field  52  produced with in the chamber  124 . The example inductive coil  112  includes portions with different numbers of windings per axial distance. The number of windings for a given current supplied by the power source  44  creates a desired magnitude of the magnetic field  52  that is produced. Increasing or decreasing the number of windings changes the strength of the magnetic field  52  that is generated. 
         [0039]    In this example, the susceptor  120  includes a fixed thickness  122  for the entire axial length of the chamber  12 . The inductive coil  112  includes three different zones each having different numbers of windings per axial length. The first set of windings  114  includes a high number of windings to produce the greatest strength of the magnetic field  52  within the chamber  12 . 
         [0040]    A second number of turns  116  produce an intermediate magnetic field strength within the chamber  12 . A third number  118  is smaller than both the second  116  and first  114  number of turns and produces the least amount of magnetic field strength. In this example, the least amount of magnetic field strength is provided by the group of windings  118  disposed at a lower portion of the furnace assembly  110 . The modification of the inductive coil  112  provides the desired tailoring and proportional reduction in magnetic field strength within the chamber  12  desired to create variable mixing dependent on the axial position of the mold as it is being lowered from the furnace assembly  110 . 
         [0041]    Accordingly, the disclosed example inductive furnace assemblies provide for the generation and control of varying amounts of magnetic stirring based on a position of the mold that in turn produce the desired grain structures with the cast part. 
         [0042]    Although an example embodiment of this invention has been disclosed, a worker of ordinary skill in this art would recognize that certain modifications would come within the scope of this invention. For that reason, the following claims should be studied to determine the true scope and content of this invention.