Abstract:
A single or multilayer smart susceptor for temperature control of part fabrication in an induction processing system generally consists of single layer or laminated layers of ferromagnetic material susceptible to heating by induction. The use of the susceptors in fabrication at elevated temperatures causes oxidation damage to the surface. The susceptor may be coated with a nickel aluminide surface coating to minimize oxidation of the susceptor outer surface.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to single or multilayer smart susceptors used for achieving thermal uniformity in induction processing of organic matrix composites or metals. The improved smart susceptor has a coating to minimize oxidation during repeated use in induction process manufacturing. 
     An induction processing system used for fabrication of material combines, metals and the like are disclosed in U.S. Pat. Ser. Nos. 5,808,281, 5,728,309, and 5,645,744 and is hereby incorporated by reference. The induction processing system uses susceptors to translate electrical energy to heat energy for fabrication of various parts and structures. The susceptors are often referred to as smart susceptors because the material composition is specifically chosen to produce a set temperature point when used in an induction processing system. 
     The sheets of material used to construct the susceptor may consist of ferromagnetic materials including a combination of iron (Fe), nickel (Ni), and/or cobalt (Co). For the higher temperature applications the alloys may be Co based with additions of Fe and Ni. Each specific composition and combination of material sheets constructed is based on the Curie temperature characteristics desired of the susceptor. The Curie Point at which there is a transition between the ferromagnetic and paramagnetic phases of a material is used to set the equilibrium temperature point caused by inductive heating in the inductive processing system. 
     The use of smart susceptor alloys such as combinations of Co, Ni and Fe material in cycling to elevated temperatures in inductive processing systems may cause oxidation of the susceptor material. It has been found that at elevated temperatures these alloys may oxidize at a relatively aggressive rate. In addition, the iron oxide formed may create a low melting point intermetallic interface when positioned in close proximity to the die material at elevated temperature. These conditions may cause the susceptors to deteriorate prematurely requiring replacement thereof. Also, the susceptors may interact with the die material, a ceramic, which necessitates die repair or replacement. 
     The known approach to reducing the effects of the oxidation of the susceptor has been to insert an intermediate metal sheet of material, such as Inconel 625™, between the susceptor and the die. Material such as Inconel 625™ is a nickel base alloy that is non-magnetic and has oxidation resistive qualities. While this solves the interaction problem, it does not eliminate the oxidation problem and adds additional parts to the system tooling requirements. 
     One solution to oxidation of the metal surface is the coating thereof. Examples include coating of steel and other materials in internal combustion engines where the high temperature combined with exposure to air create an environment that may be corrosive and cause erosion of metal surfaces. A method of flame or plasma spraying a thin coating layer on the exposed metal surfaces has been used to reduce oxidation damage. Various materials have been used to coat metals and other materials depending on the application. U.S. Pat. No. 3,762,884 discloses one such process as well as surveying other methods. These coatings were not applied to environments such as those for which the present invention is intended nor have coating compositions been formulated to solve the smart susceptor problem. 
     The coated smart susceptor and method therefore improves the control of superplastic forming, hot forming and/or heat treating processes to achieve a higher quality fabricated part. The coated susceptor use also enables cold loading and unloading of parts that improves dimensional control. The quick thermal cycles with induction processing and the improved life of the susceptor due to the coating reduces the need for batch loading of material as such is no longer required for economic viability of the fabrication processes. 
     As can be seen, there is a need for a simple, effective protectant to minimize oxidation of susceptors used in induction processing systems. 
     SUMMARY OF THE INVENTION 
     An improved susceptor construction and method according to the present invention comprises a metal alloy composition susceptor having a nickel aluminide (NiAl) coating thereon. 
     In one aspect of the present invention, a susceptor for temperature control of a part fabrication in an induction processing system comprises a single layer or a lamination of layers of ferromagnetic material susceptible to heating by induction, each having a selected Curie point. The single layer or laminated material layers are then coated with a nickel aluminide surface coat for oxidation protection during temperature cycling for part fabrication in the induction processing system. Also, the temperature at which the nickel aluminide may be sprayed to coat a susceptor and any exothermic reaction from the process generates heat creating a sintering effect to bond the coating to the susceptor outer surface. 
     In another aspect of the invention, the multiplayer susceptor may include an Al 2 O 3  adherent layer formed on the surface coating to inhibit deterioration of the surface coating. The nickel aluminide coating may create it&#39;s own Al 2 O 3  layer at the surface. This oxide film and the very stable thermally resistant nickel aluminide layer underneath may form an oxidation barrier. 
     A further aspect of the present invention involves the method for producing a single or multilayer susceptor having a surface coat oxidation protective layer comprising the steps of fabricating a single layer or a plurality of laminated material layers of ferromagnetic material, introducing a wire or powder mixture of nickel and aluminum into a flame or plasma spray gun, operating the spray gun to heat the powder or wire to form droplets and to spray the droplets on an outer surface of the single or laminated material layers, and continuing the spraying to sinter a surface coating of nickel aluminide to the outer surface. 
    
    
     These and other features, aspects and advantages of the present invention will become better understood with reference to the following drawings, description and claims. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 illustrates a schematic representation of an induction processing system according to the prior art; 
     FIG. 2 illustrates a partial cross-sectional view of the tooling with smart susceptor according to the prior art; 
     FIG. 3 illustrates a partial perspective view of an induction processing tool with temperature gradients according to the prior art; 
     FIG. 4 illustrates a transition diagram of the deposition of nickel aluminide coating of a susceptor according to the present invention; 
     FIG. 5 illustrates a cross-sectional view of an aluminum nickel formed wire for use in a plasma spray gun according to the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The following detailed description is of the best currently contemplated modes of carrying out the invention. The description is not to be taken in a limiting sense, but is made merely for the purpose of illustrating the general principles of the invention, since the scope of the invention is best defined by the appended claims. 
     Referring to FIG. 1, an induction processing system  10  has metal support plates  12  mounted on support posts  14  that may be used to constrain a tool  20  such as a phenolic box under pressure. The tool  20  includes ceramic die halves  22  which contain the part cavity  24  therebetween. A susceptor (not shown) is mounted in a cavity  24 . 
     The tool  20  includes reinforcing rods  26  for reinforcement of the ceramic dies  22 . The tool  20  also incorporates induction coils  28  for heating and coolant conduits  30  for temperature control. Flexible coil connections  32  are provided external to the dies  22  to connect induction coils  28  and coolant conduits  30 . In operation, electric power is applied to the induction coils  28  that due to magnetic induction heat the susceptor contained in the part cavity  24 . 
     Referring to FIG. 2, a portion of the tool  20  with ceramic dies  22  and reinforcing rods  26  includes part cavity  24 . A susceptor  40  with a fabrication part  42  may be contained therein. For a particular part  42  manufacture for super-plastic forming thereof a relative temperature in the part cavity  24  at the ceramic die  22  inner surface may be 1,650° F. A single layer susceptor  40  may be used when there is just one crucial processing temperature necessary for part fabrication. The susceptor  40  may be coated on all external surfaces. 
     Referring to FIG. 3, the ceramic die  22  has electric power applied at induction coil  28 . A temperature of 1,650° F. may be desired at the inner surface  36  of the ceramic die  22  in the part cavity  24  for interior point  33  heating of the susceptor (not shown). With the ceramic die  22  characteristics the temperature between the induction coil  28  and the inner surface  36  may be 333° F. at an intermediate point  38 . A susceptor may be maintained at the desired Curie Point when sufficient electric power is applied to the induction coils  28  and proper cooling is flowing through coolant conduits  30 . The environment point  34  may be room or ambient temperature. 
     In use, the induction processing system  10  may have material placed in it for heat treatment, forming, consolidation, etc. The system may then be closed and activated to raise and maintain the internal part cavity  24  at the desired temperature. The system may then be cooled and the part removed. This is a cycle process that is repeated for the production of each part. 
     Referring to FIG. 4, the susceptor  40  may consist of a single or lamination of sheets of highly ferromagnetic materials composed of a combination of Fe, Ni, and/or Co. In higher relative temperature applications the alloys may be Co based with additions of Fe and Ni. The higher the Fe content the more likely the susceptor  40  maintains a set temperature, that is, the smarter it appears. 
     The Fe and the elevated temperature may combine to form flakes of iron oxide on the susceptor  40  outer surface  44  as a result of the temperature cycling process. It has been found by experiment that coating the susceptor  40  with a nickel aluminide plasma spray coat  50  inhibits oxide formation. The approximate effective coating thickness may be about 0.005 inch to 0.010 inch. 
     To apply the coating  50 , a wire  56  consisting of a nickel powder  60  enclosed in an aluminum foil  58  as illustrated in FIG. 5 may be used in a flame or plasma spray gun. The composition of the wire  56  may be nickel powder of approximately −125 to +45 um (−120 to +325 mesh) range of granularity and represent approximately 79.5 to 80.5 percent of a unit cross section of the wire  56  by weight. The aluminum foil  58  may be approximately 0.010 to 0.015 inch thickness and approximately 19.5 to 20.5 percent of a unit cross section of the wire  56  by weight. A wire  56  that may be ⅛ inch in diameter may define a foil thickness to achieve an approximately 20 percent aluminum content wherein the foil thickness may be related to the diameter and powder density. The use of the flame or plasma spray gun as understood in the art melts the wire  56  and the resulting liquid may be sprayed on the susceptor  40  using a compressed gas. 
     A powder mixture of nickel and aluminum may also be used for spraying with a flame or plasma spray gun. The powder mixture may also have an approximate ratio of 80 percent nickel and 20 percent aluminum. 
     During the melting and spraying action the nickel and aluminum are mixed together and begin to react as droplets  54 . This may be an exothermic reaction and therefore heat may be generated. When the droplets  54  contact the susceptor  40  they may still be reacting and therefore tend to sinter to the outer surface  44  interface  52  due to the heat created. This combination of actions may result in a strong bond of converted nickel aluminide on the susceptor  40 . 
     It has been found that the nickel aluminde intermetallic is chemically very stable as evidenced by the exothermic reaction occurring during the spray coating. The nickel aluminide coating may form a stable adherent Al 2 O 3  surface layer  62  when used at the processing temperatures for the susceptor  40 . It has been found that after approximately two cycles of use of a coated susceptor  40  in manufacturing that an approximate 0.0002 inch to 0.0008 inch (5 to 20 microns) thick coating of Al 2 O 3  is formed on the nickel aluminide coating. If the Al 2 O 3  layer cracks during use, the ductile nickel aluminide coating can reform the oxide layer at elevated temperatures. The result may be a stable and strong oxidation resistant coating on the susceptor  40 . 
     It should be understood, of course, that the foregoing relates to preferred embodiments of the invention and that modifications may be made without departing from the spirit and scope of the invention as set forth in the following claims.