BACKGROUND OF THE INVENTION  
       [0001]     This invention relates generally to sintered metallic components and more particularly to components sintered by microwave heating.  
         [0002]     Metal Injection Molding (“MIM”) is a known process in which a fine metallic powder is mixed with a plastic binder and extruded to a desired shape using plastic molding equipment. The resulting preform is washed to remove a large portion of the plastic from the powder. Subsequent sintering consolidates the preform to form a finished component.  
         [0003]     Prior art methods of sintering for MIM preforms require furnace heat treatment at temperatures capable of causing the metal powders to sinter together to make the preform mechanically strong enough for further processing. This is a time consuming process that results in a non uniform product due to the heating process being “from the outside in”, meaning the outer portion of the preform gets more time at high temperature and can sinter earlier causing voids to be trapped inside the preforms. This can also result in non-uniform mechanical properties.  
         [0004]     Accordingly, there is a need for a method of sintering a metallic preform to provide a uniformly dense finished component.  
       BRIEF SUMMARY OF THE INVENTION  
       [0005]     The above-mentioned need is met by the present invention, which according to one aspect provides a method of producing a metallic component including: providing a mixture of a metallic powder and a binder; melting the binder and forming the mixture into a preform in the shape of the component; remove a majority of the binder from the preform; and heating the preform with microwave energy to remove the remainder of the binder and to sinter the metal powder together to form the component.  
         [0006]     According to another aspect of the invention, a method of producing a metallic component includes providing a mixture of a metallic powder and a binder; melting the binder and forming the mixture into a continuous preform in the shape of a desired component; removing a majority of the binder from the preform; and heating the preform with microwave energy to remove the remainder of the binder and to sinter the metallic powder together to form the component. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0007]     The invention may be best understood by reference to the following description taken in conjunction with the accompanying drawing figures in which:  
         [0008]      FIG. 1  is a perspective view of a compressor blade constructed in accordance with the present invention;  
         [0009]      FIG. 2  is block diagram of a manufacturing process carried out in accordance with the present invention;  
         [0010]      FIG. 3  is a schematic side view of an injection molding apparatus;  
         [0011]      FIG. 4  is a schematic side view of a preform being removed from the mold shown in  FIG. 3 ;  
         [0012]      FIG. 5  is a schematic cross-sectional view of a preform inside a microwave chamber;  
         [0013]      FIG. 6  is schematic side view of an apparatus for carrying out an alternative molding and sintering process;  
         [0014]      FIG. 7  is a schematic perspective view of a weld wire produced by the present invention;  
         [0015]      FIG. 8  is a schematic perspective view of the weld wire of  FIG. 7  wound onto a spindle for further processing;  
         [0016]      FIG. 9  is a schematic view of an alternative extruding apparatus; and  
         [0017]      FIG. 10  is a schematic perspective view of a metallic sheet wound onto a spindle for further processing. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0018]     Referring to the drawings wherein identical reference numerals denote the same elements throughout the various views,  FIG. 1  depicts an exemplary compressor blade  10  for a gas turbine engine. The present invention is equally applicable to the construction of other types of metallic components, non-limiting examples of which include rotating turbine blades, stationary turbine vanes, turbine shrouds, and the like. The compressor blade  10  comprises an airfoil  12  having a leading edge  14 , a trailing edge  16 , a tip  18 , a root  19 , and opposed sides  20  and  22 . An arcuate inner platform  24  is attached to the root  19  of the airfoil  12 . A dovetail  26  extends downward for mounting the blade  10  in a rotor slot. The compressor blade  10  is made from a metal alloy suitable for the intended operating conditions.  
         [0019]      FIG. 2  depicts the process for constructing the compressor blade  10  according to the method of the present invention. Initially, as shown in block  28 , a metallic powder and a suitable binder are provided.  
         [0020]     The metallic powder may be a single alloy or it may be a mechanical mixture of more than one alloy. For optimum performance in the injection molding process and also for compatibility with the microwave heating step described below, the particle size of the metallic powder should be about 100 micrometers or less. Examples of known alloys suitable for constructing compressor blades include titanium alloys such as Ti-6Al-4V, nickel-based alloys such as INCO 718 or UDIMENT 720, and iron-based alloys such as A286.  
         [0021]     The binder may be any material which is chemically compatible with the metallic powder and which allows the required processing (e.g. mixing, injection, solidification, and leaching). Examples of known suitable binders include waxes and polymer resins. The binder may be provided in a powder form.  
         [0022]     The binder and the metallic powder are thoroughly mixed together, as shown in block  30 . The mixture is then heated to melt the binder and create a fluid with the metallic powder coated by the binder (block  32 ). Next, the mixture is formed into a predetermined shape at block  34 . One way of forming the mixture is to use a known injection-molding apparatus. A schematic view of an injection molding apparatus  36  including a hopper  38  and an extruder  40  with rotating screw  42  is shown in  FIG. 3 . The mixture is extruded into the cavity  44  of a mold  46 . The mold  46  may optionally be heated to avoid excessively rapid solidification of the binder which would result in a brittle preform  48 . Instead of melting the binder in a discrete batch, the mixture could be molded in a continuous manner using known injection molding equipment capable of melting the binder as it passes through the screw  42 . Once the mixture has solidified, the mold  46  is opened as shown in  FIG. 4  and the resulting uncompacted or “green” preform  48  is removed (see block  50  in  FIG. 2 ).  
         [0023]     The preform  48  comprises metal particles suspended in the solidified binder. The preform  48  is not suitable for use as a finished component, but merely has sufficient mechanical strength to undergo further processing. At block  52  of  FIG. 2 , the preform  48  is leached to remove the majority of the binder. This may be done by submerging or washing the preform  48  with a suitable solvent which dissolves the binder but does not attack the metallic powder  
         [0024]     Next, at block  54 , the preform  48  is microwave sintered. As shown in  FIG. 5 , The preform  48  is placed in a chamber  56  which includes means for creating a suitable atmosphere to prevent undesired oxidation of the preform  48  or other reactions during the sintering process. In the illustrated example a supply  58  of inert gas such as argon is connected to the interior of the chamber  56 . The sintering could also be performed under a vacuum. A microwave source  60  such as a known type of cavity magnetron with an output in the microwave frequency range is mounted in communication with the chamber  56 . The microwave spectrum covers a range of about 1 GHz to 300 GHz. Within this spectrum, an output frequency of about 2.4 GHz is known to couple with and heat metallic particles without passing through solid metals.  
         [0025]     The microwave source  60  is activated to irradiate the preform  48 . In the illustrated example the microwave source  60  is depicted as having a direct line-of-sight to the entire preform  48 . However, it is also possible to configure the chamber  56 , which would typically be metallic, so that the preform is heated by a combination of direct and reflected microwaves. Because of the small metallic particle size in the preform  48 , the microwaves  62  couple with the particles and heat them. The preform  48  is heated to a temperature below the liquidus temperature of the metallic powder and high enough to cause the metallic powder particles to fuse together and consolidate. The high temperature also melts and drives out any remaining binder. The preform  48  is held at the desired temperature for a selected time period long enough to result in a consolidated compressor blade  10 . The heating rate (i.e. the output wattage of the microwave source) is selected depending on variables such as the mass of the preform  48 , the shape of the chamber  48  and the and the desired cycle time of the sintering process. When compared to prior art methods, the combination of the MIM-formed preform  48  with the microwave sintering step gives the compressor blade  10  a significantly greater density, that is, freedom from voids, in less time.  
         [0026]     When the sintering cycle is complete, the compressor blade  10  is removed from the chamber  56  and allowed to cool. When required, the compressor blade  10  may be subjected to further consolidation using a known hot isostatic pressing (“HIP”) process to result in a substantially 100% dense component, as noted in block  63  of  FIG. 2 . If desired, the compressor blade  10  may be subjected to additional processes such as final machining, coating, inspection, etc. in a known manner (see block  64  of  FIG. 2 ).  
         [0027]      FIGS. 6 and 7  illustrate an alterative method suitable for producing continuous components. Initially, a metallic powder and a suitable binder are provided. The metallic powder may be a single alloy or it may be a mechanical mixture of more than one alloy. For optimum performance in the injection molding process and also for compatibility with the microwave heating step described below, the particle size of the metallic powder should be about 100 micrometers or less in diameter. This process is particularly suitable for alloys which are difficult to cold work and which are ordinarily cast. Examples of such alloys include so-called “superalloys” based on nickel or cobalt and containing a high percentage of a gamma-prime phase component. Examples of such alloys include RENE 77, RENE 80, RENE 142, and RENE N4 and N5 nickel-based alloys.  
         [0028]     The binder may be any material which is chemically compatible with the metallic powder and which allows the required processing (e.g. mixing, injection, solidification, and leaching). Examples of known suitable binders include waxes and polymer resins. The binder may be provided in a powder form.  
         [0029]     The binder and the metallic powder are thoroughly mixed together. The mixture is then heated to melt the binder and create a fluid with the metallic powder coated by the binder. Next, the mixture is extruded using known injection-molding apparatus. A schematic view of an injection molding apparatus  136  including a hopper  138  and an extruder  140  with rotating screw  142  is shown in  FIG. 6 . The mixture is extruded through a die  144  of a known type to produce a continuous preform  148  of a constant cross-section. For example, a die  144  having a circular opening of about 1.27 mm (0.050 in.) in diameter may be used to produce a preform  148  for use as a welding filler wire. The die  144  may optionally be heated to avoid excessively rapid solidification of the binder which would result in a brittle preform  148 . Once the preform  148  has solidified, it passes along a conveyer belt  150  or other suitable transport mechanism.  
         [0030]     The conveyor belt  150  carries the preform  148  through a solvent bath  152  which leaches the majority of the binder out of the preform  148 . This may be done with a suitable solvent which dissolves the binder but does not attack the metallic powder.  
         [0031]     The preform  148  then passes into a sintering chamber  156  where it is microwave sintered. As shown in  FIG. 6 , The chamber  156  includes means for creating a suitable atmosphere to prevent undesired oxidation of the preform  148  or other reactions during the sintering process. In the illustrated example a supply  158  of inert gas such as argon, or a gas fore creating a reducing atmosphere such as hydrogen is connected to the interior of the chamber  156 . The processing could also be performed under a vacuum. A microwave source  160  similar to the source  60  described above is mounted in communication with the chamber  156 . The microwave source  160  is activated to irradiate the preform  148 . Because of the small metallic particle size in the preform  148 , the microwaves couple with the particles and heat them. As the preform  148  passes through the chamber  156 , it is heated to a temperature below the liquidus temperature of the metallic powder and high enough to cause the metallic powder to fuse together and consolidate. The high temperature also melts and drives out any remaining binder. The heating rate (i.e. the output wattage of the microwave source) and the speed of the conveyor belt  150  are selected so that the preform  148  is held at the desired temperature for a selected time period long enough to result in a consolidated completed component  162 .  FIG. 7  illustrates a short section of the component  162 , which in this case is a welding filler wire  162 . When compared to prior art methods, the combination of the MIM-formed preform  148  with the microwave sintering step gives the filler wire  162  a significantly greater density, that is, freedom from voids, in less time.  
         [0032]     When the sintering cycle is complete, the component  162  passes out of the chamber  156  and allowed to cool. If desired, the product  162  may be subjected to additional processes such as coating, inspection, etc. in a known manner.  
         [0033]     When required, the welding filler wire  162  may be subjected to further consolidation using a known hot isostatic pressing (“HIP”) process to result in a substantially 100% dense component. As shown in  FIG. 8 , This step may be facilitated by winding the welding filler wire  162  on to a spindle  164 , with a small spacing “S” between the individual coils. The loaded spindle  164  may then be placed into a chamber (not shown) for the HIP process.  
         [0034]     The continuous process described above may be used to produce any other type of component with a constant cross-section. For example, the process may be used to produce sheet materials. As shown schematically in  FIG. 9 , this may be done by providing a die  244  of the desired width “W” for extruding a wide, thin preform  248 . In order to supply an adequate feed of a binder-metallic power mixture to the die  244 , a plurality of side-by side injection molding apparatuses  236  may be provided. The extruded preform  248  is then leached and microwave sintered as described above, to result in a metallic sheet  262 , shown in  FIG. 10 .  
         [0035]     When required, the metallic sheet  262  may be subjected to further consolidation using a HIP process to result in a substantially 100% dense component. As shown in  FIG. 10 , This step may be facilitated by winding the metallic sheet on to a spindle  264 . A release compound may be placed between the layers of the metallic sheet  262  to prevent undesired consolidation and diffusion bonding of the layers. The loaded spindle  264  may then be placed into a chamber (not shown) for the HIP process.  
         [0036]     The foregoing has described a manufacturing process for microwave sintered components. While specific embodiments of the present invention have been described, it will be apparent to those skilled in the art that various modifications thereto can be made without departing from the spirit and scope of the invention. Accordingly, the foregoing description of the preferred embodiment of the invention and the best mode for practicing the invention are provided for the purpose of illustration only and not for the purpose of limitation, the invention being defined by the claims.
A method of producing a metallic component includes: providing a mixture of a metallic powder and a binder; melting the binder and forming the mixture into a preform in the shape of the component; removing a majority of the binder from the preform; and heating the preform with microwave energy to remove the remainder of the binder and to sinter the metal powder together to form the component. The component may be formed as an individual component or continuously.