Abstract:
In various embodiments, three-dimensional layered metallic parts are substantially free of gaps between successive layers, are substantially free of cracks, and have densities no less than 97% of the theoretical density of the metallic material.

Description:
RELATED APPLICATION 
       [0001]    This application claims the benefit of and priority to U.S. Provisional Patent Application No. 62/302,847, filed Mar. 3, 2016, the entire disclosure of which is hereby incorporated herein by reference. 
     
    
     TECHNICAL FIELD 
       [0002]    In various embodiments, the present invention relates to high-density, crack-free metallic parts fabricated via additive manufacturing techniques. 
       BACKGROUND 
       [0003]    Additive manufacturing, or three-dimensional (3D) printing, is a widely utilized technique for rapid manufacturing and rapid prototyping. In general, additive manufacturing entails the layer-by-layer deposition of material by computer control to form a three-dimensional object. Most additive manufacturing techniques to date have utilized polymeric or plastic materials as raw materials, as such materials are easily handled and melt at low temperatures. Since additive manufacturing involves the melting of only small amounts of material at a time, the process has the potential to be a useful technique for the fabrication of large, complex structures composed of metal as well. Unfortunately, additive manufacturing of metallic materials is not without its challenges. When fabricating a three-dimensional part with a metallic precursor material via additive manufacturing, the melting of the precursor material may result in sparking, blistering, and splattering (i.e., ejection of small pieces of the precursor material itself). In addition, even if the three-dimensional part is successfully fabricated utilizing conventional precursor materials, the part may exhibit excessive porosity, cracking, material splatter, and insufficient density and machinability. 
         [0004]    In view of the foregoing, there is a need for improved precursor materials for the additive manufacturing of metallic parts. 
       SUMMARY 
       [0005]    In accordance with various embodiments of the present invention, wires for use as feedstock for additive manufacturing processes are fabricated such that the amounts of gaseous and/or volatile impurities therein are reduced or minimized. As utilized herein, the term “volatile elements” refers to elements having boiling points lower than the melting point of the nominal, majority wire material. For example, the concentrations of elements such as oxygen (O), sodium (Na), magnesium (Mg), phosphorus (P), sulfur (S), potassium (K), calcium (Ca), and antimony (Sb) may be kept below a concentration of 20 ppm, below a concentration of 10 ppm, below a concentration of 5 ppm, below a concentration of 3 ppm, below a concentration of 2 ppm, or even below a concentration of 1 ppm (all concentrations herein are by weight unless otherwise indicated), and one or more, or even all, of these elements may be volatile elements in various embodiments of the present invention. The precursor wire itself may include, consist essentially of, or consist of one or more refractory metals, e.g., niobium (Nb), tantalum (Ta), rhenium (Re), tungsten (W), and/or molybdenum (Mo). The wire may be utilized in an additive manufacturing process to form a three-dimensional part, e.g., a refractory crucible. 
         [0006]    In various embodiments of the invention, the precursor wire is fabricated, at least partially, via arc melting in a vacuum or a substantially inert ambient. The arc-melting process advantageously minimizes or reduces the concentration of volatile impurities within the wire, thereby enabling successful additive manufacturing processes utilizing the wire. The resulting wire is utilized in an additive manufacturing process to form a three-dimensional part composed, at least partially, of the precursor material. In exemplary embodiments, the wire is fed toward a movable platform, and the tip of the wire is melted by, e.g., an electron beam or a laser. The platform (and/or the wire) moves such that the molten wire traces out the pattern of a substantially two-dimensional slice of the final part; in this manner, the final part is fabricated in layer-by-layer fashion via melting and rapid solidification of the wire. In such additive manufacturing processes, the wire is successfully melted during formation of the three-dimensional part with minimal (if any) sparking, blistering, and/or splattering. In addition, the finished part exhibits a high density (e.g., greater than 96%, greater than 97%, greater than 98%, or even greater than 99% of the theoretical density) without the porosity or cracking that may accompany use of conventional powder metallurgy feedstock materials, particularly those for refractory metals. 
         [0007]    Wire in accordance with embodiments of the invention may also be utilized in a variety of different wire-fed welding applications (e.g., MIG welding, welding repair) in which an electric arc is struck between the wire and a workpiece, causing part of the wire to fuse with the workpiece. 
         [0008]    In an aspect, embodiments of the invention feature a method of fabricating a three-dimensional part that includes, consists essentially of, or consists of molybdenum. In a step (a), powder is compacted to form a feed electrode. The powder includes, consists essentially of, or consists of molybdenum. In a step (b), the feed electrode is arc-melted in a processing ambient including, consisting essentially of, or consisting of a vacuum or one or more inert gases, thereby forming a billet. In a step (c), the billet is mechanically deformed into wire having a diameter (or other dimension, e.g., width) less than a diameter (or other dimension, e.g., width) of the billet. In a step (d), a tip of the wire is translated relative to a platform (i.e., all or a portion of the wire is translated, the platform is translated, or both). In a step (e), while the tip of the wire is being translated, the tip of the wire is melted with an energy source to form a molten bead, whereby the bead cools to form at least a portion of a layer of a three-dimensional part. In a step (f), steps (d) and (e) are repeated one or more times to produce the three-dimensional part (or at least a portion thereof). The three-dimensional part includes, consists essentially of, or consists of molybdenum. 
         [0009]    Embodiments of the invention may include one or more of the following in any of a variety of combinations. A concentration within the wire of sodium, calcium, antimony, magnesium, phosphorous, and/or potassium may be less than 5 ppm by weight, less than 4 ppm by weight, less than 3 ppm by weight, less than 2 ppm by weight, or less than 1 ppm by weight. A concentration within the wire of sodium, calcium, antimony, magnesium, phosphorous, and/or potassium may be no less than 0.001 ppm by weight, no less than 0.005 ppm by weight, no less than 0.01 ppm by weight, no less than 0.05 ppm by weight, no less than 0.1 ppm by weight, or no less than 0.5 ppm by weight. A concentration of oxygen within the wire may be less than 25 ppm by weight, less than 22 ppm by weight, less than 20 ppm by weight, less than 19 ppm by weight, less than 18 ppm by weight, less than 15 ppm by weight, or less than 10 ppm by weight. A concentration of oxygen within the wire may be no less than 0.001 ppm by weight, no less than 0.005 ppm by weight, no less than 0.01 ppm by weight, no less than 0.05 ppm by weight, no less than 0.1 ppm by weight, no less than 0.5 ppm by weight, no less than 1 ppm by weight, or no less than 2 ppm by weight. The density of at least a portion of the three-dimensional part may be greater than 97% of a theoretical density of molybdenum, greater than 98% of a theoretical density of molybdenum, greater than 99% of a theoretical density of molybdenum, or greater than 99.5% of a theoretical density of molybdenum. The density of at least a portion of the three-dimensional part may be no greater than 100% of a theoretical density of molybdenum, no greater than 99.9% of a theoretical density of molybdenum, or no greater than 99.8% of a theoretical density of molybdenum. Step (c) may include, consist essentially of, or consist of drawing, rolling, swaging, extruding, and/or pilgering. Step (a) may include sintering the compacted powder at a temperature greater than 900° C., greater than 950° C., greater than 1000° C., greater than 1100° C., or greater than 1200° C. Step (a) may include sintering the compacted powder at a temperature less than 2500° C. In step (e), the energy source may include, consist essentially of, or consist of an electron beam and/or a laser beam. Prior to step (a), the powder may be provided by a process including, consisting essentially of, or consisting of plasma densification and/or plasma atomization. Prior to step (a), the powder may be provided by a process including, consisting essentially of, or consisting of (i) hydrogenating metal to form a metal hydride, (ii) mechanically grinding the metal hydride into a plurality of particles, and (iii) dehydrogenating the metal hydride particles. Embodiments of the invention include three-dimensional objects or parts fabricated according to any of the above methods. 
         [0010]    In another aspect, embodiments of the invention feature a method of fabricating a three-dimensional part utilizing wire. The part includes, consists essentially of, or consists of molybdenum. The wire is produced by a process including, consisting essentially of, or consisting of (i) compacting powder to form a feed electrode, the powder including, consisting essentially of, or consisting of molybdenum, (ii) arc-melting the feed electrode in a processing ambient including, consisting essentially of, or consisting of a vacuum or one or more inert gases, thereby forming a billet, and (iii) mechanically deforming the billet into wire having a diameter (or other dimension, e.g., width) less than a diameter (or other dimension, e.g., width) of the billet. In a step (a), a tip of the wire is translated relative to a platform (i.e., all or a portion of the wire is translated, the platform is translated, or both). In a step (b), while the tip of the wire is being translated, the tip of the wire is melted with an energy source to form a molten bead, whereby the bead cools to form at least a portion of a layer of a three-dimensional part. In a step (c), steps (a) and (b) are repeated one or more times to produce at least a portion of the three-dimensional part. The three-dimensional part includes, consists essentially of, or consists of molybdenum. 
         [0011]    Embodiments of the invention may include one or more of the following in any of a variety of combinations. A concentration within the wire of sodium, calcium, antimony, magnesium, phosphorous, and/or potassium may be less than 5 ppm by weight, less than 4 ppm by weight, less than 3 ppm by weight, less than 2 ppm by weight, or less than 1 ppm by weight. A concentration within the wire of sodium, calcium, antimony, magnesium, phosphorous, and/or potassium may be no less than 0.001 ppm by weight, no less than 0.005 ppm by weight, no less than 0.01 ppm by weight, no less than 0.05 ppm by weight, no less than 0.1 ppm by weight, or no less than 0.5 ppm by weight. A concentration of oxygen within the wire may be less than 25 ppm by weight, less than 22 ppm by weight, less than 20 ppm by weight, less than 19 ppm by weight, less than 18 ppm by weight, less than 15 ppm by weight, or less than 10 ppm by weight. A concentration of oxygen within the wire may be no less than 0.001 ppm by weight, no less than 0.005 ppm by weight, no less than 0.01 ppm by weight, no less than 0.05 ppm by weight, no less than 0.1 ppm by weight, no less than 0.5 ppm by weight, no less than 1 ppm by weight, or no less than 2 ppm by weight. The density of at least a portion of the three-dimensional part may be greater than 97% of a theoretical density of molybdenum, greater than 98% of a theoretical density of molybdenum, greater than 99% of a theoretical density of molybdenum, or greater than 99.5% of a theoretical density of molybdenum. The density of at least a portion of the three-dimensional part may be no greater than 100% of a theoretical density of molybdenum, no greater than 99.9% of a theoretical density of molybdenum, or no greater than 99.8% of a theoretical density of molybdenum. Mechanically deforming the billet into wire may include, consist essentially of, or consist of drawing, rolling, swaging, extruding, and/or pilgering. The process of producing the wire may include sintering the compacted powder at a temperature greater than 900° C., greater than 950° C., greater than 1000° C., greater than 1100° C., or greater than 1200° C. The process of producing the wire may include sintering the compacted powder at a temperature less than 2500° C. In step (b), the energy source may include, consist essentially of, or consist of an electron beam and/or a laser beam. The process of producing the wire may include providing the powder by a process including, consisting essentially of, or consisting of plasma densification and/or plasma atomization. The process of producing the wire may include providing the powder by a process including, consisting essentially of, or consisting of (i) hydrogenating metal to form a metal hydride, (ii) mechanically grinding the metal hydride into a plurality of particles, and (iii) dehydrogenating the metal hydride particles. Embodiments of the invention include three-dimensional objects or parts fabricated according to any of the above methods. 
         [0012]    In yet another aspect, embodiments of the invention feature a method of fabricating a three-dimensional part that includes, consists essentially of, or consists of molybdenum. In a step (a), a wire including, consisting essentially of, or consisting of arc-melted molybdenum is provided. In a step (b), a tip of the wire is translated relative to a platform (i.e., all or a portion of the wire is translated, the platform is translated, or both). In a step (c), while the tip of the wire is being translated, the tip of the wire is melted with an energy source to form a molten bead, whereby the bead cools to form at least a portion of a layer of a three-dimensional part. In a step (d), steps (b) and (c) are repeated one or more times to produce at least a portion of the three-dimensional part. The three-dimensional part includes, consists essentially of, or consists of molybdenum. 
         [0013]    Embodiments of the invention may include one or more of the following in any of a variety of combinations. A concentration within the wire of sodium, calcium, antimony, magnesium, phosphorous, and/or potassium may be less than 5 ppm by weight, less than 4 ppm by weight, less than 3 ppm by weight, less than 2 ppm by weight, or less than 1 ppm by weight. A concentration within the wire of sodium, calcium, antimony, magnesium, phosphorous, and/or potassium may be no less than 0.001 ppm by weight, no less than 0.005 ppm by weight, no less than 0.01 ppm by weight, no less than 0.05 ppm by weight, no less than 0.1 ppm by weight, or no less than 0.5 ppm by weight. A concentration of oxygen within the wire may be less than 25 ppm by weight, less than 22 ppm by weight, less than 20 ppm by weight, less than 19 ppm by weight, less than 18 ppm by weight, less than 15 ppm by weight, or less than 10 ppm by weight. A concentration of oxygen within the wire may be no less than 0.001 ppm by weight, no less than 0.005 ppm by weight, no less than 0.01 ppm by weight, no less than 0.05 ppm by weight, no less than 0.1 ppm by weight, no less than 0.5 ppm by weight, no less than 1 ppm by weight, or no less than 2 ppm by weight. The density of at least a portion of the three-dimensional part may be greater than 97% of a theoretical density of molybdenum, greater than 98% of a theoretical density of molybdenum, greater than 99% of a theoretical density of molybdenum, or greater than 99.5% of a theoretical density of molybdenum. The density of at least a portion of the three-dimensional part may be no greater than 100% of a theoretical density of molybdenum, no greater than 99.9% of a theoretical density of molybdenum, or no greater than 99.8% of a theoretical density of molybdenum. In step (c), the energy source may include, consist essentially of, or consist of an electron beam and/or a laser beam. The process of producing the wire may include, consist essentially of, or consist of (i) compacting powder to form a feed electrode, the powder including, consisting essentially of, or consisting of molybdenum, (ii) arc-melting the feed electrode in a processing ambient including, consisting essentially of, or consisting of a vacuum or one or more inert gases, thereby forming a billet, and (iii) mechanically deforming the billet into wire having a diameter (or other dimension, e.g., width) less than a diameter (or other dimension, e.g., width) of the billet. Embodiments of the invention include three-dimensional objects or parts fabricated according to any of the above methods. 
         [0014]    In another aspect, embodiments of the invention feature a three-dimensional part manufactured by additive manufacturing using a feedstock material that includes, consists essentially of, or consists of molybdenum. The part includes, consists essentially of, or consists of a plurality of layers. Each layer includes, consists essentially of, or consists of solidified molybdenum. The part is substantially free of gaps between successive layers and/or substantially free of gaps within one or more of the layers. The part is substantially free of cracks. The density of at least a portion of the three-dimensional part may be greater than 97% of a theoretical density of molybdenum, greater than 98% of a theoretical density of molybdenum, greater than 99% of a theoretical density of molybdenum, or greater than 99.5% of a theoretical density of molybdenum. The density of at least a portion of the three-dimensional part may be no greater than 100% of a theoretical density of molybdenum, no greater than 99.9% of a theoretical density of molybdenum, or no greater than 99.8% of a theoretical density of molybdenum. A concentration within the part of sodium, calcium, antimony, magnesium, phosphorous, and/or potassium may be less than 5 ppm by weight, less than 4 ppm by weight, less than 3 ppm by weight, less than 2 ppm by weight, or less than 1 ppm by weight. A concentration within the part of sodium, calcium, antimony, magnesium, phosphorous, and/or potassium may be no less than 0.001 ppm by weight, no less than 0.005 ppm by weight, no less than 0.01 ppm by weight, no less than 0.05 ppm by weight, no less than 0.1 ppm by weight, or no less than 0.5 ppm by weight. 
         [0015]    Embodiments of the invention may include one or more of the following in any of a variety of combinations. A concentration within the part of each of sodium, calcium, antimony, magnesium, phosphorous, and potassium may be less than 5 ppm by weight, less than 4 ppm by weight, less than 3 ppm by weight, less than 2 ppm by weight, or less than 1 ppm by weight. A concentration within the part of each of sodium, calcium, antimony, magnesium, phosphorous, and potassium may be no less than 0.001 ppm by weight, no less than 0.005 ppm by weight, no less than 0.01 ppm by weight, no less than 0.05 ppm by weight, no less than 0.1 ppm by weight, or no less than 0.5 ppm by weight. A concentration of oxygen within the part may be less than 5 ppm by weight, less than 4 ppm by weight, less than 3 ppm by weight, less than 2 ppm by weight, or less than 1 ppm by weight. A concentration of oxygen within the part may be no less than 0.001 ppm by weight, no less than 0.005 ppm by weight, no less than 0.01 ppm by weight, no less than 0.05 ppm by weight, no less than 0.1 ppm by weight, no less than 0.5 ppm by weight, no less than 1 ppm by weight, or no less than 2 ppm by weight. The feedstock material may include, consist essentially of, or consist of wire. The feedstock material may include, consist essentially of, or consist of arc-melted wire (i.e., wire fabricated at least in part by arc melting). The feedstock material may include, consist essentially of, or consist of wire fabricated by a process that includes, consists essentially of, or consists of (i) compacting powder to form a feed electrode, the powder including, consisting essentially of, or consisting of molybdenum, (ii) arc-melting the feed electrode in a processing ambient including, consisting essentially of, or consisting of a vacuum or one or more inert gases, thereby forming a billet, and (iii) mechanically deforming the billet into wire having a diameter less than a diameter of the billet. 
         [0016]    In an aspect, embodiments of the invention feature method of fabricating a three-dimensional part that includes, consists essentially of, or consists of a metallic material. In a step (a), powder is compacted to form a feed electrode. The powder includes, consists essentially of, or consists of the metallic material. In a step (b), the feed electrode is arc-melted in a processing ambient including, consisting essentially of, or consisting of a vacuum or one or more inert gases, thereby forming a billet. In a step (c), the billet is mechanically deformed into wire having a diameter (or other dimension, e.g., width) less than a diameter (or other dimension, e.g., width) of the billet. In a step (d), a tip of the wire is translated relative to a platform (i.e., all or a portion of the wire is translated, the platform is translated, or both). In a step (e), while the tip of the wire is being translated, the tip of the wire is melted with an energy source to form a molten bead, whereby the bead cools to form at least a portion of a layer of a three-dimensional part. In a step (f), steps (d) and (e) are repeated one or more times to produce the three-dimensional part (or at least a portion thereof). The three-dimensional part includes, consists essentially of, or consists of the metallic material. 
         [0017]    Embodiments of the invention may include one or more of the following in any of a variety of combinations. The metallic material may include, consist essentially of, or consist of one or more refractory metals. The metallic material may include, consist essentially of, or consist of niobium, tantalum, rhenium, tungsten, and/or molybdenum. The metallic material may include, consist essentially of, or consist of niobium, tantalum, rhenium, and/or tungsten. A concentration within the wire of sodium, calcium, antimony, magnesium, phosphorous, and/or potassium may be less than 5 ppm by weight, less than 4 ppm by weight, less than 3 ppm by weight, less than 2 ppm by weight, or less than 1 ppm by weight. A concentration within the wire of sodium, calcium, antimony, magnesium, phosphorous, and/or potassium may be no less than 0.001 ppm by weight, no less than 0.005 ppm by weight, no less than 0.01 ppm by weight, no less than 0.05 ppm by weight, no less than 0.1 ppm by weight, or no less than 0.5 ppm by weight. A concentration of oxygen within the wire may be less than 25 ppm by weight, less than 22 ppm by weight, less than 20 ppm by weight, less than 19 ppm by weight, less than 18 ppm by weight, less than 15 ppm by weight, or less than 10 ppm by weight. A concentration of oxygen within the wire may be no less than 0.001 ppm by weight, no less than 0.005 ppm by weight, no less than 0.01 ppm by weight, no less than 0.05 ppm by weight, no less than 0.1 ppm by weight, no less than 0.5 ppm by weight, no less than 1 ppm by weight, or no less than 2 ppm by weight. The density of at least a portion of the three-dimensional part may be greater than 97% of a theoretical density of the metallic material, greater than 98% of a theoretical density of the metallic material, greater than 99% of a theoretical density of the metallic material, or greater than 99.5% of a theoretical density of the metallic material. The density of at least a portion of the three-dimensional part may be no greater than 100% of a theoretical density of the metallic material, no greater than 99.9% of a theoretical density of the metallic material, or no greater than 99.8% of a theoretical density of the metallic material. Step (c) may include, consist essentially of, or consist of drawing, rolling, swaging, extruding, and/or pilgering. Step (a) may include sintering the compacted powder at a temperature greater than 900° C., greater than 950° C., greater than 1000° C., greater than 1100° C., or greater than 1200° C. Step (a) may include sintering the compacted powder at a temperature less than 3500° C., less than 3000° C., or less than 2500° C. In step (e), the energy source may include, consist essentially of, or consist of an electron beam and/or a laser beam. Prior to step (a), the powder may be provided by a process including, consisting essentially of, or consisting of plasma densification and/or plasma atomization. Prior to step (a), the powder may be provided by a process including, consisting essentially of, or consisting of (i) hydrogenating metal to form a metal hydride, (ii) mechanically grinding the metal hydride into a plurality of particles, and (iii) dehydrogenating the metal hydride particles. Embodiments of the invention include three-dimensional objects or parts fabricated according to any of the above methods. 
         [0018]    In another aspect, embodiments of the invention feature a method of fabricating a three-dimensional part utilizing wire. The part includes, consists essentially of, or consists of a metallic material. The wire is produced by a process including, consisting essentially of, or consisting of (i) compacting powder to form a feed electrode, the powder including, consisting essentially of, or consisting of the metallic material, (ii) arc-melting the feed electrode in a processing ambient including, consisting essentially of, or consisting of a vacuum or one or more inert gases, thereby forming a billet, and (iii) mechanically deforming the billet into wire having a diameter (or other dimension, e.g., width) less than a diameter (or other dimension, e.g., width) of the billet. In a step (a), a tip of the wire is translated relative to a platform (i.e., all or a portion of the wire is translated, the platform is translated, or both). In a step (b), while the tip of the wire is being translated, the tip of the wire is melted with an energy source to form a molten bead, whereby the bead cools to form at least a portion of a layer of a three-dimensional part. In a step (c), steps (a) and (b) are repeated one or more times to produce at least a portion of the three-dimensional part. The three-dimensional part includes, consists essentially of, or consists of the metallic material. 
         [0019]    Embodiments of the invention may include one or more of the following in any of a variety of combinations. The metallic material may include, consist essentially of, or consist of one or more refractory metals. The metallic material may include, consist essentially of, or consist of niobium, tantalum, rhenium, tungsten, and/or molybdenum. The metallic material may include, consist essentially of, or consist of niobium, tantalum, rhenium, and/or tungsten. A concentration within the wire of sodium, calcium, antimony, magnesium, phosphorous, and/or potassium may be less than 5 ppm by weight, less than 4 ppm by weight, less than 3 ppm by weight, less than 2 ppm by weight, or less than 1 ppm by weight. A concentration within the wire of sodium, calcium, antimony, magnesium, phosphorous, and/or potassium may be no less than 0.001 ppm by weight, no less than 0.005 ppm by weight, no less than 0.01 ppm by weight, no less than 0.05 ppm by weight, no less than 0.1 ppm by weight, or no less than 0.5 ppm by weight. A concentration of oxygen within the wire may be less than 25 ppm by weight, less than 22 ppm by weight, less than 20 ppm by weight, less than 19 ppm by weight, less than 18 ppm by weight, less than 15 ppm by weight, or less than 10 ppm by weight. A concentration of oxygen within the wire may be no less than 0.001 ppm by weight, no less than 0.005 ppm by weight, no less than 0.01 ppm by weight, no less than 0.05 ppm by weight, no less than 0.1 ppm by weight, no less than 0.5 ppm by weight, no less than 1 ppm by weight, or no less than 2 ppm by weight. The density of at least a portion of the three-dimensional part may be greater than 97% of a theoretical density of the metallic material, greater than 98% of a theoretical density of the metallic material, greater than 99% of a theoretical density of the metallic material, or greater than 99.5% of a theoretical density of the metallic material. The density of at least a portion of the three-dimensional part may be no greater than 100% of a theoretical density of the metallic material, no greater than 99.9% of a theoretical density of the metallic material, or no greater than 99.8% of a theoretical density of the metallic material. Mechanically deforming the billet into wire may include, consist essentially of, or consist of drawing, rolling, swaging, extruding, and/or pilgering. The process of producing the wire may include sintering the compacted powder at a temperature greater than 900° C., greater than 950° C., greater than 1000° C., greater than 1100° C., or greater than 1200° C. The process of producing the wire may include sintering the compacted powder at a temperature less than 3500° C., less than 3000° C., or less than 2500° C. In step (b), the energy source may include, consist essentially of, or consist of an electron beam and/or a laser beam. The process of producing the wire may include providing the powder by a process including, consisting essentially of, or consisting of plasma densification and/or plasma atomization. The process of producing the wire may include providing the powder by a process including, consisting essentially of, or consisting of (i) hydrogenating metal to form a metal hydride, (ii) mechanically grinding the metal hydride into a plurality of particles, and (iii) dehydrogenating the metal hydride particles. Embodiments of the invention include three-dimensional objects or parts fabricated according to any of the above methods. 
         [0020]    In yet another aspect, embodiments of the invention feature a method of fabricating a three-dimensional part that includes, consists essentially of, or consists of a metallic material. In a step (a), a wire including, consisting essentially of, or consisting of arc-melted metallic material is provided. In a step (b), a tip of the wire is translated relative to a platform (i.e., all or a portion of the wire is translated, the platform is translated, or both). In a step (c), while the tip of the wire is being translated, the tip of the wire is melted with an energy source to form a molten bead, whereby the bead cools to form at least a portion of a layer of a three-dimensional part. In a step (d), steps (b) and (c) are repeated one or more times to produce at least a portion of the three-dimensional part. The three-dimensional part includes, consists essentially of, or consists of the metallic material. 
         [0021]    Embodiments of the invention may include one or more of the following in any of a variety of combinations. The metallic material may include, consist essentially of, or consist of one or more refractory metals. The metallic material may include, consist essentially of, or consist of niobium, tantalum, rhenium, tungsten, and/or molybdenum. The metallic material may include, consist essentially of, or consist of niobium, tantalum, rhenium, and/or tungsten. A concentration within the wire of sodium, calcium, antimony, magnesium, phosphorous, and/or potassium may be less than 5 ppm by weight, less than 4 ppm by weight, less than 3 ppm by weight, less than 2 ppm by weight, or less than 1 ppm by weight. A concentration within the wire of sodium, calcium, antimony, magnesium, phosphorous, and/or potassium may be no less than 0.001 ppm by weight, no less than 0.005 ppm by weight, no less than 0.01 ppm by weight, no less than 0.05 ppm by weight, no less than 0.1 ppm by weight, or no less than 0.5 ppm by weight. A concentration of oxygen within the wire may be less than 25 ppm by weight, less than 22 ppm by weight, less than 20 ppm by weight, less than 19 ppm by weight, less than 18 ppm by weight, less than 15 ppm by weight, or less than 10 ppm by weight. A concentration of oxygen within the wire may be no less than 0.001 ppm by weight, no less than 0.005 ppm by weight, no less than 0.01 ppm by weight, no less than 0.05 ppm by weight, no less than 0.1 ppm by weight, no less than 0.5 ppm by weight, no less than 1 ppm by weight, or no less than 2 ppm by weight. The density of at least a portion of the three-dimensional part may be greater than 97% of a theoretical density of the metallic material, greater than 98% of a theoretical density of the metallic material, greater than 99% of a theoretical density of the metallic material, or greater than 99.5% of a theoretical density of the metallic material. The density of at least a portion of the three-dimensional part may be no greater than 100% of a theoretical density of the metallic material, no greater than 99.9% of a theoretical density of the metallic material, or no greater than 99.8% of a theoretical density of the metallic material. In step (c), the energy source may include, consist essentially of, or consist of an electron beam and/or a laser beam. The process of producing the wire may include, consist essentially of, or consist of (i) compacting powder to form a feed electrode, the powder including, consisting essentially of, or consisting of the metallic material, (ii) arc-melting the feed electrode in a processing ambient including, consisting essentially of, or consisting of a vacuum or one or more inert gases, thereby forming a billet, and (iii) mechanically deforming the billet into wire having a diameter (or other dimension, e.g., width) less than a diameter (or other dimension, e.g., width) of the billet. Embodiments of the invention include three-dimensional objects or parts fabricated according to any of the above methods. 
         [0022]    In another aspect, embodiments of the invention feature a three-dimensional part manufactured by additive manufacturing using a feedstock material that includes, consists essentially of, or consists of a metallic material. The part includes, consists essentially of, or consists of a plurality of layers. Each layer includes, consists essentially of, or consists of solidified metallic material. The part is substantially free of gaps between successive layers and/or substantially free of gaps within one or more of the layers. The part is substantially free of cracks. The density of at least a portion of the three-dimensional part may be greater than 97% of a theoretical density of the metallic material, greater than 98% of a theoretical density of the metallic material, greater than 99% of a theoretical density of the metallic material, or greater than 99.5% of a theoretical density of the metallic material. The density of at least a portion of the three-dimensional part may be no greater than 100% of a theoretical density of the metallic material, no greater than 99.9% of a theoretical density of the metallic material, or no greater than 99.8% of a theoretical density of the metallic material. A concentration within the part of sodium, calcium, antimony, magnesium, phosphorous, and/or potassium may be less than 5 ppm by weight, less than 4 ppm by weight, less than 3 ppm by weight, less than 2 ppm by weight, or less than 1 ppm by weight. A concentration within the part of sodium, calcium, antimony, magnesium, phosphorous, and/or potassium may be no less than 0.001 ppm by weight, no less than 0.005 ppm by weight, no less than 0.01 ppm by weight, no less than 0.05 ppm by weight, no less than 0.1 ppm by weight, or no less than 0.5 ppm by weight. 
         [0023]    Embodiments of the invention may include one or more of the following in any of a variety of combinations. The metallic material may include, consist essentially of, or consist of one or more refractory metals. The metallic material may include, consist essentially of, or consist of niobium, tantalum, rhenium, tungsten, and/or molybdenum. The metallic material may include, consist essentially of, or consist of niobium, tantalum, rhenium, and/or tungsten. A concentration within the part of each of sodium, calcium, antimony, magnesium, phosphorous, and potassium may be less than 5 ppm by weight, less than 4 ppm by weight, less than 3 ppm by weight, less than 2 ppm by weight, or less than 1 ppm by weight. A concentration within the part of each of sodium, calcium, antimony, magnesium, phosphorous, and potassium may be no less than 0.001 ppm by weight, no less than 0.005 ppm by weight, no less than 0.01 ppm by weight, no less than 0.05 ppm by weight, no less than 0.1 ppm by weight, or no less than 0.5 ppm by weight. A concentration of oxygen within the part may be less than 5 ppm by weight, less than 4 ppm by weight, less than 3 ppm by weight, less than 2 ppm by weight, or less than 1 ppm by weight. A concentration of oxygen within the part may be no less than 0.001 ppm by weight, no less than 0.005 ppm by weight, no less than 0.01 ppm by weight, no less than 0.05 ppm by weight, no less than 0.1 ppm by weight, no less than 0.5 ppm by weight, no less than 1 ppm by weight, or no less than 2 ppm by weight. The feedstock material may include, consist essentially of, or consist of wire. The feedstock material may include, consist essentially of, or consist of arc-melted wire (i.e., wire fabricated at least in part by arc melting). The feedstock material may include, consist essentially of, or consist of wire fabricated by a process that includes, consists essentially of, or consists of (i) compacting powder to form a feed electrode, the powder including, consisting essentially of, or consisting of the metallic material, (ii) arc-melting the feed electrode in a processing ambient including, consisting essentially of, or consisting of a vacuum or one or more inert gases, thereby forming a billet, and (iii) mechanically deforming the billet into wire having a diameter less than a diameter of the billet. 
         [0024]    These and other objects, along with advantages and features of the present invention herein disclosed, will become more apparent through reference to the following description, the accompanying drawings, and the claims. Furthermore, it is to be understood that the features of the various embodiments described herein are not mutually exclusive and may exist in various combinations and permutations. As used herein, the terms “approximately” and “substantially” mean±10%, and in some embodiments, ±5%. The term “consists essentially of” means excluding other materials that contribute to function, unless otherwise defined herein. Nonetheless, such other materials may be present, collectively or individually, in trace amounts. For example, a structure consisting essentially of multiple metals will generally include only those metals and only unintentional impurities (which may be metallic or non-metallic) that may be detectable via chemical analysis but do not contribute to function. As used herein, “consisting essentially of at least one metal” refers to a metal or a mixture of two or more metals but not compounds between a metal and a non-metallic element or chemical species such as oxygen, silicon, or nitrogen (e.g., metal nitrides, metal silicides, or metal oxides); such non-metallic elements or chemical species may be present, collectively or individually, in trace amounts, e.g., as impurities. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0025]    In the drawings, like reference characters generally refer to the same parts throughout the different views. Also, the drawings are not necessarily to scale, emphasis instead generally being placed upon illustrating the principles of the invention. In the following description, various embodiments of the present invention are described with reference to the following drawings, in which: 
           [0026]      FIG. 1  is a schematic cross-sectional view of an arc-melting apparatus utilized to form metallic billets in accordance with various embodiments of the invention; 
           [0027]      FIG. 2  is a schematic view of a wire being fabricated from a metallic rod or billet in accordance with various embodiments of the invention; 
           [0028]      FIG. 3  is a schematic of an additive-manufacturing apparatus utilized to fabricate three-dimensional metallic parts in accordance with various embodiments of the invention; 
           [0029]      FIG. 4  is an image of the tip a wire fabricated in accordance with various embodiments of the invention during an additive manufacturing process; 
           [0030]      FIG. 5  is an image of a three-dimensional part fabricated by additive manufacturing utilizing the wire of  FIG. 4 ; 
           [0031]      FIG. 6  is an image of the part of  FIG. 5  after machining; 
           [0032]      FIG. 7  is an image of the tip of a conventional wire during an additive manufacturing process; and 
           [0033]      FIGS. 8A and 8B  are images of a three-dimensional part fabricated by additive manufacturing utilizing the wire of  FIG. 7 . 
       
    
    
     DETAILED DESCRIPTION 
       [0034]    In accordance with various embodiments of the invention, a billet of the desired metallic precursor material is formed via compacting powder including, consisting essentially of, or consisting of the precursor material (e.g., one or more refractory metals) into a rod or other three-dimensional structure. The powders themselves may be initially formed utilizing one or more techniques that minimize or substantially reduce the amount of oxygen and other volatile elements within the powders. In this manner, the amount of such volatile species within the wire is minimized or reduced. For example, various powders may be formed and/or treated via a hydride/dehydride process, plasma densification, and/or plasma atomization, and the powders may have low concentrations of volatile species such as oxygen (e.g., oxygen contents lower than 300 ppm, lower than 100 ppm, lower than 50 ppm, or even lower than 30 ppm). As known in the art, hydride/dehydride processes involve the embrittlement of a metal via hydrogen introduction (thereby forming a hydride phase), followed by mechanical grinding (e.g., ball milling) and dehydrogenation (e.g., heating in a vacuum). Various powders or powder precursors may even be combined with one or more materials (e.g., metals) having a higher affinity for oxygen (e.g., calcium, magnesium, etc.), deoxidized at high temperature, and then separated from the high-oxygen-affinity material via, e.g., chemical leaching, as detailed in U.S. Pat. No. 6,261,337, filed on Aug. 19, 1999 (the &#39;337 patent), the entire disclosure of which is incorporated by reference herein. As described in U.S. patent application Ser. No. 15/416,253, filed on Jan. 27, 2017, the entire disclosure of which is incorporated by reference herein, in a plasma densification process, metal particulates are fed into a plasma jet or plasma torch, at least partially melted thereby, and upon cooling tend to have a substantially spherical morphology. The plasma densification process may also reduce the concentrations of volatile elements within the metal powder. 
         [0035]    The compacted-powder rod may then be sintered to form a feed electrode for an arc-melting process. For example, the rod may be sintered at a temperature ranging from approximately 1800° C. to approximately 2200° C. As illustrated in  FIG. 1 , the feed electrode  100  is typically placed over a crucible  105  (which may include, consist essentially of, or consist of, e.g., copper) that may be cooled via flow of coolant (e.g., water or other heat-exchange fluid) circulating within and/or around the crucible  105 . As shown, the coolant may flow from a cooling inlet  110  and out of the crucible  105  via a cooling outlet  115 . In various embodiments, a small charge  120  of the precursor material is positioned at the bottom of the crucible  105 , and an electric current (e.g., hundreds, or even thousands of Amperes of DC current) is applied between the charge  120  and the feed electrode  100 . The electrical current, which may be applied by a power supply  125 , creates an arc  130  between the feed electrode  100  and the charge  120  within crucible  105 , causing the feed electrode  100  to melt, forming a billet having the shape of the interior of the crucible  105  (e.g., round and at least partially cylindrical). A vacuum or other inert atmosphere (e.g., nitrogen gas, argon gas, or other inert gas) may be present within and/or around the crucible  105 , and various volatile impurities within the feed electrode  100  may escape into that ambient during the arc melting. The resulting billet may even be subjected to arc melting one or more additional times in order to refine the billet material and further reduce the concentration of various volatile impurities. 
         [0036]    After the arc-melting process, the resulting billet is crafted into wire by one or more mechanical deformation processes. For example, the billet may be hot worked (e.g., extruded, rolled, forged, pilgered, etc.) to form a rod having a diameter smaller than that of the initial billet. The billet and/or the rod may be further densified by pressing, e.g., cold isostatic pressing or hot isostatic pressing. The rod may then be formed into a wire having the final desired diameter by, e.g., one or more of drawing, swaging, pilgering, extrusion, etc. (In various embodiments, the immediate product of the arc-melting process, i.e., the billet, may be formed into a wire directly, rather than being formed into a rod therebetween.) In an exemplary embodiment depicted in  FIG. 2 , the rod  200  is formed into wire  210  via drawing through one or more drawing dies  220  until the diameter of the wire  210  is reduced to the desired dimension. In various embodiments, the drawing is supplemented with or replaced by one or more other mechanical deformation processes that reduce the diameter (or other lateral dimension) of the rod  200 , e.g., pilgering, rolling, swaging, extrusion, etc. The rod  200  and/or wire  210  may be annealed during and/or after diameter reduction (e.g., drawing). The billet, rod, and/or wire may be heat treated during and/or after diameter reduction. For example, the billet, rod, and/or wire may be sintered at a temperature greater than 900° C. 
         [0037]    Once wire  210  including, consisting essentially of, or consisting of one or refractory metals (e.g., molybdenum) is fabricated in accordance with embodiments of the invention, the wire  210  may be utilized to fabricate a three-dimensional part with an additive manufacturing assembly  300 . For example, as shown in  FIG. 3 , the wire  210  may be incrementally fed, using a wire feeder  310 , into the path of a high-energy source  320  (e.g., an electron beam or a laser beam emitted by a laser or electron-beam source  330 ), which melts the tip of the wire  230  to form a small molten pool (or “bead” or “puddle”)  340 . The entire assembly  300  may be disposed within a vacuum chamber to prevent or substantially reduce contamination from the ambient environment. 
         [0038]    Relative movement between a substrate  350  (which may be, as shown, disposed on a platform  360 ) supporting the deposit and the wire/gun assembly results in the part being fabricated in a layer-by-layer fashion. Such relative motion results in the continuous formation of a layer  370  of the three-dimensional object from continuous formation of molten pool  340  at the tip of the wire  230 . As shown in  FIG. 3 , all or a portion of layer  370  may be formed over one or more previously formed layers  380 . The relative movement (i.e., movement of the platform  360 , the wire/gun assembly, or both) may be controlled by a computer-based controller  380  based on electronically stored representations of the part to be fabricated. For example, the two-dimensional layers traced out by the melting wire may be extracted from a stored three-dimensional representation of the final part stored in a memory  390 . 
         [0039]    The computer-based control system (or “controller”)  380  in accordance with embodiments of the present invention may include or consist essentially of a general-purpose computing device in the form of a computer including a processing unit (or “computer processor”)  392 , the system memory  390 , and a system bus  394  that couples various system components including the system memory  390  to the processing unit  392 . Computers typically include a variety of computer-readable media that can form part of the system memory  390  and be read by the processing unit  392 . By way of example, and not limitation, computer readable media may include computer storage media and/or communication media. The system memory  390  may include computer storage media in the form of volatile and/or nonvolatile memory such as read only memory (ROM) and random access memory (RAM). A basic input/output system (BIOS), containing the basic routines that help to transfer information between elements, such as during start-up, is typically stored in ROM. RAM typically contains data and/or program modules that are immediately accessible to and/or presently being operated on by processing unit  392 . The data or program modules may include an operating system, application programs, other program modules, and program data. The operating system may be or include a variety of operating systems such as Microsoft WINDOWS operating system, the Unix operating system, the Linux operating system, the Xenix operating system, the IBM AIX operating system, the Hewlett Packard UX operating system, the Novell NETWARE operating system, the Sun Microsystems SOLARIS operating system, the OS/2 operating system, the BeOS operating system, the MACINTOSH operating system, the APACHE operating system, an OPENSTEP operating system or another operating system of platform. 
         [0040]    Any suitable programming language may be used to implement without undue experimentation the functions described herein. Illustratively, the programming language used may include assembly language, Ada, APL, Basic, C, C++, C*, COBOL, dBase, Forth, FORTRAN, Java, Modula-2, Pascal, Prolog, Python, REXX, and/or JavaScript for example. Further, it is not necessary that a single type of instruction or programming language be utilized in conjunction with the operation of systems and techniques of the invention. Rather, any number of different programming languages may be utilized as is necessary or desirable. 
         [0041]    The computing environment may also include other removable/nonremovable, volatile/nonvolatile computer storage media. For example, a hard disk drive may read or write to nonremovable, nonvolatile magnetic media. A magnetic disk drive may read from or writes to a removable, nonvolatile magnetic disk, and an optical disk drive may read from or write to a removable, nonvolatile optical disk such as a CD-ROM or other optical media. Other removable/nonremovable, volatile/nonvolatile computer storage media that can be used in the exemplary operating environment include, but are not limited to, magnetic tape cassettes, flash memory cards, digital versatile disks, digital video tape, solid state RAM, solid state ROM, and the like. The storage media are typically connected to the system bus through a removable or non-removable memory interface. 
         [0042]    The processing unit  392  that executes commands and instructions may be a general-purpose computer processor, but may utilize any of a wide variety of other technologies including special-purpose hardware, a microcomputer, mini-computer, mainframe computer, programmed micro-processor, micro-controller, peripheral integrated circuit element, a CSIC (Customer Specific Integrated Circuit), ASIC (Application Specific Integrated Circuit), a logic circuit, a digital signal processor, a programmable logic device such as an FPGA (Field Programmable Gate Array), PLD (Programmable Logic Device), PLA (Programmable Logic Array), RFID processor, smart chip, or any other device or arrangement of devices that is capable of implementing the steps of the processes of embodiments of the invention. 
         [0043]    Advantageously, wires in accordance with embodiments of the invention contain reduced or minimal amounts (if any) of volatile elements such as O, Na, Mg, P, S, K, Ca, and Sb, and therefore the wire  210  melts during additive manufacturing with little if any sparking and without introducing porosity, cracks, or other defects into the printed part. After the additive manufacturing process is complete, the part may be removed from the platform and subjected to final machining and/or polishing. 
       Example 1 
       [0044]    A Mo wire (wire A) having an outer diameter of 0.062 inches was fabricated in accordance with embodiments of the present invention. Specifically, wire A was produced by (1) compaction of Mo powder having purity of 99.95% into a rod, (2) sintering of the rod to form a feed electrode, (3) arc melting of the feed electrode under vacuum within a water-cooled copper crucible to form a billet, and (4) hot working of the billet in order to reduce its diameter. For comparison, a control Mo wire (wire B) having an outer diameter of 0.062 inches was fabricated via conventional powder-metallurgy techniques. Specifically, wire B was produced by (1) compaction of Mo powder having purity of 99.95% into a billet, (2) sintering the billet in a hydrogen atmosphere to a density of at least 93%, and (3) hot working of the billet in order to reduce its diameter. Detailed compositional information for various impurity species in both wires was obtained via glow discharge mass spectrometry (GDMS) and is presented in Table 1. 
         [0000]    
       
         
               
               
               
             
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 Element 
                 Composition in Wire A (ppm) 
                 Composition in Wire B (ppm) 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 Na 
                 0.1 
                 21 
               
               
                 Mg 
                 0.04 
                 5.8 
               
               
                 Si 
                 5 
                 34 
               
               
                 P 
                 0.97 
                 3.6 
               
               
                 K 
                 0.06 
                 2.4 
               
               
                 Ca 
                 0.53 
                 21 
               
               
                 Sb 
                 0.67 
                 18 
               
               
                 O 
                 18 
                 28 
               
               
                   
               
             
          
         
       
     
         [0045]    As shown in Table 1, wire A contained significantly less of several volatile impurities than wire B. 
       Example 2 
       [0046]    Wires A and B from Example 1 were utilized in an exemplary additive manufacturing process to fabricate a three-dimensional Mo part. Each wire was heated to melting during the fabrication process via an electron beam having an average power of 35 kV at 110 mA (pulsed). The wire feed speed into the electron beam was approximately 30 in/min, and the relative travel speed between the wire and the fabrication platform was 10 in/min. The deposition rate was approximately 0.91 kg/hr. 
         [0047]    During fabrication using wire A, the deposition was smooth and substantially free of sparking, blistering, and splattering.  FIG. 4  is an image of the molten bead  400  forming at the tip of wire A during the additive manufacturing process.  FIG. 5  is an image of the resulting part  500  manufactured utilizing wire A. As shown, the part  500  is free of visible cracks or other imperfections, and the fabrication platform is free of visible splatter or other debris. As shown in  FIG. 6 , the part  500  could be machined and/or polished, in to remove any roughness resulting from the layer-by-layer fabrication process, without cracking or other damage. 
         [0048]    During fabrication using wire B, the deposition was plagued by sparking and blistering.  FIG. 7  depicts the molten bead  700  forming at the tip of wire B during the additive manufacturing process. As shown, wire B possessed a large number of pores or inclusions  710  due to, e.g., the higher concentration of volatile impurities within wire B. In addition, visible sparks  720  from wire B resulted during the melting and fabrication process.  FIGS. 8A and 8B  are images of the three-dimensional part  800  fabricated utilizing wire B. As shown, part  800  had visible cracking in its walls, and there was visible splattering of the wire material on the fabrication platform and on the part  800  itself. Attempted machining of part  800  was unsuccessful, and part  800  was unsatisfactory for use as a crucible, in stark contrast to part  500 . 
         [0049]    The densities of the finished parts  500 ,  800  were measured, and the results are presented in Table 2 below. 
         [0000]    
       
         
               
               
               
             
               
               
               
             
           
               
                 TABLE 2 
               
               
                   
               
               
                 Part/Location within part 
                 Density (g/cc) 
                 % of Theoretical Density 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 500/Top of wall 
                 10.13 
                 99.1 
               
               
                 500/Bottom of wall 
                 10.14 
                 99.2 
               
               
                 800/Top of wall 
                 9.73 
                 95.2 
               
               
                 800/Middle of wall 
                 9.81 
                 96.0 
               
               
                 800/Bottom of wall 
                 9.75 
                 95.4 
               
               
                   
               
             
          
         
       
     
         [0050]    As shown, the part  500  fabricated using wire A had a density much higher than that of the part  800  fabricated using wire B. In addition, compositional analysis of the fabricated parts  500 ,  800  was performed by GDMS, and the results are shown in Table 3 below. 
         [0000]    
       
         
               
               
               
             
               
               
               
             
           
               
                 TABLE 3 
               
               
                   
               
               
                   
                 Composition 
                   
               
               
                 Element 
                 in Part 500 (ppm) 
                 Composition in Part 800 (ppm) 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 Na 
                 0.01 
                 2.4 
               
               
                 Mg 
                 0.08 
                 0.21 
               
               
                 Si 
                 12 
                 31 
               
               
                 P 
                 0.27 
                 1 
               
               
                 K 
                 0.07 
                 1.8 
               
               
                 Ca 
                 0.06 
                 1.2 
               
               
                 Sb 
                 0.09 
                 1.3 
               
               
                 O 
                 &lt;5 (less than detection limit) 
                 5.5 
               
               
                   
               
             
          
         
       
     
         [0051]    As shown in Table 3, the concentrations of most of the impurity elements decreased during the fabrication process, presumably due to melting of the wire and volatilization of the impurities. However, the part  500  fabricated utilizing wire A had concentrations of these impurities that were much lower than those in the part  800  fabricated utilizing wire B. In addition, even though the amounts of these impurity elements within the part  800  fabricated utilizing wire B tended to be lower than the impurity levels within wire B itself, the part  800  fabricated utilizing wire B had unacceptable morphology and density, and was accompanied by splatter and sparking during fabrication, as shown in  FIGS. 8A and 8B  and discussed above. 
         [0052]    The terms and expressions employed herein are used as terms and expressions of description and not of limitation, and there is no intention, in the use of such terms and expressions, of excluding any equivalents of the features shown and described or portions thereof. In addition, having described certain embodiments of the invention, it will be apparent to those of ordinary skill in the art that other embodiments incorporating the concepts disclosed herein may be used without departing from the spirit and scope of the invention. Accordingly, the described embodiments are to be considered in all respects as only illustrative and not restrictive.