Patent Publication Number: US-2022234108-A1

Title: Method of preparing an electrode for use in forming a honeycomb extrusion die

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of priority under 35 U.S.C. § 119 of U.S. Provisional Application No. 62/851,973 filed on May 23, 2019, the content of which is incorporated herein by reference in its entirety. 
    
    
     FIELD 
     Embodiments described herein generally relate to methods for forming electrodes and electrodes so formed. Specifically, embodiments described herein relate to methods for preparing an electrode that is used to form an extrusion die having a honeycomb pattern. 
     BACKGROUND 
     Features of some honeycomb extrusion dies are formed using wire electrical discharge machining (wire EDM). However, wire EDM may be impractical or unsuitable for forming honeycomb extrusion dies with certain patterns that are not conducive to formation by the linear wires used. Plunge electrical discharge machining (plunge EDM) and electrochemical machining (ECM) are alternate processes that can be used to form extrusion dies, including honeycomb extrusion dies not conducive to manufacture by wire EDM. Plunge EDM and ECM require the use of a specially configured electrode that is shaped to provide the extrusion die with the desired pattern. As a result, forming an electrode for use in plunge EDM or ECM may require additional time, materials, and cost. 
     Accordingly, there is a need in the art for a method for forming electrodes that can be used to produce honeycomb extrusion dies having varied geometries rapidly and with reduced expense. 
     BRIEF SUMMARY OF THE INVENTION 
     In a first aspect, a method for additively manufacturing a copper-tungsten electrode includes forming an electrode as a series of layers, wherein each layer is formed by depositing a layer of powder on a manufacturing platform or a previous layer in the series, wherein the layer of powder comprises a tungsten powder and a copper powder, and fusing the layer of powder using a laser to apply a laser energy of about 1 to about 10 J/mm2 to the layer of powder, and wherein the electrode is comprised of an interconnected network of tungsten infiltrated by copper formed by the series of layers fused together. 
     In a second aspect according to the preceding paragraph, the electrode can have a relative density of at least about 75%. 
     In a third aspect according to either of the preceding paragraphs, the electrode can have a relative density of at least about 90%. 
     In a fourth aspect according to any of the preceding paragraphs, each of the tungsten powder and the copper powder can have a particle size distribution of about 15 μm to about 55 μm. 
     In a fifth aspect according to any of the preceding paragraphs, each of the tungsten powder and the copper powder can have a particle size distribution of about 25 μm or less. 
     In a sixth aspect according to any of the preceding paragraphs, the tungsten powder can be present in the layer of powder in an amount of about 70 wt. % to about 90 wt. %. 
     In a seventh aspect according to any of the preceding paragraphs, the tungsten powder can be present in the layer of powder in an amount of 75 wt. % and the copper powder can be present in an amount of 25 wt. %. 
     In an eighth aspect according to any of the preceding paragraphs, depositing the layer of powder can include depositing one of the tungsten powder or the copper powder, and subsequently depositing the other of the tungsten powder or the copper powder. 
     In a ninth aspect according to any of the preceding paragraphs, depositing the layer of powder can include depositing a mixture of the copper powder and the tungsten powder. 
     In a tenth aspect according to any of the preceding paragraphs, forming the electrode can include forming an electrode having a base having a web extending from the base, wherein the web defines a matrix of cellular openings. 
     In an eleventh aspect according to the previous paragraph, forming the electrode can further include forming one or more internal passages in the base of the electrode. 
     In a twelfth aspect according to any of the preceding paragraphs, the method can include smoothing the electrode using a secondary electrode by plunge electrical discharge machining or electro-chemical machining. 
     In a thirteenth aspect according to the preceding paragraph, smoothing the electrode can include smoothing the electrode such that the surface roughness of the electrode is about 1 to about 50 μm. 
     In a fourteenth aspect, an electrode can be formed by a method according to any of the preceding paragraphs. 
     In a fifteenth aspect, a method for manufacturing an electrode includes forming a porous tungsten structure in a layer-by-layer manner, wherein each layer is formed by depositing a layer of a tungsten powder; and selectively fusing the tungsten powder prior to depositing a subsequent layer using a laser having a laser energy of about 1 to about 10 J/mm 2 , and infiltrating the porous tungsten structure with molten copper so as to form an electrode comprising a porous tungsten structure infiltrated by copper. 
     In a sixteenth aspect according to the preceding paragraph, the porous tungsten structure can have a relative density level of at least about 75%. 
     In a seventeenth aspect according to either of the two preceding paragraphs, the tungsten powder can have a particle size distribution of about 15 μm to about 55 μm. 
     In an eighteenth aspect according to any of the three preceding paragraphs, infiltrating the porous tungsten structure with molten copper can be performed by one of spontaneous or pressure infiltration, centrifugal infiltration, ultrasound-driven or aided infiltration, or Lorenz force infiltration. 
     In a nineteenth aspect according to any of the four preceding paragraphs, the electrode can have a base having a web extending from the base, wherein the web defines a matrix of cellular openings. 
     In a twentieth aspect according to any of the five preceding paragraphs, the method can further include smoothing the electrode using a secondary electrode by plunge electrical discharge machining or electro-chemical machining. 
     In a twenty-first aspect, a method of forming an electrode for use in forming a honeycomb extrusion die includes forming, by an additive manufacturing process, an electrode comprising a base having a web extending from the base, wherein the web defines a matrix of cellular openings, and smoothing surfaces of the electrode using a secondary electrode having a plurality of pins, wherein the plurality of pins are shaped and arranged so as to mate with the matrix of cellular openings defined by the web of the electrode. 
     In a twenty-second aspect according to the preceding paragraph, forming the electrode can include forming the electrode from a powder mixture of tungsten and copper, and wherein the additive manufacturing process is laser powder bed fusion. 
     In a twenty-third aspect according to the preceding paragraph, the mixture of tungsten and copper can include tungsten particles and copper particles each having a particle size distribution of about 15 to about 55 microns. 
     In a twenty-fourth aspect according to any of the two preceding paragraphs, laser powder bed fusion can be performed using a laser energy of 1 J/mm 2  to 10 J/mm 2 . 
     In a twenty-fifth aspect according to any of the four preceding paragraphs, forming the electrode can include forming a porous body using tungsten powder in laser powder bed fusion, and infiltrating the porous body with molten copper. 
     In a twenty-sixth aspect according to any of the five preceding paragraphs, forming the electrode can include forming a first zone of cellular openings having a first size, and a second zone of cellular openings having a second size that is different from the first size. 
     In a twenty-seventh aspect according to any of the six preceding paragraphs, forming the electrode can include forming a first zone of cellular openings having a first shape, and a second zone of cellular openings having a second shape that is different from the first shape. 
     In a twenty-eighth aspect according to any of the seven preceding paragraphs, forming the electrode can include forming the web such that each cellular opening of the matrix of cellular openings has a hexagonal cross sectional area. 
     In a twenty-ninth aspect according to any of the eight preceding paragraphs, forming the electrode can include forming the base so as to define one or more internal passages configured for circulation of a fluid used in electrical discharge machining. 
     In a thirtieth aspect according to any of the nine preceding paragraphs, forming the secondary electrode can include forming the secondary electrode by machining a metal blank using the electrode in pulsed salt electro-chemical machining. 
     In a thirty-first aspect according to the preceding paragraph, smoothing surfaces of the electrode can include machining the electrode using the secondary electrode in pulsed salt electro-chemical machining. 
     In a thirty-second aspect according to any of the eleven preceding paragraphs, smoothing surfaces of the electrode can include plunge electrical discharge machining. 
     In a thirty-third aspect according to any of the twelve preceding paragraphs, smoothing surfaces of the electrode can include wire electrical discharge machining. 
     In a thirty-fourth aspect according to any of the thirteen preceding paragraphs, the method can further include applying an electrically insulating coating to the electrode after smoothing surfaces of the electrode. 
     In a thirty-fifth aspect, an electrode can be formed by the method according to any of the fourteen preceding paragraphs. 
     In a thirty-sixth aspect, a method of forming an electrode for use in forming a honeycomb extrusion die by electrical discharge machining includes forming, by an additive manufacturing process, an electrode comprising a base and a web extending from the base, wherein the web defines a matrix of cellular openings and is configured to form a pattern in an extrusion die corresponding to the matrix of cellular openings, and smoothing the electrode such that a surface roughness (Ra) of the electrode is about 1 to about 50 μm. 
     In a thirty-seventh aspect according to the preceding paragraph, smoothing the electrode can include plunge electrical discharge machining of the electrode using a secondary electrode. 
     In a thirty-eighth aspect according to either of the two preceding paragraphs, smoothing the electrode can include machining the electrode using a secondary electrode in a pulsed salt electro-chemical machining process. 
     In a thirty-ninth aspect according to any of the three preceding paragraphs, smoothing the electrode can include smoothing the electrode such that the surface roughness of the electrode is about 1 to about 20 μm. 
     In a fortieth aspect, an additively manufactured electrode capable of forming a honeycomb extrusion die includes a base having one or more internal passages configured for circulation of a fluid used in electrical discharge machining, and a web extending from the base and defining a matrix of cellular openings, wherein the matrix of cellular openings comprises a first zone of cellular openings and a second zone of cellular openings, wherein the first zone of cellular openings and second zone of cellular openings differ in at least one of size and shape, and wherein the web is configured to form a pattern on an extrusion die corresponding to the matrix of cellular openings. 
     In a forty-first aspect, a method for manufacturing an electrode, the method includes applying laser energy to an initial layer of a forming powder comprised of a tungsten powder and a copper powder to form a fused layer of tungsten infiltrated by copper, depositing an additive layer of the forming powder onto the fused layer and applying laser energy to the additive layer to form another fused layer of tungsten infiltrated by copper which is also fused to the previously fused layer, and repeating the depositing of additive layers and respective applications of laser energy to form multiple fused layers in an interconnected network of tungsten infiltrated by copper, wherein the laser energy applied is about 1 to about 10 J/mm 2 . 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS/FIGURES 
         FIG. 1  is a top-down view of an electrode according to an embodiment. 
         FIG. 2  is a perspective view of an electrode according to an embodiment. 
         FIG. 3  is a perspective view of a portion of an extrusion die formed using an electrode according to an embodiment. 
         FIG. 4  is a top-down view of an electrode according to an embodiment. 
         FIGS. 5A-5F  are views of patterns for electrodes according to embodiments. 
         FIG. 6  is a diagram of an electrode according to an embodiment in use in an electrical discharge machining operation. 
         FIG. 7  is a view of a microstructure of a mixture of tungsten and copper according to an embodiment. 
         FIG. 8  is a view of an unpolished surface of a mixture of tungsten and copper having pores according to an embodiment. 
         FIG. 9  is a view of a continuous tungsten matrix according to an embodiment. 
         FIG. 10  is a perspective view of a portion of a web of an electrode according to an embodiment after smoothing of the electrode. 
         FIG. 11  is a view of a rough surface of a web of an electrode prior to smoothing. 
         FIG. 12  is a perspective view of a secondary electrode for smoothing an electrode according to an embodiment. 
         FIG. 13  is a flow chart of a method of forming an electrode for use in preparing a honeycomb extrusion die according to an embodiment. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention(s) will now be described in detail with reference to embodiments thereof as illustrated in the accompanying drawings. References to “one embodiment,” “an embodiment,” “an exemplary embodiment,” etc., indicate that the embodiment described may include a particular feature, structure, or characteristic, but every embodiment may not necessarily include the particular feature, structure or characteristic. Moreover, such phrases are not necessarily referring to the same embodiment. Further, when a particular feature, structure, or characteristic is described in connection with an embodiment, it is submitted that it is within the knowledge of one skilled in the art to affect such feature, structure, or characteristic in connection with other embodiments whether or not explicitly described. 
     In some wire EDM manufacturing processes, a linear metal (or other conductive material) wire is used to cut features such as slots or grooves into the discharge face of the extrusion die. Since the wire is linear, only linear slots can be formed on the face of the extrusion die, which limits the types of patterns and geometries that can be formed in the die face. For example, using wire EDM, it is not possible to generate a honeycomb extrusion die having a matrix of hexagonal cellular openings. The formation of extrusion dies with different zones of cellular openings that vary in size or cross sectional shape is also limited using wire EDM. However, extrusion dies having other geometries may be desired so as to form extruded articles having complex honeycomb shapes (e.g., cells that are hexagonal, circular, varied in shape/size, etc.). Such extruded articles may be used in a variety of applications, such as for the treatment of a fluid flow, such as a vehicle engine exhaust flow. With respect to vehicle engine exhaust flow, the extruded articles can be arranged as wall-flow filters having alternatingly plugged channels at inlet and outlet ends and/or as substrates for catalysts in catalytic converters. The modification of the geometry of the extruded article can be used to provide improved performance parameters, such as air flow, soot load capacity, filtration efficiency, and pressure drop, etc. under differential operational conditions. 
     A honeycomb extrusion die having a more complex honeycomb pattern (that cannot feasibly be manufactured by wire EDM) may be formed using plunge or “sink” EDM (referred to herein simply as “plunge EDM”). In plunge EDM, an electrode is provided that has a specific configuration for forming a honeycomb pattern in the workpiece. The workpiece is positioned in a dielectric fluid, such as an oil or deionized water. A power supply generates an electrical potential between the electrode and the workpiece. As the electrode is brought towards the workpiece, a series of electrical discharges between the electrode and the workpiece erodes the surface of the workpiece to provide the workpiece with a configuration based on the configuration of the electrode (e.g., the slots are formed as a “negative” of the pattern of the electrode). 
     Similarly, electrochemical machining (ECM) may be used to prepare an extrusion die having a complex geometry. In ECM, an electrical current is passed through the electrode (cathode) and a workpiece (anode). An electrolytic fluid is introduced between the electrode and the workpiece, and the electrolytic fluid transfers the charge between the electrode and workpiece. As the electrode is moved towards the workpiece, the workpiece erodes, and the eroded portion of the workpiece is carried away in the electrolytic fluid. This process continues until the workpiece has a shape that corresponds to the electrode. ECM can be performed using a continuous current or using a pulsed current (p-ECM). 
     While plunge EDM and ECM allow for formation of honeycomb extrusion dies with complex geometries, these processes have tradeoffs. For example, preparing an electrode may be very time consuming (e.g., by a combination of drilling, milling, wire EDM and/or other operations). Further, such subtractive manufacturing methods are not capable of being used to form an electrode having internal passages. Due to the time, cost and complexity of formation of the electrodes, it is not feasible to manufacture an electrode corresponding to the entire discharge face of the prospective extrusion die. Instead, an electrode corresponding to only a portion of the discharge face can be employed that, must be repeatedly moved to individually form each portion of the extrusion die until the full face of the extrusion die has the desired pattern. As a result, the slots in the various portions of the extrusion die, or the interface or boundary between adjacent portions, may have slight variations, asymmetries, discontinuities, and other irregularities or defects, which is undesirable. 
     Embodiments herein relate to methods for forming an electrode by means of an additive manufacturing technique. The additively manufactured electrode includes a base and a web defining a matrix of cellular openings, such that the electrode can be used to form a honeycomb extrusion die. The method further includes smoothing the web of the additively manufactured electrode to reduce the surface roughness of the additively manufactured electrode. 
     An embodiment of an additively manufactured electrode  100  for forming a honeycomb extrusion die is shown for example in  FIG. 1 . Electrode  100  includes a base  120  and a web  140  extending from base  120 . Specifically, web  140  extends from a surface  122  of base  120  and can be perpendicular to surface  122  of base  120 . Base  120  can have any of various shapes such as a cylindrical block or plate, or a square or rectangular block, as shown for example at  FIG. 2 . Base  120  is configured to connect electrode  100  to an apparatus for performing EDM or ECM operations. 
     Web  140  of electrode  100  defines a matrix of cellular openings  160 , as best shown in  FIG. 1 . When electrode  100  is used in EDM or ECM, web  140  provides the workpiece, e.g., the extrusion die, with a honeycomb pattern corresponding to the shape and configuration of web  140 . As discussed herein, the additive manufacture, of electrode  100  assists in enabling the web  140  to feasibly be sized and shaped so as to form an entire face of the extrusion die at one time, greatly reducing the amount of time required to machine the extrusion die and while minimizing defects in comparison to the above-described EDM or ECM methods. The matrix of cellular openings  160  can be uniform in size and shape, such that each cellular opening  160  has the same shape and dimensions. Cellular openings  160  of web  140  can have any of various transverse cross sectional shapes, such as a triangle, a circle, an oval, a square, a rectangle, a pentagon, a hexagon, or a pie wedge, among others. In the embodiment of  FIG. 1 , electrode  100  is formed with a web  140  defining a matrix of cellular openings  160  each having a hexagonal transverse cross section. In some embodiments, however, web  140  defines a matrix of cellular openings  160  that is non-uniform. Thus, matrix of cellular openings  160  can include openings of different size, e.g., a matrix of cellular openings that alternate between a first size and a second size. In some embodiments, thickness of web  140  tapers towards base  120  which may help to improve distribution of fluid when electrode  100  is used in EDM or ECM. Further, thickness of web  140  can vary in different portions of web  140  in order to blend regions of web  140  having different geometries (e.g., to assist in transitioning from a cellular opening having a first size/shape to an adjacent cellular opening having a different size/shape). 
     Electrode  100  is configured for use in forming a honeycomb pattern in a discharge face  310  of an extrusion die  300 , a portion of which is shown in  FIG. 3 . The discharge face  310  comprises a plurality of pins  330  separated by a plurality of slots  320  through which an extrudable material can flow. While only a few pins  330  are shown in  FIG. 3 , it is to be appreciated that an extrusion die can include hundreds or even thousands of pins  330  arranged with respect to slots  320  in a desired pattern at the discharge face  310 . Extrusion die  300  further includes a plurality of feed holes  360  that supply the extrudable material to slots  320  of discharge face  310 , such that extrudable material flows through slots  320  and around pins  330  to produce an extruded article having a honeycomb pattern corresponding to the pattern of slots  320 . 
     When electrode  100  is used to produce an extrusion die, such as by EDM or ECM, the honeycomb pattern of the extrusion die corresponds to the geometry of web  140  and matrix of cellular openings  160  of electrode  100 . Specifically, web  140  of electrode  100  forms slots (e.g., slots  320  of extrusion die  300 ), and pins (e.g., pins  330  of extrusion die  300 ) are shaped as the cellular openings  160  of electrode  100 . In this way, electrode  100  can be used to form an extrusion die with a geometry that is the inverse or negative of web  140  and cellular openings  160  of electrode  100 . 
     As the use of additive manufacturing allows for manufacture of an electrode having complex geometries, an electrode can be formed with a plurality of zones each having different geometries and dimensions. For example, in  FIG. 4 , an electrode  400  is shown having a first zone  450  of cellular openings  460  and a second zone  470  of cellular openings  480 . First zone  450  is annular and concentrically surrounds a circular second zone  470 . In other embodiments, the zones of the electrode occupy various other shapes, sizes, or regions of the electrode. For example, a first zone can be a first half or a quadrant of the electrode and second or additional zones can occupy the opposing half or separate quadrants of the electrode. Further, in some embodiments, a first zone has a web that has a height, measured as the distance the web extends from the surface of the base in a direction perpendicular to the surface of the base, that is less than a height of the web in the second zone so as to promote uniform distribution of dielectric fluid or electrolytic fluid throughout the electrode during EDM or ECM, respectively. 
     In  FIG. 4 , first zone  450  of electrode  400  comprises a web  440   a  defining a matrix of cellular openings  460  comprising a first cross sectional shape and a first size or dimension. Second zone  470  comprises a web  440   b  defining a matrix of cellular openings  480  comprising a second cross sectional shape and a second size or dimension. Cellular openings  460 ,  480  of the first and second zones  450 ,  470  each have a square transverse cross sectional shape. However, cellular openings  460  of first zone  450  have a smaller size or dimension than cellular openings  480  of second zone  470 . The size of the cellular opening refers to the transverse cross-sectional area of the cellular openings defined by the webs  440   a ,  440   b , e.g., the area of the cellular openings  460 ,  480  in the plane of the view of  FIG. 4 . Alternatively stated, first zone  450  has a higher density of cellular openings  460  than second zone  470 . In some embodiments, first zone  450  has cellular openings  460  with a cross sectional shape that differs from the cross sectional shape of cellular openings  480  of second zone  470 . For example, the first zone can have cellular openings with a first cross sectional shape, e.g., a square, and the second zone can have cellular openings of a second cross sectional shape that differs from the first cross sectional shape, e.g., a triangle. 
     Using an electrode as described herein, an extrusion die can be formed. Various patterns of extrusion dies are shown for example in  FIGS. 5A-5F . In  FIG. 5A , extrusion die  510  has a plurality of hexagonal pins  515 . In  FIG. 5B , extrusion die  520  includes first pins  522  of a first size and second pins  524  of a second size, wherein the first size differs from the second size, and the first and second pins are arranged in an alternating pattern. In  FIG. 5C , extrusion die  530  has a web-shaped pattern with wedge-shaped pins  535  defining concentric circular slots and radially extending slots. In  FIG. 5D , an extrusion die  540  includes square pins  542  and triangular pins  544 . In  FIG. 5E , extrusion die  550  includes circular pins  555 , which can be connected by ribs that provide structural support to circular pins  555 . In  FIG. 5F , extrusion die  560  includes a series of square pins  562  and rectangular pins  564 . 
     In some embodiments, electrode  600  is formed so as to have a base  620  with internal passages  610 , as shown for example in  FIG. 6 . Using conventional wire EDM methods, an electrode having internal passages cannot readily be formed. Internal passages  610  can be useful for circulation of a fluid  1000 , such as a dielectric liquid that is used in electrical discharge machining using the electrode  600 . Internal passages  610  can have an inlet  612  for connection to an external device, such as a pump or vacuum source, and an outlet  614  on a surface  622  of the electrode  600  on which web  640  is positioned. Fluid  1000  can be circulated through internal passages  610  so as to flush or remove the material that has been eroded from workpiece  300  by the machining process. For example, a pump can provide dielectric fluid through an internal passage  610  to web  640  of electrode  600  so as to provide dielectric fluid to the cutting zone at which web  640  meets workpiece  300 , and a vacuum source can draw used dielectric fluid and eroded material away from the cutting zone through a separate internal passage  610 . 
     According to embodiments disclosed herein, electrode  100  having a honeycomb pattern as shown in  FIG. 1  is formed by an additive manufacturing process, also referred to as a three-dimensional (3D) printing process. Additive manufacturing allows for the formation of an electrode  100  having a complex geometry that cannot be achieved using conventional, subtractive manufacturing or machining techniques for forming an electrode, such as EDM, ECM, or milling and drilling operations. Additive manufacturing also allows for more rapid production of an electrode than can be achieved using conventional subtractive techniques. 
     Any of various additive manufacturing processes can be used to form electrode  100 , including laser or electron-beam powder bed fusion (PBF) techniques including selective laser melting (SLM), selective laser sintering (SLS), direct metal laser sintering (DMLS), and electron beam melting (EBM), powder bed-less directed energy deposition (DED) approaches, material jetting, binder jetting, and stereolithography, among other additive manufacturing techniques, and hybrid approaches combining additive manufacturing and traditional machining. Further, electrode  100  can be formed from any of various materials, including brass, bronze, copper, tungsten, nickel, rhenium, thorium, lanthanum, silver, zinc, silicon, or carbides, such as silicon carbide, brass impregnated silicon carbide, tungsten carbide, or a blend, composite, or alloy thereof, among other materials suitable for use in forming electrodes. 
     In some embodiments, electrode  100  is manufactured from a powder mixture of tungsten and copper. Tungsten-copper (W—Cu) mixtures are well-suited for use in forming electrodes for plunge EDM as copper is electrically conductive and tungsten provides wear-resistance and resistance to spark erosion. However, tungsten-copper can present difficulties for use in additive manufacturing processes. For example, copper and tungsten are immiscible and do not form a binary alloy system. Further, there is no temperature at which copper and tungsten are in equilibrium in the same phase, as copper is in a vapor or gaseous state when tungsten is in a liquid state due to the high melting point of tungsten. Current processes for forming a tungsten body and subsequently infiltrating the tungsten body with copper are time consuming and expensive. Further, such processes do not allow for selection of the resulting shape or geometry and instead result in a solid block of tungsten-copper which must be processed by additional shaping techniques, such as by wire-EDM, plunge EDM, and the like. 
     In some embodiments, electrode  100  is additively manufactured using a tungsten-copper mixture by direct additive manufacturing of a mixture of pure tungsten and copper powders, such as by a laser powder bed fusion (PBF) technique. 
     Laser PBF is a method for forming a three-dimensional object in a layer-by-layer fashion using thermal energy sufficient to induce fusion of one or more materials. A digital representation of the object is generated that represents the object as a series of layers or cross sections. In laser PBF, a layer of a first material, such as a metal powder, for example, tungsten powder or copper powder, among others, can be deposited on a surface of a manufacturing platform. A second material, such as a metal powder, for example, copper powder or tungsten powder, among others, can be deposited subsequently or simultaneously with the first material. The metal powders can also be deposited onto the platform as a mixture of metal powders, such as a mixture of copper and tungsten powders. The deposited materials are deposited as an unfused layer on the surface of the platform. A laser-based thermal source is then used to selectively direct energy to the layer to induce fusion of the metal powders, for example tungsten and copper powders, to form a first fused layer with the desired shape based on the digital representation. A subsequent layer of the metal powders can then be deposited on top of the first fused layer, and the subsequent layer selectively fused by the laser based on the digital representation of the object. The process of depositing layers and fusing each layer is repeated until all layers of the article are formed, thereby creating an object corresponding to the digital representation. 
     However, in other embodiments, alternate additive manufacturing techniques as described herein can be used. The mixture can include tungsten in about 70 to 90 weight percent, with the remainder being copper. In some embodiments, the tungsten-copper mixture can include 75 weight percent tungsten and 25 weight percent copper. One of ordinary skill in the art will appreciate that the amounts of tungsten and copper can be selected based on the desired properties for the resulting electrode. 
     In some embodiments, the particle size distribution of tungsten and copper particles used in the laser powder bed fusion technique is from about 15 to 55 microns. In some embodiments, the particle size distribution includes particles of 25 microns or less. Fine particle sizes (e.g., less than 25 microns) can be used so as to facilitate formation of a continuous network of tungsten in the electrode. The continuous network of tungsten is necessary to provide the resulting electrode with resistance to spark erosion. 
     In direct additive manufacturing techniques discussed herein, tungsten and copper experience two processing regimes: liquid phase sintering (LPS), and tungsten melting. In LPS, molten copper rearranges non-melted tungsten particles. As a result, a mixture  700  is formed that includes isolated tungsten particles  720  dispersed in a continuous copper matrix  710 , as shown in  FIG. 7 . Mixture  700  formed in LPS provides good electrical conductivity due to the copper matrix. However, in LPS, tungsten remains in a solid state and thus the tungsten particles do not fuse together and a continuous network of tungsten is not formed. 
     In tungsten melting, tungsten melts so as to form a continuous network of tungsten which is necessary to form an electrode having wear-resistance. However, at high concentrations of tungsten, tungsten may melt and form balls  760 , which result in a mixture  750  having pores  770 , as shown in  FIG. 8 . As a result, the mixture  750  may have a lower density than a mixture formed in LPS. Thus, in order to obtain a continuous network of tungsten without a significant loss of density, it can be desirable to perform additive manufacturing using a laser energy window that allows for both LPS and tungsten melting to take place. 
     According to embodiments disclosed herein, a laser energy used in laser PBF can be in the range of about 1 to about 10 J/mm 2 . In this range, a relatively dense and continuous tungsten network can be achieved. As the laser energy decreases below about 1 J/mm 2 , tungsten melting may not occur. Conversely, at laser energies higher than about 10 J/mm 2 , the tungsten-copper binary system experiences primarily tungsten melting. 
     In some embodiments, additive manufacturing of the electrode includes additively manufacturing a porous tungsten structure  900 , as shown in  FIG. 9 , using tungsten powder and subsequently infiltrating the porous tungsten structure with molten copper to form the electrode. Various methods for infiltrating the porous tungsten structure with molten copper are known, including spontaneous or pressure infiltration, centrifugal infiltration, ultrasound-driven or aided infiltration, or Lorenz force infiltration. In comparison to direct additive manufacturing using a mixture of tungsten and copper powders as described above, additive manufacturing of the porous tungsten structure with subsequent copper infiltration ensures the formation of a continuous network of tungsten  910 . Copper infiltration can be performed using any of various conventional methods, such by positioning the porous tungsten structure in a mold and filling the mold with molten copper. To facilitate infiltration of copper, the infiltration can be performed under pressure. 
     In some embodiments, a porous tungsten structure is additively manufactured using a powder bed additive manufacturing technique, such as laser PBF. The porous tungsten structure can be additively manufactured so as to provide the desired shape and configuration of the electrode, wherein the porous tungsten structure includes an interconnected network of pores. The particle size distribution of the tungsten particles can be from about 15 to about 55 microns, and can be about 25 microns or less. Further, in order to control the porosity or density of the porous tungsten structure, a range of laser energies used in laser PBF, or other similar additive manufacturing techniques can be from about 1 to 10 J/mm 2 . It has been found by the inventors that as the laser energy increases, the relative density of the porous tungsten structure generally decreases, e.g., as result of balling of tungsten. In order to allow for subsequent copper infiltration, the resulting porous tungsten structure can be produced so as to have a relative density level of about 75%, about 80%, about 85%, about 90%, or about 95%. 
     In some embodiments, the electrodes formed by the additive manufacturing processes as described herein have a desired geometry and configuration (such as a web defining a matrix of cellular openings, and any internal passages), but also have a high surface roughness, e.g., an undesirably high surface roughness for manufacturing extrusion dies for a particular application. As used herein, the term “surface roughness” or “roughness” shall refer to the arithmetic average roughness, Ra.  FIG. 10  shows a portion of a web  140  of an electrode  100  formed by additive manufacturing. Internal surfaces  144  of web  140  produced by additive manufacturing may have a surface roughness, Ra, of about 100 μm or more. A close-up of an internal surface of an additively manufactured electrode  100  having a high surface roughness is shown for example at  FIG. 11 . According to embodiments disclosed herein, in order for electrode  100  to be used in subsequent extrusion die manufacturing processes, electrode  100  formed by additive manufacturing is smoothed to reduce the surface roughness. 
     Particularly, internal surfaces  144  of the web  140 , which surround cellular openings  160 , can be smoothed. For example, if the surface roughness of electrode  100  is high, an extrusion die formed using the rough electrode will also have a high surface roughness. Smooth surfaces of features of the extrusion die (e.g., of the pins of the extrusion die proximate to the discharge face) can be useful to facilitate more uniform flow throughout the die and/or for extruding more dimensionally uniform articles, due to the interaction of the extrudable material flowing through the channels of the extrusion die defined between the side surfaces of the pins. 
     In some embodiments, additively manufactured electrode  100  is smoothed to provide web  140  with a surface roughness, Ra, of about 1 to about 50 μm, about 1 to about 40 μm, about 1 to about 30 μm, about 1 to about 20 μm, or about 1 to about 10 μm. However, other roughness values can be used in other embodiments, depending on the parameters of the extrusion process, such as the ingredients or batch mixture of the material to be extruded, the dimensions of the slots/pins in the extrusion die, among other parameters. 
     Electrode  100  formed by additive manufacturing can be smoothed by any of a variety of methods including plunge EDM, wire EDM, electrochemical machining (ECM), pulsed-salt electrochemical machining (p-ECM), milling, micro-machining, or a combination thereof. Specifically, the internal surfaces  144  of web  140  of electrode  100  can be smoothed by the above-listed methods. Operations for smoothing the additively manufactured electrode can be performed relatively quickly and with less expense in comparison to forming the entire electrode using EDM or ECM methods. Smoothing operations can also serve to reduce the dimensions of electrode  100 , such as the thickness of the web, ti, to the desired final dimensions for electrode  100  (e.g., enabling an extrusion die to be “rough cut” with dimensions that are intentionally too thick and then smoothed to reduce the thickness). 
     In some embodiments, electrode  100  is smoothed by EDM or ECM using a secondary electrode  200 , as shown for example in  FIG. 12 . Secondary electrode  200  includes a base  220  and a plurality of pins  260  extending from a surface  222  of base  220 . Pins  260  of secondary electrode  200  are shaped and arranged so as to mate with web  140  and matrix of cellular openings  160  of electrode  100 . By “mate” it is meant that pins  260  generally correspond in size and shape to cellular openings  160 . For example, if electrode  100  has a matrix of cellular openings with a hexagonal cross sectional area, pins  260  of secondary electrode  200  is also formed with a hexagonal cross sectional area. Pins  260  are dimensioned so as to fit within cellular openings  160  of electrode  100  with a close tolerance so as to smooth the interior surfaces  144  of web  140 . Thus, when pins  260  of secondary electrode  200  are inserted into cellular openings of electrode  100 , surfaces  264  of pins  260  can be adjacent to internal surfaces  144  of web  140  of electrode  100 . 
     Secondary electrode  200  can be formed by any of the various methods discussed herein, such as by additive manufacturing, wire EDM (depending on the desired pin shape), pulse EDM, ECM, or p-ECM. Further, secondary electrode  200  can be formed from any of various materials. Secondary electrode  200  can have a low surface roughness so as to produce an electrode with a low roughness. Secondary electrode can be relatively smooth, i.e., have a low surface roughness (e.g., in comparison to the surface roughness of electrode  100 ), e.g., particularly if secondary electrode  200  is to be used in a plunge EDM process for smoothing electrode  100 . 
     In some embodiments, as shown in  FIG. 13 , a method for forming an electrode by additive manufacturing  1300  is provided. The additively manufactured electrode  1310  is used to form a secondary electrode from a metal blank, such as a stainless steel blank. In such embodiments, the additively manufactured electrode is used as the cathode in a p-ECM process to machine a honeycomb pattern into the metal blank so as to form the secondary electrode having the negative pattern of the additively manufactured electrode  1320 . Secondary electrode is then used to smooth the additively manufactured electrode  1330 . Electrode can be smoothed using the secondary electrode as the cathode in a p-ECM process. The secondary electrode used in p-ECM can smooth the web of the additively manufactured electrode such that the electrode has a desired surface roughness Ra within about ±0.05 μm. Smoothing the electrode using p-ECM allows for smoothing of the electrode more rapidly than conventional machining methods, such as wire EDM which requires smoothing of each cell one at a time, and using p-ECM the matrix of cellular openings can be smoothed simultaneously. 
     In some embodiments, a coating is applied to surfaces of the smoothed electrode  1340 . In some embodiments, the coating is applied by chemical vapor deposition, physical vapor deposition, or by atomic layer deposition. The coating can also be applied by an electrostatic coating process, such as by electrostatic painting or powder coating. In electrostatic coating processes, an electric charge is used to create magnetic bonds between the paint, e.g., the coating material, and the object to be coated, e.g., the electrode. In powder coating, the coat is further baked in an oven after the coating has been applied to the substrate. In some embodiments, the coating is an electrically-insulating coating that is applied to the internal surfaces of the smoothed electrode, e.g., surfaces  144  of electrode  100  as shown in  FIG. 10 , so as to confine machining to the external surfaces  148  of electrode  100 . In some embodiments, the coating is amorphous silicon, parylene, aluminum oxide, silicon carbide, or diamond-like carbon (DLC), among various other coatings. 
     Unless otherwise expressly stated, it is in no way intended that any method set forth herein be construed as requiring that its steps be performed in a specific order. Accordingly, where a method claim does not actually recite an order to be followed by its steps or it is not otherwise specifically stated in the claims or descriptions that the steps are to be limited to a specific order, it is in no way intended that any particular order be inferred. 
     It will be apparent to those skilled in the art that various modifications and variations can be made without departing from the spirit or scope of the disclosed embodiments. Since modifications, combinations, sub-combinations and variations of the disclosed embodiments incorporating the spirit and substance of the embodiments may occur to persons skilled in the art, the disclosed embodiments should be construed to include everything within the scope of the appended claims and their equivalents. 
     Where a range of numerical values is recited herein, comprising upper and lower values, unless otherwise stated in specific circumstances, the range is intended to include the endpoints thereof, and all integers and fractions within the range. It is not intended that the scope of the claims be limited to the specific values recited when defining a range. Further, when an amount, concentration, or other value or parameter is given as a range, one or more preferred ranges or a list of upper preferable values and lower preferable values, this is to be understood as specifically disclosing all ranges formed from any pair of any upper range limit or preferred value and any lower range limit or preferred value, regardless of whether such pairs are separately disclosed. Finally, when the term “about” is used in describing a value or an end-point of a range, the disclosure should be understood to include the specific value or end-point referred to. Whether or not a numerical value or end-point of a range recites “about,” the numerical value or end-point of a range is intended to include two embodiments: one modified by “about,” and one not modified by “about.” 
     As used herein, the term “about” means that amounts, sizes, ranges, formulations, parameters, and other quantities and characteristics are not and need not be exact, but may be approximate and/or larger or smaller, as desired, reflecting tolerances, conversion factors, rounding off, measurement error and the like, and other factors known to those of skill in the art. 
     The terms “substantial,” “substantially,” and variations thereof as used herein are intended to note that a described feature is equal or approximately equal to a value or description. For example, a “substantially planar” surface is intended to denote a surface that is planar or approximately planar. 
     The present embodiment(s) have been described above with the aid of functional building blocks illustrating the implementation of specified functions and relationships thereof. The boundaries of these functional building blocks have been arbitrarily defined herein for the convenience of the description. Alternate boundaries can be defined so long as the specified functions and relationships thereof are appropriately performed. 
     It is to be understood that the phraseology or terminology used herein is for the purpose of description and not of limitation. The breadth and scope of the present disclosure should not be limited by any of the above-described exemplary embodiments, but should be defined in accordance with the following claims and their equivalents.