Abstract:
A plasma arc torch that includes a torch body having a nozzle mounted relative to a composite electrode in the body to define a plasma chamber. The torch body includes a plasma flow path for directing a plasma gas to the plasma chamber in which a plasma arc is formed. The nozzle includes a hollow, body portion and a substantially solid, head portion defining an exit orifice. The composite electrode can be made of a metallic material (e.g., silver) with high thermal conductivity in the forward portion electrode body adjacent the emitting surface, and the aft portion of the electrode body is made of a second low cost, metallic material with good thermal and electrical conductivity. This composite electrode configuration produces an electrode with reduced electrode wear or pitting comparable to a silver electrode, for a price comparable to that of a copper electrode.

Description:
RELATED APPLICATIONS  
       [0001]     This application claims benefit of U.S. Provisional Application No. 60/274,837, filed Mar. 9, 2001. The entire disclosure of this application is incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION  
       [0002]     The present invention relates to a composite electrode for a plasma arc torch. In particular, the invention relates to a composite electrode for a plasma arc torch in which a forward portion of the electrode body comprises a first metallic material having high thermal conductivity and the remaining aft portion of the electrode body comprises a second low cost, metallic material with good thermal and electrical conductivity.  
       BACKGROUND OF THE INVENTION  
       [0003]     Plasma arc torches are widely used in the cutting or marking of metallic materials. A plasma torch generally includes an electrode mounted therein, a nozzle with a central exit orifice mounted within a torch body, electrical connections, passages for cooling and arc control fluids, a swirl ring to control fluid flow patterns in the plasma chamber formed between the electrode and nozzle, and a power supply. The torch produces a plasma arc, which is a constricted ionized jet of a plasma gas with high temperature and high momentum. Gases used in the torch can be non-reactive (e.g. argon or nitrogen), or reactive (e.g. oxygen or air).  
         [0004]     In operation, a pilot arc is first generated between the electrode (cathode) and the nozzle (anode). Generation of the pilot arc may be by means of a high frequency, high voltage signal coupled to a DC power supply and the torch or any of a variety of contact starting methods.  
         [0005]     One known configuration of an electrode for a plasma arc torch includes an emitting insert (e.g., hafnium) which is press fit into a bore in the electrode. An objective in electrode design is to transfer heat from the hafnium insert and into a cooling medium, which is usually water. Another objective is to control arc root attachment to minimize erosion caused by undesirable arc root attachment to the electrode instead of the hafnium insert.  
         [0006]     Electrodes for plasma arc torches are commonly made from copper. Copper is a low cost material that offers good thermal and electrical conductivity. Electrodes for plasma arc torches can also be made from silver. While silver electrodes provide excellent heat transfer characteristics, they tend to be very expensive and not cost effective to use. Copper electrodes are cost effective, but do not have the superior heat transfer characteristics of a silver electrode and thus have a shorter electrode life than silver electrodes.  
         [0007]     Several companies manufacture silver and silver/copper composite electrodes using a variety of manufacturing techniques including brazing, soldering, swaging, press fitting and other methods. One company has developed a vacuum brazed copper/silver composite design with a through-hole hafnium insert. Another company has developed a press-fitted silver annulus design with a blind hole hafnium insert. Another company has developed a swaged silver annulus design in a copper holder with copper on the front portion. Another company has developed coined silver electrode design. However, these methods of manufacturing silver/copper electrodes do not produce a sufficiently high-strength joint at the silver/copper interface. In addition, these manufacturing methods result in electrodes that can leak cooling fluid at the silver/copper interface. More significantly, these silver/composite electrodes do not offer the heat transfer characteristics of an all silver electrode.  
       SUMMARY OF THE INVENTION  
       [0008]     It is an object of the present invention to provide an improved composite electrode, which combines the material property benefits of silver with the cost benefits of copper.  
         [0009]     Another object of the present invention is to provide an improved composite electrode that does not leak cooling fluid.  
         [0010]     In one aspect, the invention features a plasma arc torch for cutting or marking a metallic workpiece. The torch includes a torch body having a nozzle mounted relative to a composite electrode in the body to define a plasma chamber. The torch body includes a plasma flow path for directing a plasma gas to the plasma chamber. In one embodiment, the torch can also include a shield attached to the torch body. The nozzle, composite electrode and shield are consumable parts that wear out and require periodic replacement.  
         [0011]     The composite electrode has two portions made from different materials. The forward portion of the electrode comprises a metallic material with excellent heat transfer properties (e.g., high thermal conductivity) (e.g., silver). An emissive insert (e.g., hafnium, zirconium, tungsten, thorium, lanthanum, strontium, or alloys thereof) is disposed in a bore in the forward portion. The aft portion of the electrode comprises a low cost, metallic material with good heat transfer properties (e.g., good thermal conductivity) (e.g., copper).  
         [0012]     The high thermal conductivity, forward portion is joined onto an end of the good thermal conductivity, aft portion to form the composite electrode. The two portions are joined by a direct welding process, such as friction welding, inertia friction welding, direct drive friction welding, CD percussive welding, percussive welding, ultrasonic welding, or explosion welding, that forms a hermetic seal between the two portions of the electrode. To maximize cooling, the forward portion also extends back to the area of cooling fluid flow and is therefore directly cooled by the fluid. This construction, in contrast to known electrode designs having a relatively small diameter, high thermal conductivity sleeve inserted into a cavity formed in the front end for surrounding an emissive insert, is believed to provide an electrode that has superior heat transfer properties and does not leak cooling fluid.  
         [0013]     In another aspect, the invention features a composite electrode for a plasma arc torch for cutting or marking a metallic workpiece. The composite electrode includes a forward portion comprising a metallic material with excellent heat transfer material properties (e.g., high thermal conductivity) (e.g., silver). The aft portion of the electrode comprises a low cost, metallic material with good heat transfer material properties (e.g., good thermal conductivity) (e.g., copper).  
         [0014]     The high thermal conductivity, forward portion is joined onto an end of the good thermal conductivity, aft portion to form the composite electrode. In one embodiment, the forward and aft portions are in direct contact at the mating surface. To accomplish this, the two portions are joined together by a direct welding technique—such as friction welding, inertia friction welding, direct drive friction welding, CD percussive welding, percussive welding, ultrasonic welding, or explosion welding. The direct welding process forms a high strength, hermetic seal between the two portions of the electrode. To maximize cooling, the forward portion also extends back to the area of cooling fluid flow and is therefore directly cooled by the fluid.  
         [0015]     Yet another aspect of the invention features a method of manufacturing an electrode for cutting or marking a workpiece. An electrode is provided including a forward portion comprising a metallic material with excellent heat transfer material properties (e.g., high thermal conductivity) (e.g., silver). An aft portion of the electrode body is also provided, comprising a low cost, metallic material with good heat transfer material properties (e.g., good thermal conductivity) (e.g., copper). The two portions of the electrode are joined by a direct welding technique. They can be joined, for example, by friction welding, inertia friction welding, direct drive friction welding, CD percussive welding, percussive welding, ultrasonic welding, or explosion welding, thereby forming a high strength, hermetic seal between the forward and aft portions of the electrode. Cooling fluid flow can be used to cool the forward portion of the electrode, and an insert with high thermionic emissivity can be located in a bore in the forward portion of the electrode body. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]      FIG. 1  is a cross-sectional view of one embodiment of a plasma arc torch according to the invention.  
         [0017]      FIG. 1A  is a cross-sectional view of one embodiment of a composite electrode for use in the plasma arc torch of  FIG. 1 .  
         [0018]      FIG. 2  is a cross-sectional view of another embodiment of a composite electrode for use in the plasma arc torch of  FIG. 1 .  
         [0019]      FIG. 3  is a graph comparing the number of starts vs. pit wear from various electrode configurations.  
         [0020]      FIG. 4  is a graph comparing the performance of electrodes according to the present invention with other electrodes, using 4 second testing.  
         [0021]      FIG. 5  is a graph comparing the performance of electrodes according to the present invention with other electrodes, using 60 second testing.  
         [0022]      FIG. 6  is a graphical representation showing a temperature contour plot in a model of a silver tip electrode during torch operation, based on a computational fluid dynamics model.  
         [0023]      FIGS. 7A-7Q  show various embodiments of electrode tip configurations of the invention.  
         [0024]      FIG. 7R  shows an aft portion of an electrode having a receiving portion. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0025]      FIG. 1  shows a plasma arc torch  10  embodying the principles of the invention. The torch has a body  12 , which is typically cylindrical with an exit orifice  14  at a lower end  16 . A plasma arc  18 , i.e. an ionized gas jet, passes through the exit orifice and attaches to a workpiece  19  being cut. The torch is designed to pierce, cut, or mark metal, particularly mild steel, or other materials in a transferred arc mode. In cutting mild steel, the torch operates with a reactive gas, such as oxygen or air, as the plasma gas to form the transferred plasma arc  18 .  
         [0026]     The torch body  12  supports a composite electrode  20  having a generally cylindrical body  21 . A hafnium insert  22  is disposed in the lower end  21   a  of the composite electrode  20  so that a planar emission surface  22   a  is exposed. The insert  22  can also be made of other materials possessing suitable physical properties, such as corrosion resistance and a high thermionic emissivity. In one embodiment, the insert material has an electron work function of about 5.5 electron volts or less. Suitable materials include hafnium, zirconium, tungsten, yttrium, iridium, and alloys thereof. The torch body also supports a nozzle  24 , which is spaced from the composite electrode. The space between the nozzle  24  and the composite electrode  20  defines a plasma chamber  30 . The nozzle  24  has a central orifice that defines the exit orifice  14 . A swirl ring  26  mounted to the torch body has a set of radially offset (or canted) gas distribution holes  26   a  that impart a tangential velocity component to the plasma gas flow causing it to swirl. This swirl creates a vortex that constricts the arc and stabilizes the position of the arc on the insert.  
         [0027]     There are two ways to start the torch. One solution has been contact starting, one form of which is described in U.S. Pat. No. 4,791,268. However, a principal starting technique currently in use uses a high frequency, high voltage (HFHV) signal coupled to a power line from a D.C. power supply to the torch. The HFHV signal induces a spark discharge in a plasma gas flowing between the composite electrode and a nozzle, typically in a spiral path. A HFHV generator is usually incorporated in a power supply or in a “console” located remotely from the torch and connected to the torch by a lead set.  
         [0028]     The arc between the electrode and nozzle is a pilot arc, and the arc between the composite electrode and the workpiece is a transferred arc. The gas flow through the nozzle is ionized by the pilot arc so that the electrical resistance between the composite electrode and the workpiece becomes very small. Using a pilot resistor, a higher voltage is applied across the composite electrode and the workpiece to induce the arc to transfer to the workpiece after the gap is ionized. The time between starting the pilot arc and transferring to the work is a function of the distance of the torch above the work, the pilot arc current level, and the gas flow rate when the traditional start circuits are used.  
         [0029]     Electrodes have been commonly manufactured from copper. Copper has been chosen because of its good heat transfer capabilities and low cost. Applicants have determined that significant improvements in the service life of electrodes can be achieved using a high purity all-silver or coined silver electrode (e.g., 90% silver, 10% copper) with a swaged hafnium emitting element. Test results have shown over 2000 starts for such an electrode in laboratory testing with a plasma arc torch operating using a non-ramp-down process. This type of electrode allows direct water cooling of the silver surrounding the hafnium. However, due to the high material cost of silver, this electrode design is very expensive and has not achieved wide market acceptance.  
         [0030]     Applicants have achieved results comparable to an all-silver electrode using a copper/silver composite electrode in accordance with the present invention. To accomplish this, Applicants have optimized the amount of silver through material analysis, steady state heat flux modeling and empirical data collection. Applicants&#39; test results show that significant gains in electrode service life can be realized if the silver component extends from the forward portion of the electrode back into the area of the hollow mill and is directly cooled by water. In one embodiment, both the hafnium insert  22  and the silver are directly cooled by water.  
         [0031]      FIG. 1A  shows a cross-sectional view of one embodiment of a composite electrode  20 , in which the hafnium insert  22  can be directly cooled by a coolant  52  such as cooling water. The coolant circulates through an internal flow path inside of the composite electrode, including interior surfaces of the aft portion  20 B, and across interior surfaces of the forward portion  20 A, including the bottom wall  42 A and side walls  42 B. The cooling fluid exits the composite electrode via the annular passage  54  defined by the tube  58  and the inner wall  59  of the electrode  20 . The composite electrode is also preferably “hollowmilled.” That is, it has an annular recess  56  formed in the interior surface of the bottom wall  42 A, to enhance the surface area of the body material, thereby promoting a heat exchanging relationship with the coolant  52 . The planar emission surface  22 A is sized, in conjunction with the flow of coolant  52  and the surface areas of the bottom wall  42 A and the side walls  42 B and  42 C, to prevent boiling of the hafnium insert  22 . Further, although the insert  22  is illustrated as being a single cylindrical piece, other geometrys are within the scope of the invention. Use of multiple inserts is also contemplated.  
         [0032]     In its most basic form, Applicants&#39; electrode includes a forward silver portion directly joined to an aft copper portion. A hafnium insert is disposed in a bore formed in the forward portion. See  FIG. 2 , described in detail below.  
         [0033]     Applicants have recognized the difficulty in obtaining a high strength, leak-proof joint at the copper/silver interface when using conventional methods of joining, such as press-fit, soft-solder, vacuum brazing, torch brazing, threading, adhesive, ultrasonic weld, etc. Use of swaged, soft soldered, silver soldered, or induction brazed techniques used to attach the forward silver portion to the aft copper portion do not result in a reliable hermetic seal. This occurs because the joint must withstand torque during assembly, high pressure coolant during operation, heat stress, thermal expansion and contraction, shear stress, thermal fatigue, etc.  
         [0034]     Applicants&#39; invention includes techniques for efficiently and effectively joining the aft portion  20 B directly with the forward portion  20 A. The aft portion  20 B has a first mating surface  46  that is joined with a second mating surface  47  of the forward portion  20 A, using techniques such as those described below. Combination of the first and second mating surfaces  46  and  47  results in a joint. In one embodiment, the mating surfaces are planar, as illustrated. However, non-planar mating surfaces can be used as well. The term non-planar includes any contour or shape that can be used, for example, with the joining techniques described below. In one preferred embodiment, the first or second mating surface has a circular, planar cross-sectional shape. The size of each mating surface can be the same, or they can be different.  
         [0035]     In general, the invention contemplates a process to join directly (i.e., without the use of any additional material) the forward and aft portions. The first mating surface  46  is joined to the second mating surface  47 , using a direct welding technique, such as friction welding, which results in the forward and aft portions being in direct contact with each other. Friction welding is widely used to weld dissimilar materials and minimize cost per part. Friction welding is an ideal process for joining dissimilar metals and provides high reliability, low porosity, and excellent strength. Friction welding is an ideal process for forming a high strength, leak-proof weld between silver and copper, resulting in a hermetic seal. In addition, friction welding does not require the use of an additional material (e.g. solder). Friction welding, inertia friction welding, and direct drive friction welding techniques, are performed, for example, by MTI Welding of South Bend, Ind., and are described on their web site. See, for example, http://www.mtiwelding.com. Pages found at this web site describe various suitable welding techniques, and some of the associated metal combinations on which they can be used.  
         [0036]     More particularly, these web pages describe friction welding techniques, including inertia friction welding and direct drive friction welding. These techniques can be used to create a joint between dissimilar materials that is of forged quality, and can be used to create a 100% butt joint weld throughout the contact area of the two pieces being joined. These and other direct welding techniques, including CD percussive welding, percussive welding, ultrasonic welding, explosion welding, and others, utilize combinations of workpiece acceleration and deceleration, welding speed, frictional forces, forge forces, and other such physical forces, sometimes in combination with electricity at various voltages and current flows, to create and use force and/or heat in a predetermined and controlled manner, between the workpieces being joined, to create a strong, leak-proof joint without the introduction of extraneous materials (such as flux, solder, braze, or filler materials). They accomplish this utilizing rapid and efficient cycle times, and with minimal loss of the working materials. These techniques are all considered to be within the scope of the invention.  
         [0037]     Direct welding techniques, and friction welding techniques in particular, have been successfully employed to join materials such as silver and copper, but are also effective for joining various combinations, for example, of the following materials, or alloys thereof: aluminum, aluminum alloys, brass, bronze, carbides cemented, cast iron, ceramic, cobalt, columbium, copper, copper nickel, iron sintered, lead, magnesium, magnesium alloys, molybdenum, monel, nickel, nickel alloys, nimonic, niobium, niobium alloys, silver, silver alloys, steel alloys, steel-carbon, steel-free machining, steel-maraging, steel-sintered, steel-stainless, steel-tool, tantalum, thorium, titanium, titanium alloys, tungsten, tungsten carbide cemented, uranium, vanadium, valve materials (automotive), and zirconium alloys. Proper use of these techniques results in the significant electrode performance enhancements of the invention, as contrasted, for example, with conventional brazing, soldering, and other joining methods, some of which were discussed earlier.  
         [0038]     For purposes of this invention, in addition to the techniques described above, direct welding includes joining methods that create a suitable high-strength joint between the dissimilar metals of the first mating surface  46  and the second mating surface  47 , without the need to add additional materials such as braze, flux, solder, or filler materials. For purposes of this invention, direct welding includes inertia friction welding, direct drive friction welding, CD Percussive welding, percussive welding, ultrasonic welding, and explosion welding. These manufacturing methods achieve a direct metallurgical coupling between the first and second mating surfaces, resulting in a strong bond at low cost. The direct contact between the mating surfaces, especially in the absence of solder, flux, braze, filler materials and the like, contributes to the superior performance of the invention. Moreover, it is recognized that an alloy may be formed where the first and second mating surfaces meet, resulting from the combination of these different materials. This alloy may be formed either during direct welding, and/or during subsequent operation of the torch. Applicants have determined that formation of any alloy in this manner does not hinder the performance of the invention. Rather, it is the use of braze, flux, solder, welding filler materials, and the like, such as those used in other types of joining processes, that should be avoided. These types of materials are not used in the direct welding process of the invention, allowing Applicants to achieve the direct contact between the mating surfaces that is required.  
         [0039]     In one aspect, Applicants have developed an electrode with an optimal volume and geometry of a forward silver portion and an aft copper portion based on (1) performance and (2) cost and ease of manufacturing. Applicants&#39; composite electrode performs as if it is an all-silver electrode. The electrode approximates the material properties of the more expensive silver material. The electrode uses the requisite volume of silver to provide excellent heat transfer in the forward portion around the emissive insert, to achieve performance and service life equal to that of the all-silver electrode. The requisite geometry and volume can be determined through empirical data collection in the laboratory, and by computer modeling of the heat flux. These techniques can be used, for example, to design electrodes that minimize the amount of silver used during electrode fabrication, thereby reducing the cost of the electrode. Cavities or lumens can be strategically located within portions of the forward and/or aft portions of the electrode body, for example, to enhance cooling capabilities, or to reduce the quantity of material required for fabrication. Applicants have also used these techniques to determine that superior cooling of the hafnium insert  22  is achieved by providing a high thermal conductivity material, such as silver, in the forward portion  20 A to surround the circumference of the emissive insert  22 , thereby providing contact with the excellent heat transfer property of the forward portion of the electrode along the length of the insert  22 , whereby the life of the electrode is extended. Further, Applicants have determined that providing a single radial interface between the insert  22  and the forward silver portion also results in superior electrode performance.  
         [0040]     The aft portion  20 B of the electrode can be made with a lower cost copper material which still has good heat transfer properties, but results in a composite electrode with performance characteristics comparable to an all-silver electrode for a much lower cost. In addition, as the majority of heat transfer can take place in the forward portion  20 A, a higher emphasis on the machinability of the aft portion can be used as a criterion in the material selection of the aft portion. The heat transfer property of the forward and aft portions of the electrode can be, for example, thermal conductivity or thermal diffusivity.  
         [0041]     The forward and aft portions of the composite electrode can be made from various combinations of materials. In one embodiment of the invention the thermal conductivity of the forward portion of the electrode (e.g., silver) is generally greater than about 400 Watts/m/deg-K, and the thermal conductivity of the aft portion of the electrode (e.g., copper) is generally less than this amount. In another embodiment, the materials of the forward portion of the electrode have a high thermal diffusivity, generally greater than 0.1 m 2 /sec., and preferably at least about 0.17 m 2 /sec. The thermal diffusivity of the aft portion of the electrode is less than the thermal diffusivity of the forward portion. Any material, including alloys, with physical properties such as those listed above can be suitable for use with the invention and are contemplated to be within the scope of the invention.  
         [0042]     In addition to silver/copper, other composite or multi-metallic combinations with desirable characteristics for use with the composite electrode of the invention can be used. Different embodiments of the invention can use silver/aluminum, silver/brass, or brass/copper material combinations for the forward and aft portions of the electrode. Applicants usage herein of the term “composite” is intended to mean at least two metallic materials.  
         [0043]      FIG. 2  is an illustration of an embodiment of an electrode  200  embodying the principles of the present invention. The main components of the electrode  200  are a forward silver portion  210  and an aft copper portion  220 , which has been friction-welded to the forward silver portion  210 . The friction-welded joint is created where the surfaces of the forward silver portion  210  and the aft copper portion  220  meet. Although the joint is described as friction-welded, the other direct welding joining techniques such as those described above can also be used, and are considered to be within the scope of the invention. Moreover, although the forward silver portion  210  can be primarily silver, other materials such as gold, palladium, silver-copper alloys, brass, rhodium and platinum, and alloys of any of these are also suitable, and are within the scope of the invention.  
         [0044]     The joint illustrated in  FIG. 2  has a cross-sectional area that extends across the width of the electrode  200 . In other embodiments of the invention, the diameters of these portions can be different, and these cross-sectional areas can be different. Further, the shape of the forward portion  210  can be different from the shape of the aft portion  220 . For example, the forward portion can be in the shape of a disk or a square, and the aft portion can be in the shape of a tube, with the end of the tube being friction-welded to a surface of the forward portion. Many various shapes and configurations are contemplated, and provide for effective operation of the invention.  
         [0045]     In one embodiment of the invention, the forward silver portion  210  comprises or is made of silver and the aft copper portion  220  comprises or is made of copper. The forward silver portion  210  has a bore  230  into which a hafnium insert can be press fit. As illustrated in  FIG. 2 , the bore  230  can be located along a central axis of the forward portion of the electrode body. The friction weld used to attach the forward silver portion  210  to the aft copper portion  220  results in a reliable, leak-proof hermetic seal along with a high strength weld. To maximize cooling, the forward portion also extends back to the area  240  of cooling fluid flow and is therefore directly cooled by the fluid. In one embodiment, the electrode  200  is of a hollow-milled configuration. As shown in  FIG. 2 , the hollow-milled configuration results in increased surface area  250 A,  250 B,  250 C,  250 D,  250 E, and  250 F for transferring heat from a hafnium insert to cooling area  240 .  
         [0046]     Full strength welds of oxygen-free copper to coined (e.g., 90% silver, 10% copper) silver have been achieved using friction welding. Bend tests and tensile tests showed strength equal to silver material. Laboratory results comparing pit depth of an electrode against the number of pierces for a silver/copper electrode were identical to an all-silver electrode, until the depth of silver was consumed, as shown in  FIG. 3 . The foregoing are merely representative embodiments, as other configurations are possible and within the scope of the invention.  
         [0047]      FIG. 4  is a graph that shows pit depth versus the number of electrode starts for various electrodes. The performance of electrodes that are manufactured according to the invention are designated as curves  401  and  403  on the graph. This graph compares these results with those of copper electrodes ( 405  and  406 ), and with other copper-silver electrode combinations ( 408 A- 408 F) that are commercially available. The data in  FIG. 4  was obtained using 4 second life test testing measurements, i.e., multiple four second runs were made with each of the electrodes, to obtain the information displayed in this graph. The graph shows the superior longevity of electrodes manufactured according to Applicants&#39; invention.  
         [0048]      FIG. 5  shows a graph of comparable data as  FIG. 4 , but for 60 second life test measurements (i.e., multiple runs of 60 seconds duration on each of the electrodes). Electrodes according to the invention are labeled on  FIG. 5  as  501  and  503 . Copper electrode results are labeled as  505  and  506 . Results of commercially available copper-silver combination electrodes are labeled as  508 A- 508 F. Again, the results illustrate the superior longevity of the electrodes manufactured according to Applicants&#39; invention.  
         [0049]      FIG. 6  is a plot showing temperature contours in a silver tip electrode during extended operation based on a computational fluid dynamics model. This plot presents a cross-sectional view of an operating electrode comprising a hafnium insert  22  within a silver forward portion  20 A. The electrode modeled in this figure is symmetrical about a central axis  605 . The electrode is cooled by coolant that is present in the annular recess  56 . The temperature at and near the planar emission surface  22 A is hotter than the maximum temperature reading displayed by the graph (190 deg-C), and is displayed as white (area  610 ). This figure qualitatively demonstrates the degree of radial heat conduction away from the hafnium insert  22  in the electrode, and illustrates the importance of having silver available in the radial direction to enhance conduction.  
         [0050]     Radial heat conduction away from the hafnium insert  22  is an important feature of the invention.  FIGS. 7A-7Q  illustrate some different embodiments of different configurations of electrode tips that are within the scope of the invention. The diameter and/or quantity of the silver portion of the electrode tip  705  is sized to achieve the desired amount of radial cooling for a particular application, in combination with the amount and shape of the copper portion of the electrode  710 , and the size, shape, and positioning of the hafnium insert  22  or inserts (if multiple inserts are present). In the embodiments of the invention shown in  FIGS. 7A-7Q  the entire length of the hafnium insert  22  is in contact with the silver portion of the electrode tip  705 , to facilitate heat removal.  
         [0051]      FIG. 7R  shows one embodiment where the aft portion of the electrode is adapted to receive the forward portion of the electrode. The size and shape of the aft portion of the electrode  710  can be adjusted to allow the second mating surface  47  to fit within a receiving portion  715  formed by the first mating surface  46 . In this embodiment the forward portion of the electrode tip  705  has a smaller diameter than the aft portion of the electrode tip  710 , and the forward portion of the electrode tip  705  can be fabricated to fit within the receiving portion  715 . The forward portion of the electrode can occupy substantially all of the diameter of the receiving portion  715 . After friction welding, this embodiment of the invention can result in an electrode tip such as is depicted in  FIG. 7Q .  
         [0052]     The described embodiments preferably use coolant  52  to remove heat from the hafnium insert. These geometry of the forward and aft portion of the electrode can be manipulated in combination, to optimize, for example, heat conduction requirements and manufacturing costs. The silver used in the electrode tip is strategically located to optimize utilization of its heat transfer property. Use of direct welding allows less expensive materials (e.g., copper) to be used where the properties of the more expensive materials are not required.  
         [0053]     While the invention has been particularly shown and described with reference to specific preferred embodiments, it should be understood by those skilled in the art that various changes in form and detail may be made therein without departing from the spirit and scope of the invention.