Electrode for plasma arc torch and method of making the same

An electrode for supporting an arc in a plasma arc torch. The electrode includes a metallic holder having a cavity formed in a front end portion. An insert assembly is mounted in the cavity. The insert assembly includes an emissive insert composed of a metallic material having a relatively low work function. The emissive insert has a bore formed therein for securing a non-emissive core. The core has a base portion, which provides an interface between the emissive insert and the holder. The core and base portion act to thermally conduct heat out of the emissive insert so that the service life of the emissive insert is increased by the lowering of the emissive insert's operating temperature.

BACKGROUND OF THE INVENTION
 The present invention relates to a plasma arc torch and, more particularly,
 to a novel electrode for use in a plasma arc torch having an improved
 service life and a method of making the same.
 Commonly used for working of metals, plasma arc torches are used for
 cutting, welding, surface treatment, melting and annealing. These torches
 include an electrode that supports an arc that extends from the electrode
 to the workpiece in the transferred arc mode of operation. It is also
 conventional to surround the arc with a swirling vortex of gas, and in
 some torch designs, it is conventional to envelope the gas and arc with a
 swirling jet of water.
 The electrode used in a conventional torch of the type described typically
 comprises an elongate tubular member composed of a material of high
 thermal conductivity, such as copper or copper alloy. The forward or
 discharge end of the tubular electrode includes a bottom end wall having
 an emissive insert embedded therein, which supports the arc. The insert is
 composed of a material which has a relatively low work function, which is
 defined in the art as the potential step, measured in electron volts,
 which permits thermionic emission from the surface of a metal at a given
 temperature. In view of its low work function, the insert is thus capable
 of readily emitting electrons when an electrical potential is applied
 thereto, and commonly used insert materials include hafnium, zirconium,
 and tungsten.
 One of the major problems connected with the torches referred to above is
 the shortness of service life of their electrodes, especially when the
 torches are used with an oxidizing arc gas, such as oxygen or air. In
 those torches, the gas appears to rapidly oxidize the copper, and as the
 copper oxidizes, its work function fails. As a result, the oxidized copper
 that surrounds the insert begins to support the arc in preference to the
 insert. After this occurs, the copper melts, thereby causing early
 destruction and/or failure of the electrode.
 U.S. Pat. No. 5,023,425 (Severance, Jr.) which issued on Jun. 11, 1991, and
 which is incorporated herein by reference, discloses an electrode for a
 plasma arc torch wherein the electrode includes a copper holder having a
 lower end which mounts an emissive insert that acts as the cathode
 terminal for the arc during operation. A sleeve of silver is positioned to
 surround the insert and forms an annular ring on the lower end surface of
 the holder to surround the exposed end face of the emissive insert. The
 annular ring serves to prevent arcing from the copper holder, and
 maintains the arc on the insert. However, while the silver sleeve of the
 '425 patent was intended to prolong the life of the copper holder, in
 practice, this electrode suffers from problems in that the wear does not
 come from double arcing, but from the hafnium overheating and eroding.
 U.S. Pat. No. 3,930,139 (Bykhovsky et al.) which issued on Dec. 30, 1975,
 and which is incorporated herein by reference, also discloses an electrode
 for plasma arc working of materials. In the '139 patent, the holder is
 again formed from copper or copper alloys and an active insert is fastened
 to the end face of the holder and is in thermal and electrical contact
 with the holder through a metal distance piece disposed between the active
 insert and the holder and over the entire contact surface area. The metal
 distance piece is formed from aluminum or aluminum alloys and the active
 insert is formed from hafnium or from hafnium with yttrium and neodymium
 oxides as dopants therein taken separately or in combination. However,
 while the aluminum sleeve surrounding the active insert in the '139 patent
 serves to protect the copper holder surrounding the active insert, the
 aluminum distance piece or sleeve offers no advantages over the silver
 sleeve of the '425 patent to Severance, Jr.
 U.S. Pat. No. 5,676,864 (Walters) which issued on Oct. 14, 1997, which is
 incorporated herein by reference, discloses an electrode for a plasma arc
 torch wherein the electrode includes a copper holder having a lower end
 which mounts an emissive insert that acts as the cathode terminal for the
 arc during operation. A sleeve of silver is positioned substantially to
 surround the insert and form an annular ring on the lower end surface of
 the holder to surround the exposed end face of the emissive insert. The
 insert assembly further includes an aluminum face plate disposed in the
 enlarged outer portion of the cavity and which is exposed at the front end
 of the metallic holder so as to surround a front portion of the sleeve.
 U.S. Pat. No. 5,767,478 (Walters) which issued on Jun. 16, 1998,
 alternatively teaches eliminating the aluminum face plate of U.S. Pat. No.
 5,676,864 and instead provides for the front end of the holder to directly
 contact the emissive insert forming an overlay portion of the holder
 between the front face thereof and the sleeve, thus protecting the silver
 sleeve.
 SUMMARY OF THE INVENTION
 An electrode for supporting an arc in a plasma arc torch, according to the
 present invention includes, a metallic holder having a front end, and a
 cavity in the front end. An insert assembly is mounted in the cavity and
 comprises an emissive insert having an inner face, an outer face, and a
 bore formed therein. The emissive insert is composed of a metallic
 material having a relatively low work function. The insert assembly
 further has a non-emissive core positioned within the bore that acts to
 draw heat out of the emissive insert. The core has a base portion that
 substantially covers the inner face of the emissive insert forming an
 interface between the emissive insert and the metallic holder. This
 interface aids in transferring heat from the emissive insert to the
 metallic holder.
 The metallic holder is generally tubular and has a front end wall which
 defines an outer front face, wherein the outer face of the emissive insert
 and an end portion of the non-emissive insert lie in the plane of the
 outer front face of the metallic holder.
 It is also contemplated that the electrode further comprises a sleeve which
 surrounds at least a portion of the emissive insert so as to separate the
 portion of the emissive insert from contact with the holder. The sleeve
 being composed of a metal which is selected from the group consisting of
 silver, gold, platinum, rhodium, iridium, palladium, nickel, and alloys
 thereof.
 An embodiment of the invention additionally includes an overlay portion
 formed in the metallic holder at the front end. The overlay portion
 directly contacts the emissive insert so that none of the sleeve is
 exposed at the front end.
 It is also contemplated that an embodiment of the present invention
 includes, a nozzle mounted adjacent the transverse front end wall of the
 electrode and having a bore therethrough which is aligned with the
 longitudinal axis. A power supply is also provided, which creates an
 electrical arc extending from the emissive insert of the electrode through
 the bore and to a workpiece located adjacent the nozzle. Additionally, gas
 inlet holes may be provided for generating a vortical flow of a gas
 between the electrode and the nozzle and so as to create a plasma flow
 outwardly through the bore and to the workpiece.
 The invention also includes a method of fabricating an electrode adapted
 for supporting an arc in a plasma arc torch, which comprises, creating a
 bore in a substantially cylindrical emissive material and positioning in
 the bore, a non-emissive core, having a base, to form an insert assembly.
 The method further includes preparing a cylindrical metallic blank and
 forming a first cavity in an end portion of the blank. The insert assembly
 is then pressed into the first cavity of the blank so that the base of the
 core interfaces between the cylindrical emissive material and the blank.
 The blank is swaged so that the insert assembly is bound to the
 cylindrical metallic blank. Finally, threads are formed on an end distal
 to the front end, and a second cavity is formed by a boring and grooving
 process.
 The method further contemplates forming an overlay portion in the first
 cavity and inserting a sleeve within the cavity, which surrounds at least
 a portion of the insert assembly, wherein the overlay portion directly
 contacts the emissive material so that none of the sleeve is exposed at
 the front end.
 Features of the electrode of the present invention include adaptability for
 use in a plasma arc torch of the type described. The invention also
 provides a significant advantage from the perspective of improved service
 life when the torch is used in an oxidizing atmosphere

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
 In FIG. 1, a plasma arc torch 10 is shown which includes a nozzle assembly
 12 and a tubular electrode 14. The electrode 14 is made preferably of
 copper or a copper alloy, and it is composed of an upper tubular member 15
 and a lower, cup-shaped member or holder 16. More specifically, the upper
 tubular member 15 is of elongate open tubular construction and it defines
 the longitudinal axis of the torch. The upper tubular member 15 also
 includes an internally threaded lower end portion 17.
 With additional reference to FIG. 2, the holder 16 is also of tubular
 construction, and includes a lower body area 16a. A transverse front end
 wall 18 closes the front end of the holder 16, defining an outer front
 face 20. The rear end of the holder 16 is machined to be externally
 threaded and is threadedly joined to the lower end portion 17 of the upper
 tubular member 15.
 The holder 16 is formed from a blank, such as copper, and is open at the
 rear end thereof. The holder 16 has a cup-shaped configuration and defines
 an internal cavity 22 formed by a boring and grooving process. Also, the
 transverse front-end wall 18 of the holder includes a cylindrical post 23,
 which extends rearwardly into the internal cavity 22 and along the
 longitudinal axis. In addition, a generally cylindrical cavity 24 is
 formed in the lower body area 16a of the holder 16 and extends rearwardly
 along the longitudinal axis and into a portion of the length of the post
 23.
 An insert assembly 26 is provided and comprises a generally cylindrical
 emissive insert 28 having a bore 27 formed therein. The emissive insert 28
 has a circular outer end face 29 and a circular inner face 30. A
 non-emissive core 31, such as silver, is positioned inside the bore 27.
 The core 31 has a base 31a formed on an end portion thereof, so that upon
 assertion of the core 31 into the bore 27, the base 31a rests on the
 circular inner face 30 of the emissive insert 28. The bore 27 forms
 corners with the outer end face 29 and inner end face 30 of the emissive
 insert 28, having preferably, but not necessarily, radiuses between
 0.003-0.005 inches. Likewise, the base 31a has preferably, but not
 necessarily, matching radiuses so as to form a close fit with the emissive
 insert 28.
 The emissive insert 28 is composed of a metallic material which has a
 relatively low work function, in a range between about 2.7 to 4.2 ev, so
 that it is adapted to readily emit electrons upon an electrical potential
 being applied thereto. Suitable examples of such materials are hafnium,
 zirconium, tungsten and alloys thereof.
 The insert assembly 26 is positioned coaxially along the longitudinal axis
 in the cavity 24 of the holder 16 using a pressing process. The base
 portion 31a of the core 31 forms an interface between the circular inner
 face 30 of the emissive insert 28 and the holder 16. In operation, the
 core 31 and base 31a work as a conductive element to transfer heat to the
 holder 16 resulting in less heat build up in the emissive insert 28.
 An alternative embodiment is illustrated in FIG. 3 and has a relatively
 non-emissive sleeve 32 positioned in the cavity 24' coaxially about the
 emissive insert 28. The sleeve 32 has a peripheral wall, which is
 metallurgically bonded to the walls of the cavity 24'.
 The sleeve 32 is composed of a metallic material having a work function
 which is greater than that of the material of the holder 16, and also
 greater than that of the material of the emissive insert 28. In this
 regard, it is preferred that the sleeve be composed of a metallic material
 having a work function of at least about 4.3 ev. Several metals and alloys
 are suitable for the non-emissive sleeve 32 of the present invention. Such
 metals include silver, gold, platinum, rhodium, iridium, palladium,
 nickel, and alloys thereof. A summary of some of the properties of the
 above-noted materials are indicated in U.S. Pat. No. 5,023,425, which was
 previously incorporated by reference.
 In further accord with the alternative embodiment, the metallic holder 16
 forms an overlay portion 36. The overlay portion 36 is also discussed in
 detail in U.S. Pat. No. 5,767,478, the disclosure of which is incorporated
 by reference into this description. The overlay portion directly contacts
 the emissive insert 28 so that no portion of the sleeve 32 is exposed at
 the front face 20 of the holder 16. The overlay portion 36 of the copper
 holder 16 between the front face 20 thereof and the silver sleeve 32
 preferably, but not necessarily, has a thickness b' of 0.010 inches. The
 sleeve 32 preferably, but not necessarily, has an outer diameter c' of
 0.130 inches. The circular outer end face 29 of the emissive insert 28
 preferably, but not necessarily, has a diameter d' of 0.086 inches.
 Further, the axial length of the emissive insert 28 is preferably, but not
 necessarily, 0.203 inches, while the axial length of the silver sleeve is
 preferably, but not necessarily, 0.164 inches. Of course, these dimensions
 are given by way of example and are not intended to limit the present
 invention.
 The electrode according to the above described embodiment of the present
 invention provides a significantly improved service life. More
 specifically, the silver (and other suitable materials described in detail
 above) sleeve 32 provides good conductivity and a cooler flow of
 electricity to the emissive insert 28 (formed of, for example, hafnium).
 Adding to the cooling effect of the emissive insert 28 is the core 31 and
 the base 31a, according to the present invention, which both act to draw
 heat out of the emissive insert 28. Thus, the emissive insert 28 is able
 to have a longer service life since it can be maintained at a cooler
 temperature.
 The following table demonstrates the criticality of the present invention
 by comparing a number of different electrode configurations. All of the
 electrodes tested included an emissive insert formed of hafnium. The very
 first test, for example, is described as utilizing "silver surrounding
 hafnium" and refers to a silver sleeve surrounding a hafnium emissive
 insert, whereas "silver in hafnium" refers to the hafnium emissive insert
 having a silver stem and base according to the invention. The tests were
 conducted by making piercings of flagpoles having lengths of 45 feet. The
 thickness of the material used for the tests varied from 3/8" to 1".
 Piercings of the metal were conducted until the electrode failed or was
 considered worn out by the operator.
 TABLE
 Test Number Description of Electrode Pierces Plate Thickness
 1 Silver surrounding hafnium 502 3/8"
 1 Silver surrounding hafnium 487 3/8"
 1 Silver surrounding hafnium 515 3/8"
 1 Silver in hafnium 637 3/8"
 1 Silver in hafnium 596 3/8"
 1 Silver in hafnium 621 3/8"
 2 Silver surrounding hafnium 65 1"
 2 Silver surrounding hafnium 72 1"
 2 Silver surrounding hafnium 74 1"
 2 Silver in hafnium 151 1"
 2 Silver in hafnium 137 1"
 2 Silver in hafnium 143 1"
 Based on the above results of the piercing tests, it is apparent that the
 electrode configuration according to the present invention has a
 substantially longer operating life than the conventional electrode
 assembly.
 FIGS. 5-11B illustrate a preferred method of fabricating the electrode
 holder of the present invention. As shown in FIG. 5, a cylindrical blank
 94 of copper or copper alloy is provided, which has a front face 95 and an
 opposite rear face 96. A cavity is then formed in the front face 95, such
 as by drilling, which creates the above described cavity 24.
 The emissive insert 28 is formed having the bore 27 provided longitudinally
 therein. The non-emissive core 31 is formed, which may, for example, be
 composed essentially of silver, and which is configured and sized to
 substantially fit within the bore 27. The base 31a of the core 31 rests on
 the outer end face 29 of the emissive insert 28 forming the insert
 assembly 26.
 Next, the insert assembly 26 is pressed into the cavity 24, as shown in
 FIGS. 6A and 6B. The insert assembly is placed into the cavity 24 with the
 base 31a end entered first. After the insert assembly 26 is inserted into
 the cavity 24, with reference to FIGS. 7A and 7B, the front face 95 end of
 the blank 94 undergoes a swaging process wherein the insert assembly 26 is
 bonded to the cylindrical blank 94.
 As shown in FIGS. 8 and 9, the blank is turned to provide a first diameter
 body portion 98, and an externally threaded portion 100, which is formed
 on the rear face 96 end of the cylindrical blank 94. With reference to
 FIGS. 10 and 11A, the method further includes axially boring and grooving
 the blank 94 from the rear face 96 end to form a second cavity 102. The
 front face 95' of the assembly is then preferably finished by a hex
 milling procedure, the result of which is shown in FIG. 11B.
 The method also contemplates, as shown in FIG. 3, the insertion of the
 sleeve 32, which is positioned such that the overly portion 36 directly
 contacts the emissive insert 28 so that none of the sleeve 32 is exposed
 at the front face 95.
 The remaining plasma arc torch structure is conventional and is disclosed
 in the '425 patent mentioned above. More specifically, as shown in FIG. 1,
 the electrode 14 is mounted in a plasma arc torch body 38, which has gas
 and liquid passageways 40 and 42, respectively. The torch body 38 is
 surrounded by an outer insulated housing member 44.
 The tube 46 is suspended in the central bore 48 of the electrode 14 for
 circulating a liquid medium such as water through the electrode structure
 14. The tube 46 is of a diameter smaller than the diameter of the central
 bore 48 so as to provide a space 49 for the water to flow upon discharge
 from the tube 46. The water flows from an unshown source through the tube
 46 and back through the space 49 to the opening 52 in the torch body 38
 and to an unshown drain hose. The passageway 42 directs the injection
 water into the nozzle assembly 12 where it is converted into a swirling
 vortex for surrounding the plasma arc. The electrode 14 upon being
 connected to the torch body 38 holds in place the ceramic gas baffle 54
 and a high temperature plastic-insulating member 55. The member 55
 electrically insulates the nozzle assembly 12 from the electrode 14.
 With additional reference to FIG. 4, the nozzle assembly 12 comprises an
 upper nozzle member 63 and a lower nozzle member 64. The upper nozzle
 member 63 is preferably metal. A ceramic material, such as alumina or
 lava, is preferred for the lower nozzle member 64. The lower nozzle member
 64 is separated from the upper nozzle member 63 by a swirl passageway 66.
 A power supply P is connected to the torch electrode 14 in a series circuit
 relationship with a metal workpiece, which is typically grounded. In
 operation, the plasma arc is established between the emissive insert of
 the torch 10 which acts as the cathode terminal for the arc, and the
 workpiece which is connected to the anode of the power supply, and which
 is positioned below the lower nozzle member 64. The plasma arc is started
 in a conventional manner by momentarily establishing a pilot arc between
 the electrode 14 and the nozzle assembly 12.
 It is contemplated that numerous modifications may be made to the electrode
 for plasma arc torch and method of making the same of the present
 invention without departing from the spirit and scope of the invention as
 defined in the following claims.