US 3010009 A
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Description (OCR text may contain errors)
Nov. 21, 1961 A. c. DUCATI 3,010,009
METHOD AND APPARATUS FOR UNITING MATERIALS IN A CONTROLLED MEDIUM Filed Sept. 29, 1958 3 Sheets-Sheet 1 INVENTOR. AOG/A/VOC. M6477 Nov. 21, 1961 A. c. DUCATI 3,010,009
METHOD AND APPARATUS F R UNITING MATERIALS IN A CONTR LED MEDIUM d Sept. 29, 1958 3 Sheets-Sheet 2 FIG. 2-
INVENTOR. flDlQ/QIVO C. Dl/CflT/ Nov. 21, 1961 A. c. DUCATI 3,010,009
METHOD AND APPARATUS FOR UNITING MATERIALS IN A CONTROLLED MEDIUM Filed Sept. 29, 1958 3 Sheets-Sheet 3 FIG. 5.
INVEN TOR. HOP/4N0 C. M67477 United States Patent 3,010,009 METHOD AND APPARATUS FOR UNITING MATERIALS IN A CONTROLLED MEDIUM Adriano C. Ducati, Corona del Mar, Califl, assignor to Plasmadyne Corporation, Santa Ana, Califi, a corporation of California Filed Sept. 29, 1958, Ser. No. 763,574 7 Claims. (Cl. 219-76) This invention relates to a method and apparatus for uniting materials in a controlled medium, and more particularly to a method and apparatus for spraying one material onto another material in an inert atmosphere. The invention further relates to a method and apparatus for recirculating oxidation-preventing gases to an electrical plasma-flame torch, for plasma spraying at pressures much lower than atmospheric, and for plasma spraying in combination with electrostatic charging of the object to be sprayed.
In co-pending applications Serial No. 747,094, filed July 7, 195 8, for Plasma Stream Apparatus and Methods, now Patent No. 2,922,869, inventors Gabriel M. Giannini and Adriano C. Ducati; Serial No. 742,194, filed June 16, 195 8, for High Temperature Spray Apparatus and Methods, inventors Hubert C. Sullivan and Reno W. Prichard; and Serial No. 761,181, filed September 15, 1958, for Plasma-Jet Torch Apparatus and Method Relating to Increasing the Life of the Back Electrode, now Patent No. 2,941,063, inventors Adriano C. Ducati and Vernon H. Blackman, all assigned to the assignee of the present invention, there are shown and described methods and apparatus for effecting spraying or coating by the use of relatively high-temperature electrical plasma. The present application also relates to the use of electrical plasma for efiecting spraying and uniting of materials, and in a controlled medium or environment. The present application particularly relates to the spraying of oxidizable sub stances in an inert or oxidation-preventing gas. It also relates to the spraying of both oxidizable and non-oxidizable substances in a low-pressure gaseous medium or environment in order to provide numerous benefits including the achievement of a very long and spreading plasma flame the high temperature of which is maintained for a considerable distance from the torch.
Other important aspects of the present invention include achieving great economy of operation through recirculation of the gas, and achieving optimum control and spreading of the plasma flame by effecting electrostatic charging of the work. As in the case of the apparatus and methods described in the above-cited copending patent applications, it is an important advantage of the present apparatus and method that spraying may be carried out at very high controlled temperatures permitting the spraying of highly refractory materials.
It is, therefore, the object of the present invention to provide a highly efficient, efiective and economical method and apparatus for spraying and otherwise uniting mate rials in a controlled medium or environment by the use of electrical plasma.
Other objects and advantages of the present invention will be more fully set forth in the following specification and claims, considered in connection with the attached drawings to which they relate.
In the drawings:
FIGURE 1 is a schematic view, primarily in central section, illustrating apparatus constructed in accordance with the first embodiment of the invention;
FIGURE 2 is a fragmentary central sectional view of the same nature as the upper and left region of FIGURE 1, but schematically illustrating a second embodiment of the apparatus;
FIGURE 3 is a transverse section taken on line 33 ice of FIGURE 2, and illustrating the tangential inlet for the gas which surrounds the electric arc and plasma passing between the nozzle and back electrodes;
FIGURE 4 is a schematic view, both in side elevation and in vertical section, showing a third embodiment in which the apparatus is portable in order that it may be employed in spraying very large objects such as airplane wings; and
FIGURE 5 is a fragmentary plan view taken from station 55 of FIGURE 4.
Proceeding first to a description of the embodiment of FIGURE 1, the apparatus is illustrated to comprise wall means 10 to define a sealed chamber 11, torch means 12 to introduce an electrical plasma flame or plasma jet 13 into the chamber 11, and means 14 to provide a controlled atmosphere or environment in chamber 11 and to recirculate gas from the chamber to the torch means 12.
The wall means 10 for defining the sealed chamber 11 may be of any suitable type having sufficient capacity to hold the workpiece, for example the metal plate indicated at 16. Means are provided to support the workpiece 16 in chamber 11, in either fixed or movable relationship, and may comprise an insulating bracket 17 to which the workpiece 16' is removably secured in any suitable maner. Suitable means, not shown, are provided to permit access through the Wall means 10 into the chamber in order to move or replace the workpiece.
The torch means 12 is of the general type described in the above-cited patent applications, and comprises a nozzle electrode 18 and a back, base or plate electrode 19 which are mounted in coaxial relationship and separated by an insulating gasket or coupling 21. The indicated nozzle electrode 18 is generally cup-shaped and has a round nozzle opening 22 provided coaxially of its radial wall. The wall of the nozzle opening is protected by a tubular insert 23 formed ofa refractory metal such as tungsten.
The back, base or plate electrode 19 is shown as having a protuberant, generally frustoconical central portion 24 located coaxially of nozzle opening 22 and having inset at its center, adjacent the nozzle opening, a block 26 of refractory metal such as tungsten. Formed coaxially between the frustoconical back electrode portion 24 and the radial wall of nozzle electrode 18 is an annular sealed pressure chamber 27 into which gas is introduced through a tangential inlet passage 28, the latter corresponding to the passage 28a shown in FIGURE 3 with reference to the second embodiment. Gas introduced through tangential passage 28 flows vortically in the pressure chamber 27 and then passes through the nozzle opening 22 into the communicating chamber 11. A vortex is thus formed axially of the annular chamber 27, and through which the arc passes in traveling between the refractory substances 26 and 23. It is to be understood that a portion of the gas is converted by the are into the plasma flame or jet 13 which comprises neutral gas, ions and electrons having high temperature and velocity.
Additional elements of the plasma torch means 12 are illustrated to comprise a support member 29 which is formed of a good electrical conductor such as copper (as are nozzle electrode 18 and back electrode 19), the support member being threadedly associated with the gasket or coupling 21 and in surface engagement with the outer portion of back electrode 19. An external cap member 31, also formed of a good electrical conductor such as copper, is nested over the nozzle electrode 18 and is also threaded to the gasket or coupling 21, so that all of the elements are securely held in coaxial relationship. The cap member 31 includes an annular sealed chamber 32 provided externally of nozzle electrode 18 around nozzle opening 22, and into which gas and coating substance (in particulate of fluid form) are fed by means of an inlet passage 33. The gas and coating substance are thus introduced into the flame or jet 13, and flow therewith through a tubular portion 34 of cap member 31 into chamber 1.1.
Means are provided to supply current to nozzle electrode 18 and back electrode 19, and are schematically represented to comprise a current source 37 connected through cables 38 and 39 to support member 29 and cap 31, respectively. The source 37 is adapted to deliver very high currents, on the order of hundreds of amperes, although the voltage need not be high. Preferably source 37 is a direct current source and is so connected that cable 38 is positive and cable 39 is negative. The back electrode 19 and nozzle 18 are then correspondingly charged positively and negatively, respectively, since they are in electrical contact with the support 29 and cap 31. When the electrodes are thus charged, the electric arc may be initiated and passes between refractory block 26 and through nozzle opening 22 to the end portion of refractory tube 24 which is remote from the block 26, as described in the above-cited patent applications. The are is constricted to the vortex in the whirling gas in pressure chamber 27 to provide the high current-density and consequent high temperature necessary for the spraying of refractory substances.
Means are provided to effect continuous cooling of the electrodes, and may comprise an inlet Water conduit 41 adapted to conduct water into a cooling chamber 42 provided in support 29 and in a communicating chamber portion of the back electrode 19. From chamber 4-2, the water flows through openings 43 into an annular groove 44 in support 29, thence through a passage 45 therein to an insulating hose 46. Hose 46 communicates with a semi-annular chamber 47 formed between cap 31 and the outer cylindrical wall of nozzle 18, there being a second semi-annular chamber 48 corresponding to chamber 47 but separated therefrom by suitable means, not shown. From chamber 47, the water flows into a chamber 49 provided in nozzle electrode 18, and thence into the chamber 48 from which it drains through a suitable outlet 51. Suitable sealing means, such as O-rings, are provided to prevent leakage from the above-specified chambers and passages.
Means are provided to effect electrostatic charging of the workpiece 16, thereby causing attraction of charged particles in the plasma flame 13 to improve the spraying or coating action. Such means are illustrated schematically to comprise a suitable source 52 of high voltage, a lead 53 connecting the source 52 to the workpiece l6, and insulator means 54 to prevent shorting of the voltage to the wall means 10. It is pointed out that the bracket 17 is formed of insulating material and therefore does not result in shorting of the voltage from the workpiece.
The polarity, voltage. etc., of work 16 depend upon such factors as the torch polarity, the flame length, the pressure in chamber 11, etc. The voltage of the workpiece 16 should be sufliciently high (and of proper polarity) to attract flame 13 without, however, causing substantial arcing to the workpiece.
Proceeding next to a description of the means 14 for providing a controlled atmosphere or environment in sealed chamber 11, and for recirculating gas from the chamber '11 to torch 12, this is illustrated schematically to include filter means 56 to which gas is conducted from chamber 11 via a conduit 57. The filter means 56 may be of any suitable type adapted to filter out particles of spray material which do not adhere to the workpiece 16 during the spray process. For example, the filter means may comprise an electrostatic precipitator, or a liquidtype filter.
Filter means 56 communicates through a conduit 58 with a pump means 59 which may comprise one or more pumping units of any suitable type capable of recirculating gas through the torch means 12, and of lowering the pressure in chamber 11 as will be indicated hereinafter. A first conduit 61 is connected from pump means 59 through a valve 62 to the tangential inlet passage 28 to pressure chamber 27, and a second conduit 63 is connected through a valve 64 to the inlet passage 33 leading to the chamber 32 surrounding the plasma flame 13. Valves 62 and 64- are of any suitable type adapted to permit or block flow of gas through the conduits 61 and 63, and also to permit venting of gas from pump means 59 to the atmosphere while at the same time blocking inflow to torch 12.
A source of gas pressure, indicated at 66, is connected through a valve 67 to the downstream side of pump means 59, so that gas may be introduced into the conduits :61 and 63 when the valve 67 is open. Means 68 are provided to introduce a spray material or substance, such as powdered or fluid material, into the conduit 63 in order that such substance will be carried by the gas through passage 33 into chamber 32 for introduction into the plasma flame 13. The introduction of spray material is controlled by a valve 69 having one position at which it seals oif flow of powder or fluid.
Summary of the method In accordance with one of its aspects, the method com prises bringing two substances into close proximity in a controlled atmosphere, and eifecting electrical plasmaheating of at least one of the substances to a temperature sufliciently high to effect adherence or bonding thereof to the other. The present method is to be distinguished from methods in which a small region of a workpiece is surrounded by a protective gas such as helium, argon, etc., emanating from a torch, the instant method being instead directed to situations in which a very substantial portion of a workpiece, normally the total workpiece, is completely surrounded by the desired medium such as inert or oxidation-preventing gas. This invention is also to be distinguished from methods in which compounds, mixtures or impure substances are vaporized and selectively condensed in order toeffect refining or purifying thereof, such methods being disclosed and claimed in patent application Serial No. 733,726, filed May 7, 1958, for Method and Apparatus for Separating Metals from Compounds Thereof, inventors Gabriel M. Giannini, Adriano C. Ducati and P01 Duwez, assigned to the assignee of the present invention. The last-mentioned method relates to selective or separate solidification or condensation, whereas the present method relates to conjoint solidification or condensation normally without chemical change.
Stated more definitely, the present method comprises generating a plasma flame in an inert or non-oxidizing atmosphere, bringing the plasma flame into proximity with the workpiece, and introducing particulate or fluid substance into the plasma flame for the purpose of effecting spraying or coating of the substance onto the Workpiece. The plasma flame is caused to have a temperature sufliciently high to effect melting of the coating substance but normally insufliciently high to effect dissociation thereof, and the coating substance is deposited conjointly or intact on the workpiece. The high temperatures necessary to spray refractory materials are achieved by gasconstricting the electric are which forms the plasma flame. The method includes recirculating the surrounding medium from a chamber in which the workpiece is enclosed back to the torch which generates the plasma flame, and also includes electrostatically charging the workpiece in order to increase the effectiveness of the coating or spraying operation.
In accordance with another of its aspects, the present invention includes carrying out plasma-spraying operations at a greatly reduced pressure in order to result in generation of a very long plasma flame or jet the high temperature of which is substantially maintained through a considerable distance from the torch. The absolute pressure of the gas surrounding the workpiece is caused to be relatively low, and should be below ten centimeters of mercury, so that the plasma flame is a number of times longer than in situations where the torch discharges into a medium at atmospheric pressure. The relatively long flame facilitates application of the coating substance onto the workpiece, increases the heating time during which the coating substance is in the plasma flame, and results in sustained heating since the plasma flame does not rapidly decrease in temperature due to atmospheric friction. The reduced pressure also facilitates ionization in the torch, thereby increasing the ease and practicality of passing coating powders and other substances through the electric arc and plasma flame within the torch itself. Because the reduced pressure increases or improves ionization, relatively cheap gases may be employed which are less readily ionized. Thus, it becomes more practical to employ nitrogen, etc., instead of more expensive gases such as argon. Also, and very importantly, the increased ionization resulting from lowered pressure improves the stability of the arc and plasma flame.
With reference to the apparatus illustrated in FIGURE 1, the method of the invention comprises mounting the workpiece 16 on the insulating support 17 and then sealingly blocking all access openings to the chamber 11. The valves 62 and 64 are then turned to positions at which they vent to the atmosphere while at the same time blocking flow into passages 28 and 33, valve 67 from gas supply 66 is closed, and valve 69 from powder feed 68 is closed. The pump means 59 is then started, and operates to draw air from chamber 11 through conduits 57 and 58, and discharge the air to the atmosphere through valves '62 and 64. When a sufficiently high degree of evacuation has been achieved, so that there is only an insignificant amount of oxygen remaining in chamber 11, the valves 62 and 64 are turned to positions at which the pump means 59 communicates with passages 28 and 33. Valve 67 is then opened to introduce gas from supply 66 into the pump means 59 and thus into the conduits 61 and 63, causing the chamber -11 to be filled with an inert or oxidation-preventing gas such as argon, helium, nitrogen, etc.
The absolute gas pressure in chamber 11 should be maintained low, preferably between one millimeter and ten centimeters of mercury. When the desired pressure is achieved in chamber 11, the gas-supply valve 67 is closed. The relative flow rate between conduits 61 and 63 is so adjusted, by means of a pump means 59 and valves 62 and 64, that the desired pressure will be achieved in chamber 27 as the result of gas entering through the tangential inlet passage 28. The pressure of the gas entering through passage 28 is an important factor in determining the constriction of the electric arc struck between refractory inserts 23 and 26 since, as specified in the above-cited co-pending patent applications, increased constriction of the arc (with consequent higher temperature) results from increasing the absolute pressure of the entering gas above 1.6 times the absolute pressure in chamber 11. The desired inlet pressure to chamber 27, and thus the temperature of the arc and plasma, depend upon the refractory characteristic of the substance to be sprayed, it being necessary to effect melting or vaporization of the substance but not normally dissociation thereof when it is a compound.
The current source 37 is then turned on, and a highcurrent electric arc is initiated and maintained through the vortex in chamber 27 and between refractory elements 23 and 26. This results, as above-described, in generation of the plasma flame 13 which enters chamber 11 through tube portion 34. The flame is relatively long, as indicated above, because of the low pressure of the gas through which it travels. The flame is directed against the workpiece '16, which operation is aided by the high voltage source 52 as previously stated.
The powder or fluid feed means 68 is then (or previously) turned on, by means of valve 69, to effect introduction of the desired coating substance '(or substances where two or more materials are sprayed together) into the conduit 63 and thus, through passage 33, into chamber 32. The coating substance thus enters the plasma flame 13 and is carried thereby to the workpiece 16, during which time it is melted or vaporized so that upon engaging the workpiece it will solidify and adhere thereto. All of the material introduced into the flame 13, insofar as possible, is caused to solidify or condense together (conjointly) on workpiece 16, whether the spray material be an element, mixture, or compound.
A certain proportion of the coating powder or other substance does not adhere to the workpiece 16, and may be drawn from the chamber 11 through the conduit 57. Such substance is effectively separated from the recirculating gas by means of filter 56, so that'there is no damage to the pump means 59 and other components.
Because of the recirculation of the gas, and other factors, the present operation is extremely economical, it only being necessary to employ the gas supply 66 to compensate for any leaks and to re-charge chamber 11 with inert or non-oxidizing medium when necessary. As previously indicated, means may be provided to remove the workpiece 16 from the chamber 11. Means may also be provided to substitute different workpieces without admitting air into the chamber 11.
The present method is particularly suited to the application of non-oxides to oxidizable materials such as steel, one important non-oxide being titanium. The method is, however, adapted to be employed in applying a great many elements, compounds and mixtures to a base surface, for effecting alloying of materials in the vapor phase, and for effecting other processes. Other examples of the numerous materials which may be sprayed in accordance with the present method include molybdenum, borides, nitrides, silicides, carbides, phosphides, beryllium, chromium, tantalum, tungsten, and many others.
Embodiment of FIGURES 2 and 3 The torch 12a of FIGURES 2 and 3 comprises a generally cup-shaped nozzle electrode 71 and a generally disc-shaped back electrode 72 mounted in coaxial relationship, the back electrode being inserted in the body portion of the nozzle electrode but electrically separated therefrom by insulating means 73 A retaining ring 74 is threaded into the nozzle electrode to maintain a support or base member 76 in electrically-conductive surface contact with back electrode 72.
The nozzle electrode has a coaxial nozzle opening 77 the cylindrical wall of which is provided with a tubular refractory insert 78 to which the electric arc is struck from a refractory insert ring 79, the latter being coaxially inset into the central frustoconical portion of back electrode 72. The insert ring 79 encompasses a passage 81 which is formed through a stem portion 82 of the back electrode, and also through the support or base member 76, for communication with the conduit 63a corresponding to conduit 63 (FIGURE 1) and leading to pump means such as pump means 59. Correspondingly, the tangential inlet passage 28a into the gas pressure chamber 27a is connected to a conduit 61a corresponding to conduit 61 in FIGURE 1.
Means, not shown, are provided to supply current to the nozzle and the back electrodes, and cooling means are provided in the form of hoses 83-85 and chambers 86 and 87. A mounting ring 88 is provided on the nozzle 71 to secure the torch to the wall means 10a, so that the 7 plasma stream 13a may enter chamber 11a through a tubular portion 89 of ring 88.
The operation of the embodiment of FIGURE 2. is the same as that of the embodiment of FIGURE 1, except that the gas and powder are introduced axially into the base of the electric arc and plasma through the passage 81. The coating substance therefore passes through the torch itself. This, as previously pointed out, is rendered more practical because the lowered pressure in chamber 11a results in increased ionization in the torch and in increased ease of passing coating substances through the torch.
Embodiment of FIGURES 4 and 5 FIGURES 4 and 5 show a portable apparatus particularly adapted to be employed in coating large objects, for example the leading edge of the airplane wing indicated at 91.
The apparatus is illustrated to comprise wall means 92 having an open end which is so shaped that the edge portions of the wall means will fit closely against the object (such as wing 91) to be coated. Suitable sealing means, indicated at 93, are employed to provide a fluid seal between the edge of the wall means and the work 9 1.
A torch 12a is sealingly connected to wall means 92, and may be identical to the one shown in FIGURES 2 and 3. In the illustrated form, the wall means 92 is generally tapered or conical, and the torch means 120: is connected thereto at the small end relatively remote from wing 91.
Means are provided for providing a controlled atmosphere in the chamber 94 which is defined cooperatively by the wall means 92 and by the leading edge of wing 91. Such means may correspond generally to the means 14 described with reference to FIGURE 1, and is shown to include a conduit 96 connecting chamber 94 to a suitable filter means 97, and conduits 61a and 63a connecting the outlet portion of the filter means 97 to the tangential passage 28a and the axial passage 81 as indicated with reference to FIGURE 2. A suitable pump 98 and valve 99 are provided in conduit 61a, and a suitable pump 101 and valve 102 are provided in conduit 63a. Means 103 are provided to feed powder into conduit 63a, under control of a valve 104, and means 106 are provided to supply inert or oxidation-preventing gas to the downstream side of filter 97, under control of a valve 107.
In the operation of the embodiment of FIGURES 4 and 5, it is to be understood that all of the elements 97, 98, 101, etc., may be mounted on a movable platform supported on the ground, so that is is only necessary to manually or otherwise hold the wall means 92 and torch 12a in the desired position relative to the workpiece.
After the apparatus is in position, valves 99 and 102 are turned to positions at which the pumps 98 and 101 vent to the atmosphere, and inflow of air through con duits 61a and 63a is blocked. The valve 107 for gas supply 106, and valve 104 for powder feed 103, are closed. The pumps 98 and 101 are then operated to effect substantial evacuation of chamber 94, after which valves 99 and 102 are turned to positions permitting flow of gas from pumps 98 and 101 to the torch. The valve 107 for gas supply 106 is then turned on until the proper atmosphere is introduced into chamber 94, after which the torch operation is initiated to result in the generation of plasma flame 13a. At approximately the same time the plasma flame is initiated, valve 104 is opened to initiate the feed of powder into conduit 63a and thus, via passage 81 (FIGURE 2) into the arc and plasma stream 13afor transmission to the leading edge of the wing 91. Excess powdered material is drawn from chamber 94 through conduit 96 and is filtered out by means of filter 97.
After the indicated portion of wing 91 has been sufi'iciently coated, the system is shut off and the wall means 8 92 is moved to another section of wing at which the process is repeated.
Throughout this specification and claims, the word dissociation is intended to mean the separation of a chemical combination into simpler constitutents. It is not intended to denote the partial breakdown of a single element (such as argon) into electrons, etc., when the element is heated to form plasma.
The invention contemplates varying the lengths and diameters of elements 34 (FIGURE 1) and 89 (FIG- URE 2) in order to regulate the length and size of the plasma jet. Elements 34 and 89 may be water cooled.
Various embodiments of the present invention, in addition to what has been illustrated and described in detail, may be employed without departing from the scope of the accompanying claims.
1. A method of spraying a surface material of one composition onto a base element of another composition, which comprises providing wall means to enclose at least a major portion of said base element in a sealed chamber, reducing the pressure in said chamber to a small fraction of atmospheric, providing an electrical plasma-jet torch having a back electrode and having a nozzle which communicates with said sealed chamber, maintaining a high-current electric arc to said back electrode in the vicinity of said nozzle, eflecting fiow of gas through and adjacent said are and through said nozzle into said sealed chamber to thereby effect heating of said gas to form high-temperature plasma, introducing a particulate or fluid surface material into said plasma, effecting impingement of said plasma and surface material against said base element for deposition of said surface material thereon, and causing all components of said surface material to solidify on said base element to thereby coat the same.
2. The invention as claimed in claim 1, in which said method includes maintaining the absolute pressure in said sealed chamber below ten centimeters of mercury.
33. A method of spraying a coating onto a workpiece, which comprises providing an electrical plasma-jet torch having a nozzle electrode and a back electrode, maintaining a high-current electric are between said electrodes at the nozzle opening in said nozzle electrode, eflecting flow of an oxidation-preventing gas in the space between said electrodes and through said nozzle opening whereby said gas is heated by said are to form plasma, directing said plasma into a substantially sealed chamber and against a workpiece disposed therein, maintaining said chamber in substantially evacuated condition to thereby greatly increase the length of the jet of plasma emanating from said nozzle opening, and introducing powdered coating material into said plasma at said nozzle opening and at a point spaced from said back electrode, said coating material being entrained in said plasma for heating thereby and impingement against the workpiece.
4. The invention as claimed in claim 3, in which said method includes the step of maintaining said chamber evacuated to an absolute pressure less than ten centimeters of mercury.
5. The invention as claimed in claim 3, in which said method includes eflecting vortical flow of said gas in the space between said electrodes, and constricting said are to a smaller cross-sectional area than it would normally occupy in space.
6. The invention as claimed in claim 3, in which said method includes impressing an electrostatic charge on said workpiece sufficiently high to effect attraction of said plasma but insufficiently high to eflect arcing between said torch and said workpiece.
7. The invention as claimed in claim 3, in which said method includes eflecting continuous recirculation of gas from said sealed chamber to said space between said trodes.
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