US3017119A - Apparatus for metal spraying - Google Patents

Description

Jan. 16, 1962 G. J. GIBSON APPARATUS FOR METAL SPRAYING Original Filed Sept. 13, 1955 2 Sheets-Sheet 1 INVENTOR l GLENN J. GIBSON 7 BY 4 4M W, Mil/8w ATTORNEY a AGEN T Jan. 16, 1962 G. J. GIBSON 3,017,119

APPARATUS FOR METAL SPRAYING Original Filed Sept. 13, 1955 2 Sheets-Sheet 2 INVENTOR GLENN J. GIBSON BY M.

ATTORNEY a AGENT Patented Jan. 16, 1962 3,017,119 APPARATUS FOR METAL SPRAYING Glenn J. Gibson, Berkeley Heights, N.J., assignor to Air Reduction Company, Incorporated, New York, N.Y., a corporation of New York Original applications Sept. 13, 1955, Ser. No. 533,934, and Sept. 11, 1958, Ser. No. 761,278. Divided and this application June 20, 1961, Ser. No. 118,472 The terminal portion of the term of the patent subsequent to September 5, 1978 has been disclaimed 1 Claim. (Cl. 239-81) This invention relates to metal spraying or metallizing. Metal spraying, as the term implies, is the process by which molten metal is projected in a stream or spray against a surface to which the molten metal adheres the freezes to form a coating thereon. It is frequently called metallizing and includes the surfacing of both metallic and non-metallic objects.

This application is a division of my prior applications Serial No. 761,278, filed September 11, 1958 now Patent No. 2,998,922, and Serial No. 533,934, filed September 13, 1955, now abandoned.

Metal spraying is generally used where it is desired to have a product with a metallic surface having different properties than the base material. For example, it is frequently advantageous to apply a thin coating of stainless steel or the like to the interior surface of a vessel that is to be used to hold some chemically-active fluid. As another example, a wear-resistant metal may be applied to a bearing surface. Similarly, it is sometimes desirable to coat a plastic or other non-metallic surface with an adhering thin film of metal.

Metal spraying has been accomplished in the past by melting metal in powder or wire form in a gun, usually by heat derived from a gas flame, and subjecting the molten metal to a blast of gas, usually air, which atomizes the molten metal and propels it out of the gun as a spray at relatively high velocity. In such a process the metal is chilled by the propelling gas blast which affects its ability to make a fusion bond with the surface against which it is projected, and when an active gas is used as the blast gas, the metal is subject to chemical reaction with the gas. Even if a relatively inert gas is used as the blast gas, the turbulence of the required high velocity gas blast entrains air and oxidation of the metal is inevitable. This, of course, limits the usefulness of the process.

An object of the present invention is to provide a novel method and apparatus for producing and projecting molten metal droplets within a protective atmosphere from the end of a continuously-fed wire.

Another object is to provide a novel method and apparatus for depositing metal in a molten state on a surface, to adhere to said surface and form a coating thereon.

These and other objects and advantages of the invention will. be pointed out or will become apparent from the following detailed description and the accompanying drawings.

According to the present invention, a high current density electric arc is formed between the end of a consuming wire electrode fed toward the arc and a counter electrode positioned ofl center with respect to the axis of the consuming wire electrode. The are is maintained in an atmosphere comprising essentially monatomic inert gas. As the end of the consuming wire electrode melts the molten metal is projected substantially axially with considerable force and velocity from the end of the electrode and past the counter electrode in a stream of fine discrete droplets. This stream may be directed against a surface to which the metal droplets adhere and freeze to form the coating. Since the coated surface is not involved in the electrical circuit, it need not be an electrical conductor.

In order to more fully understand the novel method and apparatus reference should be made to the accompanying drawings.

FIGURE 1 illustrates generally a complete apparatus for the practice of the present invention.

FIGURE 2 illustrates in more detail the novel metal spray gun forming part of the apparatus of FIGURE 1.

FIGURE 3 illustrates a modified form of the invention.

FIGURE 4 is an enlarged view of a portion of the apparatus of FIGURE 3, showing in more detail the relation of the consuming wire electrode to the non-consuming counter electrode.

Referring to FIGURE 1 of the drawings, the metal spray gun is designated generally by the reference numeral 11. Wire W is fed to the gun 11 from a reel 12 by a feed motor 13 and feed rolls 14 driven thereby. The Wire reel and the wire feeding equipment are supported, along with other associated apparatus, in a wire feed carriage frame 16. The wire W, withdrawn from the reel by the feed rolls, is pushed through a flexible casing 17 to a spray gun. Arc current is supplied to the spray gun 11 from a power source such as a conventional welding power source 18. In the illustrated apparatus one lead 24 from the power source 18 connects with a power switch or contactor 26 which, in turn, is connected to the spray gun by conductor 27. The other lead 28 from the welding machine is connected directly to the spray gun 11. Shielding gas may be provided to the spray gun from a conventional compressed gas cylinder 19 through the necessary pressure reducing valve 21 and gas conduit 22. Cooling water is supplied to the apparatus from a pipe line source 30 through conduit 29 for circulation through the spray gun and subsequent discharge through conduit 31.

Details of the spray gun 11 are more clearly shown in FIGURE 2. The gun consists generally of a pistol grip handle 32, an outer cylindrical barrel 33, and an inner barrel 35. The inner barrel 35 terminates in a contact tube 34. The annular passage 36 between the inner and outer barrel serves as a gas passage for the flow therethrough of arc shielding gas. The contact tube 34 is in electrical circuit with conductor 27. The annular gas passage 36 communicates with gas conduit 22. Barrel 33 is formed of non-conducting material such as a moldable synthetic resin. An adapter 37 is screwed into the end of barrel 33 and provides the necessary means to attach a counter electrode and gas nozzle assembly 38. This unit consists of a cylindrical nozzle 39 of high conductivity copper which is flanged at one end to be attached to the adapter 37 by a connector nut 41. The counter electrode itself consists of a heavy copper ring 42 which is electrically and mechanically bonded to the inside of the nozzle 39 as by welding or an equivalent metal bonding process. A jacket 43 is soldered to the outside of nozzle 39 to form a cooling water passage through which cooling Water may be circulated in direct heat exchange relation with the nozzle 39. Cooling water is supplied to this jacket from conduit 29 through a copper tube 44 and it discharges from the jacket through a similar copper tube 45 into the discharge conduit 31. These copper tubes '44, 45 also serve to connect the counter electrode 42 through the cooling water jacket 43 and the nozzle '39 to the conductor 28 connected to one terminal of the power supply 18.

In operation the above described apparatus functions in the following manner. An arc is established between the end of the wire electrode W and the counter electrode 42 with current supplied from the power source 18. The are may conveniently be started by bridging the gap between the Wire electrode and the counter electrode with a carbon rod or the like, or it may be started by defleeting the wire to touch the counter electrode momentarily. Alternatively, the arc may be started by a high voltage, high frequency discharge if appropriate conventional apparatus (not shown) is employed. Wire is Withdrawn from reel 12 by the motor driven feed rolls 14 and pushed through the casing 17 and the spray gun 11 to emerge from the contact tube 34 at a rate to maintain the are as metal is melted from the end of the wire. Arc current is transferred from the contact tube 34 to the wire W as the wire passes through the contact tube in contact therewith. Shielding gas, preferably comprising essentially inert monatomic gas such as argon, is supplied to the annular gas passage 36 in the gun from cylinder 19 through conduit 22. This gas passes along the inner barrel and the contact tube to the projecting end of the electrode and passes out of the gun through nozzle 39. The power supply 18 is preferably a direct current power supply with its positive terminal in electrical contact with the electrode wire W via the contact tube 34 in the spray gun and its negative terminal in electrical contact with the counter electrode 42 in the spray gun nozzle. It has been found that when the arc current is maintained at a sufficiently high level, which in all instances results in burn-off of the electrode wire at a rate of at least 199 inches per minute, molten droplets are projected axially from the end of the wire W with substantial velocity. For example, using 20 cubic feet per hour of argon as the shielding gas through a /8 inch diameter nozzle in the apparatus shown, with a .035 inch diameter stainless steel wire W and a welding current of the order of 200 amperes, molten droplets are projected horizontally from the spray gun into the air six to eight feet fro-m the nozzle of the gun before falling to the floor when the gun is held at chest height by the operator. Obviously, since an objective of this invention is to provide a metal spraying method and apparatus, normal operation requires the gun to be held within a few inches of the surface to be sprayed. Under the recited conditions the projection velocity of the droplets is always sufficient to cause the droplets to traverse the distance to the work surface without any noticeable gravitational effect. The projected molted metal droplets are superheated and form a true fusion bond with a metal work surface, although little or no dilution of the deposited metal occurs. This spray gun may be used in all positions equally well. When the gun is held near the surface to be coated, the inert shielding gas emerging from the nozzle forms an envelope protecting the molten metal particles as they are projected across the space from the gun to the work and it also serves to prevent oxidation of the metal after it strikes the work and as it freezes thereon. The manipulation of the gun depends entirely on the job being performed, but in general it may be used in much the same manner as any other spray gun, i.e. by making a series. of traverses across the surface to be sprayed to thereby deposit a layer of substantially uniform thickness. While the specific examples described in this specification relate to the use of stainless steel electrode wire, it is to be understood that other metals and alloys can and have been used with equal success.

An important factor in this novel metal spraying method and apparatus is the combination of arc conditions that produce projected metal transfer from the end of the wire electrode and the proper orientation of the wire electrode and the counter electrode such that a stable arc is maintained while permitting unimpeded passage of the molten metal droplets in an axial direction from the end of the wire electrode.

The construction and mode of operation described above accomplishes the result in an entirely satisfactory manner. However, many modifications will become apparent to one skilled in the art. For example, under normal circumstances the arc current distributes itself substantially uniformly over the entire circumference of the water-cooled copper counter electrode. It has been found however, that due to lack of symmetry of construction or for some other reason the arc may tend to concentrate on one spot on the ring. This is undesirable because of the excess local heating that results. This may be overcome by providing a rotating magnetic field to cause the arc to rotate rapidly around the ring. It is also contemplated to reduce the heat produced at the counter electrode by adding an emissive agent to the are. In the preferred form of the invention the counter electrode is the cathode and any modification to improve its electron emission properties is desirable.

The composition of the shielding gas is important in that it determines to a large extent the nature of the metal transfer from the end of the wire as well as providing a medium to exclude air from the region of the arc. Inert monatomic gas such as argon has been found particularly well suited for use in the invention, although other inert gases as well as mixtures of inert gas and small amounts of active gas is contemplated.

Other variations in the invention include the use of an unbalanced counter electrode. Instead of having the are formed to a ring through the center of which the molten metal projects, the counter electrode may be a rod offset to one side of the wire axis. Apparatus of this type is shown in FIGURES 3 and 4.

In the particular application of the invention illustrated in FIGURES 3 and 4 a round workpiece 51 is being surfaced by rotating it under a metal spraying head made and operated according to the present invention. The metal spraying head may be fixed with respect to the rotation of the workpiece and may be provided with a longitudinal feed to move it axially along the work as the deposit is formed during successive revolutions of the work. This head comprises essentially a wire feeder and barrel for the consuming electrode and a holder for a non-consuming counter electrode. The wire feeder for the consuming electrode W may be of conventional design for inert gas shielded consuming electrode metal arc welding and consist essentially of a wire reel 52 containing a supply of electrode wire W, a wire feeding head 53, including wire feed rolls 54 driven by an electric motor 55, and a barrel 56 through which the wire is fed and which contains a contact shoe from which the electrode W collects welding current. The barrel 56 also provides an annular passage for the fiow of shielding gas to a nozzle 57 which delivers the gas as a flowing steam surrounding the arc end of the electrode wire Shielding gas may be provided to the barrel through an appropriate connection as shown in the drawing. Arc current is provided from a power source 60. In this modification of the invention the counter electrode consists of a tungsten electrode 61 or alternatively some other non-consuming electrode such as carbon, zirconium, or the like. This electrode is supported by an electrode holder 62 which may be of conventional design for inert gas shielded tungsten electrode arc welding. It consists essentially of means to conduct are current to the counter electrode and means to deliver shielding gas around the electrode to exclude air therefrom. The are current terminal of the counter electrode holder and the arc current terminal on the consuming electrode barrel are connected to the respective terminals of the power source 60 which in the preferred form of the invention is a direct current source and which preferably has its negative terminal connected to the non-consuming counter electrode and its positive terminal connected to the consuming wire electrode. In general, the orientation of the two electrodes is such that the end of the tungsten is offset from the axis of the consuming wire W. As used in the present specification and claims, the term axis as applied to the wire, means the longitudinal centerline of the wire and its indefinite projection beyond the end of the wire.

It has been found from experiment that a number of factors influence the type and direction of the metal spray produced by this apparatus. These factors include the angle between the two electrodes, the distance from the end of the counter electrode to the axis of the consuming electrode, the strength of the arc current, particularly as it relates to the diameter of the wire electrode, and the wire feed speed. FIGURE 4 shows the relationship of the two electrodes for optimum results for most metal spraying applications. The angle a between the axes of the electrodes should be approximately 90, the end of the tungsten electrode should be offset from the axis of the consuming wire electrode by about 4; inch, and the current density and wire feed speed should be adjusted so the consuming wire electrode assumes a form at the end which projects slightly beyond the end of the non-consuming electrode. This condition is attained, for example, using 385 amperes with a inch diameter stainless steel wire, with the wire the anode, an arc voltage of 37 volts, and a wire feed speed of 400 inches per minute. Under these conditions, a fine spray of discrete droplets is projected substantially axially from the end of the wire electrode W at a relatively high velocity, and well within the shielding envelope formed by the converging shielding gas streams. By placing the electrodes so that the angle a between them is substantially less than 90 or substantially greater than 100 the projection velocity is decreased and the molten metal stream becomes more dispersed. The angle a is preferably always more than 60 and less than 120.

Obviously, there are many alternatives to the modification of the invention described immediately above. For instance, it is advantageous to use as a counter electrode a material that will operate as a cathode with a minimum temperature rise, such as thoriated or zirconiated tungsten. Also, instead of having separate consuming electrode and counter electrode holders, a single head can be provided which will serve both functions, and instead of having separate gas shielding apparatus for each electrode both may be shielded by a single nozzle of proper design.

While only certain specific embodiments of the invention have been shown and described in connection with methods and apparatus for metal spraying, it is to be understood that the invention is not limited to the particular forms shown and described, but may be used in other ways Without departure from its spirit as defined by the following claim.

I claim:

Apparatus for forming a free stream of high temperature fluid products from a consumable electrode gas shielded arc comprising, a tubular gas passage terminating in a nozzle through which a stream of shielding gas is projected into the space beyond said nozzle, an annular electrode within said nozzle having an inner cylindrical surface of smaller diameter than said gas passage forming an exit orifice for said shielding gas stream, a coolant jacket surrounding said nozzle for containing a cooling medium which cools said annular electrode to prevent melting thereof, a tubular electrode supporting means within said gas passage for positioning a consumable electrode coaxially with respect to the inner cylindrical surface of said annular electrode in spaced relation thereto to form an annular arc gap therebetween through which the shielding gas flows as it passes through said exit orifice, and means for feeding said consumable electrode toward the are at a rate to maintain the are as the said electrode is consumed.

References Cited in the file of this patent UNITED STATES PATENTS 1,133,508 Schoop Mar. 30', 1915 2,196,647 Snyder et a1. Apr. 9, 1940 2,217,039 Beck Oct. 8, 1940 2,274,631 Meredith Feb. 24, 1942 2,592,414 Gibson Apr. 8, 1952

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