US2998922A - Metal spraying - Google Patents

Description

G. J. GIBSON METAL SPRAYING Sept. 5, 1961 2 Sheets-Sheet 1 Filed Sept. ll, 1958 Kim INVENTOR GLENN J. GIBSON Y HM ATTORNEY a AGENT G. J. GIBSON METAL SPRAYING Sept. 5, 1961 2 Sheets-Sheet 2 Filed Sept. 11, 1958 F I G.

INVENTOR GLENN J. GIBSON BY H. HM maflm M 5w ATTORNEY a AGENT United States Patent 2,998,922 METAL SPRAYING Glenn J. Gibson, Berkeley Heights, N.J., assignor to Air Reduction Company, Incorporated, New York, N.Y., a corporation of New York Filed Sept. 11, 1958, Ser. No. 761,278 3 Claims. (Cl. 239-13) This invention relates to metal spraying or metallizing and this application forms a continuation-in-part of my co-pending application Serial No. 533,934, filed September 13, 1955, for Method and Apparatus for Metal Spraying, abandoned. Metal spraying, as the term implies, is the process by which molten metal is projected in a stream or spray which may be directed against a surface to which the molten metal adheres and freezes to form a coating thereon. It is frequently called metallizing and includes the surfacing of both metallic and non-metallic objects.

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 effects 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 method of projecting a superheated arc eflluent into free space.

Another object of the present invention is to provide a novel method of metal spraying in which an electric are, employed for melting a supply wire, is operated under conditions which produce by are action alone a free stream of molten metal particles which are projected axially from the end of the Wire and from the path of current flow through the arc at velocities sufiicient to pass a substantial distance from the end of the wire without noticeable gravitational effects.

A further object of this invention is to provide a novel method of depositing molten metal in a molten state on a work surface for adherence thereto in the formation of 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.

In the metal spraying method of the present invention, the operating characteristics of a high current density self regulating arc in an inert shielding gas are employed as the sole means of forming and projecting a stream of molten metal droplets from the end of a consuming wire electrode instead of using a blast of gas for breaking up large globules of molten metal formed on the end of a consuming wire electrode and propelling the resultant spray from the end thereof. Such high current density self regulating arcs have heretofore been employed in inert gas shielded arc welding as disclosed and claimed in my-U.S. Letters Patent 2,504,868, Albert Muller, Glenn I. Gibson and Nelson E. Anderson, patented April 18, 1950, for Electric Arc Welding. In the method of welding disclosed in this patent the strength of the welding current, or current density, is such that a spray of fine discrete metal droplets is projected axially from the end of a consumable electrode with a force sufficient to overcome the influence of gravity.

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 off 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 by arc action alone 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 metal spray gun forming part of the apparatus of FIGURE 1.

FIGURE 3 illustrates apparatus that may also be used in practicing the method of this 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 .theswelding machine isconn ected 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 they 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 tothe 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 tothe outside of nozzle 39.0f "form a cooling water passage through which cooling water maybe circulated in direct heat exchange relation with the nozzle 39. Cooling water is supplied to this jacket from conduit 29 througha copper tube 44 and it discharges from the jacket through a similar copper tube-45 into the discharge conduit'3l. 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 deflecting the wire to touchthe 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 andpushed through thecasing 17 and the spray gun 11 to emerge from the contact tube 34 at a rate to maintain the are as metal is meltedfrom :the end of the wire. Arc current is transferred from the contact tube 34 to the wire Was 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 gaspassage36inthe 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 andapasses out of the gun through nozzle 39. At the high current densities .employed, the high temperature products formed by .the inert gas shielded are established between the electrode rod or Wire 'and the'boundary surfaces of the elongated nozzle of gun 11 are projected from the nozzle into free space as a superheated efiiuent which extends a substantial distance beyond the end of the nozzle. 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 42in the spray gun nozzle. It has been found that when the arc current is maintained at a sufficiently high level, Whichin all instances results-in burn-off of the electrode wire at=a rate of :at least 100 inches .per minute molten droplets are projected axially from the endof thewire W with substantial velocity. For example, using 20 cubic feet per hour of argon as the shielding gas through a 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 from the nozzle of the gun before falling to the floor when the gun is held at chest height by the operator. The force propelling these molten metal droplets is due to are action alone since the flow of shielding gas is only about 2.6 feet per second which is obviously incapable of acting as a blast gas such as heretofore been employed. 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 sufiicient to cause the droplets to traverse the distance to the work surface without any noticeable gravitational effect. The projected molten 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 by are action across the space from the gun to the workpiece and it also serves to prevent oxidation of the metal after it strikes the workpiece and as it freezes thereon. The manipulation of the gun depends entirely on the job being perf ormed, 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 are 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 arc. In the preferred form of the invention th 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 aspro- "viding a medium to exclude air from the region of the arc. Inert monatornic gas such as argonhas 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 as exemplified by the apparatus of U.S. Letters Patent 2,504,868 referred to above. It consists 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 fromwhich the electrode W collects welding current. The barrel 56 also provides an annular passage for the flow of shielding gas to a nozzle 57 which delivers the gas as a flowing stream 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 con sists 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 as shown, for example, in U.S. Letters Patent 2,512,705, Nelson E. Anderson and George R. Turbett, granted June 27, 1950, for Fluid-Cooled Gas-blanketed Arc Welding Torch. It con sists essentially of means to conduct arc 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 teminal on the consuming electrode barrel are connected to the respective terminals of the power source 60 which in the preferred from 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 olfset 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 inch, and the current density and wire feed speed should be adjusted so the consuming wire electrode assumes a tapered form at the end which projects slightly beyond the end of the nonconsuming electrode. This condition is attained, for example, using 385 amperes with a li 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 at angle between them is substantially less than or substantially greater than the projection ve locity is decreased and the molten metal stream becomes more dispersed. The angle on is preferably always more than 60 and less than 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 claims.

Furthermore, the invention is not limited to forming metal coatings on workpieces. It may be used for other purposes such as forming metal granules which may be used for example in shot blasting workpieces to produce suitable surface conditions thereon. The molten metal droplets formed by the process disclosed may be solidified to form the desired metal granules by air cooling, cooling in a shielded atmosphere, or by being directed into a cooling liquid such as water.

I claim:

1. The method of projecting into free space the fluid products of an electric arc struck in an inert gaseous medium between a bare wire consuming electrode connected to a source of direct current and a counter electrode also connected to said source and offset with respect to the longitudinal axis of said wire electrode, characterized by feeding said wire electrode toward said are at a rate of at least 100 inches per minute, and simultaneously supplying current from said source to said electrodes with said wire electrode at positive polarity and said current at a strength suffioient to both consume said wire electrode at said rate and also project the fluid arc products axially from said wire electrode past the said counter electrode.

2. The method which comprises flowing an inert gas through an annular passage around an electrical contact element and then over the exposed end portion of a consuming electrode projecting from said contact element and out through a nozzle having an inner annular metal surface portion which constitutes both a counterelectrode and an exit orifice through which said inert gas flows into the free space beyond said nozzle, said annular counterelectrode being positioned concentrically with respect to the axis of said consuming electrode and being spaced laterally therefrom to form an arc gap extending generally transversely across the flow path of the said inert gas flowing through said exit orifice, and establishing a direct current arc between the end of said consuming electrode and said annular counterelectrode at a current level which is sufficient to consume the said electrode at a rate of at least 100 inches per minute while projecting the hot fluid products of said are axially of said electrode out through the said exit orifice into the free space therebeyond while maintaining the said arc rooted at one end on the said electrode and at the other end on said counterelectrode.

3. The method which comprises positioning the exposed end of a first consuming electrode in coaxial relationship with the inner surface of an annular non-con- 7 t s a s suming counterelectrode to form an arc gap ,th ereberespect to said first electrode and out through the oritween, flowing an inert gas over the surface of the exfice formed by the inner surface of said counterelectrode posed end ofi-said "first electrode and the inner surface into free space therebeyond. of said 'counterelectrode and through said are gap to shield the said electrode surfaces and form an inert gas 5 References Cited in the file ofthispatent atmosphere in said-arc gap, and establishing-an arc across UNITED STATES PATENTS saidarc gap at a current 'level of the order of 200 amy I peres or more to consume said first electrodewhile mainiz fi g taining the said are rooted at one'end on said first elecer e 1950 frolic-and at'the other end on saidcounterelectrode while 10 4.1 4 3 33 X 1952 projecting-the high temperature are products axially with

Images