US3016447A - Collimated electric arc-powder deposition process - Google Patents

Description

Jan. 9, 1962 R. M. GAGE ETAL 3,015,447

COLLIMATED ELECTRIC ARC-"POWDER DEPOSITION PROCESS Filed Nov. 2, 1959 4 0 0 4 i J 54 2 3 4 J15 Z 4 K 3?. l R E 5 m 0 \E W 4 H 01 1 /R% R P i; m m Ma 4 w wu m a 4\ 2 6\ w i K E M I O w msm DAT w Wx.

POWDER 8- GAS INVENTORS ROBERT M. GAGE ONTARIO H- NESTOR DONALD M-YENNI Q WJZQ A T TORNEV United States i is This invention relates to the art of electric arc fusing and welding particles of powder together, and more particularly to building up a mass on a suitable base or workpiece by spray-casting powdered material thereon by means of a collimated arc-efiiuent or plasma.

The present application is a continuation-in-part of applications, Serial No. 706,099 and Serial No. 706,135, filed December 30, 1957; which applications are, in turn, continuations-in-part of applications Serial No. 631,557 and Serial No. 631,558, filed December 31, 1956, respectively, all now abandoned.

Space missiles equipped with graphite exhaust nozzles, for example, can have the linings of such nozzles coated with tungsten metal according to the invention here proposed to provide one of the greatest advances in the rocket art to date.

Heretofore, many processes have been proposed for producing coatings on surfaces of workpieces. Some of such processes employed oxy-fuel flames to fuse a stream of powder or a rod, and the fused particles were deposited on the surface to be coated. Such processes presented an inherent disadvantage in that the temperatures developed by a flame were not sufficiently high to fuse the higher melting point coating materials. Another disadvantage was that in such processes the control of the ambient coating atmosphere was narrowly limited to the oxidation or reduction potential of the flame chemistry. Even then, desired variations in oxidation or reduction potential of the coating atmosphere required complex adjustment of fuel and oxidant feed withthe usual undesirable resulting variation in flame temperature. Neutral or inert coating atmospheres were essentially impossible to attain when desired.

Coating processes also have been suggested employing electric arcs. Some of these involved the use of a gasshielded, non-consumable electrode-electric torch and a separate filler rod of the surfacing material. A disadvantage of such processes is that the deposited coating material alloys to a substantial degree with the base material without any ability to control the degree of this alloying. In addition, a considerable degree of skill is required by the operator in such surfacing processes.

Various other metallizing processes employing conventional electric arcs have been proposed, but the thermal energy concentrations and distributions, the energy transfer of the arc to the coating material, and the acceleration of the coating particles are usually insufficient to heat and propel the material to the extent required to form sound, dense, adherent coatings on the surface of the workpiece.

It is, therefore, the prime object of the present invention to provide a method for coating surfaces of a workpiece employing electric arcs which accomplishes the transfer of a sufficient amount of energy to the coating material to insure the formation of sound, dense, adherent coatings.

Another object is to provide a method for depositing material on a workpiece in a manner to effect a controlled degree of fusion, for example to another workpiece, wherein an electric arc is employed which is capable of transferring a sufficient amount of energy to the material to be deposited to insure substantial fusion thereof and wherein a deoxidizer may be included, if desired.

The present invention provides a novel process of'depositing powdered coating material which involves passing such material in the form of relatively fine powder into, axially through and with an arc and ionized inertgas stream of a collimated Gage-type high-pressure are which is wal-stabilized through lateral constriction, and depositing the so-accelerated and heated particles on a suitable base by directing the so-collimated efiiuent containing such material against such base. a

in accordance with the present invention, a method is provided for depositing material on the surface of a suitable base or workpiece comprising concurrently forming a high-pressure electric are between a non-consumable stick electrode and a second electrode spaced therefrom, passing a stream of gas in contact with the stick electrode to contain the arc, construing the arc-containing gas stream and wall-stabilizing at least a portion of the arc to collimate the energy of the arc and produce a high thermal content effluent, passing coating material in the form of powder through and with the high thermal content effluent to heat and propel the material, and depositing the resulting hot material on such surface. When the method of the invention is employed to effect the coating of a surface of a workpiece, the deposited material forms a sound, dense, adherent surface, consisting of irregular shaped microscopic leaves interlocked and welded with and to each other and such surface.

In one species of the invention the arc is transferred to the base or work which is in the electrical circuit. This species employs an arc of the type described in US. Patent 2,806,124. Another species of the invention employs a non-transferred arc wherein the base or work is not in the electrical circuit. This species employs an arc of the type described in US. Patent 2,858,411. The non-transferred arc species is presently preferred for coating operations, while the transferred arc species is preferred for fusion bonding and welding operations wherein controlled degree of fusion is desired between the heated particles and the base or workpiece.

The process of the present invention is capable of spraycasting such low melting point materials as iron, aluminum, nickel, and the like, as well as relatively higher melting point materials such as tungsten. molvbdenum. tu gsten carbide, hafnium carbide, and the like. Sometimes binders, such as iron, nickel, and cobalt, in amouts up to approximately 30 percent by weight may be added to the material. It is necessary, however, that at least a substantial portion of the coating mixture becomes plastic during the coating process in order to attain dense, adherent coatings.

It has been found that many gases can be employed in this process depending on the type of material being coated and on the coating desired. If a pure metal coating is desired, an atmosphere inert both to the coating material and the base or work, such as argon, helium, and in some circumstances nitrogen, hydrogen, or carbon monoxide, should be employed. Mixtures of gases, such as argon-nitrogen and argon-hydrogen, can also be used when desired to deposit the desired material. The prac tical advantage in this regard of the present are torch plating process is that the chemistry of the ambient atmosphere can be controlled without basically affecting the energy available for heating the coating material. One item which must be considered in the choice of a given atmosphere (gas) is that precautions be taken to prevent damage to the torch.

The coating material in the form of powder may be introduced into the collimated arc zone directly. It may also be introduced into gas which acts to carry the material into the arc.

For some materials it may be introduccd into and with the high thermal content .efl luent.

It has been found that a direct current source of electric power at either reverse or straight polarity, or an alternating current power source, may be employed to energize the are in accordance with the process of the invention.

In the drawings:

FIG. 1 is a schematic view, partly in "vertical crosssection, of apparatus for carrying out the invention with the work-out-of-circuit (non-transferred);

FIG. 2 is a similar view of a modification thereof;

FIG. 3 is a schematic View, partly in vertical crosssection, of apparatus for carrying out the invention with the work-in-circuit (transferred); and

FIG. 4 is a similar view of a modification thereof.

Referring to FIG. 1, a non-transferred arc torch is provided having an internal stepped-diameter cylindrical bore 12 terminating at the lower end in an arc stabilizing orifice 14 and having axially positioned therewith a non-consumable stick electrode 16, of tungsten or the like. The stick electrode may contain highly emissive material such as thoria. Inlet means 18 is provided for introducing water into an annular space 20 to cool the lower end of the torch, and outlet conduit 22 is provided to carry the cooling water from the torch body. Stick electrode 16 is electrically insulated at 28 from the upper end 30 of torch 10 and is connected, through line 32, to the negative terminal of a direct current power source 34. Line 35 connects the positive terminal of power source 34 to the torch body 10.

A powder and gas inlet conduit 38 is provided in torch body 10 and communicates with the interior of boring 12 to introduce a powder-carrying gas stream into the annular space formed between the walls of bore 12 and sleeve 11. Additional torch gas enters through conduit 46 in the upper end of torch body 10 and passes through annular space 48 to shield the stick electrode 16. Such gas flows from annular space 48 around the end portion of the electrode 16 while the gas-borne powder stream entering through annular space between thesleeve 11 and the inner cylindrical wall of torch body 10 is thereby shielded from undesirable contact with the stick electrode.

As the are 40 is established, with the gas stream, the arc effluent or plasma conforms to the shape of the arc stabilizing orifice 14, and a collimated arc is produced and the powder carried by the gas stream passes axially through such high-energy collimated arc, is heated and propelled by the high thermal content efiiuent (plasma), and is deposited as a dense, adherent coating 42 on the surface of workpiece 44. The gas flow through orifice 14 is preferably in an axial direction.

The apparatus of FIG. 2 is a double-anode modification of that shown in the embodiment of FIG. 1. In this modification a ballast resistor 50 is provided between the torch body (secondary electrode) and the positive terminal of power source 34 to maintain a pilot are between the electrode 16 and the torch body 10; and a remotely-positioned primary electrode 52, having an orifice 54 and cooling fluid inlet 56, cooling fluid passage 58 and cooling fluid outlet 60, is connected directly to the positive terminal of the power source. Coating material in the form of'powder suspended in a gas stream may preferably be introduced through the annular space between bore 12 and sleeve 11.

A Wide variety of workpieces have been satisfactorily coated with a wide variety of powdered materials, both with and without binder components.

In addition to coating operations, suitable deoxidation powders may be introduced into a treating zone in accordance with the process of the invention, or fusion of two workpieces such as in a welding opera i n y be effected in accordance with the process of the invention. In carrying out such processes, the degree of fusion between the welded particle mass and the workpiece is controlled by varying the thermal energy supplied to the workpiece.

The following table sets forth operating conditions obtained in producing three coatings of tungsten carbide- 8 percent cobalt material (325 x D mesh) for a x x 2-inch plain steel bar traversed under the torch shown in FIGURE 2.

TABLE I Anode Nozzle 1 Size, inches Coating Thickness Torch to Work Dist., inches Surface Speed, i.p.m.

c.f.h. Argon Powder low, g.p.m.

Amps. Volts xxx ,5 inch diameter, spacing between nozzles,

TABLE II Non-transf rred. arc torch coatings on steel Powder Flow, g.p.m.

Torch- Work Spacing, inches Coating Volts c.t'.h.

Argon Amps Size,

inches A1203 Cr m-F394 Stainless steel 301 stainless steel. Al

TiIIIIIIIIIIIIIIII The high-pressure arc plating process of the invention is extremely versatile. A wide variety of materials can be coated under diverse conditions by a proper combination of process variables. The energy available for heating can be controlled by varying the arc current, torch gas and gas flow rate and orifice size; coating particle velocity can be varied by gas flow rate and powder particle size; the chemical character of the coating atmosphere and resulting coating can be controlled by varying the composition of the gas employed. In order to obtain acceptable coatings, the current density and gas velocity are maintained at relatively high values. The wide variety of controllable process variables thus serves to impart extreme versatility to the process of the invention.

It has been found that for nozzle orifice should be at least A inch diameter. 'When orifice sizes smaller than this are used, it becomes difficult to properly maintain an arc in the orifice. Such small nozzles are also subject to plugging by the coating material. When tungsten powder, for example, is used as the coating material, it has been convenient to use nozzles as large as inch diameter.

For practical purposes the nozzle orifice should not be larger than about /2 inch in diameter, otherwise the extremely large arc currents that will be necessary to substantially fill the orifice passage will provide an undesirably large overall efiiuent heat output which could harm the workpiece being coated.

The total collimated are energy to which the coating material is exposed in the torch orifice passage is an important variable for successful'coating application. Relatively low melting point materials or materials of practical purposes the torch V fine particle size require a lower total energy for proper coating conditions as compared with higher melting point or larger particle size materials. One method of varying the are energy is to vary the arc current. However, only limited control is thus available since the useful range of arc current is related to the orifice size being used.

Too low a current value result in a non-collimated arc with its attendant lower arc intensity, while too high a current will damage the given orifice passage. With a /8 inch diameter orifice, the arc current should be at least 30 amperes, for example, for a useful coating of tungsten metal using a powder having an average particle size of 4.5 microns. The preferred lower limit for are current in a /8 inch diameter orifice passage is about 100 amperes for a tungsten coating. No absolute upper limit has been found, but currents as high as 400 amperes have been tested successfully in orifice passages up to about inch diameter. It has generally been found that as the arc current increases in a given apparatus, the resulting coating porosity also decreases.

An alternative and preferred method of varying the total are energy available to the coating material is to vary the dwell time that a given coating particle is exposed to the collimated are. One method of attaining this result is to vary the length of the nozzle orifice. Orifice lengths from about A; inch to about 1 inch could conveniently be used. Desirable coatings of tungsten are obtained, for example, using a inch diameter orifice 4 inch long with a 200 ampere are and 300 c.f.h. argon gas flow through the orifice. Alumina coatings are obtained under similar conditions using a A; inch long orifice.

Another method of varying the dwell time of the coating particle in the orifice is to vary the gas velocity and thus vary the particle velocity. The important criterion is that sufiicient gas velocity is attained to properly accelerate the coating particles so as to form a dense, adherent coating. In addition, when a single nozzle electrode torch, of the type shown in FIG. 1 is used, the gas velocity in the chamber between the stick electrode and nozzle electrode must be sufficiently high to force the are down into the nozzle orifice. if at any time the arc is permitted to go directly to the side of the nozzle electrode mouth, then plating ceases since the bulk of the gas and coating material can pass around the arc and out through the orifice without being heated sufficiently for proper plating.

It has been found that the gas flow should be at least 20 c.f.h. through nozzles of the above description. No upper limit has been found but flows as high as 380 c.f.h. have been successfully employed. As the gas flow increases, the coating porosity generally decreases. It is desirable that the powder velocity leaving the orifice passage be at least 500 fps. and preferably at least 1000 fps.

The rate at which a coating is deposited is dependent upon the coating material feed rate as well as the general coating conditions. As low as 0.6 gram/min. and as high as 85 grams/min. have been used with Ms-V inch diameter orifices. Such feed rates are not limiting since higher or lower amounts could be used if desired. Deposition rates of the coating on the workpiece have varied from 0.1 gram/min. to above 20 grams/min.

Another variable which has an effect upon coating quality is the stand-elf distance between the torch orifice outlet and the baseplate. Stand-off distances of about Vs to 4 inches have been used with about 2 inches being conveniently used for most coating applications. At low stand-off distances undesirable baseplate heating results. At higher stand-off values the hot torch effluent gases can cool somewhat to reduce the amount of heat reaching the workpiece without substantially reducing the coating material temperature. However, at such high stand- 6a values the coating particles have a tendency to be oxidized by air contamination.

This latter problem is alleviated by using a protective extension in the form of cylindrical barrel 64 below the torch orifice as shown in FIG. 1. Such torch extension should have an internal diameter at least about three times the diameter of the nozzle orifice passage in order to prevent coating buildup along the extension wall. Such protective extension is necessary only when oxidizable coating materials are used. It is helpful but not necessary for other coating materials.

The coating material must be of such size that it can be uniformly dispensed. Also it should be as fine as possible, compatible with uniform dispensing, so as to permit rapid heating and acceleration in the arc stream. Powders of 325 mesh and smaller are quite satisfactory.

The present invention has been found quite useful to deposit refractory metal coatings, for example metallic tungsten. Prior methods have been incapable of producing such metallic coatings due to the high melting points of the metals. Recently, however, coatings of refractory materials were found obtainable by the employment of the method of United States Patent 2,714,563 wherein a detonation phenomenon is employed to partially melt and impart velocity to powder suspended in a detonated body of gas. When tungsten is applied by such detonation process, the metal particles have a tendency to react with the products of combustion (detonation) and, depending on the oxygen-fuel ratio, will be oxidized or carburized. The resulting coating, therefore, is not a metal of'the purity required for uses such as electron emitting elements, high temperature filaments, and the like.

Pure metallic coatings can be produced in the practice of the process of the present invention and such coatings are dense and adherent to an extent not obtained heretofore.

In one example, a metallic tungsten coating was deposited employing apparatus similar to the type shown in FIG. 1. An are between the stick electrode and the nozzle electrode was maintained at 250 amperes. Argon gas at c.f.h. and tungsten powder (average particle size 4.5 microns) at about 6 grams per minute passed axially down through the /8 inch diameter anode throat. The resulting hot gas-coating particle stream was directed.

against a steel workpiece to deposit a tungsten coating having microscopic porosity below 1 percent and a VPN hardness of 470 (equivalent to swaged tungsten).

Similar equipment was used again at 55 volts (DCSP) and amperes with a gas mixture of 38 c.f.h. argon and 65 c.f.h. nitrogen passing through a $1 inch diameter nozzle anode. The tungsten coating had 3 percent porosity and VPN hardness of 320.

In another example an arc of 200 amperes and 65 volts (DCSP) was maintained between a A; inch diameter tungsten cathode and a water-cooled copper nozzle having a A; inch diameter nozzle passage. The tungsten cathode was surrounded along part of its length by a cooled copper shielding gas sleeve. Argon gas at c.f.h. passed down along the tungsten cathode in the annular space between the shielding gas sleeve and the cathode. A second argon stream of 150 c.f.h. carrying 30 grams/min. finely-divided tungsten powder (average particle size of 6 microns) was introduced along the annular space surrounding the shielding gas sleeve. The total gas and powder flow passed concurrently through the nozzle passage where they were heated by the collimated arc and then passed through a nozzle extension tube inch ID. and 2 inches long attached to the nozzle anode. The hot tungsten particles were then impinged on a workpiece positioned near the outlet of the nozzle extension tube to form a dense, adherent coating. The as-deposited coating had a modulus of rupture of 50'65,000 p.s.i.; modulus of elasticity of 22x10 p.s.i.; density about 90% of theoretical; and a hardness of 400-470 VPN. After heat treatment at U 1600 C., the coating had a modulus of rupture of 68,000 psi; modulus of elasticity of 50x10 p.s.i.; and a density of about 93% of theoretical.

In another example, a metallic tungsten coating was deposited employing apparatus similar to the type shown in FIG. 2 of the drawing except that barrel 64 was omitted. The torch was operated at 150 amperes and about 40 volts (DCSPL while argon gas at 120 c.f.h. was passed axially through the nozzle of the torch. Tungsten powder particles having a size below 11 microns was introduced into the argon stream at 2 grams per minute to produce a substantially pure tungsten metal coating on a copper workpiece. 1

In another example, the same apparatus was employed with a barrel 64 consisting of a A inch I.D. extension tube 1 inch long' attached to the lower anode as shown in FIG. 2. A steel workpiece positioned approximately inch from the end of the extension tube was rotated during the plating operation. The torch was operated at 47 volts (DCSP) and 125 amperes while argon gas at 150 c.f.h. was passed through the torch nozzle. Tungsten powder of 4.5 microns (average particle size) was intro duced into the argon stream at a rate of 5 grams per minute. The resulting tungsten metal coating had an excellent interface bond, porosity was less than 1 percent, and the hardness approached that of cold drawn tungsten rod (450 Diamond Pyramid hardness).

In still another example apparatus similar to that shown in FIG. 1 was used. Argon gas at 162 c.f.h. passed down along the tungsten cathode in the annular space between the shielding gas sleeve and the cathode. A second argon stream of 132 c.f.h. carrying 20 grams/min. finely-divided hafnium carbide powder was introduced along the annular space surrounding the shielding gas sleeve. The total gas and power flow passed concurrently through the nozzle passage where they were heated by a 200 ampere- 59 volt direct current collimated and wall-stabilized are and then passed through a nozzle extension tube about 2 inches long attached to the nozzle anode. An additional nitrogen gas stream of 30 c.f.h. was introduced near the outlet of the extension tube to aid in shielding the hot coating particles from atmospheric contamination. The hot hafnium carbide particles were then impinged on a /2 inch diameter brass tube workpiece to form a dense adherent coating 0.015 inch thick.

A wide variety of other refractory metals, such as molybdenum, tantalum, columbium, rhenium, and the like, may be applied as coatings in accordance with this aspect of the invention.

Referring to FIG. 3 of the drawing, a transferred arc torch 10 is provided having an internal stepped-diameter cylindrical bore 12 terminating at the lower end in an arc stabilizing orifice 14, and having axially positioned therewith a non-consumable stick electrode 16, of copper or the like. Inlet pipe 18 is provided for introducing water into the annular space 20 to cool the lower end of the torch, and outlet pipe 22 is provided to carry the cooling water from the torch body. Similarly, inlet conduit 24 is provided for introducing cooling water into the body of stick electrode 16, and outlet conduit 26 provided for conducting such water therefrom. Stick electrode 16 is electrically insulated at 28 from the upper end of torch 10. Electrode 16 is connected through conductor 32 to the positive terminal of a direct current power source 34. Conductor 36 connects the negative terminal of power source 34 and the base or workpiece 44 to be coated.

A ballast resistor 54 is also electrically connected between the negative terminal of power source 34 and the torch body 10. This serves to continuously maintain a pilot are between the stick electrode 16 and the torch body 10. In case the main are 40 between the electrode 16 and the workpiece 44 is interrupted, the pilot arc aids in reestablishing the main are.

A powder and gas inlet conduit 38 is provided at the upper end of torch body 10 and communicates with the interior of bore 12 to introduce the powder-carrying gas stream into the annular space 12a formed between the walls of bore 12 and the outer surface of stick electrode 16. As the main are 40 is established, with the gas stream, the arc efliuent or plasma conforms to the shape of the arc stabilizing orifice 14, and a collimated arc is produced; the powder carried by the gas stream passes axially through and along with such high-energy collimated arc, is heated and propelled thereby and is deposited as a dense adherent coating 42 on the surface of base or workpiece 44. The gas flow through orifice 14 is preferably in an axial direction.

The embodiment of apparatus shown in FIG. 4 of the drawing differs from that of FIG. 3 in that water-cooled copper electrode 16 is provided with a tungsten tip 46, and that the gas introduced through conduit 48 to annular space 50, and around the electrode tip 46, mainly provides the arc with gas that is ionized by enveloping such electrode tip 46, while the gas-borne powder stream entering through inlet conduit 42 to annular space 12a is thereby shielded from undesirable contact with such electrode tip 46. A tungsten stick electrode may be substituted for the tipped copper electrode 16, if desired. The tungsten electrode may also contain emissive material such as thoria. Another difference between the apparatus of FIG. 3 and that as shown in FIG. 4 is a reversal of the polarity of the direct current power source employed. A nozzle ex tension 64 similar to that shown in FIG. 1 can also be used with the apparatus of FIGS. 3 and 4.

A wide variety of workpieces in the are circuit have been satisfactorily coated according to invention with a wide variety of coating materials, both with and without binder components. The following table sets forth results for a few of such coating operations employing a direct current, reverse polarity power source in the manner shown in FIG. 3 of the drawing and an arc torch operated at V3 inch from A1 inch thick workpieces of the composition set forth. The resulting coatings were all dense and adherent, composed of irregularly shaped microscopic leaves welded in interlocking relation with one another.

TABLE III Powder Are Power Trav- Feed Argon erse Powder Rate, Workpiece Flow, Speed, g'n/ Volts Amps c.t.h. i.p.m. mm

WC+8% (30.. 2 Steel 28 40 15 WC+8% O0" 2 Aluninun'L- 28 80 40 15 WC|8% Co 2 Carbon 28 80 40 15 WO+8% C0 2 sttstinlpss 30 80 40 20 ee Aluminum..- 1-4 Steel 28 80 4O 15 Iron 1-4 do 28 80 40 15 C Carb0n 35-40 250 25 8 1 35-40 240 25 8 Stainless Steel. 3 Steel 26 80 40 13 Do 3 Aluminum 26 80 40 13 Enamel frit..- ElteeL 50 HastelloyG. 3 do 24 80 40 13 Do 3 Aluminum 24 8O 40 13 Hastelloy 75 K 3 Steel 25 8t) 40 1 3 The Hastelloy powder materials employed in the last three examples are nickel base alloys produced and sold by Haynes Stellite Company, Division of Union Carbide Corporation. 3

In addition to coating operations, suitable deoxidation powders may be introduced into a treating zone in accordance with the process of the invention, or fusion of two workpieces, such as in a welding operation, may be effected in accordance with the process of the invention. The following table sets forth the data for two such deoxidation and one such metal fusion applications employing apparatus and torch spacing conditions substantially identical with those employed in the examples of Table III.

Whereas argon was employed as the carrier and areenveloping gas in such examples, a wide variety of other gases such as hydrogen, helium, carbon monoxide, carbon electrical circuit. The employment of relatively low currents, such as about 80 amperes and gas flows of 40 c.f.h. argon through a /s inch orifice for a coating operation according to the invention with the work-in-circuit, has been found to result in substantially no alloying of the coating with the base metal, whereas the employment of high currents, of the order of 140-160 amperes at the lower traverse speeds employed for the deoxidation and fusion applications, results in gross melting of the base and in substantial dilution of the coating material and the base metal.

The following table sets forth various operating conditions obtained in producing coatings of tungsten carbide-8 percent cobalt material (325 x D mesh) for a /z inch diameter round bar plain steel both rotated and traversed under the various torches of the stated figures of the drawing.

dioxide, nitrogen and the like may alternatively be employed. One item which must be considered in the choice of a given torch gas is that precautions must be taken to prevent damage to the torch itself by reaction with the torch gas.

The following examples describe operation of the present invention wherein gas mixtures are used as the torch gas.

In one example, a metallic tungsten coating was deposited employing apparatus similar to the type shown in FIG. 1. An are between the stick electrode and the nozzle electrode was maintained at 190 amperes and 75 volts. A mixture of 12 e.f.h. hydrogen and 138 c.f.h. argon passed down along the tungsten cathode in the annular space between the shielding gas sleeve and the cathode. A 150 c.f.h. argon stream carrying 50 grams/ min. finely-divided tungsten powder (average particle size of 6.8 microns) was introduced along the annular space surrounding the shielding gas sleeve. The total gas and powder flow passed concurrently through the nozzle passage where they were heated by the collimated and wallstabilized arc and then passed through a nozzle extension tube. The hot tungsten particles were then impinged on a /2 inch diameter brass tube workpiece positioned near the outlet of the nozzle extension tube to form a dense adherent coating. The 0.050 inch thick as-deposited coating had a modulus of rupture of 50,000 p.s.i. and a modulus of elasticity of 22x10 p.s.i.

In another example using similar apparatus, an arc of 205 amperes and 65 volts was maintained between the stick electrode and the nozzle electrode. A mixture of 4 c.f.h. nitrogen and 150 c.f.h. argon passed down along the tungsten stick cathode in the annular space between the shielding gas sleeve and the cathode. A 150 c.f.h. argon stream carrying 28.3 grams/min. finely-divided tungsten powder was introduced along the annular space surrounding the shielding gas sleeve. The total gas and powder flow passed concurrently through the nozzle passage where they were heated by the collimated and wallstabilized arc and then passed through a nozzle extension tube. The hot tungsten particles were then impinged on a /2 inch diameter brass tube workpiece to form a dense adherent coating 0.050 inch thick.

It has been found that the speed of traverse of the workpiece required for satisfactory coating operations is affected by the electric arc currents and gas velocity employed in the process primarily when the work is in the Such coatings were all dense and well bonded to the workpiece. They were also composed of irregular shaped microscopic leaves interlocked with each other.

The workpiece on which the arc-heated coating particles are deposited is heated by the hot gas efliuent from the torch.

When the workpiece is in the arc circuit, it is heated not only by the high thermal emuent but also by the are current. Such heating of the workpiece can, to some extent, be overcome or compensated by interrupting the application of coating from time to time and permitting the workpiece to cool with or without directing a blast of cooling fluid such as air against it. A high traverse rate of the workpiece is also desirable. In transferred arc operation, the arc may desirably be interrupted to cool the Workpiece if necessary. External cooling during coating application with a liquid spray or fog, such as liquid carbon dioxide, can also be used provided the cooling fluid is not applied directly to the arc-efiiuent zone. Internal water cooling might also be used with hollow workpieces. According to the invention particles of a material such as tungsten carbide can be applied securely to a workpiece having a substantially diiferent coefficient of thermal expansion, such as steel, by cooling the workpiece as described above.

Coatings applied according to the present invention may be built up to any desired thickness by continuing the depositing operation until the desired thickness is obtained.

What is claimed is:

1. Method of welding particles of powder together to form a dense coherent mass composed of irregularly shaped microscopic leaves welded into interlocking relation with one. another, which comprises concurrently maintaining a high pressure electric are between a nonconsumable stick electrode and a second electrode spaced therefrom, passing a stream of gas in contact with said stick electrode to contain said are, passing said arc-containing gas stream through an orifice which constricts the gas stream and wall-stabilizes a portion of said are so as to collimate the energy of said are and gas stream and produce a high pressure are and high thermal content effluent, passing powdered material through and with said high thermal content erfiuent to produce a high velocity stream of gas and heated particles, impinging said gas and heated particle stream against the surface of a suitable base, thereby depositing the so-heated particles on said 11 base as a dense adherent coherent mass wherein the soheated and deposited particles are welded together.

2. Method of welding as defined by claim 1, wherein such powder is first passed through said are to take advantage of the extremely high temperature afforded thereby.

Method of welding as defined by claim 1, wherein said base is in electrical circuit with such high pressure are.

4. Method of welding as defined by claim 1, wherein said base is electrically insulated from the arc circuit;

5. Method of welding as defined by claim 1, wherein said second electrode is provided with an orifice for constricting and wall-stabilizing such arcand gas stream.

6. Method of welding as defined by claim 5, wherein said base also is in electrical circuit with such high pressure are.

7. Method of welding as defined by claim 1, wherein the value of the thermal energy supplied to the base is used to control the degree of fusion between such welded particle mass and said base.

8. Method of depositing material on the surface of a workpiece, comprising concurrently maintaining a high pressure electric are between a non-consumable stick electrode and a second electrode spaced therefrom, passing a stream of gas in contact with said stick electrode to contain said are, passing said arc-containing gas stream through an orifice which constricts the stream and wallstabilizes a portion of said are so as to collimate the energy of said are and gas stream and produce a high pressure are and high thermal content eifiuent, passing powdered material through and with said high thermal content effiuent to produce a high velocity stream of gas and heated particles, impinging said gas and heated particle stream against the surface of said workpiece, and depositing the soheated particles on the surface of said workpiece.

9. Method as defined by claim 8, wherein such powdered material is first passed through such orifice with said are.

10. Method as defined by claim 9, wherein the current is conducted to such are through the workpiece.

11. Method as defined by claim 9, wherein said second electrode is provided with an orifice for so-constricting and wall-stabilizing at least a portion of such are and gas stream containing such powdered material.

12. The method of depositing material on the surface of a workpiece comprising concurrently maintaining a high pressure electric arc of at least amperes between a nonconsumable stick electrode and a second non-consumable electrode positioned therefrom and having an orifice at least inch in diameter, passing a stream of gas hav ing a flow rate of at least 20 c.f.h. around said stick electrode and through said orifice, whereby the arc-gas stream is collimated and a portion of the arc is wall-stabilized to form a high thermal content efiluent, positioning the outlet of said orifice-containiug electrode about to 4 inches from such surface of said workpiece, passing coating material in the form of powder into said collimated high thermal content effluent, and directing the resulting collimated hot particle stream against such surface of said workpiece to deposit said powdered material thereon.

13. The method of depositing material on the surface of a workpiece comprising feeding a gas along a nonconsumable stick electrode, passing said gas through a secondary non-consumable electrode orifice and a primary non-consumable electrode orifice, establishing a pilot are between said stick electrode and said secondary electrode, establishing a main are between said stick electrode and said primary electrode, a portion of said main are being Wall-stabilized, discharging a collimated high thermal content efiluent from said primary electrode orifice, passing coating material in the form of powder into said high thermal content collimated effluent and directing the resulting collimated stream containing hot coating material particles against such surface of said workpieceto deposit said material thereon.

14-. Method of depositing material as defined by claim 8, wherein the gas is selected from the class consisting of argon, helium, nitrogen, hydrogen and mixtures thereof and the powdered material is selected from the class consisting of tungsten, molybdenum, tantalum, columbium and rhenium.

15. Method of depositing material as defined by claim 12 wherein the gas is selected from the class consisting of argon, helium, nitrogen, hydrogen and'mixtures thereof and the powdered ma erial is selected from the class consisting of tungsten, molybdenum, tantalum, columbium and rhenium.

References Cited in the file of this patent" UNITED STATES PATENTS 1,243,795 Apple Oct. 23, 1917 1,395,269 Gebauer Nov. 1, 1921 2,149,656 Armstrong et a1. Mar. 7, 1939 2,768,279 Rava Oct. 23, 1956

Claims (1)

1. METHOD OF WELDING PARTICLES OF POWER TOGETHER TO FORM A DENSE COHERENT MASS COMPOSED OF IRREGULARLY SHAPED MICROSCOPIC LEAVES WELDED INOT INTERLOCKING RELATION WITH ONE ANOTHER, WHICH COMPRISES CONCURRENTLY MAINTAINING A HIGH PRESSURE ELECTRIC ARC BETWEEN A NONCONSUMABLE STICK ELECTRODE AND A SECOND ELECTRODE SPACED THEREFROM, PASSING A STREAM OF GAS IN CONTACT WITH SAID STICK ELECTRODE TO CONTAIN SAID ARC, PASSING SAID ARC-CONTAINING GAS STREAM THROUGH AN ORIFICE WHICH CONSTRICTS THE GAS STREAM AND WALL-STABILIZES A PORTION OF SAID ARC SO AS TO COLLIMATE THE ENERGY OF SAID ARC AND GAS STREAM AND PRODUCE A HIGH PRESSURE ARC AND HIGH THERMAL CONTENT EFFLUENT, PASSING POWDERED MATERIAL THROUGH AND WITH SAID HIGH THERMAL CONTENT EFFLUENT TO PRODUCE A HIGH VELOCITY STREAM OF GAS AND HEATED PARTICLES, INPRINGING SAID GAS AND HEATED PARTICLE STREAM AGAINST THE SURFACE OF A SUITABLE BASE, THEREBY DEPOSITING THE SO-HEATED PARTICLES ON SAID BASE AS A DENSE ADHERENT COHERENT MASS WHEREIN THE SOHEATED AND DEPOSITED PARTICLES ARE WELDED TOGETHER.

Images