US3639150A - Electric explosion metal spraying for substrate - Google Patents

Abstract

This invention relates to an improved process for coating a metal substrate with metallic particles by exploding a consumable metal wire in atmosphere or an inert gas of normal pressure by a heavy current.

Description

g United States Patent 1151 3,639,150

Suhara el al. 1 Feb. 1, 1972 [5 1 ELECTRIC EXPLOSION METAL References C d SPRAYING 11 OR SUBSTRATE OTHER PUBLICATIONS inventors! Tflflhil'o Suhal'il, 9-12 Komuin Jutaku, Chace A Survey of Exploding Wire Progress" from Explod Miyuki-rnachi, Kashii; Shigehisa Fukuda, ing Wires Vol. 3, Ed. by Chace et al., Plenum Press, New York 1479-3 Hakozaki, Hakata, both of Japan P1 [22] Filed: July 1968 7 Primary Examiner-Alfred L. Leavitt 21 LN 747 675 Assistant Examiner-J. H. Nev/some 1 App 0 AttorneyLinton & Linton 1571 ABSTRACT This invention relates to an improved process for coating a [51] Int. Cl ..B44d l/02 metal Substrate with metanic particles by wading a Com sumable metal wire in atmosphere or an inert gas of normal [58] Field of Search ..117/105, 93.1, 93, 105.2, 105.1 pressure by a heavy current.

2 Claims, 15 Drawing Figures mm FEB 11972 3s39;150

sum 1 or 2 FIG. 1.,

IN TmH/f juumzn MM BY HIGEHIJF FuKupA ELECTRIC EXPLOSION METAL SPRAYING FOR SUBSTRATE The present invention relates to an improved method of coating of metals by applying on a substrate of metals, metallic compounds and metal alloys, comprising the steps of exploding fine particles of a consumable wire usable as spraying material in atmosphere or in an inert gas of normal atmospheric pressure by a heavy electric discharge, causing impingement of the atomized particles with a very high speed against the substrate surface to be coated to provide a dense, smooth and highly adherent coating layer.

Wire explosion phenomena are well known, but it has been normally caused by an electric discharge of a heavy current through a metallic wire of relatively small diameter, say smaller than 0.1 mm., and the explosion vaporizes the wire into a high-pressure gaseous state. W. M. Conn reported an explosion coating in the book, Exploding Wire, Volume 2, Plenum Press, New York (1962) or (Bulletin of the American Physical Society" Volume ll, No. 2, Feb. 1966) wherein a silver wire of 0. l 6 mm. in diameter was exploded by an electric discharge under a low pressure (about I mm. Hg) of the air or inert gases. Upon following the procedure described in the above-mentioned references the following difficulties have been found:

a. The electric discharge under the low pressure of air or inert gas used, ionized the gases around the wire through which a quantity of electric current passed, decreasing the explosive force and in turn the adhering strength of the coated particles.

b. Optimum operating conditions could not be established because of uncertainty as to the quantity of electric current passing through the wire itself.

. The thickness of a coating layer adhered on a substrate by exploding a small wire of the order of 0.l mm. in diameter was approximately 0.05 micron for a single explosion which would be an impractical value.

One of the important objects of the present invention is to provide a novel coating process by using the optimum discharging conditions i.e., using appropriate value of electric current, voltage and frequency in a discharging circuit and using a consumable wire having dimensions of from 0.5 to 2.0 mm., superheating the wire instantaneously thus forming a mixture of high-pressure gas and the melt of the wire, and scattering the atomized particles at a high speed of 1-10 Machs. A coating by the present process here described, prevents the oxidation of the metal particles because the highpressure gas produced will expel the ambient air or gas surrounding the wire.

Another object of the present invention is to provide a novel coating process as described above which produces a uniform coating layer, unidirectional scattering and high yield of product applied.

The distinctive character of the present process is to control the direction and speed of the atomized particles by utilizing the energy of the shock wave generated by the explosion of the consumable wire in the ambient atmosphere or gas.

One of the advantages of the present invention is that the process is suitable to apply the coating on the undersurface of hollow cylindrical or spherical objects.

These and other objects and advantages of the present invention will be easily appreciated by those skilled in the art from the following detailed explanation taken in connection with the accompanying drawings wherein:

FIG. 1 is a basic circuit diagram of electric exploding equipment used in the practice of the present invention;

FIG. 2 shows an arrangement of a metallic pipe section, the inner surface of which is to be coated with a refractory metal such as tungsten;

FIG. 3 is a recorded curve obtained by a surface-roughnessmeasuring instrument which shows the superficial roughness of a coated surface of the sample coating shown in FIG. 4A having the composition by weight WC 93.50 Co. 6.5;

FIG. 4A is a microphotograph of a section of a copper pipe coated with tungsten by this method;

FIG. 4B is a similar photograph to FIG. 4C showing a section of a copper pipe coated with molybdenum by this method;

FIG. 4C is a microphotograph of a section of a mild steel sample four-ply coated with WC-Co alloy by this method;

FIG. 4D is a microphotograph of a section of a graphite sample four-ply coated with tungsten by this method;

FIG. 4B is a photograph of stainless-steel-coated iron (X I00);

FIG. 4F is a photograph of titanium-coated brass (X 450);

FIGS. 5A and 5B show the interaction between the atomized particles and the shock wave reflected from a reflector by this method, FIG. 5A being an axial view and FIG. 58 a side view;

FIG. 6A and 6B are microphotographs of the atomized particles and the shock wave produced by a. wire explosion; and

FIGS. 7A and 7B are photographs of mild steel surfaces coated by a WC-Co wire explosion respectively, without and with acrylic resin reflectors.

Referring now to the drawings, FIG. 1 diagrammatically illustrates a generating circuit for an impulsive discharge current and a consumable wire to be exploded in accordance with the present invention, wherein a wire 1 is mounted between a pair of electrodes 2,2 in the atmosphere or in an inert gas, and a heavy electric current is discharged through a discharging gap switch 6 and the wire 1 from a capacitor 3 of a large capacity which is charged from a high-voltage DC generator 5 through a charging resistor 4. Because of the fact that an electrospark-forming circuit is broadly known, a more detailed explanation of the circuit will be omitted. The process illustrated by the circuit shown in FIG. 1 is suitable to coat metals on the inner surface of cylindrical and spherical objects.

A feature of the present invention involves the utilization of the shock wave generated by the explosion of a consumable wire in the atmosphere or gas of normal pressure.

An advantage obtained by adoption of an inert gas has been proved by experiments using a wire of aluminum, titanium and molybdenum, and nitrogen atmosphere which prevent the coated surface from adhesion of oxidized particles on the substrate surface.

An explosion of a wire having its diameter less than 0.5 mm. produces a thin coating layer of a lesser bonding strength because of a small explosion energy.

The shock wave propagated in the ambient gas by the explosion is reflected by reflectors which control .the flying direction and speed of atomized particles of the wire itself resulting in a uniform coating layer of a high density and good adhering strength and a good yield of applied coated material by acceleration and concentration of the atomized particles.

FIG. 4E shows mild steel coated by an explosion of stainless steel wire (2 mm. diameter and 60 mm. long) using a current of 20 kv. and a capacity of 60 microfarads.

FIG. 4F shows brass coated by an explosion of titanium wire 1.6 mm. diameter and 60 mm. long, using a current of IO kv. and a capacity of microfarads.

The principle of operation of this method is shown in FIGS. SA and 5B.

The shock wave produced by wire 11 is reflected by reflectors 12, 12 positioned at an angle of 45 apart, into reflected waves 14, 14' traveling toward a substrate 13 to be coated, which converges the atomized particles 15 and I6 in the directions shown in dashed lines in FIG. 5. Thus, the wave intensity and its direction are controlled by the reflectors which in turn control the movement of the atomized particles. This practice is suitable for coating metals onto a plane surface. A suitable distance between the wire and the substrate is 20 to 60 times the radius of the wire, preferably substantially 40 times.

The process of the present invention is conducted generally in the normal atmosphere which is rather difficult to be ionized, such as at normal air atmospheric pressure at sea level, resulting in prevention of leakage of current through the ambient atmosphere. Accordingly, the electric current concentrates in the wire I enabling the explosion of a wire of a larger diameter, up to approximately 2 mm.

The wire is exploded after the selection of the dimension and wire material together with the optimum conditions of discharging, e.g., the discharging voltage, the value of capacitor capacity and the resonant frequency of the circuit to reduce the loss of electric energy.

The following table shows the results of the explosion spraying using various kinds of substrate.

generates a superheating of the wire, producing instantaneously a mixture of a high-pressure (of the order of 1,000 atmospheres) gas (less than 30 percent of the mass of wire), and melt. The gas ejects the melt at a speed of l-lO Machs in the radial directions.

As mentioned above, in this process, a wire of diameter 0.5-2.0 mm. is used in the explosion which produces a coating layer having a thickness of more than 5 microns per one explosion.

Subtrate Al W Sprayed wire:

Brass (7:3) .1 Stainless steel (18:8). E WC-Co E Stainless Brass steel N1 M (7.3) (18.8

F E F E E E E E E E E E G E E F E E E F E E E F E E E E E E E F E E Remarks:

E =Excellent-Bond strength more than 100 kg./sq. em. G GoodBond strength more than 50 kg./sq. em.

F= Fair-Bond strength more than kg./sq. em

The optimum conditions of the present invention, for instance in exploding a tungsten wire, may be expressed by the following formulas:

These formulas have been proven to be correct by applicant, the numerical constants have been determined empirically.

Suitable materials for the substrate to be coated include brass and stainless steel as shown in the above table.

A consumable wire of a diameter from 0.5 to 2 mm. is appropriate to perform the process. With a wire of a larger diameter, when a high-frequency current or an impulsive DC current pass through the conductor, the well-known skin effect" is produced resulting in the disadvantage that uneven melting and evaporation of the wire result due to the possible uneven quality of wire material. This statement applies to most metal wires.

This disadvantage of unevenness would produce larger particles having a diameter of more than 100 microns which deteriorate the smoothness of the applied coated surface and adhering strength.

With a consumable wire of diameter less than 0.5 mm., a great portion of the wire is exploded into an explosion gas, and the diameter of the atomized particles is less than l micron, and hence the smaller energy of impingement against the substrate surface will produce a poor adhesion.

In the process of the present invention, using a consumable wire of diameter 0.5 to 2.0 mm, a discharge through the wire The following are examples illustrating the practice of the present invention.

EXAMPLE 1 At the axial position of the section of hollow copper pipe A (FIG. 2) having a length of 30 mm. and inside diameter of 30 mm., machined, polished and washed with trichloroethylene, a tungsten wire B having a length of 50 mm. and diameter of 1 mm. was mounted between a pair of electrodes C, C as shown in FIG. 2. The copper pipe A was supported on an insulating body D.

When the charged energy of 4 kilojoules in condenser 3 is discharged during about 10 microseconds. the tungsten wire 1 partly vaporizes by the sudden superheating and the remainder of the wire melts and is exploded into fine particles by the explosion gas and is ejected at a high speed of l-l0 Machs against the inner surface of the pipe producing a tungsten coating thereon.

The explosion is made generally under normal atmospheric pressure, but because of the fact that the explosion gas precedes the atomized particles pushing away the air by a distance of approximately l0 times the pipe radius, the particles are not oxidized.

The surface roughness of the applied coated layer by the present invention, is within the variation of approximately 5 microns as shown in the record curve of FIG. 3 recorded by a roughness tester and the coated surface is very smooth. The density of the layer is measured as 18 g./cc. which corresponds to 93 percent of the theoretical density and its structure is very dense as shown in the microphotograph of FIG. 4A.

The above-mentioned process was repeated with a molybdenum wire having a length of 50 mm. and diameter of 1.4 mm., and the sectional microphotograph of the coated layer obtained is shown in FIG. 4B. The result of the adhering test on the coated layer shows an excellent value of bond strength of more than 226 kg./sq. cm. as compared with an average value of 173 kgJsq. cm., as compared with an average value of 713 kg./sq. cm. which has been obtained by a prior spraying gun process of the prior art.

A voltage more than 50 kv. is apt to ionize the air and produce a corona discharge which makes the charging equipment hard to be handled. Furthermore, a high voltage will cause an electrical resistance rise during an explosion of wire and hence a direct discharge often occurs between an electrode and the substrate. No critical value exists in capacitor capacitance.

EXAMPLE 2 A schematic diagram of the operation of this example is shown in FIGS. 5A and 5B. As a consumable wire 11, a tungsten wire having a length of 30 mm. and diameter of 1 mm. was connected to electrodes at both its ends, and acrylic resin plates as reflectors 12, 12' were set apart from the wire by 5 mm., said plates being mounted in a diverging position at an angle of 45 to one another and on an angle 0 to mounting base 17. A metallic plate 13 to be coated was positioned opposite the diverged side of the reflectors 20 mm. apart from the wire. When there is an explosion by an electric discharge under conditions of charging voltage 20 kv., applied to the capacitor of 20 microfarads under a charging energy of 4 kilojoules passed through the wire, the resulting coated layer on plate 13 had a thickness of 1 1 microns. With these reflectors 12, the thickness of the obtained coated layer on plate 13 was only 7 microns.

In FIGS. 2A and SB, 10, 10 are electrodes; 11 is the wire; 12, 12' are reflectors; 13 is a substrate; 14, 14' are reflected shock waves; 15, 16 are deflected spraying particles; 17 is a mounting base for the substrate; 18 is the shock wave.

The wire llll is mounted in a position parallel to and apart from the substrate by a suitable distance e.g., 40 times of the wire radius, and the reflectors are arranged at an angle of 60-l 20bL relative to said mounting base 17.

A part of the cylindrical shock wave generated by the explosion of the wire is reflected by the reflectors 12, 12 forming the reflected waves 14, 14', which are directed towards the substrate with the inwardly inclined reflectors. A group of particles 15, 16 flying in the radial direction are deflected in a direction shown by an arrow by the waves 14, 14 and collected on the substrate surface. In this case the velocities of the shock wave and particles are approximately l,500-2,000 m./sec. and 30-1 ,000 m./sec. respectively. Thus, the direction of particle spraying may be controlled by the reflectors.

Accordingly, the thickness of coating per one explosion may be increased.

Adjusting the inclination of the reflectors 12, 12' from 60 to 120 relative to the mounting base 17 will control the flying direction of the particle toward the substrate effecting its concentration or uniform distribution of the particle.

Application of low-velocity particle or oxidized fume on the substrate is in every respect disadvantageous which will cause surface stain and obstruction to the succeeding spraying, but since the kinetic energy of these particles is relatively small, reflectors 12, 12' arranged at an angle 0 of 90-l20 relative to said mounting base 17 generate the reflected waves 14, 14 which expel these particles outside the substrate 13. Therefore, a clean surface may be obtained as shown in FIG. 7B, whereas without the reflectors the surface has been stained with these particles as shown in FIG. 7A.

The relationships between the shock wave generated by the explosion and the atomized particles are shown in FIG. 6A and FIG. 6B obtained by using a high-speed (200,000

frames/sec.) camera. The microphotograph in FIG. 6A was taken at an instant I/40,000 second after the explosion wherein the central white portion is a mixture of explosion gas I and atomized particles and the outer circle shows a wave front of the shock wave just reaching the plate to be coated. By this moment both the explosion gas and the atomized particles are expanding uniformly, but as shown in the microphotograph of FIG. 6B taken l/200,000 second after FIG. 6A, a reflected shock wave from the plate 12 is obviously striking against the front of explosion gas and atomized particles affecting their propagations. In this event, a spherical collision is caused between the reflected shock wave and the propagating gas, and a resulting plane wave reaches the metallic plate forming a uniform-coated layer.

FIG. 7A and FIG. 7B are photographs of mild steel surfaces coated by a WC-Co wire (diameter of 1 mm. and length of mm.) explosion in a nitrogen atmosphere under same conditions (1 l kv. and 80;.tf.) respectively, without and with acrylic resin reflectors l2, 12 disposed at an angle of to mounting base 17, the former (A) apparently shows black status by oxidized particles of lower velocity whereas (B) demonstrates a clear surface coated by nonoxidized particles.

The process described has proved to give results similar to the case in which the explosion was conducted under normal atmospheric pressure value of an inert gas such as nitrogen because of the fact that the explosion gas effectively expels the air or gas surrounding the wire.

We claim:

1. A method of spray coating metallic materials on a substrate comprising the steps of positioning a substrate to be coated at a distance from a wire of said metallic material mounted between a pair of electrodes under atmospheric pressure, said distance of said substrate from said wire being substantially 40 times the radius of said wire having a diameter of 0.5-2.0 mm. and a length of 20-300 mm., discharging an impulsive electric current in accordance with the formulas:

K I is a material constant (:20percent) K is a constant (:30 percent) C =capacitor capacitance (farad) V=charging voltage (volts) f =resonant frequency of the circuit (c.1lsec.)

l opt: Optimum length of a wire (mmr), through the wire,

explosively spraying 50-60 percent of the wirefused particles having I-10 microns diameter and the remaining 40-50 percent of the wire being vaporized and providing a coating having 5-15 microns thickness at a time on surfaces of the substrate.

2. The method of spray coating as claimed in claim 1 including the further step of reflecting said exploded material from inclined reflectors situated on both sides of said wire and said substrate.

Claims (1)

1. 2. The method of spray coating as claimed in claim 1 including the further step of reflecting said exploded material from inclined reflectors situated on both sides of said wire and said substrate.

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