U.S. patent number 3,914,573 [Application Number 05/386,036] was granted by the patent office on 1975-10-21 for coating heat softened particles by projection in a plasma stream of mach 1 to mach 3 velocity.
This patent grant is currently assigned to Geotel, Inc.. Invention is credited to Erich Muehlberger.
United States Patent |
3,914,573 |
Muehlberger |
October 21, 1975 |
Coating heat softened particles by projection in a plasma stream of
Mach 1 to Mach 3 velocity
Abstract
An electric arc plasma spray gun provides optimum coating of
substrates by projecting a stream of plasma at a velocity at or
about Mach two, at ambient pressure, and entraining therein
particles of material to be coated upon said substrate. Power,
pressures and temperatures are employed together with a unique set
of interchangeable supersonic nozzles to achieve particle exit
velocities of from one to ten thousand feet per second, heating
said particles to a temperature below their melting point but
sufficient to soften the particles for enhanced coating. Unique
parameters of particle size, particle injection angle and particle
injection positions are identified for different materials to be
entrained in the supersonic plasma stream.
Inventors: |
Muehlberger; Erich (Costa Mesa,
CA) |
Assignee: |
Geotel, Inc. (Amityville,
NY)
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Family
ID: |
26841547 |
Appl.
No.: |
05/386,036 |
Filed: |
August 6, 1973 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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143956 |
May 17, 1971 |
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372260 |
Jun 21, 1973 |
3823302 |
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214584 |
Jan 3, 1972 |
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Current U.S.
Class: |
219/76.16;
219/121.48; 219/121.5; 219/121.47; 219/121.49; 219/121.51 |
Current CPC
Class: |
C23C
24/04 (20130101); H05H 1/42 (20130101); B05B
7/226 (20130101) |
Current International
Class: |
B05B
7/16 (20060101); B05B 7/22 (20060101); H05H
1/42 (20060101); H05H 1/26 (20060101); B23k
009/04 () |
Field of
Search: |
;219/121P,74,75,76
;13/9,31 ;239/3,15,434 ;313/231 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Truhe; J. V.
Assistant Examiner: Peterson; G. R.
Attorney, Agent or Firm: Gausewitz, Carr &
Rothenberg
Parent Case Text
CROSS-REFERENCE TO RELATED APPLICATIONS:
This application is a continuation-in-part of Ser. No. 143,956,
filed May 17, 1971 now abandoned, for Coating Heat Softened
Particles by Projection in a Plasma Stream of Mach 1 to Mach 3
Velocity, and is also a continuation-in-part of application Ser.
No. 372,260, now Pat. No. 3,823,302 filed June 21, 1973, for
Apparatus and Method for Plasma Spraying. Said last-mentioned
application Ser. No. 372,260 is a continuation of Ser. No. 214,584,
filed Jan. 3, 1972, now abandoned, for Apparatus and Method for
Plasma Spraying. This application is related to application Ser.
No. 351,814 filed Apr. 17, 1973 now Pat. No. 3,839,618 which is a
continuation-in-part of application Ser. No. 214,584, now
abandoned.
Claims
I claim:
1. The method of projecting high velocity particles onto a
substrate to coat the same, comprising the steps of:
establishing a plasma stream,
expanding said plasma stream from the throat to the exit of a
supersonic nozzle having a ratio of exit area to throat area
sufficient to achieve a plasma stream exit velocity greater than
Mach 1 and less than Mach 3,
injecting into said plasma stream a stream of particles to be
projected, said particles having a diameter not greater than about
44 microns,
heating said particles to a temperature near but less than their
melting point, said last-mentioned step comprising controlling the
time of dwell of said particles within said plasma stream by
selectively controlling both the angle and position of injection of
said particles into said stream substantially at the throat of said
nozzle, whereby said particles will be softened, but not melted,
and will obtain a velocity that is substantially maximized with
respect to the velocity of said plasma stream, and
impinging said softened particles against a substrate to thereby
coat said substrate.
2. A supersonic plasma spray gun for projecting particles of high
velocity comprising:
means for producing a plasma arc at a pre-selected pressure and
electrical power, and including nozzle means for expanding plasma
produced in said arc to provide a plasma stream having an exit
velocity of greater than Mach 1 and less than Mach 3,
means for injecting a stream of particles of a size not greater
than 44 microns into said plasma stream to be entrained thereby,
and
means for controlling time of dwell of said particles within said
plasma stream so as to heat the particles to a temperature near to,
but below, their melting point and to achieve substantially
maximized velocity of said particles,
said means for controlling dwell time comprising a plurality of
nozzles, each including means for selective and alternative
attachment to said spray gun, and each having a bore having a ratio
of exit area to throat area sufficient to produce the
above-mentioned plasma stream exit velocity, and
each of said nozzles having a particle injection conduit providing
communication between a source of particles exterior to said
nozzles and the bore of said nozzle at a point thereof between said
exit and throat for injection of said particles into the plasma
stream,
a first one of said nozzles for use with particles of low melting
points such as bronze, copper and aluminum, having its particle
injection conduit extending at a large angle (between 25.degree.
and 49.degree.) with respect to a normal to the direction of flow
of said plasma stream, and communicating with the bore of said
nozzle at a point closely adjacent the nozzle exit to thereby
provide a short dwell time that prevents premature melting and
plating of the nozzle bore,
a second one of said nozzles for use with materials of intermediate
melting points (2000.degree.F to 3000.degree.F), such as stainless
steel, nickel, chromium, and hard facing alloys having its particle
injection conduit extending at an intermediate angle (between
7.5.degree. and 25.degree.) with respect to a normal to the
direction of plasma stream flow and communicating with the bore of
its associated nozzle at a point substantially midway between the
throat and exit, but somewhat closer to the throat
whereby the dwell time of such intermediate temperature melting
point particles is greater than the dwell time achieved by lower
temperature particles in said first-mentioned nozzle, and
a third one of said nozzles for use with high temperature melting
point particles such as tungsten carbide-cobalt, chromium carbide
nickel chromium blends, tungsten and molybdenum, having its
particle injection conduit extending at a considerably smaller
angle (between -13.degree. and 7.5.degree.) with respect to a
normal to the direction of the plasma stream and communicating with
the bore of its associated nozzle at a point thereof substantially
at its throat whereby dwell time of such high temperature particles
is at least sufficient to ensure substantial heat softening of the
particles without melting.
3. A method of projecting particles to be spray-coated upon a
substrate wherein the particles are borne by a supersonic stream of
plasma having a velocity greater than Mach 1 and less than Mach 3
and wherein the velocity of the particles is substantially
maximized as the particles are heated, said method comprising:
providing an electrical plasma jet torch having a gas vortex
chamber communicating with a nozzle passage having a throat, said
torch also having a rear electrode extending adjacent said nozzle
passage,
creating and maintaining an electric arc in said nozzle passage
between said rear electrode and a forward electrode that defines
said nozzle passage,
introducing plasma arc gas into said vortex chamber in a direction
that is substantially tangential thereto but inclined forwardly
through a relatively small angle and causing said gas to flow
vortically and helically in said vortex chamber and toward said
nozzle passage,
ionizing said plasma arc gas in said electric arc,
constricting said ionized gas as it flows from said rear electrode
toward the throat of said nozzle passage, and continuously
expanding said ionized gas as it flows from said throat to the exit
of said nozzle passage to attain said supersonic velocity,
injecting into said nozzle, through the boundary layer of said
ionized gas, a stream of particles having a size not greater than
44 microns and having melting temperatures above 3000.degree.F,
said last-mentioned step including injecting said particles into
said nozzle at an angle within the range of -13.degree. and
7.5.degree. relative to said plasma flow and at a position within
said nozzle between 0.032 and 0.142 inches downstream of the nozzle
throat, so that the particles are heated to a temperature near but
less than their melting point, and
projecting said heated particles against a substrate.
4. The method of high velocity spraying of particles comprising the
steps of:
generating a stream of plasma,
flowing said plasma stream through a converging diverging
supersonic nozzle having a throat and an exit,
causing said plasma stream to expand from the nozzle throat to the
nozzle exit to provide a plasma stream exit velocity of between
Mach 1 and Mach 3 at ambient exit pressure, and also causing the
plasma stream to decrease in density from a high density at said
throat to a lower density at said exit,
feeding to said nozzle a flow of particles having a maximum
dimension of 44 microns,
injecting said particles through the boundary layer and toward the
center of said plasma stream at a point within said nozzle where
said plasma stream has substantially said high density so as to
heat said particles to a temperature near but less than the melting
point thereof, whereby greater kinetic energy is transferred from
said plasma stream to the particles injected therein, and
impinging said heated particles against a substrate to thereby coat
said substrate.
5. The method of claim 4, wherein said step of injecting particles
into said plasma stream comprises injecting said particles into
said nozzle in a direction that is inclined rearwardly with respect
to the direction of flow of said plasma stream whereby transfer of
kinetic energy from said plasma stream to said particles is still
further increased.
6. Particle spraying apparatus for generating and projecting a
stream of plasma exiting therefrom at a Mach number exit velocity
greater than 1 and less than 3, said apparatus comprising:
a gun having front and back electrodes and a vortex chamber
extending about said back electrode,
means for feeding gas under pressure into said vortex chamber
adjacent said back electrode,
means for energizing said electrodes to form an arc therebetween to
ionize said gas and create a plasma stream flowing to and through
said front electrode,
said front electrode comprising an exit nozzle communicating with
said vortex chamber and having an exit, having a throat spaced
downstream of said back electrode, and having a portion diverging
from said throat to the exit of said nozzle to provide a ratio of
area of said exit to area of said throat sufficient to provide an
exit velocity greater than Mach 1 and less than Mach 3, said nozzle
having an axis along which said plasma stream flows,
said means for feeding gas under pressure including means for
effecting a pressure in said vortex chamber in the vicinity of said
arc having a relation to ambient pressure sufficient to provide
said Mach number exit velocity with said ratio of areas, and
means for entraining and heating in said plasma stream a flow of
particles having a size of less than 44 microns to be coated upon a
substrate against which said plasma stream impinges, said means for
entraining and heating causing said particles to be heated to a
temperature near but below their melting point,
said means for entraining comprising a particle injection conduit
intersecting the nozzle at a point adjacent the nozzle throat
wherein said plasma stream is of relatively higher density, and
means for feeding said particles through said conduit and through
the boundary layer of said plasma stream toward the center thereof
whereby both transferred kinetic energy and dwell time of the
particles in the plasma stream are increased.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention:
The present invention relates to electric plasma arc apparatus and
methods, and more particularly concerns such apparatus and methods
for coating substrates with high-velocity heat-softened particles
entrained in a supersonic plasma stream.
2. Description of Prior Art:
Arc torch devices comprising electrically powered generation of
high-temperature high-velocity plasma streams have been used for
many years as heat sources, as cutting torches, for plating or for
coating processes, and many other similar uses. High-temperature
high-velocity plasma streams projected into a vacuum test chamber
have been used throughout the industry to obtain environmental
conditions for research and laboratory testing. Many of these
devices employ convergent-divergent or supersonic nozzle to provide
the desired supersonic plasma velocity. Typical of such
environmental test apparatus are devices shown in U.S. Pat. Nos.
3,149,222, 3,075,065, 3,233,147, 3,304,774, 3,106,631, 3,301,995,
3,418,445, 3,106,632.
The high temperature of the plasma stream has enabled these devices
to be widely adapted for cutting of various types of materials.
Examples of such plasma cutting torches are shown in U.S. Pat. Nos.
3,370,148, 2,874,265, 3,366,772 and 3,106,633. In these cutting
torch arrangements, the velocity of the plasma stream is of
secondary significance as compared with the stream temperature and
density. Accordingly, high electrical input powers and high
pressures are employed to achieve the desired temperature.
Apparatus for generating plasma streams generally comprises an
arrangement for striking an electric arc between a pair of
electrodes, means for passing gas under pressure into an arc
chamber adjacent the arc and a nozzle for confining the exiting
plasma stream.
In plasma arc generators of the transferred arc type, which are
generally used as torches for cutting, welding and the like, the
arc normally is struck from a rear electrode, such as a cathode, to
the workpiece that is to be cut. The nozzle is often water cooled.
Concomitantly, a stream of gas under pressure is passed through the
nozzle with the arc.
In plasma arc torches of the nontransfer type, the arc is struck
between a rear electrode, commonly a cathode, and a forward
electrode that forms the exit nozzle for the plasma stream.
Another well-known use of the electric arc plasma stream comprises
the coating of various materials upon a substrate. For spraying
particles, as in coating, the electric arc plasma torch is provided
with means for injecting suitable particles or powder into the
exiting plasma stream, to be softened or melted and accelerated to
high velocity. Typical of such electric arc plasma spray guns are
U.S. Pat. Nos. 3,179,782 to Matvay, 3,145,287 to Siebein et al.,
3,308,623 to Ferrie et al., and 3,313,908 to R. Unger et al.
It has long been known that coating qualities, including
characteristics of bonding strengths, coating density and coating
uniformity, show marked improvement with increasing velocity of the
impinging particles. Recognizing this desideratum, much time and
effort has been expended in attempts to achieve apparatus and
methods that provide supersonic velocities in a plasma spray gun.
In fact, several of the arrangements of the prior patents are
described by the patentees, optimistically but erroneously, as
capable of providing supersonic exit velocity. For example, the
above-identified patent to Matvay states that an object of his
invention is to propel particles at supersonic velocity and further
states that the particles which are changed to molten gaseous vapor
or atomic particles are conveyed at sonic or supersonic
velocity.
Similarly, in the description of the patent to Unger et al., such
high velocities are suggested. Nevertheless, it is a well known
fact that supersonic velocity of a gas cannot be achieved without
the use of the commonly known converging-diverging supersonic
nozzle. As a practical matter particles injected into the plasma
stream will not attain a velocity equal to that of the stream
itself. Particle velocity cannot exceed gas velocity as long as the
gas velocity is in a state of acceleration, as in the divergent
nozzle. Only when the gas stream is decelerating can the particles
achieve the same velocity as the gas stream.
Thus, if devices such as shown in the Matvay and Unger et al.
patents fail to show or suggest any equipment or configuration
capable of achieving supersonic gas or plasma flow, no such
supersonic particle velocity can be accomplished with such
configuration. Attesting to this fact is the total absence of any
plasma arc equipment today (excepting that employing principles of
the present invention) that actually achieves supersonic spray
coating.
Accordingly, it is an object of the present invention to provide an
improved method of high velocity spray coating and an improved
electrical plasma torch apparatus for accomplishing such
coating.
SUMMARY OF THE INVENTION
In carrying out principles of the present invention in accordance
with a preferred embodiment thereof, certain supersonic parameters
of a plasma stream are matched to specified parameters of particle
injection. There is provided a supersonic plasma spray gun for
generating and projecting a stream of plasma comprising a
converging-diverging nozzle having a ratio of exit area to throat
area sufficient to achieve a plasma stream exit velocity between
Mach 1 and Mach 3, the nozzle including means for injecting into
the plasma stream at a point between the throat and exit of the
nozzle a stream of particles to be entrained by the plasma stream,
the particles having a size not greater than 44 microns (-325
mesh), and being injected into the plasma stream at such an angle
with respect to the direction of the stream and at such a position
between the throat and exit of the nozzle as to substantially
maximize the velocity imparted to the particles by the plasma
stream and to provide a dwell time of particles within the stream
sufficient to achieve a temperature nearly at but below the
particle melting point. Different types of materials having
different melting temperatures and different heat characteristics
may be employed in the coating process by employing a selected one
of a group of nozzles each of which has a ratio of exit area to
throat area sufficient to maintain the plasma stream exit velocity
within the indicated range of Mach numbers, and each of which has a
differently and uniquely oriented and/or positioned particle entry
conduit so that the particle dwell time within the supersonic
plasma stream will optimize velocity and temperature of different
materials to be sprayed.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a cross-sectional view of the working portions of a
high-velocity plasma gun employed in the practice of the present
invention;
FIG. 2 is an enlarged view of an anode and cathode arrangement
together with arc gas injection ports and powder injection conduit
employed for spraying relatively high melting temperature particles
in the gun of FIG. 1 to achieve plasma stream exit velocity of Mach
2;
FIG. 3 is a view of the anode and cathode employed in the gun of
FIG. 1 for use with particles of lower temperature melting
points;
FIG. 4 is a sectional view of another nozzle embodiment for
spraying at a 45.degree. angle;
FIG. 5 is a sectional view of a nozzle arranged for spraying at a
90.degree. angle;
FIG. 6 is a vertical sectional view illustrating an additional
embodiment of an electrical plasma-jet spray torch;
FIG. 7 is a sectional view taken on line 7--7 of FIG. 6;
FIG. 8 is a section taken on line 8--8 of FIG. 7;
FIG. 9 is an enlarged fragmentary sectional view showing a
lower-right portion of the torch of FIG. 6, and also illustrating
in dashed lines various alternative positions for the powder
port;
FIG. 10 is a transverse sectional view on line 10--10 of FIG.
6;
FIG. 11 is a transverse sectional view on line 11--11 of FIG. 6;
and
FIG. 12 is a transverse sectional view illustrating schematically
an embodiment wherein a plurality of powder sources are employed to
inject powder into the torch.
DETAILED DESCRIPTION
The fundamental parts and the overall arrangement, configuration
and mode of operation of the electric arc plasma torch illustrated
in FIG. 1 are substantially the same as in several prior electric
arc plasma spray guns made and sold by the assignee of the present
invention, including, for example, the gun shown in the patent to
Johnson 3,179,783, that shown in the patent to Winzeler et al.,
3,378,391, and that shown in the patent to Unger et al., 3,313,908.
The disclosures of these patents are incorporated herein by this
reference, as though fully set forth herein.
Proceeding first to a general description of the entire apparatus,
the electrical plasma torch is illustrated to comprise an
insulating handle and casing 10, of which only portions are shown,
which comprises a cup-shaped insulating body mounting a rear
housing member 12 and a front housing member 14 separated from each
other by an insulating ring 16. Forwardly projecting from the rear
housing member 12 is an insulating and rear electrode support
sleeve 18 that encompasses a forwardly projecting rear electrode
20. Preferably, the rear electrode comprises a thoriated tungsten
tip 21 and is mounted by means including a male threaded member
that extends into the threaded end of a forwardly projecting
portion 23 of rear housing 12.
The forward portion of electrode 20 has a somewhat smaller diameter
than its sleeve 18 to provide a gas entry or vortex chamber 25 (see
FIG. 2) between its outer periphery and the inner periphery of
sleeve 18. Formed in the wall of sleeve 18 and communicating with
the gas vortex chamber 25 are a plurality of arc gas entry conduits
of which only two, 24 and 26, are illustrated (see FIGS. 2 and
3).
Securely fixed to, but detachable from front housing member 14 is a
front electrode 28. The front electrode 28 forms a supersonic
converging-diverging nozzle that provides exiting expansion of the
plasma stream to achieve the desired supersonic exit velocity. The
nozzle bore includes a converging portion 30 that extends from a
straight portion co-operating with the sleeve 18 and cathode 20 to
define the annular gas entry chamber 25, and then converges,
tapering inwardly in close proximity to and along the mating
tapered tip 21 of electrode 20, to a nozzle throat 32. From the
throat, the nozzle bore diverges at an angle substantially less
than the angle of convergence of the rearward portion to its exit
port indicated at 34.
Formed in and extending from the exterior of the nozzle or front
electrode 28, for communication with the nozzle bore, is a powder
entry conduit 36. The enlarged outer end of conduit 36 is in
communication with the inner end of a passage 38 formed in the
forward housing member 14. Passage 38 is connected to a
positive-feed powder hopper 40, preferably of the type described in
detail in U.S. Pat. No. 3,517,861 to R. P. De La Vega, of which the
description is incorporated herein by this reference, as though
fully set forth herein.
Schematically illustrated in FIG. 1 is a source of electrical power
42 having electrical connections via leads 44 and 46 to the rear
and forward housing members 12 and 14, respectively, and, through
these members, to the rear and forward electrodes. Preferably, lead
44 is connected to the negative side of the power source, wherefor
the rear electrode 20 is the cathode and the forward electrode 28
is the anode.
A source 48 of arc gas, such as argon for example, is coupled by
means of a feed conduit 50 to provide the arc gas under pressure to
the group of arc gas entry conduits 24, 26 et cetera, via an
annular space 27 formed between insulating ring 16 and sleeve 18.
In a preferred embodiment, 16 arc gas entry conduits are provided
equi-spaced around the periphery of the forward end of sleeve 18.
Each of these conduits extends through the sleeve 18 in a direction
that is nearly tangent to the outer periphery of rear electrode 20.
In addition to this tangential inclination, each of the conduits is
forwardly inclined, by an angle such as 15.degree. with respect to
a plane normal to the longitudinal axis of the apparatus. The
substantially full tangential inclination of the arc gas entry
conduits creates a maximum vortex pattern within the arc gas
chamber. This produces a highly stable arc and minimized electrode
wear to thus promote longer life of the apparatus. The forward rake
of the entry conduits enhances the flow of the arc gas to and
through the nozzle.
In its broad aspects, operation of the described electric arc
plasma spray gun is substantially the same as that of previous guns
although, as will be described in detail below, the present
invention provides for selection and control of a number of
essential and variable parameters which interact and must be
collectively considered and controlled for a particular material in
order to achieve the improved apparatus and methods of the present
invention. Fundamentally, however, when power is applied from
source 42 by means of a controlling switch (not shown), an electric
arc is produced from the forward tip 21 of the rear electrode 20 to
the forward electrode 28, impinging upon the converging internal
surface 30 of the nozzle bore. Gas is fed under pressure from gas
source 48 via entry conduits 24, 26 and others (not shown) where it
proceeds in a helical vortex forwardly around the rear electrode
20. The gas is ionized and heated in the electric arc as it flows
from the converging section of the nozzle to the throat section 32.
The heated ionized gas is now a body of plasma which flows to the
exit of the nozzle as an expanding and accelerating
high-temperature stream of plasma. Powder hopper 40 supplies a fine
powder, entrained in a powder gas, via conduits 38 and 36 to be
injected into the outflowing plasma stream. The powder particles
are entrained in the plasma and carried thereby to impinge upon a
substrate (not shown) that is to be coated by the particles.
It is known from studies by numerous investigators that an optimum
particle temperature range exists for specific materials. If this
temperature range is exceeded, the particle may melt, its fluidity
becomes too great, and spattering occurs when it impacts the
workpiece. On the other hand, if the temperature is too low,
insufficient deformation of the particle occurs upon impact,
wherefor poor bonding and coating quality result. Thus, it will be
seen that the only way by which individual particle energy can be
increased is by increasing the kinetic energy of the particle. By
increasing the velocity of the individual particles, kinetic
energy, which varies as the square of the velocity, can be
significantly increased. Increased total energy of the particle
results in greater deformation upon impact and results in coatings
having improved bonding, higher densities and greater
uniformity.
To achieve this higher kinetic energy, the above-described
apparatus attains a particularly specified range of plasma
velocities by employing a converging-diverging supersonic nozzle
together with an improved arc gas injection arrangement and matches
certain critical particle parameters to these supersonic plasma
stream parameters. These critical particle parameters include those
relating to particle size, material, heat characteristics and angle
and position of particle injection.
Many different and interacting factors must be considered,
identified and controlled if the optimum particle temperature is to
be obtained concomitantly with maximized particle velocity.
Furthermore, these factors will vary with different particle
materials. In the past, plasma spray guns have attempted to solve
problems involved in spraying of different materials by varying the
power input and arc gas flow rate. The above-described patent to
Unger et al. suggests increase of residence time by increasing
nozzle bore diameter to thereby decrease gas velocity. The
above-mentioned patent to Siebein et al. suggests a forward or
rearward inclination of the powder injection inlet for the spraying
of different materials. However, there is no previous suggestion of
either recognition or solution of the problems involved in
attaining supersonic plasma flow or the requirement for varying
both injection position and inclination in accordance with
materials to be injected in such a supersonic flow.
In accordance with illustrative embodiments of the present
invention, optimum conditions of temperature and maximized particle
velocity are achieved with a variety of different materials,
without varying electrical input power, temperature, arc chamber
pressure or arc gas flow rate, by providing a group of nozzles each
of which has an area ratio that produces a plasma stream exit
velocity between Mach 1 and Mach 3, preferably, at or about Mach 2.
Different nozzles are provided with powder injection passages all
communicating with the nozzle bore between the nozzle throat and
exit and each inclined at uniquely different angles and/or
intersecting the nozzle bore at uniquely different points
thereof.
Illustrated in an enlarged view in FIG. 2 is one nozzle 28a (also
showing the rear electrode and sleeve 18 of a set of nozzles
modified to enable use of the gun with different types of powder
materials. The front electrode of FIG. 2 includes a nozzle
particularly designed for materials of intermediate temperature
melting points (between 2000.degree. and 3000.degree.F.). Typical
of such materials are stainless steels, nickel, chromium and
hard-facing alloys such as nickel-chromium-tungsten-boron,
cobalt-chromium-molybdenum-boron and iron-nickel-chromium-boron.
This nozzle is designed to achieve exit velocity of Mach 2 into
ambient atmospheric pressure and has a throat diameter of 0.140
inches and an exit diameter of 0.190 inches for a specified power
of 50 kilowatts. This provides a required area ratio that
establishes the specified Mach number and a given ratio of ambient
pressure to pressure at the throat of the nozzle. The length of the
nozzle exit cone from the forward edge of the throat to the exit is
0.477 inches. In the specified example, the area ratio is 1.813 and
the pressure ratio is 0.1314. Where ambient pressure is 1
atmosphere this entails a pressure of 7.6 atmospheres in the arc
chamber and provides a gas exit speed of approximately 10,000 feet
per second.
These plasma stream conditions are achieved by a power input of
approximately 50 kilowatts, employing 900 amperes at 55 volts and
an arc gas flow from source 48 of 225 standard cubic feet per
hour.
As shown in FIG. 2, the particle injection conduit for these
conditions and for the above-identified group of materials of
intermediate melting temperatures is inclined downstream at an
angle of 10.degree. with respect to a normal to the direction of
plasma flow (axially of the nozzle bore). The axis of the particle
injection conduit 36a intersects the outer surface of nozzle 28a at
a point that is spaced 0.375 inches rearwardly of the nozzle exit.
This positions the particle injection conduit at a point on the
nozzle bore substantially midway between the nozzle exit and throat
but somewhat closer to the throat.
The same gun, having the same power and gas inputs, may be employed
for spraying of materials having substantially higher temperature
melting points (3000.degree.F. and higher), such as tungsten
carbide-cobalt, chromium carbide-nickel chromium blends, tungsten,
and molybdenum. For such application, the nozzle 28a of FIG. 2 is
replaced by the nozzle 28 (of FIG. 1) that is identical to the
nozzle 28a in all respects except for the angle and location of the
particle injection conduit. The nozzle 28 of FIG. 1 is the "high
temperature" nozzle of the exemplary set of three interchangeable
nozzle described herein. For such high-temperature materials, the
angle of inclination of the injection passage axis with respect to
a normal to the bore axis is decreased and is 5.degree.. The point
of entry of the injection conduit 26 into the nozzle bore is moved
slightly rearwardly. This may be achieved, for example, by
retaining the same location for the intersection of the particle
conduit axis and the external surface of the nozzle, whereby the
decreased angle will move the entry port closer to the nozzle
throat. By decreasing the angle of inclination of the particle
injection conduit, dwell time of the particles in the plasma stream
is increased at least in part because the particles have a smaller
initial component of velocity parallel to the direction of plasma
flow. Further, the relatively rearward positioning of the particle
injection conduit also increases the dwell time.
This positioning of particle entry achieves other more significant
results in the transfer of kinetic energy from the plasma stream to
the particles. In particular, since in this converging-diverging
supersonic nozzle the plasma stream has its highest density at or
near the nozzle throat, the closer the particle injection is to
such point of maximum plasma density, the greater is the heating
effect of the plasma upon the particles. With greater density of
plasma, there is a much greater frequency of collision between
powder particles and plasma, this collision frequency being the
essential mechanism by which kinetic energy is imparted to the
powder particles. Thus, the rearward positioning of the particle
entry port achieves increased acceleration of the particles and
increased rate of heating, whereby for materials of the indicated
high-temperature melting point the projected particles will have
maximized velocity and, at the same time, will be sufficiently
heated.
For another group of materials having a lower temperature melting
point (2000.degree.F. and less), such as bronze, copper and
aluminum, it is necessary to provide a shortened dwell time and to
provide injection at a much increased angle and at a point closer
to the nozzle exit if optimum velocity is to be achieved without
premature melting of these particles. For such low-temperature
melting point materials, there is provided a third type of nozzle
28b, as illustrated in FIG. 3. This nozzle is interchangeable with
each of the others and also is identical to each of the others in
all respects except for the angle and position of the particle
entry port 36b. Thus this nozzle assumes the same physical and
operative position with respect to the rear electrode 20 and sleeve
18, which are also shown in FIG. 3. As illustrated in FIG. 3,
particle entry conduit 36b is inclined at an angle of 36.degree.
with respect to a normal to the axis of the nozzle bore. This
increased inclination, of course, will move the intersection of the
entry end of the injection conduit with the nozzle bore much closer
to the nozzle exit. Nevertheless, it is found that if still greater
forward displacement of the point of particle entry is desired the
point of intersection of the injection conduit with the exterior of
the nozzle insert may also be moved somewhat forwardly with respect
to the similar point of the other nozzles. The short dwell time and
injection of particles adjacent the nozzle exit with this
low-temperature nozzle prevents premature melting of the particles
and plating buildup in the nozzle bore.
From the above description, it will be seen that the exemplary set
of nozzles enables a unique co-operative action between the
elongated and smoothly diverging bore of the converging-diverging
supersonic nozzle and the particle injection passage of selectively
variable inclination and position along the bore. Although but
three nozzles are included in the described set it will be readily
understood that more nozzles may be employed and formed with still
other choices of angle and position of particle entry conduit as
determined for optimum operation with particular powders.
Illustrated in FIG. 4 is a nozzle employing principles of the
invention as described herein and formed with the specific
parameters, dimensions and shapes described above. This nozzle has
certain modifications that enable the plasma spray stream to be
projected at an169 angle of substantially 45.degree. with respect
to the axis of the rear electrode. The 45.degree. spray nozzle 28c
of FIG. 4 is substantially similar in size and nozzle dimensions to
the nozzle described above, and includes a converging nozzle
portion 30c. The nozzle also includes a throat section 32c and a
diverging portion that terminates in a nozzle exit 34c, all of
which are identical in dimensions and operation with the
corresponding parts described in connection with FIGS. 1 through 3.
In this arrangement the nozzle is formed with an intermediate
nozzle portion 31c that is interposed between converging nozzle
section 30c and throat section 32c. This facilitates the angular
transition of the plasma flow and enables the axis of the projected
plasma stream to extend at the indicated 45.degree. angle.
A powder conduit 36c is positioned at an angle and at a location
between nozzle exit and nozzle throat exactly as specified in
connection with the above described nozzles of FIGS. 1 through 3
for high, low or intermediate temperature powders, as desired.
Since the axis of the diverging nozzle portion is at 45.degree.
with respect to the axis of the converging nozzle portion, the
nozzle is formed with a projecting shoulder 33c in which the powder
conduit 36c is formed. It will be readily appreciated that the
other portions of the spray gun are suitably modified so that the
powder conduit 38c will properly deliver powder to the passage 36c.
Therefore, a slightly different configuration of the spray gun
front housing member 14 is required for the angulated spray
nozzles.
For ejecting a plasma stream and entrained particles at a
90.degree. angle, the nozzle 28d of FIG. 5 is employed. This has a
configuration quite similar to that of FIG. 4 and includes a
converging nozzle portion 30d, an intermediate nozzle passage
portion 31d, a nozzle throat 32d and a nozzle diverging portion
that terminates in nozzle exit 34d. This nozzle, like the nozzle of
FIG. 4, includes a powder entry conduit 36d that is positioned to
intersect the diverging nozzle portion at a particularly chosen
point between the throat and exit and is directed at an angle with
respect to the direction of the motion of the plasma stream in
accordance with the particular critical parameters described
herein. Again, as in FIG. 4, the nozzle throat and diverging nozzle
portion are formed within a projecting shoulder 33d, in which
shoulder is also formed the powder entry conduit 36d. The angulated
nozzle of FIG. 5 like the angulated nozzle of FIG. 4, requires
modification of the spray gun itself in order to provide flow of
powder through the gun passage 38d and thence into the powder entry
conduit 36d.
Except for the described variations in angulation of the nozzle,
the nozzles of FIGS. 4 and 5 both employ all of the critical
dimensions and operating parameters described in connection with
the nozzles of FIGS. 1 through 3 and either of the nozzles of FIG.
4 or FIG. 5 may be employed with any one of the described powder
entry conduit angles and locations for spraying of low,
intermediate or high temperature melting point particles.
For each of the described nozzles, typical dimensions and angles in
addition to those identified above are as follows: The converging
portion of the nozzle surface 30 makes an angle of 10.degree. with
respect to the gun axis. The diverging portion of the nozzle,
between its throat and exit, has a considerably smaller degree of
angulation, such as 3.degree.. The distance between the forward tip
of the rear electrode 20 and the nozzle exit (FIGS. 1-3) is 1.0
inches. The length of the nozzle throat of unvarying diameter is
0.15 inches, and the diameter of the particle entry passage is
0.062 inches. Further, all particle sizes are less than 44
microns.
Shown in Table I are typical materials, particle sizes, powder
entry velocities and powder entraining gas flow rates that
exemplify, but in no way limit, the practice of this invention.
Each of the materials of Table I has been sprayed in the described
apparatus at plasma exit velocities of Mach 2, all employing 900
amperes of current at 55 volts, with an arc gas flow rate of 225
standard cubic feet per hour. The aluminum, bronze and copper have
been sprayed with the above-described low-temperature nozzle as
illustrated in FIG. 3. The stainless steel, nickel-chromium, and
chromium carbide-nickel chromium were projected with the
intermediate temperature nozzle of FIG. 2, and the other materials
of Table I were projected with the high-temperature (5.degree.
inclination) nozzle shown in FIG. 1. It will be seen that all of
the particles were injected in sizes considerably smaller than 44
microns. The aluminum and copper particles were analyzed as having
an average size of 9 microns.
TABLE I
__________________________________________________________________________
Material Composition Particle Anode Power Power Size Flow Gas
Microns Rate scfh (lb/hr)
__________________________________________________________________________
Aluminum 99.5% Al 44 Low Temp. 0.5 20 Aluminum Bronze 10% Al, 90%
Cu 44 Low Temp. 1.7 20 Copper 99.3% Cu 4 Low Temp. 1.5 20
Nickel-Chromium 80% Ni - 20% Cr 10 - 25 Int. Temp. 1.8 25 Chromium
Carbide- 75% Cr.sub.3 C.sub.2 15 - 25 Int. Temp. 1.3 20 Nickel
Chromium 25% NiCr Molybdenum 99.5% Mo 10 - 44 High Temp. 2.9 25
Stainless Steel (304) 18% Cr, 8% Ni, 44 Int. Temp. 1.4 20 balance
Fe Tungsten 99.5% 10 - 44 High Temp. 2.6 25 Tungsten Carbide- 88%
WC, 12% Co 10 - 25 High Temp. 1.6 25 Cobalt
__________________________________________________________________________
In Table I where but a single size (not a range) of particle size
is given, this identifies a maximum, with average size in all cases
being considerably less than 44 microns.
As described above, the choice of powder entry conduit location,
together with choice of powder entry conduit angle, are critical
parameters that must be matched with the particular material being
sprayed. Nevertheless, the values specified are the preferred
values within a range of such locations and angles that may be
employed in the practice of the invention. The following ranges of
angles and locations are given by way of examples for the
illustrated gun. In these ranges, the outer end of the powder entry
conduit is retained at the same position and the entry angles are
varied. Therefore, the entry locations are also caused to vary.
Although this is a most convenient way to manufacture a group of
nozzles with different entry angles and positions within the
diverging exit cone of the supersonic nozzle, it will be readily
appreciated that either the angle only may be changed while
retaining the same position of entry of the powder entry conduit
or, alternatively, the angle of entry may remain the same and the
position of the entry of the powder conduit into the nozzle exit
cone may be varied. Within such ranges of variations of angle and
location of powder entry conduit for the respective low,
intermediate and high temperature melting point particles, the
desired end conditions will be optimized, these conditions being
maximized heating of the particles without melting and minimized
velocity differential between the plasma and the particles.
The ranges are as follows. For anode 28, designed for high
(3000.degree.F. and higher) melting point materials, the powder is
normally injected at a forwardly inclined (downstream) angle of
5.degree. and enters the exit cone of the nozzle 0.129 inches from
the forward edge of the nozzle throat. To optimize the injection of
specific high melting point materials, the injection angle can vary
from -13.degree. upstream (inclined rearwardly), entering the
nozzle exit cone 0.032 inches from the nozzle throat, to
7.5.degree. downstream (forwardly inclined), entering the nozzle
exit cone 0.142 inches from the nozzle throat.
For anode 28a, designed for intermediate (2000.degree. to
3000.degree.F.) melting point materials, the powder is normally
injected at an angle of 10.degree. downstream and enters the exit
cone of the nozzle at 0.156 inches from the nozzle throat. To
optimize injection of specific intermediate melting point
materials, the injection angle can vary from 7.5.degree.
downstream, entering the nozzle exit cone at 0.142 inches from the
nozzle throat, to 25.degree. downstream, entering the nozzle exit
cone 0.244 inches from the nozzle throat.
For the anode 28b, designed for low (2000.degree.F. and less)
melting point materials, the powder is normally injected at an
angle of 36.degree. downstream and enters the nozzle exit cone
0.324 inches from the nozzle throat. To optimize the injection of
specific low melting point materials, the injection angle can vary
from 25.degree. downstream, entering the nozzle exit cone at 0.244
inches from the nozzle exit, to 49.degree. downstream, entering the
nozzle exit cone 0.453 inches from the nozzle throat.
The above-specified preferred values for Mach number, power, gas
flow and related nozzle dimensions have been chosen for practical
purposes, and it will be readily appreciated that these may be
varied without departing from the principles of the described
invention. Nevertheless, there are certain critical limits for the
several parameters as described above without which optimization of
the specified temperature and velocity of ejected particles cannot
be accomplished. Thus, with a plasma exit velocity of less than
Mach 1, desirable density, bonding and uniformity of particle
coatings cannot be achieved. On the other hand, to attain an exit
velocity of Mach 3 or greater requires too much power. Further, at
plasma velocities of Mach 3 and greater, the energy of the stream
is much too high.
It will be readily understood that pressure of the powder gas, that
is pressure in the powder hopper from which the injected particles
are drawn, is established at a value greater than the static
pressure within the supersonic nozzle at the point of powder
injection. In the supersonic nozzle, the gas stream pressure, and
also the static gas pressure at the periphery of the nozzle bore,
varies (in a non-linear fashion) from a maximum at the entrance to
the nozzle throat, to its lowest value at the nozzle exit. For the
exemplary guns described herein, the arc chamber pressure, which is
the gas pressure at the entrance to the nozzle throat, is 7.6
atmospheres, whereas the pressure at the nozzle exit is ambient
pressure, or 1 atmosphere. These nozzle pressures are higher than
those in the subsonic spray guns and, accordingly, require higher
powder gas hopper pressures.
Various factors are involved in optimizing particle and gas
entrance velocity and hopper pressure. In general, entrance
velocity of the powder gas need not be supersonic, nor even sonic.
High subsonic entrance velocities of the powder gas have been found
to be adequate. A primary constraint is that the momentum of the
particles entrained in the powder gas must be sufficient to allow
the particles to penetrate the boundary layer of the outflowing
plasma stream and enter into the stream toward the center thereof.
In addition to the static pressure of the outflowing plasma stream
at the point of entry of the particles, the angular velocity of the
plasma stream is also a factor. It will be recalled that the arc
gas enters the arc chamber tangentially, whereby the outflowing gas
follows a helical, or spiraling path, having its greatest angular
velocity at the nozzle throat. As the nozzle and gas expand, the
angular velocity component of the exiting plasma decreases. Note
that the relatively greater angular velocity component of the
plasma at or near the throat is yet another factor (in addition to
the above-described greater plasma density) that enhances transfer
of kinetic energy to particles injected closer to the throat.
For a given entry point within the nozzle bore, injected particles
of greater mass will have greater momentum for the same velocity,
as compared with particles of lesser mass so that comparatively
lower entrance velocities of powder gas may be employed for such
higher mass particles. In any event, entrance velocities of the
powder gas at, or approaching sonic velocity, (Mach one) are
preferred. It will be understood that supersonic entrance
velocities of the powder gas may be useful under certain conditions
and can be achieved by employing appropriate area ratios and
pressure ratios of a converging-diverging powder entry conduit.
With the exemplary high temperature particle nozzle, illustrated in
FIG. 1, wherein powder conduit entry angle is 5.degree., entering
the nozzle exit cone 0.129 inches downstream of the nozzle throat,
powder gas entry velocity at, or nearly at Mach 1, is achieved with
a hopper pressure of some 40 to 50 pounds per square inch. At this
particular point of entry within the illustrated nozzle, static
pressure of the plasma has decreased from its maximum, at the
throat, to a value of approximately 25 pounds per square inch. This
achieves a pressure ratio of approximately the required 1.8 to 2.0
for sonic or near sonic entry velocity of the powder gas. The
increased powder gas pressures within the powder hopper that are
required for the greatly increased plasma pressures through a major
portion of the nozzle bore are readily available with the
above-described hopper by increasing the pressure of the powder gas
that is fed to the hopper. In general, the pressure of such input
powder gas to the hopper is maintained at least equal to, but
preferably above, the pressure required to ensure the desired
entrance of the particles into the plasma stream.
As described in detail herein, a number of critical dimensions and
operating parameters are required for the practice of the present
invention. Nevertheless, the sizes and powers described for the
exemplary guns may be scaled to provide spray guns of different
sizes and powers, still achieving the improved results of the
present invention. These varying sizes may be accomplished by
suitable scaling of several of the parameters of the described
guns. To vary the gun size and accordingly to vary the output of
the gun, it is necessary to linearly scale certain of the gun
parameters, although other essential parameters such as Mach number
and temperature of the plasma stream will remain the same. Factors
that are linearly scaled, either upwardly or downwardly from those
described herein, are the area ratio, the nozzle volume (between
throat and exit), the mass flow rate of gas, the feed through flow
rate of powder and the power applied to the gun. For example, in
order to upwardly scale the described gun by a factor of 10,
assuming for this example that the power applied to the described
gun is 50 kilowatts, one would apply 500 kilowatts to the gun,
increase the area of both nozzle throat and nozzle exit by a factor
of 10, increase the nozzle volume between throat and nozzle exit by
a factor of 10, and also increase the mass flow rate both of the
gas and of powder by a factor of 10. Even with such increases in
this upward scaling, temperature and exit speed of the plasma and
entrained particles remain substantially the same. The critical
angles and locations of the particle entry passages also remain the
same.
Scaling upwardly by a factor of 100 requires the use of 5 megawatts
of power and results in the use of a nozzle having an exit diameter
of 1.9 inches. Thus, the exiting collimated stream of coating
particles would have a diameter of 1.9 inches in such upwardly
scaled gun, as compared to a similar diameter of 0.19 inches in the
gun illustrated in FIGS. 1-3.
Similarly, the illustrated gun may be scaled downwardly by linearly
changing the identified dimensions and parameters.
The desired Mach number and critical area ratio may be obtained
with nozzle cone half angles (angle of divergence of the diverging
nozzle portion) within the range of 3 to 18.degree.. Nevertheless,
the smallest cone half angle is preferred since this smaller angle
enables use of a greater length of diverging nozzle portion
(between exit and throat). The increased nozzle length provides for
a greater dwell time which is particularly useful for high
temperature melting point particles, and thus allows for a greater
energy exchange between the plasma stream and the entrained
particles.
With the above-identified nozzle parameters and Mach number there
is provided what is termed a "matched nozzle" which results in a
substantially collimated exit stream having a minimum of a
divergence after it leaves the nozzle. Thus, upward scaling of the
nozzle and spray gun and related parameters, as described above,
will achieve a larger area of particle coating stream. Similarly,
downward scaling will achieve a smaller area of particle stream
whereby the gun may be designed in accordance with the principles
of the present invention to provide the desired coating stream
area.
There have been described an improved apparatus and method for
supersonic spraying of fine particles employing a unique
combination of electrical power and supersonic nozzle, having
particularly selected and controlled particle size, entry angle and
location. The described invention accomplishes the spray coating of
particles of many different heat characteristics with the same
spray gun operated under the same parameters merely by selecting
one of a group of supersonic nozzles having individual and uniquely
oriented and positioned powder entry passages. Thus, plasma exit
velocities of from 5 to 50,000 feet per second are achieved
entraining optimumly heated particles having velocities of 1 to
10,000 feet per second.
DESCRIPTION OF FIGS. 6-11, INCLUSIVE
Referring next to FIG. 6, a torch body is illustrated to comprise
three annular body members 10e 11e and 12e which are mounted in
closely nested relationship relative to each other. The rear body
member, numbered 10e, is formed of a suitable insulating plastic
such as a phenolic. The intermediate body member 11e is also formed
of insulating plastic but preferably one which is much stronger,
such as a fiber glass-resin composition. Front body member 12e is
formed of metal, such as brass.
Each of the annular body members defines an opening therethrough,
and the three such openings combine to produce an opening or
passage through the torch. Into such torch opening or passage are
inserted the anode and cathode means, as described below.
The annular body members 10e-12e are maintained in assembled
relationship, despite the very high pressures contained therein, by
means of three bolts 13e-15e (FIGS. 6 and 7) which are oriented
longitudinally to the common axis of the body members and are
circumferentially spaced 120.degree. from each other. The bolts
extend interiorly of the body members, instead of through external
flanges or connectors, thereby greatly improving the compactness of
the torch.
The head of each bolt 13e-15e is recessed into a cylindrical cavity
in the forward surface of front body member 12e. The threaded rear
end of each bolt is threaded into a nut 16e (FIG. 6) which is
mounted in a recess in the rear member 10e. An insulating plug 17e
is cemented into the recess in member 10e to conceal the rear end
of each bolt and to prevent the operator from making electrical
contact therewith.
It is emphasized that all portions of the torch rearwardly of the
metallic front body member 12e are insulating, except for the
connection to the cathode means described below. This relationship
makes the present apparatus relatively safe to operate.
As shown in FIGS. 6 and 8, the body members 10e-12e receive in
snug-fitting relationship an anode means 18e and a cathode means
19e, the latter having a stick (rod) electrode portion 21e which
extends coaxially into an arc chamber 22e in the anode means.
A gas-injector ring 23e, formed of a heat-resistant insulating
ceramic such as boron nitride, aluminum oxide, zirconium oxide,
etc., is mounted between the adjacent or inner end portions of the
anode means and the cathode means and in radially-outwardly spaced
concentric relationship to the stick electrode portion 21e. More
specifically, the cylindrical inner surface of gas-injector ring
23e is flush with the cylindrical surface of the side wall of arc
chamber 22e, so that the gas-injector ring and the anode means
cooperate to define a gas-vortex chamber 22f around the stick
electrode portion.
Arc gas is introduced into such gas-vortex chamber 22f through a
multiplicity of small-diameter gas-inlet passages 24e (FIGS. 6, 8
and 10) which are drilled through the gas-injector ring 23e. The
illustrated passages 24e are, in the illustrated embodiment,
tangentially oriented relative to the gas-vortex chamber 22f and,
furthermore, incline somewhat forwardly relative to a vertical
plane which is perpendicular to the axis of the apparatus. In some
instances, however, other types of gas injection may be employed,
for example injection which is not adapted to effect vortical flow
in the chamber around the stick electrode.
It is an important advantage of the present torch that the manner
of arc gas injection may be readily changed, merely by substituting
one ring 23e for another. The ring 23e also produces other
important advantages, including (a) permitting the vortex chamber
22f to be small in diameter, and (b) effectively insulating the
anode and cathode from each other.
Gas-injector ring 23e has a rectangular cross section except at the
exterior surface thereof, which is provided with an annular groove
26e communicating with the various gas-inlet passages 24e. The ring
23e is seated in a recess or counterbore which is formed in the
forward side of intermediate body member 11e. The wall of such
recess or counterbore is undercut, at the region radiallyoutwardly
of groove 26e, to provide an annular manifold chamber 27e into
which arc gas is introduced through a passage 28e (FIG. 8). Passage
28e communicates with a recess in intermediate body 11e, and into
which a tube 29e is sealingly inserted (there being an O-ring 31e).
Tube 29e is soldered to a fitting 32e adapted to be connected to a
gas source which is schematically represented at 33e.
It is emphasized that, in accordance with the present apparatus and
method, a very large amount of power is "packaged" within a very
small space, with consequent enormous generation of heat. For
example, in a torch wherein the arc chamber 22e is only about
two-thirds of an inch in diameter, the power input may be between
80 kilowatts and 100 kilowatts. The heat-resistant gas-injector
ring 23e, particularly since it is spaced rearwardly from the
arcing tip of the electrode 21e, is able to withstand the resulting
extremely high temperatures. However, means are provided to cool
the seals which prevent escape of gas from manifold chamber 27e
except through the gas-inlet passages 24e.
The inner end of anode means 18e has a radial flange the rear
radial surface of which is abutted against the radial forward
surface of gas-injector ring 23e. The flange extends outwardly to a
cylindrical element 34e the exterior surface of which abuts an
O-ring 36e which is mounted in a groove formed in the interior wall
of intermediate body 11e. Furthermore, an undercut is formed
between the element 34e and the main body of the rear portion of
anode means 18e, into which water flows to thereby maintain O-ring
36e sufficiently cool that it will not deteriorate.
In similar manner, the forward end of the stick-holder or
slug-holder portion (described hereinafter) of cathode means 19e is
provided with a radial flange and with a cylindrical element 37e,
the latter contacting an O-ring 38e which is seated in the
intermediate body. The resulting undercut receives water which
maintains the O-ring 38e relatively cool. In addition, an O-ring
39e is mounted in the radial forward surface of the slug holder, in
contact with the radial rear wall of the gas-injector ring 23e,
being cooled by water present in the undercut in the cathode
means.
There will next be described the remainder of the cooling means
which maintain the anode and cathode means sufficiently cool that
they will not melt or deteriorate excessively, despite the great
heat which is generated by the electric arc. Water from a suitable
source 41e (FIG. 6) is caused to flow rapidly through a
large-diameter conduit 42e and thence into a right-angle fitting
43e the upper end of which is brazed into a recess in front body
12e. The fitting communicates with an annular groove 44e formed in
the front body 12e. From such groove 44e, the water is forced
rearwardly through a large number of saw cuts or notches 46e (FIG.
11) which are defined by teeth 47e extending outwardly from anode
means 18e at the region around arc chamber 22e. The teeth 47e are
in surface engagement with an interior cylindrical surface 48e of
the front body 12e, so that the water is not merely caused to flow
around the saw cuts or notches 46e but instead is forced rapidly
therethrough in highly efficient cooling relationship to the anode
means.
The interior surface 48e is formed on a neck portion of the front
body 12e, such neck portion extending rearwardly into a large
counterbore in intermediate body 11e. An O-ring 49e is provided to
prevent leakage of water out of such counterbore.
The rear end of the neck portion of front body 12e is spaced
forwardly from the opposed radial surface of intermediate body 11e
at the indicated counterbore, whereby to form an annular chamber
50e into which the water flows after leaving the saw cuts or
notches 46e. It is emphasized that the water upon entering the
chamber 50e impinges against the above-described cylindrical
element 34e to aid in cooling the O-ring 36e adjacent thereto.
From chamber 50e, the water flows rearwardly through a large number
of circumferentially-spaced longitudinal bores or passages 51e in
intermediate body 11e (FIG. 10), such bores having rear portions
which incline inwardly and rearwardly to a chamber 52e which is
defined in intermediate body 11e around cathode means 19e. The
chamber 52e communicates with a plurality (for example, six) of
passages 53e which are formed through the cathode means 19e and
which communicate with a central passage 54e therein and thus with
a fitting 56e leading to a suitable drain 57e.
An O-ring 58e is provided around the cathode means 19e to prevent
leakage of water from chamber 52e. An additional O-ring, numbered
59e, is formed around the front portion of the anode means 18e
between a cylindrical external surface thereof and an interior
cylindrical surface of the front body 12e, forwardly of annular
groove 44e. Heating of the last-mentioned O-ring 59e is prevented
by water present in an undercut region 60e of the anode means, such
region being located radially-inwardly of a cylindrical portion 61e
of the anode means and which abuts the O-ring 59e.
Anode means 18e is a single element made of copper, and which is
machined or otherwise formed to contain the various cooling
portions described above. The anode means also contains the arc
chamber 22e as described, which arc chamber 22e communicates
coaxially with a smaller-sized arc chamber or counterbore 62e
located forwardly of the rounded tip of the cathode stick or slug
21e. The forward regions of the arc chamber 22e and of the smaller
arc chamber 62e are generally rounded or spherical.
The smaller arc chamber 62e communicates coaxially with a nozzle
passage 63e having a cylindrical rear portion and a somewhat flared
or conical forward portion. The illustrated nozzle passage is of
the supersonic type. Nozzle passage 63e will not be described in
detail since counterparts thereof are set forth previously in this
specification.
The present spray torch is of the non-transferred arc variety,
wherein the entire arc is contained within the torch. Thus, a D.C.
power source 64e (FIG. 6) has the positive terminal thereof
connected to conduit 42e (which is formed of copper) to thereby
supply D.C. power of positive polarity to the fitting 43e and thus
to the front body 12e and to the anode means 18e in contact
therewith. The negative terminal of power source 64e is connected
to fitting 56e and thus to the cathode means 19e. An electric arc
is thus maintained between the tip of the cathode means and the
wall of the arc chamber 62e.
Because gas is introduced at high pressure from source 33e (FIG. 8)
through the passages described above, the gas pressure in chambers
22e and 62e is high (for example, 120 psi gauge when the electric
arc is present). This high pressure cooperates with the high
electric power contained in the torch, and with the characteristics
of nozzle passage 63e, in such manner that the flow through the
nozzle passage is caused to be supersonic, for example between Mach
1 and Mach 3 (preferably about Mach 2 when spraying is being
effected in the atmosphere, as distinguished from being effected in
a vacuum chamber).
As previously indicated, the cathode means 19e comprises (in
addition to the thoriated tungsten slug, stick or rod 21e) a
slug-holder or stick-holder 66e having a radial flange 67e. Such
flange is seated between the bottom of a recess in rear body 10e
and the rear end of a neck 68e on intermediate body 11e. The slug
holder 66e is preferably formed of copper.
Referring next to FIGS. 6, 7 and 11 in particular, the torch
further comprises handle means which are screwed directly to the
torch body by means of the screws 70e. The screws project into
inserts in the intermediate body 11e, which (being formed of fiber
glass) is very strong. Screws 70e project respectively through the
upper ends of first and second handle portions 71e and 72e which
are mirror images of each other and abut at the central plane of
the torch. Portions 71e and 72e are secured together by bolts 73e
and 74e shown in FIG. 6.
The handle portions 71e and 72e have grooves therein which
cooperate to form passages through which the above-described
conduit 42e passes, as does an additional tube or conduit 76e
adapted to supply spray powder to nozzle passage 63e as described
below. Both the conduit 42e and the conduit 76e extend upwardly
through the handle and then bend forwardly to a position in advance
of the handle, whereupon they bend upwardly into forward body 12e
as shown in FIG. 6.
Powder tube 76e is supplied by a source 75e (FIG. 6) with spray
powder entrained in gas. Such a source is shown in U.S. Pat. No.
3,517,861.
The upper end of powder conduit 76e projects slidably through a
corresponding radial bore in the lower region of front body member
12e. Furthermore, as best shown in FIG. 9, the extreme upper end of
the powder conduit or tube 76e is beveled at 78e to abut the
conical wall of a recess 79e formed in anode means 18e. A
cross-member or fitting 80e (FIG. 7) is rigidly secured to the
powder tube or conduit 76e (as by brazing), and is fastened by
screws 81e (FIG. 7) to the front body 12e.
Loosening of the screws 81e permits the operator to shift the upper
end of powder tube 76e downwardly and out of the recess 79e (FIG.
9), thereby permitting rotation of the anode means 18e as described
below. Correspondingly, tightening of the screws 81e forces the
beveled upper end of tube 76e into recess 79e and effects a seal
with the wall of such recess, so that all gas and powder which flow
upwardly through the tube 76e pass into the nozzle passage 63e.
Anode means 18e is provided with a plurality, for example eight in
the illustration, of such recesses 79e (FIG. 9), in
circumferentially-spaced relationship. Each recess 79e communicates
with a port or passage which extends inwardly to the nozzle passage
63e. The various ports or passages are numbered 82e-89e in FIG. 7,
and each has at least one characteristic different from that of all
the others. Thus, for example, the passages may have different
diameters, different inclinations, etc.
As an example, the passage 85e (FIG. 7) is shown as being
tangentially related to the nozzle passage, the relationship being
such that the powder is introduced in a clockwise manner as viewed
from the front of the torch. This is opposite to the direction of
introduction of arc gas through injector ring 23e, this being
counterclockwise as shown in FIG. 10.
Referring to FIG. 9, the passages 82f and 82g correspond to passage
82e except that they are inclined at different angles relative to a
plane which is perpendicular to the nozzle passage 63e and contains
the powder tube 76e. It is pointed out that the three passages 82e,
82f and 82g (FIG. 9) are not present in the same torch (there
preferably being only one port 82e, etc., which communicates with
tube 76e at any one time). The passages 82f and 82g, each of which
communicates with the throat of nozzle passage 63e, are illustrated
herein as alternative angles of powder introduction. Various angles
and types of powder introduction are described previously in this
specification.
To cause a selected one of passages 82e-89e to register with powder
conduit 76e, the upper end of such conduit is lowered by loosening
the screws 81e (FIG. 7) as described above. Thereafter, a front
retainer ring 92e (which normally locks the anode means 18e in
position) is removed from the front of the torch by removing
mounting screws 93e (FIG. 8) therefor. After the front ring is
removed, a threaded tool is inserted into an internallythreaded
bore 94e (FIG. 6) in the forward face of the anode means 18e.
The threaded relationship between the tool and the threads in bore
94e permits the operator to pull the entire anode means 18e
forwardly for a fraction of an inch, until there is no longer any
engagement with an indexing pin 96e (FIG. 6) which is permanently
and fixedly mounted in the forward face of front body 12e in
parallel relationship to the axis of the torch. The pin 96e is
selectively received in any one of eight circumferentially-spaced
bores 97e which are provided in a flange 98e in anode means 18e.
Each bore 97e corresponds in position to one of the conical
recesses 79e described relative to FIG. 9. The flange 98e seats in
a counterbore in the face of front body 12e.
Since the indexing pin 96e is no longer inserted in one of the
bores 97e after the anode means 18e is pulled forwardly as stated
above, the operator may rotate the anode in order to cause the
desired one of passages 82e-89e to register with the upper end of
powder conduit 76e. Thereafter, the anode 18e is pushed rearwardly
until indexing pin 96e is again inserted in one of the bores 97e,
following which the front ring 92e is mounted in position by means
of screws 93e, and following which the screws 81e (FIG. 7) are
tightened to elevate the powder tube 76e and effect a seal at bevel
78e (FIG. 9) as stated above.
The described rotation of the anode 18e permits a single anode to
have a much greater utility than in the prior art, so that various
types of powders, various settings of the torch, etc., may be
employed with a single anode as necessary in order to achieve
maximum spray rates.
DESCRIPTION OF FIG. 12
The method which will next be described, in connection with FIG.
12, relates to the discovery that the combination of supersonic
plasma flow, simultaneous plural-port injection of spray powder,
and very high arc power produces spray rates which are surprisingly
high. It is emphasized, however, that the arc power must be related
to the size of the torch, since the larger torches normally
generate arc powers much higher than do the smaller torches.
The preceding part of this specification discloses supersonic flow
but does not disclose simultaneous plural-port powder injection.
Such prior-art patents as 3,114,826; 3,183,337; and 3,197,605 teach
plural-port powder injection, but only at subsonic flows and
relatively low arc powers. Patents 3,179,782 and 3,246,114 purport
to teach supersonic flow, and appear ambiguous relative to whether
or not there are plural powder ports. It has now been ascertained
that if the arc power is extremely high compared to the size of the
torch, if plural-port powder injection is employed, and if
supersonic flow is employed, then the spray rates increase
extremely rapidly.
The method will be described in connection with the torch of FIGS.
6-11, which has (as mentioned above) an arc chamber diameter of
about two-thirds inch (chamber 22e). The torch has a nozzle passage
diameter of 0.234 inch at the smallest portion thereof, and a
nozzle passage length of 0.812 inch.
For a torch of the indicated size, the arc power is in excess of 50
kilowatts, and is preferably in the range of 80 kilowatts to 100
kilowatts. As one illustrative condition, the arc current is 900
amperes and the arc voltage 90 volts.
The pressure of the arc gas (for example, argon) introduced from
source 33e (FIG. 8) is in excess of 50 psi gauge, which produces in
arc chamber 22e a gas pressure of 120 psi gauge after the arc is
initiated. The combination of the high gas pressure and the high
arc power combine with the characteristics of the supersonic nozzle
passage 63e to create a supersonic flow through the nozzle and out
the torch. The velocity of the plasma emanating from the torch may
be about 10,000 feet per second when spraying occurs in the
atmosphere. Stated otherwise, the plasma emanating from the torch
is in the range of Mach 1 to Mach 3, being preferably about Mach 2
(when spraying is in the atmosphere as distinguished from in a
vacuum chamber).
By plural-port powder injection it is meant that separate powder
sources are employed simultaneously to feed powder and gas through
separate conduits to the nozzle passage 63e, as described below
relative to FIG. 12. It is not preferred to use a single powder
source which feeds powder and gas to a plurality of ports.
Referring to FIG. 12, there is schematically represented a powder
source 101e and a powder source 102e which are connected,
respectively, to the torch by means of powder tubes 103e and 104e.
Such sources 101e and 102e combine with powder from the first
source 75e and which is connected through the tube 76e as described
relative to FIGS. 6 and 9.
The powder tubes 103e, 104e and 76e communicate, respectively, with
powder ports 106e, 107e and 82f which are provided in the anode
means 18f.
In the present example, all of the ports 106e, 107e and 82h
correspond to each other in diameter, angle, etc., and all are
constructed as shown in FIG. 9 relative to port 82e. It is to be
understood, however, that different longitudinal or circumferential
positionings may be employed, as well as different angular
relationships, etc.
In the present example, and with the torch of the size described
above, each port 106e, 107e and 82h has a diameter (for example) of
one-sixteenth inch. The rate of gas flow from each source 101e,
102e and 75e is 50 scfh. The spray coating is thus deposited on a
substrate (not shown) at a rate of, for example, 40 pounds per
hour.
Except as specifically stated above, the torch of FIG. 12 is
identical to the one previously described in detail relative to
FIGS. 6-11, inclusive.
The foregoing detailed description is to be clearly understood as
given by way of illustration and example only, the spirit and scope
of this invention being limited solely by the appended claims.
* * * * *