rrent page or check for previous versions at the Internet Archive. Yahoo! is not affiliated with the authors of this page or responsible for its content.
Electromagnetic Powder Deposition Experiments Abstract The Department of Defense (DoD) and commercial
entities are dependent on chemical plating and coating processes
to replace worn or eroded material on damaged parts. Logistics
Centers have been forced to consider replacement materials for
repair operations due to the tightening of government regula-
tions on the use of toxic and hazardous materials.
This paper describes a new process capable of fulfilling
many of these requirements. Existing state of the art thermal
spray processes (HVOF, D-gun, plasma spray) are limited to
powder velocities of about 1 km/s because they rely on the ther-
modynamic expansion of gases. A new thermal spray process
using electromagnetic forces can accelerate powder particles to a
final velocity in excess of 2 km/s. At this velocity, powder parti-
cles have sufficient kinetic energy to melt their own mass and an
equivalent substrate mass on impact. The energetics of the
process allow fusion bonding of greater strength than that creat-
ed by low velocity processes as well as improved coating density.
This paper will describe the laboratory system designed and
constructed to conduct proof of principle experiments. Results
of the experiments will be presented along with high speed pho-
tographs of powder particles confirming system modeling and
performance. The paper will conclude with a discussion of the
future direction of the program.
I
NTRODUCTION
The Electromagnetic Powder Deposition (EPD) process
was developed at The University of Texas at Austin Center for
Electromechanics (UT-CEM) as a method of imparting high
velocities to powder particles. Fig. 1 compares several ther-
mal spray processes in terms of particle velocity and gas tem-
perature and show how EPD is predicted to perform. The data
for the other processes was obtained from a paper describing
a Cold Gas-Dynamic Method.[1] What is unique to the EPD
approach is that the use of electromagnetic railgun force
means that the gas flow velocity can be as high as desired and
is not limited by any chemical or thermodynamic constraints.
The railgun process is combined with a gas-dynamic mecha-
nism called a snowplow [2] to produce controllable bursts of
gas with the speed and duration required to accelerate finite
segments of dispersed powder to the conditions required for
plating purposes. The process has promise of greatly improv-
ing bond strength and coating density if empirical scaling
trends continue with increasing velocity.
The railgun is filled with an ionizable gas, and a radio fre-
quency (rf) excited cavity at the breech of the accelerator pro-
vides a line source of plasma. A high energy electrical pulse,
provided by a pulsed energy source, expands the line source
into a planar arc which is driven forward by electromagnetic
forces. The arc is an efficient snowplow sweeping the gas in
the bore to a final velocity approximately twice desired pow-
der velocity. The gun length and current pulse is tailored such
that the plasma arc is extinguished as the gas column reaches
the powder ports into the railgun bore. The hot plasma does
not interact with the powder or the substrate surface. The
shocked gas passes over a powder cloud introduced near the
end of the gun and accelerates the powder through drag
forces. The electrical and powder discharge frequency can be
Electromagnetic Powder Deposition Experiments
R.C. Zowarka, J.R. Uglum, J.L. Bacon, M.D. Driga,
R.L. Sledge, and D.G. Davis
Center for Electromechanics, The University of Texas at Austin
5201.0141
Gas Temperature (C
°
)
10,000
Particle Velocity (m/s)
0
7,500
5,000
2,500
0
500
1,00
1,500
2,000
WIRE ARC
PLASMA ARC
HVOF, DGUN
EPD
POWDER FLAME
WIRE FLAME
Fig. 1. A comparison of particle velocity and gas tempera-
ture for several thermal spray processes.
Manuscript received May 1, 1998.
R. Zowarka may be contacted at 512-471-4496, fax 512-471-0781,
r.zowarka@mail.utexas.edu.
This research was sponsored by the United States Air Force, Tinker
Air Force Base at Oklahoma City, through a contract administered by
ARINC Corporation. adjusted so that the deposition rate and thermal input to the
substrate can be controlled.
The new process uses a regenerating line source of plas-
ma at the breech of the gun as opposed to a wire or a fuse to
start the arc. This allows the process to be rep ratable and
requires no wire feed or fuse loader. Powder in this process is
introduced continuously at the muzzle of the gun at low veloc-
ity and only accelerated powder can reach the target.
The process has the ability to control the thermal input to
the substrate because it is pulse-driven as opposed to continu-
ous. The process of forming and accelerating the snowplow
arc can be unstable. A special current pulse to drive the
process has been specified to avoid plasma instabilities. The
process has been designed to operate at atmospheric pressure
for ease of use and cost savings. A special rf excitation source
and arc generation cavity have been designed to allow the ini-
tiation plasma to form at atmospheric pressure. A special
manifold has been designed for the end of the gun to keep the
substrate flooded with inert gas thereby preventing target oxi-
dation. Because the process uses a snowplowed gas column
to accelerate the powder with drag forces, conducting and
nonconducting powders may be sprayed.
This process can be used to build up material of parent
material strength because it has the potential to create a fusion
bond with the substrate. It can build up the material with less
heat input than a welding process therefore mitigating sub-
strate warpage. The more energetic impact will create denser
coatings. It can be used to apply chrome to substrates there-
fore avoiding the generation of environmentally hazardous
hexavalent chrome, a byproduct of electroplating. Due to the
improved bond strength, material build-up may be possible,
allowing the formation of macro structures.
A
CCELERATOR
D
ESCRIPTION
The EPD spayer, a railgun, consists of two metallic rails
with insulating sidewalls separating them. The bore is filled
with an ionizable gas, and an radio frequency (rf) excited cav-
ity at the breech of the accelerator provides a line source of
plasma.[3] A high energy electrical pulse, provided by a
pulsed energy source, expands the line source into a planar arc
which is driven forward by electromagnetic forces. Fig. 2
shows a simple railgun and the electrical currents and mag-
netic fields which interact to create the Lorentz force (the
cross product of the current density vector and the magnetic
field vector). The arc is an efficient snowplow sweeping the
gas in the bore to a final velocity approximately twice the
desired powder velocity. This shocked gas passes over a pow-
der cloud introduced near the end of the gun and accelerates
the powder through drag forces. The electrical and powder
discharge frequency can be adjusted so that the deposition rate
and thermal input to the substrate can be controlled.
The proposed technical program evolved from considera-
tions of the snowplow mechanism driven by electromagnetic
Lorentz railgun forces. It can be shown [4] that current
requirements are related to gas velocity by the relation
where the current I is in kA and the gas velocity V
gas
is in
km/s. The other quantities entering this equation are : gas = the relative density of ambient gas being snow-
plow accelerated to velocity V
gas
, with unit
relative density corresponding to air at STP.
A = the cross-sectional area, in cm
2
, of the railgun
structure used to generate Lorentz force
L = the inductance per unit length, in µH/m, of the
railgun structure
For example, using argon gas at STP ( = 1.389 times air) in
a 1.6 cm
2
gun structure with inductive gradient 0.5 µH/m, to
achieve a velocity of 4 km/s requires 135 kA driving current.
The electrical pulse width required is dependent on the
fraction f of gas velocity to which the powder particle is to be
accelerated. A practical value is 50% (f = 0.5). The pulse
length is then given by the relation
where in addition to the previously defined quantities we also
have t =the electrical pulse duration in microseconds powder =the powder density relative to air at STP
Dpowder =the effective diameter of the powder particles
in microns t
D
V
f
f
powder
gas
powder
gas
=





4
3
1
I
V
A
L
gas
gas
=
16 ©
Compressed Gas
Plasma Arc
Removed
Transparent
Sidewall
B
F, v
5201.0133
Parallel Conducting Rails
Insulating Sidewall
Fig. 2.
The plasma armature is accelerated down the length
of the railgun by an electromagnetic Lorentz force
generated by the interaction of the magnetic fields
surrounding the rails and the current flow