Note: Descriptions are shown in the official language in which they were submitted.
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Jeff L. Dulaney
Allan H. Clauer
David W. Sokol
LASER PEENING PROCESS AND APPARATUS WITH
REDUCTION OF DIELECTRIC BREAKDOWN TO INCREASE PEAK PRESSURE PULSE
BACKGROUND OF THE INVENTION
1. Field of tile invention .
The present s.nvention relates to the use of coherent energy
pulses, as from high power pulsed lasers, 1x1 the shock processing
of solid materials, and, more particu:Larly, to methods and
apparatus for reducing dielectric breakdown during operation.
a_0 The invention is especially useful for enhancing or creating
desired physical properties such as hardness, strength, and
fatigue strength.
2. Description of the related art.
Known methods for the shock processing of solid materials
typically involve the use of high explosive materials in conta<~t
with the workpiece. High explosive materials or high pressure
gases are used to accelerate a plate that strikes the solid to
produce shock waves therein. Shot peeving is another widely
known and accepted process for improving tlxe fatigue, hardness,
and corrosion resistance properties of materials by impact
treatment of their surfaces.
Shoc)c processing with coherent radiation has several
advantages over what has been done before. For example, the
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source of the radiation is highly controllable and reproducible.
The radiation is easily focused on preselected surface areas and
the operating mode is easily changed. This allows flexibility in
the desired shocking pressure and careful control over the
workpiece area to be shocked. Workpieces immersed in hostile
environments, such as high temperature and high vacuum can be
shock processed. Additionally, it is easy to shock the workpiece
repetitively.
Laser peening (sometimes referred to as laser shock
processing) utilizes two overlays: a transparent overlay (usually
water) and an opaque overlay (usually an oil-based or acrylic-
based black paint). During processing, a laser beam is directed
to pass through the water overlay and is absorbed by the opaque
overlay, causing a rapid vaporization of the this overlay and the
generation of a high-amplitude shock wave. The shock wave cold-
works the surface of the part and creates compressive residual
stresses, which provide an increase in fatigue properties of the
part. A workpiece is typically processed by processing a matrix
of overlapping spots that cover the fatigue critical zone of the
part.
A problem, in utilizing transparent overlays, is that
dielectric breakdown of the overlay may occur during use. The
term dielectric breakdown, as used in this application, is the
laser-induced ionization of a transparent overlay and subsequent
formation of a plasma. It occurs during laser peening, when the
laser intensity incident on the transparent overlay causes
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ionization of the transparent overlay which subsequently produces
an avalanche of free electrons. As a result, a plasma forms that
absorbs incoming laser light, and thereby reduces the amount of
laser energy that can reach the opaque overlay. This
significantly reduces the peak pressure and duration of the shock
wave incident on the workpiece that is being processed.
In the case of a linear polarized laser field, experimental
data show that the dielectric breakdown is a probabilistic event
that depends on the rrns laser field, E, through the simple
relation P = exp (-K/E), where K is a proportionality constant.
The net result of the dielectric breakdown is that the laser
energy is not efficiently coupled into the sample surface of the
workpiece. Dielectric breakdown may occur in localized sites
within the dielectric material (transparent overlay), resulting
in a number of small ionized spots that are randomly scattered
throughout the overlay. At these spots, a localized plasma forms
which absorbs energy from the process area.
What is needed in the art is a laser shock process that is
highly repeatable without irregularities and reduction in the
applied pressure.
SUMMARY OF THE INVENTION
The present invention provides a method of laser shock
processing that can be used in a production environment that
significantly increases the peak pressure of the shock wave
applied to the workpiece. The method includes the steps of
applying a transparent overlay such as water over the workpiece
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and reducing or limiting the thickness of the transparent overlay
material. An alternate embodiment of the invention to reduce
dielectric breakdown incorporates the use of a circularly
polarized, elliptical polarized, varying polarized laser beam or
mixtures thereof, as opposed to a linearly polarized laser beam.
The invention comprises, in one form thereof, a method of
improving properties of a solid material by providing shock waves
therein, including the steps of applying a transparent overlay
material to the solid material; controlling the thickness of said
overlay; and directing a pulse of coherent energy to the solid
material through the transparent overlay material to create a
shock wave.
The invention comprises, in another form thereof, a method
of improving properties of a solid material by providing shock
waves therein, including the steps of applying a transparent
overlay material to the solid material; providing a pulse of
coherent energy with constantly changing polarization; and
directing the pulse of coherent energy to the solid material
through the transparent overlay material to create a shock wave.
The invention comprises, in yet another form thereof, an
apparatus for improving properties of a workpiece by providing
shock waves therein. The apparatus includes a transparent
overlay applicator for applying a transparent overlay to the
workpiece. The applicator can develop and maintain an overlay
layer of approximately between 0.01 mm to 3.0 mm thick. A high
intensity laser is operatively associated with said transparent
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overlay applicator to provide a laser beam through the liquid
transparent overlay to create a shock wave on the workpiece.
An advantage of the present invention is that the method
allows an even penetration of the laser beam through the
transparent overlay.
Yet another advantage of the present invention is that by
reducing the probability of dielectric breakdown in the
transparent overlay, a more uniformly shocked piece results.
A further advantage of the present invention is the
utilization of a flowing, transparent liquid overlay which
permits a thinner overlay to be created on the workpiece.
Yet a further advantage of the present invention is that
greater laser energies can be applied more efficiently to the
opaque overlay. This will result in higher shock pressures and
greater working of the workpiece.
BRIEF DESCRIPTION OF THE DRAWINGS
The above-mentioned and other features and advantages of
this invention, and the manner of attaining them, will become
more apparent and the invention will be better understood by
reference to the following description of embodiments of the
invention taken in conjunction with the accompanying drawings,
wherein:
Fig. 1A is an enlarged side view of a workpiece being
processed in the laser peening process of the present invention,
showing in an exaggerated view a number of dielectric breakdowns
occurring in the transparent overlay. The dielectric breakdowns
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are shown casting exaggerated shadows upon the opaque overlay and
workpiece;
Fig. 1B is a plan view of the processed section of the
workpiece of Fig. 1A, showing the processed area along with
unprocessed areas caused by dielectric breakdowns;
Fig. 2A is a graph showing how the onset of dielectric
breakdown can effect the laser beam intensity on the workpiece
surface during a laser pulse;
Fig. 2B is a graph of effects of the changed laser pulse on
the pressure pulse;
Fig. 2C is a graph of an example of the trend of the amount
of dielectric breakdown versus time;
Fig. 3A is a graph of experimental data collected depicting
the depth and amount of residual stress created in test
workpieces compared to the thickness (T) of the transparent
overlay;
Fig. 3B is a graph of experimental data collected depicting
the depth and amount of residual stress created in test
workpieces compared to the polarization of the laser beam
utilized;
Fig. 4 is a diagrammatical view of the polarization of a
laser beam;
Fig. 5 is a diagrammatic view of the polarization of the
laser, beam of the present invention, indicating rotation thereof;
and
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Fig. 6 is a black diagram of the laser pracessing system of
one form of the present invention.
Corresponding reference characters indicate corresponding
parts throughout the several views. The exemplification set out
herein illustrate one preferred embodiment of the invention, in
one form, and such exemplifications are not to be construed as
limiting the scope of the invention in any manner.
DETAILED DESCRIPTION OF THE INVENTION
The improvements in fatigue life produced by laser shoc)c
:LO processing are the results of residual compressive stresses
developed in the irradiated surface retarding fatigue crack
initiation and/or slowing the crack propagation rate. A crack
front is the leading edge of a crack as it propagates through a
solid material. Changes in the shape of a crack front and
slowing of the crack growth rate when the crack front encounters
the laser shocked zone in a laser shock processing condition have
been shown. Laser shock processing is an effective method of
increasing fatigue life in metals by treating fatigue critical
regions. The greater and deeper the residual stresses are
created in the workpiece, the greater the effect.
Far a more thorough background in the prior history of laser
shoclc processing and that of high power processing of engineered
materials, reference can be made to U.S. Patent No. 5,131,957. This
patent also shows a type of laser and laser configuration adaptable
for use with the present invention. Another type of laser adaptable
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for use with the present invention is that of a Nd-Glass Laser
manufactured by LSP Technologies of Dublin, Ohio.
Overlays are applied to the surface of the target workpiece
being laser shock processed. These overlay materials may be of
two types, one transparent to laser radiation and the other
opaque to laser radiation. They may be used either alone or in
combination with each other, but it is preferred that they be
used in combination with the opaque overlay adjacent to the
workpiece, and the outer transparent overlay being adjacent to
the opaque overlay.
Where used, the opaque overlay material may be strongly
absorbing to the radiation. Useful opaque overlay materials
include black paint, pentaerythritol tetranitrate (PETN);
bismuth, aluminum, iron, lead, cadmium, tin, zinc, graphite; and
mixtures of charcoal or carbon black with various transparent
materials such as mixtures of nitrocellulose and potassium
perchlorate or potassium nitrate. Optionally, a layer of another
solid overlay material may be attached to the layer of
substantially opaque material.
The transparent overlay material should be substantially
non-absorbing and/or transparent to the radiation. Useful
transparent overlay materials include water, water-based
solutions, other noncorrosive liquids, glass, quartz, sodium
silicate, fused silica, potassium chloride, sodium chloride,
polyethylene, fluoroplastics, nitrocellulose, and mixtures
thereof. Fluoroplastics, as they are known by ASTM nomenclature,
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are parallinic hydrocarbon polymers in which all or part of each
hydrogen atom has been replaced with a fluorine atom. Another
halogen, chlorine, can also be part of the structure of a
fluoroplastic. By order of decreasing fluorine substitution and
increasing processability, these materials include
polytetrafluoroethylene (PTFE); fluorinated ethylenepropylene
(FEP): the chlorotrifluorethylenes (CTFE); and polyvinylidine
fluoride (PVF2). Also available is a variety of copolymers of
both halogenated and fluorinated hydrocarbons, including
fluorinated elastomers. Further, mineral oils or other
hydrocarbon based fluids may be utilized. Additionally, the
transparent overlay could be a gel comprised of one or more of
the above materials or others.
The term "transparent" in this application is defined as
meaning non-absorbing or pervious to the laser beam utilized, not
automatically or necessarily non-absorbing to visible light. The
transparent overlay material should be substantially transparent
to the radiation as discussed above, water being the preferred
overlay material.
One physical interpretation of the dielectric breakdown
effect for a linearly polarized laser beam is known as the "lucky
electron" model. Here "lucky electrons" are those that undergo
favorable elastic collisions that reverse their momentums when
the laser field reverses. This allows the electrons to gain
energy from the alternating light field. When the collision rate
is slower than the frequency of the laser light, then the
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probability of ionization is law. On the other hand, if it is
greater than the :Light frequency, the probability of ionization.
increases.
Fig. 1A illustrates the current problem. In this
description, worlcpiece 20 has a first opaque layer 22 with a
layer of transparent overlay material 21 covering a portion
thereof. Dielectric breakdown events :L7 occur in laser beam path
16 and reduce tl~e laser beam intensity directly under the
breakdown by refracting, reflecting, or absorbing the laser beam
LO (reference arrows 15} out of the area in which the laser beam
would normally impact. Such action results in under-processed or
unprocessed areas 18 on the surface of workpiece 20 within
processed area 19 (Figs. lA and 1B}.
The present invention controls particular variables during
laser shock processing which permit the laser system to create a
larger peak pressure as shown graphically in Figure 2A which cold-
works the surface of workpiece 20 and directly creates the results
and intended effects in the warkpiece. This peak pressure is
increased when there is a reduction in the amount of dielectric:
breakdown, thereby permitting larger amounts of laser energy to be
applied to opaque layer 22 and workpiece 20.
Prior conventional thicknesses of the transparent overlay
are that of between approximately 3.0 mm. to 5 mm. of water.
This was utilized in laser shock processing because there is a
requirement to apply the developed pressure pulse as long as
possible to the workpiece. The laser pulse duration can be
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measured and the length of time the pressure pulse travels
through the transparent overlay can be calculated. Therefore,
conventional thinking required that the transparent overlay be
thick enough to maintain the pressure pulse against the workpiece
much longer than the laser pulse. This necessitated a thick
transparent overlay to maintain and cover or "hold" such pressure
pulse to the workpiece for as long a time as possible. Such
usage was though conventional.
The inventors of present invention have discovered that the
time-length of the pressure pulse is not the only limiting factor
in laser peening. Their discovery now teaches that applying the
highest peak pressure pulse to the workpiece is as significant or
more significant than providing that the duration of maintaining
the pressure pulse against the workpiece is as long as possible.
That is of course providing that the duration of maintaining the
pressure against the workpiece surface is at least equal to the
length of the laser pulse.
As shown in Fig. 3A, these conventional thicknesses (i.e., 3
mm and 5 mm) of transparent overlay result in peak pressure
pulses that created residual compressive stresses depicted via
the dotted lines. In one form of the invention, utilizing an
unconventional thickness of transparent overlay, the residual
compressive stress level within the workpiece was increased at
each identified depth level in the workpiece. In the sample
shown, an approximately 1.0 mm thickness of the transparent
overlay (in this case water) increased the residual compressive
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stress level at all depths as shown by the solid line of the
graph. It is theorized that reducing the thickness of the
transparent overlay reduces the probability of dielectric
breakdown thereby permitting more energy to couple to the opaque
layer and workpiece. Also, in the system of the present
invention, the applied peak pressure created trends upward as
laser power density is increased.
Previous conventional thought on the thickness of the
transparent overlay layer was that one needed to have it thicker
to keep the pressure waves, as shown in Fig. 2B, controllable at
a high pressure level for as long as possible against the surface
of the workpiece (workpiece 20). Conventional thought was that
by making the transparent overlay thicker it would achieve the
highest and longest total pressure.
The present invention utilizes the transparent overlay of
approximately 0.01 to 3.0 mm of water. Additionally, by causing
the transparent overlay to flow over the workpiece 20, another
way to control the overlay thickness has been developed. The
overall improvement is that of creating a higher peak pressure
pulse and greater residual stress levels by utilizing a thin
transparent overlay as opposed to a thick overlay.
It is desirable to have the transparent overlay thickness be
as thin as possible to decrease the volume of transparent overlay
exposed to the laser beam, and thereby decrease the probability
of dielectric breakdown. However, the thickness of the
transparent overlay can influence the length of the pressure
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pulse if the overlay is very thin. That is beCat~se the duration
of confinement of the plasma by the transparent overlay is
sustained only until the shock wave traveling back out through
the transparent overlay reaches the outside surface of the
overlay. At this point, the transparent overlay is "blown off"
and the plasma expands freely away from the target surface,
releasing the pressure.
This consideration limits how thin the overlay can be,
depending on the minimum length of the pressure pulse desired.
If the transparent overlay is very thick, the pressure pulse wil7_
naturally decay with increasing time. The pressure increases and
is sustained during the time the laser beam is applied. After
true laser pulse is turned off, the pressure decays due to cooling
of the plasma by thermal conduction into the target surface and
by continued adiabatic expansion of the plasma due to the
movement of the target and overlay surfaces away from each other
under the action of the plasma pressure. As can be understood from
Figure 3B, at some point, the surface pressure decreases below the
dynamic yield strength of the target material and it is no longer
effective in imparting plastic strain into the material for further
development of residual compressive stress. This point is presumed
to be no more than three to five times the length of the laser
pulse, but may be less. If the maximum effect of a selected laser
pulse was desired, then the transparent overlay thickness should be
nominally no less than that given by the simple calculation of:
sound speed in the transparent overlay multiplied by three times
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the laser pulse length. For a water overlay and a laser pulse
length of 25 nanoseconds, this calculation would be: (1.5 x 106
mm/s)'x (3 x 25 x 10-9 s) - 0.11 mm thick water overlay minimum.
However, there are possible advantages in using overlays
thinner than that provided by the above calculation. Thinner
overlays will further reduce the probability of dielectric
breakdown and will also act to limit the length of the pressure
pulse, even if the laser pulse length is longer. In this case,
the transparent overlay can be made as thin as required to reach
the length of pressure pulse desired. For example, if a 25
nanosecond laser pulse is being used, but a 10 nanosecond
pressure pulse is wanted, and the speed of sound in the
transparent overlay material is 2 x 106 mm/s, then the thickness
of the transparent overlay would be: (2 x 106 mm/s) x (10 x 10-9
s) - 0.02 mm.
Controlling the thickness (T) of the transparent overlay is
accomplished in a number of ways. Accurate control and placement
of workpiece 20 within a pool of transparent overlay would
control the effective thickness (T) of the overlay relative to
laser beam 16. Control of fluid injectors or valves bringing
liquid transparent overlay material to workpiece 20 within the
processing chamber (not shown) would also permit control of
overlay thickness. One other way to control the thickness (T) of
transparent overlay would be to add controlled quantities of
surfactants, liquefiers, or transport agents so for a known
amount of transparent overlay on a part, the overlay
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characteristics of surface tension and other physical parameters
may be controlled to ensure accurate control of overlay
thickness. Other embodiments of control of transparent overlay
thickness may comprise transparent paint or transparent tape.
The transparent overlays to minimize dielectric breakdown
and control the length of the laser beam might also be
accomplished using transparent tape of solid polymeric or mineral
materials with or without an adhesive backing.
Another way to control the thickness of transparent overlay
21 would be to constantly flow such overlay material over the
target area on workpiece 20. Such action would reduce surface
tension effects (for liquids). Controlled application of the
volume and velocity of the applied transparent overlay material
via valves, injectors, solenoids, etc., would permit control and
of the operational thickness (T) of transparent overlay.
Another embodiment of the invention, to control dielectric
breakdown, is that of controlling the polarization of the laser
light. Fig. 4 shows a diagrammatic representation of the
polarization of a cross-section of a typical laser beam. The
present invention creates a means and device to cause the
polarization of the beam to change or vary, and in a preferred
embodiment to rotate. Since the dielectric breakdown of the
material is a probabilistic effect, the inventors have determined
it is more probable if one utilizes a linearly polarized beam.
If a laser beam with changing, varying, elliptical or circular
polarization is used, the probability of such ionization will
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decrease. As used in this application, for ease of description,
elliptical or circular polarization of the laser beam is defined
to include the following types of polarization: elliptical,
circular, time varying, time changing, and mixtures thereof.
Such definition does not include pure linearly polarization of
the laser beam. In this case, the electrons (from the lucky
electron model) will not only have to reverse their direction
after a collision, but also have to change directions to remain
in phase with the electric field. Creation of the intentionally
circularly or elliptically polarized laser beam thus reduces the
probability of the breakdown in the dielectric, as compared to
the linearly polarized case. The present embodiment, then
changes the way of entering energy into the transparent overlay
in such a manner that the overlay cannot capture the energy, and
therefor dielectric breakdown is reduced.
One method of forming a elliptically or circularly polarized
laser beam is shown in the depiction of the laser system of Fig.
6. Shown there, a laser source 30 creates a pulsed laser beam 32
which impacts a mirror 34. The laser beam reflects from mirror
34 and travels to a laser pre-amp 36. Pre-amp 36 operates to
increase the amplitude of the laser beam previously created. The
amplified laser beam 38 emerging from pre-amp 36 enters into a
device 40 for changing, in the preferred embodiment of constant
rotation, the polarization of the laser beam 38. One form of
changing the polarization of laser beam 38 is to utilize a 1/4
waveplate as device 40.
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It is at this point where the linearly polarized laser beam,
as shown in Fig. 4, is changed into a elliptically or circularly
rotating laser beam as in Fig. 5. The resultant, elliptically or
circularly polarized laser beam 42 passes through and is
reflected off of beam splitter 44. The reflected laser beam 46
travels toward one particular laser amplifier 48 while the
transmitted laser beam 50 reflects from a second mirror 52
through to another laser amplifier 54. These transmitted and
reflected laser beams either or both 46 and 50, of an elliptical
or circular polarized nature, are then directed to workpiece 20
as shown in Fig. lA. Alternatively, device 40 may be placed at
different locations along the created laser beam.
In another embodiment of the present invention, both the
elliptically or circularly polarized laser energy and reduced
thickness transparent overlay material maybe used together to
further reduce dielectric breakdown effects.
The above-described process or portions of the process are
repeated to shock process the desired surface area of workpiece
20.
Depending upon the workpiece material, a particular
parameter space of the present invention may be selected to
control the shock process. For example, the operator controller
may select a particular laser pulse energy, laser pulse length,
number of laser pulses, focal length lens, working distance,
thickness of both the energy absorbing coating and transparent
overlay to control the laser shock process. More particularly,
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laser pulse energy and laser pulse width directly affect this
process. The amount of energy placed on the surface of the
workpiece and number of laser pulses affects the depth of each
shock and the speed of the shocking process. It has been found
that the energy of the laser pulse, as well as other parameters
should be controlled in order to optimize the process and prevent
surface melting of the workpiece.
While this invention has been described as having a
preferred design, the present invention can be further modified
within the spirit and scope of this disclosure. This application
is therefore intended to cover any variations, uses, or
adaptations of the invention using its general principles.
Further, this application is intended to cover such departures
from the present disclosure as come within known or customary
practice in the art to which this invention pertains and which
fall within the limits of the appended claims.
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