U.S. patent application number 12/962535 was filed with the patent office on 2011-06-09 for laser peening process and apparatus using a liquid erosion-resistant opaque overlay coating.
This patent application is currently assigned to LSP Technologies, Inc.. Invention is credited to Jeff L. Dulaney, David F. Lahrman, Richard D. Tenaglia.
Application Number | 20110132886 12/962535 |
Document ID | / |
Family ID | 34218073 |
Filed Date | 2011-06-09 |
United States Patent
Application |
20110132886 |
Kind Code |
A1 |
Tenaglia; Richard D. ; et
al. |
June 9, 2011 |
Laser Peening Process and Apparatus Using a Liquid
Erosion-Resistant Opaque Overlay Coating
Abstract
The invention relates to a method and apparatus for improving
properties of a solid material by providing shockwaves there
through. Laser shock processing is used to provide the shockwaves.
The method includes applying a liquid energy-absorbing overlay,
which is resistant to erosion and dissolution by the transparent
water overlay and which is resistant to drying to a portion of the
surface of the solid material and then applying a transparent
overlay to the coated portion of the solid material. A pulse of
coherent laser energy is directed to the coated portion of the
solid material to create a shockwave. Advantageously, at least a
portion of the unspent energy-absorbing overlay can be reused in
situ at a further laser treatment location and/or recovered for
later use.
Inventors: |
Tenaglia; Richard D.;
(Columbus, OH) ; Dulaney; Jeff L.; (Dublin,
OH) ; Lahrman; David F.; (Powell, OH) |
Assignee: |
LSP Technologies, Inc.
Dublin
OH
|
Family ID: |
34218073 |
Appl. No.: |
12/962535 |
Filed: |
December 7, 2010 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11843363 |
Aug 22, 2007 |
7868268 |
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12962535 |
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10654369 |
Sep 2, 2003 |
7268317 |
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11843363 |
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Current U.S.
Class: |
219/121.85 |
Current CPC
Class: |
B23K 26/18 20130101;
B23K 26/009 20130101; C21D 10/005 20130101; B23K 26/356
20151001 |
Class at
Publication: |
219/121.85 |
International
Class: |
B23K 26/00 20060101
B23K026/00 |
Claims
1. A method of treating a workpiece by applying shockwaves thereto,
comprising the steps of: applying an energy-absorbing overlay to a
portion of a surface of the workpiece, said energy-absorbing
overlay being composed of a liquid material resistant to
dissolution by the transparent water overlay and resistant to
drying; applying a transparent overlay upon said energy-absorbing
overlay; and directing a pulse of coherent energy to said
energy-absorbing overlay, said pulse of coherent energy causing a
portion of said energy-absorbing overlay to vaporize and thereby
generate at least one shockwave for transmission to the
workpiece.
2. The method of claim 1, wherein said liquid erosion-resistant and
drying-resistant material has a combined viscosity and level of
adherence such that said energy-absorbing overlay made thereof
tends to conform and adhere to the workpiece under substantially
static conditions yet is capable of fluid displacement when
subjected to at least one shockwave.
3. The method of claim 1, wherein said liquid erosion-resistant and
drying-resistant material is a colloidal substance having at least
one energy-absorbing particulate dispersed therein.
4. The method of claim 3, wherein said colloidal substance is a
mixture of an oil and graphite.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to the use of coherent energy
pulses, as from high power pulsed lasers, in the shock processing
of solid materials, and, more particularly, to methods and
apparatus for improving properties of solid materials by providing
shockwaves therein. The invention is especially useful for
enhancing or creating desired physical properties such as high
cycle fatigue life, fretting fatigue life, hardness, strength, and
fatigue strength.
[0003] 2. Description of the Related Art
[0004] Old methods for the shock processing of solid materials
typically involve the use of high explosive materials in contact
with the solid, high explosive materials or high pressure gases
used to accelerate a plate that strikes the solid to produce
shockwaves therein. Such methods have several disadvantages. For
example: (a) it is difficult and costly to shock process non-planar
surfaces and complicated geometries, (b) storage and handling of
the high explosive materials and high pressure gases pose a hazard,
(c) the processes are difficult to automate and thus fail to meet
some industrial needs and (d) high explosive materials and high
pressure gases cannot be used in extreme environments such as high
temperatures and high vacuum.
[0005] Shot peening is another widely known and accepted process
for improving the fatigue, hardness, and corrosion resistance
properties of materials by impact treatment of their surfaces. In
shot peening, many small shot or beads are thrown at high speed
against the surface of a material. However, the depth of treatment
using conventional shot peening is typically only 0.004 to 0.006
inches deep. This depth is only 10-20 percent as deep as achieved
with laser peening, and the surface enhancement of material
properties with shot peening is much less effective.
[0006] Laser shock processing with coherent radiation has several
advantages over what has been done before. For example, the source
of the radiation is highly controllable and reproducible. The
radiation is easily focused on preselected surface areas and the
operating conditions are 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. This is desirable where it is possible to enhance
material properties in a stepwise fashion. Shocking the workpiece
several times at low pressures can avoid gross deformation,
cracking, and spallation of the workpiece while non-planar
workpieces can be shock processed without the need of elaborate and
costly shock focusing schemes.
[0007] Laser peening (hereinafter referred to as laser shock
processing) utilizes two overlays: a transparent overlay (usually
water) and an opaque overlay, previously an oil based or acrylic
based black paint. Tapes, such as black polyvinyl chloride or
polyethylene tapes, have also been used successfully as the opaque
overlay. During processing, a laser beam is directed to pass
through the water overlay and is absorbed by the opaque overlay
(black paint or tape), causing a rapid vaporization of the opaque
overlay surface and the generation of a high-amplitude shockwave.
The shockwave cold works the surface of the part as it propagates
into the material 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.
[0008] The current laser processing of workpieces requires multiple
re-applications of the opaque overlay, which require that the
workpiece be manually removed from the laser processing station and
recoated after several non-adjacent spots have been processed. When
using paint, the old paint is sometimes removed before repainting
the part or sometimes additional paint is simply added over the old
paint. The repainting can require upwards of 12 to 15 paint cycles.
Each cycle usually requires 15-20 minutes before the part can be
returned to the processing station. This additional handling of the
part for repainting will add as much as 50% to the cost of the
processing in a production environment. In a similar manner,
re-application of tape to overcome the problem of tape damage from
the laser shot during processing adds significantly to the
processing time and cost.
[0009] Other drawbacks have been found to be associated with using
paints, both oil-based and water-based ones. These drawbacks stem
from the need to dry paint prior to use thereof. Such drying adds
to the process cycle time and/or increases system requirements in
order to promote faster drying times.
Furthermore, in the case of water-based paints, the paint can be
rewet by the transparent overlay, such as flowing water, and may be
eroded from the surface of the part before the laser beam is
applied to the part. The removal of oil-based paints, not dissolved
by a water overlay, must be achieved chemically and/or physically
in a manner that essentially dissolves the paint or results in the
flaking thereof. Additionally, there is no potential of relocating
unused dried paint, in situ, to a new location where it may be used
so as to reduce the total amount of paint to be applied. As such,
recycling and/or reuse of paints is not practical, given typical
removal methods.
[0010] A method of automatically applying the overlays in sequence
with the laser system has been developed into an applicator system.
This applicator system has, when used with the laser peening
system, reduced the time of applying the overlay coatings and
increased the throughput of the laser peen process. The opaque
overlays used tend to be eroded by the transparent overlay during
processing and have resulted in the generation of a weaker
shockwave than achieved with tape or dry paint.
[0011] What is needed in the art is a laser shock process that
utilizes an opaque overlay that does not have to dry, will not be
eroded by the transparent overlay and yet allows for the generation
of a strong shockwave that can be applied with the applicator
system.
SUMMARY OF THE INVENTION
[0012] The present invention provides a method of laser shock
processing that can be used in a production environment that
significantly reduces processing time. The method includes the
steps of coating the workpiece to be laser shock processed with a
layer of a liquid opaque overlay coating that is resistant to
erosion or dissolution by the transparent water overlay and that is
resistant to drying. The liquid overlay coating is applied to a
small area at least about 2 to 3 times the diameter of the laser
beam. A transparent overlay, such as water, is applied forming a
thin flowing layer over the previously coated portion. When the
water has totally covered the coated portion, the laser is fired
directly through the flowing water overlay and onto the coated
area. The entire sequence and event timing is controlled by a
preprogrammed microprocessor such as found in a personal computer.
The sequences are repeated for each spot to be processed along the
workpiece surface.
[0013] The invention comprises, in one form thereof, a method of
improving properties of a solid material by providing shockwaves
therein. An energy-absorbing coating that is resistant to
dissolution by the transparent water overlay and resistant to
drying is applied to a portion of the surface of the solid
material. A transparent overlay material is then applied to the
coated portion of the solid material. A pulse of coherent energy is
then directed to the coated portion of the solid material to create
a shockwave.
[0014] The invention comprises, in another form thereof, a method
of improving properties of a solid material by providing shockwaves
therein. An energy-absorbing coating that is resistant to
dissolution by the transparent water overlay and resistant to
drying is applied to a portion of the surface of the solid
material. A transparent water overlay is then applied to the coated
portion of the solid material. A pulse of coherent energy is then
directed to the coated portion of the solid material to create a
shockwave. An unspent portion of the energy-absorbing coating is
displaced by the shockwave into at least one adjacent treatment
location, effectively allowing in situ reuse of that unspent
portion.
[0015] The invention comprises, in yet another form thereof, an
apparatus for improving properties of a workpiece by providing
shockwaves therein. The apparatus includes a material applicator
for applying an energy-absorbing material on to the workpiece to
create a coated portion and a transparent overlay applicator for
applying a liquid transparent overlay to the workpiece over said
coated portion. A laser is operatively associated with the
transparent overlay applicator to provide a laser beam through the
liquid transparent overlay to create a shockwave on the workpiece.
A positioning mechanism is included to selectively position the
workpiece relative to the material applicator, the transparent
overlay applicator and the laser. Conversely, the positioning
mechanism may position the material applicator and transparent
overlay applicator correctly over the spot on the workpiece to be
treated while it is in position in the laser beam path. A control
unit is operatively associated with each of the applicators, laser,
and positioning mechanism, to control the operation and timing of
each of the applicators, laser, and the selective operation of the
positioning mechanism.
[0016] The invention comprises, in yet a further form thereof, an
energy-absorbing overlay for use in conjunction with a
laser-induced shock process. The energy-absorbing overlay includes
a base material, which is resistant to erosion or dissolution by
the transparent water overlay and resistant to drying, and at least
one energy-absorbing particulate dispersed within the base
material. Advantageously, the base material is an oil, and the
particulate material is colloidal graphite and/or fine black iron
oxide (FeA).
[0017] An advantage of the present invention is that the method
allows the use of energy-absorbing coating that is viscous,
adherent, and resistant to drying and that lends itself to
reuse/recycling. Prior laser painting processes utilized oil, water
or acrylic based paints that needed to be dried prior to use and
that did not lend themselves to reuse and/or recycling.
[0018] Another advantage of the present invention is that the
energy-absorbing coating that has an appropriate viscosity that
allows it to both conform to a workpiece shape under substantially
static conditions and be readily displaced under sufficiently
dynamic conditions. Alternately, the energy-absorbing material can
be chosen so as to be self-limiting with respect to its thickness,
thereby improving thickness uniformity and reducing the need for
thickness control measures.
[0019] Yet another advantage of the present invention is that since
the energy-absorbing coating (both the viscous and self-limiting
thickness versions) is resistant to drying and is typically oil
based (i.e., lubricant as base), such a coating material will not
tend to stop up an applicator, either during use or after an
extended period of stagnation therewith in. The use of extremely
fine or colloidal particulate prevents clogging of the applicator
and prevents problems with settling during storage or processing.
The non-drying feature permits the pre-application of the
energy-absorbing coating. This pre-application can reduce the time
and amount of spraying needed to be done in the processing cell and
can limit the opportunity of applying a laser pulse to an uncoated
area.
[0020] A further advantage of the present invention is the
utilization of a flowing, transparent water overlay for processing
of the workpiece surface. The use of the flowing water covers the
coated area uniformly while additionally ensuring that any heat
possibly transferred to the workplace by the process will be
removed.
[0021] Yet a further advantage of the present invention is that the
process eliminates the need to move the workpiece from workstation
to workstation as was previously accomplished and necessitated. The
laser shock processing system now can be adapted to manufacturing
process workloads and requirements.
[0022] A yet additional advantage of the present invention is that
the applicator(s) in the coating system can be fitted with integral
protectors that can preclude unwanted spraying of selected
workpiece portions and/or selected process apparatus components,
thereby both eliminating the need to manually mask such selected
portions/components and reducing clean-up time.
BRIEF DESCRIPTION OF THE DRAWINGS
[0023] 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 an embodiment of the invention
taken in conjunction with the accompanying drawings, wherein:
[0024] FIG. 1 is a flow chart of the method of the present
invention
[0025] FIG. 2 is a diagrammatic view of one embodiment of the
present invention; and
[0026] FIG. 3 is a diagrammatic view of another form of the present
invention utilizing a rotatable carousel to index a workpiece.
[0027] Corresponding reference characters indicate corresponding
parts throughout the several views. The exemplification set out
herein illustrates one preferred embodiment of the invention, in
one form, and such exemplification is not to be construed as
limiting the scope of the invention in any manner.
DETAILED DESCRIPTION OF THE INVENTION
[0028] The improvements in fatigue life produced by laser shock
processing are the results of residual compressive stresses
developed in the irradiated surface that retard fatigue crack
initiation and/or slow 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 shock
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. As to what effect
the tensile residual stresses surrounding the laser shocked region
would have on crack initiation, a previous study is described in
"Shockwaves and High Strained Rate Phenomena in Metals" by A. H.
Clauer, J. H. Holbrook and B. P. Fairand, ed. by M. S. Meyers and
L. E. Murr, Plenum Press, New York (1981), pp. 675-702. Described
in the above reference are the effects of laser shock processing on
fatigue of welded aluminum specimens that had used a series of
overlapping spots to cover the weld and heat-affected zones.
Significant increases in fatigue life were observed for these
specimens indicating that overlapping the spots did not
significantly reduce the effects of laser shocking. This is
supported by results on a low carbon steel that showed no change in
the compressive residual stress profile across the edge of a spot
in a spot-overlap region.
[0029] For a more thorough background in the prior history of laser
shock processing and that of high power processing of engineered
materials, reference can be made to U.S. Pat. No. 5,131,957, such
patent explicitly hereby incorporated by reference. This patent
also shows a type of laser and laser circuit adaptable for use with
the present invention. Another type of laser adaptable for use with
the invention is that of a Nd:Glass Laser manufactured by LSP
Technologies, Inc. of Dublin, Ohio.
[0030] 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 the workpiece, and the
outer transparent overlay being adjacent the opaque overlay.
[0031] The transparent overlay material should be substantially
transparent to the radiation. Useful transparent overlay materials
include water, water-based solutions, other non-corrosive 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, 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 process ability, these materials include
polytetrafluoroethylene (PTFE); fluorinated ethylene propylene
(FEP): the chlorotrifluorethylenes (CTFE); and polyvinylidine
fluoride (PVF.sub.2). Also available is a variety of copolymers of
both halogenated and fluorinated hydrocarbons, including
fluorinated elastomers. Additionally, the transparent overlay could
be a gel or a strip of tape comprised of one or more of the above
materials.
[0032] Where desired, the opaque overlay material may be
substantially opaque to the radiation. Useful opaque overlay
materials of the present invention include an energy-absorbing
dispersant and a base material. The dispersant may, for example, be
graphite, carbon black, black iron oxide (Fe.sub.2O.sub.3), and/or
mixtures of these materials. The base material for such opaque
overlay materials is advantageously an oil (e.g., mineral,
vegetable, petroleum-derived) which contributes to the viscosity
control, adherence, and drying resistance needed by the opaque
overlay material of the present invention. The mixture of the
dispersant and base material is colloidal in nature, in the sense
that the dispersant is too fine to be filtered readily from the
base material and is resistant to settling. A typical overlay is
about 10 to 20,000 micrometers (.mu.m) thick. In an advantageous
embodiment of the invention, an oil-and-graphite mixture is used to
give superior results both in terms of energy absorption during
shock processing and reuse thereafter.
[0033] Referring now to the drawings and particularly to FIG. 2,
there is shown a preferred embodiment 10 of the present invention
including a target chamber 12 in which the laser shock process
takes place. The target chamber 12 includes an opening 14 for a
laser beam 16 created by laser 18, a source of coherent energy.
Laser 18, by way of example, may be a commercially available high
power pulse laser system capable of delivering more than
approximately 40 joules in 5 to 100 nanoseconds. The laser pulse
length and focus of the laser beam may be adjusted as known in the
art.
[0034] Shown in FIG. 2, a workpiece 20 is held in position within
target chamber 12 by means of a positioning mechanism 22.
Positioning mechanism 22 may be of the type of a robotically
controlled arm or other apparatus to precisely position workpiece
20 relative to the operational elements of laser shock system
10.
[0035] System 10 includes a material applicator 24 for applying an
energy-absorbing material onto workpiece 20 to create a coated
portion. Material applicator 24 may be that of a solenoid operated
painting station or other construction such as a jet spray or
aerosol unit to provide a small coated area onto workpiece 20. The
material utilized by material applicator 24 is preferably an
energy-absorbing material that is resistant to erosion or
dissolution by the water overlay and resistant to drying, or an
energy-absorbing material that is self-limiting in thickness yet
adherent and resistant to drying. In addition, the opaque overlay
may advantageously be resistant to ignition by the plasma plume.
Each such material advantageously is composed of an oil and
graphite mixture. Various features of the oil/graphite mixture such
as the graphite particle size, graphite concentration, oil
composition, and additive usage can be adjusted to produce the
desired set of material characteristics. Alternatively, other types
of opaque coatings may be used that incorporate those materials
discussed above.
[0036] With respect to the energy-absorbing material that exhibits
self-limiting coating thickness, the oil-based nature of the
colloidal graphite coating exhibits a tendency to limit the coating
build-up during spray application. The coating wets the surface of
parts well and, due to its relatively low viscosity, it spreads out
rather uniformly when deposited on a given surface. Like the
viscous version of the energy-absorbing material, the self-limiting
coating thickness composition does not dry during application, so
it is less susceptible to clumping, clogging, or similar spray
problems encountered with water-based paints that have been
previously evaluated.
[0037] As the oil-based coating of this embodiment is sprayed, it
readily displaces laterally, and the coating thickness at the spot
where the laser beam will be applied reaches a self-limiting
maximum. Overspray of the coating simply displaces sideways and
does not affect the laser peening operation. As such, the
self-limiting nature of this coating embodiment provides improved
uniformity to the coating thickness, which theoretically should
improve the uniformity of the shockwave created at the surface
during laser peening. Further, the spreading characteristic of the
coating makes control of the coating thickness much less critical
than when using coatings, which are susceptible to overspray
irregularities.
[0038] The non-drying nature of the oil-based colloidal graphite
coating (both the viscous and self-limiting thickness compositions)
offers an opportunity to pre-coat or pre-spray the part before
laser peening. In some instances, this may reduce the amount of
overlay coating that must be applied by material applicator 24
(which may be incorporated into a RapidCoater.TM. system (not
shown).
[0039] Using this approach, the part is pre-coated, and material
applicator 24 can be used just to "touch up" the spot to be laser
peened. This procedure can reduce the coating spray cycle time in
the processing cell and potentially increase productivity in
situations where the coating application cycle time is the
time-limiting parameter. This methodology can also be used to
minimize the amount of spraying in the work cell, which can reduce
overspray on the workpiece and potential contamination of objects
associated with the process system. Furthermore, pre-coating the
part will also lessen the chances of applying a laser beam pulse to
an uncoated area, which can result in melting, "burning", and/or
"staining" of the surface
[0040] System 10 further includes a transparent overlay applicator
26 that applies a fluid or liquid transparent overlay to workpiece
10 over the portion coated by material applicator 24. Transparent
overlay material should be substantially transparent to the
radiation as discussed above, water being the preferred overlay
material.
[0041] As shown in FIG. 2, both material applicator 24 and
transparent overlay material applicator 26 are shown directly
located within target chamber 12. In a production operation
environment, only the necessary operative portions need be located
through and within target chamber 12 such as the portion through
which the materials actually flow through a flow head. The supply
tanks for the transparent overlay materials and other
energy-absorbing materials may be located outside of target chamber
12.
[0042] Both material applicator 24 and transparent overlay material
applicator 26 can each be fitted with an integral protector 37
schematically indicated in FIG. 3. Each protector 37 extends beyond
a respective spray nozzle 39 of the particular one of material
applicator 24 and overlay material applicator 26. Such protectors
37 are attached to the robotically-positionable coating system
fixturing (not labeled). Thus, the shielding protector 37
accompanies a given nozzle 39, and manual application of protectors
to the workpiece and/or specific portions of the process apparatus
can be obviated. Protectors 37 may be made of any suitable
material, such as polyethylene, Lexan.TM., or other plastics. Metal
protectors can be used, but plastics are typically preferred to
minimize the potential for scratching parts during processing or
movement of applicators 24 and 26 and such or other portions of the
RapidCoater.TM. System.
[0043] A control unit, such as controller 28, is operatively
associated with each of the material applicator 24, transparent
overlay material applicator 26, laser 18 and positioning mechanism
22. Controller 28 controls the operation and timing of each of the
applicators 24, 26, laser 18 and selective operation of positioning
mechanism 22 to ensure proper sequence and timing of system 10.
Shown in FIG. 2, controller 28 is connected to laser 18,
positioning mechanism 22, material applicator 24 and transparent
overlay material applicator 26 via control lines 30, 32, 34 and 36,
respectively. Controller 28, in one embodiment, may be a programmed
personal computer or microprocessor.
[0044] In operation, controller 28 controls operation of system 10
once initiated. As shown in FIG. 1, the method of the invention is
that first, workpiece 20 is located (38) particularly within
targeting chamber 12 by positioning mechanism 22. Controller 28
activates material applicator 24 to apply a laser energy-absorbing
coating 40 onto a particular location of workpiece 20 to be laser
shock processed. The next step of the process is that controller 28
causes transparent overlay material applicator 26 to apply
transparent overlay 42 to the previously coated portion of
workpiece 20. At this point, laser 18 is immediately fired by
controller 28 to initiate a laser beam 16 to impact the coated
portion 44. Preferably, the time between applying the transparent
water overlay and the step of directing the laser energy pulse is
approximately 0.1 to 3.0 seconds. By directing this pulse of
coherent energy to the coated portion, a shockwave is created. As
the plasma expands from the impact area, it creates a compressional
shockwave passing through and against workpiece 20.
[0045] As part of the laser peening system to be employed, it is
advantageous to provide a detection/monitoring unit 45 which can be
used to ensure that the overlay coatings for each spot during each
cycle have been appropriately applied and that the laser beam has
been applied to the part before moving on to the next spot.
Detection/monitoring unit 45 may be any one of a mass/flow meter, a
video monitoring unit, a plasma monitor, and an acoustic monitor.
Mass/flow meters may be provided on the coating and water supply
lines to applicators 24 and 26, respectively. Digital outputs from
the meters may be monitored with controller 28. In the basic form
thereof, a pulse from the meters can be used to verify application
of an overlay through a system of spray nozzles 39 or to signal an
alarm condition if no flow is registered. With more sophisticated
meters, a quantitative measure of the amount of the overlay coating
can may be registered and recorded. Other sensors for detecting the
presence of the overlay coating may also be used, such as
ultrasonic flow/motion detectors or interrupted optical
signals.
[0046] Another alternative for detection/monitoring unit 45 is a
video monitor. With such a video monitor, the spray pulse from the
system nozzles 39 may be observed with video monitors focused at
locations showing the nozzles 39 positioned to spray the part.
Image analysis can be used in real time during the spray cycle to
verify the application and the spot location. In the case of
graphite, oil-based coatings, the spray pattern may be difficult to
observe with a video monitor because of the dark colors of the
coating and the substrate.
[0047] As a yet further alternative for detection/monitoring unit
45, a plasma monitor can be used which can verify application of
the coating overlays. For example, the flash of the plasma during
the application of the laser beam has a characteristic signature
that can be analyzed in real time. This characteristic signature
can be used to determine if water overlay coating was in place as
the laser pulse was applied.
[0048] As a yet further alternative, one or more acoustic monitors
can be used for detection/monitoring unit 45. The presence of the
overlay coatings can be verified using acoustic monitors such as a
pinducer. During proper processing, the application of the laser
pulse produces a loud snap (typically >130 dB). If the overlay
coatings are not present or the laser pulse misses the part, the
acoustic signature is not present, is altered, or is greatly
diminished. Thus, a minimum sound loudness threshold can be
monitored, recorded, and used as a means of indicating that each
spot was processed properly with the overlay coatings in place.
[0049] During plasma formation due to laser impact, a first portion
of the laser energy-absorbing coated portion 44 is sacrificed,
e.g., converted to plasma or driven off workpiece 20. A second,
unsacrificed portion thereof is prevented from escaping by
transparent overlay 42 but, due to its characteristic viscosity,
instead is fluidly displaced laterally along workpiece 20 away from
the impact/impingement point of laser beam 16. An advantageous
by-product of this displacement is that an unspent/unsacrificed
portion of the energy-absorbing overlay 44 can be fluidly displaced
into one or more adjacent potential treatment locations on
workpiece 20 or recovered for reuse (i.e., Reuse/Recycle Step 46).
That the improved oil-based energy-absorbing overlay 44 spreads
well, is readily fluidly displaced, and is self-limiting in
thickness can effectively reduce or eliminate the need for further
laser energy-absorbing coating 40 to be deposited onto such a
potential treatment location. A detection/monitoring unit 45 (which
may be a direct (i.e., contact) or indirect (i.e., photo) unit as
appropriate) is provided for measuring the thickness of the coated
portion 44 at a treatment location in order to determine if more
laser energy-absorbing coating 40 (i.e., a touch-up application
thereof) need be applied thereto prior to firing of laser 18. Using
this approach of maximizing the applied energy-absorbing overlay 44
can result in reduced coating cycle times and thereby result in
increased productivity in cases where coating application cycle
times is the rate-limiting portion of the laser peening
process.
[0050] With the use of the improved oil-based overlay coating
compositions of the present invention, a step of cleaning a
laser-peened spot with an air/water blast to remove the paint and
to prepare for the next laser peening system cycle may be performed
or omitted, as desired. Because of the improved nature of the
overlay coatings of the present invention, it is not always
necessary to remove the coating between overlay spray cycles. This
is especially true when the self-limiting thickness composition is
employed. A touch-up of the overlay coating can be adequate to
replenish the overlay coating layer sufficiently to promote burning
and to generate the shockwave. Using this approach, the overall
cycle time for the laser peening process can be reduced, and
productivity can be increased in cases where the coating cycle time
is the time-limiting process step during laser peening.
[0051] In the case where the overlay coating is not removed during
processing, the part may be cleaned after laser peening has been
completed using various spray cleaning techniques such as a
high-pressure cleaning system or cleaning systems analogous to
dishwasher cleaning. Removal of the oil-based overlay coating may
be facilitated with the use of hot water and a detergent
(preferably non-foaming). The removed coating is advantageously
immiscible with water and may be skimmed or separated readily from
the waste water for disposal or recycling.
[0052] If it is desired to use the RapidCoater.TM. System with the
graphite/oil-based overlay and to remove the coating at each spot
during a given system cycle, the air/water blast portion of the
cycle may be used. It is advantageous to use hot water and a
non-foaming detergent to facilitate the removal of the overlay
coating. Many suitable detergents are available, one such
non-foaming detergent being AP-12 available from EMD Chemicals,
Inc.
[0053] The above-described process or portions of the process are
repeated to shock process the desired surface area of workpiece 20.
Depending upon the energy levels and the amount of laser shocking
desired on workpiece 20, controller 28 may position or re-index
workpiece 20 into another position using positioning mechanism 22
so that system 10 may apply coatings to and laser beam 12 may
impact a different portion which may overlap the previously impact
area.
[0054] It may be advantageous in a production environment to
separate the coating operation from the lasing operation. FIG. 3
discloses another way to reposition and re-index a workpiece 20
using a selectively rotatable carousel 50. Positioning mechanism 22
is located on carousel 50 for rotation therewith. Controller 28
will control both the operation of positioning mechanism 22 and the
location of the mechanism by selectively rotating carousel 50 to
position a workpiece adjacent either material applicator 24 or
transparent overlay material applicator 26. If necessary,
additional workstations may be utilized but these may reduce the
process efficiency.
[0055] In one form of the invention, it may be possible such that
all the steps of the process shown in FIG. 1 may be accomplished
without moving the workpiece 20. Workpiece 20 may be indexed if
necessary to cover large areas by the process. By not moving
workplace 20, additional manufacturing efficiencies are produced
while additionally reducing the time between steps. The limiting
factor in the process currently is the cycle time of laser 18.
Laser 18 may be fired about every 0.5 seconds to 10 seconds.
[0056] This minimum amount of time is required by laser 18 to
recharge while permitting selected other steps of the process shown
in FIG. 1 to be accomplished.
[0057] In another form of the invention, controller 28 can be a
smart control unit advantageously interfaced with the
RapidCoater.TM. System. This interface promotes the parameters and
timing of the coating system to be set remotely by the operator.
Potentially, the settings can be varied spot-by-spot using a
program sequence. Movement of the part and the laser peening system
robots (not labeled) can be coordinated for optimal processing.
Detection/monitoring unit(s) 45 used to validate the application of
the overlay coatings and/or the laser spot can be interfaced to
controller 28, which is specifically designed to be a smart
controller as part of this embodiment. Using this approach,
processing can be verified for each spot before proceeding to the
next. This verification system is much more efficient than
re-positioning to rework various spots after the primary processing
is completed.
[0058] In alternative embodiments, the application of transparent
overlay material (shown in step 42) may comprise of applying the
transparent overlay material continuously before and during the
directing of the laser energy pulse.
[0059] Depending upon the workpiece material, many parameters 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 time, number of laser pulses, focal lens,
working distance, thickness of both the energy-absorbing coating
and transparent overlay to control the laser shock process. More
particularly, laser pulse energy and laser pulse width directly
affect this cycle. 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 prevent surface melting of the
workpiece.
[0060] 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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