Title: Method of modifying a workpiece following laser shock processing
Abstract: A method of manufacturing a workpiece involves performing any one of various post-processing part modification steps on a workpiece that has been previously subjected to laser shock processing. In one step, material is removed from the compressive residual stress region of the processed workpiece. Alternately, the workpiece may be provided with oversized dimensions such that the removal process removes an amount of material sufficient to generate a processed workpiece having dimensions substantially conforming to design specifications. Alternately, the material removal process is adapted to establish a penetration depth for material removal that coincides with the depth at which the workpiece exhibits maximum compressive residual stress. Alternately, a first high-intensity laser shock processing treatment is performed on the workpiece, followed by the removal of material from the compressive residual stress region, and then a second low-intensity laser shock processing treatment is performed on the workpiece. Material may be removed from the compressive residual stress region through a workpiece surface different from the laser shock processed surface. Material may also be deposited onto the laser shock processed surface.
Patent Number: 6,852,179 Issued on 02/08/2005 to Toller, et al.
| Inventors:
|
Toller; Steven M. (Dublin, OH);
Clauer; Allan H. (Worthington, OH);
Dulaney; Jeff L. (Dublin, OH)
|
| Assignee:
|
LSP Technologies Inc. (Dublin, OH)
|
| Appl. No.:
|
590866 |
| Filed:
|
June 9, 2000 |
| Current U.S. Class: |
148/525; 148/565; 219/121.72; 219/121.85 |
| Intern'l Class: |
C21D 010/00; C22F003/00 |
| Field of Search: |
148/525,565
219/121.67,121.72,121.85
|
References Cited [Referenced By]
U.S. Patent Documents
| 4401477 | Aug., 1983 | Clauer et al. | 148/565.
|
| 4498917 | Feb., 1985 | Weinstein et al. | 219/121.
|
| 4982065 | Jan., 1991 | Sandaiji et al. | 219/121.
|
| 5131957 | Jul., 1992 | Epstein et al. | 148/565.
|
| 5151134 | Sep., 1992 | Boquillon et al. | 219/121.
|
| 5591009 | Jan., 1997 | Mannava et al. | 148/525.
|
| 5741559 | Apr., 1998 | Dulaney | 427/554.
|
| 5742028 | Apr., 1998 | Mannava et al. | 219/121.
|
| 5744781 | Apr., 1998 | Yeaton | 219/121.
|
| 5767479 | Jun., 1998 | Kanaoka | 219/121.
|
| 5859405 | Jan., 1999 | Golz et al. | 148/565.
|
| 5952014 | Sep., 1999 | Wada et al. | 219/121.
|
| 6359257 | Mar., 2002 | Clauer et al. | 219/121.
|
Primary Examiner: Wyszomierski; George
Attorney, Agent or Firm: Knuth; Randall J.
Claims
What is claimed is:
1. A method of processing a workpiece, comprising the steps of:
providing at least one of an energy-absorbing coating and an
energy-absorbing paint on said workpiece;
laser shock processing said workpiece to produce a processed workpiece
having at least one laser shock processed workpiece region having
compressive residual stress, said at least one of said energy-absorbing
coating and said energy-absorbing paint being at least one of ablated and
vaporized during said laser shock processing; and
subsequently removing workpiece material from the at least one laser shock
processed workpiece region of said processed workpiece.
2. The method as recited in claim 1, wherein the at least one laser shock
processed workpiece region has compressive residual stresses extending
into the processed workpiece from a laser shock processed workpiece
surface thereof.
3. The method as recited in claim 2, wherein the material removal step
removes workpiece material from the laser shock processed workpiece
surface.
4. The method as recited in claim 2, further includes the steps of:
determining a penetration depth into the processed workpiece and defining a
workpiece subsurface representative thereof at which the processed
workpiece exhibits a selective compressive residual stress upon removal of
material overlying at least part of the defined workpiece subsurface by
the material removal step, the workpiece material removal step being
sufficient to expose at least a portion of the defined subsurface.
5. The method as recited in claim 1, wherein a subsurface layer of said
processed workpiece exposed by the workpiece material removal step has a
greater compressive residual stress value than the previously overlying
surface layer removed by the workpiece material removal step.
6. The method as recited in claim 1, wherein the workpiece material removal
step is sufficient to remove at least one present residual tensile stress
field from the at least one laser shock processed workpiece.
7. The method as recited in claim 1, wherein the workpiece material removal
step removes an amount of workpiece material sufficient to produce in said
processed workpiece at least one selected dimensional characteristic.
8. The method as recited in claim 1, further includes the step of:
laser shock processing said processed workpiece following completion of the
workpiece material removal step, the laser shock processing of said
processed workpiece being performed at a second processing condition
different from a first processing condition associated with the initial
laser shock processing step which produced the processed workpiece.
9. The method as recited in claim 8, wherein the first processing condition
being associated with a lasing intensity level greater than a lasing
intensity level associated with the second processing condition.
10. The method as recited in claim 1, wherein the at least one laser shock
processed region extends into the workpiece from a first surface thereof,
the first workpiece surface having at least one laser shock processed
portion.
11. The method as recited in claim 10, wherein the workpiece material
removal step removes workpiece material from the at least one laser shock
processed portion of said first workpiece surface.
12. The method as recited in claim 1, wherein said workpiece includes a gas
turbine engine component.
13. The method as recited in claim 12, wherein said gas turbine engine
component includes an airfoil.
14. The method as recited in claim 1, wherein said workpiece includes a
mold.
15. The method as recited in claim 1, wherein said workpiece includes a
die.
16. The method as recited in claim 1, wherein the workpiece material
removal step includes the step of chemically processing a surface of said
processed workpiece.
17. The method as recited in claim 1, wherein the workpiece material
removal step includes the step of machining a surface of said processed
workpiece.
18. The method as recited in claim 1, wherein the workpiece material
removal step includes at least one of the steps of grinding, sanding,
mechanical milling, chemical milling, electro-chemical milling, chemical
etching, polishing, and thermally treating said processed workpiece.
19. The method of claim 1 wherein the workpiece material removal step
comprises removing more than 0.0005 inches of workpiece material.
20. A method of processing a workpiece, comprising the steps of:
laser shock processing said workpiece to produce a processed workpiece
having at least one laser shock processed workpiece region; and
removing workpiece material from the at least one laser shock processed
workpiece region of said processed workpiece, the at least one laser shock
processed region extending into the workpiece from a first surface
thereof, the first workpiece surface having at least one laser shock
processed portion, the material removal step removing material from a
second surface different from the first surface.
21. The method as recited in claim 20, wherein the second workpiece surface
having at least one portion being substantially unaffected by the laser
shock processing step.
22. The method as recited in claim 21, wherein the laser shock processing
step includes the step of directing energy toward the workpiece, wherein
substantially no part of the directed energy impacts the second workpiece
surface.
23. A method, comprising the steps of:
providing a workpiece having at least one dimensional characteristic
exceeding a specification;
laser shock processing said workpiece to produce a processed workpiece
having at least one laser shock processed workpiece region having
compressive residual stress, at least part of the at least one dimensional
characteristic of said workpiece lying within the at least one laser shock
processed workpiece region; and
subsequently removing workpiece material from the at least one laser shock
processed workpiece region of said processed workpiece in a manner
sufficient to bring the at least one dimensional characteristic of said
workpiece into substantial compliance with the specification.
24. A method of processing a workpiece, comprising the steps of:
providing at least one of an energy-absorbing coating and an
energy-absorbing paint on said workpiece;
laser shock processing said workpiece to produce a processed workpiece
having at least one laser shock processed workpiece region having
compressive residual stress, said at least one of said energy-absorbing
coating and said energy-absorbing paint being at least one of ablated and
vaporized during said laser shock processing; and
removing workpiece material from the at least one laser shock processed
workpiece region of said processed workpiece.
25. A method, comprising the steps of:
providing a workpiece having at least one dimensional characteristic
exceeding a specification;
laser shock processing said workpiece to produce a processed workpiece
having at least one laser shock processed workpiece region having
compressive residual stress, at least part of the at least one dimensional
characteristic of said workpiece lying within the at least one laser shock
processed workpiece region; and
removing workpiece material from the at least one laser shock processed
workpiece region of said processed workpiece in a manner sufficient to
bring the at least one dimensional characteristic of said workpiece into
substantial compliance with the specification.
26. A method of processing a workpiece, comprising the steps of:
laser shock processing said workpiece to produce a processed workpiece
having at least one laser shock processed workpiece region having
compressive residual stress; and
removing workpiece material from the at least one laser shock processed
workpiece region of said processed workpiece, said step of removing
workpiece material being at least one of separate and distinct from said
step of laser shock processing, said step of removing workpiece material
being performed to thereby render the workpiece into a final finished form
that exhibits at least one of:
a substantial absence of surface irregularities, deformations, and
distortion features;
a substantial conformity of at least one of a geometry and a corresponding
dimensional characteristic of the finished workpiece to a predetermined
specification; and
a compressive residual stress profile having a peak compressive residual
stress value immediately adjacent the workpiece surface within a fatigue
critical zone.
27. A method of processing a workpiece, comprising the steps of:
laser shock processing said workpiece to produce a processed workpiece
having at least one laser shock processed workpiece region having
compressive residual stress; and
removing workpiece material from the at least one laser shock processed
workpiece region of said processed workpiece, said step of removing
workpiece material being configured for removing an at least one of a
selected and defined amount of material, such that the workpiece thereby
exhibits at least one of a substantial absence of surface irregularities
and a substantial conformity of a geometry and a set of dimensional
characteristics of the workpiece to a predetermined set of specifications.
28. A method of processing a workpiece, comprising the steps of:
laser shock processing said workpiece to produce a processed workpiece
having at least one laser shock processed workpiece region having
compressive residual stress, said workpiece having a workpiece surface
subjected to said laser shock processing, said workpiece surface either
having one of a painting and a coating thereon or remaining uncoated, at
least a portion of said one of a painting and a coating or an uncoated
workpiece surface portion, respectively, being at least one of ablated and
vaporized during said laser shock processing; and
subsequently removing workpiece material from the at least one laser shock
processed workpiece region of said processed workpiece, said step of
removing workpiece material being performed to thereby render the
workpiece into a final finished form that exhibits at least one of:
a substantial absence of surface irregularities, deformations, and
distortion features;
a substantial conformity of at least one of a geometry and a corresponding
dimensional characteristic of the finished workpiece to a predetermined
specification; and
a compressive residual stress profile having a peak compressive residual
stress value immediately adjacent the workpiece surface within a fatigue
critical zone.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a laser shock processing operation, and,
more particularly, to a method and apparatus for modifying a workpiece
previously subjected to a laser shock processing treatment, such as by
removing material from, or adding material to, the laser shock processed
region.
2. Description of the Related Art
The use of laser shock processing has found wide success, particularly in
applications involving the enhancement of certain structural features such
as the leading and trailing edges of airfoils in integrally bladed rotor
systems. However, the high levels of compressive residual stresses that
accompany laser shock processing may at times produce unique features in a
processed workpiece. Recognition of the occurrence of one or more of
theses features has underpinned various efforts to examine the extent to
which such processing can be modified to
mitigate or remove these features, if they prove to be undesirable in a
particular application.
Laser shock processing can leave surface geometry irregularities such as
surface roughness and partially rolled-over or extruded edges, and other
undesirable features. The surface roughness may, for example, take the
form of laser-beam-spot depressions, surface melt or `staining`, pits from
collapsed sub-surface porosity in castings, and beaded surface patterns.
The surface roughness created by laser shock peening can vary from none to
0.001 to 0.002 inches in depth. Surface roughness as little as 0.0005
inches is a concern in certain applications such as airfoils, or polished
surfaces. Laser shock peening may also cause some distortion in the shape
of the part due to the compressive residual stresses created. This may
necessitate smoothing the surface of airfoils of aircraft gas turbine
engine blades and integrally bladed rotors (IBRs) after laser peening or
shot peening at high intensities. This may be desirable to increase the
aerodynamic efficiency of the airfoils after processing. In addition, the
performance of some parts is degraded by required manufacturing steps, for
example, certain machining operations that leave a rough surface, or
intensive shot peening.
In view of the foregoing, there is needed a material treatment process that
eliminates undesirable distortion and surface roughness introduced by
conventional manufacturing processes or laser shock processing, without
sacrificing the benefits of such processing.
SUMMARY OF THE INVENTION
According to the present invention there is provided a method for
manufacturing and processing a workpiece that involves performing any one
of various post-processing part modification steps on a fabricated
workpiece that has been previously subjected to laser shock processing.
One part modification procedure involves removing material from at least a
portion of the compressive residual stress region previously produced by
laser shock processing the workpiece. In one form, the fabricated
workpiece is provided with oversized dimensions such that the removal
process is adapted to remove an amount of material sufficient to generate
a processed workpiece having dimensions substantially conforming to design
specifications.
In another form, the material removal process is adapted to remove a
localized tensile stress region sometimes present immediately beneath part
of the laser shock processed surface.
In another form, the material removal process is adapted to establish a
penetration depth for material removal that coincides with the depth at
which the workpiece exhibits maximum compressive residual stress.
In another form, a first laser shock processing treatment is performed on
the workpiece at a high-intensity energy level, material is removed from
the compressive residual stress region of the processed workpiece, and a
second laser shock processing treatment is performed on the processed
workpiece.
In another form, material is removed from the compressive residual stress
region through a workpiece surface (preferably un-processed) that is
different from the laser shock processed surface.
According to another category of part modification procedures, material is
deposited onto the laser shock processed surface in the form of a material
deposition layer. Some of this layer will then be removed to form a smooth
surface.
As used herein, and well known by those skilled in the art, laser shock
processing (LSP), laser shock peening, or laser peening as it is also
referred to, is a process for producing a region of deep compressive
residual stresses in the workpiece induced by the presence of traveling
pressure or shock waves that are imparted to the surface by laser shock
peening. This form of treatment utilizes a laser beam from a laser beam
source to produce a strong localized compressive force on a portion of the
workpiece surface by precipitating an explosive force caused by
instantaneous ablation or vaporization of a painted, coated, or un-coated
surface.
In one typical form, laser peening employs two surface overlays: a
transparent overlay (usually a flowing film of water) and an opaque
overlay, such as an oil-based or acrylic-based black paint. During
processing, a laser beam is directed to pass through the water overlay to
enable the energy to become absorbed by the black paint, causing a rapid
vaporization of the paint surface, which is sufficient to generate a
high-amplitude shock wave. The water film acts as a confining agent that
contains and redirects the shock waves into the body of the workpiece,
thereby acting to cold-work the surface of the part and to create
compressive residual stresses extending from the surface into the interior
of the part.
The workpiece is typically treated by developing a matrix of overlapping or
non-overlapping laser beam spots that cover a critical zone of interest.
Additionally, the same or adjacent areas may be repeatedly processed by
cyclically directing energy pulses to the desired target area. Various
parameters may be controlled by the production manager, design engineer,
or operator to tailor the laser shock processing operation. For example,
the operational parameters that the designer can select and adjust include
(but are not limited to) the location of the incident beam spot; number
of, and spacing between, spots; distance of spots from certain workpiece
features (e.g., leading and trailing edge of an airfoil on an integrally
bladed rotor); angle of incidence of the laser pulse; laser pulse width
and repetition; and beam intensity.
Additional descriptions may be found in U.S. Pat. Nos. 5,741,559 and
5,911,890, both assigned to the same assignee as the present application
and incorporated herein by reference thereto. U.S. Pat. No. 5,131,957 is
also incorporated herein by reference thereto.
The advantage of laser shock processing relates to its ability to increase
the fatigue properties of the part by selectively developing pre-stressed
regions within certain critical areas where incipient flaws or cracks
typically appear. The technique has been applied with favorable success to
the processing of the pressure and suction sides of leading and trailing
edges of fan and compressor airfoils and blades in gas turbine engines.
The various effects of laser peening on the fatigue properties of welded
samples has been reported in "Shock Waves and High Strain 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
(incorporated herein by reference thereto).
As used herein, a workpiece refers to any solid body, article, or other
suitable material composition that is capable of being treated by laser
shock processing. The workpiece may represent a constituent piece forming
part of an in-production assembly, a final production article, or any
other desired part. Accordingly, the laser shock processing treatment may
be applied at any stage of production, i.e., pre- or post-manufacturing or
any intervening time. Preferably, in certain industrial applications, the
present invention finds significant use in processing the airfoils of an
integrally bladed rotor, most notably in the region proximate the leading
and trailing edges of airfoils where flaws and other high-cycle failures
pose serious problems affecting the performance and durability of the
engine.
The invention, in one form thereof, is directed to a method of processing a
workpiece. According to the method, a workpiece is laser shock processed
to produce a processed workpiece having at least one laser shock processed
region. The laser shock peening roughens the surface of the surface with
one or more depressions having a depth ranging of 0.0005 to 0.002 inches.
Material is removed from at least one laser shock processed region of the
processed workpiece to remove the depressions and bring the surface into
substantial compliance with predetermined dimensional and and/or surface
finish workpiece requirements. This would be a consideration when the
depressions are deeper than 0.0005 inches. In this example of the method,
0.0005 inches or greater amounts of material would be removed, thereby
making a substantially smooth surface. The laser shock processed region
has compressive residual stresses extending into the processed workpiece
from a laser shock processed surface thereof. In one form, the material
removal step removes material from the laser shock processed surface.
The method further includes the steps of determining a penetration depth
into the processed workpiece at which at least one selective compressive
residual stress level is present; and defining a subsurface of the
processed workpiece representative of the determined penetration depth.
The material removal step is sufficient to expose at least a portion of
the defined subsurface.
The material removal step, in another form, is sufficient to remove at
least one present residual tensile stress feature from the laser shock
processed region. In yet another form, the material removal step removes
an amount of material sufficient to produce in the processed workpiece at
least one selected dimensional characteristic.
The method further includes the step of laser shock processing the
processed workpiece following completion of the material removal step,
wherein laser shock processing of the processed workpiece is performed at
a second energy level different from a first energy level associated with
the initial laser shock processing step which produced the processed
workpiece. The first energy level is preferably greater than the second
energy level.
In another form of the method, the laser shock processed region extends
into the workpiece from a first surface thereof, wherein the first
workpiece surface has at least one laser shock processed portion. The
material removal step removes material from the at least one laser shock
processed portion of the first workpiece surface. Alternately, the
material removal step removes material from a second surface different
from the first surface. The second workpiece surface preferably has at
least one portion substantially unaffected by the laser shock processing
step.
The invention, in another form thereof, is directed to a method of
processing a workpiece. According to the method, a workpiece is laser
shock processed to produce a processed workpiece having at least one laser
shock processed region. Material is deposited on at least a portion of the
laser shock processed region of the processed workpiece. A portion of the
deposited material is then removed to bring at least one dimensional
characteristic into substantial compliance with the specification.
The material deposition step includes, in various forms, the step of
performing at least one of flame-sprayed coating, plasma-sprayed coating,
chemical plating, electro-plating, chemical vapor deposition and vacuum
deposition.
According to various implementations of the processing method, the
workpieces may include, without limitation, a gas turbine engine
component, a mold, and a die.
In alternative forms, the material removal step includes the step of
performing at least one of grinding, sanding, mechanical milling, chemical
milling, electro-chemical milling, chemical etching, polishing, and
thermally treating the processed workpiece.
The invention, in another form thereof, is directed to a method comprising,
in combination, the steps of providing a workpiece having at least one
dimensional characteristic exceeding a specification; laser shock
processing the workpiece to produce a processed workpiece having a laser
shock processed region, wherein at least part of the at least one
dimensional characteristic of the workpiece lies within the laser shock
processed region; and removing material from the laser shock processed
region in a manner sufficient to bring the at least one dimensional
characteristic of the workpiece into substantial compliance with the
specification.
The invention, in another form thereof, is directed to an article
manufactured by a process, wherein the article has an exposed surface and
an unexposed subsurface portion. The process involves laser shock
processing the article to produce a processed article having at least one
laser shock processed region; and removing material from the at least one
laser shock processed region of the processed article to expose at least
the subsurface portion of the article. The laser shock processed region
has compressive residual stresses extending into the processed article
from a laser shock processed surface thereof.
In one form, the material removal step induces a stress relaxation effect
in the processed article, causing a modification in the mechanical
equilibrium condition at and beneath the exposed subsurface portion of the
article.
In another form, the material removal step induces a change in the
compressive residual stress characteristics at the exposed subsurface
portion of the article. In particular, the material removal step induces
an increase in the surface compressive residual stress characteristics at
the exposed subsurface portion of the article.
In yet another form, the material removal step is sufficient to remove at
least one present residual tensile stress feature from the laser shock
processed region.
The invention, in yet another form thereof, is directed to an article
manufactured by a process, wherein the article has an exposed surface and
an unexposed subsurface portion. The process involves laser shock
processing the article to produce a processed article having at least one
laser shock processed region; and depositing material on at least a
portion of the at least one laser shock processed region of the processed
article; then removing a portion of the deposited material to bring at
least one dimensional characteristic into substantial compliance with the
specification.
One advantage of the present invention is that the various part
modification steps enable surface irregularities and deformations to be
eliminated without materially sacrificing any of the beneficial effects of
laser shock processing.
Another advantage of the present invention is that post-processing removal
of material from the compressive residual stress region of the processed
workpiece enables the designer to make selective changes to the residual
stress characteristics of the workpiece and improve the fatigue properties
thereof.
Another advantage of the present invention is that the various part
modification steps occur as part of a post-processing activity, allowing
the designer to adapt the material removal and material deposition
processes to remedy any physical disturbances introduced by the laser
shock processing treatment.
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 an embodiment of the invention taken in conjunction with
the accompanying drawings, wherein:
FIG. 1 is a fragmentary, side-elevational schematic view of a
representative workpiece illustrating in exaggerated form a type of
distortion that is removed according to one embodiment of the present
invention;
FIG. 2 is a flowchart of the processing method disclosed in FIG. 1;
FIG. 3 is a fragmentary, side-elevational schematic view of a
representative workpiece illustrating the manner of removing material from
the processed workpiece to render it compliant with predetermined
dimensional specifications, according to another embodiment of the present
invention;
FIG. 4 is a flowchart of the processing method disclosed in FIG. 3;
FIG. 5 is a fragmentary, side-elevational schematic view of a
representative workpiece illustrating the manner of removing material from
the processed workpiece by accessing the laser shock processed region
through an unprocessed surface, according to another embodiment of the
present invention;
FIG. 6 is a flowchart of the processing method disclosed in FIG. 5;
FIG. 7 is a flowchart of one alternative processing method that involves
variable-intensity laser shock processing operations, which precede and
follow part modification, according to another embodiment of the present
invention;
FIG. 8 is a graph illustrating the variation in compressive residual stress
values as a function of penetration depth below a laser shock processed
surface;
FIGS. 9A and 9B are fragmentary, side-elevational schematic views of a
workpiece illustrating the manner of depositing material onto the
processed workpiece, according to another embodiment of the present
invention; and
FIG. 10 is a flowchart of the processing method disclosed in FIG. 9.
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
By way of overview, the various processing methods disclosed herein involve
processing activities that are preferably executed upon a workpiece,
article, or other such part following the performance of a laser shock
processing operation on the workpiece. Stated otherwise, the various part
modification procedures disclosed herein are carried out on a previously
processed workpiece.
The manner of conducting such laser shock processing does not form an
essential part of the present invention as it should be apparent that the
workpiece can be subjected to any suitable type of laser peening
conditions. Additionally, the processed condition of the workpiece may be
generated in accordance with any activity involving, inter alia, laser
shock processing, shot peening, the application of a force or pressure
field to the workpiece, or the development of stress regions within the
workpiece.
The various part modification procedures of the present invention
individually endeavor in a general way to configure or otherwise render
the subject workpiece into a final finished form that exhibits, inter
alia, the substantial absence of surface irregularities, deformations, and
other such distortion features; substantial conformity of the geometry and
other dimensional characteristics of the finished workpiece to
predetermined specifications; and a compressive residual stress profile
having robust characteristics in the regions of interest, e.g., a peak
compressive residual stress value immediately adjacent the workpiece
surface within a fatigue critical zone.
Referring now to the drawings, and particularly to FIG. 1, there is shown a
representative workpiece 10 depicting the manner of eliminating a type of
distortion illustrated in exaggerated form as recess or dimple 12 and a
bump or elevated portion 14, according to one embodiment of the present
invention. Reference is also made to the flowchart of FIG. 2 depicting the
operating sequence of the part modification procedure.
The illustrated workpiece 10 has previously been subject to laser shock
processing at side 16 to produce a laser shock processed surface area 18
having the indicated distortion features 12 and 14 introduced in a known
manner by the completed laser shock processing activity (step 100). As
conventionally known, the laser shock processing induces the formation of
deep compressive residual stresses extending from surface 18 into the body
of workpiece 10 and reaching a penetration depth illustratively designated
by first subsurface 20, thereby defining an illustrative compressive
residual stress region 22 between first subsurface 20 and exposed surface
18.
According to one aspect of the present invention, a part modification
procedure is implemented with respect to workpiece 20 that involves the
removal of at least a portion of compressed residual stress region 22 in a
manner adequate to selectively eliminate the surface irregularities or
imperfections such as distortion features 12 and 14 (step 102). In
particular, a second subsurface 26 is chosen that will form the exposed
surface of processed workpiece 10 following completion of the material
removal procedure. The manner of arranging second subsurface 26 as the new
surface of workpiece 10 involves removing an amount of material from
processed workpiece 10 that is contained within and represented by surface
layer 24 disposed between surface 18 and second subsurface 26.
As shown, second subsurface 26 is preferably disposed intermediate surface
18 and first subsurface 20 (i.e., subsurface 26 lies above subsurface 20)
such that a portion 28 of stress region 26 will remain following
completion of the material removal step.
The manner of removing material from stress region 22 of workpiece 10 is
preferably conducted with a view toward developing a new surface (i.e.,
previously subsurface 26) that is polished or otherwise configured in a
finished form substantially free of surface defects. The as-modified
workpiece 10 is now preferably ready for further assembly (if a component
part) or installation in the field (if already arranged in a finished
product). Additionally, it should be apparent that any suitable method may
be used to perform the material removal procedure, including, but not
limited to, grinding, sanding, mechanical milling, abrading, chemical
milling, electro-chemical milling, chemical etching, and thermal
treatment.
A removal process having minimal target area impact is preferred (such as
chemical milling), since unlike mechanical-type treatments it does not
impart any mechanical stresses, added residual stresses, or surface
effects. As conventionally known, chemical milling treats the workpiece
with a chemical reagent that reacts with the surface layer 24 to easily
facilitate its removal. It should also be apparent that the form and
extent of second subsurface 26 is shown for illustrative purposes only
since other subsurface portions may be chosen for exposure and attendant
designation as the new surface layer of workpiece 10.
Referring now to FIG. 3, there is shown a lateral schematic view of
representative workpiece 10 provided with an upper buffer layer
(illustrated at 30) defined between surface 32 and a first subsurface 34
of predetermined location, according to another embodiment of the present
invention. Reference is also made to the flowchart of FIG. 4 depicting the
operating sequence of the part modification procedure illustrated by FIG.
3. As explained below, the upper buffer layer 30 is formed as part of a
design fabrication effort aimed at providing workpiece 10 with oversized
dimensions relative to normal part specifications (step 104). The
particular construction of workpiece 10 can be developed using any
conventional fabrication techniques known to those skilled in the art.
Fabricated workpiece 10 is subjected to a laser shock processing operation
to conventionally produce laser shock processed surface area 32 (step
106). The laser shock processing induces the formation of deep compressive
residual stresses extending from surface 32 into the body of workpiece 10
and reaching a penetration depth illustratively designated by second
subsurface 36, thereby defining an illustrative compressive residual
stress region 38 between second subsurface 36 and exposed surface 32.
Following laser shock processing, the processed workpiece 10 is further
treated by removing a portion of stress region 38 corresponding to the
material contained within buffer layer 30, thereby exposing first
subsurface 34 as the new surface of workpiece 10 (step 108). According to
another aspect of the present invention, first subsurface 34 corresponds
to a desired final dimensional feature of workpiece 10 that conforms to
design specifications or other production criteria for workpiece 10.
In effect, workpiece 10 is fabricated in an oversized configuration as
exemplified by buffer layer 30 such that following removal of the material
in buffer layer 30, the final form of workpiece 10 will exhibit a
dimensional characteristic (defined by surface 34) that complies with
certain specifications (step 104). This removal step therefore functions
to remove the portion of compressed residual stress region 38 that is
encompassed by the workpiece dimensions which exceed a part specification
(step 108).
The specific parameters for buffer layer 30 (such as depth and coverage
area) are preferably chosen such that the laser shock processing will
develop a stress region 38 that adequately extends beneath subsurface 34.
For example, the fabrication of buffer layer 30 may be tailored such that
a peak compressive residual stress is developed beneath surface 32 at a
depth substantially aligned with subsurface 34. As a result, following
part modification (i.e., removal of buffer layer 30), the processed
workpiece 10 will advantageously possess peak compressive stress levels in
the critical zone immediately adjacent its surface to thereby enhance the
retardation of crack propagation, for example.
Referring to FIG. 5, there is shown a fragmentary schematic view of a
representative workpiece 10 illustrating the manner in which the removal
of a portion of a laser shock processed region occurs via penetration
through a non-processed surface area, according to another embodiment of
the present invention. Reference is also made to the flowchart of FIG. 6
depicting the operating sequence of the part modification procedure
illustrated by FIG. 5.
Fabricated workpiece 10 is subjected to a laser shock processing operation
to conventionally produce laser shock processed surface area 40 (step
110). The laser shock processing induces the formation of deep compressive
residual stresses extending from surface 40 into the body of workpiece 10
and reaching a penetration depth illustratively designated by first
subsurface 42, thereby defining an illustrative compressive residual
stress region 44 between first subsurface 42 and exposed surface 40.
Following laser shock processing, the processed workpiece 10 is further
treated by removing a portion of workpiece 10 lying subjacent to surface
46 and extending to second subsurface 48. This removed portion is
illustratively depicted at 50. For this purpose, the part modification
procedure involves the definition of a workpiece surface 46 different from
the laser shock processed surface 40 (step 112). Associated with this
definition of workpiece surface 46 is the companion definition of a
subsurface 48 associated therewith, which together define a workpiece
portion 50 subject to removal that encompasses at least a portion 52 of
residual compressed stress region 44.
As shown, this removal of portion 50 has the effect of removing a portion
52 of stress region 44 bounded by first subsurface 42, second subsurface
48, processed surface 40, and surface 46. The removal procedure accesses
processed portion 52 of stress region 44 by penetrating through surface
46, e.g., by a machining or milling operation (step 114). This removal
mechanism differs from FIGS. 1 and 3 in which the respective stress
regions are accessed directly through laser shock processed surface areas
associated with the stress regions.
Surface 46 is preferably unprocessed by the laser shock processing activity
chiefly directed at surface 40. In one form, no part of surface 46 is
affected by the laser shock processing that is directed at surface 40 or
any other part of workpiece 10. In particular, the energy pulses directed
toward workpiece 10 to induce the stress-forming shock waves do not
impinge upon surface 46. Accordingly, surface 46 may be considered an
unprocessed area, at least with respect to the laser shock processing that
affects surface 40. Alternately, surface 46 may receive some laser shock
processing. Additionally, surface 40 and surface 46 may be distinct from
one another (i.e., non-overlapping) or they may overlap at least in part.
It is seen that the removal technique evident in FIG. 5 will typically
require that surface 40 and surface 46 be disposed in angular relationship
to one another. Additionally, as surfaces 40 and 46 become increasingly
coplanar, the removal method will correspondingly require a higher level
of directionality in the material removal process. By contrast, in the
generally orthogonal relationship depicted in FIG. 5, a simple machining
action oriented perpendicularly to surface 46 will readily accomplish the
desired removal of portion 50.
Reference is now made to FIG. 7, which sets forth a flowchart describing
the operating sequence of a part modification procedure that involves a
further laser shock processing treatment, according to another embodiment
of the present invention. This procedure may be used in conjunction with
any of the material removal techniques described above concerning FIGS.
1-6 or otherwise.
According to the part modification procedure, the fabricated workpiece is
initially subjected to a first laser shock processing treatment, which
applies a first energy level or density to the workpiece (step 116). In a
manner similar to that described hereinabove, there is removed from the
processed workpiece at least a portion of the compressed residual stress
region formed by the first laser shock processing treatment (step 118).
Following the removal step, the processed workpiece is next subjected to a
second laser shock processing treatment which applies a second energy
level or density to the workpiece, preferably at the newly exposed surface
of the processed workpiece (step 120).
In a preferred form, the first energy density is greater than the second
energy density. In particular, the first laser peening treatment
preferably involves a high-intensity lasing operation while the second
laser peening treatment involves a low-intensity laser peening operation.
An optional step may be used to remove additional material from the
compressed residual stress region that extends from the newly exposed
surface of the processed workpiece. A processing cycle involving such
iterations of material removal and low-intensity laser peening treatment
may be repeated to obtain certain compressive residual stress profiles
within the workpiece. Material may also be added to the processed
workpiece at any stage of manufacturing.
The low-intensity laser shock processing serves to provide additional
fatigue strength, hardness, and corrosion resistance properties without
further deforming the surface in any meaningful way.
Several synergistic effects have been observed in consequence of the
various removal procedures outlined above. For this purpose, reference is
made to the graph of FIG. 8 illustrating the variation in residual
compressive stress 80 as a function of penetration depth into the
workpiece as sometimes measured from the laser shock processed surface. As
shown, stress curve 80 sometimes exhibits a hook-type behavior within the
first 0.002" of penetration into the compressive residual stress region.
This hook-type feature is characterized by a short rise in the stress
value over a shallow penetration depth until reaching a maximum stress
value, at which point the stress value declines fairly rapidly with
increasing distance from the processed surface.
The presence of this sub-maximal stress range in the immediate proximity of
the laser shock processed surface is not optimal because it is precisely
within this initial depth range that the highest possible stress values
are needed to counteract or oppose any defects, such as cracks,
imperfections, and other irregularities that may contribute to or
precipitate the occurrence of failure or fatigue.
According to a preferred aspect of the present invention, the part
modification procedures described above are adapted to ensure that the
depth of material removal corresponds to the depth at which the
compressive residual stress value exhibits a maximum or near-maximum
value, as determined from graph 80 or any suitably equivalent data. Thus,
at a depth of approximately 0.002" (namely, at the newly-exposed workpiece
surface within the stress region), the workpiece will provide its maximum
resistance to the formation or propagation of defects due to the presence
of the maximum surface compressive residual stress value at this point.
According to another preferred aspect of the present invention, after
completion of the removal step, a material layer may be deposited on the
newly-exposed workpiece surface (discussed infra in connection with FIGS.
9-10), followed by an additional laser shock processing treatment that
processes the newly-deposited material layer. The result is the formation
of a new compressive residual stress region (within the deposited material
layer) that exhibits the stress behavior indicated by curve 82 adjoined to
curve 80 at its peak value. As shown, it is possible to change the
residual stress characteristics at the workpiece surface.
Returning to the stress curve 80, it has also been observed that the
near-surface portion of the compressive residual stress region that
experiences the initial sub-maximal stress range contains various local
tensile residual stresses. Accordingly, removing this leading portion of
the stress region immediately beneath the laser shock processed surface
enables the tension effects to be eliminated, thereby increasing the
average compressive surface residual stress.
However, in response to this removal, the workpiece experiences a
relaxation effect in which the existing elastic residual stresses arrive
at a new mechanically stable equilibrium condition. This relaxation may
uniformly reduce the compressive residual stress levels, as evidenced by a
shift in stress curve 80 to a relaxation curve 84.
In sum, as shown by the graph of FIG. 8, the highest value for the
compressive residual stress is sometimes found between one and three
thousandths of an inch below the laser shock processed surface of the
workpiece; however, the value for compressive residual stress may peak at
greater depths, such as five thousandths of an inch, depending on the
material used and the application of the laser peening process.
When this occurs, it may therefore be advantageous to remove a surface
layer within the laser shock processed region, such that a subsurface
portion having increased values for compressive residual stress is made
the new surface layer of the workpiece. The decision to remove a surface
layer having a sub-maximal residual stress range will typically be based
on the needs of the application. For example, when an application
necessitates a higher compressive stress immediately below the surface, it
may be advantageous to remove only a finite layer, and then subject the
workpiece to a low intensity laser peening process for further
strengthening.
Referring now to FIGS. 9A and 9B, there are shown fragmentary schematic
views of a workpiece 10 illustrating in exaggerated form the manner in
which material is deposited onto a laser shock processed surface area of
workpiece 10, according to another embodiment of the present invention.
Reference is also made to the flowchart of FIG. 10 depicting the operating
sequence of the part modification procedure.
Referring first to FIG. 9A, the illustrated workpiece 10 has previously
been subjected to laser shock processing at side 60 to conventionally
produce a laser shock processed surface area 62 (step 122). As
conventionally known, the laser shock processing induces the formation of
deep compressive residual stresses extending from surface 62 into the body
of workpiece 10 and reaching a penetration depth illustratively designated
by subsurface 64, thereby defining an illustrative compressive residual
stress region 66 between subsurface 64 and exposed surface 62.
According to another aspect of the present invention, the processed
workpiece 10 of FIG. 9A is modified by depositing a material formation or
layer 68 upon the laser shock processed surface 62, as shown in FIG. 9B
(step 124). One advantage of such part modification procedure involves the
ability to precisely form layer 68 in any suitable manner utilizing the
appropriate layer formation technology known to those skilled in the art.
For example, workpiece 10 in FIG. 9B can be provided with a highly
finished and polished upper surface 70 substantially free of defects,
irregularities, and other such imperfections. Additionally, the material,
properties, geometry, and dimensions of layer 68 may be suitably chosen to
achieve a variety purposes tailored to particular applications.
It should be apparent that any suitable technique may be used to form
material layer 68, including, but not limited to, flame sprayed coating,
plasma sprayed coating, chemical plating, electro-plating, vacuum
deposition, and chemical vapor deposition. Additionally, any of various
material finishing techniques may be used to process the surface of
material layer 68. It is also possible to process the workpiece
configuration shown in FIG. 9B in conjunction with any of the
aforementioned part modification procedures. For example, material layer
68 could be subject to a sequence of laser shock processing and material
removal and/or deposition steps.
It is a general feature of the present invention that the part modification
procedures disclosed herein may be used to change the residual stress
characteristics of the workpiece surface. Additional, the modification
procedures may be combined with another.
The present invention finds particular use in applications where the
workpiece corresponds to an assembly or a gas turbine engine component.
The workpiece may also be a mold, a die, or any other solid body.
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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