U.S. patent application number 13/452377 was filed with the patent office on 2013-05-02 for method and apparatus for peening with liquid propelled shot.
The applicant listed for this patent is Daniel G. Alberts, Thomas J. Butler, Nicholas J. Cooksey, Dan Woodward. Invention is credited to Daniel G. Alberts, Thomas J. Butler, Nicholas J. Cooksey, Dan Woodward.
Application Number | 20130104615 13/452377 |
Document ID | / |
Family ID | 48171001 |
Filed Date | 2013-05-02 |
United States Patent
Application |
20130104615 |
Kind Code |
A1 |
Butler; Thomas J. ; et
al. |
May 2, 2013 |
METHOD AND APPARATUS FOR PEENING WITH LIQUID PROPELLED SHOT
Abstract
Systems and methods for generating beneficial residual stresses
in a material by impacting the surface of the material with
particles that are softer than the material to be peened ("target
material"). Shock waves emanate through the target material from
the soft particle impacts to generate residual stresses without
significantly deforming the surface of the target material. A high
pressure liquid is accelerated through a peening nozzle to generate
a high-speed liquid jet that is used to accelerate the soft
particles that impact the surface of the target material.
Inventors: |
Butler; Thomas J.;
(Enumclaw, WA) ; Alberts; Daniel G.; (Renton,
WA) ; Cooksey; Nicholas J.; (Seattle, WA) ;
Woodward; Dan; (Maple Valley, WA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Butler; Thomas J.
Alberts; Daniel G.
Cooksey; Nicholas J.
Woodward; Dan |
Enumclaw
Renton
Seattle
Maple Valley |
WA
WA
WA
WA |
US
US
US
US |
|
|
Family ID: |
48171001 |
Appl. No.: |
13/452377 |
Filed: |
April 20, 2012 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61477255 |
Apr 20, 2011 |
|
|
|
Current U.S.
Class: |
72/53 |
Current CPC
Class: |
B24C 7/0007 20130101;
B24C 7/0076 20130101; B24C 3/02 20130101; B24C 1/10 20130101 |
Class at
Publication: |
72/53 |
International
Class: |
B24C 3/02 20060101
B24C003/02; B24C 1/10 20060101 B24C001/10 |
Claims
1. A peening system for increasing residual stresses in a target
material, the peening system comprising: a liquid pump configured
for pressurizing a liquid; a solid particles storage container
configured for storing a quantity of solid particles; a peening
head comprising: a liquid input port couplable with the liquid pump
configured for receiving the pressurized liquid from the liquid
pump; a solid particles input port couplable with the solid
particles storage container configured for receiving the solid
particles from the solid particle storage container; a liquid
nozzle coupled to the liquid input port and configured for
accelerating the pressurized liquid into a high velocity liquid
jet; a mixing chamber coupled to the liquid nozzle and the solid
particles input port, such that the solid particles are combined
with the high velocity liquid jet and accelerated thereby in the
mixing chamber; and an output nozzle coupled to the mixing chamber
configured to accelerate the velocity of the liquid jet and solid
particles mixture to permit the solid particles to impact a surface
of the target material disposed at a stand-off distance from the
output nozzle.
2. The peening system of claim 1, wherein the liquid is housed in
the liquid pump and comprises liquid water.
3. The peening system of claim 1, wherein the liquid is housed in
the liquid pump and comprises a cryogenic liquid.
4. The peening system of claim 1, further comprising a robotic
manipulator coupled to at least one of the peening head and the
target material configured to selectively provide relative motion
between the peening head and the target material in order to impart
solid particle impacts over a desired area of the surface of the
target material.
5. The peening system of claim 4, further comprising a computer
control unit operative to selectively control the movement of the
robotic manipulator according to pre-programmed instructions.
6. The peening system of claim 1, wherein the solid particles have
a hardness that is less than the hardness of the target
material.
7. The peening system of claim 1, wherein the solid particles have
a hardness equal to or less than 75% of the hardness of the target
material.
8. The peening system of claim 1, wherein the solid particles are
made from a polymer material
9. The peening system of claim 8, wherein the solid particles are
made from rubber, acrylic, Viton.RTM., or polyethylene.
10. The peening system of claim 1, wherein the solid particles are
made from brass, copper, lead, or aluminum.
11. The peening system of claim 1, wherein the solid particles are
made from an organic material.
12. The peening system of claim 11, wherein the solid particles are
made from corn husks or nut shells.
13. The peening system of claim 1, wherein the mixing chamber is
configured to draw the solid particles from the solid particles
storage container into the mixing chamber by a vacuum created by
the high velocity liquid jet passing therethrough.
14. The peening system of claim 1, wherein the solid particles are
accelerated by the high velocity liquid jet such that the solid
particles impact the surface of the target material with a
sufficient velocity to generate residual stress in the target
material.
15. The peening system of claim 1, further comprising a first
conduit operative to couple the liquid pump with the liquid input
port of the peening head and a second conduit operative to couple
the solid particles storage container with the solid particles
input port of the peening head.
16. The peening system of claim 1, wherein the solid particles have
a largest dimension that is between 0.025 mm and 1 mm.
17. The peening system of claim 1, further comprising a control
valve coupled to the solid particles storage container operative to
regulate the mass flow rate of the solid particles flowing from the
solid particles storage container to the solid particles input
port.
18. The peening system of claim 17, further comprising a computer
control unit operatively coupled to the control valve configured to
selectively control the operation of the control valve.
19. The peening system of claim 1, wherein the liquid pump is
configured to pressurize the liquid to a pressure greater than
10,000 pounds per square inch (PSI).
20. The peening system of claim 1, further comprising a robotic
manipulator coupled to at least one of the peening head and the
target material configured to selectively provide relative motion
between the peening head and the target material in order to impart
solid particle impacts over a desired area of the surface of the
target material, wherein the robotic manipulator is configured to
maintain the stand-off distance between 1/8 of an inch and 15
inches.
21. The peening system of claim 1, wherein the solid particles are
accelerated such that the solid particles impact the surface of the
target material at a velocity of at least 300 meters per
second.
22. A peening system for increasing residual stresses in a target
material, the peening system comprising: a liquid pump configured
for pressurizing a liquid to a pressure of at least 10,000 pounds
per square inch (PSI); a solid particles storage container
configured for storing a quantity of solid particles having a
hardness of no more than 75% of the hardness of the target
material; a peening head comprising: a liquid input port couplable
with the liquid pump configured for receiving the liquid from the
liquid pump; a solid particles input port couplable with the solid
particles storage container configured for receiving the solid
particles from the solid particle storage container; a liquid
nozzle coupled to the liquid input port and configured for
accelerating the liquid into a high velocity liquid jet; a mixing
chamber coupled to the liquid nozzle and the solid particles input
port such that the solid particles are combined with the high
velocity liquid jet and accelerated thereby in the mixing chamber,
the mixing chamber being configured to draw the solid particles
into the mixing chamber by a vacuum created by the high velocity
liquid jet passing therethrough; and an output nozzle coupled to
the mixing chamber configured to accelerate the velocity of the
liquid and the soft particles to permit the solid particles to
impact a surface of the target material disposed at a stand-off
distance from the output nozzle.
23. A method of peening a target material to increase beneficial
residual stresses therein, the method comprising: providing a
quantity of solid particles; pressurizing a liquid; forming a high
velocity liquid jet from the pressurized liquid; and accelerating
the solid particles using the high velocity liquid jet such that
the solid particles impact a surface of the target material to
increase beneficial residual stresses therein.
24. The method of claim 23, where the liquid comprises liquid
water.
25. The method of claim 23, where the liquid comprises a cryogenic
liquid.
26. The method of claim 23, wherein the solid particles are
accelerated within a peening head, the method further comprising
selectively moving at least one of the peening head and the target
material relative to each other to impart solid particle impacts
over a desired area of the surface of the target material.
27. The method of claim 26, wherein the selectively moving is
performed by a programmable robotic manipulator.
28. The method of claim 23, wherein the solid particles have a
hardness equal to or less than 75% of the hardness of the target
material.
29. The method of claim 23, wherein the solid particles are made
from a polymer material, metal material, or organic material.
30. The method of claim 23, wherein accelerating the solid
particles comprises drawing the solid particles into a mixing
chamber by a vacuum created by the high velocity liquid jet passing
therethrough.
31. The method of claim 23, wherein the solid particles have a
largest dimension that is between 0.025 mm and 12 mm.
32. The method of claim 23, further comprising selectively
regulating a mass flow rate of the solid particles prior to being
accelerated by the high velocity liquid jet.
33. The method of claim 23, wherein pressurizing the liquid
comprises raising the pressure of the liquid to a pressure greater
than 10,000 pounds per square inch (PSI).
34. The method of claim 23, wherein accelerating the solid
particles comprises accelerating the solid particles to a velocity
of at least 300 meters per second.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application claims priority to U.S. Provisional
Application No. 61/477,255, filed Apr. 20, 2011, entitled "Method
and Apparatus for Peening With Liquid Propelled Shot," which is
hereby incorporated by reference in its entirety.
FIELD OF THE INVENTION
[0002] The present invention relates to systems and methods for
generating beneficial residual stresses in materials by impacting
the surface of the materials with particles that are softer than
the material to be peened ("target material"). Shock waves emanate
through the target material from the soft particle impacts to
generate residual stresses without significantly deforming the
surface of the target material.
BACKGROUND OF THE INVENTION
[0003] The following description includes information that may be
useful in understanding the present invention. It is not an
admission that any of the information provided herein is prior art
or relevant to the presently claimed invention, or that any
publication specifically or implicitly referenced is prior art.
[0004] Peening is the process of inducing residual compressive
stresses in materials in order to improve properties such as
fatigue resistance and stress corrosion cracking resistance, or to
shape a part through peen forming (e.g., aircraft wing skins). The
most common conventional method is to blast the surface of a
material to be peened with small particles (referred to as "shot")
which are normally harder than the material to be peened. Shot
peening methods that utilize gas to propel hard shot into metal
materials to perform peening are known. (See e.g., U.S. Pat. Nos.
7,699,449; 6,153,023; and 4,365,493).
[0005] The shot is typically propelled with compressed air using
automated equipment to move a peening nozzle over the surface of
the part or material to be peened. The shot, frequently steel or
ceramic, is usually accelerated to 50-100 meters per second (m/s)
by the compressed air and strikes the material with enough energy
to deform the surface beyond its elastic limit (i.e., plastic
deformation). The deformed surface material yields permanently and
creates local compressive residual stresses that result in improved
fatigue life of the peened part, and can improve resistance to
stress corrosion cracking, or beneficially change the shape of the
part.
[0006] Variations on this method include striking the surface with
particles spun off from a rotating wheel (see U.S. Pat. No.
3,834,200), low plasticity burnishing with a ball that is
hydraulically pressed into the surface as it rolls across the
material, and vibrating captured balls against the surface, also
called ultrasonic peening (see U.S. Pat. No. 7,276,824).
[0007] These methods all involve plastic deformation of the top
layer of the target material to generate residual compressive
stresses. This plastically deformed surface is critical to inducing
residual compressive stresses in the material since the material
underneath the surface, which is not plastically deformed, tries to
"push" the plastically deformed material back into its original
volume. This "pushing" is the compressive stress that is beneficial
for fatigue resistance.
[0008] More recently developed alternative methods include laser
shock peening (see U.S. Pat. No. 5,932,120) and cavitation peening
(see U.S. Pat. No. 7,716,961). These processes work not by
deforming the target surface with shot or balls to produce a
plastic layer, but by inducing a shock wave that travels through
the material to be peened. The magnitude of the shock wave is
sufficient to exceed the dynamic yield strength of the material,
thereby inducing deeper residual compressive stresses than can be
produced by conventional shot peening.
[0009] Biopeening uses hard particles to embed biocompatible
materials into a target surface to encourage tissue attachment (see
U.S. Pat. No. 6,502,442).
[0010] Additionally, U.S. Pat. No. 6,153,023 discloses a method
where the shot hardness can be as low as 80% of the Vicker's
Hardness of the target material.
BRIEF DESCRIPTION OF THE DRAWINGS
[0011] Exemplary embodiments are illustrated in the referenced
figures. It is intended that the embodiments and figures disclosed
herein are to be considered illustrative rather than
restrictive.
[0012] FIG. 1 illustrates a schematic diagram of a peening system
according to an embodiment of the present invention.
[0013] FIG. 2 illustrates a soft particle feed hopper and control
valve operative to regulate the flow of soft particles to a peening
head of the peening system of FIG. 1.
[0014] FIG. 3 illustrates an embodiment of the peening head of the
peening system of FIG. 1.
[0015] FIG. 4 illustrates a graph of residual stress versus depth
profile for titanium material peened with conventional shot and
titanium material peened with soft shot.
[0016] FIG. 5 illustrates a graph of residual stress versus depth
profile for stainless steel material peened with soft shot.
[0017] FIG. 6 illustrates a graph of residual stress versus depth
profile for titanium material peened with soft shot and carburized
9310 alloy steel peened with soft shot.
DESCRIPTION OF THE INVENTION
[0018] One skilled in the art will recognize many methods, systems,
and materials similar or equivalent to those described herein,
which could be used in the practice of the present invention.
Indeed, the present invention is in no way limited to the methods,
systems, and materials described.
[0019] The inventors of the present invention have recognized that
all of the aforementioned methods have various shortcomings and
limitations. Some or all of these shortcomings and limitations are
remedied by the embodiments of the present invention discussed
below. What follows is a discussion of some of the recognized
shortcomings of past peening methods.
[0020] Conventional shot peening only produces shallow compressive
stresses, typically less than 0.0098 inches (0.25 mm) deep. It also
has the considerable drawback of roughening up the target surface,
thereby causing a limitation to the improvement in fatigue life.
The rough surface can also provide initiation sites for pitting and
other forms of corrosion.
[0021] Further, using conventional shot peening, it is impossible
to peen into tight corners of the target material because the shot
size must be small to reach small radii corners. However, shot
having this small of a size is typically ineffective because it
does not have enough energy to plastically deform the target
material.
[0022] One fundamental limitation of the conventional peening
method is that the shot is accelerated using air, so it cannot
reach very high velocities. Momentum transfer from the air to the
shot particle is inversely proportional to the ratio of the
densities of the air and the shot particle. Because the shot
particles are more than 5,000 times as dense as the air, shot
particles generally are not accelerated above 150 m/s and are
typically accelerated much slower than 150 m/s.
[0023] Another fundamental limitation is that the conventional
peening process relies on plastic deformation of the surface, so
the shot particles must be harder than the target material or very
nearly as hard and the target surface is inevitably roughened by
the peening process. Yet another limitation of conventional shot
peening is that the beneficial residual stresses can easily be
relieved by exposure to high operating temperatures, or even by
cyclic loading. Another limitation is that the surface magnitude of
the residual stress for hard, high strength materials like
carburized steel is only about 50-60% of the yield strength.
[0024] Low plasticity burnishing is limited to accessible geometry
that will allow access to the rolling ball and hydraulic actuators.
Ultrasonic peening is faced with almost identical limitations.
[0025] Laser shock peening is comparatively slow and very
expensive. The equipment costs millions of dollars per station.
Additionally, good line-of-sight is necessary so peening holes,
grooves, remote locations, or tube insides is very difficult. Thus,
the process is normally only used for high value applications that
can justify the high cost, such as some critical aircraft engine
components and in-situ nuclear reactor weldments.
[0026] Cavitation peening is lower cost that laser shock peening
but is more expensive than conventional peening. Residual stresses
are deeper than conventional peening. However, cavitation peening
must be performed submerged in a liquid.
[0027] As noted above, U.S. Pat. No. 6,502,442 discusses
biopeening, suggesting peening with garnet, where the goal is not
embedment but generation of residual stress. However, the hard
particles roughen the surface and can become embedded in the target
material. The garnet will also erode the material, machining away
the impact surface and severely limiting the effectiveness of the
peening effect. The resulting residual stresses are typically very
shallow. Additionally, the garnet is much harder than the target
material.
[0028] Waterjet peening without any particles uses high pressure to
generate fast moving droplets that impinge on the target surface.
Some beneficial peening effect has been observed, but surface
erosion is an issue. Further, higher magnitude residual stresses at
greater depths would be desirable but are not achievable using
waterjet peening.
[0029] Embodiments of the present invention are directed systems
and methods for generating improved residual stress profiles in
target materials without significant surface distortion in a range
of materials, such as metals and ceramics. In some embodiments, a
high pressure liquid is accelerated through a peening nozzle to
generate a high-velocity liquid jet that is used to accelerate
particles or shot that impact the surface of a target material. In
some embodiments, the shot or particles are significantly softer
than the material being peened, and may be referred to herein as
"soft shot" or "soft particles." Generally, the soft shot or
particles may have a Vicker's Hardness value that is less than or
equal to 75% of the Vicker's Hardness value of the target material.
These novel systems and methods rely only on particle impact shock
waves to generate residual stresses and not significant material
deformation.
[0030] When the accelerated soft particles impact the surface of
the target material, the resulting shock waves are capable of
generating beneficial residual stresses in the material. The soft
particles are accelerated to a very high speed such that they
impact the surface of the target material in a way that shock waves
are generated without damaging or significantly deforming the
surface of the material. The shock waves from the soft particle
impacts on the target material surface result in a beneficial
residual stress/depth profile that is superior over past practice
shot peening. The resulting stress/depth profile is similar to
those produced using laser shock peening or low plasticity
burnishing, but without the complex implementation, high cost, and
deployment limitations. The embodiments of the present invention
are capable of peening a range of materials, including metals,
ceramics, and polymers.
[0031] FIG. 1 is a schematic block diagram of a peening system 10
in accordance with an embodiment of the present invention. The
system 10 comprises a soft particle feed hopper or container 32
configured for storing soft peening shot or particles 50, which
pass through a control valve 34 that regulates their flow into a
rigid or flexible conduit 36 that conducts the flow of the soft
particles 50 to a peening head 22. The soft particles 50 may be
made from a wide variety of materials, such as, but not limited to,
metals (e.g., annealed copper, lead, aluminum, brass, etc.),
polymers (e.g., rubber, acrylic, Viton.RTM., polyethylene, etc.),
organic materials (e.g., nut shells or corn husks), or other
materials.
[0032] A high pressure liquid pump 26 is provided to generate
liquid pressures that are preferably 10,000 psi to 60,000 psi, up
to 100,000 psi, or higher. A rigid or flexible high pressure liquid
conduit 28 is used to couple pressurized liquid 64 from the pump 26
to the peening head 22. The liquid 64 may comprise liquid water,
cryogenic liquid, or other suitable liquid. As an example, the pump
26 may be a KMT Waterjet Streamline V, a Flow International
20.times. pump, or other suitable pumps.
[0033] The peening head 22 (or a plurality of peening heads) is
mounted to a robotic manipulator 14 configured to provide relative
motion between the peening head 22 and a target material 40 (e.g.,
the portion thereof to be peened). The relative motion is designed
such that the peening particles 50 strike a surface 42 of the
target material 40 in areas that are desired to be peened. The
robotic manipulator 14 may be coupled to a computer control unit 18
configured to preprogram and control the movement of the peening
head 22 in a plurality of dimensions and to control the starting
and stopping of the peening process (e.g., by controlling the
operation of the control valve 34, pump 26, etc.) using
pre-programmed instructions. Alternatively, the target material 40
may be mounted on the robotic manipulator 14 to provide the
relative motion with the peening head 22 being stationary. A
further alternative is that both the peening head 22 and the target
material 40 are mounted on separate robotic manipulators 14 to
provide the relative motion. Additionally, the peening head 22
could also be held by a person and a peening nozzle 68 (see FIG. 3)
of the peening head 22 could be pointed at the surface 42 of the
target material 40, the operator manually moving the peening nozzle
68 to peen the area of the material that is to be peened. As an
example, the robotic manipulator 14 may be a Flying Bridge
available from Flow International, a PAR Vector CNC, or other
suitable robotic manipulator. An additional alternative is that, if
only a small area is to be peened in one operation, peening may be
performed with no relative motion between the nozzle 68 and target
material 40.
[0034] FIG. 2 is a detailed view of the soft particle feed hopper
32 and control valve 34 that are used to supply and regulate the
mass flow rate of the soft particles 50 to the peening head 22 via
the conduit 36. The control valve 34 comprises an on/off valve
portion 34A and a variable flow control portion 34B configured to
selectively control the mass flow rate of the soft particles
50.
[0035] FIG. 3 illustrates a detailed view of the peening head 22
according to one embodiment. As shown, the peening head 22
comprises a main body 54 having a liquid input port 56 and a soft
particle input port 60. The high pressure liquid 64 is fed to the
liquid input port 56 at the top of the main body 54 of the peening
head 22 and passes through a liquid nozzle 58, where it is
accelerated to form a high velocity liquid jet 62. The high
velocity liquid jet 62 then passes through a mixing or induction
chamber 66 and an output or acceleration nozzle 68, creating a
vacuum that operates to draw the soft particles 50 into the mixing
chamber 66 of the main body 54 where the energy of the high speed
jet 62 is transmitted to the particles 50 in the acceleration
nozzle 68 to form an accelerated liquid/shot jet 70. The particles
50 are accelerated to high velocities within the acceleration
nozzle 68 and emerge from the end of the nozzle at a high enough
velocity that they are capable of generating beneficial residual
stresses in the target material 40. A typical velocity range for
the particles 50 may be above 300 m/s (e.g., 300 to 1,000 m/s, 500
to 1,000 m/s, 600 to 1000 m/s, or greater). A stand-off distance D
between the end of the acceleration nozzle 68 and the surface 42 of
the target material 40 is maintained while peening. The stand-off
distance D may range between approximately 1/8 of an inch to 15
inches, for example.
[0036] In some embodiments, instead of feeding the particles 50 to
the peening head 22 via the dry soft particle feed hopper 32, the
particles 50 may be first mixed with a carrier liquid to form a
slurry and the slurry may be fed into the peening head 22 by a
slurry feed system. In other embodiments, the particles 50 may be
fed to the peening head 22 by a pressurized feed system (e.g., a
pressure pot). It should be appreciated that other methods may be
used to feed the particles 50 to the peening head 22.
[0037] FIGS. 4, 5, and 6 illustrate graphical data depicting
examples of the types of compressive residual stresses (measured in
kilopounds per square inch (KSI)) that may be generated by use of
the embodiments of the present invention. The residual stresses may
be tailored to be higher or lower stress levels and depths within
the target material 40 than those shown in the Figures, depending
upon the selected operating parameters. The stress magnitudes and
residual stress depths that are generated can be higher, and
therefore more beneficial in many cases, than possible by past
practice shot peening methods.
[0038] More specifically, FIG. 4 illustrates a graph 80 of residual
stress (in KSI) versus depth profile for a titanium target material
peened with conventional shot (line 84) and titanium material
peened with the soft shot 50 (line 88). As shown, the titanium
material peened with the soft shot 50 has more residual stress at
greater depths than the titanium material peened with conventional
shot. FIG. 5 illustrates a graph 90 of residual stress versus depth
profile for stainless steel material peened with the soft shot 50
(line 94). FIG. 6 illustrates a graph 100 of residual stress versus
depth profile for titanium material peened with the soft shot 50
(line 104) and carburized 9310 alloy steel peened with the soft
shot 50 (line 106).
[0039] Embodiments of the present invention overcome the
difficulties of past practices by using pressurized liquid 64 to
accelerate soft shot 50 or soft particles to very high velocities.
Instead of deforming the surface 42 of the target material 40 using
hard shot as occurs with conventional shot peening, the fast moving
soft particles 50 impact the surface 42 and induce a shock wave in
the material 40 without significantly indenting or deforming the
surface. The shock waves generated by the soft particles 50 travel
into the material 40, exceeding the dynamic yield strength to some
depth, thereby inducing relatively deep residual compressive
stresses. Because the shot material 50 is much softer than the
target material 40, the shot material does not embed in the target
material or cause roughening of its surface 42. Further, because
the density of the liquid 64 is roughly 1,000 times the density of
air, it is much more efficient at accelerating the particles 50
than air-propelled methods. Because the liquid 64 is at a very high
pressure, the fluid velocity is very high--high enough to
accelerate the particles 50 to sufficient velocity to induce shock
waves in the target material 40. Because the particles 50 are
moving faster than particles move in conventional shot peening,
they can be much smaller, allowing access to tight corners and
other stress concentration areas. In other cases where large areas
are to be peened, relatively large particles can be used
efficiently if desired due to the large amount of energy available
in the liquid jet. For example, the soft particles 50 may range in
size from 0.044 mm to 12 mm, but generally are no larger than a
fraction (no larger than roughly 1/2) of the diameter of the bore
of the acceleration nozzle 68. In some embodiments, the particles
50 may range in size between 0.025 mm and 1 mm when using a 2 mm
inside diameter acceleration nozzle 68. Alternatively, particles 50
may range in size from 0.025 mm to 6 mm when using a 12 mm inside
diameter acceleration nozzle 68. As discussed below, in other
embodiments the particles 50 may be other sizes. Additionally, the
acceleration nozzle 68 may be other sizes than those discussed in
the preceding discussion.
[0040] Because the system relies on shock waves, rather than
surface deformation, the residual stresses are much deeper than
conventional shot peening and are comparable to laser shock
peening. Because there is little or no roughening of the surface 42
of the target material 40, there is reduced generation of pit
initiation sites. Thus, surfaces intended for bearings or sealing,
which require a good surface finish, can be peened without
significant roughening. Moreover, fatigue life is enhanced through
deeper residual stress and a smoother surface. Effective surface
coverage rates are even higher than conventional shot peening, so
costs can be kept very low. Additionally, the soft particles 50 are
not as brittle as conventional shot, so they do not break down
during peening and can be reused many times, thereby further
reducing costs.
[0041] The process parameters that result in successful peening can
vary widely, depending on the desired results. The liquid pressure
is preferably between 10,000 and 150,000 psi or higher. In some
embodiments, the liquid pressure is selected between 10,000 and
60,000 psi, while other embodiments may be operated using liquid
pressures between 40,000 and 90,000 psi, and other embodiments may
operate using liquid pressures between 60,000 and 125,000 psi or
higher. The size range for the liquid nozzle 58 (see FIG. 3) may be
between 0.005 and 0.060 inches in diameter or larger, however
larger liquid nozzles require more pumping horsepower and may limit
the pressure that can be used, depending upon the pump that is
available. The diameter of the accelerating nozzle 68 may range
from roughly 0.020 inches to 1 inch in diameter, but is generally
several times the diameter of the liquid nozzle 58. The soft
particles 50 may range in size from 0.044 mm to 12 mm), but
generally are no larger than a fraction (e.g., no larger than 1/2)
of the diameter of the bore of the acceleration nozzle 68. A range
of stand-off distances D between the end of the accelerating nozzle
68 and the surface 42 of the target material 40 is possible. For
example, a stand-off distance range of roughly 1/8 to 15 inches may
be used. The traverse speed of the liquid/shot jet 70 over the
target material 40 may be set to roughly 10 to 600 inches per
minute, depending on the type of the target material, the desired
stress intensity, and the other operating conditions. The resulting
peening intensity may vary, depending on the type of the target
material 40 and the peening operating parameters, but 180,000 psi
and higher have been demonstrated with surface stress. As discussed
above, the soft particles 50 may be made from a wide variety of
materials, such as, but not limited to, metals (e.g., annealed
copper, lead, aluminum, brass, etc.), polymers (e.g., rubber,
acrylic, Viton.RTM., polyethylene, etc.), organic materials (e.g.,
nut shells or corn husks), or other materials.
[0042] Generally, the selected operating parameters depend on size
constraints caused by limited access to the target material 40, the
shape of the target material, the type of the target material,
limitations in abrasive material compatibility, and desired
residual stress results. The example particle materials described
above may have a range of Durometer 30 Shore A hardness (e.g., for
softer rubbers) to 65 Shore D (e.g., for polyurethane), to 3 Mohs
for walnut shells, and to Rockwell B77 for brass particles.
Generally, as can be appreciated, there is no set preferred
hardness, because the selected particle material depends on the
hardness of the target material 40, desired residual stress
profile, and desired surface finish the target material after
peening.
[0043] As an example, in one embodiment of the present invention,
the soft particles 50 used are the polymer Acrylic with a hardness
of approximately 3.5 Mohs and size of 80 mesh (0.177 mm) travelling
at a velocity of approximately 800 m/s, where the accelerating
fluid is water pumped at 420 MPa (60,000 psi). The target material
40 may be hardened steel in this example, having a Mohs equivalent
hardness of 6. However, it should be recognized that in some
embodiments, the soft particles 50 are so much softer than the
target material 40 their hardness should be measured on different
hardness scales. For example, polymers may be measured on a
Durometer scale and metals may be measured on a Rockwell or Brinell
scale.
[0044] It should be appreciated that the size of the particles 50
may vary depending on the diameter of the acceleration nozzle 68.
For example, particles 50 having a diameter of 0.25 mm may work
well with an acceleration nozzle 68 having an inside diameter a 0.5
mm or larger. Similarly, a particle size of 0.841 mm may work well
with an acceleration nozzle 68 having an inside diameter of
approximately 1.5 mm or larger. In some embodiments, the
acceleration nozzle 68 may be larger in size (e.g., between 1 mm
and 25 mm inside diameter, or larger). For example, particles 50
having a diameter of 3 mm may work well with an acceleration nozzle
68 having an inside diameter of 6 mm, 25 mm, etc. As yet another
example, particles 50 having a diameter of 6 mm may work well with
an acceleration nozzle 68 having an inside diameter of 12 mm or
larger.
[0045] The foregoing described embodiments depict different
components contained within, or connected with, different other
components. It is to be understood that such depicted architectures
are merely exemplary, and that in fact many other architectures can
be implemented which achieve the same functionality. In a
conceptual sense, any arrangement of components to achieve the same
functionality is effectively "associated" such that the desired
functionality is achieved. Hence, any two components herein
combined to achieve a particular functionality can be seen as
"associated with" each other such that the desired functionality is
achieved, irrespective of architectures or intermedial components.
Likewise, any two components so associated can also be viewed as
being "operably connected," or "operably coupled," to each other to
achieve the desired functionality.
[0046] While particular embodiments of the present invention have
been shown and described, it will be obvious to those skilled in
the art that, based upon the teachings herein, changes and
modifications may be made without departing from this invention and
its broader aspects and, therefore, the appended claims are to
encompass within their scope all such changes and modifications as
are within the true spirit and scope of this invention.
Furthermore, it is to be understood that the invention is solely
defined by the appended claims. It will be understood by those
within the art that, in general, terms used herein, and especially
in the appended claims (e.g., bodies of the appended claims) are
generally intended as "open" terms (e.g., the term "including"
should be interpreted as "including but not limited to," the term
"having" should be interpreted as "having at least," the term
"includes" should be interpreted as "includes but is not limited
to," etc.).
[0047] It will be further understood by those within the art that
if a specific number of an introduced claim recitation is intended,
such an intent will be explicitly recited in the claim, and in the
absence of such recitation no such intent is present. For example,
as an aid to understanding, the following appended claims may
contain usage of the introductory phrases "at least one" and "one
or more" to introduce claim recitations. However, the use of such
phrases should not be construed to imply that the introduction of a
claim recitation by the indefinite articles "a" or "an" limits any
particular claim containing such introduced claim recitation to
inventions containing only one such recitation, even when the same
claim includes the introductory phrases "one or more" or "at least
one" and indefinite articles such as "a" or "an" (e.g., "a" and/or
"an" should typically be interpreted to mean "at least one" or "one
or more"); the same holds true for the use of definite articles
used to introduce claim recitations. In addition, even if a
specific number of an introduced claim recitation is explicitly
recited, those skilled in the art will recognize that such
recitation should typically be interpreted to mean at least the
recited number (e.g., the bare recitation of "two recitations,"
without other modifiers, typically means at least two recitations,
or two or more recitations).
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