Note: Descriptions are shown in the official language in which they were submitted.
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METHOD AND COMPRESSION APPARATUS FOR INTRODUCING
RESIDUAL COMPRESSION INTO A COMPONENT HAVING A REGULAR OR
AN IRREGULAR SHAPED SURFACE
[0001]
Background of the Invention
[0002] This invention relates to inducing compression along
and into the
surface of a component and more particularly to a method and apparatus for
inducing compressive residual stress along and into the surface of a workpiece
having a regular or an irregular surface topography.
[0003] Many metallic machines and various structural
components are
subject to failure by fatigue, corrosion fatigue, or stress corrosion cracking
(SCC).
Failures generally initiate from the surface of the component in highly
stressed
areas, often from scratches, corrosion pits, or other surface damage that
creates
a shallow notch or indentation that produces a local stress concentration. It
is
well known that surface enhancement, such as the introduction of a layer of
= compressive residual stress, can if of sufficient magnitude and depth,
mitigate
the stress concentration due to the damage and greatly improve the "damage
tolerance" or fatigue strength and service life of a component. Further, since
SCC requires surface tension above a threshold level, placing the surface in
residual compression can eliminate or significantly reduce SCC.
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[0004] The
introduction of compressive residual stress is achieved in all
practical cases by introducing non-uniform cold work, or plastic strain, into
the
workpiece. The resulting amount and distribution of residual stress and the
resulting change in shape depend upon both amount and distribution of plastic
strain and the original geometry of the workpiece.
[0005] The
introduction of residual stress is also used in the forming of
components, such as the curved skin of aircraft wings. For forming
applications
the magnitude, depth and distribution of the induced stress throughout the
workpiece are critical properties requiring precise control.
[0006] In some
applications, the cold working of the metallic material is
used to modify the mechanical and chemical properties of the existing surface
or
a surface layer deposited by plasma spray, cold spray, plating, or some other
process. The original surface and/or the deposited surface layer is
deliberately
cold worked to a required amount to achieve the desired properties, such as
work hardening. Cold working may be followed by heat treatment for crystalline
grain refinement or to promote diffusion and bonding of a coating to a
substrate.
For these surface modification applications the magnitude and distribution of
cold
working are the critical processing properties that require precise control.
[0007] A
variety of surface enhancement processes have been developed.
Hammer peening of welds is an ancient practice known to eliminate residual
tension caused by shrinkage of the hot weld, but is an uncontrolled manual
process. Modern processes for inducing compression along and into the surface
of a workpiece include shot peening (SP), laser shock peening (LSP), low
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plasticity burnishing (LPB), deep rolling (DR), ultrasonic peening (UP),
ultrasonic
needle peening (UNP), flapper peening (FP), and cavitation peening (CP).
However, all such methods have limitations that make automated application to
certain surfaces, such as irregular topography surfaces often found in welded
assemblies, difficult or undesirable.
[0008] Application of LPB, DR, CP and LSP all require that the surface of
the workpiece or component, to be processed be well defined geometrically so
that the mechanical burnishing tool, the cavitation zone, or the laser focal
spot
can be accurately positioned during processing. The positioning requirements
for these methods are similar to machining. Automated processing of welds or
other irregular topography surfaces using CNC control is difficult because the
workpiece shapes, surface geometries, or irregularities vary making the
process
non repeatable. Therefore, components having irregular topography surfaces,
such as manually welded assemblies, cannot be reliably treated because the
irregularities may cause the processing tool to be positioned too close or
distant
from the surface to be effective, and some regions may be missed altogether
during processing.
[0009] SP, UP and UNP all utilize a blast of shot propelled from nozzles
or
thrown from a wheel, a fluidized cloud of shot ricocheting in a chamber, or
clusters of randomly impacting needles to deform the surface by covering it
with
dimples. Programmed robotic direction of shot flow from nozzle peening systems
is a common practice. FP utilizes a rotating flexible sheet studded with
impacting
media (shot) generally positioned manually. One such flapper peening system is
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disclosed in U.S. Patent No. 7,954,348 that controls the speed of the rotating
"flapper" to regulate the impacting force and speed. While these methods can
accommodate processing of an irregular topography surface, such as a manual
weld, they impact the surface randomly thereby making it difficult to achieve
the
optimum surface processing necessary for certain applications. Further, to
achieve full coverage of the treated surface the media (shot or needles)
impact
the surface repeatedly, often as many as 16 times on some areas in order to be
sure that most of the surface has been impacted once. The repeated impacts
can highly cold work the surface which can be detrimental to work hardening
alloys, leaving a compressive layer that is subject to rapid thermal stress
relaxation or mechanical overload relaxation in service. Cold working also
work
softens hardened steels leaving a softened surface layer, and transforms
retained austenite causing slight swelling and often results in an
unacceptable
change in critical dimensions. The depth of compression achievable by shot is
limited by the size of the media used, and is generally more shallow than the
depth of compression induced by LPB or LSP. Finally, the repeated impacting
required of these methods is also simply inefficient in terms of energy usage.
[0010]
Robotically controlled hammer peening has been developed such
as for the peening of welds, where the impacting head follows a fixed path
defined by the robot control code. However, such systems do not provide an
effective method of controlling and monitoring the performance of the peening
process, or for accommodating irregularities in the surface of the workpiece
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thereby reducing or eliminating the likelihood of inducing the desired or
effective
=
compression along surfaces having such irregularities.
10011] Accordingly, a need exists for a method and apparatus of inducing
compressive residual stress along and into the surface of workpiece; that can
be
automated, such as by robotic or CNC machine tools; produces a controlled
desired depth and magnitude of compression and cold work; and can be reliably
applied with process monitoring to workpieces having an irregular topography
surface.
Summary of the Invention
[0012] The present invention is a method and compression apparatus for
inducing compression along and into the surface of a component and more
particularly, for inducing controlled compression along and into the surface
of a
component workpiece having a regular or an irregular topography surface. In a
preferred embodiment of the invention, the compression apparatus comprises a
precision control system, such as a computer numerically controlled (CNC)
robot
or machine tool, for positioning and controlling the movement of a tool head
effective for contacting the surface of the workpiece and inducing compression
along and into the surface of the workpiece and for providing an apparatus and
method for accommodating topography irregularities along the surface of the
workpiece, as may be encountered in cast, welded or similar surfaces.
[0013] Preferably, the compression apparatus for inducing compression
along and into the surface of a workpiece includes a tool head, such as an
impact tool head, having a compression element of a controlled shape, that
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operates to create a specific controlled zone of plastic deformation and
achieving
a prescribed depth and magnitude of residual compression (residual
compressive stress) along and within the surface of the workpiece.
[0014] In another preferred embodiment of the invention, the compression
apparatus for inducing compression into and along the surface of a workpiece
is
in the form of an impact apparatus and includes a control system that operates
such that the impacts of the compression element are spaced to minimize
excessive deformation (often encountered with conventional shot peening,
ultrasonic peening and needle peening) so as to minimize the cold working of
the
surface by reducing or eliminating random repeat impacts along the surface of
the workpiece.
[0015] In a preferred embodiment of the invention, the compression
apparatus for inducing compression along and into the surface of a workpiece
includes a positioning device and a flexible arm assembly extending from an
end
of the positioning device for supporting the tool head.
[0016] In another preferred embodiment of the invention, the flexible arm
assembly operates to provide one degree of freedom in flexure such that the
tool
head is free to move perpendicular (normal) to the surface of the workpiece
being worked and parallel to the impact vector of the tool head.
[0017] In another preferred embodiment of the invention, the, compression
apparatus for inducing compression along and into the surface of a workpiece
operates such that a control system, in conjunction with the positioning
device,
functions to position the tool head and move the tool head such that it
follows a
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nominal virtual control surface defined to be at a distance below the actual
surface of the workpiece.
[0018] In another preferred embodiment of the invention, the compression
apparatus for inducing compression along and into the surface of a workpiece
is
an impact tool having a control system that functions in conjunction with the
positioning device to position and move the tool head such that it follows a
nominal virtual control surface defined to be located at a distance within the
range of the tool head stroke in the direction towards the surface being
treated
(the stroke of the tool head) and below the actual surface of the workpiece
being
treated.
[0019] In another preferred embodiment of the invention, the flexible arm
assembly operates to accommodate a range of topography irregularities along
the surface of the workpiece to be treated.
[0020] In a preferred embodiment of the invention, the control surface is
selected to accommodate the range of topography irregularities along the
surface
of the workpiece to be treated.
[0021] In another preferred embodiment of the invention, the tool head is
an impact tool head driven by a trip hammer or similar device driven
pneumatically, hydraulically, magnetically, or electrically at a controlled
reciprocating rate of impact.
[0022] In another preferred embodiment of the invention, the Impact rate
or frequency of the compression element of the tool head is from about 1 to
about 100 impacts or strikes per second.
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[0023] In another preferred embodiment of the invention, the impact rate
or frequency is varied to provide a desired spacing between impacts of the
compression element of the tool head along the surface of the workpiece.
[0024] In another preferred embodiment of the invention, the speed of
positioning of the tool head along the surface of the workpiece is varied to
provide a desired spacing between impacts of the compression element along
the surface of the workpiece.
[0025] In another preferred embodiment of the invention, the impact rate
or frequency is varied along with speed of positioning of the tool head to
change
the impact spacing along the surface of the workpiece.
[0026] In another preferred embodiment of the invention the striking
force
of the compression element is selected to provide the desired depth to which
the
material forming the workpiece is deformed to ensure the depth of the residual
compression is achieved.
[0027] In a preferred embodiment of the invention, the compression
element has a spherical shape for contacting the surface of the workpiece.
[0028] In a preferred embodiment of the invention, the compression
element has a geometric shape such that the compression element does not
produce areas of demarcation along the surface of the workpiece that operate
as
stress risers.
[0029] In a preferred embodiment of the invention, the compression
element has a geometric shape with only rounded edges.
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[0030] In a preferred embodiment of the invention, the tool head
comprises more than one impacting surface having a predetermined spacing.
[0031] A preferred embodiment of the invention, is a method of inducing
compression along the surface of a workpiece comprising the steps of placing a
workpiece in position for processing, using a positioning device coupled to a
flexible arm assembly and a tool head mounted on the flexible arm assembly
such that the flexible arm assembly operates to provide one degree of freedom
in
flexure such that the tool head is free to move perpendicular (normal) to the
surface of the workpiece being worked and parallel to the stroke of the tool
head
and wherein the tool head has a compression element that operates to impact
the surface of the workpiece.
[0032] In a preferred embodiment of the invention, the method further
comprises the step of using a control device to position and move the tool
head
such that the compression element operates to impact the surface of the
workpiece.
[0033] In a preferred embodiment of the invention, the method further
comprises the step of using a control device to position and move the tool
head
such that the compression element operates to travel along the surface of the
workpiece.
[0034] In another preferred embodiment of the invention, the method
further comprises the step of using a control system to direct the movement of
the tool head.
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[0035] In another preferred embodiment of the invention, the control
system operates to monitor and record the movement of the tool head and the
force being applied against the surface during processing of the workpiece.
[0036] In another preferred embodiment of the invention, the control
system operates to monitor and record the force being applied by the
compression element along the surface of the workpiece.
[0037] In another preferred embodiment of the invention, the control
system operates to control the force being applied by the compression element
against the surface of the workpiece.
[0038] In a preferred embodiment of the invention, the tool head is an
impact tool head and the impact spacing of the compression element is varied
to
achieve different states of residual stress by changing the rate at which the
tool
head follows a control surface.
[0039] In another preferred embodiment of the invention, the speed by
which the tool head is moved along the surface of the workpiece is varied to
change the spacing between impacts or strikes of the compression element
along the surface of the workpiece.
[0040] In another preferred embodiment of the invention, the force being
applied against the surface of the workpiece by the compression element is
varied to induce the desired compressive stress along and in the surface of
the
workpiece.
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[0041] In another preferred embodiment of the invention, the striking
force
of the compression element is varied to change the depth of compression along
the surface of the workpiece.
[0042] In another preferred embodiment of the invention, the striking
force
of the impact element is varied to change the magnitude of compression along
the surface of the workpiece.
[0043] In another preferred embodiment of the invention, the impact
spacing and force are determined for inducing the desired form, depth, and
magnitude of the subsurface residual stress distribution produced and the
surface roughness.
[0044] In a preferred embodiment of the invention, the elastic deflection
of
the flexible arm apparatus is selected to accommodate any irregularities of
the
surface being worked.
[0045] In a preferred embodiment of the invention, the tool head is an
impact tool with at least one compression element each having a spherical
shape
for contacting the surface of the workpiece.
[0046] In a preferred embodiment of the invention, the tool head is an
impact tool with at least one compression element having a geometric shape
such that the impact of the compression element does not produce an area of
demarcation along the surface of the workpiece that operate as a stress riser.
[0047] In a preferred embodiment of the invention, the tool head has at
least one compression element each having a geometric shape with only
rounded edges.
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[0048] In a preferred embodiment of the invention, the virtual control
surface is defined to be within the actual surface of the workpiece such that
it
accommodates the range of topography irregularities along the surface of the
workpiece.
[0049] In a preferred embodiment of the invention, the virtual control
surface is defined to be below (further within) the actual surface of the
workpiece
and within the range of motion of the tool head.
[0050] In a preferred embodiment of the invention, one or more
compression elements are elastically pressed against the surface of the
workpiece by a force equal to the spring constant of the flexible arm
apparatus
times the distance from the virtual control surface to the point of contact of
the
one or more compression elements with the surface of the workpiece.
[0051] In a preferred embodiment of the invention, the region of the
workpiece is selected for impact surface enhancement treatment where fatigue
or SCC failures might originate.
[0052] In a preferred embodiment of the invention, the control system is
a
closed loop process.
[0053] In another preferred embodiment of the invention the closed loop
process is achieved by means of an accelerometer, load cell, microphone or
other force or impact transducer attached to the tool head or the flexible arm
assembly such that each impact or strike against the surface of the workpiece
by
a compression element produces an electrical signal that is calibrated to
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determine the force of impact or strike and monitored by the control system to
verify and record the processing.
[0054] In a preferred embodiment of the invention, the control system
functions to determine if the impact strike of the compression element is not
within a predetermined range for producing the desired compression, the
control
system operates to direct the compression apparatus to re-impact or restrike
the
surface of the workpiece at the missed position or position of an inadequate
impact or strike to ensure complete coverage of the surface, and/or rejects
the
workpiece and/or indicates that additional processing is required.
[0055] In a preferred embodiment of the invention the location of any
processing flaw or error is recorded along with the position and/or error in
the
striking force.
[0056] In a preferred embodiment of the invention, the operating
parameters are selected to induce the desired compressive stress within and
along the surface of the workpiece with the minimum amount of cold working.
[0057] A preferred embodiment of the invention is a method of inducing
compression or cold work along the surface of a workpiece comprising the steps
of placing a workpiece in position for processing, using a control system to
direct
a tool head having a compression element such that said tool head follows a
virtual control surface, wherein said virtual control surface is positioned
between
the surface of the workpiece and a distance within the workpiece, and using
the
compression element to contact the surface of the workpiece and provide
controlled cold work along and within the surface of the workpiece.
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[0058] In a preferred embodiment of the invention the method includes the
step of using the compression element to contact the surface of the workpiece
and create a specific controlled zone of plastic deformation achieving a
prescribed depth and magnitude of residual compression along and within the
surface of the workpiece.
[0059] In a preferred embodiment of the invention the method includes the
step of determining the amount of controlled cold work to achieve a desired
amount of work hardening the surface of the workpiece.
[0060] In a preferred embodiment of the invention the method includes the
step of determining the amount of controlled cold work to achieve a desired
amount of refined grain structure along and within the surface the surface of
the
workpiece.
[0061] In a preferred embodiment of the invention the method includes the
step of heat treating the workpiece.
[0062] Other advantages, objects, and embodiments of the invention will
be apparent from the following description and the accompanying drawings.
Brief Description of the Drawings
[0063] The forgoing and other features of the invention will be best
understood with reference to the following detailed description of a specific
embodiment of the invention when read in conjunction with the accompanying
drawings, where in:
[0064] FIG. 1 is a diagrammatic view illustrating the various components
of
the compression apparatus of the subject invention for inducing compression
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along and into the surface of a workpiece;
[0065] FIG. 2 is a perspective diagrammatic view of the compression
apparatus of the subject invention showing the positioning device coupled to
the
flexible arm assembly for supporting the tool head for inducing compression
along and in the surface of a workpiece;
[0066] FIG. 3 is another diagrammatic view of the flexible arm assembly
of
FIG. 2 supporting the tool head effective for inducing compression along and
into
the surface of a workpiece, the flexible arm assembly operates to maintain the
tool head having at least one compression element against the surface of a
workpiece having an irregular surface topography;
[0067] FIG. 4 is a section diagrammatic view of a portion of the flexible
arm assembly of FIG. 2 and supporting the tool head and at least one
compression element for contacting the surface of the workpiece and having an
accelerometer or other transducer for monitoring the force of the compression
element on the surface of the workpiece;
[0068] FIG. 5 is a control diagram showing connections and linkages for
sensing compression force and control of the position of the tool head of the
subject invention during processing;
[0068] FIG. 6a, 6b is a flow chart showing the method of the subject
invention for inducing compression along and into the surface of a workpiece;
[0069] FIG. 7 is a flow chart showing another preferred method of the
subject invention for inducing a controlled amount of cold work along and into
the
surface of the workpiece;
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[0071] FIG. 8 is a plot of high cycle fatigue data and damage tolerance
achieved by the apparatus and method of the subject invention illustrating the
increase fatigue strength of a workpiece having damage along the surface; and
[0072] FIG. 9 is a plot of the longitudinal residual stress distribution
as a
function of depth produced by the present invention.
Detailed Description of the Invention
[0073] The present invention relates to inducing compression and cold
work along and in the surface of component workpieces. In describing the
preferred embodiments of the invention illustrated in the drawings, specific
terminology will be resorted to for the sake of clarity. However, the
invention is
not intended to be limited to the specific terms so selected, and it is to be
understood that each specific term includes all technical equivalents that
operate
in a similar manner to accomplish a similar purpose.
[0074] The apparatus and method of the invention operate to provide
controlled plastic strain by compression to create cold work and compressive
residual stress along and in the surface of a workpiece having an irregular
surface topography, such as welds, castings or re-worked components, for which
automated processing, such as be use of CNC controlled machines or robots
was not previously practical or reliable. In a preferred embodiment of the
invention, the compression apparatus operates as an impact apparatus
comprising a precision positioning control system, such as a computer or a
numerically controlled (CNC) robot or machine tool, for controlling the
movement
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of a tool head that operates to induce compression along and into the surface
of
a workpiece.
[0075]
Preferably, the compression apparatus includes a tool head having
a compression element of a controlled or predefined shape that operates to
create a specific controlled zone(s) of plastic deformation for achieving a
prescribed depth and magnitude of residual compression (compressive stress)
along and within the surface of the workpiece. In a preferred embodiment of
the
invention, the compression apparatus comprises an elastic or spring loaded
flexible arm assembly for positioning the tool head such that it follows a
predefined virtual control surface while holding the compression element in
contact with the workpiece. It should now be understood that in a preferred
embodiment of the invention, the contour of the virtual control surface
remains
substantially constant regardless of any irregularities along the surface of
the
workpiece. The use of an elastic or spring loaded flexible arm assembly
provides
an effective system for accommodating any geometric or topography
irregularities along the surface of the workpiece, such as may be encountered
in
cast, welded or similar component surfaces. The defining and use of a virtual
control surface eliminates both the need for precise prior knowledge of the
workpiece surface topography and the need for complex and expensive systems
for locating and following surface irregularities such as a robotic vision
system or
other system requiring relatively complex sensors for scanning or detecting
surface irregularities.
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[0076]
FIGS. 1 - 4 illustrate a preferred embodiment of the invention
whereby the compression apparatus 100 for inducing compression along and
into the surface of a workpiece includes a tool head 102 for inducing
= compression along and into the surface of a workpiece. In a preferred
embodiment of the invention, the compression apparatus 100 includes a
positioning device 104 having a control system 106, such as a computer,
robotic
system, CNC system, and the like. Preferably, the control system 106 further
comprises other devices, such as a suitable input device 108, like a keypad,
touch screen, or any other suitable input device that can accept information
or
instructions from an operator (including operating parameters, instructions
directing movement or controlling the direction or path of the tool head
during
operation); one or more suitable output devices 110, such as a computer
display,
printer, image-forming or display device, and the like; and a data storage
device
112 such as any of the usual devices used for the storage of data, such as
computer hard drives, floppy discs, binary codes, optical bits, mechanical
scribes, magnetic tapes, compact discs, digital audio tapes, analog tapes,
vinyl
discs, and any device or devices capable of storing data. It should be
understood
that the control system 106 can include any combination of the above
components, or any number of different components, peripherals, and other
devices. Preferably, the control system 106 operates under the control of an
operating system, such as the open source Linux operating system, the
WINDOWS operating system developed by Microsoft Corporation or the
MACINTOSH operating system developed by Apple Computer Corporation. It
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should be understood, however, that other operating systems could be utilized
to
implement the system software 114 of the compression apparatus 100 of the
present invention. It should be understood that the control system 106 can
also
comprise conventional CNC code control, but it is understood that the control
system 106 may include any computer controlled machine programmed for
directing the positioning device 104 to move the tool head 102 along a defined
virtual control surface. The workpiece W being processed in FIGS. 2 - 4 is
shown as a right-angle butt weld of two plates, but it is to be understood
that the
workpiece processed by the invention may be any surface having a relatively
smooth surface found on machined or ground components, or an irregular
surface topography such as welded component surfaces, cast surfaces, or other
surfaces having such an irregular topography.
[0077] FIGs. 2,
3 and 4, illustrate in increasing detail, a preferred
embodiment of the compression apparatus 100 for inducing compression along
and into the surface S of a workpiece W. In FIG. 2, the preferred embodiment
the tool head 102 is in the form of an impact tool such as a peening head that
is
driven by a trip hammer or other similar device that is operated
pneumatically,
hydraulically, magnetically, or electrically at a controlled reciprocating
rate of
impact. Preferably, the tool head 102 includes an compression element 136,
shown in FIG. 4, in the form of a spherical ball or other geometrically shaped
element, having a generally rounded contour (such as an ellipsoid, cylindrical
or
other geometric shapes) so that the impact of the compression element 136
against the surface of the workpiece W does not produce any sharp surface
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demarcations that can function as stress risers along the processed surface.
It
should be understood that the specific geometric shape of the compression
element 136 as well as the specific material composition of the compression
element 136 is selected for providing a particular surface finish and residual
stress distribution for the workpiece. In a preferred embodiment of the
invention,
the compression element 136 is formed from Cr steel, which provides relatively
long tool life while reducing dust, and minimizing replacement needs and cost.
Further, the use of Cr steel compression elements reduces or eliminates the
formation of surface demarcations that can lead to fatigue crack initiation,
such
as notches often created during shot peening by broken shot. It should be
understood that the compression element can be formed from a variety of other
materials having different modules of elasticity, hardness, and other
characteristics depending on the material, the particular structure, and the
compressive residual stress to be induced along and into the surface of the
workpiece.
[0078] In
another preferred embodiment of the invention, the compression
element 136 is mounted within or to the tool head 102 such that it is free to
rotate
with respect to the tool head 102. It has been found that such rotation
permits
the compression element 136 to rotate during use thereby allowing the surface
of
the compression element 136 to be aligned for contacting the surface of the
workpiece while evenly wearing the compression element 136 to greatly extend
the life of the compression element 136.
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[0079] The positioning device 104, such as a conventional 6-axis
industrial
robot, is coupled to a flexible arm assembly 116 attached to the end of a last
axis
122 of the positioning device 104. The workpiece W is held in position for
processing by a conventional fixture (not shown) or other suitable means of
fixing
the location and orientation of the workpiece W. In a preferred embodiment of
the
invention, as shown in FIGs. 3 and 4, the flexible arm assembly 116 includes
one
or more longitudinally extending parallel springs 120, such as longitudinal
extending flat springs, having sufficient length to allow the tool head 102 to
be
positioned as needed and to access tightly spaced areas of the workpiece W.
The springs 120 operate to form a rectangular suspension that deforms to
maintain the axis 122 of the tool head 102 normal to the processing surface S
of
the workpiece W during operation when the tool head 102 is moved across the
surface S of the workpiece W being processed.
[0080] In a preferred embodiment of the invention, the operating range
124, the distance between the point of contact 126 with the tool head 102 at
the
irregular surface IS and a virtual control surface 128, is shown in FIG. 4,
and is
set to be greater than the variation in the topography of the irregular
surface IS.
The amount of spring preload of the flexible arm assembly 116 is equal to at
least the operating range 124 times the combined spring constant of the
springs
120 so that the tool head 102 is preferably held at all times in contact with
the
surface S as well as the irregular surface IS. The preload of the springs 120
are
determined to cause the tool head 102 to follow a virtual control surface 128
that
is below the actual irregular surface IS (deeper within the surface of the
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workpiece) by an amount that is greater than the deviations along the surface
S
of the workpiece W. For example, if the variation in weld topography for a
group
of welded workpieces to be processed was 0.25 inches, defining a virtual
control
surface as 0.5 inches below the surface ensures the tool head 102 remains in
constant contact with the irregular surface IS. The actual amount of spring
preload may vary from the example illustrated, and may be changed over any
range that will keep the tool head 102 in contact with and not overload the
springs 120 of the flexible arm assembly 116. It
should now be apparent that
when the flexible arm assembly 116 presses the compression element 136
against the surface S of the workpiece W in order to follow the virtual
control
surface 128 located below the actual irregular surface IS of the workpiece W,
the
reciprocating axis 130 of the tool head 102 is then held parallel to the face
132 of
the last axis 118 of the positioning device 104. In this
way, the force being
applied against the surface S of the workpiece W is at all times held normal
to
the control surface 128 so that the force deforming the irregular surface IS
of the
workpiece W remains constant even in the event a complex surface is followed.
[0081]
Referring to FIG. 5, in a preferred embodiment of the invention, the
compression apparatus includes a tool head having an accelerometer 134
attached to the tool head 102 or, alternately the end of the springs 120, and
produces an electrical signal ES proportional to the force being applied
against
the surface S of the workpiece W. The signal ES is used by the control system
106 and is calibrated to allow direct determination of the force being applied
against the surface S of the workpiece W in real time during processing. It
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should be understood that other devices such as load cells, microphonic
transducers, or other force sensing transducers may be used together with or
as
an alternative to an accelerometer for determining the force being applied
against
the surface of the workpiece. The control system 106 operates to monitor the
force, compares it to the upper and lower force operating bounds, and the
system software 114 operates to direct the data storage device 112 to record
the
processing and any detected errors in the force that falls outside of the
allowed
range. The system software 114 preferably operates to select from a range of
actions in the event of a force fault, such as, to simply indicate the
occurrence of
the fault, rejecting the workpiece, or interrupting the process and repeat the
processing of the workpiece at the location of the fault. It should now be
understood by one skilled in the art that other actions, not noted here, may
be
taken as well with respect to the manufacturing or processing of the
workpiece.
In a preferred embodiment, the system software 114 operates to provide an
output on the output device 112 to warn a system operator that a fault has
occurred. The operator can then review the workpiece and the identified fault
and using the input device 108 can provide additional processing instructions
into
the control system 106 for the workpiece which can then be implemented by the
system software 114 of the compression apparatus 100.
[0082]
Referring to FIGs. 6a ¨ 6b, the method of inducing compression
along and into the surface of a workpiece comprises the steps of defining a
region of the component for processing (step 200). In an exemplary
illustration,
such regions may include regions of high tensile residual stresses in weld
fusion
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and heat affected zones (HAZ) along the surface of a component and containing
notches at the weld toes and those formed by the overlapping passes of
welding.
The process zone of the example extends through the fusion, heat affected zone
(HAZ) and into the parent metal beyond the weld. The depth and magnitude of
residual compression needed to achieve the required fatigue performance is
determined from prior failure analysis, observation of surface flaws,
measurement of existing residual tensile stress, fracture mechanics analysis,
and/or other conventional materials performance methods (step 202). It should
be understood that processing of welded areas may be done when the surface is
at an elevated temperature, such as a result of welding or heated by other
means, to provide deeper and higher magnitude of plastic strain than produced
by treating the surface at room temperature. A tool head having a suitable
diameter to provide access to the surface features and geometry of the
workpiece to be processed and which provides a suitable surface finish is
selected (step 204). A force and the allowed upper and lower force bounds that
will produce suitable depth of compression are selected (step 206). The
workpiece is positioned on a suitable stand, conveyor or other means, so that
it is
in a fixed reproducible position, accurate within the irregularities of the
surface,
for processing of a number of like workpieces (step 208). The workpiece
surface
irregularity is determined by measuring and/or estimating the range of
variation
for the population of pieces to be processed (step 210). A virtual control
surface
is defined at a distance between the actual surface being processed and below
the actual surface as described above (step 212). The tool head is then moved
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along following the virtual control surface in a pattern such that the
compression
elements contacts the surface with sufficient compressive force to create a
desired distribution of residual stress and surface finish (step 214). It
should be
understood that the control system software, such as CNC processing control
code, is prepared for the particular positioning device that positions the
tool head
for providing compression along the virtual control surface so that the
springs
deflect forcing the compression element(s) against the surface, including any
irregular surface portions of the workpiece. The tool head is moved over the
surface at the speed and in the pattern necessary to cover the surface to be
processed and induce the desired compression along and within the surface of
the workpiece (step 216).
[0083] In a
preferred embodiment of the invention the force being applied
against the surface of the workpiece is monitored by the accelerometer, or
other
force sensor, attached near the tool head (step 218). The force measured as a
calibrated electrical signal is read by the control system and the system
software
operates to compare to a requested or predetermined desired force to determine
whether the area being processed is being treated properly, that is, within
the
range of force needed to produce the depth and magnitude of compression
required (step 220). Preferably, the processing data is recorded by the data
storage device and the system software operates to compare it to the
predetermined desired force to provide processing quality control records
(step
222). Because the measurement of the compression force is measured in real
time during the processing, if desired, the control system can operate to
CA 02845964 2014-03-13
determine if additional processing is required and if additional processing is
needed it can operate to reposition the tool to repeat the processing of the
affected area of the workpiece (step 224). Preferably, the location and the
force
and/or any deviation in force being applied along the surface are recorded and
a
determination can be made if treatment of the surface of the workpiece has
been
properly achieved (the depth and magnitude of residual compression has been
achieved) (step 226). If in the event the desired amount of compression has
not
been achieved, the control system can operate to reject or identify the
workpiece
for further processing (step 228). It should now be apparent that the
mechanical
linkages of the positioning device, flexible arm assembly, and tool head
provide
the location and speed of positioning of the tool head along the workpiece
while
following the predefine path (control surface) inputted into the control
system
using the input device, such as inputted into the system software, such as a
CNC
programmed code. An output signal from the accelerometer or feedback loop
provides closed loop control for comparing to the preset minimum and maximum
force bounds. If the force is outside of the allowed processing bounds, the
control software is programmed to record the error, reject the workpiece
(component), and/or signal the positioning device to reposition the tool head
to
repeat the treatment for inducing compression at the location where the fault
occurred. The system software of the control system can also be programmed to
record into the data storage device the entire sequence of processing
(including
for a impact tool recording information concerning the individual impacts) to
provide a detailed record of the processing. Alternately, it may simply record
the
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success or failure of the processing and indicate that to the operator. In
another
preferred embodiment of the invention, the control software of the control
system
can operate to compare the force applied at a location with the preset minimum
and maximum force bounds and determines if the amount of force applied to the
surface at the location and if additional processing is required. The system
software then operates to calculate the correction force that must be applied
to
the surface at that location to induce the desired compression (compressive
residual stress).
[0084] As shown, the mechanical linkages of the control system,
positioning device, flexible arm assembly, and tool head operate together to
provide the location and speed of positioning of the tool head along the
surface
of the workpiece while following the control surface. The output signal from
the
accelerometer or feedback loop provides closed loop control and is compared by
the control system to a minimum and maximum force boundary. If the force is
outside of the allowed processing force boundary, the system software of the
control system operates to record the error, reject the component, and/or
signal
the robot to reposition the tool to repeat the compression treatment at the
location where the fault occurred. The system software further operates to
record the entire sequence of processing and provides a detailed record of
such
,processing. Alternately, in a preferred embodiment the system records the
success or failure of the processing and indicates that to the operator.
[0085] In a preferred embodiment of the invention the method includes
determining the compression force and the allowed upper and lower force
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bounds that will produce suitable depth of compression are selected (step
302).
In a preferred embodiment of the invention, the compression force is imparted
along the surface of the workpiece by impacting the surface with the
compression element such that the impact spacing is selected to produce the
required residual compression, and/or cold work, and/or a suitable surface
finish
(step 304). The rate of reciprocation of the impacting tool is selected to
allow
calculation of the positioning speed for the desired spacing and the time
required
for processing the selected area (step 306), It has been found that preferably
the impact rate or frequency of the tool head is from about 1 to about 100
impacts or strikes per second. In a preferred embodiment the speed of moving
the tool head over the surface of the workpiece is calculated to provide the
desired impact spacing (step 308). It should be understood that the impact
spacing may be varied for the purposes of changing both the residual stress
distribution and the surface finish by altering the speed of tool positioning.
[0086] In
another preferred embodiment of the invention, as shown in FIG.
7, the method of the subject invention for inducing compression along the
surface
of a workpiece comprises the steps of placing a workpiece in position for
processing (step 400) and using a control system to direct a tool head having
a
compression element such that said tool head follows a virtual control surface
(step 402). It should be understood that the virtual control surface is
positioned
between the surface of the workpiece and a distance within the workpiece. A
compression element is then used to contact the surface of the workpiece and
provide controlled cold work along and within the surface of the workpiece
(step
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404). I a preferred embodiment, the method further comprises the step of using
the compression element to contact the surface of the workpiece and create a
specific controlled zone of plastic deformation achieving a prescribed depth
and
magnitude of residual compression along and within the surface of the
workpiece
(step 406). In a preferred embodiment, the method includes the step of
determining the amount of controlled cold work to achieve a desired amount of
work hardening of the surface of the workpiece (step 408) and/or the desired
amount of controlled cold work to achieve a desired amount of refined grain
structure along and within the surface of the workpiece (step 410). After the
surface of the workpiece has been treated, the workpiece can be heat treated
(step 412) or have other processes applied, such as plasma sprays or other
similar coatings applied (step 414).
[0087]
Referring to FIG. 8, is a plot of high cycle fatigue data and damage
tolerance achieved by the apparatus and method of the subject invention and
illustrates the increase fatigue strength of a workpiece having damage along
the
surface. The residual stress distributions unexpectedly found to be produced
by
the invention using a tool head comprising a compression element for impacting
the surface of the workpiece is shown in FIG. 9. Unlike most peening
operations,
the residual compression is found to be maximum at the surface, and to extend
nearly linearly to a depth of over two (2) millimeters. Conventional shot
peening,
deep rolling, or low plasticity burnishing deform the surface in Hertzian
loading
causing maximum compression to occur below the surface where the maximum
Hertzian shear occurs. The form of the residual distribution produced by the
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present invention using a tool head comprising a compression element (or a
peening tool) for impacting the surface of the workpiece was expected to
follow
the same Hertzian form. It was unexpectedly found however that the invention
of
the subject application produces both maximum compression at the surface and
compression deeper than typically achieved with the other available methods,
and operates to improve fatigue strength and damage tolerance in workpieces.
It
is known that because fatigue cracking initiates at the surface, having
maximum
compression occurring at the surface will provide the maximum high cycle
fatigue
performance of the component. The greater depth of compression allows deeper
damage of any sort to be tolerated by holding the notch formed in high
compression. Both the
high surface compression and greater depth of
compression are known to be beneficial in suppressing SCC by maintaining the
surface, and any near surface material affected by intergranular corrosion,
below
the tensile stress threshold for cracking.
Accordingly, the compression
apparatus and method of the subject invention produce both maximum
compression at the surface and compression deeper than typically achieved with
the other available methods, thereby operating to improve fatigue strength and
damage tolerance in components.
[0088] A method
and compression apparatus for improving the fatigue and
stress corrosion cracking performance of irregular surfaces of components,
such
as welds, using robotic or CNC machine to position a tool head for inducing
compression along and into the surface of a workpiece to automatically follow
the
surface irregularities. The tool head is operates and follows the irregular
surface
CA 02845964 2014-03-13
by a flexible arm assembly while following a virtual control surface located
below
the actual surface of the workpiece and allowing the irregular topography
surface
to be uniformly processed. In a preferred embodiment, cold working is
minimized
by using single impact spacing at controlled intervals. In another preferred
embodiment of the invention, the spacing and impact force are controlled to
introduce a desired amount and depth of cold working. Impact force is measured
and compared to target values for performance documentation and correction by
repositioning, such as through CNC control, to recover any missed or
inadequately peened region. Thus, the subject apparatus and method of the
subject invention allows for full coverage of compression without repeated
treatment (such as without impacting the surface of the workpiece repeatedly)
thereby minimizing cold work of the surface. Minimizing the amount of cold
work
eliminates or reduces softening of components formed from hardened steels and
reduces swelling and other unacceptable changes in critical dimensions due to
cold work induced phase transformations.
[0089] It
should now be apparent to one skilled in the art that the
compression apparatus and method of the subject application allows
compression to be induced along and into the surface of a workpiece using a
relatively small tool head such that compression can be induced in confined
areas, such as in weld assemblies (i.e. welded closed impeller components).
Further, during operation various surface configurations can easily be treated
without requiring modifying processing codes and other systems.
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[0090] It should also now be apparent to one skilled in the art that the
compression apparatus and method of the subject invention permits the
inducement of compression along and into the surface of the workpiece to be
performed in a controlled manner and/or with a minimum amount of cold working
of the surface thereby improving thermal and mechanical stability of the
treatment and with optimum efficiency and low cost. Further, the compression
apparatus and method of the subject invention permits the use of a tool head
in
the form of an impact tool that provides controlled impact of the compression
element against the surface of the workpiece thereby eliminating or reducing
the
possibility of laps or folds being formed along the surface of the workpiece
typically produced in shot peening caused by overlapping shot impact zones.
[0091] The use of the compression apparatus and method of the subject
invention produces nearly linear residual stress depth distribution similar to
laser
shock peening processes but to a greater depth. Further, unlike shot peening
and low plasticity burnishing that generally produces subsurface maximum
compression with reduced compression at the surface, the compression
apparatus and method of the subject invention having a tool head in the form
of
an impact tool head provides maximum compression at the surface of the
workpiece for maximum resistance to surface fatigue crack initiation or SCC.
Further, depth of compression is sufficient to prevent fatigue crack growth
from
surface flaws generally too small to be detected by NDT inspection methods
(<0.020 inches deep) while providing a safe and generally reliable means for
restoring components to service following conventional NDT inspection.
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[0092] Accordingly, the method and compression apparatus of the subject
invention for inducing compressive residual stress along and into the surface
of
workpiece; preferably is an automated system by use of a positioning means,
such as a robotic or other CNC machine tool; produces a desired depth and
magnitude of compression; and is reliably applied to work pieces having an
irregular topography surface.
[0093] The compression apparatus and method for inducing compression
along the surface of a work piece provides a unique apparatus that operates to
perform a controlled amount of cold work and surface hardening of the surface
as well as being able to operate to induce controlled compression along and
into
the surface of a work piece having surface irregularities. It should now be
apparent that by positioning the tool head to follow a predefined virtual
control
surface while holding the compression element in contact with the workpiece or
ensuring the compression element continues to strike the surface of the
workpiece (for an impact tool head), eliminates the need for precise prior
knowledge of the workpiece surface topography and the need for complex and
expensive systems for locating and following surface irregularities. It should
be
understood that the compression apparatus and the structured methodology
utilized by the control system is not limited solely to the specific design
described
herein and although the foregoing invention has been described in some detail
for purposes of clarity of understanding, it should now be apparent that
certain
changes and modifications may be practiced within the scope of the disclosure
and that the various embodiments presented can be easily modified while
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keeping within the scope and spirit of the subject invention. Accordingly, it
should also be understood that the present disclosure is to be considered as
exemplary of the principals of the invention and is not intended to limit the
invention to the embodiments and the specific examples illustrated and the
invention is not to be limited to the details given herein, but may be
modified
within the scope and equivalents of the descriptions and examples contained
herein.
[0094] What is claimed is:
34
