Note: Descriptions are shown in the official language in which they were submitted.
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Description
APPARATUS AND METHOD FOR FORMING A WELD JOINT HAVING
IMPROVED PHYSICAL PROPERTIES
Cross Reference to Related Application
This application claims benefit of International Application No.
PCT/LJS02/35214
filed November 1, 2002, under the Patent Cooperation Treaty and to U.S.
Provisional
Application No. 60/367,623 filed March 26, 2002.
Technical Field
This invention relates to an apparatus and a method for forming a weld joint
having
improved physical properties and, more particularly, to a method of forming a
weld joint
utilizing a controlled method of inducing a specific compressive residual
stress pattern and
degree of cold working along the welding line to improve the physical
properties of the
weld.
Background of the Invention
In the manufacture and construction of many types of structures, welding, such
as gas
welding, arc welding, resistance welding, thermite welding, laser welding, and
electron-
beam welding, has reduced or replaced the use of various types of fastening
methods, such
as bolting, riveting and the like. Such welding techniques either involve the
complete fusion
of material forming a liquid state which subsequently solidifies producing
altered
microstructures and properties, or they involve a solid state welding process,
but again
producing a highly altered metallurgical state. The particular welding process
best suited to
join two pieces of metal depends on the physical properties ofthe metals, the
specific use to
which they are applied, and the production facilities available.
Unfortunately, several significant problems have limited the application
ofwelding
~ . for certain manufacturing processes. One problem generally associated with
welding is that
the temperature required to melt or plasticize the parent materials typically
reduces their
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yield strength. Another common problem associated with welding is the
formation of tensile
residual stresses created in the workpieces during the welding process by the
expansion and
then contraction of the fusion or plasticized zoned and regions adjacent to
the weld joint.
Such tensile residual stresses are well known to reduce both fatigue life and
increase
sensitivity to corrosion-fatigue and stress corrosion cracking in a wide
variety of materials.
It has also been found that micro-segregation kinetics found in some aluminum
alloys,
typically used in the aircraft industry, are sufficiently rapid such that
stress corrosion
resistance is reduced even a$er a short thermal transient. Further, where two
different
workpieces having different sizes are welded together, any residual stress is
amplified due to
the difference in heat capacity between the two workpieces. Another problem
associated
with many welding processes is the production of flash or excess material at
the edge of the
fusion or stir zone. Fatigue crack initiation typically occurs out of this
area and is usually
associated with the mechanical discontinuity at the edge or "toe" of the weld.
This edge or
"toe" has been found, in virtually all types of welds, to be the area where
the highest tensile
residual stresses are found.
Unfortunately, until now, there is no direct and cost effective method of
restoring
yield strength and improving the corrosion resistance of a weld joint. While
acceptable
corrosion resistance can be achieved by post-weld induction heat treatment,
this technique is
economically and technically impractical for all but the smallest and simplest
of geometric
shapes. Induction heating is also not easily controlled spatially and often
results in
overheating the material around the weld. While tensile stresses may be
reduced or
eliminated, compressive stresses are not easily induced by heat treatment
techniques, except
in special cases such as internally cooled tubular (pipe) weldments. Other
material
properties, such as yield strength, are also difficult to improve. Further,
local heating by
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induction, or other means, can result in distortion and an increase tensile
residual stresses
elsewhere in the workpiece.
Corrosion resistance of a weld joint may also be improved by applying a
coating,
such as paint, electroplating or galvanizing, to all susceptible surfaces.
However, such
coatings also require a second independent process, which significantly
increases the cost
and production time. Further, such coatings provide only a superficial
protective layer and
do not protect surfaces that cannot be accessed, and protection of the surface
is lost if the
coating is broken or deteriorates during service.
Methods of inducing compressive stresses along the surfaces of a workpiece
have
been used to improve the fatigue life and corrosion resistance in the surface
of a final part.
One such method that has been utilized for inducing a layer of compressive
stress in the
surface of a workpiece to improve the fatigue life and corrosion resistance
ofthe final part is
burnishing. The generally accepted practice for burnishing utilizes repeated
deformation of
the surface of the workpiece, in order to deliberately cold work the surface
of the material to
increase the yield strength. Yielding the surface of the material in tension
so that it returns
in a state of compression following deformation develops compressive stresses.
Unfortunately however, excess cold working may produce tensile surface
stresses or spalling
damage and may leave the surface susceptible to overload and thermal
relaxation.
Other methods commonly used in the industry to induce compressive stress in
the
surface of a part include shot peening, whereby a plurality of metallic or
ceramic pellets are
projected mechanically or through air pressure to impinge the surface of a
workpiece, and
gravity peening, whereby pellets are dropped from a predetermined distance
onto the surface
of the workpiece. While shot peening and gravity peening may be used for
inducing
compressive residual stresses along the surface ofthe weld joint,
unfortunately, shot peening
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and gravity peening also impart an uncontrolled amount of cold work making it
difficult to
optimize the physical properties of the weld. Further, the degree of cold
working of the
material by shot peening or gravity peening is relatively high, which may be
undesirable for
many applications. The shot or gravity peening induced compressive residual
stresses are
relatively shallow, affording limited benefit in arresting fatigue or stress
corrosion cracks
because the shallow compressive layer may be lost to wear or corrosion in
service. Shot
peening and gravity peening also produce a poor surface finish further making
the processes
unacceptable for many applications. It is also known that the beneficial
effects produced by
shot peening and gravity peening are generally lost as the pattern of
compression relaxes
with time in elevated temperature service.
It should now be apparent that until now, in addition to the problems
identified
above, all post welding procedures have required a second-pass process that
significantly
adds to the cost of manufacturing, since it takes more time and effort to
produce a finished
part than the time required for those parts not needing post welding
treatment. Depending on
the size, or number of parts, or the location of the weld, such increase in
time and cost
associated with a second-pass process often makes post-welding treatment
impractical. In
addition, until now, such methods for inducing compressive stress along the
surface of a
joint line in a prescribed pattern have not been used as a facet of the
welding process.
Consequently, a need exists for a relatively inexpensive and fast method, and
an
apparatus for implementing the method, of forming a weld joint having a
selected pattern of
compressive residual stress and cold working along the surface of the weld
joint, and the
regions adjacent to the weld line, which is effective for improving the
physical properties of
the weld joint and the final part or product. In addition, a need exists for
an apparatus and
method of forming a weld joint that does not require the performance of a
second-pass
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process or require the use of a second machine.
Disclosure of the Invention
The novel method of forming a weld joint of the present invention comprises
the
steps of performing a welding operation along a weld line to join two or more
workpieces
together; and performing a compression operation to induce a deep layer of
compression in
the surfaces of the workpieces to improve the material properties of the final
product. In a
preferred embodiment of the invention, the welding operation forms regions of
elevated
surface temperature along the workpieces, and the compression operation is
performed along
the regions to produce deep compression.
In another preferred embodiment of the invention, the method of forming a weld
joint further comprises the step of using x-ray diffraction for determining
the desired
compressive stress pattern and amount of cold working and surface hardening
for optimizing
the physical properties of the weld joint and the finished product.
In another embodiment of the invention, the method of forming a weld joint
further
comprises the step of varying the amount of cold working to achieve the
desired physical
properties of the weld joint.
In another preferred embodiment of the invention, the method of forming a weld
joint comprises the step of inducing a pattern of compressive residual stress
with a minimal
amount of cold working along a selected region.
In another preferred embodiment of the invention, the method of forming a weld
joint comprises the step of inducing a pattern of compressive residual stress
with less than
about 5 percent cold working along the selected region.
In another preferred embodiment of the invention, the method of forming a weld
joint comprises the step of inducing a pattern of compressive residual stress
with less than
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about 2 percent cold working along the selected region.
In another preferred embodiment of the invention, the method of forming a weld
joint comprises the step of inducing a pattern of compressive residual stress
and varying
amounts of cold working to achieve the desired physical properties of the weld
joint and the
final part.
In another preferred embodiment of the invention, the method of forming a weld
joint comprises the step of utilizing a compression tool having a single-point
of contact
means for applying a force along the weld line to produce a zone of
deformation having a
deep layer of compression within the weld joint.
In another preferred embodiment of the invention, the method of forming a weld
joint comprises the step of passing a compression tool in a predetermined
pattern across the
weld line such that the zones of deformation formed by each pass of the
compression tool
overlap in a controlled manner.
In another preferred embodiment of the invention, the method of forming a weld
joint comprises the steps of predetermining and adjusting the application
force to be applied
along the weld line any heat affected regions; and using a programmable
control unit to
direct a compression tool in a predetermined pattern over the weld line and
regions adjacent
to the weld line to provide the maximum compressive residual stress with the
minimum
amount of cold working and surface hardening.
In another preferred embodiment of the invention, the method of forming a weld
joint comprises the step of using a control device for automatically
controlling the
movement and position of a welding tool.
In another preferred embodiment of the invention, the method of forming a weld
joint comprises the step of using a control device for automatically
controlling the
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movement, position and compression force of a compression tool.
In another preferred embodiment of the invention, the method of forming a weld
joint includes the step of performing a welding operation using a welding tool
selected from
the group consisting of gas welding tools, arc welding tools, resistance
welding tools,
thermite welding tools, laser welding tools, and electron-beam welding tools.
In another preferred embodiment of the invention comprises the step of using a
welding apparatus having a welding tool for performing a welding operation and
a tool for
inducing a layer of compressive residual stress along the weld line to form a
weld joint
having improved physical properties.
In another preferred embodiment of the invention, the method of forming a weld
joint includes the step of heating a selected region of a workpiece and
inducing compression
along the selected region.
In another preferred embodiment of the invention, the method of forming a weld
joint includes the step of cooling a selected region ofthe workpiece prior to
inducing a layer
of compressive residual stress along the surface of the selected region.
In another preferred embodiment of the invention, the welding tool is capable
of
performing at least one welding operation, the welding operation being
selected from the
group consisting of gas welding, arc welding, resistance welding, thermite
welding, Laser
welding, and electron-beam welding.
In another preferred embodiment of the invention, the apparatus for forming a
weld
joint comprises a welding tool for performing a welding operation and a
compression tool
for inducing a layer of compressive residual stress along the surface of the
weld joint and
any heat affected regions.
In another preferred embodiment of the invention, the apparatus for forming a
weld
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joint comprises a welding apparatus having a single-point of contact
compression tool.
In another preferred embodiment of the invention, the apparatus for forming a
weld
joint comprises means for controlling the movement of the welding tool.
In another preferred embodiment of the invention, the apparatus for forming a
weld
joint comprises means for controlling the movement of the compression tool.
In another preferred embodiment of the invention, the apparatus for forming a
weld
joint comprises means for controlling the pressure being applied by the
compression tool
along the surface of a workpiece.
In another preferred embodiment of the invention, the welding apparatus
comprises
means for heating a region of a workpiece.
In another preferred embodiment of the invention, the welding apparatus
comprises
means for cooling a region of a workpiece.
Another preferred embodiment of the invention comprises a structure formed by
welding and having a preferred residual stress pattern formed along the weld
line.
Another preferred embodiment of the invention comprises a structure formed by
a
plurality of plates, the plates being secured in place by welding and having a
selected
compressive residual stress pattern therein.
In another preferred embodiment of the invention, the structure is selected
from the
group consisting of aircraft structures, marine structures, construction
structures, automotive
structures, and canisters, containers, and the like.
Accordingly, it would be desirable to have a method and an apparatus for
performing
the method of forming a weld joint having an improved finish and physical
properties,
including improved corrosion resistance and fatigue life over parts formed
using
conventional welding methods and apparatus.
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It would also be desirable to have a method and an apparatus for performing
the
method of forming a weld joint that induces a selected compressive stress
pattern along a
weld line.
It would also be desirable to have a relatively inexpensive method and an
apparatus
for performing the method of forming a weld joint having a compressive stress
layer formed
along the weld line and further having a relatively well defined localized
compressive stress
zone.
It would also be desirable to have a method and an apparatus for performing
the
method of forming a weld joint which can induce a compressive stress layer
along the
surface of the weld joint and which provides a relatively smooth surface along
the weld line.
Other features and advantages of the invention will be apparent from the
following
description, the accompanying drawings and the appended claims.
Brief Description of the Drawings
FIG. 1 is a schematic of the welding apparatus for implementing the method of
forming a weld joint of the present invention showing the controller,
positioning device,
welding tool and the compression tool;
FIG. 2 is a schematic perspective view of a preferred embodiment of the
welding
apparatus of FIG. 1 showing the welding tool and the compression tool;
FIG. 3 is a partial schematic side view of the welding apparatus of FIG. 2;
FIG. 4 is a graph illustrating that a greater depth of compression can be
achieved
with increase loading in spherical ball burnishing (using a 0.75 in (1.9 cm)
ball) at an
elevated temperature of 400 °F (204 °C) as compared to the same
process at room
temperature;
FIG. 5 is a graph illustrating that an increase in surface tensile stress can
be obtained
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by cooling the surface of the workpiece (plotted as a function of the
temperature differential
between the surface and the interior of the workpiece);
FIG. 6 is a schematic of another embodiment of the welding apparatus for
implementing the method of forming a weld joint showing means for spraying a
coolant to
create a temperature gradient between the surface and the interior of the
workpiece prior to
and during the compression operation;
FIG. 7 is a schematic of another embodiment of the welding apparatus for
implementing the method of forming a weld joint showing another means for
delivering a
coolant to create a temperature gradient between the surface and the interior
of a workpiece
prior to and during the compression operation;
FIG. 8 is a cross-sectional view of the welding apparatus of FIG. 7 taken
along
section A - A;
FIG. 9 is a graph illustrating the surface residual stress distribution
induced in the
surface of a workpiece using a conventional method of welding as compared to
the method
of welding of the present application; and
FIG.10 is a graph illustrating the average percent cold work distribution
relating to
the methods of welding of FIG. 9.
Detailed Description of the Preferred Embodiment
The present invention is directed to a new and novel method and apparatus for
performing the method of forming a weld joint and, a more particularly, a
method and
apparatus for forming a weld joint which utilizes a controlled process of
inducing a specific
compressive residual stress pattern and degree of cold working and surface
hardening along
a weld line to improve the physical properties of the weld joint and the
resulting final
product. In a preferred embodiment of the invention, the welding apparatus
comprises a
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welding tool for welding one or more workpieces, and a compression tool for
inducing a
layer of residual compressive stress in the surface of a workpiece. In another
preferred
embodiment of the invention, the method utilizes a process of inducing a
specific and
selected pattern of compressive residual stress and selected amount of cold
working and
surface hardening, such as by the process of controlled low plasticity
burnishing, to improve
the physical properties of the weld joint and the resulting final product.
Referring to FIGS.1, 2 and 3, a pair of workpieces 10,12 having opposing ends
14,
16, respectively, are positioned to be mated together by welding. The welding
apparatus 100
comprises a welding tool 102 having one or more welding heads effective for
performing a
conventional welding operation such as gas welding, arc welding, resistance
welding,
thermite welding, laser welding, ultrasonic welding, friction stir welding,
and electron-beam
welding. Preferably, the welding apparatus 100 further comprises a compression
tool 106
for producing a zone of deformation and a relatively deep layer of compression
along the
weld line 18 and any heat affected regions 20, which are typically adjacent to
the weld line
18. While various compression tools have been developed for inducing a layer
of
compressive residual stress in the surface of a workpiece, preferably, the
compression tool
106 is a single-point burnishing tool for implementing the method of the
present invention.
As shown in FIG. 3, the single-point burnishing operation is performed using
the forward
most tip 108 of a burnishing ball 110 which is caused to pass over the weld
line 18 (FIG. 2)
and any heat affected regions 20 in a rolling motion to induce deep
compression. The
compression tool 106 operates by forcing the burnishing ball 110 against the
surface ofthe
workpiece 10,12 and along the weld tine 18 to produce a zone of deformation
and to induce
a deep layer of compression within the surface of the workpieces 10, 12.
The welding tool 102 and the compression tool 106 can be mounted onto a
single, or
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on separate, conventional positioning device 104 that can be manually or
automatically
operated and can be controlled using a conventional controller 116 operating
under computer
software control for automatically controlling the positioning ofthe welding
tool 102 and the
compression tool 106 . The positioning device 104 may also include belt and/or
gear drive
assemblies (not shown) powered by servomotors (not shown), as is known in the
art and can
be in operable communication with the controller 116 through suitable wiring
(not shown).
During the welding process, the welding tool 102 is moved along the weld line
18
formed by the opposing ends 14,16 of the workpieces 10,12, respectively, to
weld the ends
14, 16 of the workpieces 10 and 12 together. It should be understood that in
another
preferred embodiment of the invention, the welding tool 102 can be fixed and
the
workpieces 10, 12 can be moved relative to the welding tool 102. A layer of
residual
compressive stress is then induced along the weld line 18 and any heat-
affected regions 20
produced by the heat generated during the welding process, using the
compression tool 106.
It should be understood that the compression tool 106 can also be utilized to
induce a layer
of residual compressive stress to other regions along the surfaces of the
workpieces 10,12 to
produce a final part having a desired compressive stress pattern.
Preferably, conventional x-ray diffraction techniques are used to analyze the
area
along the weld line 18 and the heat affected regions 20, for determining a
desired
compressive stress pattern and the amount of cold working and surface
hardening required to
optimize the physical characteristics of the weld joint 22 and the resulting
final product. The
burnishing ball 110 can then be passed in a selected pattern and pressure
across the weld line
18, and any heat affected regions 20, to induce the desired compressive stress
pattern with
the desired amount of cold working and surface hardening. It has been found
that the
method of single-point burnishing, applied in a single-pass or multiple passes
of reduced
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compressive force, can be an effective method for producing compressive
residual stresses
following tensile deformation of the surface to a certain depth within the
weld joint 22, and
any heat affected regions 20, and to produce deep compression with minimal
cold working.
It has also been found that this single-point burnishing method can be used to
produce a final
part with less cold work and surface hardening than a part subjected to
conventional shot
peening or gravity peening. Further, the residual compressive stress developed
by this
method penetrates to a greater depth within the surface of the workpiece than
developed by
conventional methods, such as shot peening and conventional burnishing. The
amount of
cold working and surface hardening can also be varied as part ofthe process to
optimize the
physical properties of the weld joint and the final product and will depend on
the particular
material being welded and the environment which the part will be subjected to
during its
life. It has been found, however, that by cold working the surfaces of the
welded workpieces
10 and 12 along the weld line 18, and any heat affected regions 20, by less
than about 5%,
and preferably less than about 2%, results in a weld joint 22 having longer
retention of
compressive residual stress at elevated temperature, less rapid relaxation
under cyclic
loading, and minimizes the alteration of the residual stress field during
tensile or
compressive overload than weld joints and parts formed using conventional cold
working
and surface hardening processes.
It has also been found that by inducing a layer of compressive residual stress
in the
surface of a workpiece, such as by burnishing, along regions having elevated
temperature,
such as produced during the welding operation or by some other heating means,
produces
residual stresses that are more stable when subjected to elevated temperature.
Such stability
is believed to be attributed to strain aging which occurs during the warm
deformation
process that leads to more diffuse dislocation structures and pinning
ofdislocations by solute
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atoms and/or precipitates. It has also been found that by performing the
compression
operation with the surface of the workpiece heated to an elevated temperature,
rather than at
room temperature, produces a deeper compressive residual stress layer. Because
of the
reduction of the workpiece yield strength, plastic deformation extends to a
greater depth
thereby producing deeper compression, as well as deeper penetration of the
burnishing tool,
thereby producing more lateral flow of surface material and higher surface
compression. As
illustrated in FIG. 4, the depth of compression, calculated using conventional
finite element
methods and published yield strengths, achieved by burnishing a material, such
as 7075-T6
aluminum, at a heated temperature, such as 400°F (204°C), is
over twice the depth of
compression achieved by burnishing at room temperature. The depth of
compression
achieved increases with the increasing burnishing load.
As shown in FIGS.1, 2 and 3, a preferred embodiment of the welding apparatus
100
is shown comprising a conventional welding tool 102 effective for performing a
welding
operation. The welding tool 102 includes a welding probe 112, such as an
electrode or other
heating source, extending downwardly from the shoulder 114 of the welding tool
102.
During operation, the welding probe 112 is brought into close proximity or
contact with the
opposing ends 14 and 16 of the workpieces 10 and 12, respectively, and moved
along the
weld line 18 which heats and softens the material ofthe workpieces 10 and 12
in the vicinity
of the welding probe 112 creating heated, melted or plasticized, regions 20
along the
welding line 18 in the workpieces 10 and 12. After the workpieces 10 and 12
are welded
together, the compression operation is performed using the compression tool
106, such as
the burnishing tool previously described herein, to induce a layer of residual
compressive
stress along the surface ofthe weld line 18, and any heat affected regions 20,
to form a weld
joint 22. As previously stated, the compression operation is preferably
performed while the
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weld line 18 and any heat affected regions 20 are at their elevated
temperature produced by
the welding process. The positioning device 104 (FIG.1) can be mounted to a
conventional
controller 116 having a processor for storing system software or program (not
shown) to
automatically control the pressure being exerted by the compression tool 106
at particular
points along the welding line 18, and any heat affected regions 20, and other
selected
regions, thereby controlling the magnitude of compression being induced. The
controller
116 may also be programmed to operate the positioning device 104 to control
the direction
of movement of the compression tool 106 to produce the desired stress
distribution. In a
preferred embodiment of the invention, the compression operation can be
performed along
the surface regions of the workpieces that are at an elevated temperature
caused by the
welding process. It should be understood that the compression operation can
also be
performed along regions that are not at an elevated temperature or can be
performed along
regions that have an elevated temperature produced by other means such as by
induction
heating, torch, laser, heated fluid, and the like. For purposes of
illustration, as shown in
FIG. 3, the welding apparatus 100 is shown having a heating means 107 mounted
to the
compression tool 106 for heating and elevating the surface temperature of the
workpieces
10, 12 just prior to performing the compression operation.
Referring to FIGS.1 and 6, in another preferred embodiment of the invention, a
fluid
coolant 122 is sprayed along the weld line 18, and any heat affected regions
20, prior to
performing the compression operation. It has been found that cooling, such as
by applying a
coolant 122, the regions 20 heated during the welding operation, and other
selected regions
along the surfaces of the workpieces 10, 12, creates a tensile pre-stress
condition prior to
deformation by the compression tool 106. Tension is temporarily present in the
surface layer
while a temperature gradient within the surface is maintained by contact with
the coolant
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122. The surface layer is then more easily deformed in tension during the
compression
operation, thereby creating higher surface compression. After the compression
operation is
complete, the temperature of the workpieces will re-equilibrate and return to
ambient
temperature. Further, it has also been found that as the interior of a heated
workpiece
contracts, the surface will be drawn further into compression and that the
increase in
compression upon cooling will be approximately equal to the magnitude of the
thermal
strain induced by the coolant. FIG. 5 illustrates the tensile stress induced
at the surface of
the workpiece, such as formed from aluminum, titanium, or steel alloys, by
maintaining a
temperature gradient between the upper surface and the interior surface of the
workpiece.
The typical lower surface compression achieved by the Hertzian loading, such
as produced
with a spherical burnishing ball, is thus increased by the use of a coolant
being applied along
the heated weld line, and any other heated regions, as well as any other
surface areas of the
workpieces.
Referring now to FIGS. 1 and 6, for illustrative purposes, another preferred
1 S embodiment of the welding apparatus 100 is shown having means for cooling
118 the
formed weld joint 22, any heat affected regions 20, and other selected regions
of the surfaces
of the workpieces 10,12. In a preferred embodiment of the invention, the means
for cooling
118 comprises a conventional fluid sprayer 120 effective for spraying a
coolant 122 onto the
surfaces of the workpieces 10, 12 to be placed into compression. The fluid
sprayer 120 is
connected with a coolant supply or reservoir 124 through a hose or conduit
126. A
conventional pump 128 operates to pump coolant 122 from the coolant supply or
reservoir
124 through the hose or conduit 126 to be sprayed onto the surfaces ofthe
workpieces 10,12
prior to receiving compression. As shown, the means for cooling 118 can
further comprise
means for returning the sprayed coolant 130, such as a vacuum means, to the
fluid supply or
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reservoir 124.
In another preferred embodiment of the invention, the means for cooling 118
can be
incorporated into the compression tool 106. Referring to FIGS. 7 and 8, for
illustrative
purposes, another embodiment of the compression tool 106, such as a
conventional
burnishing apparatus is shown having means for cooling 118 incorporated
therein. As
shown, the compression tool 106 includes a socket 132 having a ball seat 134
which is
essentially spherical in shape and adapted to the surface ofthe burnishing
ball 110 which is
disposed within the ball seat 134. The socket 132 is further provided with a
fluid passage
136 in flow communication with the ball seat 134 and to an external coolant
supply or
reservoir 124. In operation, coolant 122 is fed under pressure from the
coolant supply or
reservoir 124 by a conventional pump 128 through a hose 126 to the fluid
passage 136 and
the ball seat 134. Pressure then forces the coolant 122 around the surface of
the burnishing
ball 110 and outwardly onto the surface of the workpiece 10, 12. By adjusting
the pressure
being generated by the pump 128, the desired amount of coolant 122 penetrating
around the
burnishing ball 110 and onto the surface of the workpiece 10,12 can be
obtained. It should
be understood, the coolant 122 could also be used as a lubricating fluid for
the burnishing
ball 110 and the burnishing operation. The means for cooling 118 can further
comprise
means for cooling the coolant (not shown) to a desired temperature and means
for returning
the used coolant 130, such as a vacuum means, to the fluid supply or reservoir
124. As
shown, in a preferred embodiment of the invention, the compression tool 106 is
provided
with a pad 138 having a convoluted boot 140 mounted to the socket 132 to
prevent coolant
122 from flowing across the surface of the workpiece 10, 12. As shown, the pad
opening
139 can be sized and shaped to hold more or less coolant, to optimize the
temperature
gradient through the workpieces. The boot 140 includes an outlet 142, which is
in flow
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communication with the coolant supply or reservoir 124 by a hose or conduit
144. In
operation, vacuum pressure is generated inside the coolant supply or reservoir
124 which
operates to draw outside air and coolant 122 that has been expelled from the
socket 132 onto
the surface of the workpiece 10, 12 and contained within the boot 140 back to
the coolant
supply or reservoir 124.
It should be understood that various types of coolants and methods for
distributing
such coolants onto the surfaces of the workpieces to create a surface
temperature gradient
between the surface and the interior of the workpiece may be used without
departing from
the invention. For example, the coolant may be in the form of a cooled gas
which can
I O dissipate after being directed onto the surface of the workpiece. In
addition, the temperature
and the amount of coolant used can be varied to provide the desired
temperature gradients.
Coolants in the form of liquid may also be applied and removed in various
ways, such as
evaporation, run off, or recycled.
It should also now be understood that the method and apparatus of the present
IS application provides a new and novel means for forming a weld joint having
improved
physical properties. In a preferred embodiment of the invention, compressive
residual
stresses are induced along the surfaces of the workpieces having regions of
elevated surface
temperatures as a result of the welding process or by heating using other
means, such as
induction heating, torch, laser, steam, and the like. Compressive residual
stresses may also
20 be induced along surfaces of the workpieces having regions that have been
cooled, such as
by means of a cooling fluid. By properly selecting the surface temperature
gradients and the
compression parameters, parts, including parts having weld joints, may be
formed having
improved physical properties.
Accordingly, the method and the apparatus for performing the method of the
subject
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invention is relatively inexpensive and provides an effective means of welding
which
provides a compression force along the weld line, and any heat affected
regions, to induce
compressive stress in a well defined localized area with a controlled amount
of cold working
and surface hardening. Referring to FIG. 9, the inversion into tension of the
surface of a
workpiece after a welding operation is shown compared to the surface of a
workpiece having
been treated by the method of the present application. Upon welding, the
surface may
actually invert from compression into a relative high level of tension,
thereby significantly
reducing fatigue life and stress corrosion resistance of the weld joint and
accordingly the
final part, as previously stated herein. By minimizing the amount of cold
working and
surface hardening, as shown in FIG. 10, it has been found that the method of
the present
application will induce a layer of compressive residual stress along the
surface of the weld
joint, and any heat affected regions, and will result in a weld joint and a
final part having
improved physical properties, particularly at elevated temperature, as well as
minimize the
alteration of the residual stress field during tensile or compressive
overload.
As described and shown herein above, the method of forming a weld joint of the
present application has great advantage over prior welding methods as it
enables the finished
weld joint and accordingly the final part, to achieve enhanced fatigue
strength and stress
corrosion resistance while providing a part having a good surface finish.
Further, coupling
the welding process with the compression operation into a single process,
permits effective
use of the heat generated during the welding operation resulting in a
relatively low cost
procedure, requiring no expensive and/or time consuming after-weld treatments,
and which
is effective for inducing a deep layer of compression, with a minimal amount
or a controlled
amount of cold working and surface hardening, along the surface of the weld
joint and any
heat affected regions. This is particularly significant for final parts that
were formed using
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extensive welding operations where the cost of a process requiring a second-
pass would be
prohibitive. In addition, surface roughness is also improved without requiring
a relatively
expensive and time consuming process requiring a second-pass.
In another preferred embodiment of the invention, the final part is a
structure, such as
an automobile structure, an aircraft structure, a construction structure, a
marine structure,
and the like, and is formed having a plurality of weld joints. Each weld joint
is formed by
the method and apparatus ofthe subject invention, as previously described, and
includes a
layer of compression residual stress along the surface of the joint and any
heat affected
regions.
In another preferred embodiment of the invention, a structure comprising a
plurality
of plates secured in place by the welding method and apparatus as previously
described.
It should also now be understood to those skilled in the art that the method
of
forming a weld joint and the apparatus for performing the method of the
subject application
greatly increases the type of parts that can be economically manufactured by
welding rather
than by use of bolts and rivets. Such parts are particularly found in the
aerospace industry,
such as in the manufacture of aircraft fuselage and wing skins and supports,
where weight
considerations are of the up most importance. Such parts are also found in the
marine
industry, construction industry, automotive industry, and in general
manufacturing.
It should also now be understood to those skilled in the art that the method
of
forming a weld joint and the apparatus for performing the method of the
subject application
results in final parts having weld joints with improved physical properties
and are less likely
to suffer from corrosion. This can be particularly significant for canisters
and containers that
are to be used for long periods of time and where failure can be harmful or
disastrous.
While the method and apparatus described constitute preferred embodiments
ofthe
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invention, it is to be understood that the invention is not limited to the
precise method and
apparatus, and that changes may be made therein without departing from the
scope of the
invention which is defined in the appended claims.
What is claimed is:
