Note: Descriptions are shown in the official language in which they were submitted.
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TOOL GEOMETRIES FOR FRICTION STIR SPOT WELDING OF HIGH
MELTING TEMPERATURE ALLOYS
BACKGROUND OF THE INVENTION
Cross Reference to Related Applications This document
claims priority to and incorporates by reference all
of the subject matter included in the provisional
patent application having docket number 3252.SMII.PR,
with Serial No. 60/653,158 and filed on 02/15/2005,
and the subject matter in Continuation patent
applications having docket number 1219.BYU.CN with
Serial No. 10/705,668 and filed on 11/10/2003, and
docket number 1219.BYU.CN2 with Serial No. 10/705,717
and filed on 11/10/2003.
Field Of the Invention: This invention relates
generally to friction stir welding. More
specifically, the present invention relates to spot
welding of high melting temperature alloys.
Description of Related Art: There are many
applications in a variety of industries that require
spot welding. For example, the shipyard, marine,
automotive, transportation, aerospace, nuclear, oil
and gas and other industries all need to join
together, generally using a lap configuration, high
melting temperature alloys which include, among
others, steel, stainless steel, nickel base, and other
alloys. One of the most common methods used to
perform spot welding is known in the industry as
resistance spot welding (RSW). RSW passes electric
current through the materials being joined to thereby
form a molten pool of metal at the desired joint
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location. When the molten pool cools and solidifies,
a spot weld joint is formed.
There are many drawbacks to RSW technology.
These drawbacks include high energy costs, brittle
joints that lead to cracking at the location of the
weld, hazardous fumes that are emitted, low joint
strength, susceptibility to corrosion, solidification
defects, lack of repeatability due to probe wear at
the electrode joint, and the difficulty of inspecting
the quality of the joint.
One of the more prominent applications for
resistive spot welding is joining together the pieces
of a frame for the body of cars and trucks. However,
the automotive industry continues to struggle with RSW
to reliably manufacture cars.
Of particular importance to the US government is
the crash worthiness of a car or truck body.
Accordingly, the US government requires that cars
produced for the consumer undergo destructive testing
to determine RSW quality. For example, a car body of
each car model produced is randomly selected from that
production line by a Department of Transportation
inspector after it has been spot welded. Welds are
selected to be broken, and this action is performed
with a hand-held tool similar to a screw driver.
Generally, one car body from each line is
destructively tested each month from each
manufacturer. However, manufacturers typically do
significant destructive testing on their own by -
performing the test on a vehicle as often as each
shift to make sure vehicle crashworthiness is
maintained.
This destructive inspection process is typically
used because of the unreliable nature of RSW. Some
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manufacturers are also careful to make sure that more
than one soldering machine or robot makes the welds on
any single vehicle. In this way, if a robot is
creating underperforming welds, the risk is decreased
to any particular vehicle.
The automotive industry is also pursuing the use
of Advanced High Strength Steels (AHSS) in order to
lighten vehicles and improve fuel economy. Some of
these steels are far more difficult to RSW. Some.of
the steels cannot be welded at all using RSW.
Furthermore, the AHSS pose far more process control
issues than existing steels made in today's vehicles.
For example, one process control issue is load. It is
necessary to pinch the materials that are to be
resistance spot welded. Another issue is that of the
gap between the parts to be welded. The parts need to
be flush, or the strength of the weld may be
compromised. Another issue is the amount of
electricity needed to perform RSW on AHSS.
Although a substantial weight savings can be
obtained if these advanced steels can be used in
vehicle construction, there has been very little
success because of the joining problems associated
with RSW.
It is noted that one automobile manufacturer has
used friction stir spot welding (FSSW) on aluminum
door panels. However, because of existing FSSW tool
limitations, aluminum (a low melting temperature
alloy) has been the only material that can be joined
by the RSW process. Unfortunately, aluminum cannot be
used for structural components in a vehicle such as
for the frame or body, and therefore its use is
limited not only in automotive applications, but for
other applications as well.
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Accordingly, what is needed is a tool and method
of performing friction stir spot welding (FSSW) that
can be used on AHSS to thereby enable use of high
melting temperature alloys in a vehicle frame or body.
It is useful for the understanding of the present
invention to know that friction stir welding is a
technology that has been developed for welding metals
and metal alloys. The FSW process often involves
engaging the material of two adjoining workpieces on
either side of a joint by a rotating stir pin or
spindle. Force is exerted to urge the spindle and the
workpieces together and frictional heating caused by
the interaction between the spindle and the workpieces
results in plasticization of the material on either
side of the joint. The spindle is traversed along the
joint, plasticizing material as it advances, and the
plasticized material left in the wake of the advancing
spindle cools to form a weld.
Figure 1 is a perspective view of a tool being
used for friction stir welding that is characterized
by a generally cylindrical tool 10 having a shoulder
12 and a pin 14 extending outward from the shoulder.
The pin 14 is rotated against a workpiece 16 until
sufficient heat is generated, at which point the pin
of the tool is plunged into the plasticized workpiece
material. The workpiece 16 is often two sheets or
plates of material that are butted together at a joint
line 18. The pin 14 is plunged into the workpiece 16
at the joint line 18. Although this tool has been
disclosed in the prior art, it will be explained that
the tool can be used for a new purpose. It is also
noted that the terms "workpiece" and "base material"
will be used interchangeably throughout this document.
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The frictional heat caused by rotational motion
of the pin 14 against the workpiece material 16 causes
the workpiece material to soften without reaching a
melting point. The tool 10 is moved transversely
5 along the joint line 18, thereby creating a weld as
the plasticized material flows around the pin from a
leading edge to a trailing edge. The result is a solid
phase bond 20 at the joint line 18 that may be
generally indistinguishable from the workpiece
material 16 itself, in comparison to other welds.
It is observed that when the shoulder 12 contacts
the surface of the workpieces, its rotation creates
additional frictional heat that plasticizes a larger
cylindrical column of material around the inserted pin
14. The shoulder 12 provides a forging force that
contains the upward metal flow caused by the tool pin
14.
During FSW, the area to be welded and the tool
are moved relative to each other such that the tool
traverses a desired length of the weld joint. The
rotating FSW tool provides a continual hot working
action, plasticizing metal within a narrow zone as it
moves transversely along the base metal, while
transporting metal from the leading face of the pin to
its trailing edge. As the weld zone cools, there is
typically no solidification as no liquid is created as
the tool passes. It is often the case, but not
always, that the resulting weld is a defect-free,,re-
crystallized, fine grain microstructure formed in the
area of the weld.
Travel speeds are typically 10 to 500 mm/min with
rotation rates of 200 to 2000 rpm. Temperatures
reached are usually close to, but below, solidus
temperatures. Friction stir welding parameters are a
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function of a material's thermal properties, high
temperature flow stress and penetration depth.
Previous patents by some of the inventors such as
US Patent Nos. 6,648,206 and 6,779,704 have taught the
benefits of being able to perform friction stir
welding with materials that were previously considered
to be functionally unweldable. Some of these
materials are non-fusion weldable, or just difficult
to weld at all. These materials include, for example,
metal matrix composites, ferrous alloys such as steel
and stainless steel, and non-ferrous materials.
Another class of materials that were also able to take
advantage of friction stir welding is the superalloys.
Superalloys can be materials having a higher melting
temperature bronze or aluminum, and may have other
elements mixed in as well. Some examples of
superalloys are nickel, iron-nickel, and cobalt-based
alloys generally used at temperatures above 1000
degrees F. Additional elements commonly found in
superalloys include, but are not limited to, chromium,
molybdenum, tungsten, aluminum, titanium, niobium,
tantalum, and rhenium.
It is noted that titanium is also a desirable
material to friction stir weld. Titanium is a non-
ferrous material, but has a higher melting point than
other nonferrous materials.
The previous patents teach that a tool is needed
that is formed using a material that has a higher
melting temperature than the material being friction
stir welded. In some embodiments, a superabrasive was
used in the tool.
The embodiments of the present invention are
generally concerned with these functionally unweldable
materials, as well as the superalloys, and are
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hereinafter referred to as "high melting temperature"
materials throughout this document.
While the examples above have addressed friction
stir welding, friction stir processing and friction
stir mixing are also aspects of the invention that
must be considered. It is noted that friction stir
processing and welding may be exclusive events of each
other, or they may take place simultaneously. It is
also noted that the phrase "friction stir processing"
may also be referred to interchangeably with solid
state processing. Solid state processing is defined
herein as a temporary transformation into a
plasticized state that typically does not include a
liquid phase. However, it is noted that some
embodiments allow one or more elements to pass through
a liquid phase, and still obtain the benefits of the
present invention.
In friction stir processing, a tool pin is
rotated and plunged into the material to be processed.
The tool is moved transversely across a processing
area of the material. It is the act of causing the
material to undergo plasticization in a solid state
process that can result in the material being modified
to have properties that are different from the
original material.
Friction stir mixing can also be an event that is
exclusive of welding, or it can take place
simultaneously. In friction stir mixing, at least one
other material is being added to the material being
processed or welded.
MegaStir Technologies (a business alliance
between Advanced Metal Products, Inc. and SII
MegaDiamond, Inc.) has developed friction stir welding
(FSW) tools that can be used to join high melting
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temperature materials such as steel and stainless
steel together during the solid state joining
processes termed FSW. This technology generally
involves using a polycrystalline cubic boron nitride
tip 30 (including pin and shoulder areas), insulation
behind the tip (not shown), a locking collar 32, a set
screw 34 and a shank 36 as shown in figure 2.
When this tool is used with the proper friction
stir welding machine and proper steady state cooling,
it is effective at friction stir welding of various
materials. This tool design is also effective for
using a variety of tool tip materials besides PCBN.
Some of these materials include refractories such as
tungsten, rhenium, iridium, titanium, etc.
Since these tip materials are often expensive to
produce this design is an economical way of producing
and providing tools to the market place. The design
shown in figure 2 is in part driven by the limited
sizes that can be produced by sintering, hipping, and
other high pressure equipment capabilities.
BRIEF SiIMMARY OF THE INVENTION
It is an aspect of the present invention to
provide a tool geometry that enables FSSW of high
melting temperature materials.
It is another aspect to provide a tool for
performing FSSW that includes materials that enable
FSSW of high melting temperature materials.
In a preferred embodiment, the present invention
is a tool for friction stir spot welding of high
melting temperature materials, wherein the tool
geometry includes a short pin and broad shoulder to
enhance mixing of high temperature materials, and
wherein the tool includes a superabrasive coating to
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thereby enable FSSW of high melting temperature
materials.
These and other objects, features, advantages and
alternative aspects of the present invention will
become apparent to those skilled in the art from a
consideration of the following detailed description
taken in combination with the accompanying drawings.
BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS
Figure 1 is a prior art perspective view of an
existing friction stir welding tool capable of
performing FSW on high melting temperature materials
Figure 2 is another prior art perspective view of
an existing friction stir welding tool capable of
performing FSW on high melting temperature materials.
Figure 3A is an illustration of one embodiment of
a tool that can perform the desired friction stir spot
welding of the present invention. Figure 3A is a
profile view of a tool holder and a PCBN tip disposed
therein.
Figure 3B is a first profile view of the PCBN
tip.
Figure 3C is a second profile view of the PCBN
tip.
Figure 4A is an illustration of another
embodiment of a tool that can perform the desired
friction stir spot welding of the present invention.
Figure 4A is a profile view of a tool holder and a
PCBN pin disposed therein.
Figure 4B is a first profile view of the PCBN pin
with view F circled.
Figure 4C is a close-up profile view of the
threaded PCBN pin of view F.
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Figure 4D is an end-view of the PCBN pin and
toolholder.
Figure 5 is an illustration of two FSSW spot
welds wherein parameters have been modified to obtain
5 different spot welds.
Figure 6 is an illustration of three friction
stir spot welds.
DETAILED DESCRIPTION OF THE INVENTION
10 Reference will now be made to the drawings in
which the various elements of the present invention
will be given numerical designations and in which the
invention will be discussed so as to enable one
skilled in the art to make and use the invention. It
is to be understood that the following description is
only exemplary of the principles of the present
invention, and should not be viewed as narrowing the
claims which follow.
From recent developments with tool materials such
as Polycrystalline Cubic Boron Nitride (PCBN) and
other materials which have a higher melting point than
those materials being joined, friction stir welding
(FSW) of high melting temperature materials has become
a reality. However, in recent FSSW tests, it has
become apparent that the tool geometries used for FSSW
are going to be different from those used in FSW.
Changes in tool geometry include, but should not be
considered limited to, pin length, modifying the pin
length to shoulder width ratio, the pin geometries,
shoulder geometries, the use of the shouldet without
the pin, the use of the pin only, the use of threads
on the pin, and the height of the pin.
Many of the tool geometries used for FSW need a
relatively long pin on the tool in order to join
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thicker workpieces together when making a butt joint.
In contrast, FSSW is generally going to be performed
on relatively thinner workpieces. Thus, the pin may
generally be shorter than on a tool used for FSW.
This shorter pin can be used even if the tool is going
to penetrate both materials that are being FSSW
together.
Along with pin length, another aspect of the
present invention is an understanding that the pin
length to shoulder width ratio is important to FSSW
because of friction stir mixing and welding. It is
desirable to have a broad area of the workpieces being
mixed together. FSSW of a broader area is more easily
accomplished having a shoulder that is relatively
broad.
In the present invention, a FSSW joint is
achieved using a generally solid state process with
minimal or no melting of the materials being joined.
Therefore, it is important that the tool geometry
enables the material of the workpieces to be processed
in such a way that the materials mechanically bond.
For example, when a tool having the geometry of
the tool shown in figures 3A to 3D is rotated at 1500
RPM, plunges into a lap joint of AHSS at a plunge rate
of 2 to 8 inches per minute, dwells for 1 to 10
seconds, and is retracted, a FSSW joint with a
mechanical bond is created. It should be understood
that these parameters are for illustration purposes
only, and may be varied to achieve the same or similar
results.
Figure 3A is provided as a profile view of a FSSW
toolholder 40 and a FSSW tip comprised of a shoulder
42 and a pin 44.
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Figure 3B is a profile view of the PCBN tip
wherein the shoulder 42 and pin 44 are coupled to a
short shank 46.
Figure 3C is a close-up profile view of the PCBN
tip where detail of the shoulder 42 and the pin is
more plainly visible.
Figure 4A is provided as a profile view of a FSSW
toolholder 50 and a FSSW tip comprised of a pin 54
without any shoulder.
Figure 4B is a profile view of the PCBN tip
wherein the pin 54 is coupled to a short shank 56.
Figure 4C is a close-up profile view of the PCBN
tip showing the stepped spiral threads 58 of the pin
54. The stepped spiral threads 58 are created using
two threaded starts in this particular embodiment.
This particular configuration of the pin 54 resulted
in a spot weld having the highest degree of strength
as compared to spot welds made using other FSSW tool
geometries.
Figure 4D is an end-view showing the pin 54 and
the toolholder 50.
It is one aspect of the present invention that
the area be maximized that is being processed to
create the FSSW joint. In other words, it is
desirable to maximize the amount of material that is
being stirred by the FSSW tool. One way to accomplish
this objective is to use a large shoulder on the FSSW
tool. Ideally, a tool having a shank with a
cylindrical working end that might or might not have a
pin would maximize the shoulder of the FSSW tool.
Some of the consequences of this tool geometry
are that the FSSW tool would probably experience a
large axial load, the FSSW tool would probably have to
be plunged near or at the interface of the lap joint,
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and the FSSW tool could have undesirable material
flow.
One method for overcoming these difficulties is
to increase the size of the joining area. This is
accomplished by translating the FSSW tool away from
the plunge axis during the FSSW process.
The FSSW process may also include a dwell period
in which the FSSW tool is not moved, or it may have no
dwell period and the FSSW tool is kept moving.
It is another aspect of the invention that tool
geometries that manage the flow of the material being
bonded are preferred, and should include design
criteria for the flow of the particular material type
being FSSW.
Figure 5 is an illustration of an FSSW tool
wherein FSSW parameters have been modified to obtain
different spot welds. The first spot weld 60 was made
using a cycle time of 2.1 second. The second spot
weld 62 was made using a cycle time of 1.6 second.
Figure 6 is provided as photomicrographs of spot
welds using a FSSW tool that has been performed on
DP600, and which shows three different cross sections
that were created as a result of changing parameters
of the FSSW process. Weld 1 (70) had a FSSW cycle
time of 2.5 seconds, had a 50mm/min plunge, and a
213mm/min extract. Weld 2 (72) had a FSSW cycle time
of 1 second, had a 213mm/min plunge, and a 213mm/min
extract. Weld 3 (74) had a FSSW cycle time of 1.5
seconds, had a 213mm/min plunge, included a dwell time
of 0.5 seconds, and had a 213mm/min extract.
It is noted that Weld 2 (72) shows that the two
materials being joined were not flush, and thus have a
gap between them after the spot weld is performed.
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Other aspects of the invention include the use of
disposing asymmetric features on the pin and shoulder,
using a retractable pin, having a pin with varying
degrees of taper radii, parabolic, non-linear
geometries, having threads on the pin, having threads
on the shoulder, having flats and/or threads on the
pin, and moving the FSSW tool so that the FSSW tool is
moved in any direction away from the plunge axis to
increase the area under the tool.
It is to be understood that the above-described
arrangements are only illustrative of the application
of the principles of the present invention. Numerous
modifications and alternative arrangements may be
devised by those skilled in the art without departing
from the spirit and scope of the present invention.
The appended'claims are intended to cover such
modifications and arrangements.
