U.S. patent application number 15/694565 was filed with the patent office on 2018-03-15 for system and process for joining dissimilar materials and solid-state interlocking joint with intermetallic interface formed thereby.
This patent application is currently assigned to BATTELLE MEMORIAL INSTITUTE. The applicant listed for this patent is BATTELLE MEMORIAL INSTITUTE. Invention is credited to Yuri Hovanski, M. D. Reza-E-Rabby, Kenneth A. Ross, Scott A. Whalen.
Application Number | 20180073532 15/694565 |
Document ID | / |
Family ID | 61559262 |
Filed Date | 2018-03-15 |
United States Patent
Application |
20180073532 |
Kind Code |
A1 |
Whalen; Scott A. ; et
al. |
March 15, 2018 |
SYSTEM AND PROCESS FOR JOINING DISSIMILAR MATERIALS AND SOLID-STATE
INTERLOCKING JOINT WITH INTERMETALLIC INTERFACE FORMED THEREBY
Abstract
A method for connecting two dissimilar materials having
different melting points is described wherein a the materials are
heated together to obtain plasticization of the lower melting point
material within a prefigured geometry within a first material in
such a way so as to form intermetallic features within a solid
state joint.
Inventors: |
Whalen; Scott A.; (Richland,
WA) ; Ross; Kenneth A.; (West Richland, WA) ;
Reza-E-Rabby; M. D.; (Richland, WA) ; Hovanski;
Yuri; (Mapleton, UT) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
BATTELLE MEMORIAL INSTITUTE |
Richland |
WA |
US |
|
|
Assignee: |
BATTELLE MEMORIAL INSTITUTE
Richland
WA
|
Family ID: |
61559262 |
Appl. No.: |
15/694565 |
Filed: |
September 1, 2017 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
62393409 |
Sep 12, 2016 |
|
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|
62533851 |
Jul 18, 2017 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
F16B 17/008 20130101;
B23K 2103/20 20180801; F16B 5/08 20130101; B23K 20/1265 20130101;
F16B 5/0012 20130101; B23K 20/12 20130101 |
International
Class: |
F16B 5/00 20060101
F16B005/00; B23K 20/12 20060101 B23K020/12; F16B 5/08 20060101
F16B005/08 |
Goverment Interests
STATEMENT AS TO RIGHTS TO INVENTIONS MADE UNDER FEDERALLY-SPONSORED
RESEARCH AND DEVELOPMENT
[0002] This invention was made with Government support under
Contract DE-AC0576RL01830 awarded by the U.S. Department of Energy.
The Government has certain rights in the invention.
Claims
1. A method for connecting two dissimilar materials having
different melting points comprising the steps of: placing a first
material having a higher melting point than a second material, in
contact with the second material within a groove and heating said
first and second materials to a temperature sufficient to
plasticize said first and second materials within the groove and
form an intermetallic feature therein.
2. The method of claim 1 wherein said heating is obtained by
friction.
3. The method of claim 1 wherein said heating is controlled to
prevent overheating.
4. The method of claim 2 wherein the heating step is performed
using a friction stir welding device that extends beyond the
thickness of the first material into the groove to a plunge depth
greater than a thickness of the second material.
5. The method of claim 4 wherein the intermetallic feature includes
at least one set of hooks.
6. The method of claim 5 wherein the intermetallic feature includes
at least two sets of hooks.
7. The method of claim 4 wherein the friction stir welding tool is
operated within the groove in such a position so as to contact one
or both sides of the groove and form the higher temperature
material into a hook.
8. The method of claim 1 wherein said groove contains nested
dovetail grooves and said friction stir welding tool is plunged
into to the lower of two nested dovetail grooves such that a
portion of the material defining the lower groove contacts the
friction stir welding tool and results in the forming at least one
feature of higher melting temperature material that extend upward
into the lower melting temperature material.
9. A joint comprising: a geometric shape defined by a preformed
groove in a first metal material that has been filled with a second
material, the second material having a lower melting point than the
first material, the second material having been plasticized and
heated to both fill the preformed groove and form intermetallic
containing features at the interface between the first and second
materials.
10. A method for joining of aluminum to steel comprising the steps
of: placing a piece of steel with a surface defining a dovetail
groove therein in contact with a piece of aluminum having a
thickness; and friction stir welding the aluminum and the steel
with a friction stir welding tool that plunges to a depth greater
than the thickness of the aluminum and enters the dovetail groove
so as to heat both the steel and the aluminum to a temperature
sufficient to plasticize and flow the aluminum into the dovetail
groove and to form intermetallic features therein.
11. The method of claim 10 wherein friction stir welding tool has a
scrolled shoulder and a frustum shaped threaded and flatted
pin.
12. The method of claim 11 wherein the friction stir welding is
performed using a tool rotational speed between 50 and 5000 RPM and
a welding speed ranging between 1-5000mm/min.
13. The method of claim 12 wherein the friction stir welding is
performed using a tool rotational speed between 200 and 400 RPM and
welding speed ranging between 25-50 mm/min.
14. The method of claim 12 wherein the temperature of the shoulder
of the friction stir welding tool is less than the temperature of
the tip of the friction stir welding tool.
15. The method of claim 12 wherein the groove is a double dovetail
groove.
Description
PRIORITY
[0001] This invention claims priority from and incorporates by
reference provisional patent application No. 62/393,409 entitled
System And Process For Joining Dissimilar Materials And Solid-State
Interlocking Joint With Intermetallic Interface Formed Thereby
filed Sep. 12, 2016. It also incorporates provisional patent
application No. 62/533,851 entitled The Joining Of Dissimilar
Metals Through Formation Of Dovetail Extrusions With
Metallurgically Bonded Interfaces filed: Jul. 18, 2017.
BACKGROUND OF THE INVENTION
Field of the Invention
[0003] The invention generally relates to methods for joining
dissimilar materials and more particularly to connections between
dissimilar metals having different melting points.
[0004] A world of rising energy necessitates approaches for
reducing the amount of energy needed to perform standard tasks.
Among approaches under development are lighter more fuel efficient
vehicles. Reducing the weight of vehicles can be accomplished in a
variety of ways including replacing heavier steel regions with
lighter weight materials such aluminum, plastic, carbon fiber or
other dissimilar materials. However, difficulty has arisen in
attempting to find ways to robustly join dissimilar materials in a
way that provides the needed strength and resiliency that exists in
structures that are made from the same material. Preferably, and in
some instances by requirement, these seams and interconnects must
be welded together. Welding is fairly straight forward when the two
materials have similar melting points but becomes more and more
difficult when the materials have vastly different melting points
or other characteristics.
[0005] Joining materials such as steel to aluminum, titanium,
magnesium, or copper, or any combination thereof, has proved
difficult for a variety of reasons. The prior art generally teaches
that when these materials are joined that the temperatures must be
maintained generally low so as to prevent the formation of brittle
intermetallic compounds, which are generally believed to cause the
welds to be brittle and fail. Most prior art methodologies for
joining dissimilar materials have focused on getting rid of these
brittle intermetallic portions especially when the intermetallic is
the only means of joining the two dissimilar metals together.
[0006] One of the ways that this is done is by isolating the other
metal from the molten aluminum during the arc welding process.
Techniques such as coatings, or inserting bimetallic inserts that
contain portions of each of the two types of metals and which were
formed by another process and welding the materials to the inserts
are methodologies that have been taught and practiced. However, the
needs for these additional steps increase the complexity and cost
and are generally unsuitable in a high throughput manufacturing
environment because of these issues and concerns.
[0007] Hence what is needed is a process for forming high strength
joints between dissimilar materials in ways that are simpler
cheaper and more effective than the current methodologies. The
present invention is a significant step forward in addressing these
needs.
[0008] Additional advantages and novel features of the present
invention will be set forth as follows and will be readily apparent
from the descriptions and demonstrations set forth herein.
Accordingly, the following descriptions of the present invention
should be seen as illustrative of the invention and not as limiting
in any way.
SUMMARY
[0009] In one embodiment of the disclosure a method for connecting
two dissimilar materials having different melting points is
described wherein a first material having a higher melting point
than a second material is plasticized to fill a preformed groove,
shape or depression in the surface of a second material. The first
and second materials are heated together (preferably rubbed and
heated by friction) to obtain plasticization of the lower melting
point material so as to cause the plasticization of the material
and the movement of the material into the surface feature (groove)
in such a way so as to form intermetallic features of the material
within the solid state joint. Preferably and in some embodiments
the temperature within the joint is controlled so as to prevent
overheating of the weld. Examples of how this temperature control
is achieved is described in more detail in the detailed
description.
[0010] In some embodiments the method maybe performed using a
friction stir welding device that extends to a plunge depth greater
than the thickness of the second material. Various other features
of the friction stir method may be appropriately modified so as to
obtain the desired result. This may include varying the rate of
traverse, process temperature, force pressures, rotation speeds,
tool operational orientation, tip and shoulder temperatures,
pretreatments including surface coatings, pre-fillings and other
pretreatments and other parameters. In addition, various
configurations and operations of the various apertures, features,
grooves, dovetail shaped depressions or other features of the
devices may also be employed.
[0011] In one exemplary arrangement the groove contains nested
dovetail grooves and the friction stir welding tool is plunged into
to the lower of two nested dovetail grooves such that a portion of
the material defining the lower groove contacts the friction stir
welding tool and results in the forming at least one feature of
higher melting temperature material that extend upward into the
lower melting temperature material. In addition to this single
exemplary embodiment a variety of other embodiments are also
described and set forward.
[0012] The result of the implementation of this methodology for
joining materials is the formation of a joint that has a geometric
shape defined by a preformed groove in a first metal material
having a first melting point that has been filled with a second
material that has a second lower melting point that has been
plasticized and heated to both fill the preformed groove and form
intermetallic containing features. This method and these joints can
be found in a variety of heterogeneous combinations including
combinations of aluminum to steel and other metallic and
non-metallic combinations.
[0013] Various advantages and novel features of the present
invention are described herein and will become further readily
apparent to those skilled in this art from the following detailed
description. In the preceding and following descriptions we have
shown and described only the preferred embodiment of the invention,
by way of illustration of the best mode contemplated for carrying
out the invention. As will be realized, the invention is capable of
modification in various respects without departing from the
invention. Accordingly, the drawings and description of the
preferred embodiment set forth hereafter are to be regarded as
illustrative in nature, and not as restrictive.
BRIEF DESCRIPTION OF THE DRAWINGS
[0014] FIG. 1 shows a prior art configuration of a friction stir
welding arrangement for use in connecting different materials
[0015] FIG. 2 shows failed joints created by the arrangement shown
in FIG. 1 with no metalurgically bonded interlayer.
[0016] FIG. 3 shows an example of two materials of differing
melting points joined in an overheating process where
intermetallics are intentionally formed at the dissimilar
interface.
[0017] FIG. 4 shows an example of one embodiment of the present
disclosure.
[0018] FIG. 5 shows an intermetallic reinforced connection prior to
tensile testing.
[0019] FIG. 6 shows specimens after tensile testing when an
intermetallic is intentionally formed at the dissimilar interface
performed on various disclosed examples.
[0020] FIG. 7 shows plots of the data reflected in Table 1.
[0021] FIG. 8 shows SEM photographs of the intermetallic features
in the filled dovetail sections corresponding to FIG. 7 and Table
1.
[0022] FIGS. 9-11 show various embodiments and configurations of
friction stir welding tools with mechanical contact shown being
necessary to create metallurgical bond.
[0023] FIGS. 12-14 shows various feature designs and the respective
joints formed therein.
[0024] FIGS. 15-18 show examples of such friction stir tooling.
[0025] FIG. 19 shows information of one set of process
parameters.
[0026] FIG. 20 shows load vs. extension curves for different plunge
depths.
[0027] FIG. 21 shows the failure morphologies discovered during
tensile testing having the data shown in FIG. 20.
[0028] FIG. 22 shows different interlayer thicknesses that are
generated under the present embodiment.
[0029] FIG. 23 shows an arrangement of one tested embodiment
[0030] FIG. 24 shows a cross section of one tested
configuration
[0031] FIG. 25 shows various dovetail geometries.
[0032] FIGS. 26-27 show the results of testing on the dovetail
geometries of FIG. 25.
[0033] FIG. 28 shows the maximum tensile load per unit length of
weld (i.e. specimen thickness) plotted against different dovetail
grooves and welding conditions.
[0034] FIG. 29 is a plot of a function of extension for different
dovetail geometries.
DETAILED DESCRIPTION OF THE INVENTION
[0035] The following description includes examples of various
embodiments of the present invention. It will be clear from this
description of the invention that the invention is not limited to
these illustrated embodiments but that the invention also includes
a variety of modifications and embodiments thereto. Therefore the
present description should be seen as illustrative and not
limiting. There is no intention in the specification to limit the
invention to the specific form disclosed, but, on the contrary, the
invention is to cover all modifications, alternative constructions,
and equivalents falling within the spirit and scope of the
invention as defined in the claims.
[0036] The present invention centers around the joining of
dissimilar materials by utilizing a combination of embedded
portions of a first material within a preformed geometric shape or
groove located in another material under process conditions and
tooling geometries able to form an intermetallic interconnection or
layer at the dissimilar interface within the preformed shape or
groove. Joining metals with different melting temperatures can be
accomplished by extruding a lower melting temperature material into
groves in a higher melting temperature material while
simultaneously forming a metallurgical bond within the groove at
the interface between the dissimilar metals. Joints with this
configuration exhibit superior strength and ductility compared to
other known techniques for Friction Stir Welding (FSW) of aluminum
to steel.
[0037] In one embodiment, a method for creating such a connection
using a friction stir welding tool to heat the materials, cause
plasticization and the formation of intermetallic features and
layers are described. Contrary to prior art which teaches that
intermetallics and layers should not be created within preformed
grooves, the method described herein teaches that creating
intermetallics and layers within preformed grooves significantly
improve strength and ductility.
[0038] FIGS. 3-25 demonstrate various examples and embodiments of
the invention. Referring however, first to FIG. 1, a prior art
configuration of a friction stir welding arrangement for use in
connecting different materials is shown. In such an arrangement a
friction stir welding tool (FSW) and a material are brought into
contact and the material (typically the lower melting point
material) is plasticized by the rotating tool. The tool and the
plasticized zone that the rotating tool forms (stir zone) are
traversed over a joint or along or raster path. When the lower
melting temperature material is heated by the friction stir welding
device the lower temperature materials is plasticized and flows
down into the preformed grooves in the higher temperature
material.
[0039] Typically the configuration is arranged such that the tool
does not enter into the dovetail and is far from contacting the
higher melting temperature material to prevent mixing conditions
and elevated temperatures which would form intermetallic layers at
the interface between the higher and lower melting point materials.
Generally speaking, it is believed that lower temperature welds are
stronger because of the more finely grained microstructures that
performing welds under these conditions can create. Therefore
existing teachings in the art of friction stir welding try to run
the weld as cold as possible and to avoid higher temperature
operating conditions and the formation of intermetallic interfaces.
As a result the connections that are formed by plasticizing and
pushing the softened material down into without forming an
intermetallic connection or layer results in a purely mechanical
interconnection that may provide mechanical strength in one
direction but does not include metallurgically bonded interlayer in
other direction that the present invention provides. An example of
the failure after tensile testing is shown in the photograph in
FIG. 2 for a single and double dovetail joints with no
metallurgically bonded interlayer. The lower melting point
material, aluminum in this case, easily tears out from the groove
within the higher melting point material (steel in this case).
[0040] In embodiments of the present invention, such as the example
shown in FIG. 3, two materials of differing melting points are
joined in a process wherein what is typically termed as overheating
of the joint occurs and an intermetallic layer is formed within in
the dovetail interconnect and strengthens rather than weakens the
connection between the higher and lower temperature materials.
[0041] In one example of this process called Friction Stir
Dovetailing (FSD) a custom designed friction stir welding pin
extends into the preformed feature (groove, slot, dovetail, or
other depression of a predesignated geometry) and generates heat
sufficient to both plasticize the lower melting point material such
that it flows into the preformed feature while also heating the
higher melting point material through rubbing to a point whereby
the filled feature contains intermetallic features (or layer) at
the joint interface. An example of such an arrangement for
performing this method is shown in FIG. 4. This methodology has
shown to be effective when the traverse rate is between 10 mm to
200 mm per minute, the process temperatures range from about
300.degree. C. to about 600.degree. C., the vertical force is
between about 1,000 pounds to about 25,000 pounds and the tool rpm
between about 50 rpm to about 1000 rpm. All possible parameter
combinations for all possible materials have not been examined and
parameters outside the general ranges given may also produce the
desired intermetallic. Thus the ranges given should not be viewed
as restrictive but are exemplary. In other embodiments combinations
of other heating methodologies may also be utilized whereby
insertion of the tip of the FSW into the higher temperature
material is not always necessary to achieve the formation of
intermetallic features.
[0042] Contrary to the teachings in the art, the formation of this
intermetallic connection between, for example, aluminum and steel
within these locking sections significantly improves joint
strength. This process is particularly applicable to thick section
joints where no other practical solution currently exists. An
intermetallic reinforced connection is shown in FIG. 5.
[0043] The effectiveness of FSD with an intermetallic layer for an
AA6061 and Rolled Homogeneous Armor joint is demonstrated through
tensile test data which shows specimens failing in the processed
aluminum rather than at the joint interface. (see FIG. 6).
TABLE-US-00001 TABLE 1 Extension at 75% of Extension at Maximum
Maximum Maximum 75% Maximum Weld Load Load Load Load Set N/mm mm
N/mm mm A 560 .+-. 6 1.42 .+-. 0.04 420 .+-. 6 2.57 .+-. 0.05 B
1175 .+-. 36 2.73 .+-. 0.26 881 .+-. 27 5.36 .+-. 0.32 C 797 .+-.
25 1.49 .+-. 0.04 582 .+-. 18 2.24 .+-. 0.03
[0044] The data reflected in Table 1 is plotted in FIG. 7 and
illustrates the effect of the formation of an intermetallic
interface. In samples A, no intermetallic interface was formed. In
examples B and C these intermetallic interfaces were formed to
different degrees. As the data shows the inclusion of the interface
in sample B increased the max. tensile load by 107% and extension
at max. load increased by 92%. In sample C the max. tensile load
was increased by 42% and extension at max. load increased by 5%.
The improvements attained with B (with intermetallic) compared to A
(no intermetallic) are even larger when considering the load and
extension at 75% of maximum load where failure is conventionally
defined. Contrary to the teachings of the prior art, a process that
includes infilling with intermetallic formation has shown to be an
effective process of joining and welding dissimilar materials and
does not weaken the weld as the prior art suggests. In this
described arrangement the entire dissimilar interface within the
dovetail can react to stresses in more than a mechanical interlock
in one direction. With intermetallic present, failure has been
driven into the bulk material away from the joint; a highly
desirable failure mode.
[0045] This arrangement prevents sheering of the angled lower
temperature piece such as aluminum and dovetail pullout resulting
in greatly improved strength of the joint. This results in lap
sheer samples that fail in the lower temperature material, not at
the joint. The results show that using FSP or FSW to extrude a
plasticized material into an existing feature/s in a material of
higher plasticization temperature with the intent to create a
mechanical interlock where an intermetallic is created at the
dissimilar material interface within the dovetail during the
process is superior to joints where the intermetallic interconnect
are not formed. SEM photographs of the intermetallic features in
the filled dovetail section are shown in FIG. 8. Table 2 below
shows the process conditions which generated the aluminum-steel
intermetallic described in FIG. 8 leading to the data depicted in
FIG. 7.
TABLE-US-00002 TABLE 2 Tool Avg. Avg. Avg. Plunge Shoulder Shoulder
Rotational Forge Weld Avg. WC TIP Depth Scroll Temperature Speed
Force Power Temperature Weld Set (mm) Numbers Degrees (C.) RPM kN
kW Degrees C. A 15.22 3 470 170 35 4.95 475 B 15.45 3 470 150 57
4.85 490 C 15.45 2 470 400 19 5.25 570
[0046] In addition to the various examples provided herein, a
variety of other alterations or various variations to the basic
concept are also contemplated, and various modifications to the
process and processing parameters can be undertaken. In one
embodiment of the present invention, the friction stir welding tool
is inserted or oriented so as to contact the bottom or side of the
groove and generate additional heat at these points of contact.
This method generates heat at the interface where it is needed to
form the intermetallic and is not generated in the bulk material
where overheating could degrade the properties. This rubbing
between the tool and underlying steel exposes atomically clean
surfaces which facilitate formation of intermetallics. In other
embodiments, the groove or the dovetail may contain features that
when brought into contact with the FSW tool cause this heating to
take place and enhance the formation of intermetallic features. In
other embodiments the shape of the FSW tool or tip may be modified
so as to engage selected portions of the groove or the groove may
be variously configured to engage with the FSW tool in a particular
way. Examples of various modifications are shown in FIGS. 9, 10,
and 11.
[0047] FIG. 10 for example, shows an embodiment wherein the
dimensions of the dovetails are proportioned to be generally
shallow and small as compared to the pin diameter of this tool.
Because the dovetails are shallow and small compared to the
diameter of the pin tip, the overheated area created by contact
between the pin and high temperature material is generally larger
compared to other arrangements and is sufficient to generate a hot
layer of material that can form a continuous layer of intermetallic
features above and within the dovetails. In other embodiments of
the invention induction heating is used to produce localized
heating at the interface. In other embodiments plasticized material
is forced through narrow openings between the tool and the higher
melting temperature material within the dovetail to produce high
temperature while the material flows. This localized heating within
the gaps will cause localized heating within the gaps allowing for
formation of intermetallic layers in the openings.
[0048] In other embodiments of the invention, the formation of
intermetallic hooks of higher melting material are formed by
running the tool within the dovetail while the tool is biased such
that it contacts one or both side of the dovetail joint and higher
temperature material into a hook as shown in FIG. 11. This provides
an advantage in that it increases the area of intermetallic contact
between the dissimilar materials and assists in forcing the
materials together. In other embodiments generally squared grooves
are formed in the higher temperature material and then heated with
the friction stir welding tool to cause the corners of the device
to rise and form hooks in the lower melt temperature materials. In
other embodiments the heating process forms intermetallic hooks.
These hooks are formed by plunging the tool into to the lower of
two nested dovetails (as shown in FIG. 12) such that the edges of
the tool contact the corners of the lower dovetail resulting in the
formation of two hooks of higher melting temperature material that
extend upward into the lower melting temperature material.
[0049] While this specific example is provided the particular
squared form of the groove should not be seen as limiting and it
should be understood that various other embodiments wherein the
geometry provides that pushing the tool into a fabricated groove or
slit or against the edge of a groove slit so as to cause the higher
melting temperature alloy to form a hook or other feature that
extends into the lower melting temperature material during friction
stir processing, welding or dovetailing are also contemplated.
Examples of such configurations and embodiments are found for
example in FIG. 13.
[0050] In other arrangements such as the one shown in FIG. 14
mechanical interlocking is accomplished by deforming groves that
are easier or faster to manufacture. In instances a rastering grid
can be produced. When the friction stir processing tool is
sufficiently close to or contacts these grooves during welding,
these grooves can have sections that are deformed and form
intermetallic features that fill groves and provide strong
interlocking. While straight grooves are shown for purposes of
illustration this is not meant to be limiting. Various alternatives
and modifications can be undertaken to deform the groove during
welding to create mechanical interlocking or increase the amount of
mechanical interlocking. In addition to the geometry that is shown
a variety of other geometries including nesting features, multiple
T-slots or notches or other fabricated features may be used to
created layers of interlocking features. In some embodiments the
dovetails or other mechanical interlocking features with rounded
corners improve flow of material into the dovetail and reduce
fatigue.
[0051] Preferably the tool temperature and force are maintained
constant so as to provide consistency along the weld path and
manage the strength of the various parts. This is accomplished in
one set of embodiments by controlling the tool temperature control
algorithm and a force control algorithm in conjunction with
techniques where the tool contacts the dovetails. Constant tool
temperature and position improves consistency of the intermetallic
layer and uniformity of size of generated hooks or new features
along the weld path and from part to part. In some applications
improved performance was obtained when a two piece friction stir
welding tool was utilized wherein the pin and shoulder of the tool
can move axially relative to one another.
[0052] In cases where the pin is contacting the high temperature
material within the dovetail, the pin can extend into the dovetail
as material is worn from the pin without affecting the shoulder
position. This could be done for example by having a servo actuated
pin and shoulder that allows for selective connection and release.
In another embodiment a spring loaded pin could be used to force
more material out and keep pin length relatively constant despite
wear on the pit itself. In another embodiment of the invention the
upper low melting temperature materials are being extruded into the
dovetail groves of underlying high melting temperature materials
using a counter-clock wise threaded pin within the FSW tool. Thus
clockwise rotation of the tool causes downward extrusion of the
plasticized material. Locally heating dovetail interface caused
metallurgical bonding by kneading action.
[0053] In as much as the present invention utilizes the combination
of mechanical interlocking with intermetallic formation various
modifications and alterations could be made so as to enhance and
foster the development of intermetallic interconnects at a lower
temperature. In one example, a materials such as Yttrium, Tungsten,
Molybdenum, Iron compounds and others could be applied to reduce
the temperature or improve the rate of formation of intermetallic
to the dovetail joints prior to FSD. This could be done using cold
spray, thermal spray or any other deposition method which can also
be used to tailor the composition of the intermetallic layer.
[0054] In another example pre-filled dovetails are utilized wherein
the mechanical grooves in the higher temperature material is
pre-filled with lower melting temperature material. This can reduce
or eliminate the excess material that maybe removed from the top of
the lower melting temperature material when filling the dovetail.
This prefilling can be accomplished by filling the groove with bar
stock, powder chips of other forms of the lower temperature
material. In another embodiment a laminated approach could be used
wherein arc welding, strip cladding or other fusion welding
techniques are used to bond lower temperature materials such as
aluminum inside of the dovetails and then execute friction stir
welding to create the intermetallic hooks and interconnects. This
can improve process robustness, welding speed and can prevent the
formation of a recess at the top of the weld from material lost to
fill the dovetail.
[0055] In one application friction stir welding was used to apply
cladding by creating a dovetail grid similar to the grid shown in
FIG. 14. While the term grid connotes a square or rectangular
geometry it is to be understood that the grid can be circular or
any other shape and while the grid would likely be two dimensional
on flat cladding and three dimensional on contoured cladding these
parameters are not limiting. This cladding arrangement allows for
the use of a grooved grid for forming a mechanical interlocks that
will in turn dramatically improve resistance to ballistic impact
and provides the multi directional strength and fatigue life of
thick section cladding. In one set of preferred embodiments a two
pass technique for accomplishing this was utilized wherein one pass
of the friction stir device was made to create the intermetallic
layer or layers along the dovetail interface and a second pass, run
at much cooler process conditions followed which increased the
strength of the material inside the dovetail while maintaining the
intermetallic interface.
[0056] Specialized tooling capable of 1) heating the dissimilar
metal interface within or adjacent to the dovetail to temperatures
higher than the stir zone and 2) "kneading" a thin interfacial
layer to locally mix the dissimilar metals can also assist in the
performance of the method. The simultaneous localized temperature
rise and kneading at the dissimilar metal interface are achieved by
pressing the tool against the higher temperature material during
FSD. Tool and dovetails configurations can be designed in
coordination to allow for contact anywhere or everywhere within the
dovetail. This method enables the formation of intermetallic and/or
amorphous bonding at the dissimilar interface, which reinforces the
joint, while stir zone temperatures are kept low. A low stir zone
temperature are preferable for minimizing degradation of bulk
material properties in the lower melting point material. Examples
of such tooling are shown in FIGS. 15-16. Tooling Friction stir
tools have been developed with features specifically intended to
extrude lower melting point metal into dovetail grooves in a higher
melting point material; while simultaneously forming a
metallurgical bond at the dissimilar interface. The tools contain
an insert (such as tungsten-carbide, tungsten-rhenium,
polycrystalline boron nitride, etc. . . . ) within the pin tip
which enables high wear resistance and consistency of the
metallurgical bond. For example, a tungsten-carbide insert could be
press fit into an H13 steel FSW tool. The insert rubs against the
higher melting point material, within the dovetail groove, and
gives dramatically improved tool life and wear resistance compared
to tools without a tip insert. The intent is to protect insertion
of high wear resistant materials into FSW tools as a pin, or pin
insert, for the purpose of rubbing the higher melting temperature
in a dissimilar dovetail joint--for the purpose of creating a
metallurgical bond. These illustrative examples are not intended to
restrict the possible configurations.
[0057] In one embodiment a tip insert is the tool feature that
interacts with the dissimilar material interface. The insert can be
flat or convex, and may contain scrolls, stepped spirals or other
features that enhance "kneading" of the dissimilar materials and
also expose new material and push surface impurities away from the
interface. Illustrative insert configurations are shown in FIG. 17.
The insert may be circular, hexagonal, square, or any shape
desired. These features are unique from other tip features
attempted in FSW because these features are designed to push
material outward and to encourage the formation of a metallurgical
bond at a dissimilar metal interface.
[0058] FIG. 16a shows the H13 FSW tool with circular tungsten
carbide tip insert after eight linear feet of welding. The pin is
not deformed and the tungsten carbide insert has no visible sign of
wear. By comparison, FIG. 16b shows a H13 FSW tool without a tip
insert after eight linear inches of welding. Wear and deformation
is immediate when a hardened insert is not used when rubbing to
generated an intermetallic bonding layer. Use of a tungsten carbide
insert dramatically improves tool wear for this new process. In one
set of tests two examples of FSW tools having tungsten-carbide
inserts within the pin tip were used, see FIG. 18. The upper tool
contains a cylindrical insert and the lower tool contains a
hexagonal insert. The cylindrical insert configuration was used to
join AA6061 to Rolled Homogenous Armor (RHA) MIL-DTL-12560J in a
lap weld configuration. The upper material of the lap joint was
0.5'' thick AA60601 and the lower material was 0.5'' thick RHA
containing a single dovetail. A single tool was used to weld eight
linear feet without visible signs of wear or degradation of the tip
insert. FIG. 19 shows that in use, the temperature was higher at
the face of the tip insert (area of rubbing on the RHA) than at the
shoulder which is an important for making the key feature for
forming a metallurgical bond. In traditional FSW, the shoulder is
the highest temperature--which is not desirable in the present
arrangement.
[0059] In one set of experiments nine sets of lap joints were
welded having key parameters within the following ranges. Tool
speed 100-250 rpm, federate up to 7.5 cm/min, force 25-100 kN,
torque 250-350 Nm, tip temperature 450-550 degrees C., shoulder
temp 400-500 degrees C. These samples were then tested at different
plunge depths. FIG. 20 shows load vs. extension curves for
different plunge depths (0.599'', 0.603'', 0.608'' and 0.620'') of
the FSW tool. The 0.599'' case did not involve rubbing of the tip
insert within the dovetail grooves for the express purpose of
determining baseline strength in the absence of a metallurgical
bond. The other three plunge depths were intended to impart
increasing amounts of rubbing between the tip insert and base of
the RHA dovetail. A total of 26 specimens were tensile tested (qty
6 for 0.599'', qty 5 for 0.603'', qty 6 for 0.608'', qty 9 for
0.620''). The four curves in the following plot represent an
average of each grouping. From this plot it is clear that the
highest strength and largest ductility (extension) is for a plunge
depth of 0.608''. A smaller plunge depth of 0.603'' gives lower
strength and ductility as does a larger plunge depth of
0.620''.
[0060] FIG. 21 shows that the failure morphology during tensile
testing (AA6061 being pulled to the right and RHA being pulled to
the left) is very different for each of the four curves in the
above plot. For the 0.599'' plunge depth, the aluminum simply pulls
out of the dovetail as the aluminum corner plastically deforms. For
the 0.603'' plunge depth, a weak metallurgical bond is formed which
fractures in a brittle manner and then shears at the aluminum
corner. For the 0.62'' case, the metallurgical bond does not
fracture and failure occurs in the bulk aluminum within the
dovetail resulting in higher strength and ductility. The case with
the highest strength and ductility is for the 0.608'' plunge depth
where shear failure occurs in the bulk material.
[0061] FIG. 22 shows different interlayer thicknesses (2.2 micron
on left, 1.3 micron in middle and 100 nm on right) that are
generated. The phase (for example, intermetallic or amorphous) and
strength of the metallurgical bond at the dissimilar interface are
affected by temperature as well as the strength of the heat
affected zone in the aluminum. Controlling temperature in the stir
zone and the dissimilar metal interface simultaneously can be
performed by modulating the spindle axis speed, torque, current,
power or any combination of these variables. The temperature of the
dissimilar interface is preferably controlled by modulating the
position, forge force or motor torque of the forge axis. Control
algorithms governing the temperatures in the stir zone and at the
dissimilar interface operate independently, but may be linked
together as part of multivariable control scheme.
[0062] In one embodiment the spindle axis is used to control the
temperature of the stir zone and the forge axis to control the
temperature at or near to the dissimilar interface. This could be
done with a monolithic tool or with a two piece tool where the
shoulder and pin can move relative to each other along the forge
axis. Another embodiment of this concept is to use the spindle axis
to control the temperature at or near the dissimilar interface and
the forge axis to control the temperature of the stir zone. This
could be done with a monolithic tool or with a two piece tool where
the shoulder and pin can move relative to each other along the
forge axis. Typically the spindle axis is controlled by commanding
speed, torque or power to regulate temperature and the forge axis
is controlled by commanding a force, velocity or position change to
regulate temperature. In FSW machines that allow the pin to rotate
relative to the shoulder one spindle axis can control the
temperature of the stir zone, while the other control the
temperature at the dissimilar interface.
[0063] The friction stir dovetailing process can also be used to
join dissimilar materials with a myriad of different joint
configurations. For example, metal with a higher melting point (for
example steel) can be "buttered" (coated) with a metal having a
lower melting point (for example aluminum) such that subsequent
fusion welding can be performed to form previously impossible
configurations for dissimilar metals. This "buttering" can be
single or double sided and the thicker section can be either the
higher or lower melting point material. The buttered layer, or
underlying steel, may contain features (not illustrated due to the
limitless embodiments) such as tabs, angles, holes, slots and other
features that enable subsequent fusion welding of joints having a
final configuration that is otherwise unweldable for dissimilar
metals. Buttering can also enable subsequent fusion welding of a
nearly limitless array of other structures and attachments such as
extrusions, brackets, threaded shafts, fittings and so forth (also
not illustrated here due to the numerous possibilities). Buttering
can also overcome clearance/access issues during manufacturing that
are currently preventing adoption of FSW in vehicle applications.
The buttering approach can also enable fusion welding in areas for
materials where welded properties are more beneficial than FSW; all
while simultaneously allowing a joint between dissimilar metals.
Another example is the enabling of interior joints that are
otherwise impossible for dissimilar metals.
[0064] The chemistry of intermetallic or amorphous layers/regions
affects the mechanical properties and microstructure of the
metallurgically bonded interface. The intent is to protect the use
of cold spray to deposit a layer of metal within the dovetail to
modify the chemistry of the metallurgical bond at the dissimilar
interface. One embodiment of this concept is to spray a thin layer
of cold spray material on the inner surfaces prior to friction stir
dovetailing. Alternatively, the dovetail groove could be filled
partially or fully with cold spray material prior to FSW. For
example, cold spraying 7000 series aluminum into the dovetails of
underlying steel would reduce/eliminate the presence of aluminum
alloying elements and therefore change the structure/properties of
the bonded interlayer.
[0065] The following examples are provided as illustrations of the
principles and embodiments described above:
EXAMPLE 1
[0066] Solid-state joining of thick section aluminum to steel plate
was achieved using a custom designed pin tool in a friction stir
welding device to flow a lower melting point material (AA6061) into
dovetail grooves previously machined into the surface of an
underlying material having a higher melting point (rolled
homogeneous armor [RHA]). Repeating dovetails form a mechanical
interlocking structure akin to metallic Velcro, however the forming
of intermetallic interconnects by the friction stir welding tool
strengthened this interconnection. In one example, 38.1 mm (1.5
in.) thick AA6061 was joined to 12.7 mm (0.5 in.) thick RHA plates.
Tensile test data showed specimens failing in the processed
aluminum rather than at the joint interface.
[0067] Plates of RHA procured to MIL-DTL-12560J were dual disc
ground to a thickness of 12.7 mm and pre-machined dovetail grooves
shown in FIG. 23. The RHA plates were inserted into AA6061-T651
sandwich structures having a total thickness of 38.1 mm. FSD was
performed using a tool made from H13 tool steel that was heat
treated to obtain RHC 45. The one-piece FSW tool consists of a
scrolled shoulder and a frustum shaped (6.1.degree.) threaded+3
flatted pin. FSD was performed using a tool rotational speed of 275
RPM and welding speed ranging between 25-50 mm/min. All welding was
performed using a position control mode where the forge force is a
response variable of the commanded plunge depth. Welds were made on
the top side, then machined flat, and the assembly was turned over
to weld the bottom side. Tensile specimens were cut from the welded
Al-steel to an average thickness of 12.0 mm using a water jet. A
cross section is shown in FIG. 24. Standard grinding and polishing
sequences were followed for metallographic sample preparation and
final polished surface was obtained using colloidal silica
(<0.05 .mu.m).
[0068] A scanning electron microscope (SEM) equipped with energy
dispersive spectroscopy (EDS) was employed to investigate the
intermetallic formation. Tensile testing of sandwich plates was
performed using a 50 kip MTS test frame to ascertain tensile test
and microstructural observations. The results of that testing are
shown in FIGS. 26-27. Structural analysis of a dovetail joints
between AA6061 and RHA subjected to tensile load was simulated
using LS DYNA finite element software. The simulation predicted the
failure of tensile specimens with, and without, the formation of
IMCs along Al and RHA dovetail interface. Cases for 1, 2 and 3
dovetails having the outlined geometries (shown in FIG. 25) were
structurally analyzed.
[0069] From the finite element simulations, it was observed that
shear failure of the Al dovetail occurred for configuration with
one, two and three dovetails when no intermetallic connection is
present. Therefore, simple dovetail interlock without bonding
doesn't have impact on structural integrity regardless of the
number of dovetails. The testing showed that joint strength is
improved when IMC is present at the Al-RHA interface within the
dovetail. In the case of IMC being present, only two dovetail
features are required to cause failure in the bulk Al. In general,
the results of this structural analysis indicate that, the presence
of IMCs formation improves joint efficiency in the FSD process. As
a result, steps were taken to generate an IMC at the Al-RHA
interface while simultaneously filling the dovetail grooves.
[0070] Transverse macro sections of Al-RHA joints with different
dovetail geometries are shown in FIG. 25. The macro-sections
clearly demonstrate the effective filling of Al into the dovetail
grooves regardless of dovetail geometric variations. The FSD
process is quite robust in terms being able to fully fill the
grooves. For example, welds were performed (from 200 to 275 rpm and
25 to 100 mm/min) with the tip of the tool ranging from 2 mm above
the RHA surface to having the tool tip in contact with the bottom
of the dovetails. In all cases, the grooves were fully filled with
no voids observed. While FIG. 25 provides a macro-view of weld
cross sections in terms of defect formation and dovetail filling,
metallographic analysis is needed to determine the bonding state
along the Al-RHA interface. SEM analysis at the Al-RHA interface of
specimens in are shown in FIGS. 26 and 27 respectively.
[0071] The data indicates that interfacial bonding has occurred due
to the formation of an IMC measuring 0.5 .mu.m to 1 .mu.m thick in
narrow dovetail grooves and 1.0 .mu.m to 2.0 .mu.m thick in wider
dovetail grooves. The SEM micrographs suggest that incipient
melting of AA6061 during FSD might cause bonding between RHA and Al
with the formation of an intermediate transition layer which will
be further confirmed as IMCs from energy dispersive spectroscopy
(EDS) analysis. The formation of IMCs was confirmed by elemental
quantitative analysis using EDS. The spot (area) and line scanning
energy spectrum results are combined with the SEM micrograph in
FIG. 27. The atomic percentage of corresponding line scans of Al
and Fe at the intermediate transition layer indicate a diffusion
profile of Al and Fe across the interface suggesting IMC formation.
Moreover, the EDS spot analysis of this layer showed 79 at. % Al
and 14 at. % Fe. In FSD, intense plastic deformation of AA6061 by
the stirring tool might cause incipient melting of Al in close
proximity to the RHA due to high localized temperature. The
increased heat input caused by the tool contacting and deforming
the RHA resulted in the formation of possible multiple IMCs (AlFe,
Al3Fe, FeAl2, Al4Fe, Al13Fe4, Al5Fe2 etc.) at the bonding interface
which might be further confirmed from temperature measurement
during FSD, phase diagram analysis and corresponding X-ray
diffraction analysis.
[0072] The macro cross section shows the deformed layer of RHA near
the upper region of dovetails where the stir tool intentionally
contacted the RHA during processing to locally increase temperature
and promote IMC formation. Consequently, the growing of IMCs were
evident outside the dovetail in the SEM and EDS analysis.
Frictional heating due to contact between the stir tool and RHA may
result in the Al being melted locally, thereby resulting in the
formation of IMCs. According to the EDS spectra and elemental
composition, the intermetallic compounds FeAl2, Fe3Al or Fe2Al
might form in the Al-RHA interlayer.
[0073] FIG. 28 presents the maximum tensile load per unit length of
weld (i.e. specimen thickness) plotted against different dovetail
grooves and welding conditions. It was observed that nested
dovetails result in higher strength than single wider dovetails
regardless of welding speed. The higher load carrying capacity
provided by nested dovetails is due to the additional interlocking
that resists deformation in the tensile and transverse directions.
In the absence of IMC, there does not appear to be a statistical
difference in the load at failure on the weld speed range of 25-50
mm/min. However, inclusion of the IMC within the wider single
dovetail at 25 mm/min was found to increase strength compared to
the case of no IMC. This speaks to the role of IMC formation for
improving joint strength. The narrow dovetails welded at 25 mm/min
have IMC formation outside the dovetail and interestingly show
higher strength than the wider dovetails with IMC. From this data,
we concluded that the formation of IMCs significantly improves
joint strength.
[0074] The normalized load (load per unit weld length) as a
function of extension for different dovetail geometries is plotted
in FIG. 29. Failure of the narrower dovetail specimen (A) occurred
due to fracture of the brittle intermetallic layer on one side of
the sandwich structure at peak load which is followed by ductile
failure of bulk Al due to eccentric loading. For the specimens D
and E, successive separation of dovetails occurred after reaching
the maximum load as the dovetails tend to unzip one pair after
another. This phenomenon is indicated by the changes in slope of
the load curves on their descending part as tensile testing
progress to joint failure. For the nested dovetail welded at 25
mm/min corresponding to specimen C, failure occurred in the
processed Al rather than at the dovetail interlock. As mentioned
earlier the volume of the filled Al in the nested dovetail is high
enough to encounter the tensile loading near the region of the
additional interlock, resulting in failure in the Al with the
failure plane perpendicular to the loading direction. The failure
of the tensile specimen C is similar to specimen B. However, an
additional contribution of bonding between Al and RHA with the
formation of IMCs resulted in a bulk Al failure rather than a
failure at the joint. This is indicative of the strength of the
joint and demonstrates the viability of extending to a wide range
of material stack-up (50 mm or higher thickness) to form dovetail
interlock.
[0075] While various preferred embodiments of the invention are
shown and described, it is to be distinctly understood that this
invention is not limited thereto but may be variously embodied to
practice within the scope of the following claims. From the
foregoing description, it will be apparent that various changes may
be made without departing from the spirit and scope of the
invention as defined by the following claims.
* * * * *