U.S. patent application number 10/997473 was filed with the patent office on 2006-02-02 for friction stir rivet and method of joining therewith.
Invention is credited to Robin Stevenson, Pei-Chung Wang.
Application Number | 20060024145 10/997473 |
Document ID | / |
Family ID | 35732391 |
Filed Date | 2006-02-02 |
United States Patent
Application |
20060024145 |
Kind Code |
A1 |
Wang; Pei-Chung ; et
al. |
February 2, 2006 |
Friction stir rivet and method of joining therewith
Abstract
A friction stir rivet having a body, a mandrel, and a mechanical
interface therebetween, is disclosed. The body has an elongated
cylindrical shank, a cap at a first end, and an axial hole
therethrough. The mandrel has an elongated shaft defining an axis
and a head at one end thereof, the head having an effective outside
diameter greater than the effective outside diameter of the shaft,
the shaft being disposed within the axial hole of the body, the
mandrel head being disposed at an opposite end of the body to that
of the cap, and the end of the mandrel head having a flat surface
oriented substantially perpendicular to the axis of the shaft. The
mechanical interface between the body and the mandrel is such that
the body rotates in response to rotation of the mandrel.
Inventors: |
Wang; Pei-Chung; (Troy,
MI) ; Stevenson; Robin; (Bloomfield, MI) |
Correspondence
Address: |
KATHRYN A MARRA;General Motors Corporation
Legal Staff, Mail Code 482-C23-B21
P.O. Box 300
Detroit
MI
48265-3000
US
|
Family ID: |
35732391 |
Appl. No.: |
10/997473 |
Filed: |
November 24, 2004 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60592048 |
Jul 29, 2004 |
|
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|
Current U.S.
Class: |
411/34 |
Current CPC
Class: |
B23K 20/127 20130101;
F16B 19/1045 20130101; Y10T 29/49938 20150115; B21J 15/048
20130101; B21J 15/027 20130101; Y10T 428/2495 20150115; B21J 5/066
20130101; B21J 15/043 20130101; Y10T 29/49943 20150115 |
Class at
Publication: |
411/034 |
International
Class: |
F16B 13/04 20060101
F16B013/04 |
Claims
1. A friction stir rivet, comprising: a body having an elongated
cylindrical shank, a cap at a first end, and an axial hole
therethrough; a mandrel having an elongated shaft defining an axis
and a head at one end thereof, the head having an effective outside
diameter greater than the effective outside diameter of the shaft,
the shaft being disposed within the axial hole of the body, the
mandrel head being disposed at an opposite end of the body to that
of the cap, and the end of the mandrel head having a flat surface
oriented substantially perpendicular to the axis of the shaft; and
a mechanical interface between the body and the mandrel such that
the body rotates in response to rotation of the mandrel.
2. The rivet of claim 1, wherein: the flat surface oriented
substantially perpendicular to the axis of the shaft is
perpendicular to within plus-or-minus two degrees thereof.
3. The rivet of claim 1, wherein: the flat surface has an effective
outside diameter that is equal to or greater than about 80% of the
effective outside diameter of the mandrel head.
4. The rivet of claim 3, wherein: the flat surface extends over
about 100% of the effective diameter of the end of the mandrel
head.
5. The rivet of claim 1, wherein: the shaft has a first region
having a first tensile strength and a second region having a second
tensile strength, the second tensile strength being less than the
first tensile strength, and the second region being disposed
proximate the cap.
6. The rivet of claim 1, wherein: the mandrel comprises a material
having a microstructure that substantially maintains its strength
during a short-term temperature excursion, such that subsequent to
a short-term temperature excursion, and in response to the mandrel
head being pulled into the body, the mandrel has sufficient
strength to upset the body prior to the mandrel severing.
7. The rivet of claim 1, wherein: the mandrel comprises a material
having a martensite microstructure.
8. The rivet of claim 1, wherein: the mandrel comprises a material
having a bainite microstructure.
9. The rivet of claim 1, wherein: the mandrel comprises a material
having a cold drawn microstructure.
10. The rivet of claim 1, wherein: the body comprises a deformed
region thereby defining the mechanical interface.
11. The rivet of claim 1, wherein: the mandrel comprises a deformed
region thereby defining the mechanical interface.
12. The rivet of claim 1, wherein: the mandrel and the body have
non-circular cross sections perpendicular to the axis of rotation
defining a non-circular annulus such that the outer surface of the
mandrel engages the inner surface of the body in response to
rotation of the mandrel thereby defining the mechanical
interface.
13. The rivet of claim 1, wherein: the shaft has an effective
outside diameter equal to or greater than the effective inside
diameter of the shank at a process temperature greater than room
temperature thereby defining the mechanical interface.
14. The rivet of claim 1, wherein: the mandrel and body comprise
copper, titanium, iron, or any alloy comprising at least one of the
foregoing.
15. The rivet of claim 1, wherein: the mandrel comprises medium
carbon steel or high carbon steel.
16. The rivet of claim 1, wherein: the mandrel comprises a medium
carbon steel having equal to or greater than about 0.29 weight %
carbon and equal to or less than about 0.53 weight % carbon that is
quenched to form a martensite microstructure and is then tempered
at a temperature of equal to or greater than about 450 deg-C. for
equal to or greater than about 30 minutes.
17. The rivet of claim 16, wherein: the mandrel is tempered at a
temperature of equal to or greater than about 500 deg-C. for equal
to or greater than about 30 minutes.
18. The rivet of claim 1, wherein: the mandrel comprises a material
that undergoes a change in tensile strength in response to the
friction stir process that is equal to or less than about 5%
change.
19. The rivet of claim 18, wherein: the mandrel comprises a
material that undergoes a change in tensile strength in response to
the friction stir process that is equal to or less than about 1%
change.
20. A method of joining upper and lower workpieces fluidly bondable
at a point of engagement, comprising: positioning a friction stir
rivet at the point of engagement of the workpieces, the rivet
comprising: a body having an elongated cylindrical shank, a cap at
a first end, and an axial hole therethrough; a mandrel having an
elongated shaft defining an axis and a head at one end thereof, the
head having an effective outside diameter greater than the
effective outside diameter of the shaft, the shaft being disposed
within the axial hole of the body, the mandrel head being disposed
at an opposite end of the body to that of the cap, and the end of
the mandrel head having a flat surface oriented substantially
perpendicular to the axis of the shaft; and, a mechanical interface
between the body and the mandrel such that the body rotates in
response to rotation of the mandrel; rotating the mandrel about its
rotational axis, driving the rivet toward and into the workpieces
such that resultant frictional heating between the rivet and the
workpieces causes the materials of the workpieces to soften at a
process temperature thereby providing a friction stirred
displaceable path for the rivet to traverse, and driving the rivet
along the displaceable path until the cap is seated against the
workpieces; stopping further rotation of the mandrel and allowing
the workpieces and mandrel to cool below the process temperature,
thereby permitting the softened workpieces to harden; and axially
loading the mandrel with sufficient force to drive the mandrel head
into the end of the body thereby upsetting and expanding the body
end to create an interference between the body and the underside of
the lowermost workpiece and to cause the mandrel shaft to sever at
a point internal to the body and proximate the cap, thereby
resulting in the workpieces being held together by the fluidly
bonded materials of the workpieces, the differential thermal
contraction of the workpieces and the rivet, and the mechanical
loading between the mandrel and the body, at the point of
engagement.
21. The method of claim 20, wherein the rotating comprises:
rotating the mandrel at equal to or less than about 12,000
revolutions per minute.
22. The method of claim 20, wherein the driving comprises: driving
the rivet at a rate equal to or greater than about 6 millimeters
per minute and equal to or less than about 150 millimeters per
minute.
23. The method of claim 20, further comprising: holding the mandrel
rotation at about 12,000 revolutions per minute for about 2 seconds
subsequent to the cap being seated against the workpieces.
24. The method of claim 20, wherein the resultant frictional
heating is initiated by the friction stir interaction between the
flat surface of the mandrel and the workpieces.
25. The method of claim 20, wherein the flat surface extends over
equal to or greater than about 80% of the effective diameter of the
end of the mandrel head, thereby displacing the material along the
displaceable path in such a manner as to reduce the tendency for
the displaced material to penetrate the region between the
workpieces as the rivet is driven into the workpieces.
26. The method of claim 25, wherein the flat surface extends over
about 100% of the effective diameter of the end of the mandrel
head.
27. The method of claim 20, wherein the driving the rivet into the
workpieces comprises: driving the rivet absent a preexisting hole
in the workpieces.
28. The method of claim 20, wherein the causing the materials of
the workpieces to soften at a process temperature comprises:
causing the materials to soften at a process temperature that is
substantially lower than the melting temperature of the rivet.
29. The method of claim 20, further comprising: driving the rivet
along the displaceable path until the underside of the cap is in
loaded contact with one of the workpieces; and friction stirring
and partially penetrating the cap into the one of the workpieces.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application claims the benefit of U.S. Provisional
Application Docket No. GP-305227 (GM2-0141), filed Jul. 29, 2004,
which is incorporated herein by reference in its entirety.
BACKGROUND OF THE INVENTION
[0002] The present disclosure relates generally to friction stir
riveting and a method of joining therewith, and particularly to a
friction stir rivet and method that has a reduced tendency to
undesirably displace softened friction stirred material sideways
into the joint between the workpieces being joined.
[0003] Friction stir welding (FSW) is a method used to join metal
workpieces that generally uses a cylindrical shouldered tool with a
profiled pin that is rotated at the joint line between two
workpieces while being traversed along the joint line. The rotary
motion of the tool generates frictional heat that serves to soften
and plasticize the workpieces. As the pin moves laterally, the
softened material, contributed by both workpieces, intermingles in
the wake of the traversing pin and cools and hardens due to the
absence of further frictional stirring, creating a bond between the
two workpieces.
[0004] Recent advances in friction stir processes have extended the
FSW technique to friction stir riveting (FSR), where a stir rivet
is rotated and advanced into an arrangement of workpieces to be
joined such that the material of the workpieces plasticizes around
the rivet during the friction stirring, and then hardens around the
rivet when the body of the rivet stops rotating and the workpieces
and rivet are allowed to cool.
[0005] While present devices and methods may be suitable for their
intended purposes, it is desirable to further advance the
technology of friction stir riveting in a manner that offers
opportunities for joining dissimilar materials.
BRIEF DESCRIPTION OF THE INVENTION
[0006] Embodiments of the invention include a friction stir rivet
having a body, a mandrel, and a mechanical interface therebetween.
The body has an elongated cylindrical shank, a cap at a first end,
and an axial hole therethrough. The mandrel has an elongated shaft
defining an axis and a head at one end thereof, the head having an
effective outside diameter greater than the effective outside
diameter of the shaft, the shaft being disposed within the axial
hole of the body, the mandrel head being disposed at an opposite
end of the body to that of the cap, and the end of the mandrel head
having a flat surface oriented substantially perpendicular to the
axis of the shaft. The mechanical interface between the body and
the mandrel is such that the body rotates in response to rotation
of the mandrel.
[0007] Other embodiments of the invention include a method of
joining upper and lower workpieces fluidly bondable at a point of
engagement using the aforementioned friction stir rivet. The rivet
is positioned at the point of engagement of the workpieces, and the
mandrel is rotated about its rotational axis. The rivet is driven
toward and into the workpieces such that resultant frictional
heating between the rivet and the workpieces causes the materials
of the workpieces to soften at a process temperature thereby
providing a friction stirred displaceable path for the rivet to
traverse. The rivet is driven along the displaceable path until the
cap is seated against the workpieces. Further rotation of the
mandrel is stopped, allowing the workpieces and mandrel to cool
below the process temperature, thereby permitting the softened
workpieces to harden. The mandrel is axially loaded with sufficient
force to drive the mandrel head into the end of the body thereby
upsetting and expanding the body end to create an interference
between the body and the underside of the lowermost workpiece and
to cause the mandrel shaft to sever at a point internal to the body
and proximate the cap. As a result, the workpieces are held
together by the fluidly bonded materials of the workpieces, the
differential thermal contraction of the workpieces and the rivet,
and the mechanical loading between the mandrel and the body, at the
point of engagement.
BRIEF DESCRIPTION OF THE DRAWINGS
[0008] Referring to the exemplary drawings wherein like elements
are numbered alike in the accompanying Figures:
[0009] FIGS. 1 and 2 depict in cross section longitudinal view
exemplary rivets in accordance with embodiments of the
invention;
[0010] FIGS. 3-5 depict in cross section axial view exemplary
shafts of a mandrel of a rivet for use in embodiments of the
invention;
[0011] FIGS. 6 and 7 depict in cross section longitudinal view
alternative exemplary rivets in accordance with embodiments of the
invention;
[0012] FIGS. 8 and 9 depict in cross section axial view alternative
exemplary rivet geometries having a non-circular cross section in
accordance with embodiments of the invention;
[0013] FIGS. 10-12 depict a riveting method in accordance with
embodiments of the invention;
[0014] FIG. 13 depicts a martensite microstructure for use in
accordance with embodiments of the invention; and
[0015] FIG. 14 depicts a cold drawn microstructure for use in
accordance with embodiments of the invention.
DETAILED DESCRIPTION OF THE INVENTION
[0016] Embodiments of the invention disclose a friction stir rivet
having an outer body and an inner mandrel with a mechanical
interface therebetween such that the outer body rotates in response
to the mandrel being rotated. The mandrel has a flat head that
engages the workpieces to be joined, thereby providing a friction
stir surface that has a reduced tendency to undesirably displace
the softened friction stirred material sideways in the joint
between the workpieces. Other embodiments disclose a method of
joining two or more workpieces using the friction stir rivet
disclosed herein, and doing so in the absence of a preexisting hole
in the workpieces to be joined.
[0017] FIG. 1 depicts a cross section view of an exemplary
embodiment of a friction stir rivet 100 having a body 105 and a
mandrel 110. Body 105 has an elongated cylindrical shank 115, a cap
120 at one end, and an axial hole 125 extending from one end to the
other. Mandrel 110 has an elongated shaft 130 defining an axis 135,
and a head 140 at one end. Head 140 has an effective outside
diameter D that is greater that the effective outside diameter d of
shaft 130.
[0018] As used herein, the term effective diameter refers to that
diameter that the respective cross section would transcribe if it
were rotated about its axis of rotation, such as axis 135 in the
case of shaft 130. For example, and referring now to FIGS. 3-5
which depict exemplary cross section cuts through shaft 130, a
shaft 130 having a circular cross section would have an effective
diameter defined by its own diameter d (see FIG. 3), a shaft 130
having a hexagonal cross section would have an effective diameter
defined by d2 (see dashed line, FIG. 4), and a shaft 130 having a
square cross section would have an effective diameter defined by d3
(see dashed line, FIG. 5). While only shaft 130 is depicted in
FIGS. 3-5, it will be appreciated that the same configuration may
be applied to both the mandrel head 140 and the body shank 115.
That is, a circular mandrel shaft 130 may be used with a circular
body shank 115, a hexagonal mandrel shaft with a hexagonal body
shank, and a square mandrel shaft with a square body shank, for
example. In an embodiment where both shaft 130 and shank 115 are
non-circular, the outer and inner surfaces, respectively, may
provide a mechanical interface therebetween, thereby resulting in
the rotation of shank 115 and body 105 in response to mandrel 110
being rotated. Also, while only three cross section geometries are
depicted in FIGS. 3-5, it will be appreciated that rivet 100 is not
so limited and that any cross section geometry may be applied to
rivet 100 in accordance with embodiments of the invention.
[0019] As depicted in FIG. 1, shaft 130 is disposed within axial
hole 125, and head 140 is disposed at the opposite end of body 105
to that of cap 120. In an embodiment, the end 145 of mandrel head
140 has a flat surface 150 that is oriented substantially
perpendicular to axis 135 of shaft 130, and preferably flat surface
150 is oriented substantially perpendicular to axis 135 within
plus-or-minus two degrees of axis 135. While it is desirable to
have flat surface 150 extend over about 100% of the effective
diameter D of the end 145 mandrel head 140, it may also be possible
to have and use flat surface 150 that extends over something less
than 100% of effective diameter D. In an alternative embodiment,
and referring now to FIG. 2, flat surface 150 may have an effective
outside diameter Ds that is less than the effective outside
diameter D of mandrel head 140. For example, flat surface 150 may
have an effective outside diameter Ds that is equal to or greater
than about 80% of the effective outside diameter D of mandrel head
140. Notwithstanding the foregoing, it has been observed that the
closer effective diameter Ds is to effective diameter D, the less
the tendency will be to displace the softened friction stirred
material sideways into the joint between the workpieces.
[0020] A mechanical interface 155, 160, 165, best seen by now
referring to FIGS. 6-9, between body 105 and mandrel 110 of FIGS. 6
and 7, and between shaft 130 and shank 115 of FIGS. 8 and 9,
enables body 105 and shank 115 to rotate in response to the
rotation of mandrel 110 and shaft 130. FIGS. 6 and 7 depict section
cuts containing axis 135 similar to that of FIGS. 1 and 2, and
FIGS. 8 and 9 depict section cuts perpendicular to axis 135 similar
to that of FIGS. 4 and 5.
[0021] Referring first to FIG. 6, body 105 includes a deformed
region 170 such that inner surface 175 of shank 115 is compressed
against outer surface 180 of shaft 130, thereby forming mechanical
interface 155. The compressive force at interface 155 is made
sufficient to cause body 105 to rotate in response to mandrel 110
being rotated. In an embodiment, deformed region 170 is created by
a crimping operation.
[0022] Referring now to FIG. 7, mandrel 110 includes a deformed,
upset or flared region 185 such that outer surface 190 of deformed
region 185 is compressed against inner edge 195 of cap 120 during a
friction stir riveting operation. In an alternative embodiment,
deformed region 185 and head 140 form a tight fit between top and
bottom of body 105, thereby tightly gripping and constraining body
105 during a friction stir riveting operation. In either embodiment
of FIG. 7, mechanical interfaces 160 may result. The degree of
interference at interfaces 160 is made sufficient to cause body 105
to rotate in response to mandrel 110 being rotated during a
friction stir riveting operation.
[0023] Referring now to FIGS. 8 and 9, shaft 130 of mandrel 110,
and shank 115 of body 105, each have matching non-circular cross
sections perpendicular to axis 135 that define an annulus 200
therebetween, such that the outer surface of shaft 130 engages the
inner surface of shank 115 in response to rotation (arrow A) of
mandrel 110 about axis 135, thereby forming mechanical interface
165, best seen by referring to the phantom lines 112 of each
figure, which represent a rotated mandrel 110.
[0024] In an alternative embodiment, the material and outside
diameter d of shaft 130 and the material and inside diameter B of
shank 115 are selected and sized such that diameter d is less than
diameter B at room temperature, such as 20 degree Celsius (deg-C.)
for example, and is equal to or greater than diameter B at a
process temperature, such as greater than 20 deg-C. and less than
or equal to the melt temperature of the workpieces for example. In
an embodiment where the workpieces are aluminum, the process
temperature is less than or equal to about 660 deg-C., for example,
and in an embodiment where the workpieces are thermoplastic, the
process temperature is less than the melt temperature of the
respective thermoplastic. In this manner, a thermal expansion
compression fit between shaft 130 and shank 115 at the appropriate
process temperature will produce the desired mechanical interface
between shaft 130 and shank 115 such that shank 115 and body 105
will rotate in response to the rotation of mandrel 110 and shaft
130.
[0025] In an embodiment, mandrel 110 and body 105 are selected to
be copper, titanium, iron, or any alloy having at least one of the
foregoing materials. If mandrel 110 is steel, it is preferable to
use medium or high carbon steel. As used herein, medium carbon
steel refers to a steel having equal to or greater than about 0.29
weight % carbon and equal to or less than about 0.53 weight %
carbon, and high carbon steel refers to a steel having equal to or
greater than about 0.55 weight % carbon and equal to or less than
about 0.95 weight % carbon.
[0026] The materials used for mandrel 110 and body 105 are selected
such that head 140 of mandrel 110 is strong enough to deform the
end 205 of body 105 in response to head 140 being axially pulled
into hole 125 of body 105 subsequent to the friction stirring
process, best seen by now referring to FIGS. 10-12, which depict an
exemplary method of friction stir riveting in accordance with
embodiments of the invention.
[0027] In FIG. 10, workpiece 215 is supported in an appropriate
fashion, workpiece 220 is positioned on top of workpiece 215,
friction stir rivet 100 is positioned at the point of engagement
210 of the workpieces 215, 220, and mandrel 110 is rotated about
its rotational axis 135 via a rotary drive tool 225. In FIG. 11,
tool 225 has driven rivet 100 downward towards and into workpieces
220 and 215 in a rotary fashion such that resultant frictional
heating between rivet 100 and workpieces 220, 215 causes the
materials of workpieces 220, 215 to soften, thereby providing a
friction stirred displaceable path (depicted generally as numeral
250) for rivet 100 to traverse. Rivet 100 is driven along the
displaceable path until cap 120 is seated against the top surface
230 of workpiece 220. While it may be possible to rotate and drive
rivet 100 at sufficient speed and rate to cause melting of
workpieces 220, 215, it has been observed that rotating and driving
rivet 100 to cause softening of workpieces 220, 215 is sufficient
for producing a suitable joint. In an embodiment, tool 225 rotates
mandrel 110 at a speed of about 12,000 revolutions per minute
(rpm), and drives rivet 100 downward at a rate of equal to or
greater than about 6 millimeters per minute (mm/min) and equal to
or less than about 150 mm/min. However, it is contemplated that
rotational speeds of equal to or less than about 12,000 rpm may be
suitable for the purposes disclosed herein. An exemplary drive rate
of about 12 mm/min was applied using 2024-T3 aluminum workpieces
220, 215. As a result of the rotation speed in combination with the
drive rate, the friction heating initiated between the end 140 of
mandrel 110 and the surface 230 of workpiece 220, a friction stir
process temperature is established that results in the softening of
workpieces 220 and 215, and preferably but not necessarily results
in softening without melting. As discussed previously, the process
temperature is that temperature between room temperature and the
melt temperature of workpieces 220, 215 at which workpieces 220,
215 are soft enough to provide a displaceable friction stir path
for rivet 100 to traverse. In an embodiment, the process
temperature is substantially less than the melt temperature of
rivet 100.
[0028] In an embodiment, and with reference still to FIG. 11, tool
225 drives rivet 100 toward workpieces 220, 215 until the underside
of cap 120 is in loaded contact with the topside surface 230 of
workpiece 220, resulting in friction stirring and partial
penetration of cap 120 into surface 230, holds the 12,000 rpm
rotation of mandrel 110 for a defined period of time, such as two
seconds for example, and then stops further rotation to allow
workpieces 220, 215 and mandrel 110 to cool below the process
temperature. During the cooling, the softened workpieces 220, 215
harden.
[0029] In an alternative embodiment, tool 225 holds the 12,000 rpm
rotation of mandrel 110 for a defined period of time subsequent to
the underside of cap 120 being seated against the topside surface
230 of workpiece 220, and then stops further rotation to allow
workpieces 220, 215 and mandrel 110 to cool below the process
temperature.
[0030] Subsequent to hardening, and with reference now to FIG. 12,
tool 225 applies an axial tensile load (pulling operation) to
mandrel 110 with sufficient force to drive mandrel head 140 into
end 205 of body 105, deform end 205 in an upsetting like operation,
and cause mandrel shaft 130 to sever at a location 235 internal to
body 105 and proximate cap 120. As a result, workpieces 220, 215
are held together by the fluidly bonded materials of the workpieces
220, 215, the differential thermal contraction of the workpieces
220, 215 and the rivet 100, and the mechanical loading between
mandrel 110 and body 105 and between body 105 and workpieces 220,
215, at the point of engagement 210.
[0031] To assist in the severing of shaft 130 during the pulling
operation, an embodiment of shaft 130 includes a first region of
relatively high tensile strength 240 and a second region of
relatively low tensile strength 245, depicted in FIG. 1 but also
applicable to other embodiments of shaft 130 depicted in FIGS. 2,
6, 7, and 10-12. In an embodiment, the first and second regions
240, 245 of relatively high and low tensile strength are achieved
by differing the cross section area of shaft 130 in the two
regions. Second region 245 is proximate cap 120, thereby resulting
in a substantial amount of shaft 130 remaining within body 105
after being severed, which adds to the strength of the resultant
riveted joint. It has been observed in an embodiment that by
causing a portion of mandrel 110 to be retained within body 105,
the joint strength is improved by about a factor of two compared to
the same joint absent the portion of mandrel 110.
[0032] In an embodiment, and as previously discussed, end 145 of
mandrel 110 is flat to within plus-or-minus two degrees of
perpendicular relative to axis 135, where flat surface 150 has an
effective diameter at about 100% of the effective diameter of head
140. The use of flat surface 150 provides an effective way of
initiating and generating frictional heating as the rotating flat
surface 150 of mandrel 110 is driven into workpieces 220, 215, and
the use of a 100% flat surface 150 provides an effective way of
reducing the tendency for the displaced material along the
displaceable path to penetrate the region between workpieces 220,
215 at the faying surfaces as rivet 100 is driven into and through
workpieces 220, 215.
[0033] Due to the elevated temperatures (process temperature)
associated with friction stir riveting, it is desirable to use a
material for the mandrel 110 that can withstand the elevated
temperatures without substantially losing tensile strength
qualities during the duration of the riveting process.
Metallurgical hardening processes, such as transformation hardening
or workhardening, generally develop microstructures which, due to
diffusional processes, degrade upon exposure to elevated
temperatures. The mandrel 110, is designed to upset the body 105 of
the rivet 100 before severing. To maintain this capability after
friction stir riveting, the mandrel should be strengthened using a
process that will substantially maintain mandrel strength under the
temperature-time history experienced by the mandrel during the
friction stir riveting process. Ideally this would entail that the
mandrel tensile strength be unaffected by the time-temperature
history of the riveting process, but less stringent requirements
may be imposed provided that the operational requirement of
upsetting before severing is satisfied. Since the degree of
softening is influenced by the initial choice of strengthening
mechanism, the maximum temperature experienced, and the time during
which the mandrel 110 is exposed to the process temperature, the
softening behavior of the mandrel 110 under the time-temperature
history to which it is exposed should be taken into consideration.
Thus, for example, a mandrel 110 with a tempered martensite
structure, such as is shown in FIG. 13, would generally be
preferred over a cold drawn mandrel, such as is shown in FIG. 14,
since the tempered martensite mandrel has already been subjected to
a short-term elevated temperature excursion during the tempering
process, while the cold drawn structure has not. Thus, the cold
drawn structure would be expected to soften more than the tempered
martensite structure under identical riveting process conditions.
As used herein, the term short-term temperature excursion refers to
a temperature at or below the process temperature for a period of
time equal to or less than about two minutes. As depicted, FIGS. 13
and 14 are magnified views of the respective material
microstructure, with a scale of 50 .mu.m (micro-meters) also
shown.
[0034] In an embodiment, a suitable material for mandrel 110 is a
medium carbon steel, such as having 0.40 weight % carbon for
example, that is quenched to form martensite, and is then tempered
at a temperature of not less than about 450 deg-C. for not less
than about 30 minutes, and is preferably tempered at a temperature
of not less than about 500 deg-C. for not less than about 30
minutes.
[0035] In another embodiment, a suitable material for mandrel 110
is a material that undergoes a change in tensile strength in
response to the friction stir process, which may involve an
elevated temperature at the process temperature for the period of
hold time, that is equal to or less than about 5% change, and
preferably is equal to or less than about 1% change.
[0036] In a further embodiment, a suitable material for mandrel 110
is a material that can withstand a process temperature of equal to
or greater than about 50% and equal to or less than about 100% of
the highest melt temperature of metallic workpieces 220, 215, at a
process time of equal to or less than about two minutes, such that
the end 205 of body 105 is upset prior to second region 245 being
severed during the pulling process.
[0037] In yet another embodiment, a suitable material for mandrel
110 is a material that can withstand a process temperature of equal
to or greater than the highest glass transition temperature and
equal to or less than the highest melt temperature of polymeric
workpieces 220, 215, at a process time of equal to or less than
about two minutes, such that the end 205 of body 105 is upset prior
to second region 245 being severed during the pulling process.
[0038] In yet a further embodiment having dissimilar materials for
workpieces 220, 215, such as a polymeric upper workpiece 220 and a
metallic lower workpiece 215 for example, a suitable material for
mandrel 110 is a material that can withstand the higher of the
aforementioned process temperatures for polymeric 220 and metallic
215 workpieces, respectively.
[0039] While some manufacturing processes may be desirous of short
friction stir hold cycles, such as the aforementioned two seconds
for example, other manufacturing processes may prefer longer hold
cycles, such as one minute or two minutes for example. With a two
second or longer hold cycle, it is contemplated that the use of a
martensite or bainite microstructure for mandrel 110 will result in
a stronger rivet 100 as compared to a rivet 100 having a mandrel
110 with a cold drawn microstructure.
[0040] However, it will be appreciated that overall production
process cycles may be sensitive to the drive rate of the friction
stir riveting process, with a faster drive rate being preferred,
and that a faster drive rate may result in lower temperatures for
less time, and consequently less annealing of the mandrel, thereby
enabling mandrel materials, such as cold drawn microstructures for
example, to endure the desired time-temperature process parameters
of the friction stir riveting process. Accordingly, and under
certain conditions, a cold drawn microstructure for the material of
mandrel 110 may be suitable for the purposes disclosed herein.
[0041] From the foregoing, it will be appreciated that a suitable
material for mandrel 110 is not limited to martensite, bainite, or
any other specific microstructure, but is rather determined by the
resultant material characteristics of the mandrel 110 subsequent to
exposure to the process parameters of time and temperature.
[0042] In accordance with embodiments of the invention, 3 mm thick
workpieces 220 and 215 made of 5052 aluminum have been successfully
joined. However, it is contemplated that embodiments of the
invention also offer opportunities for joining dissimilar materials
including but not limited to composites to aluminum, polymers to
aluminum, and aluminum to magnesium. For composite to aluminum or
polymer to aluminum joints, it is contemplated that the aluminum be
mounted below the composite or polymer so that the expanding rivet
during the pulling operation may engage the aluminum as it is
expanded by head 140 of mandrel 110, while the composite or polymer
is held by the larger rivet cap 120 of body 105, and thus subjected
to a lower, less localized stress.
[0043] As disclosed, some embodiments of the invention may include
some of the following advantages: the ability to join workpieces
together in the absence of a preexisting hole, thereby minimizing
clearance, tolerance, fit-up and alignment issues, particularly for
multi-member stack-ups; improved flow control of the displaced
material that reduces its tendency to penetrate the joint area
between the workpieces, thereby reducing the likelihood of the
displaced material forcing the workpieces apart as it cools and
hardens, leaving a large gap therebetween; and, the opportunity for
friction stir riveting dissimilar materials.
[0044] While the invention has been described with reference to
exemplary embodiments, it will be understood by those skilled in
the art that various changes may be made and equivalents may be
substituted for elements thereof without departing from the scope
of the invention. In addition, many modifications may be made to
adapt a particular situation or material to the teachings of the
invention without departing from the essential scope thereof.
Therefore, it is intended that the invention not be limited to a
particular embodiment disclosed as the best or only mode
contemplated for carrying out this invention, but that the
invention will include all embodiments falling within the scope of
the appended claims. Moreover, the use of the terms first, second,
etc. do not denote any order or importance, but rather the terms
first, second, etc. are used to distinguish one element from
another. Furthermore, the use of the terms a, an, etc. do not
denote a limitation of quantity, but rather denote the presence of
at least one of the referenced item.
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