U.S. patent application number 11/557605 was filed with the patent office on 2008-05-08 for method of forming a panel from a metal alloy sheet.
This patent application is currently assigned to FORD GLOBAL TECHNOLOGIES, LLC. Invention is credited to Alan Gillard, Sergey Golovashchenko, Albert Krause.
Application Number | 20080105023 11/557605 |
Document ID | / |
Family ID | 39358556 |
Filed Date | 2008-05-08 |
United States Patent
Application |
20080105023 |
Kind Code |
A1 |
Golovashchenko; Sergey ; et
al. |
May 8, 2008 |
METHOD OF FORMING A PANEL FROM A METAL ALLOY SHEET
Abstract
A method is provided for forming a panel from a metal alloy
sheet having known stress-strain forming properties including a
forming strain limit that is exceeded when a single forming
operation is used to form the panel. The method comprises
determining a stress relieving treatment and an incremental forming
treatment for forming the panel. The total incremental forming
strain comprises a first strain increment. The stress relieving
treatment results in the total incremental forming strain remaining
within the forming strain limit. A first preform shape is
determined in which the geometry of the shape does not exceed the
first strain increment. The sheet is formed into the first preform
shape. The first preform shape is then stress relieved. The first
preform shape is then formed into a panel shape having a geometry
that is within the total incremental forming strain.
Inventors: |
Golovashchenko; Sergey;
(Beverly Hills, MI) ; Krause; Albert; (Plymouth,
MI) ; Gillard; Alan; (Lincoln Park, MI) |
Correspondence
Address: |
BROOKS KUSHMAN P.C./FGTL
1000 TOWN CENTER, 22ND FLOOR
SOUTHFIELD
MI
48075-1238
US
|
Assignee: |
FORD GLOBAL TECHNOLOGIES,
LLC
Dearborn
MI
|
Family ID: |
39358556 |
Appl. No.: |
11/557605 |
Filed: |
November 8, 2006 |
Current U.S.
Class: |
72/364 |
Current CPC
Class: |
C21D 1/30 20130101; C22F
1/04 20130101; C21D 9/48 20130101; Y02P 10/25 20151101; Y02P 10/253
20151101; C21D 1/40 20130101; C21D 1/42 20130101 |
Class at
Publication: |
72/364 |
International
Class: |
B21D 31/00 20060101
B21D031/00 |
Claims
1. A method of forming a panel from a metal alloy sheet having a
grain microstructure and known stress-strain forming properties
including a monotonic ultimate elongation and a forming strain
limit, where the forming strain limit of the sheet is exceeded at a
location in the panel when a single forming operation is used to
shape the sheet into the panel, the sheet being strain hardenable
and being initially in a tempered condition for forming, the method
comprising: determining a stress relieving treatment for forming
the panel such that the stress relieving treatment does not
generate substantial grain growth of the grain microstructure;
determining an incremental forming treatment including a total
incremental forming strain comprising a first strain increment,
wherein the stress relieving treatment enables the total
incremental forming strain to be within the forming strain limit of
the sheet; determining a first preform shape as a forming precursor
of the panel, wherein the geometry of the first preform shape does
not exceed the first strain increment; forming the sheet into the
first preform shape; stress relieving at least a portion of the
first preform shape by the stress relieving treatment; and forming
the stress relieved first preform shape into the panel by the
incremental forming treatment, wherein the geometry of the panel
does not exceed the total incremental forming strain and is within
the forming strain limit of the sheet.
2. The method according to claim 1 wherein the total incremental
forming strain exceeds the monotonic ultimate elongation.
3. The method according to claim 1 wherein forming includes
stamping.
4. The method according to claim 1 wherein forming includes
hydroforming.
5. The method according to claim 1 wherein the total incremental
forming strain further comprises a second strain increment and the
method further comprising: determining a second preform shape as a
forming precursor of the panel, wherein the geometry of the second
preform shape does not exceed the sum of the first and the second
strain increment; and the step of forming the stress relieved first
preform shape includes: forming the stress relieved first preform
shape into a second preform shape; stress relieving at least a
portion of the second preform shape by the stress relieving
treatment; and forming the stress relieved second preform shape
into the panel by the incremental forming treatment.
6. The method according to claim 1 wherein the stress relieving
treatment includes a temperature-time treatment, wherein the time
for the temperature treatment is in the range of approximately 1 to
30 seconds.
7. The method according to claim 6 wherein the temperature
treatment includes contact heating.
8. The method according to claim 6 wherein the temperature
treatment includes induction heating.
9. The method according to claim 6 wherein the metal alloy sheet is
aluminum alloy of the AA6xxx family and the temperature treatment
does not heat the grain microstructure to a temperature in excess
of 345C.
10. The method according to claim 9 wherein the time for
temperature treatment is in the range of approximately 4 to 8
seconds and the temperature treatment heats the grain
microstructure to a temperature in the range of approximately 145 C
to 165 C.
11. The method according to claim 9 wherein the first strain
increment is in the range of approximately 10% to 17%.
12. The method according to claim 6 wherein the metal alloy sheet
is aluminum alloy of the AA5xxx family and the temperature
treatment heats the grain microstructure to a temperature in the
range of approximately 375 C to 465 C.
13. The method according to claim 12 wherein the time for
temperature treatment is in the range of approximately 4 to 12
seconds and the first strain increment is in the range of
approximately 10% to 17%.
14. A method of forming a panel from a metal alloy sheet having a
grain microstructure and known stress-strain forming properties
including a monotonic ultimate elongation and a forming strain
limit, where the forming strain limit of the sheet is exceeded at a
location in the panel when a single forming operation is used to
shape the sheet into the panel, the sheet being strain hardenable
and being initially in a tempered condition for forming, the method
comprising: forming the sheet into a first preform shape by an
incremental forming treatment, wherein the incremental forming
treatment includes a total incremental forming strain which exceeds
the monotonic ultimate elongation and comprises a first strain
increment, wherein the first preform shape is a forming precursor
of the panel and the geometry of the first preform shape does not
exceed the first strain increment; stress relieving at least a
portion of the first preform shape by a stress relieving treatment,
wherein the stress relieving treatment does not substantially
recrystalize the grain microstructure and enables the total
incremental forming strain to be within the forming strain limit of
the sheet; and forming the stress relieved first preform shape into
the panel by the incremental forming treatment, wherein the
geometry of the panel does not exceed the total incremental forming
strain and is within the forming strain limit of the sheet.
15. The method according to claim 14 wherein the total incremental
forming strain further comprises a second strain incremental and
the step of forming the stress relieved first preform shape
includes: forming the stress relieved first preform shape into a
second preform shape, wherein the second preform shape is a forming
precursor of the panel and the geometry of the second preform shape
does not exceed the sum of the first and the second strain
increment; stress relieving at least a portion of the second
preform shape by the stress relieving treatment; and forming the
stress relieved second preform shape into the panel by the
incremental forming treatment.
16. The method according to claim 14 wherein the total incremental
forming strain comprises a plurality of strain increments within
the range of approximately 4% to 17%.
17. The method according to claim 14 wherein the stress relieving
treatment includes a temperature-time treatment, wherein the time
for the temperature treatment is in the range of approximately 1 to
12 seconds.
18. The method according to claim 17 wherein the metal alloy sheet
is aluminum alloy of the AA6xxx family and the temperature
treatment heats the grain microstructure to a temperature in the
range of approximately 145 C to 165 C.
19. The method according to claim 17 wherein the metal alloy sheet
is aluminum alloy of the AA5xxx family and the temperature
treatment heats the grain microstructure to a temperature in the
range of approximately 375 C to 465 C.
20. The method according to claim 17 wherein the step of forming
the panel includes stamping.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates generally to incrementally
forming a panel from a strain hardenable metal alloy sheet that has
a forming strain limit that may be exceeded if the panel is formed
in a single forming operation.
[0003] 2. Background Art
[0004] Styling is an important consideration in designing a vehicle
that may be limited by the manufacturing feasibility of producing
body panel parts with complex shapes. Often the panels are made of
metal alloy sheets. Two examples of commonly used production
forming methods for these panels are metal sheet stamping and more
recently, hydroforming. Single sheet forming operations, such as
for example, progressive die stamping of a panel, may not be
feasible in producing more complex shapes. Body panels having
complex shapes may have deep draw geometries that exceed the draw
limitations of the forming operation. The ability to form complex
shapes in body panels is compounded by the fact that the metal
alloy sheets may be comprised of high strength steel or aluminum
alloys that have less formability than traditional mild steels.
[0005] Two solutions employed today to circumvent this issue are to
either limit the complexity of the geometry or produce smaller
panels which may be welded or riveted in order to create a larger
panel with more complex geometries. Both of these solutions either
add cost and complexity to the manufacturing process, or limit
design flexibility.
[0006] Accordingly, there is a need for an enhanced deep draw
forming methodology for metal alloy sheets that have limited
formability.
SUMMARY OF THE INVENTION
[0007] In one embodiment of the present invention a method of
forming a panel from a metal alloy sheet is provided. The metal
alloy sheet has a grain microstructure and known stress-strain
forming properties. Two of such properties may include a monotonic
ultimate elongation and a forming strain limit. The forming strain
limit of the sheet may be exceeded at a particular location in the
panel when a single forming operation is used to shape the sheet
into the panel. The sheet is strain hardenable and is initially
provided in a tempered condition for forming. The method comprises
determining a stress relieving treatment for forming the panel that
does not generate substantial grain growth of the grain
microstructure. An incremental forming treatment is determined that
includes a total incremental forming strain that is initially
formed to a first strain increment. After the first strain
increment the panel is stress relieved. The stress relieving
treatment enables the total incremental forming strain to remain
within the forming strain limit of the sheet. A first preform shape
is determined as a forming precursor of the panel. The geometry of
the first preform shape does not exceed the first strain increment.
The sheet is formed into the first preform shape. A stress
relieving treatment is provided to at least a portion of the first
preform shape. The stress relieved first preform shape is formed
into the panel by the incremental forming treatment, wherein the
geometry of the panel does not exceed the total incremental forming
strain and is within the forming strain limit of the sheet.
[0008] In at least one other embodiment of the present invention a
method of forming a panel from a metal alloy sheet is provided. The
metal alloy sheet has a grain microstructure and known
stress-strain forming properties. Two of such properties may
include a monotonic ultimate elongation and a forming strain limit.
The forming strain limit of the sheet may be exceeded at a
particular location in the panel when a single forming operation is
used to shape the sheet into the panel. The sheet is strain
hardenable and is initially provided in a tempered condition for
forming. The method comprises forming the sheet into a first
preform shape by an incremental forming treatment, wherein the
incremental forming treatment includes a total incremental forming
strain which exceeds the monotonic elongation. The total
incremental forming strain further comprises a first strain
increment. The first preform shape is a forming precursor of the
panel and the geometry of the first preform shape does not exceed
the first strain increment. At least a portion of the first preform
shape is stress relieved by a stress relieving treatment, wherein
the stress relieving treatment does not substantially recrystalize
the grain microstructure and enables the total incremental forming
strain to be within the forming strain limit of the sheet. The
stress relieved first preform shape is formed into the panel by the
incremental forming treatment, wherein the geometry of the panel
does not exceed the total incremental forming strain and is within
the forming strain limit of the sheet.
BRIEF DESCRIPTION OF THE DRAWINGS
[0009] FIG. 1 is a partial perspective view of a vehicle having a
body panel with a shape that has a deep draw geometry;
[0010] FIG. 2 is a sectional view of a sheet metal forming
operation performed by stamping;
[0011] FIG. 3 is a side view of a driver side fender where the deep
draw geometry of the fender has been damaged by the forming
operation;
[0012] FIG. 4 is a stress-strain diagram of one embodiment of an
incremental forming process for a metal alloy sheet;
[0013] FIG. 5 is a diagram of incremental forming results for
6016-T4 aluminum alloy of the final elongation and average drop in
yield strength as a function of temperature using strain increments
of 12%+6%+6%+ . . . etc., in accordance with one embodiment of the
present invention;
[0014] FIG. 6 is a diagram of incremental forming results for
6111-T4 aluminum alloy of the final elongation and average drop in
yield strength as a function of temperature using strain increments
of 12%+4%+4%+ . . . etc., in accordance with one embodiment of the
present invention;
[0015] FIG. 7a is a photo of the grain microstructure of 6111-T4
after 12% strain and no heat treatment in accordance with one
embodiment of the present invention;
[0016] FIG. 7b is a photo of the grain microstructure of 6111-T4
after 12%+4%+4%+4% strain and 4 heat treatments in accordance with
one embodiment of the present invention;
[0017] FIG. 7c is a photo of the grain microstructure of 6111-T4
after 12%+4%+4%+4%+4%+4%+4%+4% strain and 8 heat treatments in
accordance with one embodiment of the present invention;
[0018] FIG. 8a is a photo of the grain microstructure of 6016-T4
after 12% strain and no heat treatment in accordance with one
embodiment of the present invention;
[0019] FIG. 8b is a photo of the grain microstructure of 6016-T4
after 6%+4%+4%+4%+4%+4% strain and 6 heat treatments in accordance
with one embodiment of the present invention;
[0020] FIG. 8c is a photo of the grain microstructure of 6016-T4
after 12%+4%+4%+4%+4%+4%+4%+4%+4%+4%+4%+4% strain and 12 heat
treatments in accordance with one embodiment of the present
invention;
[0021] FIG. 9 is a diagram of the incremental forming results for
6016-T4 with heat treatments of 30 seconds in a peak specimen
temperature of 110 C in accordance with one embodiment of the
present invention;
[0022] FIG. 10 is a diagram of the incremental forming results and
the effect of duration of heat treatment on the finally elongation
of 6016-T4 and 6111-T4 in accordance with one embodiment of the
present invention;
[0023] FIG. 11 is a diagram of incremental forming results for
5754-O aluminum alloy of the final elongation and average drop in
yield strength as a function of temperature using strain increments
of 10%+10%+ . . . etc., in accordance with one embodiment of the
present invention;
[0024] FIG. 12a is a photo of the grain microstructure of 5754-O
pre-stained 17% and heated at 400 C with contact heating for 30
seconds;
[0025] FIG. 12b is a photo of the grain microstructure of 5754-O
pre-stained 17% and heated at 400 C with contact heating for 40
seconds;
[0026] FIG. 12c is a photo of the grain microstructure of 5754-O
pre-stained 17% and heated at 400 C with contact heating for 60
seconds;
[0027] FIG. 13a is a photo of a grain microstructure of 5754-O, 17%
pre-stain and heated at 600 C with contact heating for 8
seconds;
[0028] FIG. 13b is a photo of a grain microstructure of 5754-O, 17%
pre-stain and heated at 600 C with contact heating for 10
seconds;
[0029] FIG. 13c is a photo of a grain microstructure of 5754-O, 17%
pre-stain and heated at 600 C with contact heating for 12
seconds;
[0030] FIG. 13d is a photo of the grain microstructure of 5754-O
with an elongated grain structure after six strain increments of
10%, and 5 heat treatments of the peak temperature of 375 C in a
duration of 30 seconds; and
[0031] FIG. 14 is a flow chart illustrating an embodiment of the
method of forming a panel from a metal alloy sheet.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)
[0032] Detailed embodiments of the present invention are disclosed
herein. It is understood, however, that the disclosed embodiments
are merely exemplary of the invention and may be embodied in
various and alternative forms. The figures are not necessarily to
scale, some figures may be exaggerated or minimized to show the
details of the particular components. Therefore, specific
structural and functional details disclosed herein are not to be
interpreted as limiting, but merely as a representative basis for
the claims or as a representative basis for teaching one skilled in
the art to practice the present invention.
[0033] Referring to FIG. 1, a partial perspective view is provided
of a vehicle 10. The vehicle 10 includes a fender body panel 12, an
exterior door trim panel 14, an exterior lighting lens 16, and a
bumper fascia 18. The exterior lens 16 and the bumper fascia 18 are
produced by injection molding a plastic material. Other panels,
such as the fender 12, are often manufactured by stamping or
hydroforming a metal alloy sheet. Styling requirements may dictate
highly contoured geometric shapes that may exceed the forming
strain limit in single stamping or hydroforming operation.
[0034] In FIG. 2, a sectional view is provided of a sheet metal
stamping operation 30. The stamping operation 30 represents one of
several ways of forming sheet metal into a panel. Another method of
forming sheet metal into a panel is by hydroforming. A metal alloy
sheet 32 is forced into a cavity 34 by the core 36 of the tool or
die in the stamping operation 30. Clamping pads 37 with regular
springs or gas springs may be used to clamp the sheet 32 to help
prevent wrinkling of the sheet 32 during stamping. The cavity 34
had a deep draw volume 38 which is used to form a deep draw shape
within the metal alloy sheet 32. Conventional stamping operations
may have multiple stages using multiple dies for forming. One
example of a conventional stamping operation is a progressive die
stamping operation. In this operation, the metal alloy sheet 32 may
be stamped several times where each stamping performs a unique
forming step. The process of stamping and forming the metal sheet
32 multiple times may result in accumulating dislocations and
residual stress as a result of non-uniform distribution of plastic
deformation between material grains. This factor may limit the
sheet's 32 ability to be stretched or drawn. Stated another way,
strain hardening effectively reduces the forming strain limit of
the sheet 32. When this occurs, the metal sheet 32 is less able to
be properly formed into the deep draw volume 38 of the cavity
34.
[0035] In FIG. 3, a side view is provided of the driver side fender
12 with deep draw geometry. The fender 12 has a deep draw area
located in the nose 50 of the fender 12 that interfaces with the
headlamp 16. As shown, the nose 50 has a crack 54 that resulted
from the forming operation. A crack 54 may sometimes only
occasionally occur in high volume production of a highly contoured
panel. The nose 50, for example, may be susceptible to thinning out
of the material, which may also occasionally result in
cracking.
[0036] The present invention provides a method of forming a panel
from a metal alloy sheet that has a forming strain limit that is
susceptible to strain hardening. The forming strain limit of the
sheet may be exceeded at a location in the panel in a single
forming operation. The forming capability of a metal alloy sheet
may be enhanced by using incremental forming at respective strain
increments interposed with stress relieving treatments. The
following study performed by the Applicants illustrates at least
one embodiment of the present invention.
[0037] The materials used in this study are shown in Tables 1 and 2
with their respective mechanical properties and chemical
compositions.
TABLE-US-00001 TABLE 1 Gauge Y.S. U.T.S. Alloy Manufacturer (mm)
(MPa) (MPa) % EL* 6016-T4 EDT Alcan 1.02 116 230 27.9 6111-T4 PD
Alcan 0.93 145 284 26.8 5754-O Alcoa 2.00 115 220 25.0 *Monotonic
Ultimate Elongation
TABLE-US-00002 TABLE 2 6016-T4 EDT 6111-T4 PD 5754-O Mg 0.61 0.89
2.6 3.6 Si 0.97 0.54 <0.40 Cu 0.04 0.67 <0.10 Fe -- 0.19
<0.40 Mn 0.04 0.22 <0.50 Al Bal. Bal. Bal.
[0038] Interrupted tensile testing was used to simulate the
incremental forming process. Tensile specimens were cut from
aluminum sheet stock. In testing, the tensile specimens were
deformed to a specific amount of strain, then removed from the load
frame and heat treated by various methods to promote recovery and
stress relief, and then placed back into the load frame for further
deformation. This procedure was then repeated. The specimens were
put through these strain/heat treatment increments until they
ultimately failed, and the final elongation was measured and used
as an indication of the effectiveness of the heat treatment
parameters. The stress-strain data for each deformation increment
was recorded, and with this information it was possible to measure
the response of the material to the heat treatment in terms of
strength reduction, or softening. More specifically, the quantity
measured was the difference between the peak stress achieved by a
specimen at the end of a strain increment, and the yield stress of
that same specimen after being heat treated.
[0039] FIG. 4 is a schematic of the incremental forming process and
the drop in yield stress, which occurs in response to an
intermediate heat treatment as illustrated in the figure as
.DELTA..sigma..
[0040] Microstructural analysis was conducted using an optical
microscope. Samples of each alloy were exposed to various amounts
of accumulated strain in different heat treatment conditions, and
the resulting microstructures were analyzed in order to determine
if any recrystallization or grain growth had occurred.
[0041] The results of the study for the 6xxx series aluminum alloys
is discussed first. It was originally found for 6111-T4 that a heat
treatment of 30 seconds in a furnace maintained at 250 C resulted
in excellent formability for this alloy. When using strain
increments of 12%, followed by 4% in each increment after the
first, the elongation increased from 27% in the as received
condition, otherwise referred to as the monotonic ultimate
elongation, to 45% with incremental forming. These same conditions
were chosen as a starting point for experimentation with 6016-T4,
and the result was an increase in elongation from the 28% monotonic
ultimate elongation to 65% elongation with incremental forming.
[0042] Further testing was conducted with both the 6016-T4 and the
6111-T4 alloys. Using a tensile specimen with an attached
thermocouple, it was found that the specimens reached a temperature
of 110.+-.5 C after being heated in a tray for 30 seconds in a
furnace at 250C. In order to measure the effect of the peak
specimen temperature on 6016-T4, tests were run using the same 30
second heat treatment duration, but with peak specimen temperatures
of 90 C, 110 C, 130 C, 150 C and 170 C. The strain increments used
in these tests were 12% in the first increment, followed by 6% in
each increment after the first, and the results are shown in FIG.
5.
[0043] FIG. 5 illustrates that a 30 second heat treatment with a
peak specimen temperature in the range of 110 C to 150 C results in
a significant increase in the formability of 6016-T4. The results
indicate that heating the specimen to a peak temperature of 90 C
did promote some recovery and stress relief, but not nearly as much
as the peak temperatures in the range of 110 to 150 C. Heating the
specimen to a peak temperature of 170 C also promoted some recovery
and stress relief. The fact that the results were much lower than
those at 110 C to 150 C indicates that artificial aging is taking
place along with recovery and stress relief at the higher
temperatures. An overall decrease in material formability were
caused relative to the results at 110 C to 150 C. FIG. 5 also
illustrates that the average drop in yield stress follows a trend
similar to that of the elongation.
[0044] The same conditions were then applied to 6111-T4, and the
results are shown in FIG. 6. It should be noted, that the strain
increments used in this experiment, however, were 12% followed by
increments of 4%.
[0045] FIG. 6 illustrates that 6111-T4 responded similar to 6016-T4
in that there is a window of specimen peak temperatures within
which incremental forming is significantly beneficial. Under these
conditions, the 6016-T4 alloy's optimal temperature range was about
110 C to 150 C, and for the 6111-T4 alloy, this range was about 110
C to 130 C. This difference may be due to the higher Cu content of
6111-T4 relative to 6016-T4, which is known to increase
precipitation hardening kinetics. As in FIG. 5, FIG. 6 shows a
strong correlation between the final elongation and the drop in
yield stress after heat treatment. The magnitude of the drop in
yield stress of 6111-T4 is about half that of 6016-T4, with the
value becoming negative at 170 C in the case of 6111-T4.
[0046] The microstructures of the 6111-T4 and 6016-T4 specimens
were analyzed after undergoing numerous increments of deformation
followed by heat treatments. Some representative microstructures
resulting from the incremental forming of 6111-T4 and 6016-T4 are
shown in FIGS. 7 and 8, respectively. In all cases, substantial
recrystallization or grain growth were not observed. This result
was obtained because for most aluminum alloys, recrystallization
and grain growth usually occur only at temperatures greater than
about 345 C. Because neither recrystallization nor grain growth
were observed, the primary softening mechanism that was occurring
during the intermediate heat treatments of 6xxx alloys is the
relief of residual stresses, which occurs through redistribution of
those stresses from isolated grains in the entire grain
structure.
[0047] Further testing was conducted to evaluate the effect of
strain increment size. Tests were run on 6016-T4 with a common heat
treatment condition (30 seconds, peak temperature of 110 C), with
the following strain schedules (%): 12+4+4+ etc., and 12+6+6+etc.,
and 12+12+ etc. The results of these tests are shown in FIG. 9. As
illustrated, even with larger strain increments and thus fewer heat
treatments for stress relief, incremental forming still provides a
benefit to the overall forming elongation.
[0048] A typical production rate for automotive body panels with
conventional stamping processes is approximately 300 panels per
hour, or 1 panel per 12 seconds. A heat treatment time of 30
seconds between forming increments on a stamping line may limit
production output. The duration of the heat treatments may be fit
within the 12 second window preferred by conventional stamping
rates for incremental forming. The heat treatments may also be
performed using off-line batch processing to provide a greater time
window.
[0049] Testing was further conducted with tensile specimens that
were heated more rapidly to an operative temperature range of 110 C
to 170 C and within a time period range of 4 to 8 seconds,
depending upon the test. Starting with 6016-T4, specimens reached a
peak temperature of 165.+-.10 C in 8 seconds. Using strain
increments of 12%+4%+ . . . , 6016-T4 showed a final elongation of
60%. This value was almost as large as the 65% elongation achieved
with a 30 second heat treatment at a peak specimen temperature of
110 C Tests were then run using a heat treatment duration of 4
seconds and a peak specimen temperature of 157.+-.10 C. The
resulting elongation of 6016-T4 with strain increments of 12+4+ . .
. % was 52%. This value was also a significant increase over the
monotonic ultimate elongation of 28%. These results are shown in
FIG. 10.
[0050] FIG. 10 illustrates a time-sensitivity to the stress relief
and recovery mechanism for 6016-T4 within a time range of 0-10
seconds. After about 10 seconds, the amount of additional stress
relief and recovery that may be gained is minimal. There was
virtually no difference between the elongation achieved with
durations of 4, 8, and 30 seconds for 6111-T4. This difference may
be due to the higher precipitation hardening kinetics of 6111-T4
relative to that of 6016-T4.
[0051] Next, the 5xxx series aluminum alloys were studied. Original
testing on 5754-O including a heat treatment of 120 seconds in a
furnace maintained at 600 C resulted in excellent formability for
this alloy. Using strain increments of 12%, the alloy achieved an
average final elongation of 87%. This value exceeds the 65%
elongation achieved by 6016-T4 when forming with smaller strain
increments of 12%+4%. This is because much higher heat treatment
temperatures may be used with 5xxx series alloys since
precipitation hardening does not occur in 5xxx series alloys. Heat
treatment duration of 120 seconds may be undesirable for production
operations. As a result, contact heating of durations of 8 to 30
seconds were used for further testing of 5754-O.
[0052] Precipitation hardening does not occur in 5xxx alloys.
Accordingly, there is not a limited temperature window between
which final elongation would be maximized as there may be with the
6xxx series alloys. The 5xxx series alloys may use higher heat
treatment temperatures that would likely result in higher total
elongation. Incremental forming of 5754-O tests were run at
different temperatures using contact heating for 8 seconds and
strain increments of 10%. The results are shown in FIG. 11.
[0053] Grain growth may occur in aluminum alloys at temperatures in
excess of 345 C, which may produce reduced strength and poor
surface quality after forming due to orange peel. The 5754-O
specimens incrementally formed and heat treated for 120 seconds at
600 C exhibited a very rough surface finish after forming,
indicative of significant grain growth and orange peel. Experiments
were conducted with tensile samples prestrained to 17% to determine
when recrystallization or grain growth would occur in the contact
heating treatment of 400 to 600 C for 4 to 30 seconds. The
specimens were heat treated for various amounts of time, sectioned,
mounted, and polished for microstructural analysis. Specimens were
then heat treated using contact heating with aluminum plates
maintained at 400 C for durations of 12 seconds, 16 seconds, 20
seconds, 30 seconds, 40 seconds, 50 seconds and 60 seconds.
[0054] Microstructural analysis showed that no recrystallization or
grain growth had occurred through 30 seconds, but at 40 seconds
approximately half of the grains had grown to three or four times
their original size, and by 60 seconds almost all the grains had
grown to the same size. Micrographs of the resulting structures are
shown in FIG. 12. FIG. 12a is 5754-O prestrained 17% and contacted
heated at 400 C for 30 seconds; FIG. 12b is the same as 12a except
it has 40 seconds of contact heating; and FIG. 12c is the same as
12a except it has 60 seconds of contact heating.
[0055] Other prestrained specimens were heat treated using contact
heating with plates maintained at 600 C for durations of 4 seconds,
6 seconds, 8 seconds, 10 seconds, 12 seconds, 16 seconds, 20
seconds and 30 seconds. Microstructural analysis showed that no
recrystallization or grain growth had occurred through 8 seconds
(see FIG. 13a), but at 10 seconds (see FIG. 13b) grain growth was
partially complete, and by 12 seconds (see FIG. 13c) all the grains
had grown larger than their original size. No recrystallization was
observed with the prestrain level of 17% at both the 400 C and 600
C conditions, and grain growth was observed to be a somewhat rapid
at onset, but then significantly slower after all the grains had
begun to grow. Representative micrographs showing the progress of
grain growth are shown in FIG. 13. The elongated grain structure
resulting from extensive incremental forming (six increments of 10%
and 5 heat treatments of a peak temperature of 375 C and a duration
of 30 seconds--see FIG. 13d) is without recrystallization or grain
growth.
[0056] After determining the times and temperatures at which grain
growth occurs in 5754-O, incremental forming tests were conducted
with proper heat treatment conditions to determine how much
elongation could be achieved without significantly altering the
grain size of the material. At 400 C, a duration of 30 seconds is
just below the onset of grain growth, and at 600 C, a time of 8
seconds is just below the onset of grain growth. Temperature
measurements showed that the use of contact heating with plates at
400 C resulted in the tensile specimens reaching a peak temperature
of 375 C in 30 seconds. Using plates at 600 C brought the tensile
specimens to a temperature of 465 C in 8 seconds. Incremental
forming tests were run with these heat treatment parameters and
with strain increments of 10%.
[0057] The heat treatment of 30 seconds using 400 C plates resulted
in an average final elongation of 63%. Heat treatments for 8
seconds using 600 C plates resulted in an average elongation of
52%. Microstructural analysis was performed on fractured tensile
specimens deformed at these conditions and no grain growth was
observed.
[0058] Referring to FIG. 14, one embodiment of a method of forming
a panel from a metal alloy sheet is illustrated in a flow chart. In
FIG. 14, a method is provided for forming a panel from a metal
alloy sheet having a grain microstructure and known stress-strain
forming properties including a monotonic ultimate elongation and a
forming strain limit. The forming strain limit of the sheet may be
exceeded at a location in the panel when a single forming operation
is used to shape the sheet into the panel. The sheet is strain
hardenable and is initially in a tempered condition for
forming.
[0059] The metal sheet made be comprised of, for example, a steel
or aluminum alloy. For instance, low carbon steel alloys such as
SAE 1008 or 1010 may be suitable steel alloy sheet materials.
Alternatively, 6016-T4, 6111-T4 and 5754-O are examples of suitable
aluminum alloy sheet materials. Other suitable sheet materials may
also be used that are known to those skilled in the art.
[0060] The method comprises determining a stress relieving
treatment 200 for the metal alloy sheet. The stress relieving
treatment includes a temperature-time treatment. The time duration
of the temperature treatment according to the invention is
preferably at least, with increasing preference in the order given,
1, 2, 3, or 4 seconds and independently, primarily for degradation
and/or economic reasons, preferably is not more than, with
increasing preference in the order given, 30, 20, 15 or 12 seconds.
The temperature treatment, which may be unique to the respective
metal alloy used and may also be dependent on the specific time
treatment chosen, heats the grain microstructure to a temperature
which redistributes residual stresses from isolated grains into the
entire grain structure without generating substantial grain growth
or recrystallization of the grain microstructure.
[0061] The method further comprises determining an incremental
forming treatment 202 including a total incremental forming strain
comprising a first strain increment, wherein the stress relieving
treatment enables the total incremental forming strain to be within
the forming strain limit of the sheet. In at least one embodiment,
the stress relieving treatment enables the total incremental
forming strain to exceed the monotonic ultimate elongation of the
metal alloy sheet. The total incremental forming strain may be
comprised of a plurality of strain increments. In at least one
embodiment, the strain increments are within the range of
approximately 4% to 17%.
[0062] The method further comprises determining a first preform
shape 204 as a forming precursor of the panel. The geometry of the
first preform shape is such that it does not exceed the first
strain increment, wherein the sheet geometry represents a base
strain of zero. There are corresponding preform shapes for each of
the strain increments in embodiments where the total incremental
forming strain is comprised of a plurality of strain increments.
Each respective preform shape is a forming precursor of the panel,
wherein the geometry of the preform shape does not exceed the sum
of the prior and corresponding strain increments.
[0063] Incremental forming may include limiting the method to only
deeper draw areas of the panel. For instance, in FIG. 3, the nose
50 region of the fender 12 may be a local area where incremental
forming may be used. More specifically, a plurality of preform
shapes may be determined and limited to this specific area of the
panel. Alternatively, preform shapes and incremental forming may
include the entire panel. Any suitable area of the panel determined
by those skilled in the art, including the entire panel, may
benefit from various embodiments of the present invention.
[0064] The method further comprises forming the sheet into the
first preform shape 206. Various methods of forming may be used
including stamping and hydroforming. Although production forming
may benefit substantially from embodiments of incremental forming,
it is understood that prototype forming processes are also included
as suitable forming methods.
[0065] The method further comprises stress relieving the first
preform shape 208 by the stress relieving treatment. The stress
relieving treatment may be limited to a portion of the first
preform shape or may include the entire first preform shape. In one
embodiment, only a portion of the first preform shape is heated by
a heat source for a time period. After removal of the heat source,
the heated portion cools rapidly by thermal conduction into the
remaining cooler portion of the first preform shape.
[0066] Various heat sources may be employed for the stress
relieving treatment. For example, induction heating, contact
heating, electric heating or oven heating may be suitable methods
of heating depending on the desired cycle time. Other suitable
methods of heating known by those skilled in the art may also be
used.
[0067] The method further comprises forming the stress relieved
first preform shape into the panel 210, wherein the geometry of the
panel does not exceed the total incremental forming strain and is
within the forming strain limit of the sheet. Any suitable method
of forming may be employed as was previously discussed. Moreover,
forming the stress relieved first preform shape into the panel may
include in one embodiment, forming a plurality of preform shapes by
the incremental forming treatment interposed with stress relieving
treatments.
[0068] While the best mode for carrying out the invention has been
described in detail, those familiar with the art to which this
invention relates will recognize various alternative designs and
embodiments for practicing the invention as defined by the
following claims.
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