U.S. patent application number 10/337239 was filed with the patent office on 2004-07-08 for method of reducing cycle time for metal forming.
Invention is credited to Bradley, John Robert, Krajewski, Paul Edward, Morales, Arianna T., Saunders, Frederick Irvin, Verma, Ravi.
Application Number | 20040129052 10/337239 |
Document ID | / |
Family ID | 32507436 |
Filed Date | 2004-07-08 |
United States Patent
Application |
20040129052 |
Kind Code |
A1 |
Krajewski, Paul Edward ; et
al. |
July 8, 2004 |
Method of reducing cycle time for metal forming
Abstract
A method for reducing the part-to-part cycle time of quick
plastic forming and superplastic forming of a metallic sheet alloy
into an automotive sheet metal component by locally modifying a die
surface to control friction which militates against undesirable
necking and thinning.
Inventors: |
Krajewski, Paul Edward;
(Sterling Heights, MI) ; Bradley, John Robert;
(Clarkston, MI) ; Saunders, Frederick Irvin;
(Addison, MI) ; Verma, Ravi; (Shelby Township,
MI) ; Morales, Arianna T.; (Royal Oak, 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: |
32507436 |
Appl. No.: |
10/337239 |
Filed: |
January 6, 2003 |
Current U.S.
Class: |
72/347 |
Current CPC
Class: |
B21D 26/055 20130101;
B21D 37/20 20130101; B21D 22/20 20130101 |
Class at
Publication: |
072/347 |
International
Class: |
B21D 022/00 |
Claims
What is claimed is:
1. A method for tailoring friction characteristics of a die to
minimize a cycle time for metal forming, the method comprising the
steps of: providing a metal forming die having a die surface to
effect metal forming of a structure thereon, the die surface having
at least one coefficient of friction; and modifying a portion of
the die surface to change the at least one coefficient of friction
on the portion of the die surface.
2. The method according to claim 1 wherein the die surface is
modified by chemical etching.
3. The method according to claim 1 wherein the die surface is
modified by laser surface dimpling.
4. The method according to claim 1 wherein the die surface is
modified by scribing.
5. The method according to claim 1 wherein the die surface is
modified by sand blasting.
6. The method according to claim 1 wherein the die surface is
modified by laser particle injection.
7. The method according to claim 1 wherein the die surface is
modified by inserting dissimilar metal inserts therein.
8. The method according to claim 1 wherein the die surface is
modified by laser ablation.
9. The method according to claim 1 wherein the die surface is
modified by local oxidation.
10. The method according to claim 1 wherein the metal provided is
at least one of aluminum, magnesium, titanium, and stainless
steel.
11. A method of making a structure by metal forming, the method
comprising the steps of: providing a metal blank; providing a metal
forming die having a die surface to effect forming of the metal
blank thereon, the die surface having at least one coefficient of
friction; and forming the metal structure by applying pressure to
the metal blank with the metal forming die; wherein a portion of
the die surface is modified to change the at least one coefficient
of friction on the portion of the die surface to minimize a cycle
time for said forming and militate against at least one of necking,
localized thinning, and splitting of the metal blank during said
forming.
12. The method according to claim 11 wherein the die surface is
modified by chemical etching.
13. The method according to claim 11 wherein the die surface is
modified by laser surface dimpling.
14. The method according to claim 11 wherein the die surface is
modified by scribing.
15. The method according to claim 11 wherein the die surface is
modified by sand blasting.
16. The method according to claim 11 wherein the die surface is
modified by laser particle injection.
17. The method according to claim 11 wherein the die surface is
modified by inserting dissimilar metal inserts therein.
18. The method according to claim 11 wherein the die surface is
modified by laser ablation.
19. The method according to claim 11 wherein the die surface is
modified by local oxidation.
Description
FIELD OF THE INVENTION
[0001] The invention relates to a method of reducing the cycle time
for metal forming and more particularly to a method of reducing the
part-to-part cycle time of quick plastic forming and superplastic
forming of a metallic sheet alloy into an automotive sheet metal
component by locally modifying a die surface to control
friction.
BACKGROUND OF THE INVENTION
[0002] Typically, an automobile sheet metal component is made by
stamping or shaping a low carbon steel or an aluminum alloy sheet
stock into a desired shape. Automobile sheet metal components are
formed and welded or otherwise joined to form vehicle body or
closure panels. It is a goal to make the panels from as few parts
as possible in order to minimize manufacturing cost and the overall
weight of the vehicle. It is another goal to make the sheet metal
components as quickly as possible to minimize manufacturing cost.
In order to accomplish these goals, there is an incentive to devise
more formable metal alloys and better forming processes so that
fewer automobile body panels having a more complex shape can be
made and joined to form either the vehicle body or closure panels
rather than welding or bolting together a myriad of smaller,
simpler pieces.
[0003] Lubrication is a critical aspect of the forming process.
Typically, a lubricant with a low coefficient of friction is
selected to enhance material flow in a die. By minimizing friction,
sticking between a blank and the die is likewise minimized, and
part removal without distortion is facilitated.
[0004] Superplasticity is the capability of a material to develop
unusually high tensile elongation with a reduced tendency toward
local necking during deformation at elevated temperatures. Necking
is a defect that results from excessive local thinning during
forming and can ultimately lead to failure during or after forming.
Alloys which exhibit superplasticity are capable of being subjected
to superplastic forming, wherein portions of a preform are expanded
by the application of fluid pressure against the surface of a
forming member. The forming member is usually in the form of a die
which produces structures of predetermined shapes. The expansion of
the preform occurs through an increase in the surface area of the
preform produced by an elongation in the length and a reduction in
thickness of individual material elements.
[0005] The above process is typically called superplastic forming
or SPF. Recent advances have resulted in a mixed-mode deformation
process termed quick plastic forming or QPF.
[0006] In superplastic forming and quick plastic forming
operations, the preform is clamped firmly at its periphery, thus
ideally allowing for material to be stretched from the area inside
of the clamped periphery only. Thinning of a preform as a result of
stretching is highly uniform except in areas coming in contact with
the forming surface. A unique area that comes in early contact with
the forming surface is the forming member cavity entrance radius
area, i.e., the intermediate region between the peripheral portion
of the preform and the part expanding into the cavity. As the
preform drapes over the radius area there is a tendency to increase
the local rate of material elongation at sharp features which, in
turn, may produce localized thinning or necking in this area. The
necking can ultimately lead to splits or tears during subsequent
forming. If the forming cycle is too aggressive at a given
temperature, the blank will neck and/or split just below the entry
radius. Necking makes it difficult to obtain uniform thickness
profiles in the structure and can lead to failure during forming.
To reduce necking, and obtain uniform thickness profiles, a release
coating which is capable of producing a high coefficient of
friction has been used. The use of the release coatings in specific
areas results in an increase in the frictional force and a lower
net force causing material expansion, and, in turn, reduced necking
at the radius area. The alternative to preventing necking is to
form components at very slow cycle times, which is prohibitive for
high volume production.
[0007] While conventional thinking in metal forming has been that
increasing the amount of lubricant enhances forming, it has been
shown that in specific cases, using lubricants having different
coefficients of friction actually improves formability. Varying
lubricant types across a blank may be plausible for low volume
applications. However, such a method is difficult to use in high
volume automobile production. In addition, forming lubricants are
costly and can lead to surface blemishes on automobile outer body
components.
[0008] It would be desirable to produce an automobile sheet metal
component using a superplastic for quick plastic forming process
where forming time, necking, excessive thinning, and splitting are
minimized by locally modifying a die surface to control
friction.
SUMMARY OF THE INVENTION
[0009] Consistent and consonant with the present invention, a
method of producing an automobile sheet metal component where
forming time, necking, thinning, and splitting are minimized by
locally modifying a die surface to control friction has
surprisingly been discovered.
[0010] The method of producing an automobile sheet metal component
comprises the steps of:
[0011] providing a metal blank;
[0012] providing a metal forming die having a die surface to effect
forming of the metal blank thereon, the die surface having at least
one coefficient of friction; and
[0013] forming the metal structure by applying pressure to the
metal blank with the metal forming die;
[0014] forming the metal structure at an elevated temperature using
one of a superplastic forming and a quick plastic forming
process;
[0015] wherein at least a portion of the die surface is modified to
change the at least one coefficient of friction thereon to minimize
a cycle time for the forming and militate against at least one of
necking, localized thinning, and splitting of the metal blank
during the forming.
DESCRIPTION OF THE DRAWINGS
[0016] The above, as well as other advantages of the present
invention, will become readily apparent to those skilled in the art
from the following detailed description of a preferred embodiment
when considered in the light of the accompanying drawings in
which:
[0017] FIG. 1 is a top view of a panel formed by a die in
accordance with the method of the present invention;
[0018] FIG. 2 a sectional view of the panel illustrated in FIG. 1
taken along line 2-2;
[0019] FIG. 3 is a table showing pressure-time cycle data for
blanks having various friction characteristics obtained during
experimentation to arrive at the method of the present
invention;
[0020] FIG. 4 is a table showing pressure-time cycle data for die
surfaces having various friction characteristics obtained during
experimentation to arrive at the method of the present
invention;
[0021] FIG. 5 is a graphical representation of the effect of die
lubricant on a license plate pocket panel wall thickness
distribution;
[0022] FIG. 6 is a graphical representation of the effect of die
lubricant on a license plate pocket panel wall thickness
distribution shown in FIG. 5, compared with a graphical
representation of the effect of die lubricant on a license plate
pocket panel wall thickness where a die has a higher coefficient of
friction at a die entry radius;
[0023] FIG. 7 is a partial cross-sectional view of the panel of
FIGS. 1 and 2 showing a die having a surface thereof modified to
control the die friction; and
[0024] FIG. 8 is a partial cross-sectional view of the panel of
FIGS. 1 and 2 showing a die having a metal insert to control the
die friction.
DESCRIPTION OF THE PREFERRED EMBODIMENT
[0025] Referring now to the drawings, and particularly FIG. 1,
there is shown generally at 10 a license plate pocket panel formed
by a die in accordance with the method of the present invention.
The license plate pocket panel 10 was used in the development of
the present invention since there exists therein a very aggressive
die entry radius 12. A cross-section of the license plate pocket
panel 10 is illustrated in FIG. 2. All testing was performed on a
1.2 mm thick aluminum alloy 5083-H18 such as that produced by
Pechiney Rolled Products, Ravenswood, W. Va. Three lubricants were
also used in the testing, a Boron Nitride (BN) lubricant sold under
the trademark LUBRICOAT, Milk of Magnesia or Mg(OH).sub.2, and a
lubricant sold under the trademark SEALMET. The BN lubricant and
the SEALMET lubricant were supplied by ZYP coatings, Oak Ridge,
Tenn. The SEALMET lubricant contained an unspecified mixture of
metal oxides. The BN lubricant was provided in two forms: (a) as an
aerosol spray which contained 97% hexagonal BN and 3% magnesium
silicate with an alcohol/acetone carrier; and (b) as a paint which
consisted of a suspension of hexagonal BN (25 wt %) and
Al.sub.2O.sub.3 (4 wt %) in a water carrier. The Milk of Magnesia
consisted of an aqueous suspension of Mg(OH).sub.2 at a
concentration of 80 mg of Mg(OH).sub.2 per ml of water. Forming was
conducted on an up-acting, 320 ton hydraulic press designed for
superplastic forming.
[0026] Two types of experiments were performed. The first type
involved forming lubricated blanks in a bare or uncoated die to
show the effect on cycle time by varying lubricity. The blanks were
lubricated with one of two lubricants: (1) BN LUBRICOAT lubricant
or (2) Milk of Magnesia. The license plate pocket panel 10 was
formed with the two different lubricant conditions at incrementally
faster cycle times ranging from 6 minutes down to 13 seconds, to
determine when necking and splitting occurred. The difference in
cycle time was produced by changing the pressurization rate of the
forming operation, as well as by increasing the dwelling time at
the peak pressure. For the sake of clarity, results are compared
using cycle time, but could also be easily compared using
pressurization rate. All blanks were formed at a temperature of 450
degrees Celsius with a 5 minute preheat to ensure proper blank
temperature. The pressure-time cycles for each trial using the BN
LUBRICOAT lubricant or Milk of Magnesia are shown in FIG. 3. After
forming, each of the license plate pocket panels 10 was evaluated
for necks or splits at or near the die entry radius 12.
[0027] The second type of experiment consisted of forming bare or
unlubricated blanks in a selectively lubricated die to show the
effects on necking and splitting by varying die surface friction.
The friction condition was varied across the die by one of two
methods. The first method, called Pattern 1, involved spraying
lubricants with extremely different lubricity on the two halves of
the die. This was accomplished by masking one half of the die with
tape and spraying a thick layer (approx. 0.001") of BN LUBRICOAT
lubricant on the die using the BN lubricant spray. The surface was
burnished by rubbing with wax paper to create a very slippery
surface. The second half of the die was then sprayed with the
SEALMET coating without any subsequent polishing or burnishing.
This provided a very rough surface with high friction. The second
method for varying friction in the die, called Patterns 2, 3, 4,
and 5, involved masking off selected regions of the die and then
spraying with the BN lubricant aerosol spray. The die surface was
then burnished as described previously. After burnishing, the tape
was removed leaving areas of bare tool surrounded by highly
lubricious BN lubricant. The license plate pocket panel 10 was
formed under similar conditions to the previously described
lubricated blank trials to determine the onset of necking and
splitting. The pressure-time cycles for each trial are shown in
FIG. 4.
[0028] Bare Die Trials
[0029] Blanks lubricated with either BN lubricant or Milk of
Magnesia were formed in an unlubricated license pocket die at
successively faster pressurization rates (i.e. faster cycle times)
until necking or splitting occurred. The goal of these trials was
to establish the effect of lubricity on cycle time. BN lubricant
and Milk of Magnesia have been previously shown to exhibit
different lubricity during elevated temperature friction testing,
with BN lubricant giving a significantly lower coefficient of
friction.
[0030] Referring now to FIG. 3, blanks coated with Milk of Magnesia
could be formed using cycles as fast as 13 seconds without evidence
of necking or splitting. However, the license plate pocket panels
10 formed using the BN lubricant coated blanks exhibited necking,
at cycles of 4 minutes or less, and splitting at cycles of 2
minutes or less. Thus, the "poorer" lubricant, Milk of Magnesia,
actually produced a lower cycle time than the "better" lubricant,
BN. These results clearly demonstrate that frictional modifications
can significantly affect cycle time. It should be noted, however,
that the forming of parts with a poor lubricant often leads to
sticking during part release, which affects the dimensional
accuracy of the part. In addition, a poor lubricant can also cause
formability problems in very deep sections, as the material will be
unable to flow into the critical feature areas without
splitting.
[0031] Lubricated Die Trials
[0032] The goal of the second set of experiments was to determine
whether locally tailored coefficients of friction on the surface of
the die could produce the same cycle time reductions that were
observed for blanks lubricated with BN lubricant and Milk of
Magnesia. Four different lubricant test patterns were produced.
Each test pattern will be described below, followed by the forming
results for experiments using the described pattern. The
pressure-time cycles and trial results are summarized in FIG.
4.
[0033] Pattern 1
[0034] Pattern 1 was made by coating one-half of the die with the
lubricious BN lubricant and the other half with the non-lubricious
SEALMET lubricant. The purpose for testing this first pattern was
to determine whether drastically different friction conditions in
only a portion of the die produced the same effect as when the
entire die or blank consisted of the same frictional condition. The
license plate pocket panels 10 were formed using different
pressurization rates to determine the onset of necking and
splitting.
[0035] For cycle times greater than 4 minutes, no significant
difference in necking behavior was observed between the two sides
of the die. Both halves of the die successfully formed without
necking. For cycle times between 2 and 4 minutes, necking at the
die entry radius 12 was observed on the side of the license plate
pocket panel 10 that was provided with the "good" lubricant (BN).
The SEALMET lubricant side of the die showed no evidence of
necking. The difference in friction or roughness between the two
coatings is demonstrated by examining the differences on the die
side of the formed license plate pocket panels 10. The BN lubricant
side of the license plate pocket panel 10 was very clean with no
evidence of galling or scratching, indicating low friction. The
SEALMET lubricant side of the license plate pocket panel 10 showed
evidence of galling marks, scratches, and other blemishes
indicating significant interaction between the tool and the blank
during forming.
[0036] For cycle times at or below 2 minutes, splitting occurred,
but only on the side of the license plate pocket panel 10 which was
formed in the BN lubricant side of the die. These results indicate
that the presence of BN lubricant on half of the tool controlled
the ability to form the part. Necking and splitting were observed
at similar cycle times to the tests with BN lubricant coated blanks
in the bare die trial section above.
[0037] The thickness profile for each "side" of a license plate
pocket panel 10 formed in 4 minutes was also measured, as shown in
FIG. 5. The side of the license plate pocket panel 10 formed with
the SEALMET lubricant coating showed more overall variation in
thickness than the side of the license plate pocket panel 10 formed
with BN lubricant. The die entry radius 12 regions were thicker on
the SEALMET lubricant side of the license plate pocket panel 10,
but the bottom corners were thinner. The BN lubricant side showed a
more uniform thickness across the entire bottom of the license
plate pocket panel 10. The thickness profile indicates that while
the SEALMET lubricant side of the license plate pocket panel 10 did
not exhibit necking, it exhibits higher strain values at the bottom
of the pocket and a less uniform strain distribution than the BN
lubricant side of the panel 10. This would become an increased
issue of concern if the depth of the license plate pocket panel 10
were increased.
[0038] Pattern 2
[0039] Pattern 2 was produce by masking off specific regions of the
die to produce unlubricated areas, while the remainder of the die
was coated with BN lubricant. The goal of evaluating Pattern 2 was
to determine whether locally increasing friction in the vicinity of
the die entry radius 12 could prevent necking and to determine the
optimal location for increasing friction. Regions were masked off
either slightly above the die entry radius 12 on a plateau 14, on
the die entry radius 12, or slightly below the die entry radius 12
on a wall 16, as illustrated in FIG. 1. Some of the die entry
radius 12 regions were not masked off for comparison. The trials
were performed at a temperature of 510 degrees Celsius to help
exaggerate the necking phenomena. The gas pressure time cycles for
these trials are summarized in FIG. 4.
[0040] Initial trials were conducted using a seven-minute cycle
time with pressurization rates of either 90 psi/min or 30 psi/min.
In both trials, the license plate pocket panels 10 split
catastrophically in the areas where there was full BN lubricant
coverage. A subsequent trial was performed at a slower cycle of 12
minutes with a pressurization rate of 15 psi/min. In that trial,
the license plate pocket panel 10 split at the die entry radius 12
along the entire side, except the region where the die entry radius
12 was void of BN lubricant. The split occurred right up to the
point where the lubricant was removed from the die, at which point
it arrested. The split traveled into the region where the lubricant
was removed from the plateau 14. Some slight necking was observed
at the die entry radius 12. No necking was observed in the region
where the die entry radius 12 was void of lubricant. However,
removing the lubricant either above or below the die entry radius
12 did not prevent necking, indicating that the best pattern for
minimizing necking is to remove the lubricant from the region right
at the die entry radius 12.
[0041] Pattern 3
[0042] The results from lubricant Pattern 2 indicated that locally
increasing friction at the die entry radius 12 could prevent
necking. To evaluate this, a third pattern was studied where the
die entry radius 12 on half of the die did not have lubricant while
the other half of the die was completely coated with BN lubricant.
Blanks were formed using three different ramp rates. 200 psi/min,
100 psi/min, and 25 psi/min at a temperature of 450 degrees
Celsius. In all three cases, the license plate pocket panels 10
split during forming on the side of the die where the lubricant was
present on the die entry radius 12. No splitting or necking was
observed on the side of the die where the lubricant was removed
from the die entry radius 12. In fact, the split that initiated on
the side of the die where the die entry radius 12 was lubricated
did not spread into the region of the die where the die entry
radius 12 was bare. The gas pressure time cycles for these trials
are summarized in FIG. 4.
[0043] Pattern 4
[0044] The success in preventing necking demonstrated by locally
removing the lubricant at the die entry radius 12 led to testing of
a fourth pattern to see whether the locally tailored friction could
produce an improvement in cycle time. The die entry radius 12 along
both sides of the die was masked off to eliminate lubricant. Bare
blanks were formed at an aggressive ramp rate of 400 psi/min.
During testing for Pattern 4, it was discovered that the region of
the unlubricated die entry radius 12 did not extend far enough
along the side of the die. The license plate pocket panels 10 split
along the edge in the area where the die entry radius 12 was
lubricated. None of the other regions showed evidence of necking.
While this test was unsuccessful in reducing cycle time, it clearly
demonstrated the effect of friction at the die entry radius 12 on
necking and splitting. The gas pressure time cycles for these
trials are summarized in FIG. 4.
[0045] Pattern 5
[0046] Pattern 5 involved removing lubrication from the die entry
radius 12 portion of the entire die. Blanks were formed at a
temperature of 450 degrees Celsius using pressurization rates from
400 to 2000 psi/min. No splitting or necking at the die entry
radius 12 was observed in any of the trials for Pattern 5. The
fastest cycle time was 23 seconds.
[0047] The thickness distribution in the license plate pocket panel
10 formed using a cycle of 2.5 minutes with pattern 5 is shown in
FIG. 6. The license plate pocket panel 10 formed with the tailored
BN lubricant die coating showed a more gradual change in thickness
across the bottom of the license plate pocket panel 10 than the
license plate pocket panel 10 shown previously with the SEALMET
lubricant coating illustrated in FIG. 5. This clearly demonstrates
that the use of dies having tailored friction areas can
significantly reduce cycle time while also preventing localized
splitting and necking. The gas pressure time cycles for these
trials are summarized in FIG. 4.
[0048] The testing described above demonstrates that metal forming
cycle time can be significantly reduced by locally modifying the
friction characteristics of a die, i.e. the coefficient of
friction. In addition to cycle time reduction, local control of
friction characteristics in metal forming tools has other benefits
including: control of as-formed panel thickness distribution; the
prevention of localized thinning; part release is facilitated,
thereby improving the dimensional accuracy of parts; and improved
surface quality of formed parts by minimizing lubricant buildup on
die entry radii.
[0049] Tailoring the friction characteristics of a die by surface
modification has proven to be a critical enabler for cycle time
reduction. While the methods used in the present study were
excellent for characterizing the phenomenon, they are not practical
for high speed production. The lubricant sprayed on the tooling was
almost completely removed after only forming a few of the license
plate pocket panels 10 and would require reapplication. It is also
difficult and time consuming to apply the spray coating uniformly
to create a smooth surface that would be acceptable for an exterior
body panel. Thus the method of the present invention for directly
modifying the die to control friction solves this problem. Thus,
referring to FIG. 7, by directly modifying a surface 18 of a die 20
to control die friction, production can proceed without lengthy
interruptions for lubricant application and the like. Methods for
modifying the surface 18 of the die 20 may include chemical
etching, laser surface dimpling, scribing, sand blasting, laser
particle injection, laser ablation, local oxidation, and
combinations thereof, for example. Referring to FIG. 8, a
dissimilar metal insert 22 can also be used to control friction as
desired for the die 20. It is understood that other methods of
controlling die friction can be used without departing from the
scope and spirit of the invention.
[0050] From the foregoing description, one ordinarily skilled in
the art can easily ascertain the essential characteristics of this
invention and, without departing from the spirit and scope thereof,
can make various changes and modifications to the invention to
adapt it to various usages and conditions.
* * * * *