Note: Descriptions are shown in the official language in which they were submitted.
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HOT FORMING SYSTEM FOR METAL WORKPIECES
TECHNICAL AREA
The invention relates with an electrical system that includes at least one
transformer with
primary / secondary coil ratio more than one and a control unit that controls
electrical current
at the primary circuit; that closes the electrical circuit by passing the
electric current
generated at the secondary coil through the metal workpiece and preparing the
workpiece by
heating prior or simultaneous to the forming operation.
BACKGROUND OF THE INVENTION
Heating of metal workpieces prior to forming operations such as hot forging,
rolling,
extrusion etc. is a significant part of the process. Heating of the workpiece
is usually
performed in a furnace and subsequently, the workpiece is placed in the
forming machine.
'This is a series of independent operations in the sequence of heating,
handling and forming.
There are a few patents granted and technologies developed on combined heating
and
forming. In Weldon and Jains invention (US Pat No: 5.515.705) the lower and
upper dies
forging the billet in between are used as electrodes supplying electric
current. This invention
has some technical difficulties and limitations of practical implementation
due to relatively
small contact are between workpiece and dies, electrical arcs formed by sharp
features of the
klies and workpiece, localized overheating and uncontrollable deformation or
melting on the
'workpiece. In another patent in which heating by electrical resistance and
forming are
combined (Yasui, US Pat. No. 5,737,954) the sheet metal workpiece are formed
at
superplastic conditions and welded to each other using diffusion welding. The
applicant of
this patent also holds a patent (Terziakin, US Pat. No: 6,463,779) on this
technology. In the
proposed apparatus, the electrical heating is conducted inside the press table
and thus the dies
need to be designed accordingly. The press ram is stopped for a few seconds
while the sheet
metal part is being heated via conditioned electric current and the forming
process is
'performed immediately after the heating is complete. Therefore, the
electrodes need to be
isolated from the dies and the workpiece must not touch the dies during when
the electric
charge is on.
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SUMMARY OF THE INVENTION
This invention shall provide a system including at least one transformer
serving to improve
formability of metal workpieces and to increase strength rates of formed
parts. It will enable
to heat metal workpieces in combination with forming process and controlled
cooling
process after forming. Additionally generation of the heat in the workpiece
and the short
duration of heating, forming and cooling (treatment) help reduction or
elimination of scale,
while significant changes in microstructure will not occur. On the other hand,
under proper
conditions it is capable to harden metal workpieces during or after the
forming process to
obtain higher mechanical strength such as martenzitic steel or hardened
aluminum alloys.
1o The system will direct the line energy through at least one transformer
with a
primary/secondary coil ratio more than 1 and that reduces electric voltage and
increases
electric current. The electric current amplified at the secondary coil is
directed over the
metal workpiece and the required process temperature at which the material
formability is
highest is obtained. This electrical system will work at a timing tuned to
work subsequently
in coordination with the mechanical forming process. Being coupled with the
metal forming
system, this system will provide effective automation of the whole process.
Another high
current rate source is to use a homopolar generator. This DC generator type
has also capable
to generate such high current rates. Homopolar DC generator can also be used
as current
source instead of transformer. In this case timing of current feeding heating
the workpiece is
controlled by opening or closing connection between metal workpiece and DC
generator .
'This timing and magnetute control of current generated by DC generator must
also be made
in synchronization with other mechanical forming operations as a general rule.
Any figure an
arragement including DC generator about the invention has not been added
because of it is a
well known and basic technology. If DC generator is used as a current source
the invention
also includes the possibility of the system to be mounted on the material
handling system.
'This way the part is heated during transport from stock pile or between
subsequent
operations and thus the need for a furnace is eliminated. As it is known
forming of metals at
elevated temperatures can be realized as warm or hot forming depending on
recrystalisation
-r.emperature of each metalic material. Hereby hot forming expression is
generally used for
forming process at elevated temperatures at these documents.
In this invention whole system has ability to keep its own temperature between
a
predetermined range . A cooling system (or several systems) is employed to
keep cool whole
system sufficiently in spite of significiant and unpredictable heat input
occuring during
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continuous operation . Some complementary operations can be applicated during
hot
stamping operation. Elevated temperature of the workpiece simplifies
operations such as
blanking, punching etc. Suitable apparatus for such operations can be added to
the dies. The
process can also offers an effective way to increase strength rate of the hot
formed parts with
die queching for proper materials and process charecteristics.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention is explained in detail below with reference to the exemplary
embodiments and
with reference to the accompanying drawings, in which:
FIG. 1 is schematic view of the electrical circuit of the stepped system using
three phases and
providing high current rate for the process.
FIG. 2 shows application of the invention on hot tube hydroforming . In this
process, a tube
is heated by current application and formed by external dies while being
internally
pressurized by a fluid.
:FIG. 3 illustrates the application where electrical heating system is used
for bulk metal hot
forging process that integrated to the material handling system.
FIG. 4 combination of electrical heating, forming and hardening by air or
spray cooling is
illustrated.
FIG. 5 illustrates integration of heating during material handling, forming
and rapid cooling
to form hardened workpiece. FIG. 5 also illustrates integration of the
electrical heating
system to a material hardening apparatus that works in a multiple step or
progressive forming
process at elevated temperature.
3o Finally, FIG. 6 illustrates current heating and selective spray cooling
system providing
desired temperature gradient along the blank for sheet forming process at
elevated
temperatures.
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DETAILED DESCRIPTION OF THE DRAWINGS
As illustrated in FIG. 1 the current control in this system is achieved at the
primary coil
where the current value is less. An electronic circuit 2 (ECU) with thrystor
or switching
device etc. will command at the input of the primary circuit. Devices to
protect against
electrical overcharge such as thermal switches must also be place at the
primary circuit. The
magnitude of the current at the secondary circuit must be high in this system;
therefore, the
contact resistance between metals completing the circuit significant. All the
connections
including all the conductors completing the secondary circuit 3 except the one
between
electrodes 7 and the workpiece 4 can be made using soldering or copper brazing
to minimize
resistance.
In FIG. 1 illustrated system consists of three essential devices. These are
transformer group
-with auxiliaries 1, current control device 2 and last circuit consisting of
connections 3
between second coil of the transformer , electrode sets 7 contacting with the
workpiece and
the workpiece 4 itself. Workpiece 4 is replaced in each production cycle and
all other parts
of the system is cooled during production and temperature rates of the parts
are kept within
iheir predetermined temperature ranges .
As shown in FIG.1 to be able to use the process on larger workpieces 4 systems
that use the
three phases U1,V1,W1 of the mains energy are employed loading the phases
equally.
Especially in industrial scale applications the system will demand high
electric power rates.
At first stage current control system 2 is charged by three phases ( U1,V1,W1)
of the means .
'This three phases AC is converted into DC by 6-pulse bridge rectifier
arrangement 5. At the
second stage this DC is converted into AC (U2,V2) current with higher
frequency than that
of the means ( 50 or 60 Hz) by employing transistorized frequency inverter 6 .
Higher
frequency simplifies function of the last step transformer 1 by means of
increasing induction
::ate inside the core of the transformer. In this case smaller scale
transformers will be
sufficient for high power rates. Intensity of the current is controlled by
adjusting the pulse
width at the frequency inverter 6.These pulses are triggered by electronic
control unit 9
depending on desired current rate. One or more current measurement
transformers 10
situated primary or secondary circuit of the transformer 1 measures actual
current rate and
determinates deviation between desired and actual current rates . Thus ECU 9
determinates
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proper pulse width for transformer 1 feeding and adjust current flow by
employing triger
circuit and thristor set 6.
This two phases AC (U2,V2) provided by current control unit 2 is connected to
transformer
unit 1. Primary coil ( Nl) / secondary (N2) coil ratio of this transformer is
more than one .
Voltage of this AC (U2,V2) charge is reduced and current rate is also
significiantly increased
at the second coil of the transformer (U3,V3) . In some cases a group of
parallel or series
connected transformers can also be employed instead of one transformer. In
some industrial
applications required current rate may be too much for a single transformer.
As it is known altenative current causes higher impedance in a circuit loop
3,4 in high
current rates. As current rate increases empedance rate of the last circuit
3,4 also increases
and becomes an obstacle for providing desired high current rates . Because of
this reason in
order to reduce empedance of the last circuit consisting of the workpiece 4,
connections 3,
electrodes 7 etc, AC inducted at the second coil of the transformer should be
converted into
DC at the exit of the transformer by being rectified by diods 8. A cooling
system is
employed to keep temperature rates of these devices within their acceptable
ranges and to
prevent damages of heat accumulation. A coolant fluid is passed through
coolant passages of
these devices. It is also possible to arrange other different alternatives for
controlling current
flow of the system.
For small workpieces, a simpler arragement can be employed . In this system,
two of the
three line phases are connected to the primary coil. The primary circuit
current passes
through these two phases of the line. A control unit is used to control
primary current
characteristics with thrystor, switching device etc. are included. Whole
system including
control unit with thrystors, transformer, connectors and electrode sets
contacting with the
workpiece is cooled by a suitable cooling system such as closed circuit fluid
cooling. The
control system coordinates the operations of the forming process and the
magnitude and
timing of the primary current simultaneously. As an option, the primary
current may also be
3o developed between phase and ground. Primary / secondary coil ratio of this
transformer is
also more than one and that reduces the voltage and increases the current of
the received
electrical power and feed current to the workpiece is order to generate heat .
This system is
fairly simple and it is not illustrated in figures.
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In FIG 2, above electrical system is used to hot or warm hydroforming of
tubular metal
workpieces with closed sections. As it is being heated by current provided by
above system,
dies are used to compress and to form the tubular workpiece. The control
system 2 illustrated
with dashed lines (CU) controls the timing and current magnitude of the
electrical heating
system, in sequence with the mechanical forming operation(s). The control of
the mechanical
forming operations illustrated in several examples below is performed by this
system through
ciirect communication between the systems that control the hydraulic and/or
pneumatic
valves etc. For instance, the subsequent mechanical operations in tube
hydroforming process
performed using an internally pressurized fluid and both heating and forming
procesess are
performed under synchronization with this control device.
In this configuration, to reduce the contact resistance between the metal
workpiece 4 and the
electrode set 5, it is advisable to clean the contact surfaces of the
workpiece contacting the
electrodes. During rolling of coiled sheet metal or metal billets, bars etc.
the material is
li.ibricated using mill oil to reduce friction and corrosion. In addition, the
outer skin of the
metal consists of material with higher electrical resistance and lower surface
properties due
to oxidizing and other effects of air and forming operations. Depending on the
material type,
application of a chemical and/or mechanical cleaning / improvement process at
the electrode
contact area of the workpiece.
In the forming operation of an anti-corrosion coated metal at elevated
temperature, the
coating may be damaged due to high temperature. Particularly, the coating will
peel of at
electrogalvanized or galvannealled sheet steels. However, the flow stress will
be reduced at
elevated temperature and thus corrosion resistant steels with higher chromium
content and no
coating will be possible with the invention. This way, parts with both higher
strength and
corrosion resistance will be produced. Instead of lubricating the workpiece,
the die
components may be lubricated and/or the die components may be coated with heat
resistant
ceramic coatings or metal alloys.
In FIG 2, forming of welded or seamless tube, pipe etc. with closed section 4
under internal
pressure at elevated temperature using the proposed invention is illustrated.
Compared to the
tubes formed using cold tube hydroforming, tubes made of higher strength
and/or low
formability metals will possibly be formed using the invention. In this
system, an electrical
device with a transformer that has a primary coil size larger than secondary
coil size 1 and
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that reduced voltage whereas increases current as illustrated in Figures 1. In
this set-up, an
electric control circuit 2 using thrystor(s) to synchronize the mechanical
forming operations
with the feeding of the current, adjusting its magnitude and thus heating of
the workpiece 4.
The workpiece 4 shown in Figure 2, is a metal tube or pipe with closed
profile. In the
Figure, this part is initially subjected to a bending operation. Next, a
process combining the
e?ectrical heating operation with internal fluid pressurizing and external
forming operations
using dies are conducted in a synchronized manner. The internal pressure may
be applied
using pressurized gas as well as some type of liquid, preferably an insulator.
In the forming process, the heating operation by feeding electric current 21
using the
electrical system 1,2 described above, achievement of workpiece temperature
control,
pressurizing of the tube by liquid or gas pressure 12 and subsequent tube
forming using dies
are achieved. Both ends of the tube are closed by plugs 11 that function as
the electrodes 21
as well as pressurized fluid feeders 21. These plugs are supported by
pressurized hydraulic
cylinders 13. After placing the plugs into the ends of the tube, the hydraulic
cylinders are
pumped with pressurized hydraulic fluid and thus these cylinders compress the
plugs with
the necessary force.
In this figure, the forming operation is designed to be conducted by two piece
die set
containing the upper 23 and lower dies 22 linked 20 and operated using a
couple of hydraulic
cylinders 17.
In principle, there are three basic parameters in tube forming using this
process: The internal
pressure 12, the workpiece temperature, which determines the forming
properties of the
material and which is controlled by the electrical current 21 fed and the
displacements and
pressures of the forming dies 14, 15, 16, 18 that surround the workpiece. The
sequence and
magnitudes of these three process parameters are to be designed appropriately
for any given
tube geometry and other properties. To obtain internal pressure, pressurized
fluid 12 is
pumped through the component number 11 Electric charge is fed by the
electrodes 11 and
the tube is heated to a temperature at which the formability is increased to a
satisfactory level
for the forming operation. Component number 11 is made of materials such as
appropriate
copper alloys with good conductivity and high strength completely or partially
at the contact
portion. To prevent bursting due to excessive internal pressure 12 and/or
localized
overheating by electric current 21 the surrounding forming die components 14,
15, 16, 18
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must be closely located to the tube surface. The surrounding die components 14
approach
and contact the tube. The die lower 22 and upper 23 die components approach
each other in
both directions being pushed by the hydraulic cylinders 17 in the mechanical
linkage 20.
Simultaneous to this operation the internal pressure and/or the current fed to
the workpiece
may be increased gradually. Consequently, the final/required geometry of the
tube is formed
by the internal fluid pressure and dies.
The laterally moving die components 14, where necessary, are guided by the
slides mounted
to the lower die 22 or 23 and powered by the hydraulic cylinders on the rear
side 17, 19.
These channels are not shown in the figure not to make the figure more
complicated. All of
-the contacting surfaces (elements) of the die components are preferable made
by ceramic
inserts 18.
Based on the initial form and final geometry of the tube 15, 17, 16, the
lateral die
components may also be an option. If the previously bent tube fits in the die
cavity
supported by the lateral components 14, these components may be designed in
fixed
configuration and only the lower 22 and/or upper dies 23 will move 18 and form
the tube.
The tube material may be aluminum, magnesium or steel alloy as well as other
more
expensive or exotic metals.
In those steel alloys with Carbon equivalent of 0.35 or higher, after forming
operation at hot
forming temperature, rapid cooling using water, oil or air will lead to a
martensitic
microstructure and thus a higher mechanical strength. The invention proposes
two different
methods for this purpose. In the first one, the dies are retracted after the
forming operation,
and pressurized air or air-mist mixture is sprayed over the part and thus the
tube is cooled
immediately. In the other one, the pressurized fluid in the tube is drained
immediately after
the forming operation through the plugs 11, 12 and it is filled by a coolant
fluid or an air-mist
mixture is passed through the tube for rapid cooling. The details of this
cooling system are
not illustrated in the figures.
The parameters of the process described above, such as part material, size and
geometry,
?nternal pressure, forming temperature, and cooling method, are determined by
carefully
planned engineering experiments.
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Application of the invention in the hot or warm (billet) forming operation is
illustrated in
FIG.3 This process is particularly advantageous for long workpieces with small
cross section
combining the rapid heating and forming operations. The workpiece in the form
of a billet or
bar 25 is charged with the electric current generated at secondary coil of the
transformer (1)
by two electrodes/clamps 28, 24 while being transferred into the forging dies
on the
conveyor. Thus, the workpiece gains the required temperature before the
subsequent
forming operation.
This type of process may be performed in multiple dies or a progressive die
set 26, 27 in a
sequence of operations ( from 1. Press to N. Press) designed to start at the
1. th die or station
and finish at the n. th die or station. The characteristic principle of the
invention, the
electrical heating system, is used in this set-up as the rapid initial heating
and rapid
intermediate heating between subsequent forming steps. Similar to the other
applications,
the thrystor type control device 2 connected to the primary coil of the
transformer 1 controls
tlle magnitude and timing of the electrical current adjusted according to the
designed
sequence of heating and forming operations. The contact areas of the
electrodes 24 on the
'vorkpieces 25 should be subjected to a chemical and/or mechanical cleaning
operation to
r~duce contact resistance. This cleaning operation may also be integrated to
the system, if
necessary. Heating the workpiece during transport; namely, movement of the
workpiece
along with the electrodes 24, will help reduce the total process time. In this
configuration,
the connectors 28 between the transformer 1 secondary coil and the electrodes
24 must be
sufficiently long and flexible, and the clamp type electrodes 24 must hold the
workpiece
strongly and the conveyor system must be isolated from the electrode to
prevent any
shortcuts . The connectors have also a cooling system to dissipate heat
preferable with fluid
circulation or air blowing. The transport system moves the workpiece and the
electrical
heating system heats it up before next forming operation. This way, the
forming of the
previous and heating of the next workpieces will be performed simultaneously,
and thus the
heating time will not be added to the forming operation (cycle) time. In this
system, to
improve superior friction conditions, the die surfaces may be coated by
appropriate ceramics.
This system is preferably used in a forming set-up that works with an
automated conveyor
system. In the figure, the heating and forming operations take place at one
tip of the billet or
bar, and rest of the billet/bar is fed into the die set for the next workpiece
29 after the formed
portion is cut-off. Whole operation parameters of the process such as current
heating,
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transporting and forming operations must be carried out synchroniously and
should be
controlled by a central control unit . Same hot or warm forming process can be
applicated in
scew , rivet , nut, bolt etc production especially for relatively big size
parts made of high
strength metal alloys. Forming operation in these systems can be achieved as
forging
tapping, threat rolling, turning , bending etc. depending of parts to be
producted.
As illustrated in FIG.4, a set-up in which the electrical heating system is
implemented in the
handling robot 40 or material handling system. In this configuration blank
sheet 37 is heated
during handling while previous one 33 is being stamped. Whole system including
current
heating and hot or warm forming processes can be operated at same time, thus
each
production cycle takes less time. The electrodes 38, 39 that both hold and
move the sheet
rnetal workpiece 37 are clamps that have a long strip of contact surface for
sufficient
electrical conductivity. This contact surface 39 is made of the electrode
material and the
Iftigh conductivity cables and/or bars are connected to the secondary coil
output terminals.
High current rate is provided by second coil of transformer 1 as explained
above. This
electric transmission line is made of either flexible cables or rigid copper
bars etc. linked
=4vith hinges 31. The electric conductors are fitted to carrier arms 30 of the
system etc. and
should also cooled by fluid circulation. The blank sheet is heated during
transport from stock
pile 32 or waiting for subsequent forming operation.
The lower and upper components 38, 39 of the clamp type electrodes are hinged
to each
other. The open-close function is performed by reciprocal motion of a
hydraulic or preferably
pneumatic cylinder 36. The lower 38, the upper 39 or both of the clamps may be
used as the
electrode. In the figure, it is difficult to use the moving lower clamp as the
electrode;
therefore, the stable one 39 is more suitable to be an electrode. In this set-
up, while the
clamps are opening in the downward direction and the sheet workpiece is
lowered, they
rruide the part to prevent movement in the lateral direction and thus locate
it on the right
position on the die.
:Fleat transfer between hot workpiece 33 and the dies 34 influence hot forming
process
However this influence generally beneficial and leads to increase in local
strenght rates in
some critical contact areas between dies and hot sheet . At these contact
areas local stress
:rates intensify and such a local cooling can improve local strain rates by
means of strength
increase . Average temperature of the dies should be maintained between
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range because process should be repeteable and too low or too high
temperatures distort hot
forming characteristics and part dimensions. Another reason is that die
materials may be
damaged by overheating. In mass production in this system a suitable cooling
means should
be used such as blowers that could be placed around dies or a fluid
circulation system
including passages or pipes contacting dies. Temperatures of the dies are
measured and near
upper limit blowers or fluid circulation is employed to dissipate heat from
dies. In Figure 5
an example of such a system is shown . Blowers or water sprays 35 are placed
around dies
and used to dissipate heat . For example dies can be maintained between 100-
150 C range.
ECU, transformer, carrier and connector arms 30,31 and electrode sets are also
cooled by a
proper cooling means such as coolant fluid circulation . Thereby these
components are hold
within predetermined temperature ranges along mass production and overheating
damages
lcvill be prevented.
On the other hand the invention also offers another important instrument to
control such hot
or warm stamping process. By contrast of cold forming , metals at the elevated
temperatures
Y-ias high strain rate sensivity feature . At low forming speeds alongation
rates of the heated
metal can be seriously increased . Since such an hot stamping system should be
used for
various materials, temperatures and several dies , each combination of those
can require
i?.ifferent forming speeds . There are several ways to make presses with
adjustable speed. In
this invention this feature can be used easily by employing speed control
means with speed
control such as frequence inverter ( not shown in Figures) in electric feeding
of electric
rilotor of main hydraulic pump in hydraulic presses. Because of this current
heating means
can also be applicated during forming stage ( if nonconductive dies are used )
both
temperature and forming speed can be controlled together. This speed control
means with
frequence inverter should also be controlled by central control unit
controlling whole heating
and forming parameters of the process.
As illustrated in FIG.5, the system proposed in the invention is used in
combination with the
workpiece conveyor and basic process stages are shown in sequence in a double
action
hydraulic press. The metal workpiece (sheet, plate, billet, bar etc.) is
heated during transport
from storage or pallet to the forming die. Since the time interval between
heating and
forming operations is minimal heat loss particularly in workpieces with large
surface areas as
compared with their cross-sections such as sheets, plates, bars etc. and the
possibility for
;process operation is much higher. The sheet forming operation in this system
will be
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lierformed in a set-up similar to conventional systems. However, due to lower
yield strength,
higher ductility and strain rate sensitivity and the temperature gradient that
may occur on the
Nworkpiece some process and die design modifications may be necessary. For
example, a
lower blank holder force may be used. Current heating is fed by main system of
the
invention as shown in Fig. 1
In this set-up, the workpiece 41, while being transported to the forming die
set 45 on a press,
=.s held by two clamp type electrodes at two ends and is heated within a few
seconds by
feeding the regulated low voltage electric current and is placed in the die
set at the required
forming temperature. The electric current is fed from the secondary coil of
the transformer 1
by cables or conductors connected to the moving arms holding the clamps 41 to
the
~~iorkpiece. The moving arms 41 of the transport system may contain mechanical
linkages
i1-2. The motion of these linkages 42 may be obtained by conventional
hydraulic or electrical
(such as step or servo motor) systems. These linkages 41 are designed close to
each other
;;.nd as short as possible to keep the conductor lengths short, the electrical
impedance of the
lectrical system low and thus electrical efficiency of the system high.
This invention can be applicated as many different configurations . One of
these alternatives
:i to situate elecrode clips on a stable position near dies . Especially for
relatively thich
:rnaterials allowing handling operation while maintaining its temperature
sufficiently until
forrning operation this configuration may be a easy to applicate and
inexpensive alternative .
Mthough it is not shown in a figure particularly , only difference between
this configuration
.3nd that of seen in Fig. 5 is these clips 38, 39 connected to second coil of
the transformer 1
~kvill be positioned in stable places adjacent to forming tools . When the
workpiece reachs
sufficient temperature is then carried to forming position .
'?'o reduce formation of scale due to high temperature, the workpiece can be
coated with a
protective layer such as a suitable metallic coating or heat resistant oils or
ceramic coating
etc.
' he subsequent operations of this process performed in a double action
hydraulic press, are
illustrated in FIG.5. In the first part (FIG.5A), a sheet metal workpiece is
taken and
transported from a stack of sheets using vacuum cups , while at the same time
dies 33, 34 are
being cooled and cleaned by pressured air 35 or/and pulverized water blowing.
Then proper
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lubricants can also be sprayed toward die surfaces. When the sheet is located
at the holding
position the two clamp type electrodes hold the sheet at two opposite sides
and applicate the
current to heat the material. As shown in FIG.5B when the simultaneous forming
operation
is complete and the die set is empty and ready for the next cycle, the
workpiece is ready at
,:1_,e required temperature and it is located on the blank holder 41 by
releasing the clamps and
the clamps are retracted. The first contact points of the sheet metal
workpiece are designed
as a bead such that a small pointwise or curvilinear contact area rather than
a planar one is
;;enerated and thus heat loss from the workpiece is minimized. In FIG.5C upper
die 34 is
raoved down and rests blank holder 41 and thus hot sheet 37 is hold firmly and
then stamped
~~y upward action of lower die 33 . Hot stamped sheet is then quenched by air
blowing 35 or
pulverized water spraying for being hardened. In 5 D formed and hardened part
37 is
removed from die and next blank is prepared for next cycle .
[f the operation is an intermediate step in a series of operations, the part
is transferred to the
next station with a similar heating set-up and forming die. If a hardened
sheet metal part is
required at the end of the process, an air or spray quenching operation is the
final step as
explained above. The spraying may take place right after a hot finish forming
operation or it
riiay be performed after heating up to recrytallization temperature. After the
workpiece is
taken of the die set, residual water droplets on the die components are
eliminated manually or
automatically by pressurized air. In this process, the die components are
lubricated rather
than the workpieces. This way the lubricant will contact the hot workpiece
only during
forming operation. However, the forming dies need to be cleaned and re-
lubricated after a
ni.unber of cycles to be determined by experience. Both the cleaning and
relubrication may
be performed manually or by an automatic system.
One important point to pay attention in the forming processes at elevated
temperature is that
the die temperature must be controlled within an optimal range. If the die
components are at
a lower temperature, they must be heated; if they are at a higher temperature,
the excess heat
is removed by a coolant (water, oil or air) pumped through the cooling
channels made inside
the die components or the coolant may be sprayed over the die surfaces and
thus the die
temperature is controlled with the designed range.
In complex sheet forming applications of automotive industry, die geometry
often poses
restrictions on the easy flow of metal from one region of the part to another,
thus leaving
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relatively unstretched regions of the part bounded by heavily stretched areas.
In such cases
tormability of the metal is poorly utilized due to the strain non-uniformity,
and the
propensity for fracture increases. This occurs because it is difficult to
transmit stresses into
certain regions of the sheet metal workpiece due to high frictional resistance
or larger cross-
sectional area in these regions. To enhance overall formability of the blank ,
some local area
;:eeds to be softened while some other critical areas needs to be maintained
relatively harder
by means of proper temperature gradient along the hot blank.
On the other hand if such an hot stamping and die quenching process also
includes a
1o hardenning process as described above ( such as generating martenzitic
structure for suitable
steel blanks) some certain areas may be desired to be kept mild . In other
words hardening
operation may be desired to be achieved for certain areas of the blank . There
may be other
certain zones of the parts may be subjected to a subsequent cold forming
operation such as
"Dlanking, punching, bending etc.
In FIG.6 an further application area of the invention is shown to solve above
problems. The
nresent invention can be used to improve material flow with modified selective
temperature
gradient along the workpiece two dimensionally (along the surface) in order to
enhance
overall formability for hot stamping of such complex parts . The aim of this
selective heating
operation is to prevent local over stretching that resulting fractures or
overthinning at certain
.critical areas (such as 49-50) . This selective heating process is performed
by means of
interval cooling of these areas of the blank 44 during or after current
heating by spraying
pulvarized water with pressured air or pure air flow 45 toward those areas 46
for very short
time cycles. Spray nozules 45 are placed around the blank and directed toward
such critical
ti-reas such as 46 of the blank 44 . Thereby during or after current heating
of the blank,
desired portions are cooled by spray pulses of directed nozules 45 . At the
stamping moment
the blank sheet includes relatively cool/ certain areas in such critical
portions surrounded by
relatively hot and easily formable areas 47, 48 . Modifying overall
formability of the blank
by generating relatively high and low strength areas depending on form of the
part is very
easy in a few seconds.
In selective heating process shown in FIG.6A at the beginning whole workpiece
44 is
homogeniously heated by application of high current rate connected 42, 43 from
second coil
of the transformer 1 . Current rate and timing is controlled by control unit 2
situated on first
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WO 2006/124005 PCT/TR2005/000025
roil of the transformer. At second stage as seen in FIG.6B certain portions 46
of the blank
'which will involve severe strain rates are locally cooled by directed air
jets 45 . These areas
46 will be stretched between two sharp edges of two opposite dies. As seen in
FIG.6C at the
stamping stage a desired temperature gradient and harder 46 and softer 47, 48
areas are
obtained resulted by temperature gradient. These cooler and thus harder areas
46 will remain
between two opposite edges 49, 50 of upper 51 and lower 52 dies . Thus a
fairly important
instrument is provided for determining which areas will be stretched more and
which areas
=,,A/ill be stretched less . At the stage of die desing, desired temperature
gradient map is
calculated using by proper simulation program.
Instead of stable nozules are shown in FIG.6 movable nozules can be employed .
These
movable nozules can be approached toward these areas during interval cooling
stage and
narrow areas can be cooled more accurately . Movement of these nozules can be
provided by
pnomatic or hydraulic arrangement. Consequently any desired temperature and
formability
gradient along the blank sheet can be achieved in a few seconds. For example
it was
observed that yield strength of a steel alloy is approximately 2.5 times
higher at 800 C than
that of at 1000 C in our previous experiments.
For example a steel blank sheet to be hot formed can be heated to 950 C
homogeniously and
then temperature of some critical areas can be reduced to 750 C locally by
means of above
rnentioned way , these critical portions such as sharp corners edges etc. are
prevented from
being subjected of fractures or overthinning. Directions, spraying angles of
spray nozules,
air and water quantities to be sprayed and their pulse cycles are determined
and adjusted
depending on form of the part and other forming parameters.
Another important application area of the local cooling of certain portions of
the hot blank
sheet is to obtain certain soft ( unhardened ) portions at the end of the
process. For example
heat treatable ( hardenable ) steel sheets ( boron alloyed hardenable sheets
for automotive
industry in practice ) can only be hardened when it is heated up above
austenite temperature
and then be quenched between dies. If above mentioned local cooling process is
applicated
during current heating cycle ( before these zones are heated up to the
austenite temperature )
and maximum temperature rates of such certain areas are kept below austenite
temperature
( between 850-900 C) these areas will remain mild ( with ferritic and without
martenzitic or
dual phase structure ) at the end of the process .
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WO 2006/124005 PCT/TR2005/000025
Thus last stage forming operations such as bending, blanking, collar forming,
punching etc.
can be performed later for such certain areas.
Consequently maximum temperature of a certain portion of the workpiece made of
hardenable steel should be reached above austenite temperature in order to
obtain hard
martenzitic structure at the end of the process . In other terms if certain
zones of the blank are
prevented to reach austenite temperature by means of local coolant spraying
during current
heating cycle, these zones will be remained as soft portions at the end of the
process.
Both two different local cooling types can be applicated in one production
cycle.
Determining key factors of these local cooling processes are:
1) If there are critical zones of the blank which will be subjected to local
fracture and
overthining , these zones should be cooled locally ( by being sprayed coolant
locally) after
whole blank is heated up above austenite temperature limit. At the stamping
moment these
zones should be cooler considerably than that of general average temperature
of the whole
blank. In this case these critical areas will be slightly stretched during
stamping and will also
become hardened zones at the end of the process.
2) If there are certain zones of the blank which will be desired to be
subjected to be cold
forming operations after hot stamping cycle, these zones should be cooled by
spraying etc ,
daaring heating stage with current and such zones should never be allowed to
reach austenite
temperature limit while the blank is generally being heated above austenite
temperature limit.
These two local cooling types can be applicated in same hot stamping cycle .
For example a
heat treteable steel blank sheet will be heated by current application within
10 seconds and
its general temperature will be reached up to 1000 C. Pressured air and / or
coolant nozules
are employed (during) at the 5. sec of current application while actual
temperature of the
blank is about 500 C. At the end of the heating period of 10 sec. general
temperature of the
blank is 1000 C and temperature of locally cooled areas is 750 C. But there
are some other
type critical zones of the blank to be subjected to overthining or fracture.
These areas are
cooled after austenite temperature is reached then their temperature is
decreased to 700 C.
Then the blank is stamped and compressed by dies for 3 seconds and quenched.
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In this case three types of zones will occur at the end of the process:
1) Zones which were heated generally by current and were not involved any
local cooling
process: These zones are heavily stretched during stamping moment and became
hardened
areas at the end of the operation.
2) Zones which were heated by current and cooled locally during current
heating : Since
these zones is not heated up to austenite temperature limit these zones will
remain as mild
areas at the end of the process . Cold forming operations such as blanking,
punching,
bending etc. can be applicated on such zones after hot stamping
3) Zones which are heated by current and cooled locally after current heating
: These zones
will also be slightly stretched during stamping and became hardened areas at
the end of the
process.
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