TECHNICAL AREA
[0001] 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
[0002] 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 dies 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.
SUMMARY OF THE INVENTION
[0003] 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.
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 related with DC generator feeding
for the invention has not been added because of it is a well known and basic technology.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. In this configuration,
the need to modify the die design with isolation to accommodate the electrical heating
system in the previous patent granted to the inventor (Terziakin, US Pat. No: 6.463.779)
is totally eliminated due to the electric flux running outside the die(s) as well.
As it is known forming of metals at elevated temperatures can be realized as warm
or hot forming. Hereby hot forming expression is generally used for forming process
at elevated temperatures at these documents .
BRIEF DESCRIPTION OF THE DRAWINGS
[0004] To illustrate the invention, 6 figures are included. In Figure 1 A, the electrical
circuit of the stepped system using three phases; in Figure 1 B, the system charged
by two phases and forming system are provided. In Figure 2, application of the invention
on tube hydroforming is shown. In Figure 2, a tube formed by external dies while being
internally pressurized by a fluid. Figure 3 illustrates the application where electrical
heating system is used for bulk metal hot forging process that integrated to the material
handling system. In Figure 4, combination of electrical heating, forming and hardening
by air or spray cooling is illustrated. In Figure 5, integration of heating during
material handling, forming and rapid cooling to form hardened workpiece are shown.
Figure 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. Finally, Figure 6 illustrates current heating and selective
spray cooling system providing desired temperature gradient along the blank for sheet
forming process at elevated temperatures.
DETAILED DESCRIPTION OF THE DRAWINGS
[0005] As illustrated in Figure 1A and 1B the current control in this system is achieved
at the primary coil where the current value is less. An electronic circuit 2 (CU)
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 and the workpiece can be
made using soldering or copper brazing to minimize resistance.
[0006] As shown in Figure 1 A, to be able to use the system on larger workpieces systems
that use the three phases 8 of the line energy are employed loading the phases equally
to obtain the exit frequency two or three times larger can also be used. This technology
is currently used to feed energy in large welding equipment. The upper part of the
figure 1 A separated by dashed lines represents this subsystem 7. Since the output
current of it uses all three phases and it is at high frequency, current is conditioned
very well with reduced voltage and increased amperage for the last step transformers
1 (T1 - Tn). The increased frequency will ease induction at the coils and thus the
necessary ferritic core size of the last step transformers 1 can be reduced . Thereby
the last step transformer group 1 (T1 - Tn) is fed 6 by the energy at U2 potential
with 2-3 times higher ( depending on its arrangement) frequency than that of main
electric frequency. These last step transformers 1 (T1 - Tn) are the transformer group
with primary 9/ secondary 10 coil ratio more than one and that reduces the voltage
and increases the current of the received electrical power and feed current to the
workpiece in parallel via the conductors 3 ( C1 - Cn). There are clips 5 at two opposite
sides of the metal workpiece which also be used as electrodes 5 to connect secondary
coil current to the workpiece 4.
[0007] Last step transformers 1 (T1 - Tn) has equivalent characteristics working at the
same phase and voltage. Since they are wired in parallel, the total current fed by
them is the current passes through the workpiece. The larger secondary circuit impedance
is developed based on the conductor's length 3 (C1 - Cn) , curvature(s) and the resistance
at contact points, and thus this is an ideal system, because it uses the three phases
in balance and it eliminates the limitations on the current that can be applied on
the workpiece . The control system 2 closes the primary to induce the current at the
secondary circuit that passes through the workpiece 4 to increase its temperature
to the required magnitude in synchronization with the forming operation. In the figure,
it is shown that at a standard line frequency of 50 Hz, 100-150 (depending on system
arrangement) Hz frequency is generated at the secondary circuit; Similarly, at a line
frequency of 60 Hz, 120-180 Hz frequency must be expected. Increased frequency increases
system impedance and thus restricts the line current. Another approach can be to obtain
direct current at the initial step and generate alternative current at a lower frequency
(lower than 50-60 Hz), feed it to the final step transformers and eventually reduce
the system impedance.
[0008] For small workpieces, a simpler circuitry as shown in Figure 1-B is proposed. 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. In this primary circuit,
a control unit 2 is used to control primary current characteristics with thrystor,
switching device etc. are included. Especially the control unit 2 with thrystor needs
to be cooled by a closed circuit fluid cooling system. 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 developed between
phase and ground. Primary 9/ secondary 10 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 .
[0009] In Figure 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 direct 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.
[0010] 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 lubricated 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.
[0011] 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.
[0012] In Figure 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 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 power input to this system is preferably
performed by connecting the two phases of the three line phases to the transformer
1 primary coil terminals.
[0013] 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 electrical 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.
[0014] 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 12. 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 and
lower dies linked 20 and operated using a couple of hydraulic cylinders 17.
[0015] 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 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.
[0016] 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.
[0017] 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.
[0018] 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.
[0019] The parameters of the process described above, such as part material, size and geometry,
internal pressure, forming temperature, and cooling method, are determined by carefully
planned engineering experiments.
[0020] Application of the invention in the hot or warm (billet) forming operation is illustrated
in Figure 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.
[0021] 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 the 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 workpieces 25 should be subjected to a chemical
and/or mechanical cleaning operation to reduce 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 should
also have a cooling system to dissipate heat preferable with fluid circulation. 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, 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.
[0022] As illustrated in Figure 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 metal 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 high 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 with 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.
[0023] 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 guide the part to prevent movement in the lateral direction
and thus locate it on the right position on the die.
[0024] Heat 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 predetermined 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.
[0025] 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 has 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 different 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 Fig.5) in electric feeding of electric motor 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.
[0026] As illustrated in Figure 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 performed 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 workpiece 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
[0027] In this set-up, the workpiece 41, while being transported to the forming die set
(45) on a press, is 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 workpiece. The moving arms (41) of the transport
system may contain mechanical linkages (42). 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 and as short as possible
to keep the conductor lengths short, the electrical impedance of the electrical system
low and thus electrical efficiency of the system high.
[0028] This invention can be applicated as many different configurations . One of these
alternatives is to situate elecrode clips on a stable position near dies . Especially
for relatively thich materials allowing handling operation while maintaining its temperature
sufficiently until forming operation this configuration may be a easy to applicate
and inexpensive alternative . Although it is not shown in a figure particularly ,
only difference between this configuration and that of seen in Fig. 5 is these clips
38, 39 connected to second coil of the transformer 1 will be positioned in stable
places adjacent to forming tools . When the workpiece reachs sufficient temperature
is then carried to forming position .
[0029] To 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.
[0030] The subsequent operations of this process performed in a double action hydraulic
press, are illustrated in Figure 5. In the first part (Figure 5 A), 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 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. 5 B when the simultaneous forming operation is complete and the die
set is empty and ready for the next cycle, the workpiece is ready at the 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 generated and thus heat loss from the workpiece is minimized. In 5 C upper
die 34 is moved down and rests blank holder 41 and thus hot sheet 37 is hold firmly
and then stamped by 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 .
[0031] If 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 may 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 number of cycles to
be determined by experience. Both the cleaning and relubrication may be performed
manually or by an automatic system.
[0032] 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.
[0033] 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 relatively unstretched regions of the part bounded by heavily stretched
areas. In such cases formability 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 needs to be softened
while some other critical areas needs to be maintained relatively harder by means
of proper temperature gradient along the hot blank.
[0034] In Fig. 6 an further application area of the invention is shown to solve above problem.
The present 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 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 areas 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.
[0035] In selective heating process shown in Fig. 6 A 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 coil of the transformer. At second stage as seen in Fig.
6.B 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. 6 C 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 will be stretched
less . At the stage of die desing, desired temperature gradient map is calculated
using by proper simulation program.
[0036] 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.
[0037] For example a steel blank sheet to be hot formed can be heated to 900 C homogeniously
and then temperature of some critical areas can be reduced to 750 C locally by means
of above mentioned 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.
1. Device for enhancing formability of the metal workpieces 4 that will be subjected
to a subsequent forming operation, including at least one transformer 1 with primary
9 /secondary 10 coil ratio more than one and including a control unit 2 at the primary
circuit that controls magnitude and timing of electrical current to be inducted at
the secondary coil 10, which closes the electrical circuit by passing the electric
current inducted at the secondary coil through the metal workpiece 4, and enables
heating of the metal workpiece by application of the electric current generated at
said secondary coil of said transformer and enables to control temperature of the
metal workpiece by means of time and magnitude of current at the primary 9 circuit
by said control unit and have ability to be operated in cooperation with a mechanical
forming machine including a set of dies isolated from said electric current in which
at least one die can be compressed toward the workpiece and includes said control
unit having ability to be operated in coordination with control unit of said mechanical
forming machine to achieve forming operation at elevated temperature, when the workpiece
has reached the desired temperature.
2. A homopolar DC generator is used instead of the device as claimed in Claim 1 as a
source of current to be passed along the metal workpiece to heat the metal workpiece
in order to prepare the workpiece by elevating its temperature in order to prepare
subsequent mechanical forming operation which is controlled in synchronized manner
with mechanical forming operation .
3. The device as claimed in Claim 1, comprising electrical charge converting means 7
using three phases 8 at the electrical incoming lines to produce a two phase current
fed to at least one transformer 1 heating the workpiece.
4. The device as claimed in Claim 1, comprising more than one transformer 1 fed by the
same two phases connected to two ends of their primary coils and two ends of their
secondary coils 10 are connected to 3 in parallel, thereby total current inducted
at their second coils are passed through the metal workpiece 4.
5. The device as claimed in Claim 1 is employed in heating by electrical current in the
process of tube forming at elevated temperature performed by means of internal pressure
12 applied inside the tube 4 and dies 14, 15, 16, which compress the tube 4 externally,
after the metal tube has reached to proper temperature for hot or semi-hot forming.
6. The process in which the workpiece 25 is subjected to multiple forming operations
at elevated temperature using the device as claimed in Claim 1 including interval
heating processes by application of electric current while transport between subsequent
forming stations.
7. The device as claimed in Claim 1 is performed with a metal workpiece 37 handling system,
which is employed for metal workpiece transfer to a forming machine including carrier
arms 30 and 31 with electrodes 39 conducting current from second coil of said transformer
to two opposite sides of the metal workpiece in order to generate internal heat during
handling cycle to prepare it for the subsequent forming operation at elevated temperature.
8. A forming press which is configured for forming process at elevated temperature with
the device as claimed in Claim 1 having means of forming speed adjustment depending
on desired strain rate of the hot forming process.
9. The die cooling system that performs hot forming process with the device as claimed
in Claim 1 that controls the die temperature at the preset range by passing coolant
fluid through channels in the die.
10. The heat treatment process following the forming operation performed subsequent to
the heating process performed with use of device as claimed in Claim 1, achieved by
means of spraying pressurized air or air-coolant fluid mixture to control cooling
speed of the workpiece.
11. The selective stretching process along the blank during the forming operation performed
with use of the device as claimed in Claim 1 by means of obtaining a predefined temperature
gradient two dimensionally along the blank surface 44 by means of gas/fluid spraying
with a set of directed nozzles 45 between current heating and stamping cycles, toward
certain portions 46 of the hot blank, which are desired to be cooled relatively that
providing local increase in strength rates to avoid local overstraining and fractures
in such critical areas 46 in order to obtain selective stress strain curves 46, 47,
48 depending on actual local temperature varying two dimensionally throughout workpiece
area at the hot stamping moment.
12. The corrosion protection means at the metal workpiece to be formed at elevated temperature
with use of the device as claimed in Claim 1, by means of coating a protective layer
to avoid direct contact between air and the hot workpiece.
13. The temperature adjustment means at the surfaces of the dies facing the metal workpiece
by maintaining within a predetermined range that used for hot forming process performed
with use of the device as claimed in Claim 1, by means of surface heating to increase
of temperature for heating means and gas spraying toward the die surfaces for cooling
means depending on actual surface temperature.
Amended claims in accordance with Rule 86(2) EPC.
1. Method for enhancing formability of a metallic workpiece (4) by means of electric
current application to a workpiece to be formed by a subsequent forming operation
at the elevated temperature by being compressed between a set of dies, the method
characterised in that;
- controlling magnitute and timing of electric current with an electronic control
unit (2) feeding primary coil of at least one transformer (1) with primary (9) / secondary
(10) coil ratio more than one,
- generating electric current at the second coil of said transformer(s) in a controlled
manner,
- connecting said current to two electrode sets (5) contacting with two opposite sides
of the forming area of said workpiece via two sets of the connections so that heat
is generated inside said workpiece by transforming electric energy into heat energy
until predetermined temperature at the workpiece is obtained,
- keeping temperature of the dies between a predetermined temperature range by taking
away heat from the dies in a controlled manner,
- forming said workpiece between at least two dies by being compressed between said
dies,
- achieving a predetermined heat transfer amount within a certain range from the workpiece
to the dies while said workpiece is being compressed between dies ,
- cooling of electrical connections (3) from said transformer to the workpiece including
electrodes (5) contacting with said workpiece by means of cooling fluid flow .
- varying heat amount of taken away from the connectors and tools by variation of
their actual temperature such that keeping their temperatures between their predetermined
ranges.
2. A homopolar DC generator is employed instead of the transformer at the process as
claimed in Claim 1 as a source of current to be passed along the metallic workpiece
(4) to heat up said workpiece in order to prepare subsequent mechanical forming operation
which is controlled in synchronized manner with mechanical forming operation,
characterized in that;
- generating electric current by said homopolar generator in a controlled manner,
- connecting said current to two electrode sets (5) contacting with two opposite sides
of the forming area of said workpiece via two sets of the connections (3) so that
heat is generated inside said workpiece by transforming electric energy into heat
energy until predetermined temperature at the workpiece is obtained,
- keeping temperature of the dies between a predetermined temperature range by cooling
means being achieved by cooling fluid flow contacting with die masses in a controlled
manner,
- forming said workpiece between at least two dies by being compressed between said
dies,
- achieving a predetermined heat transfer amount within a certain range from the workpiece
to the dies while said workpiece is being compressed between dies,
- cooling of electrical connections from said homopolar generator to the workpiece
including electrodes contacting with said workpiece by means of cooling fluid flow
.
- varying heat amount of taken away from the connectors and tools by variation of
their actual temperature such that keeping their temperatures between a predetermined
range.
3. Method according to claim 1
characterised in that;
- converting three phase of mean (8) into one or two phase electric current,
- feeding primary circuit of said transformer(s) (1) with said one ore two phase current.
4. Method according to claim 1
characterised in that;
- converting alternating current inducted at the secondary coil of the transformer
(1) into direct current,
- applicating said direct current to said workpiece,
- reducing impedance of the last circuit including said workpiece (4) and its connections
(3).
5. Method according to claim 1
characterised in that;
- employing two or more transformers (1) featuring same characteristics as the current
source of the process,
- collecting their secondary circuit currents,
- applicating their total current to the workpiece (4).
6. Method according to claim 1
characterised in that;
- utilizing a tubular metallic part with closed section as the workpiece of the process,
applicating internal pressure (12) inside the tubular metallic workpiece, heating
said tubular metallic workpiece by current application via two sets of electrodes
(21) being contacted with its two opposite edges,
- providing a proper temperature rate at the tubular workpiece within a predetermined
range,
- forming the tubular workpiece between at least two die sets (14,15,16,18) by being
compressed by at least one die,
- holding the tubular workpiece between die sets and internal pressure until a predetermined
temperature is reached at the tubular workpiece so that sufficient dimensional stability
is obtained,
- removing the formed tubular workpiece between the die sets.
7. Method according to claim 1
characterised in that;
- measuring actual temperatures of the dies with temperature sensor(s),
- comparing with predetermined upper temperature limit whether temperature of the
die(s) is higher than predetermined upper limit
- cooling the die(s) by spraying coolant (35) toward its surfaces or by the flow of
cooling fluid being flowed in the passages placed inside the die bodies when die temperatures
is above said upper limit.
8. Method according to claim 1 characterised in that, forming operation is performed in a machine which have ability to be operated with
variable forming speeds by means of varying movement speed of at least one die against
the workpiece during compression cycle.
9. Method according to claim 1 characterised in that, temperatures of transformer , connectors and clamp type electrodes are kept within
a predetermined range by dissipating heat by a coolant flow.
10. Method according to claim 1 which is applicated in a multistage forming process performed
in a series of forming stations in sequence in a progressive manner including forming,
piercing and blanking stages and applicating interval heating process by means of
current application between forming stages, the methot is
characterized in that;
- holding the metallic workpiece (25) in a billet or a blank form with two clamp type
electrodes (24) contacting two opposite sides of the heating area,
- generating heat inside the forming portion of the workpiece by applicating current
via two clamp type electrodes,
- placing the heated portion of the workpiece between two dies of the forming station,
- preforming the heated portion of the workpiece by being compressed between two dies,
- carrying the workpiece out of the forming machine,
- applicating current to generate heat inside the forming portion of the workpiece,
- placing the heated portion of the workpiece between two dies of the next forming
station,
- repeating heating and forming cycles for each following forming station,
- finishing the completed part by piercing and blanking at the last station.
11. Method according to claim 1
characterised in that;
- combining the heating means with handling means of the workpiece by employing carrier
arms (31) with movable clamp type electrode sets (38,39) contacting with two opposite
edges of the blank (37) as the holding tools and connecting current came from two
ends of second coil of the transformer (1) to generate heat inside the blank during
handling process,
- accelerating process cycles by heating up the workpiece by current application out
of the press table while previously heated workpiece is being formed at the press
table at the same time,
- preventing overheating at the carrier arms, connectors and electrodes by cooling
means.
12. Method according to claim 1 characterised in that; the heat treatment process performed as increased cooling cycle after forming of
the workpiece in a controlled manner after hot forming process by spraying a coolant
fluid (35) toward workpiece surface (33) after the die (34) is moved back.
13. Method according to claim 1
characterised in that;
- heating the workpiece equally through its surface by application of current fed
by seconder coil of the transformer,
- cooling selected portions (46) of the workpiece surface by spraying coolant jets
by directed nozules (45) toward these certain portions which are previously determined
as critical areas subjected to potential fracture or overstraining areas depending
on die forms,
- obtaining two dimensionally predetermined temperature gradient on the workpiece
surface, strengthening such critical areas by providing selective strain stress characteristics
gradient through the workpiece,
- forming of the workpiece by being compressed between two dies (51,52)
- preventing fractures and overstraining at the such critical areas by increasing
their strength, decreasing their strain rates and facilitating material flow from
surrounding areas to these critical portions.
14. A method as claimed in claim 1 characterised in that clamp type electrodes (38,39) holding two opposite sides of the workpiece is closed
before the current application and opened between current application and forming
stages under control of central control unit.
15. A method as claimed in claim 1 characterised in that cleaning and lubricating die surfaces by spraying fluid between stamping cycles.