Patent Application
20090272168
The present invention relates to an electrohydraulic forming (EHF) tool and a method of forming a sheet metal blank in an EHF operation.
Aluminum alloys and advanced high strength steels are becoming increasingly common as materials used in automotive body construction. One of the major barriers to wider implementation of these materials is their inherent lack of formability as compared to mild steels. Incorporating lightweight materials such as advanced high strength steels (AHSS) and aluminum alloys (AA) into high-volume automotive applications is critical to reducing vehicle weight, leading to improved fuel economy and reduced tailpipe emissions. Among the most significant barriers to the implementation of lightweight materials into high-volume production are stamping issues and the lack of intrinsic material formability in AHSS and AA.
Numerous stamping challenges are associated with the implementation of AHSS and AA in automotive production. The primary method of stamping body panels and structural parts is forming sheet material between a sequence of two sided dies installed in a transfer press or a line of presses. During the era of low oil prices, most automotive parts were stamped from Deep Drawing Quality (DDQ) steel or even Extra Deep Drawing Quality (EDDQ) steel, with both alloys exhibiting a maximum elongation in plane strain above 45%. The formability of aluminum alloys, on the other hand, typically does not exceed 25%. In practice, stamping engineers do not intend to form sheet metal beyond a level of 15% in plane strain due to the much lower work-hardening modulus of metals in these strain ranges, and also due to the danger of local dry conditions on the blank surface. The formability of AHSS is typically around 30%. Insufficient formability drives the necessity to weld difficult to form panels from several parts or to increase the thickness of the blank used in forming the panels.
Electrohydraulic forming (EHF) is a process which can significantly increase sheet metal formability by forming a sheet metal blank into a female die at high strain rates. The high strain rate is achieved by taking advantage of the electrohydraulic effect, which can be described as the rapid discharge of electric energy between electrodes submerged in water and the propagation through the water of the resulting shockwave—a complex phenomenon related to the discharge of high voltage electricity through a liquid. The shockwave in the liquid, initiated by the expansion of the plasma channel formed between two electrodes upon discharge, is propagated towards the blank at high speed, and the mass and momentum of the water in the shockwave causes the blank to be deformed into an open die that has a forming surface. The shockwave forces the blank into engagement with the forming surface to form the metal blank into the desired shape.
The present invention is pointed out with particularity in the appended claims. However, other features of the present invention will become more apparent and the present invention will be best understood by referring to the following detailed description in conjunction with the accompany drawings in which:
A binder 26 defining a binder cavity 28 may optionally be included between the die 24 and the vessel 12. The binder 26 may be configured to facilitate placement and orientation of the blank 20 relative to the vessel 12 and die 24. Corresponding sealing grooves 36 may be provided between the vessel 12, binder 26, and die 24. These grooves 36 may be filled with a resilient element 38 having properties sufficient to prevent and/or limit fluid leakage from the tool 10. The binder 26 is shown to include a relatively flat upper surface for exemplary purposes. The binder 26 may include a three-dimensionally shaped upper surface having undulations or other contours. This shaping of the binder 26 can be helpful in positioning non-uniformly shaped blanks. The binder 26 is shown as a separate feature but it may be eliminated and/or integrated with either one of the vessel 12 or die 24.
A liquid supply port 40 and valves 41, 41′ may operate in cooperation with a liquid source 42. A controller (not shown) or operator may control the port 40 and source 42 to controllably add and remove liquid from the vessel 12. The liquid supply port/valve 40 may be included at a bottom end of the vessel 12 to facilitate drainage of the liquid to a tank 44. The liquid source 42 may include the water tank 44 and a pump 46. An accumulator 48 may operate with the water tank 44 and pump 46 to facilitate discharging liquid at a quicker rate and/or greater pressure than the tank 44 and pump 46 acting alone. A pressure switch 50 may be used to control a pressure of the liquid within the tool 10. A flow meter 54 may be included to monitor the flow of liquid into and out of the tool 10.
The tool 10 may be filled with liquid once or while the air is evacuated. The tool 10 may be filled with liquid until the liquid begins to press against the blank 20. The pressure of the liquid against the blank 20 may be controlled to a desired pressure. The pressure may be selected based on the material, size and other parameters of the blank 20. The pressure may be increased to an extent sufficient to deform the blank 20. This pre-forming may be helpful in forming at least a portion of the blank 20 before it is stamped with the shockwave. This can be helpful in limiting the number of pulses and the load on the die 24 and vessel 12. The pre-forming may also be helpful in limiting cycle times since it may limit the number of shockwave steps used to stamp the blank 20.
Once the tool 10 is filled with a sufficient volume of liquid, the electrodes 16, 18 may be controlled to induce the desired shockwave. If the blank 20 is pre-formed or if the liquid is otherwise maintained at too high of a pressure, the efficiency of the EHF process may be negatively influenced.
With each successive pulse the blank 20 is formed further and further into the die cavity 24, thus creating a larger cavity volume below the blank 20. Without the ability to back-fill the chamber with water after each pulse, this extra volume would be occupied by a pocket of low pressure air and water vapor that would be compressed and heated with each subsequent pulse, thereby substantially reducing the pressure that is delivered to the blank. The accumulator 48 can be used to back fill water added through the use of an appropriate water supply connected to the tool 10 through tubing and ports, and controlled by valves. The air may be evacuated from the area above and/or below the blank 20 prior to re-filling it with liquid. The re-filling process may also be completed at pressure in order to pre-form the part 20. The pressure may then be regulated, with or without the pre-forming, in anticipation of the next shockwave.
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In some cases it may be difficult to determine with desired precision whether the blank 20 was actually formed to its final shape or whether additional forming stages are needed. An embodiment of the present invention contemplates monitoring the amount of fluid within the tool 10 in an effort to assess whether the blank 20 was formed to its final shape. Depending on the shape of the die 24, the amount of fluid added to the tool after each forming stage should decrease over time until there is no more room within tool 10 to receive fluid, i.e., until the blank 20 is matched to the shape of the die 24. Once the addition of water ceases it may be determined that the blank 20 has been formed to its final shape and matches the die.
The amount/flow of liquid may also be used to assess previous forming stages. If past history indicates a certain amount of liquid is typically added after a particular forming stage, that amount of liquid can be used as a benchmark for judging a corresponding forming stage. If too little liquid was introduced, it may be assumed that the blank 20 was under-formed and if too much liquid was introduced, it may be assumed that the blank 20 was over-formed. Because of the liquid levels and the ‘black box’ nature of the tool, it may be difficult to visually inspect the forming of the die and/or to sense its formation. Reliance of the amount of liquid can help ameliorate this issue. An additional flow meter may be used to measure the amount of drained water before opening the press.
The entire EHF system on one non-limiting aspect of the present invention may be a combination of several sub-systems, comprising a pulsed current generator, a hydraulic press used for clamping dies together, the water/air management system, and the integrated hydroforming system. All three of these sub-systems may exist as stand-alone units, with each having its own set of independent push-button controls. The main function of the water/air management system is to deliver water to the electrode chamber and to apply vacuum to the volume between the die and blank. The die and electrode chamber may be mounted in a press. The press can clamp the die and binder attached to the electrode chamber together and the edges of the blank prior to forming to act as a binder or lock and also as a sealing system. The vacuum pump can work in concert with the water delivery step to completely fill the electrode chamber with water. The water/air management system can also partially drain the electrode chamber at the end of the forming process to a level just below the upper rim of the chamber so that the die can be opened without spillage.
The water/air management system may consist of a water supply tank, a supply pump, a water filter, a drain pump, a water accumulator, several flow meters, and vacuum components. The vacuum components can consist of a liquid ring vacuum pump, a water separator, and associated valves and piping. These sub-systems may be operated by solenoid valves, and controlled remotely. The separator prevents delivery of excess liquid water to the vacuum pump and provides the visual indicator for water delivery to the upper ports in the electrode chamber. This visual indicator is used to establish timing for water and vacuum valve openings and closings needed to prepare for the forming operation. An accumulator provides water at rates exceeding the pump capacity in between forming discharges and maintains design pressure to the electrode chamber.
The hydroforming subsystem described above may be used for partially forming the blank 20, as a pre-forming step, before the final forming steps are completed using electrohydraulic forming. Using a pre-forming step can be advantageous in terms of process cycle time since a pre-forming step can be accomplished in only 15 seconds, whereas the steps that it replaces can require 75-90 seconds. While hydroforming is a superior forming method for the initial forming steps, the final forming steps can only be accomplished through EHF, because very high strain rates and substantial pressure are necessary for forming the sheet metal blank completely into deep die cavities. Check valves and solenoid valves may be required to shield the other components of the water/air management system from the hydroforming pressures.
The electrode chamber may be filled to within 10 mm of the top edge of the binder 26 prior to inserting the blank 20. The blank 20 may then be inserted and the press can be closed. A vacuum pump capable of reaching a vacuum adequate to boil water at room temperature can evacuate the volume of air from between the surface of the water and the underside of the blank 20, and also simultaneously evacuate air from the binder cavity 28 between the upper surface of the blank 20 and the die surface. These two volumes may be evacuated simultaneously to prevent differential pressures from deforming the blank 20 by being sucked toward the vacuum source 64.
After air evacuation, the space below the blank 20 may be left containing low pressure water vapor only. The water supply valve can then be opened and the newly created portion of the electrode chamber filled with liquid. When the level reaches the vacuum ports and liquid water is determined in the separator 86, the vacuum supply valve to the space below the blank 20 can be closed and water can then fill in the evacuated volume. A flow meter, which determines in real time the volume of water added to the chamber, will indicate when the filling is completed. The vacuum supply can then be connected to the space above the blank 20 to evacuate the air which would otherwise be compressed by the forming operation. This vacuum should be as deep as is possible. Any air remaining in this volume can impede the high speed forming event. After a deep vacuum has been established above the blank 20, the forming steps can commence.
The blank 20 is now ready to be pre-formed using static hydroforming pressure in the water. Water can now be pumped into the chamber using the hydroforming pump, until the optimal maximum static pressure is reached. This maximum pressure will vary from part to part and will depend on the geometry and draw depth of each specific part. Proper high pressure valves and hoses may be necessary to deliver pressurized water to the chamber without harming other components in the water/air management system. After the pre-forming step is complete, the static pressure in the chamber can be bled off through bleed valves.
The final forming increments can now be accomplished using EHF. The blank may be forced into the die cavity by a pressure wave formed by an electrical discharge between the submerged electrodes 16, 18. With each successive discharge the volume inside the electrode chamber increases as the blank 20 is pressed into the die. This volume may be automatically replaced by pressurized water from the supply system. Higher chamber water pressures, such as 30-100 psi, can suppress arc formation between the electrodes 16, 18, and therefore lower the probability of a good discharge.
The electrical discharge is created by connecting a bank of high voltage capacitors to the electrodes 16, 18. The system may deliver up to 100,000 Amperes from a starting charge voltage of 15,000 volts but higher voltage systems may be employed. Stray losses aside, this discharge is governed by I=C[dV/dt], where I is the current, C is capacitance, and [dV/dt] is the time derivative of voltage. Ignitron or solid state switches that start the discharge may be controlled by a programmable operating system. This operating system may control multiple discharges at various power levels from a single ‘START’ command. The physical properties of the blank and geometry of the die 24 may dictate the regime of discharges used in the forming process. Through Programmable Logic Controller (PLC) (not shown) of the pulsed current generator, the entire EHF process can be automated so as to optimize process cycle time. Any number of process steps may be done concurrently, such as chamber back filling done in parallel with capacitor charging and discharging to reduce cycle time. Also vacuuming can be done in parallel with charging the capacitors and filling the binder are with liquid.
When the forming sequence is completed, the die opening process may be initiated. The die water supply valve is closed and the vacuum pump is shut down and the separator vent valve is opened. Before the press can be opened, the water added to fill the additional chamber volume must be removed, or otherwise spillage would occur. The fastest and most efficient way to remove this water is to pump pressurized air into the chamber and to force water out of the vacuum port and into the separator 86. Once water is no longer flowing into the separator 86 but instead only pressurized air, it is then confirmed that the water level is low enough for the dies to be opened. The press is then opened and the formed blank 20 is removed. The total time necessary for die filling, part forming, and die draining is merely dependent upon the supply pump capacity, the vacuum pump capacity, the size and power of the transformers which charge the capacitors, the drain pump capacity, and the flow and pressure limitations of the tubing and/or piping which carries water to and from the dies.
As required, detailed embodiments of the present invention are disclosed herein; however, it is to be understood that the disclosed embodiments are merely exemplary of the invention that may be embodied in various and alternative forms. The figures are not necessarily to scale, some features may be exaggerated or minimized to show details of particular components. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a representative basis for the claims and/or as a representative basis for teaching one skilled in the art to variously employ the present invention.
