Abstract:
An electrohydraulic forming (EHF) tool having a pair of electrodes that can be used to generate a shockwave to facilitate forming a sheet metal blank against a forming die. The electrodes may be adjusted during the course of operation. This may be useful should continued use cause their efficiency to drop below a desired threshold.

Description:
TECHNICAL FIELD 
   The present invention relates to an electrohydraulic forming (EHF) tool. 
   BACKGROUND 
   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 current 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. 

   
     DRAWINGS 
     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: 
       FIG. 1  schematically illustrates an EHF process; 
       FIG. 2  illustrates an electrode configuration in accordance with one non-limiting aspect of the present invention; 
       FIGS. 3-4  illustrate a spacer in accordance with one non-limiting aspect of the present invention; 
       FIGS. 5-6  illustrate an electrode configuration in accordance with one non-limiting aspect of the present invention; 
       FIG. 7  illustrates a configuration for arranging and combining pairs of electrodes in accordance with one non-limiting aspect of the present invention; and 
       FIG. 8  illustrates a paired electrode system in accordance with one non-limiting aspect of the present invention. 
   

   DESCRIPTION 
     FIG. 1  schematically illustrates the EHF process. Electrical energy may be stored in high voltage capacitors  220  with the assistance of a transformer  222  and a set of diodes  224 . A special switch or discharging device  226 , such as an ignitron, vacuum discharger, or solid state switch, may be used to close the circuit and deliver high voltage stored in the capacitor  220  to electrodes  228 ,  230 . The parameters which define the efficiency of the EHF process include the mutual position of the electrodes  228 ,  230 , the electrical properties of the liquid, the charged voltage, the capacitance, the inductance and resistance of the equipment and connecting cables, the volume of the chamber, and the distance between the discharge channel and the blank. 
   At the beginning of the discharge process, the electrical resistance of a channel between the electrodes  228 ,  230  drops by several orders of magnitude, and the electric current sharply grows due to the increasing temperature and the expansion of the plasma channel. Due to the significant amount of electric energy pumped through the small, ionized channel, the temperature may increase, and the pressure inside the channel may grow during a short time interval. Driven by such high pressure, the discharge channel is quickly expanding and creates a shockwave. 
   For some parts, a blank  232  may be clamped between a chamber  234  and die cavity  236 , which defines a forming surface  237  agent which the blank  232  is pressed during forming. However, in some cases, the outer edge of the part may have a three dimensional contour. A binder (not shown) having a corresponding shape can be employed to support the blank  232 . In order to prevent a short circuit between the electrodes  228 ,  230  (which are usually made out of steel), they should be electrically insulated from the chamber  234 , and the insulation material should be able to withstand the maximum voltage of the process. When it comes to inserting the electrodes  228 ,  230  into the chamber  234 , proper hydraulic insulation should be provided to prevent leakage of water around the electrodes  228 ,  230 . The chamber  234  should be properly sealed to avoid water leakage between the blank  232  and the chamber  234 . Air between the blank  232  and the die  236  should be evacuated in order to avoid energy losses in the EHF process due to heating and compressing of the air during the forming step. 
   The repeated discharge of high-voltage electricity between the electrodes  228 ,  230  can cause the electrodes  228 ,  230  to gradually erode. This erosion can cause the distance between the electrodes  228 ,  230  to grow slowly over time, which can have a negative effect on the efficiency of the EHF process, if the electrodes  228 ,  230  are not adjusted and repositioned periodically. Due to the need to electrically insulate the electrodes  228 ,  230  from the EHF chamber  234 , it can be difficult and cumbersome to adjust and reposition the electrodes  228 ,  230  in an attempt to regain the desired spacing and efficiency. 
     FIG. 2  illustrates an electrode configuration  10  in accordance with one non-limiting aspect of the present invention. A pair of electrodes  12 ,  14  may include consumable electrode tips  16 ,  18  at a leading end of a body portion  20 ,  22 . The tips  16 ,  18  can be replaced instead of replacing an entire solid rod forming the body portion  20 ,  22 . The consumable electrode tips  16 ,  18  may be threaded or press fit to the electrodes  12 ,  14 . A polyurethane insulation layer or other resilient material  26 ,  28  may be molded directly onto the body  20 ,  22 . The body  20 ,  22  may include an undulating outer surface with successive sections having different diameters. Some or all of the diameters may be larger than a diameter of resilient element. The diameter conflict may be sufficient to force the resilient material  26 ,  28  to fill cavities within the undulated outer surface. This allows the resilient material to electrically isolate the electrode body  20 ,  22  and to limit liquid from leaking out of the chamber  30 . 
   The press-fit nature of the resilient material  26 ,  28  allows the body  20 ,  22  to be easily inserted and extracted through an electrode shaft or collar  40 ,  42  attached to the chamber  30 . Whenever the tip  16 ,  18  needs to be replaced, or whenever the inter-electrode distance needs to be adjusted, the electrode  12 ,  14  can be removed or advanced into the chamber  30 . As shown in  FIGS. 3-4 , a spacer  44  can be positioned within the chamber  30  to facilitate advancing the electrodes  12 ,  14  to the desired position.  FIG. 3  illustrates the spacer  44  being located within a relief or other fixture  46  in the chamber, and  FIG. 4  illustrates the spacer  44  being robotically positioned with an arm of a robot (not shown). 
   Once the spacer  44  is positioned, the electrodes  12 ,  14  can be advanced into contact. If the spacer  44  is positioned at a location that is beneficial to the efficiency of the electrical discharge, the advancement of the electrodes  12 ,  14  in this manner allows the electrodes  12 ,  14  to be positioned at a desirable location relative to each other. The electrodes  12 ,  14  may be advanced manually and/or with a robot or other tool. The undulations on the electrodes  12 ,  14  and the press-fit between the resilient element  26 ,  28  and the collar  40 ,  42  may require a certain amount of force be overcome before the electrodes  12 ,  14  can be advanced. A nut  50 ,  52  and compression ring  54 ,  56  used to compress the resilient material  26 ,  28  to the electrode body  20 ,  22  and to seal the chamber  30 , can influence the amount of force needed to position the electrodes  12 ,  14 . The nut  50  may be loosened from its normally tightened state to reduce this pressure. 
     FIGS. 5-6  illustrate an electrode configuration  80  in accordance with one non-limiting aspect of the present invention. Unlike the chamber  30  shown in  FIG. 2 , a chamber  82  shown in  FIG. 6  is not angled in an upwardly sloping direction. While neither  FIG. 2  nor  6  illustrate a blank and forming die, but either would be positioned over top of the chamber  82  or over top of a binder (not shown) positioned on top of the chamber. A pair of electrodes  84 ,  86  having a tip  88 ,  90 , body  92 ,  94 , and shaft  96 ,  98  are positioned within side openings of the chamber  82 . The electrodes  84 ,  86  may be operated to discharge a shockwave within liquid to form a blank in the manner described above. 
   A resilient element  102 ,  104  may be positioned within the openings to seal the shaft  96 ,  98  and limit liquid leakage. One or more seals  110 ,  112 ,  114 ,  116 ,  118 ,  120  may be strategically positioned between compression points to help prevent leakage. A chamber fastener  124 ,  126  may be press-fit, threadably secured, or otherwise fastened to a portion of the shaft  96 ,  98  and operatively connected to press a portion of the resilient material  102 ,  104  against an outside of the chamber  82  while securing a positioning on the shaft  96 ,  98  with respect to chamber  82 . The outer diameter of the shaft  96 ,  98  may include features that limit a distance by which it can advance into the chamber  82 . It may be advantageous to fix this distance, so that the shaft  96 ,  98  is positioned at the same location each time it is removed and subsequently inserted into the openings. This can be helpful in facilitating proper positioning of the electrodes  84 ,  86 . 
   The proper positioning of the electrodes  84 ,  86  may be facilitated if the body  92 ,  94  is slidable moveable within the shaft  96 ,  98 . Optionally, an outer diameter of the body  92 ,  94  may be less than an inner diameter of the shaft  96 ,  98  so that the body  92 ,  94  can be completely removed from the chamber  82  without having to unfasten the shaft  96 ,  98 . An end of the body  92 ,  94  may be shaped to include a shoulder  130 ,  132  that extends above an end of the shaft  96 ,  98 . A body fastener  134 ,  136  can be press-fit, threadably secured, or otherwise fastened over corresponding portions of the body  92 ,  94  and shaft  96 ,  98 . The fastener  134 ,  136  may be tightened to press the shaft  92 ,  94  and body  96 ,  98  together. The seal  130 ,  132  may be positioned between the shaft  92 ,  94  and body  96 ,  98  to help prevent leakage. 
   The electrode tips  88 ,  90  may be press-fit, threadably secured, or otherwise fastened to a leading end of the body  92 ,  94 . The body  92 ,  94  may include a shoulder portion  140 ,  142  against which the tip  88 ,  90  may be secured. The consumable tip  88 ,  90  may be discarded and replaced with a new tip should corrosion or properties of the tip  88 ,  90  degrade over time due to being continuously discharged within the liquid. The tips  88 ,  90  may be easily replaced by unfastening the body fastener  134 ,  136  and slidable removing the body  92 ,  94  through the shaft  96 ,  98 . 
     FIG. 6  illustrate spacers  144 ,  146  that may be positioned between the body  92 ,  94  and shaft  96 ,  98  and/or the body  92 ,  94  and tip  88 ,  90 . The spacers  144 ,  146  may be shaped to fit over the body  92 ,  94  and/or otherwise configured into some other type of shim. The object of the spacers  144 ,  146  is to offset the body  92 ,  94  and/or the tip  88 ,  90  from the non-offset positions shown in  FIG. 6 . This allows the present invention to initially position the electrodes  88 ,  90  and then to adjust their positioning simply by adding and/or removing the spacers  144 ,  146 . While only a single spacer  144 ,  146  is shown to be included at each end of the body  92 ,  94 , multiple spacers of any shape or thickness may be include at each end to facilitate positioning the electrodes  88 ,  90 . 
     FIG. 7  illustrates a configuration  170  for arranging and combining pairs of electrodes  172 ,  174 ,  176 ,  178 ,  180 ,  182 . This arrangement essentially creates several small EHF chambers, each chamber complete with its own electrode pair. This arrangement could be attached to a single binder plate which would then serve as a large EHF chamber for forming large panels. Since the available forming pressure in a volume of water in the EHF process would decrease with increasing chamber volume, this reconfigurable and modular EHF arrangement allows for high forming pressures to be generated at all areas within the chamber volume. In addition to providing technical and physical advantages to the process, reconfigurable EHF chambers also present an opportunity for reducing capital costs spent on EHF tooling, since the same small chambers could be attached to many different binders and upper dies as necessary, i.e., multiple dies would be used with a single chamber at the same time. This allows for precise tailoring of the EHF process for a specific part, since certain parts may require more forming pressure in one area than in another. 
     FIG. 8  illustrates a paired electrode system  190  in accordance with one non-limiting aspect of the present invention. This system  190  includes a first and second set of electrodes  192 ,  194 . Each set of electrodes  192 ,  194  may operate in the manner describe above. The sets  192 ,  194  are orthogonally positioned with respect to each other so that the resulting shockwave produces substantially the same result regardless of whether it originates with the first or second set of electrodes  192 ,  194 . The positioning of each set of electrodes  192 ,  194  may be facilitated with the use of the above-described spacers. 
   Each set of electrodes  192 ,  194  may be positioned, so that either set  192 ,  194  can be used to form a blank (not shown). The present invention contemplates an arrangement when one set of electrodes  192  is used until their performance degrades below an acceptable threshold. Once this threshold is met, the electric discharge can be switched over to the other set of electrodes  194 . This allows the present invention to switch the electrodes  192 ,  194  without having to service the degraded electrodes until a later time when it may be more convenient to open the die. While the switched-in electrodes  194  are in use, the degraded electrodes  192  may optionally be removed for servicing. 
   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.