Patent Publication Number: US-10758960-B2

Title: Chamber for an electrohydraulic forming device

Description:
The present invention relates to a chamber for an electrohydraulic forming device. 
     Electrohydraulic forming devices are increasingly being used for the production of mechanical parts. Indeed, this forming technology makes it possible to obtain parts of relatively complex appearance while controlling the production costs. For example, the automotive and aerospace industries use such technologies. 
     A hydraulic forming process is a process of manufacturing by deformation. It enables plastic deformation of a metal part of relatively small thickness. To achieve this deformation, a fluid is used which, when pressurized, enables the deformation of said part on a mold. Several techniques are used to pressurize the fluid. 
     One of the processes used is an electrohydraulic forming process. This process is based on the principle of an electrical discharge in the fluid stored in a tank. The released amount of electric energy generates a pressure wave which propagates in the fluid, enabling plastic deformation of the mechanical part against the mold. To do this, electrodes positioned in the fluid are adapted to release an electrical charge stored in energy storage capacitors. 
     The use of a closed enclosure improves the forming of the part in comparison to a tank without a cover. The pressure waves are confined within the closed enclosure and reflected waves contribute to shaping the part. 
     U.S. Pat. No. 6,591,649 discloses an electrohydraulic forming device comprising a tank that is substantially elliptical and closed by a mold, a workpiece, and a set of electrodes coupled to an electric energy storage device adapted to generate a pressure wave. This pressure wave, of relatively high power, strikes both the workpiece and the tank of the electrohydraulic forming device. During production phases, these repeated impacts can lead to premature wear of the tank and issues with failing welds on certain parts of the electrohydraulic forming device. 
     Document US2010/0154502 discloses a method and a device for rapidly producing casings for medical use. The rapid formation of these casings is achieved by creating a pressure wave in a liquid contained in an enclosure. A die and a workpiece to be formed are placed in the path of the pressure wave within the enclosure and the pressure wave forces the workpiece to the contours of the die. 
     In order to improve the life of the tank, high density materials capable of withstanding such impacts are used, for example such as metal alloys of relatively large thicknesses. However, the use of thick walls results in a significant increase in the mass of the tanks, particularly for tanks of large dimensions. 
     In order to reduce this mass, stiffeners can be installed outside the walls to increase their rigidity while reducing their thickness. This technological solution does not offer satisfactory results, however. 
     The object of the present invention is to propose an electrohydraulic forming device comprising a tank of improved reliability compared to devices of the prior art, while reducing the mass of the tank and maintaining high forming efficiency. In addition, the invention will advantageously provide an electrohydraulic forming device having a controlled manufacturing cost. 
     To this end, the invention proposes an electrohydraulic forming device comprising a tank having a tank inner wall and inside of which are positioned a mold, a first electrode, and a second electrode. According to the invention, a free first reflector is placed in the tank and surrounds the mold, the first electrode, and the second electrode. 
     The reflector is free because it is not rigidly connected to the tank and/or to the elements secured to the tank. It is mounted in the tank so as to be able to move with respect to the tank. Of course, these movements must be restricted and controlled. Due to the presence of the first reflector, the tank is subjected to less stress from the pressure waves triggered by an electric arc between the first electrode and the second electrode. Indeed, the pressure waves are mostly reflected by the first reflector, which reduces the stresses on the tank inner wall. 
     In order to distribute the pressure waves uniformly, the first reflector is for example cylindrical in shape. The cross-section of this cylindrical shape then depends for example on the part to be formed. In a variant embodiment, this first reflector will be of circular cylindrical shape. 
     According to one embodiment, the first reflector is positioned concentrically with respect to the mold whose shape generally corresponds to that of the part to be formed. 
     In order to better resist the pressure waves and thus better protect the tank, the first reflector is preferably composed of a metal or a metal alloy. 
     In a variant embodiment, a second reflector is preferably placed substantially parallel to a cover, between the first electrode and said cover. The inertia of the device according to the invention is thus improved. 
     In order to improve the performance of the device, the second reflector has a disc shape for example, providing better confinement of the pressure waves when the first reflector is of circular cylindrical shape. The second reflector is connected to the cover, for example by connecting means in the form of dampers. The second reflector can thus move with at least one degree of freedom relative to the cover. 
     In one exemplary embodiment, a space separates the tank inner wall from a reflector outer wall of the first reflector and is filled with the same fluid as the tank, so that the tank is less exposed to the pressure waves during a forming process. 
    
    
     
       Features and advantages of the invention will be more apparent from the following description, made with reference to the accompanying schematic drawing in which: 
         FIG. 1  is a schematic simplified cross-sectional view of an electrohydraulic forming device according to the invention,  FIG. 2  is a partial schematic view of another embodiment of the invention, and 
         FIG. 3  is a partial perspective view of an electrohydraulic forming device according to another embodiment. 
         FIG. 1  shows an electrohydraulic forming device  2  comprising a frame  4 , a tank  6 , a mold  10 , a first electrode  11 , a second electrode  12 , and a first reflector  14 . 
     
    
    
     The frame  4  is adapted to support and hold the tank  6  on a base  16  which may be made of metal or concrete for example. The frame  4  may be made of a metal or a metal alloy, for example such as hardened steel. 
     The tank  6  is adapted to receive and contain a fluid  8  which is water in our example. Preferably, the tank  6  is of cylindrical shape of a given height and has a vertical axis of symmetry A-A′ ( FIG. 1 ). It also has a tank inner wall  18  and a tank bottom  20 . Preferably, it is made of a metal of a thickness of about 5 cm (1 cm=0.01 m). 
     A cover  30  is placed on the tank  6  and is secured by suitable fastening means (not shown in the figures) to hold the cover  30  on the tank  6  during execution of a forming process. Also, a seal  32  between the edge of the tank  6  and the edge of the cover  30  is used. 
     The mold  10  is preferably centered on the vertical axis of symmetry A-A′ of the tank  6 . It has a cavity  24  fixed to a mold support  22 , for example by means of screws. In addition, the mold  10  comprises internal piping  27  coupled to a pumping device (not visible in the figures) making it possible to obtain the desired vacuum under a part to be formed  26 . Thus, during the process of forming the part to be formed  26 , there is no counter-reaction (caused by the presence of air between the part to be formed  26  and the mold cavity  24 ) to oppose the deformation of the part. A fastening device  28  is positioned facing the mold  10  and holds the part to be formed  26  in the desired position. 
     The first electrode  11  and the second electrode  12  are positioned in the tank  6 , preferably on the vertical axis of symmetry A-A′. They are adapted to generate at least one electric arc in the fluid  8 . The first electrode  11  and the second electrode  12  are spaced apart by an adjustable inter-electrode space ( FIG. 1 ). 
     The first electrode  11  and the second electrode  12  are held in the tank  6  by means of a rod  29  ( FIG. 1 ) fixed to the cover  30 . The rod  29  has an adjustable length, making it possible to control the distance between the mold  10  and the second electrode  12 . 
     The generation of an electric arc between the first electrode  11  and the second electrode  12  allows creating pressure waves in the fluid  8 , called direct pressure waves, in order to deform the part to be formed  26 . The direct pressure waves propagate concentrically relative to the inter-electrode space (represented by solid arrows in  FIG. 1 ). 
     The first reflector  14  is positioned in the tank  6  and is preferably cylindrical in shape. It has a diameter adapted to surround the mold  10  as well as the first electrode  11  and the second electrode  12 . The first reflector  14  has a reflector inner wall  34  and a reflector outer wall  36 . The first reflector  14  can move freely within the tank  6  and can be moved in a controlled manner in the tank with at least one degree of freedom. In addition, it must be sufficiently rigid to withstand the pressure waves and reflect them. It is made for example of a metal or a metal alloy and it has a thickness for example of about 3 cm. 
     The diameter of the first reflector  14  is such that a space  38  ( FIG. 1 ) is present between the tank inner wall  18  and the reflector outer wall  36  of the first reflector  14 . In the exemplary embodiments of  FIGS. 1 and 2 , this space  38  contains the same fluid  8  as the fluid contained in the tank  6 . Thus, the tank inner wall  18  is subjected to less stress during a forming process, making it possible to reduce its thickness. 
     In this embodiment, shims  40  positioned between the tank inner wall  18  and the reflector outer wall  36  may be used to hold the first reflector  14  in place in the tank  6 . They are positioned on a lower portion and/or an upper portion of the first reflector  14  by retaining means (not shown in the figures). Thus, the shims  40  allow maintaining the first reflector  14  in an optimal position before, during, and after the process of forming the part to be formed  26 . 
     In a preferred embodiment, the diameter of the first reflector  14  is substantially larger than the diameter of the mold  10  containing the part to be formed  26 . Thus, the pressure waves are sent to a work surface corresponding to the surface of the part to be formed  26 , optimizing the forming process. The use of such a first reflector  14  makes it possible to minimize the exposure of the tank bottom  20 , and in particular the connecting region between the tank inner wall  18  and the tank bottom  20 , to pressure waves during a forming process, thereby improving the service life of the tank  6 . 
     Furthermore, in addition to the direct pressure waves, pressure waves called indirect pressure waves are applied to the surface of the part to be formed  26 . The indirect pressure waves result from the reflection of a portion of the direct pressure waves on the reflector inner wall  34  and on the cover  30 . This thus increases the time during which pressure is applied to the part  26  to be formed, improving the forming process. 
     The embodiment of  FIG. 2  adopts the same geometry as that disclosed in  FIG. 1 . In this variant, in order to reduce the impact of the pressure waves on the tank  6 , an air cushion  45  filled with pressurized air is positioned in the space  38  between the tank inner wall  18  and the reflector outer wall  36 . The air cushion  45 , made of synthetic material, is of toric shape for example ( FIG. 2 ) and can be positioned anywhere along the height of the first reflector  14 . 
     It is also possible to use a plurality of air cushions. For example, two air cushions  45  positioned respectively on an upper part of the first reflector  14  and on a lower part of the first reflector  14  may be used to reduce the impact of the pressure waves on the tank  6  while maintaining the first reflector  14  in an optimal position, as illustrated in  FIG. 2 . 
     In the embodiment of  FIG. 2 , one will also note the presence of a second disc-shaped reflector  15  with a diameter adapted to the diameter of the first reflector  14 . This second reflector  15  substantially closes off the top of the first reflector  14  and is also immersed in the tank  6  ( FIG. 2 ). The second reflector  15  is positioned between the first electrode  11  and the cover  30 . It is spaced apart from the cover  30  and is substantially parallel to the cover. It is free to move relative to the tank  6  with at least one degree of freedom. Advantageously, it makes it possible to increase the inertia of the first reflector  14 . 
     The space between the second reflector  15  and the cover  30  is filled with fluid  8  from the tank  6  but may possibly also have a pressurized air cushion. The damping or absorption of pressure waves by the device is thus improved. 
     The second reflector  15  is preferably connected to the cover  30  by suitable connecting means  44 , for example such as pneumatic or elastomeric shock absorbers. They may be arranged along the entire periphery of the cover  30  in one exemplary embodiment. 
     In another embodiment illustrated in  FIG. 3 , the space  38  between the tank inner wall  18  and the reflector outer wall  36  is filled with air that can be pressurized. In this exemplary embodiment, a circular envelope made of synthetic material can store air at a given pressure and thus provide a seal between the air (contained in the circular envelope) and the water contained in the tank  6 . Due to the greater capacity for deformation of air compared to water, the transmission of pressure waves towards the tank  6  is attenuated. The tank  6  is thus subjected to less stress, making it possible to reduce the thickness of the tank  6  and hence its mass. 
     In order to protect the base  16  from possible vibrations generated by the pressure waves, supports  42  ( FIG. 3 ) are positioned between the tank  6  and the frame  4 . They are preferably positioned along a periphery of the tank  6 . The thickness of the supports  42  as well as the material used are adapted to distribute the forces from the tank  6  onto the frame  4  during the forming process. 
     The present invention therefore proposes an electrohydraulic forming device with at least one reflector positioned in the tank, making it possible to reduce the impact of the pressure waves on the tank and thus increase its service life. Moreover, the presence of at least one reflector in the tank makes it possible to decrease the thickness of the tank and hence its mass. 
     The invention is not limited to the embodiments described above by way of non-limiting examples and to the shapes represented in the drawing and the other variants mentioned, but relates to any embodiment that is within the reach of a person skilled in the art and within the scope of the following claims.