Patent Publication Number: US-2017355007-A1

Title: Electrohydraulic forming apparatus

Description:
The present invention relates to an electrohydraulic forming apparatus. 
     Electrohydraulic forming apparatuses are increasingly being used for the production of mechanical parts. These forming apparatuses make it possible to obtain parts of relatively complex appearance while controlling production costs. For example, the automotive and aerospace industries use such apparatuses. 
     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 fluid stored in a tank. The released amount of electric energy generates a wave which propagates very quickly in the fluid and enables plastic deformation of the mechanical part against the mold. To do this, electrodes positioned in the fluid release an electric charge stored in energy storage capacitors. 
     To achieve this deformation, a non-negligible amount of water is required. The deformation of the part is proportional to the volume of water displaced during the explosion generated by the electric arc. In addition, a step of emptying the tank is necessary after each explosion in order to retrieve the formed part. Such apparatuses are therefore generally preferred for the production of parts in small series. 
     However, the generation of electric arcs causes wear to the electrodes and the appearance of small particles which, due to gravity, fall onto the part positioned at the bottom of the mold. These particles then result in defects in the formed part. 
     U.S. Pat. No. 7,493,787 discloses an electrohydraulic forming apparatus in which a membrane is used to retain two volumes of liquid. A apparatus adapted to generate a high voltage pulse is coupled to electrodes in order to create a shockwave in one of the volumes of liquid. The shockwave thus generated is transferred through the membrane to the other volume of liquid, thus deforming a part against a mold. Due to the use of a membrane separating the two volumes of liquid, only the volume of liquid in which the part to be deformed is placed must be drained when changing the part, which improves productivity. Furthermore, the part to be formed is protected from particles resulting from electrode wear. Such a apparatus is of relatively complex design however, because it is composed of at least three parts. In addition, the strength of the membrane directly impacts the reliability of the apparatus. Plus the use of a membrane allows only the simple transmission of the shock wave. 
     The object of the present invention is to provide an electrohydraulic forming apparatus of relatively simple design, meaning that it preferably comprises only two parts with improved reliability compared to apparatuses of the prior art. In addition, the present invention advantageously provides an electrohydraulic forming apparatus in which the manufacturing costs are reduced while meeting current standards. Advantageously, the part to be formed is protected from particles resulting from electrode wear, which makes it possible to obtain parts with the desired surface condition. 
     To this end, the invention proposes an electrohydraulic forming apparatus comprising a mold with an upper portion and a lower portion, an enclosure having a first region, a second region, a mold cavity positioned in the second region, and electrodes having electrode tips positioned in the first region, 
     characterized in that a piston is mounted so as to be movable in translation within a channel in a fluidtight manner and separates the first region from the second region of the enclosure. 
     Due to the use of a piston, the reliability of the electrohydraulic forming apparatus is improved. Productivity is also improved because it is not necessary to drain all the liquid contained in the mold of the electrohydraulic forming apparatus. 
     To avoid the potential risk of the piston jamming in the channel, the piston may have first guide means complementary to second guide means of the channel. 
     In one exemplary embodiment, the guide means comprise three ribs to limit the degrees of freedom of the piston to one. 
     In a variant, the second guide means comprise three grooves, thus optimizing translational movements of the piston within the channel. 
     To optimize the deformation of a part positioned on the mold cavity, the piston has a first face of planar shape and directed towards the mold cavity. 
     In a variant, the piston may have a first face of a shape selected from the set of concave and convex shapes and directed towards the mold cavity. 
     In order to optimize the movement of the piston within the enclosure, the piston has a second face adapted to the shape of the enclosure and directed towards the first region. 
     In a variant, the piston has for example a second face of a shape selected from the set of concave and convex shapes and directed towards the first region. 
     To prevent the piston from falling into the lower portion of the mold, the channel may have a stop adapted to retain the piston. In this exemplary embodiment, a spring may be positioned between the piston and the stop in order to improve the reproducibility of the details on the parts to be formed. Due to the presence of the spring, the piston is returned to a determined and identical height after each emptying of the mold. 
    
    
     
       Features and advantages of the invention will be more apparent from the following description, made with reference to the accompanying schematic drawings in which: 
         FIG. 1  is a schematic simplified cross-sectional view of an electrohydraulic forming apparatus according to the invention, 
         FIG. 2  is a simplified schematic view corresponding to  FIG. 1 , in another position, 
         FIG. 3  is an enlarged and simplified schematic view of a detail of another embodiment of the invention, and 
         FIG. 4  shows different piston shapes. 
     
    
    
       FIG. 1  is a schematic representation of an electrohydraulic forming apparatus  2  comprising a mold  4  having an upper portion  6 , a lower portion  8 , an enclosure  10 , a channel  12 , a mold cavity  14  positioned in the lower portion  8 , and electrodes  16 . 
     Such an electrohydraulic forming apparatus  2  may be arranged on a frame (not shown in the figures) made of a metal or metal alloy for example such as hardened steel. 
     The upper portion  6  of the mold  4  in the embodiment illustrated in the drawing is positioned above the lower portion  8  of the mold  4 . The lower portion  8  is fixed to the upper portion  6  by clamping means for example (not shown in the figures). Preferably, the mold  4  (comprising the upper portion  6  and the lower portion  8 ) is composed of a high-density material, for example such as a metal or metal alloy. 
     The enclosure  10  has a first region  18 , a second region  20 , and the channel  12 . As illustrated in  FIG. 1 , the enclosure  10  has a first wall  22  that is rotationally symmetrical with respect to an axis A-A′ and is for example of cylindrical shape with a determined diameter. 
     The enclosure  10  also has a second wall  24  of frustoconical shape, connected to the first wall  22  and to the channel  12 . 
     The enclosure  10  is also adapted to receive, in the first region  18 , the tips  26  of electrodes  16 . The electrodes  16  are high-voltage electrodes (several tens of kV). Here, they are held perpendicularly to the axis of revolution A-A′ ( FIG. 1 ). In order to insulate the electrodes  16  from the mold  4 , an insulating sleeve  28  is used. 
     The electrodes  16  also have an adjustable and modifiable inter-electrode space which makes it possible to control the triggering of an electric arc between them. 
     An electrical storage apparatus (not shown in the figures) is used that is suitable for storing an amount of electrical energy sufficient to generate at least one electric arc between the electrodes  16 . In order to control the amount of electrical energy delivered by the electrical storage apparatus to the electrodes  16 , a pulse generator (not shown in the figures) is coupled to the energy storage apparatus. As the pulse generator and the electrical storage apparatus are known to those skilled in the art, they are not presented in the present description. 
     In a preferred embodiment, the channel  12  has a circular cylindrical shape and has a determined length sufficient to allow movements of a piston  30  corresponding to the deformation to be made to a part placed facing the mold cavity  14 . The channel  12  is also adapted to the mold cavity  14 . 
     The lower portion  8  receives the mold cavity  14  which defines the final shape to be given to the part  32  to be produced by electrohydraulic forming (EHF). Depending on the complexity of the shape of the part  32  to be formed, the mold cavity  14  may have a large form factor with high precision details. 
     Also, the lower portion  8  may comprise tubing (not shown in the figures) coupled to vacuum means (not shown in the figures) for eliminating any air present between the part  32  and the mold cavity  14  Thus, during the process of forming the part  32 , there is no counter-reaction (caused by the presence of air between the part  26  and the mold cavity  14 ) to oppose the deformation of the part  32 . 
     The piston  30  is mounted so as to be movable in translation within the channel  12  in a fluidtight manner and forms the separation between the first region  18  and the second region  20  of the enclosure  10 . The first region  18  is filled with a first fluid and the second region  20  is filled with a second fluid. 
     In a preferred embodiment considered here, the first fluid and the second fluid are water. Advantageously, by virtue of the presence of the piston  30  in the channel  12 , the water contained in the first region  18  is isolated from the water contained in the second region  20  of the enclosure  10 . Thus, particles worn off the tips  26  of the electrodes  16  are stopped by the piston  30  and do not reach the part  32 . It should be noted, as illustrated in  FIGS. 1 and 2 , that the first region  18  and the second region  20  vary with the positioning of the piston  30  within the channel  12 . 
     The piston  30  is, for example, made of a material identical to the material of the mold  4 . Advantageously, in order to ensure the fluidtight seal between the first region  18  and the second region  20 , the piston  30  has a diameter identical to the diameter of the channel  12 . The piston  30  is mounted so as to be movable in translation within the channel  12 , thus enabling translational movements along the axis of symmetry A-A′ from a first position ( FIG. 1 ) to a second position ( FIG. 2 ). 
     To optimize the seal between the first region  18  and the second region  20 , sealing means may be used, for example such as elastic rings  38  positioned between the piston  30  and the channel  12 . 
       FIG. 3  shows a partial cross-sectional view of the electrohydraulic forming apparatus  2  with the two elastic rings  38 . In an alternative embodiment, to improve the retention of the elastic rings  38  on the piston  30 , the latter may have grooves (not shown in the figures) of a shape and depth adapted for receiving and retaining the elastic rings  38  which form a seal between the piston  30  and the channel  12 . 
     Also, to avoid rotation about axis A-A′ during the translational movements of the piston  30  within the channel  12 , and thus eliminate any risk of the piston  30  jamming, it is provided in one embodiment that the channel  12  comprises at least one groove (not shown in the figures) and that the piston  30  comprises at least one rib. The groove of the channel  12  is adapted to engage with the rib of the piston  30 . 
     In order to improve the reliability of the electrohydraulic forming apparatus  2 , the piston  30  may comprise three ribs equally distributed around the piston  30 , and the channel  12  may comprise three equally distributed grooves, the ribs being positioned to face the grooves. The piston  30  thus has a single degree of freedom and forces are better distributed during the transition from the first position ( FIG. 1 ) to the second position ( FIG. 2 ), which improves the service life of the electrohydraulic forming apparatus  2 . 
     In an alternative embodiment, to prevent the piston  30  from leaving the channel  12 , the latter has a stop  40  as illustrated in  FIG. 3 . Preferably, the stop  40  is positioned on a lower portion of the channel  12 . Furthermore, this stop  40  also prevents the piston  30  from exiting and/or falling out of the channel  12  when the second region  20  of the enclosure  10  is drained. 
     In a variant, to facilitate repositioning the piston  30 , a spring (not shown in the figures) may be used. The spring is for example positioned on an outer edge of a first face  42  of the piston  30  and bears against the stop  40 . The spring then makes it possible to return the piston  30  to its first position after a step in the process of electrohydraulic forming of the part  32  placed in the mold cavity  14 . 
     The transition from the first position to the second position of the piston  30  as presented above in the description is achieved by the propagation of a first wave generated by an electric arc at the electrodes  16 . The first wave so generated propagates in the first region  18  perpendicularly to the axis A-A′ toward the piston  30  and more precisely toward a second face  44  of the piston  30 . 
     The first wave has an energy which depends, among other things, on the power of the electric arc. The movement of the piston  30  within the channel  12  enables transferring almost all the energy of the first wave to the water contained in the second region  20 , giving rise to a second wave. The second wave thus created propagates towards the mold cavity  14  in order to deform the part  32  arranged thereon. 
     Advantageously, the use of an electrohydraulic forming apparatus  2  with a piston  30  positioned in the channel  12  to isolate the first region  18  from the second region  20  allows improving the quality of the part  32 . When the electric arc is triggered at the electrodes  16 , a relatively small quantity of material is torn from the electrodes and forms particles which fall into the fluid, in this case water. These torn-off particles fall onto the piston  30  due to gravity and do not reach the part  32 , in contrast to apparatuses of the prior art which comprise a single volume of fluid. 
     In addition, the use of a piston  30  in the channel  12  to isolate the water contained in the first region  18  from the water contained in the second region  20  advantageously reduces the time required to fill and empty the water contained in the second region  20 . 
     Thus, the movement of the piston  30  from its first position to its second position occurs without resistance and can be carried out in a relatively short time, for example less than one millisecond, which makes it possible to obtain a rapid deformation of the part  32  and thus a better deformation of the part  32 . 
     In order to improve the transfer of energy between the first wave and the second wave, the second face  44  of the piston  30  may have a concave shape for example and the first face  42  of the piston  30  may have a convex shape ( FIG. 4 d   ). Propagation of the second wave in the lower portion  8  is thus optimized, improving the deformation quality of the part  32  after deformation. 
     In other exemplary embodiments, as illustrated in  FIGS. 4 d  to 4 f   , the first face  42  may have a greater or lesser radius of curvature. Thus, depending on the radius of curvature selected, it is possible to improve the focusing of the second wave on the part  32  in order to optimize its deformation. 
     The first face  42  and the second face  44  may also have other shapes as illustrated in  FIGS. 4 a  to 4 c   , with lesser or greater curvatures, optimizing the deformation of the part  32 . Also, the piston  30  may have a rectangular shape, preventing any rotation within the channel  12 . 
     More generally, the first face  42  is shaped to match the deformation to be made to the part  32  and the second face  44  is shaped to match the shape of the first region  18 . 
     The present invention therefore proposes an electrohydraulic forming apparatus comprising a mold having an enclosure with a channel and a piston positioned in said channel as well as a mold cavity. The mold proposed here is composed of only two parts, facilitating its assembly and limiting its manufacturing cost. The use of a piston in the channel in order to separate a first region from a second region reduces the volume of fluid to be emptied between two manufacturing phases and improves the gain in productivity. Indeed, the time to manufacture a part corresponds to the time required to position a part on the mold cavity, fill the enclosure with fluid, close the enclosure, and trigger an electric arc before the fluid is emptied. Moreover, due to the presence of two separate volumes of fluid, the particles torn from the electrodes when the electric arc is triggered do not fall on the part to be formed, such that the quality is not impacted. 
     The present invention is not limited to the embodiments described above by way of non-limiting examples, to the shapes represented in the drawing, and to 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.