Indirect electrohydraulic press forming tool, device and method

Abstract

An indirect electrohydraulic draw-forming tool includes a body having a tubular cylindrical area, a chamber intended to be filled with a liquid, two electrodes, each of the electrodes having a portion arranged in the chamber, a punch. The tool also includes a piston mounted so as to slide in a sealed manner within the cylindrical area and sealingly defining the chamber. Moreover, the punch is carried by the piston on a face thereof that is opposite to the chamber.

Claims

1. A tool for indirect electrohydraulic draw-forming, the tool comprising: a body having a tubular cylindrical area; a chamber formed in the body and configured to be filled with a liquid; two electrodes, each of the electrodes having a portion arranged in, or at least partially defining, the chamber; a punch; a piston mounted so as to slide in a sealed manner within the cylindrical area and sealingly define the chamber; wherein the punch is carried by the piston on a face thereof that is opposite to the chamber; and a pump for placing the liquid of the chamber under pressure, wherein the pump is configured to place the liquid in the chamber under quasi-static pressure and to initiate movement of the piston so that the punch is able to press against a blank of material with sufficient quasi-static force to stamp the blank of material.

2. The tool according to claim 1, wherein one of the electrodes comprises an end immersed in the chamber and the other electrode comprises an inner surface of the body defining the chamber.

3. The tool according to claim 1, wherein the two electrodes each have one end in the chamber.

4. The tool according to claim 1, wherein the body has an opening through which the protruding punch can exit the body.

5. The tool according to claim 4, wherein the body includes a transverse wall through which the opening extends, the tool further comprising biasing means, arranged between said transverse wall and the piston face bearing the punch, for biasing the piston away from the transverse wall.

6. A device for indirect electrohydraulic draw-forming, comprising: the tool for indirect electrohydraulic draw-forming of claim 1; and a die, the die and the punch being of generally complementary shape, said device being configured to hold the tool and the die such that the punch and the die are opposite one another and at a predetermined distance from each other to enable placement between the punch and die of a blank of material to be deformed, the blank of material being in contact with the die and being held against the die with a predetermined force.

7. A method for indirect electrohydraulic draw-forming, comprising: placing the tool for indirect electrohydraulic draw-forming according to claim 1 such that the punch is at a predetermined distance from the blank of material to be deformed; and causing the piston bearing the punch to move by means of an electric discharge generated between the at least two electrodes, wherein the liquid of the chamber is placed under quasi-static pressure before the electric discharge, the placing under quasi-static pressure being achieved by a pump, the punch being applied against the blank of material to be deformed with a predetermined quasi-static force causing the punch to stamp the blank of material.

8. The method according to claim 7, wherein the tool is placed in contact with the blank of material to be deformed with a predetermined force.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) Details and advantages of the invention will become more apparent from the following description, made with reference to the accompanying drawings in which:

(2) FIGS. 1A and 1B show a first embodiment, FIG. 1A showing the forming device prior to electrohydraulic discharge and FIG. 1B showing the device shortly after electrohydraulic discharge, and

(3) FIGS. 2A and 2B show a second embodiment, FIG. 2A showing the forming device prior to electrohydraulic discharge and FIG. 2B showing the device shortly after electrohydraulic discharge.

DETAILED DESCRIPTION

(4) FIGS. 1A and 1B show a device for indirect electrohydraulic draw-forming according to a first embodiment. The device for indirect electrohydraulic draw-forming 100 comprises a tool 110, a die 120, and a blank holder 140. A blank of material to be formed 130, here a metal sheet, is held between the blank holder 140 and the die 120. The blank holder is used to control the drawing of the blank during the indirect electrohydraulic forming. The force of the blank holder is controlled in order to avoid wrinkles on the workpiece while permitting the material to be drawn into the die, thereby limiting the elongation of the material which could lead to ruptures. In this embodiment, the tool 110 is above the die 120 and the blank of material to be formed 130. The tool 110 includes a punch 111 of a general shape complementary to that of the die 120 in order to allow insertion of the punch 111 into the die 120 while accommodating the thickness of the sheet. The punch 111 is positioned opposite a cavity 122 of the die 120. In the embodiment illustrated with reference to FIGS. 1A and 1B, the surface 112 of the punch 111 comprises reliefs substantially identical to those present on the cavity 122 of the die 120.

(5) The tool 110 includes a body 113 and a piston 114 bearing the punch 111. The body 113 includes a tubular cylindrical area 113a closed off by an upper transverse wall 113b and a lower transverse wall 113c. The lower transverse wall 113c has an opening 113d through which the punch 111 can exit the body 113. The piston 114 is mounted so as to slide in a sealed manner within the tubular cylindrical area 113a. It has a top face 114a, a bottom face 114b, and sealing means 114c, here an O-ring. The piston 114, and more particularly its bottom face 114b, rests on biasing means, here a coil spring 118 pressing against the lower transverse wall 113c. Other biasing means may be used, such as Belleville washers or elastomers having sufficient elasticity to cause the piston to return to its initial position. The punch 111 is carried by the lower face 114b of the piston. A chamber 115 is formed in the tubular cylindrical area between the upper transverse wall 113b and the upper wall 114a of the piston 114. This chamber 115 is fluidtight due to the sealing means 114c and is completely filled with a liquid such as water.

(6) The tool 110 also comprises two electrodes 116a, 116b entering the chamber 115 through sealed feed-throughs 117a, 117b. In this embodiment, the electrodes 116a, 116b are electrically insulated on their periphery. Only the ends facing each other are bare.

(7) When both electrodes 116a, 116b are subjected to a large potential difference, an electric discharge occurs between the two electrodes as shown in FIG. 1B. This electric discharge causes a shock wave which propagates in the chamber 115 and exerts strong downward pressure on the piston 114. The punch 111 then strikes or rather presses the blank of material 130 against the cavity 122 of the die 120 at a high speed. During the drawing, the tool 110 and the die 120 are held relative to each other. The device 100 therefore comprises means for holding the tool and the die so that the punch and the die are opposite one another and at a predetermined distance from the blank of material. For this purpose, the tool 110 and the die 120 are mounted between the platens of a press or of a specific frame. To avoid tearing the sheet, a blank holder is used with a controlled force in order to lock the blank in place or control its drawing and avoid the formation of wrinkles.

(8) FIGS. 2A and 2B show a device for indirect electrohydraulic draw-forming according to a second embodiment. Some elements of this device are similar to those described with reference to FIGS. 1A and 1B. These elements will therefore be numbered in the same manner but will begin with 200 instead of 100.

(9) The device for indirect electrohydraulic draw-forming 200 again comprises a tool 210 and a die 220. In this embodiment, the tool 210 is located below the die 220 and below the blank of material when the latter is held between the die 220 and the tool 210. The cavity 222 of the die is located on the lower face of the die, on the portion in contact with the blank of material 230 when the blank is held between the tool 210 and the die 220. The tool is arranged so that the punch is facing the cavity 222 of the die 220.

(10) In this embodiment, the device 200 does not comprise a blank holder as such. The tool 210, being arranged against the blank of material to be deformed so as to exert a constant and predetermined force, acts as a blank holder. The device 200 for indirect electrohydraulic draw-forming therefore comprises means for holding the tool 210 at a predetermined distance from the blank of material to be deformed with a predetermined force. The tool 210 may be in contact with the blank to be formed or to a distance of less than 1 mm, preferably 0.1 mm. The device 200 also comprises means for holding the die 220 at a predetermined distance relative to the tool and to the blank of material to be deformed.

(11) In the variant embodiment illustrated with reference to FIGS. 2A and 2B, the surface 212 of the punch 211 and the surface of the cavity 222 of the die 220 are planar. Any other shape of the surface of the cavity 222 of the die can be considered. The shape of the surface 112 of the punch may either remain flat or have a shape substantially similar to the cavity 222 of the die.

(12) As in the first embodiment, the tool 210 comprises a body 213 and a piston 214 bearing the punch 211. In this second embodiment, the body 213 has a tubular cylindrical area 213 closed off by a lower transverse wall 213b and has no upper transversal wall. The upper portion of the body 210 is therefore not closed off. The tubular end portion of the body 210 is flat and is in direct contact with the blank of material to be deformed. The piston 214 is mounted so as to slide in a fluidtight manner within the tubular cylindrical area 213a. It has an upper face 214a, a lower face 214b, and sealing means 214c. The punch 214 is carried by the upper face 214b of the piston. The piston 214 is adapted to move longitudinally towards the blank of material. In this particular embodiment, the body 213 of the tool is in contact with the blank of material. However, in an alternative embodiment, the tool 210 could include a transverse wall having an opening to allow the punch to exit the body. As in the first embodiment, the piston 214 defines a sealed chamber 215 to be filled with a liquid and the tool 210 also comprises two electrodes 216a, 216b each having one end arranged in the chamber. In another embodiment, one of the electrodes 216a, 216b comprises an end immersed in the chamber 215 and the other electrode comprises a conductive inner surface of the body 113, 213 defining the chamber 115, 215.

(13) When the two electrodes 216a, 216b are subjected to a large potential difference, an electric discharge occurs between the two electrodes as shown in FIG. 2B. This electric discharge causes a shock wave which propagates in the chamber 215 and exerts strong upward pressure on the piston 214. The punch 211 then exerts strong pressure on the blank of material 230 with high speed. In an alternative embodiment, the movement of the piston may be initiated or facilitated by placing the liquid of the chamber under quasi-static pressure, by means of a pump 219. The pump serves to inject pressurized liquid into the chamber until the desired pressure in the chamber is reached. Valves and pipes able to withstand the pressure of the liquid injected into the chamber may be used. The pressurization must be slow enough that the liquid pressure is uniform throughout the chamber, which is what is meant by placing the liquid “under quasi-static pressure”.

(14) If the quasi-static pressure is sufficient, the punch can press against the blank of material with sufficient quasi-static force to stamp the blank of material. The electrohydraulic discharge exerts dynamic pressure to accelerate the punch.

(15) The piston 214 is then returned to its initial position, illustrated with reference to FIG. 2A, by simple gravity. The water present in the chamber 215 acts as a backstop device. In another variant, the body 213 includes a backstop device stop 218′ illustrated with dotted lines in FIG. 2B. The tool shown in this second embodiment has the advantage of being lighter, since it does not include any biasing means.

(16) The electrohydraulic discharge allows moving the punch at high speed, thereby generating strong pressure at the workpiece to be formed. The drawing thus obtained has reduced springback and/or finer embossing and/or reduced form radii, and/or improved local elongation before rupture, potentially with fewer passes in comparison to conventional drawing. Finer embossing can be obtained when the surface of the punch and/or die have reliefs.

(17) The invention is not limited to the embodiments described above nor to the variants mentioned, but also relates to any embodiment in the reach of the skilled person within the scope of the following claims.