Electrohydraulic forming method and associated device

Abstract

Method for electrohydraulically forming a blank of material wherein - a blank of material to be deformed is placed between a mould and a blank holder, - a cavity containing electrodes is filled with liquid to a predetermined liquid level, - the blank of material is placed in contact with the liquid in the cavity, - a first electric discharge is generated between at least two electrodes so as to deform the blank of material against the mould, - the mould is brought nearer to the electrodes by moving the mould so as to reduce the distance between the electrodes and the blank of material to be deformed after the first electric discharge has been generated, - at least one other electric discharge is generated between at least two electrodes so as to deform the blank of material against the mould.

Description

Background of the invention

[0001] The present invention relates to an electrohydraulic forming process and an electrohydraulic forming device. Object and summary of the invention

[0002] Electrohydraulic forming allows a blank of material to be deformed against a mold by applying dynamic pressure. To achieve this, an electrical discharge is generated between at least two electrodes in a cavity filled with liquid, such as water. An electric arc is then formed between the two electrodes, causing a high temperature gradient and vaporization of the liquid. A pressure wave, also commonly called a "shock wave," travels at high speed and presses the blank of material against the mold. Electrohydraulic forming is particularly advantageous compared to other forming processes because it allows for reduced springback and improved engraving-type details, sharp angles, and / or elongation before breakage on the parts being formed.

[0003] In some cases, particularly when the parts to be formed are especially deep, several successive electrical discharges are performed. After each discharge, the material blank moves away from the electrodes. The pressure wave then propagates over a greater distance, which reduces the impact performance and the efficiency of the process.

[0004] US8844331 proposes a solution to this problem by moving the electrodes closer to the material blank after each discharge and before each new discharge by repositioning the electrodes. In this document, the electrodes are mounted on a movable part of the cavity. Due to the high voltages used to generate an electrical discharge between the electrodes, the current-carrying conductors connecting the electrodes to the pulsed voltage generators are heavy, bulky, and prone to damage from repeated movement. Currents on the order of tens or hundreds of kA flow through these current-carrying conductors. The device proposed by US8844331 to allow the movement of the electrodes, and therefore partially of the current-carrying conductors supplying them, is thus quite complex, bulky, and raises reliability issues.Document DE 100 19 594 A1 discloses a device according to the preamble of claim 1.

[0005] The present invention aims in particular to overcome the aforementioned disadvantages of the prior art.

[0006] To this end, the present invention proposes a device as defined in claim 1 and a method as defined in claim 8.

[0007] In the process according to the invention, the mold is moved and the cavity remains fixed. Thus, the current supply conductors connecting the high-voltage generator to the electrodes are not moved, which tends to limit their damage.

[0008] In one embodiment, one or more other electrical discharges are generated when the mold is brought closer.

[0009] The mold can therefore be moved continuously while successive electrical discharges are generated. This increases the number of discharges performed within a given cycle time. It should be noted that the approach speed is not necessarily constant and that the electrical discharges can occur at intervals ranging from one hundredth of a second to several seconds, depending on the approach speed, the complexity of the part being formed, and the high-voltage pulse generator used.

[0010] To achieve such long intervals between each electrical discharge, the high-voltage pulse generator can include several modules connected to one or more pairs of electrodes. When the electrohydraulic forming device has a single pair of electrodes, the different modules connected to the same pair can be triggered to generate successive discharges. When the electrohydraulic forming device has multiple pairs of electrodes, the modules connected to the different pairs can be triggered sequentially or simultaneously. When triggered simultaneously, a larger shock wave can be generated.

[0011] Advantageously, a vacuum is created between the material blank and the mold. This improves the efficiency of electrohydraulic forming.

[0012] In the device according to the invention, by having a movable mold, with the electrodes and cavity fixed, the current-carrying conductors are not displaced, which tends to limit their damage. The device according to the invention is therefore more robust and reliable.

[0013] Furthermore, it is not necessary to drain the cavity between electrical discharges or add water between each discharge. Simply adjusting the distance between the mold and the electrodes is sufficient. This saves fluid and, more importantly, reduces cycle times for part production.

[0014] Finally, positioning the blank to be formed between the mold and the blank holder is made easier since the blank is placed directly onto the holder. The mold is then lowered until it is in contact with the blank. The pressure exerted by the holder on the blank can be selected and regulated by controlling the pressure applied by the cylinder.

[0015] Furthermore, the device preferably includes a vacuum pump.

[0016] The vacuum pump creates a vacuum between the mold and the material blank in order to improve the efficiency of electro-hydraulic forming.

[0017] In one embodiment, the blank holder extends longitudinally towards the electrodes and surrounds at least part of the electrodes.

[0018] The blank holder acts as a reflector and improves forming efficiency. The blank holder also prevents shock waves from propagating towards the walls of the cavity or frame and avoids damage to them, particularly at the welds if it is made of a welded structure.

[0019] In one embodiment, the cavity is formed at least in part by the frame.

[0020] When the cavity is formed in the frame, the device is less complex and less bulky.

[0021] In one embodiment, the electrodes are supported by a base resting on a bottom wall of the frame, the cavity then being delimited by the base and by the clamp.

[0022] The cavity has a reduced volume, which saves liquid and allows the cavity to be filled more quickly.

[0023] In one particular embodiment, at least one cylinder is a gas spring.

[0024] The pressure exerted on the blank of material is then constant regardless of the position of the mold in the frame, as long as the mold is in contact with the blank of material.

[0025] In one embodiment, the electrohydraulic forming device includes a cylindrical reflector placed between the electrodes and the side wall of the frame, preferably between the electrodes and the blank holder.

[0026] Advantageously, the cylindrical reflector has a cross-section (circular, elliptical, square, etc.) adapted to that of the workpiece being formed. Such a reflector improves forming efficiency and prevents shock waves from propagating towards the walls of the cavity or frame, thus avoiding damage to them, particularly at the welds if the cavity or frame is made of a welded structure. Brief description of the drawings

[0027] Details and advantages of the present invention will become clearer from the following description, made with reference to the accompanying drawings in which: THE figures 1 to 4 illustrate different stages of an electrohydraulic forming process, the process being implemented with an electrohydraulic forming device not forming part of the invention, the figure 5 illustrates an electrohydraulic forming device according to an embodiment of the invention, the figure 6 illustrates an electrohydraulic forming device according to another embodiment of the invention, the figure 7 illustrates an electrohydraulic forming device according to yet another embodiment of the invention. Detailed description of several embodiments of the invention

[0028] There figure 1represents an electro-hydraulic forming device 100 not forming part of the invention. This electro-hydraulic forming device 100 comprises a frame 110 and a platen 120 on which a mold 130 is mounted. The platen 120, and therefore the mold 130, are movable relative to the frame 110. The platen 120 is mounted on a press fixed to the frame 110.

[0029] A blank 150 of material to be deformed is placed between the mold 130 and a blank holder 140. In the embodiment described here, the blank holder 140 is fixed to the mold 130. The frame 110 has a bottom wall 112 and a side wall 114. The bottom wall 112, the side wall 114, and the edges of the blank holder 140 define a cavity intended to be filled with a liquid, for example, water. A pumping circuit associated with a pump 180 allows the cavity to be filled with liquid. A vacuum pump 170 makes it possible to create a vacuum in the space between the mold 130 and the blank of material 150 to be deformed and in the cavity, more particularly in the space between the blank of material 150 and the blank holder 140. On the bottom wall 112 are mounted at least two electrodes 160 connected to current supply conductors, which can be, for example, cables or insulated metal plates (not shown in the drawings).These current supply conductors can be connected to an electrical generator capable of generating high-voltage pulses sufficient to cause an electrical discharge between two electrodes 160. The current supply conductors can pass in a sealed manner through the walls of the frame or pass over the edges of the walls of the frame.

[0030] In one embodiment, one of the electrodes is formed by the bottom wall 112.

[0031] Different stages of an electrohydraulic forming process with the above device are described with reference to figures 1 to 4 .

[0032] In the first step, the blank 150 to be deformed is placed between the mold 130 and the blank holder 140, and the blank holder 140 is tightened against the blank 150, for example, using screws. The cavity containing the electrodes 160 is filled with liquid to a predetermined level. Then, the lower part of the blank holder 140 is brought into contact with the liquid in the cavity, for example, either by moving the mold 130 closer to the electrodes 160 or by refilling the cavity. Next, a vacuum is created between the blank holder 140 and the blank 150. The cavity is then filled with liquid until it is in contact with the blank 150. Finally, a vacuum is created between the blank 150 and the mold 130.

[0033] In a second step, an initial electrical discharge is applied between the two electrodes 160 to create an electric arc. Since the two electrodes 160 are immersed in a liquid, such as water, the electric arc causes a strong temperature gradient until the water between the electrodes 160 vaporizes. This vaporization generates a pressure wave, also referred to as a "shock wave," which propagates through the liquid until it reaches the blank 150 to be deformed. Under the effect of the shock wave, the blank deforms against the mold, as illustrated in the figure. figure 2 .

[0034] In a third step, the mold 130 is brought closer to the electrodes 160 by moving the mold 130 so as to reduce the distance between the material blank and the electrodes as illustrated in the figure 3 Another electrical discharge is then induced between the two electrodes 160 as illustrated in the figure 4The blank material is again pressed against the mold 130 by another shock wave, and its shape becomes even closer to that of the mold. If necessary, this third step is repeated as many times as needed until the desired shape is achieved.

[0035] It should be noted that the electrical discharges between the electrodes 160 can be generated while the mold 130 is moving continuously towards the electrodes, or after the mold 130 has moved to a predetermined distance using a sequential approach. When the mold 130 is moving continuously towards the electrodes, the approach speed is not necessarily constant, and the electrical discharges can occur at time intervals ranging from one hundredth of a second to several seconds, depending on the approach speed, the complexity of the part being formed, and the high-voltage pulse generator used.

[0036] In order to achieve such time intervals between each electrical discharge, the high-voltage pulse generator may include several modules charged simultaneously and capable of discharging simultaneously and / or successively.

[0037] In one embodiment, the different modules are connected to a single pair of electrodes and can be triggered successively to generate successive discharges.

[0038] In another embodiment, the electrohydraulic forming device comprises several pairs of electrodes, and the modules connected to the different pairs of electrodes can be triggered successively or simultaneously. When triggered simultaneously, a larger shock wave can be generated.

[0039] It should be noted that by moving the mold 130 closer to the electrodes 160 between each successive electrical discharge, by shifting the mold 130, the efficiency of the electrohydraulic forming achieved by each electrical discharge is improved. Indeed, since the material blank deforms after each new discharge, with fixed electrodes and mold, the shock wave must travel a greater distance to reach the material blank and thus loses intensity.

[0040] There figure 5 represents a preferred embodiment of the device according to the invention, which is a variant embodiment of the device described with reference to the figure 1 Most of the elements of this variant embodiment are identical to those described previously. Compared to the embodiment of figures 1 to 4For example, we note the presence of shims and centering pins 125. These shims and centering pins 125 are used to guide the movement of the platen 120 when the platen 120 is mounted on a press detached from the frame 110. The shims and centering pins 125 guide the movement of the platen 120 relative to the blank holder 140 so that the lower face of the mold 130 presses firmly against the upper face of the blank holder 140. They also limit the maximum play of the platen in order to control the minimum distance between the electrodes and the blank material.

[0041] In the 100' setup illustrated with reference to the figure 5 , the electrodes 160 are arranged at a distance from the bottom wall 112, for example, by being mounted on a base 190. Alternatively, each of the electrodes or each pair of electrodes could be carried by an individual arm (not shown here).

[0042] Furthermore, the blank holder 140' extends longitudinally towards the electrodes 160. The blank holder 140' is mounted on one or more cylinders, preferably three cylinders 142, which may, for example, be gas springs. According to the invention, one end of each cylinder is fixed to the bottom wall 112 of the frame and their other end is fixed to the blank holder 140'. The pressure of the cylinder or gas spring is managed so as to control the blank holder pressure exerted on the blank 150 regardless of the position of the mold 130 inside the cavity. Since the blank holder 140' is not fixed to the mold as before, it will suffice to place the blank of material 150 to be deformed on the blank holder 140' and then lower the mold 130 so that it is in contact with the blank of material 150 to ensure that the blank of material is held against the mold 130.

[0043] In this embodiment, the hydraulic forming process is therefore similar to that described with reference to figures 1 to 4 On the other hand, the blank of material 150 is no longer held against the mold 130 by means of a blank holder 140 screwed onto the mold 130. In this variant, the blank of material 150 to be deformed is placed on the blank holder 140' then the mold 130 is lowered to rest on the blank of material 150 and the blank holder 140'.

[0044] Advantageously, when the blank holder 140' surrounds at least part of the electrodes 160, the shock waves are reflected by the blank holder and confined to the space delimited by the blank holder inside the cavity. The propagation of the shock waves towards the frame is therefore attenuated and their effectiveness in deforming the blank of material 150 is improved.

[0045] There figure 6illustrates another variant of the embodiment in which a cylindrical tube 195, hereafter called a ferrule, preferably with a cross-section adapted to the shape of the part, is placed between the electrodes 160 and the blank holder 140', so as to surround the electrodes 160. This ferrule 195 acts as a reflector of the pressure waves generated by the electrical discharge between the electrodes 160.

[0046] The ferrule 195 can also be placed between the blank holder 140 and the side wall 114 of the frame in the embodiment of figures 1 to 4 to reflect shock waves propagating towards the building 110.

[0047] There figure 7illustrates another embodiment of an electro-hydraulic forming device 200 which comprises, as in the previous embodiment, a frame 210, a movable platform 220 on which a mold 230 is mounted, a blank holder 240 for holding the blank 250 to be deformed against the mold 230. Positioning and centering pins 225 similar to those described with reference to the figure 5 are provided between the frame 210 and the moving platform 220 to guide the movement of the platform 220 when the platform 220 is moved using a press detached from the frame 210 or a hydraulic cylinder located above the platform 220.

[0048] The frame 210 has a bottom wall 212 and a side wall 214. The blank holder 240 extends longitudinally parallel to the side wall 214 of the frame 210. The blank holder 240 is mounted on one or more cylinders 242, preferably three cylinders, the cylinders being for example gas springs. One end of each of these jacks 242 is fixed to the bottom wall 212 of the frame and the other end is fixed to the blank holder 240. The electrodes 260 are mounted on a base 290 comprising, for example, three feet 292 supporting a base 294. The electrodes 260 are connected in a sealed manner through the base 294, at least one foot 292 and the bottom wall 212 of the frame to an electrical generator capable of generating brief, high-voltage pulses of high electrical power sufficient to cause an electrical discharge between two electrodes 160.The base 290, and more particularly its base 294, the blank holder 240 and the material blank 250 define a cavity intended to be filled with a liquid, for example water.

[0049] A pumping circuit connected to a 280 pump fills the cavity with liquid. This type of cavity offers the advantage of being filled more efficiently with a smaller volume of liquid compared to a similar prior art device. Furthermore, the 240 blank holder reflects a portion of the shock wave generated by the electrical discharge between the electrodes, thus reducing stress on the frame. Indeed, if the frame is subjected to frequent shock waves, it can become weakened, particularly at the welds between its various parts if the frame is constructed using a welded structure. Therefore, a frame with thinner walls can be used.

[0050] In a first step, illustrated by the figure 7The blank 250 to be deformed is placed between the mold 230 and the blank holder 240 by depositing the blank 250 onto the holder 240. The mold 230 is then lowered so that it is in contact with the blank 250. The pressure exerted on the blank 250 by the holder 240 is controlled by the cylinder(s) 242, for example, gas springs. Next, the cavity containing the electrodes 260 is filled with liquid using the pump 280, while simultaneously creating a vacuum in the cavity using the vacuum pump 270. The vacuum created facilitates the filling of the cavity and also reduces the amount of air present in the cavity, thereby improving the efficiency of the electrohydraulic forming. The cavity is filled until the material blank 150 is in contact with the liquid in the cavity. A vacuum is then created between the material blank 250 and the mold 230 using the pump 270.

[0051] Electrical discharges are then generated between the electrodes, and the mold is brought closer to the electrodes by moving the mold between each electrical discharge as described previously, particularly with reference to the figures 2 , 3 and 4 .

[0052] The various forms of electrohydroforming device design and the forming processes described above allow for increased forming efficiency from the electrical discharges between the electrodes by adjusting the distance between the electrodes and the blank to be deformed. Incorporating a mold movement relative to the electrodes simplifies the device structure compared to simply moving the electrodes, as only mechanical components need to be moved, while the electrical connections remain fixed.

[0053] The present invention is not limited to the various embodiments described and illustrated and the variants mentioned, but also relates to embodiments within the reach of a person skilled in the art within the scope of the following claims.

Claims

1. Electrohydraulic forming device (100; 100'; 100"; 200) for electrohydraulically forming a blank of material (150; 250) comprising: - a cavity capable of being filled with a liquid, - at least two electrodes (160; 260) placed inside the cavity, - a frame (110; 210), - a mould (130; 230) mounted on a plate (120; 220), capable of moving towards the electrodes (160; 260), the plate (120; 220) being mounted such that it can move relative to the frame (110; 210), characterised in that it comprises a blank holder (140; 140'; 240) capable of holding the blank of material (150; 250) to be deformed against the mould (130; 230) when the mould is moving, the blank holder (140; 140'; 240) being placed inside the frame (110; 210), and in that the blank holder (140'; 240) is mounted on at least one cylinder (142; 242), a first end of each cylinder (142; 242) being fixed to the bottom wall (112) of the frame (110), a second end of each cylinder (142; 242) being fixed to the blank holder (140'; 240).

2. Electrohydraulic forming device (100; 100'; 100"; 200) according to claim 1, characterised in that it comprises a vacuum pump (170; 270).

3. Electrohydraulic forming device (100'; 100"; 200) according to either claim 1 or claim 2, characterised in that the blank holder (140'; 240) extends longitudinally towards the electrodes (160; 260) and at least partly surrounds the electrodes (160; 260).

4. Electrohydraulic forming device (100; 100'; 100"; 200) according to one of claims 1 to 3, characterised in that the cavity is at least partly formed by the frame (110; 210).

5. Electrohydraulic forming device (200) according to one of claims 1 to 4, characterised in that the electrodes (260) are supported by a baseplate (290), the cavity thus being delimited by the baseplate (290) and by the blank holder (240).

6. Electrohydraulic forming device (100) according to claim 1, characterised in that the at least one cylinder (142; 242) is a gas spring.

7. Electrohydraulic forming device (100") according to one of claims 1 to 6, characterised in that it comprises a cylindrical reflector (195) placed between the electrodes (160) and the side wall of the frame (114), preferably between the electrodes (160) and the blank holder (140').

8. Method for electrohydraulically forming a blank of material with an electrohydraulic forming device according to claim 1, wherein - a blank of material to be deformed is placed between a mould and a blank holder, - a cavity containing electrodes is filled with liquid to a predetermined liquid level, - the blank of material is placed in contact with the liquid in the cavity, - a first electric discharge is generated between at least two electrodes so as to deform the blank of material against the mould, - the mould is brought nearer to the electrodes by moving the mould so as to reduce the distance between the electrodes and the blank of material to be deformed after the first electric discharge has been generated, - at least one other electric discharge is generated between at least two electrodes so as to deform the blank of material against the mould.

9. Electrohydraulic forming method according to claim 8, characterised in that one or more other electric discharges are generated when bringing the mould nearer the electrodes.

10. Electrohydraulic forming method according to one of the previous claims, wherein a vacuum is created between the blank of material and the mould.

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