Patent Publication Number: US-2021187578-A1

Title: Device for incremental stamping by magnetic forming and associated method

Description:
FIELD OF THE INVENTION 
     The present invention pertains to the field of forming, more particularly stamping. 
     The present invention relates to a device and a method for stamping blanks by magnetic pulse for producing stamped parts, in particular parts called deep stamped parts. 
     BACKGROUND OF THE INVENTION 
     In the field of forming, in particular metal forming, stamping is a very often selected method because it is robust and well mastered. 
     Stamping is commonly used in industry, in particular in the automotive industry, in particular to form trim panels, such as a motor vehicle bonnet or door, because of the high admissible production rates. 
     Stamping is a forming method consisting in obtaining by plastic deformation of a blank, under the action of pressure, a part of more or less complex shape. 
     The stamping device for implementing this method consists essentially of a die and a punch of almost complementary shape, between which the blank is positioned. The shape is obtained by driving the blank under the action of the punch in the die. The movement of the blank is generally controlled by a blank holder, imposing a retaining pressure thereon, in order to reduce the appearance of wrinkles or tears on the final stamped part. 
     However, in the presence of a part which is difficult to shape, in particular a deep stamped part, the choice of the clamping force to be applied on the blank holder proves to be difficult. If the force of the blank holder is too high, the wrinkles are removed but the risk of tearing is high. If the force of the blank holder is too low, the risk of wrinkles is high. 
     To produce deep stamped parts, alternatives to the stamping method are known. 
     Among them, the hydraulic forming method can be mentioned. In this method, the blanks are formed by the action of a pressurised fluid. 
     The associated hydraulic forming device consists of a hermetically closed enclosure formed in two portions including a hollow mould having a recess complementary to the shape of the part desired to be obtained. The blank is placed inside the enclosure. A hydraulic pressure is exerted thereon pressing it against the recess of the mould. This quasi-static shaping method has the major advantage of dispensing with punching and producing complex shapes, which are in particular undercut. However, high pressures are essential for forming, which tends to require large tonnage presses for large parts. Therefore, this method is mainly used for the forming of tubular parts. One of the disadvantages of the method is the cycle time, often several tens of seconds due to the filling and pressurisation times. 
     Among the existing hydraulic forming methods, mention may be made of the Electro Hydraulic Forming method, known as the EHF method, which is, in turn, a high speed deformation method. Such a method has many advantages, in particular a significant reduction in elastic return and increased formability of metals. However, the major disadvantages lie in the need to bring the part to be formed into contact with water (corrosion possible and drying necessary) and water management. 
     Mention may also be made of hot forming methods, such as the Superplastic Forming method, known as the SPF method. This method is based on the ability of some alloys, for example titanium, to withstand significant deformation. These alloys, hereinafter called superplastic alloys, can reach elongations sometimes going beyond 1000% under certain conditions of temperature, of pressure and deformation while the conventional alloys generally deform only within a typical range of a few % to 50%. 
     The associated SPF forming device consists of a hermetically closed enclosure formed in two portions including a hollow mould having a recess complementary to the final external geometry of the part desired to be obtained. The blank is placed inside the enclosure and fixedly maintained between the two portions. A pressurised gas is injected into the enclosure and presses the blank, while deforming it, against the recess. The pressure and temperature, of the order of 900° C. for titanium alloys, must be perfectly controlled. 
     The obvious disadvantages associated with this SPF forming device and the associated method lie in the cycle time, the cost and the fact that only some materials can be used. 
     OBJECT AND SUMMARY OF THE INVENTION 
     The object of the present invention is in particular to overcome all or part of the limitations of the solutions of the prior art, in particular those set out above, by proposing a solution which allows obtaining stamped parts, and in particular deep stamped parts. 
     To this end, the invention aims firstly at a device for stamping a blank for producing a stamped part including:
         a punch including a bearing surface,   an anvil,   a die,   magnetic field generation means, disposed in the punch, at the bearing surface.       

     The term blank means a thin plate, in particular made of metallic material. A plate is said to be thin when one of its dimensions is significantly smaller than the other two, typically at least an order of size. 
     In an initial position, that is to say before the stamping phase, the stamping device is configured such that:
         the bearing surface of the punch is intended to receive a portion of a first face of the blank,   the anvil and the magnetic field generation means are intended to be disposed on either side of the same portion of the blank.       

     The magnetic field generation means are facing the first face. The anvil is facing a second face of the blank, opposite the first face, at a distance from said second face. The die is intended to be disposed facing the second face, at another portion of the blank. 
     The magnetic field generation means are intended for and configured to apply on the blank a pressure in the direction of the anvil, in a direction Z′Z. 
     The stamping device includes first displacement means arranged to displace the punch, relative to the die, in the direction Z′Z. The punch is advantageously displaced in translation. 
     The stamping device includes second displacement means arranged to displace the anvil, relative to the die, in the direction Z′Z. 
     The punch, the anvil and the die are preferably made of a metallic material to contain the high pressures generated by the magnetic field generation means. 
     The stamping device according to the invention differs from conventional stamping devices in that the stamping is not carried out by the punch itself, but by the magnetic field generation means. 
     Similarly, the magnetic field generation means are used differently from the conventional framework of a magnetic forming method which forms the entire blank at once. The magnetic field generation means are arranged so as to generate magnetic pulses only on a portion of the blank. The relative displacement of the punch relative to the generation means allows displacing the area of the blank which will be affected by the magnetic pulses. 
     Such a device thus advantageously allows to work at high speed of expanding deformation, with the advantages that the magnetic forming can bring, such as obtaining radii of curvature less than 2 mm, fine engravings, or tight tolerances, as well as the avoidance of cracking or tearing of the material in areas with high elongation, in particular for aluminium. 
     Such a stamping device is thus particularly adapted for producing stamped parts, in particular deep stamped parts, without generating tears in the part. 
     It is also adapted for making flanging edges, with the advantages of the magnetic forming mentioned above. 
     According to preferred embodiments, the invention also meets the following features, implemented separately or in each of their technically operative combinations. 
     According to preferred embodiments, the first displacement means include a linear actuator. 
     According to preferred embodiments, in order to obtain good efficiency of the method, the magnetic field generation means are in the form of a flat coil, for example in a spiral. The coil is disposed substantially in a plane parallel to the bearing surface of the punch. 
     According to preferred embodiments, the stamping device includes a blank holder configured to maintain the other portion of the blank against the die, to impose a retaining pressure on the movement of the blank, against the die in order to limit the formation of wrinkles. 
     The invention also relates to a method for stamping a blank by magnetic pulse for producing a stamped part, from a stamping device in accordance with at least one of its embodiments. The method includes the steps of: 
     a) positioning the blank in the stamping device, 
     b) subjecting the blank to a magnetic field caused by the magnetic field generation means so that a pressure is exerted on the first face of the blank in the direction Z′Z and presses said blank against the anvil, 
     c) displacing the punch by the first displacement means and the anvil by the second displacement means, relative to the die, in the direction Z′Z, 
     steps b) and c) being repeated, preferably in a synchronised manner, until the desired shape for the finished stamped part is obtained. 
     Synchronised means that the steps are carried out either successively, one after the other, or simultaneously. 
     As the punch displaces, a magnetic pulse is generated by the magnetic field generation means, exerting, on the one hand, an axial pressure on the blank in the direction of the anvil, pressing said blank on said anvil, and on the other hand, a radial pressure on the blank in the direction of the die, pressing said blank on said die. 
     This axial and radial double pressure advantageously allows deforming the blank, without the risk of generating tears in the part desired to be obtained and with excellent shaping precision. 
    
    
     
       PRESENTATION OF THE FIGURES 
       The invention will be better understood upon reading the description below, given by way of non-limiting example, and made with reference to the figures which show: 
         FIGS. 1 to 4 , schematic sectional views of an embodiment of a stamping device according to the invention showing the successive steps of stamping a blank, 
         FIG. 5 , a schematic view equivalent to  FIG. 1  illustrating a particular embodiment of the stamping device with a blank holder. 
     
    
    
     In these figures, identical references from one figure to another designate identical or similar elements. For the sake of clarity, the elements shown are not to scale, unless stated otherwise. 
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     A stamping device  10 , as illustrated in  FIGS. 1 to 4 , is intended for stamping blanks  50 , in order to produce stamped parts, in particular deep stamped parts. 
     In an exemplary embodiment, the blanks  50  are made of a metallic material, such as steel. 
     The blank  50  has a first face  51  and a second face  52 , opposite the first one. 
     In a preferred non-limiting exemplary embodiment of the invention, the stamping device  10 , as shown in section in  FIGS. 1 to 4 , is adapted for producing buckets. Bucket means a stamped part having a hollow cylindrical shape, with or without a flanged rim. 
     The person will easily understand that the teaching of the present invention can be transposed to other embodiments. 
     In the present description, the terms such as upper, lower, high, low, left, right are used for the sake of simplicity, with reference to the orientation of the various elements presented in  FIGS. 1 to 4 . However, unless otherwise indicated, these terms characterise only the relative disposition of these elements, after a possible imaginary rotation relative to the effective orientation of the assembly in space. 
     The stamping device  10  includes a first frame  20  and a second frame  30 . The first frame  20  may be a lower portion of the stamping device and the second frame  30 , an upper portion, as illustrated in the figures. Alternatively, and without departing from the scope of the invention, the first frame  20  may be an upper, left or right portion, of the stamping device and the second frame  30 , respectively a lower, right or left portion. 
     First Frame 
     The first frame  20  is in the form of a first hollow body delimiting a first open cavity  23 , which is preferably central. 
     In a particular embodiment, the first open cavity  23  has a cylindrical shape, preferably of circular cross section. 
     As illustrated in  FIGS. 1 to 4 , the first hollow body is formed of a lower portion  21  and a side portion  22 , an inner wall  221  of which delimits the first open cavity  23 . 
     A punch  40  is disposed in the first open cavity  23 . 
     In the particular example where the desired final stamped part is a bucket, the punch  40  is in the shape of a cylindrical body, preferably of circular cross section, and of longitudinal axis Z′Z. 
     In the example of  FIGS. 1 to 4 , the longitudinal axis of the punch  40  is a vertical axis, preferably coincident with a longitudinal axis of the first open cavity  23 . 
     The punch  40  is preferably full and made of a material capable of containing the high pressures generated by the magnetic field generation means, for example a metallic material. 
     Said punch includes a bearing surface  41  and a side surface  42 . The bearing surface  41  is intended to receive a portion of the blank to be stamped. The side surface  42  is intended to be facing the inner wall  221  of the side portion  22  of the first hollow body, when said punch is positioned in the first open cavity  23 . 
     The punch  40  is movable in the first open cavity  23 . The punch  40  is movable in translation along its longitudinal axis Z′Z between:
         a retracted position, wherein said punch is located in the first open cavity  23 , and   a deployed position, wherein said punch is located outside the first open cavity  23 .       

     The punch  40  displaces in a direction Z′Z, to its deployed position. The punch  40  displaces in a direction ZZ′, towards its retracted position. 
     In the example of  FIG. 4 , the punch  40  is illustrated in the deployed position. 
     First displacement means  43  are configured to displace the punch  40  between the retracted position and the deployed position. The first displacement means  43  are actuated manually or automatically. 
     In an exemplary embodiment, the first displacement means  43  include at least one linear, hydraulic or pneumatic actuator, such as a cylinder operating between the first frame  20  and the punch  40 . In this exemplary embodiment, preferably, the fixed portion of the linear actuator—for example the cylinder body—is housed in the first open cavity  23  formed in the first frame  20 . The movable portion of the linear actuator, for example the cylinder piston, is capable of displacing out of the first open cavity  23  for deploying the punch  40  in the direction Z′Z and capable of displacing to the first open cavity  23  in the direction ZZ′ to return the punch  40  to its retracted position. In a particularly advantageous manner, control means control the first means  43  for displacing the punch. 
     In a variant embodiment, the first displacement means  43  are in the form of a support carrying a thrust screw capable of cooperating with the punch  40  to displace it in translation along the longitudinal axis Z′Z. 
     The stamping device  10  also includes magnetic field generation means  60 . 
     The magnetic field generation means  60  are disposed, at the bearing surface  41  of the punch  40 , inside said punch. 
     The magnetic field generation means  60  are configured to create a magnetic field concentrated in a delimited space and over a very short period, as will be described later. 
     In a preferred exemplary embodiment, the magnetic field generation means  60  are in the form of a flat coil, for example in a spiral. The flat coil is preferably disposed in a plane substantially parallel to the bearing surface  41  of the punch  40 . 
     The magnetic field generation means  60  preferably form an integral portion of an assembly which further includes an electrical energy storage unit and one or more switches (not shown). 
     The electrical energy storage unit is configured for and intended to store moderate energy, for example of the order of a few kilojoules to a few tens of kilojoules (kJ). 
     In a preferred exemplary embodiment, the storage unit is a battery of discharge capacitors. 
     Second Frame 
     The second frame  30  is in the form of a second hollow body delimiting a second open cavity  33 . 
     In a particular embodiment, the second open cavity  33  has a cylindrical shape, preferably of circular cross section. 
     As illustrated in  FIGS. 1 to 4 , the second hollow body is formed of an upper portion  31  and a side portion  32 , an inner wall  321  of which delimits the second open cavity  33 . 
     The second frame  30  is arranged relative to the first frame  20  so that the second open cavity  33  is intended to receive the punch  40 , when the latter displaces along its longitudinal axis Z′Z, in the direction Z′Z, to its deployed position. A free end  322  of the side portion  32  of the hollow body of the second frame  30  is substantially facing a free end  222  of the side portion  22  of the first hollow body of the first frame  20 . 
     The second hollow body and the punch  40  have dimensions such that the punch  40  can displace, in translation, freely in the second open cavity  33  and allow the passage of the blank  50 , in its thickness, between the inner wall  321  of the second open cavity  33  and the side surface  42  of the punch  40 . 
     In a preferred embodiment, the side surface  42  of the punch  40  and the side wall  32  of the second hollow cavity  33  are of almost complementary shape, except for the thickness of the final stamped part and an operating clearance. 
     In a non-limiting exemplary embodiment, when the punch  40  is in the shape of a cylindrical body, of circular cross section, the second open cavity  33  is cylindrical, of circular cross section, with a diameter greater than the external diameter of the punch. 
     In the example of  FIGS. 1 to 4 , and preferably, a longitudinal axis of the second open cavity  33  is coincident with the longitudinal axis of the punch  40 . 
     The stamping device further includes an anvil  70 . 
     The anvil  70  is housed in the second open cavity  33 . Said anvil is movable in the second open cavity  33 . The anvil is movable in translation along the longitudinal axis Z′Z, Second displacement means  72  are configured to displace the anvil  70  in translation in the second open cavity  33 . 
     The second displacement means  72  are actuated manually or automatically. 
     In an exemplary embodiment, the second displacement means  72  include at least one linear, hydraulic or pneumatic actuator, such as a cylinder operating between the second frame  20  and the anvil  70 . In this exemplary embodiment, preferably, the fixed portion of the linear actuator—for example the cylinder body—is housed in the second open cavity  33 . The movable portion of the linear actuator, for example the cylinder piston, is capable of displacing in the second open cavity  33  to displace the anvil  70  in said second cavity. In a particularly advantageous manner, control means control the second displacement means  72  of the anvil  70 . 
     In a variant embodiment, the second displacement means  72  are in the form of a support carrying a thrust screw capable of cooperating with the anvil  70  to displace it in translation, along the axis ZZ′. 
     In a preferred embodiment, the first  43  and second  72  displacement means are similar. 
     The stamping device further includes a die  80 . 
     The die  80  is disposed at the free end  322  of the side portion  32  of the second frame  30 . The free end  322  of the side portion  32  of the second frame  30  forms a lower surface  81  of the die  80 . 
     In a preferred embodiment, the side portion  32  of the second hollow body of the second frame  30  forms the die  80 . 
     The anvil  70  and the die  80  are preferably made of a metallic material, for example steel, having sufficient structural strength allowing to contain the high pressures generated by the impact of the blank  50  on said anvil and said die, during the stamping method which will be described later. 
     The punch  40 , the anvil  70 , the die  80  and the magnetic field generation means  60  are disposed relative to each other so that, in an initial position of the stamping device  10  ( FIG. 1 ), that is to say before starting the stamping method, the blank  50  is positioned flat between said various elements. 
     More specifically, the punch  40  is positioned so that its bearing surface  41 , in a retracted or intermediate position of said punch, is intended to receive a portion of the first face  51  of the blank  50 . In the example illustrated in  FIG. 1 , the bearing surface  41  of the blank is intended to receive a central portion of the first face  51  of the blank  50 . The anvil  70  and the magnetic field generation means  60  are disposed on either side of the same portion of the blank  50 . The magnetic field generation means  60  are facing the first face  51  of the blank  50 . A lower surface  71  of the anvil  70  is facing the second face  52  of the blank  50 . 
     The lower surface  71  of the anvil  70  is disposed opposite the second face  52  of the blank  50 , at another portion of the blank. In the example illustrated in  FIG. 1 , the lower surface  81  of the die  80  is disposed opposite the second face  52  of the blank  50 , at a peripheral portion of said blank. 
     The lower surface  71  of the anvil  70  is disposed at a distance from the second face  52  of the blank  50 . 
     Preferably, as illustrated in  FIG. 1 , the magnetic field generation means  60 , as positioned, are thus able and intended to apply on the blank  50  a pressure mainly in the direction of the lower surface  71  of the anvil  70 , in the direction Z′Z. 
     In one embodiment, illustrated in  FIG. 5 , the stamping device  10  includes a blank holder  90 . The blank holder  90  is housed between the free ends  222 ,  322  of the side portions  22 ,  32  of the first and second hollow bodies. It is configured so that the blank is compressed between the blank holder and the die, at the free end  322  of the side portion  32  of the second hollow body, when said blank is in position on the punch  40 . The adjustment of the force of the blank holder and/or of the clearance between the blank holder and the die will condition the swallowing of the blank during its shaping by preventing or limiting the formation of wrinkles. 
     In an exemplary embodiment, the blank holder  90  is maintained pressed against the free end  322  of the side portion  32  of the second frame  30  by compression means such as for example gas springs. 
     An example of a stamping method is now described. 
     The blank  50  is intended to conform to the shape of the lower surface  71  of the anvil  70  and to the inner wall  321  of the side portion  32  of the second frame  30  to form a deep stamping, of the bucket type. The bucket obtained may or may not include a flanged rim. 
     In a prior step, the blank  50  is cut, to the desired dimensions (length and width, or diameter, and thickness), from a sheet. 
     In a first step, called step a), the blank  50  is positioned in the stamping device  10 . 
     The blank  50 , of substantially flat shape, is positioned between the first frame  20  and the second frame  30 , as illustrated in  FIG. 1 . 
     In one embodiment, the blank  50  is disposed on the one hand, at its central portion, on the punch  40 . The blank  50  is disposed so that its first face  51  bears against the bearing surface  41  of the punch. 
     The blank  50  is disposed so that its second face  52  faces the lower surface  71  of the anvil  70 . The lower surface  71  of the anvil  70  is positioned at a distance e from the second face  52  of the blank  50 . 
     The distance e defines the desired depth for the deformation of the blank at each discharge (described below). The distance e is maximised so as to reduce the number of discharges and consequently the forming time. 
     The blank  50  is disposed on the other hand, at its peripheral portion, between the lower surface  81  of the die  80 , therefore the free end  322  of the side portion  32  of the second frame  30   i , and the free end  222  of the side portion  22  of the first frame  20 . The second face  52  of the blank  50  is disposed facing the die  80 , at a distance therefrom. The first face  51  of the blank  50  is disposed facing the free end  222  of the side portion  22  of the first frame  20 . 
     In an exemplary implementation, when the blank  50  was deposited on the punch  40 , said punch is displaced, from its retracted position, in translation along the direction Z′Z, to offset the blank  50  so that the second face  52  of said blank, at the peripheral portion of the blank, is placed in the immediate vicinity, for example of the order of a millimetre, of the free end  322  of the side portion  32  of the second frame  30 , therefore of the lower surface  81  of the die  80 . 
     When the stamping device  10  includes a blank holder  90 , the blank  50  is maintained bearing against the free end  322  of the side portion  32  of the second frame  30  by said blank holder. 
     The method then includes a second step, called step b), of deforming the blank  50  by magnetic forming. 
     The central portion of the blank  50 , located in the vicinity of the magnetic field generation means  60 , is subjected to a magnetic field originating from said magnetic field generation means  60  so that an axial pressure is exerted against the first face  51  of the blank  50 , and tightly presses said blank against the lower surface  71  of the anvil  70 . The arrow illustrated in  FIG. 2  represents the direction of the axial pressure exerted on the blank  50 . 
     Consequently, the blank  50  deforms to bear against the lower surface  71  of the anvil  70 . 
     During this step b) shown in  FIG. 2 , the magnetic field generation means  60  progressively deforms the central portion of the blank  50  so as to obtain a first stamp of depth P 1 , less than a depth P of the final stamp sought. The peripheral portion is pressed against the free end  322  of the side portion  32  of the second frame  30 , therefore against the lower surface  81  of the die  80 . 
     The anvil  70  advantageously allows limiting the impact of the discharge on the blank and avoiding a tearing thereof. 
     At the end of this step b), the blank  50  is deformed and has a first stamp. 
     In a third step, called step c), the punch  40  and the anvil  70  are displaced. 
     The punch  40  is displaced by the first displacement means  43 , in the direction Z′Z, until the bearing surface  41  of the punch is pressed again against the first face  51  of the blank  50 , so that the central portion of the blank  50  returns in the immediate vicinity of the magnetic field generation means  60 . The peripheral portion of the blank  50  is maintained at a distance from the die  80 , as illustrated in  FIG. 3 . 
     The displacement of the punch  40  is carried out in the same direction as the direction of displacement of the central portion of the blank  50  during step b). 
     The anvil  70  is displaced by the second displacement means  72 , in the direction Z′Z. 
     In an exemplary implementation, the relative displacement of the punch  40  and the anvil  70  relative to the die  80  is carried out incrementally, preferably simultaneously. 
     The displacement of the punch  40  and that of the anvil  70  is not necessarily of the same amount. 
     The anvil  70  is displaced in the direction Z′Z by a sufficient distance so as to define the desired depth for the incremental deformation of the blank. 
     In an exemplary implementation, the relative displacement of the punch  40  and the anvil  70  is carried out continuously. The shaping of the blank  50  by the magnetic field generation means  60  can be considered instantaneous relative to the displacement of the punch  40  and to that of the anvil  70 . Indeed, the duration of the displacement of the punch  40 , and that of the displacement of the anvil  70 , is generally very slow (of the order of a second) compared to the duration of the magnetic pulse generated by the magnetic field generation means  60  (of the order a few micro seconds). In the particular case of this embodiment, the second and third steps are carried out simultaneously without modifying the result of said steps. 
     In a fourth step, steps b) and c) are reproduced sequentially. 
     Steps b) and c) are repeated until obtaining the depth P of the final stamped part desired to be obtained. 
     As the punch  40  and the anvil  70  relatively displace relative to the die  80 , said magnetic field generation means  60  advantageously exert an axial pressure on the central portion of the blank  50  in the direction of the anvil  70 , pressing the central portion of said blank on said anvil. Said magnetic field generation means  60  also exert radial pressure on the blank  50  in the direction of the die  80 , against the inner wall  321  of the side portion  32  of the second frame  30 , pressing said blank against said inner wall. The horizontal arrows illustrated in  FIGS. 3 and 4  show the direction of the radial pressure exerted on the blank  50 . This radial pressure of the blank  50  against the inner wall  321  of the side portion  32  of the second frame  30  advantageously allows said blank to perfectly match the shape of said inner wall. 
     The number of iterations of steps b) and c) depends in particular on the material constituting the blank, on the desired depth of the stamped part. 
     At the end of the stamping method, the blank became a deep stamped part, of the bucket type, with or without a flanged rim. 
     In the example of  FIG. 4 , the bucket is of the type of bucket without flanged rim. 
     The present invention is not limited to the preferred embodiments described above by way of non-limiting examples and to the mentioned variants. It also relates to the variant embodiments within the reach of the person skilled in the art. 
     The above description clearly illustrates that by its various features and their advantages, the present invention achieves the objectives which it had set itself. Particularly, it provides a stamping device adapted for producing stamped parts, in particular deep stamped parts, without generating wrinkles or tears in the part. Such a stamping device and the associated stamping method allows working the part at high speeds, and advantageously deforming the blank without the risk of generating tears in the part desired to be obtained and with excellent shaping precision. 
     The invention also advantageously allows producing flanging edges.