Abstract:
A method for hydroforming hollow metal workpieces includes submerging the same in a reservoir of hydraulic fluid. A press is provided having a hydroforming station located within the reservoir and includes a forming cavity configured to receive the fluid filled workpiece and create a fluid cushion spaced between the outside of the workpiece and the inside of the forming cavity. Sealing mandrels supply high-pressure hydraulic fluid to the interior of the workpiece which hydroform the same to the shape of the final part. The outward flow of fluid from the fluid cushion space is controlled, along with the associated pressure, to maintain the fluid cushion at least until such time as the mandrel pushing step concludes, thereby alleviating friction and adhesion between the formed part and the forming cavity.

Description:
CLAIM OF PRIORITY 
       [0001]    Applicants hereby claim the priority benefits under the provisions of 35 U.S.C. §119, basing said claim of priority on German Patent Application Serial No. 10 2009 030 089.9, filed Jun. 22, 2009. In accordance with the provisions of 35 U.S.C. §119 and Rule 55(b), a certified copy of the above-listed German patent application will be filed before grant of a patent. 
       BACKGROUND OF THE INVENTION 
       [0002]    The present invention relates to a method for hydroforming hollow profile elements made of a metal material using internal high-pressure hydraulics. 
         [0003]    The state of the art is to produce metal components by means of conventional hydroforming methods. Forming times are typically on the order of approx. 1.5 to 3 seconds. These production times are very long compared to so-called high-speed hydroforming (HSH). Hydroforming times using HSH methods are normally well below 0.5 seconds. Cycle times are also very different for high-speed hydroforming. While cycle times for conventional hydroforming are on the order of for example 25 seconds, cycle times for HSH methods are between 6 and 8 seconds. 
         [0004]    It is a disadvantage when using internal high-pressure hydroforming on softer materials such as for example aluminum materials because these materials tend to stick or adhere to the forming cavity. This results in aluminum accumulating on the tools. This increases maintenance costs for the hydroforming tools. Despite a theoretically faster forming speed, these advantages may be outweighed by increased tool costs, and repair downtime. 
       SUMMARY OF THE INVENTION 
       [0005]    An object of the present invention is therefore to provide a method for internal high-speed hydraulic hydroforming of hollow profile elements made of a metal material, wherein it is possible to prevent method-related material accumulation on the tool surfaces. 
         [0006]    This object is attained in a method having the features of patent claim  1 . 
         [0007]    Advantageous refinements of the present invention are the subject matter of the subordinate claims. 
         [0008]    In the present inventive method, a hollow profile element or workpiece that is to be formed is submerged in a dip tank filled with a hydraulic fluid. Thus, the hollow profile is filled not only in the hydroforming station, but in other processing stations as well. More specifically, the workpiece is filled with hydraulic fluid prior to the hydroforming station. This is how the cavity of the hollow profile element is completely filled with liquid prior to the forming process. Thus, the workpiece is transported into the actual hydroforming station through or in the hydraulic fluid. A press that has a top die and a bottom die with a corresponding forming cavity incorporates the hydroforming station. The hollow profile element, filled with hydraulic fluid, is placed in the forming cavity. After the top die has been lowered and the forming cavity is closed, the ends of the hollow profile element are closed using sealing mandrels. At the same time, internal pressure is applied for internal high-pressure hydroforming of the workpiece. 
         [0009]    Another aspect of the present inventive method is that a fluid cushion that is provided between the forming cavity and the hollow profile elements, and is maintained in a controlled manner for a period of time as the hydroforming fluid flow decreases. This control provides good lubrication when the sealing mandrels push against the hollow profile element. Therefore, hydraulic fluid is used to form a fluid cushion that must not under any circumstances be removed too rapidly. In contrast to prior high-speed hydroforming, which forms the component as rapidly as possible and brings the component to its final contour as rapidly as possible, in the present inventive method, the final contour of the workpiece is not attained at the beginning of the pushing using particularly high internal pressure, especially in those areas that the sealing mandrel is to push against. Rather, the final contour is not attained until as late a point in time as possible, more specifically, not until the insertion of the sealing mandrel has been concluded. At this moment, the fluid cushion is no longer needed. The fluid cushion should be maintained for as long as the hydroforming process is taking place, especially in those areas where the sealing mandrel pushes against the workpiece. Preferably, the thickness of the fluid cushion should decrease continuously. 
         [0010]    It would be best if the workpiece material did not come into direct contact with the tool at all while it is being pushed against. When the method is designed ideally, there is no temporal delay compared to high-speed hydroforming, which does not use such a fluid cushion. This is because the present forming process, or the pushing by the sealing mandrel, does not occur slower when the fluid cushion of the present invention is employed. 
         [0011]    It is possible to significantly reduce the friction forces between the workpiece and the forming cavity using the present invention. This has a positive effect on the force that is to be transferred via the sealing mandrel. Accumulations of adhered workpiece material are avoided. The standing time for the tools is increased and overall efficiency is improved. 
         [0012]    The inventive method exhibits its advantages in particular with materials that are softer than steel, such as for example aluminum. However, the method is also just as suitable for other metal materials, such as for example steel or even magnesium. 
         [0013]    The press used is preferably a transfer press that has automatic transport systems. The press may be either a hydraulically driven press or a mechanically driven press. It is also possible to use presses driven by servo-motors. 
         [0014]    A so-called transfer bar transports the hollow profile elements or workpieces from station to station. With the present invention, this transfer occurs entirely inside or within a hydraulic fluid bath, i.e., somewhat below the fluid level. 
         [0015]    Another process station can be used for a pre-forming station or the like in which the hollow profile element obtains a cross section that is suitable for creating a fluid cushion between the hollow profile element and the forming cavity. For instance, the hollow profile element may be given a wavy cross section, at least in those areas that are to be pushed against. This is so that there are as few points of contact as possible between the hollow profile element and the forming cavity. The goal is to create a defined fluid cushion. Therefore, the cross-sectional contour of the blank can be very different from the contour of the completed part sought through internal high-pressure hydroforming. Thus, the goal of pre-forming the hollow profile element is not to create a contour that is as close as possible to that of the finished product, but rather to create deliberate differences that facilitate forming the fluid cushion. 
         [0016]    As a rule, the hollow profile elements or workpieces prepared for internal high-pressure hydroforming are bent, or even just deformed, so that the ends are not completely even against the sealing mandrels. This necessarily results in leaks. In prior hydroforming processes, these leaks would have to be eliminated in a separate production step, wherein the pre-formed components are either compressed or trimmed. However, with the present invention, the increase in pressure during hydroforming is so high, and the corresponding flow is so great, that such leaks at the ends of the hollow profile element can be ignored, and the workpiece considered operably sealed. Therefore, it is not necessary for the end of a hollow profile element to be placed completely flat against the sealing mandrel. Because of the great excess of hydraulic fluid flow, the relatively small quantity of hydraulic fluid that escapes through leaks is negligible. High-speed hydroforming using the present invention can be performed with no problem. 
         [0017]    Thus, the intent is to pump a greater volume of hydraulic liquid into the hollow profile element during internal high-pressure hydroforming than can be accommodated therein, in addition to the hydraulic fluid already present in the filled hollow profile element. This is a function of the final contour of the completed part at the end of the internal high-pressure hydroforming process. 
         [0018]    Since from the beginning of the present process, the hollow profile element is to be surrounded as completely as possible by a fluid cushion, it is useful for the end of the sealing mandrel to be blunt. In the context of the present invention, a blunt seal shall be construed to be a sealing mandrel having an end face that is disposed perpendicular to the longitudinal direction of the workpiece without projections or depressions that are specially adapted to the inner contour of the hollow profile element. This end face, that is disposed perpendicular to the direction of advancement of the mandrel, extends across a significantly larger area than just the wall thickness of the hollow profile element to be formed. This is specifically because the shape of the hollow profile elements is not close to the final contour of the completed part, but rather is deliberately formed wavy for creating the fluid cushion, and in particular, extends at a distance from the walls of the forming cavity. Some circumferential areas of the hollow profile element are therefore displaced much farther radially outward than other areas during the internal high-pressure hydroforming. The sealing mandrel has a flat, i.e., blunt, positioning surface of a corresponding size, so that no obstructions occur in the area of the sealing mandrel. There are no special sealing mechanisms for reducing the leaks in the transition area between the sealing mandrel and the hollow profile element. This type of leak-tolerant seal has the advantage that the ends of the hollow profile element do not have to be prepared in a particular manner in order to be able to perform high-speed hydroforming using the present method. In addition, the ends of the fully hydroformed hollow profile elements do not have to be cut off. This results in material savings. 
         [0019]    It is considered advantageous when the top die displaces less than one twentieth of the quantity of fluid in which the bottom die is disposed when the forming cavity is closed. The ratio of displaced volume to bath or reservoir volume must be selected to be high enough. Very high pressures are used in the present inventive method, and fluid from leaks flows back into the fluid reservoir or bath. The flow from leaks can be damped by a corresponding quantity of hydraulic fluid, so that the hydraulic fluid does not spray out in an uncontrolled manner. To this end, the leak locations are preferably disposed deep under the fluid level of the reservoir. Additional shielding measures are also useful. 
         [0020]    Hydraulic fluid is selectively drained or metered from the fluid cushion in a controlled manner from a defined gap disposed between the top die and the bottom die. This gap borders on the forming cavity. In other words, a gap is created in the separating gap between the top die and the bottom die, and it permits exactly enough hydraulic fluid to drain off, so that only when the insertion process for the sealing mandrel has concluded, the fluid cushion has been completely removed. Alternatively, or in addition, grooves can be provided in the forming cavity, and the hydraulic fluid can be selectively drained or metered using these grooves as well. This functionally occurs towards the sealing mandrel where even larger flows from leaks may occur. 
         [0021]    The temperature of the hydraulic fluid can be controlled so that the hollow profile elements, comprised of a metal material, can be somewhat hot-formed during the present high-speed hydroforming method. Semi-hot-forming and hot-forming of metal increases formability. As the temperature of the fluid bath increases, the hollow profile element heats faster, so that the subsequent internal high-pressure hydroforming operation can also be performed at an accelerated pace. Heating the hollow profile elements in the fluid bath has the advantage that the hollow profile elements can be heated conductively with a medium that is in direct contact with the hollow profile element. This method is more effective than furnace heating because liquids conduct heat so well. 
         [0022]    These and other advantages of the invention will be further understood and appreciated by those skilled in the art by reference to the following written specification, claims and appended drawings. 
         [0023]    The invention is explained in greater detail in the following using the exemplary embodiments depicted in the drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0024]      FIG. 1  is a schematic representation of an apparatus for performing the method embodying the present invention. 
           [0025]      FIG. 2  is a section through the forming cavity of a hydroforming station. 
           [0026]      FIGS. 3   a  through  3   c  depict a segment of a longitudinal section through a forming cavity of a hydroforming station at three different times during the processing. 
           [0027]      FIG. 4  depicts a variant of the depiction in  FIG. 3   c , having leak areas on the sealing mandrel. 
           [0028]      FIG. 5  is a graph in which the movement curve for the sealing mandrel, the pressure applied for internal high-pressure hydroforming, and the thickness of the fluid cushion are plotted over time. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0029]    For purposes of description herein, the terms “upper”, “lower”, “right”, “left”, “rear”, “front”, “vertical”, “horizontal” and derivatives thereof shall relate to the invention as oriented in  FIG. 1 . However, it is to be understood that the invention may assume various alternative orientations and step sequences, except where expressly specified to the contrary. It is also to be understood that the specific devices and processes illustrated in the attached drawings, and described in the following specification, are simply exemplary embodiments of the inventive concepts defined in the appended claims. Hence, specific dimensions and other physical characteristics relating to the embodiments disclosed herein are not to be considered as limiting, unless the claims expressly state otherwise. 
         [0030]      FIG. 1  depicts a press  1  embodied as a transfer press, and having a hydroforming station  6 . The press  1  has a press table  2 . A reservoir or dip tank  3  is disposed on table  2 , and is filled with a hydraulic fluid. There are four processing stations inside dip tank  3 . The first station is a fill station  4 , which is followed by a pre-forming station  5 . Thereafter is a hydroforming station  6 , and finally a final station  7 . Raw materials  8  are transported to the fill station  4  by means of a robot  9  in the processing sequence from left to right. At the fill station  4 , the blank or raw material  8 , which constitutes a hollow profile element or workpiece  10 , is filled with hydraulic fluid, or the hydraulic fluid fills the hollow profile element  10 . Then, the hollow profile element  10  with hydraulic fluid therein is transported to the next station  5  by means of a transfer bar  11 , as shown in the  FIG. 1  drawing. Transport from one station to the next station occurs below the fluid level of tank  3  until such time as the hollow profile element  10  that has been hydroformed with internal high pressure is finally removed from tank  3  by another robot  12  at the final station  7 . The robot  12  puts the finished parts  13  away. 
         [0031]    The hollow profile element  10  is actually processed in the pre-forming station  5 , the hydroforming station  6 , and the final station  7 . To this end, the press  1  has a press ram  14 . Appropriate top dies  15  for each of the stations  5 ,  6  and  7  are arranged on the press ram  14 . A piston/cylinder unit  16  is arranged on the top die  15  of the hydroforming station  6 , and presses hydraulic fluid into the interior of the hollow profile element  10  during the internal high-pressure hydroforming. The press  1  is connected (not shown in greater detail) to a pressure control system and pressure regulator, such as that described in DE 10 2005 057 863 B3. A bottom die  17  is associated with the top die  15  in a known manner. 
         [0032]      FIG. 2  is an enlarged depiction of a cross section through the closed hydroforming station  6 . It can be seen that a hollow profile element  10 , that is to be formed, is arranged within a forming cavity  18 , which has an essentially rectangular cross-sectional shape disposed between the top die  15  and the bottom die  17 . The exterior surface of the hollow profile element  10  is in contact with the interior surface of the forming cavity  18  as little as possible, i.e., only at selected points. This is because the hollow profile element  10  has been pre-formed into a shape that creates a fluid-filled free space between the inside surface of the forming cavity  18  and the outside surface of the hollow profile element  10 . A fluid cushion  19  forms in this free space. In addition, the forming cavity  18  has grooves  20 , which in this exemplary embodiment, are disposed adjacent the center of the top die  15  and bottom die  17 , and extend generally in the longitudinal direction of the forming cavity  18 . These grooves  20  are not provided to create a contour in the hollow profile element during the internal high-pressure hydroforming process. Rather, they are provided to channel the hydraulic fluid from the fluid cushion  19  when the internal pressure “p” in the interior of the hollow profile element  10  increases during the internal high-pressure hydroforming, and the hollow profile element  10  is thereby caused to expand. In the lateral area of forming cavity  18 , the hydraulic fluid can be selectively drained or metered from the fluid cushion  19  via a gap  21  that is disposed between top die  15  and bottom die  17 . The gap  21  is so narrow that no material from the hollow profile element  10  penetrates into the gap  21  during the internal high-pressure hydroforming. The same is true of the grooves  20 . Naturally, it is also possible to provide a plurality of additional grooves  20  in the top die  15  and/or bottom die  17 . The cross section of the grooves and gaps is adapted in a particular manner, specifically such that the hydraulic fluid can only flow out of the fluid cushions  19  at a reduced, controlled flow speed. The goal is to maintain the fluid cushion  19  with a continuously decreasing quantity of fluid for a certain period of time, specifically at least until the sealing mandrel has been inserted. 
         [0033]    The cross-sectional view of  FIG. 2  depicts that stage of the present process wherein the hollow profile element  10  is only slightly expanded by the internal high-pressure hydroforming. Specifically, workpiece  10  is shown expanded only to the extent that portions of the same come to be positioned against the forming cavity  18 , without being elongated and/or experiencing a reduction in the wall thickness. The actual expansion using internal high pressure occurs in other areas, and the depicted cross-sectional contour merely indicates those portions of the aforesaid areas that are to undergo more significant expansion are being pushed against. The depicted cross section is thus especially showing deformation in those areas positioned adjacent to the sealing mandrel. The corresponding grooves  20  and gaps  21  for the fluid cushion  19  are also located there. 
         [0034]      FIGS. 3   a - c  illustrate how the hydroforming method proceeds. A longitudinal section through the forming cavity  18 , similar to  FIG. 2 , is shown. It can be seen in  FIG. 3   a  that the sealing mandrel  22  is inserted into the forming cavity  18 . Hydraulic fluid is pumped into the interior of the hollow profile element  10  via a channel  23  in mandrel  22 , and a pressure “p” builds up. In  FIG. 3   b , the sealing mandrel  22  has been urged in the direction of the arrow P 1 , in order to push against the end of hollow profile element  10 . A recess or convexity is provided in an area (not depicted in greater detail) of the forming cavity  18 . The hollow profile element  10  is to be pressed into this convexity by internal high-pressure hydroforming. Workpiece material is pushed against the end in order to prevent a reduction in the material wall thickness of the workpiece. The fluid cushion  19  is maintained in these areas of the hollow profile element  10  that are pushed against. Small quantities of the hydraulic fluid can escape from the fluid cushion  19  out of the forming cavity  18  towards the sealing mandrel  22  via grooves  20  in the top die  15  and bottom die  17 . It can also selectively escape (not shown in greater detail) from the gap  21  between the top die  15  and the bottom die  17 . The hollow profile element  10  generally floats during this phase of the hydroforming, i.e., while the sealing mandrel  22  is displaced by the path “W”, to some extent in the hydraulic fluid, and is carried by the fluid cushion  19 . The hollow profile element  10  is not forced against the inside surface of the forming cavity  18 , as depicted in  FIG. 3   c , until the pushing process is in the process of concluding or has fully concluded. At this point, the inner pressure “p” has fully expanded the hollow profile element  10 , such that the fluid cushion  19  has been removed. It can be seen that the hollow profile element  10  has not penetrated into the grooves  20 . Only the pushing process or the pushing of the sealing mandrel  22  has concluded in the depicted position. Meanwhile, the expansion of areas (not depicted in greater detail) of the hollow profile element  10  can be continued, because the inner pressure “p” is still being applied after the fluid cushion  19  has been removed. 
         [0035]      FIG. 4  depicts a hollow profile element  10 , the end of which is not positioned flat against the sealing mandrel  22 . The circled area “L” illustrates that the end face of the hollow profile element  10  is disposed a spaced apart distance from the sealing mandrel  22  in the vicinity of the top die  15 . Leaks therefore occur there. The pressure “p” for internal high-pressure hydroforming can still be effectively applied, because a very large quantity of fluid is being supplied via the channel  23  in the sealing mandrel  22 . It is to be understood that the size of the area “L” shown in  FIG. 4  is exaggerated for illustration purposes. In practice, the amount of hydraulic fluid that escapes in the area “L” is not so great that the desired hydroforming pressure “p” cannot be attained. Thus, the present inventive method can also be performed when there are leaks in the area of the sealing mandrel  22 . Therefore, the sealing mandrel  22  can have an end face that runs perpendicular to the advancing direction, without additional sealing means that would be inserted into the hollow profile element  10  to be hydroformed. Thus, even when there are larger fluid cushions, or when the spaces between the pre-formed hollow profile element  10  and the wall of the forming cavity are larger, it is possible to ensure that the hollow profile element can move uninhibited transversely to the sealing mandrel  22 , i.e., in the direction of expansion. 
         [0036]    The speed with which the fluid cushion is dissipated or removed is essential in the present inventive method, and shall be explained using the graph in  FIG. 5 . The curve K 1  represents the progression of pressure for a press  1  over time with an electronically or hydraulically controlled pressure system for hydroforming according to the prior art. The pressure begins to build at zero and climbs above the working point “A” for the internal high-pressure hydroforming method to the top dead point B 1  on the curve K 1 . Then the pressure drops again through the pressure drop point C 1  to point D 1 . 
         [0037]    Curve K 2  depicts the path, i.e., the stroke, of a mechanical press. In the press that is used here, the top die is embodied with an additional piston/cylinder unit  16  for producing pressure. After it has reached its bottom-most dead point B 1 , the press stroke moves back in the direction of the top dead point (cannot be shown in the graph) through pressure drop points C 1  and D 1 . The press is still held down between the lower dead point B 1  of the press stroke, the curve K 1 , and the pressure drop point C 1 . The press moves up starting at pressure drop point C 1 , the pressure is removed, and the press  1  opens the hydroforming tool at point D 1 . The top dead point OT (not shown) is passed through without a temporal delay. The curve K 2  depicts the pressure progression for a press as is described in DE 10 2005 057 863 B3. In that press, a pressure control system and a pressure regulator made of at least one piston/cylinder/spring unit are provided. The press is provided with another apparatus for additional production operations. Additional production operations are performed in the time window for the pressure plateau B 1 -B 2 . 
         [0038]    The curve K 3  is a movement curve for the sealing mandrel  22 . In the first movement phase, in the zero to “R” range, the hollow profile element that is to be formed provides relatively little resistance. There is an adequate fluid cushion between the hollow profile element and the forming cavity during this period of time. The R-S segment is normally the critical segment for the entire movement curve because, in this range, the hydraulic fluid drains out of the fluid cushion rapidly since the hollow profile element  10  is positioned against the forming cavity in this phase. In the S-T phase, the sealing mandrel  22  holds its position until it is finally withdrawn (T-D 2 ). 
         [0039]    The curve K 4  illustrates the thickness of a fluid cushion. It can be seen that the thickness decreases relatively quickly between the point “G” and the point “H”, and in particular approaches zero, before the sealing mandrel has completely passed through the R-S range in the movement curve. This means that the fluid flows out rapidly. There is increased friction between the workpiece and the tool, which can lead to material adhesion and the disadvantages discussed above. In accordance with the present invention, it is provided that the thickness of the fluid cushion decreases at a significantly slower rate, as is illustrated by the curve K 5 . It can be seen that the sealing mandrel has already passed through the R-S range, while the thickness of the fluid cushion has not even decreased 50 percent. It is only at point “J”, which is temporally after the end of the sealing mandrel insertion process, that the thickness of the fluid cushion approaches zero. However, at this point in time, there is no more friction between the workpiece and the tool, so that the fluid cushion is no longer needed. Thus, what is critical is that the point “J” for curve K 5  on the time axis be located to the right of the point “S”, wherein point “S” denotes the end point for the sealing mandrel insertion process. 
         [0040]    The curves depicted in  FIG. 5  are purely schematic. In practice, curves may result instead of the straight lines shown. What is essential is that the speed with which the hydraulic fluid selectively escapes the fluid cushion should be reduced substantially. The curve K 5  is therefore flatter than the curve K 4 . 
         [0041]    In the foregoing description, it will be readily appreciated by those skilled in the art that modifications may be made to the invention without departing from the concepts disclosed herein. Such modifications are to be considered as included in the following claims, unless these claims by their language expressly state otherwise.