MXPA99001827A - Hydroforming die assembly and method for pinch-free tube forming - Google Patents

Abstract

A die assembly having die structures that are cooperable to define a die cavity into which a metallic tubular blank can be disposed. A first die structure is moveable to seal the die cavity, and after the die cavity is sealed, the first and second die structures are moveable to reduce the cross-sectional area of the die cavity and thereby deform the metallic tubular blank within the die cavity.

Description

MOUNTING OF DIE FOR HYDROCONFORMATION AND METHOD FOR THE CONFORMATION OF FREE PIPE TUBE BACKGROUND OF THE INVENTION The present invention relates generally to the hydroforming of die mounting and more particularly to a hydroforming die assembly which prevents a metal tubular preform from being hydroformed from being punched during the closure of the die assembly. Hydroforming methods are commonly known as a means for forming a tubular metal preform into a tubular component having a predetermined desired configuration. In particular, the typical hydroforming operation involves placing a tubular metal preform within a hydroforming die cavity and providing a high pressure fluid to the interior of the preform to cause the preform to expand outward and conform to the surfaces that define ^ the cavity of the die. More particularly, the opposite longitudinal ends of the tubular metal preform are sealed, and high pressure water is provided through a hydroforming orifice or gate that seals one of the tubular ends. The fluid provided REF. 29579 inside the tube is pressurized by a conventional intensifier. Typically, the die assembly includes a lower die half and an upper die half. The upper die half moves down to cooperate with the lower die half to form the die cavity sealed therebetween. The tubular metal preform is placed in the lower half of the die before the upper half of the die goes down to seal the tubular preform within the cavity. For many applications, the tubular preform, which typically has a circular cross section, is hydroformed into a tubular part or component having a box-shaped or rectangular cross-section, as defined by the die cavity. Because the circumference of the tubular preform is significantly smaller than the circumferential circumference or perimeter of the surfaces defining the die cavity, it is often desirable to lightly press or deform the tubular preform within the die cavity according to the half Top of low die to seal the die cavity. The desirability of the light deformation of the tubular preform within the die cavity prior to pressurizing the riser tube, in part, of the need to shape the cross-sectional perimeter of the tubular preform closest to the cross-sectional perimeter of the circumference of the surfaces defining the die cavity to alleviate part of the need to expand or stretch the metallic material of the tubular preform during the pressurization phase of the hydroforming operation. In addition, by providing a tubular preform with a perimeter in cross section which is more closely adapted to the shape of the die cavity (which can be considered to provide some "clearance" in the metal material to facilitate its expansion and which conforms to the die cavity) facilitates the expansion capability of the tubular preform within the "hard" corners of the die cavity. One problem that arises during the deformation of the tubular preform before the closure of the die cavity is the possibility that the deformed tubular preform is folded between the upper and lower die halves as the die cavity is sealed. A solution to this potential problem is discussed in U.S. Patent No. 4,829,803. This patent discusses an arrangement in which the tubular preform must be pressurized sufficiently before the upper half of the die descends, and the outer surface of the preform must be sufficiently smoothed, so that the internal pressure within the tubular preform before the upper die half is closed is at least sufficient to overcome the frictional or frictional forces exerted on the preform by the die sections when the die sections are closed. This construction establishes a degree of critical condition on the internal pressure within the tubular preform and the uniformity of various friction surfaces. further, because the die assembly deforms the tube before the die cavity is sealed, the folding problem remains a possibility. An alternative proposal in the patent No. 5,339,667 also requires the deformation of the tubular preform prior to sealing the die cavity. Again, this generates the possibility of folding the tube when the die cavity is closed. In addition, this patent provides a die cavity with very specific contours to take into consideration the possibility of folding the tubular preform. Therefore, only limited forms of tubular components can be shaped by this process. U.S. Patent No. 5,239,852 provides another additional proposal to solve this problem. However, in this arrangement, the two die structures must meet together with a very high degree of accuracy to ensure that each of the side walls of the die cavity is in close proximity to the sealing surfaces of the structure of opposite die. In addition, this construction provides a severely sharp angle in the transition between the projection and the bead of the die structures. This corner, formed at such an acute angle, provides a relatively weak portion of the die structure which can be subjected to spalling or cracking after prolonged use. An object of the invention is to solve the difficulties of the prior art indicated above. The present invention accomplishes this by providing at least three separate die structures which cooperate to define a die cavity within which a metal tubular preform may be placed. The first die structure can be moved to seal the die cavity, and after the die cavity is sealed, the first and second die structure can be moved to reduce the cross-sectional area of the die and die cavity. This way deform the metallic tubular preform within the die cavity. Also in accordance with the present invention, two movable die structures and a single fixed die structure are provided to define the die cavity. The relative movement between the first and second movable structures seals the cavity. After the cavity is sealed, the movement of the first structure relative to the fixed die structure reduces the cross-sectional area of the die cavity to deform the metal tube in the die cavity. A further objective of the present invention is to provide a method of hydroforming a metal tube. The method comprises placing the metal tube in a hydroforming die assembly having three separate die structures, the three die structures cooperate to define a die cavity; moving the first of the die structures to seal the die cavity; then moving one of the die structures and the second of the die structures to reduce the cross-sectional area of the die cavity; and deforming the metal tube as a result of the reduction of the cross section of the die cavity. A further object of the invention is to provide a hydroforming die assembly comprising a lower die assembly that defines a lower die cavity portion into which a metal tube can be placed, the lower die assembly provides side walls that they define opposite sides of the lower die cavity portion, and a lower wall defining a lower surface of the lower die cavity; a movable upper die structure having sealing surfaces which can be moved to engage the lower die assembly on opposite sides of the lower die cavity portion to seal the lower die cavity portion and thereby provide a sealed die cavity; the lower die assembly and the upper die structure cooperate to reduce the size of the sealed die cavity to deform the metal tube after the die cavity is sealed. Other objects and advantages of the present invention will be noted in accordance with the following detailed description, appended drawings and claims. «BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is an exploded perspective view of the hydroforming die assembly according to the present invention; Figure 2 is a plan view of a longitudinal end of the hydroforming die assembly of the present invention, with the upper die structure shown in a raised or open position; Figure 3 is a plan view similar to that of Figure 2, but showing the upper die structure in an initial closed position, before the upper die structure is in a fully collapsed or closed position; Figure 4 is a cross-sectional view taken through line 4-4 in Figure 1, but showing the components completely assembled, with the upper die structure in the raised or open position, as in Figure 2; Figure 5 is a sectional view similar to that shown in Figure 4, but showing the next step in a hydroforming process in which the upper die structure is in the initial closed position, as in Figure 3; Figure 6 is a cross-sectional view similar to that shown in Figure 5, but showing the next step of hydroforming according to the present invention, wherein the upper die structure is in the fully collapsed position, and a preform tubular to be hydroformed is slightly deformed or depressed by the relative movement of the die structures that make up the die cavity, in accordance with the present invention; Figure 7 is a cross-sectional view, similar to that of Figure 6, but showing a subsequent hydroforming process in which the fluid under pressure expands the tubular preform to conform to the die cavity; and Figure 8 is a longitudinal sectional view taken through line 8-8 in Figure 1, but showing the components fully assembled, with a tubular preform placed in the lower die assembly, a pair of hydraulic hatches that they are coupled to the opposite ends of the tubular preform and the upper die structure in a raised position.

DETAILED DESCRIPTION OF THE PREFERRED MODALITIES ILLUSTRATED IN THE DRAWINGS An exploded view of a hydroforming die assembly, generally indicated with the numeral 10, according to the present invention, is generally shown in Fig. 1. The hydroforming die assembly 10 generally includes a movable die top structure 12, a movable die bottom structure 14, a fixed die structure 16 and a fixed base 18 to which the fixed die structure 16 is to be fixed, and a plurality of commercially available nitrogen spring cylinders 20 for mounting the lower die structure 14 for movement on the fixed base 18. The upper die structure 12, the lower die structure 14 and the fixed die structure 16 cooperate to define a longitudinal die cavity therebetween and - which has a substantially box-shaped cross section, as will be described with greater detail in relation to figures 5-7. Preferably, the upper die structure 12, the lower die structure 14 and the fixed die structure 16 and the fixed base are each made of a suitable steel material, such as P-20 steel. As shown in Figure 1, the upper die structure 12 has a pair of spacer or support areas 31 at opposite longitudinal ends thereof. The frame areas 31 are shaped and arranged to receive and accommodate the upper fastening structures 26 at opposite longitudinal ends of the upper die structure 12. Particularly the fastening structures 26 are each connected to the upper die structure 12 in the respective spreader areas 31 by a plurality of nitrogen spring cylinders which allow relative vertical movement between the clamping structures 26 and the structure Top 12 of die. For example, as shown in Figure 2, the nitrogen spring cylinders 27 mount the clamping structures 26 in a resilient, slightly spaced relationship with respect to the upper die structure 12. The lower die structure 14 has similar frame areas 33 at opposite longitudinal ends of the same which are constructed and arranged to adapt the lower fastening structures 28 in a similar manner. Each of the lower fastening structures 28 has an upwardly facing, generally arcuate semicircular surface extending longitudinally. The structures 34 are constructed and arranged to engage and support the lower side of the tubular preform positioned in the lower die structure. Since each of the arcuate structures 34 in the lower clamping structures 28 extend longitudinally inward toward the center points of the hydroforming die assembly 10, they transition to a substantially square or U-box surface of the configuration 36. The upper tube holding structures 26 are substantially identical to the lower holding structures 28, but inverted with respect thereto. More particularly, as can be seen from FIGS. 1-3, each upper fastening structure 26 has a downwardly facing, longitudinally arched or semicircular surface 38, which transits to a U-shaped surface configuration 39 of inverted box. The arcuate surface 38 of each clamping structure 26 cooperates with the surface 34 of one of the respective lower clamping structures 28 to form the cylindrical clamping surfaces which seal and seal the opposite ends of the tubular preform 40 when the structure Top die is initially reduced (see Figure 3). As can be seen from the cross-sectional view of Figure 4, between the upper frame areas 31 and the upper die structure 12 define a longitudinal channel 37 having a substantially inverted U-shaped cross section. The channel 37 is defined by vertical lateral surfaces 43 extending longitudinally, spaced apart, which run parallel to each other, and a longitudinally extending, generally horizontal, surface 66 between them. As can be seen from Figure 1 and from the end plan views of Figures 2 and 3, the opposite longitudinal ends of the lower die structure 14 which define the frame areas 33 have a substantial cross-section of U. However, as can be seen from the cross-sectional view of Figure 4, the lower die structure 14 has a central opening 42 therethrough between 3 longitudinal U-shaped ends. The vertical surfaces 41 interiors on the lower die structure 14 define and surround the central opening 42 mentioned above on all four sides. More particularly, a pair of longitudinally extending lateral surfaces 41 define the lateral ends of the opening 42. These surfaces are arranged vertically and in parallel, in oriented relationship with one another, as can be seen from Figures 4-7. Although not shown, it can be seen that a pair of transverse lateral surfaces 41 (not shown) define the longitudinal ends of the opening 42 and are positioned vertically in parallel, in relation oriented towards each other. It can also be seen that the four surfaces 41 provide the opening 42 with a substantially rectangular top plan view configuration. Returning now to Figure 1, it can be seen that the fixed base 18 is in the form of a substantially rectangular metal plate, and that the fixed die structure 16 is fixed to an upper surface 46 of the fixed base 18, by a plurality of bolts 44. The fixed die structure 16 is an elongated structure which extends over a substantial portion. of the length of the upper surface 46 of the fixed base 18, generally along the transverse center of the fixed base 18. The die structure 16 projects upward from the fixed base 18 and has a substantially vertical side surface 52 on opposite longitudinal sides thereof (only one such side surface is shown in Figure 1). The fixed die structure 16 also has end surfaces 54 that are substantially vertical at opposite longitudinal ends thereof (only one such side surface is shown in Figure 1). The fixed die structure 16 is constructed and arranged to extend within the opening 42 in the lower die structure 14 with minimal clearance between the generally vertical surfaces 41 defining the opening 42 and the vertical side surfaces 52 and 54 of the 16 fixed die structure. The fixed die structure 16 further includes a longitudinally extending, generally horizontal, upper die surface 56, which is constructed and arranged to extend in spaced relation to the die surface 66 extending longitudinally on the upper structure 12 of the die. die Preferably, cooperation between surfaces 41 mentioned above, the upper surface 56 and the surface 43 of the fixed die structure 16, and the lower surface 66 of the upper die structure 12 cooperate to provide a die cavity 60 having a cross-sectional configuration generally in box shape substantially throughout its longitudinal length (see Figures 5 and 6), to form a hydroformed part having a substantially closed-box cross-sectional configuration through its longitudinal extension. The die surface 56 of the die stationary structure 16 and the die surface 66 of the die top die structure 12 provides the upper and lower die surface, respectively, of the die cavity 60. Referring again to Figure 1, it can be seen that although the upper surface 56 of the fixed die structure 16 referred to above is generally horizontal, and actually has portions 62 of substantially horizontal and generally parallel end faces 62. Opposite longitudinal portions thereof, a portion 64 of arcuate downwardly extending surface is placed therebetween. Therefore it can be appreciated that the tubular hydroformed part can be provided with an irregular configuration, if desired. Figure 2 is an end plan view of the hydroforming die assembly 10, with the upper die structure 12 in an open or raised position. In this position, the hydroforming die assembly 10 allows the tubular preform 40 to be placed within the lower die structure 14. The preform 40 is preferably pre-bent in an intermediate portion thereof before it is placed in the lower die structure 14. The pre-bent configuration of the preform 40 generally follows the contour of the curved composite die surfaces 56 and 66. It can be seen from Figures 1, 4 and 5, that the tubular preform 40 to be hydroformed is suspended by the lower holding structures 28 to extend slightly above the upper surface 56 of the fixed die structure 16 when the The tubular preform 40 is first placed in the hydroforming die assembly 10. When the preform is placed in the lower die structure 14, the opposite ends of the preform 40 rest on the respective surfaces 36 of the lower holding structures at opposite ends of the lower die structure 14 (see Fig. 8). Preferably, the surfaces 36 are constructed and arranged to form an interference fit with the lower portion of the respective opposite ends of the tubular preform 40. Subsequently, the upper die structure is lowered or lowered so that the upper holding structures, which are maintained in extended position by the nitrogen cylinders 27 as shown in Figure 2, form an interference fit with the portion of the respective opposite ends of the tubular preform 40. At this point, both opposite ends of the tubular preform are retained between the jaws 26 and 28 before the upper die structure 12 comes down to its fully closed position. At this point, the tubular preform 40 is held in a substantially rigid manner to allow the hydroforming cylinders, indicated with the number 59 in Figure 8, to be telescopically and sealably inserted at both opposite ends of the tube 40, without any movement of the tube and without the need to completely lower the upper die structure 12 into its fully closed or collapsed position. The hydroforming cylinders are preferably pre-filled, but not pressurized to a large extent, and the tubular preform 40 with the hydraulic fluid (indicated by the reference character F in FIGS., 5, 6 and 7) before, or simultaneously with, the continuous folding of the upper die structure 12. Preferably, water is used as the hydraulic fluid. Although the pre-filling operation is preferred to reduce cycle times and to obtain a more uniformly contoured part, the present invention contemplates that the upper die structure 12 can be completely collapsed before any fluid is delivered internally to the tube. 40. As shown in Fig. 5, the upper die structure 12 preferably includes a pair of laterally spaced parallel ridges 70 projecting downward from opposite sides of the die surface 66 extending along the entire length of the die. the upper die structure 12. When the upper die structure 12 decreases further, after the initial engagement of the upper clamping structure 26 with the tube 40 and the lower clamping structure 28 (as shown in Figure 3), the nitrogen and gas cylinders 27 are compressed. the ridges 70 are brought into engagement with the upper die surfaces 72 of the lower die structure 12 on opposite sides of the openings 42 so as to seal the die cavity 60 (as shown in Fig. 5). The flanges 70 form a robust seal that can withstand extremely high cavity pressures in excess of 10,000 atmospheres. It may be desirable to provide similar ridges on the die surfaces 72 on opposite longitudinal sides of the aperture 42, which cooperate with the ridges 70. In any case, because the hydroforming die assembly 10 uses three (or optionally more) structure 12, 14 and 16 of die to form the die cavity 60, the folds-free hydroforming die assembly 10 in accordance with the present invention need not provide any area having a thin cross-section that may be vulnerable to the spawned or the break after several hydroforming operations. After the initial engagement of the ridges 70 with the die surface 72, the continuous movement of the upper die structure 12 downwards causes the lower die structure 14 to be forced down with it, against the force of the cylinders. of nitrogen spring on which the lower die structure 14 is mounted. The tube 40, trapped at its ends between the upper die structure 12 and the lower die structure 14, likewise moves downwards. The downward movement of the lower die structure 14 can be completed by using a cutting weight of the upper die structure 12, or by providing a hydraulic system that drives the upper die structure 12 downward. The upper die structure 12 and the lower die structure 14 continue to move downward, until such movement is stopped when the lower die structure engages with the stop structure provided by the fixed base 18. During this continuous downward movement of the upper die structure 12 and the lower die structure 14, the die surface 66 of the upper die structure 12 moves towards the die surface 56 of the fixed die structure 16, in a manner that reduces the size of the die cavity 60, and at the same time maintains a substantial peripheral seal in the cavity. Finally, the lower portion of the preform 40 moves downwardly and engages the die surface 56 of the die structure 16. After the lower portion of the preform 40 engages the die surface 56, the continuous downward movement of the die structures 12 and 14 causes the preform 40 to bend. As shown in Figure 6, when the upper die structure 12 and the lower die structure 14 are finally supported in the fully collapsed or closed position, the cavity 60 is made small enough so that the tubular preform 40 is slightly depressed. This light operation of the tubular preform is carried out so that the cylindrical tubular preform 40 can be provided with a circumference that more closely matches the final cross-sectional perimeter of the box-shaped die cavity 60. Because the tubular preform 40 is pre-filled with hydraulic fluid prior to squeezing, wrinkles in the tube are generally prevented as a result of the squeezing, and generally contoured hydroformed, generally smoothed, portions can be formed. As shown in Figure 7, after the upper die structure 12 reaches its fully collapsed position, where the lower die structure 14 engages with the fixed base 18 so that it no longer moves, the Hydraulic fluid within the depressed preform 40 is pressurized by the hydraulic system in a known manner (for example by the use of hydraulic intensifier or a high pressure pump) through one end of the tubular preform 40. Alternatively, the expansion or hydroforming of the tubular preform 40 may begin prior to the complete folding of the upper die structure 12 and thus prior to the pressing of the tubular preform 40. More specifically, the present invention contemplates that the expansion of the tubular preform 40 may begin immediately after the upper die structure 12 is brought down to the point where the sealing surface 70 thereof couples into engagement with the surface 72. of cooperating die of the lower die structure 14, as shown in Figure 5. Upon beginning the expansion at this early time, the time cycle for the entire hydroforming process can be redu In addition, because the die cavity has a larger cross-sectional area when the clamping structure 26 and the upper die structure 12 are first attached to the lower die structure 14 (see FIG. 5) compared to the moment in As the die structure 12 and the lower die structure 14 are in the fully collapsed position (see FIG. 6), this early expansion of the tubular preform allows the preform to expand radially in a vertical direction (i.e. in an oval configuration) beyond that which is possible with the upper die structure 12 in the fully collapsed position. As a result of this increased capacity of expansion, the cross-sectional circumference of the tubular preform 40 can be plain closer conformity with the final circumferential circumference with the final die cavity 60, and it becomes easier to expand the tubular preform 40 in the corners of the die cavity. In particular, because the tubular preform 40 expands to conform to its circumferential circumference as mentioned above before the tubular preform is engaged by the die surface ßß, the tubular preform can be expanded at the corners of the die cavity 60 without having to move the metal material of the preform while the outer metal surface of the preform 40 is in friction or friction engagement with the upper and lower surfaces of the die 56 and 66. As a result, the expansion in the corners of the die cavity 60 is carried out more easily, and a more uniform end portion can be formed. During the hydroforming expansion of the tubular preform 40, the fluid F is pressurized to a sufficient degree to expand the preform radially outward in accordance with the die surfaces defining the die cavity 60. Preferably, a fluid pressure of between about 2000 and 3500 atmospheres is used, and the preform is expanded in a manner that provides a hydroformed part having a cross-sectional area which is 10% or greater in comparison to the original preform. In addition, the opposite longitudinal ends of the tubular preform are pushed longitudinally inward toward each other to fill the wall thickness of the tube as it expands, as described in US Patent Application Serial No. 08 / 314,496, filed. on September 28, 1994, and incorporated herein by reference. While the preform 40 is pressurized and expanded, the upper die structure 12 continues to be driven downward to maintain the shape of the cavity 60 sealed, for example, by a hydraulically driven piston, to oppose the upward force resulting from the pressurizing the tube 40. After the tube 40 is hydroformed, the upper die structure 12 is lifted. Because the hydroformed part is driven in engagement with the peripheral die surfaces forming the cavity 60, the part can form a substantially rigid interference fit with the surfaces 41 and 43 of the upper die structure 12. In this case, the tube 40 will rise upwards with the upper die structure 12 and must be extracted therefrom. For this purpose, the upper die structure 12 is provided with an ejection structure 80, shown in Fig. 1. The ejection structure 80 is placed within a frame area in the upper die structure 12 and forms part of the die cavity 40 in a continuously contoured manner. The ejection structure 80 can be moved in a vertical direction away from its rack position in the die structure 12 to effectively eject the hydroformed part. The ejection structure can be moved by virtue of a hydraulic piston. Similarly, the lower die structure 14 can be provided with a pair of ejection structures (not shown), which are placed within the lower die structure to define part of the lateral surfaces 41 that define the opening 42 in the structure 14 of die. The ejection structures function to eject the hydroformed portion in the case where they are wedged or placed on the lower die surfaces of the lower die structure 14 after a hydroforming operation. It should be appreciated that the foregoing detailed description and the accompanying drawings of the preferred embodiment are only illustrative in nature, and that the present invention includes all other embodiments that are within the spirit and scope of the described embodiment and the appended claims. For example, although the specific illustrated embodiment provides three separate die structures which cooperate to form the die cavity, it can be appreciated that four or more die structures can be used and remain within the scope of this invention. It is noted that in relation to this date, the best method known to the applicant to carry out the aforementioned invention, is the conventional one for the manufacture of the objects or products to which it refers.

Claims (21)

1. CLAIMS Having described the invention as above, property is claimed as contained in the following: 1. A hydroforming die assembly, characterized in that it comprises: a first movable die structure; a second movable die structure; a fixed die structure; the first movable die structure, the second movable die structure and the fixed die structure cooperate to define a die cavity into which a metal tube can be placed; wherein the relative movement between the first and second movable die structures seals the die cavity; and wherein, after the die cavity is sealed, the movement of the first movable die structure with respect to the fixed die structure progressively reduces the cross-sectional area of the die cavity to deform the metal tube within the die. the die cavity.
2. 2. The hydroforming die assembly according to claim 1, characterized in that it further comprises: hydroforming orifice members constructed and positioned to provide pressurized fluid to the interior of the metal tube so as to expand the metallic tube outwardly to conform to the surfaces that define the cavity.
3. 3. The hydroforming die assembly according to claim 2, characterized in that the hydroforming orifice members are capable of relative movement therebetween to allow the hydroforming orifice members to longitudinally compress the metallic tube between them. The metal material of the metal tube flows in a longitudinal direction to fill the wall thicknesses of the tube as it is expanded. The hydroforming die assembly, according to claim 1, characterized in that the fixed die structure is received within an opening in the second movable die structure, and wherein the first die structure moves in engagement with the first die structure. the second die structure for sealing the die cavity. The hydroforming die assembly, according to claim 4, characterized in that the second movable die structure is mounted on a plurality of compressible spring members, wherein the first movable die structure moves downward in engagement with the second die structure for sealing the die cavity, and wherein the continuous downward movement of the first movable die structure after the coupling moves the second movable die structure downwardly thereof, against a deviation of the die members. spring, and wherein the downward movement of the first movable die structure and the downward movement of the second movable die structure reduces the cross-sectional area of the die cavity to deform the metal tube. 6. The hydroforming die assembly according to claim 5, characterized in that the compressible spring members comprise nitrogen spring cylinders. The hydroforming die assembly according to claim 5, characterized in that it further comprises a pair of opposed lower jaw structures mounted on the second movable die structure and constructed and arranged to engage with the lower side of the metal tube in opposite longitudinal ends thereof, and wherein the lower jaw structures suspend in metal tube in a superimposed relationship to the fixed die structure before the first movable die structure moves downwardly in engagement with the second movable die structure. . The hydroforming die assembly, according to claim 7, characterized in that the lower jaw structures are mounted on the second die structure movable by spring cylinders to allow relative movement between the lower jaw structures and the second one. Movable die structure. 9. Assembly of hydroforming die, according to claim 7, characterized in that the lower jaw structures form an interference fit with the opposite longitudinal ends of the metal tube. The hydroforming die assembly, according to claim 8, characterized in that it further comprises a pair of opposed jaw structures mounted on the first movable die structure and constructed and arranged for coupling with the upper surface of the metal tube at ends Opposite longitudinals when the first movable die structure moves in engagement with the second die structure, the opposing jaw structures mounted on the first movable die structure cooperate with the lower jaw structures mounted on the second movable die structure to retain the outer surface of the metal tube at opposite ends. 11. A hydroforming die assembly, characterized in that it comprises: a first die structure; a second die structure; a third die structure; the first die structure, the second die structure and the third die structure cooperate to define a die cavity into which a metal tube can be placed; the first die structure can be moved to seal the die cavity; and wherein, after the cavity is sealed, the first and second die structures can be moved to reduce the cross-sectional area of the die cavity to deform the metal tube within the die cavity. 12. The hydroforming die assembly according to claim 11, characterized in that the second die structure remains stationary as the first die structure moves to seal the die cavity. 13. The hydroforming die assembly according to claim 12, characterized in that the third die structure remains fixed as the first and second die structures move to progressively reduce the cross-sectional area of the die cavity. The hydroforming die assembly according to claim 11, characterized in that it further comprises: hydroforming orifice members constructed and arranged to provide pressurized fluid to the interior of the metallic tube so that it expands the metallic tube outwardly so as to be according to the surfaces that define the cavity. The hydroforming die assembly, according to claim 14, characterized in that the relative movement between the hydroforming orifice members longitudinally compresses the metal tube therebetween so that metallic material flows from the metallic tube in a longitudinal direction to Fill the wall thicknesses of the tube as it expands. 16. The hydroforming die assembly according to claim 11, characterized in that the die structure is received within an aperture in the second moveable die structure, and wherein the first die structure moves in engagement with the die structure. second die structure to seal the die cavity. 17. The hydroforming die assembly according to claim 16, characterized in that the second die structure is mounted on a plurality of compressible spring members, wherein the first die structure moves downwardly in engagement with the second die structure. die so that the continued movement downwardly of the first die structure after engagement, moves the second die structure downwardly therewith, against a deviation of the spring members, and wherein the continued downward movement of the The first die structure and the downward movement of the second movable die structure reduce the cross-sectional area of the die cavity. 18. The hydroforming die assembly according to claim 17, characterized in that the compressible spring members comprise nitrogen spring cylinders. A method for hydroforming a metal tube, characterized in that it comprises: placing the metal tube in a hydroforming die assembly having three separate die structures, the three die structures cooperate to define a die cavity; moving the first of the die structures to seal the die cavity; then moving the first of the die structures and the second of the die structures to reduce the cross-sectional area of the die cavity; and deforming the metal tube as a result of reduction in the cross-sectional area of the die cavity. The method according to claim 19, characterized in that it further comprises: providing pressure fluid to the interior of the metal tube before deforming the metal tube so that an internal support is provided to the metal tube as it deforms. 21. A hydroforming die assembly, characterized in that it comprises: a lower die assembly defining a lower die cavity portion into which a metal tube can be placed, the lower die assembly providing side walls defining opposed sides of the lower die cavity portion, and a lower wall defining a lower surface of the lower die cavity portion; an upper movable die structure having a sealing surface which can be moved to couple the lower die assembly on opposite sides of the lower die cavity portion to seal the lower die cavity portion and thereby provide a cavity of sealed die; the lower die assembly and the upper die structure cooperate to reduce the size of the sealed die cavity to deform the metal tube after the die cavity is sealed.