Patent Publication Number: US-8978432-B2

Title: Multi-stage tube hydroforming process

Description:
TECHNICAL FIELD 
     The present disclosure generally relates to tube hydroforming processes, and more particularly, methods for hydroforming tubes used, e.g., in roll over protection system structures. 
     BACKGROUND 
     Sometimes it is desired to have a tubular structure made of a malleable metal, including but not limited to steel, aluminum, etc., having a specific cross-sectional profile for specifically defined applications. However, such tubes for use as tubular blanks are generally sold by suppliers having standard circular (or other) cross-sections. Accordingly, for use in specific applications, the tubes must be formed into a desired cross-section. Furthermore, in many cases, particularly where such tubes will be used in structural applications, it is important that the structural integrity of the tube is not damaged or weakened by the desired cross-sectional forming operation. 
     Some non-limiting applications in which tubular structures such as these may be used include in the auto and biking industries. Other non-limiting examples include in the forming of cab frames of mobile machines, such as earthmoving machines, excavation-type machines, mining machines, and the like. In the case of cab frames for mobile machines, these frames may be made up of dozens of separate tubes which may be welded together to produce the desired shape of the cab frame, as well as to provide the cab frame with portions meeting different dimensional and strength requirements. In applications such as these, in order for the cab frame to be strong enough so that it may provide protection to the person in the cab during a rollover, structural integrity of the tubes being used is relatively important. 
     It has been found that hydroforming is particularly useful as a relatively cost-effective way of shaping and forming tubular blanks of ductile metals into lightweight, structurally stiff and strong pieces for the applications discussed above. In particular, hydroformed tubular structures can often be made with a higher stiffness-to-weight ratio and at a lower per unit cost than traditionally formed tubes. It has been found that virtually all metals capable of cold forming can be hydroformed, including aluminum, brass, carbon and stainless steel, copper, and high strength alloys. 
     More specifically, in a traditional tubular hydroforming process, a hollow blank tube may be placed inside a half of a negative die mold that, when combined with a complementary die portion, has the cross-sectional shape of the desired resulting part. When the dies are closed, the tube ends are sealed, generally by axial punches, and the tube is filled with pressurized hydraulic fluid. The internal hydraulic pressure then causes the tube to expand against the die. After a period of time, the pressure is released allowing some of the fluid to be released from the tube. The tube ends are then unsealed, allowing egress of the remaining hydraulic fluid, the die halves are opened, and the resulting hydroformed part may be removed. 
     Traditional hydroforming processes such as this have been found useful in the past due to the fact that they generally allow for the forming of parts with a higher stiffness-to-weight ratio and at a lower per unit cost than other techniques. However, prior art tubular hydroforming processes do have some drawbacks. 
     Specifically, when hydroforming tubes having relatively high wall-thickness and/or bends having relatively tight corner radii, it has been found that conventional low pressure hydroforming processes are inadequate to achieve the desired cross-section of the finished tubes. Conversely, it has been found that conventional high pressure hydroforming processes can result in undesirable thinning of the cross-section of the finished product, thereby weakening the structural integrity of the resulting part. Further, regardless of whether conventional low pressure or high pressure hydroforming methods have been used, there have been situations where springback (the dimensional change of the deformed part after unloading caused by elastic recovery) issues have been problematic. 
     In this regard, U.S. Pub. No. 2010/0186477 A1 entitled “Method of Forming a Flanged Tubular Member in Hydroforming” discloses a tube hydroforming method whereby a combination of low pressure and high pressure hydroforming processes are used to create a final part having a flange. However, the method disclosed in that publication is not believed to resolve the wall part thinning and springback issues that can be particularly problematic in instances when the final part is required to have significant structural integrity. Additionally, the two-step process disclosed in that publication requires the use of two separate dies, which adds to cost and complexity of the part forming process, and is specifically directed at finished tubular parts having a flange thereon. 
     SUMMARY OF THE INVENTION 
     The present disclosure is directed to a method for forming a finished tube having a desired cross-section from a tubular blank using a hydroforming process. 
     Specifically, one embodiment of the present method for hydroforming a finished tube having a desired cross-section from a tubular blank in accordance herewith may include the steps of: (1) at least partially closing the die portions of a die about a tubular blank disposed in the die; (2) introducing a hydraulic fluid into the tubular blank at a first pressure; (3) substantially closing the die portions about the tubular blank for a first instance to form a first intermediate form tube; (4) partially opening the die by moving the die portions relatively away from one another to form a gap therebetween while at least initially maintaining the pressure of the hydraulic fluid within the first intermediate form tube to allow at least a partial expansion of a cross-section of the first intermediate form tube; and (5) substantially closing the die portions around the first intermediate form tube at a second instance. Optional steps included within embodiments of the present disclosure include repeating steps 4 and 5 as necessary (depending on the application) and increasing the fluid pressure of the hydraulic fluid after either step 3 or step 5. 
     Another embodiment of the present method for hydroforming a finished tube having a desired cross-section from a tubular blank in accordance herewith may include the steps of: (1) positioning a tubular blank in a die; (2) partially closing the die; (3) introducing pressurized hydraulic fluid into the tubular blank; (4) substantially closing the die a first time; (5) partially opening the die; (6) substantially closing the die at least a second time; (7) releasing the hydraulic fluid pressure; and (8) opening the die to release a finished tube. Optional steps included within embodiments of the present disclosure include repeating steps 5 and 6 as necessary (depending on the application) and increasing the fluid pressure of the hydraulic fluid after either step 4 or step 6. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-sectional view of a prior art finished tube in a die formed via a prior art low pressure hydroforming process; 
         FIG. 2  is a cross-sectional view of a prior art finished tube in a die formed via a prior art high pressure hydroforming process; 
         FIG. 3  is a cross-sectional view of a tubular blank in a die prior to any work being done thereon in accordance with an aspect of the present disclosure; 
         FIG. 4  is a cross-sectional view of an intermediate form tube in a die in a first phase of a hydroforming process in accordance with an aspect of the present disclosure; 
         FIG. 5  is a cross-sectional view of an intermediate form tube in a die in a second phase of a hydroforming process in accordance with an aspect of the present disclosure; 
         FIG. 6  is a cross-sectional view of an intermediate form tube in a die in a third phase of a hydroforming process in accordance with an aspect of the present disclosure; 
         FIG. 7  is a cross-sectional view of a finished tube in a die in a final phase of a hydroforming process in accordance with an aspect of the present disclosure; 
         FIG. 8  is a side perspective view of an intermediate form tube in a die in a third phase of a hydroforming process in accordance with an aspect of the present disclosure; 
         FIG. 9  is a side perspective view of a finished tube in a die in a final phase of a hydroforming process in accordance with an aspect of the present disclosure; and 
         FIG. 10  is a flow chart illustrating some embodiments consistent with an aspect of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     Referring first to  FIG. 1 , the result of a prior art low pressure tubular hydroforming method is shown wherein a prior art finished tube  2  does not fully conform to a die cavity  6  leaving a gap  4  due to the fact that the walls of the prior art finished tube  2  are too thick to allow the pressure of the hydraulic fluid  8  to fully deform the prior art finished tube  2 . 
     Referring to  FIG. 2 , the result of a prior art high pressure tubular hydroforming method is shown wherein a prior art finished tube  3  has a wall  9  that has thinned due to the fact that the prior art die method has worked the wall  9  of the prior art finished tube  3  too extensively prior to the application of the hydraulic fluid  8  pressure. 
     Referring to  FIG. 3 , in a method according to the present disclosure, a tubular blank  10  is shown disposed within a hydroform die  12  composed of a first die portion  14  and a second die portion  16 , wherein the die portions may represent lower, upper, left, right, etc. portions of a die  12  depending on the application and the orientation of the die  12 . The tubular blank  10  is depicted prior to any work being performed thereon and prior to the introduction of any pressurized hydraulic fluid therein. Consistent with this embodiment, the first die portion  14  and second die portion  16 , when combined, form to create a hydroforming die cavity  18  having a cross-sectional dimension as desired for a final hydroformed part. 
     Referring to  FIG. 4 , further in accordance with an embodiment of the disclosure, an intermediate form tube  11  is formed by the initial compression of the first die portion  14  and the second die portion  16 , wherein the die portions  14 ,  16  are in an intermediate form position (i.e. not completely closed) thereby creating a gap  20  therebetween. The gap  20  created in the intermediate foam position for the die portions  14 ,  16  may vary from application to application and may be dependent on the desired amount of cold work desired on the part at any particular point in the process, but, consistent with the disclosure, may be between approximately 5 mm and approximately 20 mm, or more particularly, approximately 15 mm. The ends of the intermediate form tube  11  may then be sealed in a manner and fashion as is known in the art and a hydraulic fluid  22  at a first pressure may be introduced therein. Initially, such first pressure may be a pressure in the range generally used in low pressure hydroforming processes, namely approximately between 100 and 500 bar, and more particularly, approximately 300 bar. 
     As shown in  FIG. 5 , the die  12  may then be operated such that the first die portion  14  and the second die portion  16  are moved relatively closer to one another to a substantially closed position. For example, the first die portion  14  may be closed up on the second die  16  (or the second die  16  may be closed upon the first die  14 , or the first  14  and second  16  dies may be closed upon each other) to form die cavity  18 . The hydraulic fluid  22  at a first pressure continues work on the intermediate form tube  11  by deforming the intermediate form tube  11  within the die cavity  18 . Optionally, at this time, the pressure of the hydraulic fluid  22  may be increased to a pressure more generally associated with high pressure hydroforming processes, namely approximately between 800 and 1500 bar, and more particularly, approximately 1100 bar. 
     Next, as best shown in  FIG. 6  and  FIG. 8  (either concurrent with the rise in pressure of the hydraulic fluid  22  or shortly thereafter (if utilized), the first die portion  14  and second die portion  16  may be moved relatively away from each other, such as, e.g., separated (using any of the methods discussed above) to once again provide a gap  20  between the first and second die portions  14 ,  16 . As discussed above, this intermediate form position for the die portions  14 ,  16 , may vary from application to application, and may be dependent on the desired amount of cold work desired on the finished part (as well as the size of the intermediate form tube  11 ). As above, it has been found that a gap  20  between approximately 5 mm and approximately 20 mm is operable in the context of the present disclosure, as is a gap  20  of approximately 15 mm. 
     As shown best in  FIGS. 6 and 9 , the die  12  may be finally operated to compress the intermediate form tube  11  which still includes hydraulic fluid  22  (either at the first pressure, or if utilized, at the second pressure, or even at another pressure if desired) providing the final desired cold work on the intermediate form tube  11  to form the finished tube  13  in closer conformity to the geometry of the die cavity  18 . Depending on the final finished tube  13  geometry desired (as well as other factors), these last two steps may be performed a single time or multiple times. Further, if the final compression steps are performed multiple times, the gap  20  size may be lowered on each successive compression and/or the pressure of the hydraulic fluid  22  may be modified. Specifically, the pressure of the hydraulic fluid  22  may be maintained at whatever pressure was utilized previously (either the first pressure or the second pressure) or may be adjusted to a different pressure, including, but not limited to, the first or second pressure, or some pressure therebetween. In this manner, the desired geometry for the finished tube  13  may be achieved according to the method disclosed herein. 
     As shown in  FIG. 10 , multiple embodiments of the present disclosure are illustrated. Specifically, steps of the process in accordance with one aspect of the disclosure, may include: (1) positioning a tubular blank in a die  30 ; (2) partially closing the die  32 ; (3) introducing pressurized hydraulic fluid into the tubular blank  34 ; (4) substantially closing the die a first time  36 ; (5) partially opening the die  38 ; (6) substantially closing the die at least a second time  40 ; (7) releasing the pressure of the hydraulic fluid  42 ; and (8) opening the die to release a finished tube  44 . Optional steps included within embodiments of the present disclosure include partially opening the die  46  and substantially closing the die  48  as many times as necessary (depending on the application) and increasing the pressure of the hydraulic fluid  50 ,  52  at various stages of the process. 
     It is noted that although the method of this disclosure is explained and illustrated in conjunction with a linearly extending tube it is to be understood and appreciated by a person of ordinary skill in the art that the method in accordance with this disclosure may be used in conjunction with a tube that has been pre-bent, such as in a conventional tube bending apparatus, to have one or more bends therein. 
     Similarly, while the initial blank for the tubular blank  10  shown herein is circular in cross-section in the illustrated figures, it will be understood that initial blanks of the tubular blank  10  having other initial cross-sections (including oval, square, rectangular, etc.) would be operable in accordance with the scope of the present disclosure. In accordance herewith, it is also noted that sealing of the hydraulic fluid  22  within the intermediate form tube  11  in any manner known by those of ordinary skill in the hydroforming art including, but not limited to, the use of sealing cones or sealing tubes. 
     Further, it is to be understood that the wall thickness of the tubular blank  10  may be any suitable thickness. In an embodiment consistent with the disclosure, this wall thickness may range from approximately 4 mm to approximately 10 mm, and more particularly, between approximately 6 mm and 8 mm. It is to be further understood that any suitable materials may be used to form the tubular blank  10 . For example, suitable materials include, but are not limited to high strength low alloy steel, dual phase steel, transformation induced plasticity (TRIP) steels, and Martensite steel (as well as combinations and/or alloys thereof). 
     INDUSTRIAL APPLICABILITY 
     Mobile machines, such as earthmoving machines, excavation-type machines, mining machines, or the like, may be employed for earthmoving, excavation, mining, or other operations. Such mobile machines often require an operator who sits in a cabin or cab that is connected to the machine. Often, the frame of the cab includes an integrated roll over protection system (ROPS). As its name describes, the purpose of the ROPS is to provide a structure that may prevent the cab frame and the cab from being crushed in a rollover. 
     Often times, the cab frame in a ROPS may be constructed from numerous hollow metal tubes. Each individual tube in such a structure may generally be straight and may have a constant cross section. Tubes of different lengths, having different interior and/or different exterior dimensions, may be used. In many cases, the cab frame may be made up of dozens of these separate, differently-sized tubes. Such tubes may be created using the method in accordance with the present disclosure. Furthermore, tubes created in accordance with the method of the present disclosure may be used in many other industries and for many other purposes including the automotive industry and in connection with high strength tubular bike frames and the like. 
     Further in accordance with the foregoing, it may be desired to have a process for producing final tubular parts having a desired cross-section from tubular blanks having relatively thick walls (e.g. approximately 4 mm or greater), having a high degree of structural integrity, that conform relatively closely with the die used to form the finished tube, and that do not exhibit excessive springback issues and/or wall-thinning associated with tubular parts made by prior art processes, and in particular, prior art hydroforming processes. 
     Accordingly, the method disclosed herein has been found to lessen the bending springback of the finished part resulting in an improved finished part cross-section conformance to die cavity geometry. Further, the method in accordance with the present disclosure has been found to generally improve the conformance of the corner radii of the part in question to the die cavity while maintaining desired wall-thickness and rigidity. Thus the method in accordance with the present disclosure can potentially allow for tighter corner radii to be achieved than had previously been achieved using either a conventional low pressure hydroforming process or a conventional high pressure hydroforming process. 
     The many features and advantages of the disclosure are apparent from the detailed specification, and, thus, it is intended by the appended claims to cover all such features and advantages of the disclosure which fall within its true spirit and scope. Further, since numerous modifications and variations will readily occur to those skilled in the art, it is not desired to limit the disclosure to the exact construction and operation illustrated and described, and, accordingly, all suitable modifications and equivalents may be resorted to that fall within the scope of the disclosure.