Patent Publication Number: US-2011067467-A1

Title: Method and tool for contracting tubular members by electro-hydraulic forming before hydroforming

Description:
BACKGROUND 
     1. Technical Field 
     The present invention relates to electro-hydraulic forming to contract a tubular member in a die. 
     2. Background Art 
     In electro-hydraulic forming (“EHF”), an electric arc discharge is used to convert electrical energy to mechanical energy. A capacitor bank, or other source of stored charge, delivers a high current pulse across two electrodes that are submerged in a fluid, such as oil or water. The electric arc discharge vaporizes some of the surrounding fluid and creates shock waves in the fluid. A workpiece that is in contact with the fluid may be deformed by the shock waves to fill an evacuated die. 
     Electro-hydraulic forming may be used, for example, to form a flat blank in a one-sided die. The use of EHF for a one-sided die may save tooling costs and may also facilitate forming parts into shapes that are difficult to form by conventional press forming or hydroforming techniques. Electro-hydraulic forming also facilitates forming high strength steel, aluminum and copper alloys. For example, advanced high strength steel (AHSS) and ultra high strength steel (UHSS) can be formed to a greater extent with electro-hydraulic forming techniques when compared to other conventional forming processes. Lightweight materials, such as AHSS and UHSS and high-strength aluminum alloys are lightweight materials that are used to reduce the weight of vehicles. 
     The use of these high strength, lightweight materials is increasing and has been proposed for hydroforming tubes. Tube hydroforming is well-known technology that is currently used in production. One problem with hydroforming tubes is that the tube tends to thin in areas that are formed to a greater extent. 
     The above problems are addressed by Applicant&#39;s invention as summarized below. 
     SUMMARY 
     The method and tool disclosed and claimed in this application provide increased opportunities for hydroforming parts from ductile steel and also high strength materials that have reduced formability. By applying the method, larger diameter tubular preforms can be used to form parts having smaller diameter cross-sections in localized areas. Generally, the tube blank is selected to correspond to the average perimeter of the final part. The tube blank provides material that is worked in the hydroforming process. The hydroforming process is generally used to expand the tubular blank with pressure that is exerted from the inside of the tube. With expansion hydroforming, the size of the tube is limited to the minimum perimeter of the smallest cross-section of the finished part. This limits the quantity of material that is available for the hydroforming operation and, in turn, limits the extent to which the tube can be expanded. 
     According to the method, a tube or tubular preform is first formed to a reduced diameter in an electro-hydraulic forming process that applies an impact force to the outer surface of the tube. The partially contracted tube is then loaded into a hydroforming tool and formed by the application of fluid pressure to the inner side of the tube to expand the tube and form the tube against the hydroforming die. 
     The tool that is illustrated to compress or contract the tubular preform includes two parts that together define a chamber. A portion of the tube is first encircled with a wire and then placed in the chamber. The chamber is filled with a fluid, such as water or oil, and sealed. The wire is selectively connected to a source of stored electrical energy, such as a capacitor circuit, to cause an electrical discharge in the fluid in the chamber that forms the portion of the tube radially inward to a reduced cross-sectional area. The balance of the tube may be maintained at full cross-sectional area size. The tubular preform is later formed by expanding in a hydroforming operation in the full cross-sectional area. The portion of the tube that was compressed may be expanded from the reduced cross-sectional area. 
     Other aspects of Applicant&#39;s concept will be better understood in view of the attached drawings and detailed description of the illustrated embodiments. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagrammatic cross-sectional view of an electro-hydraulic forming tool that is used to contract the diameter of a tube prior to hydroforming. 
         FIG. 2  is a cross-sectional view taken along the line  2 - 2  in  FIG. 1 . 
         FIG. 3  is a cross-sectional view similar to  FIG. 2 , but showing an alternative embodiment wherein variable diameter coils are used to contract the tube to different extents along different portions of the tube. 
         FIG. 4  is a diagrammatic cross-sectional view of an alternative embodiment of the electro-hydraulic forming tool wherein a single loop of wire is provided in the electro-hydraulic forming tool. 
         FIG. 5  is a diagrammatic cross-sectional view of a tube showing the tube before contraction and after contraction. 
         FIG. 6  is a flowchart illustrating the steps of the method of compressing a tubular preform in an electro-hydraulic forming tool prior to forming the tubular preform by expanding the tube in a hydroforming operation. 
     
    
    
     DETAILED DESCRIPTION 
     Referring to  FIG. 1 , an electro-hydraulic forming tool  10  is used to contract a tubular preform  12  prior to hydroforming the tubular preform is diagrammatically illustrated. A wire coil  14  is wrapped in a spaced relationship around the tubular preform  12  and submerged in a liquid  18 , such as water or oil. The liquid  18  is contained within a chamber  20  defined by a first tool part  22  and a second tool part  24 . The chamber  20  must be sealed, as shown by first seal  26  and second seal  28 . The chamber  20  is filled by an upper port  30  and a lower port  32 . It should be understood that a single fill/evacuation port could be provided instead of the two ports as illustrated. 
     Tubular preform  12  and wire coil  14  are preassembled and then inserted into the chamber  20  defined by the first tool part  22  and the second tool part  24 . When assembled, the first seal  26  engages a second seal  28 . The chamber  20  is filled through the lower port until the liquid flows out of upper port  30 . 
     Referring to  FIG. 2 , the electro-hydraulic forming tool  10  is shown with the second tool part  24  (shown in  FIG. 1 ) removed. The tubular preform  12  is encircled by the wire coils  14  and immersed in the liquid  18 . The first tool part  22  retains the first seal  26  to seal the chamber  20  as described with reference to  FIG. 1  above. The seal  26  extends about the periphery of the forming chamber  20  and on one side of the tubular perform  12 . (The seal  26  is not visible behind the tubular perform  12  as viewed in  FIGS. 2-5 .) 
     A capacitor circuit  36  that comprises a stored power source is connected to opposite ends of the wire coil  14  by a positive electrode  38  and a negative electrode  40 . Alternatively, the stored power source may be an induction circuit that could be used instead of the capacitor circuit. When the capacitor circuit  36  is actuated, the wire coil  14  is energized to create a shockwave within the fluid  18  that is imparted to the tubular member  12 . The tubular member in the area where the wire coil  14  encircles the tubular member is compressed from an initial tube section  42  shown in solid line to a contracted tube section  44  shown in phantom lines. 
       FIG. 3  is a view similar to  FIG. 2  that shows an alternative embodiment wherein reduced diameter wire loops  46  are provided as part of the wire coil  14 . The tubular preform  12  is shown wrapped by the wire coil  14  including the reduced diameter wire loops  46  and is submerged in the fluid  18 . The wire coil  14  is connected to a capacitor circuit, as previously described with reference to  FIG. 2 . When the capacitor circuit  36  is discharged, the more closely wrapped wire loops  46  are closer to the tubular preform  12  and, as a result, exert a greater contraction force on the tubular member  12 . This greater contraction force compresses that portion of the tube to a greater extent compared to the contraction force applied by the other loops of the wire coil  14 . 
     Referring to  FIG. 4 , an alternative embodiment of the electro-hydraulic forming tool is shown in which a single loop wire  48  is provided. In the embodiment shown in  FIG. 4 , the same reference numerals are used as previously described with reference to  FIGS. 1-3 . The single loop of wire  48  is wrapped in a spaced relationship around the tubular preform  12  and immersed within the liquid  18  in the chamber  20 . Only one part of the chamber  20  is shown in  FIG. 4  which is that part defined first tool part  22  with its associated seal  26 . The second tool part  24  and the second seal  28  are also included in this embodiment, but are not illustrated to better illustrate the tool. 
     Referring to  FIG. 5 , the embodiment of  FIG. 4  is shown including the tubular preform  12  with a full diameter wall section illustrated by reference numeral  38  and a contracted wall section shown in phantom lines and identified by reference numeral  40 . The single loop wire  48  may be used to act on a smaller portion of the tubular member  12  than in the embodiment shown in  FIGS. 1-3 . 
     Referring to  FIG. 6 , a flowchart is illustrated that shows the steps of the process used to initially contract portions of a tube prior to hydroforming to expand the tube into a desired part shape. In many instances, the tube is preformed by bending to form the tube to a desired shape along its length. The first step in the process may follow the preform bending and comprises wrapping the coiled wire around the tube at  50 . The coil and tube are then inserted into the electro-hydraulic forming tool at  52 . The electro-hydraulic forming tool is discharged to compress a localized area of the tube at  54 . The wire is destroyed by the discharge and essentially vaporizes creating a shockwave in the electro-hydraulic forming tool chamber  20  that impacts the tubular preform to compress it in a localized area. The tube may then be removed from the electro-hydraulic forming tool at  56 . The tubular preform with the contracted localized area is then inserted into a hydroforming tool at  58 . The hydroforming tool forms the tubular preform at  60  expanding appropriate portions of the tube including portions of the tube that were not contracted. The portions of the tube that were contracted or compressed in the electro-hydraulic forming tool may also be expanded in the hydroforming operation at  60 . The tubular preform is compressed to the minimum diameter of the part to be formed. 
     While embodiments of the invention have been illustrated and described, it is not intended that these embodiments illustrate and describe all possible forms of the invention. Rather, the words used in the specification are words of description rather than limitation, and it is understood that various changes may be made without departing from the spirit and scope of the invention.