Abstract:
A superplastic forming apparatus and method for forming a sheet of material at an elevated temperatures into a workpiece having a complex geometry. The process takes advantage of a mechanical forming step, which draws material along a major axis to form a first preform. Upon completion of the first mechanical forming step, a second, initial superplastic forming step ask on the first preform to create a second preform having a plurality of channels or grooves located thereon. The grooves or channels function to draw additional material prior to the second or final gas forming step, which completes the forming process by driving the material against a forming surface. Accordingly, the method and apparatus function to reduce forming time and eliminate thinning and wrinkling of the sheet material during the forming process.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    Not applicable. 
       BACKGROUND OF THE INVENTION  
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates generally to an apparatus and method for forming a workpiece; and more particularly, to a multistage forming apparatus that forms a sheet of material at an elevated temperature into a workpiece having a complex geometry. 
         [0004]    2. Description of Related Art 
         [0005]    Superplastic forming (SPF) is a process that takes advantage of a material&#39;s superplasticity or ability to undergo large strains under certain elevated temperature conditions. Superplasticity in metals is defined by very high tensile elongation and is the ability of certain materials to undergo extreme elongation at a predetermined temperature and strain rate. A simple example of superplastic forming is gas forming a sheet of material placed within a single-sided die set having a planar seal surface. The die and sheet of material, or as sometimes referred to the blank, are heated to a superplastic temperature and a predefined gas pressure profile is applied to one side of the sheet. The gas pressure forces the sheet into a die cavity and against a mold surface while maintaining a predetermined or target strain rate during the forming cycle. 
         [0006]    Taking advantage of the superplasticity of the material enables forming of complex components not normally formed by conventional room temperature metal forming processes. For example, in the automotive industry it is often necessary to form components having deep cavities and very small radii. While superplastic forming enables forming such a component, one disadvantage is that it normally requires relatively long forming cycle times. Specifically, a conventional SPF process used to manufacture a complex part can require a forming cycle time as high as 30 minutes. Reducing these cycle times can result in necking and splitting when forming over small radii and excessive thinning of the part in certain areas such as the inside corners of concave parts or at the bottom of the mold cavity. 
         [0007]    One way to address such problems includes prethinning the blank through a separate step prior to performing the superplastic forming process. Such a step can either include a mechanical step that draws additional material into the die cavity or a pre- or initial superplastic forming process that stretches or pre-thins a portion of the blank prior to the blank undergoing the final superplastic forming step. Depending upon the depth or configuration of the part, these additional steps typically require additional forming equipment, which increases costs, and in some instances may increases the forming time of the part. 
         [0008]    While attempts have been made to combine a mechanical process for pre-forming the metal sheet or blank prior with a superplastic forming process such systems may require complicated pre-forming equipment that is expensive to manufacture, such as a double-action die set having a blank holder or binder assembly. Accordingly, there exists a need for a reduced complexity superplastic forming apparatus for forming metal sheet or blanks that combines the benefits of both mechanical and superplastic forming processes. 
       SUMMARY OF THE INVENTION  
       [0009]    The present invention is a multistage superplastic forming apparatus and method for forming a sheet of material at an elevated temperature. The apparatus includes a forming tool having an upper or pre-form die and a lower or forming die that moves between a first, open position and a second, closed position. Both the upper die and lower die include a die surface and a periphery. The periphery of each die being non-planar, wherein the non-planar periphery of the upper die is complementary to the non-planar periphery of the lower die. The forming tool further includes the upper die having a draw member and the lower die having a die cavity and a forming surface. Passageways extend through each of the upper die and lower die to the respective die surfaces and allow fluid of the type used in a superplastic forming process to travel through each of the upper die and lower die to their respective die surfaces. 
         [0010]    In a further embodiment of the invention, the upper die surface includes a recess therein forming a die cavity in the upper die surface. The recess configured such that material from the metal sheet placed between the respective upper and lower die during the first or initial gas forming process or step creates an additional preform that aids in material distribution during the final gas forming step. Depending upon the configuration and placement of the recess, initial gas forming step draws the material of the metal sheet in a multitude of directions. Accordingly, the apparatus provides a single-action forming tool operative to draw metal into the die cavity along multiple axes using upper and lower dies and without the use of a blank holder. 
         [0011]    The present invention further includes a method for multistage superplastic forming. The method includes providing a single-action forming tool having an upper die and a lower die with the upper die and lower dies operative to move between a first, open position and a second, closed and sealed position. After placing a heated metal sheet between the upper and lower dies, the dies are moved from the first, open position to the second, closed and sealed position whereby they draw metal into a die cavity of the forming tool. In one embodiment, first the single-action forming tool mechanically draws metal substantially along a major axis after which an initial superplastic forming step draws metal along a different axis. Both steps are taken before the final superplastic forming step forms a workpiece from the preform created by the foregoing steps. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]      FIG. 1  is a perspective view of a forming tool used for superplastic forming a workpiece according to the present invention. 
           [0013]      FIG. 2  is a front view of the forming tool illustrated in  FIG. 1 . 
           [0014]      FIG. 3  is a perspective view of a preform formed according to an initial step of one embodiment of the present invention. 
           [0015]      FIG. 5  is a perspective view of a preform formed according to an initial superplastic forming step of one embodiment of the present invention. 
           [0016]      FIG. 6  is a perspective view of a metal sheet formed according to a subsequent superplastic forming step of one embodiment of the present invention. 
           [0017]      FIG. 7  is a flowchart illustrating a method for designing a superplastic forming tool according to one embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0018]    Referring now to the drawings,  FIGS. 1-2  illustrate a single-action forming tool  10  according to the present invention used for forming a flat metal sheet or blank  11 . The term single-action describes the use of upper and lower dies that act on the blank when closing as opposed to a double-action tool that utilizes a separate blank holder or binder to hold the metal sheet during the draw process. The forming tool  10  includes an upper or pre-forming die  12  and a lower or forming die  14 . The upper or pre-forming die  12  and the lower or forming die  14  are operative to move between an open and a closed position. 
         [0019]    The upper die  12  includes a die surface  16 . The die surface  16  includes a punch or draw member  18  and a periphery  20  having a non-planar configuration. Wherein the periphery  20  is a region or zone of the die surface  16  adjacent, the outer edge  22  of the upper die  12 . The die surface  16  of the upper die  20  further includes a plurality of recesses illustrated as of semi-cylindrically shaped channels or grooves  24 . These recesses  24  form a die cavity in the upper die  12 . A plurality of passageways  26  extend through the upper die  12  to the die surface  16 . The passageways  26  connect to a gas pressure source (not shown) enabling pressurized gas to flow from the gas pressure source to the die surface  16  of the upper die  12 . 
         [0020]    The lower or forming die  14  includes a die surface  28 . The lower or forming die  14  further includes a die cavity  30  having a forming surface  32 . The die surface  28  further including a periphery  34  having a non-planar configuration. Once again, the periphery  34  is a region or zone of the die surface  28  adjacent the outer edge  36  of the lower die  14 . A plurality of passageways  38  extend through the lower die  14  to the die surface  28  of the lower die  14 . The passageways  38  connect to the gas pressure source (not shown) enabling pressurized gas to flow from the gas pressure source to the die surface  28  of the upper die  14 . As known in the art, the gas pressure source supplies pressurized gas used to perform a superplastic forming process according to one aspect of the present invention. Various gases can be used, the type typically depending upon the composition of the material being formed. In addition, U.S. Patent Application Publication No. US 2006/0260373 A1 discloses a method for forming a workpiece by applying pressure through passageways located in upper and lower dies of a superplastic forming tool, the disclosure and contents of which is hereby incorporated by reference in its entirety. 
         [0021]    In accordance with the present invention, the periphery  20  of the upper die  12  and the periphery  34  of the lower die  14  come together to form a non-planar binder that imparts a non-planar configuration to the metal sheet or blank. In addition, the punch or draw member  18  cooperates with the die cavity  30  to draw additional material from the periphery of the metal sheet or blank into the die cavity  30 . Accordingly, the single-action forming tool  10  mechanically pre-bends or pre-forms the metal sheet or blank  11  positioned between the upper and lower dies  12 ,  14  as the upper and lower dies  12 ,  14  move from the open position to the closed position. When placed in the closed position, the upper and lower dies  12 ,  14  sandwich the metal sheet or blank  11  between the die surface  16  of the upper die  12  and the die surface  28  of the lower die  14 . 
         [0022]    In accordance with the method of the present invention, the single-action forming tool  10  utilizes both mechanical forming and gas forming to form a metal sheet or blank. In the preferred embodiment, a three-stage process forms the metal sheet or blank into a finished workpiece. The three-stage process includes a mechanical forming step along with first and second gas forming steps. During the mechanical forming stage, a planar blank or flat sheet of material  11  is placed between the upper and lower dies  12 ,  14 . When closed, the upper and lower dies  12 ,  14  cooperate to draw material from the blank or sheet  11  to create an initial or first preform  40 , see  FIG. 3 . As the upper die  12  approaches the lower die  14 , the punch or draw member  18  engages the metal sheet  11  and draws additional material into the die cavity  30 . Since the single-action forming tool  10  does not use a blank holder, when the forming tool  10  starts to close the punch or draw member  18  engages the metal sheet  11  and pulls at least a portion of the metal sheet  11  into the die cavity  30  in an unrestricted manner. 
         [0023]    As the forming tool  10  continues to close, the periphery  20  of the upper die  12  and the periphery  34  of the lower die  14  come together and bend or form the sheet to create the initial or first preform  40 . As illustrated in  FIG. 3 , the initial preform  40  has a periphery  42  having a configuration conforming to the non-planar configuration of the peripheries  20 ,  34  of the respective upper and lower dies  12 ,  14 . Since the forming die  14  also performs the superplastic forming steps, the respective peripheries  20 ,  34  of the upper die  16  and lower die  18  are complementary to one another. Accordingly, once they form the periphery  42  of the first preform  40 , the upper and lower dies  16 ,  18  sandwich the periphery  42  of the preform  40  between them and form a seal after closing the forming tool  10 . 
         [0024]    As shown, the first preform  40  has a non-planar periphery  42  mechanically formed according to a first stage of the present invention. During the first mechanical preforming stage, the forming tool  10  engages and bends or curves the blank or sheet in a manner whereby the material of the blank is drawn primarily along one dominant axes, as illustrated by the arrows  50 , to create the first preform  40 . The dominant axes being the axis receiving the primary bend or curvature necessary to form the preform. By performing the mechanical preforming stage without a blank holder, significant mechanical pre-forming; i.e., bending or curving the blank, is limited to predominantly single-curve shapes. An attempt to draw or pull a significant amount of the material of the blank  11  in more than one direction without using a binder or blank holder typically causes material wrinkling in a flange portion of the component due to the circumferential stresses occurring there. 
         [0025]      FIG. 4  illustrates a second preform  44  created upon completion of the second stage. During the second stage, with the forming tool  10  fully closed and sealed, gas pressure is supplied to one side of the first preform  40  through the passages  38  located in the lower or forming die  14 . The gas pressure drives a portion of the first preform  40  upward against the die surface  16  of the upper or preform die  12  and into the semi-cylindrically shaped channels or grooves  24  located in the upper or prefrom die  12 . Accordingly, the second stage utilizes a superplastic forming process to form the second preform  44  with multiple semi-circular channels  46  extending along pre-determined lines or locations in the preform  44 . The semicircular channels  46  may also be located in the periphery  48  of the second preform  44 . Thus, the second stage forms a second preform  44  that is pre-stretched in certain areas. 
         [0026]      FIG. 5  illustrates a part or workpiece  50  created during the third and final forming stage of the process. The third stage includes a second gas forming stage wherein gas pressure is supplied through passageways  26  located in the upper or preform die  12 . The gas pressure acts on the side of the second preform  44  opposite the die cavity  30  and drives a portion of the second preform  44  downward against the forming surface  32  of the forming die  14 . Driving the second preform  44  into the die cavity  30 , draws or pulls the material of the semi-cylindrically shaped channels  46  into the die cavity  30  in the direction of the arrows  54 . As known with superplastic gas pressure preforming, the additional length or amount of material created by the gas pre-forming step helps to reduces excessive thinning of the final part or workpiece  50  as the workpiece is formed over small radii and at the bottom of the die cavity  30 . 
         [0027]    Combining both mechanical and gas preforming in a single-action forming tool  10  minimizes necking or excessive thinning around radii by using the combination to draw metal into the die cavity  30  along multiple axes without the use of a blank holder. While the mechanical pre-forming stage draws the material of the blank in predominantly one axis, the second gas pre-form stage creates a second preform  44  configured such that during the third or final forming stage material of the second preform  44  is drawn or pulled into the die cavity  30  along multiple axes. Thus, the forming tool  10  according to the present invention is operative to pull or draw material along a plurality of axes using a single-action superplastic forming tool  10  that takes advantage of a mechanical forming process to decrease overall forming time. Accordingly, the present invention utilizes a three-stage forming tool  10  and process that operates with a standard single-action press. Simplifying the forming tool  10  in this manner correspondingly reduces costs and complexity of the forming tool while decreasing overall forming time. Further, the present invention contemplates tailoring the preform design to prevent part thinning and improve the overall thickness profile of the part or workpiece  50 . 
         [0028]      FIG. 6  illustrates a flowchart according to the present invention setting forth the steps for establishing the die surfaces  16 ,  28  of the respective upper or preform die  12  and lower or forming die  14 . In particular, the method involves designing or configuring the respective die surfaces  16 ,  28  such that they create a preform having a configuration that controls material flow within the forming tool  10  during the forming process. Further, the preform is designed to control material distribution in the forming tool  10  and thus correspondingly control material thickness. The method utilizes in part, the step of conducting a finite element analysis to model sheet metal forming parameters in order to determine the design of the preform. The method combines the need to develop both a forming surface used with the first mechanical forming process to initially shape the preform and a forming surface used with the first gas forming processes that provides the preform with additional elements or further shape. One embodiment includes adding elements to the preform to pre-stretch the preform. Accordingly, the method includes using a gas forming process to create or form channels in the mechanically formed preform. While disclosed herein as semi-circular channels, these additional elements or structure in the preform created by the gas forming process may take a plurality of shapes, configurations or designs. The shape, configuration or design of the elements formed in the preform by the initial gas forming process depend ultimately upon the final configuration and shape of the forming surface  36  located in the lower or forming die  18 . 
         [0029]    The method  60  for creating the forming surfaces of the superplastic forming tool  10  is generally shown in  FIG. 6 . As shown therein, block  62  illustrates the first step of utilizing a computer-aided design program to design a particular to pre-selected forming surface. The forming surfaces  16 ,  32  of each of the upper or pre-form die  12  and lower or forming die  14  are typically designed with a three dimensional computer-aided design system, many of which are commercially available. While the forming surface  28  of the upper or pre-form die  12  has an empirically designed shape or configuration used to create the preform, the forming surface  32  of the lower forming die  14  is shaped or configured to produce the end part or workpiece  50 . Block  64  shows the next step of using a preprocessor such as HyperMesh®, available from Altair Engineering, Troy Michigan, to mesh the forming surfaces using shell elements. 
         [0030]    Block  66  illustrates the step of developing a meshed blank formed using shell elements and applying the appropriate boundary conditions and load steps to the blank elements. Block  68  illustrates the next step of developing an input finite element analysis cardfile and executing the meshed model. Block  70  illustrates the next step wherein upon completion of the finite element analysis of the meshed model the results thereof are analyzed for excessive thinning and wrinkling of the blank member during the forming process. As illustrated in block  72 , if the results of the analysis meet the design requirements, that is, a satisfactory workpiece  50  can be formed with the selected die surfaces  16 ,  28  then the method moves to block  74  wherein these surfaces are used on the forming tool  10 . If, however, the design of the upper or the die surfaces  16 ,  28  are not satisfactory, then the method goes to the step of block  76 , which includes redesigning the forming surfaces of the forming tool  10  after which the method starts again with use of a preprocessor to mesh the newly designed forming surfaces as illustrated in block  64 . 
         [0031]    The above method takes into consideration frictional conditions occurring during superplastic forming. The frictional conditions are highly dependent on the material of the metal sheet, die surface roughness and any solid lubricant used during the forming process. The coefficient of friction can range between 0.15 and 0.5 and is established experimentally for conditions specific to the particular forming operation. In addition, the boundary conditions are also problem or workpiece dependent. For example, modeling the clamping of the sheet between the upper or pre-form die  12  and lower or forming die  14  used to form the superplastic forming chamber or die cavity  30  can be defined by constraining the edges of the sheet in a major direction transverse the blank edges. 
         [0032]    A number of constitutive models have been developed for describing the stress, strain, strain rate and grain size relationship for superplastic and hot-plastic materials; however, with the careful development of the coefficients, the empirical power law equation given below has been demonstrated to accurately and efficiently describe the high temperature deformation behavior of aluminum and magnesium. 
         [0000]      σ =K{circumflex over (ε)}   m  ε n    
       Where σ is the flow stress, {circumflex over (ε)} is the strain rate, ε is the strain, K is a constant, m is the strain rate sensitivity exponent, and n is the strain hardening exponent. 
       [0033]    Accordingly, one aspect of the present invention enables the design of the die surfaces  16 ,  28  of the upper or preform die  12  and lower or forming die  14  to create a preform that controls material flow of the metal sheet during the final, superplastic forming portion of the workpiece thereby preventing excessive thinning and distributing material as necessary. In addition, the invention includes a single-action forming tool  10  that utilizes both a mechanical forming process and a gas forming process to control material flow within the forming tool and distribute material within the forming tool to maintain desired material thickness.