Abstract:
The present invention provides a method and apparatus for forming a sheet of ductile material by superplastic forming. Excessive wrinkling is prevented in the present invention by providing a method and apparatus which controls the amount of material flow during the forming process.

Description:
BACKGROUND OF INVENTION 
     The present invention relates generally to forming of materials, and more particularly to a method and apparatus for the superplastic forming of materials, the method and apparatus including a preforming operation. 
     Superplastic alloys have long been known to exhibit large strains to failure and strong resistance to necking during tensile elongation. Superplastic forming (“SPF”) has been developed as an effective way to form such alloys and offers several advantages over conventional stamping techniques including increased formability, zero springback, and low tooling costs. The large degree of plastic strain that can be achieved with this process (&gt;200%) makes it possible to form complex parts that cannot be shaped with conventional stamping techniques. These alloys can be formed with relatively low forces and they permit a high level of detail in the stamping design. 
     Typical superplastic forming takes place in a simple one-sided, single action tool. The blank is clamped in a heated die and then blow formed with gas pressure into a female die. The part detail is captured within a single die rather than a matched pair and therefore tooling is significantly less expensive than that of conventional stamping. Furthermore, the low forces needed to form the material at these elevated temperatures allows for the use of cast iron dies instead of the harder to work and more expensive tool steel. 
     While superplastic forming may be a viable manufacturing option for some parts, there are limitations in the economic feasibility of this technique. Superplastic response in metals is inherently coupled with the rate of deformation and there exists only a narrow range of strain rates, typically slow strain rates, in which these materials display superplastic response. This results in a relatively slow cycle time which often leaves superplastic forming as a cost prohibitive option for high volume parts. 
     Another problem related to SPF stems from the inability to draw material into the die cavity. Although the superplastic material utilized in SPF can undergo substantial deformation, its formability is limited to the amount of material in the die. After the die faces are clamped and sealed, additional superplastic material cannot be drawn into the die. This may result in tears or inconsistent wall thickness in the part being formed. To overcome this, U.S. Pat. No. 5,974,847 introduces preforming the material around a punch before gas pressure sealing the dies and completing the forming process by gas pressure injection. This approach reduces the amount of superplastic forming that takes place thereby reducing the cycle time and potentially allowing greater design freedom due to the additional material drawn into the die during the preforming step. While the method of this patent teaches preforming the material before the gas is injected, the method does not restrain the material entering the die during the preforming step. Without a restraining force on the material, such as blankholder force, the material will wrinkle around the punch in all but the simplest of formings. Wrinkling of the material during preforming will result in either the inability to complete the part during subsequent gas pressure forming or, at best, a low quality finished part. 
     Therefore, there exists a need for a method of forming superplastic materials which controls the amount of material to be drawn into the die cavity during a preforming process so as to avoid wrinkling of the material. The present invention provides such a need by controlling the material flow into the die during the preforming step, thereby eliminating wrinkles in the preformed part. An apparatus to adjust the amount of material flowing into the die is also necessary to ensure uniform preform wall thicknesses and high quality preformed parts. These preforms will lead to more consistent finished parts and will assist in increasing the speed of the forming process. 
     SUMMARY OF THE INVENTION 
     It is an advantage of the present invention to provide a method of superplastic forming which increases forming speed while reducing surface defects in the formed part. 
     It is another advantage of the present invention to provide a method of reducing tooling cost by using one-sided cast iron dies and providing a universal lower die system that can be used to form a variety of parts. 
     It is another advantage of the present invention to provide a method of restraining the sheet during the preforming step so as to produce a blankholder effect that prevents wrinkling of the sheet. 
     The present invention provides these advantages by providing a method of shaping a metal sheet into a formed product, comprising the steps of providing a first and second die member operative to move between a first open position and a second sealed position such that a die cavity is formed; providing a preforming punch disposed on one of the die members; providing a metal sheet of ductile material; and providing a blankholder engagable with a cushion system operative to move between a first material loading position and a second material loaded position. The method further comprises the steps of heating the die members and the preforming punch to a predetermined temperature, heating the metal sheet to a predetermined material forming temperature and moving the first and second die members to the open position and the blankholder to the material loading position. The method then continues with the steps of placing the metal sheet into the blankholder, moving a die member to engage the blankholder until the metal sheet contacts the preforming punch. The method further includes the steps of moving a die member, the blankholder, and the metal sheet until the die member sealed position is reached, controlling the amount of material flow into the die cavity as the metal sheet is over the preforming punch and applying gas pressure to the metal sheet after the sealed position is reached and until forming of the product is completed. Once completed, the die members are opened and the finished part removed. 
     The present invention further provides an apparatus for use with the method of the present invention, the apparatus comprising at least two die members operative to move between a first open position and second sealed position such that a die cavity is formed; a preforming punch disposed upon one of the die members, a cushioning system operative to control the amount of material flow into the die cavity as the metal sheet is formed over the preforming punch and a blankholder engagable with the cushion system. The apparatus further comprises a heating platen operative to raise the temperature of the die members to a predetermined level; a source of gas pressure and passages for directing the gas pressure into the die cavity; and wherein the cushioning system exerts a force on the metal sheet holding the metal sheet in place and permitting controlled material flow into the die until the second sealed position is reached. The present invention provides these advantages by utilizing a die cushioning system to control material flow into the die cavity during the preforming step reducing wrinkling in the preform and finished part. 
    
    
     These and other advantages of the present invention will become readily apparent by the drawings, detailed description and claims that follow. 
     BRIEF DESCRIPTION OF DRAWINGS 
     FIGS. 1-4 are cross-sectional views illustrating the relative positions of the apparatus of the present invention, the metal sheet, and the die cavities during the three forming steps utilized in the superplastic forming process of the present invention. 
     FIG. 5 is a plan view of a cushion plate of the present invention. 
     FIG. 6 is a perspective view of a preformed metal sheet formed using the method and apparatus of the present invention. 
    
    
     DETAILED DESCRIPTION 
     Referring now to the drawings, FIGS. 1-4 show an apparatus  10  for superplastic forming of a sheet of highly ductile material in accordance with the present invention. The superplastic forming apparatus  10  includes a frame  12  housing an upper platen  14 , lower platen  16 , an upper die  18  and a lower die  20 . As illustrated, the upper  18  includes a forming surface  22  against which a sheet  24  of ductile material is pressed to form the final shape of a workpiece to be formed. In an alternative configuration, the forming surface could be located in the lower die. Because the material to be formed must be highly ductile, forming typically takes place at elevated temperatures. Both of the dies  18 ,  20  and the material must be heated to a predetermined temperature prior to forming. This predetermined temperature depends on the composition of the alloy to be formed. To heat the dies, the upper  14  and lower  16  platens are heated, such as by electrical resistance, and pass this heat to each of the dies. The lower platen  16  is disposed adjacent a cooling plate  17  which acts to prevent the heat from passing below the lower die  20  to heat sensitive components of the forming apparatus  10 . A typical material to be formed in the forming apparatus  10  of the present invention is an aluminum alloy, such as alloy 5083. This aluminum alloy has a nominal composition, by weight, of 4 to 4.9% manganese, 0.05 to 0.25% chromium, about 0.1% copper and the balance aluminum. This alloy would be formed at a temperature of approximately 500° C. 
     The forming apparatus  10  further includes a cushion system  30  disposed at the base of the frame  12 . As will be described in more detail below, the cushion system operates to restrain the material  24  flowing into the die by producing a blankholder force. The cushion system includes a cushion plate  32  and a pair of nitrogen cylinders  34  disposed between the frame  12  and the cushion plate  32 . Two cylinders are shown, but it is contemplated that more cylinders can be used, depending on the need and application. Alternatively, coil springs or other such resistive devices can be used. The cushion system  30  further includes cushion pins  38  which pass through lower platen  16  and cooling plate  17  and which include cushion posts or blankholders  40  disposed on a free end thereof. In operation, the sheet  24  is placed on the blankholders  40  prior to the forming operation. FIG. 5 shows a top view of the cushion plate  32  and the respective cushion pin  38  arrangement. Cushion plate  32  includes a plurality of apertures  39  through which the cushion pins  38  can pass. By providing a plurality of these apertures, the plate  32  can be used for a variety of tool configurations. Cushion pins  38  pass through the heated lower platen  16  and the cooling plate  17  before their loads are transferred into the blankholder  40 . Positioning the cushion pins  38  in this manner avoids the heating and cooling piping imbedded in lower platen  16  and cooling plate  17 . This design also allows the same plate to be used for different die designs by inserting or removing the cushion pins  38  into the cushion plate  32 . 
     A preform punch  44  is disposed in the lower die  20 . The preform punch  44  is disposed in a recess  46  formed in the die  20 . Shims  48  may be placed between the punch  44  and the recess  46  to raise to position of the punch, depending on the forming application. The punch  44  can take a variety of different configurations depending on the final shape of the workpiece. The punch may also be placed in the upper die  18  in an alternative embodiment. 
     Lower die  20  also includes a plurality of gas passages  49  that provide pressurized gas used in the forming process. Lower die further includes a gas pressure seal  50  disposed on the mating end  52  of the die  20 . As will be described below, the gas pressure seal performs two functions: the seal prevents pressurized gas from leaking during forming, and in cooperation with upper die  18 , holds the sheet  24  in position during forming. The seal  50  can be formed integrally on the ends of the lower die  20  or secured there in a known manner, such as by welding. The seal  50  is shaped so that it matingly engages a mirror shape formed in or attached to the upper die  18 . In this way, gas pressure cannot escape the die cavity when the upper and lower dies are closed together in a sealed position. 
     A method of superplastic forming the sheet  24  of ductile material using the apparatus  10  of the present invention will now be described. Referring again to the drawings, FIGS. 1-4 show the progression of steps of the forming process in accordance with the method of the present invention. Prior to these steps, the upper  14  and lower  16  platens heat the upper and lower dies, respectively, to a predetermined temperature. The sheet  24  to be formed is also heated to this forming temperature. In FIG. 1, the sheet  24  of ductile material is loaded into the blankholder  40  in the material loading position. Movement of the dies into the second sealed position is shown in FIG. 2 wherein the upper die  18  is lowered until it contacts the sheet  24  and shapes the sheet  24  around the preforming punch  44 . The amount of deformation induced in this step is controlled by the relative height of the punch  44  to the height of the gas pressure seal  50 . This can be altered by either changing the punch within the lower die or by changing the height of the punch with the shimming system  48  within the lower die  20 . While the upper die  18  moves down into the second sealed position, the blankholder  40  exerts a controlled upward force on the sheet  24  permitting the sheet  24  to flow into the die cavity during the preforming operation. The flow of the sheet  24  into the die cavity can be seen at reference numeral  60 , wherein the ends  62  of the sheet  24  are spaced a distance from the ends of the blankholder  40 . Consequently, the amount of sheet material  24  drawn into the die cavity during this preforming stage is directly related to the amount of extensive force produced by the nitrogen cylinders  34 . The rate in which the sheet material  24  is allowed to draw-in over the blankholder  40  is controlled by the force in the cushion system  30 . This cushion force is a critical element to control the draw-in process and prevent either splits (too much force) or wrinkles (not enough force) on the end product. 
     FIG. 3 shows the next step in the method of the present invention. Once the upper die  18  reached the gas pressure seal  50  on the lower die  20 , the mechanical deformation is finished and the part can be forced into the upper die  18  with superplastic gas pressure. This is the die pressure sealed position in the method of the present invention. The cushion system  30  is no longer used when the upper die  18  descends and contacts the lower die  20 . At this time a gas pressure seal  50  is created between the two dies  18  and  20 , sandwiching the material  24  therebetween. This seal  50  holds the material in place while a high-pressure gas is injected into the underside of the material via the gas passages  49 . This pressure forces the preformed material to conform to the surface of the upper die  18  producing the shape of the finished part. The gas pressure seal  50  ensures no gas leakage between the material and the lower die in addition to allowing no further material flow. During this step, the force on the upper die scales with the gas pressure to avoid gas leakage. 
     As shown in FIG. 4, after the part is completed, the gas pressure is released and the upper die  18  is raised to the open position so that the completed part can be removed from the lower die  20 . The design of this die system allows for re-use of the bottom die system including bottom die  20 , performing punch  44 , blankholder  40  and cushion system  30 . For example, four different door inners could be produced by just changing the upper die  18 . The upper dies are relatively simple one-sided tools that can be fabricated from cast iron. This flexibility results in significant savings in tooling costs. 
     FIG. 6 illustrates a properly formed metal sheet  64  after it has been formed in the apparatus  10  according to the method of the present invention and removed from the die cavity. Without controlling the amount of sheet material flowing into the die cavity during the preforming step, this part would wrinkle around the punch and make it impossible to successfully complete the part with superplastic gas pressure. 
     It will be realized, however, that the foregoing specific embodiments have been shown and described for the purposes of illustrating the functional and structural principles of the invention and is subject to change without departure from such principles. Therefore, this invention includes all modifications encompassed within the scope of the following claims.