Abstract:
A method of forming a pack in a die by superplastic formation and diffusion bonding comprises applying a forming pressure within the pack to expand the pack within the die; and supplying gas between the die and the pack to apply a back pressure around an outside of the pack while the pack is being expanded to counteract the forming pressure to reduce surface mark off.

Description:
[0001]    This is a continuation-in-part of copending U.S. Ser. No. 11/393,114 filed Mar. 30, 2006. 
     
    
     BACKGROUND 
       [0002]    Superplastic forming and diffusion bonding (SPF/DB) may be used to form monolithic metal structures having complex shapes and deep drawn features. For example, SPF/DB may be used to form exterior assemblies of aircraft. 
         [0003]    Structures formed by SPF/DB may exhibit “surface mark-off.” Surface mark-off is a marring of the surface of the formed structure. It may appear as a crease or other surface defect that interrupts the smoothness of the surface. Surface mark-off can not only be cosmetically unsightly, but can have other consequences. For instance, surface mark off on the outer-surface of an exterior aircraft assembly may increase aerodynamic drag. 
         [0004]    It would be desirable to suppress surface mark-off in SPF/DB structures. 
       SUMMARY 
       [0005]    According to an embodiment herein, a method of forming a pack in a die by superplastic formation and diffusion bonding comprises applying a forming pressure within the pack to expand the pack within the die; and supplying positive gas pressure between the die and the pack while the pack is being expanded in order to counteract the forming pressure to suppress surface mark off. 
         [0006]    These features and functions may be achieved independently in various embodiments or may be combined in other embodiments. Further details of the embodiments can be seen with reference to the following description and drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]      FIG. 1  is an illustration of a pack before and after diffusion bonding (DB) herein, but prior to superplastic forming. 
           [0008]      FIG. 2  is an illustration of a gas inlet tube for the pack. 
           [0009]      FIG. 3  is a cross sectional view of the tube of  FIG. 2  taken at  3 - 3 . 
           [0010]      FIG. 4  is an illustration of the gas inlet tube juxtaposed for insertion into a receiving non-welded opening in the pack. 
           [0011]      FIG. 5A  is an illustration of the pack and a gas inlet tube welded in place. 
           [0012]      FIG. 5B  is an illustration of the pack and the gas inlet tube welded in place along with a temporary inert gas purge tube. 
           [0013]      FIG. 6  is an illustration of a metal forming die with the pack inserted between die halves. 
           [0014]      FIG. 7  is an illustration of an enlarged portion of the die half of  FIG. 6  showing an example of die pressurizing gas inlet ports. 
           [0015]      FIG. 8  is an illustration of a portion of the die half into which the inlet gas tube is inserted during SPF/DB herein. 
           [0016]      FIG. 9  is an illustration of a method of performing SPF/DB herein. 
       
    
    
     DETAILED DESCRIPTION 
       [0017]      FIG. 1  illustrates a two sheet pack  120  before and after its sheets  100 ,  102  are welded together and diffusion bonded. A weld  115  may be formed at regions in the vicinity of the perimeters of the sheets  100 ,  102  in a fusion weld or resistance, or laser, or electron beam welding. A gas inlet tube  150  is inserted between the sheets  100 ,  102  so that its inner tube  152  (see  FIG. 3 ) is in communication with the cavity or space (not shown) formed between the two sheets  100 ,  102 . 
         [0018]    The sheets  100 ,  102  of the pack  120  are made of material that is diffusion bondable and superplastically formable. Examples include, but are not limited to, Inconel alloys, titanium, titanium aluminides, and titanium metal matrix composites. 
         [0019]    An embodiment of the gas inlet tube  150  is shown in  FIGS. 2 and 3 . The gas inlet tube  150  includes the inner tube  152  and an outer tube  154  as shown in  FIG. 3 . In addition, the gas inlet tube  150  has a pair of opposed outward extending wings  158  of suitable shape for sealing against the perimeter of the pack  120  in a gas tight seal. The outer tube  154  may be made of a superplastic material, and may be of the same material as the sheets of the pack  120  (e.g., Ti-6Al-4V, Ti-3Al-2.5V). This promotes sealing of the outer tube  154  to the pack  120  by welding, for example, to form a gas tight seal. The inner tube  152  may be made of a material that is substantially unaffected by heat and pressure conditions of SPF/DB. For example, the inner tube  152  may be of stainless steel, such as 304, 310, 316, 321, 347, Inconel alloys, and the like. This inner tube  152  will remain open to supply gas to the cavity inside the pack  120  during SPF/DB. The inner tube  152  will not collapse under pressure and temperature of SPF/DB while in the die. The wings  158  may be of any suitable material compatible for welding to the pack  120 , and may be of the same material as the sheets  100 ,  102  of the pack  120 . This facilitates welding of the wings  158  to the edge of the pack  120 , and promotes a gas tight seal. 
         [0020]      FIGS. 4 and 5  depict sections of the pack  120  around the gas inlet tube  150 . In  FIG. 4 , the gas inlet tube  150  is juxtaposed for insertion into an unwelded space between the sheets  100 ,  102  of the pack  120 . The wings  158  extend to cover edges  122 ,  124  of the pack  120  to ensure a gas tight seal when welded in place as shown in  FIG. 5 . In  FIG. 5  the outer gas tube  154  is welded in place to the pack  120  and an end portion of the gas inlet tube  150  extends into the pack  120  for a short distance. In one embodiment, the wings  158  have a through hole  159  into which a small temporary tube  161  (see  FIG. 5B ) may be inserted and through which argon or other inert gas may be routed into the space between the sheets  100 ,  102  while the sheets  100 ,  102  are being welded into a pack  120 . After welding, the temporary tube  161  is removed and the hole  159  is welded shut to seal off the pack  120 , except for the open gas inlet tube  150 . 
         [0021]      FIG. 6  shows the pack  120  and an example of a die  300  for forming an exterior aircraft assembly having an aero-smooth outer mold line (OML) surface and an inner mold line (IML) surface having stiffeners formed integrally therewith. In this example sheet  100  of the pack  102  will be referred to as the upper sheet  100 , while sheet  102  will be referred to as the lower sheet  102  The die  300  has upper and lower die halves  310 ,  320 . The upper die half  310  is configured to form an exterior surface  101  of the upper sheet  100  into the OML surface. The lower die half  320  is patterned with a pair of parallel cavities  322 ,  324  to form an exterior surface (not visible) of the lower sheet  102  into the IML surface. 
         [0022]    To provide a positive gas pressure (also referred to as the “back pressure”) within the die  300  during superplastic forming, a back pressure gas inlet  350  supplies gas (e.g., argon) to the lower die half  320 . The gas exits the lower die half  320  via a series of internal through bores  352  (shown in  FIG. 7 , an enlarged section of the lower die half  320 ). The gas exiting the bores  352  is directed towards the lower sheet  102  of the pack  120 . As will be discussed below, the gas exiting the bores  352  creates a cushion of gas pressure that forces the pack  120  against the upper die half  310  during diffusion bonding and superplastic formation. The gas pressure may be distributed evenly across the entire die surface, including the die cavities  322 ,  324 . 
         [0023]      FIG. 8  shows a section of the lower die half  320 . The lower die half  320  has a receiving cavity  330  extending laterally across the die  300  at its peripheral edge. The receiving cavity  330  is sized to receive the gas inlet tube  150 . In one embodiment shown here, the cavity  300  is funnel-shaped having a conical area  332  with a larger dimension inboard and an apex outboard. The trough  334  of the funnel shaped cavity  330  extends from the apex of the cone  332  to the outer edge of the die half  320 . The receiving cavity  330  creates a gas tight seal between the die  300  and the gas inlet tube  150  so that the back pressure does not leak from between the die half  320  and the pack  120 . 
         [0024]    Reference is once again made to  FIG. 6 , and additional reference is made to  FIG. 9 , which illustrates a method of using the die  300  to fabricate an exterior aircraft a via SPF/DB. At block  910 , the pack  120  is placed in the lower die half  320 , with the sheet  102  covering the cavities  322 ,  324  and the gas inlet tube  150  resting in the receiving cavity  330 . 
         [0025]    At block  920 , the upper die half  310  is placed over the pack  120 , and the die  300  is closed. For instance, the die  300  may be closed (and also opened) by a hydraulically-actuated forming press. With the die  300  closed, the tip of gas inlet tube  150  extends out from between the die halves  310 ,  320  and a gas supply may be attached to it to supply pressurized gas. In closing the die  300 , the gas inlet tube  150  is pressed into the receiving cavity  330  to form a gas tight seal. Prior to applying a back pressure, there may be space between the upper die half  310  and the upper sheet  100  of the pack  120 . 
         [0026]    At block  930 , diffusion bonding is performed on the pack  120 . Portions of the sheets  100 ,  102  that are not treated with a stop-off material are joined. While maintaining a vacuum (approximately −14.5 psi at sea level) between the sheets  100  and  102  via the gas inlet tube  150 , a back pressure is applied by pressurizing the lower die half  320  via the gas tube  350 . In some embodiments, the back pressure may be between 100 and 600 psi. In addition, a gas tight seal around the gas inlet tube  150  is formed. 
         [0027]    At block  940 , after diffusion bonding has been performed, superplastic forming is performed. Positive gas pressure applied through the lower die half  320  via the gas tube  350  is substantially reduced so the back pressure will be below the forming pressure. Then, a positive gas pressure (the forming pressure) is applied (via the gas port  150 ) between the upper and lower sheets  100 ,  102 . The back pressure forces the upper sheet  100  against the upper die half  310 , while the forming pressure causes the lower sheet  102  to form into the cavities  322 ,  324 . The press that closes the die  300  may apply tonnage to counteract the force of the forming pressure within the pack  120 . 
         [0028]    Temperatures for superplastic forming vary depending upon specific properties of the sheets: alloy composition and crystalline structure, for example. Typically however, temperatures in the range from about 1400 to about 1750° F. are useful for titanium alloys, but other temperatures may be better suited to certain alloys. 
         [0029]    During the superplastic forming, the back pressure is less than the forming pressure. For instance, if the forming pressure is 50 psi, the back pressure may be 25 psi. If the forming pressure is between about 200 and 600 psi, the back pressure may be about 100 to about 500 psi less than the forming pressure. 
         [0030]    In some embodiments, the back pressure may be reduced as the pack  120  is being formed. As but one example, the forming pressure and back pressure start at 300 psi and 200 psi, respectively. As the lower sheet  200  is being formed in the cavities  322 ,  324 , the back pressure is gradually reduced to 100 psi. 
         [0031]    As the pack  120  is being expanded, the gas supplied to the back pressure inlet tube  350  creates a pressure cushion between the lower sheet  102  and the lower die half  330 . The positive gas pressure coming in through gas tube  350  places positive pressure on upper sheet  100  (transferred by its intimate contact with lower sheet  102 ), thus causing the upper sheet  100  to be forced against the surface of the upper die half  310  throughout the superplastic forming. Without the pressure cushion, the forming pressure inside the pack  120  would force the sheet  102  to fill the troughs  322 ,  324 , but in the process, the sheet  100  would move away from the upper die  310  (that is, move sympathetically with the sheet  102 ), whereby mark-off would occur. By creating the pressure cushion, the lower sheet is allowed to form into the cavities  322 ,  324 , but the upper sheet  100  is forced against the surface of the upper die half  310  and thereby prevented from moving sympathetically with the sheet  102 . Consequently, surface mark off is suppressed while lower sheet  102  is being formed, and the OML surface of the exterior aircraft assembly has a defect-free, aero-quality finish.