Patent Publication Number: US-6656603-B2

Title: Multisheet sandwich structures with throughholes

Description:
REFERENCE TO RELATED APPLICATIONS 
     The present invention is a divisional application based upon U.S. patent application Ser. No. 09/386,737, filed Aug. 31, 1999, now U.S. Pat. No. 6,508,394, which was a divisional application based upon U.S. patent application Ser. No. 09/101,688, filed Jul. 10, 1998, now U.S. Pat. No. 5,994,666, which claimed priority as a 371 from International Application PCT/US96/20106, filed Dec. 20, 1996, which claimed the benefit of U.S. Provisional Patent Application 60/010,033, filed Jan. 12, 1996. 
    
    
     TECHNICAL FIELD 
     This invention pertains to fabrication of expanded multisheet metallic structures, and more particularly to apparatus and methods for superplastic forming of a pack made up of multiple sheets of superplastic metal, welded together using laser welding and expanded in a preheated die using a precisely controlled gas forming schedule. 
     BACKGROUND OF THE INVENTION 
     Multisheet superplastically formed, diffusion bonded, expanded metal sandwich structures have been in use for many years, primarily in the aerospace industry, because of the low cost, high temperature capability and good strength and stiffness per unit weight that these structures offer. Various processes for fabricating these structures have been developed in the past, with various degrees of success, but all have proven costly and slow to produce, and often they have been prone to produce defective or unreliable parts. 
     Most of the existing techniques for fabricating such structures, including the truss core technique shown in U.S. Pat. No. 3,927,817 (Hamilton), use superplastic forming of a stack of sheets in a die having a cavity shaped like the final sandwich structure. The stack includes one or more core sheets that are selectively joined to each other, when there is more than one core sheet, and to a top and bottom sheet that form the top and bottom outside skins of the sandwich structure. The stack is inflated at superplastic temperature with gas pressure to expand the top and bottom sheets outwardly against the interior walls of the die cavity with gas pressure in a die cavity to the desired exterior dimensions. During superplastic forming, the core sheets stretch between their attachment areas to the top and bottom skins as those skins expand toward the boundary surfaces of the die cavity. 
     Early developments of techniques for fabricating multi-sheet expanded metal sandwich structures utilized diffusion bonding to join the core sheets along selective areas to produce the desired core structure. These techniques require the accurate placement of stop-off to prevent diffusion bonding in areas where adjacent sheets were not intended to bond together. Diffusion bonding is a desirable joining method because the junction retains superplastic qualities, but it has been difficult to produce a clean junction line free of stop-off that is narrow enough, and diffusion bonding can be a lengthy process with long holding times in the press at elevated temperatures, preventing the press from being used for other production. The capital intensive and time consuming nature of the diffusion bonding process lead to research into other techniques for joining the core sheets of multisheet stack that would be faster, more reliable, and less costly. 
     Another joining method, shown in U.S. Pat. Nos. 4,217,397 and 4,304,821 (Hayase et al.), uses resistance welding of the core sheets along the selected lines to establish the junction lines between the core sheets, leaving gaps in the weld lines for passage of forming gas into the cells. This process was faster than the diffusion bonding technique, but still required that the core and face sheets not be loaded into a hot die to avoid premature diffusion bonding of the core sheets to each other. After closing the die, the stack could be purged and pressurized to slightly inflate the stack and separated the sheets from one another so that they would not diffusion bond together where no bonding was desired. The die would then be heated to superplastic temperature and forming gas would be admitted under pressure into the stack to superplastically expand the top and bottom sheets against the walls of the die cavity and stretch the core sheets between the top and bottom sheets to form the desired sandwich structure. 
     To prevent premature diffusion bonding of the face sheets in the stack with the core sheets, a device is used in the apparatus of the Hayase et al. patents to hold the face sheets spaced apart from the core sheets. Eight separate tooling pieces are shown for this purpose, which increases the cost and complexity of the forming process. For a high rate production operation, it would be preferable to simplify the tooling and enable the parts to be loaded into the die while it is hot, to achieve an increased production rate and lower production cost. 
     For successful forming to occur, a pressure differential must be established between the face sheet zones and the core sheet zones, and this pressure differential must be equalized over both face sheet zones. Otherwise, the core sheets will form unevenly and will result in excessive thinning. 
     Heating titanium to a high temperature in the presence of oxygen creates a surface layer of alpha case which is a hard but very brittle composition and is unacceptable in structural parts because of its tendency to crack. Such cracks could grow in a fatigue environment and lead to failure of the part. Consequently, it is desirable to purge oxygen and moisture from the stack of sheets of before heating to elevated temperatures. An ideal process would be one in which the stack of sheets is sealed and purged of oxygen and moisture before loading so the sealed pack could be loaded into a hot die without the danger of alpha case forming before the stack is purged and without using expensive press time to purge the stack and then slowly bring the die up to superplastic temperature. 
     Another fastening technique, shown in U.S. Pat. No. 4,603,089 to Bampton, uses a CO 2  laser to weld sheets in the stack together. Bampton does not teach any way to hold the sheets together while they are being laser welded, and indeed does not disclose any apparatus at all to perform the welding operation. In fact, in a production operation for making a laser-welded multisheet expanded metal sandwich structure, such as that shown in U.S. Pat. No. 5,330,092 to Gregg et al., it is necessary to press the sheets into intimate contact to obtain a quality weld, and to do so with a high speed, efficient, high production rate apparatus in order to benefit from the potential benefits that laser welding has to offer. In addition to exerting a press-up force on the sheets during welding, such an apparatus ideally would protect the weld area from oxidation at high temperature that occurs during laser welding of titanium. 
     Weld cratering and tight radii at the start and stop of the weld are inherent limitations of laser welding. They are the consequences of the high intensity, narrowly focused nature of the beam, and have in the past resulted in sharp termination points that concentrated stresses at those points which can rip the core sheet when the core is pressurized by forming gas during superplastic forming. The laser naturally produces a “keyhole” weldment that forms a crater at the weld termination, severely undercutting the top sheet at the end point of a stitch weld. Such welds weaken the top sheet of the core stack at the weld termination at a point that experiences high stress during inflation by gas pressure during superplastic forming. A production process that optimally utilizes the potential benefits of laser welding would eliminate these weak points at the beginning and terminating ends of the weld. 
     SUMMARY OF THE INVENTION 
     Accordingly, the present invention provides an improved process for forming multisheet expanded metal sandwich structures, and the structure made thereby, having face sheets diffusion bonded to internal webs wherein the webs have reduced thinning and hence superior strength over similar structures made by prior art processes. The present invention also provides an improved method of cleaning sheets to be diffusion bonded to improve the percentage of well bonded parts produced in production. Another feature of this invention is to provide a traveling laser welding head having a pressure foot for pressing up the sheets in a stack to be laser welded. Another feature of this invention provides an improved process for laser welding a stack of sheets wherein the sheets in the stack are pressed into intimate contact around the region of the weld to ensure good weld quality. In the preferred process of laser welding superplastic sheets for later superplastic forming, stop-off is applied to the sheet interface to prevent diffusion bonding, and the sheets are laser-welded through the stop-off. In yet another aspect, this invention provides a process of forming a multisheet expanded metal sandwich structure by superplastic forming/diffusion bonding wherein the pack of sheets to be formed into the sandwich structure are loaded and unloaded into a superplastic forming die at high temperature. Yet another feature provides an improved process and structure for securely attaching a gas supply line to a pack for superplastic forming/diffusion bonding thereof into a multisheet expanded metal sandwich structure. An improved method laser welds metal sheets together avoiding cratering, undercutting and pointed laser weld end points that can cause concentration of stress in the welded structure and tearing of the sheets when the pack is inflated during superplastic forming. Finally, this invention also provides a method of making a multisheet expanded metal sandwich structure having sealed openings through the sandwich structure for access through the structure for fasteners, fluid or electric lines, control cables or the like. 
     A preferred process for making an expanded metal sandwich structure includes cleaning at least two metal sheets having superplastic characteristics for forming a core of the sandwich structure to remove metal oxides and residues that would interfere with diffusion bonding of the sheets. At least one surface of at least one of the core sheets is coated with a stop-off compound such as boron nitride to prevent that surface from diffusion bonding to other sheets. The core sheets are placed in a vertical stack, with the stop-off coated surface of the one sheet facing the other sheet. A traveling weld head presses the core sheets together and laser-welds the core sheets through the stop-off along lines which will be along one or more planes located within the metal sandwich structure intermediate the thickness thereof. The laser-welded core sheets form a core pack. A gas pressure line fitting is inserted between the core sheets along at least one edge. The fitting has a through bore that communicates between the exterior of the core sheets and an interior region therebetween. The core pack is seal welded around its peripheral edge and the gas pressure line fitting is welded to the core sheets. The gas pressure line fitting is plugged and the core pack is chemically cleaned to remove metal oxides and residues that would interfere with diffusion bonding of the sheets. 
     Two additional superplastic metal face sheets are chemically cleaned to remove oily contamination, metal oxides and residues that would interfere with diffusion bonding of the sheets to the core pack. These sheets are placed one each on the top and bottom faces of the core pack. An envelope gas fitting is positioned in a notch in the core pack between the face sheets and the peripheral edges of the face sheets and the core pack are seal welded. The gas fittings are also seal welded between the face sheets to produce a sealed envelope pack enveloping the core pack, producing a full pack, with gas fittings into the core pack and into a face sheet zone between the face sheets and the core pack. 
     A gas supply tube is connected from a gas supply control system to each of the fittings, and air and moisture are purged from the packs. The packs are pressurized with an inert forming gas such as Argon, the core pack being pressurized to a higher pressure than the full pack. A die is selected having an internal cavity with the same shape as the desired shape of the metal sandwich structure after it is expanded. Preheated to about the superplastic temperature of the metal, the die is opened to receive the full pack. In the die cavity, the temperature of the full pack raises to the superplastic temperature of the metal, and forming gas is injected through the fittings to inflate the envelope pack to the interior walls of the cavity, and to inflate the core pack to the envelope pack. The full pack also inflates around the inserts to produce a sealed hole through the sandwich structure. After forming is completed, the forming gas pressure is reduced to near ambient, and the forming gas pressure in the core pack is reduced to near ambient, just enough to ensure that the cooling of the part does not pull a vacuum that would tend to produce hollows in the part between the webs. The die is opened and the formed pack is removed from the die while still at an elevated temperature above 1600° F. The formed pack is allowed to cool below 900° F. while remaining connected to the gas supply system, and the gas supply lines are then removed from the gas fittings. Portions of the peripheral flange holding the gas fittings are trimmed off of the formed pack. 
     If sealed openings through the sandwich structure are needed for fasteners, fluid or electric lines, control cables or the like, a circular laser weld may be made in the full pack before it is superplastically expanded to seal weld around the region where a hole will be cut. The hole can then be cut inside the circular seal weld to produce a sealed opening through the full pack. Insert tubes having a length equal to the height of the die cavity are placed in the sealed holes in the full pack, and the pack forms around the inserts as it inflates, producing sealed openings of the desired opening diameter in the sandwich structure. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention and its many attendant objects and advantages will become more clear upon reading the following description of the preferred embodiment in conjunction with the following drawings, wherein: 
     FIG. 1 is a perspective view of a four-sheet expanded metal sandwich structure made in accordance with this invention; 
     FIG. 2 is a schematic exploded diagram showing the four sheets which make up the sandwich structure shown in FIG. 1; 
     FIGS. 3A-C are orthogonal views of a gas fitting used in this invention; 
     FIG. 4 is a perspective view of a seal-welded core pack for use in making the sandwich structure shown in FIG. 1; 
     FIG. 5 is a perspective view of the core pack shown in FIG. 4, partially inflated; 
     FIG. 6A is a perspective view of a motion control table and laser welding apparatus for welding the core pack for the sandwich structure shown in FIG. 1; 
     FIG. 6B is a perspective view of a motion control table and laser welding apparatus shown in FIG. 4A, but with a cover sheet over the grooves in the table to prevent loss of argon; 
     FIGS. 7 and 8 are elevation and plan views, respectively, of a pressure trolley used to press the sheets together during welding and blanket them with inert gas; 
     FIGS. 9A-9G are schematic diagrams showing the laser stitch weld process in accordance with this invention; 
     FIG. 10 is a process flow diagram showing the process of this invention; 
     FIGS. 11A-11E are schematic diagrams showing the superplastic forming of the welded pack to produce the sandwich structure shown in FIG. 1; 
     FIGS. 12A-12C are schematic sketches showing the formation of webs around the laser welds in the sandwich structure shown in FIG. 1; 
     FIGS. 13 and 14 are comparison views of resistance roller welded weldments and laser welded weldments; and 
     FIG. 15 is a photograph of a typical sandwich structure having a reinforced throughhole. 
    
    
     DESCRIPTION OF A PREFERRED EMBODIMENT 
     Turning now to the drawings, wherein like reference numerals designate identical or corresponding parts, and more particularly to FIG. 1, a four-sheet expanded metal sandwich structure  30  made in accordance with a preferred method of this invention is shown having a top skin  32 , a bottom skin  34 , and a plurality of webs  36  extending between and integrally connected to the top and bottom skins. The webs  36  are preferably arranged as indicated to form a plurality of square cells  38 , although cells of other shapes can be made, such as hexagonal cells made with webs made in a hexagonal pattern. 
     The sandwich structure shown in FIG. 1 is made from four sheets of a metal, such as titanium 6-4 alloy, which has superplastic and diffusion bonding characteristics. Superplastic characteristics include the capability of the metal to develop unusually high tensile elongations and plastic deformation at elevated temperatures, with a reduced tendency toward necking or thinning. Diffusion bonding refers to metallurgical joining of two pieces of metal by molecular or atomic co-mingling at the faying surface of the two pieces when they are heated and pressed into intimate contact for a sufficient time. It is a solid state process resulting in the formation of a single piece of metal from two or more separate pieces, and is characterized by the absence of any significant change of metallurgical properties of the metal, such as occurs with other types of joining such as brazing or welding. The characteristics of superplastic forming and diffusion bonding are now reasonably well understood, and are discussed in detail in U.S. Pat. No. 3,927,817 (Hamilton) and U.S. Pat. No. 4,361,262 (Israeli). 
     Turning now to FIG. 4, a stack  42  of four sheets that make up the sandwich structure shown in FIG. 1 are shown in exploded form to shown their relative positions and the relative positions of the features on and between the sheets. The stack  42  includes two core sheets  44  and  46  and top and bottom face sheets  48  and  50 . A core gas fitting  52  is inserted between the two core sheets  44  and  46  that are later welded together to make up a core pack  45 . The core gas fitting provides a connection to a forming gas supply system for inflation of the core pack  45  during superplastic forming, wherein the core pack  45  is superplastically inflated as illustrated in FIG.  5  and described in more detail below. An envelope gas fitting  54  is inserted into a notch  56  in the core pack  45  communicating with the interior of an envelope pack  47  made up by the welding around the periphery of the face sheets  46  and  48 . The envelope gas fitting  54  provides a gas flow path into the space above and below the core pack  45  and between the face sheets  48  and  50  for superplastic forming, as described below. The gas fittings  52  and  54  are welded into the stack  42  later in the process. 
     The sheets  44 - 50  are cut to the desired size, which is the size and shape of the plan form of the sandwich structure part, plus about 2″-6″ for a flange  58  around the part by which the part may be clamped in the superplastic die, and by which it may be attached into the assembly for which it is intended. After cutting, the sheets are wiped with acetone to remove ink markings printed on the sheets by the manufacturer. The sheets are then chemically cleaned. As used herein, the term “chemically cleaned” refers to subjecting the surface to reactive agents that by immersion, first in an alkaline bath to remove grease and other such contaminants, and then in an acid bath, such as 42% nitric acid and 2.4% hydrofluoric acid to remove metal oxides from the titanium alloy sheets. The cleaned sheets are rinsed in clean water to remove residues of the acid cleaner, but residues from the rinsing solution remain on the sheets after removal from the rinsing bath. These residues are removed from the sheets by wiping with a fabric wad, such as gauze cloth, wetted with a reagent grade solvent such as punctilious ethyl alcohol. The sheets are wiped until the gauze comes away clean after wiping. The alcohol evaporates leaving no residue and leaving the sheets free of contaminants that would interfere with a complete and rapid diffusion bond when the conditions for such a bond are established. 
     One side of at least one of the core sheets  44  and  46  is coated with a stop-off compound such as boron nitride. For large area surfaces, the boron nitride may be dissolved in a solvent such as a mixture of water and alcohol and sprayed with an electrostatic sprayer onto the entire surface area of the one side of the one sheet. The water and alcohol evaporate, leaving a thin even coating of boron nitride on the surface of the titanium sheet. For smaller surfaces, the stop-of may be sprayed from an aerosol can of a solution of boron nitride in an alcohol solution that is commercially available from the Cerac Company in Milwaukee, Wis. 
     The coated sheet is aligned with and abutted against the other sheet, with the boron nitride coated face facing the other sheet. The two core sheets  44  and  46  are laser welded in the pattern shown in FIG. 4 on a laser welding apparatus shown in FIGS. 6A and 6B, purchased from Convergent Energy Corp. in Sturbridge, Mass. The apparatus  59  includes a CNC motion control table  60  on which the sheets  44  and  46  are placed and secured in an aligned stack. A vertically movable plunger is mounted over the table  60 . The plunger has a fitting on which a pressure trolley  62 , shown in more detail in FIGS. 7 and 8, is mounted for exerting a vertical force on the sheets to press them into intimate contact during laser welding by a laser beam aimed vertically downward through the center of the trolley  62  at the table. 
     The motion control table  60  has a series of parallel grooves  64  opening upwardly in its upper surface, and a perforated tube  66  in each groove. The tubes  66  are connected to a manifold that is connected through a pressure control valve (not shown) to a supply of Argon gas. Argon is admitted through the tubes to flood the area between the table  60  and the bottom core sheet  46  and displace oxygen and nitrogen from the area. As shown in FIG. 6B, the area of the table  60  not covered by the sheets  44  and  46  may be covered by another sheet which is held down on the table  60  by weights or magnets to save argon. The motion control table  60  is connected to actuators, such as ball screws or the like, to move the table  60  on slides in the X-Y directions on a horizontal plane under the pressure trolley  62 . 
     The laser apparatus (not shown) has an optical system for directing and focusing the laser beam from the a laser generator vertically through a threaded connector  68  on the trolley  62 . The connector  68  can be loosened to turn the trolley  62  to point in a desired direction and then tightened with a jam nut to remained aimed in that direction. A gas flood nozzle  70 , best shown in FIG. 7, is mounted at the lower end of the connector  68  for directing a wide, low pressure stream of helium onto the top core sheet  44  at the weld point where the laser beam impinges the top sheet  44 . The helium gas displaces oxygen and nitrogen from the weld point and prevents formation of titanium oxides and nitrides over the laser weldment. Helium is used instead of argon because it enhances the plasma, whereas argon gas tends to suppress the plasma. The gas flooding technique is used to avoid disturbing the puddle of molten titanium alloy, instead of a high pressure narrow gas jet which is used in laser cutting to displace the molten metal out of the kerf to leave a narrow cut through the metal. 
     The laser welding pressure trolley  62  has a top plate  72  with a center hole  74  around which the threaded connection  68  is welded. Four sheet metal sides  76  depend from the top plate  72  to provide an enclosure for a cloud of helium to blanket the weld point and surrounding area. A pair of spring loops  78  is fastened one each on each side of the trolley  62 , and supports a pair of bars  80  attached, as by screws  82  to the spring loops  78 . Two spaced axles  84  are journaled in holes in holes in the bars  80 , spanning the bars and supporting cylindrical wheels  86 . 
     For curved laser welds such as circular welds used to make “egg crate” three sheet expanded metal sandwich structures or sinusoidal welds used to make four sheet sinusoidal web sandwich structures, an omnidirectional capability for rolling the trolley on the core pack would be desirable. The cylindrical wheels  86  roll best in the plane normal to their axis of rotation. Omnidirectional rolling capability is provided by spherical ball rollers, each mounted in the end of a piston spring or air pressure biased in a cylinder opening in the underside of the trolley  62 . The ball rollers should be fairly large, preferably about an inch in diameter, and should be at least six in number spaced uniformly in a circle around the gas flood nozzel  70 . 
     In use, a pair of core sheets  44  and  46  are laid on the table  60  and secured thereon by clamps  88 . The argon gas is turned on to flood the underside of the bottom core sheet  46  through the tubes  66  with inert gas and displace the oxygen and nitrogen from the region between the table  60  and the underside of the sheet  46 . The flow rate will depend on the table size, but for a table 36″ square, a flow rate of about 20 cubic feet per hour is sufficient. The controller for the table actuators is programmed with the speed and dwell of the table movement and the spacing between adjacent welds. The trolley  60  is aligned with the plane of its wheels  86  parallel with the grooves  64  in the table  60 , and the vertical motion mechanism on which the trolley  62  is mounted is lowered to engage the wheels  86  with the top surface of the top sheet  44 . The vertical extension of the vertical motion mechanism is selected to deflect the spring loops  78  to the extent necessary to produce the desired compressive force exerted by the wheels  86  on the top sheet  44 . For example, a 38 pound force can be exerted by two titanium spring loops ½″ wide and 3½″ long when deflected about ¾″. This would be sufficient force to press two titanium alloy sheets 0.025″ thick into such intimate contact that an excellent laser weld, substantially free of porosity, would be obtained. 
     The helium gas is turned on at a flow rate of about 40 cubic feet per hour and after purging the air from the sheet metal enclosure  76 , the laser is turned on with a power of about 650 watts, continuous wave. At the start of the weld, the laser is allowed to dwell for about 0.25 seconds at the beginning point in the weld, as shown in FIG.  9 A and FIG.  9 B. The dwell time melts a small puddle of titanium and forms a bulbous node  90  at the start of the weld instead of the usual sharp point at the start of a laser weld which can concentrate forces at a small point and initiate a tear in the metal which could then propagate through the weldment or elsewhere in the material. 
     After the dwell period, the table  60  is moved as indicated in FIG. 9C to cause the weld point to travel along the sheets  44  and  46  to produce a weld line  92 . The speed of travel is selected on the basis of the laser power and thickness of the sheets. For two sheets of titanium 6-4 alloy 0.025″ thick and a laser power of 650 watts, a table speed of 60 in/min produces a complete weld of excellent quality. As the weldpoint passes the midpoint of the weld line  92 , the laser power is ramped down gradually until it eventually decouples from the sheets  44 / 46 , as indicated in FIG.  9 D. The table  60  is now moved to position the laser point at the other end of the weld line and the laser is turned on again and allowed to dwell as before to create another bulbous node  90  at the other end of the weld line, as indicated in FIG.  9 E. After the dwell, the table is moved to move the laser weld point toward the first end of the weld, as indicated in FIG. 9F, and as the laser reaches the weld line midpoint, the laser power is ramped down gradually so that the laser welds back over the portion previously welded, as shown in FIG.  9 G and then decreases in power until the laser eventually decouples from the sheets  44 / 46 . The gradual ramp-down in power prevents the formation of an undercut terminal portion in a laser weld which otherwise occurs when the laser power is terminated abruptly. 
     The weld pattern, shown schematically in FIGS. 2 and 4, is in the form of a series of orthogonally aligned crosses  94 , or considered differently, is a grid pattern with interruptions or gaps  96  in the weld lines midway between each intersection  98  in the weld lines. The gaps  96  in the weld lines  92  provide a passage through which forming gas can flow when the core pack  45  is superplastically formed by heating to about 1760° F. in a die and injecting forming gas through the core gas fitting  52 , as illustrated schematically in FIG.  5 . When the core pack  45  is inflated, the gaps  96  open to provide near circular openings  100  in webs  102  formed by the material of the top and bottom core sheets  44  and  46  as the material stretches superplastically away from the laser welds  92 . 
     After the grid pattern is laser welded into the sheets  44  and  46 , the sheets  44  and  46  are seal welded completely around their periphery and around the core gas fitting  52  to fully seal the periphery of the core pack  45 . A convenient type of welding for this purpose is gas tungsten arc welding (also referred to as TIG welding) wherein the welding arc can be directed into the edge face of the sheets  44  and  46 . A conventional stainless steel compression coupling such as a Swagelock coupling (not shown) is attached to the gas fitting  52 , and one end of a short length of stainless steel gas tubing is attached to the compression coupling. The other end of the tube is pinched shut and welded closed to seal off the interior of the core pack  45  against intrusion of cleaning solution for the following cleaning operation. 
     The sealed core pack  45  is cleaned by immersion in the alkaline bath and the pickling bath as describe above and is wiped with a fabric wad wetted with punctilious alcohol, as also described above. The cleaned core pack  45  is assembled between the cleaned face sheets  48  and  50 , with the envelop gas fitting  54  positioned in the notch  56 , and the periphery of the two face sheets  48  and  50  plus the core pack  45  is seal welded all around and around the envelop gas fitting  54  to produce a full pack  110  which is completely sealed, except for the gas flow path provided into the envelop pack  49  between the face sheets  48  and  50  through the envelop gas fitting  54  and the notch  56 . 
     The envelop gas fitting  54  is sealed with another pinched and welded tube in a compression coupling, as described above for the core pack  45 , and the full pack is cleaned as before. After cleaning, the full pack is now ready for superplastic forming and diffusion bonding to produce the expanded metal sandwich structure of this invention. The process is schematically illustrated in FIGS.  10  and  11 A-E and described below. 
     The external surface of the pack  110  are coated with a parting agent, such as the boron nitride stop-off described above. Compression fittings are attached to the gas fittings  52  and  54  and gas lines from a forming gas control system, such as that described in U.S. Pat. No. 5,419,170 (Sanders) are connected to the compression couplings. The full pack is purged with inert gas, such as argon, to remove air and moisture from inside the envelop pack  49  and the core pack. This may be accomplished with several cycles of alternate vacuum suction and backfilling with argon under a pressure of about 0.5 psi in the envelop pack  49  and about 10 psi in the core pack  45 , until the interior of the packs  45  and  49  are purged clean of air and moisture. The packs  45  and  49  are now pressurized with argon to separate the surfaces from each other. The pressure inside the core pack  45  is preferably higher than the pressure in the envelop pack  49  because the grid welds  92  tend to hold the core sheets  44  and  46  together more tightly than the peripheral weld holds the face sheets  48  and  50  together. The initial pressure is about 0.1 psi in the skin zone within the envelop pack and about 10 psi in the core pack  45 . This provide sufficient pressure to prevent contact and premature diffusion bonding between the facing surfaces of the sheets, but not so high as to cause premature pillowing of the core envelop or tearing of the sheets at the laser welds or the peripheral welds. The pressurized pack  110  is placed in a die  112  preheated to about 1760° F., and the die is closed with a superplastic forming press (not shown). The die may be provided with grooves extending from an internal cavity to the exterior in which the gas fittings  52  and  54  lie to avoid squeezing shut the gas passages through the flange  58 . After closing the die, the pressure of the forming gas in the envelop pack  49  is immediately increased to ensure expansion of the face sheets  48  and  50  away from the core pack  45 , and the pressure in the core pack  45  is also increased to resist the compression of the gas pressure in the envelop pack  49 . 
     After the pack reaches forming temperature inside the die  112 , the pressure in the envelope pack  49  and the core pack  45  is increased to forming pressure, and the sheets  44 ,  46 ,  48  and  50  superplastically stretch as shown in FIGS. 11C and 11D, and diffusion bond as shown in FIGS. 11D and 11E. The superplastic forming of the core sheets  44  and  46  around the laser welds  92  is shown in FIGS. 12A-C and in FIG.  8 . The narrow width of the laser weldment  92  provides a small region around which the sheets  44  and  46  must wrap when they fold back over the weldment  92  to diffusion bond together to form the web  102 . This is contrasted with a relatively wide weldment  92 ′ shown in FIG. 13 that is made with a conventional resistance welding wheel. In the web  102 ′ shown in FIG. 13, the thinning in the region above and below the weldment  92 ′ can be seen to be considerably greater than it is for the laser weldment  92  shown in FIG.  14 . 
     After the pack  110  is fully formed, as shown in FIG. 11E, the pressure is reduced to near ambient, about 0.05 psi and the press is opened to open the die  112 . The sandwich part is removed from the die cavity  114  and is allowed to cool while the gas pressure is maintained slightly above ambient to prevent the cooling part from pulling a vacuum and collapsing under air pressure. After cooling below 900° F., and preferably under 500° F., the gas lines are removed from the compression couplings, and the part is sealed with pinched and welded gas lines in the couplings for cleaning any external alpha case that may have formed on the part from high temperature contact of the external surfaces with air. After cleaning, the part may be trimmed to remove the gas fittings  52  and  54 , and the part is completed. 
     If it is desired to have sealed through openings in the part, the full pack  110  may be laser welded in a circle around the region where the opening is to be located. The interior of that region may be cut out with a laser cutter, which uses the same laser and table  60 , but uses a high pressure gas nozzle instead of the gas flood nozzle shown in FIG.  3 . The full pack is placed on a protective mat to prevent cutting or splattering of the surface of the table  60  when the laser cuts through the pack when cutting out the hole in the center of the seal weld region. A sleeve having the same height as the sandwich structure part is inserted in the opening cut out by the laser, and the full pack  110  with the sleeves installed in the openings is placed in the die cavity  114  and is superplastically formed around the sleeves. If the sleeve is a non-superplastic material such as stainless steel, it may be removed or retained, whichever is desired for the application. If a superplastic material such as titanium is use, it will diffusion bond in place in the opening and form the interior of the opening. The sleeve may be provided with a threaded insert for threaded connection to the part later, or the “sleeve” may be a solid titanium slug that can be drilled and tapped after the part has been formed for a connection hardpoint to the sandwich structure. 
     Obviously, numerous modifications and variations of the preferred embodiment described above will occur to those skilled in the art in light of this disclosure, Accordingly, it is expressly to be understood that these modification and variation, and the equivalents thereof, are to be considered to be within the spirit and scope of our invention, wherein we claim: