Superplastically Formed Structure and Method of Making

Diffusion bonding at selected points between three sheets of material followed by introducing inert gas between the sheets while held at superplastic forming temperatures makes a lightweight structure with formed outer sheets and the inner sheet acting as a reinforcing core extending between the outer sheets.

A grid patterned core is joined at opposite sided alternate grid intersections to face sheets and the sheets with joined core superplastically formed into a lightweight structure.

FIG. 1 shows a fragmented perspective view of the structure of this invention.

FIG. 2 shows a side elevational sectional view of the invention of FIG. 1 in a mold prior to forming.

FIG. 3 shows the view of FIG. 2 with the structure formed.

FIG. 4 shows a fragmented perspective view of the core of FIG. 1 prior to forming into the structure.

FIG. 5 shows a different embodiment of the core of FIG. 4.

FIG. 6 shows a fragmented side elevational view of the core in FIG. 5 between face sheets and prior to forming.

FIG. 7 shows the view of FIG. 6 but after the part is formed.

FIG. 8 shows a partial view of FIG. 6 showing welding electrodes.

FIG. 9 shows a fragmented perspective view of a structure of a different embodiment of this invention.

FIG. 10 shows a fragmented perspective view of the core of FIG. 9 prior to forming into the structure.

FIG. 11 shows a different embodiment of the core of FIG. 10.

FIG. 12 shows a fragmented side elevational view of the core of FIG. 11 between face sheets and prior to forming.

FIG. 13 shows the view of FIG. 12 but after the part is formed.

FIG. 14 shows a fragmented perspective view of yet another core.

FIG. 15 shows a sectional view taken along line 15--15 of FIG. 14.

FIG. 16 shows the section of FIG. 15 after resistance welding the joints.

Lightweight superplastically formed structure 10 is made up of an upper face sheet or plate 12, a lower face sheet or plate 14, and a grid patterned core material 16. These materials are all from metal alloys having superplastic characteristics to permit forming the structure while at an elevated temperature in the superplastic forming range. Various alloys may be formed in the superplastic state with a titanium alloy preferred due to its lightweight and high temperature resistance.

FIG. 4 shows the core 16 as it looks prior to formation into the structure 10. The grid patterned core is a wire mesh that in FIG. 2 is shown located between the face sheets 12 and 14 to make up an assembly which is placed in a mold 18 made up of formed lower mold 20 and upper cover 22.

The core is preferably resistance spot welded at the intersections and treated with a stop-off compound prior to being placed between the cover sheets with the stop-off covering the core at all places except at alternate crossover joints 24 adjacent the top cover sheet, and alternate crossover joints 26 adjacent the bottom cover sheet. This localized heating of the core material, at the grid intersection, prior to covering with the stop-off compound, converts the metal at the intersection to a Widmanstatten grain structure. Spacers 28 of alloys of the superplastic material are located on each side to permit introduction of an inert gas through line 30a and 30b. The mold also has lines 32a and 32b for introduction of gas into the mold volume at 34.

The assembly is placed in the mold and inert gas introduced to purge the air from the mold. Then the mold is raised to and held at the diffusion bonding temperature to diffusion bond the core at each location where the stopoff compound was not applied. Next, the pressure of the inert gas is raised through line 30a, and with the mold assembly at the superplastic forming temperature face plate 14 moves down to surface 36 taking the joined connections 26 of the core 16 with it to form a truss type member 10.

FIGS. 5 through 8 show a different embodiment of the invention. In this embodiment the superplastically formed titanium structure 38, has upper face plate 40, lower face plate 42, a grid patterned wire core 44, a titanium shim 46 at alternate core joint locations on one side and at each of the other joint locations on the opposite side of the core, and an aluminum shim 48 at the opposite side of the core joints from each titanium shim. In making up the assembly with cover sheets, core and shims the core is placed between the cover sheets with the shims preferably tack welded in place and spot welding electrodes 50a and 50b placed over each joint and each joint is welded together. In another preferred method of joining the core to the face plates, the core is first spot welded to the lower face plates at each intersection where the titanium shims contact that face plate. Aluminum shims are then inserted between the core and the lower face plate at the unwelded intersections, the upper face plate placed over the core, and the previously unwelded intersections are welded between the welding electrodes. It is not required to use the aluminum shims between the upper plate and the core when welding the cover plates to the core by this two step process. The side of the welded joint having the titanium shims will remain welded when the part is superplastically formed. The aluminum shims each provide a path for the electrical energy when spot welding, and convert to titanium aluminide; which readily separates under the conditions when the part is being superplastically formed to destroy any attachment on the aluminum side of the joint. When the joints are being spot welded, the titanium is heated to create a Widmanstatten microstructure. This microstructure in the area of the joint adds a stiffness to prevent deformation of the core and face plates at the joints during superplastic forming.

FIGS. 9 through 13 show yet another embodiment. In this embodiment the superplastically formed titanium structure 52, has upper plate 54, lower plate 56, a grid patterned core 58 prepared from a sheet or plate, a titanium shim 60 at alternate core intersection locations on one side and at each of the other intersections on the opposite side of the core, and an aluminum shim 62 at the opposite side of the intersections from each titanium spacer. The intersections are each spot welded to permanently join the core to the plate at each location having a titanium spacer, and the structure formed by superplastic forming, at which time the core and plate separate at each intersection having an aluminum spacer.

Welding of the core to the cover plates may be accomplished by welding both plates to the core by the one step method; or first welding the core to the lower plate then to the upper plate by the two step method.

FIGS. 14 through 16 show yet another core that is superplastically formed into a truss structure using the methods as set out above. In this embodiment titanium wires 64 are laid out parallel to each other. Then a line of parallel titanium wires 66 are set at an angle of 90 degrees to the first wires, placed on top, and tack welded at each joint. This core may be used for diffusion bonding and superplastic forming. However, it is preferred the core have titanium spacers 68 at alternate joint locations on one side and at each other joint location on the opposite side, and that each joint location then be spot welded together while under pressure to flatten the core and merge the joints as shown in FIG. 16.