Electric heating – Metal heating – By arc
Patent
1998-07-10
1999-11-30
Ryan, Patrick
Electric heating
Metal heating
By arc
228157, 228193, 228206, 228265, B23K 2600, B23K 2000
Patent
active
059946660
DESCRIPTION:
BRIEF SUMMARY
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 to Hamilton, utilize 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 to 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 comple
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Buldhaupt Frederick W.
Gane David H.
Kistner Matthew G.
Will Jeffrey D.
Knapp Jeffrey T.
Neary J. Michael
Ryan Patrick
The Boeing Company
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