Stock material or miscellaneous articles – Structurally defined web or sheet – Honeycomb-like
Reexamination Certificate
1999-12-07
2001-10-09
Copenheaver, Blaine (Department: 1775)
Stock material or miscellaneous articles
Structurally defined web or sheet
Honeycomb-like
C428S131000, C264S239000, C029S423000
Reexamination Certificate
active
06299963
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a panel that may be formed by diffusion bonding and superplastic forming (DB/SPF). The invention has particular application in the aerospace industry in the production of panels and structures for constructing aircraft.
BACKGROUND ART
Combined diffusion bonding and superplastic forming is an established technique for making structural components, particularly lightweight components requiring complex internal structure, from materials that exhibit superplastic properties at elevated temperatures. These materials are primarily titanium, aluminium and alloys of both these metals.
In established DB/SPF processes, for example see U.S. Pat. Nos. 5,143,276, 4,534,503, GB-2030480, GB-2129340, U.S. Pat. Nos. 4,607,783, 4,351,470, 4,304,821 and EP-0502620, it is known to apply stop-off material to selected areas of two or more sheets of superplastic material; several sheets, including the sheets to which stop-off material has been applied, are then assembled into a pack with the stop-off material lying between adjacent superplastic sheets. The assembled pack is then heated and compressed until the sheets are diffusion bonded together; however, the sheets are not bonded in the selected areas covered by stop-off material since the stop-off material prevents diffusion bonding in those areas. The superplastic forming step is then conducted by heating the bonded pack, usually in a mould, to a temperature at which the components exhibit superplastic properties. An inert gas is then injected in a controlled manner into the unbonded areas of the pack under high pressure so as to “inflate” the sheets gradually into a three dimensional structure having an outer shape corresponding to the shape of the mould. The configuration of the final composite structure is dependent upon, among other things, the number of sheets in the pack, the location of the stop-off material and the shape of the mould.
It is known, for example from GB-1495655, to form a composite panel from a pack comprising a pair of opposed face sheets and a core sheet sandwiched between, and bonded at selected points to, the face sheets; in the superplastic forming process, the face sheets are forced apart and because the internal core sheet is selectively attached to both of the face sheets, the core sheet adopts a zigzag shape that, in effect, constitutes struts extending from one face sheet to the other.
U.S. Pat. Nos. 4,304,821 and 5,143,276 each describes the making of a panel from four sheets of superplastic material from a pack comprising a pair of opposed face sheets and two core sheets sandwiched between the face sheets; the two core sheets are bonded to each other at selected points by linear welds. The face sheets are superplastically formed by injecting gas into the area between each face sheet and the adjacent core sheet to expand the face sheets into the shape of a mould; gas is then injected between the two core sheets. Because the core sheets are selectively joined by the linear welds, the core sheets expand to form cells extending between the face sheets; the side walls of the cells are formed by U-shaped doubled-back sections of the two core sheets.
The superplastically formed panels produced using these known techniques have many advantages but they are not suitable for withstanding localised high loads, for example where other external components will bear on or are to be attached to the panels.
EP-754098 proposes a process for superplastically forming a part for use as an aircraft component, in which localised pre-thinning of a sheet is employed to facilitate superplastic forming in areas where forming tends to be slow and thus to avoid excess thinning in other areas of the part. In this way, the overall thickness of the sheet can be controlled during forming and hence the strengthening of given areas of the final part is possible.
Another process for stiffening a superplastically formed panel is described in U.S. Pat. No. 4,632,296. In this process, the initial thickness of predetermined areas of the sheets to be formed is selected to control the rate of superplastic deformation of the sheet during forming. This can be used both to avoid areas of malformation and to produce reinforced areas of extra strength in the final panel.
Nevertheless, neither of these two prior references addresses the problem of providing in a superplastically formed panel localised areas capable of withstanding substantial point loads, for example where other components will bear on or require load bearing attachment to the panel.
Furthermore, none of the prior methods provides a superplastically formed panel suitable for machining, post forming, in order to enable the attachment of other components.
The present invention addresses these problems and seeks to overcome them by providing a superplastically formed structure that is more robust than prior art panels and that has localised load bearing areas.
DISCLOSURE OF THE INVENTION
According to one aspect of the present invention, a superplastically formed composite structure comprises:
a panel provided by a pair of superplastically-formed sheets, which together form a plurality of cells, the panel having an insert opening therein defined by a side wall of a respective one, or side walls of respective ones, of the cells; and
an insert plug received in the insert opening and bonded to the said side wall or walls.
The panel may further include face sheets forming the outer faces of the panel.
The insert plug itself is intended to provide a solid relatively hard region in the panel, which is capable of sustaining substantial point loads without collapse, buckling or racture. For example, the structure may in use be subjected to such loads at areas of contact with other components, and the insert plug may be employed in these areas to act as a hinge point or an attachment point for the other component.
The insert plug may accordingly be substantially solid and of generally cylindrical or frusto-conical shape. It may also be pre-formed with a central bore and/or machined after forming according to its intended purpose.
The material forming the cell side wall or walls should have superplastic properties at an elevated temperature, and may for example be titanium or aluminium or alloys thereof. The insert plug may also have superplastic properties and be made from a similar material but this is not essential.
The bonds between the superplastically formed sheets and the insert plug may be brought about by diffusion bonding, explosive bonding, welding or indeed any other process that forms strong bonds to retain the insert plug in place.
In one of the embodiments described below, the insert plug is made from the same material as the superplastically formed sheets and is located in place and diffusion bonded to them during forming.
In another of the embodiments described below, the insert plug is made from a different material and is inserted into the panel opening after forming and first edge welded and then HIP (Hot Isostatic Pressure) bonded in place.
The structure of the present invention may be used to form a beam, bar, strut or frame or some such similar structure, particularly for use in constructing aircraft.
According to another aspect of the present invention, a method of producing a superplastically formed composite structure comprises:
providing a pack or stack of sheets of superplastically formable material;
defining an insert location in the pack and joining the sheets together by bonds at least in the region of the insert location;
placing the pack in a forming tool and superplastically forming the sheets by heating the sheets to a temperature at which they exhibit superplastic properties and by injecting gas so as to expand the pack into a three dimensional panel having a plurality of cells, wherein the insert location becomes an insert opening defined by a side wall of a respective one, or side walls of respective ones, of the cells; and
locating an insert plug in the insert opening and bonding the plug to the said side wall or wall
Banks Samual James
Pilling John Ross Mark
Boss Wendy
British Aerospace, plc.
Copenheaver Blaine
Pillsbury & Winthrop LLP
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