GB2295980A

GB2295980A - Superplastically formed panel

Info

Publication number: GB2295980A
Application number: GB9525520A
Authority: GB
Inventors: Alan Derek Collier; Stephen Harold Johnston; John Eastham
Original assignee: British Aerospace PLC
Current assignee: BAE Systems PLC
Priority date: 1994-12-16
Filing date: 1995-12-14
Publication date: 1996-06-19
Anticipated expiration: 2015-12-14
Also published as: GB2295980B; GB9525520D0; GB9425461D0

Abstract

A composite superplastically formed panel comprising a pair of superplastically-formed sheets (4, 8) and a further sheet (6) generally conforming in shape to the overall contour of the panel and being sandwiched between the superplastically-formed sheets and joined thereto by bonds (14). The shape-conforming sheet and the superplastically-formed sheets together form a plurality of cells (16) on either side of the shape-conforming sheet (6); the side walls (20) of the cells being formed by one of the said superplastically-formed sheets (4, 8) that is doubled back upon itself about one of the said bonds to form double thickness of the said superplastically-formed sheet. <IMAGE>

Description

Superplasticallv Formed Panel Technical Field The present invention relates to a panel that may be formed by diffusion bonding and superplastic forming (DB/SPF).

Background Art Combined diffusion bonding and superplastic forming is an established technique for making composite articles from materials which 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 US-5 143 276, US4 534 503, GB-2 030 480, GB-2 129 340, US4 607 783, US-4 351 470, US-4 304 821 and EP-0 502 620, 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 to, the face sheets; in the superplastic forming process, the face sheets are forced apart and because the internal core sheet is 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.

US4304821 and US-5143276 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 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 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 present invention provides a panel that is more robust than prior art panels by the provision of an internal shear web that conforms to the overall shape of the panel and provides resistance to shear forces acting on the panel.

Disclosure of the Invention According to the present invention, there is provided a composite superplastically formed panel comprising: a pair of superplastically-formed sheets and a further sheet generally conforming in shape to the overall contour of the panel (the "shape-conforming sheet") and being sandwiched between the superplastically-formed sheets and joined thereto by bonds, wherein the shape-conforming sheet and the superplastically-formed sheets together form a plurality of cells on either side of the shape-conforming sheet, the side walls of each cell being formed by one of the said superplastically-formed sheets that is doubled back upon itself about one of the said bonds to form a double thickness of the said superplastically-formed sheet.

The "shape-conforming sheet" is such that, if the panel is substantially planar, then the shape-conforming sheet is also substantially planar and if the panel is curved or domed in shape, then so is the said core sheet. Thus the said sheet cannot zigzag between the outer sheets of the panel.

The panel may further include face sheets forming the outer faces of the panel.

The bonds between the shape-conforming sheet and the superplastically-formed sheets may be brought about by diffusion bonding, explosive bonding, welding or indeed any other process that forms strong bonds between the sheets.

The use of the said shape-conforming sheet within the panel stabilises the cell structure and hence increases the strength of the panel both with regard to the shear forces parallel to the plane of the panel as well as forces acting perpendicularly to the panel. It is known,e.g. from US-5 143 276, to provide planar face sheets on SPF panels with the intention of providing the panel with a planar outer face. However, such face sheets do not provide the resistance to shear forces provided by the present invention.

The material forming the side walls of the cells should have superplastic properties at an elevated temperature, for example titanium and aluminium and alloys thereof. The shapeconforming sheet may also have superplastic properties but it is not essential that it does.

The two individual thicknesses of superplastic material that constitute the doubled-back walls of the cells are preferably bonded together to form a single composite structural wall; this is preferably achieved during the superplastic forming process by holding the panel at an elevated temperature for sufficiently long for the diffusion bond between the two wall sections to form. The two thicknesses may lie adjacent to each other or a spacer or reinforcing member may be included between them, as described in US-A9351470.

The panel may be used to form a beam, bar, strut or frame or some such similar structure, particularly for use in constructing aircraft, as described in a copending patent application claiming priority from UK Patent Application No 9425447.1.

Description of the Drawings The invention will be further described, by way of example only, with reference to the accompanying drawings in which: Figure 1 shows a pack of titanium sheets that can be used in the present invention; Figure 2 shows the pack of sheets of Figure 1 after they have been bonded together; Figure 3 shows a panel that has been superplastically formed from the bonded pack of Figure 2; Figure 4 shows a detailed cross section of part of the panel of Figure 3; Figure 5 shows an isometric view of the panel of Figures 3 and 4; Detailed Description of the Preferred Embodiments Referring to the accompanying Figures, and initially to Figure 1, a stack or pack 10 composed of three sheets 4,6,8 is formed, the sheets being made of a material that has superplastic properties at elevated temperature, for example titanium, aluminium or alloys thereof.Stop-off material 12, e.g. silica, is applied to selected areas between adjacent sheets of the pack to prevent diffusion bonding of the pack in those selected areas. Other areas 14 are not covered by stop-off material. The uncovered areas 14 preferably form an array of grid lines, which if the grid lines are in a rectangular array, results (see below) in the panel having rectilinear cells.

The assembled pack of sheets 10 is then placed in a heated press (not shown) and compressed at a temperature and for a time sufficient to diffusion bond the sheets of the pack together in areas 14 that are not covered by stop-off material. Instead of diffusion bonding, the sheets of the pack may be bonded together in the said selected areas by other means, for example explosion bonding or welding but diffusion bonding is preferred.

Gas supply pipes (not shown) are provided in the bonded pack 10 to supply inert gas to the selected areas 12 within the pack for superplastic forming. In order to facilitate the supply of inert gas to all the areas 12 within the pack, adjacent areas can be connected together, as is known, by openings within the pack 10; alternatively an external manifold could be used.

The bonded pack 10 is then placed in a superplastic forming mould (not shown) and using well known superplastic forming techniques, inert gas is injected into the stopped off areas 12 of the pack to "inflate" the outer sheets 4,8 of the pack to conform to the internal shape of the superplastic forming mould. During superplastic forming a number of generally rectilinear closed cells 16 are formed on either side of the inner core sheet 6, the cells having side walls 20 and an outer wall 22. As can be seen in Figure 4, which shows a detailed section of the panel, the superplastic forming process forces part of the outer sheets 4 and 8 away from the central core sheet 6; however, in the linear regions 14 where the outer sheets 4,8 are bonded to the core sheet, the outer sheets cannot move away from the core sheet 6 and so the outer sheets stretch and form folded-back doublethickness side walls centred about the bonds 14. The superplastic forming process is performed in such a way that the two thicknesses of the side walls 20 are diffusion bonded together to form a single composite wall.

The core sheet 6 remains during the superplastic forming process in substantially its original planar shape and provides increased rigidity to the panel, as compared to a conventional superplastic formed panel.

An isometric view of the panel is shown in Figure 5.

Claims

1. A composite superplastically formed panel comprising: a pair of superplastically-formed sheets and a further sheet generally conforming in shape to the overall contour of the panel and being sandwiched between the superplastically-formed sheets and joined thereto by bonds, wherein the shape-conforming sheet and the superplastically-formed sheets together form a plurality of cells on either side of the shape-conforming sheet, the side walls of each cell being formed by one of the said superplastically-formed sheets that is doubled back upon itself about one of the said bonds to form a double thickness of the said superplastically-formed sheet.

2. A panel as claimed in claim 1, wherein the two individual thicknesses of superplastic material that constitute the doubled-back walls of the cells are bonded together to form a single composite structural wall

3. A panel as claimed in claim 2, wherein the two thicknesses that constitute the doubled-back walls lie adjacent to each other.

4. A panel as claimed in claim 2, wherein a spacer or reinforcing member is included between the two thicknesses that constitute the doubled-back walls and the spacer or reinforcing member is bonded to the said two thicknesses.

5 A panel as claimed in any one of claims 1 to 4, wherein the cells are rectilinear in shape.

6 A panel as claimed in any one of claims 1 to 4, wherein the shape-conforming sheet is made of a material having superplastic properties.

7. A method of forming a panel as claimed in any one of claims 1 to 6, which method comprises: forming a stack or pack comprising the shape conforming sheet sandwiched between a pair of sheets of superplastically formable material, bonding the shape conforming sheet to each of the superplastic sheets around the perimeter of at least one area, the interface between the shape conforming sheet and the sheets in the or each area including stopping-off material, superplastically forming the superplastically formable sheets by injecting inert gas into the stopped-off areas to form a composite structure comprising the shape-conforming sheet and the superplastically-formed sheet that together form a plurality of closed cells on each side of the shape-conforming sheet, the cells comprising side walls and an outer wall, the side walls of each cell being formed by the said superplastically-formed sheet that is doubled back upon itself about bonds between the shape-conforming sheet and the superplastically-formed sheets to form double thicknesses of the said superplastically-formed sheet.

8. A panel substantially as hereinbefore described in connection with and as illustrated in Figures 1 to 5 of the accompanying drawings.

9. A method of forming a panel substantially as hereinbefore described in connection with Figures 1 to 5 of the accompanying drawings.