DOMED STRUCTURES AND A METHOD OF MAKING THEM BY SUPERPLASTIC FORMING AND DIFFUSION BONDING

Abstract: The present invention provides a method of making a domed structure having a peak (20)using superplastic forming and diffusion bonding techniques. The method includes the step of forming a stack (19) of two or more sheets (10,11,12; 14,16,17; 22,23) made of superplastic material and the sheets of the stack being joined together in face to face contact by metallic bonds (13) such that neither of the sheets so joined is bonded to any other sheet in the stack; the metallic bonds (13) preferably lie on lines extending radially from and/or circumferentially around the said peak. The resulting stack (19) is heated to superplastic temperatures and a pressure differential is established across the stack to form it into a domed shape (19a). The resulting dome is placed in a mould (32) and heated; an inert gas is injected between the sheets to inflate the stack superplastically thereby forming a space between the sheets which is divided into cells (45) by webs (39) formed from two of the said sheets that have been superplastically formed by the inflation process.

Description: This invention relates to domed structures manufactured by a superplastic forming and diffusion bonding process. More particularly, it concerns deep domed structures of cellular construction and of regular or irregular form. As used herein, the term 'domed structure' and like expressions means a structure having two (or more) opposed external surfaces one being generally convex, the other being generally concave. The domed structure will also have a peak on its convex surface, the structure sloping away from the peak in all directions, that is to say the peak is not formed as a straight ridge; likewise, the concave surface is not formed by a straight furrow. The domed structure can be regular in shape, e.g. a hemisphere or a hemi-elipsoid, or it can be irregular; the peak need not be a point but rather can be a general area at the top of the dome. The term 'domed structure' also includes structures in which the dome forms only a part of a larger structure, the remainder having any desired shape. BACKGROUND ART Metals having superplastic characteristics have a composition and micro-structure such that when heated to within an appropriate temperature range and when deformed at a strain rate within an appropriate range they exhibit the flow characteristics of a viscous fluid. With such metals, large deformations are possible without fracture. Diffusion bonding is a process by which a metallurgical bond is formed by the application of heat and pressure to metallic pieces held in close contact for a specific length of time. Bonding is thought to occur by the movement of atoms across adjacent faces of the pieces. The process allows metals to be joined without significantly changing their physical or metallurgical properties. The temperature ranges at which superplasticity and diffusion bonding occur may or may not be the same depending upon the material joined. It is known from GB-A-1,378,421 to form an inflatable envelope by metallurgically bonding together peripheral regions of two substantially flat sheet member of metal having superplastic characteristics, heating the envelope to within the temperature range for superplasticity of the metallic alloy, and applying a differential pressure between the interior and the exterior of the envelope while it is within the said temperature range such that the envelope expands as a balloon. The end product is a spherical container suitable for use as a pressure vessel. In GB-A- a method of forming a stiffened panel is taught in which an interior sheet is placed between two face sheets and the inner faces of each of the face sheets are attached directly to the respectively adjacent faces of the interior sheet by metallurgically bonded regions. At least the interior sheet is of superplastic material. The blank sheets are welded around their assembled peripheries to form a sealed envelope which is fed internally with a pressurised inert gas such that when the interior sheet is superplastic the outer sheets are moved apart by a predetermined amount to effect corrugation of the interior sheet thus effectively achieving a cellular structure of 'zig-zag' construction. In yet a further arrangement, as exemplified by US-A-4,217,397 and 4,351,470, twin interior sheets forming a sealed envelope and being joined to each other by metallurgically bonded regions are supplied with inert gas under pressure, thereby causing the sheets to serve away from each other within a limiting fixture, which may include face sheets, the envelope being expanded against the face sheets to form a series of cavities between the sheets, the said cavities being preferably of substantially rectangular form. Finally, US-A-4,526,312 teaches a method of making metallic sandwich structure by superplastic forming and diffusion bonding using a three sheet stack assembly of blanks but whereas in the previous examples of prior art the expanded assembly remains substantially flat, in this example the sandwich structure is required to achieve single curvature configuration, that is to say the structure has a straight ridge along an external face. The flat stack is placed in a mould and simultaneously or sequentially formed to a desired curvature; it is then trimmed and the trimmed stack is placed in a further mould defining the boundaries of the required component form and then formed into the expanded sandwich structure. In the arrangement as taught, the corrugations or 'zig-zag' stiffeners lie transversely to the radius of curvature, i.e.the stiffeners lie parallel to the ridge. Such a process cannot, however, be used for forming domed structures. US-A-4 304 821 describes a method of making a panel of cellular construction having two face sheets and at least one core sheet by superplastically expanding the one or more core sheets within a cavity formed by the face sheets that are held spaced apart in a fixture. The core sheet(s) are welded to each other or to a face sheet before superplastic forming. GB-A-1 495 655 describes a method of forming a planar panel of cellular construction by superplastically expanding a stack of sheets within a mould cavity; the sheets are welded together in the stack prior to superplastic forming. Although the referenced prior art teaches a deep drawn pressure vessel and alternative arrangements of cellular or sandwich structures using superplastic forming and/or diffusion bonding techniques, there appears to be no known structure or method of construction in the art for producing a deep domed cellular structure, for example a pressure bulkhead in an aircraft fuselage. Hitherto it has been practice to fabricate such bulkheads by conventional means by the use of a multiplicity of component parts riveted together and requiring the use of proprietary sealants to ensure a pressure resistant structure. It is, therefore, the object of the present invention to provide a method of making domed cellular structures by a method of superplastic forming and diffusion bonding. DISCLOSURE OF THE INVENTION According to the present invention there is provided a a method of manufacturing a domed cellular metallic structure comprising a convex surface having a peak and a concave surface opposed to the convex surface, which method is as defined in the accompanying claims 1-7. The present invention also provides a domed structure as defined in the accompanying claims 8-11. BRIEF DESCRIPTION OF THE DRAWINGS Alternative embodiments of the present invention will now be described, by way of example, with reference to the following drawings in which:- Figure 1 illustrates, in cross-section, a portion of a four-sheet assembly prior to forming; Figure 2 illustrates, cross-sectionally, the four sheet assembly of Figure 1 in free blown domed configuration; Figure 3 illustrates, cross-sectionally, the free blown configuration of Figure 2 mounted within a forming tool; Figure 4 illustrates, cross-sectionally, the free blown configuration of Figure 3 after forming into an expanded cellular structure; Figure 4a illustrates a detail of Figure 4; Figure 5 illustrates, in cross-section, a portion of a three-sheet assembly prior to forming; Figure 6 illustrates, cross-sectionally, the three sheet assembly of Figure 5 in a free blown domed configuration; Figure 7 illustrates, cross-sectionally, the free blown configuration of Figure 6 mounted within a forming tool; Figure 8 illustrates, cross-sectionally, the free blown configuration of Figure 7 after forming into an expanded cellular structure; Figure 9 illustrates, in cross-section, a portion of a two-sheet assembly prior to forming; Figure 10 illustrates, cross-sectionally, the two sheet assembly of Figure 9 in a free blown domed configuration; Figure 11 illustrates, cross-sectionally, the free blown configuration of Figure 10 mounted within a forming tool; Figure 12 illustrates, cross-sectionally, the free blown configuration of Figure 11 after forming into an expanded cellular structure; Figure 13 illustrates diagrammatically in plan view a circular assembly of sheets prior to forming; Figure 14 illustrates, in plan view, a typical blank of irregular configuration; Figure 15 illustrates, cross-sectionally, a four sheet assembly in free-blown partially inflated configuration; Figure 16 illustrates, cross-sectionally, the partially inflated component of Figure 15 mounted within a forming tool of irregular form; Figure 17 illustrates, cross-sectionally, the fully inflated component of Figure 15 after forming into an expanded irregular structure of cellular form in the forming tool of Figure 16; Figure 18 illustrates a typical section through the edge of a fully formed domed structure. DETAILED DESCRIPTION OF THE DRAWINGS Referring to the drawings, three possible laminate lay up arrangements are first discussed with reference to Figures 1, 5, 9 and 13. Figure 1 shows a four sheet lay up; two interior sheets 10 are placed between two outer (or face) sheets 11 and 12 to form a stack 19. The interior sheets 10 are metallurgically bonded together at 13 but are not bonded to outer sheets 11 and 12. The position of the bonds is shown in Figure 13 and, as can be seen, the bonds 13 are formed along lines running radially and circumferentially with respect to a point (or area) 20 which will form the peak of the desired domed structure. The bonds 13 need not be formed continuously all the way along the lines indicated and indeed, as indicated below, preferably they are not continuous; suitable point bonds formed along the indicated lines will be satisfactory. The metallurgical bonds indicated may be resistance welds, electron beam welds or diffusion bonds and may be of line or spot form in accordance with requirements. The four sheets 10, 11 and 12 are circular in order to make a dome of circular plan and are made of superplastic material, that is to say material that when heated to a suitable temperature exhibits superplastic properties. In Figure 5, a three sheet arrangement is illustrated in which a superplastic interior sheet 14 is sandwiched between two superplastic face sheets 16 and 17 to form a stack 19. The interior sheet 14 is metallurgically bonded to the lower face sheet 17 as shown by bonds 13, the positions of which are the same as in the four sheet arrangement, i.e. as shown in Figure 13. Sheets 14 and 16 are not joined joined together. In Figure 9, a two sheet arrangement is illustrated in which superplastic sheets 22 and 23 are joined together by bonds 13 (the positions of which are as shown in Figure 13) to form a stack 19. Three examples of methods of producing a cellular domed structure in accordance with the invention are illustrated in connection with Figures 2 to 4 (illustrating the structure achieved with the four sheet lay up of Figure 1), Figures 6 to 8 (illustrating the structure achieved with the three sheet lay up of Figure 5) and Figures 10 to 12 (illustrating the structure achieved with the two sheet lay up of Figure 9). The optimum cellular structure that is chosen in practice will be determined by structural requirements. Referring initially to Figures 2, 6 and 10, the circular stack 19 (shown in ghost lines) composed of two, three or four sheets is placed in a sealed platen fixture 25 to which it is secured by means of a peripheral clamp 26. A cut out switch 27 is located above the platen coincident with the centre line 28 and at a height corresponding to the desired dome height. The peripheral edge 30 of the blank assembly is TIG (tungsten inert gas) welded to form a sealed envelope and further incorporated into the edge are twin gas inlet ports 21 (see Figure 13) for the supply of inert gas under pressure to the interspaces between the respective sheets that are connected by bonds 13. With the circular blank assembly 19 clamped into the sealed fixture 25 it is heated to within the temperature range at which the material exhibits superplastic characteristics; pressurised inert gas is injected via the inlet port 31 which inflates the assembly until it attains the desired dome size 19 a . In the illustrated embodiments, the process is free blown and the pressure will be cut off as the inflated envelope comes into contact with a cut out switch 27. Alternatively, the dome may be achieved by blowing into a female mould or template. During the above-described formation of the dome 19 a , gas is injected through the gas inlet ports 21 at a pressure that is just sufficient to initiate a slight expansion of the envelope (i.e. the pressure is insufficient to cause complete superplastic forming); this gas pressure maintains a small separation between the sheets during formation of the dome and so serves to deter diffusion bonding between abutting faces of the sheets. Diffusion bonding between sheets that are joined by bonds 13 can also be prevented by the application of a known stopping-off composition to the interface between these sheets but such a remedy is not available to prevent diffusion bonding in other interfaces in the stack 19 because such measures would also prevent diffusion bonding at a later stage of the process when it is necessary to bring about diffusion bonding at these other interfaces (see below) in order to form a stiff, unified domed structure. Deleterious diffusion bonding may be avoided in these cases by forming the dome at a temperature lower than that at which diffusion bonding will occur. Once the stack 19 has been formed into the initial dome shape 19 a , it is transferred to a forming mould tool 32 (see Figures 3, 7 and 11) which comprises a lower male tool 33 and an upper female tool 34, the male tool having a convex domed surface 35 and the female tool a concave surface 36 of inverted form configured such that when they are assembled around the domed component 19a they form a parallel sided cavity having a depth corresponding to the required cellular structure. In operation, the domed component 19a is peripherally clamped within the mould tool assembly 32 and the whole is heated to within a temperature range at which the component sheets exhibit superplastic characteristics. An inert gas, e.g. argon, is admitted under pressure into the sealed envelope formed between those sheets (10; 14 and 17; 22 and 23) connected by the metallic bonds 13, for example, via the inlet tubes 21 as previously referred to. As stated above, the bonds 13 are preferably not formed continuously all the way along the lines shown in Figure 13, but rather are interupted by unbonded gaps that allow the inert gas from inlet tubes 21 to pass through them to reach all areas of the blank 19. The inert gas causes the stack to be superplastically formed into the shape of the mould cavity as described below. In the case of the four sheet arrangement of Figures 1 to 4, gas is admitted to the interface between the sheets 10 which inflates these sheets superplastically to form internal cavities in the manner of a balloon. The expansion of the sheets 10 pushes the outer face sheets 11 and 12 into contact with the mould surfaces 35 and 36. Further inflation urges the portions 38 of the interior sheets 10 against the outer sheets 11, 12 until they conform to shape of the inner surfaces of these outer sheets. However, the areas of the sheets 10 adjacent to bonds 13, which because of the bonds 13 are unable to be forced against the outer sheets, form abutting folded-back side wall portions 39 and the side wall portions on the two sheets 10 together form webs 40 separating the interior of the dome into cells 45 (see Figure 4a). As the respective component elements meet, diffusion bonding occurs producing an homogeneous cellular structure of deep domed construction. The structure is held at a temperature for a sufficient time for the abutting parts of the structure to be diffusion bonded together. With the three sheet arrangement of Figures 5 to 9, the flow of pressurised inert gas is admitted to the space between sheets 14 and 17 causing the sealed envelopes to expand, i.e.sheets 14 and 17 are inflated. The inflation of sheet 17 causes it to be urged against the male mould surface 35 such that portions 38 of that sheet conform to the shape of that surface. Expansion of interior sheet 14 causes the outer sheet 16 to be urged against the female mould surface 36 so that it conforms in shape to the said mould surface. Continued expansion of interior sheet 14 urges the portions 38 of that sheet to conform to the shape of the inner surface of sheet 16 (see Figure 8). In the area of the bond 13, webs 40 are formed by bent back portions 39 of sheets 14 and 17, which portions are connected by the metallic bonds 13. Again, the structure is held at a temperature for a sufficient time for the abutting parts of the structure to be diffusion bonded together. With the two sheet embodiment, the inert gas is fed into the interspace between the two sheets 22 and 23 and inflates the sheets about bonds 13 until the portions 38 of these sheets conform to the surfaces 35 and 36 of the mould. Such a structure will have webs 40 formed by bent-back portions 39 of the sheets 23 and 24. It is not necessary that the domed structures are of regular form nor that the precise stages of formation previously referred to are followed. As illustrated in Figures 14 to 18 inclusive, the blank 19 may be of irregular plan form and the method of forming may be adopted to produce a component having an irregular outer profile 43 and a different inner irregular profile 44 as shown in Figures 16 and 17. Similarly, referring to Figure 15, the blank may (as in the previously described embodiment) be free blown within the bounds of the clamping fixture 40 and the limiting block 41 (or cut-out switch) but the stack may simultaneously be partly expanded as shown. This partially expanded component is then clamped within a mould tool assembly 45 having appropriately shaped upper and lower mould surfaces 46 and 47 then further expanded to attain the structure of Figure 13. Particularly when the dome is of irregular shape, the dome may not have a precisely defined peak but rather it may have a general peak area in which case the bonds 13 (see Figure 13) will extend generally around the peak area and/or extend radially therefrom. The peak thus need not be a point and it could be a flat plateau at the top of the dome. Furthermore, the circumferentially extending metallic bonds need not be circular in shape but could be composed of a number of straight lines. One of the benefits of this method of manufacture is that it enables attachment faces angularly disposed to the forming plane to be formed as an integral part of the domed structure and which may, for instance, in the case of an aircraft pressure bulkhead be utilised for attachment to fuselage skin panels. One example is illustrated in Figure 18 in which the domed structure is formed with an integral attachment face 48 as an extension of the peripheral clamping flange 49, the clamping flange 49 being removed along a trim line 50 on completion of production. Although in this instance the attachment face is shown at right-angles to the clamping face, it may be inclined where it has to conform to a tapering fuselage profile, for example.

Claims: A method of manufacturing a cellular metallic structure, the method comprising the steps of: (A) providing at least two metal sheets (10,11,12; 14,16,17; 22,23) having superplastic characteristics; (B) joining two of said sheets in face to face contact by metallic bonds or welds (13) and forming the sheets into a stack (19) such that neither of the sheets so joined is further joined to any other sheet in the stack; (C) sealing the perimeter edges ((30) of said stack (19) but including means (21) for the admission of pressurised inert gas between said sheets thus forming an inflatable envelope assembly; (D) opening a mould tool (32) having opposed upper and lower surfaces (35,36) defining a cavity and placing said stack (19a) in the cavity; and (E) heating said stack (19a) and said mould tool (32) to a temperature suitable for superplastic forming and diffusion bonding, supplying inert gas under pressure between said sheets, thereby expanding by superplastic forming the outermost sheets (10,12;16,17;22,23) of said stack (19a) to conform to the upper and lower surfaces (35,36) of said mould tool and further causing the said sheets to expand about said metallic bonds to form webs (39) extending between the outermost sheets thereby defining a cellular structure; characterised in that, in order to make a a domed cellular metallic structure comprising a convex surface having a peak and a concave surface opposed to the convex surface, the method further comprises the following steps between steps (C) and (D): (C1) placing said stack in a fixture (25) including means (26) for sealingly clamping said stack (19) within said fixture at or about its perimeter and means (31) for the admission of a supply of pressurised inert gas to one external face of said stack (19); (C2) heating said stack (19) and said fixture (25) to a temperature suitable for superplastic forming, applying gas pressure to the said one external face of said stack (19) such that the stack is expanded superplastically to a pre-defined domed shape (19a); and further characterised in that, said upper and lower surfaces of the said mould tool form a cavity of domed form, whereby in step (E), the outermost sheets (10,12;16,17;22,23) of said domed stack (19a) are expanded superplastically to conform to the upper and lower surfaces (35,36) of said mould tool to form the said convex and concave surfaces of the domed structure. A method as claimed in claim 1, characterised in that two metal sheets (22,23) are provided and wherein the two sheets are joined in face to face contact by the said metallic bonds (13). A method as claimed in claim 1, characterised in that three metal sheets (14,16,17) are provided which are formed into a stack (19) having two outer sheets (16,17) and one internal sheet (14) sandwiched between the outer sheets and wherein the internal sheet (14) is joined to only one of the outer sheets (17) by the said metallic bonds (13). A method as claimed in claim 1, characterised in that four metal sheets (10,10,11,12) are provided which are formed into a stack (19) having two outer sheets (11,12) and two internal sheets (10,10) sandwiched between the outer sheets and wherein the internal sheets are joined together by the said metallic bonds (13). A method as claimed in any one of claims 1 to 4, characterised in that the metallic bonds (13) lie on lines extending radially from and/or circumferentially around the said peak (20). A method as claimed in claim 5, characterised in that the said circumferentially-extending metallic bonds or welds are circles that are concentrically spaced about a point and wherein the radially extending metallic bonds or welds extend radially away from that point, which point forms the peak (20) of the final domed structure. A method as claimed in any one of claims 1 to 6, characterised in that it includes injecting inert gas between the sheets during the said step (C2) in which gas pressure is applied to one external face of said stack. A domed metallic structure made of two or more sheets (10,11,12; 14,16,17; 22,23) of superplastic metal, the structure comprising: a convex surface having a peak (20), a concave surface that is opposed to the said convex surface, webs (39) extending between the convex and the concave surfaces thereby forming a cellular core within the domed structure, which webs are composed of portions of two of the said sheets that are joined by metallic bonds (13), characterised in that the metallic bonds (13) lie on lines extending radially from and/or circumferentially around the said peak (20). A structure as claimed in claim 8, characterised in that it is made from two sheets (22,23), a first one of the sheets (22) forming the convex surface and part of each of the said webs (39) and the second of the said sheets (23) forming the concave surface and the remainder of the said webs (39), the webs each being formed by a portion of the first sheet that has been bent back on itself to form a ridge and a portion of the second sheet that has also been bent back on itself to form a ridge, the two portions being joined along the said ridges by the said metallic bonds (13). A structure as claimed in claim 8, characterised in that it is made from three sheets 14,16,17), a first one of the sheets (16) forming the convex surface and a second one of the sheets (17) forming the concave surface and wherein the said webs (39) are each formed partly by a portion of one of the said first and second sheets that has been bent back on itself to form a ridge and partly by a portion of a third one of the said sheets (14) that has been bent back on itself to form a ridge, the two portions being joined along the said ridges by the said metallic bonds (13). A structure as claimed in claim 8, characterised in that it is made from four sheets (10,10,11,12), a first one of the sheets (11) forming the convex surface and a second one of the said sheets (12) forming the concave surface and wherein each of the said webs is formed partly by a portion of a third one of the said sheets (10) that has been bent back on itself to form a ridge and partly by a portion of a fourth one of the said sheets (10) that has been bent back on itself to form a ridge, the two portions being joined along the said ridges by the said metallic bonds (13).

Inventors: MANSBRIDGE, MARTIN, NORTON, JOHN, STEPHEN, DAVID
Applicants: BRITISH AEROSPACE PUBLIC LIMITED COMPANY
Application year: 1989
Main classification: B23K31
IPC codes: E04B1, B23K20, B04C1, B21D26, B21D47