GB2276840A

GB2276840A - Method of bonding and superplastic forming

Info

Publication number: GB2276840A
Application number: GB9405807A
Authority: GB
Inventors: Ian Edward Bottomley; N D Moulson; D France
Original assignee: British Aerospace PLC
Current assignee: BAE Systems PLC
Priority date: 1993-03-25
Filing date: 1994-03-24
Publication date: 1994-10-12
Also published as: GB9306175D0; GB9405807D0; FR2702974A1

Abstract

The present invention provides a gas injection system for a bonding/superplastic forming process; an insert (12) is included in a pack of components that are to be subjected to diffusion bonding and superplastic forming (DBISPF). The insert (12) includes a bore (20) that has a blind end (22). After the pack has been subjected to diffusion bonding, the blind end (22) of the insert is drilled out, a gas injection fitting (30) is screwed into the insert from the edge of the pack and gas is injected into the pack via the fitting. The arrangement of the present invention has the advantage that the gas injection bore (20) does not become blocked during diffusion bonding. The insert has a coefficient of thermal expansion such as to form a seal with the pack at superplastic forming temperatures. <IMAGE>

Description

METHOD OF BONDING AND SUPERPLASTIC FORMING Technical Field The present invention relates to a method of bonding and superplastic forming of components to manufacture composite articles and, more particularly, relates to the arrangement whereby a fluid can be injected into a pack of components to inflate the components during superplastic forming.

Background Art Combined diffusion bonding and superplastic forming (DB/SPF) 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 these metals). In a known DB/SPF process, stop-off material may be applied to selected areas of one or more sheets of a 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 stopoff material prevents diffusion bonding. The superplastic forming step is then conducted by heating the bonded pack in a mould to a temperature at which the components exhibit superplastic properties and an inert gas is injected into the unbonded areas of the pack under high pressure so as to "inflate" the sheets into a three-dimensional structure having an outer shape corresponding to the shape of the mould. As is well known, the configuration of the final composite structure is dependent upon, amongst other things, the number of sheets in the pack, the location of the stop-off material and the shape of the mould.

In order to inject gas into the pack, it is known to create a cavity within the assembled (but unbonded) pack and to place an insert into the cavity that includes at least one bore; the assembled pack is then subjected to diffusion bonding, which secures the insert within the cavity. During superplastic forming, gas is injected through the bore(s) in the insert and the cavity is so placed that the injected gas penetrates into the unbonded areas of the bonded pack to inflate the pack, as described above.

We have discovered that a major disadvantage of the above-described method for injecting gas, particularly when using aluminium and aluminium alloys as the superplastic material, is that the bore(s) in the insert become blocked in the bonding process, which restricts or prevents gas injection. The present invention overcomes the above problem.

US-5 205467 describes a DB/SPF method in which pre-formed passageways are protected from collapse during DB by inserting solid needles into them; after DB, the passageways still have to be reamed out before gas injection needles can be inserted into the passageways where they are welded in place.

US4 220 276 describes a DB/SPF method in which a pack is formed having two grooves extending to unstopped-off areas, a gas inlet tube is welded in place in communication with one of the grooves and a gas outlet tube is welded in place in communication with the other. Gas is injected through the gas inlet during DB and at a greater pressure during SPF.

The present invention provides a simpler way of injecting gas into the pack than is afforded by the above two specifications.

Disclosure of the Invention According to the present invention, there is provided a method of forming a composite structure from components exhibiting superplastic properties at elevated temperatures, which method comprises: (a) . assembling the components into a pack, the pack including a cavity, and a fluid injection insert that comprises: a head portion within the cavity, and a bore extending at least partially through the insert, the arrangement being such that the cavity and the head portion are so shaped that the head portion is held captive by the cavity and the coefficients of thermal expansion of the insert and the rest of the pack are such that a fluid-tight seal is formed between the insert and the rest of the pack at superplastic forming temperatures, (b) forming a bonded pack by bonding together the components in selected areas while leaving at least one area between adjacent components in the pack unbonded, (c) heating the bonded pack to a temperature at which the components exhibit superplastic properties, and (d) injecting fluid into said unbonded area(s) to superplastic form the components into the said composite structure, the fluid being injected into the said area(s) via the bore, wherein the bore either extends to or through the edge of the pack or can be accessed via a passage that extends to the edge of the pack, and wherein the bore together with any such passage extends inwardly into the pack in a substantially straight line, and wherein any blockages in the bore are removed prior to the injection of fluid to provide fluid communication between the bore and the unbonded area(s).

The passage between the unbonded area(s) and the exterior of the pack may be deliberately blocked and only opened after the components have been bonded and prior to the superplastic forming step. Preferably, the insert includes a blind bore and wherein the method includes the step of drilling out the blind bore to establish communication between the unbonded area(s) and the exterior of the pack.

The insert may be wholly composed of the said head portion or may include a head portion located within the said cavity and another portion located outside the perimeter of the components.

The present invention also provides a pack comprising: components exhibiting superplastic properties at elevated temperatures, which components are bonded together in selected areas while remaining unbonded in at least one area, a cavity within the pack, a fluid injection insert that comprises a head portion within the cavity and a bore, the arrangement being such that the cavity and the head portion are so shaped that the head portion is held captive by the cavity and the coefficients of thermal expansion of the insert and of the rest of the pack are such that a fluid-tight seal is formed between the insert and the rest of the pack at superplastic forming temperatures, wherein the bore extends to or through the edge of the pack or is accessible via a passage extending to the edge of the pack, the arrangement being such that the bore together with any such passage extends inwardly into the pack in a straight line so that a blockage in the bore can be accessed in order to remove it.

Brief Description of the Drawings There will now be described, by way of example only, a method of forming a composite structure with reference to Figures 2 to 4 of the accompanying drawings; in the drawings: Figure 1 is a cut-away sectional view of a diffusion bonded pack which shows the prior art arrangement for injecting gas into a bonded pack during a superplastic forming step; Figure la is a perspective view of the insert shown in Figure 1; Figure 2 is a cut-away sectional view through a pack of assembled components prior to diffusion bonding; Figure 3 is a cut-away sectional view of the pack shown in Figure 2 but after it has been diffusion bonded but prior to it being superplastically formed; Figure 4a is a perspective view of an insert for use in accordance with the present invention, and Figure 4b is a sectional view through the insert of Figure 4a and through part of a pack of assembled components.

Detailed Description of the Invention Referring initially to Figure 1, there is shown the edge region of an unbonded pack that is formed by stacking a top sheet 2, a bottom sheet 4 and a core sheet 6, all of which are made out of an aluminium lithium alloy that exhibits superplastic forming properties at temperatures of approximately 530"C. Layers of known stop-off material, e.g. silica or zirconia, such as those shown at 8 and 10, have previously been applied to selected areas of the sheets so that in the assembled pack the stop-off material lies between, on the one hand, top sheet 2 and the core sheet 6 and, on the other hand, between the core sheet 6 and the bottom sheet 4; the stop-off material prevent diffusion bonding of the sheets in those areas to which it has been applied.

A cavity is formed in the three sheets 2,4 and 6 that has a size and shape to accommodate a steel insert 12, which is shown in perspective in Figure la. The insert 12 is placed in the cavity during the assembly of the components into the unbonded pack.

A steel washer 14 is placed on top of the insert to provide a gas-tight seal during gas injection. The insert 12 includes a central opening 16 and several bores 18, extending radially inwards from the opening 16. As shown in Figure la, four such bores are provided.

The assembled pack with the insert in place is then subjected to a diffusion bonding step which essentially consists of compressing the pack at high temperatures and pressures until the interfaces between the top sheet, core sheet and bottom sheet that are not covered by stop-off material are diffusion bonded together.

The bonded pack is then transferred to a mould, where it is heated to a temperature at which the aluminium alloy exhibits superplastic properties. During superplastic forming, gas is injected into the opening 16 at high pressure and passes through the bores 18 into the unbonded area between the bottom sheet 4 and the core sheet 6. If gas cannot reach the unbonded area provided by stop-off material 8 directly from the gas injected through bores 18, it can be passed into such unbonded areas by means of holes (not shown) between the unbonded area provided by stop-off material 10 and that provided by stop-off material 8. In this way, gas can be fed to all unbonded areas of the pack to inflate the pack. The gas inflates the bonded pack until it conforms to the shape of the mould.

The arrangement of the present invention is shown in Figure 2, the difference between the arrangement in Figure 2 and that in Figures 1 and la being the shape and positioning of the insert 12 and accordingly a further description of the pack will not be given but similar reference numbers will be used to indicate corresponding features in all of Figures 1 to 3.

Figure 2 shows the arrangement of a pack after it has been assembled from the sheets 2,4 and 6 but prior to bonding. Prior to assembly, a cavity is formed in the sheets 2,4 and 6 that is so shaped to accommodate the insert 12, which has a bore 20 having a blind end 22 and an open end; the bore is provided with a female screw thread 23 in the region of the open end. Before the insert is placed in the cavity, a passage 24 is drilled from the edge of the assembled pack through to the cavity so as to be in register with the open end of the bore 20. The pack is then dismantled and the insert 12 placed in the cavity and a steel plug 26 is inserted into the passage 24 to prevent deformation of the hole during diffusion bonding.

The assembled pack is then heated in a press, the temperature and pressure exerted on the pack being sufficient to diffusion bond adjacent surfaces of the sheets together except in those areas to which stop-off material has been applied.

Other methods of bonding the pack, e.g. explosive bonding, can be used instead of diffusion bonding.

After bonding, the steel plug 26 is removed and the blind end 22 of bore 20 is drilled out to establish fluid communication between the unbonded areas of the pack, such as areas 8 and 10, and the exterior of the pack.

Turning now to consider Figure 3, which shows the arrangement of Figure 2 but with the blind end 22 drilled out; the outer end of the passage 24 is chamfered as shown at 28, and a gas injection tube 30 is screwed into the screw threaded end 23 of the insert 12 until the body 32 at the distal end of the tube engages with the chamfered end 28 of the passage to form a gas-tight seal. The body 32 includes an adaptor 34 of standard design (for example a "Swage Lok" fitting) into which a male gas connection fitting (not shown) can be secured for the supply of gas from a gas source directly to the interior of the pack.

The pack is then transferred to a superplastic forming mould of the desired shape, the pack is heated to a temperature of about 530"C and gas is injected through the tube 24 and the bore 20 in the insert 12 so that the gas can break out into the unbonded areas of the pack to inflate the latter. If only some of the unbonded areas are accessible to inert gas injected via the insert 12, more than one insert can be included in the superplastic forming pack and/or the holes may be provided in the core sheet to allow the passage of gas from one unbonded area to another.

The gas tight seal between the insert 12 and the pack is maintained as a result of the differential thermal expansion of the insert, which is made of steel, and the bonded pack, which is made of aluminium lithium alloy. The coefficients of expansion are such that, at superplastic forming temperatures (e.g. 530"C), the insert and the adjacent cavity are brought into close engagement to providethe required gas-tight seal.

After superplastic forming, the composite article is removed from the mould and trimmed to remove, inter alia, the portion of the pack including the insert 12.

Because the pack must be sufficiently thick to accommodate the insert 12, the system illustrated can only be used in packs above a minimum thickness which is generally 7mm or above.

Figures 4a and 4b show an alternative type of insert having an internal head part 50, an external part 52 and a connecting part 54. A bore 56 extends through all three parts of the insert and it should be noted that thre is no blockage along its length. the head part 50 is held captive in the cavity in the pack.

The insert is incorporated into a pack in an analogous manner to the insert described in connection with Figures 2 and 3 (see Figure 4b), with the internal and the connecting parts 50 and 54 located within the pack and the external part 52 located outside the pack.

Despite the fact that the bore 56 extends unblocked through the insert, it is liable to become blocked during diffusion bonding by ingress of material from the pack and this blockage must be drilled out before superplastic forming takes place. Since the bore 56 is straight, a drill can be inserted into the bore to drill out the blockage.

A "Swage-lok" fitment could be attached directly to the external part 52 of the insert for injection of superplastic forming gas.

Although the present invention has been described in connection with DBJSPF of aluminium and its alloys, it is equally applicable to titanium and its alloys.

Claims

1. A method of forming a composite structure from components exhibiting superplastic properties at elevated temperatures, which method comprises: (a) assembling the components into a pack, the pack including a cavity, and a fluid injection insert that comprises: a head portion within the cavity, and a bore extending at least partially through the insert, the arrangement being such that the cavity and the head portion are so shaped that the head portion is held captive by the cavity and the coefficients of thermal expansion of the insert and the rest of the pack are such that a fluid-tight seal is formed between the insert and the rest of the pack at superplastic forming temperatures, (b) forming a bonded pack by bonding together the components in selected areas while leaving at least one area between adjacent components in the pack unbonded, (c) heating the bonded pack to a temperature at which the components exhibit superplastic properties, and (d) injecting fluid into said unbonded area(s) to superplastic form the components into the said composite structure, the fluid being injected into the said area(s) via the bore, wherein the bore extends to or through the edge of the pack or can be accessed via a passage that extends to the edge of the pack, and wherein the bore together with any such passage extends inwardly into the pack in a substantially straight line, and wherein any blockages in the bore are removed prior to the injection of fluid to provide fluid communication between the bore and the unbonded area(s).

2. A method as claimed in claim 1, wherein communication between the unbonded area(s) and the exterior of the pack along the bore and any passage is established only after the components have been bonded and prior to the superplastic forming step.

3. A method as claimed in claim 1 or claim 2, wherein the bore in the insert is closed at its radially inner end by a blockage, which blockage is removed prior to the injection of fluid.

4. A method as claimed in any one of claims 1 to 3, wherein the insert is made of a material that has a lower linear coefficient of thermal expansion than the material from which the components are made.

5. A method as claimed in any one of claims 1 to 4, wherein the insert includes securement means to engage corresponding securement means on a fluid injection fitment and the method includes engaging the injection fitment with the insert by way of the said engagement means and injecting fluid into the bore of the insert by way of the said fitment.

6. A method as claimed in claim 5, wherein the securement means comprises a screw thread in the bore to accept a correspondingly threaded fluid injection fitment.

7. A method as claimed in any one of claims 1 to 6, wherein the insert comprises the said head portion and a further portion that is located outside the edge of the pack semponents.

8. A method as claimed in any one of claims 1 to 7, wherein the bonding is performed by diffusion bonding.

9. A method as claimed in any one of claims 1 to 8, wherein the components are made of aluminium or titanium or an alloy of these metals.

10. A pack comprising: components exhibiting superplastic properties at elevated temperatures, which components are bonded together in selected areas while remaining unbonded in at least one area, a cavity within the pack, a fluid injection insert that comprises a head portion within the cavity and a bore, the arrangement being such that the cavity and the head portion are so shaped that the head portion is held captive by the cavity and the coefficients of thermal expansion of the insert and of the rest of the pack are such that a fluid-tight seal is formed between the insert and the rest of the pack at superplastic fbrming temperatures, wherein the bore extends to or through the edge of the pack or is accessible via a passage extending to the edge of the pack, the arrangement being such that the bore and any such passage extends inwardly into the pack in a straight lin so that a blockage in the bore can be accessed in order to remove it.

11. A pack as claimed in claim 10, wherein the bore in the insert is formed as a blind bore.

12. A pack as claimed in claim 11, wherein the blind bore is closed at the radially inner end of the bore.

13. A pack as claimed in any one of claims 10 to 12, wherein the insert is made of a material that has a lower linear coefficient of thermal expansion than the material from which the components are made.

14. A pack as claimed in any one of claims 10 to 13, wherein the insert includes securement means to engage corresponding securement means on a fluid injection fitment.

15. A pack as claimed in claim 14, wherein the securement means comprises a screw thread in the bore of the insert.

16. A pack as claimed in any one of claims 10 to 15, wherein the components are made of aluminium or titanium or an alloy of these metals.

17. A pack as claimed in any one of claims 10 to 16, wherein the insert comprises the said head portion and a further portion that is located outside the edge of the pack components.

18. A method of forming a composite structure substantially as hereinbefore described in connection with Figures 2 and 3 or Figures 4a and 4b of the accompanying drawings.

19. A pack including an insert substantially as hereinbefore described in connection with and as illustrated in Figures 2 and 3 or Figures 4a and 4b of the accompanying drawings.

20. A composite structure made by the method as claimed in any one of claims 1 to 9 or 17.