Diffusion bonding and superplastic forming

Abstract: A process is described whereby an article is formed by a superplastic forming/diffusion bonding a stack of sheets (1) made in particular from an aluminium-lithium alloy; in the diffusion bonding step, adjacent sheets are prevented from bonding together in selected areas by placing lamellae (2) of an inert porous material in those areas. The lamellae are preferably made of glass cloth or vermiculite paper.

Description: This invention relates to diffusion bonding and in particular to a stop-off material for use in the production of combined diffusion bonded and superplastically formed structures which are preferably made of aluminium or an aluminium alloy BACKGROUND ART Combined diffusion bonding and superplastic forming (DB/SPF) has recently become established as a production technique, in particular with titanium alloys. To summarise the DB/SPF process: several sheets of a superplastic metal or alloy are formed into a stack and compressed at high temperature and pressures until the sheets are diffusion bonded together. In selected locations, a stop-off material is applied to the sheets to prevent bonding in these locations while permitting bonding at the remaining locations. After diffusion bonding, and also at high temperature, the superplastic forming step is conducted that is to say an inert gas is supplied under high pressure to the interior of the stack so as to 'inflate' the sheets into a three-dimensional structure. As is well known, the shape and structure of the final product is dependant on, amongst other things, the number of sheets in the stack and the location of the stop-off material. In DB/SPF of titanium alloys a suitable stop-off material is fine particles of yttria in an organic resin binder, which may be applied to the sheets by conventional silk-screen printing techniques (see for example GB 2 095 137 and 1 495 655). During the DB step, which is carried out at a temperature greater than 900°C for titanium, evaporation of the binder can contaminate the surfaces of the alloy sheets; similar problems occur in the diffusion bonding of aluminium aloys. In order to produce structures, particularly those made of an aluminium base alloy, by DB/SPF techniques satisfactorily, there remains the need for a suitable stop-off material. DE-A-37 10 680 describes a process of explosion bonding foils in a packet; paper or fibre sheets can be interposed between the sheets to prevent bonding in the areas covered by the sheets. EP-A-0 350 220 describes a process of diffusion bonding aluminium components together after a surface treatment to improve diffusion bonding. There is an example of using a glass cloth as a stop-off material. DISCLOSURE OF THE INVENTION According to the invention there is provided a method of forming a structure, which method comprises arranging a plurality of sheets of a superplastic metal (e.g. aluminium or an aluminium alloy) in a stack, interposing a lamella of inert porous material between adjacent sheets in a predetermined pattern to prevent bonding between said adjacent sheets in selected regions, diffusion bonding said stack of sheets together and then subjecting the thus bonded stack to superplastic forming. The interposed lamella must meet certain requirements to act as a successful stop-off material. The material must be inert at the elevated temperatures concerned, typically in the region of 500 to 600°C depending on the alloy. In addition, the material must be porous to permit gas to flow between the alloy sheets during the SPF step. Examples of suitable materials are glass cloth and vermiculite paper. It is preferred to preheat the lamella(e) prior to said diffusion bonding to drive off any possible contaminants and preferably this is achieved at a temperature above the temperature used in the diffusion bonding step. It is also preferred that prior to the diffusion bonding step, the stack of alloy sheets and interposed lamella(e) is sealed under vacuum. Suitable aluminium alloys are any that exhibit superplastic properties, and in particular aluminium-lithium alloys such as 8090 aluminium-lithium. In contrast to the prior art, the present invention achieves the stopping-off of the superplastic sheets by the simple expedient of placing a self-supporting lamella of inert porous stop-off material between adjacent sheets prior to the diffusion bonding step and so does away with the necessarity of applying stop-off material in a liquid vehicle, e.g. by silk screen printing, and so avoids the consequent problems of contamination by the liquid vehicle used in such printing. BRIEF DESCRIPTION OF THE DRAWINGS An embodiment of the invention will now be described by way of example and with reference to the accompanying drawings, in which:- Figures 1(a) and (b) are views in plan and section respectively of an apparatus for the diffusion bonding of a stack of alloy sheets; Figures 2(a), (b) and (c) are plan views of an alloy core sheet and two stop-off masks, respectively; Figures 3(a) and (b) are views of upper and lower superplastic forming tools; Figure 4 is a plan view of a diffusion bonded stack ready for superplastic forming; and Figure 5 is section through a structure formed by the process of the present invention. BEST MODE OF PERFORMING THE INVENTION Referring firstly to Figure 1(b), which is a section along line A-A of Figure 1(a), there is shown a diffusion bonding pack. The pack comprises a stack of two outer or face sheets 1 of 8090 aluminium-lithium alloy, a core sheet 1′ also made of 8090 aluminium-lithium alloy, the core sheet 1′ being separated from the face sheets 1 by two thin masks 2 formed of glass cloth. The stack of alloy sheets is sandwiched between spacer elements 3, and is located in the cavity 15 in the base 16 of a press 17 between a base element 4 and an upper diaphragm 5, which is made of a superplastic material. Before being formed into the bonding pack as shown in Figure 1, the alloy sheets 1,1′ and the glass cloth masks 2 are pretreated. In particular the sheets are chemically cleaned and the surfaces to be bonded are grit blasted using aluminium oxide. The glass masks are pre-baked at 590°C for one hour to drive off any organic binders or other possible contaminants. The outer two alloy sheets 1 are uninterrupted squares or rectangles, but the inner or core sheet 1′, as shown in Figure 2(a), has two cut-away portions 6 which are partly complementary in shape to the two masks 2 that are shown in Figures 2(b) and 2(c). Figure 2(b) shows the upper of the two masks (as viewed in in Figure 1(b)), and Figure 2(c) the lower of the pair. It will be appreciated that when the alloy sheets and masks are superimposed in the bonding pack, the masks will prevent bonding of the alloy sheets in selected locations by stopping the sheets from contacting one another. The shapes of the masks 2 and the core sheet 1′ are shown by broken lines in Figure 1(a). The prepared sheets are formed into the bonding pack along with the glass cloth masks as shown in Figure 1(b). The bonding pack is then loaded into the cavity 15 of the press which is preheated to 560°C, the membrane is placed over the pack and the press is sealed by clamping the top 18 of the press against the base 16 thereby copmpressing the edges of the membrane 5 and sealing the cavity 15. Whilst heating up to the required bonding temperature, the cavity 15 is evacuated for 10 minutes to achieve a vacuum of 10⁻⁶ mbar or better. This evacuation is carried out via a vacuum pipe 7 passing through the wall of the base 16 and connected to a pump (not shown). After the pack has reached the required temperature, a bonding pressure of 1000 psi (7 MPa) is applied to the pack by supplying pressurised gas through inlet 8 to the space 14 above the diaphragm 5. The bonding pressure and temperature are maintained for as long as is necessary to achieve diffusion bonding; typically a period of 3 hours. After diffusion bonding has taken place, the pack is removed from the press and allowed to cool. After cooling, a gas-supply hole 19 (see Figure 4) is drilled in the pack of bonded sheets (the shapes of the masks 2 and of the core sheet 1′ are shown by broken lines in Figure 4); the hole 19 passes through a tail part 6′ in the cut-out section 6 of the core sheet 1′ (see also Figure 2(a)) and through a tail part 2′ in the stop-off masks 2 (see also Figure 2(b) and (c)) to allow gas to be injected into the interior of the pack, as described in more detail below; the hole 19 does not, however, extend all the way through the stack of sheets and does not pass through the lower face sheet 1. The pack is then loaded into the superplastic forming (SPF) tooling shown in Figures 3(a) and (b). The SPF tooling includes an upper tool part 9, shown in Figure 3(a), and a lower tool part 10 shown in Figure 3(b). The upper tool part 9 is provided with a supply line 11 for the supply of an inert gas, such as argon or compressed air, to the diffusion bonded pack during the SPF step; the outlet 11′ of the gas supply line 11 is located in register with the above-described gas-supply hole 19 that has been drilled in the pack. The lower tool part 10 is formed with an internal surface 12 corresponding to the outer surface of the structure to be formed. After the bonded stack has been loaded into the SPF tool, the tool is placed in a preheated press (530°C) and the temperature allowed to stabilise (which takes approximately 20 minutes). Compressed air is then injected into the interior of the pack via supply line 11 and the gas-supply hole 19 drilled in the pack, the gas pressure being controlled to produce an optimum pressure-time cycle which may be calculated in advance using computer modelling techniques. The porous nature of the masks allows the gas under pressure to permeate from the gas-supply hole 19 to the interface between the sheets of the bonded stack where they are not bonded, and the gas pressure, in conjunction with the high temperature, causes the stack to inflate superplastically into the recess 12 formed in the lower tool 10 until the final structure is obtained, which is then allowed to cool. Figure 5 shows in section the structure obtained with the sheets and masks of Figures 1 and 2. In addition to the above method it may be advisable to seal the stack under vacuum prior to diffusion bonding by means of electron beam welding. This would reduce the possibility of the porous lamellar masks trapping appreciable quantities of air that would result in the subsequent formation of undesirable oxides. Such sealing would also increase substantially the 'shelf-life' of the stacks of sheets once they have been cleaned and prepared for bonding, that is to say the time between, on the one hand, the pre-treatment of the alloy sheets and the assembly of the bonding pack and, on the other hand, the diffusion bonding of the pack. Furthermore, such sealing of the sheets under vacuum prior to bonding would allow for the option of carrying out the diffusion bonding by hot isostatic pressure techniques (HIP).

Claims: A method of forming a structure, which method comprises arranging a plurality of sheets (1, 1') of a superplastic metal in a stack, interposing inert stop-off material between adjacent sheets in a predetermined pattern to prevent bonding between said adjacent sheets in selected regions, diffusion bonding said stack of sheets together and then subjecting the thus bonded stack to superplastic forming, characterised in that the interposed inert stop-off material is a lamella (3) of porous material but subject to the constraint that the stop-off material is not a glass cloth when (1) at least one of the sheets is made of aluminium or an aluminium alloy and which is liable to form a surface coating of aluminium oxide and (2) prior to arranging the sheets in a stack, the sheet(s) made of aluminium or aluminium alloy are grit blasted in areas that are to be bonded to other sheets, and (3) prior to arranging the sheets in a stack, the grit blasted sheets are chemically treated to remove surface aluminium oxide, and (4) the sheets are diffusion bonded together by heating and pressing them together without substantial plastic deformation of the sheets. A method according to claim 1, characterised in that said lamella (3) comprises a mat, paper or cloth. A method according to claim 1 or claim 2, characterised in that the said lamella of stop-off material (3) comprises a filamentary material. A method according to claim 3, characterised in that the lamella (3) of stop-off material comprises a glass cloth. A method according to claim 1 or claim 2, characterised in that said lamella of stop-off material (3) comprises vermiculite paper. A method according to any preceding claim, characterised in that said lamella of stop-off material (3) is preheated prior to said diffusion bonding to drive off any contaminants. A method according to claim 6, characterised in that the said lamella of stop-off material (3) is preheated prior to said diffusion bonding step to a temperature in excess of the temperature of the diffusion bonding step. A method according to any preceding claim, characterised in that said stack, prior to said diffusion bonding, is sealed under vacuum, e.g. by means of electron beam welds, around the edge regions of the sheets. A method according to any preceding claim, characterised in that the metal is aluminium or an aluminium alloy. A method according to claim 9, characterised in that said aluminium alloy is an aluminium-lithium alloy.

Inventors: BOTTOMLEY IAN EDWARD, COOPER GRAHAM ALAN, BOTTOMLEY, IAN EDWARD, COOPER, GRAHAM ALAN
Applicants: BRITISH AEROSPACE, BRITISH AEROSPACE PUBLIC LIMITED COMPANY
Application year: 1990
Main classification: B23K20
IPC codes: B23K20