British Aerospace Plc

British Aerospace Public Limited Company

Ian E Bottomley

Graham A Cooper

George Strickland

Duncan R Finch

Steven Harris

Christopher Somerton

The diffusion bonding of aluminum and aluminum alloys presents problems because, when left exposed to the atmosphere even for a short time, they acquire an extremely tenacious surface oxide layer that inhibits or prevents satisfactory diffusion bonding. Unlike titanium, this oxide layer is not absorbed at diffusion bonding temperatures. The present invention comprises a method of reducing oxidation of a surface of aluminum or aluminum alloy by causing a nitride film to form on the surface. This film is believed to prevent oxidation of the aluminum or aluminum alloy, but to be absorbable into it at diffusion bonding temperatures. In the preferred embodiment shown in FIG. 1, the nitride film is applied to the piece of aluminum or aluminum alloy (10) in a vacuum chamber (4) by sputtering with nitrogen ions, or by heating the piece (10) in a nitrogen gas atmosphere.

Diffusion bonding of aluminum and aluminum alloys

Described herein is a method of manufacturing components having 'warren girder' or 'X' core structures, i.e. those having internal support walls extending between their outer surfaces, from superplastically formable and diffusion bondable materials having different flow stress characteristics. By using materials of relatively high flow stress characteristics for the outer skin sheets of the component, and relatively low flow stress characteristics to form the internal support walls, the tendency for the sheets outer skin to bow outwards during superplastic formation at a faster rate in regions where they are the greatest distance from support walls is reduced. Consequently, the exterior of the outer skin sheets is less susceptible to the formation of recesses thereon at the point where the corresponding interior surface meets the inner support walls, i.e. the phenomenon of quilting is substantially eliminated.

Manufacture of articles by diffusion bonding and superplastic forming

A process is disclosed of explosive bonding two or more components together by forming the components into a stack, placing the stack into an envelope made of a deformable material, sealing the envelope except for an evacuation port, evacuating the interior of the envelope through the said port and subjecting the stack to explosive bonding within the envelope, thereby joining the components together; by conducting the explosive bonding in an evacuated envelope, not only are the components protected from damage during the explosion but the bonding is enhanced due to the reduced pressure of air between the components. After bonding, the stack can be subjected to superplastic forming.

Explosive bonding

A process is described of forming a diffusion bonded article from components made of aluminum or an alloy thereof that forms a protective coating of an oxide that inhibits such bonding. The process comprises removing the oxide layer and immediately forming the components into a stack, placing the stack in the chamber of an electron beam welding device, evacuating the chamber and electron beam welding the edge regions of the stack; the welded stack can then be diffusion bonded at leisure without the oxide re-forming. The diffusion bonded article can be superplastically formed to a finished or semi-finished shape.

In a superplastic forming/diffusion bonding process, in particular for aluminum-lithium alloys, the stop-off material in the diffusion bonding step is a porous lamellar material such as glass cloth or vermiculite paper. In one embodiment, prior to diffusion bonding, the stack of alloy sheets and lamellar material may be sealed under vacuum.

Diffusion bonding and superplastic forming

A method is described of diffusion bonding and superplastic forming components made of aluminium or an aluminium alloy to form a composite formed article. The method involves applying stopping-off material to selected areas of the components to prevent diffusion bonding in those areas, the stopping-off material being applied by plasma spraying.

Stopping-off method for use with diffusion bonding

The present invention provides a process in which components made of aluminum or an alloy thereof can be diffusion bonded together; the process involves chemically treating the components to remove aluminum oxide on the surface of the components, subjecting them to grit blasting and to a second chemical oxide removal step and finally pressing the components together under heat until they form diffusion bonds. The resulting composite can be superplastically formed to shape.

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Preston, United Kingdom

Ian E Bottomley