Combined diffusion bonding/superplastic forming DB/SPF is a well established technique for making composite articles from materials that exhibit superplastic properties at elevated temperatures. These materials are primarily titanium, aluminium and alloys of these metals. In established DB/SPF processes, for example see U.S. Pat. No. 5,153,276, 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 the 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 will not be bonded in the selected areas covered by the stop-off material since the stop-off material prevents the diffusion of metal atoms between adjacent sheets and so prevents diffusion bonding in the selected areas.
The diffusion bonded pack may then be subject to superplastic forming by heating the pack, usually in a closed mould, to a temperature at which the metallic components exhibit superplastic properties. An inert gas is then injected at high pressure in a controlled manner into the unbonded areas of the pack so as to "inflate" the sheets gradually into a three dimensional structure having an outer shape corresponding to the shape of the mould. The unbonded areas of the pack are the selected areas covered by stop-off material during diffusion bonding.
DB/SPF has particular application in the aircraft industry where complicated high strength lightweight structures can be manufactured using this technique.
Known stop-off materials are generally refractory materials, such as yttria, boron nitride, graphite or alumina. Yttria and alumina are the most commonly used stop-off material.
The stop-off material may be applied by various techniques, for example silk screen printing or spraying through a mask. Commercially, stop-off material is applied using silk screen printing.
EP-A 2 006 073 describes a composition that may readily be used to apply stop-off material in DB/SPF techniques.
The manufacture of the silk screen is complex and time consuming. The handling and storage of silk screens is troublesome. The silk screen can become damaged in use. The silk screen does not have a very high resolution and the printing of the stop-off pattern using silk screens can be inaccurate. A brief description of the manufacture of the silk screens and their use in applying stop-off materials will now be given.
In complex aerospace design, computer aided design (CAD) techniques are usually employed. Using such techniques, a required stop-off pattern can be generated within a computer memory. Using this stored pattern data a polymer film (e.g. a Rubylith film) can be cut out for example using computer control cutting techniques. A silk screen mesh is then tensioned and coated with a photosensitive emulsion. The film is then placed over the emulsion and the emulsion is exposed to light. The light hardens the emulsion except in the areas in the shadow of the film. The unexposed emulsion underneath the film is then rinsed off leaving a silk screen that after developing and fixing, is permeable in the areas corresponding to the stop-off pattern but otherwise is impermeable. In order to apply stop-off to a sheet of superplastic material, the silk screen, which is held in tension by an aluminium frame, is placed over the superplastic material and the stop-off composition is applied through the open pores of the silk screen by drawing a squeegee soaked in stop-off material across the silk screen to apply the stop-off material to the underlying superplastic sheet.
It can be seen that the manufacture of a silk screen is time consuming and therefore expensive and also leads to a delay in transferring the design into a manufactured article while the silk screen is being produced. It would be advantageous to reduce or eliminate this time delay. Silk screens are delicate and can easily be damaged by impact with a sharp object. This is particularly true when the silk screens are held in tension on a frame. If damaged, a fresh silk screen must be produced.
The accuracy of the stop-off pattern applied by silk screen printing techniques is limited by the diameter and pitch of the strands of silk, which are usually larger than is desirable. Further inaccuracies can be introduced in the various manufacturing stages required to produce the silk screen.
When a large silk screen is used to apply a large stop-off pattern, the silk can stretch as the stop-off material is applied by drawing the squeegee over the silk screen. This can cause a displacement of the pattern and/or inaccuracies in the shape of the stop-off pattern applied.
Before storing the silk screen, it must be cleaned using solvents and this is time consuming and troublesome.
It is an object of the present invention to provide a simpler technique for applying stop-off material in a predefined pattern.