GB2259038A - Diffusion bonding aluminium and its alloys - Google Patents

Abstract

Oxidisation of aluminium or its alloys during diffusion bonding is reduced or eliminated by placing a substance such as magnesium, which is more susceptible to oxidisation than aluminium, in the chamber in which the components are diffusion bonded. Any oxygen present oxidises with the more highly reactive substance in preference to the aluminium. The substance 3 may be in the form of a foil wrapping about the components 1 being bonded. <IMAGE>

Description

DIFFUSION BONDING OF ALUMINIUM AND ALUMINIUM ALLOYS This invention relates to processes for diffusion bonding aluminium and aluminium alloys to form a composite structure.

Diffusion bonding is an extremely useful technique for joining metal components, particularly in the aircraft industry, and involves the pressing together of the components when heated so that their atoms inter-diffuse at the interface, thus forming a metal-to-metal bond between the components. Diffusion bonding can be combined with the process of superplastic forming if certain types of metal are used that have a specific uniform grain structure (for example, titanium or aluminium), thus enabling the manufacture of multi-sheet components of complex structure.

The superplastic forming process is a technique in which a heated metal is subjected to slow deformation during which the metal stretches (up to several hundred percent), and is thinned out in the deformed areas, but does not neck or fracture.

Aluminium and many of its alloys (all of which are hereinafter referred to as simply "aluminium") if left exposed to the atmosphere even for a short time acquire an extremely tenacious surface oxide layer that inhibits or prevents satisfactory diffusion bonding. This oxide layer forms quickly even at very low partial pressures of oxygen and therefore it is difficult to remove and maintain the surface totally oxide free prior to and during diffusion bonding.

However, the physical properties of aluminium, i.e. low density and high strength, make it an ideal material for use in aircraft components used by the aircraft industry, and therefore several prior proposals for overcoming the problem of the oxide layer and enabling the diffusion bonding of aluminium have been made. For example, in our earlier British patent application number 8815663.3 (corresponding to EP-A-0350220 and US patent number 4 948 457) we described a method of removing the oxide layer of an aluminium component by grit blasting it and then subjecting it to a chemical treatment. Although this technique greatly reduces the problem of the oxide layer, it has been found that, despite the fact that oxide is removed before diffusion bonding takes place, an extremely thin oxide layer is still present in the component in areas where bonding has occurred.This is thought to occur due to trace oxygen present during the bonding process. The result is a fairly poor diffusion bond which has a low peel strength.

Various other proposals have been made for diffusion bonding aluminium, and include: (i) removing or disrupting the oxide film in-situ as part of the bonding process (see Metrigger, Welding Journal, January 1978, pages 37-43); (ii) the application of a coating to the surface which will help prevent oxidation of the aluminium surface prior to bonding, and then will diffuse away from the interface (see GB 1 533 522, 1 485 051, 2 167 329, 2 117 691 and 1 544 201); or (iii) the application of a coating which will form å liquid phase at the bond interface which may help disrupt the oxide film and act as a carrier for atoms to diffuse across the interface.

British Patents Nos. 1,488,984 and 998,081 claim that good diffusion bonding can be achieved between an aluminium component and a component made of another metal by mechanical working of the components which involves passing them between rollers to reduce their thickness by 20%-70%.

However, such working adds to the complication and expense of the processes and makes it difficult to obtain a precisely-dimensioned final product.

It is an object of the present invention to substantially prevent the oxidisation of aluminium during the diffusion bonding process.

According to the present invention there is provided a process of diffusion bonding components, at least one of which is made of aluminium or aluminium alloy, and which is liable to form a surface coating of aluminium oxide, which process includes heating and pressing the components together in a chamber, characterised in that a substance more likely to oxidise than said aluminium or aluminium alloy is also present in said chamber.

Preferably, the substance comprises magnesium and preferably the magnesium is in contact with the surface of the aluminium or aluminium alloy.

For a better understanding of the invention two examples of our process will now be described with reference to the accompanying drawings in which: Figure 1 shows a cross-sectional view through a stack of aluminium sheets to be diffusion bonded; Figure 2 shows a cross-sectional view of an envelope containing aluminium sheets to be diffusion bonded in accordance with a first example of the process; Figure 3 shows a cross-sectional view of a diffusion bonding tool containing aluminium sheets to be diffusion bonded in accordance with a second example of the process; and Figure 4 shows a cross-sectional view through the pack of Figure 1 after diffusion ' bonding and superplastic formation.

To improve understanding of the drawings, like elements which appear in more than one figure are designated by the same reference numeral.

Prior to assembly of aluminium sheets 1 into a pack (such as the one shown in Figure 1) where they lie one on top of the other for diffusion bonding, the aluminium sheets 1 (which may be, for example, alloy 8090 commercially available from British Alcan) are subjected to the chemical cleaning and mechanical abrasion process described in our aforementioned British patent application number 8 815 663.3 and its corresponding foreign equivalents. Briefly, this process includes removal of the oxide layer from the aluminium sheets using either an acid etch (for example sulphuric and/or chromic acid) or a de-oxidising solution such as Alprep 290 (commercially available from Lee Chemicals of Buxton, UK).The sheets 1 are then grit blasted using various grit sizes and then finally immersed again in a de-oxidising solution in an ultra-sonic cleaner for five minutes in order to dislodge any trapped grit and to remove any oxide which may have built-up on the prepared surface. After this process has been completed the panels are rinsed with distilled water and dried with acetone. The aluminium sheets must then be placed in a vacuum or an inert gas atmosphere within 20 minutes if significant oxidisation is not to recur.

If required, bond inhibitors (commonly known as stop-off materials) are applied to selected areas 2 of the de-oxidised aluminium sheets 1 by, for example, a silk screen printing process. Alternatively, instead of actually applying a stop-off material to the aluminium sheets 1, stopping-off can be achieved by only removing the oxide layer from the areas of the sheets 1 where bonding is required. The required selective removal of the oxide layer could be done by masking off areas of the sheets 1 during the chemical cleaning and mechanical abrasion process, thus stopping the removal of the oxide layer in these areas. The absence of diffusion bonding in the stopped-off areas 2 allows cellular structures such as that shown in Figure 4, to be produced by superplastically forming the selectively diffusion bonded aluminium sheets 1.

In a first example of our process, a pack of aluminium sheets 1 de-oxidised as described above and selectively interlaid with stop-off material 2 is assembled as shown in Figure 1. The pack is then wrapped in magnesium foil 3 as shown in Figure 2. As an alternative to wrapping in foil, the pack could be backed with magnesium sheet, plate, or strips; or covered in magnesium powder. This assembly is then encapsulated in an envelope which comprises respective upper and lower parts 5 and 6 made of stainless steel or any other material having properties similar to steel. If magnesium powder is being used, backing plates (not shown) should be positioned between the inner surfaces of the envelope 4 and the powder. The envelope 4 and its contents are then placed in (as far as is practically possible) a vacuum or an inert gas atmosphere, and the upper and lower parts 5 and 6 are welded together at points 7 to form a gastight seal for the envelope 4 and thus retain the vacuum orinert gas atmosphere therein. Any suitable welding method that works in a vacuum/inert gas atmosphere could be used, such as electron-beam (EB) - see our patent application EP-A-0-398 760, tungsten inert gas (TIG) or resistance welding.

The envelope 4 and its contents are then subjected to hot isostatic pressing (a technique well known in the field of powder metallurgy) which involves the application of an isostatic pressure to the envelope 4 while maintaining it at a required constant temperature. This technique enables pressures of several million Pascals (several thousand pounds per square inch) to be achieved. When this process is completed each aluminium sheet 1 is inter-diffused with its adjacent sheet 1 except in the areas where the stop-off material 2 inhibits bonding.

As an alternative to the steps of encapsulation and hot isostatic pressing, our new process can be used with conventional diffusion bonding techniques. This second example is illustrated in Figure 3. A tool, shown generally at 8, for performing such a method includes bottom tool 9 and top tool 10 which together define a cavity 11 in which the magnesium and the pack of aluminium sheets 1, cleaned and assembled as described above with reference to Figures 1 and 2,is positioned. The cavity 11 is pressurised by a gas (shown by vertical arrows in Figure 3) from a pipe 12 connected to a pressure pump (not shown). The gas enters space 13 and exerts pressure on a diaphragm 14 made of, for example, Supral alloy (which is superplastically formable) which, in turn, presses on the pack of aluminium sheets 1.

A pipe 15 is connected to a vacuum pump (not shown) to evacuate the part of the cavity 11 below the diaphragm 14 containing the pack, although it should be noted that this evacuation is unnecessary if the pressurising gas is inert.

Heaters (not shown) are provided in the walls of top tool 10 so that aluminium sheets 1 can be sufficiently heated for diffusion bonding to occur when pressure is exerted by the gas from the pump (not shown).

In either of these alternative processes described with reference to Figures 1, 2 and 3, any oxygen that is present in the envelope 4 or cavity 11 will react with the magnesium rather than the aluminium because the former has a much higher reactivity and will therefore oxidise preferentially to the aluminium. The magnesium acts as a "getter" for any oxygen present and removes the traces of oxidisation that would otherwise be present in the diffusion bonded component.

A cellular structure such as that shown in Figure 4, may then be produced by superplastically forming the diffusion bonded pack. In this process, the pack is clamped between two halves of a nickel chromium steel mould which is 0 heated to 930 C. An inert gas is introduced under pressure to stopped-off areas 2 between the sheets 1 of the pack. As the inert gas is introduced, the sheets 1 deform superplastically and bow out towards the inner mould surfaces. The sheets do not separate from one another at the diffusion bonds. Eventually the pack superplastically deforms to produce a structure which substantially corresponds to the inner shape of the mould and, because of the diffusion bonding of selected areas, the inner sheet 1 of the pack forms supporting walls 16 at regular intervals throughout the final structure, the walls 16 extending from one outer sheet to the other. The structure can then be removed from the mould.

Various other techniques that could be used for performing diffusion bonding of the aluminium sheets 1 will now suggest themselves to those skilled in the art.

Although embodiments of the invention have been described which use magnesium as a "getter" for oxygen, many other materials which are more highly reactive than aluminium would also be suitable.

Claims (4)

1. A process of diffusion bonding components, at least one of which is made of aluminium or aluminium alloy, and which is liable to form a surface coating of aluminium oxide, which process includes heating and pressing the components together in a chamber, characterised in that a substance more likely to oxidise than said aluminium or aluminium alloy is also present in said chamber.

2. A process according to Claim 1, characterised in that said substance comprises magnesium.

3. -A process according to Claim 1 or Claim 2, characterised in that said substance contacts a surface of said aluminium or aluminium alloy.

4. A process substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.