GB2203376A

GB2203376A - Diffusion bonding apparatus

Info

Publication number: GB2203376A
Application number: GB08812538A
Authority: GB
Inventors: Michael John Ball; Brian John Turner; Kevin Stephen Broad
Original assignee: British Aerospace PLC
Current assignee: BAE Systems PLC
Priority date: 1984-11-28
Filing date: 1988-05-26
Publication date: 1988-10-19
Also published as: GB2167329A; GB8812538D0; GB2203376B; GB8529235D0; GB8429979D0; GB2167329B

Abstract

An apparatus for diffusion bonding composite articles comprises two envelopes 15, 16 made up of metal sheets 14 welded about their periphery. The envelopes are located between cooperating mould parts 11,11' and subjected to pressure and vacuum respectively. The composite is located in the vacuum envelope and the mould parts are heated. The pressure in envelope 16 is maintained to effect diffusion bonding. Two separate envelopes 15, 16 may be employed. <IMAGE>

Description

DIFFUSION BONDING OF METALS This invention relates to apparatus for, effecting diffusion bonding of metals. In particular, though not exclusively, the invention is concerned with diffusion bonding of parts formed of aluminium or aluminium alloy.

This is a patent application divided out of our co-pending patent application number 8529235.

According to the present invention, there is provided apparatus for effecting diffusion bonding of sheets to form a composite article, said apparatus comprising:i) a mould tool comprising two complementary parts, ii) a pressure pack adapted to contain the plurality of sheets and adapted to fit between said complementary parts, said pressure pack including three sheet elements provided one on top of the other and being sealed around their periphery to form first and second adjacent chambers, each having means for connection to pressure increasing means and pressure reducing means respectively, said second chamber having an edge region for being opened and resealed to allow the insertion and removal of said sheets.

The invention will be further described,by way of example, with reference to the accompanying drawings, in which: Figure 1 is a perspective view of a first embodiment of mould apparatus for use in a diffusion bonding process, the apparatus being partly disassembled, Figure 2 is a perspective view similar to that of Figure 1, of a second embodiment of mould apparatus for use in a diffusion bonding process, and Figure 3 is a schematic section view of the mould apparatus of Figure 2 in use.

The invention will also be further described with reference to the following examples in conjunction with the drawings: Examples A study was conducted to investigate methods of achieving an improved form of diffusion bond between articles of aluminium matrix composite in which the metallurgical properties in the interface region are substantially unchanged with respect to the parent material. Specimens of the aluminium matrix composite were cleaned and degreased and then introduced to a vapour deposition chamber 4 at 2 maintained at a low pressure (104 to 102 torr) where they were subjected to a glow discharge process in which the reformed oxide on the surface of the specimen was removed by ion bcttardment and the specimens were then coated with copper, in a vapour deposition process.Specimens were coated with thicknesses of 6-, 12-, and 24 x 106 of copper.

The specimens were then introduced into the mould apparatus illustrated in Figure 1 for bonding. Referring to Figure 1 the apparatus comprises a pressure pack 10, and a mould device comprising complementary mould portions 11, Ill, rear clamps 12 and front clamps 13. The pressure pack is of the form illustrated in Figure 3 except in that it is generally flat. The pack comprises three sheets 14 of mild steel material laid one on top of the other and sealed around their periphery to form two chambers; a vacuum chamber 15 and a pressurising chamber 16, each chamber having fluid inlet means for connection to a vacuum pump and a pressure pup respectively.The mould device has a relatively high thermal mass and in use is preheated to the bonding temperature required in an oven and 10 then removed. The end region 17 of the pressure pack is cut open and the component to be formed, together with any stopoff or lubricant is slid into vacuum chamber 15. The chamber is then resealed by welding and the pressure within the pressure pack reduced to the required level and then outgassed. The pressure pack is then placed between the portions 11, 111 and the clamps 12, 13. The pressure is released after the required period and the clamps 12, 13 removed to allow disassembly of the mould device. The pressure in vacuum chamber 15 is allowed to return to atmospheric and the end region 17 is then cut open to allow withdrawal of the formed part.In the meantime, the mould tool may be replaced in an oven to return it to the required preheat temperature in readiness for bonding a further component.

The specimens coated as above were then bonded using the above apparatus, undergoing bonding cycles at temperatures up to 6000C for times of up to 60 minutes, and at a bonding pressure of 100 psi achieved by pressuring the pressure chamber 16 and exhausting the vacuum chamber 15. Metallurgical sectioning and microscopic analysis were then carried out to find parameters for producing the most effective diffusion bond. Not surprisingly it was evident that the thinnest coating 6 x 106", the highest temperature 6000C, for the longest period, gave the most satisfactory results but these parameters were not considered practical.Further review of the specimens revealed that although the extremes of temperature and time produced with regard to the joint line the best specimens, a good sample was obtained using 6 x 106 of copper at 5650C for 15 minutes and this area of the parameters was then investigated 10 more thoroughly.

A further series of samples coated with 6- to 24 x 106 of copper was bonded at 5650C for times of up to 75 minutes. Of the copper thicknesses investigated, 12 x 106 produced the most consistent and regular results. The period for which the bond was maintained at 5650C was not found to produce significant differences in the distribution of copper and it was concluded therefore that for the particular simple bonding cycle under consideration to consolidate aluminium matrix composite using copper coatings, the optimum parameters were 12 x 106 of copper, a bonding temperature of 5650C for a period of between 5 and 15 minutes.

Having investigated the coating and bonding parameters it was decided to fabricate a Z-section stringer by consolidation of 5 sheets of aluminium matrix composite. The moulding apparatus used was similar to that of Figures 1 and 2, except that the components were configured to give a Z-section. Similar items are given the same reference. The pieces of aluminium matrix composite were cleaned and deoxidised and introduced into a vapour deposition chamber where they were initially subjected to a glow discharge process to remove the reformed oxide layer and then a thickness of 12 x 10 6 of copper was deposited on the cleaned surface.

The coated pieces of composite were then formed into Z-sections by pressing them over the pressure pack whilst laid on the lower tool 111. The preformed sheets were then introduced with the pressure pack and the pressure pack sealed by welding and then outgassed. The components of the mould device were preheated in an oven to a uniform 0 temperature of 565 C. The components were then removed from the oven, the vacuum chamber 15 placed under the vacuum, the pressure pack placed between the mould portions 11, 111, clamps 12 and 13 applied, and the pieces of composite subjected to a consolidation process in which they were bonded at the developed parameters. The pack was then removed from the mould device and the formed Z-section stringer removed. The stringer so formed was of good quality with no evidence of voids or interlamellar defects.

In the above examples a pressure pack arrangement employing three sheets has been used to define a pressure chamber and a vacuum chamber. Instead, a separate pressure pack and a separate vacuum pack may be employed for particular applications.

In the above examples, the parts to be bonded have been placed in contact prior to introduction into a low pressure atmosphere. The method of bonding may be used, however, with processes where this is not the case, for example, where a sheet of aluminium moves into contact with another sheet or blank of aluminium during a superplastic forming process. In this instance, the starting blanks or sheets are coated, as described where bonding will eventually occur. The parts are then superplastically deformed to move into contact, and bonding will either occur during the superplastic forming cycle or during a modified version of this cycle.

In addition, in these examples, the coating comprises a thin layer of copper; the coating may however comprise multiple layers, for example of magnesium covered by a very thin layer of copper as a protective film. The coating materials are not limited to magnesium and copper, for other materials such as copper, magnesium, Germanium, zinc, tin, gold, and silver may be used. Finally, the material of which the metal parts are formed does not need to be the same.

Claims

1 Apparatus for effecting diffusion bonding of sheets to form a composite article comprising: i) a mould tool comprising two complementary parts, ii) a pressure pack adapted to contain the plurality of sheets and adapted to fit between said complementary parts, said pressure pack including three sheet elements provided one on top of the other and being sealed around their periphery to form first and second adjacent chambers, each having means for connection to pressure increasing means and pressure reducing means respectively, said second chamber having an edge region for being opened and resealed to allow insertion and removal of said sheets.

2 Apparatus according to Claim 9 where the pressure pack is formed of three sheets of metal material joined around their periphery, one side of said pack being adapted to be cut and rejoined to allow insertion and removal of said sheets.

3 Apparatus according to Claim 8 where said first and second adjacent chambers are separately formed to form a separate pressure pack connected to pressure increasing means and a separate vacuum pack connected to pressure reducing means.

4 Apparatus for effecting diffusion bonding substantially as hereinbefore described with reference to the accompanying drawings.