GB2167329A - Diffusion bonding - Google Patents

Abstract

In a method of diffusion bonding metal parts, particularly of aluminium or aluminium alloy the parts are subjected to an oxide removing process including ion cleaning, depositing on the surface of the parts a relatively thin layer of material relatively immune to oxidation and then subjecting the parts to a bonding process. The parts are urged together at a temperature and pressure sufficient to allow diffusion bonding and the relatively thin layer is effectively dispersed on each of the parts without significantly affecting the physical properties of the metal. In apparatus for performing the method the coated parts are enclosed in a metal envelope which is evacuated. An adjacent envelope is pressurised, the whole unit being constrained between mould parts 11, 11'. <IMAGE>

Description

SPECIFICATION Diffusion bonding of metals This invention relates to a method of, and 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.

Diffusion bonding of titanium articles is known in which the articles are urged together under controlled conditions of temperature and pressure until solid state diffusion occurs between the articles so that a durable bond is formed. However, for several metals other than titanium, such as aluminium and magnesium, diffusion bonding is difficult to achieve at a reasonably low pressure because a tenacious oxide layer readily forms on the surface of the metal and hampers diffusion bonding.

It has been suggested in UK Specification NO. 1,533,522, that parts of commercially pure aluminium can be diffusion bonded if the parts are initially cleaned and then plated with copper. In this arrangement a relatively thick layer of copper is provided on each article, and the process is akin to a conventional brazing process. Although there is no discrete layer of copper in the finished bond, there is still a copper-rich region at the interface, which means that this region will have metallurgical properties which are substantially different to those of the parent material.

Moreover, it is acknowledged in the specification that traces of oxide remain on the surface of the articles, and it is believed that these traces of oxide would degrade the integiity of the bond.

UK Specification No. 1,485,051 discloses a process for diffusion bonding aluminium parts in which the articles are initially cleaned and then plated with zinc; the treated parts are then provided with a coating of one or more materials selected from copper, magnesium and tin and urged together. In this process it is a prerequisite that the parts be coated with zinc and the layers of material are relatively thick. Thus bonds effected by this process using realistic bonding times will have a discrete layer of interface material remaining. In addition, the amount of material which does diffuse into the parent aluminium material from the relatively thick layers will cause substantial changes in the metallurgical composition in the interface region and hence alter the metallurgical properties.

According to one aspect of this invention there is provided a method of diffusion bonding metal parts in which method each of said parts is subjected to a pretreatment process which includes the steps of (i) cleaning the part to remove the oxide layer (ii) introducing the part into an atmosphere maintained at a low pressure and removing the reformed oxide layer by means of an ion cleaning process and immediately thereafter depositing on the surface of the part a relatively thin layer of material comparatively immune to oxidation, and then subjecting said parts to a bonding process which includes the steps of (i) urging the parts together at a temperature and relatively low pressure sufficient to allow diffusion bonding, (ii) effecting dispersion of said relatively thin layer on each of said parts.

It will be appreciated that dispersion may be effected prior to, during or after the respective parts are urged together.

In one particular method, the relatively thin layer comprises a metal comparatively immune to oxidation and dispersion of the layer is effected by diffusion of the layer into the metal part, and solid state diffusion bonding occurs between the metal parts with no liquid phase occuring.

In another method, where the relatively thin layer is of metal, an initial liquid phase may occur between the metal parts during the bonding process.

It is believed that a relatively thin layer of non-metallic material immune to oxidation may be employed, and in this case it is proposed that the layer may be vaporised prior to contact of the metal parts.

Where the parts to be bonded are of aluminium or aluminium alloy, it is preferred for the relatively thin layer to be of a metallurgically compatible metal or metals, selected from the group comprising copper, magnesium, Germanium, zinc, tin, gold and silver. For example, where the layer is of copper, the relatively thin layer may be coated to a thickness of between 6- and 24- X 10-6 (15 to 60 X 10 8 M); alternatively, the aluminium or aluminium alloy parts may be provided with multiple layers, for example a layer of magnesium on which is deposited a layer of copper.

Where the metal parts are formed of aluminium or aluminium alloy and the relatively thin layer is of copper metal, the bonding process may comprise urging the parts together at a pressure in the range from 10 psi to 1000 psi and at a temperature of up to 6000C for a period of between 5 and 60 minutes.

In another aspect of this 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 con nection 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.

Preferably said 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.

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 into a vapour deposition chamber maintained at a low pressure (10 4 to 10 2 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 bombardment and the specimens were then coated with copper, in a vapour deposition process. Specimens were coated with thicknesses of 6-, 12-, and 24 X 10 bof 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, 11', 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 pump 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 stop-off 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 pressurising 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 10 6, the highest temperature 600 C, 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 10-6" 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 10 6of copper was bonded at 5650C for times of up to 75 minutes. Of the copper thicknesses investigated, 12 X 10 - 6'prnduced 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 10-6of 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 Zsection stringer by consolidation of 5 sheets of aluminium matrix composite. The moulding apparatus used was similar to that of Figures 1 and 2, except in 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 de position 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 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, 11', 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 (14)

1. A method of diffusion bonding together metal parts in which method each of said parts is subjected to a pretreatment process which includes the steps of (i) cleaning the part to remove the oxide layer, (ii) introducing the part into an atmosphere maintained at a low pressure and removing the reformed oxide layer by means of an ion cleaning process and immediately thereafter depositing on the surface of the part a relatively thin layer of material comparatively immune to oxidation, and then subjecting said parts to a bonding process which includes the steps of (i) urging the parts together at a temperature and relatively low pressure sufficient to allow diffusion bonding, (ii) effecting dispersion of said relatively thin layer on each of said parts.

2. A method of diffusion bonding according to Claim 1 in which the relatively thin layer comprises a metal comparatively immune to oxidation.

3. A method of diffusion bonding according to Claim 2 where dispersion of the layer is effected by diffusion of the layer into the metal part.

4. A method according to Claim 1 where the relatively thin layer is of non-metallic material immune to oxidation and dispersion of the layer is effected by vapourisation prior to contact of the metal parts.

5. A method according to Claim 1 where the parts to be bonded are of aluminium or aluminium alloy and the relatively thin layer is of a metallurgically compatible metal or metals selected from the group comprising copper, magnesium, germanium, zinc, tin, gold and silver.

6. A method according to Claim 5 where the parts are provided with multiple layers of metallurgically compatible metals.

7. A method according to Claim 5 where the relatively thin layer is of copper having a thickness in the range between 6- and 24- X 10 6(15 to 60 X 10 6M) and said parts being urged together are maintained at a pressure in the range of 10 psi to 1000 psi at a temperature of up to 600 C for a period of between 5 and 60 minutes.

8. A method according to Claim 1 or Claim 5 or Claim 6 where the relatively thin layer is of copper having a thickness of 12 X 10 6", and the said parts being urged together are maintained at a temperature of 5650C for a period of between 5 and 15 minutes.

9. Apparatus for effecting diffusion bonding of sheets to form a composite article comprising (i) a mould tool comprising two compiementary 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.

10. 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.

11. 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.

12. A method of diffusion bonding together metal parts substantially as hereinbefore described with reference to any one of the examples.

13. A method of diffusion bonding together metal parts substantially as hereinbefore described with reference to the accompanying drawings.

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