WO1992007681A1

WO1992007681A1 - Bonding metals

Info

Publication number: WO1992007681A1
Application number: PCT/GB1991/001865
Authority: WO
Inventors: Jonathan Ball; Ricky Arthur Ricks; Peter-Juergen Winkler
Original assignee: Alcan International Limited; Messerschmitt-Bolkow-Blohm Gmbh
Priority date: 1990-10-24
Filing date: 1991-10-24
Publication date: 1992-05-14
Also published as: AU8750691A

Abstract

This invention relates to the bonding together of composite metalbodies by diffusion bonding whereby a bond of sufficient strength is obtained that the bonded bodies can be used to form mechanical structures, such as aircraft sub-assemblies. A metal body is discribed which is capable of being diffusion bonded to another metal body to form a mechanical structure characterised in that the body carries bonded thereto two superposed layers comprising an outer liquation layer and an inner buffer layer wherein the body is capable of being diffusion bonded to the other body via its liquation layer, and wherein the compositions of the body, the buffer layer and the liquation layer are selected such that under the conditions of diffusion bonding the constituents of the liquation layer will diffuse more readily into the buffer layer than into the body. Preferably the body is of an aluminium alloy, for example one that is superplastically deformable.

Description

BONDING METALS

This invention relates to the bonding together of composite metal bodies by diffusion bonding whereby a bond of sufficient strength is obtained that the bonded bodies can be used to form mechanical structures, such as aircraft sub-assemblies.

The technique of diffusion bonding by which two metal objects are joined by the application of modest pressure at an elevated temperature, which is lower than the melting point of either metal, has been known for many years. An. article by E.J. Clark in Welding Journal, 1959, Volume 38 pp 251s-258s described such a technique. However, the number of metals and alloys that can be bonded in this way is limited.

It has therefore been proposed to produce a composite body by providing a metal sheet which cannot readily be diffusion bonded, with a different metal coating layer - generally referred to as the "liquation layer" - of lower melting point which readily alloys with the metal of the sheet. During subsequent bonding of the composite bodies, coating layer to coating layer, the layers melt and a new alloy is formed at the interface of the sheet. This operation is in many v/ays analogous to brazing and has been described as transient liquid phase bonding or TLP. An example of such a proposal is disclosed in GB-A-1, 485, 051. However, such bonding has been found to be inconsistent in performance. The term "diffusion bonding" as used herein is intended to encompass the term "transient liquid phase bonding".

It is particularly difficult satisfactorily to diffusion bond readily oxidisable metals such as aluminium and magnesium where an extremely adherent, impermeable and chemically complex coating is formed very rapidly upon exposure to atmosphere containing even minute amounts of oxygen.

Notwithstanding the difficulties inherent in the process various attempts to produce diffusion bonds have been made. In several of these, attempts are made to remove the oxide layer from the aluminium by, for example, aqueous pretreatments prior to coating with materials such as silver. US-A-3 , 180, 022 discloses one such attempt. In an article in the Welding Journal, August 1980, pages 29 to 34, Morley and Caruso described a method for joining valve bodies consisting of either silver plating or copper plating or employing a zinc shim at the interface where joining is to occur. Larcfe clamping pressures, of the order of 26.5 MPa, are applied for extended times and yet the bond tensile strengths achieved were only of the order of 69 MPa.

In more recent attempts to produce a satisfactory diffusion bond more complex procedures of oxide removal from the surface of the aluminium have been employed. Thus in GB-A-2 , 117 , 691 the oxide is removed by a technique of ion bombardment under high vacuum. The clean., oxide free surface is then coated without delay and in the same high vacuum chamber with a ^"metal, such as silver, by an ion plating technique before bringing together the surfaces that are to be bonded at elevated temperature and high pressure. While reasonable bond strengths are quoted, very high bonding pressures were employed in the recited example. The basic disadvantages in a process such as that described lie in its complexity and inevitable associated high cost together with a limitation in the ability to scale up the process to the size necessary for commercial exploitation.

A method which relies on the use of a liquid phase is disclosed in US-A-3 , 633 , 266 in which a zinc alloy is coated onto a tapered aluminium tube. This tapered tube is fitted into a corresponding female tube and breaking up of the oxide film formed on the zinc coating is achieved by employing an interference fit between the co-operating components. Subsequent to joining, however, a distinct layer of the "soldering material" remains between the joined components whereas in a truly diffusion bonded joint there should ideally be no such interfacial layer. Thus the method has only limited application as a joining method.

US-A-3 , 180, 022 (Briggs et al) discloses a method of diffusion bonding an aluminium alloy with silver and other metals. The disclosure contains examples of bonding plugs into the ends of cylinders." No attempt is made to remove the oxide coating from the aluminium alloy. Such a method would produce a weak bond that may be satisfactory in the examples described where separation of the tubes and plugs would require shear forces to be exerted on the bond.

However, the technique described would demonstrably be unsatisfactory with bonded bodies which could be subjected to separating forces normal to the plane of the bond tending to "peel" one from the other.

US-A-3 , 046, 640 (Singleton) discloses cladding aluminium alloy sheets -with a layer of zinc and subsequently soldering the zinc layers together. The resultant bond would have poor strength characteristics and between each sheet there would be two discrete layers of zinc and a layer of solder.

One method of bonding together bodies which readily oxidise which does give consistent, high strength results is described in EP-B-0117671. There is disclosed a method of bonding together tv/o bodies of the same or different metals, at least one of which forms an oxidised coating thereon so readily as to be inherently incapable of being diffusion bonded to the other body, which method comprises creating two composite bodies by coating the bodies with metallic layers - liquation layers - which are capable of being bonded together by the application of pressure at an elevated temperature lower than the melting point of either

body, wherein each layer is applied to its associated body by a process in which: -

1) the oxidised coating on each body is physically broken up and, at the same time,

2) a new metal surface of increased superficial area is formed on the body while it is in intimate contact with its associated layer ro prevent the ingress of air therebetween and the oxidation of the new metal surface, so that

3) the body and its associated layer become bonded together in metal-to-metal contact and the layer diffuses into the body over substantially the whole of said increased area and wherein the composite bodies are bonded together with at least predetermined parts of their layers in direct metal-to-metal contact with one another by a process which does not increase said surface area and is selected from diffusion bonding and transient liquid phase bonding whereby the layers diffuse into one another and further diffuse into the bodies with any excess metal from the layers being expelled from between said predetermined parts until the discrete identity of the layers has been lost and the bodies are effectively bonded directly together.

Although the method of EP-B-0117671 is effective in producing strong bonding between metal bodies, a difficulty has been found to arise when trying to bond alloyed metal bodies in which, because of a high content of the alloying components, the diffusion thereinto of the bonding species is hindered. Not only are extended times required in order for satisfactory bond strengths to develop, but also the maintenance of the metal bodies at the required relatively high bonding temperatures for extended periods of time can -

adversely affect the microstructure of the bodies and hence their physical properties, especially when the bodies are of superplastic alloy. Furthermore relatively thick liquation layers were sometimes needed which were found not to diffuse completely into the bodies, thereby reducing the bonding strength achieved.

There is therefore a need for a method of the type disclosed in EP-B-0117671 but which can be used successfully for. joining highly alloyed components. The present invention is based upon the use of a buffer layer between the metal body and its overlying liquation layer, whose composition is such that under the bonding condition its components will diffuse more readily into its adjoining buffer layer than into the underlying metal body.

In accordance with one aspect of the present invention there is provided a metal body capable of being diffusion bonded to another metal body to form a mechanical structure, characterised in that the body carries bonded thereto two superposed layers comprising an outer liquation layer and an inner buffer layer wherein the body is capable of being diffusion bonded to the other body via its liquation layer, and wherein the compositions of the body, the buffer layer and the liquation layer are selected such that under the conditions of diffusion bonding the constituents of the liquation layer will diffuse more readily into the buffer layer than into the body.

In accordance with a second aspect of the present invention there is provided a method of diffusion bonding together two bodies of the same or different metals to form a mechanical structure, v/hich method comprises creating from at least one of the bodies a composite body by bonding thereto a coating comprising two superposed layers on its surface to be diffusion bonded, the two layers comprising an outer liquation layer and an inner buffer layer, and bringing the bodies together under conditions of pressure and elevated temperature such that diffusion bonding occurs via the at least one liquation layer, wherein the compositions of the at least one body, the at least one buffer ^"layer and the at least one liquation layer are selected such that under the conditions of diffusion bonding the constituents of the at least one liquation layer will diffuse more readily into its associated buffer layer than into its associated body. The method described in EP-B-0117671 - as modified by the present invention becomes a method of bonding together two bodies of the same or- different metals to form a mechanical structure, at least one of which bodies forms an oxidised coating thereon so readily as to be inherently incapable of being diffusion bonded to the other body, which method comprises creating from the at least one body a composite body by coating the body with a multi-layered metallic coating, the coatings when more than one are present being the same or different and the or each coating comprising an inner buffer layer in contact with and bonded to its corresponding body and an outer liquation layer overlying and bonded tb^" its corresponding adjacent buffer layer and not in contact with its respective body, wherein the or each liquation layer is capable of being bonded to either the other liquation layer or the other body by the application of pressure at an elevated temperature lower than the melting point of either body, wherein the compositions of each body and any associated buffer and liquation layer are selected so that under the bonding conditions the constituents of the or each liquation layer will diffuse more readily into its respective buffer layer than into its associated body, and bonding the bodies together with at least predetermined parts of the or each multi-layer coating in direct metal-to-metal contact with either one another or with the other body by diffusion bonding.

In the preferred embodiment of the modified method at least one layer of the coating is applied to its associated body by a process in which the inherent oxidised coating on the at least one body is physically broken up, and at the same time a new metal surface of increased superficial area is formed on the body while it is in intimate contact with its associated coating layer to prevent the ingress of air therebetween and the oxidation of the new metal surface. The liquation layer can be formed prior to the bonding step or be formed as a result of the elevated temperature used during the bonding step. It can consist of a single layer which is capable under the bonding condition of diffusing into an adjoining layer or of two, or possibly more, layers which are capable under the bonding conditions of alloying together and of diffusing into an adjoining layer.

When the metal of the liquation layer has a melting point below the melting point of the metal body to be bonded then the elevated temperature used may be between said melting points and the composite bodies may be joined by transient liquid phase bonding. Preferably the liquation layer is formed from aluminium, zinc, silver, copper, silicon, magnesium or an alloy thereof which may include other constituents. It may be desirable to use an alloy for this layer if, for example, it is desired to have a particular melting point in the layer or if particular strength requirements are required from the bond. Thus, for example, alloys of zinc with copper and/or magnesium may be selected or alternatively an alloy of zinc and silver could be attractive if a layer with a melting point higher than that of zinc is required. For example a zinc alloy may contain at least one of 0-25% Al; 0-20% Cu; 0-3%

Mg and 0-20% Ag by weight. It may be desirable to prevent surface oxidation of the liquation layer by applying a very thin silver coating. Not only does this result in a more stable multi-layered coating on the bodies to be bonded, but also since silver oxide is unstable at the temperatures normally used for diffusion and transient liquid phase bonding the bonding process can be carried out in air without the need for any inert gas blanketing. Furthermore, since bonding times are dependent upon the thickness of the liquation layer, those of copper and/or silver allows optimisation of the bonding times since the thickness of such layers can be controlled accurately by known methods.

When copper and/or silver is used as the liquation layer then preferably an intermediate layer of a silicon- containing aluminium alloy is also used located between the copper and/or silver layer and its associated buffer layer. By using this combination of layers a transient liquid phase layer of an aluminium-silicon-copper alloy of relatively low melting point will be formed as a result of the elevated temperature used in the bonding step, to act as the bonding medium. The liquation layer can also be formed by surface modification, for example by ion plating, the underlying buffer layer. The liquation layers on the two bodies to be bonded together may be the same or different. Preferably the buffer layer is of a substantially pure metal, especially aluminium having a total alloy content of not more than 5%, preferably not more than 2.5%, and more preferably not more than 0.5% by weight. Such an alloy allows the diffusing species from the liquation layer, such as zinc or the combination of silicon and copper from the above-mentioned low melting point alloy, to be taken up substantially within the buffer layer. It will be appreciated that the more dilute the buffer layer is the more readily will the diffusing species be taken up thereby and hence the shorter will be the bonding time needed. An advantage of a relatively thick buffer layer is that it will take up any species from the underlying body which may under the bonding conditions tend to diffuse into the liquation layer with perhaps deleterious effects on the resulting bonding strength and required bonding times.

It will be appreciated that the buffer layer must be sufficiently dilute in composition to permit rapid diffusion of the bonding species, such as copper, silicon and/or silver, but have sufficient strength not to affect adversely the mechanical properties of the bond. Examples of suitable buffer layer materials are a dilute aluminium alloy of composition AA "-1050A, a dilute manganese- containing aluminium alloy, such as that of the 3,000 series of compositions, particularly AA 3003, and a dilute magnesium-containing alloy, such as that of the 5,000 series of compositions, particularly AA 5005.

It has been found that there is particular advantage in using the magnesium-containing alloy as a buffer layer where the liquation layer contains a significant proportion of silicon, such as those aluminium alloys of the 4,000 series. It has been found that, under the conditions of diffusion bonding with^" the components of the liquation layer diffusing into the buffer layer, the magnesium of the buffer layer and the silicon from the liquation layer form a solid solution in the buffer layer. On cooling to room temperature and/or holding at an intermediate temperature, the magnesium and silicon can form a precipitate of Mg₂Si in the buffer layer and the possibility then exists of precipitation hardening the buffer layer so as to improve even more the mechanical strength of the resulting diffusion bond.

This precipitation hardening effect can be achieved by other species diffusing into the buffer layer, either from the liquation layer or from the body itself, where the diffusing species can combine v/ith another species in the buffer layer to form a combination capable of giving a precipitation hardening effect. Alternatively a solution strengthening effect can be achieved by the diffusion of species from the liquation layer or from the body into the buffer layer. In the alternative, the buffer layer can be an alloy which is inherently capable of a precipitation hardening effect, such as an alloy of the AA 6000 series of compositions, for example alloy AA 6063.

^•The buffer . layers on the two bodies to be bonded together may be the same or different and may be composed, to at least a substantial part of the same metal as the, or each of the, liquation layers.

The base metal is preferably of aluminium, magnesium or an alloy of either metal, such as those of the AA 2000, 6000 or 7000 series of compositions. The alloy may be superplastically deformable so that an integrally stiffened structural component may be formed by selectively bonding parts of two composite bodies together and superplastically deforming the non-bonded parts of the bodies. Aluminium- lithium alloys are particularly preferred and the base metals of the bodies may be the same or different. The formation of the composite body may be achieved by mechanical means such as roll-bonding, explosive bonding or extrusion, particularly hydrostatic extrusion, by chemical or electrochemical methods, spluttering, or even by ion plating, or any combination thereof. Where an oxidised layer could interfere with the bonding process the said mechanical means should preferably be such as to be able to break up physically this layer. The different layers that make up the buffer and liquation layers can either be applied sequentially to the body or can be pre-formed and then applied as a multi-layered coating.

When a coated body of the present invention is to be bonded to another alloy body which itself contains a sufficient amount of a species capable of diffusing and .1 -

forming a low melting point alloy when brought into contact with the liquation layer of the coated body under the bonding conditions, such as an aluminium/silicon alloy, then there is no need to put on that other body a buffer layer or a liquation layer, as is mentioned in GB-A-1532628.

Bonding by the TLP (transient liquid phase) technique with a rolled-on liquation layer of zinc can be achieved more satisfactorily and the resulting bond can be of higher strength if the atmosphere in which the bonding operation takes place is controlled to reduce oxygen and water vapour contents to very low levels.-

The metal bodies may be in the form of sheets, plates, bars, or any other shape of structural component. Methods of bonding together metal bodies will now be described by way of illustration in the following Examples which include comparative data.

The accompanying drawings show graphically the experimental results achieved, Figures l, 3 and 5 illustrating the room temperature shear strengths of the bonds tested and Figures 2 and 4 indicating the corresponding high temperature peel strengths. Example 1

A sample of an aluminium-lithium alloy of composition AA 8090 was rolled into a sheet and cut up into three plates. Each of the three plates was then given a different surface treatment as described below and then equal sized pairs of test pieces were cut out from each plate for bonding together, treated surface to treated surface.

The surface treatment applied to each of the three plates consisted of roll bonding at least one metallic surface layer thereonto and then degreasing the faying surfaces ready for bonding together. Three different metallic surface layers v/ere used for the three plates as follows:-

A. A multi-layered coating in accordance with the present invention consisting of in order:-

(i) A roll-bonded layer of dilute aluminium alloy of composition AA 1050A,

(ii) A roll-bonded layer of aluminium-silicon alloy of composition AA 4343,

(iii) An electroplated layer of substantially pure copper, and (iv) An electroplated layer of substantially pure silver.

B. A multi-layered coating in accordance with the invention consisting of in order:-

(i) A roll-bonded layer of dilute aluminium alloy of composition AA 1050A, and

(ii) A roll-bonded layer of zinc containing 1% by weight of copper.

C. A single layer not in accordance with the invention consisting solely of a roll-bonded layer of zinc containing 1% by weight of copper.

Each of the three pairs of test pieces were put together, coated surface to coated surface, and heated between hot platens in a press to a temperature of 540°C and held for 60 minutes at a pressure of 5 MPa. The resultant bonded test pieces were then tested to assess the strengths of the resultant bonds both for air room-temperature shear strengths and their high temperature peel strengths. The peel strength tests were T-peel tests carried out according to ASTM D-1376-69 using tests pieces 15 mm wide and a head speed of 20 mm min.^". For all of the tests a deformation of about 5% was used, except for the room temperature shear strength test for coating C where the deformation was about 20%.

The results of these tests are tabulated in Figures 1 and 2 wnere, because of the variation in measured strengths, maximum and minimum values are given. For the high temperature peel strength of coating c no detectable peel strength was measurable at about 5% deformation but a strength of about 4 N/mm was measured when the deformation was 20% or more.

The results show that it is only when a dilute alloy is present as a buffer layer between the base alloy and the coating layer or layers does the advantageous combination of good room temperature shear strength and good high temperature peel strength arise.

Metallurgical examination of the bonded test pieces showed that where a buffer^ layer was present there was essentially no diffusion of lithium into the outer liquation layer or layers. Furthermore, because of the relatively short bonding time, the microstructural stability of the base AA 8090 alloy was found not to have been adversely affected.

Example 2 Following the procedures described in Example 1, a number of further tests were carried out on test pieces using coatings A and B with varying degrees of deformation. The results are shown in Figures 3 and 4 and demonstrate the ability of the method according to the present invention of producing good bonding at relatively low deformations. Example 3

A sample of an aluminium-lithium alloy of composition AA 8090 was rolled into a sheet and cut up into six plates. Each plate was coated by roll bonding with a buffer layer of AA 5005 to a thickness of approximately 180 μm overlaid with approximately 20 um of AA 4343 which was then electroplated with a layer of copper and then a layer of silver. Equal sized pairs of test pieces were then cut out from each plate and bonded together, treated surface to treated surface, oetween two 120 x 120 mm heated steel platens under normal atmospheric pressure with the application of a holding pressure using hydraulic cylinders. Bonding was performed at about 540°C under a pressure of 1 MPa for 30 minutes, and the bonded structure air cooled to room temperature and then aged for 16 hour at 170°C. The room temperature lap-sheer strength of the six bonded test specimens was then measured at three locations in the bonded area. The results are presented in Figure 5 of the accompanying drawings. Example 4

Samples were prepared and tested as in Example 3, except that for the buffer layer a layer of an aluminium/magnesium alloy of composition 5754 was used. Lap shear tests carried out at room temperature gave results comparable to those obtained in Example 3. Example 5

Samples were prepared and tested as in Example 3, except that for the buffer layer a layer of an aluminium/magnesium alloy of composition 3003 was used. Lap shear tests carried out at room temperature gave average shear strengths of approximately 80 MPa. Example 6 Samples were prepared and tested as in Example 4 using bonding temperatures of between 520°C and 560°C. Surprisingly, a bond strength of about 160 MPa was obtained by bonding at 520°C, which is below the liquation level predicted from the Al-Si-Cu phase diagram. It is believed that magnesium diffusing into the liquation layer from the buffer layer lowered the liquation temperature.

Raising the bonding temperature to 560°C did not significantly alter the bond strength achieved. Example

Samples were prepared and tested as in Example 4, except that the bonding time was varied from 0 minutes

(corresponding to spontaneous bonding identified by the appearance of liquid metal at the edge of the bond line) to

60 minutes. A bond strength of about 110 MPa was achieved on spontaneous bonding. A bonding time of 15 minutes resulted in bond strength of about 180 MPa which was reduced to about 160 MPa when the bonding time was increased to 30 and 60 minutes, respectively.

Claims

— z. o —

CLAIMS :

1. A metal body capable of being diffusion bonded to another metal cody to form a mechanical structure characterised in mat the body carries bonded thereto two superposed layers comprising an outer liquation layer and an inner buffer layer wherein the body is capable of being diffusion bonded to the other body via its liquation layer, and wherein the compositions of the body, the buffer layer and the liquation layer are selected such that under the conditions of diffusion bonding the constituents of the . liquation layer will diffuse more readily into the buffer layer than into the body.

2. A body as claimed in claim 1 wherein the body forms an oxidised coating thereon so readily as to be inherently incapable of being diffusion bonded to the other body.

3. A body as claimed in claim 1 or claim 2 wherein the body is of a material selected from aluminium and its alloys and magnesium and its alloys.

4. A body as claimed in any one of the preceding claims wherein the body is of a superplastic alloy.

5. A body as claimed in claim 4 wherein the body is of a superplastic aluminium-lithium alloy.

5. A body as claimed in any one of the preceding claims -.-/herein the buffer layer is of aluminium having a total alloy content of not more than 5% by weight.

7. A body as claimed in claim 6 wherein the buffer layer is of aluminium naving a total alloy content of not more than 2.5% by weiqht. 8. A body as claimed in any one of the preceding claims wherein the buffer layer is capable of being precipitation or solution hardened.

9. A body as claimed in any one of the preceding claims wherein the liquation layer is formed from aluminium, zinc, silver, copper, silicon, magnesium, or an alloy thereof which may include other constituents.

10. A body as claimed in any one of the preceding claims wherein the liquation layer consists essentially of a single layer which is capable under the bonding conditions of diffusing into an adjoining layer.

11. A body as claimed in claim 10 wherein the liquation layer is composed of zinc or an alloy of zinc.

12. A body as claimed in any one of claims 1 to 9 wherein the liquation layer consists essentially of two layers which are capable under the bonding conditions of alloying together and of diffusing into an adjoining layer.

13. A body as claimed in claim 12 wherein the innermost of the two layers is composed of an aluminium/silicon alloy and the outermost of the two layers is composed of copper or a copper alloy.

14. A body as claimed in any one of the preceding claims wherein the liquation layer is protected from surface degradation by an additional layer.

15. A body as claimed in claim 14 wherein the additional layer is composed of silver. 16. A method of diffusion bonding together two bodies of the same or different metals to form a mechanical structure, v/hich method comprises creating from at least one of the bodies a composite body by bonding thereto a coating comprising two superposed layers on its surface to be diffusion bonded, the two layers comprising an outer liquation layer and an inner buffer layer, and bringing the bodies together under conditions of pressure and elevated temperature such that diffusion bonding occurs via the at least one liquation layer, wherein the compositions of the at least one body, the at least one buffer layer and the at least one liquation layer are selected such that under the conditions of diffusion bonding the constituents of the at least one liquation layer will diffuse more readily into its associated buffer layer than into its associated body.

17. A method as claimed in claim 16 wherein at least one body forms an oxidised coating thereon so readily as to be inherently incapable of being diffusion bonded to the other body.

13. A method of bonding together two bodies of the same or different metals to form a mechanical structure, at least one of which bodies forms an oxidised coating thereon so readily as to be inherently incapable of being diffusion bonded to the other body, which method comprises creating from the at least one body a composite body by coating the body with a multi-layered metallic coating, the coatings when more than one are present being the same or different and the or each coating comprising an inner buffer layer in contact v/ith and bonded to its corresponding body and an outer liquation layer overlying and bonded to its corresponding adjacent buffer layer and not-in contact with its respective body, wherein the or each liquation layer is capable of being bonded to either the other liquation layer or the other body by the application of pressure at an elevated temperature lower than the melting point of either body, wherein the compositions of each body and any associated buffer and liquation layer are selected so that under the bonding conditions the constituents of the or each liquation layer will diffuse more readily into its respective buffer layer than into its associated body, and bonding the bodies together v/ith at least predetermined parts of the or each multi-layer coating in direct metal- to-metal contact with either one another or with the other body by diffusion bonding.

19. A method as claimed in claim 13 wherein at least one of the layers of one or both coatings is applied to its associated body by a process in which the inherent oxidised coating on the at least one body is physically broken up, and at the same time a new metal surface of increased superficial area is formed on the body while it is in intimate contact v/ith its associated coating layer to prevent the ingress of air therebetween and the oxidation of the new metal surface.

20. A method as claimed in any one of claims 16 to 13 wherein one or each composite body and at least one of its associated coating layers are roll bonded together.

21. A method as claimed in any one of claims 16 to 18 wherein one or each composite body and at least one of its associated coating layers are bonded together by explosive bonding.

22. A method as claimed in any one of claims 16 to 18 wherein one or each composite body and at least one of its associated coating layers are bonded together by extrusion. 23. A method as claimed in claim 22 v/herein one or each composite body and at least one of its associated coating layers are bonded together by hydrostatic extrusion.

24. A method as claimed in any one of claims 16 to 23 wherein the bodies are of a material selected from aluminium and its alloys and magnesium and its alloys.

25. A method as- claimed in any one of claims 16 to 24 wherein one or both of the liquation layers is formed from aluminium, zinc, silver, copper, silicon, magnesium, or an alloy thereof which may include other constituents.

26. A method as claimed in claim 25 wherein one or both of the liquation layers is composed of zinc or a zinc alloy.

27. A method as claimed in any one of claims 16 to 25 wherein one or both of the liquation layers composed of two layers consisting of an outer layer of silver overlying and in contact with an inner layer of copper or a copper alloy.

23. A method as claimed in any one of claims 25 to 27 including an intermediate layer of a silicon-containing aluminium alloy located between the said inner layer and its associated buffer layer.

29. A method as claimed in any one of claims 16 to 28 wherein one or both of the buffer layers is of aluminium having a total alloy content of not more than 5% by weight.

30. A method as claimed in any one of claims 16 to 29 v/herein one or both of the puffer layers is of aluminium having a total alloy content of not more than 2.5% by v/eight. 31. A method as claimed in any one of claims 16 to 30 including the step of precipitation hardening the bonded structure.

32. A method as claimed in any one of claims 16 to 31 wherein the metals of the bodies are superplastically deformable, parts only of the bodies are selectively bonded together and the non-bonded parts of the bodies are superplastically deformed.

33. A method as claimed in any one of claims 16 to 32 wherein one or both of the bodies is of an aluminium- lithium alloy.

34. A method as claimed in any one of claims 16 to 33 wherein the bonding process is carried out in a controlled atmosphere selected from an inert gas, carbon monoxide and nitrogen containing water vapour.