BONDING METALS

Abstract: A method of bonding together bodies which may be sheets of the same or different metals which form an oxidised layer thereon so readily as to be inherently incapable of being dif­ fusion bonded together. The method comprises the steps of causing each body to be coated with a different metallic layer which is capable of being bonded to a similar layer by the appli­ cation of pressure at an elevated temperature lower than the melting point of the metal of either body while at the same time breaking up and dispersing the oxidised layer on each body so that the latter and its metallic layer are in metal to metal contact and thereafter bonding the bodies together with the metallic layers in contact. Bonding may be effected by diffusion bond­ ing or transient liquid phase bonding. The breaking up and dispersing of the oxidised layer may be effected by hot roll bonding the metallic layer and the body together, but the same result may be achieved by other processes such as explosive bonding or hydrostatic or conventional extrusion. The metals of the sheets can be superplastically deformable and parts only of the bodies selectively bonded together, the non-bonded parts of the bodies being superplastically deformed to provide an integrally stiffened component.

Description: Bonding metals This invention relates to the production of composite metal bodies and to bonding such bodies together. 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 the 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 provide a metal sheet which cannot readily be diffusion bonded, with a different metal coating of lower melting point which readily alloys with the metal of the sheet. During bonding, coating to coating, the latter melts and a new alloy is formed at the interface of the sheets. This operation is in many ways analogous to brazing and has been described as transient liquid phase bonding or TLP. An example of such a proposal is disclosed in G.B.1,485,051. There are no known techniques however that enable satisfactory diffusion bonding of readily oxidisable metals such as aluminium and magnesium where an extremely adherent, impermeable and chemically complex coating is formed very rapidly upon exposure to atmospheres 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 siiver. US-3,180,022 discloses one such attempt. In an article in the Welding Journal, August 1980 Page 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. Large 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 69MPa. 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-2,117,691A 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 example quoted. The basic disadvantages in a process such as that described lie in the 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-3,633,266 in which a zinc alloy is coated on a tapered aluminiun 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. Additionally, subsequent to joining, 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. According to one aspect of the present invention there is provided a method of bonding together bodies of the same or different metals, at least one of which forms an oxidised layer thereon so readily as to be inherently incapable of being diffusion bonded to another body, comprising causing each body to be coated with a different metallic layer which is capable of being bonded to a similar layer by the application of pressure at an elevated temperature lower than the melting point of the metal of either body while at the same time breaking up and dispersing the oxidised layer on each body so that the latter and its metallic layer are in metal to metal contact and thereafter bonding the bodies together with the metallic layers in contact by a process selected from diffusion bonding and transient liquid phase bonding. The breaking up and dispersing of the oxidised layer may be effected by hot roll bonding the metallic layer and the body together, but the same result may be achieved by other processes such as explosive bonding or hydrostatic or conventional extrusion. The metals of the bodies may be superplastically deformable and parts only of the bodies may be selectively bonded together, the non-bonded parts of the bodies being superplastically deformed to provide an integrally stiffened component. According to another aspect of the present invention there is provided a method of producing and joining composite metallic bodies of a base metal, selected from aluminium and its alloys and magnesium and its alloys, and a coating layer of a different metal which coating layer is capable of being bonded to a similar layer by the application of pressure at an elevated temperature lower than the melting point of the base metal comprising hot rolling a block of the base metal having an oxidised layer thereon with a plate of the different metal to produce the composite body, the area of contact between the block and the plate being increased by the rolling process to an extent sufficient to break up and disperse the oxidised layer so that metal to metal contact between the base metal and the coating layer is obtained and subsequently bonding the bodies together with the metallic layers in contact. The invention also includes joining such composite bodies by diffusion bonding. When the different metal has a melting point below the melting point of the base metal the elevated temperature may be between said melting points and the composite bodies may be joined by transient liquid phase bonding. Preferably the different metal is selected from zinc, silver, copper, titanium, nickel and alloys thereof which may include other constituents. It may be desirable to use an alloy for the cladding material if, for example, it is desired to have a particular melting point in the cladding 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 cladding with a melting point higher than that of zinc is required. The base metal may be of an alloy which is superplastically deformable so that an integrally stiffened component may be formed by selectively bonding parts of two composite bodies together and superplastically deforming the non-bonded parts of the bodies. In the above bonding proposals the base metals of the composite bodies may be different. The metallic layers applied to the two bodies to be bonded together may be composed of the same metal or alloy or they may be different. For example one layer may consist of copper and the other may comprise silver, the copper and silver being alloyed together on bonding. Bonding by the TLP (transient liquid phase) technique with a rolled-on cladding 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. A similar effect can also be achieved by depositing a very thin silver layer on top of the zinc to protect the zinc from oxidation as the latter melts. The metal bodies may comprise metal sheets. Methods of bonding together metal bodies will now be described by way of illustration in the following Examples which numbers 2 and 3 are comparative. Figures 1-3 are backscattered electron micrographs showing some of the joints obtained in the Examples. Example 1 A cast block of 2004 aluminium alloy having dimensions 165mm x 120mm x 55mm was clad with a sheet of l.25mm commercial purity zinc by hot rolling. Having reduced the gauge to 3.3mm in this operation the blank was cold rolled to l.Omm thickness. The clad surfaces of pieces of the sheet obtained were pressed together at a pressure of 37 MPa and held at a temperature of 4500C for 30 minutes in air. Example 2 A foil of commercial purity zinc of 0.1mum thickness was pressed between two sheets of 2004 aluminium alloy #or of 1.9mm thickness at a pressure of 37 MPa and held at a temperature of 450 C for 30 minutes in air. Example 3 Two sheets of 2004 aluminium alloy of thickness l.9mm were degreased in Genklene, a proprietary chlorinated solvent, then immersed in a 4% sodium hydroxide solution at 50 0C for 30 seconds, then rinsed in tap water, then immersed in a 30% nitric acid solution at 20 0C for 30 seconds and finally rinsed. The sheets were then immersed in a solution containing 425g of sodium hydroxide and 45g of zinc oxide per litre of demineralised water at 200C, then rinsed and dried. The two sheets were pressed together at a pressure of 37 MPa and held at a temperature of 450 C for 30 minutes in air. Example 4 Pieces of 1.0mm-thick zinc clad 2004 aluminium alloy were prepared as described in Example 1. The clad sheets were sputter coated with silver to a thickness of 0.1 um. The pieces were pressed together at a pressure of 0.67 MPa and held at a temperature of 450 0C for 2 hours in air. The results of the above Examples are shown in Table 1. Table 1 Quality of bond produced with alloy 2004 at 450 C in air using various interlayers as described in Examples 1 to 4 (bond tested at ambient temperature) EMI7.1 <tb> Example <SEP> Time <SEP> Pressure <SEP> Bond <tb> <SEP> number <SEP> Interlayer <SEP> (mins) <SEP> (MPa) <SEP> Achieved <tb> <SEP> 1 <SEP> Roll <SEP> bonded <SEP> zinc <SEP> 30 <SEP> 37 <SEP> G <tb> <SEP> 2 <SEP> Zinc <SEP> shim <SEP> 30 <SEP> 37 <SEP> p <tb> <SEP> 3 <SEP> Zincate <SEP> coating <SEP> 30 <SEP> 37 <SEP> p <tb> <SEP> 4 <SEP> Roll <SEP> bonded <SEP> zinc <SEP> 120 <SEP> 0.67 <SEP> <SEP> G <tb> <SEP> sputter <SEP> coated <tb> <SEP> with <SEP> silver <tb> P= poor, failed with ease on peeling G= good, on peeling failure occurred in aluminium alloy substrate. Example 5 Zinc clad aluminium alloy sheets were prepared as in Example 1 but with a gauge of 5 mm after hot rolling and a gauge of 1.5 mm after cold rolling. The 2004 alloy substrate had the same tensile properties as the materials used in Examples l to 4. The clad surfaces of the pieces were pressed together at a pressure of 3MPa and held at a temperature of 450 0C for 1 hour in air. The bonded joint thus formed was then solution treated by subjecting it to a temperature of 5300 C for 60 mins. and rapidly quenched. It was then age hardened at a temperature of 165 0C for 16 hours. Example 6 By employing a zinc alloy containing a trivalent metal such as aluminium it is known that the rate of oxidation of the alloy is greatly reduced compared to that of pure zinc. This is because oxidation proceeds by the migration of interstitial zinc atoms and the number of those available are significantly reduced if the lattice is doped with a tripositive ion in order to maintain electrical neutrality. Moreover, the duration of the liquid phase will be shorter for a cladding layer of a Zn-Al alloy than if a pure zinc alloy is employed. Space charge effects at the oxide-air interface of the zinc cladding layer can also limit the rate of oxidation, thus water vapour in the region of the bond can greatly reduce the rate of oxidation of the zinc while reducing gases such as hydrogen or carbon monoxide can reverse the reaction at elevated temperatures. The same conditions as described in Example 5 were used to prepare two sheets of 1.5mm thick 2004 aluminium alloy clad with a 99% Zn-lX Al alloy. The pieces were pressed together at a pressure of 3 MPa and held at a temperature of 450 0C for 1 hour in air. The joint thus formed was solution treated and aged as in Example 5. Fig.1 is a backscattered electron micrograph of the joint obtained. It shows that the joint is free from porosity and non-bonded areas and has a continuous structure across the bond. Example 7 The main advantages of using a solid state diffusion bondable metal such as silver as a cladding metal, are that it has a high melting point so that aluminium alloys can be processed at temperatures well above that of even high melting point zinc alloys and without the possibility of embrittlement of the aluminium alloy by zinc. As a consequence of this it is possible to envisage the use of forming processes during which diffusion bonding occurs part way through the forming cycle. The use of silver as a cladding metal at the thicknesses required is not inordinately expensive and prior to bonding it can be reclaimed by electrochemical processes from areas on an article that are not to be bonded. A cast block of 2004 aluminium alloy having dimensions 165mm x 120mm x 55mm was clad with a sheet, 0.33mm thick of 99.1% purity silver by hot rolling. Having reduced the gauge to 5.0mm in this operation the block was cold rolled to 1.5mm. The clad surfaces of the sheet obtained were pressed together at a pressure of 12MPa while heated over a 1 hour period to a temperature of 380 0C in air. The pressure was then reduced to 3MPa and the temperature raised to 4500 C and held for 1 hour. The joint thus formed was solution treated and aged as in Example 5. Fig.2 is a micrograph similar to Fig.1 of the joint obtained and shows absence of porosity and non-bonded areas. Example 8 A composite material comprising electrical purity grade copper (BSS. 1036) coextruded on electrical purity grade aluminium and which was rectangular in cross section was cold rolled to a gauge of 1.5mum. Pieces of this material were held together at a pressure of 3mPa and a temperature of 450 C for l hour in vacuum. On testing the bonds produced in the above examples using a T-peel test it was found that in all cases those bonds which had been formed by the methods embodied in this patent application (i.e. examples 1 and 4 to 8) were strong enough to cause failure of the aluminium substrate rather than the bonds themselves e.g. for the 2004 material this was recorded as greater than 70 Nmm 1 whereas the use of a zinc shim resulted in a T-feel strength of 4 Nmm 1 and zincate coating of 1 Nmm#1.TensiIe failure annealed 2004 material typically occurs at 200Nmm 2. Fig.6 is a micrograph showing the bond after the T-peel test. It can be seen that peeling failure occurred in the aluminium alloy substrate and not in the bond. The T-peel test was carried out according to ASTM D-1876-69 using test pieces 15 mm wide and a head- speed of 20 mm mien.¯1. It should be noted that 2004 sheet prepared in a manner to that described in Example 1 exhibits superplastic behaviour at 450 0C and is, therefore, fairly soft at this temperature. Nevertheless because of the low pressures employed in Examples 4 to 7 the reduction in gauge resulting from the bonding operation was less than 5%. The relatively high pressure employed during the pressure temperature cycle described in Example 7 was designed to forge together asperities in the surface of the materials at lower temperatures when the aluminium alloy substrate is relatively hard and aids the expulsion of air from the forging surfaces. A typical sequence for fabricating an article would include the following steps according to the invention. 1) Cast and scalp an ingot of readily oxidisable metal. 2) Place a plate or plates of cladding metal next to the scalped surface or surfaces and preheat the assembly. 3) Hot roll the assembly to the required gauge. 4) Cold roll the composite material thus formed to the required gauge. 5) Cut the composite material into sheets of the required shape. 6) Mark out and stop-off areas where bonding is not required. 7) Assemble together the desired configuration of sheets in a sandwich form. 8) Heat and apply pressure, either by gas or mechanic ally, until the sandwich has been at the bonding temperature for sufficient time to bond the sheets together. Later stages of fabrication may require superplastic forming as disclosed in, for example, GB-A-2,071,547A and/or solution treating and ageing, and/or machining or further welding processes depending upon design requirements.

Claims: Claims 1. A method of bonding together bodies of the same or different metals, at least one of which forms an oxidised layer thereon so readily as to be inherently incapable of being diffusion bonded to another body, comprising causing each body to be coated with a different metallic layer which is capable of being bonded to a similar layer by the application of pressure at an elevated temperature lower than the melting point of the metal of either body while at the same time breaking up and dispersing the oxidised layer on each body so that the latter and its metallic layer are in metal to metal contact and thereafter bonding the bodies together with the metallic layers in contact by a process selected from diffusion bonding and transient liquid phase bonding. 2. A method according to Claim 1, wherein the metallic layer and the body are roll bonded together. 3. A method according to Claim 2, wherein the metallic layer and the body are hot roll bonded together. 4. A method according to Claim 1, wherein the metallic layer and the body are bonded together by explosive bonding. 5. A method according to Claim 1, whereit the metallic layer and the body are bonded together by extrusion. 6. A method according to Claim 5, wherein the metallic layer and the body are bonded together by hydrostatic extrusion. 7. A method according to any one of the preceding claims, wherein the bodies are of a material selected from aluminium and its alloys and magnesium and its alloys. 8. A method of producing and joining composite metallic bodies of a base metal, selected from aluminium and its alloys and magnesium and its alloys, and a coating layer of a different metal which coating layer is capable of being bonded to a similar layer by the application of pressure at an elevated temperature lower than the melting point of the base metal comprising hot rolling a block of the base metal having an oxidised layer thereon with a plate of the different metal to produce the composite body, the area of contact between the block and the plate being increased by the rolling process to an extent sufficient to break up and disperse the oxidised layer so that metal to metal contact between the base metal and the coating layer is obtained and subsequently bonding the bodies together with the metallic layers in contact. 9. A method according to Claim 8, comprising joining said composite bodies by diffusion bonding. 10. A method according to Claim 8, comprising joining said composite bodies by transient liquid phase bonding. 11. A method according to any one of the preceding claims, wherein said different metal layer is selected from zinc, silver, copper, titanium, nickel, and alloys thereof which may include other constituents. i2. A method according to any one of the preceding claims, wherein the different metal layer is zinc or a zinc alloy containing at least one of 0-25% aluminium, 020% copper, 0-3% magnesium, 0-20% silver by weight. 13. A method according to any one of the preceding claims, wherein the said different metal layer is coated with a protective layer of an inert metal. 14. A method according to Claim 13, wherein said inert metal is silver. 15. A method according to any one of the preceding claims, wherein the metal bodies are superplastically deformable. 16. A method according to any one of the preceding claims, 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. 17. A method according to any one of the preceding claims, wherein the bodies are bonded together by diffusion bonding and a higher pressure followed by a lower pressure are applied to the sheets during a heating period. 18. A method according to any one of the preceding claims, wherein the bonding process is carried out in a controlled atmosphere. 19. A method according to Claim 18, wherein the controlled atmosphere is an inert gas. 20. A method according to Claim 18, wherein the controlled atmosphere is carbon monoxide. 21. A method according to Claim 18, wherein the controlled atmosphere is nitrogen containing water vapour. 22. A method according to any one of the preceding claims further comprising the steps of solution treating and precipitation hardening the sheets after the bonding process. 23. A method according to any one of the preceding claims in which said bodies comprise sheets. 24. A bonded metal article made by a method according to any one of the preceding claims.

Inventors: BALL, JONATHAN, GRIMES, ROGER
Applicants: ALCAN INTERNATIONAL LIMITED
Application year: 1984
Main classification: B23K20
IPC codes: B21C23, B23K35, B23K20, B32B15