PROCESS FOR THE DIFFUSION BONDING OF ALUMINIUM BASED MATERIALS

Abstract: A process and apparatus for the diffusion bonding of aluminium based materials are described. The process comprises the steps; i removing the oxide film by an ion bombardment technique, ii depositing a coating of a suitable metal wherein at least the initial stages of the coating deposition is carried out by an ion plating method, iii diffusion bonding the cleaned and coated aluminium alloy components together and wherein there is no time interval between the completion of step (i) and the commencement of step (ii). A suitable metal for the deposited coating is silver applied in the range 10 to 1000nm thickness.

Description: PROCESS FOR THE DIFFUSION BONDING OF ALUMINIUM BASED MATERIALS The present invention relates to a process for diffusion bonding aluminium based materials. High strength precipitation hardening aluminium alloys are difficult to join by fusion welding techniques. Brazing alloys have been developed for many of these aluminium alloys but, however, the joint strengths do not approach the parent metal strength. An alternative jointing technique to either fusion welding or brazing is diffusion bonding. Some of the advantages of the latter method which have been demonstrated on alloys other than aluminium based include: 1. Parent metal strength may be obtained. 2. Large area joints are possible. 3. Bonding involves minimum distortion and deformation and close dimensional control is possible. 4. No corrosion arises from fillers or fluxes at the joint interface. 5. Thick and thin sections can be joined to each other. 6. Cast, wrought and sintered powder products and dissimilar metals can be joined. 7. Machining costs may be reduced. 8. More efficient design may be possible. Previous attempts to diffusion bond aluminium alloys have nearly always been thwarted by the tenacious aluminium oxide film present on the alloys under virtually all conditions. Work has been carried out in the past to chemically remove the oxide film and to electroplate or vacuum deposit various coating metals onto the aluminium alloy surface to prevent re-oxidation. The coating metal which has attracted most attention is silver since its oxide decomposes before substantial diffusion of the coating into the alloy occurs. Previous attempts at silver coating after partial removal of the aluminium oxide have involved relatively thick coating layers. Bond strengths obtained have depended critically on the residual oxide layer thickness between alloy and coating. The oxide film thickness can be reduced by stable pretreatments and rupture of the oxide film during bonding can be promoted by the use of a transient liquid phase, a soft interface layer or large plastic deformations. However, all of these techniques only achieve partial success and the resulting bond strengths never approach the parent metal strength. According to the present invention a diffusion bonding process for aluminium based materials comprises the following steps; i removing the oxide film by an ion bombardment technique, ii depositing a coating of a suitable metal wherein at least the initial stages of the coating deposition is carried out by an ion plating methods iii diffusion bonding the cleaned and coated aluminium alloy components together and wherein there is no time interval between the completion of step (i) and the commencement of step (ii) Alluminium alloys have been prepared for diffusion bonding by the process of the invention and have given bond shear strengths at least as strong as the parent metal. According to a feature of the present invention the diffusion bonded component is given a suitable heat treatment to diffuse away from the joint interface relatively high concentrations of the coating metal which may give rise to brittleness of the interface. Such a heat treatment could, for example, be a solution treatment of the type normally given to a precipitation hardening alloy. The coating metal chosen will be one where any coating oxide formed after the coating operation may be removed by a relatively simple treatment prior to the diffusion bonding step. Preferably the chosen coating metal will have a substantial solubility i aluminium at the diffusion bonding temperature. Suitable elemental coating metals may comprise magnesium, zinc, silver and copper. Of this group silver offers the most favourable properties having an oxide which decomposes at 1900C. In order that the invention may be more fully understood an example will now be described with reference to the accompanying drawings, of which: Figure 1 shows a schematic view of the apparatus used to clean and coat aluminium alloy samples prior to diffusion bonding. Figure 2 shows a schematic view of the apparatus used to diffusion bond the cleaned and coated aluminium alloy samples, and Figure 3 shows a photomicrograph of the bond interface area after post diffusion bonding heat treatment showing grain structure to be continuous across the interface. Samples of 7010 alloy 12 x 110 x 3.24 mm thick clad with aluminium - 1% zinc to a thickness of 5% on each surface were coated with silver by the process of the invention. The clad alloy was to a specification X166 - T7651 and was used in the as-received condition apart from degreasing in trichloroethane and acetone. The silver coating was obtained by evaporation from molten 99.95% silver. With reference to figure 1. Cleaning and ion-plating of the aluminium alloy specimens is carried out in a triode sputtering apparatus. The apparatus comprises a glass lowpressure vessel 10 with a heated tungsten filament 11 on one' side of the vessel 10 and an anode 12 mounted on the opposite side of the vessel. Mounted on a vertically adjustable support which forms the cathode 13 are the aluminium alloy specimens 14. Circular electromagnetic coils 15 are mounted on the horizontal limbs of the vessel 10 such that they individually encircle the tungsten filament 11 and anode 12. Vertically below the specimens 14 is a carbon-glass crucible 16 containing silver heated by a radiant heater 17, above the crucible 16 is a removable shutter 18, the temperature of the crucible 16 is monitored by a thermocouple 19. Mounted on the base support of the low pressure vessel 10 is an argon gas inlet valve 20 and an air inlet valve 21. A vacuum pump 22 (not shown) is used to evacuate the vessel 10. In operation degreased specimens 14 of the aluminium alloy are placed on the specimen support and cathode 13. The glass low pressure vessel 10 is pumped down using the vacuum pump 22 (not shown) to a pressure of 1.3 x 10 3 Pa (1 x 10 5 torr), argon is then leaked continuously into the apparatus through the valve 20 until the pressure has risen to 0.27Pa (2x10#3torr). The anode 12 is maintained at a potential of +30V with respect to the heated tungsten filament 11 and a plasma is then generated having the parameters given below in Table 1. TABLE 1 EMI5.1 <tb> <SEP> I <tb> Magnetic <SEP> Filament <SEP> (11) <SEP> Argon <SEP> DISCHARGE <SEP> HT <SEP> ON <SEP> SPECIMEN <SEP> (14) <SEP> <tb> coil <SEP> (15) <SEP> current <SEP> gas <SEP> Sputtering <tb> current <SEP> pressure <SEP> Volts <SEP> AmpVolts <SEP> mA <SEP> time <tb> <SEP> (Amp) <SEP> (Amp) <SEP> <SEP> (Pa) <SEP> <SEP> (min) <tb> <SEP> 2 <SEP> 20 <SEP> 0.27 <SEP> 30 <SEP> 4 <SEP> 1000 <SEP> <SEP> 140 <SEP> 20 <tb> 1 kV is applied to the specimen support/cathode 13 and, as a result of the plasma present the current flow is 140mA to the 2 specimen 14, corresponding to a current density of 1mA/cm for the size of specimen used due to impingement of argon ions. Sputtering is continued for 20 minutes whereupon the shutter 18 above the crucible 16 is opened, the crucible and silver contents having already been heated to the required temperature by means of the radiant heater 17. The initial layers of silver deposited on the specimens 14 are thus produced under ion-plating conditions where some of the silver atoms evaporated from the crucible 16 are ionised by the plasma and accelerated onto the specimens 14, the process enhancing adhesion of the initial silver layers. After the shutter 18 has been open for 30 seconds the negative potential of 1 kV applied to the specimen support/ cathode 13 is removed, the argon leak via the valve 20 closed and the gas discharge thus extinguished. Evaporation of silver is continued for 12 minutes after which a coating of approximately 0.7 m thickness is achieved. After coating and cooling the specimens 14 the vacuum is released by admitting air into the vessel 10 through the valve 21. Conditions used to deposit the coating are summarised in Table 2 below. TABLE 2 EMI6.1 <tb> Specimen <SEP> Evaporation <tb> distance <tb> <SEP> from <SEP> . <SEP> Tempera- <SEP> Coating <tb> source <SEP> Time <SEP> ture <tb> <SEP> 65 < <SEP> t2 <SEP> 1li0 <SEP> Ag <tb> Referring now to Figure 2 aluminium alloy specimens coated as above are now diffusion bonded using the following procedure. The coated specimens 14 are held in a stainless steel jig 30 with a gap of approximately 3 mm between them, the specimens being supported and held apart by aluminium foil spacers 31. A 10 mm overlap is provided between the specimen 14 ends giving an eventual bonded area of approximately 120 sq mm. A thermocouple 32 is attached to the jig 30 specimen 14 assembly to control the temperature thereof. The jig 30 carrying the specimens 14 is enclosed in a silica glass tube 33 which is supported on a ceramic insulator 34 which in turn rests upon a stainless steel pillar 35. The pillar 35 rests upon a water cooled copper plate 36 but is insulated therefrom by a ceramic spacer 37. The bonding load is applied to the specimens by a steel push rod 38. A feed pipe 39 supplies argon gas to the silica glass tube 33 and is arranged to direct gas into the gap between the specimen 14 surfaces to be bonded. The assembly described above is surrounded by an electrical resistance heated furnace 40. Alumina insulating blanket 41 is placed in the neck of the furnace 40 and around the steel push rod 38. The total apparatus described above is supported on the load cell 42 of an Instron testing machine. The air in the glass tube 33 is displaced before bonding by argon gas introduced through the pipe 39. The specimens 14 are heated by the furnace 40. At about 1900C the silver oxide decomposes and the decomposition products are dispersed by the argon gas jet. The temperature of the specimens is further 0 raised to 220 C and a maximum load of 18.9 kN is applied via the push rod 38 over a period of 2 minutes (the load of 18.9 kN corresponds to a pressure of 158 MPa). The load of 18.9 kN is maintained for 5 minutes by which time the specimen temperature 0 has risen to approximately 320 C. Deformation of the specimens under these conditions is a compressive deformation of approximately 5% of the thickness at the joint area. After the above bonding sequence the applied load is removed and the bonded sample allowed to air cool. After the bonding treatment the samples are heat treated, both to solution treat the alloy and to diffuse away silver from the bond interface. Heat treatment comprises essentially 0 of heating for 3 hours in a salt bath at 480 C followed by water quenching. Specific heat treatment conditions, however, will depend to a great extent on the requirements of the actual alloys being joined. Lap joints prepared by the above process were tested in shear in a Hounsfield Tensometer testing machine. Results of tests gave a shear strength of 105 MPa. It will be appreciated that this figure is higher than the nominal shear strength of Al-1% An alloy (approximately 80 MPa), this is attributed to diffusion of strengthening elements from the 7010 alloy into the cladding and also to some solid solution strengthening due to the diffused silver coating. Figure 3 shows the homogeneity of the resulting interface, showing uninterrupted grain growth across the joint interface. This amply demonstrates that the aluminium oxide film has been adequately removed and, moreover, prevented from reforming by the process of the invention since grain growth across the joint interface. This amply demonstrates that the aluminium oxide film has been adequately removed and. moreover, prevented from reforming by the process of the invention since grain growth across the interface would have been impeded. To prevent the formation of undesirable compounds at the interface the coating metal must be kept sufficiently thin whilst still retaining its function of preventing reoxidation of the aluminium. Preferably the thickness of a silver coating should be in the range 10 to 1000nm, since below 10nm the control of coating thickness becomes difficult. It will be appreciated that thinner silver coatings than the 700 nm cited in the example will require less time to diffuse away from the bond interface. The example quoted above was an elemental coating of silver wherein the initial stages of coating are effected by silver ions and hence is termed ion-plating. After the initial stage, however, the bulk of the coating may be applied by evaporative deposition, instead of ion-plating. The above method is suitable for elemental coatings but as the person skilled in the art will realise the invention is not restricted to elemental coatings. The invention may be effected by coatings based on binary, ternary or any other suitable alloy systems. When depositing a particular binary or ternary alloy control of the composition is extremely difficult when using a molten metal source. A more reliable alternative method would be that of sputter ion plating of the aluminium based alloy using a solid source of the desired coating alloy. An advantage of sputter ion plating is that sources may be made from other than melted and prealloyed constituents. A source could be made from physical combinations, for example, a bundle of wires of the required constituent elements in the correct proportions, the end face of the bundle being the sputtering source. Coatings, which of course may include elemental coatings, applied by the alternative method of sputter ion plating will self evidently be wholly applied by ion plating. Particularly attractive alloy systems are those based on magnesium-lithium, magnesium-silver and magnesium-zinc. The use of other than elemental coatings would allow the use of transient liquid phases during the diffusion bonding step. This would offer the dual advantage of rapidly increasing the diffusion rate and hence bonding rate thus dispersing the coating constituents more rapidly from the interface area and also dispersing any residual oxides there may be on the coatings themselves. Although the invention has been described above with reference to an example utilising clad aluminium alloys the invention is not limited to such and may equally well be applied to unclad alloys. Dissimilar alloys may also be joined provided that no brittle phases are formed at the interface during bonding and/or subsequent heat treatment. A reliable process for the production of high strength diffusion bonded joints in aluminium alloys such as the invention provides makes feasible the process for simultaneous diffusion bonding and superplastic forming as is employed for titanium alloys. Aluminium alloys suitable for superplastic forming have been reported in the technical press.

Claims: CLAIMS 1. A diffusion bonding process for aluminium based materials characterised by the steps of: i removing the oxide film by an ion bombardment technique, ii depositing a coating of a suitable metal wherein at least the initial stages of the coating deposition is carried out by an ion plating method and, iii diffusion bonding the cleaned and coated aluminium alloy components together, and wherein there is no time interval between the completion of step (i) and the commencement of step (ii). 2. A diffusion bonding process according to claim 1 characterised in that the deposited coating of step (ii) is silver. 3. A diffusion bonding process according to claim 1 characterised in that the deposited coating of step (ii) is an alloy. 4, A diffusion bonding process according to any preceding claim characterised in that the deposited coating thickness is between 10 and 1000 nm. 5. A diffusion bonding process according to claim 5 characterised in that the diffusion bonding step (iii) has a transient liquid phase. 6. A diffusion bonding process according to claim 1 characterised in that the source of the deposited coating is vapour from a molten metal. 7. A diffusion bonding process according to claim 1 characterised in that the coating is applied by a sputter ion plating technique. 8. A diffusion bonding process according to any preceding claim characterised in that the aluminium based alloys to be joined are clad. 9. A diffusion bonding process according to any preceding claim characterised in that the aluminium alloys to be joined are different.

Inventors: HARVEY, JOHN, PARTRIDGE, PETER GEORGE
Applicants: THE SECRETARY OF STATE FOR DEFENCE IN HER BRITANNIC MAJESTY'S GOVERNMENT OF THE UNITED KINGDOM OF GREAT BRITAIN AND
Application year: 1983
Main classification: B23K20
IPC codes: C25D5, B23K20, C23C14, C23C10, B23K35