BONDING OF METALLIC ELEMENTS

Abstract: A method of bonding two metallic elements together. The method comprises the steps of placing the two elements adjacent to one another, with a metallic interlayer therebetween to form a bonding region and applying an external load to the bonding region. The region is heated such that the temperature of the region increases at a rate sufficient to minimise oxidation of the elements and the thickness of any oxide layer formed on the surface thereof.

Description: BONDING OF METALLIC ELEMENTS This invention relates to diffusion bonding. Diffusion bonding can be used to join advanced materials for particular applications when conventional welding methods have proved unsuccessful. The materials joined by diffusion bonding include titanium alloys, aluminium-based alloys or composites, nickel-based superalloys and intermetallics. Diffusion bonding can be carried out in the solid or liquid state. In solid state diffusion bonding, bonds form as a result of the interdiffusion of atoms across the interface due to an applied pressure at elevated temperature. In contrast, Transient Liquid Phase (TLP) diffusion bonding relies on the formation of a liquid phase at the bond line during an isothermal bonding cycle as a result of interdiffusion between an interlayer, inserted at the interface, and the base material. The liquid phase forms generally as a consequence of the reduced liquidus temperature in either eutectic or peritectic alloy systems. This liquid phase eventually solidifies at constant temperature as a consequence of continued diffusion of solute from the liquid phase into the adjacent solid phase. Some materials have stable oxide layers which can inhibit the interdiffusion of atoms across the interface. Therefore, diffusion bonding of these materials is normally conducted in vacuum in order to minimise further oxidation of the faying surfaces. Aluminium-based materials, in particular, are notoriously difficult to diffusion bond due to the presence of tenacious and chemically stable surface oxide layers. Transient liquid phase (TLP) diffusion bonding (in which the formation of a liquid phase enhances the disruption of the oxide layer and consequently improves the bond strength) has the potential to overcome the problem caused by the presence of stable surface oxides. However, the bonding process usually has to be carried out in vacuum, typically 10 -10 bar, to prevent any increase in the thickness of surface oxide layers and also to avoid high temperature oxidation of the interlayer. The necessity of joining in vacuum in order to form bonds with reasonable mechanical strengths has restricted the application of TLP diffusion bonding. According to the present invention there is provided a method of bonding two metallic elements together, the method comprising the steps of: placing the two elements adjacent to one another, with a metallic interlayer therebetween to form a bonding region; applying an external load to the bonding region; and heating the region, such that the temperature of the region increases at a rate sufficient to minimise oxidation of the elements and the thickness of any oxide layer formed on the surface thereof. Preferably the heating is such that the heating rate is at least 20°C per second. The elements may be formed from aluminium or an alloy thereof . The heating rate and load may be such that solid phase diffusion bonding occurs. Alternatively the heating rate and load may be such that liquid phase diffusing bonding occurs. The interlayer may be formed, by the use of a foil, sputtering or a similar technique, on one of the elements. The method may be performed in a vacuum or, advantageously can be performed in air. The metallic interlayer may be formed from copper or other suitable metal or alloy thereof. According to the invention a new method for diffusion bonding has been developed which is capable of reliably producing high strength aluminium bonds in air. The use of this method drastically reduces both the initial investment and the running costs of the bonding process, therefore a much wider range of applications for TLP diffusion bonding is possible. This method is applicable to the diffusion bonding in air of other materials in addition to aluminium- based materials. One example of the present invention will now be described with reference to the accompanying drawings, in which: Figure 1 is a schematic diagram of an arrangement for preparing elements to be joined; Figure 2 is a schematic diagram showing an apparatus for performing the invention; and Figure 3 is a graph showing shear strength for sample elements bonded according to the invention and compared with a continued element formed from the same material. The method of the invention is described below in terms of the major features of aluminium-based materials (as the parent material) and copper (as the interlayer) . The method is, however, applicable to a much wider range of parent materials and interlayers. The background to the invention will now be described. Aluminium and oxygen have such a high chemical affinity that a new oxide layer forms immediately after surface cleaning. A few nanometres of oxide are sufficient to isolate the base materials from ambient oxygen, therefore the rate of oxidation decreases rapidly following initial oxidation (in a fraction of a second) . This intrinsic feature of aluminium-based materials has been considered to be an obstacle when diffusion bonding these materials. When TLP diffusion bonding aluminium alloys using a copper interlayer in vacuum, the liquid phase forms within a few seconds after reaching the eutectic temperature of the Al- Cu binary system (i.e. -548 °C) . The interface becomes sealed by liquid or a compliant solid (for solid phase bonding) and so a vacuum is not required for the rest of the bonding process. It can be concluded that any increase in the thickness of the oxide layer from its initial condition occurs mostly during the heating stage and ceases when the liquid phase seals the interface. Theoretically, the relatively impermeable and chemically stable surface aluminium oxide layers protect minimise further oxidation of the faying surfaces during the short heating stage. Accordingly, the higher the heating rate, the less the increase in the thickness of oxide layers. The present invention takes advantage of this. Copper is extensively used as a sealant in vacuum and pressurised systems. Hence the copper interlayer, particularly used for TLP diffusion bonding, can act also as a sealant and reduce the oxidation of itself and the aluminum alloy during the heating stage, provided the thickness of interlayer is adequate. The faying surfaces should be flat and smooth to ensure proper contact between the interlayer and alloy. In the method of the invention, good contact is required to seal the bond and hence reduce the oxidation due to the exposure of the interface to air. Also, the higher the bonding pressure, the better is the sealing of the interface, providing that the bonding pressure does not exceed the yield stress of the parent material at the bonding temperature. It should be noted that lack of good contact at the initial stage of conventional TLP diffusion bonding in vacuum can be tolerated as the liquid phase forms, normally with a thickness of a few times that of the thickness of the interlayer, and consequently fills any crevices. A method according to the present invention will now be described. Precision is required in the surface preparation stage. The faying surfaces should be ground to produce flat surfaces without rounded edges. Elements to be bonded are manually ground on a rotary wheel 1 with silicon carbide paper 1 of 1200 grit. A steel jig 3 and retaining screw 4 is used to hold the elements 2 to be in order to produce flat surfaces, see figure 1. All elements 2 and the copper interlayers are degreased, for example by acetone and then methanol, before being inserted in the diffusion bonder. The design of the bonding set-up is shown schematically in figure 2. Insulation rods 6 are machined precisely to produce parallel ends. A load 8 is applied by an electric motor via a transmission system including a steel rod 9 with a round tip. The point contact between the rod and the centre of the upper insulator prevents any undesirable eccentric loading. An r.f. induction coil 10 is kept as close as possible to the elements 2 in order to increase the coupling effect and consequently to achieve the highest possible heating rate. Other forms of heating, e.g. laser sources, are suitable. Other methods of pressure generation, such as differential thermal expansion may be employed. In an example of the method, aluminium alloy 6082 was used as the parent material and copper foils as the interlayer. With a fairly constant heating rate of 20°C per second and a constant bonding temperature of 550°C for all elements 2, the effects of bonding pressure (5 and 7 MPa) , time (5 and 10 minutes including 30 seconds for the heating stage) and the thickness of copper interlayer (0.7, 1,7 and 12.5μm) on bond strength were investigated. The rapid thermal expansion of the specimens during the heating stage increased the pressure up to 10 MPa for a short time before the loading system resumed the pre-set pressure (5 or 7 MPa) . Having selected bonding conditions for Al-Mg-Si alloy 6082 (using a 12.5μm copper interlayer with a bonding time of 5 minutes and bonding pressure of 7 MPa) , samples were produced and shear tested in the fully aged condition. In Fig. 3, the shear strengths of Al-6082 bonds are compared with that of the parent material subjected to the same thermal cycle. Bonds with shear strengths as high as 206 MPa (90% of that of the parent material) were achieved. The minimum bond strength was 173 MPa (75% of that of the parent material) . The effects of various bonding conditions on bond strength have been investigated and it seems that increasing both the bonding pressure (from 5 to 7 MPa) and the thickness of interlayer (within the range of 1 to 12.5μ) increase the bond strength. It should be noted that the bonds made using 0.7xm thick interlayers resulted in lower strengths, suggesting that a minimum thickness of interlayer is required to achieve high strength bonds. The thickness of interlayer should be optimised for each particular alloy and interlayer combination. Increasing the bonding time up to 10 minutes did not seem to improve the bond strength. The bonding pressure should ideally be set as close as possible to the maximum pressure that the parent material can withstand without substantial deformation at the bonding temperature. A high bonding pressure not only reduces the likelihood of oxidation but also expels excessive liquid from the interface during the bonding cycle.

Claims: CLAIMS 1. A method of bonding two metallic elements together, the method comprising the steps of: placing the two elements adjacent to one another, with a metallic interlayer therebetween to form a bonding region; applying an external load to the bonding region; and heating the region, such that the temperature of the region increases at a rate sufficient to minimise oxidation of the elements and the thickness of any oxide layer formed on the surface thereof . 2. A method according to claim 1, wherein the heating rate is at least 20°C per second. 3. A method according to claim 1 or claim 2 , wherein the elements are formed from aluminium or an alloy thereof. 4. A method according to any of claims 1 to 3, wherein the heating rate and load are such that solid phase diffusion bonding occurs. 5. A method according to any of claims 1 to 3 , wherein the heating rate and load are such that liquid phase diffusing bonding occurs . 6. A method according to any of claims 1 to 5 , wherein the interlayer is formed on one of the elements. 7. A method according to any of claims 1 to 6 , wherein the method is performed in air. 8. A method according to any of claims 1 to 7, wherein the metallic interlayer is formed from copper or alloy thereof .

Inventors: SHIRZADI-GHOSHOUNI AMIR ABBAS, WALLACH ERIC ROBERT, SHIRZADI-GHOSHOUNI, AMIR, ABBAS, WALLACH, ERIC, ROBERT
Applicants: SHIRZADI GHOSHOUNI AMIR ABBAS, UNIV CAMBRIDGE TECH, WALLACH ERIC ROBERT, CAMBRIDGE UNIVERSITY TECHNICAL SERVICES LIMITED, SHIRZADI-GHOSHOUNI, AMIR, ABBAS, WALLACH, ERIC, ROBERT
Application year: 1999
Main classification: B23K20
IPC codes: B23K20, B23K35