GB2386578A

GB2386578A - Bonding of aluminium-based material using gallium

Info

Publication number: GB2386578A
Application number: GB0128623A
Authority: GB
Inventors: Amir Abbas Shirzaki Ghoshuni
Original assignee: CAMBRIDGE JOINING TECHNOLOGY L; GHOSHUNI AMIR ABBAS SHIRZADI
Current assignee: CAMBRIDGE JOINING TECHNOLOGY L; GHOSHUNI AMIR ABBAS SHIRZADI
Priority date: 2001-11-29
Filing date: 2001-11-29
Publication date: 2003-09-24
Anticipated expiration: 2021-11-29
Also published as: GB0128623D0; GB2386578B

Abstract

This invention provides a new method for flux-free brazing of aluminium using gallium. This method requires non-abrasive smearing of liquid gallium 1 onto a faying surface of a piece of aluminium-based material 2. Preferably, the aluminium pieces 2, 3 are then quenched, eg in liquid nitrogen, to stop further diffusion of the liquid gallium into the aluminium pieces. Then bonding is carried out in air by pressing the pieces of aluminium together with fast heating, ( / 200{C/minute) and maintaining them at a temperature between 200 - 600{C for a few seconds under a controlled pressure.

Description

BONDING OF ALUMINIUM-BASED MATERIAL

The present invention relates to the bonding together of pieces of aluminium 5 based material such as metallic alloys and composites. Aluminium-based metallic alloys and composites oxidise readily because of the high chemical affinity of aluminium for oxygen. This property has, for many years, been employed to particular advantage in that the almost instantaneous formation of oxide layers on the surfaces of such materials formed an excellent barrier to further oxidation. However, the 10 almost instantaneous formation of the oxide layer can be a significant obstacle when wishing to join such materials. For example, it is a major obstacle during diffusion bonding and brazing.

Many different approaches to oxide layer removal have been suggested as removal of the oxide layer is a necessary step in many processes, such as in the 15 brazing of aluminium alloys. One approach to removal is to heat the aluminium-basedt material to a brazing temperature in the presence of a flux in order to remove the oxide layer. The flux is usually toxic and corrosive and must be removed after brazing and typically is chloride or fluoride based for aluminium alloys. Chemical treatment with an acid or alkaline simply results in the replacement of the aluminium oxide layer 20 with various types of sulphide, nitride, hydroxides, etc., rather than producing the desired oxide free surface.

Another known method for the removal of the oxide surface is the use of ion beam cleaning in a vacuum, which must then be followed by in situ sputter-coating of another metal (such as copper or silver) on a clean surface to prevent re-oxidation of 25 the aluminium when the surface is exposed to air. As will be appreciated, this approach is expensive, requires complex equipment and procedures and therefore is of restricted use.

There have also been proposals to employ gallium in combination with aluminium for bonding or brazing soldering. For example, EP-A-0123382 proposes 30 several different methods for bonding aluminium using gallium by rubbing molten gallium or an aluminium-gallium alloy directly on to the surface of aluminium or by employing chemical deposition by dipping aluminium in a solution of nitric acid containing gallium nitrates followed by lengthy heat treatment. There is also discussion in this document of an electrochemical disposition using a gallium nitrate 35 electrolyte. However, as has been known for a number of years, aluminium is -1

À attacked by molten gallium which then embrittles the aluminium to an extent that it can be damaged even by simple touch with a fingernail. So, this prior art indicates that it

is necessary to employ a long bonding and/or heat treatment times (for example 70 80 hours) making the bonding a lengthy and impractical process.

5 In relation to soldering, US-A-2824365, for example, proposes the rubbing of gallium onto an aluminium surface in order to improve joint properties prior to soldering the aluminium by using a lead-tin alloy. The detrimental effect of rubbing gallium is referred to in this prior art document to the extent that it is recommended to

wipe off the crumbled surface of aluminium prior to soldering.

10 The present invention provides a method of bonding pieces of aluminium based material which solves some of the problems associated with the formation of an aluminium oxide layer on the surface of the aluminiumbased material and the embrittlement associated with the use of gallium.

This invention provides a method of bonding two pieces of similar or dissimilar 15 aluminium-based material, the method: comprising the steps of: non-abrasive smearing of liquid gallium onto a raying surface of one or both of the pieces, and pressing of the raying surface together and heating the pieces at a rate equal to or 20 greater than 200 C/minute to reach a temperature in the range 200 - 600 C to generate a bond between the two pieces.

This invention provides a method which does not require abrasive removal of the aluminium oxide, unlike previous methods which require abrading or grinding of aluminium or rubbing gallium on the aluminium surface. The aluminium oxide layer is 25 not removed from the aluminium-based material and therefore the aluminium itself is not exposed to the liquid gallium. The surface aluminium oxide remains intact in order to act as a barrier between the liquid gallium and the bulk aluminium prior to the bonding stage. This prevents liquid gallium penetrating into the aluminium grain boundaries which would result in immediate embrittlement of the aluminium and also 30 prevents loss of the liquid gallium at the joint interface (which is required for the bonding process).

Before the step of smearing, the piece of aluminium-based material may be heated to a temperature in the range of 50 C to 150 C. The aluminiumbased material can be heated to this temperature to improve wetting of liquid gallium on the 35 aluminium surface.

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The gallium can be applied by smearing a thin layer of liquid gallium on the aluminium-based material faying surface using a soft tissue or cloth containing a very small but controlled amount of liquid gallium. The nonabrasive smearing of liquid gallium onto a raying surface may apply 0.5 to 1.5 mg/cm2 of gallium. Liquid gallium 5 may also be smeared onto a raying surface of the second piece of aluminium-based material. The pressure used to generate the bond may be above 1 MPa and preferably about 20 MPa. The bonding load should preferably be as high as possible, but should be less than the maximum yield strength of the aluminium-based material at 10 the bonding temperature, unless deformation of the component being joined is designed to be part of a bonding process such as simultaneous bonding and superplastic forming.

It is preferred that the rate of heating in the pressing step of the method of this invention is at least200 C/minute and preferably is above 1000 C/minute. Rapid 15 heating of the pieces being joined is one of the main requirements and features of this; method. Due to the presence of the very small amount of gallium at the joint interface, any prolonged duration in the heating stage will result in grain boundary diffusion of the gallium and therefore deficiency of the liquid gallium at the interface when it is later needed to promote bonding. Rapid heating can be achieved in a 20 number of ways including pressing the aluminium pieces between two pre-heated hotplates (that may be made of steel or any other suitable material). In this case, the temperature of the hotplates may be between 200 - 600 C depending on the type of aluminium-based material used. Other fast heating methods, such as laser beam or induction heating can be used.

25 If the bonding step does not follow immediately after the smearing step then the piece of aluminium-based material can be immediately quenched, to stop further diffusion of the liquid gallium into the aluminium pieces. The quenching occurs by reducing the temperature and comprises cooling a piece of aluminium-based material with liquid gallium smeared on its raying surface to below the melting point of gallium 30 and maintaining the piece at this temperature. The maximum temperature at which the sample should be kept is below the melting point of gallium- aluminium alloy which is 26 C. The cooling rate must be as high as possible to minimise the reaction of aluminium with the liquid gallium prior to pressing. In practice temperatures considerably lower than 26 C are favourable as they further reduce the solid state 35 diffusion of gallium into aluminium. It is preferred that the piece of aluminium-based -3

material is cooled to below 0 C, preferably below -100 C and more preferably at or below -196 C. By rapid cooling below the melting temperature of gallium (using liquid nitrogen), any inter-granular attack of aluminium by liquid gallium is prevented.

An example of the present invention will now be described with references to 5 the accompanying drawings, in which: figure 1 shows the process of using a layer of gallium to bond together pieces of aluminium-based material; and figure 2 shows a three point bend test of bonded aluminium pieces.

10 Example 1

A small amount of gallium was melted onto a piece of soft cloth/tissue using a heat gun or ordinary hair dryer. The surface of aluminium was also heated to 100 C to improve wetting of liquid gallium on the aluminium surface. A thin layer of gallium was gently smeared on the aluminium and the parts were immediately quenched in 15 liquid nitrogen for a few seconds to solidify the liquid gallium. The gallium-coated pieces of aluminium were then put together and pressed between two hot steel plates for 20 seconds. The temperature of the plates was about 500 C and the bonding pressure was about 20 MPa. The procedure described is shown in Figure 1. Step 1 shows a very thin layer of gallium [1] applied to the aluminium surface [2]. Step 2 20 involves solidifying the gallium layer [3] by quenching the aluminium in liquid nitrogen.

This stage can be omitted if bonding is carried out immediately after applying the liquid gallium. Step 3 shows bonding the aluminium pieces using a heat source with a fast heating rate, eg hot plates [4], induction system or laser beam (not shown).

Pressure (P) is also applied at this stage. In some cases, a post-bond heat treatment 25 (such as 500 C for 30 minutes) is recommended for further improvement of the bond quality. This method can also be used for any combination of aluminium-based alloys or composites.

The as-bonded sample was bent at room temperature to assess the bond strength. Figure 2A shows the three point bond test (as shown by the arrows) of 30 bonded pieces [5] of pure aluminium. Figure 2B shows that no joint failure or embrittlement was observed.

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Claims

1. Method of bonding two pieces of aluminium-based material each having a 5 raying surface, the method comprising the steps of: non-abrasive smearing of liquid gallium onto a raying surface on at least one of the pieces, and pressing the raying surfaces together and heating the pieces at a rate equal to or greater than 200 C/minute to reach a temperature in the range of 10 200 - 600 C to generate a bond between the two pieces.

2. The method of claim 1 wherein before the smearing step the piece of aluminium-based material is heated to a temperature in the range of 50 C to 150 C.

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3. The method of claim 1 wherein the non-abrasive smearing of liquid gallium -

onto a raying surface applies between 0.5 to 1.5 mg/cm2 of gallium.

4. The method of claim 1 wherein the pressure used to generate the bond is above 1 MPa, and preferably about 20 MPa.

5. The method of claim 1 wherein the rate of heating is at or above 200 C/min and preferably is above 1000 C/min.

6. The method of any one of the preceding claims which additionally includes a 25 procedure between the smearing step and the bonding step which comprises cooling the piece of aluminium-based material with liquid gallium smeared on its raying surface to below the melting point of gallium and maintaining the piece at this temperature. 30

7. The method of claim 6 wherein the piece is cooled to and kept below 27 C, preferably below-100 C and more preferably at or below -196 C.

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