New method for flux free diffusion brazing of aluminium alloys using liquid gallium (UK patent application 0128623.6)

摘要

Removal of the oxide film is usually mandatory before brazing aluminium alloys. Typical fluxes contain toxic and corrosive chemicals such as chlorides and fluorides, which must be removed after brazing, for instance to minimise subsequent corrosion. Therefore, developing a brazing process that is capable of joining aluminium based alloys without the use of any flux is of interest. A new method for flux free brazing of aluminium in air, using gallium, has been developed and is capable of providing strong bonds, having strengths comparable to that of the parent material. The key feature of this new method is that the thickness of the liquid gallium at the joint interface is minimised in order to reduce grain boundary attack and subsequent embrittlement by liquid gallium. Hence in this method a very thin layer of gallium is applied to the faying surfaces of the parts being joined, and these are then immediately quenched in liquid nitrogen to stop further diffusion of the liquid gallium into the aluminium. Bonding is subsequently carried out by rapidly heating the gallium treated aluminium parts, and holding them at a temperature between 400 and 550 °C for a few minutes under a controlled pressure. Bonding can be improved by a subsequent heat treatment at elevated temperatures to enhance the diffusion of gallium into the aluminium and so reduce further its concentration at the interface and grain boundaries. Such post-bond heat treatments may not be required if the bonding time is increased for a few minutes.