Question

Which of the following is NOT a component of a commonly used aluminium brazing alloy?
1. Copper
2. Silicon
3. Aluminium
4. Iron

Answer 1

Correct Answer - Option 4 : Iron

Explanation:

Aluminum brazing:

There is a distinction between the brazing of aluminum and the brazing of other metals. For aluminum, the brazing alloy is one of the aluminum alloys having a melting point below that of the parent metal.  For other metals, the brazing alloys are often based on copper-zinc alloys (brasses - hence the term brazing) and are necessarily dissimilar in composition to the parent metal.

Wetting and fluxing

When a surface is wetted by a liquid, a continuous film of the liquid remains on the surface after draining. This condition, essential for brazing, arises when there is a mutual attraction between the liquid flux and solid metal due to a form of chemical affinity. Having accomplished its primary duty of removing the oxide film, the cleansing action of the flux restores the free affinities at the surface of the joint faces, promoting wetting by reducing the contact angle developed between the molten brazing alloy and parent metal. This action assists in spreading and the feeding of brazing alloy to the capillary spaces, leading to the production of well-filled joints. An important feature of the brazing process is that the brazing alloy is drawn into the joint area by capillary attraction: the smaller the gap is between the two metal faces to be joined, the deeper is the capillary penetration.

The various grades of pure aluminium and certain alloys are amenable to brazing. Aluminium-magnesium alloys containing more than 2 percent magnesium are difficult to braze, as the oxide film is tenacious and hard to remove with ordinary brazing fluxes. Other alloys cannot be brazed because they start to melt at temperatures below that of any available brazing alloy. Aluminum silicon alloys of nominal 5 percent, 7.5 percent or 10 percent silicon content are used for brazing aluminium and the alloy of aluminium and 1.5 percent manganese.

The properties required for an effective flux for brazing aluminium and its alloys are as follows:

1. The flux must remove the oxide coating present on the surfaces to be joined. It is always important that the flux be suitable for the parent metal, but especially so in the joining of aluminum-magnesium alloys.
2. It must thoroughly wet the surfaces to be joined so that the filler metal may spread evenly and continuously.  
3. It must flow freely at a temperature just below the melting point of the filler metal.
4. Its density, when molten, must be lower than that of the brazing alloy.
5. It must not attack the parent surfaces dangerously in the time between its application and removal.
6. It must be easy to remove from the brazed assembly.

Many types of proprietary fluxes are available for brazing aluminum. These are generally of the alkali halide type, which is basically mixtures of the alkali metal chlorides and fluorides. Fluxes and their residues are highly corrosive and therefore must be completely removed after brazing by washing with hot water.