3 - Metallic alloys and equilibrium diagrams

Summary

Metallic alloys

The metals with which a welder may have to deal are often not pure metals but alloys consisting of a parent metal with one or more alloying elements added in various proportions, e.g. mild steel is an alloy mainly of iron with a small amount of carbon, while brasses are alloys of copper containing up to 45% of zinc.

The properties of such alloys vary according to the form in which the alloying element is present. There are several possible forms:

(1) The alloying element may be present in an unchanged form in a state of fine mechanical mixture with the parent metal so that it can be seen as a separate particle or crystal (constituent) under the microscope.

Examples of this are:

1. (a) Carbon present as flakes of graphite in grey cast irons.

2. (b) Silicon present as fine silicon crystals in the aluminium–silicon alloys.

3. (c) Lead present as round particles in free cutting brasses. Alloying elements present in this condition do not generally produce a great increase in strength, but they increase or decrease the hardness, reduce the ductility and improve the machinability of the parent metal.

(2) The alloying element may be present in solid solution in the parent metal, i.e., actually dissolved as salt or sugar dissolves in water, so that under the microscope only one constituent, the solid solution, can be seen, similar in appearance to the parent metal except that the colour may be changed.