Solidification

All metals solidify after fusion, and the temperature at which they change from the liquid to the solid state is known as the"freezing-point". Alloys in solidifying very often alter, as the heavier constituent separates, and consequently a metal is obtained which is not uniform in structure. It may be taken as a general rule that the melting-point of an alloy is below that of its most fusible constituent.

Contraction

Nearly all metals contract in cooling. So when making patterns for articles that have to be cast, allowance must be made for the contraction or shrinkage; this depends to a large extent on their form and the distribution of the metal. Sand cores have a tendency to retard shrinkage; cylindrical or box-shaped articles shrink more in their length than in their diameter. The approximate contraction of various metals is as follows:-

Hardness

This is a relative term, and is the resistance offered by the molecules of a metal to their separation by the penetrating action of another body, and is affected considerably by the presence of impurities. The presence of another metal often tends to increase both the brittleness and hardness.

Softness

This is also a relative term, but in many instances is a test of purity.

Aluminium

A bluish-white but somewhat soft metal, very malleable, and takes a good polish. Can be cleaned by immersion in caustic potash and soda; unaffected by exposure to the atmosphere. Annealed only at a low temperature. Difficult to solder owing to insolubility of oxide, but can be welded by means of the oxy-acetylene blow pipe. There are many patent solders for soldering this metal.