How To Make Aluminum Alloys

The natural abundance and low density of aluminium prompt designers to identify multiple applications. Given that small quantities of other metals are added, the material becomes sufficiently hard to serve as a construction material.

Aircraft manufacturers have utilised aluminium for many years. They required that the materials be lightweight, stiff and resistant to temperature variations. Lithium, the lightest metallic element, can act as a substitutional impurity in an aluminium lattice. Research shows that the addition of copper increases the strength of Aluminium-Lithium alloys. Magnesium can also be introduced as an alloying element in place of lithium. These alloys exhibit lower corrosion resistance. On pure aluminium surfaces, an aluminium oxide layer develops, although impurities disrupt its continuity.
The mere combination of lithium or magnesium with aluminium does not yield a metal with high stiffness. In bronze, large tin atoms obstruct the movement of copper atoms along their slip planes. In steel, small carbon atoms occupy interstices in the iron lattice and thereby restrict atomic movement. Lithium and magnesium are not larger than aluminium and, when occupying lattice sites, do not prevent atomic displacement.
A process called precipitation hardening reduces the deformability of an aluminium alloy. The material is heated to a temperature below its melting point. Consequently, the quantity of impurity that dissolves in the aluminium increases. The alloy is cooled at a specified rate and aged for several days.

Although a lithium or magnesium atom fits into the aluminium lattice, its bond with the surrounding aluminium atoms is weaker than that of a standard aluminium atom. This condition imposes stress on the lattice and causes slight distortion. Thermal energy enables the foreign atoms to migrate within the crystal. When several lithium atoms encounter one another, they coalesce and form a precipitate that comprises an accumulation of atoms with a different, smaller lattice pattern. These precipitates obstruct the slip planes in the aluminium lattice, thereby increasing the hardness of the material.
Aluminium alloys can also be processed by cold working. Rolling and pressing deform the grains and reduce their size.