Aluminium is a silvery white reactive metal which is usually covered in oxide. The oxide makes it inert to acids, but it is attacked by alkali. It is extracted from Bauxite by electrolysis of the oxide dissolved in molten sodium hexafluoroaluminate. The metal has good thermal properties and is malleable and ductile. During the 19th century aluminium was the most expensive metal and some jewellery was made from it.
Abundance
The most common metallic element in the earth’s crust at 82000 ppm.
Alloys
Aluminum containing specified quantities of alloying elements added to obtain the necessary mechanical and physical properties. There are a number of grades of aluminium designed for specific applications.
Bauxite
An ore of aluminum consisting of moderately pure hydrated alumina. Symbol Al2O32H2O
Bright Dip
For high-purity aluminium an acid/copper solution is used. The pieces are dipped in order to obtain a clean, bright surface. The reaction is entirely chemical with no external source of electrical current.
Discovered
The element was known in prehistoric times. In 1825 Hans Christian Oersted isolated impure aluminium. The pure metal was first isolated by Friedrich Wohler in 1827.
Grades
There are a number of different grades that have some significantly different properties and machining qualities. Aluminium also forms a number of different alloys.
Joining
Brazing – You can braze aluminium with zinc/aluminium alloys – one commercially available brazing rod is Durafix. Issue is this is more of a filler than a structural join.
TIG Welding – The best way to join aluminium parts structurally.
Machining
Parting Aluminium on the lathe can be tricky, so some simple rules to make this easier.
Image shows a parting tool being used to machine deep cooling fins into the cylinder barrel of a small engine. Lightweight and thermal conductivity make this a perfect material selection.
Oxides
Alminium Oxide (Al2O3) – the most widely used oxide ceramic, either in pure form or as raw material to be mixed with other oxides.
Temper
The heat treatment and coding as applied to aluminium.
| F | As fabricated |
| H | Strain hardened by cold working with or without thermal treatment |
| H1X | Strain hardened without cold working |
| H2X | Strain hardened with partial annealed |
| H3X | Strain hardened and stabilized with low temperature heating |
| HX2 | ź Hard |
| HX4 | ˝ Hard |
| HX6 | ž Hard |
| HX8 | Full Hard |
| HX9 | Extra Hard |
| O | Annealed (Full Soft) or T0 |
| T1 | Cooled from hot working and naturally aged (at room temperature) |
| T2 | Cooled from hot working, cold worked and naturally aged |
| T3 | Solution heat treated, cold worked and naturally aged |
| T4 | Solution heat treated and naturally aged |
| T5 | Cooled from hot working and artificially aged (at elevated temperatures) |
| T6 | Solution heat treated and artificially aged |
| T7 | Solution heat treated and stabilized |
| T8 | Solution heat treated, cold worked and artificially aged |
| T9 | Solution heat treated, artificially aged and cold worked |
Use
The alloys are widely used for various applications including aircraft assemblies and engine parts.
Pistons – W.O. Bentley experimented with aluminium pistons in 1913, he designed new alloys to add strength and to stop the aluminium from melting at high temperatures, finally settling on a formula of 88% aluminium and 12% copper.
The Bentley BR1 rotary engine designed by W. O. Bentley as an improvement on the Clerget 9C. It first ran as a prototype in 1916 and had aluminium cylinders, cast iron liners, aluminium pistons and dual ignition.
References
- Aluminium World – the aluminium industry aims to promote a wider understanding of its activities and demonstrate both its responsibility in producing the metal and the potential benefits to be realised through their use in sustainable applications and through recycling.