Aluminium and its alloys are internationally classified into eight groups identified by a four figure series number.
The first digit indicates the principal alloying element. For example any alloy in the 2000 series such as 2117 or 2024 has copper as its main alloying element. 7075 has zinc as its main alloy.
Aluminium Alloys grouped by main alloying elements. | 99.0% minimum | 1000 |
Copper | 2000 | |
Manganese | 3000 | |
Silicon | 4000 | |
Magnesium | 5000 | |
Magnesium and Silicon | 6000 | |
Zinc | 7000 | |
Lithium and others | 8000 |
The second digit identifies the alloy modification state, 0 indicates that the alloy is original. 1 indicates that the alloy has been modified once. The third and fourth digits identify the specific aluminium alloy. In the case of 2024, the alloy consists of 4.5% copper, 1.5% magnesium, 0.1% chromium and the remainder aluminium.
There are two basic divisions of aluminium alloys:
- NON-HEAT TREATABLE – those that can be softened but not hardened by heat treatment. These alloys are strengthened by controlled working, eg cold rolling.
- HEAT TREATABLE – those that can be softened and hardened by heat treatment.
If non-heat treatable alloys are heated to their annealing temperature and allowed to cool slowly they will be softened to their annealed, or –0 condition.
The 2000, 4000, 6000, 7000 and 8000 series alloys are strengthened by solution treatment followed by age hardening.
The 1000, 3000 and 5000 series alloys are cold worked to increase their strength.
IDENTIFYING STRAIN HARDENING PROCESSES
Non-heat treatable aluminium alloys can be hardened by strain hardening. This is usually done by rolling the sheets of metal.
The cold worked temper of wrought alloys is indicated by the letter –H followed by a number. Tempers in the –H series are not applicable to castings.
The first digit following –H indicates the form of the strain hardening.
A second digit 2, 4, 6 or 8 indicates the final degree of hardness. The fully hard condition is 8.4 indicates material having a strength midway between fully annealed –0 and full hard i.e. half hard. -0 Indicates the soft, annealed condition. Applies only to the wrought alloys.
5052 – H24----------->Strain hardened + Half hard Then partially annealed
IDENTIFYING HEAT TREATMENT PROCESSES
The heat treatable alloys can be hardened and strengthened by solution heat treatment or by precipitation heat treatment (artificial ageing).
The heat treated temper of aluminium alloys is indicated by the letter –T followed by one or more numbers. The number following the letter T shows the type of heat treatment. Any variations are indicated by a second number.
-T3 Solution heat treated and strain hardened (cold rolled).
-T36 Solution heat treated and cold rolled to reduce the thickness of the sheet by 6 per cent.
-T4 Solution heat treated then naturally aged.
-T42 Solution heat treated by the user regardless of the previous temper.
2024 – T4--------------> Heat treated alloy + Solution treatand +naturally aged
