Heat treatment can multiply the mechanical properties of many aluminium alloys. The code after the alloy number (for example 6061-T6) tells you exactly what treatment the part has been through.
Which alloys can be heat-treated?
The 2xxx, 6xxx and 7xxx series and cast alloys such as A356 are strengthened by precipitation hardening. The 1xxx, 3xxx and 5xxx series cannot be heat-treated; they are hardened by cold work (rolling, drawing).
The steps of precipitation hardening
- Solution treatment: heating to a high temperature so the alloying elements dissolve in the aluminium matrix.
- Quenching: rapid cooling to keep a supersaturated solid solution.
- Ageing: natural (at room temperature) or artificial (at a controlled temperature), so that fine particles precipitate and strength increases.
Common temper codes
| Code | Meaning |
|---|---|
| F | As fabricated (no special control) |
| O | Annealed; the softest condition |
| H | Strain-hardened (for non-heat-treatable alloys), e.g. H14 |
| T4 | Solution treated + naturally aged |
| T5 | Cooled after hot forming (e.g. extrusion) + artificially aged |
| T6 | Solution treated + artificially aged; the usual maximum strength |
| T7 | Solution treated + over-aged for dimensional stability or stress-corrosion resistance |
To choose a heat-treatable casting alloy, use the alloy finder.