Heat Treatment of Aluminium Alloys
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Aluminium Alloys
HEAT TREATMENT OF ALUMINIUM CASTING ALLOYS
Most aluminium castings are supplied in the as-cast condition, but some applications require enhanced mechanical properties or different characteristics than those provided by the untreated casting. Heat treatment is used to alter these properties by exposing the casting to one or more controlled thermal cycles.
For sand and gravity-die castings, a full range of heat treatments is generally possible, although not all are formally standardised. Pressure die-cast components, however, are rarely suitable for heat treatment.
The following suffixes are used in the UK to denote the BS1490 heat-treatment conditions for aluminium alloys:
- M — As cast
- TS — Thermally stress-relieved or annealed
- TB — Solution-treated and naturally aged to a stable condition
- TB7 — Solution-treated and stabilised
- TE — Artificially aged
- TF — Solution-treated followed by artificial ageing
- TF7 — Solution-treated, artificially aged, and stabilised
The following alloys are not fully heat-treatable: LM0, LM2, LM5, LM6, LM20, LM24.
Some alloys may undergo heat treatment, but their properties are not standardised in the recognised heat-treated condition: LM12, LM21, LM27-TF, LM30-TF.
The alloys listed below are standardised in the heat-treated conditions indicated:
LM4-TF, LM9-TE, LM9-TF, LM13-TE, LM13-TF, LM16-TB, LM16-TF, LM22-TB, LM25-TE, LM25-TB7, LM25-TF, LM26-TE, LM28-TE, LM28-TF, LM29-TE, LM29-TF, LM30-TS.
LM31 is a naturally ageing alloy that may also be artificially aged when required.
HEAT TREATMENT OF ZINC CASTING ALLOYS
Zinc-based casting alloys are generally not heat-treated, but annealing ZA27 can significantly enhance ductility.