Hardening aluminum
Hardening aluminum means increasing the hardness and strength of an aluminum alloy. In castings, this is most often done by precipitation hardening, i.e., a heat treatment consisting of solution treatment and aging. Other methods – solid solution strengthening, grain refinement or strain hardening – are of lesser importance in foundry practice or take effect already during melting and solidification.
Methods of hardening aluminum
| Method | How it works | Use in castings |
|---|---|---|
| Precipitation (age) hardening | solution treatment and aging – fine particles of strengthening phases precipitate from the solution | the main method for Al-Si-Mg and Al-Si-Cu alloys (T4, T5, T6, T7 tempers) |
| Solid solution strengthening | atoms of alloying elements, e.g., magnesium, dissolved in aluminum hinder deformation | e.g., aluminum-magnesium alloys, used in the as-cast condition |
| Grain refinement and modification | additions to the molten metal and faster solidification produce a finer structure | at the melting and casting stage, e.g., in a permanent mold |
| Work hardening | cold plastic deformation – rolling, drawing | applies to sheet, bars and profiles, not castings |
Precipitation hardening is also sometimes called dispersion hardening, because it is strengthened by fine particles dispersed in the metal. In strict terminology, however, dispersion strengthening refers to foreign particles that are insoluble in the matrix (e.g., oxides in aluminum matrix composites), not particles precipitated from solution during aging.
How precipitation hardening of aluminum works
- Solution treatment – soaking the casting at approx. 480–545°C (depending on the alloy) and rapid cooling, usually in water. The alloying elements are retained in solution. Details: aluminum solution treatment.
- Aging – natural at ambient temperature or artificial in a furnace, usually at approx. 150–180°C. Fine particles precipitate from the solution, increasing hardness and strength. Details: aluminum aging.
Essential condition: the solubility of the alloying elements in aluminum must decrease as the temperature drops. Alloys with magnesium and silicon or with copper meet this condition; pure aluminum and EN AC-51300 (AlMg5) do not. The whole cycle is also commonly called hardening of aluminum.
What hardening of aluminum castings provides
- higher tensile strength and yield strength,
- greater hardness and wear resistance, as well as better machinability – a harder material smears the tool less,
- repeatable properties across the entire series, verified at acceptance according to PN-EN 1706 requirements.
The trade-off is usually lower elongation and, with rapid quenching, a risk of distortion in thin and long castings. Heat treatment has little effect on corrosion resistance; here the alloy composition and coating matter more, e.g. powder coating.
Surface hardening of aluminum
When only the surface needs to be hard, a surface treatment is used, e.g., hard anodizing, which forms a hard layer of aluminum oxide. On castings with a high silicon or copper content, however, the anodic layer can be thinner and less uniform, so the alloy should be selected with the planned finish in mind.
In short: aluminum castings are hardened mainly by precipitation – through solution treatment (approx. 480–545°C and rapid cooling) and aging (natural or at approx. 150–180°C). This requires a precipitation-hardenable alloy, e.g., EN AC-42000 (AlSi7Mg).
Hardening castings at our foundry
We harden aluminum castings at the customer's request – we perform solution treatment and aging, as well as annealing. We combine heat treatment with casting in permanent mold or in sand molds and with machining. We also perform strength testing on request.
More: heat treatment of castings i aluminum casting alloys.
Do you need hardened aluminum castings? Send a drawing or 3D model to odlewnia@adjatech.pl or call: +48 61 662 43 37 (Mon–Fri 8:30 a.m.–4:30 p.m.).
Scope of services: foundry production capabilities.