Solution Heat Treatment of Aluminum
Solution heat treatment is the first stage of precipitation hardening of aluminum alloys. The casting is heated to a temperature at which the alloying elements dissolve in the aluminum (approx. 480–545°C, depending on the alloy), held at that temperature, and then rapidly quenched – most often in water. This produces a supersaturated solid solution, from which the casting reaches its full strength only during aging.
Only alloys in which the solubility of the alloying elements decreases as temperature falls can be solution treated. In foundry practice, these are primarily aluminum alloys with silicon and magnesium (Al-Si-Mg) and with silicon and copper (Al-Si-Cu). Solution treatment alone does not harden the casting – immediately after quenching, the material is ductile and relatively soft, but ready to be strengthened.
What Solution Heat Treatment of Aluminum Involves
- Heating to the solution treatment temperature. The temperature is selected so that the strengthening phases (e.g., Mg2Si or Al2Cu) dissolve in the matrix without incipient melting of the grain boundaries. The margin can be narrow, so the furnace must maintain a uniform temperature throughout the entire load.
- Soaking. The casting stays at this temperature for typically several to a dozen or so hours. The time depends on the alloy, wall thickness and structure – thick sections and a coarse-grained structure require longer soaking. In silicon-containing alloys, the silicon crystals also become rounded, which improves ductility.
- Rapid quenching. The load is transferred from the furnace to the quench bath as quickly as possible, usually within a few to a dozen or so seconds. Rapid cooling "freezes" the dissolved elements in solution, producing a supersaturated solution.
Solution Treatment Temperature and Time
The parameters depend primarily on the alloy composition. Approximate values for common casting alloys according to PN-EN 1706:
| Alloy | Solution treatment temperature | Notes |
|---|---|---|
| EN AC-42000 (AlSi7Mg) | approx. 530–545°C | the most commonly solution treated casting alloy; T6 temper after aging |
| EN AC-46200 (AlSi8Cu3) | approx. 480–500°C | lower temperature, because the copper-rich phase begins to melt earlier |
| EN AC-51300 (AlMg5) | not solution treated | an alloy without strengthening phases, used in the as-cast condition |
Water at ambient temperature cools fastest and gives the highest properties, but also the highest stresses and risk of distortion. Castings with complex shapes are therefore sometimes quenched in heated water or in polymer solutions – at the expense of slightly lower strength. Exact parameters are set for the specific alloy, casting size and required properties.
Most Common Solution Treatment Errors
- Temperature too high – melting of the grain boundaries (known as burning), which cannot be repaired. The casting loses strength and ductility.
- Temperature too low or soak time too short – the phases do not dissolve completely, so aging does not achieve its full effect.
- Quench delay too long – some of the elements precipitate before quenching, which reduces hardness after aging.
- Distortion – thin and long castings may warp. Proper placement in the load, a milder quenchant or straightening of the casting helps.
- Surface blisters – gas trapped in the casting expands at high temperature. For this reason, conventional high-pressure die castings are usually not solution treated.
Solution Treatment and Aging – T4, T6, T64 and T7 Tempers
After solution heat treatment, the casting is aged. The aging method determines the temper of the material, designated in EN 1706 by a letter-number symbol:
| Temper | What it means | Typical result |
|---|---|---|
| T4 | solution heat treatment and natural aging | good ductility, moderate strength |
| T6 | solution heat treatment and full artificial aging | highest strength and hardness |
| T64 | solution heat treatment and partial artificial aging | slightly lower strength than T6, higher elongation |
| T7 | solution heat treatment and overaging (stabilization) | stable dimensions and properties at elevated temperatures |
The T5 temper – artificial aging only, directly after casting – does not require solution heat treatment and is typical, among others, for die castings.
In short: Solution heat treatment of aluminum is soaking at approx. 480–545°C (depending on the alloy) followed by rapid cooling, most often in water. It produces a soft, supersaturated solid solution that hardens only during aging – natural (T4) or artificial (T6).
Solution heat treatment of aluminum vs. steel
The name is the same, but the purpose is different. In aluminum alloys, solution heat treatment prepares the material for hardening by aging. In austenitic steels, such as stainless steels, solution annealing is soaking at typically approx. 1000–1150°C followed by rapid cooling. It dissolves chromium carbides, restores resistance to intergranular corrosion and softens the steel. Stainless steel after solution annealing is therefore soft and ductile, not harder. In our foundry, solution heat treatment applies to aluminum castings.
Solution heat treatment of castings in our foundry
We perform solution heat treatment and aging of castings at the customer's request – together with casting and machining of castings. We also carry out strength testing on request. It is best to specify the alloy and temper (e.g. T6) already in the request for quotation, as they affect the choice of process – permanent mold casting or sand casting.
More about heat treatment: aluminum hardening, aluminum aging, aluminum hardening, heat treatment of castings and aluminum casting alloys.
Do you need aluminum castings in the T6 temper? 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.