Aluminum Hardening

Aluminum hardening is a heat treatment in which a casting is heated to a temperature just below the alloy's melting onset (approx. 480–545°C, depending on composition), soaked, and then rapidly cooled, most often in water. In aluminum alloys this stage is technically called solution treatment – the casting reaches its full hardness and strength only after the aging that follows.

The purpose of hardening is to give the casting properties matched to its application: higher hardness, strength and dimensional stability. Unlike in steel, rapid cooling alone does not harden aluminum – it only holds the alloying elements in solution, from which fine strengthening particles precipitate during aging.

How Aluminum Hardening Works

1. Heating and soaking. The casting is soaked at a temperature selected for the alloy, usually for several to a dozen or so hours – until the alloying elements (magnesium and silicon, or copper) dissolve in the aluminum and the structure becomes homogeneous.
2. Rapid quenching. After removal from the furnace, the casting goes into the quench bath as quickly as possible – usually within a few to a dozen or so seconds. The longer the transfer takes, the more elements have time to precipitate and the weaker the result.
3. Aging. Right after quenching, the alloy is ductile but not yet very hard. Strength increases during aging: natural aging at ambient temperature or artificial aging in a furnace, usually at 150–180°C.

Aluminum Hardening Temperature

The temperature must be high enough to dissolve the strengthening phases and low enough to avoid melting the grain boundaries. It therefore depends primarily on the alloy composition:

These are indicative values. The exact temperature, soaking time and quenchant are determined for the specific alloy, wall thickness and required casting properties.

Which Aluminum Castings Can Be Hardened

• Precipitation-hardening alloys – primarily aluminum with silicon and magnesium, and with copper. These alloys respond well to hardening and aging (T6 temper).
• Alloys without strengthening phases – e.g. the magnesium alloy EN AC-51300 (AlMg5) or pure aluminum – do not gain hardness this way.
• High-pressure die casting are usually not hardened: gas trapped inside them expands at high temperature and forms blisters. They are used in the as-cast condition or after aging only (T5).

What Hardening Does for Castings

• higher tensile strength and yield strength,
• higher hardness and wear resistance,
• a more homogeneous structure and predictable properties across the whole batch.

The minimum properties of a casting in the as-cast (F) condition and after heat treatment, e.g. T6, are specified for each alloy in PN-EN 1706 – they are the basis for acceptance. Hardening usually reduces elongation, and rapid quenching can distort thin and long castings, so they must be positioned properly in the furnace and sometimes straightened.

Hardening Castings in Our Foundry

We perform heat treatment – annealing, hardening (solution treatment) and aging – at the customer's request, together with casting and machining of castings. We also carry out strength testing on request. We select the alloy so that it can be hardened – in permanent mold casting this is often EN AC-42000.

More about the individual stages: aluminum solution treatment, aluminum aging, aluminum hardening, heat treatment of castings and aluminum casting alloys.