General (Topic: 10041)

The heat-treatable alloys get their high strength by precipitation of hardening elements. This hardening mechanism is called precipitation hardening (also age hardening or ageing).

As a general rule there are two conditions that has to be satisfied for an alloy to be age hardenable:

1. The alloy must contain elements that, at a specific temperature, enters a state of solid solution in aluminium and at another temperature precipitates as another phase. In other words the solid solubility must be higher at high temperatures than at low temperatures.

2. The phase that precipitates must create a hard, coherent or semi-coherent precipitate. Coherent means that the precipitate must be connected with the aluminium lattice.

This is the case for the intermetallic compounds Mg2Si and MgZn2 and also for Cu. In this manual the examples will be connected to Mg2Si since this is the hardening element in the most frequently used alloys for extrusion, namely the AlMgSi-alloys (6000-group).

A simplified quasi binary phase-diagram of Al-Mg2Si is shown in the figure below.

Figure. Schematic quasi binary phase-diagram Al-Mg2Si. (The diagram is only for illustration).

To explain the age hardening phenomenon we will take hold of the phase-diagram and use 6000-alloys to exemplify the different steps.

Solution heat-treatment

The area of the phase-diagram marked solid solution shows the necessary temperature to solve precipitated Mg2Si particles for an alloy of given composition. It is evident from the phase-diagram that if one for instance heats an alloy with 1% Mg2Si (i.e. an average AW-6063 alloy) to between 510 and 600°C and hold it there until equilibrium is attained we will have a solid solution of Mg and Si in aluminium (see figure below).

Figure. Mg and Si in solid solution in aluminium.

Figure. Coherent Mg2Si particle (schematic).

If we lower the temperature to below 510°C the solid solution becomes supersaturated and the most stable condition for the Mg and Si is to precipitate as Mg2Si. What actually happens depends on the cooling from this state:

ñ Slow cooling will result in the equilibrium phase Mg2Si precipitating as relatively coarse incoherent non hardening particles.

ñ If we quench the alloy the Mg and Si atoms will be frozen in the solid solution state and the result is a supersaturated solid solution (SSS) of Mg and Si in Al.

With the latter cooling sequence the Mg and Si atoms will be available for age hardening.

The next step in the age hardening sequence is to produce, in a controlled manner, a large number of small Mg2Si particles that are coherent or semi coherent with the aluminium lattice (see figure above). This can be done by heating the AW-6063 in our example to 185°C and hold it there for 5ñ6 hours. This will give a high increase in mechanical strength.

This example shows the ordinary sequences of age hardening, namely solution heat-treating, quenching and artificial ageing. We will look more closely into the various steps of heat-treatment that is used after the solution heat-treatment in the following topics.