The properties of metals are more or less temperature-dependent. Thus strength decreases with increasing temperature, whereas the solubility of most alloying elements increases. Both of these factors are widely used in metal processing, above all to influence the strength in a controlled manner using heat treatment.
The five most important variants are:
- Preheating: forming of aluminium and its alloys is much easier at temperatures above 200 °C, which is why continuously cast billets, for example, are heated to 400 to 500 °C prior to rolling or extrusion. Preheated materials are also used in other solid-state forming processes, such as forging.
- Homogenisation or solution heat treating: especially if they have a high alloying content, aluminium-based materials have a grain structure after casting in which the alloying elements are not uniformly distributed and have often precipitated out as a second phase. In order for a product to have material properties that are as homogeneous as possible, these inhomogeneities have to be removed. By "annealing" at temperatures between 500 and 600 °C (depending on the alloy), any possible precipitates of elements that are readily soluble in aluminium, such as copper, magnesium, silicon or zinc, are dissolved and distributed uniformly. Precipitates of difficult to dissolve elements like iron and manganese become coarser and lose their angular shape ("spheroidising"). Both effects improve formability. The dissolved elements only remain in solution, however, if cooling is rapid; if cooling is slower, they precipitate out again. Homogenisation is often combined with preheating prior to forming.
- Annealing: in cold working, such as the rolling of strip and foil, the material becomes stronger and more brittle with every pass. In order to be able to process it further without any difficulty, one "anneals" it at a temperature of 300-400 °C until it is again in the same "soft" condition (temper) and can be readily processed further. At these temperatures recrystallisation occurs: new crystal grains form from nuclei (the smallest possible ideal crystal grains) in the cold-worked and thus highly deformed grain structure. This results in a completely new grain structure, which is favourable for further forming.
- Softening, an annealing treatment ("tempering") at about 250 °C, and thus below the recrystallisation temperature of aluminium (about 300 °C depending on the alloy), causes a reduction in the strength to the "three-quarters hard" or "half-hard" temper while at the same time increasing the ductility (malleability).
- Age hardening or precipitation hardening, which is only possible with so-called "age-hardenable alloys": as with homogenisation, the alloying elements are first taken into solution at 450-550 °C. The work piece is then cooled or quenched to a temperature below 200 °C, with the speed of cooling depending on the alloy, for example in moving air, water mist or cold water. The alloying elements do not have enough time to precipitate out again so for the time being they remain in solution. However, the solution is "supersaturated" because a larger amount of the elements is dissolved than natural equilibrium would allow at these low temperatures. Thus, a slow precipitation process begins and increases the strength to a level well in excess of that achievable by cold working. This age hardening takes several days at room temperature ("natural ageing"), but can be intensified and accelerated by using temperatures between 140 and 190 °C ("artificial ageing").
For each of heat treatment there are special furnaces available with gas, oil or electrical heating and automatic temperature control. There is usually hot air or a protective atmosphere around the work piece.