To achieve the required strength characteristics, aluminum profiles undergo various types of heat treatment after extrusion. Thermal treatment of aluminum is a process that enhances the strength and rigidity of a specific subgroup of aluminum alloys, namely processed and cast alloys that undergo dispersion hardening. Dispersion-strengthened aluminum alloys include the 2XXX, 6XXX, 7XXX, and 8XXX series. In addition, heat treatment may be necessary for parts that have undergone deformation strengthening during the extrusion (or pressing) process.
Typical thermal treatments of aluminum include annealing, homogenization, solution heat treatment, natural ageing, and artificial ageing (the latter is also referred to as "dispersion strengthening"). The oven temperature depends on a specific process used in each case and ranges between 115 and 540°C. It is important to remember the significant differences in the heat treatment of aluminum and steel.
Annealing is a high-temperature treatment process that leads to changes in the physical, and in some cases, chemical properties of materials, aimed at increasing plasticity and reducing brittleness to make it more suitable for work. Strengthening or deformation strengthening contributes to increasing the resistance of aluminum alloys to fracture and the development of plastic deformation. This occurs when the formation of aluminum alloy is carried out through plastic deformation, during which the grain structures of aluminum must move and interact with each other along the crystallographic plane, which is also referred to as the "slip plane."
As more and more plastic deformation occurs, there are fewer and fewer slip planes that can be easily deformed. As a result, greater effort is required to achieve further deformation. When a part reaches this state, it is called hardened. The purpose of annealing is to significantly restore the crystalline grain structure, recover the slip planes, and allow for the continued shaping of the part without applying excessive force. For the annealing of hardened aluminum alloys, the metal needs to be heated from 298°C to 410°C for a specific period, which can range from thirty minutes to three hours. The choice of heating duration and temperature regime is determined by the sizes and chemical composition of the alloy of the parts to be annealed.
Annealing also relieves the internal stress that forms in a part during cold metal processing processes, such as forging or casting, stabilizes the dimensions of the parts, and addresses issues arising from internal deformation (for example, warping). Moreover, annealing can be successfully performed on aluminum alloys, which are considered non-heat-treatable. It is usually used for working with forged, extruded, or cast aluminum parts.
Homogenization
Homogenization is the process of heating metal to a temperature very close to its melting point, followed by slow cooling. Homogenization is used to ensure that the precipitated elements are more evenly distributed throughout the aluminum part. Often, this is necessary when we have parts made of cast aluminum alloy in our work. When the part cools down, the outer edge that is in direct contact with the die cools first. As a result, a shell of aluminum grains or crystals is formed. As the part continues to cool inside, the result is a fairly clean metal at the surface and in some areas near the centre. Alloying elements form a sediment, which causes the aluminum grains to be fixed in place. The casting process ends up with some areas being soft while others are strong. This division between the parts can be reduced, and the resulting portion can be made more suitable for homogenization.
The aluminum part is homogenized by heating it to a temperature slightly below the melting point, which typically ranges from 480°C to 537°C. After the entire part reaches this homogenization temperature, it must cool down slowly. As a result, we will obtain a cast part that will have a homogeneous internal structure.
Thermal treatment by dissolution
For the annealing process, the rate at which the part cools down is not an important factor. In another similar process called solution heat treatment, its significance is great. In this thermal processing, the elements responsible for ageing (which complicates work with the metal part over time) dissolve. Then these dissolved elements become spheroids, and in the final result, we obtain a homogenized structure. To preserve the effect achieved during high-temperature treatment, namely to fix the final distribution of dissolved elements in the alloy, the part must be subjected to quenching or rapid cooling. Then it's much easier to work with the detail. However, after some time, these fixed elements will settle again, leading to ageing.
The exact temperature for thermal treatment by solution heat treatment is determined by the composition of the aluminum alloy, but it typically ranges from 440°C to 526°C. In cases where such a temperature is not reached, the heat treatment process through dissolution will not be successful: at too low a temperature, strength will be lost; at too high a temperature, the part may lose its color, critical elements may melt, or increased stress may develop within the part.
When the part reaches this narrow window for the target temperature, it needs to be soaked. Soaking time can range from ten minutes (for thin parts) to twelve hours (for larger and thicker parts). However, heat treatment specialists have a general rule: one hour for each inch of cross-sectional thickness.
Next comes the phase of extinguishing. The goal of tempering here is to "freeze" the captured elements in place or to cool the aluminum part quickly enough so that the alloying elements do not precipitate during the cooling of the part. Water is the most common and usually the most effective hardener for aluminum alloys.
Any shaping that needs to be done after the dissolution process must be carried out very quickly after the quenching is completed. Otherwise, natural ageing will begin, and it will become more difficult to work with the part. This is the opposite of what happens with heat-treated steels, which are extremely brittle and hard after quenching.
Natural aging
Over time, the elements dissolved during the heat treatment of aluminum alloys begin to precipitate. Thus, the grains are fixed in place, which in turn leads to an increase in the natural strength of aluminum and is called ageing.
The temperature regime necessary for the process of natural ageing is simple – just room temperature is needed; the entire process lasts up to 5 days. It should be noted that ninety per cent of the hardening occurs on the first day. Therefore, it is necessary to start the process of shaping aluminum parts fairly quickly after the solution heat treatment process.
Artificial aging
For some aluminum alloys, a necessary condition for achieving the maximum possible hardness is the complete precipitation of dissolved elements. Not every aluminum alloy is capable of achieving a sufficient level of hardness during natural ageing at room temperature. Some of them can harden only up to a certain point, but this can be achieved through artificial ageing.
During dispersion strengthening, aluminum is heated from 115°C to 237°C. After that, it is soaked for a period of six to twenty-four hours, after which it is cooled to room temperature. The result will be characterized by a significant increase in the yield strength of aluminum, a somewhat smaller increase in tensile strength, and a decrease in ductility.
