Aluminum profiles are treated to various sorts of treatment following extrusion, which often involves employing high temperatures, to provide the required strength qualities. The strong point and stiffness of a specific subclass of aluminium alloys, namely precipitation hardened and cast alloys, are increased through the heat treatment of aluminium profiles or alloys. The 2XXX, 6XXX, 7XXX, and 8XXX series of precipitation-strengthened aluminium alloys are among them. In order to harden pieces over the course of the extrusion (or pressing) process, heat treatment procedures could be required.
Annealing, homogenization, solution heat treatment, and both natural and artificial ageing are examples of common heat treatments for aluminium (the latter also has the definition of "dispersion strengthening"). The precise procedure performed in a given situation dictates the oven temperature selection; the temperature range is between 115 and 540°C. It's crucial to keep in mind that aluminium and steel undergo very different heat treatments.
Annealing
In order to increase flexibility and reduce stiffness and make materials more appropriate for work, annealing is a high temperature treatment technique that affects the physical and, in some situations, chemical properties of the material. The resistance of aluminium alloys to fracture and the emergence of plastic deformation is increased by hardening or strain hardening. This occurs when an aluminium alloy is formed through plastic deformation, which requires the aluminum's granular structures to advance and interact with one another along the crystallographic plane, also known as the "slip plane."
There are less and less slip planes that are simple to distort as there is more and more plastic deformation. More force is therefore required to achieve greater deformation. This stage of the part's development is referred to as hardening. Restoration of slip planes, substantial restoration of the crystal grain structure, and the ability to continue forming the component without using excessive force are the goals of annealing. Hardened aluminium alloys must be heated for a specific amount of time—up to 30 minutes or 3 hours—between 298°C and 410°C in order to anneal them. The measurements and chemical composition of the alignment of the details to be annealed dictate the length of the heating and temperature regime.
Annealing also eliminates internal strain created in the component during cold metal working procedures like forging or casting, stabilizes part dimensions, and addresses problems brought on by internal deformation (e.g. warping). Additionally, an aluminium alloy that is thought to be non-heat treatable can be effectively annealed. It is frequently used to repair aluminium pieces that have been cast, extruded, or forged.
Homogenization
When a metal is heated to a point extremely close to its melting point and then progressively cooled to room temperature, the process is known as homogenization. To make sure that the sedimentary components are dispersed more uniformly throughout the aluminium section, homogenization is performed. When we use cast aluminium alloy parts, this is frequently necessary. The exterior side of the component, which is in direct touch with the mould, cools first when it is cooling. In response, a skin made of aluminium crystals or grains forms. The outcome is relatively clean metal at the part's surface and in other places at the centre as the part continues to cool down internally. The alloying components precipitate, which fixes the aluminium granules in place. The casting partially results in some regions being soft and others being hard. Through homogenization, this segregation between the pieces can be reduced and the final part can be improved for moulding.
A section of aluminium is homogenized by heating it to a temperature just below the melting point, which normally falls between 480C and 537C. Heating an aluminium detail just below its melting point, which commonly ranges from 480C to 537C, homogenizes the metal. The part should be slowly cooled once it has reached the homogenization temperature throughout. We shall thus obtain a cast component with a uniform interior structure.
Heat Solution Treatment
The rate at which the component cools down during the annealing procedure is unimportant. But it has enormous use in a different, related procedure known as dissolution heat treatment. The components that cause ageing are eliminated via this heat treatment process (making the metal piece more hard to operate with over time). These solute components eventually become spheroids, giving rise to a homogenized form. The item should be quickly hardened or chilled in order to preserve the effect obtained during high-temperature processing, specifically to stabilize the final distribution of soluble elements in the alloy. The item is then easily operated upon after that. However, these permanent components eventually precipitate once more, which causes ageing.
The aluminium alloy's composition determines the exact temperature mode for heat treatment dissolution, which is typically in the range of 440 C to 526 C. If this temperature is not reached, the solution heat treatment will not be successful. If the temperature is too high, the component may experience heightened stress or lose colour, and if it is too low, the hardness will be lost.
One should immerse the detail in the water once it has reached the desired narrow window of heat. The time needed to soak a piece might range from ten minutes (if it's thin) to twelve hours (for larger and thicker parts). However, the industry standard for heat treaters is one hour for every inch of cross-sectional thickness.
The extinction stage follows. Here, hardening serves to "freeze" the trapped elements in situ or to swiftly cool the aluminium component so that the alloying elements won't precipitate out when the part cools. The most popular and frequently used hardener for aluminium alloys is water.
After the dissolution process, whatever shape or form that needs to be completed must be completed very soon after which the hardening is completed. Otherwise, the artwork will start to naturally age and become more challenging to work with. The converse is true for heat-treated steels, which after hardening become incredibly brittle and rigid.
Natural Aging
The components that were soluted during the heating of aluminium alloys start to sludge with time. Aging is the process by which aluminum's intrinsic hardness increases as a result of the grains becoming set in place.
The only temperature regulation required for this process is the ambient temperature; the complete procedure can take up to 5 days. Take note that 90% of curing happens on the first day. Therefore, after completing the dissolution heat treatment procedure, it is imperative to immediately begin manufacturing aluminium parts.
Ageing Artificially
For some aluminium alloys, attaining the highest level of hardness necessitates the entire liberation of dissolved elements. Not all aluminium alloys can naturally age at the temperature of the environment and achieve a sufficient level of hardness. Some of them can only harden to a certain amount, although artificial ageing can fix this.
Metal is heated at temperatures between 115°C and 237°C during dispersion strengthening. Following that, it is submerged in water for 6 to 24 hours before cooling to room temperature. Aluminum's yield strength will significantly increase as a result, while its tensile strength will increase considerably less slowly and its ductility will diminish.
