Heat Treatment of Aluminium and Aluminium Alloys
20 October 2017When a discussion turns to heat treatment, aluminium isn’t the first metal that comes to mind. We think of its lightweight features, the uses of aluminium alloys as an aeronautics material and a commercial commodity. Be that as it may, there are thermally conditional aluminium alloys out there, forms of the abundant metal that can be heat treated. Starting with homogenizing, it’s time to investigate the heat treatment of aluminium alloys.
Aluminium Alloys: Heat Treatment Suitability
In its purest form, this is a soft metal. Composite variants alloy the pure metal so that it gains the capacity to achieve a transformative state when heat is applied. Interestingly, when alloyed with manganese or magnesium, the newly amalgamated alloy does not respond well to heat treatment. We need to turn to other metallic elements if we’re to accomplish this thermally transformative operation. These are the copper, magnesium silicide, and zinc alloying materials that enhance aluminium and gift the normally soft metal with greater strength.
Thermally Redistributing the Grain
Aluminium alloys undergo a physical change when they’re processed, but that material alteration tends to precipitate outwards. Imagine an aluminium casting, a product that’s cooling. Due to the innate characteristics of the material, grainy crystals form against the cool casting surface. Basically, the metal substructure is unevenly distributed. It’s soft on the outside and harder in the centre of the casting. The product is not ready for application. The heat homogenizing process cooks the aluminium just below its melting point. The thermal energy then redistributes the alloying elements until all of the soft precipitates are eliminated.
Annealing and Solution Heat Treatment
If the lightweight metal alloy becomes work hardened, it’s no longer workable. Annealing heats the workpiece beyond its upper critical temperature (300°C to 400°C), then it holds that temperature for a predetermined period. The resulting microcrystalline ‘reset’ reintroduces slip planes, an important malleability feature that’s lost if the workpiece is work hardened. As for solution heat treatment, there are similarities between this method and annealing. The major difference, a quenching phase, stops age hardening while promoting a homogenized material structure.
Time is also a strong ally here, which means the hardening process can continue after the solution heat treatment work. Stored in sheds for several days, the age hardening occurs at room temperature as the grains “lock” in place. In conclusion, though, there are heat-treatable aluminium alloys. They’re divided into several numerically labelled groups. Copper alloyed aluminium forms the core of the 2xxx series, zinc hosts the 7xxx series, and magnesium/silicon amalgams occupy the 6xxx series. Before considering one of the above heat treatment methods, the correct aluminium alloy series must be sourced.
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