Aluminum Heat Treating and Annealing
Heat treating of aluminum is a precision process applied to improve the mechanical properties of the metal.
Heat treatment and the subsequent annealing process is achieved in specialized furnaces that provide the correct thermal qualities required. These furnaces are designed to maintain heat continuity, uniform temperatures, and the required cycle times.
Heat treating aluminum increases both the strength and hardness of certain aluminum alloys, primarily the wrought and cast alloys. Cast aluminum is melted in a furnace and poured into a mold, while wrought aluminum is cold formed using specific tools.
Wrought aluminum material is used for roll forming parts.
Heat treatment is used to anneal aluminum parts that have experienced strain hardening during their forming process. In addition, typical aluminum heat treatments include homogenizing, natural aging, artificial aging or precipitation hardening, and solution heat treatment.
The Purpose of Aluminum Heat Treating and Annealing
Annealing is done to soften the material, reduce stress, and to give it increased workability. This is often needed because aluminum alloys are subject to work hardening, or strain hardening.
Strain hardening occurs when the aluminum alloy is being shaped by plastic deformation, the permanent distortion that occurs when a material is subjected to tensile, compressive, bending, or torsion stresses. This causes the grain structures within the aluminum to slide against each other and, as more plastic deformation takes place, there is less capacity in the grain structures for deformation.
As a result, as more force is required to achieve further deformation, the part reaches a state of being work hardened. In order to apply further plastic deformation of the aluminum, the strain hardening has to be removed from the part.
Consequently, the initial purpose of annealing is to essentially reset the crystalline grain structure. This once again allows the grain structures to slide so the part can be shaped without requiring excess force.
Annealing aluminum hinges on temperature and length of time.
To anneal a work hardened aluminum alloy, the metal must be heated to somewhere between 570°F to 770°F for a specific period of time, typically between thirty minutes and three hours. The actual time and temperature required depends on two factors: the size of the part that is being annealed and the composition of its alloy.
In addition, annealing by heat treating relieves internal stresses in a part that may have developed during cold forging or casting. It also works to stabilize the dimensions of a part and resolve problems from internal strains such as warping, for example.
The Benefits of Aluminum Heat Treating and Annealing
Annealing aluminum parts restores their ductility, or ability to stretch under tensile strain, following cold working. This quality allows for additional processing of a part without cracking.
Annealing can also be applied to a part for the release of mechanical stresses created by grinding, machining, and so on. This helps prevent distortion of a part during subsequent higher temperature heat treatment operations.
In addition, annealing makes aluminum more formable. In other words, when the aluminum is stronger and more ductile, it gives manufacturers more latitude in the fabrication process as there is less risk of the material fracturing while bending or pressing.
Annealing can also improve a metal’s ability to be machined, which extends the lifespans of machining tools.
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