What Are the Main Annealing Methods for Wrought Aluminum?

Annealing is a heat-treatment process used to obtain uniform composition, a stable microstructure, or favorable processing properties. Depending on the objective, wrought aluminum annealing is divided into homogenization, recrystallization, intermediate, and final annealing.

Homogenization Annealing

Homogenization annealing is performed mainly at aluminum smelters and alloy producers. Cooling too quickly can create a quenching effect. To prevent this, the material should cool in the furnace or be stacked together for air cooling after removal.

Recrystallization Annealing

Recrystallization annealing removes crystal defects and strain hardening caused by plastic deformation and improves ductility and toughness. Recrystallization proceeds through nucleation and growth of new grains.

Intermediate Annealing

During cold working, a large required deformation often cannot be achieved in one operation. Intermediate annealing removes work hardening and restores ductility so that deformation can continue to the required size and shape.

Final Annealing

Depending on the alloy and service requirements, final annealing may be partial, or low-temperature, annealing or full, high-temperature, annealing.

Full annealing heats a cold-work-hardened or partially quench-hardened alloy above its transformation temperature and holds it until a single-phase solid solution forms. Slow cooling, generally in the furnace, allows the solid solution to decompose and second-phase particles to coalesce by diffusion.

Partial annealing heats the alloy to a suitable temperature below the critical transformation point, holds it, and then cools it relatively quickly, generally in air. It removes part of the work-hardening effect to support a subsequent forming operation with limited deformation or improves ductility while retaining some strengthening from cold deformation, producing a half-hard condition.

Most deformation work is dissipated as heat, but a small portion remains in the metal as stored energy in lattice distortion and defects such as point defects, dislocations, subgrain boundaries, and stacking faults. This stored energy, expressed as the increase in free energy after cold deformation, drives microstructural change.

When plastically deformed metal is heated and held, its structure changes through three stages. During recovery, the elongated fibrous grains remain nearly unchanged under the microscope. During recrystallization, new small grains nucleate within the deformed grains and grow until the fibrous structure is completely replaced by equiaxed grains. During grain growth, the new grains consume one another until they reach a relatively stable size. Holding time at a progressively higher temperature produces the same sequence.

During recovery, hardness falls only slightly while ductility improves; strength changes similarly because it generally tracks hardness. During recrystallization, hardness and strength decrease significantly and ductility rises sharply. The hardness and strength increase produced by plastic deformation is associated with increased dislocation density. Dislocation density decreases only slightly during recovery but falls substantially during recrystallization.