Work Hardening Aluminum Alloys: Part One

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Introduction to Strain Hardening in Aluminum

Strain hardening occurs naturally during most working and forming operations of aluminum and its alloys. In pure aluminum and non-heat-treatable alloys (such as aluminum-manganese and aluminum-magnesium varieties), strain hardening significantly increases the strengths already achieved through solid solution and dispersion hardening. For heat-treatable alloys, strain hardening not only supplements the strengths achieved by precipitation but also enhances the material's response to precipitation hardening processes.

Temper Designations and Applications

Work hardening is used extensively to produce strain-hardened tempers of non-heat-treatable alloys. The severely cold worked or full-hard condition (H18 temper) is typically obtained with cold work equal to approximately 75% reduction in area. For applications requiring even greater strength, the H19 temper identifies products with substantially higher strengths achieved through greater reductions in area. The H16, H14, and H12 tempers are obtained with progressively lesser amounts of cold working, representing three-quarter-hard, half-hard, and quarter-hard conditions, respectively.

Manufacturers also employ combinations of strain hardening and partial annealing to produce the H28, H26, H24, and H22 series of tempers. In this process, products are strain hardened beyond the required level to achieve desired properties and then partially annealed to reduce strength to the target specifications.

For aluminum-magnesium alloys, which tend to age soften at room temperature when in the strain-hardened condition, a series of strain-hardened and stabilized tempers (H38, H36, H34, and H32) are employed. These alloys are typically heated at a low temperature to complete the age-softening process, providing stable mechanical properties and improved working characteristics.

It's important to note that products hardened by cold working can be restored to the O temper—a soft, ductile condition—through annealing. This process eliminates strain hardening and the structural changes resulting from cold working.

Table 1. Temper Designations for Strain-Hardened Alloys

Temper	Description
F	As-fabricated. No control over the amount of strain hardening; no mechanical property limits.
O	Annealed, recrystallized. Temper with the lowest strength and great- est ductility.
H1	Strain hardened. H12, H14, H16, H18. The degree of strain hardening is indicated by the second digit and varies from quarter-hard (H12) to full- hard (H18), which is produced with approximately 75% reduction in area. H19. An extra-hard temper for products with substantially higher strengths and greater strain hardening than obtained with the H18 temper.
H2	Strain hardened and partially annealed. H22, H24, H26, H28. Tempers ranging from quarter-hard to full- hard obtained by partial annealing of cold worked materials with strengths initially greater than desired.
H3	Strain hardened and stabilized. H32, H34, H36, H38. Tempers for age-softening aluminum-magnesium alloys that are strain hardened and then heated at a low temperature to increase ductility and stabilize mechanical properties.
H112	Strain hardened during fabrication. No special control over amount of strain hardening but requires mechanical testing and meets minimum mechanical properties.
H321	Strain hardened during fabrication. Amount of strain hardening con- trolled during hot and cold working.
H321	Strain hardened during fabrication. Amount of strain hardening con- trolled during hot and cold working.
H323, H343	Special strain hardened, corrosion-resistant tempers for aluminum- magnesium alloys.

Microstructural Changes During Deformation

The deformation of aluminum and its alloys proceeds by normal crystallographic slip processes. Evidence of such slip can be observed in single crystals and coarse-grained materials when surfaces are metallographically polished before deformation.

More severe cold working produces increasingly higher dislocation densities and further reduction in fragment size. The lattice distortions associated with these dislocations and the interaction stresses between them constitute the principal sources of strain hardening resulting from cold work.

Texture Development and Directionality

Cast aluminum typically has a random distribution of grain orientations, except in cases where columnar grains form. This random character of the cast structure rapidly transforms during hot or cold working into crystallographic "textures," where considerable numbers of grains and grain fragments assume specific orientations. These textures develop because slip occurs on restricted crystallographic planes and in certain crystallographic directions. At room temperature, slip occurs on the {111} planes in the (110) directions. Deformation on these planes produces a gradual rotation of grains and grain fragments into specific orientations relative to the workpiece surface and working direction.

The final textures achieved with substantial deformation vary depending on the working process, changes in the workpiece shape, and to a lesser extent, the alloy composition. Aluminum wire, rod, and bar typically develop a "fiber" texture, where a {111} direction runs parallel to the product axis, with random orientation of crystal directions perpendicular to this axis. In rolled sheet, the deformation texture may be described as a mixture of more complex textures.

Although aluminum is generally considered an isotropic material, the textures developed through various working practices introduce some directionality in properties. This directionality is typically much less pronounced than in hexagonal metals and some other cubic metals. The primary challenge associated with directionality in strain-hardened aluminum alloys is the formation of ears during deep drawing of sheet.