Work Hardening Aluminum Alloys: Part One

Strain hardening is a natural consequence of most working and forming operation aluminum and its alloys. In pure aluminum and the non-heat-treatable aluminum-manganese and aluminum-magnesium alloys, strain hardening increases the strengths achieved through solid solution and dispersion hardening. In heat treatable alloys, strain hardening not only supplements the strengths achieved by precipitation but also increases the response to precipitation hardening.

Work hardening is used extensively to produce strain-hardened tempers of the non-heat-treatable alloys (Table 1). The severely cold worked or full-hard condition (H18 temper) is usually obtained with cold work equal to about 75% reduction in area. The H19 temper identifies products with substantially, higher strengths and greater reductions in area. The H16, H14, and H12 tempers are obtained with lesser amounts of cold working, and they represent three-quarter-hard, half-hard, and quarter-hard conditions, respectively.

A combination of strain hardening and partial annealing is used to produce the H28, H26, H24, and H22 series of tempers; the products are strain hardened more than is required to achieve the desired properties and then are reduced in strength by partial annealing. A series of strain-hardened and stabilized tempers -H38, H36, H34, and H32 -are employed for aluminum-magnesium alloys. In the strain-hardened condition, these alloys tend to age soften at room temperature. Therefore, they are usually heated at a low temperature to complete the age-softening process and to provide stable mechanical properties and improved working characteristics.

Products hardened by cold working can be restored to the O temper, a soft, ductile condition, by annealing. Annealing eliminates strain hardening, as well as the changes in structure that are the result of cold working.

Table 1. Temper Designations for Strain-Hardened Alloys

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

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

Cast aluminum tends to have a random distribution of grain orientations, except where columnar grains are formed. The random character of the cast structure is rapidly lost during hot or cold working; it is replaced by crystallographic "textures", in which considerable numbers of the grains and grain fragments assume, or approach, certain orientations. Such textures are caused by slip on restricted crystallographic planes and in certain crystallographic directions. At room temperature, slip occurs on the {111} planes in the (110) directions. Deformation on sets of such planes produces a gradual rotation of grains and grain fragments into specific orientations with respect to the surface of the workpiece and the direction of working.

The final textures achieved with large amounts of deformation vary with the nature of the working process, with the changes in shape of the workpiece, and, to a lesser extent, with the composition of the alloy. Aluminum wire, rod, and bar usually have a "fiber" texture, in which a {111} direction is parallel to the axis of the product, with a random orientation of crystal directions perpendicular to the axis. In rolled sheet, the deformation texture may be described as a mixture of the more textures.

Although aluminum is usually considered to be an isotropic material, the textures developed by various working practices produce some directionality in properties. Generally, this directionality is much less than that observed in hexagonal metals and in some other cubic metals. The principal problem associated with directionality in strain-hardened aluminum alloys is the formation of ears during deep drawing of sheet.