Work Hardening Aluminum Alloys: Part Two


The internal structural changes produce substantial changes in the mechanical properties of aluminum and its alloys. Tensile properties are among those most affected.
Work-hardening curves for several non-heat-treatable alloys illustrate the increase in strength that accompanies cold work. This increase is obtained at the expense of ductility, as measured by the per cent elongation in the tensile test and by reduced formability in operations such as bending and drawing.

Mechanical Properties

The internal structural changes described previously produce substantial changes in the mechanical properties of aluminum and its alloys. Tensile properties are among those most affected. Work-hardening curves for several non-heat-treatable alloys illustrate the increase in strength that accompanies cold work. This increase is obtained at the expense of ductility, as measured by the per cent elongation in the tensile test and by reduced formability in operations such as bending and drawing.

Cold working also increases shear strength, creep strength at low temperatures, and smooth-specimen fatigue strength. It has little effect on notch fatigue strength, but increases notch tensile strength in about the same proportion as smooth-specimen tensile strength.

Limited use is made of strain hardening to increase the strength of heat treatable alloys. The principal applications are in extruded and drawn products, such as wire, rod, and tube. Heat treated Al-Mg-Si alloys are employed extensively in such products, which are sometimes drawn after heat treatment to increase strength and improve surface finish. The low ductility and poor workability of other artificially aged, heat treatable alloys have restricted cold working as a procedure for obtaining higher strengths. In the aluminum-copper alloys, however, small amounts of cold work are employed after solution heat treatment to obtain increased response during artificial aging.

Work-hardening curves for annealed, recrystallized aluminum alloys, when plotted as a function of true stress and true strain, are approximately parabolic and usually can be described by:

σ = kεn (1)

where σ is the true stress, k is the stress at unit strain, ε is the true strain , and n is the strain-hardening exponent.

As a close approximation, all of the alloys obey Eq (1) over the range of strains employed here. There is evidence that the lopes of the lines decrease as the initial strengths of the alloys increase, indicating a decrease in the value of n. At the same time, there is an increase in k.

Non-heat-treatable alloys initially in a cold worked or hot worked condition have rates of strain hardening substantially below those of material in the annealed temper. For the cold worked tempers, this difference is caused by the strain necessary to produce the temper.

If this strain equals ε0, then the equation for strain hardening becomes:

σ = k(ε0 + ε)n

A similar situation exists for products initially in the hot worked condition. The strain hardening resulting from hot working or forming is assumed to be equivalent to that achieved by a certain amount of cold work. From the tensile properties of the hot worked product, the amount of equivalent cold work can be estimated, using the work-hardening curve for the annealed temper. By such procedures, it is usually possible to calculate, work-hardening curves for hot worked products that are in reasonable agreement with those for annealed products.

The work-hardening characteristics of aluminum alloys vary considerably with temperature. At cryogenic temperatures, strain hardening is greater than it is at room temperature. The gain in strength by working at -320°F (-195°C) is about 40% but is accompanied by a significant reduction in ductility.

The work-hardening characteristics of aluminum alloys at elevated temperatures vary both with temperature and with strain rate. Strain hardening achieved by rolling of Al-5 Mg alloy decreases progressively as rolling temperature increases, until at 700°F (700°C) and above no effective strain hardening occurs. These results are similar to those observed in many commercial operations, although the exact strength-temperature relationship varies with the method and amount of deformation, time at temperature, and other factors.

Numerous studies of the effects of strain rate show that the yield strength of aluminum alloys increases as strain rates increase. These effects are not large at room temperature; rate changes of several orders of magnitude are required to produce an appreciable increase in yield strength. At elevated temperatures, the relative increase in yield strength is much larger.

Although the yield strength of aluminum increases as strain rates increase, this should not be interpreted as a substantial, or necessarily permanent, increase in strain hardening. Except for shock loading, the effects are small and may be offset by recovery phenomena resulting from the heat generated by the rapid plastic deformation.

Physical and chemical properties

Physical and chemical properties of aluminum and its alloys are affected by strain hardening. Usually, however, the changes are small and of academic interest only. In some instances, notably the resistance to stress corrosion of certain alloys and alloy types, the effects of strain hardening are of commercial importance.

The effect of strain hardening on the electrical conductivity of aluminum is small and generally less than that of alloying or of the heat treatments used for many aluminum alloys. The electrical conductivity of conductor-grade aluminum is decreased from a typical value of 63% IACS in the annealed condition to 62.5% in the strain-hardened H19 temper.

Density is also decreased slightly by cold working. This change may amount to 0.2% with severe cold working of aluminum.

Little is known about the density changes of various alloys through cold working. The change in density of cold worked aluminum-magnesium alloys is greater than that of unalloyed aluminum. This suggests that magnesium increases the number of dislocations and of point defects produced by the cold working of aluminum.

The elastic module of aluminum and its alloys are affected only slightly by cold working. These slight changes result principally from variations in texture and crystal anisotropy. As a result, elastic constants, such as shear modulus and modulus of elasticity, have the same engineering value in both annealed and cold worked tempers.

Anelastic properties, such as internal friction and damping, are influenced by strain hardening. Damping usually is greater in annealed aluminum alloys than in strain hardened alloys. Results vary, however, with the conditions of testing and the applied stress. Where the applied stress is large and mechanical hysteresis is observed, annealed alloys damp better than strain-hardened alloys. But when applied stresses are very small and mechanical hysteresis is not a factor, strain hardening may increase damping.

The effects of strain hardening on the chemical properties of aluminum are usually quite small. Where substantial effects are encountered, they can often be traced to secondary reactions from the effects of strain hardening on the metallurgical structure of the alloy. It is expected that aluminum alloys would react with specific environments at an increased rate because of the greater strain energy stored in the metals by deformation. But, there is much evidence to show that cold work has little effect on the resistance to corrosion of most aluminum alloys in a variety of exposure conditions.

In some special situations the resistance to corrosion of certain aluminum alloys may be decreased by cold working. Cold working can cause residual tensile stresses and consequent stress-corrosion cracking of some heat treatable alloys exposed to corrosive environments. Cold work also may induce or accelerate grain boundary precipitation in the non-heat-treatable aluminum-magnesium alloys; alloys containing more than 4% Mg may thereby become susceptible to stress-corrosion cracking.

Generally, only long aging at room temperature or heating at elevated temperatures would produce sufficient grain boundary precipitation to induce susceptibility to stress-corrosion cracking. However, in most commercial aluminum-magnesium alloys, the amount of cold work is intentionally limited; special corrosion-resistant tempers are recommended.

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