Work Hardening Aluminum Alloys: Part Two

---

Mechanical Properties of Work Hardened Aluminum Alloys

The internal structural changes produced by work hardening create substantial modifications in the mechanical properties of aluminum and its alloys. Among all property changes, tensile properties experience the most significant alterations. Work-hardening curves for several non-heat-treatable aluminum alloys clearly illustrate the strength increases that accompany cold work processes. However, this strength enhancement comes at the expense of ductility, as evidenced by reduced percentage elongation in tensile testing and decreased formability during operations such as bending and drawing.

Cold working processes also enhance shear strength, creep strength at low temperatures, and smooth-specimen fatigue strength in aluminum alloys. While cold work has minimal effect on notch fatigue strength, it increases notch tensile strength proportionally to smooth-specimen tensile strength improvements.

Strain Hardening Applications in Heat Treatable Alloys

Limited applications exist for strain hardening to increase strength in heat treatable aluminum alloys. The primary uses occur in extruded and drawn products, including wire, rod, and tube manufacturing. Heat treated Al-Mg-Si alloys find extensive use in such products, which sometimes undergo drawing after heat treatment to increase strength and improve surface finish quality. The low ductility and poor workability of other artificially aged, heat treatable alloys have restricted cold working as a method for achieving higher strengths. However, aluminum-copper alloys utilize small amounts of cold work after solution heat treatment to obtain increased response during artificial aging processes.

Mathematical Relationships in Work Hardening

Work-hardening curves for annealed, recrystallized aluminum alloys, when plotted as functions of true stress and true strain, exhibit approximately parabolic behavior. These relationships can typically be described by the equation:

σ = kεⁿ                  ...(1)

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

As a close approximation, all aluminum alloys obey Equation (1) over the strain ranges typically employed in industrial applications. Evidence suggests that line slopes decrease as initial alloy strengths increase, indicating a decrease in the n value while simultaneously showing an increase in k.

Non-heat-treatable aluminum alloys initially in cold worked or hot worked conditions demonstrate strain hardening rates substantially below those of materials in annealed temper. For cold worked tempers, this difference results from the strain necessary to produce the specific temper.

When this strain equals ε₀, the strain hardening equation becomes:

σ = k(ε₀ + ε)ⁿ                  ...(2)

Similar situations exist for products initially in hot worked conditions. The strain hardening resulting from hot working or forming is assumed equivalent to that achieved through specific amounts of cold work. From tensile properties of hot worked products, the equivalent cold work amount can be estimated using work-hardening curves for annealed temper. These procedures typically enable calculation of work-hardening curves for hot worked products that reasonably agree with annealed product curves.

Temperature Effects on Work Hardening Aluminum Alloys

The work-hardening characteristics of aluminum alloys vary considerably with temperature changes. At cryogenic temperatures, strain hardening exceeds room temperature levels. Strength gains from working at -320°F (-195°C) reach approximately 40% but accompany significant ductility reductions.

Work-hardening characteristics of aluminum alloys at elevated temperatures vary with both temperature and strain rate. Strain hardening achieved through rolling of Al-5 Mg alloy decreases progressively as rolling temperature increases, until at 700°F (371°C) and above, no effective strain hardening occurs. These results mirror observations in many commercial operations, although exact strength-temperature relationships vary with deformation method and amount, time at temperature, and other factors.

Numerous strain rate effect studies demonstrate that aluminum alloy yield strength increases with increasing strain rates. These effects remain modest at room temperature, requiring rate changes of several orders of magnitude to produce appreciable yield strength increases. At elevated temperatures, relative yield strength increases become much larger.

Although aluminum yield strength increases with increasing strain rates, this should not be interpreted as substantial or necessarily permanent strain hardening increases. Except for shock loading conditions, effects remain small and may be offset by recovery phenomena resulting from heat generated by rapid plastic deformation.

Physical and Chemical Properties

Physical and chemical properties of aluminum and its alloys experience effects from strain hardening, though changes are typically small and primarily of academic interest. In specific instances, notably stress corrosion resistance of certain alloy types, strain hardening effects carry commercial importance.

Strain hardening effects on aluminum electrical conductivity remain small and generally less significant than alloying or heat treatment effects used for many aluminum alloys. Conductor-grade aluminum electrical conductivity decreases from typical values of 63% IACS in annealed condition to 62.5% in strain-hardened H19 temper.

Cold working also slightly decreases density, with changes potentially reaching 0.2% through severe cold working of aluminum. Limited knowledge exists regarding density changes of various alloys through cold working processes. Cold worked aluminum-magnesium alloys show greater density changes than unalloyed aluminum, suggesting that magnesium increases dislocation numbers and point defects produced by aluminum cold working.

Elastic Properties and Anelastic Behavior

The elastic modulus of aluminum and its alloys experiences only slight effects from cold working. These minor changes result principally from texture variations and crystal anisotropy. Consequently, elastic constants such as shear modulus and modulus of elasticity maintain the same engineering values in both annealed and cold worked tempers.

Anelastic properties, including internal friction and damping, are influenced by strain hardening processes. Damping typically shows greater values in annealed aluminum alloys compared to strain hardened alloys. Results vary with testing conditions and applied stress levels. When applied stress is large and mechanical hysteresis occurs, annealed alloys demonstrate better damping than strain-hardened alloys. However, when applied stresses remain very small and mechanical hysteresis is not a factor, strain hardening may increase damping characteristics.

Chemical Properties and Corrosion Resistance

Strain hardening effects on aluminum chemical properties are usually quite small. When substantial effects occur, they often trace to secondary reactions from strain hardening effects on alloy metallurgical structure. While aluminum alloys might be expected to react with specific environments at increased rates due to greater strain energy stored through deformation, substantial evidence shows that cold work has little effect on corrosion resistance of most aluminum alloys under various exposure conditions.

In specialized situations, corrosion resistance of certain aluminum alloys may decrease through 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 may also induce or accelerate grain boundary precipitation in non-heat-treatable aluminum-magnesium alloys, with alloys containing more than 4% Mg potentially becoming susceptible to stress-corrosion cracking.

Generally, only extended aging at room temperature or heating at elevated temperatures would produce sufficient grain boundary precipitation to induce stress-corrosion cracking susceptibility. However, in most commercial aluminum-magnesium alloys, cold work amounts are intentionally limited, and special corrosion-resistant tempers are recommended for critical applications.