Heat Treatable Aluminum Alloys

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Understanding Heat Treatment in Aluminum Alloys

Heat treating, in its broadest sense, refers to any heating and cooling operations performed to change the mechanical properties, metallurgical structure, or residual stress state of a metal product. When applied to aluminum alloys, this term is frequently restricted to specific operations that increase strength and hardness in precipitation-hardenable wrought and cast alloys.

These alloys are commonly referred to as "heat-treatable" alloys to distinguish them from those in which no significant strengthening can be achieved through heating and cooling. The latter group, generally called "non heat-treatable" alloys, depends primarily on cold work to increase strength. Heating to decrease strength and increase ductility, known as annealing, is used with both types of alloys, though metallurgical reactions may vary with the type of alloy and degree of softening desired.

Precipitation Hardening Systems in Aluminum Alloys

One essential attribute of a precipitation-hardening alloy system is temperature-dependent equilibrium solid solubility characterized by increasing solubility with rising temperature. The major aluminum alloy systems with precipitation hardening capabilities include:

Aluminum-copper systems achieve strengthening from CuAl₂ formation, while aluminum-copper-magnesium systems benefit from magnesium's ability to intensify precipitation. Aluminum-magnesium-silicon systems derive strengthening from Mg₂Si, and aluminum-zinc-magnesium systems utilize MgZn₂ for enhanced properties. Additionally, aluminum-zinc-magnesium-copper systems combine multiple strengthening mechanisms.

The general requirement for precipitation strengthening of supersaturated solid solutions involves forming finely dispersed precipitates during aging heat treatment, which may include either natural aging or artificial aging. The aging process must occur below the equilibrium solvus temperature and below a metastable miscibility gap called the Guinier-Preston (GP) zone solvus line.

Commercial Heat-Treatable Alloy Classifications

Commercial heat-treatable alloys are, with few exceptions, based on ternary or quaternary systems regarding the solutes involved in developing strength through precipitation. Commercial alloys whose strength and hardness can be significantly increased by heat treatment include 2xxx, 6xxx, and 7xxx series wrought alloys and 2xx.0, 3xx.0, and 7xx.0 series casting alloys.

Some alloys contain only copper, or copper and silicon as primary strengthening additions. Most heat-treatable alloys, however, contain combinations of magnesium with one or more elements including copper, silicon, and zinc. Characteristically, even small amounts of magnesium working with these elements accelerate and accentuate precipitation hardening. Alloys in the 6xxx series contain silicon and magnesium approximately in proportions required for magnesium silicide (Mg₂Si) formation. Although not as strong as most 2xxx and 7xxx alloys, 6xxx alloys offer good formability, weldability, machinability, and corrosion resistance with medium strength.

In heat-treatable wrought alloys, with notable exceptions including 2024, 2219, and 7178, solute elements are present in amounts within mutual solid solubility limits at temperatures below the eutectic temperature. In contrast, some casting alloys of the 2xx.0 series and all 3xx.0 series alloys contain soluble elements that far exceed solid-solubility limits. In these alloys, phases formed by combining excess soluble elements with aluminum will never completely dissolve, although undissolved particle shapes may change through partial solution.

The Three-Step Heat Treatment Process

Heat treatment to increase aluminum alloy strength follows a three-step process: solution heat treatment for dissolving soluble phases, quenching to develop supersaturation, and age hardening through precipitation of solute atoms either at room temperature (natural aging) or elevated temperature (artificial aging or precipitation heat treatment).

Solution Heat Treatment

To utilize the precipitation hardening reaction, producing a solid solution is essential. Solution heat treating accomplishes this by taking maximum practical amounts of soluble hardening elements into solid solution. The process involves soaking the alloy at sufficiently high temperature for adequate time to achieve a nearly homogeneous solid solution.

Precipitation Heat Treatment Without Prior Solution Heat Treatment

Certain alloys that are relatively insensitive to cooling rate during quenching can be air cooled or water quenched directly from final hot working operations. In either condition, these alloys respond strongly to precipitation heat treatment. This practice is widely used in producing thin extruded shapes of alloys 6061, 6063, 6463, and 7005.

Upon precipitation heat treating after quenching at the extrusion press, these alloys develop strengths nearly equal to those obtained by adding separate solution heat treating operations. Property changes during precipitation treatment follow principles outlined in solution heat-treated alloy discussions.

Quenching: The Critical Step

Quenching represents the most critical step in the heat-treating operation sequence. The objective involves preserving the solid solution formed at solution heat-treating temperature by rapidly cooling to lower temperature, usually near room temperature.

In most instances, to avoid precipitation types detrimental to mechanical properties or corrosion resistance, the solid solution formed during solution heat treatment must be quenched rapidly enough to produce supersaturated solution at room temperature – the optimum condition for precipitation hardening.

The resistance to stress-corrosion cracking of certain copper-free aluminum-zinc-magnesium alloys improves with slow quenching. Most frequently, parts are quenched by immersion in cold water, or in continuous heat treating of sheet, plate, or extrusions in primary fabricating mills through progressive flooding or high-velocity spraying with cold water.

Age Hardening: Natural and Artificial Aging

After solution treatment and quenching, hardening is achieved either at room temperature through natural aging or with precipitation heat treatment via artificial aging. In some alloys, sufficient precipitation occurs within days at room temperature to yield stable products with properties adequate for many applications. These alloys are sometimes precipitation heat treated to provide increased strength and hardness in wrought or cast products. Other alloys with slow precipitation reactions at room temperature always receive precipitation heat treatment before use.

In some alloys, notably those of the 2xxx series, cold working freshly quenched material greatly increases its response to later precipitation heat treatment.

Natural Aging Characteristics

The more highly alloyed members of the 6xxx wrought series, copper-containing alloys of the 7xxx group, and all 2xxx alloys are almost always solution heat treated and quenched. For some alloys, particularly 2xxx alloys, precipitation hardening from natural aging alone produces useful tempers (T3 and T4 types) characterized by high tensile-to-yield strength ratios and high fracture toughness and fatigue resistance.

For alloys used in these tempers, relatively high supersaturation of atoms and vacancies retained by rapid quenching causes rapid GP zone formation, with strength increasing rapidly and attaining nearly maximum stable values in four or five days. Tensile-property specifications for products in T3- and T4-type tempers are based on nominal natural aging time of four days. In alloys where T3- or T4-type tempers are standard, further natural aging changes are relatively minor, and products are considered essentially stable after approximately one week.

Contrasting the relatively stable condition reached in days by 2xxx alloys used in T3- or T4-type tempers, 6xxx alloys and especially 7xxx alloys are considerably less stable at room temperature and continue exhibiting significant mechanical property changes for many years.

Artificial Aging Parameters

Precipitation heat treatments are generally low-temperature, long-term processes. Temperatures range from 115 to 190°C, with times varying from 5 to 48 hours.

Time-temperature cycle selection for precipitation heat treatment requires careful consideration. Larger precipitate particles result from longer times and higher temperatures; however, larger particles must be fewer in number with greater distances between them.

The objective involves selecting cycles that produce optimum precipitate size and distribution patterns. Unfortunately, cycles required to maximize one property, such as tensile strength, usually differ from those required to maximize others, such as yield strength and corrosion resistance. Consequently, used cycles represent compromises providing the best property combinations.

Production of material in T5- through T7-type tempers necessitates precipitation heat treating at elevated temperatures through artificial aging.

Temper Classifications and Applications

Differences in type, volume fraction, size, and distribution of precipitated particles govern properties and changes observed with time and temperature, all affected by initial structure state. Initial structure may vary in wrought products from unrecrystallized to recrystallized and may exhibit modest strain from quenching or additional strain from cold working after solution heat treatment. These conditions, along with precipitation heat treatment time and temperature, affect final structure and resulting mechanical properties.

Precipitation heat treatment following solution heat treatment and quenching produces T6- and T7-type tempers. Alloys in T6-type tempers generally have the highest practical strengths without sacrificing minimum levels of other properties and characteristics found satisfactory for engineering applications. Alloys in T7 tempers are overaged, meaning some strength has been sacrificed to improve one or more other characteristics.

Strength may be sacrificed to improve dimensional stability, particularly in products intended for elevated temperature service, or to lower residual stresses for reducing warpage or distortion in machining. T7-type tempers are frequently specified for cast or forged engine parts. Precipitation heat-treating temperatures used to produce these tempers are generally higher than those used for T6-type tempers in the same alloys.

Two important T7-type temper groups—the T73 and T76 types—have been developed for wrought alloys of the 7xxx series containing more than approximately 1.25% copper. These tempers improve resistance to exfoliation corrosion and stress-corrosion cracking, but as a result of overaging, they also increase fracture toughness and, under some conditions, reduce fatigue-crack propagation rates.