Aluminum and aluminum alloys are widely used throughout industry. While their heat treatment might seem relatively straightforward, in reality, it is a simple process that must be very carefully controlled. This blog series is intended to increase our knowledge of this important alloy. We begin with a discussion of wrought aluminum alloys.
In high-purity form, aluminum is soft and ductile. For use as an engineered material, however, greater strength is normally required. This is achieved in aluminum first by the addition of certain elements to produce alloys of aluminum. These elements act singly or in combination with one another to impart greater strength to the material. Further strengthening is possible either by mechanical methods (i.e. cold working) or heat treatment. There are two basic categories for aluminum alloys.
The method used to impart strength to non-heat-treatable alloys is primarily cold working (e.g., drawing, stretching).
Alloying elements that produce a hardening effect include manganese (Mn), silicon (Si), iron (Fe) and magnesium (Mg), which act singly or in combination with one another. The non-heat-treatable alloys include wrought alloys in the 1xxx, 3xxx, 4xxx or 5xxx series, and cast aluminum alloys include the 1xx.x, 4xx.x, 5xx.x and 8xx.x series (see Part 2).
Since these alloys are work-hardenable, strengthening depends on the degree of cold working. Alloys containing appreciable amounts of magnesium when supplied in strain-hardened tempers are usually given a final elevated-temperature treatment called “stabilizing” to ensure the effect of these properties is consistent over time.
All wrought aluminum alloys can be supplied in the annealed (or “O”) condition. It may be desirable to anneal an alloy from any other initial temper, after working or between successive stages of working (e.g., deep drawing).
Alloying elements such as copper (Cu), magnesium (Mg), zinc (Zn) and silicon (Si) respond to heat treatment to change the strength characteristics of these alloys by increasing solid solubility (in aluminum) with increasing temperature. Thus, it is possible to use various treatments to improve the strength and hardness of these alloys. The heat-treatable alloys include wrought alloys in the 2xxx, 6xxx and 7xxx series and cast aluminum alloys include 2xx.x, 3xx.x and 7xx.x series (see Part 2).
Typical treatments include solution heat treatment, quenching and age hardening (aka aging or precipitation hardening). The optimum strength is achieved by the proper combination of these treatments.
2. Herring, Daniel H., "Fundamentals of Aluminum Heat Treatment," white paper, 2004.