We continue with our alphabetical list of major and minor alloying additions to aluminum alloys and their effects.
Cobalt: Not a common addition to aluminum alloys. Adding cobalt to some aluminum-silicon alloys containing iron can improve strength and elongation. Aluminum-zinc-magnesium-copper alloys produced by powder metallurgy typically contain 0.2-1.9% cobalt.
Copper: Cast and wrought aluminum-copper alloys respond to solution heat treatment and subsequent aging by increasing strength (and hardness) with a decrease in elongation. Maximum strengthening usually occurs between 4-6% copper, depending on the influence of other constituents in the alloy. Aluminum-copper alloys with magnesium exhibit increased strength following solution heat treatment and quenching. For wrought aluminum-copper alloys, an increase in strength as well as high ductility occurs on natural aging. When artificially aged, a further increase in strength is obtained, especially in yield strength, while sacrificing tensile elongation. Aluminum-copper-magnesium alloys containing iron have high strength and hardness values at elevated temperatures and are categorized by improved dimensional stability.
Gallium: An impurity in aluminum (0.001-0.02%) with a limited effect on mechanical properties. At 0.2% or above, gallium can affect the corrosion properties of certain alloys.
Hydrogen: Solubility is greater in its liquid state at the melting point than in the solid state, resulting in gas porosity during solidification. Hydrogen can also cause secondary porosity, blistering and high-temperature deterioration (advanced internal gas precipitation) during heat treatment.
Indium: Small amounts (0.05-0.2%) of indium have a marked influence on the age hardening of aluminum-copper alloys, particularly at low copper contents (2-3% Cu).
Iron: The most common impurity found in aluminum. Iron has high solubility in molten aluminum and can easily be dissolved at all molten stages of production. However, iron solubility in aluminum in the solid state is very low (~0.04%). Because of this, most of the iron present in aluminum over this amount appears as an intermetallic second phase in combination with aluminum and other elements.
Lead: Found usually only as a trace element in commercial-purity aluminum. The addition of lead (0.5%) with the same amount of bismuth improves machinability in some alloys.
Lithium: Lithium is an impurity of aluminum, present in the order of only a few ppm. At a level of less than 5 ppm, it can promote the discoloration (blue corrosion) of aluminum foil under humid conditions. Lithium traces can greater increase the rate of oxidation of molten aluminum and alter surface characteristics of wrought products.
The list continues in our next installment.
1. ASM Handbook, Volume 2, Properties and Selection: Nonferrous Alloys and Special Purpose Materials, ASM International, 1990.
2. Key-to-Metals (www.keytometals.com)
3. Herring, Daniel H., Fundamentals of Aluminum Heat Treatment, white paper, 2004.