We conclude our alphabetical listing of major and minor alloying additions to aluminum alloys and their effects.
Magnesium: A major alloying element in the 5xxx series of wrought alloys. Adding magnesium (up to 5.5%) to aluminum increases strength without decreasing ductility. These alloys also have good corrosion resistance and weldability. Wrought alloys of the 6xxx group contain up to 1.5% each of magnesium and silicon. The maximum solubility of magnesium silicon (Mg2Si) is 1.85%, and the percentage decreases with temperature. Magnesium-manganese combinations have high strength in the work-hardened condition. Increasing amounts of either magnesium or manganese will intensify fabrication difficulty and also increase the tendency toward cracking during hot rolling, especially if traces of sodium are present.
Manganese:Commonly found in primary aluminum as an impurity (5-50 ppm). Manganese increases strength either in solid solution or as a finely precipitated intermetallic phase. Manganese decreases the alloys resistivity but has no adverse effect on corrosion resistance. Manganese has very limited solid solubility in aluminum in the presence of normal impurities but remains in solution when chill cast so that most of the manganese added is substantially retained in solution, even in large ingots.
Mercury: Mercury in aluminum or in contact with it as a metal or salt will rapidly corrode most aluminum alloys.
Molybdenum: An impurity in aluminum (only 0.1-1.0 ppm), but it can be used as a grain refiner (0.3%).
Nickel: Nickel (up to 2%) increases strength of high-purity aluminum but also reduces ductility. Nickel is added to aluminum-copper and to aluminum-silicon alloys to improve hardness and strength at elevated temperatures as well as reducing the alloy’s coefficient of expansion.
Niobium: Added as a grain refiner in castings.
Phosphorus: A minor impurity (1-10 ppm) in commercial-grade aluminum. Phosphorous solubility in molten aluminum is quite low (~0.01% at 660ºC).
Silicon: The second-highest impurity element (after iron) in commercial aluminum (0.01-0.15%). For wrought alloys, silicon is used in combination with magnesium at levels up to 1.5% to form Mg2Si in the 6xxx series of heat-treatable alloys.
Vanadium: Present in commercial-grade aluminum (10-200 ppm). Vanadium addition lowers conductivity.
Zinc: In aluminum-zinc alloys, zinc additions produce the good tensile properties in wrought aluminum alloys. Magnesium additions to the aluminum-zinc alloys develop the full strength potential, particularly in the 3-7.5% zinc range. Copper addition to aluminum-zinc-magnesium, together with small amounts of chromium and manganese, results in the highest strength aluminum base alloys available commercially. For this alloy, the zinc and magnesium control the aging process, and the copper increases the aging rate and quench sensibility upon heat treatment. Generally, copper reduces corrosion resistance of aluminum-zinc-magnesium alloys but increases its resistance to stress corrosion.
Zirconium: Added to the aluminum-zinc-magnesium family of alloys to increase the recrystallization temperature as well as control the grain structure in wrought alloys.
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.