We begin an alphabetical list of major and minor alloying elements for aluminum alloys. The influence of each alloying element, whether it be as an alloying addition or as an impurity, is briefly discussed.
Antimony: Found in commercial-grade aluminum (0.01-0.1 ppm), antimony has a very limited solid solubility in aluminum (<0.01%). Adding antimony to aluminum-magnesium alloys can help counteract hot cracking.
Arsenic: Arsenic (as AsO3) is highly toxic and must be controlled to very low limits in aluminum, especially when used for applications involving food.
Beryllium: Added to aluminum alloys to reduce oxidation at elevated temperatures. It is often combined with magnesium for this purpose.
Bismuth: Added to aluminum to make free-machining alloys given bismuth’s low melting point. Bismuth has a restricted solubility in solid aluminum and forms a soft, low-melting phase that helps lubricate cutting tools and promotes chip breaking. The expansion of bismuth on solidification compensates for the shrinkage of lead in aluminum parts.
Boron: Added (0.005-0.1%) for grain refinement in aluminum and aluminum alloys, and it can be used to improve conductivity by precipitating elements such as chromium, molybdenum, titanium and vanadium. It becomes more effective (a titanium-to-boron ratio of 5 to 1 is common) when used with an excess of titanium.
Cadmium: A relatively low-melting element that has only limited uses. Cadmium can be added to aluminum-copper alloys (up to 0.3%) to accelerate the rate of age hardening, increase strength and increase corrosion resistance. Cadmium (0.005-0.5%) reduces the time of aging of aluminum-zing-magnesium alloys.
Calcium: Has low solubility in aluminum, forming the intermetallic compound CaAl4. Calcium can combine with silicon to form CaSi2, a compound that is insoluble in aluminum and, therefore, will slightly increase the conductivity of commercial-grade metals. Decreases age hardening when used in aluminum-magnesium-silicon alloys. The effect of calcium on non-heat-treatable aluminum-silicon alloys is an increase to strength and decrease to elongation.
Carbon: An impurity of aluminum that may lead to surface pitting.
Cerium: Added to increase fluidity and reduce sticking in castings.
Chromium: A minor impurity in commercial-purity aluminum (5-50 ppm). Has a significant effect on electrical resistivity. Chromium is a common addition to many alloys of the aluminum-magnesium, aluminum-magnesium-silicon and aluminum-magnesium-zinc type. Typically, it is not added in excess of 0.35%. Exceeding this limit tends to form very coarse constituents with other impurities or additions such as iron, titanium and manganese. In wrought products, chromium has a slow diffusion rate and forms fine dispersed phases that inhibit grain growth and nucleation. In aluminum-magnesium alloys, chromium is utilized to control grain structure and prevent grain growth. In aluminum-magnesium-silicon or aluminum-magnesium-zinc alloys, chromium is used to prevent recrystallization during hot working or heat treatment.
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.