Aluminum Bronzes: Part One

Bronze is a metal alloy consisting primarily of copper, usually with tin as the main additive, but sometimes with other elements such as phosphorus, manganese, aluminum, or silicon. It was particularly significant in antiquity, giving its name to the Bronze Age.

Bronze was significant to any culture that encountered it. It was one of the most innovative alloys of mankind. Tools, weapons, armor, and various building materials like decorative tiles made of bronze were harder and more durable than their stone and copper predecessors.

Initially bronze was made out of copper and arsenic to form arsenic bronze. It was only later that tin was used, becoming the sole type of bronze in the late 3rd millennium BC. Tin bronze was superior over arsenic bronze in that the alloying process itself could more easily be controlled (as tin was available as a metal) and the alloy was stronger and easier to cast. Also, unlike arsenic, tin is not toxic.

Aluminum bronze is a type of bronze in which aluminum is the main alloying metal added to copper. A variety of aluminum bronzes of differing compositions have found industrial use, with most ranging from 5% to 11% aluminum by weight, the remaining mass being copper; other alloying agents such as iron, nickel, manganese, and silicon are also sometimes added to aluminum bronzes.

Compositions

The following table lists the most common standard aluminum bronze wrought alloy compositions, by ISO 428 designations. The percentages show the proportional composition of the alloy by weight. Copper is the remainder by weight and is not listed (Table 1).

Table 1: Aluminum bronze wrought alloy compositions

Forged aluminum bronze products, heat treated as needed, offer significant advantages over cast products. Because of their fine equiaxed grain structure, forged metals are particularly recommended for pieces that will undergo detailed machining. Cast products are more likely to have random inclusions and porosity that may result in inconsistent surfaces, machining limitations and excessive tool wear.

The Effects of Alloying Elements

In addition to aluminum, which ranges from 5% to 14% in these alloys, the alloying elements most commonly used in aluminum bronzes are nickel, iron, manganese, silicon and tin. The mechanical properties of aluminum bronze depend primarily on aluminum content; however, varying proportions of these secondary additions result in sub-classifications of the family, as described below.

The four principal types of aluminum bronzes are:

• The low alloy, single-phase (face-centered cubic) alpha alloys containing less than 8% aluminum. These alloys have a good ductility, both hot and cold, and are well suited for cold working into tube, sheet, strip and wire. Alloys of this type containing 3% iron are single-phase at compositions exceeding 9% aluminum.
• The more highly alloyed, two-phase (duplex) alloys containing from 8% to 11% aluminum and, usually, additions of iron and nickel, for higher strength. As aluminum content is increased to between 8% and 10%, the alloys are progressively strengthened by appearance of the harder body-centered cubic beta phase, which additionally makes the bronzes more suitable for hot working and casting. Even greater strength and hardness is developed in alloys containing more than 10% Al. Such alloys are favored for specialized applications requiring superior wear resistance.
• The copper-aluminum-silicon alloys, or silicon-aluminum bronzes. These are mainly alpha-phase alloys and therefore have good strength and ductility: Alloys having silicon contents ranging up to about 2% and aluminum to about 6% are known as aluminum-silicon bronzes; these alloys are stronger than unmodified single-phase aluminum bronzes and can be cast and hot-worked more readily. Like other aluminum bronzes, they have a low magnetic permeability and excellent resistance to shock loading. Silicon also improves machinability. The alloys are available in wrought and cast forms.
• The copper-manganese-aluminum alloys, or manganese-aluminum bronzes. These alloys have good castability and were, in fact, developed primarily for the manufacture of propellers. Manganese, at about 13%, is the major alloying addition in a series of manganese-aluminum bronzes in which aluminum levels range between 8 and 9%. Although not so strong as other aluminum bronzes, the alloys' foundry properties are better. They also have good resistance to impingement and cavitation and can be heat treated to give low magnetic permeability. They have excellent weldability.

The other alloying elements mentioned earlier also modify the structure and thereby increase strength and corrosion resistance: iron improves tensile strength and acts as a grain refiner; nickel improves yield (proof) stress and corrosion resistance and has a beneficial stabilizing effect on the metallurgical structure; manganese also performs a stabilizing function.