Aluminium Bronzes

Composition - Properties

Aluminium Bronzes are alloys mainly consisting of Copper ranging from 77% to 93% and Aluminium from 6% to 12% together with small amounts of Iron, Nickel, Manganese and Silicon.

There are three major types of Aluminium Bronze:- Copper Aluminium; Nickel Aluminium Bronze; and Silicon Aluminium Bronze.

Given the inherent corrosion resistance of Copper the addition of Aluminium improves the strength and corrosion resistance. Nickel further improves the corrosion resistance and raises the proof stress. Manganese stabilises the metallurgical Beta phase and improves the tensile strength. Iron refines the grain size, again improving the strength. Silicon improves the ductility, hot working and machinability of the alloy and enhances its good corrosion resistance.

Corrosion Resistance

The excellent corrosion resistance of the Aluminium Bronzes, particularly in sea water, is attributable to a hard, thin tenacious film of Aluminium Oxide which forms on the surface of the alloy by taking Oxygen from the surrounding medium and which re-forms immediately if removed. This makes the alloys preferable to Stainless Steels in many marine applications, particularly where crevice corrosion conditions are prevalent.

High Strength

The high strength properties mean that often Ferrous componets which suffer from corrosion can be directly replaced with Aluminium Bronzes with ultimate tensile values typically in excess of 700 N/mm (45 tonnes/sq inch). If ductility is the criteria then the Silicon Aluminium Bronzes offer elongation figures of 35% to 40% coupled with good impact values.

Shock, Wear and Abrasion Resistance

Although the coefficient of friction of Aluminium Bronze is inferior to Phosphor Bronzes its underlying high strength means that in well lubricated applications it can be used for much higher stresses with thinner sections, particularly where shock loading and abrasion are involved and where rubbing speeds are low and intermittent.

High and Low Temperature Performance

Aluminium Bronzes are not prone to scaling and can be used in applications up to 300 deg Celcius if moderately stressed. This can be increased for use with steam or air to 450 deg Celcius where components are lightly stressed. As with most Copper based alloys the tensile strength of Aluminium Bronzes improves with decreasing temperatures whilst the elongation and impact strength are only slightly reduced at temperatures down to -180 deg Celcius.

Magnetic Permeability

Silicon Aluminium Bronzes are available with exceptionally low magnetic permeabilities and are ideal for use in instrumentation such as compasses and in defence applications such as mine sweeping vessels and their equipment which is permanently exposed to salt water.

Resistance To Sparking

Excellent resistance to sparking makes Aluminum Bronzes suitable for use in tools and equipment used in the manufacturing, processing and handling of volatile gases, petroleum chemicals and explosives. They are widely used in bulk oil carriers and liquified gas storage plants.

Welding and Fabrication

Aluminium Bronzes can be readily welded and deposited using conventional MIG and TIG equipment.

Hot and Cold Working

Aluminium Bronzes generally can be hot worked by forging, stamping, extrusion, induction heating and upsetting. The Aluminium Bronzes and Silicon Aluminium Bronzes can be cold worked to a limited extent.

Machinability

Conventional tungsten carbides and high speed tool steels cope easily with the machining of Aluminium Bronzes although it must always be noted that they are far tougher than Brasses which they resemble in appearance only.