Aluminum Bronzes: Part Two

요약:

Aluminum bronzes are most valued for their higher strength and corrosion resistance as compared to other bronze alloys. These alloys are tarnish-resistant and show low rates of corrosion in atmospheric conditions, low oxidation rates at high temperatures, and low reactivity with sulfurous compounds and other exhaust products of combustion. They are also resistant to corrosion in sea water.

General Material Properties

Aluminum bronzes are most valued for their higher strength and corrosion resistance as compared to other bronze alloys. These alloys are tarnish-resistant and show low rates of corrosion in atmospheric conditions, low oxidation rates at high temperatures, and low reactivity with sulfurous compounds and other exhaust products of combustion. They are also resistant to corrosion in sea water.

Aluminum bronzes resistance to corrosion rests in the aluminum component of the alloys, which reacts with atmospheric oxygen to form a thin, tough surface layer of alumina (aluminum oxide) which acts as a barrier to the corrosion of the copper-rich alloy.

Another notable property of aluminum bronzes are their biostatic effects. The copper component of the alloy prevents colonization by marine organisms including algae, lichens, barnacles, and mussels, and therefore can be preferable to stainless steel or other non-cupric alloys in applications where such colonization would be unwanted.

This feature often makes aluminum bronzes the first choice - and sometimes the only logical choice - for demanding applications. The main attributes of aluminum bronzes are:

  • Excellent strength, similar to that of low alloy steels
  • Excellent corrosion resistance, especially in seawater and similar environments, where the alloys often outperform many stainless steels
  • Favorable high temperature properties, for short or long term usage
  • Good resistance to fatigue, ensuring a long service life
  • Good resistance to creep, making the alloys useful at elevated temperatures
  • Oxidation resistance, for exposure at elevated temperatures and in oxidizing environments
  • Ease of casting and fabrication, when compared to many materials used for similar purposes
  • High hardness and wear resistance, providing excellent bearing properties in arduous applications
  • Ductility, which, like that for all copper alloys, is not diminished at low temperatures
  • Good weldability, making fabrication economical
  • Readily machined, when compared with other high-duty alloys
  • Low magnetic susceptibility, useful for many special applications
  • Ready availability, in cast or wrought forms.

Simply put, aluminum bronzes are used where other materials might fail prematurely or would be more expensive to buy or fabricate. For example, the alloys find widespread applications in chemical, petrochemical and desalination plants; in marine, offshore and shipboard hardware and equipment; in power generation, aircraft, automotive and railway engineering; and in the iron and steel-making, electrical manufacturing and building industries.
These areas of application are best discussed with reference to the primary service property required of the alloy, allowing for the fact that it is normally a combination of several properties that finally governs the selection of a particular material. As to the choice of product form, it is worth noting that the excellent mechanical properties obtainable from both cast and wrought aluminum bronzes makes it possible in many applications to interchange the manufacturing method in order to achieve maximum economy.

As mentioned above main mechanical properties are

Tensile strength. Some aluminum bronzes exhibit strengths comparable to low alloy steels and many are stronger than most stainless steels. Furthermore, the alloys retain a substantial proportion of their strength at elevated temperature, and at low temperatures, they gain strength slightly while retaining ductility. Shear strength can be estimated as being two thirds of the tensile strength.

Yield (Proof) strength is a more useful property than tensile strength since it is a measure of the stress needed to cause a measurable permanent (non-elastic) deformation, i.e., far lower than the stress needed to cause failure.

Hardness. The hardness of aluminum bronzes increases with aluminum (and other alloy) content as well as with stresses caused through cold working. Some manganese- and manganese-nickel-aluminum bronzes exhibit martensitic transformations similar to those seen in steels, but while these reactions produce higher mechanical properties, they are not generally thought of as primary strengthening mechanisms.

Ductility. Most aluminum bronzes show ample ductility to provide adequate service life and to resist fatigue. Again, values attainable vary with alloy content and amount of prior cold work.

Wear Resistance. From the standpoint of wear resistance, aluminum bronzes often provide excellent service in both cast and wrought forms. Metal-sprayed or welded overlay deposits of aluminum bronze on steel also provide effective wear-resistant surfaces. At the high end of the wear- and abrasion-resistance spectrum are special aluminum bronze alloys containing up to 14% aluminum, whose applications include dies for deep drawing and molds for die casting, casting of glass bottles and pressing of vinyl records.

Fatigue Strength. Aluminum bronzes possess exceptional resistance to fatigue, which is one of the most common causes of deterioration in marine engineering equipment. This property helps to give the alloys their excellent resistance to corrosion fatigue that makes them suitable for use as propellers and in pumps.

Magnetic Permeability. Aluminum bronzes can be made with exceptionally low magnetic permeability and are therefore ideal for non-magnetic instrumentation, survey vessels, mine counter-measure craft and other marine parts where permeability must not exceed 1.05 H/m.

Corrosion Resistance Aluminum bronzes can be used in environments that are far more aggressive than are tolerated by most other metals, including even copper and the brasses used for general purposes. They can provide heavy-duty service at higher temperatures, in seawater environments and in the presence of many chemicals and acids. Much useful work has been done to characterize the corrosion resistance of these alloys so that best use can be made of them.

Applications

Aluminum bronzes are most commonly used in applications where their resistance to corrosion makes them preferable to other engineering materials. These applications include bearing bushes and landing gear components on aircraft, engine components (especially for seagoing ships), underwater fastenings in naval architecture, and ship propellers. The attractive gold-toned coloration of aluminum bronzes has also led to their use in jewelry.

Like many copper alloys, aluminum bronzes resist biofouling in both fresh and saline waters. This property is useful in propellers and especially so in seawater piping systems aboard ships and offshore platforms where, for example, fire safety systems depend on fouling-free piping for maximum flow.

Aluminum bronzes are in the highest demand from the following industries and areas:

  • General sea water-related service
  • Water supply
  • Oil and petrochemical industries e.g. tools for use in non-sparking environments
  • Specialized anti-corrosive applications
  • Certain structural retrofit building applications

Aluminum bronze alloys may be considered for service with some chemicals, particularly where there is a combination of stress and erosion, but selection must take account of the anticipated temperature, concentration and other service conditions.

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