15-10-2012, 04:41 PM
Aluminum Bronzes
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Introduction
The aluminum bronzes are a family of copper-based alloys offering a combination of
mechanical and chemical properties unmatched by any other alloy series. This feature
often makes aluminum bronzes the first choice and sometimes the only logical
choice for demanding applications. What are these attributes?
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.
Applications
The aluminum bronzes comprise a wide range of compositions, and alloys can be
chosen with a correspondingly wide range of properties to suit many types of duty. In
fact, the mix of properties available is so varied that alloy selection needs to be carefully
considered, and expert advice is always useful.
Where are all these properties best employed?
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 (Photos 1-4); in power generation (Photo 5),
aircraft, automotive and railway engineering; and in the iron and steel-making, electrical
manufacturing and building industries (Photo 6).
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.
Effect 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.
Mechanical Properties
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
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. However, yield
strength is not quite so easy to measure as is tensile strength in this case because,
unlike steels, copper alloys do not show a sudden "yield" deformation when stress is increased past a critical value, so "proof strength" is a more appropriate term. Values
may be given with respect to various percentages of permanent deformation; in the
U.S.A., for example, yield stress for most copper alloys is expressed as the stress
corresponding to 0.5% extension under load, whereas for certain high-strength alloys
such as C63020, it is given as the stress corresponding to a 0.2% strain offset to the
linear (Hooke's Law) portion of the engineering stress-strain curve. Comparison of
values therefore needs care.
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. Elongation figures decrease as the alloys get harder, the ranges shown in the
table below reflect this.