Aluminum Bronzes - Part I

Metallurgy of Copper & Copper Alloys

By Vin Callcut

• Aluminum Bronzes - Part II

• Introduction
• Applications
• Aluminum Bronzes
• Effect of Alloying Elements
• Mechanical Properties
• Corrosion Resistance
• Freedom from Oxide Flaking
• Cavitation Erosion

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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
• 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, and
• Ready availability, in cast or wrought forms.

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.

Photo 1. Centrifugally cast nickel-aluminum bronze high-pressure flange for a sub-sea weapons ejection system. Alloy C95800 was selected for this application for its combination of high yield strength (35 ksi, 241 MPa minimum as cast), excellent seawater corrosion resistance and galvanic compatibility with mating materials. (Photo courtesy MetalTek. Inc.)

Photo 2. Clutch components for a shipboard winch, centrifugally cast in aluminum bronze alloy C95400. The inside diameter surfaces of these castings are actually metal-matrix composite structures containing non-metallic anti-wear particles, while the outer diameter is cast to net shape. Application of the continuous casting process to produce this innovative technique extended the life of the clutch component by 20-fold, and net shape near casting reduced the cost of finish machining. (Photo courtesy MetalTek. Inc.)

Photo 3. Propeller shaft seal, centrifugally cast in nickel-aluminum bronze. Alloy C95800 was specified for this application for its seawater corrosion resistance, non-magnetic properties and good anti-galling characteristics against steel (the propeller shaft). Small flats were intentionally cast into the blank, which was then cut transversely and finish machined to the proper inside diameter. Note that the outside diameter is cast to net shape and does not require further machining. (Photo courtesy MetalTek. Inc.)

 

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.

Photo 4. Nickel-aluminum bronze (alloy C95800) hub for a variable pitch naval propeller. The alloy was selected for its high strength, non-magnetic properties, cavitation resistance and galvanic compatibility with the propeller blades. Nickel-aluminum bronze exhibits good anti-galling characteristics against itself, an important property in this application since the propeller blades, which are also cast in nickel-aluminum bronze, rotate within the cylindrical openings one the sides of the hub. (Photo courtesy MetalTek. Inc.)

Photo 5. Wear rings for a large hydroturbine, centrifugally cast in nickel-aluminum bronze, alloy C95800. Nickel-aluminum bronze was selected for these rings, which operate under a hydrostatic head as large as 400 ft (122 m), for the alloy's proven anti-galling properties and excellent galvanic corrosion resistance with respect to the austenitic stainless steel (Type 304 and Nitonic 60) rotating runner components against which the rings operate. Passive nickel-aluminum bronzes are galvanically compatible with several corrosion-resistant ferrous materials, including titanium and its alloys. Rings of the type illustrated here have achieved more than 20 years of service in the Hoover, Grand Coulee, and other dams, and in numerous hydroturbine installations around the world. (Photo courtesy MetalTek. Inc.)

Photo 6. Continuous cast gear-wheel blanks, aluminum bronze, alloy C95400. This alloy, like other aluminum bronzes, is known for its high as-cast strength (35 ksi, 221 MPa YS; 85 ksi, 586 MPa UTS) and favorable anti-wear properties. Note that the gear-wheel spokes are cast as part of the blank, thereby reducing the extent of finish machining operations. (Photo courtesy MetalTek. Inc.)

 

Aluminum Bronzes are Finding an Increasing Number of Applications

Table 1 Contains a list of typical products, both cast and wrought, manufactured from aluminum bronzes. It is by no means an exhaustive list as new applications are constantly being developed. Note that many applications are related to marine environments because of the outstanding performance of aluminum bronzes in seawater.

In addition, aluminum bronzes are extensively used as metal-sprayed or weld-deposited surfacing materials, generally over steel substrates, in order to provide wear, corrosion and sparking resistance.

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.

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 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.

• 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.

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.

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. Such alloys are quite brittle and are exclusively used as overlays.

Bearing and Frictional Properties

Aluminum bronzes are used as bearing materials and heavy-duty guides where other materials would fail quickly. They thrive on heavy loads, shocks and harsh working environments and so are found in steel works, in agricultural, mining and earthmoving equipment, and as bridge bearings and other structural and architectural applications. In rotating applications, best results are usually achieved when running aluminum bronzes against hardened surfaces. When lubrication of sliding surfaces is less than ideal, aluminum bronzes are superior to ferrous materials. Both wrought and cast alloys are used. Permanent mold (also known as gravity or low-pressure die casting) provides an excellent production method for the quantity production of such low-friction items as selector forks for gearboxes. Additional information about bronze sleeve bearings can be found in the Industrial section.

Shock Resistance

Aluminum bronze alloys, and in particular the wrought products, have excellent resistance to shock provided, as always, that the material is sound, and undue stress concentrations are avoided in design.

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.

Damping Capacity

Aluminum Bronzes are twice as effective as steel in their ability to dampen vibrations.

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. The magnetic permeability of certain aluminum bronzes is often less than 1.01 H/m, whereas that of austenitic stainless steels can be higher than this value if excessive ferrite is present.

Non-Sparking Characteristics

Excellent non-sparking characteristics make aluminum bronzes suitable for the manufacture of tools and equipment used in the handling of explosives, in mines, petroleum and chemical plants, gas-handling equipment and similar applications.

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.

Like many copper alloys, aluminum bronzes also 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 Bronze alloys may be considered for service with the following 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.

Freedom from Oxide Flaking

The outstanding corrosion resistance of aluminum bronzes in marine and chemical processing environments is due to the formation of an intrinsic, thin but tough adherent film of aluminum oxide. This film is self-healing and, once formed, prevents further oxidation and consequently eliminates the exfoliation of oxides so often encountered with ferrous alloys. This property, combined with the alloys' corrosion resistance and creep and fatigue properties at elevated temperature makes aluminum bronzes ideal for high temperature service. The aluminum bronzes have excellent resistance to stress corrosion cracking and corrosion fatigue; in fact, some alloys are especially chosen for these attributes. They are also rarely, if at all, susceptible to pitting and are generally far more resistant to selective attack than brasses.

Because strength is comparable to many ferrous alloys, it is possible, without radical re-design, to substitute aluminum bronzes where even mild corrosion of ferrous components is a potential problem. This applies particularly in critical areas of plant operation such as pumps and valves. Table 5 gives a comparison of the resistance to corrosion of various ferrous and non-ferrous alloys.

Corrosion rate = kt^-0.2

Impingement resistance: a maximum of 14 ft/s (4.3 m/s) is an appropriate design value for aluminum bronzes used in pipe systems carrying clean flowing seawater.

When Crevice corrosion occurs in nickel aluminum bronze, it takes the form of selective phase de-alloying and is usually of little practical significance since it has only a minimal effect on the surface finish. Crevice corrosion of austenitic stainless steels 304 and 316, although shallower, takes the form of pitting with consequent serious deterioration of surface finish.

The data in Table 5 are taken from U.K. Defense Standard 01/2 Guide to Engineering Alloys Used in Naval Service: Data Sheets. The figures for general corrosion rate and crevice corrosion were determined using samples fully immersed beneath rafts in Langstone Harbor for one year. General corrosion results were provided by freely exposed specimens; crevice corrosion results by specimens held in Perspex jigs providing crevice conditions between the metal sample and the Perspex. The corrosion/erosion resistance tests were carried out using the Brownsdon and Bannister test, the specimens were fully immersed in natural sea water and supported at 140°F (60°C) to a submerged jet, 0.016 in (0.4 mm) diameter placed 0.04-0.08 in (1-2 mm) away, through which air was forced at high velocity. The minimum sea water velocity required to produce erosion-corrosion under service conditions was estimated from the minimum air jet velocity required to produce erosion-corrosion in a fourteen-day test, and on the basis of known service behavior of some of the materials.

Cavitation Erosion

Nickel-aluminum bronze has a greater resistance to cavitation erosion than cast steel, Monel alloys and the 400 and 300 series stainless steels. By the same token, it has excellent resistance to impingement attack by gas bubbles. These characteristics make it particularly suitable for propellers, pump impellers, casings and turbine runners, enabling long service lives and optimum operating efficiency. Although more resistant to impingement attack by abrasive substances than gray cast iron and gunmetal, the aluminum bronzes are more vulnerable in this regard than cast steel and stainless steels. Filtration is sometimes necessary, therefore, in the case of high-speed pumps requiring the otherwise good corrosion and cavitation-erosion resistance properties of aluminum bronze but which have to handle water badly contaminated by suspended solids such as sand or other abrasives.

Tables 6 and 7 present comparisons of cavitation erosion of various ferrous and non-ferrous alloys in fresh water and in 3% NaCl (salt) solution.

• Aluminum Bronzes - Part II

Also in this Issue:

• Aluminum Bronzes - Part II
• Aluminum Bronzes - Part I

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