High Strength Casting Copper Alloys

Aluminum Bronzes

Structurally, alloys such as AB1 consist of two principal phases, alpha and beta, the iron addition appearing as a separate minor phase, while those alloys such as AB2, containing nickel and iron, have more complex structures.

Aluminum bronzes AB1 are alloys of medium strength, with good ductility and resistance to shock; they also retain a useful proportion of their strength at elevated temperatures. Corrosion resistance is reasonable, but dependent on casting section and rate of cooling as well as environment.

The complex alloys are alloys of much higher strength, with superior corrosion resistance and good resistance to erosion, fatigue and corrosion fatigue. These alloys also retain well their properties at elevated temperatures with a good resistance to creep at temperatures up to 400°C, and have useful strength at considerably higher temperatures when creep resistance is not of over-riding importance. The complex alloys have properties which make it frequently possible to use them instead of stainless steel, with the added advantage that they are more suitable for the production of castings.

Both types of alloys are suitable for both die-casting and sand casting. The alloy AB1 is more suitable for the economic production of die-castings and is, therefore, more normally used if its properties are adequate for the service requirements.

These alloys can be welded and are, therefore, suitable for castings which have to be included in fabrications.

Because of their inherent resistance to corrosion these alloys find wide application in marine engineering and in the petroleum, oil and chemical industries. They are used in engineering generally for their high strength, resistance to fatigue and wear resistance.

In the sand casting field, particular reference might be made to pressure-tight castings, such as for pump work, particularly where resistance to erosion is required, for water turbine impellers and casings, marine castings generally and gearwheels, when loads are heavy and speeds slow, or where high resistance to shock is required.

Die-castings are widely used in the automobile industry, for cars, commercial vehicles, tractors, heavy military vehicles and motorcycles. Typical applications include gearbox parts, clutch shoes, rocker brackets, impellers, tractor glands, locking bars, housings for gear levers, bearing cages and hot spot tongues. Other items are cylinders for heavy duty hydraulic brakes and mounting brackets. In marine engineering the main applications are buckets, pistons for feed water pumps, clutch plates, glands, nuts, seats, and sleeves for valves, and various components for sanitary fittings. In the aircraft industry components are supplied for radar equipment, stowage buckles, and high pressure oil feed buckets, but the use of the alloy is limited because of its high specific gravity.

Aluminum bronze die-castings are also finding application in atomic engineering for heavy components for high duty valve gears; they are widely used in electrical engineering for brush boxes, switchgear components and coal mine switch and control gear. In general engineering the applications are too numerous for a representative list to be given, but they include examples such as components for bottle-washing machinery, papermaking machinery, printing presses, sight feed lubricators, flue gas recorders, metal window furniture and bevel gears.

Manganese Bronzes (High Tensile Brasses)

The structure of the brasses (copper-zinc-alloys) containing up to about 36.5% zinc, consists of a single phase, alpha. With more than 36.5% zinc a second phase, beta, is formed which increases in proportion as the zinc increases, until at about 46.5% zinc the alloy consists of the beta phase only.

Some of the added elements (e.g. aluminum and tin) have a considerable influence on the structure in that they act in equivalence to zinc and alter the proportion of the alpha and beta phases; others, particularly manganese and iron, can, if present in sufficient quantities, lead to the formation of separate phases. Normally, the amount of manganese added is not sufficient to form a separate phase, but it is important that sufficient iron is present to produce the iron-rich phase which is essential to proper grain refinement.

The high tensile brasses which are in general use, i.e. HTB1 and HTB2, have alpha-beta structures, throughout which is distributed the complex iron-rich phase.

There is a third higher strength alloy, HTB3, which is of all-beta structure (also with the grain refining phase) which is susceptible to stress corrosion cracking, as are all the all-beta alloys.

It will probably be observed that the specified composition limits for these alloys are widely set; they have been developed during the last 70 years, often for specific applications and it is important that a particular alloy should have a suitable proportion of copper and zinc, together with a combination of added elements designed to give the tensile strength and other characteristics which might be required, such as resistance to sea water corrosion. To obtain the varied properties which may be required wide ranges of composition are permitted by the specification and it is strongly recommended that castings in these alloys should be obtained only from those foundries which have the necessary experience in their manufacture.

Whilst these alloys have excellent strength characteristics, which can be superior to those of the aluminum bronzes (particularly with regard to proof stress), the fatigue and corrosion fatigue values cover a considerable range from not quite as good as aluminum bronze to very much inferior. These alloys are characterized by a rapid falling in properties with increase in temperature and should, therefore, not be used for applications involving temperatures in excess of 150°C.

These alloys can be tinned although some compositions are more suitable than others. They are all slightly ferro-magnetic to varying degrees. HTB1 is suitable for sand and die-casting; HTB2 and HTB3 can be sand cast only.

Typical applications for which high tensile brasses HTB1 and HTB2 are employed are components highly stressed at normal temperatures, marine propellers and cones, rudders and rudder posts, gun mountings, hydraulic equipment, water turbine equipment, locomotive axle boxes, pump casings and marine castings and fittings.

The high strength all-beta alloy, HTB3, is used for heavy rolling mill housing nuts, rolling-mill slipper castings, and spur and gear wheels which are heavily loaded and slow moving. The warning is repeated that HTB3 is susceptible to stress-corrosion cracking, that is, if it is subjected simultaneously to the influence of stress and corrosion (including certain atmospheres) it is liable to crack and for this reason great caution must be exercised in its use.