Alloys of Copper | Metals | Industries | Metallurgy

In this article we will discuss about the alloys of copper. Also learn about the fabricating characteristics, designation and Indian standard specification of copper alloys.

1. Copper-Zinc Alloys-Brasses:

These are very important alloys having high resistance to corrosion, easily machinable and acting as good bearing materials. The percentage of zinc varies from 5 to 45% in these alloys. However, commonly used alloys contain 57% to 70% of copper. Besides zinc, a little quantity of metals such as tin, manganese, aluminium, iron and lead are also added to give special properties to these alloys.

Brasses have greater strength than copper metal and by adding the appropriate quantities of constituent metal, mechanical properties such as ductility, strength, machinability, etc., can be changed according to the desired limits. For example, ductility of alloy increases by addition of zinc upto 37% but after it the ductility falls sharply. Machining properties can be improved considerably by including small amounts of lead.

Leaded brass bars are frequently used, to produce highly loaded components on automatic screw machines, because by addition of lead, tensile strength remains same, machinability improves, but shock resistance is lowered. Season cracking may occur with high zinc brasses but rarely with 15% zinc or below.

Cracking occurs spontaneously on exposure to atmospheric corrosion in brass objects with high residual tensile stresses at the surface. It may be prevented by avoiding the induction of internal macro stresses or by removing such stresses by relief annealing at 250 to 280°C without softening the work.

It should be noted that alloys susceptible to spontaneous season cracking, even if free from internal strains, will crack when exposed to corrosive conditions under high service stresses. Following types of brasses are very common.

(i) 60% Copper and 40% Zinc (Muntz metal):

It has high tensile strength and less ductility and is suitable for hot working by rolling, stamping or extruding, and used for certain marine fittings and pump parts. This alloy containing small quantities of other metals to modify the properties according to requirements is very useful for castings, hot stampings or extrusion and machining. So it is used for fittings, valves and other such parts.

Mn and Fe are added to increase tensile strength and resistance to salt corrosion. With addition of Mn, it is known as manganese bronze (typical composition: Cu-65%, Zn-35% and Mn-5%, and it can be rolled, drawn and cast; and is used for under water shafts and heavy duty bearings and gears.

(ii) 70% Copper and 30% Zinc:

It is strong, highly ductile alloy. It is very much useful for cold working. So it is largely used for making tubes, sheets and cold press work.

(iii) 70%, Copper, 29% Zinc and 1% Tin. (Admirality Brass):

It has high corrosion and abrasion resistance. So such a brass is used for marine work, and steam condenser tubes.

(iv) 76% Copper, 22% Zinc and 2% Aluminium:

Such an alloy is used for condenser tubes in marine and other installations as it is highly resistant to corrosion and impingement attack of sea water.

(v) 95% Cu, 5% Tin:

Used for worms, gears, pump bodies and bushes.

(vi) 5% to 20% Zinc alloys. (Known as red brass):

These find application because of freedom from season cracking due to their red colour. These have desirable melting point in brazing operation. Used for plumbing of pipe end connections, rivets, hardware etc.

(vii) Delta metal (Cu-55%, Zn-41%, Pb-2%, Fe-2%):

It is used for parts of marine engines, screw propellers, and in chemical, hydraulic and mining plants.

Brass is used for the manufacture of evaporators in sugar and salt industries as brass has got high corrosion resistance and heat conductivity. The addition of small quantities of tin to brass (as shown above) increases the corrosion and abrasion resistance of brass.

Addition of small quantity of aluminium to brass (as shown above) increases corrosion-resistance and resistance to impingement attack by sea water. Addition of lead to brass renders it free cutting and remarkably machinable. Addition of 0.75 to 1.25% tin improves the corrosion resistance.

Dezincification takes place in brasses containing high percentage of zinc and brass containing 85% or less of copper. Dezincification if visible by the formation of spongy areas of copper in the form of layers or plugs. Brass for springs should be rolled as hard as consistent with the requirements of subsequent forming operations.

For articles requiring sharp bends, or for deep-drawing operations, annealed brass must be used. All the brasses may be hot worked if they are free from lead, particularly those alloys containing less than 60% or less of copper. Extruded sections of many copper alloys are made in a wide variety of shapes.

2. Copper-Tin Alloys-Bronzes:

Tin content varies from 2 to 12% but generally bronzes having 5% tin and 10% tin are commonly used. Bronzes containing 5% tin are used in wrought form and those containing 10% tin are used in the cast condition. Addition of tin in bronze makes it harder and stronger alloy than brass. Bronze is formed by casting (brass is formed by working).

As the tin content of simple bronzes oxidises quickly, when the metal is hot, resulting in brittleness; it is usual to add various de-oxidisers like Zn or phosphorus. Phosphorus in small quantities (max. 4.0%) is added to bronzes and such bronzes are called phosphor bronzes. Addition of phosphorus improves the casting qualities and hardens the bronzes and increases the resilience. Phosphor bronzes are used for bearings, worm wheels, rods, sheets.

Bronzes can be readily cast and machined. So these are widely used for the manufacture of pumps, valves, flanges, etc., where resistance to corrosion is required.

If zinc (as de-oxidiser) is added to bronzes, gun metal is formed. A common composition of gun metal is 83% copper, 10% tin and 2% zinc. Gun metals are very useful for foundry work. Gun metal being highly resistant to corrosion, is used for marine fittings, valves and fittings for water and general service and other castings requiring good mechanical and corrosion-resisting properties.

Small quantities of other metals such as nickel and lead are also added to the copper-tin alloys in order to modify their mechanical or corrosion-resisting properties to make them useful for some special purposes. Engineer’s bronzes containing 88% Cu, 10% tin, and 2% Zn is used for engine parts, steam fittings and hydraulic machinery.

The aluminium bronzes with 5-8% aluminium find application because of their high strength and corrosion- resistance, and sometimes because of their golden colour. If aluminium content exceeds 10% then the bronze becomes very plastic when hot and has exceptionally high strength, particularly after heat treatment.

Bell metal is formed by adding 75-80% tin and such an alloy is used for making bells and gongs.

A number of alloys are made with silicon as the primary alloying agent and using appreciable amount of zinc, iron, tin, or manganese. These alloys are as corrosion- resistant as copper and possess excellent hot workability with high strength. These alloys are extensively fabricated by arc or acetylene welding like tanks and vessels for hot-water storage and chemical processing.

3. Copper-Aluminium Alloys-Aluminium Bronzes:

These alloys contain 5 to 11% of aluminium. The common alloys contain about 10% of aluminium. These have got high tensile strength, as high as 5600 kg/cm² when they are in cast state. By heat treatment tensile strength can be increased to over 6400 kg/cm². The heat treatment consists in heating the alloy to 850°C, quenching in water, reheating to about 650°C and then allowing it to cool slowly.

Small quantities of other metals such as iron, manganese and nickel are also added to aluminium bronzes in order to improve certain mechanical properties. Aluminium bronzes are highly corrosion-resistant due to the formation of protective film of aluminium oxide. These are resistant to action of acids also.

These make good castings when properly cast. These have got hot forging properties and mechanical strength. So these alloys are used for valves, pumps and for other parts where corrosive liquids act. These alloys are resistant to oxidation at high temperatures.

4. Copper-Nickel Alloys:

Even when small percentages of nickel arc added to copper there is a definite affect on the mechanical and physical properties.

The following are the effects produced in mechanical properties of copper when nickel is added to it:

(i) Copper-nickel alloys have high tensile strength. With increase in nickel content, there is increase in tensile strength up to 60 to 70% of nickel. However, it has been found that if the alloy is properly annealed, there is a very little difference in the tensile strength of alloys containing 50 to 80% nickel.

(ii) Copper-nickel alloys have high ductility and malleability. Alloy which has highest ductility contains 80% nickel and 20% copper. These may be worked extensively without annealing.

(iii) Such alloys retain their strength even at high temperature. The alloy which has maximum strength at high temperature contains approximately 70% nickel. This alloy has limiting creep stress of 3200 kg/cm² at a temperature of 400°C. This alloy has high resistance to shock also, having an impact value of 16 kg-m at 500°C.

(iv) Copper-nickel alloys show great resistance to corrosion. With increase in nickel-content, the resistance to corrosion increases. Such alloys resist the attack of alkalies like sodium hydroxide, sea water and also organic acids, and hydrochloric acid. However, these are prone to attack of nitric and sulphuric acids. These are used for condenser tubes for most severe service.

(v) Copper-nickel alloys are quite resistant to impingement corrosion called erosion. These being white in colour find applications as base for most silver-plated ware.

The following are some of the copper-nickel alloys. Copper-nickel alloys with 2, 20, 30, 45 and 68% nickel are commonly used.

Alloy containing about 2% of nickel is used for manufacture of wire for electrical purposes. This alloy has trade names such as Constantan, Eureka, Ferry, etc. This alloy has high specific resistance and has almost zero temperature coefficients. So such an alloy is used for making resistors for speed and voltage regulators.

This element is also used in thermocouple pyrometers in conjunction with copper. Copper-nickel alloy, to which 5 to 6% of zinc is added is used for making condenser tubes as by doing so resistance to corrosion increases.

Copper-nickel alloys are used for heat exchanger tubes in oil and other processing industries.

A common alloy known as copper-nickel containing 75% Cu and 25% Ni is used for coinage, casing of rifle bullets and condenser tubes.

5. Copper-Silicon Alloys:

Copper-silicon alloys have corrosion resisting properties of copper but physical properties and structural qualities are comparable with those of mild steel.

In copper-silicon alloys, the thermal and electrical conductivities are very much less than copper. But their strength is comparable to mild steel. Welded joints are highly resistant to corrosion. Copper-silicon alloys are used to manufacture bolts, screws, tie rods, etc. because of their high resistance to corrosion.

6. Copper-Beryllium Alloys:

These are expensive but their strength and elastic properties are far superior to other materials. These are ideal for springs, diaphragms and the like where high fatigue resistance under some-what corrosion conditions is needed. Cobalt provides a substitute for beryllium and produces a cheaper alloy of only slightly inferior properties.

7. Copper-Uranium Alloys:

This is used principally for electrodes in resistance welding machines. It has high conductivity of the order of 80% of that of copper and does not soften on prolonged heating at temperatures around 450°C. This alloy is also used in the strip form for electrical switch parts because of its high softening temperature.

Fabricating Characteristics of Copper Alloys:

Machinability Ratings of Various Copper Alloys:

Copper alloys are generally machined at as high cutting speed as possible with a light feed and moderate depth of cut. Sand cast alloys are either sand blasted or pickled in acid to remove abrasive surface scale or finished by high-speed tool with low speed and coarse feed to remove hard scale.

Free cutting brass (61.5% Cu, 35.25% Zinc and 3.25% Pb) is the most machinable non-ferrous alloy.

If it is given a machinability rating of 100% then other brasses and bronzes have machinability ratings as under:

Leaded commercial bronze—90%

Leaded naval brass, and Selenium Copper—80%

Admiralty brass, leaded phosphor bronze, and leaded nickel silver—50%

Yellow brass and tin bronze—40%

Manganese bronze—30%

Formability:

The forming qualities are dependent upon the composition of alloy, grain size, and temper. Alloys with more than 63% copper are easily press worked but with less than 63% copper, the beta phase which is brittle produces surface defects in the form of fracture or waviness. Pure copper is very ductile and gets work hardened less rapidly than brass or bronze.

Copper alloys with fine grains (0.01 mm diameter grain size) are very smooth after forming operation and can be buffed to a high luster. As grain size increases, the ductility also increases. For deep drawing and heavy drawing, bigger grain size (0.05 to 0.12 mm) is preferred.

Weldability:

Copper can be welded by gas, metal arc, carbon arc or gas-shielded arc. It is also joined by brazing or ultrasonic welding.

Castability:

Copper can be cast in sand mould. Copper alloys are also easily cast and handled in foundry.

Designation of Copper and Its Alloys as per IS: 2378-1974:

Copper is designated by Cu followed by group of symbols like CATH, ETP, FRHC, DPH for cathode copper, electrolytic tough pitch copper, fine refined high conductivity copper, and phosphorised high residual phosphorus non- arsenical grade respectively.

Depending on method of casting, following symbols are used:

G – for sand cast, GD – for die cast, GK – for chill cast, GZ – for continuous cast, and no symbol for wrought form.

If nominal or average alloy content is upto 1%, then index number for copper alloys is designated by alloy symbols in the descending order of % age content. Average alloy can be expressed upto one decimal place, the decimal digit being underlined by a bar.

If nominal or average alloy content is more than 1%, then index number for copper alloys is designated by average or nominal alloying content being rounded to the nearest whole number.

As above, one decimal digit can be used. Surface finish and heat treatment are designated as:

a – annealed, d — cold worked, e – extruded, f- forged, H -full hard, 1/4 H – Quarter hard, 1/2 H – half hard, 3/4 H – three fourth hard, J – bright rolled as drawn, J7 – bright pickled, J8 – bright annealed, M – machined.

Indian Standard Specifications Relating to Copper and its Alloys:

IS: 28 Phosphor cast bronze.

IS: 191 Pure copper (Min. 99.3% copper)

IS: 291 Naval wrought brasses.

IS: 292 Brass castings.

IS: 304 High tensile cast brasses.

IS: 305 Cast aluminum phosphor bronzes.

IS: 306 Cast tin phosphor bronzes.

IS: 319 Free cutting bars, rods and sections made of wrought naval brasses.

IS: 407 Tubes of wrought naval brasses.

IS: 410 Cold rolled sheet strip and foil of wrought naval brasses.

IS: 531 Wrought leaded brass instrument parts.

IS: 1028 Cast silicon phosphor bronzes.

IS: 1958 Cast phosphor bronzes for Railways.

IS: 1972 Pure copper plate, sheet and strip.

IS: 2283 Wrought nickel silver sheet and strip.

IS: 2501 Copper Tubes.

IS: 2704 Wrought brass wires for cold headed and machined parts.

IS: 2826 Wrought copper alloy rods and bars.

IS: 3051 Wrought copper alloy plates.

IS: 3052 Wrought copper alloy sheet, strip and foil.

IS: 3487 Copper strip and foil for gasket washers and eyelets.

IS: 3488 Wrought brass bars, rods and sections for forgings.

IS: 4076 Wrought brass wires for springs and special purposes.

IS: 4131 Cast nickel copper castings.

IS: 4170 Wrought brass rods for general engineering.

IS: 4171 Copper rolls.

IS: 4412 Copper wires.

IS: 4413 Wrought brass wires for general engineering.

IS: 5493 Wrought copper tubes.

IS: 5494 Wrought copper tube well strainers.

IS: 6912 Copper forgings.

IS: 7608 Wrought bronze wires.

IS: 7811 Wrought bronze rods and bars.

IS: 7814 Wrought bronze sheet strip and foil.

IS: 8328 Copper free cutting bars, rods and sections.

IS: 8362 Copper rolled plates for condensers and heat exchangers.