Copper has a pleasing colour, but the most important properties of copper are its high electrical and thermal conductivity, good corrosion resistance, strength, machinability with ease of fabrication, and is non-magnetic. Copper finds applications as electrical conductors, automobile radiators, gaskets, kettles, vats, pressure vessels, fuel and oil lines, in electronic tubes, contact pins, switch gears, relays, condensers, electric motors for rail road and aircrafts.
Copper finds large applications in the form of its alloys:
1. Brasses:
These are alloys of copper and zinc.
ADVERTISEMENTS:
(a) Alpha Brasses:
These are copper alloys having zinc up to 36%. Cartridge brass (70/30Cu/Zn), is one of the important alloys.
(b) Alpha + beta brasses having more than 36% to 50% Zn. Muntz metal (60/40) is an important alloy.
2. Bronzes:
ADVERTISEMENTS:
Alloys of copper with any other metal except zinc are called bronzes. Though bronze are commonly means copper-tin alloys.
Bronzes have up to 12% of alloying element in copper:
(a) Tin bronzes
(b) Silicon bronzes
ADVERTISEMENTS:
(c) Aluminium bronzes
(d) Beryllium bronzes
(e) Phosphor bronzes
3. Cupronickels:
ADVERTISEMENTS:
These are alloys of copper and nickel. Cu-Ni phase diagram shows complete solid solubility.
4. Nickel Silvers:
These are copper, nickel and zinc alloys, also called German silver.
Heat Treatment Processes for Copper Alloys:
Copper alloys are given the following heat treatment:
ADVERTISEMENTS:
(a) Homogenising
(b) Annealing
(c) Stress relieving
(d) Precipitation hardening
(a) Homogenising:
It consists of heating to a high temperature (depending on the composition of the alloy, nature of alloy, etc.) for a long period of time (which also depends on section size, type and nature of alloy) to remove, or reduce segregation of castings, particularly which are to be hot or cold worked.
Coring in brasses can be normally eliminated by ordinary process annealing. As diffusion is a slower process in the tin-bronzes, silicon bronzes, and cupronickels, coring can be removed by prolonged homogenising treatments.
Phosphor bronzes (particularly having > 8% Sn) in cast state are heavily segregated and cored. Homogenising at 760°C, or so for 3-10 hours eliminates the segregation. These alloys can be then, even cold worked.
(b) Annealing:
Annealing is basically a recrystallisation annealing process of cold worked metal. It is thus heating to a temperature to induce recrystallisation and if necessary to a higher temperature to allow grain growth to occur.
Normally (α + β) alloys and precipitation hardenable type of alloys may be cooled fast. Even copper is quenched in water. The amount of cold works of metal, its grain size, impurities in it etc. all effect the grain size and mechanical properties obtained after proper recrystallisation annealing.
Copper alloys, when a series of intermittent annealings are required, are usually annealed at successively lower temperatures as the metal approaches the final anneal after final cold reduction of 50-60%.
The initial higher temperatures accelerate homogenising and the resulting coarse grains (thus, higher ductility) induce economic reduction. Grain size should be controlled to be made fine and uniform at later stages. Heavy reductions and lower annealing temperature produce fine and uniform grain size. Fine grains produce good final press-formed-products.
(c) Stress Relieving:
The aim of this annealing is to reduce, or remove residual stresses. Residual stresses cause stress corrosion cracking in brasses (Zn > 20%), Al-bronzes, Si-bronzes, and all copper alloys. Even phosphor bronze and cupronickels too suffer from such defects but milder cracking occurs.
The treatment consists of heating to lowest temperature for a long time to relieve these residual stresses. If possible, corrosive atmosphere should be avoided during the treatment. Stress relieving does not decrease strength or hardness rather, slight increment in these properties may occur. Stresses of welded, or cold formed parts may be relieved by heating to 80 to 110°C more than their normal stress relieving temperatures.
(d) Precipitation Hardening:
The important copper alloys which respond to age hardening are- Beryllium copper, aluminium bronze, chromium copper, zirconium copper, copper-nickel-silicon and copper-nickel-phosphorus alloys.
Al-bronzes containing more than 10% Al undergo martensitic hardening on quenching, which is then tempered, Table 14.18 gives ageing cycles and properties attained of some beryllium copper castings.
Heat Treatment of Brasses:
1. Alpha-Brasses:
As for pure copper, the single-phase alpha brasses are given only the recrystallisation annealing treatment. It consists of heating the cold worked brasses up to 700 to 730°C. The original ductility and toughness are restored. Careful control of atmosphere of the furnace for annealing is needed to avoid excessive oxidation and discoloration of the surfaces.
A low temperature annealing treatment may be given to cold worked brass-objects to prevent cracks being developed spontaneously, without any external stress from outside. Cracks are seen in components being kept in stores, such as brass caps, cartridge cases for rifles, caps for incandescent lamps, pipes for air-coolers, etc.
This cracking takes place in corrosive atmospheres, even a very weak ordinary industrial atmosphere containing small amounts of ammonia and sulphur dioxide. The crack propagates along the grain boundaries. The presence of the residual stresses along-with even a mild corrosive atmosphere causes this cracking, called season cracking.
It is difficult to reduce this tendency of brasses to season cracking by annealing from industrial practice point of view. The reason is, that to avoid this cracking, the internal stresses are to be removed completely, and to remove the internal stresses completely, the brass has to be annealed well above 300°C, which is also the recrystallisation temperature of the brass.
As the recrystallisation takes place, the hardness (for which the cold working was also done) of the brass becomes less. If this decreased hardness can be tolerated in the applications during service, then brasses can be annealed at 300°C for one hour, in order to relieve residual stresses and thus reduce the tendency to season cracking. After annealing, the articles may be cooled at any rate.
2. Alpha-Beta Brasses:
Cold worked alpha-beta brasses can be given the recrystallisation annealing. But these are mainly hot worked brasses.
Hardening can be by heating the alpha + beta brasses to a temperature of 850°C, and then cooled fast to room temperature. The brass is then tempered. Fig. 14.3 illustrates the change in hardness during tempering. Hardness has a peak when tempered at 300°C.
Heat Treatment of Bronzes:
Recrystallisation annealing can be given to most of the wrought bronzes. Beryllium bronze containing 2% or 2.5% beryllium is given precipitation hardening treatment as it shows decrease of solid solubility with the fall of temperature. The beryllium bronze is solutionised between 760 to 780°C and then quenched fast in water. Ageing is done between 310 to 330°C for 2 to 2½ hours. This increases the strength and hardness of the alloys. Hardness obtained is 320 to 400 VPN.
Beryllium bronzes are used for making non-sparking tools and instruments for high temperatures and corrosive conditions. These alloys have fairly good electrical conductivity and corrosion resistance. These are used for spring contacts exposed to high temperatures. Table 14.18 gives the composition, heat treatment of such cast alloys.
Heat Treatment of Zirconium Copper:
An alloy (99.7 Cu, 0.13-0.30 Zr) can be age hardened to get some increase in strength, primarily due to cold work before ageing, but the important effect of ageing is the increase in electrical conductivity.
The following treatment produces best combination of mechanical strength and electrical conductivity:
Heat Treatment of Aluminum Bronzes:
I. Alpha Single Phase Aluminium Bronze (up to 10% Al):
These alloys can be strengthened only by cold working, and can be made soft by recrystallisation annealing at 425°C to 760°C.
The non-age hardenable single phase alpha can be made age-hardenable by adding cobalt, titanium and nickel.
The following table gives properties of two such alloys:
II. Two-Phase Alloys:
Copper with 9.5 to 16% aluminium shows two phases at room temperature, and there occurs in this range, eutectoid reaction. Considerable hardening is obtained by ‘martensitic’ hardening and tempering (just as in steels). Here, high temperature phase, beta on quenching transforms to martensite. Some other alloying elements increase the range of Al from 8 to 14%.
Following table gives treatment and properties of two such cast alloys:
