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Tuesday, 23 July 2019

Alloys of Copper and their Uses

Some of the alloys of copper are:


(a) Cadmium copper alloy
(b) Chromium copper alloy
(c) Brass         
(d) Bronze

Cadmium Copper Alloy : 

Cadmium is whitish in colour and similar to tin in appearance. It is present in traces with zinc ore. It is corrosion resistance. When maximum electrical conductivity in conjunction with high tensile strength is required, the alloy of cadmium copper with 0.55% to 1.04% of cadmium is used. The tensile strength of this type of alloy is about 65 kgf / sq. mm with 1% cadmium, the conductivity falls only to 94% to that of pure copper. For most purposes the alloy containing about 0.8% of cadmium is found to have most favourable combination of tensile and fatigue strength with good conductivity. The fatigue strength of alloy increases which is almost proportional to the cadmium content, but without the increase of brittleness. This is suitable for flexible telephone cords and trolley wires. This is also used as electrodes due to its hardness and high annealing temperature. This alloy is also used for switchgear contacts, commutator segments and conductor for overhead transmission lines. It is costlier than copper.



Chromium Copper Alloy :

Chromium is a hard and white metal. Where optimum conditions of a conductivity, strength and hardness in a casting are required, the chromium copper alloy is ideal. By simple heat treatment of alloy, the hardness can be increased to 100, tensile strength more than 40 kgf / sq.mm and conductivity 80% that of pure copper can be achieved. At higher temperatures, chromium copper alloys are far superior to any other copper alloys.

Brass :

Brass is an alloy of copper (Cu 60% to 80%) and zinc (40% to 20%). It has greater mechanical strength and wear resistant than copper but has considerably lower conductivity. It is malleable and ductile, can be casted, and is resistant to corrosion. It is used as current carrying structural material in plug points, socket outlets, switches, lamp holders, fuse holders, knife switches, sliding contacts for starters and rheostats etc. Its melting point is lower than copper. Some of the physical constants of Brass is shown in Table 2.1. The amount of copper in brass varies its properties.


Percentage Composition

Cu 60 + Zn 38 + Pb 1 + Sn 1
Cu 67 + Zn 29 + Pb 3 + Sn 1
Yellow Brass
Cu 66 + Zn 34
Specific Gravity
8.3
8.3
8.3
Young’s Modulus (kg/sq mm)
9800
9500
7300
Ultimate tensile strength  (kg/sq mm)
56
48
54
Hardness
65
50
62.7
Resistivity at 20o C in micro ohm-cm
7.5
6.78
6.29
Melting point (0o C)
890
840
930












When the alloy has 70% copper, it bears a clear golden yellow colour and is known as yellow brass or high brass, 80% of copper changes the colour to reddish and the metal becomes softer with more copper characteristics. Addition of zinc makes metal tougher and stronger.

When the alloy contains 57 to 63% of copper and 43 to 37% of zinc, it is called MUNITZ metal. The metal is malleable, ductile, hard and corrosion resistant and is used for bolts, nuts, rods and tubes. Lead is added for better machinability. Tin or nickel improves the resistance to corrosion or wear. Welding rods, condensers, springs, bases and caps of valves are some of the application of brass.



Bronze :

Bronze is an alloy of copper and tin. Other materials like phosphorous, silicon, aluminium, berilium are alloyed with copper and is also called Bronze. The physical constants of a typical Bronze is shown in Table.

TABLE - Physical Constants of a Typical Bronze


Cu 88 + Sn 10 + P 2
Temperature Co-efficient of resistance at 0o C
0.0005
Resistivity at 0o C
17.8 micro ohm-cm
Specific gravity
8.89
Ultimate tensile strength
70 kg/sq mm
Young’s Modulus
9400 – 11000  kg/sq mm
Melting point (0o C)
950o C









Phosphorous Bronze: It contains 10 to 15% of tin and upto 0.5% phosphorous. It has high tensile strength and elasticity but fairly low conductivity. Its resistivity is 6 to 12 micro ohms-cm and temperature co-efficient at 0°C is 10 x 10-4 /degree centigrade. This resistance to corrosion is due to dissimilar metals. Although the impurities of metals in copper decreases the conductivity, the conductivity of bronze containing 2% iron and 0.5% phosphorous can be increased to more than 90% by correct heat treatment. This is used for making current carrying springs, brush holders, knife switch blades etc.

Berilium Bronze: It is another alloy whose mechanical strength is higher than cadmium bronze and is also used for making current carrying springs, sliding contacts, knife switch blades etc.

Silicon Bronze: It contains 90 to 96% of copper, 3 to 5% silicon, 0.5 to 2% manganese or zinc. It is resistant to corrosion and contains chemicals also. Soldering and brazing are not possible on this. This has more electrical conductivity than phosphorous bronze. This has very good tensile strength. It is used for boiler parts, aerial wires and spring materials.

Aluminium Bronze: It contains 85 to 90% of copper, 6 to 8% aluminium, 3% iron and 0.5% tin. It has beautiful golden colour. It is light, strong and resistant to oxygen and chemical actions. It is brittle, strong, ductile and shock proof. It is used for gear drives, sliding parts, springs, brush holder frames, die castings, parts coming in touch with saline or sea water.

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