Non-ferrous metals and alloys
Non-ferrous metals and alloys do not contain any significant quantity of iron. The most common nonferrous metals used in engineering applications are copper, aluminium, tin, lead and zinc. Nickel, magnesium and antimony are also used for alloying the aforesaid non-ferrous metals.
PROPERTIES AND USES OF NON-FERROUS METALS
Copper is a corrosion resistant metal of an attractive reddish brown colour. It is an extremely good conductor of heat and electricity. It can also be drawn in wires, beaten into sheets and plates. Hence, it is extensively used in electrical industry for making armature coils, field coils, current carrying wires, household utensils etc. But its great usefulness lies in the fact that it alloys with zinc, tin and nickel to yield brass, bronze and cupro-nickels respectively which are widely used in engineering industry.
Copper, as such, is used for many decorative items.
Not much of copper is available in India. We import at least 50–60% of our requirement every year.
Aluminium metal is difficult to extract from its main ore called bauxite. However, bauxite is available in India very plentifully and we have a thriving aluminium industry. Aluminium is also very corrosion resistant (because an adherent oxide layer protects it from further oxidation). It is again a very good conductor of heat and electricity (although not as good as Cu). It is ductile and malleable and is much cheaper than copper. Hence, it has all but replaced copper wires for transmission of electricity. It is also used for household utensils including pressure cookers. However, since it can be converted into thin foils, it is now extensively used for beverage cans and in packaging industry. Its density is about a third of steel, hence it is also used for aircraft and helicopter frames and in transport vehicles.
Sometime ago, in India, 1, 2, 5, 10 and 20 paisa coins were made of an aluminium-magnesium alloy. Aluminium forms a series of alloys with magnesium, which are harder and stronger than pure aluminium.
It has an attractive silvery white colour. It has very good resistance to acid corrosion.
Before the advent of plastic tin coated steel sheets of thin gauge were used for manufacture of tin-containers for storage of ghee, mustard and other oils. Today tin is mostly used for alloying purposes.
Tin and lead melted together give a series of soft-solders. Tin has a low melting point.
Lead is a heavy metal with dull grey appearance. It has good corrosion resistance and has got good malleability. In Europe, it was extensively used for roof protection. It was also used in plumbing.
It can withstand sulphuric acid and previously this acid used to be stored in lead lined vessels. It has self lubricating properties. It was therefore used in lead-pencils.
Sometimes, a small quantity of lead is added to steel and tin bronze to impart free cutting properties.
Zinc possesses a bluish grey metallic appearance. It has high corrosion resistance. In fact, steel sheets are often covered by a thin coating of zinc. Such zinc coated sheet are known as galvanised iron sheets (G.I. sheets). The zinc coating provides protection to steel sheets from corrosion for many years.
Zinc has a low melting point and high fluidity making it suitable for items to be produced by diecasting process. Zinc is incidentally much cheaper than either copper or tin; making brass, an alloy of copper and zinc much cheaper than copper or tin-bronze. Zinc is also used in torch light batteries.
In the following table, colour, tensile strength, melting point–specific gravity and important properties of some non-ferrous metals are given.
Note: For comparison, tensile strength of Iron is 270 N/mm2.
ALLOYS OF COPPER
Brass is an alloy of copper and zinc. Commercially, two types of brasses are most important:
1. Alpha brass. It contains up to 36% zinc and remainder is copper.
2. Alpha-Beta brass. It contains from 36% to 46% Zn, remainder is copper.
Alpha and Beta are names given to different phases of brasses. Alpha-Beta brass contain both alpha and beta phases.
The tensile strength and ductility of brass both increase with increasing Zn content up to 30% zinc. If zinc content increases beyond 30%, the tensile strength continues to increase up to 45% Zn, but there is a marked drop in ductility of brasses. β-phase is much harder and stronger but less ductile than α-phase. α-phase has excellent cold-formability and is used where the parts are wrought to shape.
The mechanical properties of α-brasses also change with the amount of cold-work done on them. α-β brasses are fit for hot working.
α-brasses can be sub-divided into two groups—
(i) red-brasses containing up to 20% Zn, and
(ii) yellow brasses containing over 20% Zn.
Red brasses are more expensive and are primarily used where their colour, greater corrosion resistance or workability are distinct advantages. They have good casting and machining properties and are also weldable. One well-known red-brass is ‘‘gilding-brass’’ or gilding metal with 5% Zn. It is used for decorative work.
Yellow brasses are most ductile and are used for jobs requiring most severe cold forging operations. The cartridges are made from a 70% Cu, 30% Zn brass by a deep drawing process, hence this composition of yellow brass has come to be known as cartridge brass.
Other famous compositions of brasses are:
containing 29% Zn, 1% Tin, remaining copper.
contains 40–45% Zn, remainder is copper.
contains 39% Zn, 1% Tin, remainder is copper.
Admiralty brass, naval brass and muntz metal are all used for ships-fittings, condenser tubes, preheaters, heat exchangers etc.
Bronze is an alloy of copper and tin although commercial bronzes may contain other elements besides tin. In fact, alloys of copper with aluminium, silicon and beryllium, which may contain no tin are also known as bronzes.
Tin bronzes are of a beautiful golden colour. As in brasses, both tensile strength and ductility of bronzes increase with increases in tin content. However, more that 10% tin is not used in bronze as it results in the formation of a brittle intermetallic compound, Cu3Sn. Addition of tin to copper up to 10% increases the strength, hardness and durability to a much greater extent than the addition of zinc to copper.
The following varieties of tin bronzes are commonly used:
Addition of 0.5% phosphorous to tin bronze results in production of phosphorous bronze. Phosphorous increases fluidity of molten metal and fine castings can be made.
Addition of lead to tin bronze, results in production of leaded bronze. Lead is actually a source of weakness, but adds to machinability and has self lubricating properties.
Usually, lead percentage does not exceed 2%.
It contains 2% zinc, 10% tin and 88% copper. It is a very famous composition.
This bronze is used for bearing bushes, glands, pumps, valves etc.
It is a tin bronze but having a very high percentage of tin (20–25%). It gives a good tinkling sound on being struck with a hammer.
Bronzes having no Tin.
The following bronzes contain no tin and are commercially well-known:
(i) Aluminium bronze.
Composition: 14% Aluminium, rest copper. It possesses good strength and good corrosion resistance. Colour: golden yellow. Often used for costume jewellery.
(ii) Silicon bronze.
Composition: 1–4% Silicon, rest mainly copper. Possesses extremely good corrosion resistance. Can be cold worked and strain-hardened. Used for boiler fitting and marine fittings.
(iii) Manganese bronze.
Composition: 40% zinc and 55–60% copper with 3–5% manganese. It is essentially a brass to which manganese has been added. It is used for ship’s propellers.
(iv) Beryllium bronze.
Beryllium is very costly. So is this alloy. It contains about 2% Be. It has very good mechanical properties and can be cold worked and age-hardened. It is mainly used for bellows, bourdon gauge tubes etc.
Cupro-nickels are alloys of copper and nickel. Copper and nickel, when melted together in any proportion are perfectly miscible and dissolve each other. When the alloy solidifies, the solubility continues forming a solid solution.
Cupro-nickels are silvery white in colour and have extremely good corrosion-resistance. They are extensively used for marine fittings. They also possess good strength, hardness and ductility. Coins of rupee five are made of 75% copper and 25% nickel. However, another alloy containing 45% Ni and 55% copper is called ‘‘constantan’’. It is used for manufacture of thermocouples, low temperature heaters and resistors.
Aluminium as such is a soft metal of relatively low strength. Most of the alloys of aluminium are made by alloying it with various percentages of magnesium; these are harder and stronger. These alloys known as L-M series alloys can be extruded and are used extensively for structural work.
A famous alloy of Aluminium containing 4% copper, 0.5% magnesium, 0.5% manganese, a trace of iron and rest aluminium is called DURALUMIN. It has high strength and a low specific gravity. However, its corrosion resistance is much lower as compared to pure aluminium. Sometimes, duralumin is covered or clad by thin aluminium layer on all sides. Such material is called ALCLAD and is used in aircraft industry.
If 5–15% silicon is alloyed with aluminium, we get alloys which are temperature resistant. Castings made of Al-Si alloys are used for manufacture of pistons of two wheelers on a large scale.
ALLOYS OF NICKEL
(i) German silver.
It is a cupro nickel to which zinc has been added. A typical composition is 60% copper, 30% nickel and 10% zinc. Addition of zinc brings down the cost. Its colour is silvery with a slight pale tinge. It is very ductile and malleable and corrosion resistant. It is used for making electrical contacts, costume jewellery and high quality taps etc. Before the advent of stainless steel, it was also used for household utensils and coinage.
(ii) Monel metal.
Its composition is 68% nickel, 30% copper, 1% iron, remainder manganese etc.
Alloy of nickel and chromium, which is used as heat resistant electrical wire in furnaces, and electrical heating devices like geysers, electric iron etc.
(iv) Inconel and incoloy.
Alloys principally containing, nickel, chromium and iron. Used in electrical industry.