ALLOYS

Alloys are substances formed by the combination of two or more elements in any desired ratio out of which at least one is metal.

Alloys are obtained as results of melting two or more components. Certain physical and chemical properties such as a hardness,, tensile strength, resistance to corrosion of a metal can be changed by mixing it with a small quantity of one or more elements. Metals like Cu, Zn, Sn, Ni, Al, and Cr have been used for alloying.

NEED FOR ALLOYING

1.another or non-metal. For example, Au and Ag are alloyed with Cu to make them hard for making ornaments. The hardness of pure iron can be increased by alloying with carbon. Hence we can conclude that alloys are harder than the individual elements present in them.

2.To lower the melting point: The melting point of an alloy is lower than the individual element present in it. For example, the melting point of an alloy made of lead, tin; bismuth and cadmium called woods metal melts are higher than this temperature.

3.To resist corrosion: Metals in pure form get corroded easily since they are highly reactive to the gas and To increase the hardness of metal: The hardness of pure metals can be increased by alloying it with moisture in the surrounding. For example pure iron gets corroded where as stainless steel, alloys of iron, and resists corrosion. Hence alloys resist corrosion than pure metals.

4.To alter the chemical activity: The chemical activity of metal can be altered by alloying. For instance an alloy of Na with Hg is less active than Na.

BEARING ALLOYS

Bearing is used to give a rolling or sliding contact to support the load at the operating temperature with friction. Bearings can be either ball and roller bearings (rolling contact bearing) or bush bearing (sliding contact bearing)

Sn and Pb are two important bearing metals. However Sn is superior to lead because of its higher resistance to corrosion and thermal conductivity than lead.

Babbit metals are white metals which are alloys of either tin base or lead base alloys.

PROPERTIES OF BEARINGS ALLOYS

1. The alloy should have high compressive strength.
2. It should have low co-efficient of friction.
3. It should have melting point.
4. It should be able to withstand vibrations.
5. It should not get deformed at the operating temperature while an alloy of Al with Hg is more active than Al.
6. It should have high melting point and high thermal conductivity.
7. To improve the colour: an alloy has a different colour than that of the metal. Alloys of Cu and Al called aluminum bronze has an attractive golden yellow colour.

NON-FERROUS ALLOYS

As the name indicates these alloys do not have iron as one of the main constituents. Non-ferrous alloys are used for special purposes in engineering and technology.

BRASS: It is an alloy of copper and zinc. The property of this alloy varies with the percentage of Zn. The percentage of Zn is Red Brass is less than 20. It is used for plumbing pipes. Another variety of Brass named muntz metal contains 40% Zinc. It has greater tensile strength and lesser ductility. It is used for marine fitting fittings. Dutch metal contains Cu 80% and Zn 20%. It is golden yellow in colour and used for making jewellery, name plates and musical instruments.

BRONZE: Bronze is a copper tin based alloy. If Zn is added to it Gun metal is formed. Generally Bronze contains Cu-80-95%. Bronze is strong and is highly corrosive resistant. Coinage has a percentage composition of Cu-89-92% and Sn-11-8%. It is used in the manufacture of coins, statues and ornaments.

GUN METALS: Another variety of bronze is obtained by the addition of Zn to the general composition of Bronze. Hence its percentage composition is Cu-85%, Sn-5%, Zn-5% and Pb-5%. It is used as parts for water fillings, marine pumps and also in high pressure steam plants since it can resist the force of explosion. When phosphorous is added to Bronze, it is termed as phosphorus Bronze. Depending on the % of phosphorous in it, it is classified as low phosphorous Bronze and high phosphorous bronze. The % composition of low phosphorous bronze has a % composition of Sn-10-13%, P-0.4-1% and remaining is Cu.

Duralumin: It is a mainly an alloy of Aluminium. It has a percentage composition of 94% Al, 4% Cu and 0.5% each of Fe, Mn, Si, Mg. it is used for making nuts, bolts, tubes, sheets, non magnetic parts of instrumental and air ships.

Nichrome: It is a Nickel chromium alloy and has a composition of Ni-55-78%, Cr-5-23%, Mn-0.7-1.5%, Si-0.4-1.3%, Al-0.2%. it is highly chemical as well as electrical resistant. Hence it is used as heating elements in electric iron and other electrical appliances. This alloy is present in the form of heating elements inside a muffle furnace.

Solder: It is a tin lead based alloy. This contains tin and lead in various proportions. Depending on its proportions, its application varies. Tinmanns solder contain Sn-66%, Pb-34% and it is used for joining articles made of tin. Silver solders contain Ag-10-80%, Cu-15-42%, and Zn-4-48%. Soft solder a variety of solder has a percentage composition of Pb-37%, Sn-31-60% and Sb-0-0.12%. it has very low melting point and is used as soldering or joining electrical connections, lead pipes etc.