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5) PHYSICAL PROPERTIES OF METALS
What is a metal? Technically, it is an element which has the following properties: It is solid at room temperatures (mercury is an exception). / It is opaque (that is, you can't see through it). It conducts heat and electricity. It reflects light when polished.
It usually has a crystalline structure.
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Some metals - gold, silver, copper and zinc, for example, are often used in almost pure form. However, most metals used for industrial purposes are actually alloys, not pure metals. An alloy is a metal to which another metal (or metals) or a non-metallic clement such as carbon or silicon has been added tov modify the physical or mechanical properties of the pure metal. Iron, aluminium, titanium and magnesium are used mainly in an alloy form. Pure iron, in fact, is sometimes, of a laboratory curiosity. Steel and cast iron are jprofterly considered alloys of iron. Even thouuK a low-carbon steel may contain more than 99% iron, and not more than 0.3%
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carbon, that little amount" of carbon makes a lot of difference.
The physical properties of a material are properties not related to the ability of the material to withstand external mechanical forces such as pushing, pulling, twisting, bending... These properties include density, melting point, specific heat, thermal conductivity, thermal expansion, electrical conductivity.?
Density. The measure of a unit mass; or the weight of a unit volume. Density is expressed as grams per cubic centimetre (g/cm3), kilograms |>er cubic metre (kg/trr'), pounds per cubic foot (lb/ftJ). Density is often expressed as specific gravity, the ratio of the density of the material to the density of water. The specific gravity of aluminium is 2.70 - in other words, it is nearly three times as heavy as water. The specific gravity of iron is 7.86; for gold, the value fsf 19.3. /, /,....
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Melting Point. Every pure metal has a specific melting point. If you apply heat to a solid specimen, its temperature will rise until it reaches that melting point. It will then start to melt, and it will remain at the melting point temperature, even though the heating is continued, until the specimen is completely melted. Then, and only then, will the temperature of the liquid metal start to rise once move. -,
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Specific Heat. The amount of heat required^to^raise a unit mass of a solid metal one degree in temperature is termed specific heat. The ligliter the metal, the greater the specific heat. In other words, it takes more heat to raise the temperature of one kilogram of aluminium one degree than it takes to raise the temperature of one kilogram of iron one degree.
Thermal Conductivity is measured as the heat-conducting ability of a unit length or thickness of a substance per unit of a cross-sectional area. The relative heat conductivities of some metals and alloys are based on the conductivity of silver as 100%.
Thermal Expansion. The increase in dimensions of a solid body due to an increase in temperature is termed thermal expansion. This property is of much significance in welding operations, since the metal close to the weld zone is heated to a higher temperature and therefore expands more than the metal at a greater distance from the weld zone.
Electrical Conductivity. As stated earlier, a metal must be a conductor of electricity. Some are much better than others; generally, the metals which are the best conductors of heat, such as copper, silver and aluminium are also the best conductors of electricity.
