21. Study the texts and do different exercises.

I. Aluminium. (Read the text and make logical paragraphing of it.)

The silvery-white chemical element aluminium ranks among the most industrially important metals. It is the lightest structural metal and is highly ductile, capable of being cast, rolled, stamped, drawn, machined, or extruded. Moreover, it is corrosion resistant, heat reflective, and an excellent conductor of electricity. Although aluminium is soft and has relatively low tensile strength in its pure form, it can be made much harder and stronger if alloyed with copper, magnesium, or zinc. Aluminium is more widely used than any other metal except iron and steel. Pure aluminium metal is utilized in electronic components, reflectors, fine jewellery. It is also converted into a powder that can be mixed with other substances to produce metallic paints, rocket propellants, flares, and solders. Aluminium alloys have a far wider range of applications. Aluminium-copper alloys, which have mechanical properties superior to those of certain forms of steel, are employed extensively as structural components of buildings, aircraft, space satellites, railroad cars, and boats. In addition, the growing emphasis on improved fuel economy has stimulated the widespread use of these high-strength/low-weight alloys in the manufacture of automobiles and other motor vehicles. Aluminium alloyed with boron conducts electricity nearly as well as does copper; but it is much lighter, making it the preferred material for overhead transmission cables. Aluminium-manganese alloys exhibit exceptional resistance to weathering and corrosion and so are commonly used for siding, roofing, window frames, and other construction hardware as well as for storage tanks and highway signs. Aluminium-based magnesium alloys have many of the same properties plus superior weldability. The most important commercial applications include the manufacture of appliances and food and beverage packaging, principally in the form of foil wrappings and cans.

II. Types of Cast Iron. (Define grammar structures.)

Iron that is poured into a mould of sand or other material is called cast iron. Pig iron is a crude form of cast iron poured from the blast furnace. There are five major types of cast iron: white, malleable, gray, ductile, and high alloy. In general they contain 2 percent or more of carbon plus silicon.

Products or parts with complex shapes may be cast in one step, the pouring of molten metal into moulds. The cost of this procedure is much less than that of cutting the same items from a hard piece of metal with machine tools. This saving is one of the prime advantages of cast iron. Motor blocks, crankshafts, cogwheels, pipe fittings, valves, and guided-missile parts are some of the items that are made of cast iron.

White and gray cast irons are named for the general shade of the portion exposed when the iron is fractured. The whitish appearance is caused by substantial amounts of iron carbides. The gray tone is caused by particles of flake graphite. There is also a mottled cast iron, with distinct patches of white and gray.

The relatively high carbon content of cast iron lowers its melting point. The carbon also reduces the shrinkage of the metal during cooling and hardening and improves its machinability.