Answer the following questions:
1. What specific gravity has magnesium?
2. What is the melting point of magnesium?
3. Why is pure magnesium not employed in engineering?
4. What are the principal alloying elements in magnesium alloys?
5. How much aluminium is added to magnesium?
6. How much zinc is added to magnesium?
7. How much manganese is added to magnesium? 8. For what purpose are small additions of cerium, zirconium and beryllium added to magnesium?
COPPER AND ITS ALLOYS
Copper is a valuable metal. Its wide application in many fields of engineering is due to its exceptionally high electrical and thermal conductivity, low oxidisability, good ductility and to the fact that it is the basis of the important industrial alloys, brass and bronze.
The raw materials for the production of copper are sulphide or oxide copper ores. Most of the copper is smelted from sulphide ores (about 80 per cent) while oxide ores account for only 15 to 20 per cent. Sulphide ores are more wide-spread in nature due to the higher affinity of copper for sulphur than for oxygen.
The most abundant copper sulphide ore is copper pyrite containing the mineral chalcopyrite (Cu2Fe2S4). In some cases, the so-called copper glance is used; it contains the mineral chalcocite (Cu2S). All copper ores are very lean as they contain only from 1 to 5% Cu. Therefore, before smelting they must be concentrated by flotation. Flotation converts lean copper sulphide ores into a concentrate containing from 15 to 20% Cu.
Before smelting, the copper concentrate and rich copper sulphide ores are subjected to an oxidising roasting process at 600—900° C thereby part of the sulphur is removed in the form of a gas. This gas is trapped and utilised in the production of sulphuric acid.
Various grades of copper are used for engineering purposes. It must be noted that even a minute amount of impurities sharply alters the properties of pure copper.
The mechanical strength of pure copper is not high and depends upon the degree of deformation (reduction in working). Pure copper is used chiefly for electrical engineering products such as cables, busbars and wire.
The copper alloys are more widely employed. The alloying of copper with other elements increases the strength of the metal in some cases and improves the anticorrosive and antifriction properties in others. Copper alloys comprise two main groups — brasses and bronzes. Alloys of copper and zinc are called brasses. The addition of appreciable amount of tin, nickel, manganese, aluminium and other elements to copper-zinc alloys imparts higher hardness, strength and other desirable qualities. Complex copper-zinc alloys comprising three, four or more components are special brasses.
In Russia brasses are identified by means of the Russian letter Л (the first of the Russian word for brass) followed by letters designating the chief elements and numbers which indicate percentage content of these elements. Thus, grade ЛT 96 is the brass tombac (T) containing 96% Cu and Zn. The designation of gradе ЛЖМЦ-59-l-l indicates that the brass contains 59% Cu, 1 % Fe, 1 % Mn, the remainder is Zn.
Alloys of copper with a number of elements including tin, aluminium, silicon, manganese, iron and beryllium are called bronzes. Tin bronzes are divided into two groups: wrought bronzes, containing up to 6% Sn, and casting bronzes, containing over 6% Sn. Special bronzd are copper-base alloys in which the principal admixtures are Al, Ni, Mn, Si, Fe, Be and others. Special bronzes are fully equivalent substitutes for the more expensive tin bronzes and, therefore, have great economical value. These bronzes are designated on the same principle as brasses. The designation begins with the Russian letters Бp (the first two letters of the Russian for bronze) which are followed by letters indicating the main elements and numbers showing the average percentage of these elements.
Certain grades of special bronzes deserve more detailed consideration. Aluminium bronzes contain from 4 to 11% Al; their high mechanical properties and corrosion resistance considerably surpass those of tin bronzes and brasses. The castability of aluminium bronzes is good and the are frequently used in foundry practice. Sheets, strips, bars and wire are made of grades БpA5 and БpA4 by the rolling process. Aluminiur bronzes with admixtures of iron and manganese, grades БpAЖ9-4, БpAЖMЦ10-3-1.5 and БpAMЦ9-2, are suitable for castings and for working, especially for smith and closed-die forging.
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