ALUMINIUM - INDUSTRIAL ALLOYS These alloys may be divided into two main classes : those in which small proportions of aluminium modify the properties of other metals and those in which other metals are added for this purpose to aluminium. The first category includes the aluminium bronzes (q.v.) which are mainly copper with up to i i% alumin ium, aluminium amalgam (or aluminium dissolved in mercury) used in organic chemistry, and some aluminium solders which are mainly zinc or tin with varying proportions of aluminium. Finally, some super-light alloys consist of magnesium with small propor tions of aluminium (see MAGNESIUM).
The present section deals with the second category, the light alloys of aluminium, containing 96-65% aluminium with copper, magnesium, manganese, nickel, silicon, tin and zinc.
Preparationof these alloys follows customary lines where, as with zinc and tin, the alloying metal melts at as low a temperature as does alumin ium. Where, as with copper, manganese and nickel, the melting point is much higher than that of aluminium and the latter would have to be seriously overheated before solution took place, it is customary first to prepare hardeners or alloys containing high pro portions of the alloying metal. Such alloys not only dissolve readily in aluminium, the main bulk of which need not, therefore, be overheated, but can also be easily broken to sizes suitable for weighing and adding to the crucible. Special methods are adopted in the case of magnesium and silicon.
The light alloys are mechanically stronger than pure aluminium and especially have a much wider elastic range, but they are for the most part far less resistant to corrosion and have lower elec trical and heat conductivities. Magnesium and silicon lower the specific gravity of aluminium, the other metals mentioned raise it.
Theuseful light copper alloys contain 4 to 14% copper and their utility is due to the fact that the two metals form a definite compound,
Aluminium can hold 2% (re cent work places this figure lower) of copper dissolved at ordinary temperatures, so that alloys containing up to this proportion are composed of one constituent only, a solid solution of copper in aluminium. If, however, an alloy with 5% of copper is held at 525° C. all of this copper becomes dissolved. Such alloys, slowly cooled, deposit their excess copper in large crystals of
mainly between the grains of the metal, which is strengthened thereby. If instead of being slowly cooled the hot alloy be sud denly quenched, the compound is not given time to separate and the alloy is still further strengthened but remains ductile. On the temperature being slightly raised the
is precipitated, but in a very finely divided state instead of in large crystals. As with many alloys here also such fine structures are accompanied by good mechanical properties. Thus the 5% alloy in the form of hard rolled sheet, as normally prepared, has a tenacity of 35– 40,000 lb. but is brittle (3% elongation). Annealed the tenacity falls to 25,000 lb. whilst the elongation rises to 3o%. Heat treated as described above, the tenacity may rise to 45-50,000 lb. whilst the ductility is only slightly impaired (25% elongation). As the specific gravity is about 2.8 such an alloy has the high specific tenacity of 7.5. A casting alloy containing 4-5% copper with other and minor constituents is widely used in America where it is known as No. 195. As cast, the tenacity is 17,000 lb. with 4% elongation. H. eat-treated and quenched the tenacity rises to about 30,000 lb. with 8% elongation. Subsequent re-heating increases the tenacity to 40,000 lb., but lowers the elongation to 1.5%. Sus ceptibility to heat treatment is of great importance in the alloys of the duralumin type (q.v.) in which it has long been known but only recently explained. Copper in excess of 5% cannot be dis solved and alloys with more than this amount are increasingly brit tle and can, therefore, not be wrought, although still suitable for castings. In America the alloy with 8% copper, known as No. 12, is the most popular alloy for general automobile castings (crank cases, etc.) and similar purposes where high mechanical properties are a secondary consideration. It has a tenacity of 20,000 lb. with low ductility (I-2% elongation). The English alloy L.8 contains I I-13 % copper and is specially suited and largely used for cast ing in permanent iron moulds and notably for the pistons of in ternal combustion engines. All these alloys corrode much more rapidly than pure aluminium. The alloy containing 1.5 % man ganese, known as 3.S, is widely used in America, where it is rolled into sheet, to be spun and pressed into cooking utensils. Without increased tendency to corrosion it is considerably stronger than pure aluminium, breaking under a load of 30,000 lb. when work hardened and 15,0o0 lb. when annealed. It apparently owes these properties to the simultaneous (eutectic) crystallization of pure aluminium and a compound,
which produces a fine grained structure. No other binary manganese alloys are used.
The presence of silicon was formerly considered harmful in every way to aluminium. Recently, alloys with I I-13 % have been introduced and promise to be the most valuable light casting alloys offered to engineers for a generation. Normally aluminium and silicon crystallize simultaneously in large plates, giving coarse structures and, with high silicon content, brittle and useless alloys. Various agencies, notably metallic sodium, have been found to cause the silicon to separate in an extremely fine state of division, even, as has been suggested, colloidally (fig. 2). Such modified alloys have high tenacities (25-30,000 lb.), low specific gravity (2.66) and, therefore high specific tenacity (5) . High ductility (12-14% elongation) is frequently attained in practice but can not be relied on at present and the British Air Ministry specifica tion only calls for not less than 7%. They are very fluid when molten and contraction on solidification is low (4%) whilst they resist corrosion well. Alloys containing up to 5% silicon, with or without the addition of other metals, are largely used when their great fluidity is of special advantage. Such alloys are as a rule not modified. Alloys with up to 3 5 % zinc have been used. They are very weak at high temperatures and, therefore, somewhat difficult to cast ; they may, however, be rolled to sheets and bars of great strength.
Aluminiumwith 2 % copper and 1 % manganese is used in America for light forgings, having a tenacity of 2 7-28,000 lb. and 12% elongation. Alloys with I2-14% copper and 4-2% manganese maintain their strength well at high temperatures and are, therefore, specially suitable for the pistons of internal combustion engines. An alloy with 1 % tin and 7% copper (the English 3.L.I I) is considered specially suitable for articles subject to hydraulic pressure. In Europe, and especially England, copper and zinc alloys of aluminium are preferred for sand castings for automobile and general use. Three per cent. copper and 1 3 % zinc (2.L.5) give a tenacity of 2 5,000 lb. with 3% elongation. The zinc content makes for cheapness but raises the specific gravity to 3, so that the specific tenacity is only 3.7. With 2o% zinc (L 28) these alloys can still be rolled and after suitable heat treatment have a tenacity of 55,000 lb. with 15% elongation and a specific tenacity of about 7.6. Alloys with copper and zinc are not well suited to casting in permanent moulds owing to weakness at high temperatures.
Aluminium with 7-9% copper and I-3% nickel are favoured in America for pressure die casting. An alloy with 4% copper and 2% silicon known as lautal is used, mainly in Germany, in sheet form. Its mechanical properties approach those of duralumin.
Minute quantities of magnesium and silicon, when associated, confer on aluminium the property of age hardening. Their com pound
is soluble to about 1.6% in solid aluminium at 58o° C. but only to O.5% at ordinary temperature. If, therefore, aluminium containing sufficient magnesium and silicon be held at high temperatures and suddenly quenched the excess magnesium silicide is thrown out, but not in its normal stable condition. In course of time (3-4 days), the newly precipitated crystals undergo secondary changes, which are not yet fully explained. When first quenched these alloys are soft and can be worked in various ways. During the subsequent changes they stiffen rapidly and, finally, give products with very great strength and other valuable characteristics. Magnesium silicide confers this property on various light alloys as well as on pure aluminium. (See DURA
