ALUMINIUM - PROPERTIES Form and Structure.—Aluminiumwhen cast from the fur naces solidifies in crystal masses, as may be seen if an ingot be broken at temperatures just below the melting point. Mechanical working deforms and partly shatters the original crystals, but subsequent heating causes recrystallization. When the degree of deformation and temperature of heating are suitable, some crys tal grains grow at the expense of others and, under carefully selected conditions, one grain alone may grow and thus convert Iarge pieces of metal into a single crystal. Exaggerated grain size such as this is avoided in practice, metal showing this phenome non being defective in mechanical properties.
Composition.—Whereas such metals as copper and zinc can be deposited electrolytically from aqueous solutions almost chemi cally pure, aluminium which can only be deposited from fused electrolytes carries with it whatever metallic impurities the raw materials and electrodes used in its manufacture contain. The best metal leaves the furnaces with not less than o.2 % of impurities, whilst commercial metal generally contains only of aluminium. The remainder consists mainly of iron and silicon in approximately equal proportions, with o•o i–o• i % of copper derived from the furnace equipment. Small quantities of nitrogen are regularly found, as also of carbon, although the latter cannot be accurately estimated ; but sodium, contrary to earlier statements, is never present in appreciable quantity. The same holds good of oxygen and aluminium oxide, but hydrogen may be a cause of serious trouble. Dissolved in overheated metal it separates on slow cooling, producing porosity, which can only. be eliminated by remelting and so allowing the gas to escape. The following analysis (W. H. Withey) is typical of high grade com mercial aluminium: Iron 0.183 Silicon - o• 154 Copper o•026 Nitrogen o•040 Zinc o•oo6 Sodium . . . . . . . . . Trace Carbon . Not estimated Aluminium (diff.) 99.591 Until recently no practicable method of purifying aluminium was available. During the last few years several methods have, however, been elaborated and, by one, considerable quantities of refined metal have been made. A heavy alloy of aluminium and copper is melted under fused cryolite to which barium fluoride is added. When an electric current is passed from the alloy below to an electrode above, aluminium alone passes out from the alloy. In the reduction furnaces we have seen that aluminium sinks. Here, however, the gravity of the bath has been raised by the barium fluoride, so that the aluminium floats on the surface and can be removed. Metal containing 99.98% aluminium has been prepared in this way. The following is a typical analysis of purified aluminium : Iron o.oi4 Silicon . . . . Copper • . . . . . . . 0.020 Zinc . . . o•oo Aluminium (dill.) 99.954 Physical Properties of Commercial Aluminium.—The specific gravity is 2.70-2.71. Hence, equal volumes of the com mon metals have the following relative weights: aluminium 1, magnesium 0.65, zinc 2.6, steel 2.9, brass 3, copper 3.25, nickel 3.25, lead 4.2. The specific gravity of aluminium at its melting point is about 2.4o. Cast or annealed wrought aluminium breaks under a load of 11-13,000 lb. per sq.in., but when hardened by working under 22-28,000 lb. Hence the specific tenacity or break ing load for unit weight (tenacity), which largely determines the (sp. gr.) relative values of light constructional materials, is 2 and 4 in the two cases, compared with 3.3 for hard copper, 4.2 for mild steel and 5.9 for tempered nickel steel. At high temperatures alumin ium is very weak, whilst after being heated for a few hours to C. work hardness is permanently lost. At 250° C. this takes longer whilst after 5 years at 200° C. and ioo° C. respectively 5o% and 86% of work hardness still remain. Aluminium ranks as a soft metal, its hardness being about half that of copper and zinc but double that of tin. The ductility and malleability of an nealed aluminium are very high and it may, therefore, be drawn to the finest wire or beaten to the thinnest leaf. In tension a tin. length may be elongated by 30% before fracturing, but heavy cold working reduces this figure to 3% or less. Elasticity is very low. Work hardened aluminium ceases to be elastic when loaded to 7-8,00o lb. per sq.in., and to 2-2,500 lb. when in the soft state. Moreover, as with other soft metals, the effect of loading develops slowly and recovery therefrom is very slow. Thermal expansion (0.0000237 per degree from io° C.–ioo° C.) is two and a half times that of steel, double that of nickel and one and a half times that of copper, whilst shrinkage on solidification is high (about 7%). Specific heat (about 0.24 from normal temperature to melting point) and latent heat of fusion (probably 90 calories per gram) are both high, so that notwithstanding the low melting point of 658.7° C. it requires much heat to melt aluminium. Heat con ductivity (0.504 calories per second per c.c. per ° C.) is about three times that of tin, nickel and iron but little more than half that of copper and silver. Electrical conductivity is also high, namely about 6o% of that of copper and resistivity (2.6-2.8 microhms per c.c.) therefore low. Aluminium is non-magnetic. In the electrochemical series the position of aluminium is not quite certain. Probably it is less noble than all the common metals except magnesium and the alkali and alkaline earth metals.
The physical properties of the purest metal so far made by re fining vary somewhat from those given above. It may be said that the strength and hardness are definitely lower, and that the melting point is about I° C. higher and the electrical conductivity about 4% higher.