About Aluminium . . L
Page 91
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ALLOYS
n this series of articles we deal very briefly with those characteristics of luminiurn that are important to the student, believing that although the ight metal is now second only to steel in structural significance, its nature nd behaviour are not, perhaps, correspondingly known.
IHEIVIICALLY pure aluminium is soft, ductile and of little structural e but, as extracted, it normally con ; up to impurities, mainly iron and in. These have a marked effect on properties of the metal, so that, with :nailer hardness acquired during roll" commercial purity" aluminium has eful degree of strength and is widely luccd in sheet form.
the early years, however, casting was principal outlet for the metal, and, out the benefit of work-hardening, igth and good foundry characteristics sought by alloying, which was soon ied to the wrought forms. Alloying in
f provided a moderate improvement in ierties, but the discovery, about 1939, heat treatment could raise the strength :rutin alloys far more than could worklening, while retaining a fair ductility, ked the real entry of aluminium into structural field. This treatment was ied to rolled, extruded and cast alloys, Aing them for the first time to compete L structural steel on a strength basis. ,ay, a wide range of alloys is available leet the varied needs of industry, each ng been developed for its particular bination of properties.
he alloying elements that arc now I include copper, magnesium, silicon, iganese, zinc, and nickel; chromium, num, cadmium, columbium, cerium, lead and other metals are also ern • ed in small quantities. The effect of e additions is generally to increase the ngth and decrease the ductility of the mercially pure metal, but ease of ication, corrosion-resistance, and other -acteristics are affected by their pre:e, singly or in combination.
he composition suitable for a wrought ied, extruded, or forged) aluminium y seldom fits it for casting, owing to completely different conditions of tufacture. Casting alloys have theredeveloped along separate lines, and it be found that all aluminium fabriors offer these two main groups of ys, wrought and cast, with distinct lenclature systems. distortion of the structure of the metal during the mechanical shaping processes of manufacture. In this class are commercially pure aluminium and the alloys containing, singly or in combination, manganese, magnesium and silicon. The desired degree of strength and hardness is achieved by controlling the amount of working during fabrication, and the metal can be fully or partly softened at any stage by annealing (at 350 — 400-C).
Work hardening alloys can be given quite high strength (especially those with high magnesium content) and are generally cheaper than those requiring heat treatment. They are used for building and other constructional purposes, in hollow-ware, in packaging, and in general sheet metal work.
In the second class are the heat treatable alloys, in which copper, magnesium, zinc and silicon are important constituents. These alloys make it possible to use aluminium in many instances where the work-hardening type would not do. Some loss of ductility is inseparable from a gain in strength by cold-working, so that the more fully strain hardened materials will not tolerate much forming
but the temporary softness of the heat treated alloys gives an opportunity for severe forming with assurance that the full properties will be attained before the part is put into service.
Casting Alloys
Satisfactory casting in aluminium demands that the metal should flow readily to all parts of the mould; that, on solidifying, high shrinkage and low hot-strength should not produce fractures; and that the cast structure should be sound. These considerations decide the choice of alloying constituents as much as do strength requirements. Silicon is used in some alloys to enhance the castability and to reduce thermal expansion in service (important in pistons); copper, magnesium and the other elements are used, singly or in combination, to develop particular characteristics, such as susceptibility to heat treatment, good high temperature properties, and corrosion resistance.