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Aviation History
1951
1951 - 0534.PDF
FLIGHT 23 MARCH 19 About Aluminium ALLOYS In this scries of articles »c deal very briefly with those characteristics of aluminium that are important to the student, believing that although the light metal is now second only to steel in structural significance, its nature and behaviour are not, perhaps, correspondingly known. 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. /CHEMICALLY pure aluminium isV_j soft, ductile and of little structural value but, as extracted, it normally con-tains up to J" o impurities, mainly iron andsilicon. These have a marked effect on the properties of the metal, so that, withthe further hardness acquired during roll- ing, " commercial purity " aluminium hasa useful degree of strength and is widely produced in sheet form. In the early years, however, casting wasthe principal outlet for the metal, and, without the benefit of work-hardening,strength and good foundry characteristics were sought by alloyings which was soonapplied to the wrought forms. Alloying in itself provided a moderate improvemem inproperties, but the discovery, about 1909, that heat treatment could raise the strengthof certain alloys far more than could work- hardening, while retaining a fair ductility,marked the real entry of aluminium into the structural field. This treatment wasapplied to rolled, extruded and cast alloys, enabling them for the first time to competewith structural steel on a strength basis. Today, a wide range of alloys is availableto meet the varied needs of industry, each having been developed for its particularcombination of properties. The alloying elements that are nowused include copper, magnesium, silicon, manganese, zinc, and nickel; chromium,titanium, cadmium, columbium, cerium, tin, lead and other metals are also em-ployed in small quantities. The effect of these additions is generally to increase thestrength and decrease the ductility of the commercially pure metal, but ease offabrication, corrosion-resistance, and other characteristics are affected by their pre-sence, singly or in combination. The composition suitable for a wrought(rolled, extruded, or forged) aluminium alloy seldom fits it for casting, owing tothe completely different conditions of manufacture. Casting alloys have there-fore developed along separate lines, and it will be found that all aluminium fabri-cators offer these two main groups of alloys, wrought and cast, with distinctnomenclature systems. Wrought Alloys distortion of the structure of the metalduring the mechanical shaping processes of manufacture. In this class are com-mercially pure aluminium and the alloys containing, singly or in combination,manganese, magnesium and silicon. The desired degree of strength and hardnessis achieved by controlling the amount of working during fabrication, and the metalcan be fully or partly softened at any stage by annealing (at 350 -4OOCC). Work - hardening alloys can be givenquite high strength (especially those with high magnesium content) and are gener-ally cheaper than those requiring heat treatment. They are used for buildingand other constructional purposes, in hollow-ware, in packaging, and in generalsheet metal work. In the second class are the heat treat-able alloys, in which copper, magnesium, zinc and silicon are important constitu-ents. These alloys make it possible to use aluminium in many instances wherethe work-hardening type would not do. Some loss of ductility is inseparable froma gain in strength by cold-working, so that the more fully strain - hardenedmaterials will not tolerate much forming Casting Alloys Satisfactory casting in aluminium de-mands that the metal should flow readily to all parts of the mould; that, on solidify-ing, high shrinkage and low hot-strength should not produce fractures; and thatthe cast structure should be sound. These considerations decide the choiceof alloying constituents as much as do strength requirements. Silicon is used insome alloys to enhance the castability and to reduce thermal expansion inservice (important in pistons); copper, magnesium and the other elements areused, singly or in combination, to develop particular characteristics, such as sus-ceptibility to heat treatment, good high temperature properties, and corrosionresistance. Although it has been possible only to outline the complex subject of alloying, it will be seen that aluminium is largely dependent on this technique for its engineer- ing value and that the choice of the right alloy for each task is very necessary. The third article in this series will be concerned zvith heat treatment. There are two major classes of wroughtalloys. In the first, strength is invested by the work-hardening that results from MAIN ALLOYING ELEMENTS CHARACTERISTICS OF GROUP AN EXAMPLE OF THE GROUPWITH ITS TENSILE PROPERTIES NORALDesigna- tion • 1 % ProofStress tons,sq.in UltimateStress tons/sq.in Elongat'n% on 2 in. WROUGHT ALLOYS Mn 1 to IJ% Mg 2 to 7°o Cu IJ to 5% Mn I '„ Mg JtolV Si itoj:0 Mg 1 tol% si j to 1 :o Zn 5to7l'o Mg2to2J% Cu 1 to 2% A work-hardening alloy in wide use for building, packaging and hollow-ware. Strong, tough, work-hardening alloys; exceptional corrosion resistance. Heat treatable, widely used in aircraft and other stressed structures. Fair corrosion resistance, often improved in sheet by coating with pure aluminium (Alciad). Heat treatable, less strong than the copper group but easier to produce and form. Better corrosion resistance. Heat treatable, difficult to produce and confined to aircraft and other structures demanding the highest possible strength/ weight ratio. 3S|H (iJo Mn A56S JH 5% Mg 26S WP *i"u Cu %',, S, 2 ',, Mn 4"o Mg SIS WP /"„ Si i';u Mg C77S WP 7% Zn 2 '„ Mg 11':'» Cu 8i I7i 26 18 T 38 n 22J 31 20 42 9 II 9 13 II CASTING ALLOYS (Chill-cast test bars) Si 5 to 12% Cu 4 to 10% Mg IJ to 10% Easily cast with good pressure-tightness, and heat treatable when magnesium is present. Heat treatable alloys with good machining properties. Exceptional corrosion resistance, shock resistance, and machinability. 125 WP 5",-, Si ' i"o Cui% Mg 226 WP 4£"n Cu 350 W I0"o Mg 16 24 13 19 27 21 2 5 18 NORTHERN AOTMINIUM COMPANY LIHITED TECHNICAL DEVELOPMENT DIVISION BANBURY OXON
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