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Aviation History
1953
1953 - 0175.PDF
FLIGHT, 6 February 1953 . . . AND NOW TITANIUM 173 Aircraft Potentialities of the "New" Metal: Major Teed's R.Ae.S. Lecture AS we briefly recorded last week, on January 29th, Major P. L. • Teed, A.R.S.M., M.I.M.M., F.R.Ae.S., deputy chief of aeronautical research and development, Vickers-Armstrongs, Ltd., read a paper to the Royal Aeronautical Society on the subject of that interesting "newcomer" to the metallurgical field—titanium. Readers of Flight may recall that in the issue of June 27th, 1952, we discussed at some length the fabrication characteristics of this material, and some of its possibilities for aircraft structures and engine components. Major Teed's very thorough paper was devoted mainly to the properties of the titanium alloys and to economic considerations of their use, and we reproduce here a digest of the principal points made. In an historical introduction to his subject, the lecturer said that it seemed strange that, with over 70 metals available to mankind, only four should at the present time be used on a substantial scale as structural materials. These metals were copper and iron and—of far more recent introduction—aluminium and magnesium. For the aircraft industry at least, there now seemed to be a definite possibility that titanium might one day become a fifth. The metal was discovered in 1789 by Gregor, and named five years later by a German, Klaproth, who was much struck by the strength of its chemical bond with other elements, and therefore named it after Titan. Contrary to popular belief, titanium occurred abundantly in the earth's crust, as could be seen from Table I. SOME METALLIC CONSTITUENTS OF THE EARTH'S CRUST, PARTS PER MILLION 300 200 80 70 40 16 4 0.1 81,300 50,000 36,300 28,300 25,900 20.900 4,400 1,000 310 Strontium Chromium Nickel Copper Tin ... Lead ... Uranium Silver ... TABLE I: Aluminium Iron Calcium Sodium Potassium Magnesium Titanium Manganese Rubidium The ores, said the lecturer, were all oxides, the two most important being Ilmenite (FeO. TiCh) and Rutile (TiCh). Deposits were widely spread, notably along the eastern side of North America, and in Australia, India and Malaya. Furthermore, the total world annual output of titanium- containing materials was, in 1950, no less than 900,000 tons—repre senting a content of about a quarter of a million tons of the metal. Unfortunately, all this was combined titanium, and it was in the extraction of the pure metal from its components that immense difficulties arose. Two processes had been evolved. Of these, the Van Arkel method produced a high-purity laboratory product, but was unsuitable for even medium-scale operations. The other process was the Kroll, and virtually all the titanium so far produced for engineering purposes had been extracted by this means. At present the process was being operated on a batch basis, but it seemed likely that, given enough clever metal lurgical engineering, continuous flow would soon be possible. It was clear from Fig. 1 that the elementary basis of the process was the reduction of titanium tetrachloride by the more electropositive metal magnesium, yielding a titanium sponge which was useless metallurgically TITANIUM DIOXIDE COKE rp=t> CHLORINE |7=i> MACNESIUM Treatment at 700° C T TITANIUM TETRACHLORIDE JZ_ Reaction in Steel Chamber Mixture of SPONGY TITANIUM & MACNESIUMCHLORIDE XL Separation by Liquation, and Vacuum Distillation VKMr.irr„,i,i MACNESIUM MAGNESIUM CHLORIDE Electrolytic Cell CHLORINE ~^^ TITANIUM SPONGE Melting in ArgonAtmosphere j Arc Furnace ^^ TITANIUM INCOTS and h»d to be melted—under inert conditions, since molten titanium combined with all gases except the inert gases. The present cost of various forms of titanium could be seen from Table II. TABLF APPROXIMATE COST* OF METALLIC TITANIUM PER TON IN VARIOUS FORMS Material Ilmenite concentrate (45 per cent Tr02) Titanium tetrachloride ... Titanium sponge ... Bars, hot-rolled Forgings, rounds, discs, etc. Shest and strip Cost (£ sterling) 37 500-600 4,000 7,900 7,900 11,900 * On current American prices converted on the basis of S2.82/£1 sterling. These costs compared with £167 a ton for aluminium—although in fairness it must be recalled that a century ago the comparable price for aluminium was £125,000 a ton. Major Teed then went on to review the properties of the material, firstly when commercially pure (i.e., containing not more than one per cent of carbon, iron, oxygen and nitrogen), and secondly when alloyed. The mechanical properties of the "pure" metal included an ultimate tensile strength of 39.0 tons/sq in, 0.2 per cent proof stress °f 33-5 tons/sq in, and elongation of 23 per cent. Its tensile properties were further indicated by Fig. 2, which was based on work by Lee Williams. 100 200 300 400500 O TEMPERATURE^ C) IOO 200 300 400 TEMPERATURE (dig C) Fig. 1. Flow of materials in the manufacture of titanium ingots. Fig. 2. Tensile properties at elevated temperatures of arc-melted com mercially pure titanium, containing 0.31 per cent C, 0.002 per cent N2, 0.07 per cent Mn, and an undetermined percentage of 02. The density of the pure metal was about 57 per cent that of steel and 1.7 times that of aluminium, whilst its Young's modulus was about half that of iron and one and a half times that of aluminium. At atmospheric temperature it was entirely free from corrosion by air, or fresh or salt water, in spite of its immense reactivity at higher temperatures. After examining a series of specific cases and taking both cost and properties into account, Major Teed said that, none the less, nothing had so far been revealed about pure titanium to indicate that, under present conditions, it had any commercially justified applications in the air-frames of present air craft. For two reasons at least, however, it must not be swept aside. Certain possible uses had so far received no consideration, and no attempt whatever had been made to evaluate the potentialities of the material—it was at the beginning of its metallurgical career. Very pure iron, be it remembered, had an ultimatwensile stress of well under 20 tons/sq in, whilst a number of alloy steels of reasonable ductility, containing over 95 per cent of iron, had more than five times the strength of the pure metal. There were, nevertheless, at least four ways in which commercially pure titanium might well be employed with technical, if not with certain economic, advantage. The lecturer believed that metal having approxi mately the mechanical properties described above could be employed in the form of rivets, closeable by cold heading and with—partly as a result of cold work—a pin shear ultimate as high as 35 tons/sq in. Experiments in America, with this object in view, had not as yet suc ceeded, but their measure of ill-success had not been such as to eliminate hope. While not yet an article of commerce, titanium tubing had been made with a 0.2 per cent proof stress of about 33 tons/sq in and an elongation of over 20 per cent. If it could be manufactured with the uniformity of quality required for aircraft hydraulic and pneumatic services, there would certainly appear to be a field of application in this sphere, for its specific proof and ultimate stress were markedly better than those of such alloys as D.T.D. 310, 328, 503 and T.26. As a fireproof bulkhead, commercial titanium could replace stainless steel with a saving in weight, for equal thicknesses, of 37 per cent. If the oxygen content did not exceed 0.15 per cent, that of the nitrogen 0.05 per cent, and that of the carbon 0.15 per cent, the metal was readily
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