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Aviation History
1955
1955 - 1055.PDF
FLIGHT, 29 July 1955 167 TITANIUM in AIRCRAFT THE reduction of titanium ores to metal and its subsequentmelting present extremely difficult and complex technicalproblems. The root of the trouble is that titanium is extremely sensitive to impurities, nearly all of which affect itsproperties and behaviour. The melting point is some 200 deg C above that of steel, and the molten metal attacks all normalrefractories. Molten titanium will absorb oxygen and nitrogen from the atmosphere, and both these impurities affect theproperties of the metai. Some two years before the outbreak of the second world war,Dr. W. J. Kroll solved the problem of preparing reasonably pure titanium in sufficient quantities to be useful. Apprehensive ofthe growth of Nazi Germany, he closed his laboratory in Luxemburg and brought samples of titanium to Britain in 1939.At that period, however, Britain was too pre-occupied with rearmament to be actively interested in the long-term prospectsof a new metal. Dr. Kroll therefore went to the United States and showed his samples to many firms, but no company would takethe risk of developing his process. The United States Bureau of Mines gave him the opportunity he sought, and in 1946 theannouncement was made that the process had proved successful. Though it was only in 1948 that metallic titanium was firstproduced commercially, it is already being widely hailed as the "wonder metal." Last year more than 5,000 tons of commerciallypure sponge titanium were produced in the United States. The expansion programme sponsored by the U.S. Government aimsat an annual output of 35,000 short tons by 1957. Yet even this ambitious target has been described as totally inadequate by U.S.Air Force experts, who claim that to keep pace with aircrcaft development an annual production of at least 150,000 tons willbe required within the next five years. Though Britain lags behind the United States in the production of titanium, the metalis now being prepared on a pilot-plant scale and an establishment capable of producing 1,500 tons of sponge titanium a year willshortly be in operation. Despite this rapid progress, it may well be many years beforetitanium becomes as familiar a metal as aluminium is today. When aluminium was first made about a century ago, it was sold at £54a pound. Though the problem of cheap production was solved some seventy years ago, this metal was still in its industrial infancywhen the first world war broke out. In view of the immense scientific and technological resources at the disposal of modernindustry, titanium's progress might well be more rapid than that of its predecessors, aluminium and magnesium, but the productionon a really substantial scale is still some distance off. The future of metallic titanium depends on the cheapness withwhich it can be prepared and on the accuracy with which alloys with desirable properties can be reproduced. Higher outputsshould have a favourable effect on manufacturing costs, but the root causes of titanium's high cost are the difficulties inherent inexisting processes, none of which is really suitable for large-scale production. The reduction and casting processes must be carriedout in a complete vacuum, and very special melting techniques must be adopted. The processes used involve heavy capitalinvestment and high operating costs. Throughout the world a search is in progress for alternative processes that will maketitanium really cheap. The price of first-grade sponge metal has recently beenreduced from five dollars to four dollars a pound, but it will have to fall very steeply before titanium can compete with otherstructural materials. Meanwhile the use of titanium sheet in aircraft production is confined to highly specialised applicationswhere the increased cost is justified by the technical gains. According to General Metzger, Chief of the Production andResources Division of the U.S. Air Force and Director of the Aircraft Production Resources Agency, the use of titanium incommercial planes could increase the payload up to 30 per cent, or more. To military aircraft the favourable strength/weightratio at elevated temperatures is very attractive. As an example, General Metzger instanced a four-engined combat aircraft flyingtoday, in which engineering design coupled with the use of titanium could lead to a saving of 500 lb. per engine. Roughly, this savingcould increase the speed 15 knots or its range by 100 miles. Had it been possible to make use of titanium when the aircraftwas designed, a far greater saving could have been made, because the rule of thumb in aircraft manufacture is that 1 lb of dead-weight removed from an aircraft permits a reduction of 10 lb of structural weight in design. It takes about five years from thedrawing board to the production of a heavy bomber, so that when the aircraft above was designed, titanium was in its infancy. Apart from the high strength/weight ratio, the properties oftitanium that appeal most to the aircraft industry are heat resistance, corrosion resistance, and ability to retain strength atelevated temperatures. Under existing price conditions, titanium strong enough to displace aluminium on a strength/weigntis not ratio basis, while no titanium alloy at present available can with-stand very high temperatures. For temperatures between 250 deg F and 800 deg F, either titanium or stainless steel couldbe employed, but above the latter temperature it is necessary to revert to stainless steels, despite their greater weight. Senator Malone, chairman of the Committee on Interior andInsular Affairs, United States Senate, presided over an inquiry into the titanium situation in the United States, which was heldin 1953/54 and was concerned mainly with the requirements of the aircraft industries. In his opening address Senator Malone described titanium asa "must" for air power in the future. He stated that its use in high subsonic and supersonic aircraft was essential and that therewas no other known metal so uniquely fitted for the construction of a major part of airframes and engines. This view was supportedby much of the evidence given by leading aircraft manufacturers. The North American Aviation Corporation uses titanium inboth the F-86 and F-100. In the F-86 the quantity used is equivalent to approximately 1 per cent of the aircraft, measured byweight. In the F-86H, a major redesign of the same aircraft, the percentage has been roughly doubled. The F-100, now beingbuilt in large numbers, incorporates 5 per cent of titanium in the finished aircraft weight. Mr. William J. O'Donnell, chief development engineer of theRepublic Aviation Corporation, has stated that, without much change in production, a saving of some 50 lb could be effectedin the F-84 by the use of titanium, and this saving could be converted into additional external loads. In the case of aircraftto replace the F-84F, it is planned to use approximately 400 1b of titanium, representing a saving of about 60 lb, compared withthe weight of the equivalent quantity of stainless steel. This company has been experimenting with titanium and itsalloys since 1951 and is confident that a complete aircraft could be made from titanium if the need arose. Flying at Mach 2above 35,000ft (about 1,350 m.p.h.), the skin temperature of an aircraft would be approximately 250 deg F, and at this point theuse of titanium could result in very large savings in weight. Despite the high cost of the metal, about 200 lb of titanium isbeing used by Douglas Aircraft in the production of a new com- mercial transport, the DC-7C. The use of titanium for certainapplications in the engine nacelles, firewall, and elsewhere is reducing the weight of the aircraft by the equivalent of almost1^- passengers. By 1957, the company expect to be making structures 20 per cent (by weight) of titanium alloys. The Lockheed Aircraft Corporation claim to have been amongthe first companies—if not the first—to build and fly an aircraft having titanium components in the structure. The com-pany holds that the extent of titanium usage will be controlled by die effect on cost per weight empty and on cost per pound ofdisposable load. If the average cost of the raw material could be brought down to two dollars per lb—half the present price—titanium would compete with stainless steel and might be used for about 21 per cent (by weight) of the aircraft. If and whentitanium alloys are reduced to 40 cents per lb, they will probably be used for some 40 per cent of aircraft weight. Lockheed's own requirements of titanium in 1955 werereported to be 76 tons, compared with 31 tons last year. If the price were reduced by 25 per cent, this company would probablydouble its present rate of consumption in a very short time. The Committee was informed that engine manufacturers could use50,000 tons of titanium in 1955, if it were available, plus an addi- tional 50,000 tons in each successive year thereafter until minimumrequirements of 250,000 tons per year were satisfied. Military requirements for 1956 are estimated at 35,000 tons. In the absence of any radical transformation in productionmethods, only minor price reductions are expected during the next three or four years. Mr. Rex R. Lloyd, Branch Chief of theBureau of Mines, Boulder City, N.Y., states, however, that a selling price of $3 per lb is not out of the question as improve-ments are effected in the Kroll process. Recent technical developments suggest the possibility of entirely new processes bywhich metallic titanuim could be produced more cheaply, but it has been predicted that, due to handling and extraction diffi-culties, titanium will never become a low-cost material. Apart from price, there are several difficult problems to besolved before titanium can be regarded as a firmly established material for aircraft production. In the first place, users in theUnited States complain that the quality of titanium sponge is not yet sufficiently uniform and that alloys are often unsatis-factory for the end use. These difficulties are common to the development of any new material. A second deterrent is the lack of processing and fabricating"know-how." The aeronautical industry is still learning how to make the best use of titanium, how to process it, and for whatapplications it can safely be employed. A.G.T.
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