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Aviation History
1937
1937 - 0272.PDF
04 FLIGHT. JANUARY 28, 1937. • MAGNESIUM ALLOYS Their Use in Aircraft Construction : Many Difficulties Still to be Overcome : Beryllium Found too Brittle : A Lecture by Professor C. H. Desch A S the standards of aircraft performance are raised, possible /\ saving of weight is becoming of ever greater importance, A\ SO that more and more interest is being taken by aircraft constructors in metals lighter than aluminium. Recently Professor C. H. Desch, Superintendent of the Metallurgical Department of the National Physical Laboratory discussed, lx;fore the Royal Aeronautical Society, recent work on mag nesium and other light alloj^s, much of which has been carried out at the National Physical Laboratory. "Since," Professor Desch said, "the light metals of the alkalies and alkaline earths are ruled out for constructional work by their chemical activity as well as by their softness, only beryllium and magnesium are left among the elements lighter than aluminium. Beryllium has unfortunately dis appointed expectations, for although its crystal structure in dicates that it should be possible to render it ductile, all the metal so far produced has proved to be brittle, nor has any means been found of alloying it with small quantities of another metal so as to make it ductile. With a specific gravity of only 1.84, an elastic modulus which is believed to be very high, and a melting point of I278°C, beryllium would seem to offer remarkable advantages, but the long investigations carried out in Berlin and at the National Physical Laboratory have not so far resulted in the production of new light beryllium alloys, although attention is now being again directed to the alloys with aluminium. At the other end of the series alloys of copper and of nickel, containing small quantities of beryl lium, have shown remarkable properties and have found technical application." Atom Diameters " It might have been expected," he continued, " that a useful alloy could be made by combining beryllium with mag nesium." This had not proved to be the case, however, and he suggested that the reason lay in the fact that the effective diameter of the magnesium atom is much larger than that of the beryllium atom, spheres of such different sizes not being able to pack closely together. "It is remarkable,'" he said in this connection, "how often purely geometrical factors outweigh those of chemical similar ity in determining the structure of alloys, and new light is constantly being thrown on metallurgical problems by the application of ideas of atomic arrangement in metallic crystals, It is becoming more and more possible to decide beforehand which elements may with advantage be added to a given metal to produce a useful alioy with given properties, although the complexity of most of the mechanical properties prevents the process from being quite simple. It may, however, be said that theoretical knowledge plays an ever increasing part in the development of alloys." Magnesium is too weak a metal to be used in the unalloyed state, and the metals which may be added to it to produce strong alloys are limited in number. In fact, taking all the factors into account, -only aluminium and cadmium are suit able for alloying in comparatively large quantities, although calcium, cerium, nickel, cobalt and manganese have proved useful in small quantities. It is rather surprising to find that magnesium alloys are remarkably easy to cast. This comes about, Professor Desch said, from the fact that molten magnesium has very little power of dissolving gases, so that magnesium alloys do not suffer from the sponginess caused by the liberation of gases at the moment of freezing. n casting it is necessary to use a suit able flux and a protective atmosphere during pouring. These conditions are now well understood and no difficulties are ex perienced in the foundry. In annealing processes similar pro tection is necessary, and the special technique adopted has resulted in making a magnesium alloy foundry a remarkably clean and inoffensive place. Excellent castings of magnesium for the production of air craft parts which are not too highly stressed can be produced, and it is possible to make intricate shapes without great diffi culty. Both sand and chill castings are used. By casting in dies very satisfactory castings, involving a minimum of machining, are obtained with a higher strength than sand castings. " One of the most valuable means of increasing the strength of an alloy," Professor Desch continued, "is by the process known as age-hardening .... Put briefly, the process depends on finding some constituent which may be added to an alloy, capable of passing into solid solution at a high temperature and being retained on quenching, but separating, at first in an ultra-microscopic form, when the quenched alloy is re heated to a lower temperature, or in some instances even spon taneously at the temperature of the air." Exhaustive search at the N.P.L. for suitable magnesium alloys suggested that those containing silver were the most promising. The fact that silver was relatively costly is of little importance, since in the articles of the type produced the cost of manufacture was much greater than that of the material used. In discussing the "working properties" of magnesium alloys, Professor Desch described how the crystalline structure of the ingots affected the process of working. The form of the crystals in magnesium alloys was such that it was possible to produce material which was ductile in one direction and brittle in another. Such difficulties were overcome by making the crystal grains as small as possible by casting the ingots at a high temperature. Extruded Sections Extrusion and pressing between heated dies have both been applied successfully in commercial practice. Rolling, Professor Desch said, gives a structure with highly directional properties. Rolling at a very low speed allows much more change of section without cracking. To overcome this difficulty, mills run at very low speed are used industrially. Although data as to the actual properties of magnesium alloys under prolonged stress at high temperatures are some what scanty, the indications are that the new alloys retain their strength remarkably well at high temperatures. Pistons made from these alloys have given excellent results in trial runs under overload. Concerning the vexed question of corrosion, Professor Desch was not able to announce complete cures. Three anti-corrosion treatments had been devised, viz., using chromate, fluoride and selenium respectively. The fluoride and selenium processes did not, he said, seem to have got beyond the experimental stage as yet, and the industrial processes in use all involved the pro duction of a chromate film in one way or another. For the protection of airscrews he thought a hard metallic coating of some substance such as chromium was the most promising line of attack, ft was not practicable to deposit chromium directly on the magnesium alloy, but the production of an in termediate metallic coating capable of receiving an electrolytic deposit of chromium seemed promising, and some success had already been achieved. De Haviltand's A.CM. AT the seventeenth annual general meeting of the De Havil-land Aircraft Co., Ltd., held last week, the chairman, Mr. A. S. Butler reported a net profit on the year's working of ^71,709, compared with ^77,367 for the previous year. The small decrease was largely due to the fact that provision for the future expanded programme had involved increased overhead expenditure which was not immediately productive. Among the interesting figures quoted by Mr. Butler was the final payment of ^5,000 made in respect of the Hamilton «ur- screvv licence, bringing the total cost up to ^16,030. Satisfactory progress was reported in the sales of various types of the company's aircraft and Gipsy engines, and other activities, such as the training schools at Prestwick and Maiden head, were showing excellent results. The present programme of Government work, said Mr. But ler, did, to some extent, inevitably interrupt their establishment policy in concentrating on civil aviation in its manifold ana promising phases with the result that the development ol new types of civil and commercial machines might be retarded. They were confident, however, that any lack of progress in this au- important side of their business was only a temporary phenomenon.
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