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Aviation History
1915
1915 - 0005.PDF
JANUARY I, 1915. 1/liCHT) WELDING AND HOW IT IS DONE. OF late years the art of welding has played such an important and constantly increasing part in connection with aeroplane construction and repair that it cannot be without interest to readers of FLIGHT to understand something of the nature of this art as so applied. Many of the small metal fittings of an aeroplane that were at one time aluminium castings are now, owing to the welders' skill, made both lighter and stronger from sheet and tubular steel, even where these parts are of most intricate form, one example of this being the almost universal use of steel strut sockets. Even parts that were formerly constructed of wood are now largely replaced by steel, as in the case of the crank-levers on the elevator and rudder, these being in the majority of cases a hollow steel tapering tube welded along the join and welded to a base plate, the whole unit being in the opinion of many constructors stronger weight for weight than is possible with wood. Welding also enters largely into the construction of many of the larger metal portions of a modern machine, such as the engine frames, the chassis structure and the top and bottom pylons. Of equal importance is the part welding plays in the repairs that are so frequently required to such fragile constructions as an aeroplane and its engine; indeed it is perhaps in this regard that the utility of the welding process is most pronounced. Welding is the art of intimately and permanently joining pieces of metal (which may be like or unlike) without the use of an adhesive metallic cement, and in Two aeroplane strut sockets, showing the part played by the welding process in their manufacture. this respect differs fundamentally trom the processes commonly recognised as soldering and brazing. Speaking in a broad sense, there are two distinct forms of weld, one of which is obtained by reducing the sur faces to be joined to a plastic state, and then by hammering or pressure to unite these surfaces by the cohesion of the molecules. In its most familiar form an example of this is the manner in which a blacksmith repairs a broken iron hoop. The second method is to cause a union of the metals by fusion : that is, by bringing the immediate parts to be joined to a liquid state and allowing them to flow together. This is what in common parlance is called autogenous welding, and is that with which we are now concerned. The actual melting of the metal is usually obtained under the action of an intensely hot flame from a blowpipe; in fact when speaking of autogenous welding it is this method that is always implied, though strictly speaking the term is just as applicable to several other processes. It will probably come as a big surprise to learn that antogenous welding is in all probability almost as old a craft as that employed by the blacksmith; it is known to be a fact that it was commonly practised by the ancient Egyptians in dealing with metals of a low melting point, lead in particular. Only of comparatively recent years, however, has the process been possible with those metals having a high melting point: to be more precise, since the industrial manufacture of oxygen. The introduction of this gas as a commercial com modity permitted the production of a flame of far greater heat intensity than any known up to then except in the chemical laboratory, and used in conjunction with hydrogen was the first step in modern developments of the art of welding. The product of combustion of these two gases being exclusively water vapour, tended, how ever, to a considerable oxidation of the metal in working, and made a sound job impossible without resorting to the expensive artifice of using excess hydrogen. Even then the difficulty was not entirely overcome, and as a conse quence other combinations with oxygen were tried, notably oil-gas, benzol vapour, and acetylene, and at the present time the last-named is that most widely adopted for the purpose. It is little more than thirteen years or so ago that the oxygen and acetylene flame was first employed. So immeasurably superior in every respect was it found to be that its application constitutes the next great step in the development of the art, and from this time dates the beginning of the rapid progress that has since led up to its present position of high esteem in the industrial world. Practically every repair executed to the fractured metal parts of aeroplanes or motor cars is effected by this process, and we shall therefore confine our remarks to this method. Without going more than superficially into the chemistry of the combustion of the flame produced by these two gases, it may be stated that the reasons for their almost universal use are, first, that the products of the first phase of combustion are carbon monoxide and hydrogen, these two being what are known as " reducing agents," which is to say they are the exact antithesis of oxidising agents, while the final products are carbon dioxide and water vapour, both of which are non- poisonous to the operator; and secondly, the intensely hot flame produced, which is about i,ooo° C. above that of oxy-hydrogen. The necessary plant for the production of the flame consists of a cylinder of oxygen, and either a self- generating acetylene installation, or a dissolved acetylene cylinder, also a number of blowpipes, or alternatively one with a series of interchangeable nozzles for use according to the nature of the work and variety of metal. The high-pressure system from a dissolved acetylene cylinder is preferable for important work, in spite of the fact that the cost of the acetylene is from four to five times as great, inasmuch as a nearly perfect mixture of gas can be delivered at the nozzle of the blowpipe ; and, more important still, certain impurities such as are always present in gas delivered from generators, consisting mainly of ammonia, sulphuretted and phosphoretted hydrogen, are absent from dissolved acetylene. The presence or otherwise of any of these impurities has a very strong bearing on the nature of the weld, and is of prime importance. This consideration of purity is of equal moment in respect to the oxygen, and because of this only that of a guaranteed quality of at least 98-5 per cent, is suitable for first-class work. As regards the blowpipes, these are of a type deter mined by many factors, the most important being the pressure of the acetylene; but whatever the type, their fundamental purpose is the same, i.e., to intimately mix C 2
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