  Metal Spraying. Although the spraying of metal does not really come within the scope of this book, the process is so closely allied to paint spraying that it is considered advisable to devote a chapter to the subject, particularly as the perfected process is of very recent date, and bids fair to be used successfully in many industries. It should be stated at once that a metallic coating may be applied to practically any surface, and that almost any metal or alloy may be employed. Stated briefly, the process consists in melting metal in the form of a rod or wire, by means of oxygen and coal gas, or other gas, depending upon the metal used. The molten metal is sprayed at a high pressure, and a surface may be quickly covered with the metal of any desired thickness. A remarkable fact concerning the process is that the metal is cooled to an extent that renders it possible to hold the hand in the jet so as to receive a coat of metal without inconvenience, and samples of wood and fabrics may be coated with metal without injury. A moment of consideration will render it clear that there is an immense field for this process; for example, as aluminium can be sprayed, a lining of that metal might be given to brewers' and cooking utensils, etc., while tanks, barrels, reservoirs, intended to contain acids and oils, can also be treated. In the production of blocks for printing, in decorative work and mural decoration, there is an immense field, while ships' bottoms, instead of being painted with composition, can be copper-plated or sprayed with any other suitable metal, in order to prevent incrustation. No doubt some very beautiful effects can be produced by means of the process. It will be convenient now to describe the machine used for the application of the metals. It consists of a pistol Fig. 142.—The Metal Spraying Machine or "Pistol." In Fig. 143 is shown very clearly the construction of the sprayer, which it will be understood comprises a combined melting and spraying jet and a feed mechanism. The metal, in the form of rod or wire, is fed to the melting flame. This, as already stated, is formed by coal gas burned in the air, or oxygen, water gas, acetylene, hydrogen, etc., may be employed instead of the coal gas. The gases are supplied at such a pressure as to prevent blowing out and to ensure a highly deoxidizing flame. The spraying jet can be of carbon dioxide, nitrogen, air, steam, etc., and it must be fed at such a pressure as to produce a sufficiently high velocity for successful coating. The usual gauges and reducing valves will, of course, be employed. The feeding of the wire is accomplished by a small pneumatic motor, driven by the spraying medium, either in series or parallel with the main Fig. 145 shows an enlargement of the nozzle with the different parts marked. The cost of the process is not prohibitive; the cost of the metal only on one square foot of a thickness of 0·001 inch is quite small with the cheaper metals. The process is put on the market by the British Metal Spray Co., Ltd., Queen Anne's Chambers, Tothill Street, Westminster, London, S.W. In a paper read by Mr. R. K. Morcom before the Institute of Metals, the following interesting information was given:— Fig. 144.—Sectional Drawing of the Metal Spraying Machine or "Pistol." With a given design of jet there is only a certain volume left by the air-jet which can be filled with flame, and this flame has a limiting temperature which cannot be exceeded. The wire, passing through this cone of flame receives heat, partly by radiation, but chiefly by conduction, and becomes melted; but there is a definite limit to the amount of heat which can be picked up by the wire passing through the flame, and a definite limit to the rate at which it can be melted. This cannot be increased by forcing more gas into the flame, as the extra gas is merely blown away by the air-jet. It is possible to increase the rate of melting by shaping the nozzles so as to leave room for a larger cone of flame, and experiments are in progress on this point. There is, therefore, a most definite economical quantity of gas which should be used in the pistol, this quantity being In refractory metals these quantities may be increased slightly, as a slightly higher temperature can be obtained if the burning gases are under a pressure greater than atmospheric, and this occurs if the gas quantities are increased, the inner surface of the air-jet acting to some extent as an enclosing wall to the flame. On the other hand, for the more easily fusible and oxidizable metals, such as tin, lead, and zinc, it is advisable to keep the gas quantities rather below the figure given, so as to avoid any possibility of overheating and burning any portion of the wire. The outer jet performs a threefold purpose: it keeps the nozzles and wire cool, it cools the object, and it produces the requisite velocity. Fig. 145.—Diagrammatic Representation of Melting and Spraying Jets in Action. The velocity of the air leaving the jet will be independent of the volume discharged, and depends only upon the pressure at the jet, so long as there is no disturbance due to the entraining of air from the surrounding atmosphere. This, of course, will actually occur in practice, and the layer of air must have a certain thickness in order to prevent its being broken up, and its velocity destroyed by mixing with the surrounding atmosphere. As at present constructed the standard pistol uses about 0·55 to 0·6 cubic foot per minute for every 1 lb. per square inch air pressure, so that with an air supply at 80 lb. per square inch, which is a very suitable figure for ordinary spraying, the air consumption will be from 45 to 50 cubic feet per minute. The bulk of this will be from 830 to 920 grammes, and the mass of metal sprayed by this air will be from about 8 grammes in the case of iron to about 200 grammes in the case of lead. The action of deposition is probably a complex one. The minute particles of solid metal are driven with such force against the object that, in some cases, they fuse, but owing to their small relative size, are promptly chilled by the object to which they adhere. If any of the particles are molten or gaseous they will adhere. In addition, the suddenly chilled particles are possibly, or even probably, in the state of unstable equilibrium found in "Prince Rupert's Drops," and act like so many minute bombs, bursting on impact into almost molecular dimensions, and penetrating the smallest cracks and fissures of the object. The process requires some care in manipulation, as, by varying the conditions, it is possible to spray porous or non-porous coatings, and, with some metals, anything from a pure metal to a pure oxide. With care, however, non-porous, oxide-free, adherent coatings can be produced, of almost any metal on almost any solid. In addition to metals, it is possible to spray fusible non-metals, or, by stranded wires, alloys of metals or mixture of metals with non-metals. The process is so new that its uses are still partly to be developed. But it is easy to see that it may have far-reaching value for protective coatings against weather or fire, for ornament, for electrical resistance and conductors, for the production of special alloys, for joint making, and for many other purposes. Quite in a different category comes that of very fine casting. The surface of a pattern, polished or slightly greasy, is most minutely copied, and it is possible to produce process blocks very rapidly. It may be useful to line moulds before pouring in a metal. The application of the process to the production of very fine or coarse metallic powders is being investigated. The bulk of the work has hitherto been carried on in laboratories, but the apparatus is gradually becoming used in the more progressive factories, where extended facilities, and the knowledge of specialised requirements, will ensure a rapid improvement in technique and results. The research on the lower melting point metals has been greater than on the others, and undoubtedly the economy with them both can be greatly improved. Pre The following metals are among those which have been successfully sprayed by this process:—Aluminium, brass, bronze, copper, cupro-nickel, iron, gold, nickel, silver, tin, zinc, lead. It will be obvious that this method is a most useful one to employ when it is desired to prevent iron from rusting on machine parts which cannot possibly be treated by chemical or other anti-rust processes, and be rendered immune from rusting by treatment where they stand. To give some idea of the cost, it may be said that if the thickness of 0.001 of one inch of zinc is deposited, the cost of metal for coating 10 square feet would only be a fraction over 4d.; while for lead the cost would be about 2d. The amount of gas used is not a large item, being at the rate per minute of 0.50 cubic foot of oxygen and 0.55 cubic foot of coal gas when spraying zinc, and 0.101 cubic foot less in each case where lead is being used in the pistol. These figures are given on the authority of the "Daily Telegraph." Fig. 148.—Show Card.
Fig. 149.—A Sprayed Frieze (Aerograph). |
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