heated in a furnace to plastic condition and formed to the required shape by upsetting. Then they are brought together and hammered, so as to get the finished joint similarly, a butt joint can be prepared by forge welding as shown in Fig. 17.3 . Before joining the two pieces, their ends are formed to the required shape according to the type of joint. The forge welding is a manual process and is limited to light work because all forming and welding are done with a hand sledge. It is a slow process and there is considerable danger of an oxide scale forming on tile surfaces. The tendency to oxidize can be counteracted somewhat by using a thick fuel bed and by covering the surfaces with a fluxing material, which dissolves the oxides. Borax in combination with salt ammoniac is commonly used as flux. The forge welding is recommended to such metals, which have a large welding temperature range like low carbon steel and wrought iron. By the increase of carbon content, this range decreases rapidly. High carbon steels alloy steels require considerably more care in controlling temperature and producing the welds. Large work may be welded in hammer forges driven by steam. Welded steel pipe is made mechanically by running the preheated strips through rolls, which form the pipe to size and apply the necessary pressure for the weld.
Welding 29 24. 17.8.2 Friction Welding 25. In this process, the heat for welding is obtained from mechanically induced sliding motion between rubbing surfaces of work-pieces as shown in Fig. 17.27. In friction welding, one part is firmly held while the other (usually cylindrical) is rotated under simultaneous application of axial pressure. As these parts are brought to rub against each other under pressure, they get heated due to friction. When the desired forging temperature is attained, the rotation is stopped and the axial pressure is increased to obtain forging action and hence welded joint. Most of the metals and their dissimilar combinations such as aluminium and titanium, copper and steel, aluminium and steel etc. can be welded using friction welding. 26. 27. 17.8.3 Explosive Welding 28. In explosive welding, strong metallurgical bonds can be produced between metal combinations which cannot be welded by other methods or processes. For example, tantalum can be explosively welded to steel although the welding point of tantalum is higher than the vaporization temperature of steel. Explosive welding process is shown in Fig. 17.28. It is carried out by bringing together properly paired metal surfaces with high relative velocity at a high pressure and a proper orientation to each other so that a large amount of plastic interaction occurs between the surfaces. The work piece, held fixed is called the target plate and the other called flyer plate. While a variety of procedures have been successfully employed, the main techniques of explosive welding can be divided into contact techniques and impact techniques. In critical space and nuclear application, explosive welding permits fabrication of structures that cannot be made by any other means and, in some commercial applications, explosive joining is the least costly method. The main advantage of
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