In general, welding may be described as a process of uniting two pieces of metal or alloy by raising the temperature of the surfaces to be joined so that they become plastic or molten. This may be done with or without the application of pressure and with or without the use of added metal. This definition excludes the more recently developed method of cold-welding, in which pressure alone is used. Cold-welding, however, has a limited application, and is used principally for aluminium and its alloys, and not for steel.
There are numerous methods of welding, but they can be grouped broadly into two categories. Forge welding is the term covering a group of welding processes in which the parts to be joined are heated to a plastic condition in a forge or other furnace, and are welded together by applying pressure or impact, e.g. by rolling, pressing, or hammering. Fusion welding is the process where the surfaces to be joined are melted with or without the addition of filler metal. The term is generally reserved for those processes in which welding is achieved by fusion alone, without pressure.
Forge welding will be dealt with first. Pressure welding is the welding of metal by means of mechanical pressure whilst the surfaces to be joined are maintained in a plastic state. The heating for this process is usually provided by the process of resistance welding, where the pieces of metal to be joined are pressed together and a heavy current is passed through them.
Projection welding is a resistance-welding process in which fusion is produced by the heat obtained from the resistance to flow of electric current through the work parts, which are held together under pressure by the electrodes providing the current. The resulting welds are localized at predetermined points by the design of the parts to be welded. The localization is usually accomplished by projections or intersections.
Spot welding is a resistance-welding process ofjoining two or more overlapping parts by local fusion of a small area or 'spot'. Two copper-alloy electrodes contact either side of the overlapped sheets, under known loads produced by springs or air pressure. Stitch welding is spot welding in which successive welds overlap. Seam welding is a resistance-welding process in which the electrodes are discs. Current is switched on and off regularly as the rims of the discs roll over the work, with the result that a series of spot welds is at such points. If a gas-tight weld is required, the disc speed and time cycle are adjusted to obtain a series of overlapping welds.
Flash-butt welding is a resistance-welding process which may be applied to rod, bar, tube, strip, or sheet to produce a butt joint. After the current is turned on, the two parts are brought together at a predetermined rate so that discontinuous arcing occurs between the two parts to be joined. This arcing produces a violent expulsion of small particles of metal (flashing), and a positive pressure in the weld area will exclude air and minimize oxidation. When sufficient heat has been developed by flashing, the parts are brought together under heavy pressure so that all fused and oxidized material is extruded from the weld.
Fusion-welding processes can now be dealt with. The heat for fusion welding is provided by either gas or electricity. Gas welding is a process in which heat for welding is obtained from a gas or gases burning at a sufficiently high temperature produced by an admixture of oxygen. Examples of the gases used are acetylene (oxy-acetylene welding), hydrogen (oxy-hydrogen welding), and propane (oxy-propane welding). In air-acetylene welding, the oxygen is derived from the atmosphere by induction.
Electrical fusion welding is usually done by the process of 'arc welding'. Metal-arc welding is welding with a metal electrode, the melting of which provides the filler metal. Carbon arc welding is a process of arc welding with a carbon electrode (or electrodes), in which filler metal and sometimes flux may be used. Submerged-arc welding is a method in which a bare copper-plated steel electrode is used. The arc is entirely submerged under a separate loose flux powder which is continually fed into and over the groove which is machined where the edges to be welded are placed together. Some of the flux powder reacts with the molten metal: part fuses and forms a refining slag which solidifies on top of the weld deposit; the remainder of the powder covers the weld and slag, shielding them from atmospheric contamination and retarding the rate of cooling.
Argon-arc welding is a process where an arc is struck between an electrode (usually tungsten) and the work in an inert atmosphere provided by directing argon into the weld area through a sheath surrounding the electrode. Heliarc welding uses helium to provide the inert atmosphere, but this process is not used in the United Kingdom, because of the non-availability of helium. Several proprietary names are used for welding processes of this type, e.g. Sigma (shielded inert-gas metal-arc) welding uses a consumable electrode in an argon atmosphere. Atomic-hydrogen arc welding is a process where an alternating-current arc is maintained between tungsten electrodes, and each electrode is surrounded by an annular stream of hydrogen. In passing through the arc, the molecular hydrogen is dissociated into its atomic state. The recombination of the hydrogen atoms results in a very great liberation of heat which is used for fusing together the metals to be joined. Stud welding is a process in which an arc is struck between the bottom of a stud and the base metal. When a pool of molten metal has formed, the arc is extinguished and the stud is driven into the pool to form a weld.
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