Welding is the joining of metals by the application of heat and/ or pressure, with or without the addition of a similar metal i.e filler metal. A welding technique may be designated according to the purpose for which it is used, or the procedure employed, or the nature of the heat source i.e gas welding, electrical-resistance welding, arc welding, welding based on chemical reactions, etc.

As regards the purpose, a distinction is to be made between connective welding – i.e., the forming of joints and connections – and build-up welding or surfacing, which is the process of reconditioning damaged or worn engineering components by the application of weld metal or the protection of components against corrosion or wear by the application of an armoring layer of more resistant metal (hard surfacing). As to the nature of the welding process itself, a distinction may be made between pressure welding and fusion welding.

A pressure welding process in which macrodeformation of the base material to produce coalescence results from the application of heat and pressure.

A weld occurs when pieces of metal are joined by causing the interface to melt and blend prior to solidifying as a uniform metal joint. This process may be caused by heat, pressure or a combination of both. When heat alone is used the process is called fusion welding.

Pressure welding usually involves heating the surfaces to a plastic state and then forcing the metal together. The heating can be by electric current of by friction resulting from moving one surface relative to the other.

In pressure welding, the parts to be jointed are first locally heated at the place where the joint is to be formed and are then squeezed together in the plastic state so that they are united. In general, no filler metal is employed. Cold pressure welding makes use of high pressure, without the help of heat, to unite the parts. Related to this process are ultrasonic welding and explosion welding.

The oldest welding technique, still used in art ironwork and smith’s work, is forge welding. This is the process of joining steel or iron parts by heating them in a forge until they reach a plastic state and are then united by hammering, by pressure, or by rolling.

In gas pressure welding (Fig.1) the parts to be joined are heated by a gas-and-oxygen flame and are united by the exertion of continuous or of impact-type pressure. This principle is applied, for example, in the manufacture of small-diameter tubes from steel strip (Fretz-Moon process). By means of a special die, called a bell, and shaping rollers the continuous strip is formed into a tube. The edges of the strip are heated to welding temperature by gas burners, and the edges are pressed together and united by pressure rollers. In the case of arc pressure welding and the special techniques derived from it, the heat is generated by an electric arc briefly produced between the parts to be joined, which are then united by impact action.

In the resistance pressure-welding process (Fig.2) the heat is generated by the resistance encountered by an electric current which is passed through the material, especially the high resistance at the contact faces of the parts to be jointed. The current is applied through electrodes or generated in the parts by induction. Heating based on electrical resistance, is, for example, utilized in the process known as resistance butt welding (Fig.2a) : the two parts to be joined end to end are gripped, in contact with each other, in copper jaws which serve as electrodes for the passage of current across the joint. When the metal at the joint has reached a sufficiently high temperature, the current is switched off and the contact pressure is increased to unite the parts.

Similar in principle to resistance butt welding is spot welding (Fig .2b,P.131), which is a method of uniting by means of localized welds and is employed more particularly for sheet metal and wire. The sheets are gripped between copper electrodes through which a heavy current is passed; fusion occurs at the spots where the electrodes are thus applied. Seam welding (Fig.2c Pg 131) is the process of closing a seam by a continuous resistance weld formed between two copper-roller electrodes. The principle is the same as in spot welding. The process called projection welding (Fig.2d. pg.131) is used mainly for joining sheet-metal parts : prior to welding, projections are raised on the surfaces of the sheets, and the welds occur at these places. The current is introduced through flat electrodes.

Resistance pressure welding is widely used in the manufacture of tubes. More particularly, three techniques are employed. In all three, the initial material is a continuous strip of steel sheet preformed into a tubular section, but as yet with an open joint. In the conventional resistance-welding process of tube manufacture (Fig.3) the tube is heated at the joint with low-voltage high-intensity alternating current by means of a large-diameter copper-roller electrode. The heated edges of the joint are then forced together by pressure rollers and thus united. With this process it is possible to produce tubes of 6mm to 500 mm diameter and 0.6 mm to 10mm wall thickness.

A newer process is a contact-electrode high-frequency welding, using alternating current of 100 to 450 kilocycles/sec. Which is supplied to the edges of the joint through sliding contact electrodes. The method is employed mainly for welding the longitudinal seams of thin-walled tubes and other hollow sections. A third process widely employed in present-day tube manufacture is induction welding (Fig.4), in which the joint is heated by induction produced by a medium-frequency (10 kilocycles/sec.) alternating current. The inductor is in the form of a coil which encloses the tube or is a linear inductor placed on the joint.

In the process known as thermit pressure welding, the heat is generated by a chemical reaction between powdered aluminum and iron oxide that develops a temperature of 3000oC. The aluminum powder is converted into aluminum oxide, and molten iron is formed by the reduction of the iron oxide. A recently developed method is friction welding (Fig.5). The parts to be joined are mounted, with the joint faces in contact with each other, in a device somewhat like a lathe. One of the parts is then set in rotation.

The friction generated heat, and when the requisite welding temperature has been reached, the rotating part stopped and the two parts are then forced together with increased pressure so that they unite. Explosion welding (Fig.6) is a form of cold pressure welding in which the pressure is produced by the shock wave from the detonation of an explosive. It is sometimes used for the joining of thin overlapping plates. The plates are gripped together and are covered with a “buffer layer” of rubber sheet and a layer of a special explosive, which is detonated electrically. Another new method is ultrasonic welding (Fig.7) which is effected by high energy concentrations developed at the joint by ultrasonic vibrations, in combination with pressure.