The term arc welding applies to a large and varied group of processes that use an electric arc as the source of heat to melt and join metals. In arc welding processes, the joining of metals, or weld, is produced by the extreme heat of an electric arc drawn between an electrode and the workpiece, or between two electrodes. The formation of a joint between metals being arc welded may or may not require the use of pressure or filler metal. The arc is struck between the workpiece and an electrode that is mechanically or manually moved along the joint, or that remains stationary while the workpiece is roved underneath it. The electrode will be either a consumable wire rod or a non-consumable carbon or tungsten rod which carries the current and sustains the electric arc between its tip and the workpiece. When a non-consumable electrode is used, a separate rod or wire can supply filler material, if needed. A consumable electrode is specially prepared so that it not only conducts the current and sustains the arc, but also melts and supplies filler metal to the joint, and may produce a slag covering as well.

Metal Electrodes. In bare metal-arc welding, the arc is drawn between a bare or lightly coated consumable electrode and the workpiece. Filler metal is obtained from the electrode, and neither shielding nor pressure is used. This type of welding electrode is rarely used, however, because of its low strength, brittleness, and difficulty in controlling the arc.

Stud welding. The stud welding process produces a joining of metals by heating them with an arc drawn between a metal stud, or similar part, and the workpiece. The molten surfaces to be joined, when properly heated, are forced together under pressure. No shielding gas is used. The most common materials welded with the arc stud weld process are low carbon steel, stainless steel, and aluminum. Figure 6-2 shows a typical equipment setup for arc stud welding.

Arc welding equipment : Setup for arc stud welding.

Gas shielded stud welding. This process, a variation of stud welding, is basically the same as that used for stud welding, except that an inert gas or flux, such as argon or helium, is used for shielding. Shielding gases and fluxes are used when welding nonferrous metals such as aluminum and magnesium. Figure 6-3 shows a typical setup for gas shielded arc stud welding.

Arc welding equipment : Setup for gas shielded arc stud welding.

 Submerged arc welding. This process joins metals by heating them with an arc maintained between a bare metal electrode and the workpiece. The arc is shielded by a blanket of granular fusible material and the workpiece. Pressure is not used and filler metal is obtained from the electrode or from a supplementary welding rod. Submerged arc welding is distinguished from other arc welding processes by the granular material that covers the welding area. This granular material is called a flux, although it performs several other important functions. It is responsible for the high deposition rates and weld quality that characterize the submerged arc welding process in joining and surfacing applications. Basically, in submerged arc welding, the end of a continuous bare wire electrode is inserted into a mound of flux that covers the area or joint to be welded. An arc is initiated, causing the base metal, electrode, and flux in the immediate vicinity to melt. The electrode is advanced in the direction of welding and mechanically fed into the arc, while flux is steadily added. The melted base metal and filler metal flow together to form a molten pool in the joint. At the same time, the melted flux floats to the surface to form a protective slag cover. Figure 6-4 shows the submerged arc welding process.

Submerged arc welding process.

Gas tungsten arc welding (TIG welding or GTAW). The arc is drawn between a nonconsumable tungsten electrode and the workpiece. Shielding is obtained from an inert gas or gas mixture. Pressure and/or filler metal may or may not be used. The arc fuses the metal being welded as well as filler metal, if used. The shield gas protects the electrode and weld pool and provides the required arc characteristics. A variety of tungsten electrodes are used with the process. The electrode is normally ground to a point or truncated cone configuration to minimize arc wandering.  Figure 6-5 shows the relative position of the torch, arc, tungsten electrode, gas shield, and the welding rod (wire) as it is being fed into the arc and weld pool.

Gas tungsten arc welding :  TIG Welding equipment.

Gas metal arc welding (MIG welding or GMAW). In this process, coalescence is produced by heating metals with an arc between a continuous filler metal (consumable) electrode and the workpiece. The arc, electrode tip and molten weld metal are shielded from the atmosphere by a gas. Shielding is obtained entirely from an externally supplied inert gas, gas mixture, or a mixture o f a gas and a flux. The electrode wire for MIG welding is continuously fed into the arc and deposited as weld metal. Electrodes used for MIG welding are quite small in diameter compared to those used in other types of welding. Wire diameters 0.05 to 0.06 in. (0.13 to 0.15 cm) are average. Because of the small sizes of the electrode and high currents used in MIG welding, the melting rates of the electrodes are very high. Electrodes must always be provided as long, continuous strands of tempered wire that can be fed continuously through the welding equipment. Since the small electrodes have a high surface-to-volume ratio, they should be clean and free of contaminants which may cause weld defects such as porosity and cracking. Figure 6-6 shows the gas metal arc welding process. All commercially important metals such as carbon steel, stainless steel, aluminum, and copper can be welded with this process in all positions by choosing the appropriate shielding gas, electrode, and welding conditions.

Gas metal arc welding : GMAW, MIG welding equipment

Shielded metal arc welding (SMAW). The arc is drawn between a covered consumable metal electrode and workpiece. The electrode covering is a source of arc stabilizers, gases to exclude air, metals to alloy the weld, and slags to support and protect the weld. Shielding is obtained from the decomposition of the electrode covering. Pressure is not used and filler metal is obtained from the electrode. Shielded metal arc welding electrodes are available to weld carbon and low alloy steels; stainless steels; cast iron; aluminum, copper, and nickel, and their alloys. Figure 6-7 describes the shielded metal arc welding process.

Shielded metal arc welding : SMAW welding equipment.

 Atomic hydrogen welding. The arc is maintained between two metal electrodes in an atmosphere of hydrogen. Shielding is obtained from the hydrogen. Pressure and/or filler metal may or may not be used. Although the process has limited industrial use today, atomic hydrogen welding is used to weld hard-to-weld metals, such as chrome, nickel, molybdenum steels, Inconel, Monel, and stainless steel. Its main application is tool and die repair welding and for the manufacture of steel alloy chain.  Arc spot welding. An arc spot weld is a spot weld made by an arc welding process. A weld is made in one spot by drawing the arc between the electrode and workpiece. The weld is made without preparing a hole in either member. Filler metal, shielding gas, or flux may or may not be used. Gas tungsten arc welding and gas metal arc welding are the processes most commonly used to make arc spot welds. However, flux-cored arc welding and shielded metal arc welding using covered electrodes can be used for making arc spot welds. Arc seam welding. A continuous weld is made along faying surfaces by drawing the arc between an electrode and workpiece. Filler metal, shielding gas, or flux may or may not be used.

Carbon Electrode.

 Carbon arc welding. In this process, the arc is drawn between electrode and the workpiece. No shielding is use. Pressure and/or filler metal may or may not be used. Two types of electrodes are used for carbon arc welding: The pure graphite electrode does not erode away as quickly as the carbon electrode, but is more expensive and more fragile. Twin carbon arc welding. In this variation on carbon-arc welding, the arc is drawn between two carbon electrodes. When the two carbon electrodes are brought together, the arc is struck and established between them. The angle of the electrodes provides an arc that forms in front of the apex angle and fans out as a soft source of concentrated heat or arc flame, softer than a single carbon arc. Shielding and pressure are not used. Filler metal may or may not be used. The twin carbon-arc welding process can also be used for brazing.  Gas-carbon arc welding. This process is also a variation of carbon arc welding, except shielding by inert gas or gas mixture is used. The arc is drawn between a carbon electrode and the workpiece. Shielding is obtained from an inert gas or gas mixture. Pressure and/or filler metal may or may not be used. Shielded carbon-arc welding. In this carbon-arc variation, the arc is drawn between a carbon electrode and the workpiece. Shielding is obtained from the combustion of a solid material fed into the arc, or from a blanket of flux on the arc, or both. Pressure and/or filler metal may or may not be used.