Arc weding processes use a welding power supply to create and maintain an electric arc between an electrode and the base material to melt metals at the welding point. The intense heat produced by the arc quickly melts a portion of the base metal, resulting in the formation of a weld. This electrical arc is around 3590°C in its center. Filler metal is added in most welding processes to increase the volume and strength of the weld joint. A pool of molten metal, consisting of base and filler metal is formed near the tip of the electrode. As the electrode is moved along the joint, the molten metal solidifies in its wake.
The welding power supply can use either direct current (DC) or alternating current (AC), and consumable or non-consumable electrodes. The welding region is sometimes protected by some type of inert or semi-inert gas, known as a shielding gas. In arc welding, the length of the arc is directly related to the voltage, and the amount of heat input is related to the current. The voltage supplied by power companies for industrial purposes-120 volts (V), 230 V, 380 V, or 480 V is too high for use in arc welding. Therefore, the first function of an arc welding power source is to reduce the high input or line voltage to a suitable output voltage range, 20 V to 80 V. Constant current power supplies are most often used for manual welding processes such as gas tungsten arc welding and shielded metal arc welding, because they maintain a relatively constant current even as the voltage varies. This is important because in manual welding, it can be difficult to hold the electrode perfectly steady, and as a result, the arc length and thus voltage tend to fluctuate.
Gas Metal Arc Welding – GMAW
Gas metal arc welding (GMAW), also known as metal inert gas or MIG welding, is an arc welding process in which the electrode is a consumable bare wire and shielding is accomplished by flooding the arc with a inert gas. Metal inert gas (MIG) welding, differs from the SMAW process in that its electrode is a bare solid wire that is continuously fed to the weld area and becomes the filler metal as it is consumed. In contrast, SMAW electrodes must be discarded when they reach a minimum length. Gas metal arc welding is widely used in semiautomatic, machine, and automated modes. The gas shield must provide full protection, because even a small amount of entrained air can contaminate the weld deposit. Originally, only inert gases such as argon and helium were used for shielding. Today, carbon dioxide is also used and may be mixed with the inert gases. Because GMAW is continuously wire fed, the electrode do not need replacing at regular intervals such as in the case of SMAW, making this process suitable for automated welding.
A related process, flux-cored arc welding (FCAW), uses similar equipment but uses wire consisting of a steel electrode surrounding a powder fill material. This cored wire is more expensive than the standard solid wire and can generate fumes and/or slag, but it permits even higher welding speed and greater metal penetration.
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