<P> Arc welding is a process that is used to join metal to metal by using electricity to create enough heat to melt metal, and the melted metals when cool result in a binding of the metals . It is a type of welding that uses a welding power supply to create an electric arc between an electrode and the base material to melt the metals at the welding point . They can use either direct (DC) or alternating (AC) current, and consumable or non-consumable electrodes . The welding region is usually protected by some type of shielding gas, vapor, or slag . Arc welding processes may be manual, semi-automatic, or fully automated . First developed in the late part of the 19th century, arc welding became commercially important in shipbuilding during the Second World War . Today it remains an important process for the fabrication of steel structures and vehicles . </P> <P> To supply the electrical energy necessary for arc welding processes, a number of different power supplies can be used . The most common classification is constant current power supplies and constant voltage power supplies . In arc welding, the voltage is directly related to the length of the arc, and the current is related to the amount of heat input . 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 . Constant voltage power supplies hold the voltage constant and vary the current, and as a result, are most often used for automated welding processes such as gas metal arc welding, flux cored arc welding, and submerged arc welding . In these processes, arc length is kept constant, since any fluctuation in the distance between the wire and the base material is quickly rectified by a large change in current . For example, if the wire and the base material get too close, the current will rapidly increase, which in turn causes the heat to increase and the tip of the wire to melt, returning it to its original separation distance . </P> <P> The direction of current used in arc welding also plays an important role in welding . Consumable electrode processes such as shielded metal arc welding and gas metal arc welding generally use direct current, but the electrode can be charged either positively or negatively . In general, the positively charged anode will have a greater heat concentration (around 60%). "Note that for stick welding in general, DC+ polarity is most commonly used . It produces a good bead profile with a higher level of penetration . DC - polarity results in less penetration and a higher electrode melt - off rate . It is sometimes used, for example, on thin sheet metal in an attempt to prevent burn - through ." "With few exceptions, electrode - positive (reversed polarity) results in deeper penetration . Electrode - negative (straight polarity) results in faster melt - off of the electrode and, therefore, faster deposition rate ." Non-consumable electrode processes, such as gas tungsten arc welding, can use either type of direct current (DC), as well as alternating current (AC). With direct current however, because the electrode only creates the arc and does not provide filler material, a positively charged electrode causes shallow welds, while a negatively charged electrode makes deeper welds . Alternating current rapidly moves between these two, resulting in medium - penetration welds . One disadvantage of AC, the fact that the arc must be re-ignited after every zero crossing, has been addressed with the invention of special power units that produce a square wave pattern instead of the normal sine wave, eliminating low - voltage time after the zero crossings and minimizing the effects of the problem . </P>

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