The difference between AC and DC welding is the polarity of electrons that is emitted from the welding tool during the process. DC, also known as direct current, has a constant, single-directional, polarity. AC, known as alternating current, changes polarity evenly; on 60-hertz it will change polarity 120 times.
When the polarity of a welding tool is electrode-positive, the current will typically have a deeper penetrative impact. Electrode-negative welding, on the other hand, generates a faster deposition rate as electrodes are being melted off faster. Welding polarity needs to be matched to whatever metal is being welded as incorrect polarity can result in overheating and poor flame penetration.