The electric current used in a welding arc may be either direct current (DC) or alternating current (AC). Direct current flows constantly in one direction. Alternating current is continually changing direction. When direct current is used for welding, the welding electrode (consumable or nonconsumable) can be the positive pole or negative pole in the electrical circuit. The workpiece will have the opposite polarity. These two arrangements for current flow are called DC electrode positive (DCEP) and DC electrode negative (DCEN), respectively (Figure 1.9). The type of current selected and its polarity can have a significant influence on the shape and penetration of the weld bead .
For example, in gas tungsten arc (GTA) welding, a non consumable electrode welding process, direct current electrode negative (DCEN) is the polarity used most often. Electrons are easily emitted from the tungsten electrode (cathode). When the electrons travel through the arc they accelerate to very high speed. About 70% of the arc heat is released at the workpiece (anode or positive pole) due to electrons striking the surface at high speed. This produces a weld bead with greater penetration. When the polarity is reversed (DCEP) the workpiece becomes the cathode. The weld pool cannot easily emit electrons because the molten pool is at a much lower temperature than the tungsten and will resist the release of electrons. While DCEP is helpful in cleaning the weld pool by removing the oxides, about 70% of the arc heat is now generated at the electrode (anode). This reduces the life of the tungsten electrode and the weld bead has reduced penetration. The use of alternating current provides arc characteristics that are average of those for DCEN and DCEP (Figure 1.10).
The heat balance in consumable electrode processes differs from that in tungsten arcs. Thus, a greater amount of heat is generated at the cathode rather than the anode.
When using the gas metal arc process, direct current electrode positive is the polarity of choice as it leads to greater heat generation at the workpiece (cathode) and therefore greater penetration. Conversely, DCEN polarity produces more heat at the electrode (cathode), and therefore increases the electrode melt-off rate and reduces penetration.