Direct current welding is when the current flows in one direction only. Compared with AC welding the current once flowing will not go to zero until the welding has ended. In general TIG machines are capable of welding in either DC or AC/DC with very few offering AC only.
The TIG welding process offers two types of DC (direct current) modes, electrode negative (DCEN) and electrode positive (DCEP).
Direct Current – Electrode Negative (DCEN)
This method of welding can be used for a wide range of materials. The welding torch is connected to the negative output (-) of the tig welder and the earth return cable to the positive output (+).
When the arc is established the current flows in the circuit and the heat distribution in the arc is approximately 33% in the negative side of the arc (the welding torch) and around 67% in the positive side of the arc (the work piece). This balance provides deep arc penetration of the arc and reduces heat in the electrode. This reduced hear in the electrode allows more current to be carried by smaller electrodes compared to other polarity connections. This method of connection is often referred to as ‘straight polarity’ and is the most common connection in DC welding.
Direct Current – Electrode Positive (DCEP)
When welding in this mode the welding torch is connected to the positive output of the tig welder and the earth return cable to the negative output. When the arc is established the current flows in the circuit and the heat distribution in the arc is approximately 33% in the negative side of the arc (the work piece) and around 67% in the positive side of the arc (the welding torch).
This means the electrode is subjected to the highest heat levels and therefore must be much larger than with DCEN mode even when the current is relatively low to prevent the electrode over heating or melting. The work piece is subjected to the lower heat level so the weld penetration will be shallow. This method of connection is often referred to as ‘reverse polarity’.
It may be questioned what use is this mode when tig welding? The reason is that some non ferrous materials such as aluminum on normal exposure to atmosphere form an oxide on the surface. This oxide is created due to the reaction of oxygen in the air and the material similar to rust on steel. However this oxide is very hard and has a higher melting point than the actual base material and therefore must be removed before welding can be carried out. The oxide may be removed by grinding, brushing or chemical cleaning but as soon as the cleaning process ceases the oxide begins to form again. Therefore ideally it would be cleaned during welding. This effect happens when the current flows in the DCEP mode when the electron flow will break down and remove the oxide. The DCEP method is an ideal mode for welding these materials with this type of oxide coating. Unfortunately because of the exposure of the electrode to the high heat levels the electrode size would have to be large and arc penetration would be low.
The solution for these types of materials would be the deep penetrating arc of DCEN mode plus the cleaning of DCEP. To obtain these benefits AC welding mode is used.