Direct current TIG (DC) welding is when the current flows in one direction only. Compared with AC (Alternating Current) TIG welding the current once flowing will not go to zero until welding has ended. In general TIG inverters will be capable of welding either DC or AC/DC welding with very few machines being AC only.
DC is used for TIG welding Mild Steel/Stainless material and AC would be used for welding Aluminium.
The TIG welding process has three options of welding current based upon the type of connection. Each method of connection has both advantages and disadvantages.
Direct Current – Electrode Negative (DCEN)
This method of welding can be used for a wide range of materials. The TIG welding torch is connected to the negative output of the welding inverter and the work 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 around 33% in the negative side of the arc (the welding torch) and 67% in the positive side of the arc (the work piece).
This balance gives deep arc penetration of the arc into the work piece and reduces heat in the electrode.
This reduced heat 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 used in DC welding.
Direct Current – Electrode Positive (DCEP)
When welding in this mode the TIG welding torch is connected to the positive output of the welding inverter and the work 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 around 33% in the negative side of the arc (the work piece) and 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 overheating 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.
Also, with this mode the effects of magnetic forces can lead to instability and a phenomenon known as arc blow where the arc can wander between the materials to be welded. This can also happen in the DCEN mode but is more prevalent in the DCEP mode.
It may be questioned what use is this mode when welding. The reason is that some non ferrous materials such as aluminium 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 some 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. It could therefore be assumed that DCEP would be the 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 in this mode the electrodes 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 mode. To obtain these benefits AC welding mode is used.
Alternating Current (AC) Welding
When welding in AC mode the current supplied by the welding inverter operates with either positive and negative elements or half cycles. This means current flows one way and then the other at different times so the term alternating current is used. The combination of one positive element and one negative element is termed one cycle.
The number of times a cycle is completed within one second is referred to as the frequency. In the UK the frequency of alternating current supplied by the mains network is 50 cycles per second and is denoted as 50 Hertz (Hz)
This would mean that the current changes 100 times each second. The number of cycles per second (frequency) in a standard machine is dictated by the mains frequency which in the UK is 50Hz.
It is worth noting that as frequency increases magnetic effects increase and items such as transformers become increasingly more efficient. Also increasing the frequency of the welding current stiffens the arc, improves arc stability and leads to a more controllable welding condition.
However, this is theoretical as when welding in the TIG mode there are other influences on the arc.
The AC sine wave can be affected by the oxide coating of some materials which acts as a rectifier restricting the electron flow. This is known as arc rectification and its effect causes the positive half cycle to be clipped off or distorted. The effect for the weld zone is erratic arc conditions, lack of cleaning action and possible tungsten damage.
Alternating Current (AC) Waveforms
The Sine Wave
The sinusoidal wave consists of the positive element building up to its maximum from zero before falling back to zero (often referred to as the hill).
As it crosses zero and the current changes direction towards its maximum negative value before then rising to zero (often referred to as the valley) one cycle is completed.
Many of the older style TIG welders were only sine wave type machines. With the development of modern welding inverters with increasingly more sophisticated electronics came development on control and shaping of the AC waveform used for welding.
The Square Wave
With the development of AC/DC TIG welding inverters to include more electronics a generation of square wave machines was developed. Due to these electronic controls the cross over from positive to negative and vice versa can be made almost in an instant which leads to more effective current in each half cycle due to a longer period at maximum.
The effective use of the magnetic field energy stored creates waveforms which are very near square. The controls of the first electronic power sources allowed the control of a ‘square wave’. The system would allow control of the positive (cleaning) and negative (penetration) half cycles.
The balance condition would be equal + positive and negative half cycles giving a stable weld condition.
The problems that can be encountered are that once cleaning has occurred in less than the positive half cycle time then some of the positive half cycle is not productive and can also increase potential damage to the electrode due to overheating. However, this type of machine would also have a balance control which allowed the time of the positive half cycle to varied within the cycle time.
This can be achieved by placing the control to a position which will enable more time to be spent in the negative half cycle with respect to the positive half cycle. This will allow for higher current to be used with smaller electrodes as more
of the heat is in the positive (work). The increase in heat also results in deeper penetration when welding at the same travel speed as the balanced condition.
A reduced heat affected zone and less distortion due to the narrower arc.
This can be achieved by placing the control to a position which will enable more time to be spent in the positive half cycle with respect to the negative half cycle. This will allow for very active cleaning current to be used. It should be noted that there is an optimum cleaning time after which more cleaning will not occur and the potential of damage to the electrode is greater. The effect on the arc is to provide a wider clean weld pool with shallow penetration.