Arc welding power sources can supply either AC or DC or both forms of current. In case of DC polarity, current flows only in one direction; whereas, in case of AC, current flow direction reverses in every cycle (number of cycles per second depends on the frequency of supply). Now, in arc welding, base metals are connected with one terminal and the electrode is connected with other terminal. Under presence of sufficient potential difference, continuous flow of electrons between them through a small gap constitutes the arc (prime source of heat in arc welding).
Depending on the connection of base metals and electrode with the ports of power supply, DC polarity can be subdivided into two categories—Direct Current Straight Polarity (DCSP) and Direct Current Reverse Polarity (DCRP). It is to be noted that for AC supply, both polarities occur one after another in every cycle for a number of times (equals to frequency of supply).
- DCSP or DCEN—Base plate is positive and electrode is negative.
- DCRP or DCEP—Base plate is negative and electrode is positive.
- AC or Reversing Polarity—In one half of cycle base plate is positive and electrode is negative; and in next half of the cycle base plate is negative and electrode is positive. This repeats in every cycle.
Reversing polarity or alternating polarity in welding
Irrespective of the connection made, electrode will be repeatedly positive and negative one after another with reversing polarity. The number of such change basically depends on the frequency of power supply. For example, a 50Hz power supply will make 50 cycles in a second and in every cycle electrode will become positive for 50% of the time (in this case 0.01 second) and for rest 50% of the time electrode will be negative. Consequently, base plates will have opposite polarity at any instant. Apart from changing frequency, few welding power sources also provide facility to alter the duration of these two cycles, such as for 70% of the cycle time electrode will be negative and for rest 30% of the cycle time electrode will have positive polarity.
When electrode acts as a positive polarity, the electrons, liberating from the base plate surface, are accelerated towards electrode due to potential difference between them and finally strike the electrode at a very high velocity. Upon striking, kinetic energy of the electrons is converted into thermal energy and therefore high heat is generated at the electrode tip. It is considered that about 2/3rd of total arc heat (i.e., around 66%) is generated at the electrode tip; whereas, rest of the heat is generated near base plate. This results in quick melting of the electrode and high filler deposition rate. However, it may not provide sufficient heat for complete fusion of the base plates and this can lead to various welding defects like lack of fusion, incomplete penetration, high reinforcement, etc.
In the opposite case when electrode acts as a negative polarity, electrons, liberating from the electrode, are accelerated towards base metal due to potential difference between them and finally strike the base metal surface at a very high velocity. Therefore high heat is generated at the base metal surface. It is considered that about 2/3rd of total arc heat (i.e., around 66%) is generated at the vicinity of base metal; whereas, rest of the heat is generated near electrode. This results in quick melting of the base metal.
Advantages of AC polarity on arc welding
Since both the polarities occur one after another so AC polarity provides advantages of both straight polarity and reverse polarity; however, only to some extent. In most of the influencing factors, effect of this polarity is moderate. It gives moderate depth of penetration, moderate level of oxide cleaning action, moderate rate of filler deposition, moderate level of distortion and HAZ. You may read: