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.
Direct Current Reverse Polarity (DCRP) in welding
When electrode is connected with positive terminal of the welding power source (DC type) and base metals are connected with the negative terminal, then the connection is termed as Direct Current Reverse Polarity (DCRP). It is also called Direct Current Electrode Positive (DCEP) as electrode is positive terminal. Therefore, electrons emit from base plate (negative terminal) and flow towards the electrode (positive terminal) through the small gap between them. Avalanche of flow of such electrons ultimately constitutes the electric arc.
Here electrons, liberating from the base plate surface, are accelerated towards electrode due to potential difference between them and finally strike at electrode tip 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.
Advantages of Direct Current Reverse Polarity (DCRP) in arc welding
Better arc cleaning action—When electrons liberate from base plate, it removes dirt, coating or oxide layer present on work surface and this action is termed as arc cleaning. DCRP polarity provides excellent arc cleaning action and thus reduces the chance of inclusion defects. Read: Arc cleaning phenomenon in welding.
High volume deposition rate—Since higher heat is generated near electrode tip, so filler metal deposition rate increases if the electrode is consumable type. This sometime is beneficial as it could reduce the welding passes requirement. For non-consumable electrodes, volume deposition rate can be controlled as per the requirement.
Preferred for thin plate joining—Due to comparatively lesser heat generation at base metal, various defects that occur while joining thin plates can be eliminated by employing reverse polarity. Such defects include distortion, undercutting, residual stress, full cutting, etc.
Suitable for joining metals having lower melting point—Welding of copper, aluminum, etc. requires lesser heat for proper fusion of base plates as their melting point is comparatively lower. In such cases DCRP is better choice; however, it needs consideration on other factors.
Disadvantages of Direct Current Reverse Polarity (DCRP) in arc welding
Life of electrode reduces—In case of non-consumable electrode (such as in TIG welding), use of DCRP may result in fast melting (ball formation at tip) of the electrode and thus life of electrode reduces. It can even lead to tungsten inclusion defect.
High reinforcement—Welding with consumable electrodes, filler metal deposition rate increases. So if the speed or edge preparation is not selected optimally, then reinforcement may increase. Reinforcement does not contribute any strength to the joint; instead, it hampers appearance and causes wastage of filler metal.
Insufficient melting and lack of penetration—Lower level of heat generation at base metals may result in various defects, such as insufficient melting, lack of penetration, etc.
Not suitable for metals with high melting point—Stainless steel, titanium, etc. require high heat input for proper fusion and thus DCRP is not suitable for welding of such metals.