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 Straight Polarity (DCSP) in arc welding
When base metals are connected with positive terminal of the welding power source (DC type) and electrode is connected with negative terminal, then the connection is termed as Direct Current Straight Polarity (DCSP). It is also called Direct Current Electrode Negative (DCEN) as electrode acts as negative terminal. Therefore, electrons emit from the electrode (negative terminal) and flow towards the base plate (positive terminal) through the small gap between them. Avalanche of flow of such electrons ultimately constitutes the electric arc.
Here 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. Upon striking, kinetic energy of the electrons is converted into thermal energy and therefore high heat is generated at the vicinity of base metal surface. It is considered that about 2/3rd of total arc heat (i.e., around 66%) is generated on base plate; whereas, rest of the heat is generated near electrode. This results in quick melting of the base metal but low filler deposition rate (for consumable electrode).
Advantages of Direct Current Straight Polarity (DCSP) in arc welding
Elimination of insufficient melting—Lack of penetration, high reinforcement, insufficient melting, etc. are basically welding defects found in arc welding when base plates are not allowed to fuse properly. Since majority of heat is generated near base plates, so these defects can be eliminated by utilizing DCSP.
Ability to fuse metals with high melting point—While welding stainless steel, titanium, etc. metals, higher heat input is required in order to properly fuse them for coalescence formation. For welding such metals, DCSP is suitable.
Suitable for thick plates joining—Thick plates require higher level of heat input for proper fusion heat spreads quickly. In such cases, DCSP polarity provides better result.
Suitable for certain electrodes—Based on the electrode type and its coating, few grades of electrode (usually EXXX2 type, such as E6012) provide better performance with DCSP.
Disadvantages of Direct Current Straight Polarity (DCSP) in arc welding
No arc cleaning action—If electrons liberate from base metal then it removes any dirt, coating, oxide layers present on the plate surface, which is known as arc cleaning action. However, with DCSP no arc cleaning action occurs as electrons liberate from electrode. So there exists a tendency of inclusion defect with DCSP.
- Read more: Arc cleaning phenomenon in welding.
High distortion—Since rate of heat input at the base metal is quite large, so distortion may arise if proper clamps and supports are not used. Distortion can even result in rejection of welded plates.
High residual stress—Although common is all arc welding processes, DCSP polarity is found to induce more residual stress on the welded component.
Broader HAZ—High heat input on base metal also increases the width of Heat Affected Zone (HAZ).
Cutting action in thin plates—For welding thin plates, DCSP is not a good choice as it may lead to cutting instead of welding. Distortion will also increase.
Not suitable for metals with low melting point—If welding speed is not adjusted then use of DCSP for welding metals having low melting point (such as copper, aluminum) will produce defective welding with undercut and high weld bead width.
Low rate of metal deposition—If the electrode is of consumable type, as in case of MMAW, GMAW, etc., then filler metal deposition rate reduces. In some cases it may lead to reduction in productivity. However, if the electrode is non-consumable type (for example, TIG) then filler metal deposition rate can be controlled as per requirement.