Polarity in arc welding processes indicates the direction of flow of current (in other word – electrons) in the external circuit. Basically arc welding power source can provide either DC or AC type current. Some modern power sources also contain switch to convert one from another (integrated AC-DC converter), so these sources can supply both AC and DC power. Now in arc welding, the electrode is connected with one terminal of the power source and the base metal is connected with the other terminal. Depending on the connections, two cases are possible:
- Direct Current Straight Polarity (DCSP) or Direct Current Electrode Negative (DCEN)—When electrode is connected with the negative terminal of the power source and base metals are connected with the positive terminal.
- Direct Current Reverse Polarity (DCRP) or Direct Current Electrode Positive (DCEP)—When base metals are connected with the negative terminal of the power source and electrode is connected with the positive terminal.
However, if the power source provides alternating current (AC) then both the conditions occur one after another in every cycle. All these three polarities have respective pros and cons. A comparison among AC, DCSP and DCRP polarities in arc welding is provided in the following sections. Before going to the comparison, you may read the following articles related to arc welding polarity.
- Direct Current Straight Polarity (DCSP) in Arc Welding
- Direct Current Reverse Polarity (DCRP) in Arc Welding
- Alternating Current (AC) Polarity in Arc Welding
Comparison among DCEN, DCEP and AC polarities of welding
In DCSP, when high velocity electrons strike the workpiece its kinetic energy is converted into thermal energy, which results in higher heat generation nearer to workpiece. This causes quick and complete melting of base metal and hence better penetration. Unlike DCSP, in DCRP base plate is made negative so higher heat is generated at the tip of the electrode. Due to lack of heat, DCRP may result in incomplete fusion and lack of penetration. AC offers moderate level of penetration.
- DCRP (DCEP)—Minimum penetration.
- DCSP (DCEN)—Maximum penetration.
- AC—Moderate penetration.
Oxide cleaning action
When electrons flow from base metal surface to the electrode in the external circuit, it remove dirt, coating or oxide layer from the work surface (this is known as arc cleaning action). This prevents inclusion defects in the welded components. DCRP provides best arc cleaning action as electrons flow from base plate (negative polarity) to the electrode (positive polarity). Obviously, DCSP offers worst arc cleaning as the polarity is opposite to DCRP. In fact DCSP increases the tendency of inclusion defects by embedding foreign particles inside the molten metal pool. AC provides moderate arc cleaning action.
- DCRP (DCEP)—Best cleaning.
- DCSP (DCEN)—Worst cleaning.
- AC—Moderate cleaning.
Filler deposition rate
Due to higher heat generation at the vicinity of electrode, the DCRP polarity melts down consumable electrode quickly and thus increases filler metal deposition rate. On the other hand, DCSP produces lesser heat at electrode and thus filler deposition rate is less. AC once again takes moderate stand here.
- DCRP (DCEP)—High filler deposition rate.
- DCSP (DCEN)—Low filler deposition rate.
- AC—Moderate filler deposition rate.
Potential difference requirement
In case of AC the open circuit voltage (that is the voltage required to initiate the arc) is quite large, usually in between 80-100 volt. However, in case of DC, open circuit voltage is almost half of that for AC. In DC, open circuit voltage is around 40-60 Volt, whereas arc voltage is about 18-30 Volt.
Compatibility with electrode
Various electrodes need specific polarity for better performance, as provided below. The types of coating and corresponding polarity requirement are usually determined by the last digit of coating code.
- EXXX0 (such as E6010)—DCEP only
- EXXX1 (such as E6011)—AC or DCEP
- EXXX2 (such as E6012)—AC or DCSP
- EXXX3 (such as E6013)—AC or DC
- EXXX4 (such as E6014)—AC or DC
- EXXX5 (such as E6015)—DCRP only
- EXXX6 (such as E6016)—AC or DCRP
Distortion, HAZ and residual stress
Due to high rate of heat input per unit length, distortion of the welded components may increase if DCSP polarity is applied. Similarly, width of the heat affected zone (HAZ) and level of residual stress may also increase. However, all these welding defects can be kept under control if DCRP is applied. In this case also, AC provides an optimum result.
- DCRP (DCEP)—Low distortion, narrow HAZ, low residual stress.
- DCSP (DCEN)—High distortion, wide HAZ, high residual stress.
- AC—Moderate levels of distortion, HAZ and residual stress.