Arc welding is one type of fusion welding process where base metals are fused by applying heat in order to form coalescence. Required heat is supplied by an electric arc constituted between positive and negative terminals of an electric circuit inbuilt with a power source. For welding purpose work metal is made one terminal and the electrode is made another, and thus arc constitutes in between them in the external circuit. Since electrons always flow from negative terminal to positive terminal of any external circuit, so based on the connection made, two cases are possible:
- Electrode is connected with negative terminal of the power source; whereas, base metals are connected with positive terminal.
- Base metals are connected with the negative terminal of the power source; whereas, electrode is connected with positive terminal.
However, if the power source provides alternating current (AC) then both conditions occur one after another in every cycle. Basically arc welding power sources can provide either DC or AC type current. Some modern power sources also contain facility to convert one from another (integrated with AC-DC converter), so these sources can supply both AC and DC power. Therefore, arc welding can be carried out in any of the following three polarities; however, each has certain advantages over others as elaborated in the successive sections.
Polarity indicates direction of flow of current (in other word – electrons) between base plates and electrode in the external circuit. Remember, current flow direction is considered as opposite to flow of electrons.
Three different polarities used in arc welding
- Direct Current Straight Polarity—occurs when electrode is made negative and base plates are made positive. Thus electrons flow from electrode tip to base plates.
- Direct Current Reverse Polarity—occurs when electrode is made positive and base plates are made negative. Thus electrons flow from base plates to electrode.
- Alternating Current Polarity—if power source provides AC current then above two cases will occur one after another in every cycle. In one half of the cycle, electrode will be negative (so base plates will be positive) and in the next half, electrode will be positive (so base plate will be negative). Number of cycles per second depends on frequency of supply. For example, with a 60Hz supply, 60 cycles occur in every second.
Direct Current Electrode Positive (DCEP) polarity in arc welding
With direct current (DC) power supply, when electrode is connected with positive terminal and base plates with the negative terminal, then it is termed as Direct Current Electrode Positive (DCEP) or Direct Current Reverse Polarity (DCRP). So electrons liberate from base plate and flow towards electrode via the outer circuit. Continuous flow of avalanche of electrons in a small passage produces the arc (source of heat).
Electrons, emitting from the base plates (negative polarity), are accelerated due to presence of potential difference and are allowed to strike the electrode (positive polarity) at a very high velocity. Upon striking, kinetic energy of electrons is converted to thermal energy, which ultimately results in high heat generation at the vicinity of electrode tip. As a thumb rule, it is considered that two-third (66%) of the entire arc heat is generated at electrode; whereas, only one-third (33%) heat is generated at base plate. As a result, electrode melts down quickly and metal deposition rate increases (for consumable electrodes only). On the other hand, base plates do not fuse properly due to lack of sufficient heat and thus various defects arise, such as insufficient fusion, lack of penetration, high reinforcement, etc. However, stream of electron flow from base plate remove oil, coating, oxide layer or dust particles present on the base plate surface (termed as oxide cleaning action).
Advantages of DCEP polarity in arc welding
- Better arc cleaning action so less chance of inclusion defects.
- High volume deposition rate for consumable electrode, so faster welding.
- Better performance for welding thin plates. It reduces distortion level, residual stress, complete cutting, etc.
- Suitable for joining metals with low melting points, such as copper and aluminum.
Disadvantages of DCEP polarity in arc welding
- Shorter electrode life for non-consumable electrodes.
- Higher level of reinforcement if speed is not adjusted properly.
- Insufficient melting and incomplete penetration.
- Cannot fuse properly thick plates or metals having high melting point.
Direct Current Electrode Negative (DCEN) polarity in arc welding
Opposite to DCEP, when the electrode is connected with negative terminal and base plates with the positive terminal, then it is termed as Direct Current Electrode Negative (DCEN) or Direct Current Straight Polarity (DCSP). So electrons flow from electrode to base plates. Consequently, more heat generates at the base plate as compared to electrode, so metal deposition rate reduces. Also various defects caused by insufficient fusion of base metal eliminate. But DCEN lacks cleaning action, so inclusion defects may arise if base plates are not cleaned properly prior to welding. Pros and cons of DCEN polarity are discussed below.
- Read more: Direct Current Straight Polarity (DCSP) in arc welding.
- Read more: Difference between DCEN and DCEP in arc welding.
Advantages of DCEN polarity in arc welding
- Sufficient fusion of base metals and thus proper penetration can be achieved.
- Less chance of tungsten inclusion (with TIG welding) and also low reinforcement.
- Better choice for welding metals with high melting point, such as titanium, stainless steel, etc.
- Thick plates can also be joined properly.
Disadvantages of DCEN polarity in arc welding
- No arc cleaning action so chances of inclusion defects.
- High distortion level.
- High residual stress generation at welded components.
- Broader heat affected zone (HAZ).
- Lower productivity due to lower deposition rate.
- Not suitable for welding thin plates.
Alternating Current (AC) polarity in arc welding
AC polarity offers advantages of both DCEN and DCEP; however, to some extent only. With AC power source, in half of the cycle electrode becomes negative and in next half of the cycle, electrode becomes positive. This cycle repeats 50 or 60 times in a second depending on frequency of supply (50Hz or 60Hz). Some power sources also provide provisions to alter this frequency.
- Read more: AC polarity in arc welding.
- Read more: Comparison among DCEN, DCEP and AC polarities of welding.
Advantages of AC polarity in arc welding
- Moderate arc cleaning action.
- Compatible with most of the electrode types (but not all).
- Better fusion and weld metal penetration.
- Suitable for a wide range of plate thickness.
How polarity affects arc welding performance?
Polarity is one of the crucial factors that influences quality of welded joints. Prior to welding, the welder must select appropriate polarity depending on requirement, type of filler, electrode type, and base material. The following list shows the parameters that are commonly affected by weld polarity. For detail, read: How polarity affects arc welding performance?
- Filler deposition—With consumable electrode, DCEP polarity increases metal deposition rate. Read: Which polarity gives maximum deposition rate in arc welding and why?
- Weld penetration—DCEN polarity increases weld penetration. Read: Which polarity gives better penetration in arc welding and why?
- Base plate cleaning—DCEP helps in cleaning base plates during welding, thus reduces chance of inclusion defects. Read: Which polarity gives better oxide cleaning in arc welding and why?
- Reinforcement—DCEP causes globular mode of metal transfer, so increases weld bead width.
- HAZ—DCEN polarity heats up the base plates quickly and if speed is not adjusted then HAZ becomes broader.
- Weld bead appearance—AC, highly depends on many other factors.
How to select welding polarity correctly?
It is to be noted that selection of welding polarity needs consideration on a large number of factors; however, only few basic factors are discussed below. Proper care should be taken for selecting polarity for a particular application.
- If your base metal is aluminum or magnesium then DCEP is better option because it can rupture the oxide layer (alumina—Al2O3) present on the plate surface. Also melting point of aluminum is quite small (660ºC), so high heat generation near base plate does not require.
- If you are welding titanium or stainless steel, then AC is better option as it will give you all the desired advantages. Here DCEN can increase HAZ zone.
- If work material has poor electron emissivity or need high voltage for electron emission then DCEP is wrong choice as it may result in instable arc.
- If base plate thickness is more (>6mm) then DCEN is preferable choice. Edge preparation is also required. Similarly, for thin plates, DCEP should be selected.
- In TIG welding, use of DCEP polarity may result in ball formation at electrode tip leading to lower electrode life. It can also result in tungsten inclusion defect.