In the context of arc welding, polarity indicates the way electrical connections are made with power source. Arc welding power sources consist of one positive terminal and one negative terminal. Again these can be of two types—AC and DC. With DC power sources, when electrode is connected with positive terminal of power source and work plates are connected with negative terminal, it is termed as Reverse Polarity. Alternatively, if electrode is connected with negative terminal and work plates are connected with positive terminal, it is called Straight Polarity.
However, if the power source delivers alternating current (AC) then both conditions occur one after another. Number of times such change occurs in every second depends on and equals to frequency of supply. For example, in a 60Hz AC power supply each polarity will occur 60 times in a second. Polarity is one crucial factor in arc welding as it influences many parameters as enlisted below.
- Filler deposition rate
- Depth of penetration
- Reinforcement
- Width of weld bead
- Voltage requirement
- Arc cleaning action (oxide cleaning)
- Non-consumable electrode life
- Arc scattering (rarely)
- Various defects like lack of penetration, insufficient melting, inclusions, etc.
How polarity affects filler deposition rate?
When electrode is given positive polarity and work is given negative polarity (reverse polarity), electrons will liberate from work surface under presence of sufficient potential difference and they will accelerate within the small gap before bombarding on electrode tip. Upon striking, the kinetic energy of electrons will be converted into thermal energy and thus excessive heat will generate near electrode tip. This will quickly melt electrode and thus higher deposition rate will be noticed. Alternatively, straight polarity will provide minimum deposition rate. However, such effect is palpable with consumable electrodes only.
Higher deposition rate can be advantageous and disadvantageous also. It can increase reinforcement, which hampers weld appearance and wastage of costly filler. Post-processing (such as grinding) is also required for removal of reinforced layer. However, higher deposition rate facilitates use of high travel rate, which can significantly improve productivity. It is also beneficial when thicker deposition is required, notably when base plates have V or U shape edges. Thus proper planning must be carried out prior to welding considering both polarity and travel speed.
How polarity affects depth of penetration?
It is axiomatic from above explanation that straight polarity will generate excessive heat at work surface and thus base plate will melt down at a faster rate. Here a constricted arc can help fusing layers of material deeper into the root gap. Consequently, higher depth of penetration can be achieved without having prepared edges. On the other hand, reverse polarity reduces melting depth and thus requires proper edge preparation if base plates are thick. However, reverse polarity can be appropriate choice if base plates are thin and process utilizes a non-consumable electrode, as the opposite one can lead to cutting instead of welding.
How polarity affects reinforcement and weld bead width?
Reinforcement is measured by the height of excess material deposited on weld bead from work surface. Reverse polarity causes high filler deposition rate but low work melting depth—both may result in high reinforcement as well as wider bead width. However, heat affected zone (HAZ) may be narrow as heat input to base metal is small.
How arc voltage depends on polarity?
When sufficient potential difference is applied, electrons liberate from negative electrode and these are then accelerated towards positive counterpart. With reverse polarity, electrons need to emit from work surface by rupturing contamination layer deposited on it and then travel towards electrode tip. Emissivity of work material plays crucial role here. If work material has high emissivity, then arc voltage will drop and vice versa. Alternatively, electrode material usually has high emissivity and thus straight polarity reduces close circuit voltage.
How polarity controls oxide cleaning?
Presence of oxide, dirt, oil, grease, etc. on work surface during welding can lead to various inclusion defects. Such contaminations can be removed prior to welding; however, removing oxide layer from surface is quite difficult task, especially when work material has high affinity to oxygen. Welding polarity can also contribute in removing such oxide layers. When electrons liberate from work surface, they rupture any insulate layer including oxides present on the work surface. Oppositely, when electrons are bombarded on work surface, they embed oxide particles into the molten metal pool. This leads to various inclusion defects. Thus reverse polarity provides best oxide cleaning action; while straight polarity gives poor oxide cleaning.
How electrode life is affected by polarity?
As discussed earlier, reverse polarity increases filler deposition rate if electrode is consumable type. However, polarity also influences life of non-consumable electrodes (such as in TIG welding). With reverse polarity, excessive heat is generated nearer electrode tip. When welding is performed continuously with non-consumable electrode, it melts down automatically and a bubble of molten metal forms at electrode tip. Sometime this droplet deposits on weld bead leading to inclusion defect; sometime it retains on electrode tip, which is removed by grinding before initiating next welding pass. Both cause loss of electrode material and thus life of non-consumable electrode reduces with reverse polarity.