Welding is technically manual labor, but welders still need to have a fair amount of technical knowledge to be able to do their job properly.
One of the things you would hear a lot if you were to take a welding class, or even just go to the shop is ‘welding current.’ A lot of welding machines are labeled AC or DC, which describes the polarity of the machine’s current.
So what is polarity in welding?
The electrical circuit that forms when you turn on the welding machine has a negative and a positive pole. This property is called polarity.
Polarity is extremely important in welding because choosing the right polarity is what affects the strength and quality of the weld. With the wrong polarity, you could get lots of spatter, bad penetration, and lose control of your welding arc.
What are AC and DC current?
AC stands for alternating current, while DC stands for direct current. In AC current, the direction of the flow alternates, while there is only one direction in DC. Therefore, welding machines and electrodes that have the DC label have constant polarity, while AC labels mean that the polarity changes.
How do AC and DC currents differ in welding?
When it comes to shielded metal arc welding, DC is widely used because it has many benefits. Welding with DC results in a smoother and more stable arc, and you can strike the arc easier.
It results in fewer outages and less spatter, and reduces complications in vertical up and overhead welding.
However, while DC has its benefits, AC can be the preferred choice for other cases, such as for welding training because it is frequently used with low cost and entry-level machinery. AC is also preferred for use in shipbuilding welding or any conditions where the arc may blow side to side.
Three types of polarity
Three different types of polarity are used in welding.
Direct current straight polarity
This occurs when the electrode is made negative, and the plates are positive. As a result, the electrons flow from the electrode tip to base plates.
Direct current reverse polarity
This occurs when the electrode is made positive, and the plates are negative. The electrons then flow in the reverse direction from the base plates to the electrode.
Alternating current polarity
If the power source provides an AC current, then both straight and reverse polarity will occur one after the other in each cycle. For half the cycle, the electrode will be negative, and hence the base plates are positive. In the other half, the base plates will be negative and the electrode, positive. The number of cycles that occur within a second depends on the frequency of the supply.
Direct Current Electrode Positive (DCEP) polarity in arc welding
With a direct current (DC) power supply, if the electrode is connected to the positive terminal and the base plates to the negative terminal, it is termed as Direct Current Electrode Positive – or Reverse Polarity.
Electrons liberate from the base plate and flow towards the electrode via the outer circuit. The continuous flow of electrons in the small passage produces the arc.
The electrons being emitted from the base plates are accelerated due to the potential difference and hit the electrode at a very high velocity. This causes kinetic energy in the electrons to be converted to thermal energy, which results in heat generation at the electrode tip.
It is generally accepted as a rule that about two-thirds of the entire arc heat is generated at the electrode while the rest is generated at the base plate. This causes the electrode to melt quickly, and the deposition rate for consumable electrodes increases.
On the other hand, the base plate does not fuse properly because there is a lack of sufficient heat, and this can result in defects such as lack of penetration or high reinforcement.
However, the stream of electrons that flow from the base plate remove oil, coating the oxide layers or dust particles present on the surface of the base plates. This is called oxide cleaning action.
Pros of DCEP
DCEP has a better arc cleaning action, so there is less chance of inclusion defects. The high volume deposition means that welding is faster.
It reduces distortion, residual stress, and complete cutting, so there is better performance for welding-thin plates. It is also suitable for joining metals with low melting points such as copper.
Cons of DCEP
DCEP has a shorter electrode life for non-consumable electrodes. There is a high level of reinforcement if speed is not adjusted properly.
With insufficient melting and low penetration, you cannot fuse thick plates or metals with high melting points together properly.
Direct Current Electrode Negative (DCEN) polarity
In stark contrast to DCEP, when the electrode is connected to the negative terminal and the base plates to the positive one, it is termed Direct Current Electrode Negative – or Straight Polarity. Electrons flow from the electrode to the base plates.
This causes more heat to be generated at the base plate than at the electrode – again, the opposite of DCEP – and this means that the metal deposition rate at the electrode reduces.
This also means that defects caused by insufficient fusion are eliminated. DCEN, however, lacks cleaning action, so inclusion defects can pop up if you don’t clean the base plates properly before welding.
Pros of DCEN
DCEN means that sufficient fusion of base metals and, therefore, proper penetration is possible. The chance of tungsten inclusion and low reinforcement also lessens. DCEN is the better choice for metals with high melting points such as stainless steel. Thick plates are also able to be joined properly.
Cons of DCEN
There is no arc cleaning action in DCEN, so the chances of inclusion defects rise. There is also a high distortion level and a high residual stress generation.
There is a broader heat-affected zone, which causes low productivity because of the low deposition rate. It is not suitable for the welding of thin plates.
Alternating current polarity
AC Polarity offers advantages of both DCEP and DCEN since both are occurring in a cycle, but only to some extent.
AC polarity offers moderate arc cleaning action and is compatible with most electrode types, but not all. It allows for better fusion and metal penetration and is suitable for a wide range of plate thicknesses.
How does polarity affect arc welding performance?
Polarity is one of the crucial factors that influence the quality of welded joints. Before you begin welding, you must select the right polarity depending on the requirements, the fillers, electrode type, and the base material.
The parameters commonly affected by weld polarity are:
- Filler deposition. With consumable electrodes, DCEP polarity increases the deposition rate.
- Weld penetration. DCEN polarity increases weld penetration.
- Base plate cleaning. DCEP makes cleaning base plates easier and reduces the risk of inclusion defects.
- Reinforcement. DCEP causes a globular mode of metal transfer and increases weld bead width.
- Heat-affected zone (HAZ). DCEN polarity causes the base plates to heat up quickly, and if the speed is not adjusted, the HAZ becomes wider.
- Weld bead appearance. This depends on many other factors but occurs mostly on AC current function.
How do you select polarity correctly?
The selection of welding polarity requires careful consideration of a large number of factors. Some of the basic ones are:
- Aluminum or magnesium as your base metal would better work with DCEP because it can rupture the oxide layer that is present on the plate surface. The melting point of both is quite low, so you do not require high heat generation near the base plate.
- For titanium or stainless steel, AC is a better option since it can give you all the advantages. However, DCEN can increase the HAZ.
- If the work material has poor electron emission or needs high voltage, then DCEP may result in an unstable arc.
- If the base plate is too thick, then DCEN is preferable, and edge preparation is required. Similarly, for thin plates, DCEP is better.
- In TIG welding, DCEP may result in ball formation at the electrode tip, which can lead to lower electrode life and result in tungsten inclusion defect.
What is the difference between straight and reverse polarity? In straight polarity, the electrode is negative, and the base plates are positive. In reverse polarity, the electrodes are positive, and the base plates are negative. Straight polarity offers high penetration, while reverse polarity offers a higher deposition rate.
Is reverse or straight polarity better? Since different materials have different requirements, either one of these two types of polarity may suit different materials.
What happens if you weld with the wrong polarity? Using the wrong polarity could result in spatter, bad penetration, and lack of control over your arc.