If you’ve been working in welding, or are even slightly familiar with the way it works, you might have heard the terms ‘AC’ and ‘DC’ in welding. AC and DC are different types of currents that are used in the welding process. Because welding uses an electric arc that creates the heat needed for the metal to melt, it would need a stable current flow that has a differing polarity, which depends on the material being welded.
To make a good weld, you would first need to understand what these two currents would mean on the welding machine, as well as on the electrodes.
But first: what is the difference between AC and DC welding?
Both AC and DC welding refer to the polarity of the current that is running through the electrode of the machine. AC stands for Alternating Current, where DC stands for Direct Current. The strength and quality of the weld you make will depend on the polarity of the electrode.
In this article, I’m going to unpack this a little more and explain why it’s important for you to understand.
What is polarity?
You are likely familiar with the term ‘poles.’ For example, a magnet has north and south poles.
Similarly, electrical circuits will also have poles – negative and positive. In a circuit, direct current (DC) will flow in a single direction, which results in constant polarity – one side will remain negative, with a larger number of negative ions, while the other side stays positive, with a larger number of positive ions throughout the time the circuit is active.
On the other hand, alternating current (AC), as the name suggests, alternates in the direction in which it flows. Half the time, it flows in one direction, and the other half, in the opposite direction. AC current will change its polarity about 120 times within one second with a 60Hz current.
AC current, also known as electrode positive or reversed polarity, will provide a deeper penetration, where electrode negative or straight current – DC current – results in a faster rate of deposition because the melting at the electrode is faster. Different types of electrodes can affect these conditions, as well as different kinds of electrode shielding.
Shielded electrodes can sometimes use either kind of polarity, while some will only operate on one.
A good weld will involve proper penetration and uniform beading, and the right polarity must be used to achieve this. With the wrong polarity, you don’t just get poor penetration and irregular beads but also get excessive spatter and overheating and, in some cases, may even lose control of the arc.
The electrode may also burn quickly.
Most arc welders will have terminals or directions marked clearly to make sure that users know how to set the welding machine to either AC current or DC current. Some welding machines also use switches to change the polarity, and some require you to make changes to the cable terminals.
Welding with different currents
Different kinds of welds are better suited for different current types, due to their nature, and the effects of that specific kind of current.
AC welding is considered to be inferior to DC welding, and therefore it is used rarely. AC welding machines are mostly used only when there are no DC machines available and are sometimes known as ‘buzz boxes.’
AC welding is most commonly used for downhand heavy plate welds, fast fills, and aluminum TIG welding with high frequency, though it is also sometimes used to fix problems with arc blow. Arc blow problems occur when an arc blows out the joint that is being welded at higher current levels, which happens mostly when working with electrodes that have a large diameter.
AC welding can also be used for magnetized metals, which is not possible with DC welding. The constant change in current direction flow in AC welding means that the magnetized metal will not affect the electric arc.
AC current is also better suited for higher temperatures. Since it provides a higher current level, it allows for deeper penetration and is thus used for seam welding for building ships.
AC welding is good for repairing machinery since many of them have magnetized fields and areas that have rusted.
However, the directional instability in AC welding can also be a disadvantage in that the process has a lower product yield than DC welding.
DC welding, just like AC welding, has its own benefits, and it is used in a few specific cases where AC welding would not be useful, such as vertical welding, single carbon brazing, or stainless steel TIG welding.
Since DC welding has a higher deposition rate, it is best suited for users who need a better build-up of larger deposits. Though AC welding has better penetration, it has a lower deposition rate, which may not be suitable.
DC welding also produces less spatter than AC welding, which makes the weld bead uniform and smoother, and also leads to higher product yield. Direct current is also more reliable, and thus it becomes easier to work with since the electrical arc remains constant and stable.
DC welding is better for welding thinner metals, which makes it desirable to many welders. The machinery used with this type of current is also cheaper, which helps cut costs and makes them even more desirable.
However, although the machinery itself has a lower cost, the process of actually using DC is a bit more costly.
This is because you need special equipment to switch AC to DC because the later is likely not provided by any electrical grid. However, because DC current is better suited for most kinds of welding processes, these are considered necessary costs.
Though DC welding is better for many different metals, it is not recommended when working with aluminum, because this requires a high-intensity production of heat, which is not possible with DC. On top of that, if a magnetic field were to build up when working with DC, there would be a higher risk of arc blow, which could be dangerous.
What electrode to use?
Because the kind of current used will affect the polarity at the electrode, you’d also have to consider the electrode being used. For DC welding, look for an electrode with a high cellulose sodium-type coating, which is meant for direct current use only, and provides deeper penetration.
For AC welding, since the arc tends to go out and re-establish itself due to the alternating current, electrodes have specific elements that keep them ignited. Electrodes for AC welding should have a high cellulose potassium-type coating, which can work for all positions, and also work on rusty or dirty metal.
There are a number of different kinds of electrodes available for AC welding, but many are available for use with both AC and DC welding.
As with anything, there is no way to say one thing is ‘better’ than the other. Both kinds of welding are used to accomplish different things.
In many cases, DC welding is better suited to the situation – but not inherently ‘better’ – than AC welding, but in other cases, AC welding can be a more advantageous choice. Choosing the right polarity and current, as well as the right electrode to use, can be crucial to making a good weld, and therefore, should be done carefully.
Is MIG welding AC or DC?
In MIG welding, the polarity settings should be at direct current electrode negative – or direct current – where the negative terminal in the machine is hooked to the electrode, while the positive terminal is connected to your ground.
What is penetration in welding?
In welding, penetration refers to the distance the fusion line extends to below the surface of the material that is being welded. This is usually a result of the current being used.
What is deposition in welding?
Deposition rate – or just deposition – is the amount of filler metal that gets melted and mixed into the weld joint, and is usually defined by pounds per hour. It depends greatly on the wire size, type, and polarity.