Is TIG welding AC or DC?

When you start TIG welding, the first major setting you have to set up is the current. Using AC or DC has a hugh impact on your weld results. In this article I will give you a best practice guideline that should help you make the right choice for your welding tasks in the future. 

TIG welding can be done with both, alternating current (AC) or direct current (DC). The type of current to choose depends on the workpiece material. Generally, Steel would be welded with DC and Aluminium with AC.

In the following sections, I will get into a more detailed answer, which should enable you to make the right choice with ease, the next time you have to pick a type of current for your weld job. 

AC, DC, whats the difference?

In case you are not familiar with the terms AC and DC yet, they describe the way electricity is flowing. Electricity is a stream of electrons, comparable with water in a river. In this example, a river would be a direct current power source. Water is only flowing downstream and does not constantly change the direction of flow. Examples of DC power sources would be batteries for example. They release a constant stream of electrons in one direction until drained. 

Alternating current on the other hand would be more like the tide, moving in and going out. But in the case of electricity a million times faster. Examples for everyday AC power sources would be your wall plug. You might ask yourself now why going through the trouble of switching direction of travel for electrons a couple of thousand times per second? There are deeper underlying physical laws behind this, but the main takeaway for this article is that electricity can be transported with less resistance and therefore less losses. AC can be transformed back into DC, that is basically what is happening when you switch the welding plant to DC, even though your wall plug provides AC coming from your regional power plant. 

Why AC is probably the right choice when welding Aluminium

Aluminum is a tricky one. It looks shiny, but looks can be deceiving! When you try to join two pieces of Aluminium together, you are more likely to weld Aluminium with a splash of Aluminium oxide. Aluminum oxide is a ceramic, does not melt at the 660 °C Aluminium does, it melts around 2000 °C! That this will cause trouble is probably easy to spot. With a three times higher melting point, Aluminium oxide laughs at the temperature you are applying with your TIG torch. The result is a poor, unclean weld with a lot of oxides (very much unwanted in welding!). Increasing the temperature would not work either, because then the Aluminium will just evaporate, leaving you with a hole in the workpiece you were meant to join together.  

Normally, the electrode is poled electron negative (EN), which allows you to weld ferrous metals like steel. Heat is generated in the workpiece which creates a weld pool. But to clear the oxide layer, using the electrons to “shoot” a clearing in the oxide layer for you to melt is a tried and tested solution. This “shooting” of electrons occurs when you change the polarity of your TIG torch to electron  positive  (EP). Downside is, that a bulk of the heat is now generated on the tip of your tungsten electrode, which leads to “balling up” of the tip. So when welding with direct current (DC), you only have the option between either melting or cleaning. And since you probably not fast enough to change the polarity a couple of thousand times every second, you won’t get the benefit of both without the downsides.

Enter AC! AC takes over the switching from electron negative to electron positive by default. So you have cleaning and melting in one operation. The balance between these two phases might be adjusted if more cleaning (EP) or a deeper penetration is necessary (EN). 

Iron or Steel? DC!

Note: Recently I added an article to go into more details for Aluminium TIG welding with DC, which you can find here.

If you are not welding Aluminium, DCEN is most likely your best option. Note that if you experience heavy wear or “balling up” on your tungsten electrode, you might have clamped DCEP. When welding iron or steel, the workpiece should be cleaned manually with a grinder or metal brush. Rust is also an oxide layer, but because rust is formed much slower than aluminum oxide, it can be mechanically removed. And it should be! Welding results will be equally poor than when welding aluminum with DC. The same principles apply, the rust is basically another oxide with a much higher melting point than the iron or steel you are about to weld. The oxides won’t melt and will be included in your weld bead with catastrophic effects on the mechanical properties. It is like tying razor knives in a rope. It might look fine on the outside, but you wouldn’t want to trust your life on a rope-like that!  

How do I spot if I chose the wrong type of current?

In the previous section we covered the difference between Aluminium and other metals to weld from a current type perspective. If you are not sure what kind of material you are welding, first do the magnet test. When your material is magnetic, it is a ferrous metal and should generally be welded with DCEN. If the material you are about to weld is not magnetic, generally speaking, you are between stainless steel and aluminum. In case it is possible to safely lift the material, the weight might give you a good indication if it’s Aluminium or steel, as Aluminium is three times lighter than steel. If lifting is not a viable option, trying the softness of material by indention could be an option.  

When we look at the results, welding Aluminium with DCEN will cause a very dirty looking weld. Most likely the flow of the weld pool will be poor and the resulting bead unacceptable. If you have tried to weld Aluminium with DCEP, you probably see the tungsten tip starting to glow and then balling up or melting. This is first of all annoying because tungsten electrodes are expensive and it takes time to grind them back to shape. But also you are risking contamination of your weld with tungsten. This is similar to the oxide inclusions mentioned earlier and should be prevented at all times for any weld.  

On the other side, when welding other metals with AC, first of all the pitched noise of the frequency should be audible. This is your first indication that you might use the wrong type of current for this material. Also, the arc is probably not very calm and the resulting bead will have a poor penetration, while your tip might have slightly melted. 

I hope this article will help you to confidently chose and adjust your current type in the future. Be aware that I can give only general guidelines, and a proper weld is depending on a multitude of things. For a further deep dive, I recommend this book. If you have feedback or comments on this article, please leave me a message.