AC TIG Welding Basics: How It Works and When to Use It

TIG welding is one of the most complex welding methods, but it yields some of the highest-quality results and most aesthetically pleasing welds. Although welders rarely use TIG in high-production manufacturing, GTAW finds its way where quality and appearance matter the most.

Heat control and reliable low-amp welding make TIG welding perfect for thin sheets and heat-sensitive metals. Additionally, GTAW is one of the few methods that can run at alternating current (AC).

In this article, we will thoroughly explain AC TIG welding basics and cover some frequently asked questions, such as how it works and when to use it. So, let's learn more.

TIG Welding Aluminum in AC Mode
Photo by @vanthefabricator (TikTok)

What is TIG Welding (Gas Tungsten Arc Welding)?

Gas Tungsten Arc Welding (GTAW), also known as TIG welding, is a metal joining process that utilizes the heat of an electric arc to melt and fuse pieces. The arc forms between a non-consumable tungsten electrode and a base metal. Tungsten has the highest melting point of all metals, so it transfers the current without melting; hence, the term non-consumable.

When heated, tungsten and a molten weld pool tend to pick up contamination from the atmosphere. That's why welders use inert shielding gas. Inert gases are non-reactive and create a protective atmosphere around the weld.

Depending on the thickness, TIG welders can weld with or without the use of additional filler rods. On thin sheet metals with tight fit-up, the heat is enough to melt the edges, and they fuse as they solidify. For thicker metals, welders use additional filler metal, in the form of TIG rods, to fill the joint.

These are the very basics of TIG or GTAW welding. But to harness its power, we must explain further.

How TIG Welding (Gas Tungsten Arc Welding) Works
Source: https://www.sciencedirect.com/

TIG Welding Current

TIG welding is one of the few welding methods that can work well at both direct current (DC) and alternating current (AC). Most processes today work with DC, either positive or negative polarity (DCEP or DCEN). Stick welders sometimes use AC when dealing with arc blow, whereas it is less common with MIG or flux-core welders.

However, that's not the case with TIG welding. GTAW offers good arc stability and heat control, enabling it to produce a smooth arc even at alternating current. Today, AC TIG has become a must-have for anyone planning to TIG weld aluminum, nickel alloys, magnesium, or any metal with a surface oxide layer that requires cleaning action.

Due to versatility, AC/DC TIG welders are gaining popularity on the market. If you are looking for a suitable, capable, and reliable AC/DC TIG welder, we strongly recommend YesWelder TIG200P AC/DC Pro.

TIG-200P-ACDC-PRO AC/DC Aluminum TIG Welder

Let's further explain the basics of current in TIG Welding.

DC TIG Welding

DC stands for direct current, which means current runs in one direction. You connect your TIG torch to one side of the terminal and the ground cable to the other side of the terminal. Direct current (DC) flows in one direction, concentrating more heat onto the electrode or base metal.

Direct Current (DC) vs Alternating Current (AC)
Source: https://www.circuitbasics.com/what-is-current/

Depending on the position of the TIG torch and ground cable, you can get different DC TIG polarities. The two main polarities in TIG welding and welding overall are the following:

• DCEP, electrode positive (EP) or reverse polarity

• DCEN, electrode negative (EN), or straight polarity

Each polarity has its benefits and drawbacks, which we will briefly explain below.

Source: https://melezy.com/electrode-polarity-welding-difference-dcen-dcep-ac/

DCEN (Direct Current Electrode Negative) in TIG Welding

In DCEN or straight polarity, you connect the TIG torch to the negative terminal (-), and the ground cable to the positive terminal (+). As the electrons flow from the negative to the positive side, the electrode faces roughly 33% of the heat, while the base metal meets 67% of the heat.

This heat distribution yields deep penetration into the base metal and reduces heat in the electrode. As a result, most DC TIG welding is carried out in straight polarity (DCEN). Due to the high penetration, welders use DC TIG to weld mild steel or stainless steel.

TIG Welding Steel in DCEN Mode
Photo by @vanthefabricator (TikTok)

DCEP (Direct Current Electrode Positive) in TIG Welding

In DCEP or reverse polarity, you connect the TIG torch to the positive side of the terminal and the ground cable to the negative side. As the current flows, the tungsten electrode faces approximately 67% of the heat, while the base metal faces only 33%.

As a result, EP forces much lower penetration into the base metal, yielding shallow welds. The electrode withstands a larger portion of the heat, meaning you'll need a larger diameter than you would ordinarily use. Reverse polarity also poses a higher risk of arc blow, a phenomenon where the arc does not follow the shortest path between the electrode and the joint.

Since DCEP has many disadvantages, welders rarely use it as a standalone tool. However, EP provides cleaning action, which is necessary when welding non-ferrous metals such as aluminum. Aluminum forms an oxide layer on the surface, and reverse polarity removes it as you weld.

TIG Welding Aluminum with DCEP
Source: https://www.youtube.com/watch?v=CHkx8cca-SQ&t=220s

How Does AC TIG Welding Work?

AC TIG welding works by using an alternating current (AC) that oscillates between the positive (EP) and negative (EN) cycles. Switching from EN to EP in one cycle provides the best of both worlds: deep penetration from EN and cleaning action from EP.

Differences between AC and DC Polarity Output
Image by: Lincolnelectric

The number of current switches per second is expressed in frequency (Hz). Older AC TIG welders had a fixed frequency based on input household current (60 Hz for the US). 60 Hz means the current changes 120 times in one second. However, new AC TIG welders allow you to, among other parameters, set the desired frequency to smoothen or stiffen the arc and get better results.

Besides frequency, the first AC TIG welders were basic. But newer welding machines brought tons of improvements, including the ability to control:

• Frequency

• AC waveform

• Balance

• AC amperage

Let's further explain how setting up each can affect your AC TIG Welding.

Source: https://www.youtube.com/watch?v=l0pTD3NSecQ&t=1041s

AC TIG Frequency Adjustments

As noted, older transformer AC welders had a default frequency based on the household AC frequency (50- 60Hz). Newer but simpler inverter AC TIG welders offer a fixed frequency of 120 Hz. While this is often sufficient to produce strong welds, newer welders enable you to adjust the frequency manually.

The higher the AC TIG frequency, the narrower, more stable, and more precise your arc becomes. For example, a frequency range of 150 to 250 Hz is ideal for precise welds on thin materials. A narrow and directed heat-affected zone reduces heat build-up, thereby decreasing the risk of overheating, distortion, or burn-through on thin sheets.

Conversely, an AC TIG frequency of 80 to 120 Hz is typically more suitable for materials 3/8 inch and thicker. Lower frequencies produce a wider arc cone, which is beneficial when filling larger joints on thicker metals.

AC Frequency in TIG Welding
Source: https://www.perfectwelders.com/ 

AC Weave Form

Technological improvements and inverter technology allowed welding manufacturers to modify AC TIG waveforms for different TIG welding applications. As a result, we are seeing several different patterns today:

AC sine waveform: The sinusoidal wave, or sine wave, is the first-ever used waveform. The current gradually rises to its maximum positive value, then drops down to zero, forming a hill. Next, the current gradually drops below zero to its maximum negative value, then rises to zero again, creating a valley to complete the cycle. Sine waves produce a very smooth welding arc, which is perfect for traditional AC TIG aluminum welding.

AC Sine Waveform
Source: https://vtoman.com/blogs/news/pure-sine-waves-guide

AC Square waveform: The AC square wave was later introduced, followed by improvements in electronic controls. The first square-wave TIG welders allowed instantaneous switching between EP and EN. The instant switches improve current efficiency and speed due to a longer period at maximum. A graphical representation is very near square, ergo the name square weave. Further improvements enable welders to create asymmetrical squares by controlling the balance, which we will discuss in more detail later.

Photo Credit: Vecteezy 

AC triangular waveform: Inverter technology has also brought the AC triangular waveform. The triangular wave transfers a minimal amount of energy into the metal, as it spends very little time at maximum. The wave takes the shape of a triangle, and it is best suited for thin-sheet aluminum. By limiting the heat, this waveform reduces the risk of distortion, warping, and burn-through, but it lacks penetration on thicker pieces.

Photo Credit: Vecteezy 

AC Balance Control

As noted, the first AC square wave had equal squares on both the positive and negative sides of the cycle. This means that the current spends 50% of the time in EP and 50% of the time in the EN cycle. But further improvements allow welders to adjust the balance and create asymmetrical squares.

AC Balance Control
Source: https://www.millerwelds.com/

Adjusting the AC TIG balance enables you to enhance penetration by allocating more time to the EN portion of the current. Conversely, you can enhance the cleaning action by increasing the time spent on the welding current in the EP portion of the cycle.

For example, if you need maximum penetration, you can adjust the balance to 75% EN and 25% EP. Doing so focuses the arc, produces deeper penetration, and allows you to use a higher current with smaller electrodes. However, a lack of cleaning action can cause "peppering" or small black flecks of oxidation.

If you are about to weld a piece with a thick oxide layer that's been in service or exposed to the elements, you can adjust the AC balance to 75% EP and 25% EN. Doing so favors cleaning action over penetration, allowing the arc to remove the oxide layer as you weld. However, you should expect a lack of penetration and a lot more heat onto your tungsten. Although tungsten has a high melting point, prolonged exposure to high heat can cause it to ball back, affecting your ability to control and direct the weld.

Comparison of high (left) and low (right) EN levels. 
Source: https://www.millerwelds.com/

Keep in mind: Although adjusting the balance can improve penetration or cleaning action, this setting cannot replace proper weld preparation and amperage dial-in.

AC Amperage

Some state-of-the-art AC TIG welders will allow you to set the AC amperage independently during the EN cycle to improve penetration. This feature is often reserved for more experienced welders who prefer to fine-tune and tailor their TIG welding setups.

Welders mostly use independent AC amperage settings in high-amp applications to reduce the need for a larger tungsten or affecting tungsten geometry. For example, AC TIG welding at 300 amps can cause significant heat exposure to tungsten in a 50/50 balance setting.

Instead of tweaking the balance, you can adjust the EN current to 300 amps while leaving the EP current to 200 amps. That way, you achieve higher current and better penetration in EN while maintaining tungsten heat exposure in EP at 200 amps. The purpose of the independent AC amperage feature is to provide greater penetration while maintaining tungsten geometry in applications where high cleaning action is not required.

Source: https://weldingpros.net/tig-welder-settings-and-waveform-controls

When to Use AC TIG Welding?

You should use AC TIG welding whenever you need to join non-ferrous metals such as aluminum or nickel and their alloys. Generally, AC TIG works well with any metal that forms an oxide layer on its surface when exposed to air. A perfect example and the most common application is AC TIG welding aluminum.

The big challenge of welding aluminum lies in the oxide that forms once the base metal is exposed to air. While this oxide protects the aluminum, it has a melting point three times higher than that of the base aluminum. Trying to weld through it will require high heat, which will later burn through the base aluminum.

Thus, it is impossible to weld aluminum without first removing the oxide layer with a stainless steel brush. However, even if you remove the oxide layer, it reforms due to the present air.

Remove the oxide layer with a stainless steel brush before TIG welding aluminum.
Source: https://www.youtube.com/watch?v=smwB3pO-Lr0&t=1528s

That's where the cleaning action of the EP cycle kicks in. Once the current flows to the electrode, it attracts the newly formed oxide. As it switches to EN, the heat concentrates on the base metal, producing enough penetration to melt and fuse aluminum pieces.

As a result, AC current in TIG welding combines the advantages of negative polarity in terms of heat and penetration, as well as cleaning action, with the positive polarity required to remove surface oxides.

“Cleaning Action” Created by the AC Arc
Source: https://www.youtube.com/watch?v=nsdSX2OinVQ

Can You Weld Steel with AC TIG?

In theory, yes. You can weld steel with AC TIG, but it is not a preferred choice. The main reason AC TIG welding steel is unsuitable lies in the very nature of the process.

As noted, the current crosses the zero line two times as it switches from EP to EN. These short periods can affect arc stability, causing the arc to stutter or extinguish. In an ideal 50/50 balance, the overall penetration of AC is lower compared to DCEN, as the current spends half of the time in the EP cycle.  

Mild steel doesn't have an oxide layer, so you don't need a cleaning action of the EP. Instead, it simply requires the penetration of EN to melt and fuse the steel.

TIG Welding Mild Steel on DC Mode
Photo by @allkraftdesign (TikTok)

If you don't have any other current option, you must weld mild steel with AC TIG, tune up the power, and adjust the balance. To achieve maximum penetration, adjust the balance to 75% EN and 25% EP.

However, even then, you might not get usable results. We recommend using DC TIG welding for mild steel and stainless steel, while reserving AC TIG for aluminum, nickel, or magnesium. If you don't want to choose between these two, we recommend getting an AC/DC TIG welder such as YesWelder CT2050.

FIRSTESS CT2050 Powerful 7-in-1 Welder & Cutter

Pros And Cons of AC TIG

AC TIG welding has some distinct advantages and disadvantages. These depend primarily on the material type and the features of the welder. Here is a quick rundown.

Pros of AC TIG:

• Great for Aluminum and Magnesium: AC current alternates polarity, providing a cleaning action that helps remove the oxide layer on aluminum and other non-ferrous metals.

• Balanced Heat Distribution: In the 50/50 balance, AC evenly distributes heat between the electrode and the workpiece, preventing electrode wear and proper penetration.

• Balance Settings: Newer TIG welders enable you to adjust the balance between penetration (EN) and cleaning action (EP) according to your specific needs.

• Perfect for Thin Aluminum: AC TIG produces a smooth, clean weld on thin aluminum sheets without distortion, burn-through, or warping.

AC TIG Welding Thin Alimunum
Photo by @doughtywelders (TikTok)

Cons of AC TIG Welding:

• Not Ideal for Steel or Stainless: AC lacks the focused arc and deep penetration that DC provides on ferrous metals.

• Noisy and Less Stable Arc: The arc "wobbles" more in AC mode as it switches. It can be noisy compared to the steady hum of DC TIG. 

• More Demanding on Power Source: AC welding draws more current and may stress smaller machines or generators.

DC TIG Welding Stainless Steel
Photo by @challenge_the_build (TikTok)

Final Thoughts

TIG welding is one of the few methods that can successfully utilize alternating current. As the current oscillates between positive and negative polarity, it combines the best of both. EN provides enough penetration, and EP provides the cleaning action.

As a result, AC became a must-have when TIG welding aluminum or non-ferrous metals that form an oxide layer on the surface. The cleaning action removes the newly formed oxides, while EN provides enough heat to melt and fuse the metal pieces properly.

👏 You may be interested in the following:

• How To Weld Galvanized Steel
• Hot start, Arc Force & Anti-Stick in Welding
• Common TIG Welding Defects And How To Deal With Them