Navigating the world of TIG welding can be complex, and understanding the role of shielding gas is fundamental to achieving high-quality welds. As the name GTAW (Gas Tungsten Arc Welding) implies, this method uses a tungsten electrode to create an arc, all while being protected by an inert shielding gas. Because of this, questions like "Can you TIG weld without gas?" are easily answered: without shielding gas, the tungsten electrode would immediately oxidize and burn away, making it impossible to weld.
This overview will clarify two of the most critical aspects of TIG welding—the types of shielding gases and their corresponding flow rates. By mastering these concepts, you can ensure a clean, strong, and visually appealing weld every time.
Types of TIG Welding Gas
TIG welding primarily relies on pure, inert gases for shielding. In specialized applications, a mixture with active gases like hydrogen or nitrogen might be used. Unlike MIG welding, TIG and MIG gases are not interchangeable. While MIG welding may use CO2 to increase penetration, TIG welding never does. At high temperatures, CO2 breaks down and can cause the tungsten electrode to oxidize and burn away.
Related Article: MIG Welding Gas and Flow Rate Explained
Selecting the right shielding gas is crucial for TIG welding. While many options exist, these five are the most common and versatile choices for welders.
1. Argon (Ar)
Pure argon is the most widely used TIG shielding gas. It's a heavy, inert gas that provides excellent protection for the weld pool. Argon produces a stable, focused arc and is suitable for most applications, including welding steel, stainless steel, aluminum, and copper.
However, pure argon welding has limitations. Its low penetration depth per pass often requires complex groove designs and multiple passes to weld thick sections, which can decrease productivity and increase cost. Even increasing the welding current with argon can lead to excessive widening of the weld with no significant increase in penetration.
2. Helium (He)
Helium is a lighter, inert gas that produces a hotter, broader arc than argon. This increased heat is ideal for welding thick, high-thermal-conductivity materials like thick aluminum, magnesium, and copper.
However, helium is more expensive than argon. Its lighter weight also means you need a higher flow rate to ensure proper weld protection, which leads to higher gas consumption and increased costs.
Interestingly, helium can sometimes be used to reduce porosity when you're welding in extremely humid environments. Its higher thermal conductivity helps to vaporize moisture on the workpiece more effectively, preventing it from getting trapped in the weld pool as a gas pocket.
3. Argon/Helium Mix
This blend combines the best qualities of argon and helium. Common mixes, like 75% Argon/25% Helium or 50% Argon/50% Helium, provide a hotter arc with more penetration than pure argon while maintaining better arc stability and lower cost than pure helium. This makes argon/helium mixes an excellent choice for welding thicker materials where pure argon might not provide enough heat.
4. Argon/Hydrogen (Ar/H2) Mix
Although hydrogen is an active gas, it will not damage the tungsten electrode. Many studies have found that hydrogen, when added to argon, is effective for welding materials with nickel as the main ingredient, such as stainless steels, nickel-copper alloys, and nickel-based alloys.For other metals, it is not recommended, as it can produce hydrogen cracks and other defects in the welded metals.
The addition of hydrogen to argon has also been found to promote increased welding speed, as hydrogen significantly increases the volume of molten material in the weld pool. Using hydrogen can also effectively improve pitting corrosion resistance.
Additionally, hydrogen also affects the tensile strength of the metal; the higher the hydrogen content, the lower the tensile strength. This is due to the microstructure formed during the re-solidification of the weld zone and the formation of delta-ferrite.
Metal tensile strength is inversely proportional to hydrogen content
5. Argon/Nitrogen (Ar/N2) Mix
Nitrogen (N2) is generally considered an inert gas at normal temperatures and pressures, but it's technically a semi-inert gas because it can become active and participate in chemical reactions under specific high-temperature and high-pressure conditions. For this reason, Argon/Nitrogen mixtures have very limited use in TIG welding, primarily for austenitic stainless steels like 201 stainless steel, which contain nitrogen.
During the welding process, a loss of nitrogen can occur from both the weld pool and the heat-affected zone (HAZ). This nitrogen loss can decrease the mechanical properties and corrosion resistance of the material. Using an Ar/N2 mixture helps to compensate for this loss. This is due to nitrogen’s low ionization potential and good de-oxidation properties.
Increasing the amount of nitrogen in the shielding gas not only increases the tensile strength and hardness of the welding joint but also improves the weldment's angular distortion. Additionally, the dissolution of nitrogen in the weld metal improves its resistance to pitting corrosion and stress corrosion cracking.
How to Choose the Right Shielding Gas?
Determining the right shielding gas ultimately depends on the metal you're welding. While argon is the most widely used choice, its effectiveness can be limited by the properties and thickness of the material. For this reason, other gases or gas mixes are often necessary.
Here is a breakdown of the best shielding gases for some of the most common metals welded with TIG.
What Gas for TIG Welding Aluminum?
For welding aluminum, you'll want to use either pure argon or an argon/helium mix for thicker sections to increase penetration.
Using 100% argon with an AC (Alternating Current) polarity is highly effective for cleaning aluminum. When welding, you'll see the puddle "sweat" as the arc’s positive cycle burns off surface oxides in a process known as cathodic cleaning. This action must happen wherever the puddle touches the metal; otherwise, the molten metal won't freeze correctly and will suffer from rapid oxidation, resulting in a poor weld.
What Gas for TIG Welding Stainless Steel?
While there are many types of stainless steel, argon or an argon/helium mix remains your safest bet for TIG welding them. This combination will handle 99% of your welding tasks with great results.
For more specialized applications, you have a few other options. An argon/hydrogen (Ar/H2) mix (1%-5% H2) is used for austenitic stainless steels. This mixture creates a hotter arc, changing the weld penetration profile and producing a brighter, cleaner weld color.
Another option is an argon/nitrogen (Ar/N2) mix (1%-10% N2), which is suitable for welding austenitic, duplex, and super-duplex stainless steels. Welding these materials with pure argon can lead to a loss of nitrogen from the weld pool, which results in a ferrite-rich weld metal with poor corrosion properties. Using an Ar/N2 mix helps to compensate for this nitrogen loss.
The ideal nitrogen content in the shielding gas depends on the base metal's original nitrogen content:
- For duplex stainless steel with a typical nitrogen content of 0.16%, the shielding gas should contain 1.0%-1.2% N2 to obtain a similar nitrogen content in the weld metal.
- For super duplex stainless steel with a typical nitrogen content of 0.25%, the shielding gas should contain 2.0%-2.5% N2 to obtain a similar nitrogen content in the weld metal.
What Gas for TIG Welding Titanium?
Argon is the only acceptable gas for welding titanium. Titanium is extremely reactive, and at high temperatures, it will react violently with oxygen, even to the point of combustion. It also reacts with nitrogen and hydrogen, causing the weldment to become brittle.
When welding titanium, the argon must have a higher purity, and more extensive protective measures are required. The argon purity should be at least 99.995%, with oxygen content controlled to between 2-20 ppm. It's crucial to measure this throughout the process.
Any part of the weldment exposed to temperatures above 520℃ will absorb oxygen and nitrogen and must be protected until it has cooled below this critical temperature. This is why it's essential to add a trailing shield to your torch, which provides secondary protection to the area behind the weld. Setting the torch and trailing shield gas flow at 20 cfh typically provides the best coverage.
Protecting the backside of the weld is also critical, especially for pipes and thin sheets. Welding heat can cause the back of the material to react with oxygen, nitrogen, and water vapor in the air, even if it doesn't reach the melting point. The resulting oxides and nitrides will make the root of the weld brittle, leading to stress concentration and cracking.
There are two primary methods for backside protection:
- Purging: This is convenient for pipes, which can be sealed with water-soluble dams, rubber gaskets, specialty tape, or inflatable bladders. Argon is then purged from one end to push out all oxygen from the other. For non-pipe applications, high-temperature tape (like aluminum foil tape) can be used to create a sealed chamber on the backside, which is then filled with argon.
- Glovebox: For the highest level of purity, welding can be performed in a glovebox, a completely sealed, argon-filled environment. All welding work is done using the gloves integrated into the box.
Summary
Overall, pure argon is the universal go-to for most applications, but knowing when to switch to helium or specialized mixes like argon/hydrogen and argon/nitrogen can be the difference between a good weld and a perfect one. Always remember to match your gas to the metal's unique properties, paying close attention to thickness and reactivity. By making informed choices, you'll not only achieve stronger, cleaner welds but also save time and resources in the long run.
For a quick reference, see the table below.
