TIG Shielding Gas Guide: Argon, Argon-Helium Mixes & Back-Purging

Why 100% Argon Is the Default for TIG

Argon is the workhorse TIG gas for essentially every base metal, and there are sound physical reasons it earned that spot. It is monatomic and fully inert, so it will not react with the weld pool or oxidize the tungsten. It has a low ionization potential (about 15.8 eV), which means the arc strikes easily and stays stable at low current, a critical trait when you are laying down a root on 22-gauge stainless or fusing thin aluminum trim. And because argon is roughly 1.4 times denser than air, it blankets the puddle and drops into open joints and cups instead of drifting away, giving you good coverage even in a shop with a little air movement.

The classification you want is AWS A5.32 / A5.32M welding-grade argon, and for TIG you should specify a minimum 99.997% purity (often labeled 4.7 or 'four-nines-seven'). That grade holds oxygen and moisture down in the low single-digit parts-per-million range, which is what protects your tungsten from spitting and keeps aluminum and titanium welds bright. Do not substitute 'balloon grade' or general-purpose argon; the ppm contaminants that a balloon shrugs off will contaminate a titanium root pass.

On carbon and low-alloy steel, straight argon gives a clean, controllable arc with excellent starts. On stainless, it produces a tight, cosmetically clean bead. On aluminum and magnesium with AC output, argon delivers the cathodic etching (oxide-cleaning) action that makes AC TIG possible, and it gives better cleaning action than helium does. That combination of easy starts, arc stability, cleaning action, and universal metal compatibility is why a shop can run one cylinder of argon across mild steel, chromoly tube, stainless, and aluminum and never touch anything else.

When to Add Helium: Thick Aluminum, Copper, and Travel Speed

Helium changes the arc's character. It has a high ionization potential (about 24.6 eV), so it takes more voltage to sustain the arc, and that higher voltage dumps more energy into the joint. The practical result is a hotter, more penetrating, more fluid puddle at a given amperage. That is exactly what you want on heavy, high-thermal-conductivity metals that suck heat out of the weld zone faster than pure argon can supply it.

The classic candidates are aluminum over roughly 3/8 in. (10 mm), copper and copper alloys, and heavy heat-sink parts like thick bus bar or cast aluminum housings. On 1/2 in. to 1 in. aluminum, an argon-helium blend can be the difference between chasing a cold, sluggish puddle and welding it at a productive travel speed with proper fusion. Helium additions also raise deposition speed on long production stainless seams, and they can reduce or eliminate preheat on copper.

The tradeoffs are real, so add helium deliberately rather than by default. Helium arcs start harder (many shops start on argon-rich mixes and rely on high-frequency start), the arc is broader and a bit less pinpoint, and because helium is far lighter than air it floats away, so you need noticeably higher flow, commonly 1.5 to 2 times the argon rate, to maintain coverage. On AC aluminum work, helium also reduces the oxide-cleaning action, so you trade some surface cleaning for penetration. Helium is also the most expensive and supply-volatile of the common welding gases, which matters to a shop's gas budget.

• 75% Ar / 25% He: Mild penetration and speed boost with argon-like starts and arc stability; a good first step for aluminum 3/8 to 1/2 in. and general DCEN stainless production.
• 50% Ar / 50% He: Balanced blend for aluminum in the 1/2 to 3/4 in. range and copper; meaningfully hotter puddle while retaining workable AC cleaning.
• 25% Ar / 75% He: Maximum heat for very thick aluminum (3/4 in.+), heavy copper, and automated high-speed seams; expect hard starts and high flow demand.

Argon-Hydrogen for Austenitic Stainless

Small hydrogen additions are a specialty tool for austenitic (300-series) stainless steel, and they can transform the look and speed of a stainless weld. Hydrogen is a reducing gas: it scavenges oxygen from the arc atmosphere, which suppresses surface oxide and yields a brighter, cleaner bead with less post-weld pickling. It is also thermally aggressive like helium, raising arc voltage and heat input so you can travel faster and get deeper penetration, which is why it is popular for mechanized tube and pipe mills and sanitary stainless fabrication.

Common blends are Ar + 2% H2, Ar + 5% H2, and up to Ar + 7.5% H2 for automated work; 5% is a widely used shop default. The higher-hydrogen blends are typically reserved for mechanized processes where fit-up and parameters are tightly controlled.

The hard rules: use argon-hydrogen only on austenitic stainless and some nickel alloys. Never run it on carbon steel, ferritic or martensitic stainless, duplex stainless, aluminum, or titanium, where dissolved hydrogen causes porosity and hydrogen-induced cracking. And treat the cylinder as flammable, because a blend with more than about 5% hydrogen can form an ignitable mixture. Store and handle it per NFPA and CGA guidance for flammable gases, keep it away from ignition sources, and confirm the blend with your distributor before it goes near a critical joint.

Back-Purging Roots on Stainless and Titanium

Shielding gas protects the face of the weld; it does nothing for the back side. On an open-root pipe or tube weld in stainless, titanium, duplex, or nickel alloy, the hot underside of the root will oxidize badly if left exposed to air. On stainless that shows up as heavy 'sugaring' (a granular, chromium-depleted oxide) that destroys corrosion resistance; on titanium, an unshielded root goes from silver to blue, gray, and finally a powdery white that is brittle and must be cut out. The fix is back-purging: displacing the air on the root side with inert gas before and during the root pass.

Argon is the standard purge gas for both stainless and titanium and is the only correct choice for titanium and reactive metals. It is inert, heavier than air, and settles into the pipe. Nitrogen, or nitrogen blends such as N2 + 10% H2 (sometimes sold as a forming gas), is an economical purge option for austenitic stainless where a slightly nitrided root is acceptable, and nitrogen is actually beneficial on duplex stainless because it helps maintain the austenite-ferrite balance. Never purge titanium or carbon-critical work with nitrogen.

Practical technique matters as much as gas choice. Dam the pipe with water-soluble purge paper or inflatable purge bladders to shrink the volume you have to fill, feed gas from the low side, and vent from the high side so heavier argon pushes air ahead of it. Purge at a gentle rate, roughly 5 to 15 CFH, high enough to hold a positive atmosphere but low enough that you do not pressurize and blow out the molten root or create turbulence that pulls in air. For stainless, weld color is your gauge: bright silver to light straw is fully protected, while blue, gray, and black indicate rising oxygen and, on many code jobs, a reject. Titanium is far less forgiving, so many titanium roots require an oxygen analyzer reading below roughly 50 ppm at the vent before striking an arc, plus trailing shields on the face for the hot metal following the torch.

Flow Rates, Cup Size, and Gas Lens vs Standard Collet Body

Gas flow in TIG is set in cubic feet per hour (CFH), and more is not better. Too little flow leaves the pool exposed; too much creates turbulence that aspirates air into the shield and actually contaminates the weld, so the goal is smooth laminar coverage sized to the cup. A useful field rule is to run CFH at roughly the same number as the cup size in 1/16-in. increments, then adjust for conditions. A #6 cup wants about 12 to 16 CFH, a #8 about 15 to 20 CFH; drafty conditions or long stickout push you up, tight indoor work lets you back off.

The single biggest upgrade to coverage is a gas lens. A standard collet body dumps gas straight out of the cup as a fairly turbulent stream. A gas lens replaces the collet body with a stack of fine stainless screens that straighten the flow into a stable laminar column. That lets you extend the tungsten stickout for tight-access joints and lets you run lower flow while getting better coverage, which is a direct savings on gas over time. For stainless, titanium, aluminum, and any critical or hard-to-reach joint, a gas lens is worth every penny; the tradeoff is slightly higher consumable cost and needing larger cups to exploit it.

Match flow to current and cup so coverage extends past the pool without going turbulent. The table below is a solid starting point for DCEN steel and stainless with straight argon; add roughly 50 to 100% for helium blends and bump up in any draft.

• Cut post-flow, not shielding: If tungsten discolors on stop, raise post-flow time (5-10 s at 3/32 in.) before raising CFH; the tungsten and pool need coverage until they cool below oxidation temperature.
• Check for leaks first: A pool that will not stay clean at correct flow is often a bad O-ring, cracked cup, or loose back cap pulling air, not too little gas. Verify the gas path before chasing flow numbers.

Purity, Cylinders, Cost, and Sourcing from US Distributors

Specify 99.997% (4.7) welding-grade argon as your baseline, and step up to 99.999% (5.0 'ultra-high purity') for titanium, exotic reactive metals, and critical aerospace or nuclear work where every ppm of oxygen counts. For blends, buy certified premixed cylinders from the distributor rather than trying to blend on site; a factory mix with a certificate of analysis is traceable, which matters when a weld procedure or code inspector asks. Blends drift if mixed with cheap flowmeter-style mixers, and on a coded job that drift is a liability.

Cylinder sizing drives your cost per cubic foot. A common shop-size argon cylinder is the 'size 300' / T-cylinder at roughly 330-337 cu ft; the 250 cu ft 'Q' and the 125 cu ft 'S' are popular for smaller shops and portability. Bigger cylinders and, above all, owning rather than renting cut your effective gas cost. Renting is convenient and low-commitment but you pay monthly demurrage on every bottle; buying your own cylinders eliminates rent but makes you responsible for hydrostatic testing (DOT requires periodic requalification, typically every 5 or 10 years depending on the cylinder). High-volume shops with steady argon burn should price out bulk liquid argon (a MicroBulk or bulk tank), which drops cost per cubic foot dramatically once you are past a few hundred cylinder-fills a year.

As a rough planning range, expect a refill on a 300-size argon cylinder to run roughly $50-$110 depending on region and volume, with the argon itself often cheaper than the monthly cylinder rent over a year of light use. Argon-helium and argon-hydrogen blends carry a premium, and helium-rich mixes swing with the volatile helium market, so lock pricing where you can. Because gas is heavy and freight is expensive, the economics almost always favor a local distributor over shipping: welding gas is sold through a nationwide network of independent distributors and the majors (Airgas, Linde, Matheson, nexAir and regional players) with branches serving all 50 states. Get quotes from two or three local distributors, compare gas price, cylinder rent, delivery/hazmat fees, and contract length, and ask about a supply agreement if your volume justifies it.

Two compliance notes a 20-year distributor will hammer on. First, follow OSHA 1910.253 for the storage and handling of compressed gas cylinders: chain them upright, keep caps on when not in use, separate them from ignition sources, and never let a regulator or bottle get contaminated with oil. Second, honor DOT and CGA rules for transport and cylinder markings, and keep an SDS on file for every gas, including inert ones, since argon and helium are simple asphyxiants that can displace oxygen in a confined space or a poorly ventilated purge tent.

Frequently Asked Questions