Pure argon is the most popular shielding gas for welding aluminum and works well for both MIG and TIG processes. For thicker material, typically above 1/4 inch (6 mm), adding helium to the mix delivers more heat and better penetration. The right choice depends on your welding process, material thickness, and budget.
Why Argon Is the Standard Choice
Argon handles aluminum well across the board. For TIG welding on alternating current (AC), pure argon provides stable arc starting, consistent cleaning action that breaks through aluminum’s oxide layer, and a smooth, controllable arc. For MIG welding, pure argon also works as the default gas and produces clean welds on thinner stock.
Argon is widely available, relatively affordable, and forgiving for beginners. If you’re welding aluminum that’s 1/4 inch or thinner, pure argon is all you need for either process.
When to Add Helium
Helium has much higher thermal conductivity than argon, which means it transfers more energy into the weld. The result is a hotter weld pool that melts thicker material more effectively and freezes more slowly, giving trapped gas bubbles time to escape before the metal solidifies. On thick aluminum sections, pure argon alone can lead to porosity, incomplete penetration, and fusion defects. Adding helium significantly reduces these problems.
The most common blend is 75% argon and 25% helium. This is a good starting point for material over 1/4 inch and lets you increase travel speed without sacrificing penetration. For heavier sections, 50% to 75% helium is preferred, according to guidance from the Aluminum Association. More helium means more heat input, but you lose some of argon’s arc stability, and the gas costs more.
At 100% helium with MIG welding on thick aluminum, you get what’s called a “buried arc” transfer mode, where the arc digs deep into the joint. This is specialized territory, not a general-purpose setup. For most hobby and shop work, the 75/25 argon-helium blend covers the heavy end of the spectrum.
Gas Behavior Differs Between MIG and TIG
With TIG welding, helium raises the arc voltage directly, which increases arc power (watts) and delivers a noticeable jump in heat. This makes helium blends especially effective for TIG on thick aluminum, where you need that extra energy without cranking amperage to extremes. A 75% argon, 25% helium mix can meaningfully boost travel speeds on AC TIG.
With MIG welding, helium’s contribution works a bit differently. The voltage is held constant by the machine, so the thermal conductivity effect is the primary benefit rather than a direct voltage increase. The practical result is still more heat in the weld, but the arc characteristics change. Higher helium percentages make the arc dig deeper and can shift the metal transfer mode, which takes some adjustment to your technique and settings.
One thing that stays the same for both processes: never use carbon dioxide or oxygen-containing mixes on aluminum. Those reactive gases are common for steel MIG welding, but they’ll contaminate and oxidize an aluminum weld. Aluminum requires inert gas only.
Flow Rate Basics
For TIG welding, gas flow rates typically fall between 10 and 35 cubic feet per hour (CFH). The right setting depends on your nozzle size and welding position. A larger nozzle diameter produces longer, smoother (laminar) gas flow at a given rate. A smaller nozzle at the same flow rate pushes gas out faster, creating turbulence that can pull in outside air and compromise your shielding.
If you’re using a helium blend, you may need to bump the flow rate up slightly. Helium is lighter than argon and disperses faster, so it takes a bit more volume to maintain consistent coverage over the weld pool. Start at the higher end of the range and adjust based on your results.
Avoiding Porosity Problems
Porosity is the most common gas-related defect in aluminum welding. It happens when gas gets trapped in the weld pool and the metal freezes before the bubbles can escape. The biggest culprit is hydrogen, which dissolves easily in molten aluminum but has almost no solubility once the metal solidifies. As the weld cools, hydrogen forces its way out and leaves tiny voids behind.
Hydrogen contamination comes from moisture, oil, and the hydrated oxide layer that naturally forms on aluminum surfaces. Clean the joint with a stainless steel brush and solvent (acetone works well) right before welding. Wipe TIG filler rods with a clean rag dipped in acetone before use. Keep filler wire in its packaging until you need it, because wire left out in a shop absorbs moisture into its oxide layer surprisingly fast.
Your gas itself can also introduce problems. Leaks in hoses or fittings pull in humid air. Cheap or improperly stored gas can contain trace moisture. Use welding-grade gas from a reputable supplier and check your lines for leaks periodically.
On the MIG side, welding current matters too. Current that’s too low creates large, erratic droplets that churn up the weld pool and trap gas. Current that’s too high deposits metal over bubbles before they can rise out, creating irregular voids. Finding the sweet spot where droplet transfer is smooth and stable is key. For TIG, most porosity traces back to contamination or shielding gas loss rather than current settings.
Quick Reference by Situation
- Thin aluminum (under 1/4 inch), MIG or TIG: Pure argon. Reliable, affordable, easy to work with.
- Medium aluminum (1/4 to 1/2 inch): 75% argon, 25% helium. Better penetration and faster travel speeds.
- Thick aluminum (over 1/2 inch): 50% to 75% helium, balance argon. More heat for full fusion on heavy joints.
- Budget-conscious or occasional welding: Pure argon covers most jobs. It’s the one gas to stock if you only keep one.
Helium-content blends cost noticeably more than pure argon, and helium supply has historically fluctuated in availability. If you’re not regularly welding thick sections, the added expense usually isn’t justified. Pure argon will handle the vast majority of aluminum welding you’re likely to encounter.