Guide
Shielding Gas Selection Guide
For carbon steel MIG the default is 75% argon / 25% CO2; for stainless and aluminium TIG it is pure argon; and the rest of the choices trade penetration, spatter and cost against each other.
Shielding gas keeps atmospheric oxygen and nitrogen out of the molten weld. Which gas you choose also changes arc shape, penetration profile, spatter, travel speed and how the bead wets out — so the decision is not purely about protection.
What each gas does
| Gas | Behaviour | Notes |
|---|---|---|
| Argon | Inert, heavy, low ionisation potential | Stable arc, easy starts, narrow finger-shaped penetration. Heavier than air so it blankets the puddle well. |
| Carbon dioxide | Reactive, breaks down in the arc | Deep broad penetration and low cost, but a harsher arc and considerably more spatter. Cannot support spray transfer. |
| Helium | Inert, light, high ionisation potential | Raises arc voltage and heat input for better penetration on thick aluminium and copper. Light, so it needs higher flow. |
| Oxygen | Reactive, added in small amounts | 1–5% stabilises the arc and improves wetting on steel. Too much oxidises the weld. |
| Hydrogen | Reactive, added in small amounts | 2–5% raises travel speed and improves bead wetting on austenitic stainless only. Never on carbon steel or any hydrogen-sensitive material. |
Carbon steel MIG
- 75% argon / 25% CO2 — the shop standard. Smooth arc, minimal spatter, good appearance, works in short circuit and spray.
- 100% CO2 — cheapest, deepest penetration, most spatter. Short-circuit transfer only. Common where cost and penetration beat appearance.
- 90% argon / 10% CO2 — for spray and pulsed transfer on heavier plate.
- 98% argon / 2% oxygen — spray transfer with excellent wetting on plate.
Stainless steel
- TIG: 100% argon, always. Add an argon back purge on pipe and tube roots to prevent sugaring.
- MIG short circuit: 90% helium / 7.5% argon / 2.5% CO2 — the classic tri-mix, good wetting with minimal carbon pickup.
- MIG spray: 98% argon / 2% CO2, or argon with 2% oxygen. Keep CO2 low so the weld does not gain carbon and lose corrosion resistance.
- Argon with 2–5% hydrogen raises travel speed on austenitic grades, but never use it on ferritic or duplex stainless.
Aluminium
- 100% argon for both TIG and MIG up to about 1/2 in.
- 75% argon / 25% helium for thicker plate and castings, where argon alone runs out of heat.
- 50/50 argon-helium for heavy sections.
- Never use any CO2-bearing mix on aluminium — it will oxidise the weld badly.
Flux-cored welding
Gas-shielded flux-cored wire runs on 100% CO2 or 75/25. CO2 penetrates deeper and costs less; the argon mix gives a smoother arc, less spatter and better out-of-position control. Self-shielded flux-cored wire takes no gas at all, and adding gas does nothing useful — it is formulated to shield itself.
Flow rates
| Process | Typical flow | Notes |
|---|---|---|
| MIG, solid wire | 20–25 CFH (9–12 L/min) | Raise for larger nozzles and draughty shops |
| MIG, aluminium | 20–30 CFH (9–14 L/min) | Aluminium needs more coverage than steel |
| TIG | 15–20 CFH (7–9 L/min) | A gas lens lets you run lower flow with better coverage |
| Flux core, gas shielded | 35–45 CFH (16–21 L/min) | Needs noticeably more than solid wire |
| Back purge | Enough to displace air, then reduce | Purge until residual oxygen is under about 0.1% |
More gas is not better. Excessive flow creates turbulence at the nozzle that draws air into the shielding envelope, producing exactly the porosity you were trying to prevent.
Common gas problems
- Porosity in a shop that used to weld fine: check for a leaking hose or fitting before changing settings.
- Porosity outdoors: any breeze over about 5 mph will strip the shield. Put up a screen or switch to a self-shielded process.
- Spatter building up in the nozzle restricts flow and causes intermittent porosity — clean it and use anti-spatter.
- Sugaring on the inside of a stainless pipe root: the back purge was inadequate or was released too early.
- Black sooty aluminium welds: usually a CO2-bearing gas rather than pure argon, or a contaminated joint.
Frequently asked questions
What is the best shielding gas for MIG welding steel?
75% argon / 25% CO2 for most work — smooth arc, low spatter, good bead appearance, and it supports both short circuit and spray transfer. Straight CO2 costs less and penetrates deeper but spatters considerably more.
Can you use CO2 for TIG welding?
No. CO2 breaks down in the arc and releases oxygen, which would destroy the tungsten electrode almost immediately. TIG requires an inert gas — argon, or an argon-helium mix.
What gas do you use for aluminium?
100% argon for TIG and MIG up to about 1/2 in. Argon-helium mixes add heat for thicker sections and castings. Never a CO2-bearing mix.
Why does higher gas flow cause porosity?
Too much flow turns the gas stream turbulent at the nozzle, and turbulence entrains surrounding air into the shielding envelope. The result is the same porosity you get with too little gas.
Do you need gas with flux core welding?
Only with gas-shielded wire such as E71T-1. Self-shielded wire like E71T-11 generates everything it needs from the flux, which is what lets it work outdoors.