Welding Shielding Gas Mixture Chart
Shielding gas selection for MIG / TIG / Flux-core — Argon, CO2, Helium, Oxygen, and common blends (75/25, C25, Tri-mix, ArHe). Process compatibility, flow rates, and effects on bead profile, penetration, and spatter.
Reference
Shielding gas pushes air (oxygen + nitrogen + moisture) away from the molten weld puddle. Wrong gas = porosity, brittle welds, or no arc at all. The two big rules: (1) TIG always uses pure inert gas (Argon or Argon/Helium — never CO2 or oxygen), (2) MIG and FCAW use active mixtures with CO2 or O2 to stabilize the arc. Aluminum is special — only pure Argon, regardless of process.
Common shielding gas blends — what to use
| Gas / blend | Process | Material | Effect on weld |
|---|---|---|---|
| 100% Argon (Ar) | GTAW (TIG), aluminum MIG | All TIG · Aluminum / Magnesium MIG · Reactive metals (Ti, Zr) | Stable arc; clean appearance; mandatory for non-ferrous TIG. |
| 100% CO₂ | GMAW short-circuit, FCAW | Mild / low-alloy steel only | Deep penetration, lots of spatter, rougher bead. Cheapest option. |
| 75% Ar / 25% CO₂ (C25, "MIG mix") | GMAW short-circuit | Mild / low-alloy steel | Workhorse mix — cleaner than 100 CO2, less spatter, slightly less penetration. Default for hobby + light commercial. |
| 90% Ar / 10% CO₂ | GMAW spray transfer | Mild / low-alloy steel (heavier plate) | Spray transfer threshold lower; flatter bead. |
| 98% Ar / 2% O₂ | GMAW spray transfer | Stainless / mild steel | Smoothest spray bead, tightest weld toe. Standard for stainless production. |
| 98% Ar / 2% CO₂ | GMAW spray transfer | Stainless | Common stainless mix; less heat than O2 blends. |
| Tri-mix (90% He / 7.5% Ar / 2.5% CO₂) | GMAW | Stainless, especially food-grade | "Heli-Star" — very smooth fillet, low spatter, excellent for production stainless. |
| 98% Ar / 2% N₂ | GMAW | Duplex stainless | N2 stabilizes austenite-ferrite balance. |
| 75% Ar / 25% He | GTAW + GMAW | Aluminum 1/4" - 1" | Hotter arc; better penetration on heavy aluminum. |
| 25% Ar / 75% He | GTAW + GMAW | Heavy aluminum > 1" | Very hot arc; high travel speeds; expensive. |
| 100% He | GTAW | Heavy copper / aluminum (specialty) | Maximum heat input. Hard arc start; rarely used outside of specialty work. |
| Ar + 1-2% H₂ | GTAW (austenitic stainless) | 300-series stainless, Ni alloys | Hydrogen reduces oxide; smoother weld face. NOT for ferritic / martensitic steel — embrittlement risk. |
| Ar + 30-50% N₂ | GTAW (root pass) | Duplex / super-duplex stainless | Replaces lost nitrogen on hot side; backing gas on root. |
| 100% N₂ | Plasma cutting, copper TIG | Specialty (back purge, plasma) | Cheap purge; not for ferrous welding. |
Flow rate (CFH / L/min) by application
| Process | Position | Flow CFH | Flow L/min |
|---|---|---|---|
| MIG indoor, no draft | Flat / horizontal | 20-30 | 9-14 |
| MIG indoor, no draft | Vertical / overhead | 25-35 | 12-17 |
| MIG outdoor / windbreak | Any | 30-45 | 14-21 |
| TIG light gauge | Flat | 8-15 | 4-7 |
| TIG general (1/16-1/8") | Flat | 15-20 | 7-9 |
| TIG heavy / aluminum | Flat | 20-35 | 9-17 |
| TIG with gas lens | Flat (longer stick) | 12-18 | 6-9 (lower than reg cup) |
| Back purge (root pass) | Pipe | 5-10 | 2-5 |
| FCAW dual-shield | Flat | 35-50 | 17-24 |
| FCAW self-shielded | Any | 0 (no gas) | 0 — gas built into wire flux |
Effects on weld — Ar vs CO₂ vs He
- 100% Argon
- Cool arc, narrow bead, deep finger-like penetration. Cleanest arc. Mandatory for TIG and aluminum MIG.
- 100% CO₂
- Hot arc, wide bead, shallow but wide penetration ("blob" shape). High spatter. Cheap. Steel only.
- 75/25 Ar/CO₂
- Compromise: cleaner than 100 CO2, hotter than 100 Ar. Workhorse for steel MIG.
- + Helium
- Each % He adds heat. Use for heavy aluminum / copper. Expensive — leaks fast (small molecule).
- + O₂ (1-5%)
- Stabilizes spray-transfer arc. Cleaner bead than CO2. Used in stainless and steel production MIG.
- + H₂ (1-5%)
- Adds heat + reduces oxide. Stainless / nickel TIG only. NEVER on carbon steel (cracks).
Stainless steel — 300 series details
| Stainless grade | Best gas (TIG) | Best gas (MIG short) | Best gas (MIG spray) |
|---|---|---|---|
| 304 / 304L | 100% Ar (most common); Ar + 2-5% H2 for higher quality | 90% He / 7.5% Ar / 2.5% CO2 (tri-mix) | 98% Ar / 2% O2 |
| 316 / 316L | Same as 304 | Tri-mix or 75% Ar / 25% He | 98% Ar / 2% O2 |
| 309 / 310 | Same as 304 | Tri-mix | 98% Ar / 2% O2 |
| Duplex 2205 | 98% Ar / 2% N2 | Tri-mix + N2 supplement | 98% Ar / 2% N2 |
| Super-duplex 2507 | Ar + 5% N2 | Specialty mix | Specialty mix |
Aluminum — keep it simple
- Up to 1/4" thickness
- 100% Argon, both MIG and TIG.
- 1/4" - 1/2"
- 100% Argon usually OK; 75% Ar / 25% He gives better penetration if welder available.
- 1/2" - 1"
- 50/50 Ar/He or 25/75 Ar/He. Helium dramatically increases heat without increasing amps.
- > 1"
- 25% Ar / 75% He or pure He for absolute max penetration. Use AC TIG with high amps + preheat.
- NEVER use
- CO2, O2, or N2 with aluminum. CO2 reacts with aluminum forming oxide carbonates — porosity / cracking.
Cylinder identification
| Gas | Cylinder color (US/CGA) | Common sizes | Pressure (full) |
|---|---|---|---|
| Argon | Brown body / brown shoulder | 40 / 80 / 125 / 330 cuft | 2200-2640 psi |
| CO₂ | Gray / aluminum | 20 / 50 lb | ~830 psi at 70°F (liquid in cylinder) |
| Helium | Brown body / orange-brown top | 125 / 200 / 300 cuft | 2200 psi |
| Oxygen | Green | 20 / 40 / 80 / 125 / 251 | 2200 psi |
| 75/25 Ar/CO₂ | Gray (often) | 40 / 80 / 125 / 250 cuft | 2200 psi |
| Tri-mix | Light blue / silver / variant | 125 / 250 cuft | 2200 psi |
| Nitrogen | Black | 125 / 230 cuft | 2200 psi |
Common gas problems / fixes
| Symptom | Cause | Fix |
|---|---|---|
| Porosity (worm-hole / pinhole) | No gas / drafts / contaminated joint | Verify gas flow at the gun (15-25 CFH); block drafts; clean joint to bright metal. |
| Excessive spatter (MIG) | CO2 too high in mix; voltage too low | Switch to 75/25 or 90/10; tune V/A. |
| Black sooty deposit on aluminum | Wrong gas (CO2 or O2); damp wire | Switch to 100% Ar; new wire; check torch O-rings. |
| Tungsten contaminates (silver puddle) | Touched workpiece OR backflowing air | Re-grind tungsten; verify gas pre-flow ≥ 0.5s; check for cracked cup. |
| Sugary / oxidized stainless root | No back-purge on stainless / Ti pipe | Add nitrogen or argon back-purge to root side. |
| Hard to start TIG arc | High flow blowing arc / contaminated tip | Reduce flow to 12-15 CFH; re-grind tungsten; clean joint. |
| Yellow / brown discoloration on Ti | Oxygen contamination | Increase post-flow to 15-30s; use trailing shield; back-purge required. |
Cost reference (US, ~2025 retail)
- 100% Argon (125 cuft)
- $50-90 fill + $250-400 cylinder lease/purchase
- 100% CO₂ (20 lb)
- $25-40 fill — cheapest shielding gas
- 75/25 Ar/CO₂ (125 cuft)
- $60-100 fill
- Tri-mix (125 cuft)
- $90-130 fill — premium for stainless
- Helium (125 cuft)
- $200-350 fill — leaks fast; expensive
- Pure Argon vs MIG mix
- For TIG you MUST use pure Argon. Don't try to use MIG mix on TIG — CO2 destroys tungsten.
Safety reminders
- All shielding gases are simple asphyxiants — they displace oxygen. Ventilate enclosed spaces; never enter a tank or pipe filled with argon (heavier than air, settles in low spots).
- CO₂ is heavier than air and settles into pits / floor drains; sustained leaks in basements can cause CO₂ poisoning.
- Helium is lighter than air — accumulates near ceiling. Less of an immediate hazard but still asphyxiating.
- Never store flammable gases (acetylene, propane) in the same room as oxygen cylinders — separate by 20 ft or a 1/2-hour fire wall.
- Always chain or strap cylinders upright; never lay on side (especially CO2 — siphons liquid through the regulator).
- Hydrogen-mix cylinders are flammable. Flash-back arrestors required on the regulator.
Notes
- Mix names like "C25" (75/25 Ar/CO2), "Tri-mix" (90/7.5/2.5 He/Ar/CO2), and "Argamix" (manufacturer-specific) vary by region. Always verify composition on the cylinder label.
- Cylinder colors per CGA C-9 (US). European standards (EIGA) use different colors — green for Argon in EU, for example. Always read the label.
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