Mig welding gas: which gas, which cylinder, and how to buy it
Which shielding gas a mig welder needs for mild steel, stainless and aluminum, how cylinders are sold, what else to buy, and how to handle one.
Your welder came with a regulator and a gas hose, and no gas. That is normal. Shielding gas is bought locally, from a welding supply or industrial gas store, and the counter staff will ask you two questions: which gas, and which size.
For mig welding mild steel, ask for 75/25: a mix of 75 percent argon and 25 percent carbon dioxide, often sold as C25. Get it in a cylinder you own and can exchange, in a size you can lift and strap upright. Aluminum needs pure argon instead.
Which gas for which metal
Shielding gas keeps air away from the molten weld. Without it the weld fills with tiny holes. The gas also shapes the arc, which is why the choice changes with the metal.
| Metal | Common mig gas | Notes |
|---|---|---|
| Mild steel | 75 percent argon, 25 percent CO2 (C25) | The all-round choice. Smooth arc, modest spatter |
| Mild steel, lowest cost | 100 percent CO2 | Deeper penetration, harsher arc, more spatter. Needs a CO2 regulator |
| Stainless steel | A three-part mix, commonly helium, argon and CO2 | Ask the supplier for their stainless mig mix. C25 is not the usual choice here |
| Aluminum | 100 percent argon | Also needs a spool gun or a suitable liner, and a machine rated for it |
Tig uses pure argon for every metal. If you run both processes on steel, that means two cylinders. The processes are compared in mig vs tig vs stick.
Your welder’s manual or the chart inside its door names the gas its settings assume. Settings charts written for C25 will be off on straight CO2.
Cylinders: size and ownership
Gas cylinders are sized by how much gas they hold, in cubic feet. Common small sizes are 20, 40, 80 and 125 cubic feet. A 40 is easy to carry and empties quickly. An 80 is the usual recommendation for a home shop: it lasts a useful while and one person can still move it. A 125 lasts longer and is heavy.
How long a cylinder lasts depends on your flow setting and how much you weld. Your manual gives a flow range for your machine. Flow is set on the regulator in cubic feet per hour, and more is not better: too much flow pulls air into the gas stream.
Ownership works differently from most things you buy:
- Owned, exchanged. You buy a cylinder once. When it is empty you swap it for a full one and pay for the gas. You do not get your own cylinder back, and that is fine.
- Leased. Common for large cylinders. You pay a yearly fee plus gas.
- Check before you buy. Ask the supplier which sizes they sell outright and exchange. If you buy a cylinder elsewhere, ask first whether they will fill or swap it.
What else you need
- A regulator or flowmeter that matches the gas. Argon and argon mixes share a fitting. CO2 uses a different one.
- A gas hose from regulator to machine, usually supplied with the welder.
- Solid wire, such as ER70S-6 for mild steel, in the diameter your machine’s chart calls for. Gas mig uses solid wire, not flux core.
- The right polarity. Gas mig and flux core usually use opposite polarity. The manual shows the lead positions.
- A way to secure the cylinder: a welding cart with a chain, or a wall bracket.
If you are still choosing a machine, check that it has a gas valve at all. Some low cost welders are flux core only. See flux core welders explained and mig welders explained.
Handling a cylinder
A full cylinder holds gas at very high pressure. Shielding gases are not flammable or toxic, and they can still hurt you in two ways: a cylinder that falls and breaks its valve, and gas that displaces the air in a small closed space.
- Keep the cylinder upright and chained or strapped, in use and in storage.
- Keep the protective cap on whenever the regulator is off, and always when moving it.
- Transport it secured and with ventilation. Do not leave it in a closed car.
- Open the valve slowly, standing to one side of the regulator.
- Close the cylinder valve when you finish for the day.
- Do not weld in a small unventilated room.
Your supplier will have handling instructions for the cylinders they sell. Follow those over anything general, including this page.
If the weld is full of holes
Porosity with gas mig nearly always means the gas is not reaching the puddle: the valve is closed, the cylinder is empty, the flow is set too low or far too high, there is a draft, the nozzle is clogged with spatter, or a hose fitting leaks. The full symptom table is in beginner welding mistakes.
Frequently asked questions
Can I use pure argon for mig welding steel?
It is not the normal choice. Pure argon on mild steel tends to give an unstable arc and a tall, narrow bead with poor wetting at the edges. Steel mig is usually run on an argon and carbon dioxide mix or on straight carbon dioxide. Pure argon is the gas for mig on aluminum and for tig.
Is straight CO2 cheaper than a 75/25 mix?
Usually, per fill, and a CO2 cylinder holds a lot of gas for its size because it is stored as a liquid. The trade is a harsher arc and more spatter. Check that your regulator suits CO2, because the cylinder fitting differs from an argon mix cylinder and CO2 can freeze a regulator at high flow.
Should I buy or lease a cylinder?
Small cylinders are commonly sold outright and then exchanged: you hand over your empty and pay for a full one. Large cylinders are often leased. Before buying a cylinder anywhere, including online, ask your local supplier whether they will exchange or fill it. Some will not touch cylinders they did not sell.
Do I need gas if I use flux core wire?
Not with self-shielded flux core wire, which is what small machines use. The shielding is inside the wire. If you switch between flux core and solid wire, you also swap the machine's polarity and turn the gas on or off to match.