When you’re MIG welding, you’re going to need a shielding gas to keep your weld clean. (Unless you’re going with a flux-cored wire, but that’s not what we’re talking about).
There are a few different gas mixes that you can get, and they all do slightly different things. Unfortunately, there’s not a one size fits all when it comes to getting gas for a weld, so which one is going to work best will depend. The type of metal you use, the metal transfer method, how thick your workpiece is – it all makes a difference.
Gas Types
Inert Gas
Inert gases don’t react withexternal elements (like oxygen and nitrogen), which makes them great shieldinggases. Only argon and helium are cost-efficient enough to be used for welding.
Active Gas
Active gases react with externalelements, which can affect the arc’s stability, weld penetration, and spatteramount. Active gases are only used on ferrous metals and only in smallquantities (as large ones would be damaging).
Semi-inert gases, like carbondioxide, are a combination of inert and active gases. Because they react withthe weld pool, they’re classified as active gases.
MIG Gases
If you’re going to be MIG welding, there are two main gasesthat are used: pure argon and an argon/carbon dioxide mix. The most common (andrecommended) Ar/CO2 mix is a 75/25 ratio, with 75% of it being argon and 25% ofit being CO2.
Which one isused for what?
When you’re working with mild steel or stainless steel,you’ll want to use an Ar/CO2 mix.
If you’re working with aluminium, then you’ll want to use pure argon.
Pure argon (an inert gas) gives shallow penetration but a widerbead profile in a weld. It also results in less spatter.
CO2 (a semi-inert gas) can be used on its own, as it givesdeeper penetration than argon (plus it’s cheaper). However, it produces aharsher, less stable arc, which results in a lot of spatter.
Combining the two gives you a fully penetrative weld andgood arc characteristics without a crazy amount of spatter. That’s why a 75/25Ar/CO2 gas mix is the best for mild and stainless steel. These days it is oftenadvertised as ‘MIG gas’ as well.
Other gasesand their mixes
Argon/CO2 mixes:
While argon and CO2 can both be used on their own depending on the application, and a 75/25 mix produces great results on most things, there are a few other combinations that work.
98% Ar/2% CO2
This mix is called C2 and is generally used on stainless steel. It’s a cheaper option than a tri-mix (see below) but works in the same way.
82%-92% Ar/18%-8% CO2
This mix (anywhere in the range works) is used when usingthe spray transfer method. Straight CO2 or the standard Ar/CO2 mix is limitedto short-circuit MIG, so the gas needs to be adjusted in order to spray transfer.
Helium
Like argon, helium is an inert gas and is most commonly usedin colder climates. It burns hotter than other gases, so it gives deeppenetration to the weld. That’s why it is often added in small dosages toargon, which allows you to work on thicker materials with ease.
However, helium can be quite expensive, so even though itcan be used on its own, it’s more often only used as a mix. The only exceptionto this is when it is used in tri-mixes for stainless steel.
Helium mixes:
- Ar/He
- He/Ar/CO2
- He/Ar/O
A tri-mix can be used with stainless steel for extrapenetration, with 90% helium, 7.5% argon and 2.5% CO2/oxygen. The heliumprovides the needed heat while the CO2/oxygen helps with penetration andstabilising the arc.
Oxygen (an active gas) can be added in very small amounts(1%-5%) to increase the penetration of a weld and stabilise the arc. It is acheaper alternative to helium and works better on ferrous metals.
Oxygen mixes:
- Ar/CO2/O
- Ar/O
Oxygen is only added in small amounts, thanks to excessamounts being damaging. Shielding gas is needed to keep oxygen (and the resultingporosity) out of the weld pool. Adding too much to your shielding gas will havethe same effect as having no shielding gas.
What happenswhen you get it wrong?
Choosing the wrong shielding gas isn’t recommended, andit’ll leave you with some unwanted weld defects.
If you were to use pure argon as your shielding gas for aweld on mild steel, more often than not, you’ll be left with an inconsistentbead and lots of spatter.
If you used a highly penetrative mix, like an argon/heliummix, on really thin metal, then you’ll spend more time blowing holes through itthan you would actually welding it.
