Ernie.....Question??

Ernie's on the road. See his post below. I am sure he will answer ASAP. I'll take a stab at it, too, while you are waiting.

AR shielding gas produces a very stable transfer of metal across the arc. It is also inert, the "I" in MIG (metal inert gas) welding. That is, it won't form compounds with other elements, al least under any cicrumstances a welder is likely to encounter. So, it is very good at doing what a shielding gas is supposed to do, keep unwanted atmospheric gases out of the molten weld metal. Unfortunately, MIG in short circuit mode with AR has very shallow penetration and is prone to lack of fusion. Adding CO2 to AR gives better penetration when welding carbon and low alloy steel. Using straight CO2 gives even better penetration. It also cheaper and can result in faster welding speed. The downside is spatter is increased, substantially so with straight CO2. CO2 is also a reactive gas. Its use is why MIG is now called GMAW (gas metal arc welding). CO2 can form compounds with other elements. And it can break down into C, CO, O2 and monatomic O in the arc. So you get some extra carbon and oxygen in the weld metal, mostly O IIRC. If oxygen is trapped in the solidifying weld metal, it will cause porosity.

CO2 is also used in MIG spray mode on steel for the same reasons. But the mix generally has to contain at least about 80% AR for spray mode to be stable.

With TIG, penetration is not usually a problem. The quality of the weld is usually very important, including minimizing contamination of the weld metal with gases that will create porosity or cracking problems. A stable arc and little to no spatter is desireable. AR is the best choice. When penetration is an issue, for example when welding thick aluminum, HE can replace part or all of the AR. But HE is much more expensive than CO2 or AR.

That, too.

Doesn't the CO2 react with the tungsten as well?

Shawn

Yeah what he said.

Using AR/CO2 with MIG for steel...

When welding steel, it matters *&^$-all about the oxygen level (more accurately, "oxygen potential", because 2CO2 dissociates to O2 + 2CO). There's a unique physical reason for that. Iron oxide melts at a lower temperature than iron itself. So any oxide will float out as "slag". Different to all (?) other engineering metals. And there are "deoxidisers" like Al which are harmless if remain in solution in the weld metal instead of being used up - well, within reason, which is to say, as long as you know roughly how much oxygen you are getting in the weld-pool, you are just fine fine fine.

As one of the helpful replies said (well, all were helpful!), having an active gas like CO2 increases pen. - in fact, it makes the process usable. Ar/20%CO2 is a favourite - smooth arc with reasonable pen.

Have tried MIG with pure Ar - and I can tell you, penetration is "not good", to say the least. Why do it? 'cos you can't use Ar/CO2 for stainless. Chromium (and nickel?) oxides are refractory (high melting point) plus you are denuding the s'less of these, so it is no longer stainless. Apparently for s'less MIG, you use AR/1%oxygen, which gives you a usable process with insufficient alloying element loss to wreck things.

TIG is penetrative when using pure Ar. It's a fundamental of the process.

An aside - "TIG is penetrative when using pure Ar":

I tried explaining that to some managerial types who were trying to recruit this forceful "MIG - stainless welding expert" to kick their "useless" welding guy into using MIG for chemical-plant fab. Pity the folk at that company. And funnily enough, the management couldn't find anyone who would leave their current MIG-stainless job to come and join them! So sad - they manage yet people won't do as they say! They need to write to the Government about a "skills gap". Anyway, they were sitting right on the thickness where TIG weld speed can just about match MIG weld speed (one deep-pen pass to completion, vs. more than one MIG) - then TIG you don't have to worry about lack-of-fusion defects. Richard Smith (the mad scientist who messes around in welding)