Why 100% Argon or Argon/CO2 75/25%

[mdennis]

Recently acquired an Everlast PowerPro to TIG and Have a Miller MIG and both are light enough to put on a cart and be somewhat portable compared with my old monster TIG machine which took 3 men and a boy to move. This isn't for a welding shop, this is for my shop at home where I only go through a couple tanks of each type of gas a year.

Question is:
Why do we use 75/25 Argon/CO2 mix for MIG, (on steel), and straight Argon on TIG, (both steel and alum)?
(I realize other specialized gases are available and used, but typically that's what I use.)

In the interests of possibly only having one tank of gas on my cart;
Why not just use 100% Argon to MIG and TIG or use 75/25 Argon/CO2 if you are only going to TIG Steel?
Any problems or disadvantages either way?

I am guessing that there is some reason dealing with shielding, but in many years of welding don't ever recall seeing why we use what we do.

[Jason]

I don't remember the exact reasoning..

I do know that I have run a MIG using 100% argon (praxair gave the wrong tank) and it did work. It did not work as well but we did make it through the entire bottle and we didn't even know it was wrong until we went to refill it and they asked if we knew we had the wrong tank. (friend only has MIG and he never got 100% Argon)

[cornonda]

Without getting crazy about it, there are many reasons for using different gases for the different processes.

MIG:
100% pure Argon would be fine, from a "metallurgical" aspect. Although Argon does yield a somewhat unstable arc (mild steels, etc) - personally, it's not my first choice for MIG. It's acceptable - when I'm aware of what the arc is going to do & watch my puddle. The puddle tends to "freeze" more quickly with 100% Ar than a mix gas. Example, if you had a spool gun for your MIG (for aluminum), you'd be best with 100% Argon. On mild steels, I've seen people use 100% CO2, but the splatter was terrible - so don't consider 100% CO2. The 75/25 Ar/CO2 is somewhat of a compromise, giving you better weld appearance, reduced splatter, and bead control (helps to keep warpage to a minimum too, from what I was once told).

TIG:
Oxygen and nitrogen are not your friends here (leading to oxidation & embrittlement). 100% Argon is the ticket for what you want to do, keeps the tungsten from oxidizing, shields your bead (preventing oxidation, etc), prevents cats & dogs from mating, etc - CO2 is less inert. Weld appearance, and many of the benefits of TIG (argon helps keep contamination off of tungsten & filler rod) will be lost if you attempt to use CO2 or a mix bottle with it - your welds will look like a monkey's butt.

Hope this helped - I may have left out important items of info - it's just after 3am here and frankly, I'm surprised I can string two sentences together!

[DaveO]

First of all, welcome MDennis! Congrats on the new machine, hope you're enjoying the PowerPro- what are you working on?

Without getting crazy about it, there are many reasons for using different gases for the different processes.

Here's where to get crazy about it: there's an excellent discussion of shielding gases here, in a Lincoln document called "C4.200 GMAW Welding Guide". This is a link to search results: click on the c4.200 link to view the PDF:
http://www.lincolnelectric.com/en-us...elding%20guide

I've just bookmarked that file: it takes a deep-dive into GMAW that definitely bears further reading.

The answer to "why use co2 in MIG" is that co2 is a reactive gas and produces desirable results in MIG welds (full details within that PDF). Still looking for a similar document that discusses TIG, and the short answer to "can I use only one type of shielding gas". Miller's TIG handbook says "Gas ionization is a fundamental requirement for starting and having a stable arc." and the reason for "no co2 with TIG" >>>may<<< have something to do with co2 disrupting or not accommodating that kind of ionization, or co2's thermal conductivity- but I'm not good enough with either the weld engineering or the chemistry implications of that sentence to validate it either way.

[mdennis]

Thanks for the help guys, I did do some digging and found out all kinds of technical details about why we use separate gasses that I had forgotten about, but as Cornonda pointed out, it's primarily arc stability with MIG.

I am going to build a cart to hold both welders and since neither one is as wide as two bottles of gas side by side, I jumped to thinking about only one tank instead of two.

Using two tanks just changes my cart design a little. I am thinking of putting the TIG on the bottom and the MIG on the top with a shelf and/or drawer in between to hold stuff. Since the tank shelf in the back will be wider than the welder shelves by probably 4 inches or so, I will put some hooks on the side of the welder shelves to hold coiled up wires, hoses and such. Maybe put a tube or two to hold filler rods.

Like a lot of home designs, I tell myself that I can build it cheaper than anything I can go buy, and truthfully it will probably cost twice as much. But it will also be twice as sturdy and just about exactly what I want. (I will figure out exactly what I don't want just after I make it permanent on my cart.)

Mark

[jakeru]

You don't want to use CO2 when TIG'ing because it will oxidize your super (white) hot tungsten electrode. You need to keep that electrode oxidation-free, to keep your TIG welds clean and happy.

"MIG" welding with a reactive gas like CO2 would technically be called "MAG" welding (not MIG) due to the active CO2 gas.

For wire-feed welding use, as the electrode gets rapidly consumed by design, steel welding is tolerant of some oxidation, and also given how high the travel speeds and heat input of the wire-feed process are, the extra heat imparted by mixing in some CO2 is generally desirable. CO2 is also inexpensive.

CO2 is not desirable for wire-feed welding metals that are not tolerant of oxidation, such as aluminum (which would call for an inert gasses like pure argon, or an argon/Helium mix, such as would be used for TIG.)

So in summary, if you wanted one gas with the most versatility, and didn't care about cost, it would probably be Argon/Helium mix, because Helium also provides extra heat, while simultaneously being completely inert. Too bad Helium is very expensive. If the reason you just want to pick "one gas" is to save cost, you probably won't acbhieve that goal with Ar/He mix. If you have some other goal for picking "one gas" like portability of space savings, etc, Argon/Helium may achieve your goal. I love the stuff for TIG'ing aluminum.

[joshuab]

Heck, just go pure He and be done with. As an added bonus, you can huff it to make your voice sound funny when you're not welding. And no need for AC to TIG aluminum. It's a winner all around!

(This post is a joke in case you couldn't tell.)