Shielding Gas Selection

There's a good bit of confusing information on the internet about the selection and use of shielding gas. So-called experts share their opinions and experiences, telling what they use and believe to be fact. Others have gone so far to believe that shielding gas isn't absolutely necessary.

Let's start right there. Shielding gas use in true MIG and TIG is required. The acronyms MIG and TIG stand for Metal Inert GAS and Tungsten Inert GAS. The same is true if you use the AWS acronyms G-MAW(Gas Metal Arc Welding and G-TAW (Gas Tungsten Arc Welding). As you can see, the focus word of these processes is GAS...the pressurized, cylinder kind. Of course there's always the one who says, "I 'migged' my lawn mower deck with my welder I bought from XXXXXX with out shielding gas and it works just fine." Well to that, I would say, either he/she used a flux cored wire, which is technically Flux core Arc welding (F-CAW) or he/she should be really glad that a lawn mower deck is all that was welded on. Any one claiming to have used the TIG process without some form of shielding gas is president of the local liars club...or should be elected to that position at their next meeting in the local Waffle House.

Shielding gas is a gas or gas mixture that is used to surround the weld pool as welding takes place. The gas flows out from the source, usually from the torch nozzle, or cup and covers the area with concentrated gas that excludes the oxygen from the weld. The primary gas that is generally used in blends for this kind of welding is Argon. It is a dense gas that is heavier than air which makes it ideal for this because it "hangs" around the gas puddle where it is placed for a longer period of time. It is inert and will not interact with the metal as other elements in the atmoshpere.

Generally, in TIG welding, pure Argon is used. Back when the process was known as "Heli-Arc" Helium was originally used. Although Helium is a inert gas and produces deeper penetration, it wasn't ideal, because it did not provide the best coverage because it was lighter than air and quickly left the weld pool exposed and required much higher flow rates. It was also more expensive to use...It still is. Argon was a "left" over by-product of the air separation process and it began to be subtituted for helium as a cheaper alternative because it allowed better "cleaning" action on aluminum when welded with High Frequency AC. It soon found favor as a superior, all around shielding gas. Pure Argon is suitable for all forms of TIG welding. While some people use more exotic blends of Helium/Argon or or Argon/Hydrogen or Argon/Nitrogen for highly specialized applications, Argon is generally recognized as the best all around, best gas to use in the TIG welding. The Hydrogen and Nitrogen percentages are relatively low and are added only for working with a specific kind of metal and cannot be used with other types of metal because of unfavorable interactions.

For MIG welding, the picture becomes much more complex. In steel, Argon/CO2 blends are widely used. The most common blend of Argon/CO2 is 75% Argon and 25% CO2, referred to as 75/25. This allows for good bead contour, lower amounts of spatter and easy out-of-position welding on most steels during short circuit transfer. It has a fairly economical price point for most people only concerned with steel fabrication and repair. For spray transfer in MIG welding, relatively low amounts of CO2 are used. Generally below 20% concentration of CO2 is needed for the metal wire to spray. Also for spray, an Argon/O2 may be used, especially for welding Stainless Steels in either spray or short circuit modes, depending upon mix concentrations of O2. Ideally mixes that are less than 15% are the best, however, the lower the amount of CO2, the more difficult the weld pool becomes to control in out of position welding. There is some use of pure CO2 in the industry, because of its reduced cost. The large amounts of welding wire loss through spatter, time spent in clean-up and general aggravation usually offsets the money that is saved through pure CO2 use. Pure CO2 is used in applications where no concern is given to clean up or weld quality or deep penetration is required. Pure CO2 is not inert. While it does a good job of exluding the atmosphere from the weld, it does interact with the molten metal. This is why you often see the acronym MAG (Metal Active Gas) used to alert the reader to CO2 use. The dull gray, spattered weld is a tell-tale sign of pure CO2 use in the welding process (or lack of shielding gas!). For other metals, pure Argon is often used. Aluminum welds require the use of pure Argon or a Argon/Helium mixture. In fact, it is possible to weld nearly every metal EXCEPT steel with a Ar/He mixture. There are also tri-blends of gases with Ar/CO2/O2 which work well for stainless and steel uses, also for pure stainless and aluminum work, Ar/CO2/He.

While the list seems to be long on possible combinations of Argon blends to use in welding with MIG, it is not necessary to keep a complex "cocktail" of gases available. Generally, a 75/25 blend will be all that is necessary, unless aluminum is frequently welded, then pure argon may be used. My personal choice in my own shop is not a 75/25 however. While I have nothing against it, I use a proprietary blend from a company that is 82% Ar/18% CO2. I have found that it is the best type of gas for my type of welding, because I frequently use spray tranfser and short circuit transfer equally.

A little experimentation may be required, as it was in my case, to come up with the weld qualities that you desire. Don't be too concerned with getting the wrong "kind" of shielding gas as long as it is specified for the type of metal you are welding. It may have a little different quality than you are used to, but you should be able to manage until you consume it all and are ready for an exchange to another blend. In the meantime, you will learn to adapt your welding style and possibly learn to appreciate the weld qualities the gas blend affords you.

That was a good post!

I found it especially informative.

Thanks!

Mark been welding all my heads at the shop tried pure argon , then 75 per cent argon 25 percent helium and then tried 50 percent argon and 50 percent helium mix . since i was using the 200dx the straight argon just didnt cut it on the vw heads even with the heads heated to 500 degrees in my head oven. So tried the 50 /50 mix worked great when first welding but after about a couple of minutes welding was hard to get a arc . the straight argon never had a problem with a starting arc . So tried the 75 argon 25 helium and found I had no problem with starting arc ether . so going to stick with that mix for thicker aluminum . Have you had that proublem with the 50 / 50 mix with a rc start problem . Rod

Hello,

Yes, this was very helpfull. I did notice when I bought one of my tanks used it has some pure helium in it. I had it at the same amperage rating. It pretty much blew a hole trough the stainless I was tinkering with. I am no expert and an admitted novice. Ive used pure argon just fine. Still learning. Took awhile to figure out the flow. Perhaps one day I will be giving the advice. I was curious about titanium. I bought some titanium awhile back for a project and ended up cancelling that design. However, I still want to learn more about it for the future. I hear its simalar to stainless with a lot more cleaning involved. Would you be better off using a helium concentration with argon on that? Its grade 2, So I not building a spaceship or anything.

Thanks

We have always used 100% argon with titanium. Titanium is a bit "sticky" to weld, as in the filler tends to want to attract to the puddle and get stuck. Cleaning isn't necessarily any different than stainless, but you do need to shield it more, often with a trailing shield to prevent oxidation. Highly scrutinized welds are supposed to be "light straw" in color, but pretty anything up to dark purple will not be a problem with less critical parts. If the weld is ashy, gray, or black, then you can have issues with it being extremely brittle as it is mostly oxidized.

If it is for something like exhaust tubing or similar, welding it as if it were stainless (with titanium filler of course) will be fine. One thing to be cautious of is that titanium has a very low heat transfer and hot items will stay very hit for longer than you might expect.

John

Hello,

Thanks for the reply John. I had read that about the oxidization of titanium. Also heard that about shielding. I figured I was gonna actually build a vessel to weld it all in. Thought I would cut a viewing window and some arm ports out of a steel drum. Then run an argon line to it. I figured it would be cheaper than trying to buy all this different shielding equipment. I was curious about the oxidization. If its colored does that mean the weld is toast? I was more or less just thinking about the outside of it? Is the actual inside of the weld ruined? I know each color has to do with the penetration of the oxidized layer. I was curious if that mattered as long as your not welding that into your puddle? Thanks for all the other info. Some good and usefull stuff.