Since we're delving deeply into the discussion of MIG shielding gas, I thought I'd post an excerpt from my welding book. It's long but it sounds like a popular topic with some of you and for you it will be of interest. I found it very useful.
Choosing shielding gas
By now, you should have an idea of what level of MIG machine will suit your purposes. If you are like most home/shop users who aren't going to be welding every day, your needs may be best served by a typical, entry-level 140-amp, 110V MIG machine using shielding gas. You now have to choose a shielding-gas bottle (not included with most MIG machines), some wire (a small sample roll may be included with your machine), and the type of shielding gas you want to use. This sounds like a lot of choices, but the field is narrowed down by what kind of work you want to do and how often you need to do it. In all cases, the choice of shielding gas and wire type must be matched to the kind of material you will be welding.
Capacity of the gas bottle is important, and most first-time weldors make the mistake of buying less capacity because the bottles are cheaper, only to find themselves someday working on a project over a weekend (when there is little chance of getting a bottle refilled) and they are out of gas. Gas bottles are avail-able in sizes from 20 cubic feet to 40, 60, 80 and 125 cubic feet of gas. This roughly translates into welding time of from four hours for the 20 cu.ft. bottles to 14-16 hours for the 125 cu.ft. when used on a small machine like the 140-amp unit in our example. There's a number of factors involved in how long a particular bottle may last under different welding conditions, so these are just rough guide-lines. Every time you use your MIG machine, you should look at your tank gauge to see how much more gas is left. You may even want to just lease a small cylin-der of gas when your first get your machine just to see how long such a tank would last you, and then buy a bigger one if you think you need it.
The small bottles fit nicely under a small welding cart, making for a very portable arrangement for occasional home use (see illustration). The Iarger bot-tles are heavy and bulky, so you'll have to build a welding cart (most first-time weldors' first project) that can accommodate one. Of course, bottles of com-pressed gas are dangerous, even if the gas is non-flammable, due to the pres-sures involved, and any welding gas bottle must be securely mounted with' chains or sturdy clamps. It's important to note that welding bottles for electric welding machines should be ideally mounted in an insulated manner, such as with rubber underneath and rubber hose over any restraining chains. This pre-vents the remote possibility of arcing against the cylinder, which could damage it.
Next there is the choice of shielding gas to use. One of the widely used shielding gasses for MIG work is plain carbon-dioxide, or CO2. While it is in the bottle, CO2 is really an inert gas, but technically it turns into carbon monoxide and oxygen under the intense heat of the arc process. The oxygen can combine with other elements in the air and in the parent metal to form undesirable oxides. Thus, plain CO2 will not usually result in as clean a weld as with other gasses, but it is used often because it is the least expensive shielding gas for welding steel. It will weld very fast, with good penetration, but you should use a welding wire that contains deoxidizing elements to counteract the effect of pure CO2, Argon is a versatile inert shielding gas that is often used by itself, or in com-bination with other gasses to produce certain weld characteristics. For instance, welding of non-ferrous materials like aluminum is usually done with pure argon. It makes for good penetration patterns and a concentrated arc.
To weld ferrous materials, like the mild steel we will use for most home/shop projects, argon is usually mixed with another gas, or sometimes two other gasses, to provide special characteristics. While there are some esoteric mix-tures of gasses in different percentages for specific purposes, a mixture of 75% argon and 25% CO2 has become pretty much the standard for welding mild steel with short-arc MIG machines with wire diameters of .035-inch or less (see illus-tration). This mixture, often abbreviated as C-25, is more expensive than the plain CO2 but produces much less spatter and consistently better looking welds, even on materials that exhibit minor rust or scale. It is what we would recom-mend for most of your welding needs. If you were doing a lot of thicker materials, overhead or other out-of-position welding and pipe welding, you might use a 50-/50 mix of argon and CO2 which offers good wetting and bead shape without the. excessive fluidity that causes a bead to droop or fall when doing out-of-position
welding.
If you are working in a body shop, you might be interested in a mixture that Airco calls Argoshield LG@, which stands for light gauge. It consists of mostly ar-gon with small additions of CO2 and oxygen, The combination is designed for metals down to 20.-gauge, and produces good penetration, but with a smaller weld bead than normal for less sheet-metal finishing, less spatter and smoke and good arc starting. It is specifically formulated to work well on the thin, low-alloy steels now used on auto sheet metal.
For welding of non-ferrous materials, either straight argon or mixtures of ar-gon and helium are used in various combinations which provide higher heat to a MIG arc. Usually, the thicker the material to be welded, the higher the percentage of helium that is included in the mix, and the HE-75 gas, which is 25% argon and 75% helium is typically used in industry to weld thick aluminum. If you were interested in welding stainless-steel, you can actually use the C-25 gas' we recommended above for mild steel, but a mixture of 90% helium, 7.5% argon and 2.5% CO2 is widely used in stainless-steel MIG welding because it offers a higher heat for the normally sluggish weld puddle on stainless, as well as offering good stability, penetration and resistance to corrosion. If you have to pick just one type of gas, ,the C-25 works great on steel, can be used on stainless-steel and is also useable on cast iron.
