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There is a lot more to MIG or GMAW welding than choosing a welder and wire size. The type of gas you use makes a huge difference. Not only are there a variety of gases you can use, there are also a variety of gas mixtures. Choosing the right gas product can make your job both easier and neater. But more importantly, you will get the best weld possible and that should be your primary goal, particularly if you're welding safety equipment such as roll cages. We asked Dave Comer, of Air Liquide, to provide us with information on how to choose a welding gas and here's what he had to say:
The key advantage of using one of our mixed gases for GMAW over straight C02 are:
- Less Spatter
- Less burn through on thin material ( bodywork etc.)
- Higher mechanical properties and higher low temp impact strengths
One analogy that I have used in the past for training is, "C02 is to Argon mixtures as E6010 is to E7018." That is to say, you could compare C02 to E6010 type electrodes, and mixtures to nice smooth E7018. The right mixture of gases will often provide a far superior result over straight gases such as Argon or CO2.
Now here are the details:
There are many shielding gases and shielding gas mixtures in common use with the gas metal arc welding process. Some have a broad range of applications while others serve specialized applications.
For any specific application it is possible to find several gas mixtures that will produce good welds, but it is impossible to develop an all-purpose mixture that will give good welding characteristics under all conditions.
To help simplify the specification and ordering of shielding gas mixtures, Air Liquide has developed a series of 9 BLUESHIELD gas mixtures that offer optimum performance over a range of welding applications. For highly specialized applications Air Liquide offers sound technical advice and preparation of special gas mixtures to suit. The chart included in this brochure is intended to aid in the selection of the correct BLUESHIELD mixtures.
Air Liquide technical representatives and specialists are always available to provide more detailed information and advice.
Shielding Gas Selection
Initially, shielding gases were used to avoid weld pool contamination by oxygen, nitrogen and air humidity by using inert gas such as argon and helium. However, the protection of the weld pool is not the only criterion for selecting a shielding gas. Factors like arc stability, metal transfer, penetration profile, welding parameters (current voltage), width of weld pool, bead shape, wettability and welding speed are also influenced by the shielding gas.
Properly selected, a shielding gas can make welding easy and fast, it can control bead contour, and it can even help to control deposit chemistry, penetration and weld defects.
The primary factors in the selection of shielding gas mixtures are:
- Characteristics of the assembly
- type of base metal
- thickness of material
- joint design
- welding position
- Metal transfer mode
- spray
- globular
- short-circuit
- Quality of the welded assembly
- mechanical and chemical requirements
- solidification rate of the weld pool
- surface finish
- penetration
- Economics of the weld
- acceptable spatter level (post weld clean-up)
- travel (welding) speed
- price of gases
- volume of gases
Shielding Gas Cost
BLUESHIELD gas mixtures are more expensive then straight CO2, but often they can save money on the total operation due to lower post weld clean-up costs, faster welding speed, etc.
Shielding Gas Properties
Argon
Argon is a chemically inert gas that will not combine with any element in the weld zone. It has a low ionization potential (15.7 eV) that provides easy arc initiation and a stable arc. The gas metal arc welding process with argon as shielding gas has a constricted arc column caused by the low thermal conductivity of argon. The argon arc consists of a center core of molten filler metal droplets, intensely bright. The high-density arc permits more of the available arc energy to go into the base metal workpiece as heat.
The result is a narrow weld bead width with a finger-shaped penetration (deep central penetration) of the weld deposit.
Argon causes the weld to freeze quickly. If the metal is not molten enough to wet out to the weld toe, undercutting can result, (e.g. use of argon for mild steel applications).
Argon is used as the shielding gas for many of the commonly welded active metals such as aluminum, magnesium, copper and their respective alloys.
Helium
Helium is an inert gas. It is also the simplest of the gases and the second lightest. Its high ionization potential of 24.5 eV (compared to 15.7 eV for argon) results in high arc voltage. The high heat input of the helium arc allows deep penetration and rapid travel speed. Fusion is excellent, bead width and penetration pattern are wider. The helium density is lower than that of air and argon. Because it is so light, helium requires a larger flow rate than the other gases in order to adequately protect the molten weld metal.
High heat input and current density make helium ideal for welding thick sections and for materials with high melting temperatures or high thermal conductivities. Helium allows joining with narrow weld gaps.
Helium is used primarily for the welding of the non-ferrous metals, such as aluminum, magnesium, copper and their alloys. It is also used in gas mixtures with other shielding gases such as argon.
Carbon Dioxide (CO2)
Although argon and helium are used for gas metal arc welding of most metals, carbon dioxide has become widely used for the welding of mild steels. Carbon dioxide is a compound gas which is composed of carbon and oxygen. At high temperatures the carbon dioxide dissociates into carbon monoxide and free oxygen. This free oxygen is used to superheat the weld puddle, thus helping eliminate undercut. Based on manganese and silicon losses, a shield for carbon dioxide has an oxidation potential of 10%.
CO2 has a tendency to cause a spattering, unstable arc when used for open arc transfer of weld metals. The weld spatter may be contained to some degree by maintaining a short arc: i.e., by burying the arc in the workpiece.
Argon-Helium
Argon-helium mixtures are usually used to combine the best characteristics of each gas. The addition of helium varies between 20% and 90%. These mixtures are normally used for welding thick non-ferrous materials. As a general rule, the thicker the welding section, the higher the percentage of helium is needed.
Argon-CO2
Welding under the protection of CO2 doesn't always give the desired arc characteristics. For some applications, the problem resides in excess spatter which can scar an exposed metal surface. In low alloy steels it can manifest itself by excessive oxidation of the alloying elements. In such cases the use of a mixture of argon-CO2 has usually eliminated the trouble.
When good impact properties are essential, argon-CO2 mixtures are employed. The percentage of CO2 in these mixtures ranges between 2 and 25%. As an exception we can find mixtures containing 50% and even 75% of CO2.
The argon-CO2 mixtures are used for welding carbon steel or horizontal welds on steel or stainless steel, the quick-freeze characteristics of an argon shielded weld do not permit the molten metal to wet-out to the toes of the weld, causing undercutting at the edges of the weld bead.
To minimize the tendency to undercut ferrous metal welds, an addition of 1 - 5% oxygen to argon is recommended. The oxygen superheats the weld metal transferring across the welding arc column. It increases the metal wettability and minimizes the possibility of undercut. Oxygen action also controls the weld bead profile by flattening the weld.
To choose the best mixture, keep in mind that the reason for using oxygen additions is to superheat the molten metal. Thus a higher concentration of oxygen must be used when there is a larger volume of molten metal to be superheated.
Other Mixtures
For several years, research has been carried out on new types of shielding gas mixtures. Several gas combinations result in ternary or quaternary mixtures. For the most art these mixtures are very expensive and have a limited range of applications.
Argon-CO2-O2, Argon-helium-CO2, Argon-helium-CO2-hydrogen are the most well known mixtures.
The BLUESHIELD 9 gas mixture was developed primarily for welding austenitic stainless steels with the short-circuit method of metal transfer.
For quick reference, I've provided a couple of charts which summarize the gas products. Click on your choice to view the file:
For general welding use that most fourwheelers would be doing, the two gases that would be most recommended are as follows:
- ALMIG This is Air Liquide’s new optimized shielding gas for GMAW (MIG) applications on carbon steels and some stainless steels. It can be used in all transfer modes — short circuit, globular, spray, pulsed spray transfer — with low levels of smoke, low spatter, excellent bead appearance, reduced burn through on light gauge material, and excellent spray arc with good penetration on thicker material. Click HERE to view the data sheet (in Adobe Acrobat format).
- ALTIG If you're doing TIG welding, then we recommend Air Liquide’s new optimized inert shielding gas for GTAW (TIG) welding of all base metals and for GMAW (MIG) welding of non-ferrous metals such as aluminum. It offers increased heat content of gas, faster travel speed, faster puddle initiation, and more welding power at equivalent amperages compared to argon, as well as smooth welds. Click HERE to view the data sheet (in Adobe Acrobat format).
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