Often overlooked as a factor of weld quality, shielding gas can play a significant role in improving, or impeding, welding performance.

If you’re reading this, you probably already know that shielding gas plays a critical role in achieving a strong, visually appealing GMAW weld. That’s the easy part. What’s not so easily understood, however, is why shielding gas is necessary, the practical differences between mixtures, which gases are appropriate for which applications and how consumables and technique affect gas coverage.

The purpose of shielding gas is to keep the air atmosphere away from the weld pool. Without it, the weld pool reacts with the oxygen, nitrogen and hydrogen that comprise the air atmosphere as well as the microscopic particles present in the atmosphere. This reaction can lead to porosity in the finished weld, which means that air bubbles get trapped inside the weld bead and can significantly reduce the strength of the weld (See Figure 1).

Excessive spatter is another result of inadequate or improper shielding gas. Although spatter does not necessarily weaken a weld, it does lead to decreases in productivity due to man-hours spent grinding and chipping it away.

Although it’s the primary function of shielding gas, keeping the air environment away from the weld is far from its only function. Shielding gas also plays an important role in determining a number of factors in the welding process, including penetration, arc stability, mechanical properties of the finished weld, transfer process and more. In fact, all other variables being the same, shielding gas alone can determine the difference between a good weld and a bad weld. That’s also why it is so important to use consumables that effectively direct and protect the shielding gas, helping you to achieve the best performance from the mixture you’ve chosen for your application.

Porosity, as can be seen on the face and interior of the weld bead, can be caused by inadequate shielding gas and can dramatically weaken the weld.

Choosing The Right Gas

The four most common gases used in GMAW welding are Argon, Helium, CO2 and Oxygen, each producing specific welding characteristics that make them suitable for a variety of situations.

By far the more common of the reactive gases, CO2 is also the only one that can be used in its pure form without the addition of an inert gas. As a general rule, whether mixed with other gases or in its pure form, CO2 tends to produce very deep penetration but a relatively unstable arc and excessive spatter. When used by itself, CO2 is limited to short circuit welding process. Mixed with at least 85 percent Argon, CO2 can be used in a spray transfer process.

Oxygen, the second most common reactive GMAW gas, is usually used in quantities of 9 percent or less. Useful for spray transfer welding on mild carbon steel, low alloy and stainless steel, oxygen tends to improve weld pool fluidity, penetration and arc stability. Because it causes oxidation of the weld metal surface, it is not prescribed with aluminum, magnesium, copper and other exotic metals.

Argon, the most common of the inert gases, can be used as a shielding gas for nearly any material, although it’s only appropriate in its pure form with non-ferrous metals such as aluminum, magnesium and titanium. Argon tends to produce a very narrow arc cone, and consequently a very narrow penetration profile. When mixed with oxygen or carbon dioxide on ferrous metals, Argon greatly improves arc stability and puddle control and reduces spatter.

With a more limited scope of applications, helium is primarily used on stainless steel, normally as a tri-mix gas with Argon and CO2, and aluminum and other non-ferrous metals. Helium produces a wide, deep weld bead, and for that reason typically works well for thick sections of aluminum and other non-ferrous metals. Combining helium and Argon tends to bring out the best characteristics of both gases. Because it is more expensive than Argon and because, being lighter than air, it requires a higher flow rate to adequately protect the weld puddle, helium is often not a very cost effective shielding gas in production environments.

The Final Shielding Gas Variable

Choosing the right shielding gas, unfortunately, is not enough to guarantee a weld free from impurities and with the desired arc characteristics. In addition to selecting the ideal shielding gas for your application, choosing the right consumables is also important to achieving good gas coverage.

GMAW consumables — contact tip, diffuser and nozzle — direct the gas to the weld puddle. Choosing the right consumables will depend on the application, but the end goal is always the same: successfully blocking out the external air environment.

Consumables can impact gas coverage in a number of ways. A gas diffuser that is partially clogged with spatter, for example, will restrict the air flow and cause the gas that is emitted to flow at a higher rate than it should. Gas that flows with too much turbulence around the weld pool will mix with the air environment and create impurities and porosity in the weld.

This graphic shows the difference that consumables can make in shielding gas coverage. The illustration on the left shows good coverage, while the coverage in the illustration on the right allows the air environment to seep into and contaminate the gas.

 

Likewise, consumables that, by virtue of their design, create a turbulent gas flow also risk allowing outside air to contaminate the weld pool. Consumables that have a built in spatter guard produce a smoother, more uniform gas flow and thus offer superior weld pool protection.

Another factor that can impact gas coverage is the size of the nozzle. Higher deposition and wider welds generally require a wider nozzle to ensure complete coverage of the weld pool.

Combined, selecting the right shielding gas for your application and using high quality consumables that help prevent blockages will help ensure all of your welds are mechanically sound and visually appealing.