The Importance of Shielding
Shielding is an essential component of any arc welding process. Shielding, whether in the form of flux or gas, prevents harmful elements such as oxygen and hydrogen from getting into the weld pool. Without shielding, the welding arc becomes completely erratic, leading to a globby mess of metal riddled with holes and contaminants.
While self-shielded welding is convenient and straightforward, gas shielded welding introduces more customizability by allowing for separate selections of filler material and shielding gas. A well tailored gas shielded process leads to cleaner welds that are easier to control with potential to increase productivity.
To benefit from gas shielded welding, each situation should be considered carefully, weighing costs with the benefits. Below are some factors to consider before selecting a shielding gas.
Factors to Consider when Choosing a Shielding Gas
The choice of shielding gas depends on several factors. The importance of each depends on the situation. For example, MIG (GMAW) welding steel has several types of shielding gas to choose from, but when cost is the biggest concern, CO2 will be the reasonable choice over other argon blends even if there’s an increased amount of spatter. Here are some things to consider before selecting a shielding gas:
The most common arc welding processes include MIG (GMAW), TIG (GTAW), Flux Cored - Self Shielded(FCAW-SS), Flux Cored - Gas Shielded (FCAW-GS), and Stick (SMAW). Stick and Flux Cored - Self Shielded require no shielding gas since the flux is in contact with the electrode and will vaporize into a protective gas while welding. Self-shielded processes are convenient since no additional gas is required, but convenience comes with drawbacks. Self-shielded processes produce a less stable arc, resulting in excessive spatter. In addition to spatter, additional time and effort must be taken to remove the protective layer of slag off the weld bead.
The advantages of welding with a gas shielded process is that the arc, and therefore the puddle, is easier to control. There is also less spatter to clean and no slag to remove (with the exception of gas-shielded FCAW). MIG and Flux Cored-Gas Shielded welding use mixes of argon, CO2, and helium. TIG typically uses 100% argon, but can also be used with helium, argon-helium mixes, and argon-hydrogen mixes.
Material is critical in selecting the correct shielding gas. While shielding gas is typically beneficial, welding with an incompatible gas for the material will result in poor welds, riddled with contamination and defects. A good example of this is with argon-hydrogen gas. Adding hydrogen to argon for TIG welding stainless steels results in welds with increased penetration. Use that same gas for TIG welding aluminum and you will end up with holes scattered through the weld, known as porosity. One of the first things to always check is the compatibility of the gas with the material.
Not all welders are created equal. One of the main benefits of newer, inverter-based technology is increased flexibility to tailor the arc in new and inventive ways. One example is the autoset feature in Miller Electric’s Millermatic and Multimatic lines. Each setting category includes a suggested gas blend to use. This is because Miller’s weld engineers tailor each setting to compliment the specified gas or gas blend. The result is a fine-tuned arc that welds well without much effort. It’s important to understand how each welder is intended to be operated, particularly if there are gas blends recommended in the manual or door chart.
Transfer mode is a term used in wire welding processes to describe how filler metal is deposited into the weld pool. Short circuit mode melts the wire end into a ball, then releases it upon contact with the weld pool, causing a “short.” During spray transfer, the wire never touches the puddle, instead the melted wire is expelled down into the puddle in the form of tiny droplets. Spray transfer can be desirable because it produces an easy to control puddle with zero spatter. Spray is not always the best choice, because it’s limited to horizontal and flat positions.
Short circuit transfer is used for welding small to medium thick materials and has the potential to become spray on thicker materials, depending on the shielding gas. Shielding gas rich in argon is capable of producing spray. Those that do not contain argon, such as 100% CO2, are incapable of spray transfer, no matter how high you turn up the voltage. For welding on thicker materials, it’s important to choose the correct shielding gas for the desired mode of transfer.
Penetration in welding refers to how deep the weld pool seats itself into the material. It’s essential in determining whether a weld will hold together or eventually fail. The type or mixture of shielding gas directly affects weld penetration. This is because the gas itself becomes part of the welding arc. Plasma (in this case the arc) is generated through ionizing gases. Each gas has its own unique set of properties that provides the arc with it’s own signature shape and heat distribution. The result is different penetration profiles for each unique combination.
Penetration doesn’t necessarily have to appear circular or uniform. Some profiles appear “pinched,” having deeper penetration directly in the center. Argon rich gases will result in this pinched, uneven profile; while CO2 or helium rich gases will have a more even distribution of penetration. Penetration is most important in initial (root) passes and weldments that undergo a large amount of stress. For welds not exposed to heavy or repeated loads, penetration is much less of a concern.
Cost is one of the biggest decided factors when selecting a shielding gas. Helium is expensive when compared to argon for TIG and in wire welding aluminum and stainless steels.100% Co2 is more cost effective for wire welding steel than any argon-CO2 blend. Regardless of material or process, there’s always a trade off for welding with cheaper gases. CO2 tends to produce more spatter, which takes time to remove, resulting in additional production costs. If spatter or extra time is not a concern, CO2 is typically the preferred option.
The same idea goes for helium and helium mixes. Helium based mixes for MIG on stainless steel will provide a smoother material transfer onto thin materials compared to non-helium mixes. 98/2 is an ideal gas for spray transfer. So those who weld exclusively on thicker stainless steel may benefit as much from helium mixes, and can save money by using 98/2..
After selecting a process and material type, then weighing the importance of each factor, it’s time to choose a shielding gas. The next section discusses the most common gases used in the welding industry, ones that a welding gas retailer should typically have on-hand.
Most Common Shield Gas Combinations
Shielding Gas Types
25% CO2, 75% Argon (C25)
C25 shielding gas is an ideal gas for MIG welding because it is well balanced. The argon provides the inert environment while the active carbon dioxide gas stabilizes the arc. The addition of CO2 produces a weld with a well-rounded penetration profile. Added CO2 helps burn off contaminants such as oil and grease at the base metal's surface that would otherwise get into the weld pool. Lastly, CO2 also helps with deposition in terms of droplet transfer for short circuits. CO2 creates an environment in which the droplet is easier to detach, reducing the potential for globular transfer.
10% CO2, 90% Argon (C10)
C10 gas is ideal for those who want to achieve spray or pulsed spray transfer at lower current levels than C25. A general rule of thumb is that the more argon a shielding gas is, the less energy is required to enter spray transfer. The downside to this is what many welders refer to as argon being a "heavy" gas, preferring to form larger balls of molten metal before releasing, making it difficult to weld thinner material.
100% CO2 (C100)
CO2 is the most common gas used for welding carbon steel because it costs significantly less than C25 or other argon mixes. The tradeoff is loss in arc stability. Using only CO2 produces a more erratic, harsher looking and sounding arc, resulting in an increased spatter. This can pose an issue in industrial settings because it takes time to grind off all that extra spatter. For those who don’t need to comply with welding standards or aren't concerned about aesthetics, CO2 is a good alternative. Keep in mind that 100% CO2 is incapable of spray transfer.
100% Argon (Argon)
100% argon is only used when wire welding aluminum or any TIG process because aluminum is a very different metal in nature. Aluminum can oxidize quickly during welding and is sensitive to moisture and impurities. Inert gasses such as argon and helium are non-reactive, blocking out unwanted elements..
Tri-Mix (Mixture of helium, argon, and CO2)
Helium makes up a majority of the composition of trimix gases, followed by argon, CO2, or sometimes O2 (oxygen). Helium is inert, just like argon, which helps in preventing contamination. Helium is considered a “hotter” gas, leading to increased weld pool fluidity and the ability to travel faster. It’s also easier to make starts with helium. Stainless steel, like aluminum, is very sensitive, which limits CO2 or O2 additions to a small percentage (around 2%).
98% Argon, 2% CO2 (98/2)
98/2 is used in welding stainless steel. It is more affordable than tri-mix because it does not have helium in the mix, making it the more popular choice. It is also ideal to use on thicker materials because of the high concentration of argon, which, just like in C10, allows for spray transfer at lower current levels compared to its tri-mix counterpart. Also, like C10, the downside to welding with 98/2 is when welding on thinner materials in short circuit transfer mode because of the large, sticky droplets that hang off the end of the wire.
Summary: Choosing the Right Gas for Your Application
The choice of welding gas heavily depends on the welding process, base material type, and material thickness (targeted transfer mode). While TIG only relies on inert gas such as argon and helium, MIG shielding gas can contain several different combinations of inert and active gasses. Argon-heavy gases are ideal for welding in spray transfer. In contrast, helium-heavy gases are suitable for short circuits in stainless steel. Price typically can be the deciding factor in the final choice.
Shielding gases are a vital component of welding processes because they stabilize the arc, protect the weld from oxidation, and define penetration geometry. They’re important for both home and commercial projects, with home use focused more on the economical choice with a wider window of application.
Even with all this newfound knowledge, it can be challenging to choose a specific gas that will work well for your project while aligning with your preferences. Below is a table to help you pick a gas that works while highlighting strengths and weaknesses.