AT Air Products we understand the importance of protecting a molten weld pool from atmospheric contamination and oxidation, either by a flux or with a shield gas.
A flux is used in shielded metal arc (SMA) electrodes or submerged arc processes. Shielding gases, on the other hand, are used in gas metal arc welding (GMAW), gas tungsten arc welding (GTAW) and most of the flux-cored processes.
Reviewing the specific criteria related to a customer’s processes and providing guidance is essential. An assessment of the procedure, material and its thickness, and the preferred metal transfer should be conducted. Furthermore, to optimize the choice and ultimate quality of the weld, the porosity, spatter, penetration, bead profile and fusion should be considered.
The most frequently used mixtures and shielding gases with respective applications are:
- Ar: used for most GTAW application and GMAW of non-ferrous materials
- CO₂: used mostly for GMAW in dip transfer mode of steels
- Ar/CO₂, Ar/O₂ and Ar/CO₂/O₂ gas mixture: used for GMAW of stainless steel and other steels
- Ar/He and Ar/H₂ mixtures: used for advanced GTAW applications
- Ar/He/CO₂ and Ar/H₂/CO₂ mixtures: used for GMAW
Argon and carbon dioxide both play a core shielding role in GMAW applications as they protect the weld metal from atmospheric oxygen or other contamination.
Pure CO₂, which is the original shielding gas for GMAW, is widely used for general purpose welding of steel and is an effective shielding gas that prevents porosity. Due to its inclination to cause large droplet formation, it is restricted in its use in the dip-transfer mode. This shielding gas on its own causes large volumes of spatter which reduces the efficiency and arc time of the welder and increases the welder’s post weld activities.
Argon, on the other hand, is generally the dominant gas in any shielding gas mixture cylinder. In its pure form, argon is an inert gas which is purely used to keep other gases out and has no chemical effect on the deposited weld metal. Argon is used in almost all GTA welding as well as for GMAW of aluminium alloys copper, its alloys and other oxygen sensitive material.
The value of adding active gases
Adding small quantities of shielding gases could achieve improved welding results, which means that the metal transfer is stable, there is reduced spatter and a soft arc. An improved transfer stability makes the welding process less sensitive to welding parameters and more tolerant to voltage and current variations. This ultimately results in a vast reduction in machine set-up time and a noticeable improvement in the overall productivity.
It is important to note that when welding with cored wires, and the oxygen content is too high, there is an increased risk for porosities as the extra oxygen the flux. Argon/oxygen mixtures with oxygen of up to 2% are ideal for stainless steel applications.
To improve the penetration of carbon steel joints, CO₂ should be added to the argon in a two-part mix. An increase in CO₂ increases the heat of the welding arc, increases penetration and increases the welding speed.
However, 20% CO₂ in argon should not be exceeded, as it causes spatter and instability. It is important to note that 15% is optimal, and in the case of thinner materials where you don’t need penetration, 5% CO₂ is sufficient.
Benefits of 3-part mixtures
A combination of shielding gases plays a role in providing the best weld properties for particular applications. Three-part mixtures consist of argon and CO₂ (maximum 15%) and O₂ (maximum 3%). In an instance where all three of these gases are used, arc stability is improved, penetration and bead profiles are improved, minimum spatter is generated, and metal transfer characteristics are optimised.
The three-part mixtures are extremely easy to use, and the welding stability allows for more variations in setting parameters. We have seen how the use of
these gases have increased the quality, productivity and efficiency, and ultimately, measurable increases in profitability.
Welding stability is extremely important as it has a direct impact on the total welding costs – it is interesting to see how small improvements can make a big difference.
Sean Young is a Welding Specialist at Air Products