Understanding Shielding Gas Flow in GTAW
BY ANDREW PFALLER, an American Welding Society Certified Welding Inspector and segment manager at Miller Electric Mfg. LLC, Appleton, Wis. Reprinted with permission: The AWS Welding Journal Using shielding gas flow rates that are too high for gas tungsten arc welding (GTAW) applications can double gas costs, resulting in upwards of $3300 in added annual costs per welder. Many operations may not realize there is huge potential for immediate payback and reduced costs by using a better gas consumable. Improper shielding gas flow can drive up costs through wasted gas and added rework. When there is a lack of knowledge about shielding gas best practices, operations might also use temporary solutions that don’t address the root issue and simply push these hidden costs higher. This article will help you learn the importance of using the right shielding gas consumables and how following best practices can reduce operational costs. Standard Collet Bodies for GTAW Many operations use GTAW in high-profile applications where the greatest weld aesthetics, quality, and integrity are required — Fig. 1. The GTAW process requires an inert atmosphere to protect the tungsten (where the arc is generated) and, more importantly, the molten pool of the metal being welded. One consumable used for this in many GTAW applications is a collet body, which provides shielding gas coverage over the part being welded.
Companies often choose collet bodies because they are a less expensive method of delivering the shielding gas compared to other options. However, collet bodies can be the source of problems that end up costing time and money — much more than the initial small amount saved up front.
When using a standard collet body in GTAW, a considerable amount of turbulence and atmospheric contamination are introduced into the stream. This can cause issues such as porosity, base metal oxidation (also called sugaring on stainless steel), poor welding performance, and arc flutter — problems that are unacceptable in applications that require high quality.
The turbulence in the shielding gas flow also limits how far the tungsten can be extended past the end of the nozzle, which may restrict the operator’s ability to complete the weld.
Adjusting the Shielding Gas Rate
To overcome these quality or access issues, operations may take the common step of increasing the shielding gas. However, this only exacerbates problems because increasing the flow rate increases the instability in the gas column. Increasing the flow rate also causes more shielding gas to be consumed than before — a point that is often overlooked.
This can drive up costs in the following three ways:
- Expenses associated with the consumable shielding gas. A 300-ft3 tank costs on average $35, or $0.11 per ft3. If an operation uses one tank per welder per week, that’s $1750 per year spent on shielding gas. Some operations studied have used as much as 60 ft3/h of gas in a high duty cycle application, with approximately 50% arc-on time. This would mean shielding gas costs are about $6600 per welder shift per year. If that company had 25 welders on two shifts, that’s about $330,000 per year spent on shielding gas alone.
- Time and materials lost to rework. Greater instability in the gas column can result in problems like porosity, poor welding performance, or arc flutter. In high-purity applications, this often means rework, which can take three to seven times longer to complete compared to a weld where no rework is required.
- Downtime to change gas cylinders. If the operation is using shielding gas cylinders, going through the gas at a faster rate results in more downtime for changing out the gas cylinders.
- Gas flow rate. As previously mentioned, a gas flow rate that is too high causes more instability. But be aware that too low of a flow rate can make the gas more susceptible to interference from outside forces, such as wind or fans. Moderate rates, such as 10 to 20 ft3/h, are recommended. Lower flow rates with a gas lens can save 50% or more in consumable shielding gas spend, which could eliminate more than $3000 annually per welder at 20% duty cycle at 60 ft3/h. Going back to the earlier example of an operation with 25 welders, this is more than $75,000 in annual savings.
- Nozzle diameter. The study validated that larger-diameter, converging-type nozzles provided a longer laminar shielding gas column. This is because larger nozzles produce lower gas velocities, resulting in lower K-H Instabilities. Many companies will use the smallest consumable possible to get into tighter spaces. However, in actuality, a larger nozzle can improve access by allowing the welder to stick the tungsten out more, thus increasing visibility and access to hard-to-reach areas.
- Nozzle shape and lens design. Larger converging lenses provide longer laminar flow, but that doesn’t mean any lens with a large opening is a good choice. Nozzle shape and lens design are very important to performance. The ultimate goal is to guide the shielding gas to the weld zone and provide a gentle blanket over the weldment. Regarding nozzle shape, a converging style gives coverage over the entire orifice and helps transition the plug flow into a developed flow, which reduces K-H Instabilities or makes a stable column. A diverging or “champagne-style” nozzle gives a false sense of security due to the lack of shielding in the outer portion of the cup, like putting a funnel on the end of a garden hose. The gas will continue to flow out in a straight path, causing a false perception of gas coverage that could compromise the weld material. The plenum/screen design also is important. Look for a lens with multiple screens that vary in mesh count to achieve optimal flow profile.
- Nozzle length. Another important factor is nozzle length. A longer nozzle helps further transition the plug flow into a fully developed flow, which describes a slower flow on the outer portions of the stream and higher flows in the middle. With plug flow, K-H Instabilities are decreased, and the stable laminar region of the shielding gas column is longer. Similar to the larger lens diameter, a longer converging-type nozzle also provides a longer laminar shielding gas column.
