Cover me: proper shielding gas coverage is key to GTAW success

Proper shielding gas coverage is critical to achieving high-quality results in GTAW applications. Many factors have an impact on shielding gas coverage, including the type of consumables used and the shielding settings.

For pinpoint control, high quality, and aesthetics, gas tungsten arc welding (GTAW) is often a good process choice. However, proper shielding gas coverage is critical to achieving the best results.

Many factors affect shielding gas coverage, including the type of consumables used and the gas flow settings.

Choosing the right gas, flow, and consumables for the job depends on the desired performance and, in some cases, operator preference. Keep these considerations in mind to obtain optimal results with GTAW.

Shielding Gas Options

In GTAW, an inert gas protects the molten weld pool and tungsten from surrounding atmospheric gases. These atmospheric gases can react with the weld pool, causing contamination.

Although the primary purpose of a shielding gas is to shield the weld pool and tungsten from atmospheric gases, the shielding gas also influences heat inputs and arc-starting characteristics. Because of the influence the shielding gas has on an arc, it is critical to adhere to any welding procedure specification (WPS) that is issued.

The three most common shielding gas options for GTAW are 100 per cent argon, 100 per cent helium, and an argon/helium mix. These shielding gases can be used for all materials.

1. 100 per cent argon: Argon is primarily used for GTAW because of its availability, cost, and arc-starting characteristics. Argon produces consistent high-frequency arc starts because of its lower ionization potential and produces a more stable arc compared to that of helium.
2. 100 per cent helium: Because it has higher thermal conductivity than argon, helium can be used for GTAW to produce higher heat inputs. These higher heat inputs result in faster travel speeds and higher depth-to-width ratios and are good for welding thicker materials. Helium does have a higher ionization potential than argon, resulting in inconsistent arc starts.
3. Argon/helium mixture: An argon/helium mix typically is used to achieve the higher heat inputs of helium while maintaining the superior arc starts offered by argon. These mixes commonly contain 25 to 75 per cent helium. As helium content increases, the arc becomes hotter, but high-frequency arc starting performance and stability decrease.

To determine the best shielding gas for your application, consider the cost, required heat, and high-frequency arc-starting consistency needed.

Gas Flow Rates

The optimal gas flow rate varies with the type of consumables used and atmospheric conditions. GTAW flow rates are typically between 10 and 35 cubic feet per hour (CFH).

Consumables used in GTAW include a nozzle and a collet paired with a gas lens or a collet body. If the weld is critical or requires high quality, a gas lens is the better choice because it provides better shielding gas coverage. This image shows the gas flow with a collet body (on the left) and gas flow with a gas lens (on the right), which shows a longer laminar flow column.

When the shielding gas exits the nozzle, it has a different velocity than that of the atmospheric gases surrounding it. The different velocity and density between these gases can cause currents to form, which can potentially turn the shielding gas column from a laminar flow (which is desirable) to a turbulent flow (less desirable). As the flow becomes turbulent, atmospheric gases can be pulled into the shielding gas column, leading to contamination of the weld and/or tungsten.

As shielding gas flow rate is increased, the laminar flow column becomes more turbulent, increasing the chances for the weld and/or tungsten to become contaminated. As the flow rate is decreased, the shielding gas column becomes more laminar and less turbulent. Although a higher flow rate produces a turbulent shielding gas column and isn’t necessarily better, a flow rate that is too low can be easily disturbed, breaking down the shielding gas column and potentially contaminating the weld and/or tungsten as well. To achieve the greatest laminar flow, use the lowest gas flow rate possible for the application and conditions.

The CFH is measured by either a regulator or a flowmeter regulator. A flowmeter regulator is recommended because of its accuracy. Place the regulator as close as possible to the welding power source for the best results and easy adjustments.

Gas Lens or Collet Body?

Consumables used in GTAW include a nozzle and a collet paired with either a gas lens or a collet body. Consider the requirements of the finished weld when choosing between the two. If the weld is critical or requires high quality, a gas lens is the best option. For non-critical or practice welds, a collet body is sufficient. Complete proper testing to verify that consumable combinations work for your application, and always follow the WPS.

A collet body has four holes that introduce the shielding gas to the inside of the nozzle. The four holes tend to be perpendicular to the nozzle, causing the gas to spiral or be more turbulent exiting the nozzle. When using a collet body, you should not extend the tungsten outside the nozzle more than the distance of the inside diameter of the nozzle.

A gas lens increases shielding gas coverage and reduces turbulence compared to that of a collet body because it has several screens inside that produce a more uniform laminar flow. The gas lens allows the tungsten to extend farther than the inside diameter of the standard collet body.

Nozzle Options

The nozzle, also called the cup, screws onto the collet body or gas lens and introduces the gas to the weld. Nozzles are available in various diameters, lengths, and shapes to produce different shielding profiles or laminar flow lengths. Remember, a longer laminar flow is desirable.

As nozzle diameter increases, a longer laminar flow is produced. A smaller-diameter nozzle with the same gas flow rate produces a more turbulent flow because of the gas velocity as it exits the nozzle.

This image demonstrates gas flow rate increases (from left to right). As shielding gas flow rate is increased, the laminar flow column becomes more turbulent, increasing the chances for the weld and/or tungsten to become contaminated. As the flow rate is decreased, the shielding gas column becomes more laminar and less turbulent.

Nozzle options include standard, long, and extra-long. Longer nozzles provide longer laminar flow columns compared to shorter nozzles with the same flow rate and cup diameter. This is due to the flow being more developed before exiting the nozzle, reducing the shear between the flow and the surrounding atmosphere. Longer nozzles also allow for better access to tight joints.

Nozzle shapes include straight, converging, and champagne. A converging nozzle starts with a larger diameter and neck and decreases to a smaller diameter. This shape is recommended to achieve the longest laminar flow. A champagne nozzle is the opposite shape of converging, starting with a small diameter and increasing to a larger diameter. This nozzle is not beneficial, as the shielding gas exits at the gas lens or smaller diameter of the nozzle and does not disperse within the larger diameter before exiting.

To achieve the best laminar flow, use a converging-shape nozzle in the largest diameter and longest length practical for the job.

Best Practices for Success

In addition to making proper consumable and gas choices, following best practices can minimize common mistakes and improve success in GTAW applications:

• When assembling the torch, tighten the collet body or gas lens before the back cap. If the order is reversed, the torch can take on atmospheric gases that result in contamination.
• Missing or incorrect insulators can cause shielding gas contamination, so inspect insulators frequently.
• Don’t use a green oxygen hose — typically used in oxyfuel applications — to deliver shielding gas; it can increase gas contamination risk. A vinyl or braided rubber hose is acceptable in most applications.
• A pre-flow of shielding gas helps shield the tungsten and weld area and initiate the arc start. A minimum pre-flow of 0.2 seconds is recommended.
• Gas post-flow is also beneficial because it ensures the weld is protected from atmospheric gases as the weld pool solidifies. Hold the torch over the end of the weld until post-flow stops to ensure coverage of the area and the tungsten. Proper post-flow time in seconds is determined by dividing welding amps by 10. A minimum of eight seconds is recommended.
• When running long gas lines, the initial shielding gas released upon starting the arc will be at a much higher flow rate. Decrease this flow rate by using shorter gas lines, or by increasing pre-flow time to purge the lines before arc start.

In GTAW applications, choosing consumables and using flow rates that produce the longest laminar flow can help you achieve success. These factors reduce the risk of weld contamination and allow greater extension of the tungsten from the nozzle for better weld access. Follow best practices for proper shielding gas coverage to help prevent porosity and other weld defects.