Gas Lens Basics for TIG (GTAW) Applications
Courtesy of Weldcraft
Most welders, even novices, are familiar with the traditional components of a GTAW torch-nozzles, collets and collet bodies, back caps and tungsten-but the gas lens may be more of a mystery. Because gas lenses are often reserved for use on high performance, specialty alloys or low-access joints and tend to cost more than other components, not every welder uses them. However, gas lenses can also be used on general-purpose applications and have a definite place among repair or fabrication shops, and even in the home hobbyist's garage.
Like other GTAW components, choosing the right gas lens, whether for a professional or hobby application, depends on the material to be welded, amperage requirements, joint configurations and tungsten size. It also depends on the type of GTAW torch being used-air- or water-cooled-but in both cases, the addition of a gas lens can provide definite benefits in GTAW welding performance.
What is a Gas Lens?
A gas lens replaces the collet body that is standard in a GTAW torch. In conjunction with a collet, it helps hold the tungsten in place and creates the electrical contact necessary for proper current transfer. It also serves a much more important function: it improves shielding gas coverage and joint accessibility.
A typical gas lens is composed of a copper and/or brass body with layered mesh screens of steel/stainless steel (stainless steel offers greater durability and resistance to rust and corrosion than steel) that helps evenly distribute the shielding gas around the tungsten and along the weld puddle and arc. Gas lenses can be used with all shielding gases and are available for both air- and water-cooled GTAW torches.
The most durable-but also the more expensive gas lenses-feature an engineered porous filter media (Figure 1) that improves laminar flow compared to conventional designs. Other gas lenses, such as those composed entirely of brass and with multiple screens (Figure 2), can serve the welding purposes of less demanding applications, but they can be less conductive and could hinder gas flow after multiple uses.
|Figure 1. Some higher-priced gas lenses use a porous filter media to improve gas flow and extend product life.|
Gas Lenses Offer Distinct Benefits
Gas lenses reduce shielding gas turbulence and provide longer, undisturbed laminar flow of the gas to the weld pool. The gas lens also allows the welder to move the nozzle further away from the joint and extend the tungsten electrode past the nozzle by one inch or more(1). This extension helps minimize tungsten inclusions and improves visibility of the arc and the weld puddle without sacrificing shielding gas coverage, especially on joints with limited access.
Gas lenses are specifically helpful when GTAW welding on alloys that are highly reactive to atmospheric contaminants or on materials used in high temperature applications, as poor gas coverage on these alloys can lead to porosity and material degradation, which may negatively impact the weld's strength. The gas coverage provided by gas lenses also helps prevent oxygen contamination on materials such as stainless steel, titanium and aluminum by minimizing weld discontinuities. In more basic GTAW applications, such as on steel, gas lenses simply improve shielding gas coverage to ensure more consistent welding performance.
The ability to stick the electrode out further (thanks to the laminar flow created by the gas lens) is also a major benefit of gas lenses. This additional electrode extension provides better visibility of the joint and arc, and can improve a welder's ability to lay a proper weld in critical applications and/or hard-to-reach areas such as "T", "K" and "Y" joints.
In some applications, a gas lens can reduce shielding gas consumption, but welders should not use gas lenses solely for this purpose. With all things being equal (amperage, torch-to-work distance and tungsten extension, for example), the laminar flow provided by the screens in gas lenses creates a wider, more even gas coverage area using equal (as in a non-gas lens application) or lesser (three to five cubic feet per hour less) amounts of gas.
Gas Lens Size Selection
Gas lenses come in a number of sizes for both air- and water-cooled torches. The type of gas lens a welder chooses for a given application will depend first and foremost on the type of front-end parts (10N or 13N) required for a given torch. A welder also needs to assess the amperage requirements (and therefore, tungsten size) for the material being welded, along with the joint configuration and amount of joint access. For example, lower amperage applications on non-reactive materials require a different type of gas lens than higher amperage applications on specialty and/or reactive materials.
Gas lenses for 10N series front-end parts (common styles include 17, 18 and 26 series torches) are available in large, extra large and stubby types. Gas lenses for 13N front-end parts (common styles include 9 and 20 series torches) are available in standard, extra large and stubby types.
A standard gas lens for a torch with 13N front-end parts is sufficient in basic, lower amperage GTAW applications, as is a large type gas lens for a torch with 10N front-end parts. Extra large gas lenses for both 13N and 10N front-end parts provide improved gas coverage on specialty metals and materials that tend to react to atmospheric contaminants. These larger gas lenses also improve gas coverage on complex and hard-to-reach joints by allowing greater tungsten stick-out for extra visibility of the weld puddle and increased access to the joint.
Stubby gas lenses for both 13N and 10N series feature the same physical orifice, screen size and diameter as the extra large type lenses, but are noticeably shorter. The smaller torch profile/length increases operator comfort by reducing the overall weight of the torch and allows better access to confined joints. Welders can also gain better torch balance and control if they combine a stubby gas lens with a short back cap(2).
Figure 2. Some gas lenses have several screens and spacers to direct gas flow.
Proper Installation, Care and Monitoring of Gas Lenses
The only differences when assembling the front end of a GTAW torch with a gas lens instead of a collet body is the replacement of the existing nozzle (also referred to as a cup) with a larger nozzle(3) and the addition of a transition insulator. The transition insulator seals the area where the nozzle screws into the gas lens and is an important element to welding success. Without an added insulator to accommodate for the larger gas lens and nozzle, air can enter into the shielding gas stream and cause porosity or other weld discontinuities.
While there is no specific length of time a gas lens will last, as with any consumable, it is important to monitor for signs of wear. Gas lenses can become contaminated with filler metal due to spatter or weld popping, which can lead to screen degradation, along with shielding gas flow blockages that ultimately effect weld quality.
On aluminum, for instance, welders may see a negative change in cleaning action on both sides of the weld bead if their gas lens has become clogged or worn. Or weld pool characteristics may change on ferrous metals-the weld puddle may become erratic, for instance. Visually inspecting the gas lens for spatter or wear is the best defense against encountering these problems.
Final Comments on Gas Lenses
Adding a gas lens is not the answer for every GTAW application, especially given that they cost more than standard collet bodies, but these components can offer distinct benefits. When welders encounter applications that require extra gas coverage or better accessibility to complicated joints, the best defense is to consult a welding distributor or torch parts manufacturer to determine whether the addition of a gas lens would be advantageous. Distributors and equipment manufacturers can assist in determining which type of gas lens is best suited for a given application, material and joint configuration.
(1) Specific applications may dictate that the electrode may stick out further than 1-in. when used with a gas lens, but it is not recommended in most applications.
(2) A back cap is the torch piece that applies pressure to the back end of the collet and collet body/gas lens. It holds the tungsten in place and seals the torch head from the atmosphere.
(3) Nozzle size and length can vary with each application. Discuss the best gas lens/nozzle combo with the manufacturer or your distributor.