July 12, 2005
Several small but crucial components—back caps, collets and collet bodies, gas lenses, and nozzles—are key components in enhancing GTAW productivity. These components are available in a variety of styles and sizes. Optimizing these consumables can help to optimize welder uptime.
Editor's Note: This article, which covers consumables for gas tungsten arc welding, is the second installment of a two-part article. Part I discusses gas metal arc welding consumables.
Consumables comprise only a small portion of the total cost of gas tungsten arc welding (GTAW), but they are an important factor in creating quality welds. The characteristics of GTAW torch consumables—including back caps, collets and collet bodies, gas lenses, and nozzles—affect welding performance, expenses, productivity, and downtime.
Back caps apply pressure to the back end of the collet to force it against the collet body. This pressure holds the tungsten in place and seals the torch head from the atmosphere. Back caps thread into the back of the torch to help create a viselike hold that prevents the tungsten from slipping.
Back caps are made of various phenolic compounds, each of which has a different resistance to temperature. With these varying characteristics, back caps can be matched to specific applications.
For example, a back cap made from a low-temperature phenolic compound is suitable for general applications, but it could shrink, crack, or split in a demanding or high-duty-cycle application. For those applications, a back cap with a high thermal resistance is required to prevent weld discontinuities that can result from shielding gas leaks.
Collets and collet bodies are available as two-piece systems (top) and single-piece systems (bottom).
Back caps come in three styles—short (or button), medium, and long—all of which compare in performance but differ according to application. All types are available for both air- and water-cooled torches.
A short, or button, back cap is the smallest type. Its physical size allows for welding in restricted areas, but it requires nonstandard tungsten that is 2 inches or less in length. Welders may have to fabricate this tungsten size from a longer piece, as shorter tungsten can be costly and difficult to find.
If joint access is not a factor, a medium or long back cap is appropriate. A medium back cap generally accommodates tungsten up to 3 in. long. Long back caps use the industry-standard tungsten of up to 7 in. in length. They also are the most commonly used by welders.
Collets hold the tungsten in place as the back cap is tightened and create the electrical contact necessary for good current transfer. They generally are made of standard-grade copper or tellurium copper.
Collet bodies screw into the GTAW torch and accommodate various tungsten sizes and their respective collets, each of which range in size from 0.020 to 1/4 in., depending on the torch model. Like the collets, they are made of standard-grade copper or tellurium copper.
Two main factors to consider when choosing collets and collet bodies are:
Typically, manufacturers sell collets and collet bodies separately to match a specific tungsten size. For example, a 1/16-in. collet and collet body matches 1/16-in. tungsten. This system works well and accommodates a range of tungsten sizes, especially small ones.
Single-piece systems combine the collet and collet body. This combination provides a strong securing force and allows for simplified removal in demanding applications because the collet mechanism is located away from the heat and thus is less susceptible to distortion.
Standard gas lenses generally have several screens and spacers.
Gas lenses are available for both air- and water-cooled torches. Generally made of brass with stainless steel mesh screens, they replace the collet body to increase shielding gas coverage and reduce turbulence. They also reduce weld discontinuities associated with atmospheric contaminants.
The least expensive gas lenses typically have fewer screens, coarser mesh configurations, or both. The higher-quality gas lenses often require several layers of screens (see Figure 2), but most expensive gas lenses incorporate an engineered porous filter media in lieu of multiple screen layers (see ).
Application and performance expectation dictate gas lens choice. For welding of materials that react to atmospheric contaminants, large gas lenses provide improved gas coverage. On complex joints, large gas lenses also allow greater tungsten stick-out to gain visibility of the weld puddle or to increase the access to the joint.
For example, a smaller torch profile reduces weight and facilitates access to tight joints, so a 9 series air-cooled torch or a 20 series water-cooled torch equipped with a small gas lens or an appropriate large gas lens is a good choice. If the torch profile and weight are not concerns, a 17 series air-cooled torch or an 18 series water-cooled torch with a large or extra-large gas lens is suitable.
Some higher-priced gas lenses employ more mesh configurations or a porous filter media instead of multiple screen layers.
Nozzles (also called cups) provide a given amount of shielding gas coverage to the weld pool according to their size (anywhere from 1/4 to 3/4 in. ID). Nozzles also vary in length (short to extra-extra long), price, and performance.
The least expensive nozzles, which are 90 percent or 95 percent alumina oxide, work well for low-amperage applications. The thermal shock of high-amperage applications, however, can cause these nozzles to crack or fall off.
Lava nozzles cost more than alumina oxide nozzles and are more resistant to cracking. They are most suitable for medium-amperage applications. Lava nozzles are machined, so they tend to have varying wall thicknesses around the inside diameter, which could lead to nonuniform gas coverage.
Silicon nitrate nozzles are the most expensive. They resist heat and cracking in high-amperage, high-duty-cycle applications and last longer than other nozzle types.
The bottom line when choosing GTAW consumables is to find the price and performance combination that best fits the application at hand. For noncritical applications, the less expensive consumables perform adequately. If high performance is required, the higher-priced consumables likely will be necessary to prevent weld discontinuities and resulting rework.
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