How to Choose the Right TIG Torch Welding Consumable
Part Ii: Consumables for Gtaw
Consumables comprise only a small portion of the total cost of TIG welding, but they are an important factor in creating quality welds. When TIG welding, you want to choose torch consumables that offer both consistency and durability. Doing so improves performance and also increases cost-savings and productivity by reducing downtime associated with rework.
To help you make an informed decision about TIG torch consumables, this article focuses on how to choose backcaps, collets and collet bodies, gas lenses and nozzles.
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 simply twist into the back of the torch to help create a “vise” that prevents the tungsten from slipping.
Back caps are made of a phenolic compound, each of which vary in temperature resistance. You need to match the type of back cap to your application. For example, using a back cap comprised of a low temperature phenolic compound works for general applications. On demanding or high duty cycle applications, however, low temperature back caps can shrink, crack or split. You need to choose a back cap with a high thermal resistance for these applications, so as to avoid weld discontinuities that can result from shielding gas leaks.
There are three types of back caps: 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 non-standard tungsten that is two inches or less in length. You may have to fabricate this size tungsten from a longer piece, as shorter tungsten is more difficult to find and more costly.
If joint access is not a factor, use a medium or long back cap. A medium back cap generally accommodates tungsten up to three inches long. Long back caps use the industry standard tungsten of up to seven inches in length. They are also the most commonly used and often most preferred by welding operators.
Collets and Collet Bodies
Collets directly hold the tungsten in place when you tighten the back cap and create the electrical contact necessary for good current transfer. They are generally made of standard grade copper or tellurium copper.
Collet bodies screw into the TIG torch and accommodate various size tungsten and their respective collets, each of which range in size from .020 to 1/4 inches. They are also made of standard grade copper or tellurium copper.
When choosing collets and collet bodies, you need to consider two main factors.
First is price. Standard grade copper collets and collet bodies are less expensive, but they also tend to be less durable. Less expensive collet and collet bodies are also prone to failure under high temperature applications. After extended use, they do not secure the tungsten as reliably.
Conversely, the more expensive tellurium copper collets and collet bodies have better heat resistance on higher amperage applications. These consumables generally resist twisting or elongating and hold the tungsten more securely after extended periods of use. They also have more tensile strength and less ductility.
You also need to decide between a single- or two-piece system (See figure 1). Typically, manufacturers sell collets and collet bodies separately to match a specific tungsten size. For example, you would purchase a 1/16-inch collet and collet body to match a 1/16-inch tungsten. This system works well and accommodates a wide range of tungsten sizes, especially smaller ones.
|Fig. 1 Choosing between a standard two-piece system (top) or a single-piece system (bottom) is one of the factors in determining which collets and collet bodies are best for you.|
There are also single piece systems that combine the collet and collet body together. From a performance standpoint, they provide better securing force and are easier to remove when used in a demanding application. The reason for this factor is because the collet mechanism is further away from the heat and less susceptible to heat distortion. They also reduce the possibility of mismatching collet and collet body sizes, allow for quicker tungsten changeover and help simplify parts management.
Gas lenses replace the collet body to increase shielding gas coverage and reduce turbulence. They also reduce weld discontinuities associated with atmospheric contaminants and are generally made of a copper/brass combination with stainless steel mesh screens.
The least expensive gas lenses typically have fewer screens and coarser mesh configurations, which tend to be less durable and can negatively affect gas flow. The higher quality gas lenses often require several layers of screens (See Figure 2). However, the most durable and the more expensive gas lenses incorporate an engineered porous filter media in lieu of multiple screen layers (See figure 3). This type of filter media provides the best performance by improving the laminar flow compared to conventional designs.
Gas lenses are available for both air- and water-cooled torches.
Fig. 2 Many standard gas lenses have several screens and spacers.
|Fig. 3 Many of the more expensive gas lenses have more mesh configurations or have a porous filter media in lieu of multiple screen layers.|
Application and/or performance expectation dictate gas lens choice. When welding material that tends to react to atmospheric contaminants, larger gas lenses provide improved gas coverage. On complex joints, larger gas lenses also allow greater tungsten stick-out to gain visibility of the weld puddle or to increase the access to the joint. As an example, you could use a standard or a large gas lens on a 17 series air-cooled torch or on an 18 series water-cooled torch.
For 9 series air-cooled torch or a 20 series water-cooled torches, a standard or large gas lens (commonly called a stubby) provides good gas coverage. This smaller torch profile increases operator comfort by reducing the overall torch weight and allows access on tighter joints.
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-inch ID). For example, a smaller nozzle provides less gas coverage than a larger one. Nozzles also vary in length (short to extra, extra long), price and performance.
The most cost effective are 90 or 95 percent alumina oxide nozzles, which work adequately for lower amperage applications. These nozzles, however, do not resist thermal shock on higher amperage applications very well and tend to deteriorate or crack and fall off.
Lava nozzles cost more than alumina oxide nozzles and are more resistant to cracking. These nozzles work well on medium amperage applications, but tend to have varying wall thicknesses around the inside diameter—a factor that may lead to unequal gas coverage. This varying wall thickness is the result of the machining of the nozzle.
Silicon nitrate nozzles are the most expensive nozzles and also the highest performing. These nozzles resist heat and cracking on higher amperage, higher duty cycle applications and last longer than either lava or alumina oxide nozzles. For precision TIG welding, silicon nitrate offer the consistency and durability needed to achieve quality welds and avoid rework. In fact, the higher initial purchase price for silicon nitrate nozzles may be worthwhile to help avoid the ongoing cost of replacing inexpensive, lesser quality nozzles.
The bottom line when choosing TIG consumables is to look for items that provide you with the durability and performance you need. A wide variety of options exist for back caps, collets and collet bodies, gas lenses and nozzles, all of which offer distinct advantages, disadvantages and cost considerations. For less critical applications, the less expensive TIG consumables provide adequate performance. However, if you need high performance, it is well worth your while to purchase more expensive, long-lasting consumables. Doing so minimizes the chance of consumable failures that could ultimately increase the cost for rework associated with weld discontinuities.