Answers and explanations for precision welding
February 8, 2005
Experienced welders know that without the right information, it's easy to sacrifice quality, lose time, and generally become frustrated with gas tungsten arc welding (GTAW). And while there is merit in learning by trial and error, if you want to move toward precision GTAW, getting answers to 10 very basic questions can ease the transition.
For low-amperage applications, an air-cooled torch cooled by shielding gas works well. These torches are simple to operate and require minimal setup. For high-amperage applications, you can still use an air-cooled torch, but the cable and torch must be much heavier and may be cumbersome to manipulate.
A water-cooled torch, which circulates water through the torch and cable, works equally well, but requires additional equipment and maintenance. These torches use clean, de-ionized water with filters that prevent contaminants from entering the cooling supply or the inner-diameter tube of the torch. You also may need to use additives to prevent algae growth.
Whether you choose a water-cooled system depends on your willingness to invest in additional equipment, as well as additional time and money for maintenance. Many welders, however, prefer these systems because the smaller torch configuration provides better maneuverability and reduces operator fatigue.
A gas lens should be used when your application requires increased shielding gas coverage. The gas lens reduces turbulence and provides lengthier, undisturbed gas flow and allows you to move the nozzle farther away from the workpiece while still keeping the arc or weld puddle in view. Using a larger nozzle with a gas lens—which consequently produces a larger blanket of shielding gas—can help when welding on materials such as stainless steel and titanium.
A gas lens allows more direct and broader gas coverage on tight joints, such as an inside corner, where access is limited. In critical applications that have potential for atmospheric contamination, a gas lens can help to reduce the likelihood of weld discontinuities.
Using the wrong size tungsten, whether in AC or DC applications, is one of the more common causes of an unstable arc. If the tungsten is too large for the amperage, the arc may rotate around the end of the tungsten. Conversely, if the tungsten is too small for the amperage, the current can melt the electrode and cause an erratic arc. To remedy either of these conditions, match your welding current to the tungsten size recommended by the manufacturer.
Contaminated tungsten—caused by debris on the base metal, oxidation from inadequate shielding gas, or gas impurities from a leak—also can cause an unstable arc in both AC and DC applications. To resolve this problem, replace or regrind the tungsten, make sure the base metal is clean, or increase the shielding gas flow after making sure all your hoses are intact and leak-free.
Allowing the tungsten to touch the weld pool is one of the most common causes of contamination. This problem can be resolved by moving your torch farther away from the workpiece, which in turn lengthens the arc. Touching the filler metal to the tungsten also can be a source of contamination, but there is no clear solution to this problem other than practice; trial and error will determine your best technique.
If you feel your technique is not the culprit of tungsten contamination or discoloration, check to see that you have adequate gas flow, and allow adequate postflow time as well. Allow several seconds of postflow—about 1 second for every 10 amps of weld current.
Finally, you may want to consider using a power supply that offers high-frequency starts if you think using the scratch-start method is causing the contamination.
Using too much current on a given application is a major cause of excessive electrode consumption and is most easily solved by increasing the tungsten size, changing the type of tungsten you are using, or decreasing the amperage.
Using the wrong polarity also can result in excessive tungsten consumption. During AC welding, for example, using more electrode-positive current may provide more cleaning action, but it also subjects the tungsten to more current and thus consumption. Instead, it is best to set the power source more toward electrode negative on the balance control to minimize the amount of current and time spent on the electrode.
Using an incorrect or contaminated shielding gas can lead to high electrode consumption. Be sure to use pure argon, and check for leaks in the hoses, either from cracks or loose fittings.
A loose hose or torch component is a primary cause of porosity. It is remedied by tightening the fittings. If you cannot find loose hose connections by doing a visual check, you might want to place the hose in soapy water until you find the leak and tighten the fittings accordingly.
Using the wrong shielding gas or one that has impurities also can lead to porosity. This is remedied by using pure argon after using nitrogen to purge the line of air and any condensation that may have accumulated. Drafts from fans or open doors also can lead to porosity, so be sure that your working environment is well-isolated from drafts or use a gas lens to provide better gas coverage.
To prevent porosity caused by inadequate shielding gas flow, follow the recommended flow rate, which is approximately 10 to 20 cubic feet per minute (CFM), depending on the application. Doing so helps ensure quality welds.
The four main tungsten choices for GTAW are 2 percent thoriated tungsten, pure tungsten, 2 percent ceriated tungsten, and 1.5 percent lanthanum tungsten.
Two percent thoriated tungsten is a good choice when using a DC power source because it maintains a pointed shape, resists melting, and has a high current-carrying capacity. When welding thin aluminum—0.09 in. or less—thoriated tungsten is also a good alternative to pure tungsten because it creates a more focused arc. It is recommended that thoriated tungsten be used in a ventilated area and measures be taken during preparation to capture dust from grindings.
Pure tungsten performs well when welding aluminum using a conventional AC power source.
Two percent ceriated tungsten is a good alternative to thoriated tungsten and provides good arc starts at low currents, along with greater arc stability. This type of tungsten is recommended for aluminum AC welding with an inverter-based power source.
Last, a 1.5 percent lanthanum tungsten is most commonly used for applications in which long weld times and multiple arc restarts are necessary.
The base metal should be free of all contaminants, including dirt, paint, and oil. Wipe the base metal with a cloth or scrape it with a wire brush dedicated for use on a particular material. Before welding on aluminum, in particular, you need to remove oxides with a stainless steel brush manually; using a power brush is not recommended as it can re-embed contaminants into the metal. You can also use a caustic solution to clean aluminum. If you are considering using this method, your local welding distributor is the best resource to provide you with product options. You can also discuss several scraping methods with your distributor if you do not want to use chemicals.
Malfunctioning electrical equipment, such as computers, telephones, and radios, is often a sign that you are experiencing high-frequency interference from your welding power source.
To remedy such high-frequency interference, start by verifying that the power source is grounded according to the installation instructions provided in the operator's manual. Keep your torch cables and work cables as short as possible, and place them close together. Physically separating your welding equipment from devices that may experience interference is also an option, but doing so can be time-consuming and space-prohibitive.
If all else fails, you could switch to an inverter-based power source that provides a high frequency for arc starting only.
Arc rectification occurs when the surface oxide of a nonferrous metal acts as a barrier, making it more difficult for electrons to flow from the workpiece to the tungsten than from the tungsten to the workpiece. Excessive arc noise, unstable weld pools, or a weld pool that appears to dry up are all signs of arc rectification.
You can either increase your travel speed or decrease the amperage for the application. Another option is to adjust the balance control on your power source toward electrode negative, which provides more penetration.
As a last resort, switch to an inverter power source that offers advanced square wave technology.