Gas-Tungsten-Arc Welding: What You Need to Know

Monday, September 1, 2008
Unlike other arc-welding processes, gas-tungsten-arc welding (GTAW) is slow business. Not surprisingly, excessive downtime for troubleshooting makes the process even slower, a costly and frustrating proposition.
Gas-Tungsten-Arc Welding: What you need to know

In addition to common weld discontinuities such as porosity, undercutting or lack of fusion that can occur with all arc-welding processes, two other pitfalls can arise during GTAW: dulling of the tungsten electrode, and arc-starting troubles. Having the know-how to quickly identify and solve these problems is critical. It’s also relatively easy to obtain.

Keep on the Point

To effectively create an arc for GTAW and transfer the welding current to the base material, GTAW requires the use of a tungsten electrode. The electrode is nonconsumable, and tungsten has the highest melting point of any metal (6170 F). So in theory, it can’t melt, right? Wrong.

One of the most common tungsten troubles during welding is excessive consumption. On alternating-current (AC) applications, typically used to weld aluminum alloys, operators will set the balance control on the welding power supply toward electrode positive, to effectively remove the oxides from the weld area. However, using this setting also adds heat to the tungsten and can cause it to melt. To prevent the tungsten from melting, welders should set the balance control toward electrode negative, increasing the amount of heat going into the workpiece rather than the tungsten.

A good rule of thumb: Set the balance control on the welding power supply at 70 to 80 percent electrode negative. And, when using GTAW on ferrous alloys such as carbon and stainless steels using direct current (DC), set the power source to the electrode-negative mode.

Tungsten-electrode selection based on the application
Another cause of tungsten consumption is using excessive welding current for a given electrode diameter, regardless of whether using AC or DC. Als follow the tungsten manufacturer’s recommended operating parameters for the electrode diameter being used (see table above). If the electrode is too large for the given amperage, the arc may rotate around the tip. A too-small electrode may melt and cause the arc to become unstable.

Finally, incorrect or contaminated shielding gas and loose or cracked hose fittings also can cause excessive tungsten consumption. Take care to use clean, pure argon, and be certain that all hoses are intact and fittings tightened prior to welding.

Tungsten Contamination

Another common pitfall associated with GTAW is contamination of the tungsten electrode during welding. Suspect this condition if the weld puddle appears dirty, the filler rod does not join or melt smoothly into the weld puddle, or the arc becomes erratic.

The electrode can become contaminated if the welder inadvertently touches the tungsten electrode to the workpiece or weld puddle. To avoid this, the welder should adjust the angle of the torch and move it farther a from the workpiece. If the power source requires use of the scratch-start method to initiate the arc and tungsten contamination continues to be a problem, consider using a machine with the ability to perform a high-frequency or lift-arc start.

Another cause of tungsten contamination is touching the filler rod to the tungsten—the only solution to this problem is more practice to improve the operator’s skill level in feeding the rod into the weld puddle. And, in certain cases, inadequate shielding-gas flow, including lack of post-flow shielding after welding has stopped, can cause tungsten contamination. A rule of thumb: Maintain a shielding-gas flow rate of 10 to 20 ft.3/hr., and a post-flow of around 1 sec. for every 10 A of weld current used. If the tungsten turns from its normal color of light gray to a shade of purple or black, you’ll need to increase post-flow time.

 Weld-Repair of Tooling: Proceed with Caution
Repairing metal-stamping dies and tooling using gas-tungsten-arc welding requires extraordinary manual dexterity, attention to detail and an understanding of metallurgy that goes beyond most other welding applications. Even then, success is not guaranteed.
Here are some tips to help ensure success.
• Cleanliness is critical. Be sure to clean within 5 in. of the area to be welded using an acetone cleaner, or by burning off any plastic or silicone residue with a preheating device.
• Use a DC inverter-based welding machine, which will provide optimum low-amperage arc stability and control. An air-cooled torch rated at 50 A will provide the best maneuverability.
• Als use a gas lens—gas lenses smooth out the shielding-gas flow, greatly reducing the possibility of weld defects due to exposure to the air. Some newer gas lenses feature a single filter that also provides improved gas-flow regulation compared to older-style lenses with as many as five layers of screens. Gas lenses also allow for greater tungsten stick-out, which can provide better access to tight joint configurations.
• Select 2 percent ceriated tungsten to provide optimum low-amperage arc starts. A small-diameter electrode, 0.040-in. or 1⁄16-in., will do the best job of keeping the heat restricted to the area on the die that needs to be welded.
• Use a smooth, pointed tungsten to provide optimum arc control. Also, wear an auto-darkening welding helmet with a shade-9 or lighter lens to ensure accurate arc starts and arc visibility.
• Because the base material has been hardened and tempered, selecting the optimum filler material should only be done with the assistance of a reference manual or by consulting with a welding distributor or filler-metal manufacturer.
• Instead of dipping the filler metal into the weld pool, keep the filler metal in the pool while welding, being careful to insert the minimum amount of filler metal necessary.
• To avoid creating stresses in the tool or die that could lead to future cracking, the workpiece should be kept at a temperature specified by a reference guide before and during welding, and brought back to room temperature gradually.

Maintain Focus, Stability

In addition to causing various weld-puddle troubles, contaminated tungsten also can cause an unstable arc. To remedy this problem, remove the tungsten from the torch, clip the end and regrind it. An important rule to remember: Als grind the tungsten lengthwise, not circumferentially since this creates ridges on the electrode and will lead to an erratic or wandering arc. Also, grind a taper on the tungsten that spans a distance no greater than 2.5 times the electrode diameter. For example, a 1⁄8-in.-dia. electrode would have a taper 1⁄4 to 5⁄16 in. long. Lastly, use a grinding wheel specially designed for grinding tungsten, to avoid further contamination.

Weld-puddle troubles also emanate from welding with an excessively long arc—the arc becomes unstable and wanders offline from the weld joint. Here, the welder should shorten the arc by moving the torch and tungsten closer to the workpiece, but taking care not to touch it with the electrode.

Dirty base materials and insufficiently pure shielding gas can cause an unstable GTAW arc, so welders must remember to wipe workpiece materials free of oil, dirt or debris, and, when necessary, use a wire brush to clean the materials—particularly aluminum alloys—before welding. Shield the welds using pure argon; in some cases, a mixture of argon and helium will suffice, but minimize the amount of helium for best results. Shielding gas should flow at a rate of 10 to 20 ft.3/hr.

A final means to remedy arc-starting trouble is to ensure that the ground clamp is secure and check for and replace loose or damaged cables. Keep torch and work cables as short as possible, yet long enough to reach the work area, and place them close together. Check that all power cables have been installed according to the power-source manufacturer’s recommendations.

Tungsten Selection

Tungsten alloys are not created equal. When welding at low amperages, select a 2 percent ceriated tungsten. This electrode type provides good arc starting at low amperages, and can be used for AC and DC welding of carbon or stainless steels, nickel, aluminum and titanium alloys. The same is true for 1.5 percent lanthanated tungsten electrodes.

For higher-current applications or for low-amperage AC applications, a 2 percent thoriated tungsten electrode provides good arc starting. Note: Thorium is radioactive, so welders must follow manufacturer’s warnings, instructions and the Material Safety Data Sheet for its use. MF


See also: Weldcraft

Related Enterprise Zones: Welding

Reader Comments

There are no comments posted at this time.


Post a Comment

* Indicates field is required.

YOUR COMMENTS * (You may use html to format)




Visit Our Sponsors