Welding Well



Getting the Most from Advanced GTAW Controls

By: Doug Smith

Thursday, March 1, 2018

A car without power steering will get you from point A to point B, but the driver works much harder to make tight turns at slow speeds. The same holds true for gas-tungsten-arc (GTA) welders. While operators can achieve good results using conventional technology, inverter-based GTA welders with advanced controls help operators achieve the same good results, but with greater ease.

This article covers the hows and whys of inverter-based GTA welders with advanced controls.

Tungsten Electrode Points the Way

Electricity likes to come off a point. Using a pointed tungsten electrode promotes a narrower arc cone and more precise weld bead placement, and advanced GTA controls enable using a pointed tungsten. Most manufacturers now recommend using a ceriated (orange band) or lanthanated (blue band) electrode, as adding 1.5 to 2 percent of these rare-earth elements helps the electrode to maintain a point. As an added benefit, these tungstens enable users to switch between the AC and DC welding outputs without changing electrodes. Thoriated tungstens (red band) also are suitable for DC welding, but because thorium is radioactive, it may pose a health hazard.

To prepare the tungsten, grind (with the grain) to create a taper that is about 2 or 2.5 times the diameter of the electrode. Use a sharp point for welding below 40 A. Otherwise, put a small flat on the end of the tungsten to prevent the tip from becoming included in the weld bead. Do not ball the tungsten for welding aluminum, an old-school practice when using pure (green band) tungsten electrodes.

Pulsed DC TIG for Steel, Stainless and Alloyed Metals

In pulsed GTA welding, the power source typically has controls that adjust peak current, background current (often set as a percentage of peak current) and the number of pulses per second. Some systems also enable the operator to adjust the duration of the peak or background current (again, often set as a percentage, such as from one to 99 percent of total cycle time).

  • Benefits of pulsing depend on the specific application and control setting. Generally speaking, pulsing a DC output:
  • Focuses the arc without increasing total heat input. A more focused arc provides the operator with more control over the weld puddle size and direction, promoting better weld quality and weld-bead appearance.
  • Narrows weld-bead width to 50 percent, and helps increase penetration by as much as 30 percent.
  • Reduces total heat input, possibly by 50 percent or more, as well as reduces the size of the heat-affected zone.
  • May allow increasing travel speeds to 30 percent.

Directing the heat more precisely can help prevent warping and burnthrough on thin materials, edges, outside corners, autogenous welds and assemblies prone to warping, and when filling a hole or joining thin-to-thick sections. Reducing heat input can help preserve metallurgical and mechanical properties, notably on stainless and other alloyed materials. When heat input stays localized (as it does when welding stainless steel), it not only promotes warping, it can affect chemical composition and grain microstructure.

Pulsing at a lower frequency (10 pulses/sec. or less) will create a broader weld bead with shallower penetration. Pulsing at a higher frequency narrows the weld bead and increases penetration. Start by experimenting with frequencies of 100 to 250 pulses/sec. Note that the benefits of pulsing in manual applications cease at about 500 pulses/sec. To measure benefits, make welds with and without pulsing; cross-section and polish the beads; and compare travel speeds and calculate total heat input.

Experimenting with Pulsed DC GTA Welding

Every application requires its own settings. Here are some general guidelines for setting pulsing parameters:
  • Peak current—Current affects penetration, so use the traditional rule of thumb: 1 A for every 0.001 in. of thickness, increasing current as necessary to achieve good penetration. If using a foot control, add 20 percent more amperage to provide wiggle room at the top end.
  • Background current—Start at about one-quarter to one-third of the peak current, adjusting upward to 45 or 50 percent if needed.
  • Pulse frequency—Start at 100 pulses/sec. and adjust upward from there without changing any other variables. Higher frequencies increase penetration and narrow the bead width without increasing total heat input. Applications that benefit from a narrower bead with deeper penetration often use a frequency of 200 to 250 pulses/sec.
  • Pulse width (technically percentage of time at peak/background amperage)—Between 40 and 65 percent of time at peak current works well in most applications, using less time on thinner metals.

AC GTA Welding Essentials

All advanced inverters provide extended balance control and an adjustable AC output frequency. Balance control adjusts the amount of time the AC wave spends in the electrode-negative (EN) phase, which mostly affects fusion and the electrode-positive (EP) phase, which provides the cleaning action necessary to remove surface oxides). With inverters, operators can extend balance control from 10 to 90 percent EN. Try 75 percent as a starting point. This will allow you to remove heat from the electrode and maintain a pointed tungsten. Greater amounts of EN are possible, especially with well-cleaned aluminum. However, if black pepper-like flakes appear, the waveform needs more cleaning action.

Focusing the Arc

Adjustable AC-output-frequency functions control the shape and force of the arc cone to optimize it for the joint configuration at hand. Lower frequencies, such as the 60 Hz output of a conventional GTA welder, create a broader cone and create a wider bead. While handy in a few applications, a broad cone often provides less control (e.g., when watering a wide area instead of directing the spray where needed).

Increasing frequency concentrates and narrows the arc column and prevents the arc from wandering. By increasing frequency, operators can increase travel speed and/or penetration, narrow the oxide etch zone and put in a narrower weld bead. On a fillet weld, the operator can keep the arc force directed into the bottom of the joint to ensure good fusion. With a conventional GTA welder and balled tungsten, the arc tended to wander and dance from plate to plate. The lack of precise control sometimes resulted in over-welding and poor fusion at the root of the joint.

To start, make a test plate for comparison, setting balance control at 60 percent and output frequency at 60 Hz. For the next plate, increase balance control to 75 percent and set the frequency to 100 to 150 Hz. For the next plate, increase frequency another 50 Hz, or, if possible, have a coworker increase frequency while you weld. Several high-end GTA units can boost the frequency to 400 Hz, creating a highly focused arc cone. Note: Because high frequencies increase the intensity of the buzzing sound that occurs when AC GTA welding, use hearing protection.

Independent Amplitude

A few advanced GTA inverters also enable operators to adjust amperage independently during the EN and EP portions of the AC cycle. Doing so enables them to direct more heat where they want it—into the work—and reduce heat on the tungsten.

For example, when welding 1⁄4-in.-thick aluminum, an operator could set EN at 200 A and EP at 70 A and subsequently increase penetration and/or travel faster, add more filler rod, reduce or eliminate the need to pre-heat the weldment and use 100-percent argon instead of a higher-energy (and more costly) argon/helium blend.

Increasing the EN-to-EP ratio constricts the arc cone, which in turn enables the operator to narrow the weld puddle and better direct weld bead placement. The ability to reduce EP values may also permit the use of a smaller-diameter electrode. With these benefits, independent amplitude control may be of value for welding thinner section or when extreme precision counts.

For testing EN and EP values, one way to start is by using standard EN setting recommendations (e.g., 1 A per 0.001 in.) and by reducing EP values to about one-third of EN values. If the weld shows signs of insufficient cleaning action, increase EP values.

Total Recall

At the start of the article, we noted that optimum settings depend on the actual application. For that reason, experiment on scrap and practice parts. Conduct nondestructive and destructive tests as necessary, and document parameters and the results. Note that just about all advanced GTA welders enable users to store and recall weld parameter settings with the push of a few buttons. After finding ideal settings, you can quickly call up a saved program for the next time you encounter the same application. MF


See also: ESAB Welding & Cutting

Related Enterprise Zones: Welding

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