Page 36 - MetalForming-Mar-2018-issue

Welding Well
By Doug Smith
Getting the Most from Advanced GTAW Controls
Acar without power steering will get you from point A to point B, but the driver works much hard- er 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 pro- motes 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 per- cent 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 chang- ing 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
Doug Smith, product business manager--arc equipment, ESAB Weld- ing & Cutting Products, shares his arc-welding insights in MetalForming magazine’s new Welding Well column every-other month.
Doug Smith
Product Business Manager- Arc Equipment, ESAB Welding & Cutting Products
dsmith@esab.com
2 or 2.5 times the diameter of the elec- trode. 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 cur- rent) 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 set- ting. Generally speaking, pulsing a DC output:
• Focuses the arc without increasing total heat input. A more focused arc provides the operator with more con- trol over the weld puddle size and direction, promoting better weld qual- ity 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 burn- through on thin materials, edges, out- side 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 pro- motes 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 penetra- tion. Start by experimenting with fre- quencies of 100 to 250 pulses/sec. Note that the benefits of pulsing in manual applications cease at about 500 puls- es/sec. To measure benefits, make welds with and without pulsing; cross- section and polish the beads; and com- pare travel speeds and calculate total heat input.
Experimenting with Pulsed DC GTA Welding
Every application requires its own settings. Here are some general guide- lines 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 nec- essary 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 vari- ables. Higher frequencies increase pen- etration and narrow the bead width without increasing total heat input. Applications that benefit from a nar- rower bead with deeper penetration often use a frequency of 200 to 250 pulses/sec.
34 MetalForming/March 2018
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