Page 37 - MetalForming-Mar-2018-issue

• Pulse width (technically percent- age of time at peak/background amperage)—Between 40 and 65 per- cent 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. Bal- ance control adjusts the amount of time the AC wave spends in the elec- trode-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 per- cent 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 fre- quencies, 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 pre- vents the arc from wandering. By increasing frequency, operators can increase travel speed and/or penetra- tion, 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 con- ventional GTA welder and balled tung- sten, 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 com- parison, 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 fre-
quency 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
www.metalformingmagazine.com
MetalForming/March 2018 35
Welding Well