GTAW Process Fundamentals, Power Sources, and Parameter Selection
This domain tests how GTAW works and how to set polarity, current, gas, tungsten, and travel variables for a sound weld.
Core rules
- GTAW uses a nonconsumable tungsten electrode and an inert shielding gas, usually argon. The arc melts the workpiece; filler is added separately when required.
- Use a constant-current power source with a drooping characteristic. Current is set at the machine or remote control; arc voltage changes mainly with arc length.
- DCEN places heat at the workpiece and is standard for carbon steel and stainless steel. AC is used for aluminum because the electrode-positive half-cycle provides oxide cleaning. DCEP is rarely used for GTAW because it puts heat at the electrode.
- Select current by thickness, joint, position, and root condition. Match tungsten diameter and tip shape, filler diameter, gas flow, and travel speed. Too long an arc widens the bead and reduces protection.
- Set preflow and postflow. Postflow protects the hot electrode and weld until they cool. Check gas flow with a flowmeter, not by sound alone.
Worked example
A 3 mm carbon steel butt joint is welded in the flat position. The welder selects DCEN, a 2.4 mm tungsten electrode, 3.2 mm ER70S-2 filler, argon at about 10 L/min, and a starting current near 80 to 100 A. The arc is kept short. If the root is too cold, current is increased slightly or travel is slowed; if the pool becomes too fluid, current is reduced. AC would not be selected for carbon steel because it reduces workpiece heat and does not improve this weld.
Traps that cost marks
- Using AC for carbon steel or DCEN for aluminum cleaning.
- Selecting a constant-voltage source instead of constant current.
- Setting gas flow by guessing; too much flow can cause turbulence and porosity.
- Using a tungsten too large or too small for the current.
- Omitting postflow and oxidizing the tungsten or weld.