AC GTAW Porosity on 6061-T6 Tubing
A welder is joining 1/8-inch-wall 2-inch-OD 6061-T6 aluminum tube to a cast aluminum elbow. The machine is set to AC, 60 Hz, 80% cleaning action, 90 A, and 12 cfh argon through a #6 cup. The tungsten is 3/32-inch 2% lanthanated, ground to a sharp point. The welder cleaned the joint with a stainless steel brush that is also used on carbon steel, then wiped it with a rag. The arc wanders, the tungsten balls heavily and turns black, the bead is wide and shallow with a heavy black smut along the toes, and porosity appears at the stop. The welder increases argon to 20 cfh and the porosity decreases slightly but the tungsten still contaminates. The tubing is clean on the outside but has a dull gray oxide film at the joint.
1. Diagnose the cleaning, shielding, and machine-setting problems. Which single change would you make first, and why?
Read the worked answer
The cleaning is wrong before the machine settings are even considered. A carbon-steel brush transfers iron to aluminum, and a dry rag does not remove the oxide film; aluminum oxide melts far above the base metal and causes smut, poor fusion, and porosity. The 80% cleaning action is excessive for 1/8-in tube: too much electrode-positive time balls and overheats the tungsten, widens and shallows the bead, and creates a heavy etched/smut zone. A needle-sharp point on 3/32-in tungsten is also poor for AC; it should be tapered with a small flat. 90 A is low for 1/8-in aluminum, and 12 cfh is low for a #6 cup. The first change should be proper cleaning with a dedicated stainless brush and solvent, then reduce cleaning action and set current/gas by test coupon.
2. Describe the correct preparation and settings for this joint, including how you would verify the cleaning action is adequate.
Read the worked answer
Degrease with acetone or an approved cleaner, then brush only with a clean stainless brush reserved for aluminum until the oxide is gone; wipe and weld promptly. Use 3/32-in 2% lanthanated tungsten ground with a truncated point, AC, about 60–120 Hz, balance around 30–35% cleaning (or about 65–70% EN), and current in the 120–150 A range for 1/8-in wall, adjusting for the fitting. Use 15–20 cfh argon with a #7 cup or gas lens, and maintain preflow/postflow. Verify on scrap: the etched zone should be narrow, the bead shiny, and the tungsten should not ball excessively. If the arc still wanders, check gas, tungsten, and ground.
3. What causes the black smut and tungsten contamination, and how does each affect the weld?
Read the worked answer
Black smut is aluminum oxide and hydrocarbon residue that was not removed and was not adequately cleaned by the arc; excessive cleaning action can also contribute. Tungsten contamination comes from dipping the electrode into the puddle, touching it with filler, or overheating it with too much EP; a contaminated tungsten causes arc wander and tungsten inclusions. Porosity comes from hydrogen sources-moisture, oil, low or turbulent shielding gas, contaminated filler-and from an unstable arc at the stop. Each defect matters: smut and oxide prevent fusion, tungsten inclusions are sharp discontinuities, and porosity reduces load-bearing area and can link into cracks. Increasing gas alone does not fix contaminated cleaning or technique.