Aluminum Porosity, Smut, and Tungsten Spitting
A 6 mm thick 6061-T6 aluminum butt joint is being welded with AC GTAW using a 3.2 mm 2% zirconiated tungsten, 2.4 mm 4043 filler, 180 A, 60 Hz, 30% cleaning balance, and 15 CFH argon through a gas lens. The joint was cleaned with a stainless brush, but the filler rod has been lying open on the bench for two weeks. Symptoms appear on every pass: black smut along the cap, porosity at each arc start, the tungsten tip mushrooms and occasionally spits, and the crater cracks. The welder wants to increase current to 210 A and add more cleaning action. Ambient temperature is 18°C and there is no preheat.
1. Rank the likely causes of the porosity and black smut, and state what you would check first. Justify your ranking.
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Ranking:
- Contaminated filler and base metal - the filler has been open for two weeks, so it can absorb moisture and pick up dust; aluminum oxide also re-forms quickly on the cleaned plate. Moisture and oxide are common sources of hydrogen porosity.
- Shielding gas coverage or leaks - 15 CFH through a gas lens is at the low end for this current; a loose torch cap, damaged O-ring, or draft can cause black smut and porosity at starts.
- Start technique - porosity at every arc start suggests insufficient preflow or striking on a contaminated area rather than on a clean run-on tab.
- Cleaning balance - 30% cleaning may be marginal for heavily oxidized 6061, but changing balance before fixing contamination would only mask the problem.
Check first: filler condition and storage, gas flow at the cup with a flowmeter, torch connections, preflow time, and base metal cleanliness. Then verify balance calibration. Increasing current to 210 A would add heat and tungsten erosion without removing the moisture or oxide.
2. Recommend specific parameter and technique changes. Explain the trade-off between cleaning action and tungsten life.
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Changes:
- Clean the filler with stainless wool and acetone, then store it in a sealed dry tube. Clean the joint again immediately before welding and wipe with acetone.
- Set argon to 18–20 CFH with the gas lens; check for leaks and drafts. Use 2–3 s preflow and 8–10 s postflow.
- Reduce current to about 150–165 A and increase travel speed; 180 A is high for 6 mm and contributes to tungsten mushrooming and a wide, hot bead.
- Set cleaning balance around 35–40% EP only after contamination is controlled. Use the lowest cleaning that gives good wetting and a bright bead.
- Use a run-on/run-off tab and start on scrap or the tab, not on the joint.
Trade-off: more EP cleaning removes aluminum oxide and improves wetting, but it puts more heat into the tungsten, erodes and mushrooms the tip, and can spit metal into the weld. Too much cleaning also reduces penetration because more heat goes into the electrode side. Too little cleaning leaves oxide and causes smut and incomplete fusion. The balance must be optimized with a test coupon, not guessed.
3. What explains the crater crack, and how should the weld be finished to prevent it?
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The crater crack is a solidification crack caused by terminating the arc abruptly and leaving a concave crater. As the last pool shrinks, the thin crater section cannot accommodate the contraction strain, and 6061-T6 is sensitive to hot cracking in that condition. The crack is a weld defect, not just a surface mark, and it must be removed before the joint is accepted.
Finishing technique:
- Add filler while reducing current in steps so the crater remains slightly convex.
- Use the machine’s slope-out or crater-fill function rather than breaking the arc instantly.
- Finish on a run-off tab where possible, then remove the tab.
- Maintain postflow until the weld is no longer bright; premature gas shutoff can also leave a brittle oxide at the crater.
- If a crack is already present, grind it out completely to sound metal and re-weld; do not cap over it.