Porosity and root lack of fusion in a 5083 aluminium tank
An 8 mm 5083-H111 aluminium tank is welded by MIG with 5356 filler, argon 99.99% at 20 L/min, 180 A, 22 V, 45 cm/min and no preheat. The shop is at 12 °C with 85% relative humidity. The fitter cleaned the joint with a stainless steel brush that had also been used on carbon steel, then wiped it with acetone from an open drum. Root gaps varied from 0 to 2 mm. Radiography shows rounded porosity in 20% of the butt welds and lack of fusion at the root; penetrant testing shows linear indications at one toe. The contract requires EN ISO 10042 quality level B and procedure qualification to ISO 15614-2. The shop wants to increase the argon flow to 30 L/min and continue.
1. Diagnose the likely causes of porosity and root lack of fusion.
Read the worked answer
Porosity. Aluminium porosity is usually hydrogen from moisture, oil, paint or hydrated oxide. The cleaning is the main problem: a carbon-steel brush embeds iron and contamination, and acetone from an open drum absorbs water, especially at 85% RH. The oxide film on 5083 is tenacious and must be removed mechanically with a dedicated stainless brush or scraper immediately before welding.
Root lack of fusion. No preheat at 12 °C and low heat input at 45 cm/min promote fast solidification and poor root fusion. The 0–2 mm gap variation causes inconsistent penetration.
Increasing argon to 30 L/min will not fix contaminated metal; it may cause turbulence and draw in air. The linear penetrant indication at the toe suggests a crack or lack of fusion and must be investigated, not ground blindly.
2. Specify the corrective actions and the requalification you would require.
Read the worked answer
Segregate aluminium tooling: dedicated stainless brushes, carbide burrs and clean gloves. Degrease with clean solvent, dry, then scrape or brush the weld zone immediately before welding. Preheat to 80–100 °C to drive off moisture and improve fusion, but verify with a contact thermometer. Use high-purity argon at a sensible flow, about 18–22 L/min with a gas lens, and a backing strip or consumable insert to control the root. Control the gap with tack welds and a fixture.
Qualify the procedure to ISO 15614-2 with the actual thickness, position and edge preparation; include bend and tensile tests and, if required, radiographic examination. Qualify welders for the process and position. Re-cut and re-weld the affected seams, then inspect to ISO 10042 level B.
3. How would you disposition the existing tank?
Read the worked answer
Do not accept the tank on the basis of the current RT. Map all welds, mark the porosity and the linear indication, and remove the defective areas to sound metal. Re-weld under the qualified procedure, then repeat RT or UT and PT. The linear indication at the toe requires excavation and PT confirmation. If the tank is a liquid-retaining component, leak testing after repair is necessary. Document the nonconformance, repair and re-inspection, and issue a revised inspection report. The repaired areas must meet the same level B acceptance criteria.