Aluminum GMAW Porosity on a D1.2 Chassis
A shop welding a 6061-T6 aluminum chassis to AWS D1.2 reports scattered porosity on fillet welds made with ER5356 filler. The welds are 5 mm fillets on 6 mm plate, welded with pulsed GMAW. The CWI sees a radiograph showing scattered rounded indications in one 300 mm weld length, plus some surface porosity visible under magnification. The shop recently switched shielding gas suppliers and is using a push-pull gun with a 3 m torch. Ambient humidity is high. The CWI must decide whether the porosity is acceptable, what the likely cause is, and what corrective action to require before production resumes.
1. How should the CWI classify and evaluate the porosity against D1.2?
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D1.2 acceptance criteria for porosity depend on the weld type and the inspection method. For visual inspection, surface porosity is generally rejectable if it exceeds the code's limits; for radiographic inspection, the code sets size and distribution limits for rounded indications. The CWI should:
- Identify the applicable D1.2 acceptance class and inspection method specified by the contract.
- Measure indication size and count per unit length against the code table.
- Separate surface from subsurface indications; surface porosity is often more strictly limited.
- If limits are exceeded, reject and require repair; if within limits, document and accept.
The CWI must not accept based on 'scattered' appearance alone; the code limits are quantitative.
2. What root causes should the CWI investigate given the process details?
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Aluminum GMAW porosity commonly traces to hydrogen sources and shielding problems. The CWI should investigate: (1) shielding gas purity and dew point - a supplier change is a red flag; verify certs and flow rate; (2) moisture on filler wire or base metal, especially in high humidity; (3) torch length and liner condition - a 3 m push-pull torch can draw in air if the liner or O-rings leak; (4) wire feed consistency and contact tip condition; (5) base metal surface contamination (oil, oxide, moisture); and (6) travel speed and arc stability. The CWI should require the shop to verify gas dew point, purge and store wire properly, check torch seals, and run a test coupon before resuming.
3. What corrective-action plan should the CWI approve?
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A defensible plan: (1) quarantine suspect welds and radiograph or visually inspect per contract; (2) verify gas certs and dew point, and set flow with a calibrated meter; (3) inspect and replace torch liner, O-rings, and contact tip; (4) require wire storage in sealed containers with desiccant and limit exposure; (5) clean base metal per D1.2 (degrease, remove oxide); (6) weld a qualification coupon and inspect it before production; (7) update the WPS if parameters change; (8) re-inspect first production parts. The CWI should document the root cause, actions, and verification evidence, and close the NCR only after the coupon and first-article inspection pass.