Weld Discontinuities, Troubleshooting, and Quality Control

What Is a Weld Discontinuity?

A discontinuity is any interruption in the intended physical structure of a weld. Not every discontinuity is automatically a defect — AWS D1.1 defines a defect as a discontinuity that exceeds the code's acceptance criteria and must be repaired or rejected. Understanding the difference matters: your goal is to produce welds that meet the acceptance criteria every time, and when they do not, to diagnose why quickly and correct the process before more material is wasted.

AWS A3.0 Standard Welding Terms and Definitions is the reference for precise terminology. Using the correct term — not shop slang — is required on qualification paperwork, inspection reports, and when communicating with inspectors or engineers.

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The Eight Common GMAW Discontinuities

1. Porosity

Round or elongated gas pockets trapped in the solidified weld metal. Causes include contaminated base metal (oil, rust, moisture, paint), insufficient shielding gas coverage, too-high travel speed, damaged or kinked gas hose, and wind drafts blowing gas away. Correction: clean the base metal, verify flow rate (typically 35–50 CFH for solid wire GMAW), check for leaks, and shield the work area from drafts.

2. Undercut

A groove melted into the base metal along the weld toe that is not filled by weld metal. It reduces the effective throat and creates a stress riser. Causes include excessive voltage, too-fast travel speed, or improper torch angle. Correction: reduce voltage, slow travel speed, and adjust the work angle so the arc does not wash metal away from the toes.

3. Cold Lap (Incomplete Fusion at the Toes)

Weld metal that rolled over the base metal surface without fusing to it. The bead looks wide and flat but the bond at the edge is missing. Causes include too-low voltage, too-low heat input, or contaminated surface. Correction: increase voltage and preheat if required; ensure the work surface is clean.

4. Incomplete Fusion (Lack of Fusion)

A broader term than cold lap: weld metal that did not fuse to the base metal or to a previous pass anywhere in the joint. Causes include improper torch angle directing heat away from the joint sidewall, travel speed too fast, or wire feed speed (amperage) too low. Correction: direct the arc into the joint sidewall, reduce travel speed, and increase wire feed speed.

5. Burn-Through

A hole or collapse of the weld pool through the base metal. Causes include excessive heat input — voltage too high, wire feed speed too high, or travel speed too slow — and joint fit-up gaps that are too wide. Correction: reduce wire feed speed or voltage, increase travel speed, and improve fit-up to close excessive root gaps.

6. Excessive Spatter

Droplets of weld metal expelled from the pool and deposited on the surrounding base metal. While primarily a cosmetic and productivity issue, heavy spatter can conceal cracks or undercut during inspection. Causes include voltage too low for the wire feed speed (the arc is too short and stubby), contaminated wire, or incorrect polarity. Correction: increase voltage to stabilize the arc, use clean wire, and confirm DCEP (electrode positive).

7. Overlap

Weld metal that extends beyond the weld toe onto the base metal surface without fusing to it. It creates a notch effect similar to undercut but on the opposite side of the toe. Causes include too-low travel speed and too-high wire feed speed. Correction: increase travel speed and reduce wire feed speed.

8. Weld Crack

The most serious discontinuity. Cracks may be hot cracks (solidification cracks forming while the weld cools) or cold cracks (hydrogen-assisted, forming after cooling). In GMAW on structural steel, hot cracking in the crater is common when the weld is stopped abruptly. Correction: use crater-fill technique to backfill before breaking the arc; control preheat and interpass temperature as required by D1.1 for the material and thickness.

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Troubleshooting Method: Systematic Parameter Review

When a defect appears, work through parameters in order:

1. Shielding gas — flow rate, hose condition, contamination

2. Wire feed speed (amperage) — too high or too low for joint thickness

3. Voltage — arc length and bead profile

4. Travel speed — bead width and penetration

5. Torch angle and position — work angle and travel angle

6. Base metal condition — cleanliness, fit-up, preheat

Document every change. Change one variable at a time so you know what fixed the problem.

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Visual Inspection and AWS D1.1 Acceptance Criteria

AWS D1.1 Table for visual inspection acceptance criteria sets limits on undercut depth, porosity frequency and size, crack prohibition (cracks are never acceptable), and other surface conditions. You must know these limits before signing off on any weld. Visual inspection is the first and most economical quality control tool; it requires good lighting, clean weld surface (slag and spatter removed), and a weld gauge.

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Safety During This Lesson

• PPE is required at all welding stations: helmet with appropriate shade lens, leather gloves, leather jacket or sleeves, and closed-toe leather boots.
• Even pre-welded coupons have sharp edges and residual heat from prior grinding — handle with gloves.
• When producing intentional defects, the machine is still live; maintain all standard GMAW electrical safety practices. Never touch the contact tip or nozzle without first releasing the trigger and de-energizing the machine.
• Ventilation must be running before any arc is struck. GMAW fumes from A36 plate contain manganese; adequate fume extraction is not optional.