Common Welding Defects

Welding defect identification and prevention: porosity, undercut, lack of fusion, incomplete penetration, cracking, slag inclusion, and spatter. Root causes and corrective actions for each defect type.

Every welding defect has a cause, and every cause has a fix. The trick is identifying what went wrong from the visual evidence. Most defects trace back to one of four things: wrong settings, poor joint prep, contamination, or bad technique. Fix the root cause and the defect stops repeating.

Porosity shows up as round or elongated gas pockets in or on the weld surface. Scattered porosity means general gas coverage problems. Clustered porosity means a localized issue (a gap in gas coverage, a spot of contamination). Linear porosity along the joint often means the root opening is too wide and shielding gas can’t protect the root. Fix: verify gas flow at 20-25 CFH, eliminate drafts, clean the joint, check for leaks in gas lines.

Undercut is a groove melted into the base metal along the weld toe that isn’t filled by weld metal. It reduces the base metal cross-section and creates a stress riser. Causes: excessive amperage, wrong electrode angle, too-fast travel speed, or too-long an arc. Fix: reduce heat, angle the electrode to direct the arc into the joint, slow down slightly.

Lack of fusion means the weld metal didn’t melt into the base metal or the previous pass. It looks like the bead is sitting on top of the plate without bonding. Causes: insufficient heat, wrong electrode angle, too-fast travel speed, or oxide contamination (especially on aluminum). This is a serious structural defect because the joint has no strength at the unfused area.

Cracking is the most critical defect. Hot cracks form during solidification. Cold cracks (hydrogen cracks) form hours or days after welding in the HAZ. Centerline cracks split the weld bead down the middle. Root cracks form at the root. Every type has different causes: hot cracks from restraint and impurities, cold cracks from hydrogen, hard HAZ, and residual stress.

Slag inclusions trap slag between passes on stick and flux-core welds. Clean each pass completely before welding the next. Incomplete penetration means the root wasn’t fully fused, usually from insufficient heat, too-thick a root face, or too-small a root opening.

The guides below cover each defect type in detail with visual identification, root cause analysis, and corrective actions.

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