In mechanical fabrication, the welding arc is a rapid metallurgical casting operation executed under dynamic manual or robotic conditions. Within a fraction of a second, molten base metal and filler wire combine, freeze, and cool under severe thermal gradients.
When welding parameters—shielding gas flow, voltage, travel speed, joint fit-up, or surface cleanliness—drift outside acceptable tolerances, discontinuities develop. While minor cosmetic irregularities may be harmless, structural defects act as stress concentrators that initiate fatigue fractures or catastrophic rupture under load.
1. Porosity (Gas Pockets)
What It Is: Cavities or voids formed by trapped gas bubbles within the solidifying weld metal. Can occur as uniformly scattered porosity, concentrated clusters, or elongated "wormhole" piping porosity.
Root Causes:
- Inadequate shielding gas coverage due to drafts, excessive travel speed, or a blocked welding torch nozzle.
- Surface contaminants on the joint face: oil, cutting fluid, grease, mill scale, moisture, or rust releasing hydrogen and carbon monoxide into the molten puddle.
- Contaminated or damp filler wire (particularly hygroscopic flux in FCAW/SMAW electrodes).
Prevention: Thoroughly degrease and wipe joint seams with solvent prior to welding; shield indoor welding bays from ambient shop floor fans; maintain shielding gas flow rates at 12–18 L/min; store low-hydrogen electrodes in heated holding ovens at 120°C.
Under ISO 5817 Quality Level B (stringent structural requirements), the total projected area of isolated pores must not exceed 1% of the weld area, and no individual pore diameter may exceed $0.2 \times s$ (where $s$ is the throat thickness, maximum 3 mm).
2. Undercut: The Fatigue Killer
What It Is: A groove or trench melted into the base metal adjacent to the weld toe that remains unfilled by deposited filler metal.
Why It Is Dangerous: Undercut creates a sharp notch at the exact point of highest stress concentration between the weld reinforcement and the parent plate. Under cyclic or dynamic loads, fatigue cracks almost always initiate from the root of an undercut notch.
Root Causes:
- Excessive welding voltage or current paired with an overly rapid travel speed (the arc melts the parent groove faster than the puddle can wet out).
- Incorrect torch angle pointing disproportionately toward the vertical plate in horizontal fillet welds.
Prevention: Reduce arc voltage and travel speed; allow the molten puddle to pause momentarily at the weld toes during torch weave manipulation; hold the gun angle at a balanced 45° bisecting angle on fillet joints.
3. Lack of Fusion & Incomplete Penetration
What It Is: Lack of fusion (cold lap) occurs when the weld metal fails to coalesce and fuse with the parent sidewall metal or previous weld bead. Incomplete penetration occurs when the weld root fails to extend through the full depth of the prepared joint root face.
Root Causes:
- Insufficient heat input (amperage too low for plate thickness).
- Root face land too thick, or root gap too tight on V-groove preparations.
- Welder allowing the molten puddle to race ahead of the arc ("riding the puddle"), which insulates the underlying cold plate from direct arc impingement.
Short-circuit GMAW transfer is notorious for cold-lap defects on steel plate thicker than 6 mm. The weld bead can look visually smooth and complete on the surface while having zero metallurgical fusion to the parent metal beneath. For heavy structural joints, always mandate spray transfer, pulsed MIG, or flux-cored welding.
4. Weld Defect Reference & Prevention Table
Reference this diagnostic table when conducting visual welding inspections (VT) on the manufacturing floor:
| Defect Name | Visual Characteristics | Primary Root Cause | Corrective Action |
|---|---|---|---|
| Porosity | Round pinholes or Swiss-cheese voids on or beneath bead | Loss of gas shield; oily/rusty joint | Check gas flow (15 L/min); clean joint to bare shiny metal |
| Undercut | Sharp groove melted into plate toe along the weld edge | Excessive current / travel speed too fast | Slow travel speed; adjust gun angle to fill toes evenly |
| Lack of Fusion | Weld sits on plate like cold butter; planar flaw | Heat input too low; arc not directed at root | Increase amperage; keep arc on leading edge of puddle |
| Burn-Through | Holes blown completely through thin sheet metal | Current too high; excessive root gap | Tighten fit-up; switch to pulsed MIG or handheld laser |
| Hot Cracking (Solidification) | Centerline longitudinal split along the weld bead | High weld restraint; sulfur/phosphorus impurities | Modify joint design to allow expansion; use lower restraint |
| Slag Inclusions | Non-metallic glassy particles trapped in multi-pass welds | Inadequate interpass slag chipping in SMAW/FCAW | Grind and wire brush completely between each pass |
Defect-Free Welding with Certified Procedures
FabXE welders operate under qualified Welding Procedure Specifications (WPS) with procedure qualification records (PQR). All structural and pressure assemblies undergo 100% visual inspection and non-destructive testing (NDT) as required.
Frequently Asked Questions
A discontinuity is any interruption in the typical structure of a weldment (such as a tiny pore or ripple). It only becomes a defect when its size, quantity, or location exceeds the allowable threshold established by the applicable code (e.g. AWS D1.1 or ISO 5817), rendering the part non-compliant.
Hydrogen cracking (delayed cold cracking) requires three factors: a susceptible hardened microstructure (martensite), diffusable hydrogen, and high tensile stress. It is eliminated by preheating the steel (150°C–250°C) before welding, using low-hydrogen electrodes, and applying post-weld heat treatment (PWHT).