Fixing Common Weld Defects: Porosity, Undercut & Warping

Most welding troubleshooting advice fails in the same way: it lists every possible cause of a defect without telling you which one it usually is. That turns a five-minute fix into an afternoon of changing variables at random. What follows is organised the other way round — by what you are looking at, in the order the causes actually turn up, with the specific adjustment that addresses each one.

Porosity

What it looks like: small round holes in the weld face, or a scattering of pinholes revealed when you grind the cap back. Sometimes it is entirely internal and only shows up when the weld is cut or broken.

What it is: gas that was dissolved or trapped in the molten puddle and did not escape before the metal solidified. The gas came from somewhere — finding where is the whole diagnosis.

Undercut and overlap

These two are opposites and they sit at the toe of the weld — the line where weld metal meets base metal.

Undercut is a groove melted into the base metal alongside the weld and left unfilled. It reduces the effective thickness of the part and concentrates stress exactly where you least want it. Causes, in order of frequency: current too high; travel speed too fast for the current; work or gun angle throwing the arc onto the base metal rather than into the joint; and on weaving passes, failing to pause at the edges long enough to fill.

Overlap (sometimes called cold lap) is the reverse — weld metal spilling over the base metal without fusing to it. It looks like a bead sitting on the surface rather than blended into it. Causes are typically current too low, travel speed too slow, excessive wire feed for the voltage, or a work angle that lets the puddle run ahead of the arc.

• Undercut: reduce amperage first, then slow travel slightly, then check your angle. Pause at the toes on a weave.
• Overlap: raise amperage or reduce wire feed, speed up travel, and keep the arc on the leading edge of the puddle rather than letting the puddle get ahead of it.
• Both: a weld pool that is running downhill on a vertical or overhead joint is telling you the heat input is too high for the position.

Lack of fusion and penetration

This is the defect that matters most and shows least. Lack of fusion is weld metal adjacent to base metal, or to a previous pass, without a metallurgical bond. Lack of penetration is weld metal that has not reached the root of the joint. Both can produce a weld that looks entirely acceptable from the outside and carries a fraction of the intended load.

Causes cluster around insufficient energy delivered to the right place: current too low; travel too fast; the arc aimed at the filler or the previous bead rather than the joint root; joint preparation too tight or with the wrong included angle; and welding over mill scale, slag or a previous pass that was not cleaned between runs.

Warping and distortion

Warping is not really a defect in the weld; it is the part responding to heat. Metal expands where the arc heats it, is restrained by the surrounding cold material, and then contracts as it cools — pulling the part with it. You cannot eliminate this, only manage it, and the management is mostly about sequence and heat input rather than technique.

• Reduce total heat input. Smaller beads, faster travel, appropriate current. More heat is more movement.
• Use intermittent welding where the design permits, rather than one continuous pass.
• Alternate sides and ends. Balancing where the contraction pulls is the single most effective technique available.
• Back-step. Weld short segments in the opposite direction to overall travel to distribute heat along the joint.
• Pre-set the joint so the part pulls into alignment rather than out of it — this is a skill that comes from knowing your own material and joints.
• Clamp and fixture properly, accepting that heavy restraint reduces movement but increases residual stress and, on some materials, cracking risk.
• Let it cool between passes. Impatience is a leading cause of distortion.

A common instinct is to cool the part quickly with water or compressed air to stop the movement. On many carbon steels this is at best unhelpful and at worst introduces hardness and cracking risk in the heat-affected zone. Check what your specific material requires before quenching anything.

Spatter, slag and surface problems

Spatter rarely compromises the weld itself but it wastes filler, sticks to everything, and signals that some parameter is off. The usual causes are an incorrect relationship between voltage and wire feed speed, wrong polarity for the wire type, dirty base metal, and shielding gas choice — carbon dioxide inherently produces more spatter than argon-rich mixes.

Slag inclusions are a different problem, specific to stick and flux-cored processes. Slag that gets trapped between passes because the previous run was not chipped and brushed clean becomes a permanent defect inside the weld. On multi-pass work, cleaning between passes is not housekeeping, it is part of the procedure.

Cracking

Cracking is the defect worth being conservative about, because the causes are metallurgical rather than purely technique-based and the consequences are the most serious. Broadly, cracks that appear while the weld is still hot tend to relate to weld metal composition, joint restraint and unfilled craters; cracks that appear hours after the job is finished tend to relate to hydrogen, hardenable material and cooling rate.

• Fill every crater. An unfilled crater at the end of a bead is a ready-made crack starter.
• Match filler to base material properly — on alloy steels and aluminium this is a genuine engineering decision.
• Keep hydrogen out: dry electrodes, clean surfaces, no oil, no damp.
• Respect preheat and interpass temperature requirements where the material specifies them.
• Reduce restraint where you can, and be aware that heavily clamped joints in thick section concentrate stress.

How to repair a defect properly

1. Identify the cause first. Repairing without changing the parameter that caused the defect reproduces the defect.
2. Remove the defective metal. Grind, gouge or cut back to sound material, with generous margin at each end of the affected area.
3. Clean thoroughly. Remove slag, oxide, grinding debris and any contamination from the prepared area.
4. Reprofile the joint so the repair area has a shape you can actually weld — a narrow gouged slot with vertical walls invites fresh lack of fusion.
5. Reweld with the corrected parameters, and inspect again.

The habit worth building is running test coupons on offcuts before committing to real work, then destroying those coupons. Bend them, break them, look at the fracture surface. Ten minutes on scrap tells you more about your settings than an hour of inspecting finished welds you are reluctant to cut apart.