Common Welding Defects & How to Fix Them

Every welder produces defects — the difference between a beginner and an experienced welder is recognizing what went wrong and knowing how to fix it. This guide covers the most common welding defects across MIG, TIG, and stick, with their causes and practical solutions.

Porosity

What it looks like: Small round holes (gas pockets) scattered through the weld bead or clustered at the start/end of a weld.

Causes:

• Contaminated base metal — oil, grease, moisture, paint, or rust on the weld surface
• Insufficient shielding gas coverage — flow rate too low, nozzle clogged, drafts blowing gas away, or gas line leaks
• Damp or contaminated filler wire/electrodes
• Excessive arc length (especially in TIG and stick)
• Wrong shielding gas for the process or material

Fix: Clean the base metal thoroughly before welding. Check gas flow rate and connections. Shield the weld area from wind. Store filler wire and electrodes sealed in dry conditions. Shorten the arc length.

Undercut

What it looks like: A groove melted into the base metal along the toe (edge) of the weld bead that is not filled by weld metal. It creates a notch that weakens the joint.

Causes:

• Excessive amperage for the material thickness
• Travel speed too fast — the arc melts the base metal edges but the weld pool cannot fill the groove before solidifying
• Improper torch/electrode angle — directing arc energy at the base metal edge rather than the weld pool
• Excessive weaving width

Fix: Reduce amperage, slow travel speed slightly, correct torch angle to direct heat into the joint center, and narrow the weave pattern. On multi-pass welds, pause briefly at the toes of the weave to allow filler to fill the edges.

Lack of Fusion (Cold Lap)

What it looks like: The weld bead sits on top of the base metal without actually melting into it. The weld may appear acceptable from the surface but has no structural bond at the interface.

Causes:

• Heat input too low — amperage or voltage insufficient to melt the base metal
• Travel speed too fast for the amperage setting
• Oxide layer not removed (especially on aluminum and stainless)
• Wrong torch angle — directing heat away from the joint surfaces
• Weld pool running ahead of the arc (in TIG push technique or MIG on thin material)

Fix: Increase amperage, reduce travel speed, clean the joint surfaces, and direct the arc into the base metal — not just on top of the previously deposited weld metal.

Cracking

What it looks like: Visible cracks in the weld bead, HAZ, or base metal. Can be longitudinal (along the weld), transverse (across the weld), or crater cracks (at the end of a weld).

Causes:

• Hot cracking: Occurs during solidification. Caused by high sulfur or phosphorus in the base metal, wrong filler selection, or excessive joint restraint
• Cold cracking: Occurs hours or days after welding. Caused by hydrogen in the weld (from moisture), excessive residual stress, or hardened microstructure in the HAZ
• Crater cracking: The shrinkage crack that forms in the crater at the end of a weld when the arc stops abruptly

Fix: Use low-hydrogen electrodes (7018) and dry filler, preheat when welding thick or high-carbon steel, reduce joint restraint where possible, and taper off arc energy at the end of each weld to fill the crater. For crater cracking specifically, many machines have a crater-fill function that automatically reduces amperage at the end of a weld.

Excessive Spatter

What it looks like: Metal balls and droplets scattered around the weld bead on the base metal surface.

Causes:

• Voltage too low or wire feed speed too high (MIG) — causing short-circuit transfer that is too violent
• Shielding gas issues — wrong mix, insufficient flow, or contaminated gas
• Arc length too long (stick) or contact-tip-to-work distance too long (MIG)
• Dirty base metal or rusty wire

Fix: Adjust voltage/wire speed balance, check gas mix and flow rate, reduce arc length, clean the base metal, and apply anti-spatter spray on the surrounding surface. In MIG, switch to a spray or pulse transfer mode if your machine supports it.

Burn-Through

What it looks like: A hole melted completely through the base metal. Common on thin material.

Causes: Too much heat for the material thickness — amperage too high, travel speed too slow, or excessive dwell time on one spot.

Fix: Reduce amperage, increase travel speed, use skip welding (short beads in alternating locations), add backing material, or switch to a process with better heat control (TIG with foot pedal, pulse MIG). On very thin material, tacking heavily at close intervals acts as a heat sink.

Incomplete Penetration

What it looks like: The weld does not melt through the full thickness of the joint — the root of the joint remains unfused.

Causes: Heat input too low, joint gap too tight, root face too thick, or travel speed too fast.

Fix: Increase amperage, open the root gap, reduce the root face, or slow travel speed. For groove welds, ensure the electrode/wire reaches the root of the joint — sometimes angling the torch differently or using a smaller electrode solves the problem.

Slag Inclusions

What it looks like: Irregular dark spots or lines visible on the weld surface or revealed when grinding. Slag from flux-coated electrodes or flux-core wire trapped inside the weld.

Causes: Incomplete slag removal between passes, improper torch angle trapping slag under the puddle, travel speed too slow (slag runs ahead of the arc), or undercut from previous passes that traps slag in the groove.

Fix: Chip and wire-brush all slag completely between every pass. Adjust torch angle so the arc pushes slag behind the puddle, not under it. Correct any undercut before laying the next pass.