6 Common Weld Defects and Fixes

Every welder has been there—you flip your hood up, inspect your bead, and spot something that shouldn't be there. Weld defects aren't just cosmetic problems; they compromise structural integrity, lead to costly rework, and can tank your shop's reputation for quality.

The good news? Most weld defects stem from a handful of root causes, and once you understand what's happening at the arc, you can make targeted adjustments to eliminate them. Whether you're running MIG, TIG, or stick, mastering weld defects troubleshooting is essential for producing code-quality work consistently.

In this guide, we'll break down the six most common weld defects you'll encounter in fabrication work, explain exactly why they happen, and give you the specific parameter adjustments and technique corrections to fix them for good.

Porosity appears as small holes or voids scattered throughout your weld bead, resembling a sponge or Swiss cheese when sectioned. These gas pockets form when atmospheric gases get trapped in the molten weld pool before it solidifies.

Root Causes: - Inadequate shielding gas coverage or flow rate - Contaminated base metal (oil, rust, paint, moisture) - Dirty or damp filler wire - Excessive travel speed - Wind or drafts disrupting gas coverage - Worn or clogged gas nozzle

Fixes: - Check gas flow rate—typically 25-35 CFH for MIG welding - Clean base metal thoroughly with a grinder or wire brush within 1" of the weld zone - Store filler metals properly in dry conditions - Reduce travel speed to allow gases to escape before solidification - Use wind screens outdoors or increase gas flow slightly - Inspect and replace consumables regularly

Undercut presents as a groove or channel melted into the base metal along the toe of the weld that hasn't been filled by weld metal. This creates a stress concentration point and reduces the effective throat thickness of your weld.

Root Causes: - Excessive amperage or voltage - Travel speed too fast - Incorrect electrode angle - Arc length too long - Improper weave technique on vertical or overhead positions

Fixes: - Reduce amperage by 5-10% and reassess - Slow your travel speed to allow filler metal to fill the groove - Maintain a 10-15° drag angle for most applications - Keep a tighter arc length—approximately the diameter of your electrode - On weave beads, pause briefly at the toes to allow fill - Consider multiple passes instead of one heavy pass

Lack of fusion occurs when the weld metal doesn't properly bond with the base metal or previous weld passes. The surfaces may appear to be joined visually, but there's no metallurgical bond—making this a particularly dangerous defect that often requires NDT to detect.

Root Causes: - Heat input too low (insufficient amperage) - Travel speed too fast - Incorrect torch angle directing heat away from joint - Improper joint preparation or fit-up - Oxide layer or contamination preventing fusion - Weld pool running ahead of the arc

Fixes: - Increase amperage to ensure adequate penetration - Slow travel speed to allow heat to transfer into base metal - Direct the arc into the joint root, not just onto the weld pool - Ensure proper bevel angles and root opening per WPS - Grind joints clean immediately before welding - On aluminum, use a stainless steel brush to remove oxides

Cold lap, also called overlap, happens when weld metal flows over the base metal surface without fusing to it. The weld bead literally laps over onto cold base metal, creating a mechanical rather than metallurgical joint at that location.

Root Causes: - Travel speed too slow - Excessive wire feed speed or deposition rate - Incorrect torch angle - Amperage too low for the amount of filler being deposited - Poor technique on out-of-position welds

Fixes: - Increase travel speed to prevent excessive buildup - Reduce wire feed speed to match your travel rate - Angle torch to direct heat into base metal, not just onto the pool - Increase amperage to properly wet the toes - On vertical-up welds, use a slight weave to distribute heat evenly - Balance your parameters—wire feed and voltage should be matched

Spatter consists of small droplets of molten metal that expel from the weld zone and adhere to surrounding surfaces. While not always a structural defect, excessive spatter wastes filler material, requires time-consuming cleanup, and often indicates unstable arc conditions that may cause other defects.

Root Causes: - Voltage too high or too low for wire feed speed - Arc length excessive - Contaminated or rusty base metal - Wrong shielding gas mixture - Poor work cable connection - Worn contact tip causing erratic feeding

Fixes: - Fine-tune voltage—start at manufacturer recommendations and adjust in 0.5V increments - Maintain consistent, proper arc length - Clean base metal before welding - Verify correct gas mixture (75/25 Ar/CO2 typical for mild steel MIG) - Ensure solid work clamp connection close to the weld - Replace contact tips regularly—they're consumables, not permanent parts - Apply anti-spatter compound to surrounding areas when cleanup is critical

Cracking is the most serious weld defect and an automatic rejection in any code work. Cracks can occur in the weld metal, heat-affected zone, or base metal, and may appear immediately (hot cracking) or hours to days after welding (cold cracking or hydrogen-induced cracking).

Root Causes: - High carbon or alloy content in base metal - Hydrogen contamination from moisture - Excessive restraint or residual stress - Rapid cooling rates - Incorrect filler metal selection - Poor joint design concentrating stress

Fixes: - Preheat according to material thickness and carbon equivalent - Use low-hydrogen electrodes (E7018) and store properly in rod ovens - Allow for movement in joint design; avoid over-constraining - Control cooling rate with preheat and post-heat when required - Match filler metal to base metal chemistry and mechanical requirements - Use proper welding sequence to minimize stress buildup - For crack-sensitive materials, consider PWHT (post-weld heat treatment)

Create a simple defect log in your shop. When you or your team encounters a weld defect, note the material, process, parameters, and the fix that worked. Over time, you'll build a troubleshooting reference specific to your equipment and typical jobs. This is especially valuable when training new welders or when returning to a material you haven't worked with in months.

Many modern welding power sources can log parameters automatically—take advantage of this data to correlate settings with results.

Weld defects troubleshooting comes down to understanding the relationship between your parameters, technique, and material conditions. Porosity, undercut, lack of fusion, cold lap, spatter, and cracking all have identifiable causes—and more importantly, reliable fixes.

The key is systematic diagnosis: examine the defect, consider the possible causes, and adjust one variable at a time until you dial in the process. With practice, you'll start anticipating problems before they happen based on how the arc sounds and the puddle behaves.

Want to dive deeper into welding fundamentals and advanced techniques? Check out our complete welding guide for more in-depth coverage of processes, equipment selection, and professional tips from experienced fabricators.