Cold lap is a weld defect where molten filler metal flows onto the base metal without actually fusing to it. The result is a thin, crack-like gap running parallel to the surface of the plate, typically along the weld toe. It can look fine from the outside, which makes it particularly dangerous. The weld bead appears to connect to the base metal, but underneath that contact point, there’s no real bond.
How Cold Lap Forms
During welding, the arc is supposed to melt both the filler wire and the surface of the base metal so they mix together and solidify as one piece. Cold lap happens when the arc doesn’t generate enough heat to melt the base metal surface. The filler metal, already molten, simply rolls over the cooler base metal and sits on top of it. Think of it like pouring syrup onto a frozen pan: the syrup spreads out and hardens in place, but it never becomes part of the pan.
The defect is especially common in MIG welding (GMAW) when the process is running in short-circuiting transfer mode. This mode uses lower voltages, typically 22 volts or less, and produces a fast-freezing weld puddle. That quick freeze is useful for controlling the puddle in tricky positions, but it also means the molten metal can solidify against the base plate before true fusion occurs. Without careful technique and the right settings, the puddle essentially laps over cold metal and locks in place.
Common Causes
Voltage that’s too low is one of the most direct causes. When welding voltage drops, the arc doesn’t produce enough heat to properly melt the base metal at the edges of the weld. This is a bigger problem than many welders realize, because voltage can drop between the power source and the weld itself. A machine set to 25 volts might only deliver 23 volts at the tip if the power cables are long, and that two-volt difference can be enough to cause cold lap at the toes of the weld.
Surface contamination is another major contributor. Mill scale, the dark oxide layer on hot-rolled steel, acts as an insulating barrier between the arc and the base metal. Macro cross-sections of welds made on scaled steel show no fusion for roughly an eighth of an inch inward from the weld toes. Rust, oil, and paint create similar problems. Some flux-cored wires are designed to burn through light mill scale, but even those struggle with heavy scale or multiple contaminants. The weld may wet out across the surface but never actually bond at the edges.
Travel speed matters too. Moving too fast means the arc spends less time heating any one spot, reducing penetration. Moving too slowly with a fillet weld, paradoxically, can also cause cold lap on the lower toe in the horizontal position, because the puddle grows large and rolls downward under gravity before the base metal beneath it is hot enough to fuse.
Why It’s Hard to Spot
Cold lap is deceptive because the weld often looks acceptable on the surface. The filler metal makes full contact with the base plate, and the bead profile can appear smooth and properly shaped. But the defect sits right at the boundary between the weld and the base metal, hidden from plain sight. The gap it creates is very small, often running parallel to the plate surface, making it easy to miss during a quick visual inspection.
Standard visual inspection alone is generally not reliable for catching cold lap. Several non-destructive testing methods can find it. Liquid penetrant testing, where a fluorescent dye is applied to the surface and drawn into cracks by capillary action, can detect surface-breaking defects down to about 750 microns. Magnetic particle testing works similarly by revealing where magnetic flux leaks through a gap, catching flaws as small as 1,000 microns. Ultrasonic testing, which sends sound waves into the metal and listens for reflections off internal boundaries, can detect defects down to around 500 microns. For critical structural work, ultrasonic methods are often preferred because cold lap can extend below the surface where penetrant testing can’t reach.
Structural Risks
The real danger of cold lap is that it acts as a pre-existing crack. The unbonded edge creates a sharp stress concentration point, and under repeated loading, that’s exactly where fatigue cracks initiate. In a welded lap joint, stress naturally concentrates at two locations: the weld toe and the weld root. Cold lap at the toe makes this concentration dramatically worse.
When a load is applied, the forces converge toward the weld toe, and any gap there becomes a starting point for crack growth. Under high cyclic stress, microcracks form at the toe, then propagate through the weld until the joint fractures. Research on aluminum alloy lap joints shows that fatigue life drops significantly as stress levels increase, with weld samples fracturing from the entire toe line in a large-area brittle failure pattern. At lower stress levels, cracking shifts to the root, following 45-degree shear planes. Either way, the joint fails well before an equivalent defect-free weld would.
This is why cold lap is classified as a rejectable defect in most structural welding codes. A joint with cold lap has not achieved fusion, and a weld without fusion cannot carry its designed load over time.
How to Prevent Cold Lap
The most effective prevention starts with heat input. Make sure voltage at the arc is high enough for the process and transfer mode you’re using. If you’re running long cables, measure voltage at the gun rather than trusting the machine readout. For MIG welding, voltages above 25 typically indicate spray or globular transfer, which generally produce better fusion than short-circuit transfer. When short-circuit mode is necessary, tighter control of travel speed and gun angle becomes critical.
Surface preparation is equally important. Grinding off mill scale, rust, and any coatings before welding gives the arc direct access to clean base metal. This alone can eliminate the fusion failures that mimic or contribute to cold lap. On structural steel, removing scale back at least an inch from the weld zone is standard practice for good reason.
Technique adjustments help too. Directing the arc toward the base metal rather than letting the puddle lead ensures the heat goes where fusion needs to happen. A slight weave or pause at the toes of the weld gives the arc time to melt the plate edges before the puddle washes over them. For horizontal fillets, watch the lower toe carefully, as gravity pulls molten metal down onto base metal that may not be hot enough to fuse. Reducing travel speed slightly or using a smaller bead size with multiple passes gives better control in those situations.