A weld bead is the strip of metal deposited along a joint when you weld two pieces together. It’s the visible result of melting filler material (and sometimes the base metal itself) into a seam, then letting it cool and solidify. Every welding process produces a bead, and its shape, width, and surface pattern tell you a lot about the quality of the weld and the technique used to make it.
What a Weld Bead Looks Like
A finished weld bead is a raised ridge of metal that follows the path the welder traveled along the joint. In cross-section, it has three key dimensions: width (how far it spreads across the joint), height or reinforcement (how much it rises above the base metal surface), and penetration (how deep it extends into the base material below). A good bead is consistent in all three dimensions from start to finish, with smooth edges that blend into the surrounding metal.
The surface texture depends on the welding process. TIG welding, done by a skilled operator, produces some of the cleanest beads: tight, evenly spaced ripples that look almost decorative. MIG welding creates a smoother, more uniform appearance because the wire feeds continuously, reducing stops and starts. Stick welding leaves behind a layer of hardened slag on top of the bead that must be chipped away before you can see the actual weld surface underneath. That slag forms from the electrode’s flux coating and protects the molten metal from contamination while it cools.
Stringer Beads vs. Weave Beads
There are two fundamental bead types, defined by how the welder moves the torch or electrode.
A stringer bead is a narrow, straight-line pass with little to no side-to-side motion. Stringers keep heat input low, which matters when welding stainless steel, high-strength alloys, or any situation where excessive heat could weaken the material. They also minimize the risk of slag getting trapped inside the weld, making them the go-to choice when the joint will be X-rayed for quality inspection. After the 1994 Northridge earthquake, extensive testing revealed that large weave beads in structural steel connections didn’t perform as well as stringer beads, which shifted industry practice for high-restraint joints.
A weave bead is wider because the welder moves the torch in a side-to-side pattern (zigzag, crescent, or figure-eight) while traveling along the joint. Weaving covers more area in a single pass, which makes it faster on time-sensitive jobs. It also helps reduce porosity and incomplete fusion because there are fewer stops and starts, and each stop/start point is a potential weak spot. Weave beads are commonly used for cover passes, the final visible layer of a multi-pass weld.
The general rule of thumb for weave width is no more than three to four times the diameter of the electrode being used. Some industrial standards, like those used in Middle Eastern oil and gas operations, allow up to five times the electrode diameter depending on size. Going wider than that increases the chance of cracking or uneven fusion at the edges.
How Settings Change the Bead
Three main variables shape a weld bead: travel speed, current (amperage), and voltage. Adjusting any one of them changes the bead’s profile in predictable ways.
Travel speed has a counterintuitive effect on penetration. Moving faster actually increases how deeply the bead penetrates into the base metal. At slower speeds, the arc hovers directly over the center of a large molten pool, and incoming metal droplets land on that pool like a cushion, limiting how far they dig into the base material. At higher speeds, the arc rides the leading edge of a smaller pool, so metal droplets strike the base material more directly and drive deeper. Slower speeds produce a wider, taller bead with a rounded solidification pattern. Faster speeds produce a narrower bead with a V-shaped ripple pattern on the trailing edge.
Current controls how much heat goes into the weld. Higher amperage means more heat, a wider bead, and deeper penetration, but too much current melts away the edges of the joint and causes defects. Lower amperage produces a smaller, cooler bead with less penetration. In MIG welding, current and wire feed speed are linked: you can’t raise one without raising the other through the machine’s controls alone. One workaround is adjusting the distance between the contact tip and the workpiece. Moving the torch farther from the work increases electrical resistance in the wire, which drops the current while keeping the wire feed speed the same.
Voltage primarily affects bead width and arc stability. Higher voltage spreads the arc wider, producing a flatter, broader bead. Lower voltage narrows the arc for a taller, more convex profile.
Common Bead Defects
When something goes wrong with technique or settings, the bead shows it. These are the most common problems:
- Porosity: Tiny cavities or gas pockets trapped inside or on the surface of the bead. They look like small holes or pits and are typically caused by contamination on the metal surface (oil, rust, moisture) or insufficient shielding gas coverage. Porosity weakens the joint because those voids can’t carry any load.
- Undercut: A groove melted into the base metal along one or both edges of the bead. It happens when heat input is too high or travel speed is too fast, melting away material at the edges without depositing enough filler to replace it. Undercut creates a stress concentration point that can become the starting place for a crack.
- Overlap: The opposite of undercut. Excess weld metal spills over onto the base metal surface without actually fusing to it. This usually results from moving too slowly or using too much current. The bead looks like it has a lip or ridge hanging over the edge. Because that overflowing metal isn’t bonded to the base, it offers no structural benefit and can trap moisture or debris.
- Spatter: Small droplets of molten metal that land on the surrounding surface outside the bead. They appear as raised dark globules or jagged round spots. Spatter doesn’t weaken the weld itself, but it looks messy, can interfere with paint or coatings, and requires extra cleanup time.
Multi-Pass Beads
On thicker materials, a single bead can’t fill the entire joint. Welders build up the weld in layers, with each pass depositing one bead on top of or beside the previous one. The first pass at the bottom of the joint is called the root pass, and its penetration into both sides of the joint is critical for overall strength. Intermediate passes are called fill passes, and the final visible layer is the cap or cover pass.
Each pass reheats the metal beneath it, which can refine the grain structure and improve toughness in some steels but cause problems in heat-sensitive materials. This is one reason stringer beads are preferred for alloys and stainless steel: each narrow pass adds less heat per unit area than a wide weave, giving the welder more control over how much thermal cycling the surrounding metal experiences.
The cap pass is where aesthetics matter most. Many welders prefer a slight weave on the cover pass to create a smooth, flat profile that blends into the base metal, even if the underlying fill passes were all stringers.