Undercut in Laser Welding: Causes, Inspection and Prevention

Undercut is the defect that looks tidy. Where spatter leaves obvious debris, undercut leaves a smooth groove melted into the base metal along the weld edge, so it is easy to walk past. That groove is a stress concentration and an easy place for corrosion to start, and on fatigue-loaded or coated parts it is often rejectable. This article explains what creates undercut, how the standards treat it, and how to inspect for it.

What undercut is and why it matters

Undercut is a groove melted into the base metal along the toe, or edge, of a weld and left unfilled, because metal consumed at the edge was not replaced by the pool before it solidified. The result is a notch at the toe that concentrates stress and can reduce fatigue life, especially under cyclic loading. On coated or corrosion-resistant parts the groove also traps moisture. Many criteria treat undercut as rejectable above a certain depth or length, so confirm the requirement for your part.

How beam welding standards treat undercut

Standards give you the frame for deciding how much undercut is acceptable. ISO 13919-1:2019 sets quality levels for imperfections in electron and laser beam welded joints in steel, nickel, titanium and their alloys, and works with quality levels B, C, and D, where B is the most demanding. ISO 13919-2:2021 applies the same approach to aluminium, magnesium and pure copper. Both describe quality levels rather than prescribing an inspection method, so defining the defect and looking for it are two separate decisions.

Why arc and beam welding acceptance differ

It is common to see limits borrowed from arc welding and applied to a laser weld, and that is a mistake. ISO 5817:2023 sets quality levels for fusion-welded joints in steel, nickel, titanium and their alloys, but its scope explicitly excludes beam welding, so it does not govern a laser or electron beam joint. Its value here is contrast: arc and beam welding use different acceptance systems. For aerospace work the governing document is often AWS D17.1/D17.1M:2024.

The three mechanisms behind undercut

Undercut forms in three common ways, and each calls for a different fix. The first is excess energy at the edge, where the beam melts the edge faster than the pool can supply metal to fill it. The second is insufficient energy, where the pool never reaches the edge with enough heat to fuse it, so the edge stays sharp. The third is edge overheating, because a free edge sheds heat more slowly than the middle of a plate. Adding power helps the second case and worsens the first and third.

Edge preparation and fit-up

The condition of the edge is often the real cause. A sheared edge carries a burr and a work-hardened layer, while a laser-cut edge has its own oxide and re-melted layer, so each melts differently from machined stock. Because the toe follows this edge, any variation along it becomes a variation in the weld, so check that edge quality is consistent from part to part. An opening gap is the second classic cause, because it pulls the pool away and leaves the groove unfilled.

Energy distribution at the toe

The way energy is distributed at the edge is the main lever you control. Spot size and focus position set how concentrated the beam is at the toe, so a tight spot running close to the edge will cut into it, while a broader spot spreads the heat and gives the pool time to wet the edge. Wobble welding is usually the most effective single change, because beam oscillation widens the heat distribution and lets the pool wet the edge more evenly.

Autogenous versus filler welds

An autogenous weld has only the metal already in the parts, so if the process melts the edge it has nothing extra to put back into the groove, which is why thin edges are so sensitive. Filler changes the balance twice over: the wire supplies extra metal that can fill the edge, and the added heat improves wetting. The choice between the two is often about tolerance and appearance, as this laser welding versus MIG welding comparison sets out.

Thickness and edge conduction

Thickness changes how the edge behaves, because it changes how quickly heat is carried away from the toe. On thin sheet there is little material around the edge, so heat builds quickly and the edge can fall away with little warning. On thicker material the edge is more robust, but a larger heat-affected zone means the two sides of the joint are more likely to heat at different rates. Thinner material favours lower heat input and tighter control.

Fixturing and clamping

Fixturing matters more for undercut than most people expect. If the edge can lift or move as the part heats and cools, the gap at the toe changes during the weld and pulls the pool away from the edge you are trying to protect. Clamping should hold the joint closed along its whole length rather than at a few points, and sit close enough to control the gap without acting as a heat sink, as this guide to specifying a laser welding cell sets out.

How to inspect for undercut

Visual inspection catches undercut on the accessible side, but it easily misses shallow grooves and the inside of edges that cannot be seen directly, where a magnifier or borescope helps. Depth and length are the two measurements that matter, and both are taken at the toe with a depth gauge or weld gauge. Where access allows, a macro-section through the toe is the most reliable way to confirm the profile, so sample several locations along a seam.

Undercut is almost always a question of balance at the edge: too much energy melting it away, or too little metal flowing back to fill it. Before you reach for the power setting, check the edge condition and how the part is clamped. Fix the edge and the fixture first, add oscillation next, and only then adjust power and speed. The wider set of welding problems rewards the same approach, and so does a structured look at welding defects.

Sources and references

• ISO 13919-1:2019 Electron and laser-beam welded joints - Quality levels for imperfections - Part 1: Steel, nickel, titanium and their alloys - defines the B, C and D quality levels used to judge beam-weld imperfections including undercut View source
• ISO 13919-2:2021 Electron and laser-beam welded joints - Quality levels for imperfections - Part 2: Aluminium, magnesium and their alloys and pure copper - extends the same quality-level framework to light alloys and copper View source
• ISO 6520-1:2007 Welding and allied processes - Classification of geometric imperfections in metallic materials - Part 1: Fusion welding - defines undercut as a named geometric imperfection View source
• ISO 5817:2023 Welding - Fusion-welded joints in steel, nickel, titanium and their alloys (beam welding excluded) - Quality levels for imperfections - shows that arc welding uses a separate acceptance system because it excludes beam welding View source
• AWS D17.1/D17.1M:2024 Specification for Fusion Welding for Aerospace Applications - sets fusion welding requirements for aerospace work, where undercut limits are often imposed by contract View source