Laser Welding Defects and How to Fix Them: Porosity, Cracks & Burn-Through (2026)

Almost every laser welding defect has a fixable cause, and most fixes cost nothing: porosity comes from dirty metal or weak gas coverage, cracks usually mean wrong filler wire or too-fast cooling, and burn-through on thin sheet is a power and speed problem, not a machine problem. In our factory we see the same six defects on returned samples every month — porosity, hot cracks, burn-through, black seams, spatter, and undercut. This guide covers what causes each one and the exact settings that stop them, based on production work with handheld fiber laser welders from 1000W to 3000W.

Why Does Porosity Appear in Laser Welds?

Porosity is the most common defect customers send us photos of. You see small round holes on the surface, or when the bead is cut open, a line of bubbles hiding inside. Both hurt the joint — porosity lowers strength, fatigue life, and elongation, which matters if the part carries a load or holds pressure.

Researchers split porosity into two types. Metallurgical porosity comes from surface contamination and from elements that boil at low temperature. Keyhole-induced porosity comes from the deep-penetration weld pool itself: the keyhole wobbles, bubbles form at the bottom of the pool, and the solidifying metal traps them before they float out. A 2023 study on 5kW fiber laser welding of 304 stainless steel confirmed that shielding gas choice changes the keyhole stability directly. The same study found nitrogen-based shielding raised dissolved nitrogen in the weld by 36% compared with the base metal and gave a visibly calmer keyhole.

For a handheld machine the practical causes come first:

• Dirty base metal. Oil, rust, paint, moisture, or machining fluid on the surface boils into gas. Wipe the joint area with acetone or a clean solvent, and dry it, before welding.
• Gas flow too low or too high. Below roughly 12-15 L/min the pool is not covered. Above 25 L/min you pull in air. Most handheld units run best at 15-20 L/min.
• Gas leaks. A loose fitting at the machine or the gun feeds air into the line. Check couplings under pressure; even a small leak shows up as gray or porous seams.
• Drafty work area. A fan or open door blows the gas shield away from the weld pool. Screen the area or slow the gas a little to compensate.
• Damp or wrong filler wire. Keep wire on the spool covered. Rusty or oily wire feeds contamination straight into the pool.

On thicker material, joint preparation matters. The older CO2 laser studies on stainless already showed that partial penetration with a slight forward beam tilt produced fewer bubbles, and full-penetration welds released gas better in any shielding gas. The lesson transfers to fiber lasers: leave a small root gap on butt joints instead of forcing a tight fit, so gas can escape, and weld thick plate in a single deep pass rather than stacking shallow beads.

Why Do Laser Welds Crack on Stainless Steel and Aluminum?

Cracks are the defect that scares shops the most, and for good reason — a cracked weld can fail weeks later, not on the bench. Two crack types show up in laser welding: hot cracks during solidification, and crater cracks at the end of the weld.

Hot cracking on stainless steel usually traces to the filler wire. Welding 304 or 316 with the wrong grade leaves the pool short of ferrite-forming elements, and the weld solidifies with a crack-prone structure. Use ER308L for 304 base metal and ER316L for 316. If cracks keep coming on stainless, the usual causes are contamination (sulfur from cutting oil is a classic), too-high travel speed, or welding a highly restrained joint where the metal cannot shrink freely.

Aluminum is a different story. It conducts heat fast and forms a thick oxide layer, so hot cracking is common when the operator welds cold or uses the wrong filler. ER4043 (AlSi5) fills and flows better and tolerates more crack sensitivity; ER5356 gives higher strength but less crack tolerance on some alloys. Preheat thick aluminum sections to 100-150°C before welding, and use a wobble head — the oscillation mixes the pool and helps gas escape, which is why most of our aluminum customers run 2000W or higher with the wobble function on.

Crater cracks are easier to spot and easier to fix. They form at the end of the weld when the beam stops and the pool cools too fast without filler. Most handheld machines have a crater-fill or slow-down function at the end of the seam. Turn it on, or simply hold the trigger a fraction of a second longer and let the wire fill the crater before you release.

What Causes Burn-Through on Thin Sheet Metal?

Burn-through happens when the energy going in is more than the sheet can handle. The classic case is a 0.8-1.5mm stainless sheet welded at 1500-2000W with a slow hand — the laser blows a hole before the operator can move. It is not a sign of a bad machine. It is a sign the power, speed, and focus are not matched to the thickness.

For thin sheet, drop the power and speed up the travel. A 1000-1500W machine on 1mm stainless should run around 1200W with a steady, fairly quick pass. If the machine has pulse mode, use it — pulsing puts less average heat into thin material and gives the pool a moment to solidify between pulses. Keep the focal point slightly above the surface so the beam spreads a little instead of stabbing a deep keyhole into thin metal.

Why Is My Laser Weld Black or Gray?

A black or gray seam means oxidation. The gas is not covering the molten pool, so oxygen reaches the metal while it is hot and forms oxide scale. This is the defect operators notice first, and it is also the easiest to fix.

Check the gas flow first — most handheld units need 15-20 L/min of argon (or the mix in your manual). Then check the hose and fittings for leaks. Then look at nozzle distance: if the gun is held more than 15mm off the part, the gas disperses before it reaches the pool. We tell customers to keep the nozzle 10-15mm away and to angle the gun only slightly in the direction of travel. A clean, silvery bead is the sign the shield is working.

How Do You Stop Spatter and Rough Beads?

Spatter — small drops of metal thrown around the seam — usually comes from three things: a dirty surface, power that is higher than the material needs, or an unstable wire feed. Clean the metal, drop the power 10-15%, and confirm the wire feeds smoothly without birdnesting or burnback.

Wire feeding problems are rarely the motor. Wrong liner for the wire diameter, wrong tension, or a kinked hose cause most of them. Match the liner to the wire, keep tension light enough that the wire does not slip but not so heavy it crushes, and replace the contact tip when the hole wears oval.

A rough, uneven bead with undercut along the edges usually means the operator is moving too fast or holding the gun at too steep an angle. Slow the travel, hold the gun closer to upright, and let the filler wire wet into the edges. On a handheld laser welding machine, a small wobble setting widens the bead and helps it wet out evenly — most operators find a middle oscillation setting produces the cleanest cosmetic seam.

In production, spatter and porosity are the two laser welding defects that cost shops the most rework time, because both are easy to ignore on the first pass and expensive to fix after the part moves down the line. Catch them early on a test coupon and the settings fix takes minutes.

Parameter Settings That Prevent Defects

Every material and thickness needs its own starting point. These are the settings we ship with our machines and use in our own training shop. Start here, then adjust in small steps — change one parameter at a time and write down what works.

Keep a settings card near the machine. The difference between a clean seam and a defect is often one parameter — 100W of power or 3 L/min of gas. Operators who log what works on each job stop repeating the same porosity and burn-through mistakes within the first week, and most laser welding defects disappear once the root cause is on that card.

How to Prevent Laser Welding Defects Before They Happen

Prevention is faster than rework. Before every job, run this short checklist:

• Clean the joint. Acetone wipe, dry, no oil or rust within 20mm of the seam.
• Check gas. Flow at 15-20 L/min, no leaks, nozzle 10-15mm from the part.
• Match the wire. ER308L/ER316L for stainless, ER70S-6 for carbon steel, ER4043/ER5356 for aluminum.
• Set power for thickness. Thin sheet = lower power + pulse; thick plate = higher power + slower travel.
• Practice on scrap first. Ten minutes on offcuts beats redoing a customer part.
• Keep the cooling loop healthy. An overheating source changes beam behavior and shows up as inconsistent seams. Follow the daily and weekly maintenance checklist.

The handheld laser welding market keeps pulling new shops in — global market figures put it around USD 1.05 billion in 2025, heading toward USD 2.31 billion by 2034 at a 9.2% compound annual growth rate (Dataintelo), with other analysts tracking the handheld system segment from USD 612.5 million in 2025 to USD 1.1 billion by 2032 at 8.75% (PW Consulting). Most of those new operators come from TIG or MIG, and they all hit the same defect learning curve. The good news: the defects are predictable, the causes are known, and the fixes are settings you can write on a card.

At FANY LASER we build handheld laser welding machines from 1000W to 3000W with Raycus or MAX sources, water cooling, wobble heads, and automatic wire feeders. Every machine ships with a parameter sheet for stainless, carbon steel, and aluminum, and our engineers walk new operators through the settings over a live video call. If a defect shows up in your first week, send us a photo of the seam and the settings you used — we will tell you which parameter to change. Contact our team for the parameter sheet or a machine quote.

Written by the FANY LASER engineering team. Follow us on LinkedIn for the latest laser industry updates.