Sizing a Laser Weld: Power Density, Spot Size and Travel Speed

The first question about a laser welding job is almost always about watts, and watts on their own say very little. A thousand watts focused into a spot a fifth of a millimetre across and the same thousand watts spread over a millimetre are two different processes with different penetration, different gap tolerance and different spatter. What decides a weld is power per unit area at the workpiece, and how long that area is held over the joint.

This article sets out the three variables we actually set when a job is quoted, the reason a bigger source is not automatically the answer, and what we record on the bench before a power figure goes into a quotation.

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A platform-type laser welding machine in our workshop, set up with an articulated arm over a fixture table.

Watts Are Not Power Density

The spot that reaches the workpiece is an image of the fibre core, scaled by the optics. A two-hundred-micron fibre through a two-hundred-millimetre collimator and a two-hundred-millimetre focusing lens delivers a spot close to the core diameter; change the focusing lens to a shorter focal length and the spot shrinks, and the same power now sits in a quarter of the area. Power density goes up four times without the source changing at all.

This is why two machines rated identically can differ on the same part. It is also why we ask for the joint type, the material and the gap before quoting a power figure: a butt joint closed up tightly, a lap joint with a controlled gap and a fillet on a thick section are three different optical setups, and the wattage follows from the setup rather than the other way round.

The fibre core is the other half of that choice. A fifty-micron core through the same optics gives a spot a quarter of the diameter and four times the density, which suits thin sheet and fine seams and leaves almost no tolerance for fit-up. A two-hundred-micron core spreads the same power over a wider spot, welds a gappier joint, and needs more total power to reach the same depth. Swapping cores is not a workshop adjustment: it changes the optics, the programme and often the head, which is why the core belongs in the specification with the job.

The Three Variables, and What Each One Buys

Every laser weld we run is set by power, spot size and travel speed. The interactions are consistent enough to be worth writing down, and the table below is the working summary we use when a joint needs adjusting rather than redesigning.

Variable	What it changes	Raise it and you get	Raise it too far and you get
Power	Energy entering the part	Deeper penetration, faster travel	Spatter, keyhole instability, burnt edges
Spot size (defocus)	Power density and gap tolerance	Wider bead, more tolerant fit-up	Shallow penetration, wide heat-affected zone
Travel speed	Heat input per millimetre	Less distortion, narrower zone	Lack of fusion, humping, incomplete penetration

Speed Converts Power into Heat Input

Line energy is power divided by speed, in joules per millimetre. Two kilowatts at twenty millimetres per second puts a hundred joules into every millimetre of seam; the same two kilowatts at forty millimetres per second puts in fifty. That number, not the wattage, is what predicts distortion on thin sheet and what predicts whether a given thickness can be joined in one pass.

There is a useful trap here. Slowing down seems like the safe way to get more penetration, and up to a point it is. Past that point the keyhole becomes unstable, the pool grows wider instead of deeper, spatter climbs and the heat-affected zone spreads. The weld looks bigger and is not stronger. We see this most often on stainless in the three to four millimetre range, where the answer is usually more power density at higher speed rather than less power at lower speed.

Aluminium behaves differently again. Its reflectivity at the welding wavelength means a share of the power never enters the part, and its thermal conductivity pulls heat away from the joint faster than steel does. The practical consequence is that a source sized for four millimetres of stainless will not do four millimetres of aluminium, and a quotation that carries a wattage across materials without new trials is one worth questioning.

Penetration Is a Shape, Not a Number

A penetration figure in a quotation is a statement about a section, and the section is what should be asked for. A deep narrow profile and a shallow wide one can reach the same depth and behave completely differently in fatigue and in distortion. On a single pass the shape is largely set by power density: a tight spot with high density drills down, a defocused spot spreads heat sideways.

Where a section is required, we cut a coupon and measure it. A photograph of the surface tells you whether the process was stable; it does not tell you how deep the fusion went, and the two answers come from different checks.

The photograph below is a butt joint from a customer trial in stainless sheet. What makes it acceptable is not that it is visible from the top, but that the bead is even along the whole length, with no step where the speed changed and no undercut at the edges. An even bead at a given speed is the sign that the power was matched to the thickness.

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A butt joint in stainless sheet from a customer trial. The bead is narrow and even, which is what a correctly sized single pass looks like on this thickness.

Focus Position Belongs in the Setup, Not in the Operator Hands

Focus position is the last variable and the easiest to lose. A focus set at the surface, above it or below it changes the spot size and the penetration without touching any setting on the screen, and it drifts when a nozzle is changed, a lens is cleaned or a fixture is re-shimmed. On a production cell we set focus with a gauge and record it on the setup sheet, and the standoff from the workpiece is treated as a fixture dimension rather than an operator preference.

The same discipline applies to the parts of the process that get blamed on power. Contamination on the protective lens absorbs energy and shows up as a weld that suddenly needs more current; a shielding gas flow that is too high pulls air into the plume and produces porosity that looks like a power problem; a gap that opened because a clamp was not seated produces lack of fusion that no wattage will fix.

What We Record Before Quoting a Power Figure

Our bench record for a laser job is short, and it exists so that the quotation can be defended later. It has five lines: material and thickness as received, joint type and gap measured on the real part, power and spot size, travel speed, and the result of a section or a peel test. When a customer asks why a machine was specified at a particular wattage, that record is the answer.

It also protects against the most expensive mistake in this business, which is quoting from a drawing. A drawing says three millimetres and a butt joint; the part that arrives is three millimetres with a two-tenths gap and mill scale on one face. The power that would have worked on the drawing does not work on the part, and the difference is found in the first week of commissioning rather than on our bench.

Conclusion

Sizing a laser is not a matter of buying more watts than the job needs. It is a matter of putting a known power into a known spot at a known speed, and then holding those three constant while the real parts arrive. A smaller source set up properly will outweld a larger one that was specified from a drawing.

Requirements vary by destination market. Always confirm against the current edition of the applicable standard and with your certification body.

PDKJ continues to provide customized laser welding and resistance welding solutions based on different materials, workpiece structures, welding requirements, and production applications.

Sizing A Laser Weld: Power Density, Spot Size And Travel Speed