Laser engraving metal: what diode, fibre and MOPA lasers can really do
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Metal is the material where expectations differ the most. A diode laser can label many metal parts, but can hardly engrave bare metal directly. Fibre and infrared lasers exist for that. And cutting metal is a different league altogether.
Why bare metal is difficult
Whether a laser works depends on whether the material absorbs its wavelength. Bare metal reflects a large part of the light and conducts heat away quickly.
| Laser type | Wavelength | On bare metal |
|---|---|---|
| Diode | 450 to 455 nm | only a weak mark, usually with an aid |
| CO₂ | 10,600 nm | strongly reflected, no mark without a coating |
| Fibre and infrared | 1,064 nm | absorbed well, marks and engraves directly |
That is why, with a diode or CO₂ laser, you mainly work on coatings on the metal. With fibre or infrared, you work on the metal itself. An overview of all laser types is given in Laser types compared.
What a diode laser can do
Anodised aluminium: The anodised layer is dyed. The laser removes the colour, and a light, silvery white mark appears underneath. This works well with a diode, especially on dark colours such as black, dark blue or red. Typical uses are aluminium business cards, torches and knife handles.
Painted and powder-coated metal: The laser burns away the coating and the bare metal underneath comes to light. Powder-coated mugs and bottles are a classic. Know your coating: some paints contain substances that are hazardous when burned.
Stainless steel with marking spray: A special laser marking spray is sprayed on thinly. The laser fuses the layer to the surface, producing a black, high-contrast mark. The rest is washed off after lasering. This works with diode and CO₂. It is not engraving but an application onto the metal.
Stainless steel directly: According to manufacturers, stronger diodes can produce a dark annealing mark directly on stainless steel, at slow speed and often with several passes. Results vary greatly depending on the alloy and surface. Test this before promising it to customers.
Cutting thin foil: Some diode lasers cut very thin stainless steel sheet. With the xTool S1 40W, the manufacturer states up to 0.1 mm, with the 20 W version 0.04 mm. That is foil, not sheet metal.
Good to know: many diode lasers offer an optional infrared module with 1,064 nm and 1.2 to 3 W. With it you mark bare metal directly, but much more slowly and shallower than with a fibre laser.
Fibre and infrared lasers for metal marking
A fibre laser produces light at 1,064 nm in a glass fibre doped with ytterbium. Metals absorb this wavelength well. Fibre lasers almost always work as galvo machines: mirrors steer the beam very quickly, and the work area is small as a result.
What a fibre laser can do on metal:
- Marking: The surface is darkened by annealing or lightened by roughening, without removing any significant material. Ideal for serial numbers, logos, QR codes.
- Deep engraving: With many passes it removes metal. This works on stainless steel, brass, aluminium, silver and gold.
- Anodising and coatings: cleaner and faster than with a diode.
Examples from the machine overview are the ComMarker B4 20W as a pure fibre laser and combination machines such as the xTool F series, which combine fibre and diode. More on designs is in Gantry or galvo.
Colour on stainless steel with MOPA
A MOPA fibre laser lets you adjust the pulse duration. According to the marking specialist Pryor, it is fixed at about 100 to 120 ns on standard fibre lasers (Q-switch), while with MOPA sources it can be chosen from roughly 4 to 200 ns. Higher pulse frequencies come on top of that.
This control allows colour marking on stainless steel. The laser heats the surface in a targeted way, and a thin, transparent oxide layer forms. Light is reflected at the top of this layer and at the metal beneath it, and the two portions overlap. Depending on the layer thickness, you see gold, bronze, blue, violet or green. No material is removed in the process.
What you should know:
- The colours depend heavily on the alloy, surface and machine. A colour grid is a must, and it only applies to exactly that material.
- Manufacturers usually supply colour templates in their software. They are a starting point, not a result.
- MOPA can also produce deep black marks on anodised aluminium, which is harder to achieve with standard fibres.
One machine with a MOPA source is, for example, the xTool F2 Ultra.
Cutting metal is for professionals
Hobby machines cut metal only in very thin thicknesses at best. A fibre engraver such as the F2 Ultra manages up to 2 mm of brass and stainless steel according to the manufacturer, and that slowly in many passes. For sheets in the millimetre range in a reasonable time, you need industrial fibre laser cutting systems with outputs in the kilowatt range, cutting gas such as oxygen or nitrogen and a cabin with extraction. That is a different class of machine with different prices and different safety requirements.
If you need metal parts, an online laser cutting service is usually the cheaper route.
Starting values
Metal reacts very differently, and values differ even more between machines than with wood. As a rough guide for your first grid:
| Task | Diode 10 to 40 W | Fibre 20 to 30 W |
|---|---|---|
| Anodised aluminium, remove colour | 80 to 100 %, 200 to 400 mm/s | low power, high speed, manufacturer profile as a start |
| Stainless steel with marking spray | high power, slow, follow the spray manufacturer’s instructions | not needed |
| Stainless steel, dark mark | slow, several passes, test | 60 to 80 %, 300 to 600 mm/s |
| Stainless steel, deep engraving | not possible | 90 to 100 %, 50 to 100 mm/s, 3 to 5 passes |
Create a test grid for every new metal. How to do that is explained in Material test.
Galvanised steel: only with extraction
Galvanised sheets have a thin zinc layer as rust protection. A fibre laser marks them directly, without primer. In the process, though, part of the zinc vaporises and combines with oxygen to form zinc oxide. When welding galvanised sheets, zinc oxide is a main component of the fume, and laser engraving produces it in the same way.
Anyone who breathes in zinc oxide fume can get metal fume fever. The symptoms resemble flu, start a few hours later and usually subside within one to two days. Repeated inhalation can permanently damage the lungs.
In practice, that means:
- Extraction right at the point of origin. In occupational safety this is regarded as the most effective measure. Exhaust air to the outside, or a filter that the manufacturer explicitly approves for metal fume.
- Prefer an enclosed machine. An enclosure keeps the fume and the invisible laser light together. An open fibre laser is only an option with its own enclosure, extraction and suitable safety glasses, and not in a living space.
- If the extraction is not enough, wear respiratory protection with a suitable particle filter. This is the rule for work on galvanised workpieces in commercial settings.
- Small area, little depth. A mark is enough for a logo or a QR code. Deep engravings remove more zinc and produce more fume.
Anyone who regularly lasers galvanised parts in production should agree the protective measures with the Berufsgenossenschaft (employers’ liability insurance association) or an occupational safety specialist.
Safety
Light at 1,064 nm is invisible. You cannot see where a reflection goes. Safety glasses for diode lasers do not automatically protect against infrared. They must have the right optical density for 1,064 nm, which is stated on the glasses. This applies especially to open machines with an IR module or fibre source. Metal also reflects more strongly than wood. More in Laser safety.
Continue with: Laser types compared and Laser safety.
Sources
- DocCheck Flexikon: Metal fume fever
- KomNet NRW: Welding galvanised sheets, protective measures (with reference to DGUV Information 209-011)