Dark Marking on Bare Aluminium with Small Line Interval Spacing

userfour · January 5, 2024, 7:20am

Hi, can anyone help me understand why very small line interval spacing creates dark marks on bare aluminum (as opposed to the usual lighter marks created by larger line interval spacing)?

I’m working with a Triumph 30W fibre laser, 1064nm, ~0.08mm dot size. While trying to find good settings for dark markings, I ran the following tests:

In the image above, text was engraved with 0.01mm line interval. It has a very white marking compared to the base metal.

I don’t understand why using a tiny line interval creates a dark marking. At 0.0015mm, the laser dot passes over any given point of the material ~53 times before advancing, vs ~8 times at 0.01mm.

Does small line interval create some kind of “structural colour” effect like butterfly wings?
Or is it a chemical process where the laser blasts material off the base metal, oxidizes it, and then immediately bakes it back onto the base metal in the next pass?

The test above seems to indicate there may be some kind of speed/frequency sweet spot for optimal dark marking.

I’m particularly interested in figuring out the mechanism of dark marking formation, because I’m hoping it can be applied to raster image engraving.
At 220 DPI (the finest resolution I can engrave an image at), the laser marking is light compared to the base metal.
Consequently, I’m currently looking at dithering images onto dark anodized/powder coated aluminum, to maximize contrast.

It would be preferable, however, if I could dither raster images with dark markings on bare metal. This way I can achieve good contrast, while avoiding the excess steps/work of anodization/powder coating.

I’m not sure how this could be accomplished. Perhaps if the laser rapidly pulsed each “pixel” of the dither ~53 times? …

Any insight/advice about the mechanism of dark markings on aluminium is appreciated!

jkwilborn · January 5, 2024, 1:10pm

You are oxidizing or annealing (depending on who you talk to) the surface. You can’t lase in color, so changing the way the surface layer is made.

Some light passes through the top layer and other frequencies are blocked. When the light reflects off the metal it mixes with the other light and produces an addition/subtraction of the frequencies. This causes what we view as color.

If you can feel the image, it’s engraving, not creating an oxidation layer… these cannot be felt with your fingers when it’s done properly.

Here is a dog tag done on thin stainless steel… Maybe some of the settings will be useful for you. As the thickness changes, so do the settings… As you add heat from the laser, the metal sinks it away from the surface where it’s applied…

I have an M60 MOPA…

These are dollar store stainless steel kitchen knives.

Might check out Laser Tools → Materials Test

It’s much quicker to setup and test different settings…

Good luck

userfour · January 8, 2024, 12:11am

Hi Jack, thanks for your response.
It raises more questions for me though.
If the dark marking is created by oxidizing/annealing the metal, why does line interval spacing seem to be the single parameter that causes it to occur?

If my goal was to oxidize/anneal the metal, I would try dumping as much heat into the metal as possible by running at high power, and low speed. But I don’t understand why a tight line interval would make any difference.

With respect to the goal of annealing the surface, for any given combination of line interval, power, and speed, there should be other equivalent combinations of (larger) line interval, (higher or similar) power, and (slower) speed. Through experimentation however, I have not been able to discover these other equivalent combinations.

Do you have any idea why tiny line interval in particular seems to be the magic parameter for achieving a dark marking?

Also I am very envious of your MOPA laser

jkwilborn · January 8, 2024, 12:59am

I’m not a wizard with these either…

In simple terms you want the surface to heat… Metal conducts heat away very fast, how fast depends on the metal. Copper is one of the most conductive, but all metals by nature are usually good conductors.

You don’t want to heat through the metal. That’s a problem with thin material, it usually seems to warp from the heat. As the metal thickens the same setting used on thinner metal don’t seem to work as expected.

Russ Sadler explained it to me and I use his data the best I can, that pulses mm is a good measure … along with the ratio of on to off times is also a factor.

I have a spread sheet Russ sent me when I purchased mine that he made for someone else with the same model of machine, that relates frequency to pulses/mm and on/off ratio.

I will use crazy values, but I think you can follow… if the laser kerf it 1mm then an interval of 0.10mm will pass over the same area ~10 times. This dramatically changes how much heat is delivered to that piece of metal. During the operation, how much of this heat is conducted away from the surface also effects the results.

Does this help at all?

I’m still wondering about a lot of things, especially when something should work and it doesn’t…

Although my camera has a PHD (Push Here Dummy), I don’t have any real education on the low level operation of these when it’s applied to metals.

You can have one too, just take a 2nd out on your home

It’s probably my last big purchase, spousal unit will retire in a couple of years and I hope to have it paid off by then…

Thanks I paid enough to make you envious and still paying it off…

Good luck

maggymagster · April 23, 2024, 6:15am

Hey I’m new in the fiber laser game and i try to engrave black on bare aluminium without any powder or spray. My laser is a JPT Mopa 60W.

So, i tried similar settings like above: 1000mm/s, 50% Power (it’s then 30W) and 70%, 32 KHz. But the engraving is really light and has no deep. And it takes a long time to engrave.

I was asking myself, what should i set in the “Q Puls Width” field?

Maybe someone has the same fiber laser and can help?

Best regards, Mag

maggymagster · April 23, 2024, 6:38am

Hey I’m new in the fiber laser game My laser is a JPT Mopa 60W

I try to engrave black color on bare aluminium without any powder or spray.

So, i tried settings like this: 1000mm/s, 50% Power and 70%, 32 KHz. But the engraving is really light and has no deep. And it takes very long to engrave.

What i’m doing wrong?
… and i was asking myself, what should i set in the “Q Puls Width” field?

Best regards,
Mag

sutick · April 23, 2024, 12:33pm

You will never achieve a dark black color on aluminum. The best you’ll ever get is a dark grey color. Best advice is to test test test. Each and every fiber laser is a bit different and takes testing. That’s what the Material Test was built for in LIghtBurn:

jkwilborn · April 23, 2024, 3:24pm

This is from my library… It originally came from Laser Everything site. I suggest, if you haven’t, peruse their site for a free copy… it’s a good baseline. The library they give out is for their 30W, but there is software on the site to allow you to change it to be more relative to a 60W.

Most of what I do is stainless or regular steel …

Q-pulse is how long the beam lasts… Here is a graph of the power out for different values of the Q-pulse. This is from my 60W JPT M7 MOPA manual.

The longer the q-pulse, the more heat you will apply to the material… yours is, what I’d consider, as too short for this … I usually run in the 200 to 400 width doing this… I want to put heat into the material, so a longer q-pulse.

Do you want to engrave into the metal or mark it by creating an anneal/oxidation layer to make it appear black?

This mark can’t be felt with your finger and it’s a marking not an engraving… these photos are on very low cost stainless steel kitchen knives.

Might want to try laser tools → material test to find the optimum settings… This dog tag is from the materials test…

Please don’t double post. It is difficult and time consuming for us and takes that time away from others… it’s also hard on people trying to find an answer.

60W JPT MOPA - Dark Marking on Bare Aluminium

Maybe @JohnJohn can merge them into the link post.

Good luck

maggymagster · April 24, 2024, 5:54am

Thank you very much @jkwilborn for explaining and helping.
I will check the site and the settings.

And sorry for the double post. I will delete the other one.

Best regards,
Mag