One of the big claims often made by sellers of ‘optically compressed’ (*) diode lasers is that they can mark stainless steel, I was sceptical but decided to give it a go recently on my ‘40W’ unit.
I was pleasently surprised, and this is slow, 50mm/min several passes slow for the kitty bowls, but I can’t deny that it works…
I’m not convinced the lid from my cat hair jar is actually stainless, though it looks like it and is non-magnetic. You can see that it heat-buckled badly as it was engraved, even though I used a higher speed than for the rest.
What this is doing is oxidation marking the surface of the metal, not engraving. The effect is in the surface layer and you can just about feel the marks with your fingernails. Very durable, but you are introducing surface defects, and that could be a bad thing for some applications. Also it involves deliberately pointing a laser at a reflective surface… what could possibly go wrong?
It’s clear that marking compounds produce better looking results, without adding surface defects and work on a far broader range of materials, but at a price.
As far as I can tell this works because enough of the blue light is absorbed by the nickel and carbon in the surface layer, it is this being heated, not the iron. So the laser is shock heating the surface, removing the nickel and carbon briefly under the heat, which allows the warmed iron in the material to instantly oxidise, leaving a mark.
The highly focussed (‘optically compressed’) blue light diode lasers are just about powerful to do this at the very centre of their focus, IR lasers don’t get enough absorption for it to work despite their power advantage.
You can also find people using MDS (moly-disulfide) sprays and mustard (yes, the condiment) with various types of laser for this, the goal is to add a very thin layer to the surface which will increase light absorption as the beam hits.
Finally; at a totally different price-point, blue light fibre lasers can actually be pulse-and-power modulated well enough to control the structure of the mark, allowing colour markings:
As soon as I saw that it was oxidizing the iron, I started to wonder about color. (We have christmas ornaments made from the most beautiful examples of blue and straw steel chips collected from the lathe chip tray…)
For anyone who sees this and is interested in experimenting with this using open source control software of fiber lasers, see @Bryce’s Balor software for control:
