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problem with anodize over laser engraving, causing blemish

Stirling

Cast Iron
Joined
Dec 11, 2013
Location
Alberta canada
I have a product being developed made for 6061 t6 that is type 3 hardcoat anodized.
Samples where sent out to anodizing with different tumbling media used, sandblasted, raw machined ect. to see the effects of post processing.

All of the samples came back great with no issues EXCEPT where I had laser engraved some text. The text was engraved using a desktop fiber laser with a approximatly 0.002" deep removal, engraved pre tumble finishing. At these points there is some blotching of the anodyzing, it reminds me if when the acid is not rinsed well and the color does not take in spots. but it is only at the laser sights.
both tumbled and raw machined samples xibit this same issue at the laser text on multiple surfaces.
my initial thoughts where surface tension held some acid and it was not washed well, but no other features have caused this including stamped text or sand blasted surfaces. so i would rule out surface tension. all other samples with no laser on them came out fantastic.

any input from the commmunity? i love the look of the lasered text where it did not blemish, but most had issues. i can laser post anodizing, but then it is white and "pops" visually. id prefer the more subtle look IMO

both lasered after machining, worse blemished one was also tumbled after laser engraved
IMG_2432.jpg
 

Gordon Heaton

Stainless
Joined
Feb 19, 2007
Location
St. George, Utah
A bit more light on the subject:
Remap.jpg

Doesn't look to me like the laser beam is at fault. The pattern looks more like a 'fog' effect of some kind. I'm absolutely not a laser guy, but is there a mechanism to clear the vaporized material away immediately as the engraving progresses?
 

SAG 180

Titanium
Joined
Sep 17, 2007
Location
Cairns, Qld, Australia
When I chemically etch letters and text in aluminium it leaves a black, very rough texture that can soak up and retain the electrolyte if I don't soak them in water afterwards. I would say the anodizer didn't fully rinse the items and left some sulfuric acid in the letters that interfered with the dye.
 

Garwood

Diamond
Joined
Oct 10, 2009
Location
Oregon
I have had parts rushed through type 3 with blind tapped holes and the area surrounding the holes looked like that from not flushing the holes well enough.

I would bet the laser is leaving a surface porous enough to retain electrolyte and that's causing the issue.
 

Stirling

Cast Iron
Joined
Dec 11, 2013
Location
Alberta canada
i will have to send some more sample coupons ot test these theorys out.

i wonder if the laser is not baking on some type of residue blocking the oxide growth? a better cleaning process before laser or maybe an acid etch before anodizing may be in order
 

Perry Harrington

Titanium
Joined
Oct 7, 2006
Location
Klamath Falls, Oregon
Try putting some tape over the surface prior to engraving, then remove the tape and anodize. If the cause is residue, the tape will provide a superficial protective layer that should keep stuff from settling. Paint would work even better. Then anodize the different samples and see if they behave the same.
 

SAG 180

Titanium
Joined
Sep 17, 2007
Location
Cairns, Qld, Australia
i will have to send some more sample coupons ot test these theorys out.

i wonder if the laser is not baking on some type of residue blocking the oxide growth? a better cleaning process before laser or maybe an acid etch before anodizing may be in order


I strongly suspect your anodizer is not rinsing the parts properly which allows the rough etched surface to carry the sulphuric acid electrolyte into the dye bath where the remaining acid removes/prevents the dye from working. If the worker dipped the parts up and down a few times in the post anodize rinse water rather than a short quick dip it would likely cure the problem.
 

jz79

Stainless
Joined
Mar 21, 2017
as others have stated - rinsing is probably the issue, though for typeIII I would have expected the blotch to be of a darker shade if the oxide is over 30 micron in thickness, that is a consequence of the natural color the oxide becomes when it gets thicker, 50+ micron would be nearly opaque dark greenish/grayish color

that being said - sulfuric acid is quite difficult to rinse with just water, especially if the laser leaves sponge like oxidized mess in the burnt areas, 5% nitric acid dip (~10 seconds) after anodizing then regular rinsing can help, but if the anodizer has removed all nitric acid processing in his line, they might not want to reintroduce it due to costs involved

a bit longer etch time might also help to remove all traces of the oxides left by laser, though I've processed some laser cut pieces that had very stubborn dross left on them even after long etch times...

it will be a trial and error to solve this, and little guarantee that it will work the same way in 3 or 6 months when the next batch of parts are processed

have you considered laser marking the parts after anodizing? perhaps do away with the depth requirement and rely only on the laser etching the dye, to preserve the corrosion resistance properties of the oxide layer
 

neilho

Titanium
Joined
Mar 23, 2006
Location
Vershire, Vermont
The good news is that some of the letters came out just fine, so it is possible to get what you want.

What does the anodizer have to say? They're the experts, or should be.
 








 
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