Why Laser Marks on Anodized Aluminum Are White

The question comes up on nearly every first aluminum job: "Can you mark it black?" On black anodized aluminum, the honest answer is that the laser mark will be white, and that this is the best mark aluminum offers. Understanding why makes the rest of the decisions easy.

What anodize is

Anodizing grows the aluminum's own oxide into a thick, hard, porous layer, a few ten-thousandths of an inch for Type II and a few thousandths for Type III hardcoat. Fresh anodize is clear and the pores are open. A dye soaked into those pores gives the color: black, red, blue, gold, and so on. The pores are then sealed to lock the dye in. Clear anodize skips the dye.

So a black anodized part is bright aluminum under a thin, transparent oxide layer that has been filled with black dye.

What the laser does to it

A 1064 nm fiber laser passes through the clear oxide and is absorbed in the dye and at the metal beneath. Two things happen in the mark area, depending on the parameters:

1. The dye is destroyed. Bleached or vaporized, it no longer colors the oxide, and the layer turns back to clear or frosted white over bright metal.
2. At higher energy, the oxide itself is removed, exposing the bare aluminum underneath, which a laser roughens into a frosted, light-scattering surface.

Either way the mark is light against the surrounding dyed anodize. On black, that is a white to silver legend with the highest contrast of any common laser-marking substrate. On red, blue, or gold anodize, the legend is the same white against the color. The surrounding anodize is untouched, so the part keeps its corrosion protection everywhere except inside the mark.

That is why anodized legend plates and tags are black with white text, and why a black-anodized housing is the usual specification when a Data Matrix needs to read well.

Why clear anodize marks poorly

Clear anodize has no dye to destroy, so the first mechanism does nothing visible. Removing the oxide exposes aluminum that looks much like the clear-anodized aluminum around it: light on light. Contrast is low and inconsistent. A MOPA fiber laser can produce a gray mark on clear anodize by oxidizing the surface under tuned parameters, but it is not black and it varies with the alloy and finish. If a part will be laser-marked, specify a colored anodize.

Why bare aluminum is not much better

Bare aluminum reflects most of the 1064 nm beam, and what it absorbs tends to melt and roughen the surface rather than darken it. The result is a frosted, light-gray mark: visible, but low contrast. A MOPA laser with long pulses and high frequency can push that toward dark gray, which is enough for text at arm's length and sometimes enough for a Data Matrix on a smooth finish, but it is not a black-on-silver mark and should not be promised as one. Alloy, temper, and surface finish all move the result.

How to get a dark mark on aluminum

There are two reliable ways, and both involve a decision upstream of the laser.

Anodize first, mark the anodize. Even a light or clear-to-light-gray anodize marked by the laser gives a white legend, so this does not produce a dark mark either. What it does is give you a color to mark against: black anodize with a white legend reads as high contrast, which is what the drawing usually wants. If a dark mark on a light background is genuinely required, dye the anodize a light color and the white legend will still be the mark.

Engrave and fill. For a true black mark on bare or clear-anodized aluminum, the laser engraves the characters to a shallow depth and the recess is filled with black paint or epoxy. The fill is what is dark; the laser provides a crisp, repeatable recess for it. This adds a step and the fill is a coating that can wear, so it suits nameplates and panels more than fasteners.

A third option is to change substrate. If the drawing needs a dark laser mark with no fill, stainless steel annealed by a MOPA laser produces a black oxide mark with nothing removed and nothing added. Annealing vs etching vs engraving covers what each process does.

Specifying it on the drawing

Two details that save a round trip: say after anodize when the mark is on a part that will be anodized (marking before anodize buries the mark under the new oxide), and send a sample if the anodize is a light dye or undyed hardcoat, because contrast is harder to predict there.

The aluminum marking page has the process notes by alloy and anodize type, and the material marking guide compares aluminum with the other substrates.