Wire-cut bluing of titanium is often mistaken for thermal damage, but the coloration you sometimes see at the edges of a wire-cut titanium surface is actually nothing but anodic oxidation created by the field of electrical current around the wire electrode during wire cutting. This ionized field produces a thin, and transparent oxide film on the top and bottom surfaces adjacent to the wire path. This oxide can be in the atomic range to several hundred nano meters thick. The colors produced in this manner have no pigments, dies or chemicals in them and are known as interference colors.
Interference colors are created when light strikes an oxidized surface. Part of the light is reflected and part of it passes through the oxide film to be reflected off the metal surface beneath it. As the light bounces back and passes through the oxide layer, it is slowed or interfered with, hence its name. This interference creates a color similar to the way a prism bends white light and breaks it into colors depending upon the angle. Every color in the rainbow can be achieved by this effect with the thickness of the oxide film (amount of interference) determining the color. Like anodizing, voltages, current density, water chemistry and pH can all affect the colors produced. The visible oxide colors that WEDM current typically generates are mostly in the blue and red spectrums, being a definite blue along the wire path and often fading to a tinge of red on the outer edges of the discoloration.
Being aware of all this may sound rather obscure, but we actually had to learn about this out of necessity. In a galaxy far, far away, in another lifetime when we had our shop (pre-AC generators and AE technology), we had to explain what the bluing was to many a freaked-out know all aerospace engineer, who upon seeing a WEDM'ed titanium part for the first time, would mistakenly think we had somehow "thermally damaged" the cut surfaces of his part to a depth of 2mm and it was now expensive scrap and started to jump up and down.
<-- Engineer's reaction, Seriously.This may sound amusing today, but if we weren't able to explain bluing to the at-that time to those EDM-paranoid engineers, we would have never received another titanium job from any of them. After this incident, we quickly learned to buff the top and bottom surfaces a bit to camouflage the bluing and eliminate any frantic questions or arguing for another titanium job.
As a side note, this color proves that your material is real titanium, as no other material turns blue after wire-cut. Happy Wire-cutting Titanium, with blue. :)
Titanium Metal & Alloy Supplier - Singapore
