There are several kinds of streaking that appear on aluminum profiles. AACOA Inc. has a very good and simple description in their glossary on the various streaking phenomenons.
Streaking can be divided into three categories:
- Billet structure streaks
- Extrusion process streaks
- Die streaks
Extrusion process streaks are usually due to process features or process parameters. A common process streak is found on the welding seam of billet-to-billet extrusions and is often concentrated in the beginning part of the extrusion. The surface defects often appear as white dots.
Die streaks depend on the strain and strain rate of the profile, the temperature in the deformation zone in the emerging extrusion and on the cooling rate afterwards.
All these streak defects occur because of a difference in the intermetallic phases, resulting in a different microstructure and grain sizes. These defects are often not discovered before etching and anodizing of the material because they are related to how the different alloying elements behave during the anodizing process.
The profile in the picture shows a discoloration which appears lighter than the rest of the profile, called bearing streaks or die design streaks.
These streaks are usually found in areas where the profile has different wall thicknesses, which results in an uneven cooling.
A H-profile is an example where these streaks could be found. The area where the vertical aluminum meets the horizontal will give a different cooling rate, which allows the grains to differ in size and orientation. These streaks are normally narrow and uniform in shape.
As mentioned above these defects occur because of the variation in response during etching and anodizing. The major intermetallic phases in the 6xxx series alloys are Mg2Si and primary Fe-rich phases.
During etching the Fe-rich phase will act as a more noble area than the aluminum matrix, which will stimulate the dissolution of the surrounding aluminum. This dissolution can result in deep pits that will have a major impact on the optical appearance of the surface.
The Mg2Si will act the other way around by being less noble that the aluminum matrix resulting in small pits that makes the surface look matt.
This is purely a problem for the extruder and there will not be much to do for the anodizer. Some would suggest to re-anodize the work to decrease the defect or to use a lower current density.
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