Stripes on anodized profile: the role of the matrix and optical effects

stripes on anodized aluminum profile

The “bandedness” defect is one of the common defects of anodized profiles from 6xxx series aluminum alloys, for example, alloys 6060 and 6063 according to international and European classification or AD31 alloy according to domestic GOST 4784. In english this defect aluminum anodizing called "streaking». It usually appears after anodizing., which makes it difficult to identify its causes. The essence of this defect is, that narrow stripes are visually visible on the surface of the profile with different contrast of gloss or dullness as between themselves, and compared to the surrounding normal anodized surface. An example of this anodizing defect is shown in the figure..

polochatost-alyuminievyx-profiley-foto

Causes of anodizing defect “foliation”

Known, least, There are three reasons for the streak defect:

  • Ingot surface metal
  • Press Welds
  • Incorrect matrix design

Ingot surface metal

The first reason is that the surface profile of the metal, the so-called segregation zone of the original ingot, enriched intermetallic compounds and oxides.

Press Welds

The second reason is technological, "Press" profile welds, longitudinal and transverse. Cross seams occur during the usual practice of pressing the joints “butt-to-butt”, when the metal of two consecutive workpieces is welded directly in the matrix and the metal of the surface layer from the rear end of the previous workpiece gets into it. Longitudinal seams occur on hollow profiles when connecting metal flows after passing through adjacent matrix ports. When joining two workpieces, contamination material from the rear end of the workpiece can also get into longitudinal seams. To eliminate the first two reasons, apply: 1) better quality ingots with a minimum segregation zone thickness and 2) increased thickness of the press residue.

Incorrect matrix design

More difficult is the third reason, associated with matrix design flaws. In this case, the bands arise due to strong differences in plastic deformation or its speed in different zones of section profiles with a complex or hollow section. As a result, metal zones arise with sharply differing microstructure characteristics, such as grain size, grain orientation, as well as the size and number of precipitates of the secondary phases. Moreover, longitudinal joints can also be etched non-uniformly and can be seen even without contaminated metal from the surface layer. This may again be due to inadequate matrix design in size., shape or arrangement of welding chambers.

Anodic defect optics “foliation”

Optical entity banding It is, that different bands have a different degree of gloss or dullness. Gloss and dullness of the surface depend on its reflective properties., that, first of all, depend on microtopography of the surface. This topography is determined by various surface imperfections., which are formed mainly by etching.

Often the surface of defective strips has a coarser structure, than a normal surface and therefore looks more matte, so its surface imperfections increase the diffusion part of the reflected light. at anodizing aluminum profile an oxide layer is formed on the surface of the starting metal. The optical properties of the surface after anodizing mainly depend on the topography of the surface of the starting metal and little depends on the oxide film itself, because it is transparent.

Matte alkaline etching of aluminum profiles

The purpose of surface etching before anodizing – get a homogeneous matte surface. Etching changes the microstructure and optical properties of the surface by creating additional surface imperfections: etching pits, grooves of grain boundaries and grain etching steps.

The role of iron in aluminum alloy

The size and distribution of the etching pits depends mainly on the size and distribution in the surface layer of intermetallic phases: primary particles Al3Fe, a-ALFEU и b-ALFEU and secondary phase Mg2Si. Iron particles have a higher electrochemical potential, than the aluminum surrounding them, therefore, they do not dissolve, and the aluminum around them. With a sufficiently long etching, these particles completely fall out and the pit size is often larger, than their original size, sometimes up 10 microns in diameter. Therefore, the iron content in the alloy has a significant effect on the optical appearance of the surface after etching.. in front of, particles Mg2Si act as anodes, which leads to their complete etching with the formation of pits, repeating particle shape. Therefore, the small particle size β-Mg2Si and their high distribution density make a significant contribution to the formation of a matte surface, therein, Primary fossa Fe-particles. Such a high density of fine particles Mg2Si achieved by effective hardening and artificial aging of profiles.

The role of grain boundaries

Another important parameter of the surface microstructure, affecting its optical properties, are grain boundary grooves. Grain boundaries are generally more susceptible to alkaline etching.. However, is, that inside on the “banded” surface of the groove the grain boundaries are barely visible, whereas on a normal surface - the grain boundaries are deep and visible very clearly. This "blur" of the grooves of the grain boundaries is considered one of the reasons, why defective streaks look lighter: small grooves reduce the diffusion fraction of reflected light.

Grain steps

Another element of the surface microstructure, affecting its optical properties, are so calleddull grain etching. The thing is, that when pressing aluminum alloys, some preferred grain orientations are formed, that is, a certain grain texture. established, that in defective strips most grains are oriented in the direction of pressing, and on a normal surface the grains are randomly oriented. The difference in texture of defective and normal areas leads to different light reflection intensities.

The role of the matrix design

established, that the features of the surface microstructure of the profiles described above arise during the pressing of complex and, in addition, hollow profiles, when complex metal flows arise with large deformations and strain rates. This is precisely what leads to the appearance of an inhomogeneous surface microstructure of the surface of the profiles, which causes the “bandedness” defect.

The source: X. Zhang et al, Aluminum Extrusion Technology Seminar, 2008.