Streaking on Anodized Extrusions

Introduction

Streaking defect is one of the common defects in anodized profiles made of 6xxx series aluminum alloys, for example, alloys 6060 and 6063. 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. 1.

Fig. 1 – Streak defects in an anodized extrusion with arrows indicating streak bands [1]

Causes of Streaking Defects

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

Ingot surface metal
Press welds
Incorrect die design

Metal from the surface of the ingot

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.

Fig. 2 – Inverse Segregation Zone (ISZ) or “billet skin” [2]

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:

higher quality ingots with a minimum thickness of the segregation zone and
increased thickness of the press residue.

Fig. 3 – [2]

Fig. 4 – [2]

Fig. 5 [3]

Die design

More difficult is the third reason, associated with matrix design flaws.

In this case, the stripes appear due to strong differences in plastic deformation or its rate in different cross-sectional zones of profiles with a complex or hollow section (Fig.. ).
As a result, metal zones arise with sharply differing microstructure characteristics, such as:
– grain size
– grain orientation
– size and number of secondary phases.
In alkaline pickling, longitudinal welds are not uniformly etched and are therefore almost always visible.. This can occur even without the participation of contaminated metal from the surface layer..
One of the reasons for the excessive inhomogeneity of the metal flow may be the non-optimal design of the matrix, including:
– in size, the shape or arrangement of the welding chambers (Fig.. )
– configuration of the length of the working belts (Fig.. ).

Fig. 6 – Some examples of die streaking and typical areas where it might occur [1]

Fig. 7 [2]

Fig. 8 – Sharp changes in bearing may cause streaks, due to uneven flow of the
metal or to inadequate filling of the die opening. Variations in bearing
lengths at the junction points must be properly blended to prevent streaking.
Three different types of blending processes [3]

Streaking optics

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.

Fig. 9 – Schematic diagram showing specular reflection of light from a rough surface [1]

Fig. 10 – Relationship between arithmetical mean roughness (Ra) and glossiness (Gs) [1]

Matt etching

Target alkaline etching of aluminum before anodizing – get a homogeneous matte surface. Etching changes the microstructure and optical properties of the surface by creating additional surface imperfections:

pit etching
grain boundary grooves
grain etching steps.

Iron content

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.

Fig. 11 – Morphology of (a) intermetallics (arrowed) in a metallographically prepared anodized
surface and (b) etch pits (arrowed) in an anodized section [1]

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.

Fig. 12 – Morphology of grain boundary grooves in an anodized extrusion with a bright streak in
(a) streaked region (LOM), (b) normal region (LOM) [1]

Stepped grains

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.

Fig. 13 – LOM image of cross-section of an anodized extrusion showing surface defects
due to a pronounced attack on a particular crystal plane
with arrows show the grain etching steps [1]

Conclusion

The origin of stripe defects on the surface of anodized profiles is due to the difference in intensity and diffuse reflection of light compared to the surrounding material due to the non-uniform distribution of surface defects..
Surface imperfections, consisting of etch pits, grooves along grain boundaries and grain etching steps, created on the extrusion surface during alkaline etching. Etching pits are formed either by dissolving the aluminum matrix, surrounding iron-rich intermetallic particles, or by direct dissolution of magnesium silicide particles.
The size, the number and distribution of these intermetallic compounds determine the etch pit morphology. The predominant chemical action on the grain boundaries leads to the formation of grooves along the grain boundaries.
Grain size and grain boundary characteristics have a great influence on the depth of the grooves. Moreover, a brightly directed chemical action on certain crystal planes leads to the formation of grain etching steps.
The formation and severity of surface defects depend on the features of the microstructure before etching. An inhomogeneous microstructure is formed due to the localization of deformation, caused by uneven metal flow.
Die hole geometry, as well as the length of the working bearings of the die determine the flow of metal and also affect the formation of bands.
Die design is a key factor in initiating and controlling streaking.

Sources:

X. Zhang et al, Aluminum Extrusion Technology Seminar, 2008.
Influence of Al Microstructure on Hard Anodising Quality – Profile Material / Tom Hauge, Hydro Aluminium // IHAA Symposium, 25th of September 2014, New York
Aluminum Extrusion Technology / P. Saha