Streaking defects of anodised aluminium
Discover the different types of streaking defects in anodised aluminium and their impact on extruded profiles. Understand how non-uniform metallurgical macrostructure and microstructure influence appearance of anodised aluminium profiles.
Streaking defects of extruded aluminium
Streaking is one of the major cause of non-uniformity on anodised extrusion aluminium profiles. Streaking is related to non-uniform metallurgical structure on the defect aluminium surface. This non-uniform metallurgical structure results in variations in the etching response of the material. These variations are appeared, for example, as a difference in brightness or reflectivity on various areas of the aluminum product (Figure 1).
Figure 1 – Streaking defects in an anodised extrusion with arrows indicating anodising streaking defects [1]
Categories of streaking defects
The following categories of streaking defects of anodised aluminium are distinguished [1-4]:
- Billet structure streaks.
Occur as a result of surface segregation or a surface chill zone of the billet, and also of melal quality of billet. - Extrusion process streaks.
Are due to inproper extrusion process parameters. - Die design streaks.
Are formed by localized differences in strain or strain rate, which occur in different locations of the extruded profole with complex or hollow sections.
Exhibit a different microstructure such as grain size, grain orientation, and precipitates compared with the surrounding material.
Each of these categories can include several types of streaking defects, which are listed below.
Billet structure streaking
Segregational
Segregational streaking manifests itself as non-uniform metallurgical macrostructure due the entrainment of extrusion ingot inverse segregation into extruded profile.
Intermetallic particles
Clasters of intermetallic partices can produce streaking defects. This may be due to poor quality of grain refiner rod or inadequate filterng of the molten metal [3].
Extrusion process streaking
Shell-zone
Non-uniform metallurgical macrostructure due the entrainment of the ingot oxide shell-zone into the extruded profile. The analogue of segregational streaking.
Back-end
Non-uniform metallurgical macrostructure due the entrainment of segregate or oxide from the billet back into the extrudate.
Die lines
Die lines are a isolated line or band of lines in the extrusion direction. Die lines are usually the grooves in the extruded surface caused by damage to the die bearing or the entrapment of dirt, inclusions, steel, e.g. particles of nitrided bearing and so on [2].
Depending on the depth, die lines might still appear after alcaline etching. There may be some metallurgical strucrure variations associated with groups of die lines that generates streaks when the extruded profile is anodized [2, 3].
Longitudinal weld
Longitudinal weld is a junction between the metal flowing through adjacent ports of hollow or semi-hollow die. They is often visible after anodising. These weld can be worse by oxides and intermetallics, for example, associated with back-end defect.
Transverse weld
The transverse weld anodising streaking is caused by the ivetable oxide layer that occures at the junction between the rest of just extruded billet in the die and front of the next billet. This defect have the form of a dark V-shaped streak with the tip of the V pointing towards the front end of extruded profjle [2].
Carbon marking
Longitudinal black marks in the extrusion direction can be visible on one face of an extruded profile. These called carbon marks and are due to the extruded profile rubbing on the carbon blocks of the run-out table. This rubbing results the localised cooling of profile and the precipitation of large Mg2Si particles. The metal in this area reacts differently in etchingto produce a streak.
Die design streaking
Die design streaking is also called “structural streaking”. Die design streaking is also called “structural streaking”.
Structural streaking results from:
- variations in grain size and orientation
- the precipatate microstructure in the final aluminium extruded product.
This type of streaking is usually assocated with a change in in the extruded profile thickness. This often leads to a change in grain structure and texture in the region of a change in the extruded profile thickness. See the example of structural streaking defects of anodised extruded profile in Figure 1.
Optics of streaking defects
The optical essence of most streaking defects is that different streaks have different degrees of gloss. The gloss of a surface depends on its reflective properties, which primarily depend on the macro- and microtopography of the surface. This topography is determined by various surface imperfections, which are formed during casting, extruding, heat treating and etching.
Often the surface of defective streaks has a rougher structure than that of a normal surface and therefore appears more matte, since its surface imperfections increase the diffusion portion of the reflected light. When anodizing an aluminum profile, an oxide layer is formed on the surface of the original metal. The optical properties of the surface after anodisation mainly depend on the surface topography of the original metal and depend little on the oxide film itself, because it is transparent [4].
Fig. 2 – Schematic diagram showing specular reflection of light from a rough surface [1]
Fig. 3 – Relationship between arithmetical mean roughness (Ra) and glossiness (Gs) [1]
Sources:
- Mechanisms of Streaking on Anodized 6xxx Series Extrusions / X. Zhang, H. Zhu, A.K. Dahle, M.J. Couper – Proceedings of the 9th International Aluminum Extrusion Technology Seminar, Chicago, 2008.
- Defect Analysis in Anodizing / Ellard, B.R., Proceedings of the 7th International Aluminum Extrusion Technology Seminar, Chicago, 2000.
- The Identification and Prevention of Defects on Anodized Aluminum Parts / T. Short – Metal Finishing Information Service Ltd, 2003.
- Anodizing Defects Catalogue – Qualanod